Trinity College Dublin

Oxides of the type (La1-xAx)MnO₃ ; A=Ca, Ba, Sr ; x~0.3 with the perovskite structure show a metal-insulator transition at the Curie temperature T_C. This is accompanied by colossal magnetoresistance, an intrinsic effect associated with a field-induced increase of spontaneous magnetisation near T_C. (Mixed Valence Manganites; J.M.D. Coey, M. Viret, S. von Molnar, Advances in Physics 48 (2): 167 (1999))

Chromium dioxide has the distinction of being the only simple oxide that is a ferromagnetic metal. The resistivity of CrO₂ increases rapidly as the temperature approaches the Curie point (T_C=398K), and the mean free path of the electrons is reduced to the scale of the interatomic spacing by strong spin-flip scattering. (Magnetic Properties of a New Series of Rare Earth Iron Nitrides - R₂Fe₁₇N_Y² (30): 6465 (1990))

Double Perovskites such as Sr₂FeMo₆ and Sr₂FeReO₆ are claimed to be half metals with TC higher than 400K. As the T_C of mixed-valence manganites cannot be increased above 400K, and the T_C of CrO₂ is also 400K, these materials, along with Fe₃O₄, currently offer the best prospects for high temperature applications. (R. P. Borges, R. M. Thomas, C. Cullinan, J. M. D. Coey, R. Suryanarayanan, L. Ben-Dor, L. Pinsard-Gaudart and A. Revcolevschi. Magnetic properties of the double perovskites A₂FeMoO₆ ; A = Ca, Sr, Ba. J. Phys. : Cond. Matter, 11, L445 (1999). )

The oldest magnetic material known to man, Fe₃O₄ is also the half-metal with the highest Curie temperature 870K. The electrons form small polerons and hop among the B sites. The formula can be written [Fe³⁺](Fe²⁺, Fe³⁺)O₄.
A sites gold, B sites yellow

Half metallic ferromagnets may be usefully applied in magnetoresistive devices. They can exploit the intrinisc colossal magnetoresistance of the materials, the magnetoresistance which arises from a polycrystalline grain structure or the response of a spin valve sandwich structure, where the direction of one ferromagnetic layer is pinned and the other is free to flip in a small magnetic field.

The spin valve is the simplest magnetoresistive device. It consists of two ferromagnetic layers seperated by a metallic spacer, one of which is free to switch between parallel and antiparallel alignments corresponding to the low and high resistivity states, respectively.

A variant of the spin valve is the magnetic tunnel junction, where the ferromagnetic layers are separated by an insulator just a few atoms thick. Tunnel junctions are the basis of the new Magnetic Random Access Memory chips (MRAM).

Multiple electrical contacts are needed to measure the electrical properties of this patterned polycrystalline CrO₂ film. The wire is 50 microns (mm) long and 3 mm wide.