High frequency measurements of CoFeHfO thin films

High-frequency measurements of the transverse susceptibility and damping constant of CoFeHfO thin films have been made over a frequency range of 0.1 GHz to 6 GHz as a function of film resistivity, thickness, and temperature. The film resistivity varied from 250 /spl mu//spl Omega/cm to 2100 /spl mu//spl Omega/cm. The films show relatively low damping at high frequencies with the damping constant /spl alpha/ ranging from 0.01 to 0.06. The damping constant increases with film resistivity and, for the highest resistivity films, the damping constant decreases as the thickness increases. The damping constant, induced anisotropy, and film resistivity show weak temperature dependence over a temperature range from 4 K to 300 K. The low damping constant, in conjunction with the high anisotropy, large resistivity, and large spin-dependent-tunneling magnetoresistance, makes this material attractive for several high-frequency magnetic device applications.

I. Introduction

Ferromagnetic CoFeHfO thin films (and related oxide systems) exhibit several properties, including high resistivity, large high-frequency susceptibility, and large spin-dependent tunneling [1], [2], which indicate that this material may be useful in high-frequency magneto-electronic applications. The resistivities of CoFeHfO films range between 250 and 2500 cm which places these materials in a regime between that of good metals and insulators. The device and high-frequency properties of this type of intermediate-impedance magnetic materials have not yet been fully explored. Fig.  1. (a) Easy and hard axis hysteresis loops of a 1 m thick CoFeHfO film. The coercive field is kA/m and the induced anisotropy is  kA/m. (b) Temperature dependence of the saturation magnetization () and the induced anisotropy field () for 500 nm thick CoFeHfO films with low and high resistivity.