Nanoparticle morphology in FeCo–Al 2 O 3 granular films with tunneling giant magnetoresistance

A series of FeCo–Al 2 O 3 granular films were prepared by a magnetron‐controlled sputtering system. The tunneling giant magnetoresistance and nanoparticle morphology of FeCo particles in FeCo–Al 2 O 3 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–Al 2 O 3 films consist of FeCo nanoparticles with bcc structure or amorphous FeCo phase dispersed in amorphous or crystalline Al 2 O 3 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)