No abstract

A nanocomposite material has been characterized that contains nanometer size magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5-10 nm crystals of ␥-Fe 2 O 3 dispersed in a solid methanol polymer matrix. The material was prepared by freezing a methanol-based ferrofluid of ␥-Fe 2 O 3 and subjecting it to a magnetic field applied in alternate directions to anneal the matrix. Before the field treatment, the solid displays magnetic behavior characteristic of an ordinary nanoscopic magnetic material. It is superparamagnetic above the blocking temperature ͑160 K͒ and hysteretic below, showing magnetic remanence and coercivity. 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 fields as low as 1.59 kA/m. The data are consistent with a solid containing rotationally free, nanoscopic magnets encased in cavities of very small dimensions. The free rotation of the particles precludes the observation of magnetic relaxation phenomena that are characteristic of magnetic solids and ferrofluids. The present solid portends a class of magnetic materials with very little or no electrical and magnetic loss.

No abstract

No abstract

We report measurements of the magnetic relaxation rate versus temperature for ferrofluid and magnetic-glass samples, which are formed by a modification of nanocomposite material consisting of nanocrystalline CoFe 2 O 4 and polymer. 1 The magnetic properties of the samples have also been studied by using SHE-SQUID at different temperatures ͑1.8 -300 K͒ with low and high applied magnetic field ͑Ϫ5 T to 5 T͒. The magnetic relaxation in two samples show a perfect logarithmic dependence on the time, i.e., M (t)ϭM (t 0 )[1ϪS ln(t/t 0 )], in accordance with the ZFC-FC results which indicate that there is wide energy distribution. The temperature independence of magnetic viscosity Sw[1/M (t 0 )dM /d]ln t below several Kelvin for the two samples gives clear evidence of macroscopic quantum tunneling of magnetization, in accordance with current theories of quantum tunneling of magnetization.