A simple low-frequency thin-film magnetometer

The usual way of plotting hysteresis curves involves the integration of
dB/dt
, which has many disadvantages at low frequencies, and calls for complicated and expensive instrumentation. The instrument described here is not as accurate as a good vibrating sample magnetometer, but is very simple, inexpensive, and easy to construct and to use. It involves no integration and therefore its operation is independent of the frequency of the applied magnetic field, and can be readily applied for observing time effects. The magnetic induction
B
is measured directly by a Hall probe placed in a 20-mil gap between two pieces of the same thin-film sample mounted in a 4-inch Varian magnet. A second Hall probe measures the applied field
H
, and by subtracting the voltages,
B - H
is plotted vs.
H
on an
x-y
recorder. With commercial Hall probes, the voltage obtained is sufficient to apply directly to commercial
x-y
recorders, without further amplification, even for rather thin films. At 100 mA through each of the probes, the
H
scale was 10μ V/Oe, while the M scale was 80μV/Mx.