Nanoparticle morphology in FeCo-Al2O3granular films with tunneling giant magnetoresistance

Abstract

A series of FeCo–Al2O3 granular films were prepared by a magnetron-controlled sputtering system. The tunneling giant magnetoresistance and nanoparticle morphology of FeCo particles in FeCo–Al2O3 granular films were directly determined utilizing a conventional four-probe method and TEM (HRTEM) observation, respectively. The results indicated that the tunneling giant magnetoresistance can reach a maximum of 6.9% at about 32.8 vol% FeCo particles, so far the highest value reported at room temperature and under an applied field of 12.5 kOe. Meanwhile, the sensitivity of TMR also reaches a maximum at about 32.8 vol% FeCo particles. In addition, TEM and HRTEM observation disclosed that FeCo–Al2O3 films consist of FeCo nanoparticles with bcc structure or amorphous FeCo phase dispersed in amorphous or crystalline Al2O3 matrix. For films with lower volume fraction of FeCo particles, the size distribution of FeCo particles satisfied a log-normal function. With increasing volume fraction of FeCo particles, the size distribution of FeCo particles deviated gradually from a log-normal function. Meanwhile, the average size of FeCo particles increased monotonically with increasing volume fraction of FeCo particles, leading to the fact that TMR can reach a peak value at a certain middle particle size. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)