10-10-2012, 03:50 PM
AC HYSTERESIS LOOP TRACER FOR SOFT MAGNETIC THIN FILMS
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INTRODUCTION
There is an increasing interest in soft magnetic thin
films both from the point of view of fundamental research
as well as practical applications. Because of very small
volume of samples special experimental techniques are
required for their characterization. The basic magnetic
characteristic of a ferromagnetic material is the hysteresis
loop. With decreasing film thickness the sensitivities of
conventional inductive techniques for the hysteresis loop
measurement, however, become insufficient and the more
sensitive methods have to be used. The commercial vibrating
sample, magnetic force or SQUID magnetometers,
which provide the suitable sensitivity, are usually combined
with electromagnets or superconducting coils, the
remanent or residual magnetic fields of which are inappropriate
for the low-field measurements of soft magnetic
materials. Very sensitive and widely used for thin films
investigation is the magnetooptic Kerr effect (MOKE)
technique. It, however, takes hysteresis loops from a thin
surface layer (typically few tens nm in metals) but the
interior of thicker films is not available by this method.
The classical “induction coil technique” is therefore still
important for thin films investigations. The main problem
of the DC loop tracers – the zero offset – can hardly be
overcome in high sensitive measurements. Solely AC loop
tracers are therefore used for thin film measurements.
MEASUREMENT PROCEDURE
One of the pick-up coils is used to sense the variation
of magnetic flux due to the sample. The second coil of the
pair serves for the compensation of air flux. If a differential
measurement is required, for example to eliminate the
paramagnetic moment of the substrate, a dummy sample
can be inserted in the second coil. The inner dimensions
of pick-up coils allow to measure samples in the shape of
ribbons, wires or thin films. The maximum dimensions of
thin film substrate are 15´15´1 mm. For the investigation
of in-plane magnetic anisotropy a rotatable sample holder
can be used, which allows the measurement of samples
with maximum dimensions of 10´10´0.5 mm.
SENSITIVITY ESTIMATION
The sensitivity of equipment was tested on a very thin
mumetal film prepared by plasma jet deposition. Hysteresis
loop measured on the film with the thickness of about 20 nm
is shown in fig. 4. The measurement was done following the
procedure described in Section 3 with 500 samples/cycle and
4000 cycles averaged. As can be seen, nearly noise-free hysteresis
loop was obtained. The signal-to-noise ratio (S/N)
better than 30 was estimated from this measurement. If we
accept the definition of sensitivity as the magnetic moment
for which S/N = 1 [2], then the sensitivity of our loop tracer
is better than 10-5 emu, which is equivalent to an iron film
with the thickness of 6 nm and the area of 1 mm2. Though
this sensitivity is about 2 orders lower than that reported by
Haughdal and Miller [4], it can be already compared with the
sensitivities of some VSM and SQUID magnetometers.
CONCLUSION
We have shown that the signal processing abilities of
modern digital oscilloscopes can substantially increase the
sensitivity of classical induction AC hysteresis loops tracers.
Using this method quite sensitive equipment can be
built with commercial electronic instruments.