Half metallic fe3o4 an experimental study on impurity doping and the giant magnetoresistance effect

The GMR effect is systematically studied in Fe3O4/Cu/Ni80Fe20 spin valve structures as a function of varying spacer layer and Fe3O4 layer thicknesses for both current-in-plane CIP and cu

IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE

EFFECT

DEBASHISH TRIPATHY

NATIONAL UNIVERSITY OF SINGAPORE

2007

To my father…

IMPURITY DOPING AND THE GIANT MAGNETORESISTANCE

EFFECT

DEBASHISH TRIPATHY

(B Eng.(Hons.), BITS Pilani, India)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2007

– “The Alchemist” by Paulo Coelho

Acknowledgements

I feel deeply indebted to several people who have contributed in different ways towards the work accomplished in this thesis First and foremost, I would like to express my sincerest gratitude towards my supervisor, Assoc Prof Adekunle Adeyeye for giving me the opportunity to work on this exciting topic His constant motivation, support, guidance and encouragement in all aspects varying from research to personal life, have made my candidature a truly enriching experience

I would also like to acknowledge Dr Christopher Boothroyd and Dr Santiranjan Shannigrahi from Institute of Materials Research and Engineering (IMRE),

Dr S N Piramanayagam from Data Storage Institute (DSI), and Dr Xingyu Gao from Singapore Synchrotron Light Source (SSLS), for their professional help and enlightening discussions regarding my experimental work

I would also like to express my appreciation towards Ms Kelly Low and Dr Jixuan Zhang from Department of Materials Science and Engineering for their invaluable help in letting me use the TEM sample fabrication and imaging facilities I would like to acknowledge the help of Ms Van Li Hui from Faculty of Science for SQUID measurements and also like to thank the ISML lab officers, Ms Loh Fong Leong and Mr Alaric Wong, and Ms Ah Lian Kiat from MOS Device Laboratory, for their help and support during the last four years

During the course of my PhD, I have had the privilege of working closely with the students and staff in Assoc Prof Adekunle’s group, which has benefited me immensely I would like to acknowledge Goolaup for always being such a great companion and helping me out with the cryostat on innumerable occasions I would

also like to thank Chenchen, Yunsong, Wangjin and Kiam Ming for all the enjoyable moments we have shared in ISML Outside the group, I would like to mention my friends “CX” Xingzhi, Sreenivasan, Lalit, and Saurabh for always making the lab a fun place to work

I would like to acknowledge the NUS research scholarship, and NUS Research Grant No R263-000-283-112 for providing financial support to this project

I would like to thank my entire family in India for all their support, faith and advice during my stay in Singapore I especially owe this thesis to my late father who always believed in me, and who has been the source of inspiration, perseverance, and determination for all my endeavours

Finally, but most importantly, I would like to mention my pillar of strength; my wife Shikha For proof reading this thesis and for your unwavering help, emotional support and understanding in all matters; in the lab and at home – thank you so much!

2.3.4 Substitutions in the Spinel Structure 21

3.4.2 Superconducting Quantum Interference Device Magnetometer 49

3.5 Magnetoresistance and Electrical Measurements 52

4.6 Magnetoresistance Behavior of Co Doped Fe3O4 Films 70

4.10 Magnetoresistance Behavior of Cu Doped Fe3O4 Films 80

4.11 Summary 82References 84

Chapter 5 Giant Magnetoresistance in Fe 3 O 4 /Cu/Ni 80 Fe 20 Spin Valve

5.5.1 Room Temperature Longitudinal and Transverse MR 94

Chapter 6 Magnetic and Giant Magnetoresistive Properties of an all

oxide Fe 3 O 4 -Al 2 O 3 Granular System

116

6.3.2 Transmission Electron Microscopy Analysis 119 6.3.3 X-Ray Photoelectron Spectroscopy Analysis 120

Over the last decade, the emerging field of spintronics has spurred renewed interest in the magnetic and spin dependent transport properties of magnetic oxide materials possessing a high degree of spin polarization Magnetite (Fe3O4) is a ferrimagnetic spinel with a high Curie temperature of 860 K, and a half metallic structure with a gap in the density of states of the majority carriers at the fermi level; thus making it extremely attractive for various fundamental and technological studies

in the area of spintronics

The first part of this thesis presents a detailed study on the evolution of the structural, magnetic and magnetotransport properties of Fe3O4 films with impurity doping such as Co and Cu in order to find suitable magnetic materials for potential applications The impurity doped films show a preferential growth direction and the microstructure is characterized by a large density of grain boundaries; a feature which dominates the transport properties of the doped films as well The magnetic properties and magnetoresistance (MR) behaviour at various temperatures are markedly sensitive

to the doping concentration and the distribution of impurity ions on the tetrahedral and octahedral interstitial sites of the cubic spinel structure of Fe3O4

The second part of this thesis investigates the magnetization reversal processes and the giant magnetoresistance (GMR) effect in Fe3O4 incorporated spin valve structures The GMR effect is systematically studied in Fe3O4/Cu/Ni80Fe20 spin valve structures as a function of varying spacer layer and Fe3O4 layer thicknesses for both current-in-plane (CIP) and current perpendicular to plane (CPP) configurations The magnetic properties of the spin valve structures are strongly dependent on the Cu spacer layer thickness due to interplay of various interlayer coupling mechanisms The

magnetoresistance (AMR) and GMR effects The CPP GMR is also compared with the existing theoretical models, and a good agreement is established with the Valet-Fert model for the long spin diffusion length limit A comparison between the CIP and CPP GMR effects is also presented across varying temperatures and spacer layer thicknesses

The final part of this thesis extends the study of GMR effect in Fe3O4 based structures to metal-insulator granular systems in which the ferromagnetic grains are replaced by highly spin polarized Fe3O4 grains The structural, magnetic and tunneling transport properties of Fe3O4-Al2O3 granular films prepared by cosputtering are systematically studied and drastic changes are observed as the Al2O3 fraction in the films is varied The Fe3O4 grain size decreases with increasing Al2O3 fraction and the magnetic properties are directly influenced by these structural changes The spin dependent tunneling of electrons across insulating barriers dominates the transport properties, and results in the observation of a tunneling GMR effect in the granular films This tunneling GMR effect is strongly dependent on the Al2O3 fraction in the granular films and also on temperature

Table 2.1 Classification of various Half Metals 14

Fig 2.1 Typical density of states for a non-magnetic and magnetic metal 12Fig 2.2 Schematic of density of states for Fe and Co 13Fig 2.3 Density of states for different categories of half metals 15Fig 2.4 Schematic of interstitial sites in a cubic spinel structure 16Fig 2.5 Schematic of a Fe3O4 unit cell with the double octant structure 17Fig 2.6 Schematic of superexchange interactions via oxygen anions 19Fig 2.7 Schematic illustration showing that APB shifts can be formed

for different translation and rotation symmetry of the first Fe3O4

monolayer and the MgO substrate

Fig 3.5 Schematic of a x-ray photoelectron spectrometer 43Fig 3.6 Schematic illustration of the operation of a transmission electron

microscope

45

Fig 3.7 Ray diagrams showing (a) bright field, and (b) and dark field

Fig 4.3 TEM bright field images and SAED patterns for undoped and

17% Co doped Fe3O4 films

63

Fig 4.4 In-plane and out-of-plane magnetization curves for Co doped

Fe3O4 films measured at room temperature

Fig 4.7 Temperature dependence of resistance as log R vs T-1/2 for

doped Fe3O4 films

69

Fig 4.8 MR curves for undoped Fe3O4 films as a function of temperature

with (a) H parallel to the plane of the film, and (b) H

perpendicular to the plane of the film

71

Fig 4.9 MR curves for 17% Co doped Fe3O4 films as a function of

temperature with (a) H parallel to the plane of the film, and (b)

H perpendicular to the plane of the film

74

Fig 4.11 TEM bright field images and SAED patterns for undoped and

17% Cu doped Fe3O4 films

Fig 5.1 Schematic of (a) spin valve structure grown by dc magnetron

sputtering, and (b) cross section of the CPP spin valve structure

88,89

Fig 5.2 XRD patterns for Fe3O4 (60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin

valves

90

Fig 5.3 Normalized in-plane magnetization curves measured at 300 K

for Fe3O4 (60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve

structures as a function of tCu

92

Fig 5.4 Normalized in-plane magnetization curves measured at 10 K for

Fe3O4 (60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures

as a function of tCu

93

Fig 5.5 Normalized in-plane magnetization curve measured at 300 K for 94

a Fe3O4 (60 nm)/Cu (10 nm)/ Fe3O4 (15 nm) spin valve

structure

Fig 5.6 Longitudinal magnetoresistance curves measured at 300 K for

Fe3O4 (60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures

as a function of tCu

96

Fig 5.7 Transverse magnetoresistance curves measured at 300 K for

Fe3O4 (60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures

as a function of tCu

97

Fig 5.8 GMR curves measured at 300 K for Fe3O4 (60 nm)/Cu (tCu)/

Ni80Fe20 (15 nm) spin valve structures as a function of Cu spacer

layer thickness tCu

99

Fig 5.9 Room temperature CIP GMR ratio for Fe3O4 (t)/Cu (tCu)/

Ni80Fe20 (15 nm) spin valve structures as a function of Fe3O4

layer thickness t

100

Fig 5.10 GMR curves measured as a function of temperature for Fe3O4

(60 nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures for (a)

tCu = 5 nm, (b) tCu = 10 nm, and (c) tCu = 30 nm

102

Fig 5.11 GMR ratio measured as a function of temperature for Fe3O4 (60

nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures for tCu = 5

nm, 10 nm and 30 nm

103

Fig 5.12 Room temperature CPP GMR curves for Fe3O4 (60 nm)/Cu

(tCu)/ Ni80Fe20 (15 nm) spin valve structures for (a) tCu = 5 nm,

(b) tCu = 10 nm, and (c) tCu = 30 nm

105

Fig 5.13 CPP GMR ratio and [(Rmax-RH=1T)/Rmax]-1/2 as a function of tCu

for the spin valve structures The dotted line is only a guide to

106

the eye

Fig 5.14 Room temperature CPP GMR ratio for Fe3O4 (t)/Cu (5 nm)/

Ni80Fe20 (15 nm) spin valve structures as a function of Fe3O4

layer thickness t

108

Fig 5.15 Temperature dependence of CPP GMR ratio for Fe3O4 (60

nm)/Cu (tCu)/ Ni80Fe20 (15 nm) spin valve structures as a

Fig 6.3 (a) Al 2p core level XPS spectrum characteristic of all

(Fe3O4)1-x(Al2O3)x films with x>0, and (b) Fe 2p core level XPS spectrum

for (Fe3O4)1-x(Al2O3)x films

121

Fig 6.4 (a) In plane experimental magnetization curves measured at

room temperature, and (b) fitted magnetization curves for

(Fe3O4)1-x(Al2O3)x films as a function of x

temperature for (Fe3O4)1-x(Al2O3)x films for x=0.11 with current

(a) parallel to applied magnetic field, and (b) perpendicular to

applied magnetic field

Fig 6.10 GMR ratio for (Fe3O4)1-x(Al2O3)x films measured at 300 K as a

Fig 7.1 Schematic representation of a spin valve structure that consists

of two ferromagnetic electrodes separated by an organic spacer

layer

143

Fig 7.2 Schematic illustration of devices involving multiferroic films as

(a) junction barrier, and (b) pinning layer

144

Alq 3 8-Hydroxy-Quinoline Aluminium

AMR Anisotropic Magnetoresistance

DOS Density of States

ESCA Electron Spectroscopy for Chemical Analysis

Fe 3 O 4 Magnetite

FET Field Effect Transistor

La 1-x Sr x MnO 3 Manganite Perovskite

LCC Leadless Chip Carrier

Li Lithium

lsf Spin Diffusion Length

NiMnSb Semi Heusler Alloy

OLED Organic Light Emitting Diodes

OSE Organic Semiconductors

rf Radio Frequency

RKKY Ruderman-Kittel-Kasuya-Yosida

SAED Selected Area Diffraction

SQUID Superconducting Quantum Interference Device

Sr 2 FeMoO 6 Double Perovskite

tCu Cu spacer layer thickness

TEM Transmission Electron Microscopy

The author claims the following aspects of this thesis to be original contributions to scientific knowledge

• A systematic study of the drastic modifications in the microstructure, magnetic

properties, and temperature dependent magnetoresistance (MR) behaviour of Fe3O4 films as a consequence of doping with Co and Cu impurities by cosputtering technique

[1] D Tripathy, A O Adeyeye , S N Piramanayagam, C S Mah, X Gao, and

A T S Wee, Thin Solid Films 505, 45 (2006)

[2] D.Tripathy, A O Adeyeye, C B Boothroyd, and S N Piramanayagam,

Journal of Applied Physics 101, 013904 (2007)

[3] D Tripathy, A O Adeyeye, C B Boothroyd, and S Shannigrahi, Journal

of Applied Physics (in press)

• A detailed understanding of the magnetization reversal process and the

temperature dependent giant magnetoresistance (GMR) effect in Fe3O4/Cu/Ni80Fe20 spin valve structures as a function of spacer layer and Fe3O4 layer thicknesses for both current-in-plane (CIP) and current-perpendicular-to-plane (CPP) configurations

[4] D Tripathy, A O Adeyeye, and S Shannigrahi, Physical Review B 75,

012403 (2007)

[5] D Tripathy and A O Adeyeye, Journal of Applied Physics 101, 09J505 (2007)

• An extensive study of the structural behaviour, magnetic properties, and

tunneling GMR effect in a completely oxide based Fe3O4-Al2O3 granular system prepared by cosputtering technique

[7] D Tripathy, A O Adeyeye, and S Shannigrahi, Physical Review B 76,

174429 (2007)

Introduction

1.1 Background

For most of the twentieth century, it was known that electrons which mediate the current in electronic circuitry also possess angular momentum This spin degree of freedom however, was not put into any technological use With the dawn of the new millennium, a novel paradigm of electronics emerged which exploits the electron spin and uses it to differentiate electrical carriers into two different categories, depending

on whether their spin projection on to a given quantization axis is +½ (spin ) or -½ (spin ) This new technology, also known as spintronics [1,2], offers opportunities for

a new generation of devices combining standard microelectronics with spin dependent effects that arise from the interaction between spin of the carrier and magnetic properties of the material The advantages of these new devices are non-volatility, increased data processing speed, reduced electric power consumption, and higher integration densities as compared with conventional semiconductor devices [3,4]

The basic spintronic devices are based on simple two terminal trilayer elements using either the giant magnetoresistance (GMR) effect [5], or the tunnel magnetoresistance (TMR) effect [6], and three terminal devices such as the Johnson transistor [7,8] So far, the most extensively used applications of TMR and GMR devices are in read heads for hard disk drives [9,10], and magnetic random access memories (MRAM) [11-13], which exploit the existence of two non volatile resistance states Information stored in MRAM cells can be held indefinitely without power, with switching speeds and densities projected to beat conventional memories [2]

The technological basis for spintronics is almost as old as the concept of electron spin itself In the 1930s, Mott postulated that certain electrical transport anomalies in the behaviour of metallic ferromagnets arose from the ability to consider the spin and spin conduction electrons as two independent families of charge carriers, each with its own distinct transport properties [14] The other necessary ingredient of this model is that the two spin families contribute very differently to the electrical transport processes This may possibly be due to the different densities of each carrier type, or different electron mobilities In either case, the asymmetry which makes spin electrons behave differently from spin electrons arises because the ferromagnetic exchange field splits the spin and spin conduction bands, leaving different band structures at the fermi surface As a result, the number of electrons participating in the conduction process is also different for each spin channel

1.2 Motivation

The ferromagnetic materials primarily used in first generation spintronic

devices are 3d transition metals such as Ni, Fe, Co and their alloys These

ferromagnets serve as sources and conduction channels for spin polarized electrons, as well as magnetic flux paths and shields In the diffusive limit, the spin polarization for these materials is always positive and typically close to 40% [15] The use of highly spin polarized materials could provide effective spin injection and large magnetoresistance (MR) effects [16], both of which are essential for the non-volatile applications discussed in § 1.1

Dilute magnetic semiconductors (DMS) are one of the candidates that have shown potential for next generation spintronic devices A DMS is a nonmagnetic III-V (GaAs) or II-VI (CdTe) semiconductor doped with a few percent of magnetic

impurities (such as Mn) [17-22] The most useful kind of DMS is one in which the net electronic (spin and orbital) angular momentum of the individual magnetic dopants are coupled ferromagnetically by free carriers Such materials are expected to have spin polarized states in either the valence or conduction bands The formation of interfaces with nonmagnetic semiconductors allows for the possibility of spin polarized carrier injection into semiconductor heterostructures and the fabrication of a variety of device structures that utilize spin [23] One of the major impediments however, to most III–V and II–VI DMS materials is their low Curie temperature Thus, practical spintronic devices using such DMS are precluded because of the need for cryogenic cooling Although oxide based DMS such as Co-ZnO and Co-TiO2 have been predicted to exhibit ferromagnetism at or above room temperature, the origin of ferromagnetic behaviour in these materials is still contentious [24-27]

At the same time, considerable research activity was also devoted towards another new class of spintronic materials known as half metals These materials are extremely promising for incorporation in practical spintronic devices because they have electrons at the fermi level in a single spin state, either spin or spin , which theoretically results in a spin polarization of 100% [28,29] The most cited candidates for 100% spin polarization are the semi-Heusler alloys (NiMnSb) [28-32], full Heusler alloys [33,34], zinc-blende structure materials [35-37] colossal magnetoresistance materials (La1-xSrxMnO3) [38,39], and Sr2FeMoO6 [40]), and magnetic oxides (CrO2 [41-45], and Fe3O4 [46,47]) Among these half metals, Fe3O4 has been the focus of most fundamental and technological studies for applications related to read heads for high density magnetic recording and MRAM, due to its high Curie temperature of 860

K and the predicted 100% negative spin polarization at the fermi level by band structure calculations [48]

While there have been a growing number of studies on the behaviour of epitaxial and polycrystalline Fe3O4 thin films and compact powders [49-54], doping highly spin polarized Fe3O4 films with impurities for fabricating new magnetic materials, presents an entirely different challenge Moreover, a detailed study of the magnetization reversal processes and GMR effect in Fe3O4 based spin valve structures

at various temperatures is also essential for gaining a comprehensive understanding of the finite temperature effects in these structures At the same time, Fe3O4 has tremendous potential for the well known metal-insulator granular systems which are highly suitable for sensor applications These granular films can be regarded as an assembly of small tunnel junctions and the tunneling GMR effect can be thus enhanced

by using Fe3O4 as the ferromagnetic component These key issues are the basis for all investigations presented henceforth in this thesis

1.3 Focus of Thesis

The main focus of this thesis is to provide a detailed investigation and understanding of the issues enumerated above in § 1.2 The first part of this thesis addresses the effects of impurity doping on the inherent properties of Fe3O4 films This will provide a knowledge of how doping concentration and distribution of impurity cations across the interstitial sites of the Fe3O4 crystal structure results in a drastic modification of the microstructure, magnetic properties, and MR behaviour of Fe3O4 films The conclusions drawn from this study will aid in the fabrication of modified Fe3O4-based materials with tailor-made magnetic and transport properties The second part gives a detailed insight into the effect of spacer layer and Fe3O4 layer thicknesses

on magnetization reversal mechanisms and GMR effect in Fe3O4 based spin valve structures for both current-in-plane (CIP) and current-perpendicular-to-plane (CPP)

configurations Magnetization measurements and MR studies will demonstrate unequivocally that such spin valve structures are markedly sensitive to the measurement configuration used, and also to variations in layer thicknesses and temperatures The final part of this thesis is dedicated to an extensive study of the magnetic properties and tunneling GMR effects in a completely oxide based Fe3O4-Al2O3 granular system Detailed microstructural analysis and magnetotransport studies are used to elucidate the underlying physical mechanism for such a granular system and thus establish a correlation between the tunneling transport behaviour and the observed magnetic and structural properties.

1.4 Organization of Thesis

Chapter 1 discusses the background and motivation for the work presented in this thesis Chapter 2 reviews the band diagrams of various categories of half metals followed by an insight into the inverse spinel structure of Fe3O4 and the resulting magnetic and electrical behaviour Spin dependent transport phenomena, interlayer coupling mechanisms, and the GMR effect in heterogeneous alloys are also discussed

to provide a theoretical framework for the experimental work presented subsequently Chapter 3 presents the various fabrication and characterization techniques utilized for the experimental work presented in this thesis In chapter 4, the structural, magnetic and magnetotransport properties of Co and Cu doped Fe3O4 films prepared by cosputtering techniques, are investigated in detail Chapter 5 presents a systematic study of the magnetization reversal process and the temperature dependent GMR effect

in Fe3O4/Cu/Ni80Fe20 spin valve structures as a function of spacer layer and Fe3O4 layer thicknesses for both CIP and CPP configurations The strong dependence of microstructure, magnetic and tunneling transport properties of a Fe3O4-Al2O3 granular

system on the Al2O3 volume fraction, is examined in detail in chapter 6 Finally, chapter 7 summarizes the main experimental results presented in the thesis and also provides recommendations for further work in this area

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Theoretical Background

2.1 Introduction

This chapter introduces some basic theoretical concepts and reviews previous work pertaining to the main research topics presented in this thesis § 2.2 elaborates on the importance of half metals in the field of spintronics and establishes why Fe3O4 is the focus of several studies involving highly spin polarized materials The crystallographic structure of Fe3O4, the resulting magnetic behaviour and electrical conductivity and the possibility of ion substitutions are presented in § 2.3 Spin dependent transport phenomenon, with emphasis on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) effects, are discussed in § 2.4 This is followed by § 2.5 which describes the various coupling mechanisms for a typical magnetic multilayer structure Finally, § 2.6 discusses the rich variety of magnetic and giant magnetotransport properties exhibited by granular films

2.2 Half Metals

2.2.1 Spin Polarization and Band Structure of Ferromagnetic Metals

The spin dependent conduction in spintronic devices such as spin field effect transistors [1], spin valve read heads [2], and non volatile magnetic random access memory (MRAM) [3], depends strongly on the spin polarization of the ferromagnetic

materials constituting these devices The spin polarization P of a material is defined as,

F F

N ( E ) N ( E ) P

where EF is the fermi level, and N is the density of states of majority ( ) or minority ( )

electrons at the fermi level Fig 2.1 shows the typical density of states (DOS) for a non-magnetic metal and a magnetic metal Since there are equal number of spin and

spin electrons at EF for a non magnetic metal, the net spin polarization P=0 For a magnetic metal however, there is splitting in the density of states at EF, thus resulting

in a finite net spin polarization (P≠0)

Fig 2.1 Typical density of states for a non-magnetic and magnetic metal

Conventional ferromagnets based on 3d transition metals such as Fe, Co or Ni

and their alloys are the most commonly used materials in the current generation of spintronic devices Fig 2.2 shows the schematic DOS for Fe and Co It can be clearly

seen that even though the 3d band is strongly spin split for Fe, the conduction electrons are not fully spin polarized This is due to the simultaneous presence of 4s electrons at the fermi level For Co, the 3d band is split by on-site exchange interactions in such a way that all five 3d subbands are filled, and only the 3d subbands cross the fermi level [4]; however, an unsplit 4s conduction band means that both and electrons are present at EF In 3d transition metals, the spin polarization is always positive and

EF

typically close to 40% The positive spin polarization is associated with the more

mobile 4s electrons, which are polarized by hybridization with the 3d states

Fig 2.2 Schematic of density of states for Fe and Co

2.2.2 Band Structure and Classification of Half Metals

From the previous section, it is clear that materials with higher spin polarization can dramatically enhance device performance and are necessary for a new generation of spintronic devices Half metals are materials with an unusual band structure in which only half of the electrons are conducting For electrons of one spin (either spin or spin ), they are normal metals with a fermi surface, but electrons of the opposite spin have a gap in their density of states at the fermi level With only one

available spin band at EF, half-metals are 100% spin polarized The existence of this

new class of magnetic materials representing metallic properties for one spin direction

and insulating properties for the other was discovered by Groot et al [5], during

theoretical band structure calculations for the ferromagnetic Heusler alloy NiMnSb

Half metals can be classified into various categories as shown in Table 2.1 A stoichiometric oxide of type I or type II will have a spin moment that is precisely an

integral number of Bohr magnetons per unit cell The total number of electrons (n + n )

is an integer On account of the gap in the spin polarized density of states, these

subbands are completely full or empty, so n or n is also an integer Since both are integers, the difference (n - n ) which is the spin moment in units of Bohr magnetons

is also an integer The integer spin-moment is a necessary condition for Type I or Type

II half metals [6]

Type of Half Metals Materials Classified

IA CrO2 , (Co1-xFex)S2 , NiMnSb

IB Sr2FeMoO6 , NiMnV2 IIB Fe3O4

IIIA (La0.7Ca0.3)MnO3 IVB Tl2Mn2O7

Table 2.1 Classification of various Half Metals

Half metals with fermi level in the spin band are examples of Type IA These include CrO2 [7], (Co1-xFex)S2 [8] and the ordered half heusler alloy NiMnSb [5] Those with fermi level in the spin band, such as the double perovskite Sr2FeMoO6 [4], and the heusler alloy NiMnV2 [9], fall into the Type IB category The concept of half metallicity can be extended to cover narrow bands where the electrons are localized Magnetite (Fe3O4) is an example of a Type IIB half metal [10] The suffix A

or B indicates whether the conduction electrons are spin or spin The band diagrams for type I and type II half metals are shown Fig 2.3 It must be emphasized that the definitions of half metallicity in terms of band structure can only be strictly applied at 0 K An extension of the concept embraces materials in which the spin electrons are localized and the spin electrons are delocalized or vice versa There is a large difference between the mobilities and effective masses of the two, rather than

their densities of states at the Fermi level The integer spin moment criterion is therefore relaxed Such a half metal, for example optimally doped manganite, is classified as type IIIA [11] When the bands are spin-split, it is possible to envisage a ferromagnet with mobile spin holes and immobile spin electrons or vice versa (a type IV half-metal) Tl2Mn2O7 is a good example of a type IVB half metal [12]

Fig 2.3 Density of states for different categories of half metals

For practical applications in spintronics, devices are expected to operate around

or even above room temperature This necessitates the Curie temperature Tc of the materials to be greater than 500 K [6] As discussed in § 1.2, Fe3O4 has the highest