Handbook of Magnetic Materials, Volume 8
Transcript Of Handbook of Magnetic Materials, Volume 8
Handbook of Magnetic Materials, Volume 8
Elsevier, 1995 Edited by: K.H.J. Buschow ISBN: 978-0-444-81974-1
by kmno4
PREFACE TO VOLUME 8
The Handbook series Magnetic Materials is a continuation of the Handbook series Ferromagnetic Materials. The original aim of Peter Wohlfarth when he started the latter series was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth's classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross-section of magnetism than Bozorth's book.
Magnetism has seen an enormous expansion into a variety of different areas of research in the last few years, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason the Editor and Publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 8 of this Handbook series.
Artificial multilayered structures are prominent examples of such classes of novel materials. Progress in the field of molecular beam epitaxy has made it possible to tailor-make layered metallic materials having sharp interfaces, crystalline coherence and superlattice periods of the order of lnm. These materials have opened a new field of magnetism that permits detailed studies of the propagation of magnetic order as a function of separation and crystallographic orientation, as well as studies of the interplay of strain and magnetic properties. A detailed account of achievements on rare earth based artificial multilayered structures is presented in the first chapter of this volume.
Magnetostriction refers to any dimensional changes of a magnetic material caused by changes in its magnetic state. Magnetostriction can originate from changes in magnitude or direction of the applied field or from changes in temperature. The former type of magnetostriction is particular pronounced in rare earth compounds of the type RFe2, as has been described in detail in Chapter 7 of Volume 1. The second type of magnetostriction is largest near the Curie temperature in ferromagnetic materials. This volume magnetostriction gives rise to the technically important Invar alloys, and the associated moment-volume instabilities in transition metal alloys have extensively been discussed in Chapter 3 of Volume 5. The large body of experimental results that have become available for the many intermetallic compounds in which
vi
PREFACETO VOLUME8
rare earths are combined with 3d transition metals is described in the third chapter of the present volume.
The ferrites form a large class of magnetic materials and some of these materials are of considerable technical importance. The properties of hard ferrites as well as soft ferrites have been described in several chapters in Volumes 2 and 3. Since the appearance of these chapters substantial progress has been made in the understanding of the physical and chemical properties of these materials which made it necessary to update the results described in the preceding chapters. New results obtained on ferrites are described in Chapter 3, where the emphasis is on spinel ferrites.
Of substantial technical importance is, furthermore, the group of so-called soft magnetic materials. A detailed description of several important classes of soft magnetic material has been presented already in Chapter 6 of Volume 1 and Chapter 2 of Volume 2. Supplementary results, dealing mainly with laminated amorphous alloys and electrical steels and the problem of the loss producing effect of the rotational magnetisation are highlighted in Chapter 4.
A survey of the magnetic properties of various types of rare earth intermetallics was given already in Volume 1 of the Handbook series. Since then proliferation of scientific results, obtained with novel techniques, and made for a large part on single crystals, have led to a more complete understanding of the basic magnetic interactions in these materials. This requires a major updating of the experimental results presented in Volume 1. However, the experimental and theoretical material that has accumulated over the years is so extensive that it is hardly possible to condense it in a single chapter. In the preceding volume, Vol. 7, supplementary information was presented already for intermetallics in which rare earths are combined with 3d transition metals. In the present volume the updating process has been continued by means of a chapter on rare earth copper compounds of the type RCu2.
Volume 8 of the Handbook on the Properties of Magnetic Materials, as the preceding volumes, has a dual purpose. As a textbook it is intended to be of assistance to those who wish to be introduced to a given topic in the field of magnetism without the need to read the vast amount of literature published. As a work of reference it is intended for scientists active in magnetism research. To this dual purpose, Volume 8 of the Handbook is composed of topical review articles written by leading authorities. In each of these articles an extensive description is given in graphical as well as in tabular form, much emphasis being placed on the discussion of the experimental material in the framework of physics, chemistry and material science.
The task to provide the readership with novel trends and achievements in magnetism would have been extremely difficult without the professionalism of the NorthHolland Physics Division of Elsevier Science B.V., and I wish to thank Joep Verheggen and Wim Spaans for their great help and expertise.
K. H.J. Buschow Van der Waals-Zeeman Laboratory University of Amsterdam
CONTENTS
Preface to Volume 8 . . . . . . . . . . . . . . . . . . . .
v
Contents . . . . . . . . . . . . . . . . . . . . . . . .
vii
Contents of Volumes 1-7 . . . . . . . . . . . . . . . . . .
ix
List of Contributors . . . . . . . . . . . . . . . . . . . .
xi
1. Magnetism in Artificial Metallic Superlattices of Rare Earth Metals
J.J. RHYNE and R.W. ERWlN . . . . . . . . . . . . . .
1
2. Thermal Expansion Anomalies and Spontaneous Magnetostriction in
Rare-Earth Intermetallics with Cobalt and Iron
A.V. ANDREEV . . . . . . . . . . . . . . . . . . .
59
3. Progress in Spinel Ferrite Research
V.A.M. BRABERS . . . . . . . . . . . . . . . . . .
189
4. Anisotropy in Iron-Based Soft Magnetic Materials
M. SOINSKI and A.J. MOSES . . . . . . . . . . . . . .
325
5. Magnetic Properties of Rare Earth-Cu2 Compounds
Nguyen Hoang LUONG and J.J.M. FRANSE . . . . . . . . .
415
Author Index . . . . . . . . . . . . . . . . . . . . . .
493
Subject Index . . . . . . . . . . . . . . . . . . . . . .
521
Materials Index . . . . . . . . . . . . . . . . . . . . .
527
vii
CONTENTS OF VOLUMES 1-7
Volume 1
1. Iron, Cobalt and Nickel, by E.P. Wohlfarth . . . . . . . . . . . . . . . .
1
2. Dilute Transition Metal Alloys: Spin Glasses, by J.A. Mydosh and G.J. Nieuwenhuys
71
3. Rare Earth Metals and Alloys, by S. Legvold . . . . . . . . . . . . . . . .
183
4. Rare Earth Compounds, by K.H.J. Buschow . . . . . . . . . . . . . . . .
297
5. Actinide Elements and Compounds, by W. Trzebiatowski . . . . . . . . . . . .
415
6. Amorphous Ferromagnets, by F.E. Luborsky . . . . . . . . . . . . . . . .
451
7. Magnetostrictive Rare Earth-Fe 2 Compounds, by A.E. Clark . . . . . . . . . .
531
Volume 2
1. Ferromagnetic Insulators: Garnets, by M.A. Gilleo . . . . . . . . . . . . . .
1
2. Soft Magnetic Metallic Materials, by G.Y. Chin and J.H. Wernick . . . . . . . . .
55
3. Ferrites for Non-Microwave Applications, by P.L Slick . . . . . . . . . . . .
189
4. Microwave Ferrites, by J. Nicolas . . . . . . . . . . . . . . . . . . . .
243
5. Crystalline Films for Bubbles, by A.H. Eschenfelder . . . . . . . . . . . . .
297
6. Amorphous Films for Bubbles, by A.H. Eschenfelder . . . . . . . . . . . . .
345
7. Recording Materials, by G. Bate . . . . . . . . . . . . . . . . . . . .
381
8. Ferromagnetic Liquids, by S. W. Charles and J. Popplewell . . . . . . . . . . .
509
Volume 3
1. Magnetism and Magnetic Materials: Historical Developments and Present Role in Industry
and Technology, by U. Enz . . . . . . . . . . . . . . . . . . . . . .
1
2. Permanent Magnets; Theory, by H. Zijlstra . . . . . . . . . . . . . . . .
37
3. The Structure and Properties of Alnico Permanent Magnet Alloys, by R.A. McCurrie
107
4. Oxide Spinels, by S. Krupidka and P Novcik . . . . . . . . . . . . . . . .
189
5. Fundamental Properties of Hexagonal Ferrites with Magnetoplumbite Structure,
by H. Kojima . . . . . . . . . . . . . . . . . . . . . . . . . . .
305
6. Properties of Ferroxplana-Type Hexagonal Ferrites, by M. Sugimoto . . . . . . . .
393
7. Hard Ferrites and Plastoferrites, by H. Stiiblein . . . . . . . . . . . . . . .
441
8. Sulphospinels, by R.P. van Stapele . . . . . . . . . . . . . . . . . . .
603
9. Transport Properties of Ferromagnets, by LA. Campbell and A. Fert . . . . . . . .
747
ix
x
CONTENTS OF VOLUMES i-7
Volume 4
1. Permanent Magnet Materials Based on 3d-rich Ternary Compounds, by K.H.J. Buschow
1
2. Rare Earth-Cobalt Permanent Magnets, by K.J. Strnat . . . . . . . . . . . . .
131
3. Ferromagnetic Transition Metal Intermetallic Compounds, by J. G. Booth . . . . . .
211
4. Intermetallic Compounds of Actinides, by V Sechovsk~ and L. Havela . . . . . . .
309
5. Magneto-optical Properties of Alloys and Intermetallic Compounds, by K.H.J. Buschow
493
Volume 5
1. Quadrupolar Interactions and Magneto-elastic Effects in Rare-earth Intermetallic
Compounds, by P. Morin and D. Schmitt . . . . . . . . . . . . . . . . .
1
2. Magneto-optical Spectroscopy of f-electron Systems, by w. Reim and J. Schoenes . .
133
3. INVAR: Moment-volume Instabilities in Transition Metals and Alloys, by E. E Wasserman 237
4. Strongly Enhanced Itinerant Intermetallics and Alloys, by P.E. Brommer andJ.J.M. Franse 323
5. First-order Magnetic Processes, by G. Asti . . . . . . . . . . . . . . . . .
397
6. Magnetic Superconductors, by 0. Fischer . . . . . . . . . . . . . . . . .
465
Volume 6
1. Magnetic Properties of Ternary Rare-earth Transition-metal Compounds, by H.-S. Li and
J.M.D. Coey . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2. Magnetic Properties of Ternary Intermetallic Rare-earth Compounds, by A. Szytula
85
3. Compounds of Transition Elements with Nonmetals, by O. Beckman and L. Lundgren
181
4. Magnetic Amorphous Alloys, by P. Hansen . . . . . . . . . . . . . . . .
289
5. Magnetism and Quasicrystals, by R. C. O'Handley, R.A. Dunlap and M.E. McHenry
453
6. Magnetism of Hydrides, by G. VCiesinger and G. Hilscher . . . . . . . . . . .
511
Volume 7
1. Magnetism in Ultrathin Transition Metal Films, by U. Gradmann . . . . . . . . .
1
2. Energy Band Theory of Metallic Magnetism in the Elements, by V.L. Moruzzi and
P.M. Marcus . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
3. Density Functional Theory of the Ground State Magnetic Properties of Rare Earths and
Actinides, by M. S. S. Brooks and B. Johansson . . . . . . . . . . . . . . .
139
4. Diluted Magnetic Semiconductors, by J. Kossut and W. Dobrowolski . . . . . . . .
231
5. Magnetic Properties of Binary Rare-earth 3d-transition-metal Intermetallic Compounds,
by J.J.M. Franse and R.J. Radwahski . . . . . . . . . . . . . . . . . .
307
6. Neutron Scattering on Heavy Fermion and Valence Fluctuation 4f-systems,
by M. Loewenhaupt and K.H. Fischer . . . . . . . . . . . . . . . . . .
503
chapter 1
MAGNETISM IN ARTIFICIAL METALLIC SUPERLATTICES OF RARE EARTH METALS
J.J. RHYNE
Research Reactor Center and Department of Physics University of Missouri-Columbia Columbia, Missouri 65211 U.S.A.
and
R.W. ERWlN
Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, Maryland 20899 U.S.A.
Handbook of Magnetic Materials, Vol. 8 Edited by K. H.J. Buschow ©1995 Elsevier Science B.V. All rights reserved
CONTENTS
1. Introduction to superlattices and rare ealths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2. Neutron scattering and artificial metallic superlattices ...............................
6
3. Magnetic scattering, structure, and coherenee .....................................
11
3.1. Superlattices with e-axis growth directions ...................................
11
3.2. Superlattices with a basal plane growth direction ..............................
26
4. Interlayer magnetic coupling in superlattices ......................................
30
5. Effect of applied magnetic fields on interlayer coherence ............................
32
6. Incommensurate magnetic periodicity - interplanar turn angles, spin slips, and their
temperature dependence .......................................................
35
7. Magnetoelasticity in lare earth superlattices and films and the suppression of
ferromagnetism ..............................................................
41
7.1. Theory of magnetoelasticity in superlattices and films ..........................
43
7.2. Magnetoelastie energies in Dy superlattices and films ...........................
44
8. Coherent magnetic moments and disorder at interfaces ..............................
48
9. Residual moment effects in superlattices .........................................
50
10. Summary ...................................................................
54
11. Acknowledgements ..........................................................
55
References .....................................................................
55
1. Introduction to superlattices and rare earths
Intense interest has been generated over the past several years in the growth and properties of layered magnetic materials, both from a fundamental point of view and for applications. Layered structures have been prepared by a variety of techniques such as sputtering, electro-deposition, and evaporation, and include semi-conducting, metallic, and insulating materials. These systems can consist of crystalline layers of one element or compound interleaved with layers of a different element or compound, or alternatively may be built of amorphous layers or amorphous layers alternated with crystalline layers. Depending on the materials and growth techniques, these multilayers may be produced (a) with no uniform crystallographic alignment or coherence from layer to layer, (b) with alignment of one specific crystallographic axis direction along the growth (stacking) direction, or (c) with true three-dimensional atomic order (epitaxy) in which there is multilayer atomic registry both along the growth axis and also within the growth planes. For the purposes of this review, the term artificial metallic superlattice will be reserved for this latter category of true threedimensionally coherent layered structures, while the term multilayer will be used for layered structures in which coherence is present in less than three dimensions.
Refinements in computer-controlled molecular beam epitaxy (MBE) techniques for the growth of single crystal artificial superlattices of two or more distinct compositions have unearthed vast possibilities for the production of tailor-made artificial superlattices with controlled film thicknesses down to atomic dimensions and with highly reproducible stacking sequences. This has provided previously unavailable opportunities to examine problems of interaction ranges, tunneling distances, and other coherent phenomena which are dependent on the superlattice periodicity.
The lanthanide elements and their alloys with non-magnetic but chemically and electronically similar elements such as yttrium, scandium, and lutecium (collectively known as the rare earths), have long provided a fertile area for the study of longrange indirect exchange interactions, crystal field anisotropy, and magnetostrictive effects. The elements have weak exchange compared to the 3d transition elements as illustrated by their low ordering temperatures (e.g., Tb, TN = 230 K) and have anomalously large crystal field and magnetoelastic interactions that arise from the strong spin orbit coupling and highly non-spherical 4f charge distribution. Below their magnetic ordering temperatures, many of the lanthanide elements exhibit one or more forms of periodic incommensurate spin structures such as helices or longitudinal spin density waves, etc., and transitions among them as the temperature is varied. The occurrence of these periodic magnetic orderings below their Nrel
4
J.J. RHYNEand R.W.ERWIN
temperatures in the heavy lanthanides (except for the S state ion Gd) is a consequence of electronic effects, in particular the occurrence of nearly two-dimensional parallel sections (nesting) on the hole Fermi surface that are spanned by a Q-vector whose magnitude determines the initial stable periodicity of the magnetic ordering. At lower temperatures, the free energy associated with the magnetostrictive and anisotropy interactions becomes significant and strongly perturbs the basic periodic magnetic orderings leading to phase transitions to a ferromagnetic state in Tb, Dy, Ho, and Er. These transitions are largely driven by a lowering of the magnetoelastic energy arising from a coupling of the local moments to the hexagonally symmetric lattice strains. It has been suggested by Larsen, Jensen, and Mackintosh (1987) that dipole-dipole interactions of extremely long range may be also responsible for perturbing the e-axis modulated moment states found in Ho and probably Er.
The development (Nigh 1963) of techniques for producing high-quality large single crystals of the heavy rare earth elements and alloys provided an opportunity to study the anisotropy in the magnetic properties of the rare earths that led to much of our current insight into the fundamental orderings and interactions in the rare earth metals. For details and references on these works, one is referred to the many review works available, including, but certainly not limited to, Elliott (1972), Gschneidner and Eyring (1979), Coqblin (1977), and the extensive treatise by Jensen and Mackintosh (1991).
A major breakthrough in rare earth materials occurred in 1984 with the development (Durbin, Cunningham, and Flynn 1982, Kwo et al. 1985a, 1985b) of MBE growth procedures for single crystal rare earth metal superlattices. These exotic materials have made possible prototypical tests for verifying many of the theoretical concepts of magnetic exchange, anisotropy, and magnetostrictive effects in the rare earths that could not previously be examined in as controlled a way using conventional bulk materials. Superlattices consisting of magnetically concentrated layers (e.g., Dy) interleaved in a controlled fashion with magnetically 'dead' layers (e.g., Y) offer a near-ideal opportunity to investigate these basic interactions. It should be noted that such a system is unique and can never be simulated by bulk dilute alloys because of the attendant reduction in the average exchange interaction with the decreased density of magnetic ions and the probability of some nearest neighbors even in very dilute samples. Y and Lu have similar physical and electronic properties to the magnetic heavy rare earths and good epitaxial growth is achieved because of the relatively small mismatch between the basal plane lattice parameters (e.g., 1.6% for Dy and Y). The key to the growth (see fig. 1) of rare earth superlattices lies in the use of a [110] Nb buffer layer evaporated onto a [1120] sapphire substrate beneath the rare earth metals. This buffer layer prevents a chemical reaction between the sapphire and the rare earths during growth. A strain-relieving Y overlayer is placed between the Nb and the rare earth bilayers. Y and Nb have a nearly perfect 3:4 atomic registration sequence that allows good epitaxial growth in spite of the 33% lattice parameter mismatch. Lattice parameter mismatch is a moderately serious constraint for MBE-produced materials, since this mismatch must generally be taken up by lattice dislocations. As shown in the figure for a [DyIY] superlattice, the thick Y layer is followed by a constant bilayer repeat sequence of I atomic planes of Dy and
Elsevier, 1995 Edited by: K.H.J. Buschow ISBN: 978-0-444-81974-1
by kmno4
PREFACE TO VOLUME 8
The Handbook series Magnetic Materials is a continuation of the Handbook series Ferromagnetic Materials. The original aim of Peter Wohlfarth when he started the latter series was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth's classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross-section of magnetism than Bozorth's book.
Magnetism has seen an enormous expansion into a variety of different areas of research in the last few years, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason the Editor and Publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 8 of this Handbook series.
Artificial multilayered structures are prominent examples of such classes of novel materials. Progress in the field of molecular beam epitaxy has made it possible to tailor-make layered metallic materials having sharp interfaces, crystalline coherence and superlattice periods of the order of lnm. These materials have opened a new field of magnetism that permits detailed studies of the propagation of magnetic order as a function of separation and crystallographic orientation, as well as studies of the interplay of strain and magnetic properties. A detailed account of achievements on rare earth based artificial multilayered structures is presented in the first chapter of this volume.
Magnetostriction refers to any dimensional changes of a magnetic material caused by changes in its magnetic state. Magnetostriction can originate from changes in magnitude or direction of the applied field or from changes in temperature. The former type of magnetostriction is particular pronounced in rare earth compounds of the type RFe2, as has been described in detail in Chapter 7 of Volume 1. The second type of magnetostriction is largest near the Curie temperature in ferromagnetic materials. This volume magnetostriction gives rise to the technically important Invar alloys, and the associated moment-volume instabilities in transition metal alloys have extensively been discussed in Chapter 3 of Volume 5. The large body of experimental results that have become available for the many intermetallic compounds in which
vi
PREFACETO VOLUME8
rare earths are combined with 3d transition metals is described in the third chapter of the present volume.
The ferrites form a large class of magnetic materials and some of these materials are of considerable technical importance. The properties of hard ferrites as well as soft ferrites have been described in several chapters in Volumes 2 and 3. Since the appearance of these chapters substantial progress has been made in the understanding of the physical and chemical properties of these materials which made it necessary to update the results described in the preceding chapters. New results obtained on ferrites are described in Chapter 3, where the emphasis is on spinel ferrites.
Of substantial technical importance is, furthermore, the group of so-called soft magnetic materials. A detailed description of several important classes of soft magnetic material has been presented already in Chapter 6 of Volume 1 and Chapter 2 of Volume 2. Supplementary results, dealing mainly with laminated amorphous alloys and electrical steels and the problem of the loss producing effect of the rotational magnetisation are highlighted in Chapter 4.
A survey of the magnetic properties of various types of rare earth intermetallics was given already in Volume 1 of the Handbook series. Since then proliferation of scientific results, obtained with novel techniques, and made for a large part on single crystals, have led to a more complete understanding of the basic magnetic interactions in these materials. This requires a major updating of the experimental results presented in Volume 1. However, the experimental and theoretical material that has accumulated over the years is so extensive that it is hardly possible to condense it in a single chapter. In the preceding volume, Vol. 7, supplementary information was presented already for intermetallics in which rare earths are combined with 3d transition metals. In the present volume the updating process has been continued by means of a chapter on rare earth copper compounds of the type RCu2.
Volume 8 of the Handbook on the Properties of Magnetic Materials, as the preceding volumes, has a dual purpose. As a textbook it is intended to be of assistance to those who wish to be introduced to a given topic in the field of magnetism without the need to read the vast amount of literature published. As a work of reference it is intended for scientists active in magnetism research. To this dual purpose, Volume 8 of the Handbook is composed of topical review articles written by leading authorities. In each of these articles an extensive description is given in graphical as well as in tabular form, much emphasis being placed on the discussion of the experimental material in the framework of physics, chemistry and material science.
The task to provide the readership with novel trends and achievements in magnetism would have been extremely difficult without the professionalism of the NorthHolland Physics Division of Elsevier Science B.V., and I wish to thank Joep Verheggen and Wim Spaans for their great help and expertise.
K. H.J. Buschow Van der Waals-Zeeman Laboratory University of Amsterdam
CONTENTS
Preface to Volume 8 . . . . . . . . . . . . . . . . . . . .
v
Contents . . . . . . . . . . . . . . . . . . . . . . . .
vii
Contents of Volumes 1-7 . . . . . . . . . . . . . . . . . .
ix
List of Contributors . . . . . . . . . . . . . . . . . . . .
xi
1. Magnetism in Artificial Metallic Superlattices of Rare Earth Metals
J.J. RHYNE and R.W. ERWlN . . . . . . . . . . . . . .
1
2. Thermal Expansion Anomalies and Spontaneous Magnetostriction in
Rare-Earth Intermetallics with Cobalt and Iron
A.V. ANDREEV . . . . . . . . . . . . . . . . . . .
59
3. Progress in Spinel Ferrite Research
V.A.M. BRABERS . . . . . . . . . . . . . . . . . .
189
4. Anisotropy in Iron-Based Soft Magnetic Materials
M. SOINSKI and A.J. MOSES . . . . . . . . . . . . . .
325
5. Magnetic Properties of Rare Earth-Cu2 Compounds
Nguyen Hoang LUONG and J.J.M. FRANSE . . . . . . . . .
415
Author Index . . . . . . . . . . . . . . . . . . . . . .
493
Subject Index . . . . . . . . . . . . . . . . . . . . . .
521
Materials Index . . . . . . . . . . . . . . . . . . . . .
527
vii
CONTENTS OF VOLUMES 1-7
Volume 1
1. Iron, Cobalt and Nickel, by E.P. Wohlfarth . . . . . . . . . . . . . . . .
1
2. Dilute Transition Metal Alloys: Spin Glasses, by J.A. Mydosh and G.J. Nieuwenhuys
71
3. Rare Earth Metals and Alloys, by S. Legvold . . . . . . . . . . . . . . . .
183
4. Rare Earth Compounds, by K.H.J. Buschow . . . . . . . . . . . . . . . .
297
5. Actinide Elements and Compounds, by W. Trzebiatowski . . . . . . . . . . . .
415
6. Amorphous Ferromagnets, by F.E. Luborsky . . . . . . . . . . . . . . . .
451
7. Magnetostrictive Rare Earth-Fe 2 Compounds, by A.E. Clark . . . . . . . . . .
531
Volume 2
1. Ferromagnetic Insulators: Garnets, by M.A. Gilleo . . . . . . . . . . . . . .
1
2. Soft Magnetic Metallic Materials, by G.Y. Chin and J.H. Wernick . . . . . . . . .
55
3. Ferrites for Non-Microwave Applications, by P.L Slick . . . . . . . . . . . .
189
4. Microwave Ferrites, by J. Nicolas . . . . . . . . . . . . . . . . . . . .
243
5. Crystalline Films for Bubbles, by A.H. Eschenfelder . . . . . . . . . . . . .
297
6. Amorphous Films for Bubbles, by A.H. Eschenfelder . . . . . . . . . . . . .
345
7. Recording Materials, by G. Bate . . . . . . . . . . . . . . . . . . . .
381
8. Ferromagnetic Liquids, by S. W. Charles and J. Popplewell . . . . . . . . . . .
509
Volume 3
1. Magnetism and Magnetic Materials: Historical Developments and Present Role in Industry
and Technology, by U. Enz . . . . . . . . . . . . . . . . . . . . . .
1
2. Permanent Magnets; Theory, by H. Zijlstra . . . . . . . . . . . . . . . .
37
3. The Structure and Properties of Alnico Permanent Magnet Alloys, by R.A. McCurrie
107
4. Oxide Spinels, by S. Krupidka and P Novcik . . . . . . . . . . . . . . . .
189
5. Fundamental Properties of Hexagonal Ferrites with Magnetoplumbite Structure,
by H. Kojima . . . . . . . . . . . . . . . . . . . . . . . . . . .
305
6. Properties of Ferroxplana-Type Hexagonal Ferrites, by M. Sugimoto . . . . . . . .
393
7. Hard Ferrites and Plastoferrites, by H. Stiiblein . . . . . . . . . . . . . . .
441
8. Sulphospinels, by R.P. van Stapele . . . . . . . . . . . . . . . . . . .
603
9. Transport Properties of Ferromagnets, by LA. Campbell and A. Fert . . . . . . . .
747
ix
x
CONTENTS OF VOLUMES i-7
Volume 4
1. Permanent Magnet Materials Based on 3d-rich Ternary Compounds, by K.H.J. Buschow
1
2. Rare Earth-Cobalt Permanent Magnets, by K.J. Strnat . . . . . . . . . . . . .
131
3. Ferromagnetic Transition Metal Intermetallic Compounds, by J. G. Booth . . . . . .
211
4. Intermetallic Compounds of Actinides, by V Sechovsk~ and L. Havela . . . . . . .
309
5. Magneto-optical Properties of Alloys and Intermetallic Compounds, by K.H.J. Buschow
493
Volume 5
1. Quadrupolar Interactions and Magneto-elastic Effects in Rare-earth Intermetallic
Compounds, by P. Morin and D. Schmitt . . . . . . . . . . . . . . . . .
1
2. Magneto-optical Spectroscopy of f-electron Systems, by w. Reim and J. Schoenes . .
133
3. INVAR: Moment-volume Instabilities in Transition Metals and Alloys, by E. E Wasserman 237
4. Strongly Enhanced Itinerant Intermetallics and Alloys, by P.E. Brommer andJ.J.M. Franse 323
5. First-order Magnetic Processes, by G. Asti . . . . . . . . . . . . . . . . .
397
6. Magnetic Superconductors, by 0. Fischer . . . . . . . . . . . . . . . . .
465
Volume 6
1. Magnetic Properties of Ternary Rare-earth Transition-metal Compounds, by H.-S. Li and
J.M.D. Coey . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2. Magnetic Properties of Ternary Intermetallic Rare-earth Compounds, by A. Szytula
85
3. Compounds of Transition Elements with Nonmetals, by O. Beckman and L. Lundgren
181
4. Magnetic Amorphous Alloys, by P. Hansen . . . . . . . . . . . . . . . .
289
5. Magnetism and Quasicrystals, by R. C. O'Handley, R.A. Dunlap and M.E. McHenry
453
6. Magnetism of Hydrides, by G. VCiesinger and G. Hilscher . . . . . . . . . . .
511
Volume 7
1. Magnetism in Ultrathin Transition Metal Films, by U. Gradmann . . . . . . . . .
1
2. Energy Band Theory of Metallic Magnetism in the Elements, by V.L. Moruzzi and
P.M. Marcus . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
3. Density Functional Theory of the Ground State Magnetic Properties of Rare Earths and
Actinides, by M. S. S. Brooks and B. Johansson . . . . . . . . . . . . . . .
139
4. Diluted Magnetic Semiconductors, by J. Kossut and W. Dobrowolski . . . . . . . .
231
5. Magnetic Properties of Binary Rare-earth 3d-transition-metal Intermetallic Compounds,
by J.J.M. Franse and R.J. Radwahski . . . . . . . . . . . . . . . . . .
307
6. Neutron Scattering on Heavy Fermion and Valence Fluctuation 4f-systems,
by M. Loewenhaupt and K.H. Fischer . . . . . . . . . . . . . . . . . .
503
chapter 1
MAGNETISM IN ARTIFICIAL METALLIC SUPERLATTICES OF RARE EARTH METALS
J.J. RHYNE
Research Reactor Center and Department of Physics University of Missouri-Columbia Columbia, Missouri 65211 U.S.A.
and
R.W. ERWlN
Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, Maryland 20899 U.S.A.
Handbook of Magnetic Materials, Vol. 8 Edited by K. H.J. Buschow ©1995 Elsevier Science B.V. All rights reserved
CONTENTS
1. Introduction to superlattices and rare ealths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2. Neutron scattering and artificial metallic superlattices ...............................
6
3. Magnetic scattering, structure, and coherenee .....................................
11
3.1. Superlattices with e-axis growth directions ...................................
11
3.2. Superlattices with a basal plane growth direction ..............................
26
4. Interlayer magnetic coupling in superlattices ......................................
30
5. Effect of applied magnetic fields on interlayer coherence ............................
32
6. Incommensurate magnetic periodicity - interplanar turn angles, spin slips, and their
temperature dependence .......................................................
35
7. Magnetoelasticity in lare earth superlattices and films and the suppression of
ferromagnetism ..............................................................
41
7.1. Theory of magnetoelasticity in superlattices and films ..........................
43
7.2. Magnetoelastie energies in Dy superlattices and films ...........................
44
8. Coherent magnetic moments and disorder at interfaces ..............................
48
9. Residual moment effects in superlattices .........................................
50
10. Summary ...................................................................
54
11. Acknowledgements ..........................................................
55
References .....................................................................
55
1. Introduction to superlattices and rare earths
Intense interest has been generated over the past several years in the growth and properties of layered magnetic materials, both from a fundamental point of view and for applications. Layered structures have been prepared by a variety of techniques such as sputtering, electro-deposition, and evaporation, and include semi-conducting, metallic, and insulating materials. These systems can consist of crystalline layers of one element or compound interleaved with layers of a different element or compound, or alternatively may be built of amorphous layers or amorphous layers alternated with crystalline layers. Depending on the materials and growth techniques, these multilayers may be produced (a) with no uniform crystallographic alignment or coherence from layer to layer, (b) with alignment of one specific crystallographic axis direction along the growth (stacking) direction, or (c) with true three-dimensional atomic order (epitaxy) in which there is multilayer atomic registry both along the growth axis and also within the growth planes. For the purposes of this review, the term artificial metallic superlattice will be reserved for this latter category of true threedimensionally coherent layered structures, while the term multilayer will be used for layered structures in which coherence is present in less than three dimensions.
Refinements in computer-controlled molecular beam epitaxy (MBE) techniques for the growth of single crystal artificial superlattices of two or more distinct compositions have unearthed vast possibilities for the production of tailor-made artificial superlattices with controlled film thicknesses down to atomic dimensions and with highly reproducible stacking sequences. This has provided previously unavailable opportunities to examine problems of interaction ranges, tunneling distances, and other coherent phenomena which are dependent on the superlattice periodicity.
The lanthanide elements and their alloys with non-magnetic but chemically and electronically similar elements such as yttrium, scandium, and lutecium (collectively known as the rare earths), have long provided a fertile area for the study of longrange indirect exchange interactions, crystal field anisotropy, and magnetostrictive effects. The elements have weak exchange compared to the 3d transition elements as illustrated by their low ordering temperatures (e.g., Tb, TN = 230 K) and have anomalously large crystal field and magnetoelastic interactions that arise from the strong spin orbit coupling and highly non-spherical 4f charge distribution. Below their magnetic ordering temperatures, many of the lanthanide elements exhibit one or more forms of periodic incommensurate spin structures such as helices or longitudinal spin density waves, etc., and transitions among them as the temperature is varied. The occurrence of these periodic magnetic orderings below their Nrel
4
J.J. RHYNEand R.W.ERWIN
temperatures in the heavy lanthanides (except for the S state ion Gd) is a consequence of electronic effects, in particular the occurrence of nearly two-dimensional parallel sections (nesting) on the hole Fermi surface that are spanned by a Q-vector whose magnitude determines the initial stable periodicity of the magnetic ordering. At lower temperatures, the free energy associated with the magnetostrictive and anisotropy interactions becomes significant and strongly perturbs the basic periodic magnetic orderings leading to phase transitions to a ferromagnetic state in Tb, Dy, Ho, and Er. These transitions are largely driven by a lowering of the magnetoelastic energy arising from a coupling of the local moments to the hexagonally symmetric lattice strains. It has been suggested by Larsen, Jensen, and Mackintosh (1987) that dipole-dipole interactions of extremely long range may be also responsible for perturbing the e-axis modulated moment states found in Ho and probably Er.
The development (Nigh 1963) of techniques for producing high-quality large single crystals of the heavy rare earth elements and alloys provided an opportunity to study the anisotropy in the magnetic properties of the rare earths that led to much of our current insight into the fundamental orderings and interactions in the rare earth metals. For details and references on these works, one is referred to the many review works available, including, but certainly not limited to, Elliott (1972), Gschneidner and Eyring (1979), Coqblin (1977), and the extensive treatise by Jensen and Mackintosh (1991).
A major breakthrough in rare earth materials occurred in 1984 with the development (Durbin, Cunningham, and Flynn 1982, Kwo et al. 1985a, 1985b) of MBE growth procedures for single crystal rare earth metal superlattices. These exotic materials have made possible prototypical tests for verifying many of the theoretical concepts of magnetic exchange, anisotropy, and magnetostrictive effects in the rare earths that could not previously be examined in as controlled a way using conventional bulk materials. Superlattices consisting of magnetically concentrated layers (e.g., Dy) interleaved in a controlled fashion with magnetically 'dead' layers (e.g., Y) offer a near-ideal opportunity to investigate these basic interactions. It should be noted that such a system is unique and can never be simulated by bulk dilute alloys because of the attendant reduction in the average exchange interaction with the decreased density of magnetic ions and the probability of some nearest neighbors even in very dilute samples. Y and Lu have similar physical and electronic properties to the magnetic heavy rare earths and good epitaxial growth is achieved because of the relatively small mismatch between the basal plane lattice parameters (e.g., 1.6% for Dy and Y). The key to the growth (see fig. 1) of rare earth superlattices lies in the use of a [110] Nb buffer layer evaporated onto a [1120] sapphire substrate beneath the rare earth metals. This buffer layer prevents a chemical reaction between the sapphire and the rare earths during growth. A strain-relieving Y overlayer is placed between the Nb and the rare earth bilayers. Y and Nb have a nearly perfect 3:4 atomic registration sequence that allows good epitaxial growth in spite of the 33% lattice parameter mismatch. Lattice parameter mismatch is a moderately serious constraint for MBE-produced materials, since this mismatch must generally be taken up by lattice dislocations. As shown in the figure for a [DyIY] superlattice, the thick Y layer is followed by a constant bilayer repeat sequence of I atomic planes of Dy and