Key concepts in spin tunneling : amorphous ferromagnets for spintronics

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Key concepts in spin tunneling : amorphous ferromagnets for

spintronics

Citation for published version (APA):

Paluskar, P. V. (2008). Key concepts in spin tunneling : amorphous ferromagnets for spintronics. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR635545

DOI:

10.6100/IR635545

Document status and date: Published: 01/01/2008 Document Version:

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Key Concepts in Spin Tunneling

Amorphous Ferromagnets for Spintronics

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prof.dr.ir. H.J.M. Swagten 2 promotor, Techn. Universiteit Eindhoven

Dr.rer.nat. J.T. Kohlhepp copromotor, Techn. Universiteit Eindhoven

prof.dr. R. Coehoorn lid kerncommissie, Techn. Universiteit Eindhoven

en Philips Research Laboratories

dr. C.F.J. Flipse lid kerncommissie, Techn. Universiteit Eindhoven

prof.dr. R.A. de Groot lid kerncommissie, Radboud Universiteit Nijmegen

Dr. J.S. Moodera lid kerncommissie, Massachusetts Inst. of Tech.

The work described in this thesis has been carried out in the group Physics of Nanos-tructures, at the Department of Applied Physics, Eindhoven University of Technol-ogy, the Netherlands.

This research was supported by NanoNed, a national nanotechnology program co-ordinated by the Dutch Ministry of Economic Affairs. Flagship NanoSpintronics. Project number 6474/7152 - 1B1.

The cover shows the k-resolved density of states of fcc Co at the Fermi level. Artists impression by P.V. Paluskar and data from G.A. de Wijs, J.J. Attema and R.A. de Groot (Radboud Universiteit Nijmegen).

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Key Concepts in Spin Tunneling

Amorphous Ferromagnets for Spintronics

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven

op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College

voor Promoties in het openbaar te verdedigen op dinsdag 1 juli 2008 om 16.00 uur

door

Paresh Vijay Paluskar

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prof.dr. B. Koopmans en

prof.dr.ir. H.J.M. Swagten Copromotor:

Dr.rer.nat. J.T. Kohlhepp

CIP- DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Paluskar, Paresh Vijay

Key Concepts in Spin Tunneling : Amorphous Ferromagnets for Spintronics by / Paresh Vijay Paluskar. Eindhoven : Technische Universiteit Eindhoven, 2008. -Proefschrift.

ISBN: 978-90-386-1296-6 NUR 926

Trefwoorden: spinpolarisatie/supergeleiding/tunneljuncties/amorfe ferromagneten Subject Headings: spin polarization/superconductivity/tunnel junctions/amorphous ferromagnets

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my parents Prabha and Vijay,

my wife Sonu,

and my brother Parag

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Chapter 1 Introduction to spin tunneling

Ferromagnets and magnetic tunnel junctions in spintronics

Abstract: In this chapter1 _{we will introduce some relevant aspects of the}

elec-tronic structure of ferromagnets, and how spinelec-tronic devices like MTJs employ this electronic structure for device operation. Then we will introduce a few experiments from which we derive our existing notions about the physics of spin tunneling. No attempt will be made to be complete or exhaustive in this section. Instead, the reader is referred to suitable reviews which embark on such an exhaustive overview. Subsequently, we will talk about a novel ferromagnetic material − CoFeB − which has the potential for advancing the application of spintronic devices. In the last part of the chapter, we will outline this thesis.

1_{A large part of this chapter will appear as a review in the Encyclopedia of Materials Science}

and Technology authored by H.J.M. Swagten and P.V.Paluskar [62].

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1.1 Spintronics in daily life

The fact that electrons have a spins, i.e., an intrinsic magnetic moment, plays an important role in our everyday life. Technologies that use these electron-spins are not completely unknown in the daily life of a common man. One example is the magnetic strip on credit cards, another is a magnetic compass which navigates automobiles. A recent discovery which uses these electron-spins in electronic devices has spurred another wave of technology in the realm of data storage and sensing. It has, in essence, revolutionized the way we carry personal digital information, and therefore, not surprisingly, has been awarded the Nobel prize for Physics in 2007. Indeed, here one refers to the giant magnetoresistance effect (GMR) and the field of research that engenders from it - spintronics / magnetoelectronics. The most popular product that uses this technology is a computer hard-disk where information is read using a GMR sensor. Due to the use of these sensors, the density of information that can be stored on a hard-disk has increased substantially, allowing the emergence of products like the i-POD. This thesis is placed in this field, where the physical effects and devices based on electron-spins are explored.

Let us have a look at the essentials of spin-transport in such devices. A sketch of GMR device is shown in Figure 1.1(a). Here, two ferromagnetic layers (for example, Co or Fe) are separated by a non-magnetic layer (for example, Cu or Cr). Assume that, using an external magnetic field, the magnetization of these two layers can be aligned parallel to each other [see Figure 1.1(a)] or antiparallel to each other [see Figure 1.1(b)]. When a current flows through this trilayer, the electrons which have their spins aligned with the magnetization of the layer experience less scattering events. On the other hand, the electrons with spins pointing opposite to the layer magnetization experience more scattering events. Therefore, in a parallel configura-tion, there are always electrons of one spin type that can easily travel through the trilayer. In Figure 1.1(a), this would be the case with the spin pointing right, which we call a spin-up or majority electron. We call the other electron, with spin pointing left, the spin-down or minority electron. Coming to the antiparallel configuration shown in Figure 1.1(b), one notices that although the spin-up electron manages to reach the top layer, its magnetization is aligned opposite to the local magnetiza-tion. Therefore, this electron too experiences more scattering events. Now, more scattering events implies that the electrons ‘feel’ a higher resistance while travers-ing the trilayer. Since in the antiparallel configuration, both spin-up and spin-down electrons experience more scattering events, the resistance of the trilayer in this con-figuration is high. On the contrary, the parallel concon-figuration allows easy transport of spin-up electrons, and the device resistance is comparatively low. This change in resistance which depends on the relative alignment of the magnetization of the two ferromagnetic layers is called magnetoresistance, and is defined as

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MR = RAP − RP RP

× 100% (1.1)

where P and AP denote parallel and antiparallel configuration. The resistance of such a device is shown in Figure 1.1(c) where MR is plotted as a function of the ap-plied external field. At very high positive or negative fields (in this case, ±30 kA/m), the layers are aligned parallel, the resistance is low and the MR is zero. However, these layers are so engineered that in external fields of 7−10 kA/m, the antiparallel configuration is achieved, and the observed resistance and MR is high.

In 1988−89, two groups (that of Albert Fert and Peter Gr¨unberg) reported the observation of a such a magnetoresistance effect [1, 2]. The change in resistance

observed in [Fe/Cr]n multilayers was almost 50%, which led to the name giant

mag-netoresistance effect. Since then, significant progress has been made in enhancing the observed effect, as well as in understanding the origin of the effect. The reader may refer to extended reviews on this topic [3, 4].

Such spin-dependent electronic transport was subsequently observed in another type of device called a magnetic tunnel junction (MTJ). In this case, the

non-magnetic spacer layer of Figure 1.1(a-b) is replaced by a thin insulator (∼25 ˚A thick).

Given the fact that quantum mechanics allows electrons to tunnel through such a thin insulator, one may imagine that electronic transport from one ferromagnetic layer to the other across such a tunnel barrier would also lead to a magnetoresis-tance effect. In 1995, Moodera et al. [5] and Miyazaki et al. [6] reported such a magnetoresistance effect which is appropriately called tunneling magnetoresistance (TMR). Considering that a decade later TMR effects above 200% have been re-ported, the application potential of such devices has not gone unrecognized. In fact, many technological devices which envision the use of this effect have been proposed, and some are already commercially available.

This thesis investigates the properties and fundamental aspects of electron, and consequently, spin tunneling in such tunnel junctions. In the rest of this chapter, we will briefly review some basic ideas in this field and introduce some existing notions which constitute the basis of our understanding of this effect.

1.2 Basic aspects

1.2.1 Electronic structure of 3d TM FMs

Elemental 3d transition metal (TM) ferromagnets (FM) like Fe, Co, and Ni and alloys derived from them have intrigued humans from time immemorial. From primeval amazement regarding the magnetic compass and its implication that the earth itself was a giant magnet, and the apparent magical power of magnets in at-tracting and sticking to metals like iron, to existing controversies on magnetorecep-tion in animals and birds, and the intriguing field of planetary magnetism, humans

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Figure 1.1: Origin of spin dependent transport. Schematic representation of spin dependent transport and the origin of the GMR effect in magnetic multilayers. (a) parallel configuration: majority electrons (spins aligned with local magnetiza-tion) traverse layers with lower scattering events as compared to minority electrons (spins aligned against local magnetization) (b) antiparallel configuration: each spin species scatters in one of the two ferromagnetic layers. Therefore, the comparative resistance in this antiparallel configuration is higher than the parallel configuration. (c) Example MR curve where the resistance is plotted as a function of the applied field. At large fields, positive or negative, the resistance is low due to the parallel configuration. Closer to zero, the trilayer is engineered to achieve antiparallel con-figuration in one field direction (negative field in this case). In Figure (c), note that although the MR curve is not measured on a GMR stack but a TMR stack, the primary difference is only in the magnitude of the MR effect.

are yet to conquer the mysteries cast by magnetism. However, all throughout, our search for answers has been fervent, to say the least. In this section, we will try to briefly sketch the basic concepts on the question “why metals like Fe, Co and Ni are ferromagnetic?”

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The essential aspects are that electrons have intrinsic spins, and their wave func-tions have different spatial symmetries. These wave funcfunc-tions are allowed to ac-commodate only a certain number of electrons. When placed in a solid, the wave functions form bands which electrons occupy in k -space. In 3d TMs, the d-bands lie close to the Fermi level and may accommodate 10 electrons. Their band widths are in the order of 5 eV; much smaller than the band widths of spherically symmetric de-localized s-bands. Because of this narrow band width which needs to accommodate 10 electrons, the electronic density is high, and the Fermi surface is dominated by contributions from the d-bands. Naturally, this high density of states of 3d-electrons at the Fermi level also greatly influences the electronic and magnetic properties of the solid.

The magnetic properties of 3d TMs are a consequence of the fact that the elec-tronic wave function is required to be antisymmetric, either in its spin or spatial part. This, together with the narrow band width of 3d TMs which allows greater electron density, is the cause of a collective magnetic moment in 3d TM FMs. In order to minimize coulomb repulsion, the electrons tend to couple with their spins parallel, which forces antisymmetric spatial wave functions. This is, in essence, Hund’s first rule for free atoms which renders almost 80% of the periodic table in a high spin-state. In a solid however, electrons become delocalized and the gain in exchange energy which aligns spins parallel must overcome the additional kinetic energy to put the spins in the same spin-band. Therefore, the more the electronic system becomes delocalized, the smaller the chance to display ferromagnetism. For, Fe, Co and Ni, the narrow d-band comes to rescue where the large density of states

(DOS) at the EF satisfies the Stoner criterion for ferromagnetism N(EF) · I > 1,

where I is the Stoner parameter and represents intra-atomic exchange and

correla-tion effects. In other words, for these elements, the DOS [N(EF)] is large enough

for parallel (ferromagnetic) coupling of spins without increasing the kinetic energy of the d-bands considerably. This energy is called the exchange splitting and is typically ∼1 eV. As an example the DOS of Co in a ternary alloy of CoFeB is shown in Figure 1.2 [7]. The resulting spin magnetic moment is given by

ms = (N↑− N↓) µB (1.2)

that is, the difference between the occupation of the spin-up and spin-down bands. Analyzing the contribution of the various types of electrons in 3d TM FMs (different spatial symmetries of the wave function), one finds that the spin moments of the d-electrons contribute ∼90% to the total moment, while their orbital moment is almost completely quenched in a solid. The 4sp electrons carry no orbital moment [8, 9], and their spin moments contribute ∼5% to the total moment.

1.2.2 Electron and spin tunneling

It is well-known that when an insulator is made very thin, of the order of a few nanometer, electrons can tunnel through this thin insulator according to laws of

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-10 -8 -6 -4 -2 0 2 4 -2 -1 0 1 2 -0.10 -0.05 0.00 0.05 0.10 d d d -D O S ( s t a t e s / e V / a t o m ) Energy (eV) s s s -D O S ( s t a t e s / e V / a t o m )

Figure 1.2: Density of states of a ferromagnet. Representative DOS of Co in CoFeB which shows s-DOS and d-DOS. Here the states/eV/atom are plotted as a function of energy and EF is set to zero. The s-DOS is magnified ∼20 times

for comparison with the d-DOS. Please refer to Paluskar et al. [7] or Chapter 4 for details.

quantum mechanics. Regarding the spin of these tunneling electrons, it is assumed to conserve if the electron tunnels elastically. Spin tunneling becomes relevant in the case of magnetic tunnel junctions (MTJs), where the insulator is sandwiched between two ferromagnets, as shown in Figure 1.3. In such a device, the magnitude of the tunneling current depends on the relative orientation of the magnetization of both electrodes. When the magnetization of the two electrodes is aligned parallel, a large current flows, while an antiparallel alignment of the two electrodes results in a small current. This can be understood from a few elementary arguments. (i) The tunneling current is in first order proportional to the product of the electrode’s density of states

at the Fermi level [N(EF)]. (ii) As we noted in the previous section, in a ferromagnet,

the ground-state energy bands in the vicinity of the Fermi level are shifted in energy due to exchange splitting, yielding unequal majority and minority bands for electrons with opposite spins. (iii) Assuming spin conservation for the tunneling electrons, there are two separate currents of spin up and spin down character. As a result of these ingredients, the current between electrodes with the same magnetization direction should be higher than for oppositely magnetized electrodes. This is further illustrated in the right panel of Figure 1.3. Within this simple so-called Julli`ere model, the resistance change is called tunneling magnetoresistance (normalized to the lowest resistance) is given by:

T MR = 2P1P2

1 − P1P2

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EF

N₁maj N₁min N₂maj N₂min

barrier

large current

small current

Figure 1.3: Spin-polarized tunneling in MTJs. Schematic illustration of the physics behind tunnel magnetoresistance. Top: for parallel aligned magneti-zation as sketched in the left, electrons around the Fermi level with spin-up (↑) and spin-down (↓) are allowed to tunnel from majority → majority bands, and from minority ⇒ minority bands. Bottom: when the magnetization of the two ferromagnets is anti-parallel, tunneling takes place for majority → minority and minority → majority bands, leading to a reduction of total tunneling current. In terms of electrical resistance, this corresponds to a higher resistance when the mag-netization of the two layers are oppositely aligned.

with

P1,2 =

N_1,2maj − N_1,2maj

N_1,2maj + N_1,2maj (1.4)

where P1,2 is the so-called tunneling spin polarization (also called TSP in this thesis)

determined by the relative difference in DOS at the Fermi level (for each electrode). However, it is crucial to realize that not all electrons present at the Fermi level can efficiently tunnel through the barrier and that this simple equation is not able to capture the physics behind a number of observations in MTJs. As we shall see later, the spherically symmetric s-like electrons which have a much lower DOS at the Fermi level dominantly tunnel through the barrier, and the interface between the insulating tunnel barrier and the ferromagnets plays an essential role. Nonetheless, this expression clearly demonstrates the presence of a magnetoresistance effect and the relevance of the magnetic character of the electrodes. Moreover, it shows that so-called half-metallic ferromagnets which have only one spin species available at the Fermi level [10], may in principle engender infinitely high TMR. Indications for

such anomalous behavior have indeed observed, for instance in LaSrMnO3 / SrTiO3

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Al O2 3 Co Co Normal MTJ Al O2 3 Co Co Ru Exchange coupled Al O2 3 Co IrMn Exchange biased Co Co (a) (b) (c) Al O2 3 Al O2 3 Al O2 3 Al O2 3 Co Co Co LSMO LSMO Co / Fe Co / Fe 3 SrTiO MgO 3 SrTiO Co Co-Gd Co Cu / Cr _Co _Co TMR > 0 TMR»0 TMR>>0 TMR < 0 TMR > 0 TMR >> 0 (d) (e) (f) (g) (h) (i)

Figure 1.4: Materials used in MTJs. (a-c) Achieving parallel or antiparallel configurations. (a) Two ferromagnetic layers with different thicknesses resulting in different coercivities. (b) Exchange coupling across Ru, where the trilayer Co / Ru / Co acts as the bottom electrode. (c) Exchange biasing the bottom Co layer with IrMn which results in the shift of the center of the hysteresis loop from zero field. (d-h) MTJs engineered with various types of ferromagnetic, non-magnetic and barrier materials. These stacks were used in experiments to understand spin tunneling (see text).

An important aspect for the presence of magnetoresistance is the ability to in-dependently manipulate the direction of the magnetization of the electrodes. In other words, have easy access to a parallel or antiparallel configuration of the two magnetic electrodes. This can be accomplished by a number of methods which are schematically shown in Figure 1.4. All these methods use specific materials and their properties to change the hysteresis loop of one magnetic electrode in comparison to the other. The easiest method one can imagine is to use two different thicknesses for the two electrodes [see Figure 1.4(a)], which renders two different coercivities and switching fields. Another way to change the switching fields is to use exchange coupling across a thin metallic layer like Ru [see Figure 1.4(b)]. At certain thick-nesses of Ru, it couples the two adjacent Co layers anti-ferromagnetically, and allows easy switching between the two states of the MTJ. Here, the trilayer Co / Ru / Co acts as the bottom electrode. Another method commonly used is to fix or pin the direction of one of the ferromagnetic layers with an antiferromagnet like IrMn [see Figure 1.4(c)]. In this case the hysteresis loop of the pinned layer shifts away from zero [14]. With the loop of the other electrode centered around zero, this too allows switching between the parallel and antiparallel configuration.

In Figure 1.5, we show another example of a TMR measurement. Here the first type of stack shown in Figure 1.4(a) with two soft-magnetic CoFeB electrodes having different coercivities is used to create a clear distinction between the resistance levels in parallel and anti-parallel alignment of the magnetization. As the field is swept, there are sharp changes in resistances when one switches from a parallel to an antiparallel configuration or vice versa. The TMR reported here is ∼500% at room temperature, underlining the application potential of such a device, especially if one considers two distinctly different resistance states at two different external fields.

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Magnetic field (kA/m) Resistancechange(%) MgO CoFeB capping layers substrate buffer layers CoFeB -10 -5 0 5 10 0 100 200 300 400 500 600

Figure 1.5: Example of a TMR measurement. Resistance change in a mag-netic tunnel junction consisting of (Co25Fe75)80B20 / 21 ˚A MgO / (Co25Fe75)80B20

as shown at right. The data are taken at room temperature. The arrows at left indicate the orientation of the CoFeB magnetization. Adapted from [15].

1.3 Contemporary notions on spin tunneling

Next we will discuss some of the experiments which shed new light on the physics of magnetic tunnel junction. As mentioned in the abstract of this chapter, no attempt is made to be complete here. Please refer to the review by Swagten et al. for an exhaustive account together with a description of recent advances [13].

In 1971, Tedrow and Meservey reported the first experiments [16] on spin tunnel-ing [see Section 2.5.1]. In their case, only one electrode was ferromagnetic (Ni), the other being a superconductor (Al). They found that though minority electrons dom-inate the DOS at the Fermi level of Ni, majority electrons were tunneling through

the thin AlOx barrier. Later it was suggested by Hertz and Aoi (1973) [17] and

by Sterns (1977) [18] that, although the dominant species of electrons at the Fermi level of transition metal ferromagnets were spin-down d electrons, they did not cou-ple well with the states over the barrier. Instead, highly dispersive s-like electrons had a much larger overlap integral with states in the barrier which led to a larger transmission probability for these electrons. Moreover, they also realized that the interaction between the s and d-electrons (s-d hybridization) leads to a suppression of the s-DOS in regions of large d-DOS, which is also the case at the Fermi level of a 3d transition metal ferromagnet [17, 18]. Consequently, this induces a spin polariza-tion of the s-DOS at the Fermi energy. After these initial experiments, Julli`ere [19] made the first prediction of a TMR effect. Given these demonstrations and predic-tions in spin tunneling, mainly due to technical difficulties, it took almost 25 years to do the first successful experiment with two ferromagnetic electrodes adjacent to a tunnel barrier. Two research groups, that of Moodera et al. at MIT [5] and that of Miyazaki et al. at Tohoku Japan [6], then reported the first TMR measurements

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on MTJs.

Please note that in all these experiments AlOx was preferred as barrier material,

primarily since it allowed easy growth of a pin-hole free thin barrier by natural, thermal or plasma oxidation of Al thin films. This was particularly convenient for the experiments of Tedrow and Meservey, as they used Al as a superconducting bottom electrode anyway. On the theoretical side too, there was considerable effort

to model tunneling through AlOx [20, 21]. However, due to its amorphous

struc-ture which hinders ab-initio calculations, despite persistent effort, our theoretical

understanding of tunneling through AlOx has remained limited [22, 23]. Therefore,

many experimental attempts were made to achieve this fundamental understanding, which we will discuss below. Nevertheless, theory has provided vital evidences that the interface between the barrier and the ferromagnet, and the relevant chemistry or bonding at such an interface, is crucial for spin tunneling [22–24]. For example, using first principles calculations Belashchenko et al. predicted a sign change for the spin polarization of tunneling electrons depending on where oxygen atoms sit on a Co surface [25].

1.3.1 AlOx: Relevant experiments

Earlier, we defined TMR with a simple equation [see Eqn. 1.3] which included the spin polarization (P ) of the ferromagnetic DOS. One may imagine that P is not constant over the whole Fermi surface, and varies depending on which direction in

k -space one probes, that is, on the crystallographic orientation of the electrode at the

interface with the tunnel barrier. The demonstration of such a crystal anisotropy of the TMR was given by Yuasa et al. [26], who showed that the use of single-crystalline Fe electrodes of different crystal orientations in MTJs resulted in a substantially different TMR.

After the demonstration of TMR in MTJs, there were various attempts to verify the simple equation 1.3 given by Julli`ere. As shown in Figure 1.4(d-h), many of these experiments involved inserting an additional layer at the barrier-ferromagnet interface or changing either the barrier material, or the ferromagnetic material, or both. To begin with, equation 1.3 predicts a zero TMR if any of the two electrodes has zero P . A simple test would be inserting a non-magnetic “dusting” layer at the barrier-ferromagnet interface and measuring TMR, as shown in Figure 1.4(e). LeClair et al. [27] showed that, surprisingly, inserting one monolayer of Cu between

the bottom Co electrode and the AlOx barrier showed a finite TMR. Their results

are shown in Figure 1.6(a). This indicated that a part of spin current retained its spin orientation while traversing the non-magnetic Cu layer. Moreover, while the

TMR exponentially decayed with a length scale of 2.6 ˚A for a Cu layer, a similar

layer of antiferromagnetic Cr induced an even faster exponential decay on a length

scale of 1.2 ˚A [28]. Not only do these results clearly demonstrates the limited

appli-cability of equation 1.3, but also the truly interfacial nature of the tunneling spin polarization P , illustrating that only a few monolayers adjacent to the tunnel barrier

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0 2 4 6 0 1 0 5 10 15 20 25 0 2 4 6 8

Thickness dusting layer (Å) Thickness dusting layer (Å)

Norm.tunnelmagnetoresistance T unnelmagnetoresistance(%) maj. min. NiFe Al O2 3 Co(001) Cu(001) T = 2 K T = 10 K T = 300 K Cu Cr Ru (a) (b)

Figure 1.6: Oscillations in TMR. TMR when incorporating ultrathin layers at the ferromagnet-barrier interface of a MTJ. (a) Normalized TMR data at T = 10 K for sputtered Co / X / AlOx/ Co junctions, with interfacial layers X = Cu, Cr, and

Ru [27–29]. (b) TMR at T = 2 K and T = 300 K as a function of the thickness of the Cu interface layer thickness in an epitaxial junction of Co(001) / Cu(001) / AlOx/

NiFe. The inset schematically shows quantum well reflections for minority electrons in the Cu layer, only when propagating along k_||= 0; adapted from [30].

are important for tunneling.

In Figure 1.6(a), one notices that although the insertion of a Ru layer at the interface also results in a exponential decay of the TMR as rapid as that due to the Cr layer, in case of the Ru layer, LeClair et al. observed a change in sign of the TMR. [29]. Although they demonstrated that the sign reversal of TMR was directly related to a change of the electrode DOS due to the interfacial mixing between Co and Ru, an alternative explanation would have been the formation of quantum well states in Ru if sharp, almost single crystalline, Co/Ru interfaces could be achieved. Later Yuasa et al. achieved such sharp interfaces between single crystalline Co (001) and Cu (001) by using molecular beam epitaxy [30]. Their MTJ stack and the corresponding TMR measured on it are shown in Figure 1.6(b). Here it is noteworthy that the amplitude of the TMR oscillation is large enough to allow the sign of the TMR ratio to alternate between positive and negative value. Yuasa et al. explained that majority electrons tunneling from NiFe into Co would transmit easily as compared to minority electrons which have a higher probability to be reflected at the Co-Cu interface. If multiple scattering occur between the Co-Cu and

Cu-AlOx interfaces, the minority electrons would form resonant quantum well states

(QW states) in the Cu layer, resulting in the oscillatory behavior of TMR. From

the period of the oscillation, they could argue that the QW states formed in the ∆1

band of Cu. The importance of the dominant contribution of this highly dispersive

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who fabricated single crystalline MTJs with Cr (001) inserted at the interface [see Figure 1.4(e)], similar to the work of LeClair et al. They argued that, since the band

structure of an epitaxial Cr layer has no band of ∆1 symmetry at the Fermi level

in the k||= 0 direction, the electrons from one electrode can tunnel only if they are

scattered at the interface of the other electrode due to the presence of the Cr layer. These above results clearly show the importance of the spherically symmetric s-like

electrons in tunneling through AlOx. We will return to this point in Chapter 4.

Although most ferromagnets display a positive P in conjunction with AlOx,

Kaiser et al. reported that Co-Gd alloys [see Figure 1.4(f)] can exhibit both signifi-cant positive and negative P systematically depending on the alloy composition [32]. It is known that in these alloys there exist independent subnetworks of Co and Gd magnetic moments which are individually aligned ferromagnetically, but align an-tiferromagnetically with respect to each other. Now the sign of P depends on the orientation of the respective subnetwork magnetization with respect to the applied field. The P from either of these subnetworks will be positive when its magnetization is aligned with the applied magnetic field. However, since the moments of the other subnetwork will consequently be antiparallel to the field, it give rise to negative P . Kaiser et al. argued that the measured P is the sum of independent spin-polarized tunneling currents from the Co and Gd subnetworks, resulting in a sign change of

P with alloy composition. When combined with traditional ferromagnetic materials

with positive P in a MTJ, these alloys lead to a positive or negative TMR depending on the sign of Co-Gd polarization [32].

As we clarified earlier, chemical bonding at the interface has been predicted to have a great influence on P . Such bonding would influence the tunneling matrix element occurring in Fermi’s golden rule which couples initial and final state wave functions depending on symmetry and overlap arguments. Consider the case of Co-Pt alloys studied by Kaiser et al. [33]. They observed that the measured P did not change after alloying ferromagnetic Co with up to 40 at.% of non-magnetic Pt, while the magnetic moment of the alloy reduced by ∼40% of its initial value for Co. They argued that (i) the robust magnetic moment of Co in Co-Pt alloys which did not change much from its value for pure Co and (ii) the higher tunneling rate from Co atoms at the interface as compared to Pt atoms was responsible for the robust P of Co-Pt alloys. The higher tunneling rate was argued to arise from the larger affinity

of Co to bond with oxygen at the Co-Pt / AlOx interface. Kaiser et al. estimated

that the tunneling probability from the Pt sites at the interface was ∼3.8 times lower than from the Co sites. This study suggests that it is possible to form MTJs with high P and TMR with low magnetic moment alloys by utilizing interface bonding effects and manipulating the tunneling rates of the alloy constituents [33].

Arguably the most decisive experiments demonstrating the relevance of interface bonding effects were those of Sharma et al. [34] and De Teresa et al. [35, 36]. De

Teresa et al. studied MTJs with Co / I / La0.7Sr0.3MnO3 (LSMO), where I could

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these experiments, the effective polarization of Co was found to be positive (major-ity electrons tunnel) with ALO as barrier, and negative (minor(major-ity electrons tunnel) with STO or CLO as barrier. As the P of the STO-LSMO interface was known to be positive, the inverse TMR observed in Co / STO / LSMO junctions was a signature of a negative polarization of the Co-STO interface. This inversion of the sign of P for the the Co-STO interface with respect to the P in Co-ALO interface was confirmed by growing Co / ALO / STO / LSMO junctions [see Figure 1.4(h)] which again revealed a positive P for the Co-ALO interface. De Teresa et al. argued that the negative P of Co when the barrier is STO or CLO could be viewed as a preferential selection of electrons of d-character at the Co-STO and Co-CLO interfaces, as com-pared to the positive P in Co-ALO where the selection of electrons with s-character occurred at the interface. This negative P of the Co-STO interface has later been verified from first principles by Velev et al. [37]. These results again show that P , and consequently TMR should be viewed as a property predominantly determined by barrier-ferromagnet interface which is strongly influenced by the chemistry at the interface.

1.3.2 MgO: Relevant experiments

As we have mentioned, due to the amorphous nature of AlOx, ab-initio studies

aimed to fundamentally understand spin-dependent transport in tunnel junctions have been difficult to perform [22, 23]. Therefore, there has been a continuous effort to develop crystalline barriers which allow coherent electron transport [13]. Below the use of MgO barriers (and the observation of giant TMR) is discussed specifically due to the paramount role it plays in our fundamental understanding of tunneling and due to its technological impact on MTJs.

Concept of coherent tunneling

One aspect which is highly unlikely in tunneling through an amorphous barrier is

k|| conservation of the electron wave vector. On the contrary, in a crystalline

bar-rier, k|| conservation (also known as coherent tunneling) is a distinct possibility.

This also implies that a wave vector selected at one interface efficiently couples to a corresponding wave vector at the other interface. Keeping in mind that P is not constant over the whole Fermi surface, and the possibility of coherent tunneling, one may imagine that using a certain electrode-barrier interface in a certain crystal-lographic orientation would result in efficient electron tunneling for wave functions which have specific symmetries. Among other systems, such coherent spin tunneling behavior has been theoretically predicted [38, 39] for epitaxial Fe(001) / MgO(001) / Fe(001), and later, also for other bcc ferromagnetic electrodes based on Co, and CoFe alloys. In these tunnel junctions, one describes three kinds of evanescent

states (∆1, ∆5, ∆20) which coherently tunnel between the MgO barrier and

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2 3 4 5 6 7 8 9 10 11 12 13 14 15 10-25 10-20 10-15 10-10 10-5 100 2 3 4 5 6 7 8 9 10 11 12 13 14 15 10-25 10-20 10-15 10-10 10-5 100 Majoritydensity-of-states Minoritydensity-of-states

Layer number Layer number

D1(spd) D5(pd) D5(pd) D2_’(d) D2(d) D2_’(d) Fe Fe Fe Fe MgO Fe Fe D1 D5 D2 MgO

Figure 1.7: Origin of giant TMR in MgO based MTJs. Layer-resolved tunneling DOS for k_||= 0 in Fe(001) / 8 monolayers MgO / Fe(001) for majority electrons when the magnetization of the Fe layers is parallel oriented (left). Each curve is labelled by the symmetry of the incident Bloch state in the left Fe electrode, showing, for example, the slow decay of the states with ∆1 symmetry. The strong

differences in decay is schematically illustrated in the right panel. Adapted from [38].

along the Γ−X direction in k -space. The choice for Fe (001) is made on the basis

of the fact that the highly dispersive ∆1 is present at the Fermi level only in the

majority spin channel, and absent in the minority spin channel. Moreover, as shown in Figure 1.7, this band has a relatively small attenuation coefficient in MgO (001),

as compared to the ∆5, ∆20 bands. In a tunnel junction, these two factors play

a key role in determining the tunnel conductance for the parallel and antiparallel configuration. For instance, in the antiparallel configuration, the fact that majority

∆1 states efficiently tunnel through the barrier but cannot couple to the DOS of the

other electrode due to the absence of such a band at the Fermi level. This is shown in Figure 1.7. In the case of bcc Co (001), the situation is even more interesting.

Here, for the majority channel, only the ∆1 states lie at the Fermi level. Therefore,

it is theoretically expected that all the states are completely reflected at k||= 0 in

antiparallel configuration, resulting in a giant TMR. Discovery and impact of giant TMR

After a number of initial efforts to observe this enormous selectivity of the wave function symmetry in epitaxial junctions, two breakthroughs were reported. One for epitaxial (001)-oriented Fe / MgO / Fe junctions [40] and the other for highly-textured sputtered CoFe / MgO / CoFe [41], showing TMR ratios well above 100%,

thereby substantially exceeding the magnetoresistance of AlOx based devices. Since

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by using ternary CoFeB alloys as ferromagnetic electrodes [41, 42]. It is believed that high-quality MgO can be adequately stabilized between the as-grown, amorphous

CoFeB electrodes, which, after annealing at temperature up to almost 400◦_C,

crys-tallize in the required bcc character. An example of a TMR measurement of around

500% at room temperature is shown in Figure 1.5 for an annealed (Co25Fe75)80B20 /

MgO / (Co25Fe75)80B20 junction. Today, such junctions inspire novel ideas for

var-ious spintronics devices [43]. For example, spin-torque based MTJs where, instead of the application of an external magnetic field, the angular momentum of a spin polarized current is used to switch the magnetization of one of the ferromagnetic electrodes. Such devices aim to be the basis of future random access memories [43].

1.4 Relevance of amorphous ferromagnets

We hinted the emerging and unquestionable importance of amorphous CoFeB alloys in spintronics. Let’s briefly look at amorphous alloys in general, and later, the relevance of CoFeB in particular.

The first demonstration of noncrystalline Au75Si25 alloy in 1960 by [44] was

fol-lowed by the discovery of a stable ferromagnetic state in Fe80P13C7amorphous alloys

by the same group in 1967 [45, 46]. These observations opened up a new avenue in both, solid state physics and materials research. The fact that many phenomena remain essentially unaltered by the absence of a periodic lattice and the consequent inapplicability of Bloch’s theorem has forced a reappraisal of the theoretical frame-work of solid state physics [47–49]. On the materials research side, it was quickly realized that these amorphous alloys showed excellent magnetic, mechanical and corrosion resistant properties. For example, the unusually low coercivities and high resistivities of Fe-B-Si alloys allowed the reduction of core losses in power transform-ers by a factor of 5 over contemporary materials. Concerning mechanical properties,

Inoue et al. recently demonstrated that Co43Fe20Ta5.5B31.5 glassy alloys exhibit a

fracture strength, and a Youngs modulus which are higher than previous values reported for any bulk crystalline or glassy alloys [50]. There are numerous other aspects like fatigue life, magnetostriction and coercivity of these alloys which make them technologically relevant; please see references [47–49] for more details.

Regarding the application of amorphous ferromagnets in spintronics, to the best of our knowledge, the first use of an amorphous ferromagnetic layer was made in 1995 by Jimbo et al. [51] who reported a GMR of 5.4% in CoFeB/ Cu / Co trilayers. These CoFeB alloys were first investigated in the late 1970’s, for example by O’Handley et

al. and by Heiman et al. [52, 53]. Subsequently, Jimbo et al. also reported studies

of exchange biased CoFeB spin valves together with an anneal study of such spin

valves where they showed that annealing these trilayers up to 300◦_{C enhanced the}

observed value of the GMR [54, 55]. In 2002, Kano et al. reported a TMR value

of 59% in AlOx based MTJs [56]. For MTJs based on AlOx barriers, there were

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temperature by Wang et al. [57] and Wei et al. [58], respectively. Concerning MgO based MTJs, Parkin et al. reported a room temperature TMR of more than 200% in CoFe / MgO / CoFeB MTJs [41]. Since these reports there have been many reports of increasingly higher TMR values with CoFeB-MgO based MTJs [15, 42]. These alloys have also facilitated record-low switching currents in spin-torque based MTJs [59]. Consequently, they were employed to observe the novel spin-torque diode effect [60], used in junctions to measure the strength, or even the direction, of the associated spin torque [61]. In this thesis, we will venture to remind the reader about this application potential of CoFeB alloys from time to time.

1.5 This thesis

From the experiments discussed above, the emerging importance of CoFeB in spin-tronics and its considerable impact for various spinspin-tronics applications were obvi-ous [43] during the time of this thesis. So also was the necessity for a thorough experimental and theoretical analysis of its atomic and electronic structure and their combined impact on its tunneling spin polarization (P or TSP). Therefore, this thesis is devoted to the fundamental understanding of the properties of ternary CoFeB alloys, and is an endevour to explore open questions in spin tunneling by using these properties.

After this first introductory chapter (Chapter 1) which deals with a few contem-porary notions regarding spin tunneling, Chapter 2 addresses the various deposition and experimental analysis tools used in this thesis. Here, to exemplify the vari-ous techniques, a few experimental results relevant to later chapters will also be presented.

In Chapter 3, we will investigate some structural aspects of CoFeB alloys. In particular, we will investigate the influence of crystallization of these amorphous alloys on their structural and magnetic properties after a single anneal. We will use

this information in later chapters as a starting point for further experimental work.

In Chapter 4, we will investigate the atomic and electronic structure of a single CoFeB composition from first principles. Also, we will specifically investigate the TSP of an amorphous ternary alloy, an issue never addressed before, and compare it with its crystalline counterpart. Surprisingly, we find that the TSP of the amor-phous alloy is larger than its crystalline counterpart. We also show that for these amorphous alloys, the spin polarization of the s-electron DOS at the Fermi level is

a very good representative of the TSP in AlOx based junctions.

In Chapter 5, we probe some aspects of inelastic tunneling of electrons when a sharp contrast – structural change from amorphous to crystalline electrode – is induced at the barrier-ferromagnet interface. In particular, the changes in the low energy magnetic excitations induced by inelastically tunneling electrons are investi-gated.

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CoFeB alloys. Such a correlation has been an outstanding issue in spin tunneling since its first observation in 1976. We find that the amorphous CoFeB alloys are very suitable to address this issue. We will focus on properties of d-electrons probed by synchrotron radiations in relation to the properties of s-electrons probed by electronic transport measurements. Our data support the conjecture that such a correlation between the d and s-electrons may exist.

Finally, in Chapter 7, we will investigate the thermal stability of MTJs and the effect of high-temperature annealing. Specifically, the role of Mn diffusion from the antiferromagnets used to exchange bias one of the ferromagnetic layers is probed. We find that though Mn diffuses after annealing, it does not seem to influence the TSP.

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Chapter 2 Probing electronic, magnetic and

structural properties

Experiments analyzing CoFeB

Abstract: This chapter1 _{presents brief but requisite information on the various}

experimental techniques used in this thesis. While doing so, we also present some relevant but occasionally unpublished results on materials like CoFeB and MgO obtained using some of these techniques. Most of these results will be of relevance in later chapters. The chapter is divided in five main sections: sample preparation, structural characterization, in-situ measurements of electronic properties, magnetic characterization and electronic transport. No attempt is made to be complete or exhaustive. Instead, the reader is referred to suitable references which do justice to and explain in detail the particular technique under question.

1_{A part of the last section of this chapter is under review.}

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2.1 Sample fabrication

We begin this chapter with the essential procedure followed for samples preparation in this thesis which mainly involves deposition of various materials and oxidation of Al thin films. Prior to this discussion, lets summarize the choice of substrates used and substrate cleaning procedures followed during this thesis.

2.1.1 Substrate and substrate cleaning considerations

The tunnel junctions are deposited on glass substrates, in particular 1 mm thick barium borosilicate glass sheets provided by Corning Inc. (glass code 7059). A crucial point to be considered is the roughness of the substrate for the spin polarized

tunneling measurements which use very thin (35 ˚A) aluminum films as electrodes.

From previous experience [1], the glass substrates were found to allow easy deposition of closed Al layers, as compared to silicon wafers. This difference may be ascribed to lower surface roughness, since the glass substrates are manufactured using the fusion process [2] where the glass is slowly cooled from the liquid phase to the glass

phase. On the contrary, the surface of the Si wafer consists of SiOx formed at room

temperature during the first exposure of the wafer to air. For all other purposes, Si (001) substrates were used due to the easy of cleaving, cheap and wide availability, and relatively good surface smoothness.

For the removal of any organic material on the substrate, we first ex-situ im-mersed the substrate in ammonia and placed the beaker in an ultrasonic bath for 10 minutes. Subsequently, the substrate was immersed in ethanol and the proce-dure was repeated. Then the substrate was placed in a closed isopropanol chamber where isopropanol was being constantly evaporated. In these ex-situ cleaning steps, ammonia dissolves organic molecules and the alcohols allow removal of residue ac-cumulated during the ammonia dip. This ex-situ cleaned substrate was stuck with silver paint to the substrate holder and loaded in the system load-lock. The final cleaning step was in-situ plasma-cleaning in an oxygen plasma. This step allows the conversion of any residual hydrocarbons from the ex-situ cleaning procedures into volatile carbon oxides and water vapor leaving a significantly cleaner substrate after the chamber is pumped to UHV. See Section 2.1.3 for details of the plasma-cleaning procedure.

2.1.2 Deposition: Sputtering

One of the most important research tool of this thesis is the ultra-high vacuum (UHV) deposition system used - EUFORAC (Eindhoven University nano-Film de-pOsition Research and Analysis Center). A picture of this facility is shown in Fig-ure 2.1. It consists of a 6 target sputter deposition chamber, an oxidation chamber, an organic molecular beam epitaxy (MBE), a metal MBE, an in-situ photoelectron spectroscopy characterization tool, and an in-situ scanning tunneling microscope,

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U H V T r a n s p o r t

O x i d a t i o n C h a m b e r

S p u t t e r C h a m b e r

L o a d - L o c k

S T M

M B E P r e p a r a t i o n

O r g a n i c C h a m b e r

M B E

X P S

U P S

Figure 2.1: UHV deposition system. This picture shows the EUFORAC system where a large number of UHV deposition and characterization tools are implemented making this an extremely powerful nano-tool.

all connected to each other via transport chambers held at UHV. The capabilities of the system in growing and analyzing various sorts of thin films go hand in hand with its versatility. For more details on the capabilities of the EUFORAC, please refer to the thesis of P. LeClair [3].

In this thesis, all the samples were grown using sputter deposition. Although, in the context of magnetic films, there have been some reports of the growth of epitaxial films using sputter deposition [4, 5], generally, sputtering implies that the

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O 2 a t m o s p h e r e ( 1 0- 1_{m b a r )} s u b s t r a t e r i n g - s h a p e d e l e c t r o d e s O * e -+ _ O + A B C ( d ) p l a s m a o x i d a t i o n A r a t m o s p h e r e ( 1 0- 2_{m b a r )} s u b s t r a t e s h a d o w m a s k A r+ _ e -+ t a r g e t m a t e r i al t a r g e t m a t e r i a l ( a ) s p u t t e r d e p o s i t i o n m a g n e t t a r g e t m a t e r i a l w e d g e m a s k ( c ) w e d g e g r o w t h s u b s t r a t e ( b ) s h a d o w m a s k A l / A l O x C o F e B / A l

Figure 2.2: Schematic of various deposition and oxidation techniques. (a) Sketch exemplifying sputter deposition. (b) Sketch of the shadow masks used to deposit tunnel junctions. (c) Growing wedge shaped samples for thickness dependent studies. (d) Sketch of the plasma oxidation technique.

layers are either polycrystalline or amorphous depending on the material. Never-theless, magnetic tunnel junctions and a variety of sensors based on the GMR effect are popularly and conveniently grown by sputtering. Our system is a 6 source Kurt J. Lesker sputter tool equipped with a home-built load-lock. Typical base pressure

after a bake-out is 5×10−10_{mbar. However, following a target change which requires}

breaking vacuum, the system readily achieves a base pressure of 2×10−8 _mbar

with-out bake-with-out after pumping for 48 hours. Residual partial gas pressures in the chamber can be monitored with a remote gas analyzer based on mass spectrometry. This analyzer was installed on the system during this thesis.

Although exhaustive reviews on sputtering are available [6, 7], let us briefly summarize the basic physical aspects of the technique, as shown in Figure 2.2(a). The material to be deposited is produced in the form of a palette and attached to an anode which is typically held at -100 to -1000 V. When a gas, typically a nobel gas like Ar, is inserted in the UHV chamber, it gets ionized. The positively

Key concepts in spin tunneling : amorphous ferromagnets for spintronics

Key concepts in spin tunneling : amorphous ferromagnets for

spintronics

Key Concepts in Spin Tunneling

Amorphous Ferromagnets for Spintronics

Key Concepts in Spin Tunneling

Amorphous Ferromagnets for Spintronics

Paresh Vijay Paluskar

my parents Prabha and Vijay,

my wife Sonu,

and my brother Parag

Contents

Chapter 1

Introduction to spin tunneling

Ferromagnets and magnetic tunnel junctions in spintronics

1.1

Spintronics in daily life

1.2

Basic aspects

1.3

Contemporary notions on spin tunneling

1.4

Relevance of amorphous ferromagnets

1.5

This thesis

Bibliography

Chapter 2

Probing electronic, magnetic and

structural properties

Experiments analyzing CoFeB

2.1

Sample fabrication

U H V T r a n s p o r t

O x i d a t i o n C h a m b e r

S p u t t e r C h a m b e r

L o a d - L o c k

S T M

M B E P r e p a r a t i o n

O r g a n i c C h a m b e r

M B E

X P S

U P S