High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix

Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al2O3 matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe-Al2O3 sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, T-B, (481 K), magnetic anisotropy K (4.8 x 10(5) erg/cm(3)) for Fe, and the Boltzmann constant k(B) (1.38 x 10(-16) erg/EL) from T-B = KV/25k(B), the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies. (C) 2001 Published by Elsevier Science B.V.