Magnetization decay in Co-Cr films

Abstract

We have measured magnetization time decay in perpendicularly oriented Co-Cr films over seven time decades as a function of field and temperature. The anomalous Hall coefficient of the sample itself, which is proportional to the magnetization within the sample, is used to determine the magnetization. Magnetization time decay measurements made with this technique, for times>10 s, agree with vibrating-sample magnometer (VSM) data taken at 77 and 300 K, but can be made as soon as 2 ms after turning off the external magnetic field used to saturate the sample. The principle features of magnetization decay in Co-Cr are as follows: (1) The magnetization time decay in Co-Cr films is nearly independent of external field over a range of several kG, unlike particulate media, where the magnetization decay rate is sharply peaked around the coercive field. (2) At 300 K, the magnetization time decay deviates markedly from log t, with the decay per decade of time dropping by almost a factor of 2 between 10−3 and 104 s. (3) For zero field, the deviation from a linear log t decay decreases with decreasing temperature, with a nearly constant magnetization time decay observed in any decade of time between 15 and 200 K. (4) At negative fields and temperatures below 15 K, a second region of log t decay with a larger decay rate is seen for t<100 ms. (5) For temperatures between 200 and 10 K, decay rates decrease linearly with decreasing temperature, and then below 10 K decay rates decrease more rapidly, and appear to approach zero when extrapolated to zero temperature.