Half-metallic Alloys: Fundamentals and Applications

319 Pages·2005·20.09 MB·English

by  Iosif Galanakis

Lecture Notes in Physics
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I. Galanakis P.H. Dederichs (Eds.)
Half-Metallic Alloys
Fundamentals and Applications
ABC
Editors
I.Galanakis ProfessorDr.P.H.Dederichs
ForschungszentrumJülichGmbH ForschungszentrumJülichGmbH
InstituteFestkörperforschung InstituteFestkörperforschung
Leo-Brandt-Str.,52428Jülich Leo-Brandt-Str.,52428Jülich
Germany Germany
E-mail:[email protected] E-mail:[email protected]
I.GalanakisandP.H.Dederichs,Half-MetallicAlloys,
Lect.NotesPhys.676(Springer,BerlinHeidelberg2005),DOI10.1007/b137760
LibraryofCongressControlNumber:2005930448
ISSN0075-8450
ISBN-10 3-540-27719-6SpringerBerlinHeidelbergNewYork
ISBN-13 3-540-27719-4SpringerBerlinHeidelbergNewYork
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Preface
In1903Heusler,inhisattempttosynthesizenewmaterialsforthesteelindus-
try,discoveredCu MnAl.Thereafter,awholeclassofnewmaterialswiththe
2
samelatticehavebeensuccessfullygrownandtheywerenamedafterHeusler
himself. They attracted considerable attention mainly due to the diversity
of the magnetic phenomena which they presented like itinerant and local-
ized magnetism, antiferromagnetism, helimagnetism, Pauli paramagnetism
or heavy-fermionic behavior. Recently, the rapid development of the field
of magnetoelectronics revived the interest on such materials since some of
them where found to possess a newly discovered property, the so-called half-
metallicity.
Half-metals are particular ferromagnetic materials, which can be consid-
ered as hybrids between metals and semiconductors. The existence of these
half-metallic materials was predicted using abinitio calculations by de Groot
etal.in1983.TheyfoundthatintheHeusleralloyNiMnSbthemajorityspin
band structure shows a metallic behavior, while the minority band structure
exhibits a band gap at the Fermi level like in a semiconductor. Due to the
gap for one spin direction, electrons at the Fermi level show a 100% spinpo-
larization.
Foralongtimesuchhalf-metalswereconsideredasexoticmaterials,inter-
esting,butnotimportant.Thiscompletelychangedduetotheappearanceof
thenewfieldofmagnetoorspinelectronics.Whileinconventionalelectronics
the charge of the electrons plays the essential role and the spin is only used
for information storage, in the future spin electronics both the charge and
the spin will play an equally important role. The development of this field
was initiated by the discovery of the giant magnetoresistance effect in 1988
by the groups of Fert in Paris and Gru¨nberg at Ju¨lich. It soon became clear
that in such a spin-dependent electronics half-metals can play an important
role, since the efficiency of any spin-dependent device will be largest, if the
currentwillbe100%spinpolarized.Therefore,half-metalscanbeconsidered
as the ideal materials for spin electronics.
Inthelast10–15yearsmanymaterialshavebeenfoundtobehalf-metallic.
Besides the so-called half-Heusler alloys like NiMnSb a large number of full-
Heusler alloys like Co MnGe are predicted to be half-metals. Ab-initio cal-
2
culations also yield half-metallicity for some oxides like CrO and Fe O
2 3 4
VI Preface
(magnetite), for manganites (e.g. La Sr MnO ), double perovskites (e.g.
0.7 0.3 3
Sr FeReO ), pyrites (e.g. CoS ), transition metal chalcogenides (e.g. CrSe)
2 6 2
and pnictides (e.g. CrAs) in the zinc-blende or wurtzite structures. Also the
much debated diluted magnetic semiconductors belong to this class of mate-
rials.
This book is fully devoted to the half-metallic Heusler alloys and the
structurallyverysimilartransitionmetalchalcogenidesandpnictides.Heusler
alloys are particularly interesting for realistic applications due to their very
high Curie temperatures and the similarity between their crystal structure
and the zinc-blende structure adopted by the III–V and II–VI compound
semiconductors widely used in applications. However, these alloys also offer
many problems and challenges for research and applications. Typically, the
alloys have a lot of point defects and substitutional disorder. These lead
to impurity states in the gap which, due to the high defect concentration,
strongly affect the half-metallic properties. Another problem occurs at the
interfaces to the semiconductors. Here dangling bond states, i.e. interface
statesintheminoritygap,occurwhichstronglyreducethespinpolarization.
Basically, due to these problems it is at present not clear if they can be
overcomeandifHeusleralloyswillbeusefulmaterialsforspinelectronicsand
also whether they will find applications, e.g. in spin injection, in magnetic
tunnel junctions or in GMR sensors.
For these reasons this book summarizes the latest advancements in the
understanding and applications of Heusler alloys and related compounds. In
Chap. 1 a theoretical study of the electronic and magnetic properties of the
Heusleralloysispresented,andinChap.2theeffectofdefectsandinterfaces
on the properties of these compounds is investigated. Chapter 3 discusses
the properties of the spinvalves and Chap. 4 analyzes the magnetic prop-
erties of films. Chapters 5 and 6 are devoted on the growth mechanisms of
half-Heusler alloys (NiMnSb) and full-Heusler alloys Co MnGe. Chapter 7
2
investigates the phenomena occurring during the creation of surfaces and
Chap. 8 the properties of the tunnel junctions with a Heusler alloy as the
magnetic electrode. Finally, Chaps. 9 and 10 present a theoretical and ex-
perimental investigation of the transition-metal chalcogenides and pnictides
which crystallize in a similar structure with the Heusler alloys. We should
also note that each chapter is self-contained and can be read independently.
The reader is referred to introductions in Chaps. 1 and 3 for a survey of the
theoretical and experimental studies on Heusler compounds, while Chap. 9
includes a historical survey of the research on the transition-metal pnictides.
We hope that the book encourages further research on these complex
materials leading to successful applications.
Ju¨lich, Iosif Galanakis
May 2005 Peter H. Dederichs
List of Contributors
Iosif Galanakis and Peter H. Claudia Felser
Dederichs Institut fu¨r Anorganische Chemie
Institut fu¨r Festk¨orperforschung, und Physikalische
Forschungszentrum Ju¨lich, D-52425 Chemie, Johannes Gutenberg-
Ju¨lich, Germany Universit¨at Mainz, 55099 Mainz,
[email protected] and Germany
[email protected] [email protected]
Hans-Joachim Elmers
Silvia Picozzi Institut fu¨r Physik,
CASTI-INFM Regional Lab. and Johannes Gutenberg-Universita¨t
Dip. Fisica, Mainz, 55099 Mainz, Germany
Univ. L’Aquila, 67010 Coppito (Aq), [email protected]
Italy
Gerhard H. Fecher
Institut fu¨r Anorganische
Alessandra Continenza Chemie und Physikalische Chemie,
CASTI-INFM Regional Lab. and Johannes Gutenberg-Universita¨t
Dip. Fisica, Mainz, 55099 Mainz, Germany
Univ. L’Aquila, 67010 Coppito (Aq), [email protected]
Italy Willem Van Roy
IMEC, Kapeldreef 75, B-3001
Arthur J. Freeman Leuven, Belgium
Dept. of Physics and Astronomy, [email protected]
Northwestern University, Marek Wo´jcik
60208 Evanston, Il, USA Institute of Physics-Polish Academy
of Sciences, Al. Lotnikow 32/46,
02-668 Warszawa, Poland
K. Westerholt, A. Bergmann,
[email protected]
J. Grabis, A. Nefedov, and H.
Zabel Thomas Ambrose and Oleg
Institut fu¨r Experimentalphysik IV, Mryasov
Ruhr-Universita¨t Bochum, D-44780 Seagate Research, 1251 Waterfront
Bochum, Germany Place, Pittsburgh, PA, USA
kurt.westerholt@ruhr-uni- [email protected]
bochum.de and [email protected]
VIII List of Contributors
Hae-Kyung Jeong Hiro Akinaga
Department of Physics, University Nanotechnology Research Institute,
of Nebraska-Lincoln, USA National Institute
[email protected] and of Advanced Industrial Science and
[email protected] Technology, 1-1-1 Umezono,
Tsukuba, Ibaraki 305-8568, Japan
Anthony Caruso
[email protected]
Department of Physics, University
of Nebraska-Lincoln, USA
[email protected] and Masaki Mizuguchi
[email protected] Nanotechnology Research Institute,
National Institute
Camelia N. Borca
of Advanced Industrial Science and
Institute of Applied Optics, Swiss
Technology, 1-1-1 Umezono,
Federal Institute of Technology,
Tsukuba, Ibaraki 305-8568, Japan
Switzerland
[email protected]
[email protected]
Andreas Hu¨tten, Sven
Kazutaka Nagao
K¨ammerer, Jan Schmalhorst,
Research Institute of Electrical
and Gu¨nter Reiss
Communication, Tohoku
Department of Physics, University
University, 2-1-1 Katahira, Aoba-ku,
of Bielefeld, P.O.
Sendai 980-8577, Japan
Box 100131, D-33501 Bielefeld,
[email protected]
Germany
[email protected]
Yoshio Miura
Bang-Gui Liu
Research Institute of Electrical
Institute of Physics, Chinese Acad-
Communication, Tohoku
emy of Sciences, P.O. Box 603,
University, 2-1-1 Katahira, Aoba-ku,
Beijing 100080, China;
Sendai 980-8577, Japan
Beijing National Laboratory for
[email protected]
Condensed Matter Physics, Beijing
100080, China
[email protected] Masafumi Shirai
and Research Institute of Electrical
Department of Physics, University Communication, Tohoku
of California at Berkeley, Berkeley, University, 2-1-1 Katahira, Aoba-ku,
CA 94720, USA Sendai 980-8577, Japan
[email protected] [email protected]
Contents
List of Contributors .......................................... VII
Half-Metallicity and Slater-Pauling Behavior
in the Ferromagnetic Heusler Alloys
Iosif Galanakis and Peter H. Dederichs ............................ 1
1 Introduction ................................................ 1
2 Electronic Structure and Magnetism of Half-Heusler Alloys ....... 4
3 Full Heusler Alloys .......................................... 14
4 Effect of the Lattice Parameter................................ 21
5 Quaternary Heusler Alloys.................................... 22
6 Point Defects in Half-Metals .................................. 24
7 Effect of Spin-orbit Coupling.................................. 25
8 Surface Properties........................................... 27
9 Summary and Outlook....................................... 32
References ..................................................... 34
Role of Structural Defects on the Half-Metallic Character
of Heusler Alloys and Their Junctions with Ge and GaAs
Silvia Picozzi, Alessandra Continenza, Arthur J. Freeman............ 41
1 Introduction ................................................ 41
2 Brief Review of Bulk Properties of Co MnX (X = Si, Ge, Sn)
2
Compounds ................................................ 43
3 Electronic and Magnetic Properties of Co MnGe/GaAs Interfaces . 45
2
4 Structural Defects ........................................... 52
5 Conclusions and Outlook ..................................... 64
References ..................................................... 65
Magnetism and Structure of Magnetic Multilayers Based
on the Fully Spin Polarized Heusler Alloys Co MnGe
2
and Co MnSn
2
K. Westerholt, A. Bergmann, J. Grabis, A. Nefedov, H. Zabel ........ 67
1 Introduction ................................................ 67
2 Single Co MnGe and Co MnSn Thin Films..................... 75
2 2
3 Multilayers [Co MnSn(Ge)/Au(V)] ........................... 84
2 n
4 Magnetotransport Properties ................................. 102
X Contents
5 Summary and Conclusions.................................... 108
References ..................................................... 110
The Properties of Co2Cr1−xFexAl Heusler Compounds
Claudia Felser, Hans-Joachim Elmers, Gerhard H. Fecher............ 113
1 Introduction ................................................ 113
2 Theory..................................................... 115
3 Bulk Properties ............................................. 130
4 Thin Film Properties ........................................ 144
5 Summary and Outlook....................................... 148
References ..................................................... 149
Epitaxial Growth of NiMnSb on GaAs
by Molecular Beam Epitaxy
Willem Van Roy, Marek Wo´jcik .................................. 153
1 Introduction ................................................ 153
2 Challenges.................................................. 154
3 Growth Results ............................................. 159
4 Nuclear Magnetic Resonance Study of the Local Structure........ 168
5 Spin Injection Results........................................ 181
6 Conclusions................................................. 182
References ..................................................... 183
Growth and Magnetotransport Properties
of Thin Co MnGe Layered Structures
2
Thomas Ambrose and Oleg Mryasov............................... 187
1 Introduction ................................................ 187
2 MBE Growth of Co MnGe on GaAs (001)...................... 192
2
3 Growth of Co MnGe by Magnetron Sputtering.................. 202
2
4 GMR in Co MnGe Layered Structures ......................... 205
2
5 Conclusions................................................. 217
References ..................................................... 218
Surface Segregation and Compositional Instability
at the Surface of Half-Metal Ferromagnets
and Related Compounds
Hae-Kyung Jeong, Anthony Caruso, Camelia N. Borca .............. 221
1 Introduction ................................................ 221
2 Characterization of Surface Composition ....................... 224
3 Free Energy in Segregation ................................... 229
4 Magnetic Ordering in Segregation ............................. 231
5 Discussions and Summary .................................... 234
References ..................................................... 236