Magnetics Business & Technology - Summer 2017 - 8

FEATURE ARTICLE
field. The sample experiences a force f along the axis of the field gradient (dH/dz), which
is given by f = m(dH/dz) where m is the magnetic moment. The equipment required
for such force methods are either an electro- or superconducting magnet, and a balance for force measurements. A commercial variant of these methods is the alternating
gradient magnetometer[12]. AGMs are capable of achieving sensitivities in the 10-8 to 10-9
emu range, and like the VSM, the AGM is a very fast measurement; a typical hysteresis
loop takes seconds to minutes. Commercial AGM systems can be used for ambient temperature measurements to the moderate 2 to 3 T fields achievable with electromagnets.
Since a series of FORCs contain many data points the measurement can be very
time consuming when using superconducting magnet based systems (days to weeks),
whereas electromagnet based systems can acquire the data much more quickly (minutes to hours). In the FORC measurement it is necessary to fully saturate a material
after each reversal field excursion, thus superconducting magnet systems must be
used for some materials.

Electromagnet Based VSM: Sensitivity and Speed
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Sensitivity
A VSM's sensitivity depends on a number of factors:
Electronic sensitivity.
Noise rejection through signal conditioning.
Amplitude and frequency of mechanical drive.
Thermal noise of sensing coils.
Optimized design and coupling (proximity) of sensing coils to the sample under test.
Vibration isolation of the mechanical head assembly from the electromagnet and
VSM sensing coils.
Figure 1. Noise at 100 ms/point (top) and 10 s/
Minimization of environmental mechanical and electrical noise sources which can
point (bottom) averaging. The observed noise is
deleteriously effect VSM sensitivity.
119.5 nemu and 13 nemu RMS respectively.
The voltage induced in the VSM sensing coils is given by:
Where:

Vemf = mAfS
m = magnetic moment
A = amplitude of vibration
f = frequency of vibration
S = sensitivity function of VSM sense coils.

It's clear from this equation that increasing A, f or S, will improve moment sensitivity.
S may be increased by either increasing the coupling between the sense coils and the
sample under test (i.e., minimize gap spacing), or by optimizing the design of the sense
coils (i.e., number of windings, coil geometry, etc.) And, of course, signal averaging also
improves sensitivity. The data shown in this article were recorded using a Lake Shore
Model 8600 VSM.
Figure 1 shows typical noise measurement results at 100 ms/point (top) and 10 s/point
(bottom) averaging. Note that the vertical axis is expressed in nemu (10-9 emu). The RMS
noise values are noted in the figure caption.
As an illustration of the sensitivity of the VSM, typical low moment measurement
results are presented for a CoPt thin film with saturation moment msat = 20 µemu.
Figure 2 shows hysteresis loops as a function of signal averaging. Note that the vertical
axis is expressed in μemu (10-6 emu). These loops were recorded for ±5 kOe in 25 Oe
steps at signal averages of 100 ms/point (top) and 1 s/point (bottom), which equates
to total loop measurement times of 1 minute 25 seconds and 13 minutes 30 seconds,
respectively. The peak-to-peak noise in the saturated region of the hysteresis loops are
completely consistent with the measured RMS noise values at the same signal averages.
Speed
The VSM has been designed for fast measurements, providing field ramp rates to 10
kOe/s, and data acquisition as fast as 10 ms/point. Figure 3 shows typical hysteresis loop
measurement results for a magnetic stripe (top) with saturation moment of 14 memu
(10-3 emu), and CoPt thin film (bottom) with saturation moment of 80 µemu. The magnetic stripe loop was recorded for ±10 kOe in 50 Oe steps at 10 ms/point in 13 seconds,
and the loop for the CoPt thin film was recorded for ±4 kOe in 25 Oe steps at 100 ms/

Magnetics Business & Technology * Summer 2017

Figure 2. A 1 minute 25 second at 100 ms/point
(top) and 13 minutes and 30 second at 1 s/point
(bottom) hysteresis loops for a 20 µemu CoPt
thin film.

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