University of Groningen

Spin transport across oxide semiconductors and antiferromagnetic oxide interfacesDas, Arijit

DOI:10.33612/diss.150692255

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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-twentiethcentury, a digital revolution was witnessed with the discovery of electronicsin the form of transistors and semiconductor technology. Within a decade ofthe 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’slaw that states that the number of transistors on a chip will double everytwo years. Moore’s law has been a source of continuous innovation andtechnology for many decades[2]. In the past 50 years, we have benefitedfrom an increasing number of electrical components in an integrated circuitenabled 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 chargetunneling, increasing leakage current and cross-talk between neighboringtransistors, large power consumption, thermal dissipation etc. As the fun-damental limits of downscaling are approached, new technologies are arising

Chapter 1. Introduction

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 electronics, also known as “spintronics,” has been a focus ofresearch 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 spinvalve devices in 1988, for which the Nobel Prize in Physics was awarded toAlbert 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 observedmagnetoresponses with external field was first observed across a multilayerstack of alternating thin layers of Fe and Cr, constituting a spin valvedevice. With varying magnetic field, the relative magnetization of the fer-romagnets (Fe in this case) are altered, leading to a large change in thecharge resistance[3, 4]. This resistance can be stored as a bit, hence pavingthe way for the downscaling of non-volatile memory elements in electronicdevices. This is how spintronics has found application in design of the mag-netic read-heads of hard disk drives and magnetic random access memory(MRAM)[5]. Later, GMR variation was further strengthened by addinga 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 MRAMtechnology.So far, spintronics is based on the flow of spin polarized charge carriers dueto an application of charge current. Hence, the transport of spins requireslower power compared to the flow of electrons. This idea has led to therealization 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

2

1.2. Spintronics

n / p - Si / SiO2

Source Drain

VG (Gate)

Thin 2D semiconducting channel

VG = 0 (ON)

VG > 0 (OFF)

MetalInsulator (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 theinjector and detector. The third terminal acts as the manipulator, that exerts anelectric field (VG > 0) across the 2-dimensional electron gas (2-deg) channel andgradually dephases the spins that are flowing across the channel leading to a zerodetection 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 thirdterminal can be thought of as the gate voltage. the idea of this sketch is adaptedfrom [11, 12].

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

Driven by the success of spintronics, significant interest has been shownin the manipulation of data stored in logic-based applications. This hascreated 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 logicand memory, the non-volatile nature of spin states, and the effective andfaster switching of spin / magnetization states (order of GHz for ferromag-nets and THz for antiferromagnets), laid the foundation of spintronics: inmetals[13]; semiconductors; two-dimensional materials such as graphene[14,15]; transitional metal dichalcogenides[16]; and, very recently, in magneticinsulators[17–19].One of the important proposition on the possible integration of non volatile

3

Chapter 1. Introduction

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 suchtransistor. It constitutes of 3 terminal leads of source, drain and gate,similar to the working principle of a field effect transistor. The injectedspin polarized current via source, introduces spins to the 2-Dimensionalelectron gas (2-DEG) channel shown in yellow. In the channel, the phaseof the spins are disturbed by the gate channel that introduces an electronfield that results in an effective magnetic field in the reference frame of thediffusing spins in the 2-DEG channel. This causes a spin dephasing andno 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 alongthe channel and is detected as a spin voltage response at the drain whichis an ON state. This proposition highlights the consideration of an electricfield manipulation of spin transport which in conventional semiconductorsystem is not demonstrated. This has opened a research opportunities tolook for semiconductors with tunable electronic properties.This thesis explores the possibilities of efficient spin injection-detection insemiconductors 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 wasachieved by electrical means on silicon. But the spin detection was doneoptically, using a Light Emitting diode (LED) structure, where the circu-larly polarized luminescence is produced when the injected spin-polarizedcarriers recombine[21–24]. All electrical means of spin injection-detectionwas realized by Appelbaum et. al. by sourcing hot electrons (with anenergy higher than the Fermi energy level, E ≥ EF + kBT ) through fer-romagnetic thin films on Si[25]. However, hot electron approach had amajor drawback, i.e. the detection current was 10−4 times smaller than

4

1.2. Spintronics

Current source Voltage Current source Voltage (Non-local)

p -type

n -type

Inje

ctor

cur

rent

Emitter current

Collector current

(a) (b)

(c) (d)

FM

FM

Silicon

LED

tunnel barrier

FM polarizer

FM analyzer

collector semiconductor

e-

h+

FM FM

Silicon Silicon

(1)

(2)

Silicon

injector detector

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

5

Chapter 1. Introduction

the injection current. Spin injection-detection was further realized in n-GaAs at low temperatures again by Lou et. al. In this case, they used anon-local four terminal (NL-4T) electrical geometry for injection-detectionof spin signals[26]. This geometry ensures the detection of pure spin sig-nals, without background charge related signals, and has demonstratedspin injection-detection in graphene with a spin relaxation time as largeas 24 µm. The first electrical means of spin injection-detection in n-Si atroom temperature was realized by Dash et. al. by employing three terminal(3T) electrical contacts[27]. The injector-detector consists of a ferromagnetand a thin oxide tunnel barrier in order to prevent conductivity mismatchproblems. 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 dueto the Hanle effect by applying a perpendicular magnetic field. This is aneffect where a coherent ensemble of spins that are injected into semicon-ductor dephase due to applied magnetic field out-of-plane. More details onHanle effect is discussed in the chapter 2. There have been reports of spinsignals in GaAs[28, 29], Germanium (Ge)[30] using different spin injectioncontacts (ferromagnets and oxide tunnel barrier). One of the advantagesof using the 3T geometry over NL-4T, is that it can detect spin accumula-tion in semiconductors that possess low spin lifetime and relaxation 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 interfacelocalized 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 thespin signals with junction resistance, has led to considerable debate aboutthe origin of the spin signals. It is possible that either a more thoroughtheoretical understanding about the signals are missing or the signals donot originate from spin accumulation[31–33].

Impurity assisted tunneling magnetoresistance (iaTMR) : Tx-

6

1.2. Spintronics

Δμ

Δμ

Semiconductor

localised states

IDC

SC

- -+

FM/AlOx

VDC(a)

(b) (c)

SemiconductorFM

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 toanomalous increase of detected spin voltages (adapted from [34]) (c) shows theinfluence of local magnetostatic stray fields on spin accumulation.

operena et al. showed a new kind of magnetoresistance that occurs acrosstunnel barriers that are grown with impurities[31, 35]. They report theappearance of a Lorentzian magnetoresistance with both in- and out-of-plane applied magnetic field. This kind of magnetoresistance is shown tooccur not only with ferromagnetic contacts but also with non-magneticcontacts. This has created a huge amount of controversy within the 3Tcommunity about the origin of spin signals. However, it is to be notedthat this kind of magnetoresistance occurs only when the tunnel barriersare grown with defects. In the same paper, Txoperena et al. did not findany spin signals with magnetic field across well oxidized tunnel barriers.Other groups performed experiments across well oxidized Cu/SiO2/n-Siand CoFe/Ta/SiO2/n-Si junctions[36]. These studies did not find any suchiaTMR effects but did observe Hanle effects. Moreover, Jansen et al. arguedthat the spin injection devices that use thermally driven Seebeck tunnelingare 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

7

Chapter 1. Introduction

the direction of the spin quantization axis. However, it has been shownthat interface roughness across the ferromagnetic / tunnel barrier can in-duce local magnetostatic stray fields in the plane, that partially dephasethe spin accumulation leading to a Lorentzian lineshape with an in-planemagnetic field. This is known as the “inverted Hanle effect”[34].Junction limited spin lifetime: In many experiments, the spin lifetimeis reported to be smaller than expected from theory. This raises questionson the reported spin lifetimes observed with 3T geometry. Is it the spinlifetime of the bulk semiconductor, limited by spin injection contacts, orthe spin lifetime of interface states that is observed? The presence of aninverted Hanle effect would indicate that the spin lifetime is limited bymagnetostatic fringe fields.Electric field induced new spin transport phenomena : Due toall the longstanding issues of three terminal (3T) geometry regarding theorigin of the detected signals, the realization of the proposition of Datta-Das spin field effect transistor (SFET) is still elusive[11]. Not only the spindetection signals but also the added functionalities and tailoring propertiesrequired to achieve that, is missing in the conventional semiconductors.This requires an extensive study and research on different material systemsfor an investigation on the tunability of the spin transport parameters withelectric 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 spinorbitronics. Generation of spin current in non-magnetic materials by ap-plying an electric charge current in a non-magnetic metal, is the spin HallEffect (SHE)[39–42]. Such generation of spin current in nonmagnetic ma-terials, relies on the intrinsic spin-orbit interaction. Demonstration of SHEand inverse-SHE (generation of detectable charge voltage from spin cur-rent) assisted in the spin transport phenomena in magnetic insulators. Thetransfer of information as a spin current generated from the excitation of thequantized spin waves (magnons) across a magnetic insulator necessitates anexternal stimulus, where charge current injection is not possible[17–19, 43].

8

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 detectcurrent in a magnetic insulator, but also the spin information can be carriedover long distances (orders of a micron) via excitation of magnons. Thisobservation led to a branch of spintronics called ’magnonics’ that deals withthe transport of spin current in magnetic insulators via magnons. Addi-tionally, it is also demonstrated that locally the spin current from magneticinsulators 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] andthe latter bears the information of magnons in a magnetic insulator[51–53]. Such local techniques provide understanding on the magnetic orderingand orientations by electrical transport even without employing a non-localcontacts.

1.3 Complex Oxide thin films and bulk

The strongly correlated interplay of lattice, spin and orbital degrees offreedom in complex oxide materials, has given rise to tunable physicalproperties, 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 generalchemical formula of complex oxide perovskite materials is ABO3, where Ais a metal (generally metals with smaller atomic number), B is a heavytransition metal (3d-5d electron systems) and O3 is the oxygen octahedronaround the transitional metal B. Hence, this class of complex oxides is of-ten called transition metal oxides or correlated oxides with a crystallinestructure of a perovskite. The materials belonging to this class of oxidesare generally insulating but can be made semiconducting / conducting bychemical doping. An introduction to the materials that have been investi-gated for spin transport in this thesis, is presented below.

9

Chapter 1. Introduction

1.3.1 Undoped and doped SrTiO3

SrTiO3 (STO) is an insulator with a band gap of 3.2 eV and a conductionband dominated by d-orbitals derived from Ti4+. Doping Nb5+ at the Ti4+

site results in an n-type semiconductor, Nb-doped SrTiO3 (Nb:STO), withunconventional charge transport characteristics[58–60]. The electric fieldmodulation of εr in semiconducting Nb:STO provides a useful means oftuning 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 thesurface of STO results in a Rashba spin-orbit coupling that can be tunedwith 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 isdesirable for spintronic applications as it offers the tunability of RashbaSOC and the realization of Datta and Das Spin polarized field effect tran-sistors (SFET)[11]. However, growing research in the field of 2DEG, hasdemonstrated 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 ofselecting 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 asthe core of spin-orbit coupling; (ii) the Rashba spin-orbit coupling arisingat the inversion broken symmetry can be modulated by electric field; and(iii) the strong tunability of dielectric permittivity with temperature andelectric field, this happen because STO is a quantum paraelectric material.The dielectric permittivity increases to around 30,000 at 4K and therebysaturates due to setting in of quantum tunneling and suppressing a ferro-electric ground state[66, 71]. As a consequence of the increasing dielectricpermittivity in STO, the donor levels in Nb:STO remains activated evenat low temperatures as the donor binding energy near the conduction bandis 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

10

1.3. Complex Oxide thin films and bulk

are important for realizing spins with longer spin lifetime. However, thecommercially available single crystalline substrates of Nb:STO are oftenprepared with iron (Fe) impurities that limit the spin lifetime in Nb:STO.Spin injection-detection across the semiconducting interface of Nb:STO wasshown 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 andfound that the spin lifetime decreases with increasing doping concentration.Increasing doping concentration increases the density of scattering centersin the Nb:STO crystals and the displaced Ti4+ acts as magnetic centers.This limits the spin lifetime in Nb:STO introducing additional dephasingof spins. Moreover, spin lifetime in Nb:STO is larger than in La:STO as Lais a heavier atom compared to Nb. This results in additional spin dephas-ing due to increasing spin-orbit interaction. However, the study of Han etal. did not address the tunability of the Schottky interface formed by thespin injection contacts and large room temperature dielectric Nb:STO (εr∼ 300)[72].Engineering of a Schottky interface with spin injection contacts lead tounconventional charge transport characteristics. Maximum voltage dropacross the Schottky interface and, due to high εr compared to tunnel bar-rier (AlOx), its bias modulation, allows tunable charge and spin transportproperties at the interface of Nb:STO. Kamerbeek et al., demonstrated abias dependent modulation of spin transport properties across the interfaceof 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 fieldsbrought 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. Thespin lifetimes reported by Han et al. (65 ps) and Kamerbeek et. al. (2-17ps) are very low[73]. Moreover, whereas Rasbha spin orbit field limits theratio 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 ofan additional anisotropic spin transport. The tunneling anisotropic magne-toresistance (TAMR) is as high as 1.6% at room temperature and it displaysbias dependent modulation across the Schottky interface of Co/Nb:STO us-

11

Chapter 1. Introduction

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 biasdependent modulation of both spin injection and TAMR properties. Thiswould expand the discussion beyond discrepancies of the origin of the spinsignal using 3T geometry and offers an opportunity for research in thedirection of new electric field induced spin transport properties.

1.3.2 Manganites and SrMnO3

The rare earth manganites with a chemical formula Re1−xAxMnO3, whereRe is the rare earth element that is chemically doped in the sites of A andMn3+/4+ being the transition element that bears the unpaired d-orbitals,leading to magnetism.The popular material for spintronics amongst the rare earth manganitesis La0.66Sr0.33MnO3 (LSMO), due to its ferromagnetism extending aboveroom temperature (in bulk) and half metallicity. The ferromagnetic ex-change and metallicity in LSMO, is governed by a Double Exchange (DE)mechanism postulated by Zener[74]. One of the interesting magnetore-sistance features in LSMO (also observed in other hole doped rare earthmanganites), around the Curie temperature resulting from the DE mecha-nism, is the colossal magnetoresistance (CMR)[75]. CMR results in a largedecrease in the magnetoresistance with applied magnetic field. The magne-toresponse is largest at the Curie temperature (Tc). Besides the observationof CMR, such materials, especially thin films, have shown tunable magneticproperties due to the possible entanglement of strain and chemical doping.The Mn atom consists of 3d-orbitals, that split into five-fold degeneratelevels of t2g (lower level) and eg (excited level) orbital states due to crystalfield splitting and Hund’s rule. The orbital occupation depends on the Mnvalence state (3+/4+) and chemical doping in manganites. For the rareearth manganites, La1−xSrxMnO3 family, the first member of the familyis LaMnO3 (xSr = 0), is an antiferromagnetic insulator, due to a superex-change interaction between Mn3+ eg orbitals, as postulated by Goodenoughand Kanamori[76–80]. With increasing chemical doping (xSr), a mixed va-

12

1.3. Complex Oxide thin films and bulk

lence state of Mn develops (both Mn3+ and Mn4+) leading to an increasingDE 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 groundstate 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 generalstoichiometric at room temperature but increasing temperature by heatingleads to loss of its stoichiometry. Both H and C -SMO are antiferromag-netic insulators below room temperature with Neel temperatures rangingfrom 260 K (C-SMO) to 290 K (H-SMO), the saturation magnetic momentbeing 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-parallelaligned, i.e. antiferromagnetically. This kind of ordering is quite in con-trast with the antiferromagnetic insulator LaMnO3 (LMO) (the left end inthe phase diagram in Fig.) LMO exhibits A-type ordering, i.e. the intra-planes are antiferromagnetically coupled and the inter-planes are coupledferromagnetically. The orbital ordering of Mn3+ induces an anisotropy inthe magnetic ordering in LMO and is stabilized by Jahn-Tellar distortion.On the other hand, no Jahn Tellar distortion takes place in SMO due toorbital occupation in stable t2g orbitals of Mn4+. This makes the G-typeordering 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 withthe introduction of strain, the orbital occupation and the magnetism asso-ciated with such occupation is affected. The bond angle between Mn-O-Mnin an ideal cubic SMO is 180o allowing 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 byGoodenough and Kanamori[76–80]. Ferromagnetic exchange is strongestwith a bond angle of 90o. This already shows the versatility of the mag-

13

Chapter 1. Introduction

0 2 % 3 %-1 %

Compressive Tensile

G-type C-type A-type

4 %

FMFM

FEFE

SrMnO3 : 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 schematicis 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 beengineered across thin film hetero-structures by introducing lattice strain(epitaxial strain) from the substrate. A tensile strain is shown to inducelarger antiferromagnetic exchange and a compressive strain induces a weakferromagnetism or a change in the magnetic ordering in SMO films. Suchkinds of tunable magnetic ordering are observed in hole-doped manganitethin films and in SrMnO3[82, 83]. This is sketched in Fig. 1.4.Researchers have used detection methods such as magnetic exchange biasobserved in magnetization measurements using Superconducting QuantumInterference Device (SQUID), to show the existence of brownmillerite andperovskite 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, Maurelet al have shown with muon spectoscopy technique that with strain, theG-type antiferromagnetic order can be transformed to A-type[82]. Fur-ther interesting features such as interfacial spin glass is observed at theLSMO/SMO interface. The exchange bias is attributed to spin glass like

14

1.4. This thesis

features at the interface between the magnetic moments of total compen-sated SMO and ferromagnetic uncompensated LSMO at the interface[85].It has been shown by other researchers that this exchange bias is driven bythe Dzyaloshinkii-Moriya interaction (DMI) due to the oxygen octahedraltilting across the G-type SMO and LSMO. Researchers have also pointedout 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 magnetization[86, 87]. SRO on the other hand, due toits multi-axial magnetic anisotropy and larger tendency of a perpendicularmagnetic anisotropy (PMA) due to the compressive strain from the sub-strates, has been investigated to be a potential candidate for observationof topologically non-trivial magnetic features like skyrmion, bubbles andskyrmion-bubble by magnetotransport[88, 89]. Hence, the integration ofSMO with SRO is interesting as it alters the hybridization of Ru-O bondsin SRO alone to Ru-O-Mn bonds at their interface[90, 91]. The presenceof a G-type AFM (SMO) is highly favorable to reduce any magnetostaticdipolar interaction and additionally, an exchange bias driven DMI acrossSRO-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. Thework includes a selection of different doping concentrations of Nb-dopedSrTiO3 and the development of a tunable Schottky interface where thebuilt-in electric field is tailored with applied bias and temperature. Thisgives rise to different magnetoresistive effects that are controlled by theelectric field, demonstrating a new approach to tune the spin transportsignals across the Schottky junction using 3T geometry.The next part of the thesis deals with the growth and characterization of theantiferromagnetic SrMnO3 thin films for magnon based spin transport andintegration with the ferromagnet SrRuO3 for investigation of topologicallynon-trivial features in magneto-transport measurements.

15

Chapter 1. Introduction

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) Halltransport 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 interfaceacross 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 thecharge 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 TunnelingAnisotropic Magnetoresistance (TAMR) observed across the Schot-tky interface of Ni and Nb:STO; and (iv) a further investigation intothe Schottky dependence of TAMR by fabricating and characterizingan 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 barrierof AlOx (7 A- thick) on low doped Nb:STO (0.01 wt%) and (ii) theevolution of magnetoresistance lineshapes due to the convolution ofHanle and TAMR signals and the creation of a new spin transportparameter.

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

(SMO) thin films on SrTiO3 (STO), and (ii) characterization of thethin films using bulk magnetic measurements (SQUID) and interfacesensitive spin transport measurements by Spin Hall Magnetoresis-tance (SMR) and Spin Seebeck effect (SSE).

• Chapter 6 presents (i) an investigation of altering magneto-transportproperties in thin films of SrRuO3 (SRO) grown on SrTiO3 sub-strates by anomalous Hall effect, (ii) Magnetic properties and charac-

16

1.4. This thesis

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

17

Bibliography

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