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Microscopic origin of the reduced magnetocrystalline anisotropy with increasing oxide content
in Co80Pt20 : oxide thin films
View the table of contents for this issue, or go to the journal homepage for more 2013 J. Phys. D: Appl. Phys. 46 405001
(http://iopscience.iop.org/0022-3727/46/40/405001)
J. Phys. D: Appl. Phys. 46 (2013) 405001 (5pp) doi:10.1088/0022-3727/46/40/405001
Microscopic origin of the reduced
magnetocrystalline anisotropy
with increasing oxide content
in Co
80
Pt
20
: oxide thin films
Wen Zhang
1,2,6, Simon A Morton
3, P K Johnny Wong
1, Bin Lu
4,
Yongbing Xu
2, Michel P de Jong
1, Wilfred G van der Wiel
1and Gerrit van der Laan
51NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente, Enschede 7500
AE, The Netherlands
2Spintronics and Nanodevices Laboratory, Department of Electronics, University of York, York YO10
5DD, UK
3Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 4Seagate Technology, 47010 Kato Road, Fremont, CA 94538, USA
5Diamond Light Source, Magnetic Spectroscopy Group, Chilton, Didcot OX11 0DE, UK
E-mail:xiaotur@gmail.com
Received 17 June 2013, in final form 19 August 2013 Published 17 September 2013
Online atstacks.iop.org/JPhysD/46/405001
Abstract
Angle-dependent x-ray magnetic circular dichroism at the Co L2.3edges has been utilized to
systematically study Co80Pt20: WO3perpendicular magnetic recording thin films, in which the
magnetocrystalline anisotropy significantly drops as the oxide volume fraction increases. The microscopic origin of this phenomenon in the studied films can be mainly attributed to an increase in orbital moment normal to the grain–oxide interface, with increasing oxide volume fraction, which arises from a more pronounced effect of symmetry breaking at the grain–oxide interface in smaller grains.
(Some figures may appear in colour only in the online journal)
A well-known dilemma in state-of-the-art composite
CoPt : oxide thin films for perpendicular magnetic recording is that while the desirable intergrain magnetic decoupling is improved with increasing oxide volume fraction the magneto-crystalline anisotropy, K1, is adversely reduced [1]. Our
previous experimental study of CoPt : oxide thin films has also shown that the magnetic anisotropy of the magnetic grains (K1g)itself drops when the oxide volume fraction is increased,
causing the reduction in K1 [2,3]. Revealing the underlying
physical mechanism of such a drop in K1is not only of great
scientific importance, but could also provide technological benefits, in deciding the optimum oxide volume fraction in the
6Author to whom any correspondence should be addressed.
Present address: Department of Physics, Southeast University, Nanjing 211189, People’s Republic of China.
manufacturing process for high-density storage applications [4,5]. Several mechanisms have been proposed to explain the decrease in K1—including the possible impact of intergrain
exchange decoupling [6], surface anisotropy arising from the grain–oxide interface [7], and stacking faults within the mag-netic grains [8–10]—while it remains ambiguous which one plays a more important role or whether they have contributed equally in the thin films.
Spin–orbit interaction is believed to play a dominant role in the magnetic anisotropy [11,12]. Particularly in magnetic grains with dimensions of a few nanometers, the orbital contribution to the total magnetic moment is no longer negligible, which could influence the spin–orbit interaction and in turn has an impact on the magnetic anisotropy [13–15]. It is therefore important to study the orbital moment and the spin–orbit interaction in such granular CoPt : oxide thin
J. Phys. D: Appl. Phys. 46 (2013) 405001 W Zhang et al
Figure 1. Experimental geometry (left) and schematic diagram
(right) showing the grain–oxide boundary and the easy axis of K1.
films, in order to provide a clear picture of the underlying mechanism governing the above-mentioned decrease in the
magnetocrystalline anisotropy. X-ray magnetic circular
dichroism (XMCD) is an ideal tool for this purpose due to its unique capability of separating spin- and orbital moments.
Combined with sum rules analysis [16], XMCD has been
shown to be a powerful tool to study subtle changes in spin and orbital moments and magnetic anisotropy of Co particles [17–19].
In this work, we use angle-dependent XMCD to quantify the spin and orbital moments of Co, which represent the main source of magnetization in CoPt : oxide thin films [20]. By studying the dependence of the microscopic magnetic moments on the oxide volume fraction, we have observed an increase in orbital moment normal to the grain–oxide interface with increasing oxide volume fraction, caused by the symmetry breaking at the grain–oxide interface, which plays a dominant role in the K1drop.
A series of samples, glass/Ta(5 nm)/Ru(13 nm)/Co80Pt20
+WO3(total of 13 nm)/C(7 nm), have been deposited at room
temperature using dc and RF magnetron sputtering in a Unaxis
M12 sputter tool with base pressure below 1× 10−6Pa. The
magnetic layer was formed by co-sputtering Co, Pt, and oxide targets. The oxide volume fraction (OVF), ranging from 0.08 to 0.19, was controlled by the oxide co-sputtering power. The microstructural and magnetic characterization has been described in our previous publications [2,3].
XMCD measurements, including Co L-edge XMCD spectra as well as XMCD hysteresis loops, were carried out in total-electron-yield mode on bending magnet beamline 6.3.1 at the Advanced Light Source, Berkeley, using circularly polarized x-rays with a degree of polarization of 55%. Two different experimental geometries were used, as illustrated in the left panel of figure 1, i.e. at normal incidence (γ = 0◦) and at grazing incidence (γ = 60◦) of the x-ray beam, which allows for extracting the anisotropic behaviour of the magnetic moments. In each case the magnetic field was always along the incident x-ray beam, and all measurements were taken at room temperature.
Figures2(a) and (b) display the angle-dependent XMCD
hysteresis loops of the samples with OVF = 0.08 and 0.15,
respectively, which were obtained by recording the peak height of the Co L3signal at∼778 eV divided by the Co L2signal at
∼793 eV as a function of the applied magnetic field. The loops show a pronounced hysteresis, and the difference between the
curves at γ = 0◦ and 60◦ shows that the easy axis of K1
is normal to the film plane, as depicted in the right panel of figure1.
From the above hysteresis measurements, we have obtained the coercivity of the samples, i.e. HC,0◦ (γ = 0◦)
and HC,60◦(γ = 60◦), as shown in figure2(c). The calculated
values of HC,60◦from HC,0◦, based on the domain-wall-motion (DWM, HC,60◦ = HC,0◦/cos60◦) and the Stoner–Wohlfarth (SW, HC,60◦ = HC,0◦cos60◦sin60◦)models are also plotted
in the figure for comparison [21,22]. We found that the
experimentally obtained HC,60◦ value agrees well with the
calculation by the SW model. This indicates that the
reversal mechanism of these samples tends to comply with
the SW model wherein coherent rotation dominates [22],
which suggests that the grains are magnetically decoupled and their magnetization reversal is independent of each other [23]. More importantly, we noticed that such a relationship exists for all OVF values, which rules out the effect of intergrain coupling/decoupling as the main cause for the change of K1in
the studied thin films.
From the angle dependence of the XMCD hysteresis loops, we calculated the Co magnetic anisotropy energy (MAE) by using MAE= MS 0 HdMθ1− MS 0 HdMθ2 sin2(θ1− θ2) [24],
where H is the applied magnetic field, θ1 = 0◦, θ2 = 60◦
and MSis the total magnetic moment involving the orbital and
spin magnetic moments of Co, which can be obtained from the XMCD spectra and will be shown in the following paragraphs.
As shown in figure2(d), the value of the Co MAE decreases
from∼0.18 meV/atom to ∼0.08 meV/atom as OVF increases
from 0.08 to 0.19. Although the Co MAE is estimated from the model with domain nucleation involved, which may not be remarkably accurate, its decreasing trend with increasing OVF is consistent with that of K1,2thus suggesting that the
main cause of the K1reduction is the drastically reduced Co
MAE in the studied thin films [3].
A detailed analysis of the Co orbital and spin moments as a function of OVF is required, in order to extract the microscopic origin of the drop in K1. Accordingly, angle-dependent x-ray
absorption spectra (XA) and XMCD spectra were taken at the Co L2,3edge by reversing the magnetic field of 2 T. As shown
in figure3, the spectra have been normalized to the step height of the absorption spectra at 815 eV, i.e. well above the L2edge,
where dichroic effects are negligible. From the XA spectra, we see that there is no signature of oxide formation, which would give otherwise multiplet features [25].
The Co 3d effective spin moment, mS, and orbital moment,
mL, have been evaluated from the XMCD spectra using the sum
rules analysis [16]. The mSand mLvalues obtained at different
x-ray incidence angles, are plotted in figures 4(a) and (b), respectively, as a function of OVF. Note that the values at
γ = 60◦, mS,60◦and mL,60◦, have been corrected for incomplete
magnetic saturation, i.e. by multiplication by a factor of
M2T,0◦/M2T,60◦, where M2T,0◦ (≈ saturation magnetization)
and M2T,60◦ represent the 0◦ and 60◦ magnetization obtained
at a 2 T field by vibrating sample magnetometry. 2
Figure 2. XMCD hysteresis loops for (a) OVF= 0.08 and (b) OVF = 0.15 measured at the Co L2,3edges with the x-ray incidence angles γ = 0◦(red, open circles) and γ = 60◦(green, filled circles). (c) Coercivity of the XMCD hysteresis loops at γ = 0◦(red, open circles) and
γ = 60◦(green, filled circles) with variable OVF. Also plotted are the calculated values of HC,60◦from HC,0◦using the DWM model (blue,
downward triangles) and the SW rotation model (pink, upward triangles). (d) Co magnetic anisotropy energy (purple, filled squares) and K1
(orange, open squares) versus the OVF. The grey lines are guides to the eye.
Figure 3. Normalized XA and XMCD spectra recorded at the Co L2,3edge for OVF= 0.15, with the x-ray incidence angles (a) γ = 0◦and
(b) γ = 60◦.
As seen in figure4(a), mS,0◦≈ mS,60◦, indicating that mS
is almost isotropic. This also means that the magnetic dipole term in the sum rules, which causes an anisotropy in the spin distribution, can be safely neglected here [26,27]. In sharp contrast, mL is highly anisotropic, as shown in figure4(b):
mL,0◦ > mL,60◦, which is consistent with the hysteresis loops in
figures2(a) and (b), where the easy axis of K1is perpendicular
to the film plane, i.e. along the direction of γ = 0◦.
Considering the different trends of mL,0◦ and mL,60◦ in
figure4(b), it can be concluded that mL,0◦is nearly independent
of the OVF, suggesting that the effect of stacking faults could be negligible here, since it has been reported that a considerable reduction in the orbital moment along the easy axis of K1is
expected due to the collapse of the stacking ordering [10]; while mL,60◦shows a considerable increase of∼0.06 µB/atom
J. Phys. D: Appl. Phys. 46 (2013) 405001 W Zhang et al
Figure 4. (a) Spin moment mS,0◦(red, open downward triangles)
and mS,60◦(green, filled upward triangles), (b) orbital moment, mL,0◦
(red, open circles), mL,60◦(green, filled circles) and mL,90◦(black,
open squares), extracted from
mL,60◦= mL,0◦cos260◦+ mL,90◦sin260◦and (c) orbital anisotropy, mL(blue, filled pentagons), as a function of OVF. Inset: schematic
diagram indicating the directions of mL,0◦and mL,90◦with respect to
the magnetic grain. The grey lines are guides to the eye.
OVF= 0.19), which is an essential result of this work, as we will discuss in the following paragraphs.
Based on the relation mL,60◦ = mL,0◦cos260◦ +
mL,90◦sin260◦, where mL,0◦ and mL,90◦ refer to the orbital
moments parallel and perpendicular to the sample surface normal, respectively [28], we extracted the value of mL,90◦, as also shown in figure 4(b). In our case, the direction of
mL,90◦ is normal to the grain–oxide interface, as illustrated in
figure4(c), with the orbital anisotropy, mL = mL,0◦−mL,90◦,
shown as well.
Figure 4(b) shows that mL,90◦ increases rapidly with
increasing OVF, reaching a value of ∼0.20 µB/atom at
OVF= 0.19, where the orbital moment is nearly isotropic.
This high value corresponds to a large orbital-to-spin moment
ratio of ∼0.15, comparable to values reported for various
CoPt thin films [20,29]. Since mL,0◦ is virtually constant,
the strongly increasing trend of mL,90◦ leads to a distinctly
decreasing behaviour of mL with increasing oxide content.
As shown in figure4(c), mLdecreases from∼0.12 µB/atom
(OVF = 0.08) to ∼0.02 µB/atom (OVF = 0.19), which can
be correlated to the reductions in the Co MAE (from∼0.18 to ∼0.08 meV/atom) and K1(from∼0.8 to ∼0.6 × 107erg cm3)
when the OVF increases. Thus, the above analysis provides a microscopic origin, namely that the orbital moments become increasingly isotropic, with increasing OVF, for the reduction of K1in the films we have characterized.
The underlying mechanism can be understood as follows: as the OVF increases, the magnetic grains become smaller, such that the grain–oxide interface, i.e. the interface between the grains and WO3, starts to play an increasingly important
role. At the grain–oxide interface, the effective coordination number of Co atoms is reduced. Compared to bulk Co, which features a high crystal symmetry, the lower symmetry at the interface produces a strong uniaxial crystal field. This may be expected to lift the orbital degeneracy, resulting in different occupations of the in-plane and out-of-plane 3d orbitals [30]. In turn, an enhancement in mL,90◦would result [13], which has
been predicted theoretically and found experimentally in low-dimensional systems like nanoclusters [13,31,32], surfaces and interfaces [33–35]. Consequently, the higher the OVF is, the stronger the symmetry breaking at the interface, and the larger the mL,90◦. Hence, our observed mL,60◦ enhancement,
or equivalently the increase of mL,90◦, with increasing OVF, is
mainly ascribed to the reduced symmetry at the grain–oxide interfaces, as was also demonstrated in Co nanoparticles on Pt(1 1 1) surface showing a giant magnetic anisotropy [36]. This kind of anisotropy has its easy axis orthogonal to the interface, caused by an enhanced spin–orbit interaction in that direction, which partly counteracts the overall perpendicular magnetic anisotropy K1here [7,8].
Summarizing, the present work clarifies the microscopic origin of the reduced magnetocrystalline anisotropy in Co80Pt20: oxide thin films when the oxide volume fraction
increases. For the studied thin films, the angle-dependent XMCD analysis showed that the orbital moment becomes increasingly isotropic with the increasing oxide content, caused by a symmetry reduction at the grain–oxide interface, which is the main reason for the drop in the magnetocrystalline
anisotropy. Keeping in mind that the effect of symmetry
breaking is more pronounced in smaller grains with higher surface-to-volume ratio, our results reveal an intimate correlation between the grain size and the magnetocrystalline anisotropy, which is instructive for obtaining an optimum oxide volume fraction, beneficial for the stable [37] and high-density recording simultaneously.
Acknowledgments
The authors would like to thank Dr Elke Arenholz and Dr Catherine Jenkins for their kind support at BL 6.3.1. 4
The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No DE-AC02-05CH11231.
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