Chapter 10 - Thin films for high-density magnetic recording

Abstract

The most commonly used instruments for MH & BH loop measurement are the BH looper, the vibrating sample magnetometer (VSM)—an instrument based on the principle of electromagnetic induction—and the alternating gradient force magnetometer (AGFM). These instruments are divided into two categories by the techniques used—(1) inductive techniques that measure the voltage induced by a changing flux and (2) force techniques that measure the force exerted on a magnetized sample in a magnetic field gradient. A BH looper usually employs a pair of Helmholtz coils driven by an ac field of frequency from 1 to 100 Hz. The test sample is placed in a pair of pickup coils, which are connected differentially so that the voltages induced by the flux change because of the applied ac field, which are canceled and the voltages induced by the flux change of the test sample are enhanced. Knowledge of the permeability of soft magnetic films over a wide frequency range is important for recording head applications. However, conventional permeability measurement methods—such as the toroidal method or the impedance bridge method—are not suitable for measurement of thin film head materials because thin films for heads are usually anisotropic, whereas these methods can only be used for isotropic materials.