Heusler Alloys: Properties, Growth, Applications

492 Pages·2016·23.46 MB·English

by  Claudia Felser

Springer Series in Materials Science 222
Claudia Felser
Atsufumi Hirohata Editors
Heusler
Alloys
Properties, Growth, Applications
Springer Series in Materials Science
Volume 222
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Claudia Felser Atsufumi Hirohata
(cid:129)
Editors
Heusler Alloys
Properties, Growth, Applications
123
Editors
ClaudiaFelser Atsufumi Hirohata
Max-Planck-Institut für ChemischePhysik Department ofElectronics
festerStoffe University of York
Dresden York
Germany UK
ISSN 0933-033X ISSN 2196-2812 (electronic)
SpringerSeries inMaterials Science
ISBN978-3-319-21448-1 ISBN978-3-319-21449-8 (eBook)
DOI 10.1007/978-3-319-21449-8
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Preface
SincethediscoveryofCu MnSnbyFriedrichHeuslerin1903,thistypeofternary
2
alloys has been intensively investigated due to its ferromagnetism without con-
taining any ferromagnetic elements, such as Fe, Co, Ni and Gd. Such an alloy,
known as Heusler alloy, has been demonstrated to form thermal antiphase
boundaries, which can be utilised for shape-memory materials and magnetic
refrigeration. The Heusler alloys have then attracted more attention from the
spintronics community since the first prediction of the half-metallicity on a
half-Heusler alloy, NiMnSb, by de Groot et al. in 1983. The Heusler alloys are
thereforeknowntobeoneofthebesthalf-metallic ferromagnetstoachieve100%
spinpolarisationatroomtemperatureduetotheirhighCurietemperatureandlarge
intrinsic magnetic moments. In order to implement the Heusler alloys into spin-
tronic devices, the half-metallicity needs to be maintained in their thin film form.
This requires to overcome major challenges; the formation offully-ordered phase,
the avoidance of atomic disorder at surfaces and the interfaces and introduction of
strong magnetic anisotropy.
Thesethreechallengeshavebeeninitiallydiscussedfromthetheoreticalpointof
view,followedbyexperimentalanalysisonthepropertiesoftheHeusleralloysina
bulk form. Based on the technical advancement in growth, these alloys have been
growninathin-filmformoverthelastdecades.Thesefilmshaverecentlypatterned
into a nanometre-scale junction for their device implementation. In such a
nano-junction, the surface contributions dominate the overall properties and hence
emphasisetheabovechallengesevenmorethanever.Itisthereforeagoodtimeto
overview the development of the theory of the Heusler alloys as well as under-
standing and improvement of the properties of the alloys.
This book consists of seven parts from fundamental theory to device applica-
tions. The first part focuses on the theory of Heusler compounds, including two
chapters by the pioneering research groups on half-metallic Heusler alloys; theory
ofHeuslerandfull-HeuslercompoundsbyGalanakisandbasicsandperspectivesof
magnetic Heusler compounds by Felser. The properties of the alloys are described
in the following part. Two techniques are used to characterise the alloys, i.e.,
spin-resolved photoemission (Aeschlimann) and nuclear magnetic resonance
v
vi Preface
(Wurmehl) in the first two chapters. Their properties in nanometric scale (Hütten)
aswellastheiratomicinteractions (Grin)areshownafterwards.Themagneticand
electric properties of binary-form Heusler alloys (Coey) are then discussed. Their
off-stoichiometric properties (Leighton) are also discussed at the end of the part.
The third part highlights three major applications of the Heusler alloys; spin-
tronics (Hirohata), thermoelectrics (Balke) and thermodynamics (James). These
chaptersrefertotheadvantagesoftheusageofthesealloysaswellastheobstacles
for their applications as related to the above challenges. Such applications rely on
the two-dimensional form of the alloys, which is covered by the following part
(Hono). The evaluation of the magnetic properties of these films is reviewed in
Part V, including ultrafast optics (Hillebrands), Mössbauer spectroscopy (Mibu)
and X-ray magnetic circular dichroism (Elmer). Inthe next part, further evaluation
of these films in multilayered junctions is shown to study their giant magnetore-
sistive (Takanashi) and tunnelling magnetoresistive behaviours (Mitani). The
influence of their interfacial disorder (Yamamoto) and exchange coupling
(O’Grady)ontotheirbehavioursarealsoexplainedinthelatterhalfofthispart.At
theendofthisbook,newemergingapplicationsoftheHeusleralloysarediscussed,
especially topological insulators (Chadov).
Onbehalfoftheauthors,wesincerelywishthisbookservesasacomprehensive
handbook of Heusler alloys in a timely manner and contributes to further devel-
opment in this exciting research field. We are grateful to all the authors who spent
their precious time and knowledge to realise this book. We also thank the tre-
mendous technical support from Springer.
Dresden Claudia Felser
York Atsufumi Hirohata
Contents
Part I Theory of Heusler Compounds
1 Theory of Heusler and Full-Heusler Compounds . . . . . . . . . . . . . 3
Iosif Galanakis
1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Semi-Heusler Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 Band Structure of Semi-Heusler Compounds . . . . . . . 6
1.2.2 Origin of the Gap. . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.3 Role of sp-Elements . . . . . . . . . . . . . . . . . . . . . . . . 11
1.2.4 Slater-Pauling Behavior. . . . . . . . . . . . . . . . . . . . . . 12
1.3 Full Heusler Compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.1 Electronic Structure. . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.2 Origin of the Gap in Full-Heusler Alloys. . . . . . . . . . 16
1.3.3 Slater-Pauling Behavior of the Full-Heusler Alloys. . . 17
1.4 Inverse Full-Heusler Compounds . . . . . . . . . . . . . . . . . . . . . 19
1.5 LiMgPdSn-Type Heusler Compounds . . . . . . . . . . . . . . . . . . 20
1.6 Disordered Quaternary Heusler Alloys. . . . . . . . . . . . . . . . . . 22
1.7 Half-Metallic Antiferromagnets. . . . . . . . . . . . . . . . . . . . . . . 23
1.8 Magnetic Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.9 Special Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.9.1 Exchange Constants and Curie Temperature. . . . . . . . 26
1.9.2 Defects and Vacancies. . . . . . . . . . . . . . . . . . . . . . . 27
1.9.3 Surfaces and Interfaces . . . . . . . . . . . . . . . . . . . . . . 28
1.10 Summary and Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2 Basics and Prospectives of Magnetic Heusler Compounds. . . . . . . 37
C. Felser, L. Wollmann, S. Chadov, G.H. Fecher
and S.S.P. Parkin
2.1 Basics and Prospectives of Magnetic Heusler Compounds . . . . 37
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
vii
viii Contents
Part II Properties
3 Spin-Resolved Photoemission Spectroscopy of the Heusler
Compound Co MnSi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2
Roman Fetzer, Martin Aeschlimann and Mirko Cinchetti
3.1 A Short Introduction to Spin-Resolved Photoemission
Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 SR-PES at Co-based Full Heusler Surfaces . . . . . . . . . . . . . . 53
3.2.1 Spin- and Symmetry-Resolved PES
at the Off-Stoichiometric Co2MnaSi Surface. . . . . . . . 54
3.3 SR-PES at the Co MnSi/MgO interface. . . . . . . . . . . . . . . . . 68
2
3.3.1 Low Energy SR-PES at the Off-Stoichiometric
Co2MnaSi/MgO Interface. . . . . . . . . . . . . . . . . . . . . 70
3.3.2 Low Energy Spin- and Symmetry-Resolved
PES at the Stoichiometric Co MnSi/MgO Interface. . . 76
2
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4 Structural Order in Heusler Compounds. . . . . . . . . . . . . . . . . . . 87
S. Wurmehl and M. Wójcik
4.1 Heusler Compounds—A Versatile Class
of Functional Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.2 Types of Structural Order in Heusler Compounds. . . . . . . . . . 88
4.3 Methods for the Investigation of (local) Order
in Heusler Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.3.1 Nuclear Magnetic Resonance Spectroscopy (NMR). . . 91
4.3.2 Mößbauer Spectroscopy. . . . . . . . . . . . . . . . . . . . . . 91
4.4 Examples of Structural Order in Heusler Compounds . . . . . . . 92
4.4.1 X YZ Heusler Compounds . . . . . . . . . . . . . . . . . . . 92
2
4.4.2 Heusler Compopunds with Inverse Structure . . . . . . . 97
4.4.3 Pseudo-Ternary Heusler Compounds
with 4 Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.4.4 Co2Mn1(cid:1)xFexSi . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.4.5 Co2Mn1(cid:1)xFexAl . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.4.6 Co2Cr1(cid:1)xFexAl. . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.4.7 Co2Cr1(cid:1)xFexGa. . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.4.8 Co2FeAl1(cid:1)xSix . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.4.9 YXZ Heusler Compounds . . . . . . . . . . . . . . . . . . . . 102
4.5 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Contents ix
5 Heusler Compounds Go Nano. . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Judith Meyer, Niclas Teichert, Alexander Auge, Changhai
Wang, Andreas Hütten and Claudia Felser
5.1 Identifying the Future Role of Heusler Nanoparticles
for Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.1.1 Progress in Synthesizing Heusler Nanoparticles . . . . . 117
5.2 Nanoparticular GMR-Effect Based on Co FeGa
2
Nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3 The Limits for the Austenite-Martensite Transformation
in Ultra-Thin Films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.3.1 The Martensitic Transformation in Real Crystals
and Thin Films. . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.3.2 The Martensitic Transformation
in Ultra-Thin Films. . . . . . . . . . . . . . . . . . . . . . . . . 128
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6 Chemical Bonding in MgAgAs-Type Compounds. . . . . . . . . . . . . 133
D. Bende, Yu. Grin and F.R. Wagner
6.1 Crystal Structure and Structural Relations . . . . . . . . . . . . . . . 133
6.2 Notions on Chemical Bonding in MgAgAs-Type
Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.3 Real-Space Analysis of Chemical Bonding
in MgAgAs-Type Compounds of the Main-Group
Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.3.1 QTAIM Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.3.2 Electron Localizability Analysis . . . . . . . . . . . . . . . . 142
6.3.3 ELI-D/QTAIM Intersections. . . . . . . . . . . . . . . . . . . 144
6.4 Bonding Analysis of Some Related Compounds. . . . . . . . . . . 147
6.5 A Unified Bonding Concept. . . . . . . . . . . . . . . . . . . . . . . . . 148
6.6 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7 Magnetic and Electronic Properties of Thin Films
of Mn-Ga and Mn-Ge Compounds with Cubic, Tetragonal
and Hexagonal Crystal Structures. . . . . . . . . . . . . . . . . . . . . . . . 157
Huseyin Kurt and J.M.D. Coey
7.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
7.2 Crystal Structure and Magnetic Order . . . . . . . . . . . . . . . . . . 161
7.3 Thin Film Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.4 Structural Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . 168
7.5 Magnetic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.6 The Zero-Moment Half-Metal. . . . . . . . . . . . . . . . . . . . . . . . 178
7.7 Electronic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179