Aspects of Modern Magnetism

The following sections are included:

• Contributors

• Preface

• Contents

After a sketch of the historical background to modern magnetism, some recent developments in experimental techniques, magnetic materials, nanostructures and spin electronics are reviewed. New magnet applications are presented, and the survey ends with some remarks on the broader social perspectives of research in applied science.

Micro- and nanomagnetism has become a very efficient technique to study spinstructures and magnetization processes in modern magnetic materials. In particular micromagnetism allows a quantitative treatment of the interaction between magnetism and the microstructure of ferromagnetic materials. In high-coercivity materials the deterioration of the ideal nucleation fields by misaligned grains, magnetic stray fields at sharp edges, and inhomogeneous grain surfaces may be determined quantitatively.

The complex magnetization processes in two- and three-dimensional ensembles of grains may be treated by computational micromagnetism using the method of finite elements. The role of the grain size and of the magnetic properties of the grain boundaries are shown to govern the properties of hard as well as soft magnetic properties. Composite ferromagnetic nanocrystalline materials with excess α-Fe content are shown to be hopeful future high-remanence materials.

What we shall discuss are the macroscopic quantum effects caused by the quantum tunneling in magnetic solids. Such effects are of great interest not only due to their fundamental importance in physics, but also due to the wide scope for technological innovation. In this article a few basic conceptions are interpreted, some relevant theoretical methods outlined, and a part of experimental results showing evidence of the effects reviewed.

A brief overview of magnetism in metals, a concise introduction to four kinds of exchange interactions in metals and the size effect on magnetism are presented in the lecture.

The present state of the art of theoretical studies of magnetism in artificial low dimensional transition metal materials, e.g., surfaces, overlayers and multilayers, is reviewed here by means of some illustrative examples. Strong magnetic moment enhancements are found for Fe, Co and Ni atoms at surfaces or interfaces contacting with inert substrates. By contrast, the spin polarization is strongly frustrated in systems with strong hybridization, which usually leads to more complex antiferromagnetic ground states. As a major progress for first principles determination of the magneto-crystalline anisotropy (MCA), the state tracking and the torque approaches enable us to obtain very stable and reliable results of the MCA energy and other subsequent effects such as magnetostriction. Magneto-optical effects including Kerr and dichroism spectra, their anisotropies and possibilities to be applied for determining the spin, orbital and dipole moments and surface and interface states are also discussed.

The RKKY and quantum well approaches to exchange coupling in magnetic multilayers are compared. The concept of magnetic contrast is explained and its important influence on coupling strength is discussed. A brief review is given of general features of exchange coupling which have been established by quantum well theory based on one-band tight-binding models. The calculational method for a realistic multi-band model is outlined and the results of applying it to Co/Cu/Co (001) trilayers are compared with experiment. The work reviewed here results from a collaboration between groups at City University, Imperial College and UFF Niteroi (Brazil).

The following sections are included:

• Introduction

• Giant Magnetoresistance
  • Magnetic Multilayers: Examples-Polycrystalline Co/Cu and Fe/Cr
  • Granular Alloys- An example: (111) Oriented Co-Cu

• Oscillatory Interlayer Coupling

• Simple Model of Giant Magnetoresistance

• Inverse Giant Magnetoresistance in Spin-Engineered Multilayers

• Giant Magnetoresistance at Low Fields in Magnetic Multilayers
  • Dependence of Giant Magnetoresistance on Spacer Layer Thickness
  • Giant Magnetoresistance in Permalloy/Au Multilayers
  • Giant Magnetoresistance in Exchange Biased Sandwiches

• Dependence of Giant Magnetoresistance on Structure
  • Dependence of GMR on Number of Bilayers
  • Dependence of GMR on Magnetic Layer Thickness

• Conclusions

• Acknowledgements

• REFERENCES

Our current understanding of magnetically controlled electrical transport in magnetic multilayers and granular films is discussed. The origins of the spin-dependence of scattering in these material are reviewed and outstanding issues are outlined.

The following sections are included:

• Introduction

• Perovskite ABO₃ Structures
  • Orthorhombic Structure
  • Rhombohedral Structure
  • Structure Transition

• Magnetic Properties

• Electric Properties

• CMR in Doped Thin Films
  • Sample preparation and Experiment
  • Epitaxial Thin Films
  • Influence of Substrate Temperature
  • Annealing Effect
  • Sample Preparation and Oxygen Composition

• Possible Origin of The CMR

• Acknowledgments

• References

In this chapter a review is given of recent research on nanostructured magnetic films of interest for future peripheral microelectronics or magnetoelectronics applications. Various classes of materials are discussed with emphasis on the design of the nanoscale structure to achieve needed physical properties. Requirements for future ultra high density recording media are discussed, and several novel materials, with complex nanostructures, are presented as candidates. New high coercivity films with about 10 nm grains are discussed along with challenges and opportunities for further advances.

The following sections are included:

• Introduction

• Principles of magneto-optic storage
  • Kerr rotation and ellipticity
  • Materials requirements

• Nanostructured magneto-optic materials
  • Amorphous Rare-earth/transition-metal multilayers
  • Rare-earth/transition-metal garnets
  • Mn - based materials

• Summary

• Acknowledgments

• References

The growth of monocrystals of magnetic systems is an essential step towards the understanding of their anisotropic properties. This task is particularly difficult in the case of rare earth compounds owing to their complex phase diagrams and high chemical affinity. Levitation techniques in cold crucibles are at the basis of the synthesis of polycrystals and of some crystal growth methods. The Czochralsky and Bridgman techniques will be discussed as the solid state electrolysis. In order to study the intrinsic properties of “pathological” systems, a great effort has been undertaken to lower the rate of defects. Some emphasis will be put on Solid State Electrolysis as a refining and crystal growth technique.

The strong hyperfine interaction that the 3d or 4f electrons have with the nucleus in a magnetically-ordered material and its correlation with the local environment surrounding the atom make hyperfine interaction experiments, such as nuclear magnetic resonance (NMR) and Mössbauer effect (ME), useful in the exploration of structural and magnetic properties of magnetic materials on a very local scale. This paper presents several examples showing how NMR experiments can be used in studies of materials, either crystalline or amorphous and either bulk or nanoscale size, with the goal of stimulating a more extensive application of the technique.

In this paper, we discuss experimental studies of magnetization processes in different types of magnetic systems under very high magnetic fields in the mega-gauss range. In particular, we present the recent results on the magnetization processes and magnetization saturation of randomly distributed paramagnetic ions, antiferromagnetic systems, ferrimagnetic systems, and one-dimensional antiferromagnets,as well as experimental techniques for the generation of the high magnetic fields and for the magnetization measurements.

The following sections are included:

• Author Index