University of Groningen Spin transport across oxide semiconductors and antiferromagnetic oxide interfaces Das, Arijit

Spin transport across oxide semiconductors and antiferromagnetic oxide interfaces

Das, Arijit

DOI:

10.33612/diss.150692255

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.

Document Version

Publisher's PDF, also known as Version of record

Publication date: 2021

Link to publication in University of Groningen/UMCG research database

Citation for published version (APA):

Das, A. (2021). Spin transport across oxide semiconductors and antiferromagnetic oxide interfaces. University of Groningen. https://doi.org/10.33612/diss.150692255

Copyright

Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).

Take-down policy

If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.

Chapter 1

Introduction

1.1

Beyond Moore

In the last three centuries, mankind has beheld industrial revolutions grow-ing from new discoveries in science and technology. In the mid-twentieth century, a digital revolution was witnessed with the discovery of electronics in the form of transistors and semiconductor technology. Within a decade of the first transistor being demonstrated, Gordon E. Moore suggested that “the future of integrated electronics is the future of electronics itself”[1]. This statement is still applicable today and led to the well-known Moore’s law that states that the number of transistors on a chip will double every two years. Moore’s law has been a source of continuous innovation and technology for many decades[2]. In the past 50 years, we have benefited from an increasing number of electrical components in an integrated circuit enabled by downscaling and increasing computational power.

However, such miniaturization of circuit components faces a bottleneck. As the size limits of modern manufacturing approaches that of transistors, devices are susceptible to quantum mechanical effects leading to charge tunneling, increasing leakage current and cross-talk between neighboring transistors, large power consumption, thermal dissipation etc. As the fun-damental limits of downscaling are approached, new technologies are arising

that circumvents size limitations and promise “Beyond Moore” applica-tions.

1.2

Spintronics

In the effort to find alternative, faster and more energy efficient electron-ics, spin-based electronelectron-ics, also known as “spintronelectron-ics,” has been a focus of research for more than two decades. In spintronics, the spin angular mo-mentum carried by each electron is used for information storage and ma-nipulation. Since the discovery of giant magnetoresistance (GMR) in spin valve devices in 1988, for which the Nobel Prize in Physics was awarded to Albert Fert and Peter Grunberg in 2007, spintronics have been a stimulat-ing research field in both academia and industry.

The rich physics associated with the underlying mechanism of observed magnetoresponses with external field was first observed across a multilayer stack of alternating thin layers of Fe and Cr, constituting a spin valve device. With varying magnetic field, the relative magnetization of the fer-romagnets (Fe in this case) are altered, leading to a large change in the charge resistance[3, 4]. This resistance can be stored as a bit, hence paving the way for the downscaling of non-volatile memory elements in electronic devices. This is how spintronics has found application in design of the mag-netic read-heads of hard disk drives and magmag-netic random access memory (MRAM)[5]. Later, GMR variation was further strengthened by adding a thin insulating film between two ferromagnets (spin polarizer/analyzer) leading to an even larger magnetoresponse known as Tunneling Magne-toresistance (TMR). This is the basis of non-volatile memory and MRAM technology.

So far, spintronics is based on the flow of spin polarized charge carriers due to an application of charge current. Hence, the transport of spins requires lower power compared to the flow of electrons. This idea has led to the realization of spin transfer of torque random access memory (STT-RAM), which is a fast and efficient way to switch the magnetization in ferromag-netic layer by spin polarized current. Realization of STT-RAM technology

1.2. Spintronics

n / p - Si / SiO2

Source Drain

VG (Gate)

Thin 2D semiconducting channel

VG = 0 (ON)

VG > 0 (OFF)

Metal Insulator (Oxide)

Figure 1.1: A schematic illustration of a Spin Field Effect Transistor (SFET). This consists of a three terminal configuration with source and drain being the injector and detector. The third terminal acts as the manipulator, that exerts an

electric field (VG > 0) across the 2-dimensional electron gas (2-deg) channel and

gradually dephases the spins that are flowing across the channel leading to a zero

detection or OFF state, which was initially an ON state before any bias (VG = 0)

was applied to the third terminal. This mimics a FET operation where the third terminal can be thought of as the gate voltage. the idea of this sketch is adapted from [11, 12].

supplemented by the idea of vertical stacking of conventional CMOS with memory and logic is nowadays the state-of-art for low power consumption and high-density non-volatile memory applications[6–10].

Driven by the success of spintronics, significant interest has been shown in the manipulation of data stored in logic-based applications. This has created research opportunities in the integration of memory and logic op-erations in complementary metal-oxide semiconductors (CMOS) architec-tures. The advantages of vertical stacking of CMOS with spin based logic and memory, the non-volatile nature of spin states, and the effective and faster switching of spin / magnetization states (order of GHz for ferromag-nets and THz for antiferromagferromag-nets), laid the foundation of spintronics: in metals[13]; semiconductors; two-dimensional materials such as graphene[14, 15]; transitional metal dichalcogenides[16]; and, very recently, in magnetic insulators[17–19].

data storage with logic operation is the Datta - Das Spin field effect Tran-sistor (SFET)[11, 12]. Fig. 1.1 shows a schematic of the operation of such transistor. It constitutes of 3 terminal leads of source, drain and gate, similar to the working principle of a field effect transistor. The injected spin polarized current via source, introduces spins to the 2-Dimensional electron gas (2-DEG) channel shown in yellow. In the channel, the phase of the spins are disturbed by the gate channel that introduces an electron field that results in an effective magnetic field in the reference frame of the diffusing spins in the 2-DEG channel. This causes a spin dephasing and no response is detected in the Drain. This is considered as the OFF state. But if the gate voltage was not applied, the spins flow undisturbed along the channel and is detected as a spin voltage response at the drain which is an ON state. This proposition highlights the consideration of an electric field manipulation of spin transport which in conventional semiconductor system is not demonstrated. This has opened a research opportunities to look for semiconductors with tunable electronic properties.

This thesis explores the possibilities of efficient spin injection-detection in semiconductors and in magnetic insulators. A summary of the earlier at-tempts of spin injection-detection is presented below.

1.2.1 Spin Injection-detection in semiconductors

The key cornerstones of semiconductor spintronics are: (i) creation of spins, (ii) manipulation of spins across the semiconducting channel, and (iii) de-tection of spins. In the beginning, spin injection in semiconductors was achieved by electrical means on silicon. But the spin detection was done optically, using a Light Emitting diode (LED) structure, where the circu-larly polarized luminescence is produced when the injected spin-polarized carriers recombine[21–24]. All electrical means of spin injection-detection was realized by Appelbaum et. al. by sourcing hot electrons (with an energy higher than the Fermi energy level, E ≥ EF + kBT ) through

fer-romagnetic thin films on Si[25]. However, hot electron approach had a major drawback, i.e. the detection current was 10−4 times smaller than

1.2. Spintronics

Current source Voltage Current source Voltage (Non-local)

p -type n -type Injec tor cur ren t Emitter current Collector current

(a)

(b)

(c)

(d)

Figure 1.2: Schematic of different electrical methods for spin injection and detec-tion in semiconductors, (a) shows spin injecdetec-tion by electrical means and detecdetec-tion by optical LED, (b) sourcing hot electrons through ferromagnetic films into Si and detection using the collector current. In the diagram (1) depicts the hot-electron generation and (2) depicts the hot-electron detection, (c) electrical methods of spin injection and detection using three (3T) terminal geometry. The broad central electrode is used for both injection and detection, (d) spin injection and detection using non-local (NL) four terminal method, where the spin is injected from the ferromagnetic contact (FM) in the left and detected using the right ferromagnetic (FM) electrode. This schematic is adapted from [20]

the injection current. Spin injectiodetection was further realized in n-GaAs at low temperatures again by Lou et. al. In this case, they used a non-local four terminal (NL-4T) electrical geometry for injection-detection of spin signals[26]. This geometry ensures the detection of pure spin sig-nals, without background charge related sigsig-nals, and has demonstrated spin injection-detection in graphene with a spin relaxation time as large as 24 µm. The first electrical means of spin injection-detection in n-Si at room temperature was realized by Dash et. al. by employing three terminal (3T) electrical contacts[27]. The injector-detector consists of a ferromagnet and a thin oxide tunnel barrier in order to prevent conductivity mismatch problems. The various device schemes for spin injection-detection in semi-conductors are shown in Fig. 1.2.

Since the first demonstration of spin signals in silicon at room temperature, a lot of researchers have used the 3T geometry to detect spin voltage due to the Hanle effect by applying a perpendicular magnetic field. This is an effect where a coherent ensemble of spins that are injected into semicon-ductor dephase due to applied magnetic field out-of-plane. More details on Hanle effect is discussed in the chapter 2. There have been reports of spin signals in GaAs[28, 29], Germanium (Ge)[30] using different spin injection contacts (ferromagnets and oxide tunnel barrier). One of the advantages of using the 3T geometry over NL-4T, is that it can detect spin accumula-tion in semiconductors that possess low spin lifetime and relaxaaccumula-tion length. However, spin voltage obtained from 3T-geometry has its own drawbacks, that are summarized as follows:

Importance of the junction : Spin transport / hopping, via interface localized states that are likely to form across tunnel barrier /semiconduct-ing interface, can lead to a large enhancement in the strength (amplitude) of the Lorentzian Hanle responses[29]. Such enhancement in spin signals, deviating from the linear spin injection theory and anomalous scaling of the spin signals with junction resistance, has led to considerable debate about the origin of the spin signals. It is possible that either a more thorough theoretical understanding about the signals are missing or the signals do not originate from spin accumulation[31–33].

Tx-1.2. Spintronics Δμ Δμ Semiconductor localised states IDC SC - + -FM/AlOx VDC (a) (b) (c) Semiconductor FM FM B

Figure 1.3: (a) Displays two major issues with spin injection-detection using three (3T) terminal geometry. The issues essentially arises across the spin

injection-detection interface of a ferromagnet (FM) / AlOx on semiconductor (SC). (b)

shows the spin transport/hopping via localized states that eventually lead to anomalous increase of detected spin voltages (adapted from [34]) (c) shows the influence of local magnetostatic stray fields on spin accumulation.

operena et al. showed a new kind of magnetoresistance that occurs across tunnel barriers that are grown with impurities[31, 35]. They report the appearance of a Lorentzian magnetoresistance with both in- and out-of-plane applied magnetic field. This kind of magnetoresistance is shown to occur not only with ferromagnetic contacts but also with non-magnetic contacts. This has created a huge amount of controversy within the 3T community about the origin of spin signals. However, it is to be noted that this kind of magnetoresistance occurs only when the tunnel barriers are grown with defects. In the same paper, Txoperena et al. did not find any spin signals with magnetic field across well oxidized tunnel barriers. Other groups performed experiments across well oxidized Cu/SiO2/n-Si

and CoFe/Ta/SiO2/n-Si junctions[36]. These studies did not find any such

iaTMR effects but did observe Hanle effects. Moreover, Jansen et al. argued that the spin injection devices that use thermally driven Seebeck tunneling are not compatible with iaTMR theory[37, 38].

Interface roughness : In theory, by applying magnetic effects in-the-plane of transport should not affect the spin accumulation as it points in

the direction of the spin quantization axis. However, it has been shown that interface roughness across the ferromagnetic / tunnel barrier can in-duce local magnetostatic stray fields in the plane, that partially dephase the spin accumulation leading to a Lorentzian lineshape with an in-plane magnetic field. This is known as the “inverted Hanle effect”[34].

Junction limited spin lifetime: In many experiments, the spin lifetime is reported to be smaller than expected from theory. This raises questions on the reported spin lifetimes observed with 3T geometry. Is it the spin lifetime of the bulk semiconductor, limited by spin injection contacts, or the spin lifetime of interface states that is observed? The presence of an inverted Hanle effect would indicate that the spin lifetime is limited by magnetostatic fringe fields.

Electric field induced new spin transport phenomena : Due to all the longstanding issues of three terminal (3T) geometry regarding the origin of the detected signals, the realization of the proposition of Datta-Das spin field effect transistor (SFET) is still elusive[11]. Not only the spin detection signals but also the added functionalities and tailoring properties required to achieve that, is missing in the conventional semiconductors. This requires an extensive study and research on different material systems for an investigation on the tunability of the spin transport parameters with electric field and to unravel the mysteries and intricacies of spin transport.

1.2.2 Spin Hall effect and Magnonics

The last decade has witnessed a significant enhancement in the field of spin orbitronics. Generation of spin current in non-magnetic materials by ap-plying an electric charge current in a non-magnetic metal, is the spin Hall Effect (SHE)[39–42]. Such generation of spin current in nonmagnetic ma-terials, relies on the intrinsic spin-orbit interaction. Demonstration of SHE and inverse-SHE (generation of detectable charge voltage from spin cur-rent) assisted in the spin transport phenomena in magnetic insulators. The transfer of information as a spin current generated from the excitation of the quantized spin waves (magnons) across a magnetic insulator necessitates an external stimulus, where charge current injection is not possible[17–19, 43].

1.3. Complex Oxide thin films and bulk

By engineering an interface with ’heavy’ normal metals on magnetic insu-lators, it has been shown that it is not only possible to inject and detect current in a magnetic insulator, but also the spin information can be carried over long distances (orders of a micron) via excitation of magnons. This observation led to a branch of spintronics called ’magnonics’ that deals with the transport of spin current in magnetic insulators via magnons. Addi-tionally, it is also demonstrated that locally the spin current from magnetic insulators can be detected in the normal metal by Spin Hall Magnetore-sistance (SMR) and Spin Seebeck effect (SSE). The former have been suc-cessful in probing the magnetic ordering in a magnetic insulator[44–50] and the latter bears the information of magnons in a magnetic insulator[51– 53]. Such local techniques provide understanding on the magnetic ordering and orientations by electrical transport even without employing a non-local contacts.

1.3

Complex Oxide thin films and bulk

The strongly correlated interplay of lattice, spin and orbital degrees of freedom in complex oxide materials, has given rise to tunable physical properties, both in bulk and in thin film heterojunctions, including mag-netism, electronically rich two- dimensional electron gas (2DEG), high Tc-superconductivity, ferroelectricity and multiferroics[54–57]. The general chemical formula of complex oxide perovskite materials is ABO3, where A

is a metal (generally metals with smaller atomic number), B is a heavy transition metal (3d-5d electron systems) and O3 is the oxygen octahedron

around the transitional metal B. Hence, this class of complex oxides is of-ten called transition metal oxides or correlated oxides with a crystalline structure of a perovskite. The materials belonging to this class of oxides are generally insulating but can be made semiconducting / conducting by chemical doping. An introduction to the materials that have been investi-gated for spin transport in this thesis, is presented below.

1.3.1 Undoped and doped SrTiO3

SrTiO3 (STO) is an insulator with a band gap of 3.2 eV and a conduction

band dominated by d-orbitals derived from Ti4+_{. Doping Nb}5+_{at the Ti}4+

site results in an n-type semiconductor, Nb-doped SrTiO3 (Nb:STO), with

unconventional charge transport characteristics[58–60]. The electric field modulation of r in semiconducting Nb:STO provides a useful means of

tuning charge transport when a metal is interfaced with it[61–63].

Bulk STO is reported to have a large value of spin-orbit coupling (SOC), typically around 10 meV[64–66]. The breaking of inversion symmetry at the surface of STO results in a Rashba spin-orbit coupling that can be tuned with electric field[67]. This kind of SOC is found in the 2-dimensional elec-tron gas (2DEG) formed between LaAlO3 (LAO) and STO[55]. 2DEG is

desirable for spintronic applications as it offers the tunability of Rashba SOC and the realization of Datta and Das Spin polarized field effect tran-sistors (SFET)[11]. However, growing research in the field of 2DEG, has demonstrated magnetism and different magnetic textures across such in-terfaces that are detrimental to spin transport[68–70]. On the other hand, doped STO, especially, Nb-doped STO, a commercially available substrate, has shown promising spin transport properties. The strategic advantages of selecting this material are: (i) the conduction band is populated by Ti 3d-orbitals, that are more localized compared to s and p-orbitals and serves as the core of spin-orbit coupling; (ii) the Rashba spin-orbit coupling arising at the inversion broken symmetry can be modulated by electric field; and (iii) the strong tunability of dielectric permittivity with temperature and electric field, this happen because STO is a quantum paraelectric material. The dielectric permittivity increases to around 30,000 at 4K and thereby saturates due to setting in of quantum tunneling and suppressing a ferro-electric ground state[66, 71]. As a consequence of the increasing diferro-electric permittivity in STO, the donor levels in Nb:STO remains activated even at low temperatures as the donor binding energy near the conduction band is inversely proportional to the square of the dielectric permittivity[58]. Hence, Nb:STO experiences an absence of carrier freeze out at low tem-peratures. This also enhances electron mobility at all temperatures that

1.3. Complex Oxide thin films and bulk

are important for realizing spins with longer spin lifetime. However, the commercially available single crystalline substrates of Nb:STO are often prepared with iron (Fe) impurities that limit the spin lifetime in Nb:STO. Spin injection-detection across the semiconducting interface of Nb:STO was shown for the first time using 3T geometry by Han et al.[72]. They em-ployed a CoFe/MgO spin injection contact on Nb and La-doped STO and found that the spin lifetime decreases with increasing doping concentration. Increasing doping concentration increases the density of scattering centers in the Nb:STO crystals and the displaced Ti4+ acts as magnetic centers. This limits the spin lifetime in Nb:STO introducing additional dephasing of spins. Moreover, spin lifetime in Nb:STO is larger than in La:STO as La is a heavier atom compared to Nb. This results in additional spin dephas-ing due to increasdephas-ing spin-orbit interaction. However, the study of Han et al. did not address the tunability of the Schottky interface formed by the spin injection contacts and large room temperature dielectric Nb:STO (r

∼ 300)[72].

Engineering of a Schottky interface with spin injection contacts lead to unconventional charge transport characteristics. Maximum voltage drop across the Schottky interface and, due to high r compared to tunnel

bar-rier (AlOx), its bias modulation, allows tunable charge and spin transport

properties at the interface of Nb:STO. Kamerbeek et al., demonstrated a bias dependent modulation of spin transport properties across the interface of Nb:STO at room temperature, using a spin injection contact of Co/AlOx

(1.1 nm)[60, 73]. Bias dependent modulation of Rashba Spin orbit fields brought about by the built-in electric field across the interface of Nb:STO, leads to a bias dependent modulation of spin lifetime from 2 to 17 ps. The spin lifetimes reported by Han et al. (65 ps) and Kamerbeek et. al. (2-17 ps) are very low[73]. Moreover, whereas Rasbha spin orbit field limits the ratio between in-plane and out-of-plane spin accumulation voltages to 0.5, a bias dependent modulation of the anisotropy of the spin transport pa-rameter varies the ratio from 0.25 to 0.75. This indicates the presence of an additional anisotropic spin transport. The tunneling anisotropic magne-toresistance (TAMR) is as high as 1.6% at room temperature and it displays bias dependent modulation across the Schottky interface of Co/Nb:STO

us-ing 3T geometry.

The aforementioned observations suggest an opportunity to tune the Schot-tky interface of Nb:STO with a spin injection contact that leads to a bias dependent modulation of both spin injection and TAMR properties. This would expand the discussion beyond discrepancies of the origin of the spin signal using 3T geometry and offers an opportunity for research in the direction of new electric field induced spin transport properties.

1.3.2 Manganites and SrMnO3

The rare earth manganites with a chemical formula Re1−xAxMnO3, where

Re is the rare earth element that is chemically doped in the sites of A and

Mn3+/4+ being the transition element that bears the unpaired d-orbitals,

leading to magnetism.

The popular material for spintronics amongst the rare earth manganites is La0.66Sr0.33MnO3 (LSMO), due to its ferromagnetism extending above

room temperature (in bulk) and half metallicity. The ferromagnetic ex-change and metallicity in LSMO, is governed by a Double Exex-change (DE) mechanism postulated by Zener[74]. One of the interesting magnetore-sistance features in LSMO (also observed in other hole doped rare earth manganites), around the Curie temperature resulting from the DE mecha-nism, is the colossal magnetoresistance (CMR)[75]. CMR results in a large decrease in the magnetoresistance with applied magnetic field. The magne-toresponse is largest at the Curie temperature (Tc). Besides the observation

of CMR, such materials, especially thin films, have shown tunable magnetic properties due to the possible entanglement of strain and chemical doping. The Mn atom consists of 3d-orbitals, that split into five-fold degenerate levels of t2g (lower level) and eg (excited level) orbital states due to crystal

field splitting and Hund’s rule. The orbital occupation depends on the Mn valence state (3+/4+) and chemical doping in manganites. For the rare earth manganites, La1−xSrxMnO3 family, the first member of the family

is LaMnO3 (xSr = 0), is an antiferromagnetic insulator, due to a

superex-change interaction between Mn3+eg orbitals, as postulated by Goodenough

va-1.3. Complex Oxide thin films and bulk

lence state of Mn develops (both Mn3+ _{and Mn}4+_{) leading to an increasing}

DE mechanism. The hopping of electrons via O-2p orbitals leads to in-creased conductivity and hence the material becomes metallic with xSr ∼

0.33. On further increasing xSr, increasing superexchange interaction

medi-ated by Mn4+ t2g orbitals finally lead to another antiferromagnetic ground

state SrMnO3.

SrMnO3 : SrMnO3 (SMO) in bulk is stable in two crystalline forms,

Cubic SMO (C-SMO) and hexagonal (H-SMO) SMO. H SMO is in general stoichiometric at room temperature but increasing temperature by heating leads to loss of its stoichiometry. Both H and C -SMO are antiferromag-netic insulators below room temperature with Neel temperatures ranging from 260 K (C-SMO) to 290 K (H-SMO), the saturation magnetic moment being 2.6 ± 0.2 µB. Ideal C-SMO has a space group Pm3m[81]. The

mag-netic ordering is G-type, i.e. both intra and inter-planes are anti-parallel aligned, i.e. antiferromagnetically. This kind of ordering is quite in con-trast with the antiferromagnetic insulator LaMnO3 (LMO) (the left end in

the phase diagram in Fig.) LMO exhibits A-type ordering, i.e. the intra-planes are antiferromagnetically coupled and the inter-intra-planes are coupled ferromagnetically. The orbital ordering of Mn3+ induces an anisotropy in the magnetic ordering in LMO and is stabilized by Jahn-Tellar distortion. On the other hand, no Jahn Tellar distortion takes place in SMO due to orbital occupation in stable t2g orbitals of Mn4+. This makes the G-type

ordering in SMO very stable in bulk.

SrMnO3 thin films, however, show more opportunities to tailor magnetic

properties assisted by growth and lattice strain with the underlying sub-strate. It is well known from orbital physics of the manganites, that with the introduction of strain, the orbital occupation and the magnetism asso-ciated with such occupation is affected. The bond angle between Mn-O-Mn in an ideal cubic SMO is 180oallowing superexchange interaction as postu-lated by Goodenough. However, with decreasing bond-angle from linearity, i.e. by canting, a ferromagnetic superexchange is induced as postulated by Goodenough and Kanamori[76–80]. Ferromagnetic exchange is strongest with a bond angle of 90o. This already shows the versatility of the

mag-0 2 % 3 % -1 %

Compressive

G-type C-type A-type

4 %

FM FM

FE

FE

SrMnO

: a nominal AFM insulator

Figure 1.4: Schematic showing the strain induced different magnetic and

ferroelec-tric (FE) order in a nominal antiferromagnetic insulator : SrMnO3. This schematic

is sketched from the experimental findings and ab-initio calculations presented in [82, 83].

netic exchange affected by orbital and lattice degrees of freedom and can be engineered across thin film hetero-structures by introducing lattice strain (epitaxial strain) from the substrate. A tensile strain is shown to induce larger antiferromagnetic exchange and a compressive strain induces a weak ferromagnetism or a change in the magnetic ordering in SMO films. Such kinds of tunable magnetic ordering are observed in hole-doped manganite thin films and in SrMnO3[82, 83]. This is sketched in Fig. 1.4.

Researchers have used detection methods such as magnetic exchange bias observed in magnetization measurements using Superconducting Quantum Interference Device (SQUID), to show the existence of brownmillerite and perovskite features in SrMnO3 in a single film due to the competing

in-teractions of DE mechanism due to Mn3+ in brownmillerite and superex-change mechanism due to Mn4+ in perovskite[84]. Additionally, Maurel et al have shown with muon spectoscopy technique that with strain, the G-type antiferromagnetic order can be transformed to A-type[82]. Fur-ther interesting features such as interfacial spin glass is observed at the LSMO/SMO interface. The exchange bias is attributed to spin glass like

1.4. This thesis

features at the interface between the magnetic moments of total compen-sated SMO and ferromagnetic uncompencompen-sated LSMO at the interface[85]. It has been shown by other researchers that this exchange bias is driven by the Dzyaloshinkii-Moriya interaction (DMI) due to the oxygen octahedral tilting across the G-type SMO and LSMO. Researchers have also pointed out exchange biasing across ferromagnetic SrRuO3 (SRO) and SrMnO3

su-perlattices due to the hybridzation of Ru-O-Mn bonds, leading to a reduc-tion of the effective magnetizareduc-tion[86, 87]. SRO on the other hand, due to its multi-axial magnetic anisotropy and larger tendency of a perpendicular magnetic anisotropy (PMA) due to the compressive strain from the sub-strates, has been investigated to be a potential candidate for observation of topologically non-trivial magnetic features like skyrmion, bubbles and skyrmion-bubble by magnetotransport[88, 89]. Hence, the integration of SMO with SRO is interesting as it alters the hybridization of Ru-O bonds in SRO alone to Ru-O-Mn bonds at their interface[90, 91]. The presence of a G-type AFM (SMO) is highly favorable to reduce any magnetostatic dipolar interaction and additionally, an exchange bias driven DMI across SRO-SMO interface.

1.4

This thesis

In this thesis, an exploration of the applicability of three terminal (3T) geometry on the semiconducting platform of Nb:STO is presented. The work includes a selection of different doping concentrations of Nb-doped SrTiO3 and the development of a tunable Schottky interface where the

built-in electric field is tailored with applied bias and temperature. This gives rise to different magnetoresistive effects that are controlled by the electric field, demonstrating a new approach to tune the spin transport signals across the Schottky junction using 3T geometry.

The next part of the thesis deals with the growth and characterization of the antiferromagnetic SrMnO3 thin films for magnon based spin transport and

integration with the ferromagnet SrRuO3 for investigation of topologically

The thesis is arranged as the following:

• Chapter 2 deals with the theoretical concepts that aided the devel-opment of (i) spin injection-detection across semiconductors, (ii) Hall transport and a brief history of the Anomalous Hall Effect (AHE), and (iii) spin Hall effect assisted Spin Hall Magnetoresistance (SMR) and the Spin Seebeck Effect (SSE).

• Chapter 3 deals with: (i) the development of a Schottky interface across Nb:STO and the characterization of charge transport charac-teristics by thermal assisted emission and quantum tunneling; (ii) an electrostatic modelling using the classical Gauss theorem for the charge distribution across the Schottky interface performed to ex-tract the bias dependent modulation of the built-in electric field; (iii) Electric field induced new spin transport phenomena of Tunneling Anisotropic Magnetoresistance (TAMR) observed across the Schot-tky interface of Ni and Nb:STO; and (iv) a further investigation into the Schottky dependence of TAMR by fabricating and characterizing an exchange biased Co-CoO interface with Nb:STO.

• Chapter 4 deals with: (i) spin injection-detection across a predom-inantly Schottky interface with Ni and an ultrathin tunnel barrier of AlOx (7 ˚A- thick) on low doped Nb:STO (0.01 wt%) and (ii) the

evolution of magnetoresistance lineshapes due to the convolution of Hanle and TAMR signals and the creation of a new spin transport parameter.

• Chapter 5 deals with: (i) growth of antiferromagnetic SrMnO3

(SMO) thin films on SrTiO3 (STO), and (ii) characterization of the

thin films using bulk magnetic measurements (SQUID) and interface sensitive spin transport measurements by Spin Hall Magnetoresis-tance (SMR) and Spin Seebeck effect (SSE).

• Chapter 6 presents (i) an investigation of altering magneto-transport properties in thin films of SrRuO3 (SRO) grown on SrTiO3

charac-1.4. This thesis

terization using SQUID magnetization and magnetotransport across SRO/SMO interface. Finally a ’closing remark’ section is also added.

Bibliography

[1] G. Moore, “Progress in digital integrated electronics,” Electronics 38, pp. 114–117, 1975. [2] G. E. Moore, “Cramming more components onto integrated circuits, Reprinted from

Elec-tronics, volume 38, number 8, April 19, 1965, pp.114 ff.,” IEEE Solid-State Circuits Society Newsletter 11(3), pp. 33–35, 2009.

[3] M. N. Baibich, J. M. Broto, A. Fert, F. N. Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, “Giant magnetoresistance of (001)Fe/(001)Cr magnetic su-perlattices,” Physical Review Letters 61(21), pp. 2472–2475, 1988.

[4] G. Binasch, P. Gr¨unberg, F. Saurenbach, and W. Zinn, “Enhanced magnetoresistance in

layered magnetic structures with antiferromagnetic interlayer exchange,” Physical Review B 39(7), pp. 4828–4830, 1989.

[5] C. Chappert, A. Fert, and F. N. Van Dau, “The emergence of spin electronics in data storage,” Nature Materials 6(11), pp. 813–823, 2007.

[6] B. Behin-Aein, S. Salahuddin, and S. Datta, “Switching energy of ferromagnetic logic bits,” IEEE Transactions on Nanotechnology 8(4), pp. 505–514, 2009.

[7] P. Stanley-Marbell, V. C. Cabezas, and R. P. Luijten, “Pinned to the walls Impact of packaging and application properties on the memory and power walls,” Proceedings of the International Symposium on Low Power Electronics and Design , pp. 51–56, 2011. [8] H. S. Wong and S. Salahuddin, “Memory leads the way to better computing,” Nature

Nanotechnology 10(3), pp. 191–194, 2015.

[9] A. Chen, “Emerging research device roadmap and perspectives,” ICICDT 2014 - IEEE International Conference on Integrated Circuit Design and Technology , pp. 13–16, 2014. [10] K. Bernstein, R. K. Cavin, W. Porod, A. Seabaugh, and J. Welser, “Device and architecture

outlook for beyond CMOS switches,” Proceedings of the IEEE 98(12), pp. 2169–2184, 2010. [11] S. Datta and B. Das, “Electronic analog of the electro-optic modulator,” Applied Physics

Letters 56(7), pp. 665–667, 1990.

[12] I. ˇZuti´c, J. Fabian, and S. D. Sarma, “Spintronics: Fundamentals and applications,” Reviews

of Modern Physics 76(2), pp. 323–410, 2004.

[13] F. J. Jedema, H. B. Heersche, A. T. Filip, J. J. A. Baselmans, and B. J. Van Wees, “Electrical detection of spin precession in a metallic mesoscopic spin valve,” Nature 416(6882), pp. 713– 716, 2002.

[14] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. Van Wees, “Electronic spin transport and spin precession in single graphene layers at room temperature,” Na-ture 448(7153), pp. 571–574, 2007.

Bibliography

[15] M. Gurram, S. Omar, and B. J. Wees, “Bias induced up to 100% spin-injection and de-tection polarizations in ferromagnet/bilayer-hBN/graphene/hBN heterostructures,” Nature Communications 8(1), pp. 1–34, 2017.

[16] T. S. Ghiasi, J. Ingla-Ayn´es, A. A. Kaverzin, and B. J. Van Wees, “Large Proximity-Induced

Spin Lifetime Anisotropy in Transition-Metal Dichalcogenide/Graphene Heterostructures,” Nano Letters 17(12), pp. 7528–7532, 2017.

[17] L. J. Cornelissen, J. Liu, R. A. Duine, J. B. Youssef, and B. J. Van Wees, “Long-distance transport of magnon spin information in a magnetic insulator at room temperature,” Nature Physics 11(12), pp. 1022–1026, 2015.

[18] R. Lebrun, A. Ross, S. A. Bender, A. Qaiumzadeh, L. Baldrati, J. Cramer, A. Brataas,

R. A. Duine, and M. Kl¨aui, “Electrically controlled long-distance spin transport through an

antiferromagnetic insulator,” 2018.

[19] K. Oyanagi, S. Takahashi, L. J. Cornelissen, J. Shan, S. Daimon, T. Kikkawa, G. E. Bauer, B. J. van Wees, and E. Saitoh, “Spin transport in insulators without exchange stiffness,” Nature Communications 10(1), pp. 3–8, 2019.

[20] R. Jansen, S. P. Dash, S. Sharma, and B. C. Min, “Silicon spintronics with ferromagnetic tunnel devices,” Semiconductor Science and Technology 27(8), p. 083001, 2012.

[21] G. Kioseoglou, A. T. Hanbicki, R. Goswami, O. M. Van T Erve, C. H. Li, G. Spanos, P. E. Thompson, and B. T. Jonker, “Electrical spin injection into Si: A comparison between Fe/Si Schottky and Fe/ Al2 O3 tunnel contacts,” Applied Physics Letters 94(12), 2009. [22] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, and L. W.

Molenkamp, “Injection and detection of a spin-polarized current in a light-emitting diode,” Nature 402(6763), pp. 787–790, 1999.

[23] Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno, and D. D. Awschalom, “Elec-trical spin injection in a ferromagnetic semiconductor heterostructure,” Nature 402(6763), pp. 790–792, 1999.

[24] B. T. Jonker, G. Kioseoglou, A. T. Hanbicki, C. H. Li, and P. E. Thompson, “Electri-cal spin-injection into silicon from a ferromagnetic metal/tunnel barrier contact,” Nature Physics 3(8), pp. 542–546, 2007.

[25] I. Appelbaum, B. Huang, and D. J. Monsma, “Electronic measurement and control of spin transport in silicon,” Nature 447(7142), pp. 295–298, 2007.

[26] X. Lou, C. Adelmann, S. A. Crooker, E. S. Garlid, J. Zhang, K. S. M. Reddy, S. D. Flexner, C. J. Palmstrøm, and P. A. Crowell, “Electrical detection of spin transport in lateral ferromagnet-semiconductor devices,” Nature Physics 3(3), pp. 197–202, 2007. [27] S. P. Dash, S. Sharma, R. S. Patel, M. P. De Jong, and R. Jansen, “Electrical creation of

[28] F. Rortais, C. Vergnaud, C. Ducruet, C. Beign´e, A. Marty, J. P. Attan´e, J. Widiez, H. Jaffr`es, J. M. George, and M. Jamet, “Electrical spin injection in silicon and the role of defects,” Physical Review B 94(17), pp. 1–7, 2016.

[29] M. Tran, H. Jaffr`es, C. Deranlot, J. M. George, A. Fert, A. Miard, and A. Lemaˆıtre,

“Enhancement of the spin accumulation at the interface between a spin-polarized tunnel junction and a semiconductor,” Physical Review Letters 102(3), pp. 1–4, 2009.

[30] Y. Zhou, W. Han, L. T. Chang, F. Xiu, M. Wang, M. Oehme, I. A. Fischer, J. Schulze, R. K. Kawakami, and K. L. Wang, “Electrical spin injection and transport in germanium,” Physical Review B - Condensed Matter and Materials Physics 84(12), pp. 1–7, 2011. [31] O. Txoperena, M. Gobbi, A. Bedoya-Pinto, F. Golmar, X. Sun, L. E. Hueso, and

F. Casanova, “How reliable are Hanle measurements in metals in a three-terminal geome-try?,” Applied Physics Letters 102(19), pp. 1–6, 2013.

[32] K. R. Jeon, B. C. Min, Y. H. Jo, H. S. Lee, I. J. Shin, C. Y. Park, S. Y. Park, and S. C. Shin, “Electrical spin injection and accumulation in CoFe/MgO/Ge contacts at room temperature,” Physical Review B - Condensed Matter and Materials Physics 84(16), pp. 1– 10, 2011.

[33] S. Sharma, A. Spiesser, S. P. Dash, S. Iba, S. Watanabe, B. J. Van Wees, H. Saito, S. Yuasa, and R. Jansen, “Anomalous scaling of spin accumulation in ferromagnetic tunnel devices with silicon and germanium,” Physical Review B - Condensed Matter and Materials Physics 89(7), pp. 1–10, 2014.

[34] S. P. Dash, S. Sharma, J. C. Le Breton, J. Peiro, H. Jaffr`es, J. M. George, A. Lemaˆıtre,

and R. Jansen, “Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface,” Physical Review B - Condensed Matter and Materials Physics 84(5), pp. 1–11, 2011.

[35] O. Txoperena and F. Casanova, “Spin injection and local magnetoresistance effects in three-terminal devices,” Journal of Physics D: Applied Physics 49(13), p. 133001, 2016.

[36] S. He, J. H. Lee, P. Gr¨unberg, and B. K. Cho, “Angular variation of oblique Hanle effect in

CoFe/SiO2/Si and CoFe/Ta/SiO2/Si tunnel contacts,” Journal of Applied Physics 119(11), 2016.

[37] J. C. Le Breton, S. Sharma, H. Saito, S. Yuasa, and R. Jansen, “Thermal spin current from a ferromagnet to silicon by Seebeck spin tunnelling,” Nature 475(7354), pp. 82–85, 2011. [38] K. R. Jeon, B. C. Min, A. Spiesser, H. Saito, S. C. Shin, S. Yuasa, and R. Jansen, “Voltage

tuning of thermal spin current in ferromagnetic tunnel contacts to semiconductors,” Nature Materials 13(4), pp. 360–366, 2014.

[39] M. D’yakonov and V. I. Perel’, “Possibility of orienting electron spins with current,” 1971. [40] S. O. Valenzuela and M. Tinkham, “Direct electronic measurement of the spin Hall effect,”

Bibliography

[41] Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, “Observation of the spin hall effect in semiconductors,” Science 306(5703), pp. 1910–1913, 2004.

[42] S. Murakami and N. Nagaosa, “Spin Hall Effect,” Comprehensive Semiconductor Science and Technology 1-6, pp. 222–278, 2011.

[43] K. S. Das, J. Liu, B. J. Van Wees, and I. J. Vera-Marun, “Efficient Injection and Detection of Out-of-Plane Spins via the Anomalous Spin Hall Effect in Permalloy Nanowires,” Nano Letters 18(9), pp. 5633–5639, 2018.

[44] Y. T. Chen, S. Takahashi, H. Nakayama, M. Althammer, S. T. Goennenwein, E. Saitoh, and G. E. Bauer, “Theory of spin Hall magnetoresistance,” Physical Review B - Condensed Matter and Materials Physics 87(14), 2013.

[45] T. Shang, Q. F. Zhan, H. L. Yang, Z. H. Zuo, Y. L. Xie, L. P. Liu, S. L. Zhang, Y. Zhang, H. H. Li, B. M. Wang, Y. H. Wu, S. Zhang, and R. W. Li, “Effect of NiO inserted layer on spin-Hall magnetoresistance in Pt/NiO/YIG heterostructures,” Applied Physics Let-ters 109(3), 2016.

[46] G. R. Hoogeboom, A. Aqeel, T. Kuschel, T. T. Palstra, and B. J. Van Wees, “Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO,” Applied Physics Letters 111(5), 2017.

[47] N. Vlietstra, J. Shan, V. Castel, J. Ben Youssef, G. E. Bauer, and B. J. Van Wees, “Exchange magnetic field torques in YIG/Pt bilayers observed by the spin-Hall magnetoresistance,” Applied Physics Letters 103(3), pp. 1–5, 2013.

[48] N. Vlietstra, J. Shan, B. J. Van Wees, M. Isasa, F. Casanova, and J. Ben Youssef, “Simulta-neous detection of the spin-Hall magnetoresistance and the spin-Seebeck effect in platinum and tantalum on yttrium iron garnet,” Physical Review B - Condensed Matter and Mate-rials Physics 90(17), pp. 1–8, 2014.

[49] A. Aqeel, N. Vlietstra, J. A. Heuver, G. E. W. Bauer, B. Noheda, B. J. Van Wees, and T. T. M. Palstra, “Spin-Hall magnetoresistance and spin Seebeck effect in spin-spiral and paramagnetic phases of multiferroic CoCr2O4 films,” Physical Review B - Condensed Matter and Materials Physics 92(22), pp. 1–8, 2015.

[50] A. Aqeel, N. Vlietstra, A. Roy, M. Mostovoy, B. J. Van Wees, and T. T. Palstra, “Electrical detection of spiral spin structures in Pt—Cu2OSeO3 heterostructures,” Physical Review B 94(13), pp. 1–11, 2016.

[51] S. M. Rezende, R. L. Rodr´ıguez-Su´arez, and A. Azevedo, “Theory of the spin Seebeck effect

in antiferromagnets,” Physical Review B 93(1), pp. 1–9, 2016.

[52] S. M. Rezende, A. Azevedo, and R. L. Rodr´ıguez-Su´arez, “Introduction to antiferromagnetic

magnons,” Journal of Applied Physics 126(15), 2019.

[53] K. Uchida, T. Ota, H. Adachi, J. Xiao, T. Nonaka, Y. Kajiwara, G. E. W. Bauer, S. Maekawa, and E. Saitoh, “Thermal spin pumping and magnon-phonon-mediated spin-Seebeck effect,” Journal of Applied Physics 111(10), 2012.

[54] R. Ramesh and N. A. Spaldin, “Multiferroics: Progress and prospects in thin films,” Nature Materials 3, pp. 20–28, 2009.

[55] A. Ohtomo and H. Y. Hwang, “Surface depletion in doped SrTiO3 thin films,” Applied Physics Letters 84(10), pp. 1716–1718, 2004.

[56] Y. Z. Chen, F. Trier, T. Wijnands, R. J. Green, N. Gauquelin, R. Egoavil, D. V. Chris-tensen, G. Koster, M. Huijben, N. Bovet, S. Macke, F. He, R. Sutarto, N. H. Andersen, J. A. Sulpizio, M. Honig, G. E. Prawiroatmodjo, T. S. Jespersen, S. Linderoth, S. Ilani, J. Ver-beeck, G. Van Tendeloo, G. Rijnders, G. A. Sawatzky, and N. Pryds, “Extreme mobility en-hancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping,” Nature Materials 14(8), pp. 801–806, 2015.

[57] P. Zubko, S. Gariglio, M. Gabay, P. Ghosez, and J.-M. Triscone, “Interface Physics in Com-plex Oxide Heterostructures,” Annual Review of Condensed Matter Physics 2(1), pp. 141– 165, 2011.

[58] A. Spinelli, M. A. Torija, C. Liu, C. Jan, and C. Leighton, “Electronic transport in doped

SrTiO3 : Conduction mechanisms and potential applications,” Physical Review B 81(15),

p. 155110, 2010.

[59] K. G. Rana, V. Khikhlovskyi, and T. Banerjee, “Electrical transport across Au/Nb:SrTiO 3 Schottky interface with different Nb doping,” Applied Physics Letters 100(21), pp. 98–101, 2012.

[60] A. M. Kamerbeek, E. K. De Vries, A. Dankert, S. P. Dash, B. J. Van Wees, and T. Banerjee, “Electric field effects on spin accumulation in Nb-doped SrTiO3 using tunable spin injection contacts at room temperature,” Applied Physics Letters 104(21), 2014.

[61] H. Hasegawa and T. Nishino, “Electrical properties of Au/Nb-doped-SrTiO ₃ contact,” Journal of Applied Physics 69(3), pp. 1501–1505, 1991.

[62] S. Suzuki, T. Yamamoto, H. Suzuki, K. Kawaguchi, K. Takahashi, and Y. Yoshisato, “Fab-rication and characterization of Ba1xKxBiO3/Nb-doped SrTiO3 all-oxide-type Schottky junctions,” Journal of Applied Physics 81(10), pp. 6830–6836, 1997.

[63] A. Kamerbeek, T. Banerjee, and R. Hueting, “Electrostatic analysis of n-doped SrTiO3

metal-insulator-semiconductor systems,” Journal of Applied Physics 118(22), pp. 3–8, 2015. [64] G. Khalsa, B. Lee, and A. H. Macdonald, “Theory of t2g electron-gas Rashba interactions,”

Physical Review B - Condensed Matter and Materials Physics 88(4), pp. 1–5, 2013. [65] H. Nakamura, T. Koga, and T. Kimura, “Experimental evidence of cubic Rashba effect in

an inversion-symmetric oxide,” Physical Review Letters 108(20), pp. 1–5, 2012.

[66] J. A. Sulpizio, S. Ilani, P. Irvin, and J. Levy, “Nanoscale Phenomena in Oxide Heterostruc-tures,” Annual Review of Materials Research 44(1), pp. 117–149, 2014.

Bibliography

[67] A. D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J. M. Triscone, “Tun-able Rashba Spin-Orbit Interaction at Oxide Interfaces,” Physical Review Letters 104(12), pp. 1–4, 2010.

[68] J. A. Bert, B. Kalisky, C. Bell, M. Kim, Y. Hikita, H. Y. Hwang, and K. A. Moler, “Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface,” Nature Physics 7(10), pp. 767–771, 2011.

[69] M. Gabay and J. M. Triscone, “Oxide heterostructures: Hund rules with a twist,” Nature Physics 9(10), pp. 610–611, 2013.

[70] J. Rowland, S. Banerjee, and M. Randeria, “Skyrmions in chiral magnets with Rashba and Dresselhaus spin-orbit coupling,” Physical Review B 93(2), pp. 1–5, 2016.

[71] R. C. Neville, B. Hoeneisen, and C. A. Mead, “Permittivity of strontium titanate,” Journal of Applied Physics 43(5), pp. 2124–2131, 1972.

[72] W. Han, X. Jiang, A. Kajdos, S. H. Yang, S. Stemmer, and S. S. P. Parkin, “Spin injection and detection in lanthanum-and niobium-doped SrTiO 3 using the Hanle technique,” Nature Communications 4, p. 2134, 2013.

[73] A. M. Kamerbeek, P. H¨ogl, J. Fabian, and T. Banerjee, “Electric Field Control of Spin

Lifetimes in Nb-SrTiO3 by Spin-Orbit Fields,” Physical Review Letters 115(3), pp. 1–5, 2015.

[74] “Interaction between the d-shells in the transition metals. ii ferromagnetic compounds of manganese with perovskite structure,” Physical Review 82, pp. 403–405, 1951.

[75] J.-P. Renard and a M Haghiri-Gosnet, “CMR manganites: physics, thin films and devices,” Journal of Physics D: Applied Physics 36(8), pp. R127—-R150, 2003.

[76] J. B. Goodenough, “Theory of the role of covalence in the perovskite-type manganites [La,M(II)]MnO3,” Physical Review 100(2), pp. 564–573, 1955.

[77] J. B. Goodenough, “An interpretation of the magnetic properties of the perovskite-type mixed crystals La1-xSrxCoO3-λ,” Journal of Physics and Chemistry of Solids 6(2-3), pp. 287–297, 1958.

[78] J. Kanamori, “Superexchange interaction and symmetry properties of electron orbitals,” Journal of Physics and Chemistry of Solids 10(2-3), pp. 87–98, 1959.

[79] J. Kanamori, “Crystal distortion in magnetic compounds,” Journal of Applied

Physics 14(5), 1960.

[80] P. W. Anderson, “Antiferromagnetism. Theory of Suyerexchange,” Phys. Rev. 79(2), pp. 350–356, 1950.

[81] R. Sønden˚a, P. Ravindran, S. Stølen, T. Grande, and M. Hanfland, “Electronic structure

and magnetic properties of cubic and hexagonal SrMn O3,” Physical Review B - Condensed Matter and Materials Physics 74(14), pp. 1–12, 2006.

[82] L. Maurel, N. Marcano, T. Prokscha, E. Langenberg, J. Blasco, R. Guzm´an, A. Suter,

C. Mag´en, L. Morell´on, M. R. Ibarra, J. A. Pardo, and P. A. Algarabel, “Nature of

antifer-romagnetic order in epitaxially strained multiferroic SrMnO3 thin films,” Physical Review B - Condensed Matter and Materials Physics 92(2), pp. 1–9, 2015.

[83] J. H. Lee and K. M. Rabe, “Epitaxial-strain-induced multiferroicity in SrMnO3 from first principles,” Physical Review Letters 104(20), pp. 2–5, 2010.

[84] F. Wang, Y. Q. Zhang, Y. Bai, W. Liu, H. R. Zhang, W. Y. Wang, S. K. Li, S. Ma, X. G. Zhao, J. R. Sun, Z. H. Wang, Z. J. Wang, and Z. D. Zhang, “Oxygen vacancy formation, crystal structures, and magnetic properties of three SrMnO3-δ films,” Applied Physics Letters 109(5), 2016.

[85] J. F. Ding, O. I. Lebedev, S. Turner, Y. F. Tian, W. J. Hu, J. W. Seo, C. Panagopoulos, W. Prellier, G. Van Tendeloo, and T. Wu, “Interfacial spin glass state and exchange bias in manganite bilayers with competing magnetic orders,” Physical Review B - Condensed Matter and Materials Physics 87(5), pp. 1–7, 2013.

[86] S. Dong, K. Yamauchi, S. Yunoki, R. Yu, S. Liang, A. Moreo, J. M. Liu, S. Picozzi, and E. Dagotto, “Exchange bias driven by the dzyaloshinskii-moriya interaction and ferroelec-tric polarization at G-type antiferromagnetic perovskite interfaces,” Physical Review Let-ters 103(12), pp. 5–8, 2009.

[87] S. Dong, Q. Zhang, S. Yunoki, J. M. Liu, and E. Dagotto, “Ab initio study of the intrinsic exchange bias at the SrRuO 3/SrMnO 3 interface,” Physical Review B - Condensed Matter and Materials Physics 84(22), pp. 1–6, 2011.

[88] J. Matsuno, N. Ogawa, K. Yasuda, F. Kagawa, W. Koshibae, and N. Nagaosa, “Interface-driven topological Hall effect in,” Science Advances (July), pp. 1–8, 2016.

[89] P. Zhang, A. Das, E. Barts, M. Azhar, L. Si, K. Held, M. Mostovoy, and T. Banerjee, “Robust skyrmion-bubble textures in SrRuO3 thin films stabilized by magnetic anisotropy,” arXiv 2(3), pp. 1–6, 2020.

[90] P. Padhan and W. Prellier, “Direct observation of pinned/biased moments in magnetic superlattices,” Physical Review B - Condensed Matter and Materials Physics 72(10), pp. 1– 5, 2005.

[91] P. Padhan and W. Prellier, “Coercivity enhancement in the SrRuQ3/SrMnQ3 superlat-tices,” Applied Physics Letters 88(26), pp. 2004–2007, 2006.