Report to the 1999 General Assembly for 1996-99
Associate Member 94-97:
The Magnetism Commission C9 met during the International Magnetism Conference (ICM) at Cairns, Australia in July 1997. All members bar one were present. The commission received the final report on ICM94 (Warsaw), a preliminary report on ICM97, and heard a report on the state of preparations for ICM2000 (Recife). There had been 1150 participants from 47 countries at ICM94, and there were 850 participants at ICM97. The Commission appreciated the particular difficulties that the Australian organizers had had to face, and considered that ICM97 had been a scientific success, in interesting surroundings.
The Commission discussed improved procedures for choosing invited speakers at ICM meetings, and the rules concerning the number of publications to be accepted for the proceedings. Advice on a number of other points was given to the organizers of the next conference. The adverse effect of specialist meetings not co-ordinate with ICM was considered a real danger, and there was some discussion as to whether satellites detracted from the excitement of the main conference. The ICM2000 Organizing Committee was strongly advised to maintain close contact with C9 concerning decisions to be made during the run up to the Conference.
ICM2003 and other meetings
The Commission heard proposals from four different national delegations for hosting ICM in 2003. Ultimately it was decided to invite the Italian magnetism community to organize the conference in Rome. C9 members were asked to attract the attention of organizers of other international magnetism meetings to the possibility of IUPAP sponsorship.
After discussion it was decided that the Magnetism Prize Committee would be made up of all C9 members plus all former recipients of the prize. The C9 chairman would be automatically chairman of the Prize Committee. Members of the Prize Committee could make nominations before the final deadline. The procedure for consulting the International community should be improved.
The issue is whether IUPAP should play a more active role in attempting to orient the policy concerning access to large instruments, particularly in the condensed matter field. After discussion the Commission passed a motion to support the present IUPAP guidelines. There was also some support for IUPAP to take a more active role in the development of these facilities if it had the necessary expertise and effort to make a considered contribution.
Renewal of the Commission
After an initial discussion at Cairns and further email contacts later the Commission prepared a balanced slate of new officials and members, to be proposed at the IUPAP Atlanta meeting in 1999.
1997 Magnetism Award
The 1997 Magnetism Award of Commission C-9 was presented at the ICM conference in Cairns, Australia to Professor R. J. Birgeneau of the Massachusetts Institute of Technology. The Award was made in recognition of his achievements in magnetism, and in particular for the identification of model magnetic systems and the experimental elucidation of their properties using neutron and x-ray scattering techniques. The Commission is grateful to North Holland for their generosity in donating the prize.
New Developments in Magnetism
Magnetism continues to be an important subject both for the insights it provides into the understanding of condensed matter and co-operative phenomena and for the development of technologically important materials and devices. Many aspects of the recent developments are discussed in detail in the Proceedings of the International Conference on Magnetism held in Australia in 1997 and published in the Journal of Magnetism and Magnetic Materials. Highlights of that conference and of other meetings and developments are briefly described below.
The properties of low dimensional systems and particularly materials with one-dimensional chains of magnetic ions continue to provide important information about strongly interacting systems. Theoretical and experimental work has shown there are three types of one-dimensional systems with Heisenberg antiferromagnetic interactions that exhibit spin gaps. Chains with alternating exchange constants and s=½ have a dimer-like ground state, chains with s=1 has a Haldane gap and s=½ two or four leg spin ladder compounds. Experimental progress on these types of materials has been stimulated by the growth of new oxide materials particularly in Japan by the use of infrared furnaces. Experiments have shown that the properties of s=½ Heisenberg chains in an applied magnetic field are different from those of classical chains in that when the field is applied the spin correlations are short-ranged and incommensurate transverse to the field instead of long ranged and antiferromagnetic. All of these results show how quantum fluctuations can have a large effect on the properties of low-dimensional systems.
Quantum Critical Points
The study of magnetic materials has played an important role in the understanding of thermal phase transitions by enabling experiments to be done on model systems. The properties of materials can however be changed by doping or by the application of applied fields or pressure at zero temperature. Examples of such transitions are the doping of silicon to produce a metal, the doping of high temperature superconductors to change their properties from an antiferromagnetic insulator to a superconductor and the effect of pressure in suppressing the magnetic transition of MnSi or ZrZn2. It is now realized that the quantum fluctuations occurring at these phase transitions lead to different behavior from the thermal fluctuations at thermal phase transitions and the past few years have seen considerable progress in the theory and experiments on a wide variety of different systems to test the predictions.
Thin Films and Superlattices
There has been considerable activity in the field of thin films and superlattices. It is now possible to grow a wide variety of novel materials and their properties are studied by microscopic measurements and by microscopic methods such as neutron and x-ray diffraction. This activity has led to an increasingly detailed understanding of their properties and of the magnetic interactions in these systems. During the last three years there have been many experiments on systems with 3d transition metals and 4f rare earths and a much better understanding of their properties achieved. Developments have also been made in the use of the giant-magneto-resistance effect in superlattices and read-heads for fixed discs in computers are now being built using these devices. Another development us the integration of magnetic materials with electronics to produce spin-electronics. This is electronics in which use is made of the information carried by the spin of the electrons. The first spin transistors have been demonstrated.
High Tc Superconductors
Although there is as yet no consensus about the microscopic theory of superconductivity in the cuprates, there continues to be an immense amount of experimental and theoretical work. One of the important results of the past few years has been that a considerable number of experiments have shown that the superconducting gap is anisotropic and predominantly of dx2-y2 character. This is very different from the wave character of conventional superconductors. It is worth noting that Sr2RuO4 has been shown to be a p wave superconductor so that the number of different types of superconductivity is increasing rapidly.
High Tc superconductors are being developed towards technological applications. SQUIDS are already available and it is anticipated that communications will soon make use of superconducting technology. The development of tapes has enabled the construction of small high Tc magnet coils with properties that are already comparable to those of Nb3Sn. It can be anticipated that high Tc inserts into superconducting crystals will become available and useful in the near future.
Random Magnets and Frustration
Experiments continue to be performed on systems with frustrated interactions. Particular progress has been made on spin glasses below the freezing temperature where very long and indeed infinitely long relaxation times have been found. The phenomena associated with these long relaxation times is known as ageing and this has been studied both experimentally and theoretically. The properties of geometrically frustrated systems such as the antiferromagnetic pyrochlores have attracted considerable attention. One surprising result is that long-range order is suppressed even if the exchange interactions are ferromagnetic in character when there are strong crystal field interactions.
Research into magnetic materials requires the use of large facilities. Firstly the largest magnetic fields are produced only in a few laboratories in the world and so experiments are performed in these laboratories. The largest continuous fields are provided at NHMFL at Tallahassee, Florida and NRIM in Tsukuba, Japan by using hybrid superconducting and conventional magnets. Pulsed field facilities are less expensive and the optimum compromise between pulse length and peak field depends on the particular experiment. Studies are underway to produce 100 T pulsed magnets (non-destructive) and much higher fields destructively.
The microscopic magnetic structure of materials is most often studied by using neutron scattering techniques and experiments are performed to measure the structure and excitation for all classes of magnetic materials. More recently synchrotron sources have been used to study the magnetic scattering. In the past few years these techniques have been extended to study 3d transition metals while the high resolution of x-ray scattering techniques has been used to probe phase transitions and surface effects.
A. Cowley Chairman