Recently a very large number of new materials have been discovered in which
the magnetic atoms are locked cooperatively into a non-magnetic spin-singlet
ground state. The signature of this state is that the spin-susceptibilities
develop an activated temperature dependence at low temperatures. Such a
spin-gap phase can arise from the high temperature paramagnetic phase
through a sharp phase transition or through a gradual crossover as the
temperature is lowered. Materials of interest include Calcium and Sodium
Vanadates, Copper Germanates, Strontium Cuprates, Strontium Boron Cuprates
and a number of organic materials. Many of these systems exhibit
a complex interplay of spin, phonon and orbital degrees of
freedom. Understanding the magnetic behavior of these systems requires
input from quantum chemistry, electronic structure, many-body model
Hamiltonians and quantum field theory. The physics of insulating spin-gap
materials may also have implications for high temperature superconductivity.
These systems will be one of the central topics of the ITP program.
Manipulation of the spin degree of freedom of conduction
electrons leads to a new form of
electronics, now dubbed Spintronics. This form of current and
voltage control uses low resistance (hence low voltage) magnetic metals
rather than high resistance (high voltage) semiconductors such as Si.
Thhe spin-polarized current also offers entirely new possibilities, such as
manipulations of electronic signals by magnetic fields (or vice versa), or
novel effects in ferromagnet/superconductor/ferromagnet sandwiches or
multilayers. Quantum information storage and quantum computation are
related phenomena that require further study.
Recent advances in fabrication of nanomaterials raises several important
questions. How does one understand the magnetism at decreasing length
scales and at what length scale will the magnetism become unstable?
What role do quantum fluctuations play in the dynamics of magnetic
nanoparticles? How does one characterize the behavior of individual
nanoparticles and what is its relation to the behavior of an ensemble
of nanoparticles? How does one determine their size distributions?
What novel applications may result from manipulations of magnetism
at mesoscopic and smaller length scales in semiconductors, molecular
magnets and other related systems?
Whether a material is a conductor or an insulator is one of its most
fundamental characteristics. Ferromagnets are usually metallic, but metallic
insulators also exist. In either case both up and down spin electrons
have the same electrical behavior: both spins are metallic, or both
are insulating. Half-metallic ferromagnets, however, are schizophrenic:
electrons with one spin direction (say, up) are metallic, while
spin-down electrons
are insulating. This peculiar electronic state has become the focus of
recent research, both for its intrinsic scientific interest and because
of the promise of optimum `spin-electronics' devices that could provide an extra
magnetic degree of freedom in manipulating electrical signals.
Half-metallicity, and no doubt many other consequences, can arise in materials
in very high magnetic fields. The quantum Hall effect is one
consequence that has been thoroughly studied. How to describe a metal, and
what phenomena to expect, in high fields needs theoretical attention.
Very unusual magnetotransport behavior discovered in Mn-based perovskites
has stimulated intensive study of the many phonomena that occur, not only
in this system but in a variety of transition metal oxides. The seed of the
phenomenon seems to lie in the double exchange mechanism of
Zener, but the remarkable range of other phenomena observed in these
systems (such as magnetic field induced structural transitions) indicates that
these materials will provide great scientific interest for some time.