Solid containing rotationally free nanocrystalline gamma-Fe2O3: Material for a nanoscale magnetic compass?

A nanocomposite material has been characterized that contains nanometer siz e magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5-10 nm crystals of gamma-Fe2O3 dispersed in a s olid methanol polymer matrix. The material was prepared by freezing a metha nol-based ferrofluid of gamma-Fe2O3 and subjecting it to a magnetic field a pplied in alternate directions to anneal the matrix. Before the field treat ment, the solid displays magnetic behavior characteristic of an ordinary na noscopic magnetic material. It is superparamagnetic above the blocking temp erature (160 K) and hysteretic below, showing magnetic remanence and coerci vity. After the field treatment to anneal the matrix, the same solid shows only Curie-Weiss behavior above and below the blocking temperature over the temperature range from 4.2 to 200 K and in response to applied magnetic fi elds as low as 1.59 kA/m. The data are consistent with a solid containing r otationally free, nanoscopic magnets encased in cavities of very small dime nsions. The free rotation of the particles precludes the observation of mag netic relaxation phenomena that are characteristic of magnetic solids and f errofluids. The present solid portends a class of magnetic materials with v ery little or no electrical and magnetic loss. (C) 2000 American Institute of Physics. [S0021-8979(00)06911-5].