Muons and magnets

Abstract:

TmAgGe is an antiferromagnet based on the ZrNiAl structure. At low temperatures the spins are confined to distorted kagome-like planes, wherein the magnetisation is strongly anisotropic. A previous study has shown that a series of stepped magnetic transitions are apparent in low, in-plane magnetic fields and can be explained using a three-fold Ising-like model. Here we present high-magnetic-field magnetisation experiments showing that further stepped transitions are observed when the field is directed out of the kagome planes. Angledependent measurements in fields of up to 55 T show that there are at least two distinct and separate energy scales present in this system; the weak exchange interactions and the strong crystalline electric field interactions. Simulations of the magnetisation using a three-dimensional, free-energy minimisation technique allow us to suggest the nature and hierarchy of the forces acting on the Tm3+ moments.

Abstract:

Most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. Purely organic ferromagnets, based upon nitronyl nitroxide radicals, show long range magnetic order at very low temperatures in the region of 1K, while sulphur-based radicals show weak ferromagnetism at temperatures of up to 36K. It is also possible to prepare molecule-based magnets in which transition-metal ions are used to provide the magnetic moment, but organic groups mediate the interactions. This strategy has produced magnetic materials with a large variety of structures, including chains, layered systems, and three-dimensional networks, some of which show ordering at room temperature and some of which have very high coercivity. Even if long range magnetic order is not achieved, interesting materials displaying the spin crossover effect may be prepared and these can have useful applications. Further magnetic materials may be obtained by constructing charge-transfer salts, which can produce metallic molecular magnets. A very exciting recent development is the preparation of single molecule magnets, which are small magnetic clusters. These materials can show macroscopic quantum tunnelling of the magnetization and may have uses as memory devices or in quantum computation applications. These systems can be powerfully studied using various experimental methods, including magnetometry, neutron scattering, muon-spin rotation and synchrotron radiation techniques.