Operation: When the program is entered there are two choices.
The first is called SET. In this mode any field up to I Tesla can be
specified. When it is specified the field is set to this value and a
measurement made. Thus trials of the material can be made before starting
the other option which is LOOP. In this mode a series of fields is specified.
When the field is taken up to the maximum and then measurements are
made at all fields until zero is reached when the fields are repeated
with the opposite polarity. At the end of the procedure the results
are stored. Once the results are stored they can be plotted. The plot
is scaled to fit the screen but can be modified to examine parts of
it in greater detail. When the plot is on the screen a short line is
shown. This line can be moved anywhere and matched in slope to any part
of the loop. Once a match has been made the susceptibility at that point
is displayed. With or without the susceptibility measurement the crossing
points of the loop on the axes are found and displayed in terms of remanence
and coercive force. For weak samples where the magnetism of the holder
may be important the results from the empty holder can be subtracted
from the sample results.
Field Setting Method: The field in the gap is measured by a
Hall probe. When a field is selected the current is increased then a
measurement made of the field. If the field is not yet great enough
the field is increased by a factor that depends on the difference between
the two, and the iterative process repeated until the target field is
reached.
Vibrator and measurement technique: The vibrator is an electromagnetic
type similar in the mechanism of a moving coil loudspeaker. The drive
waveform (at about 70 cps) is derived from the computer via a digital-to-
analogue converter. The waveform sent to the vibrator is interleaved
with measurements of the amplified coil voltage via an analogue-to-digital
converter. In this way lock-in amplifiers with their long settling times
are not necessary and measurements can be made very quickly. It takes
about 10 seconds per point measured.
Sensitivity: Because there is a computer in the system the output
from the coils when digitized can be multiplied by any factor. What
limits the factor is the noise of the system. Noise is defined as the
signal that occurs when a measurement is made with no sample present
and this becomes meaningful when it is compared with the signal from
a sample. Water makes a good demonstration subject. Since the holder
is slightly magnetic the empty holder is measured first. Then a measurement
of the holder filled with 0.46 ml of water is measured. The empty holder
is then subtracted and the susceptibility of water can be measured and
compared with the known result. The water trace gives a good idea of
the noise level. The noise level is about 0.04 x 10-6 A m2 this is equivalent
to the EMU value of 40 x 10-6 G cm3.
There is a 4 decade attenuator and on the setting of 1 the maximum
moment is around 100 x 10-6 A.m2 on range 1000, the maximum moment is
around 0.1 A.m2 or 100 G cm3.
The screen below shows the performance of the new system where measurements
may now be made on thin films -- the noise level is now 0.04 uA.m2 and
the programs are now in windows format --
|
The
screen for a typical measurement
|
|
|
Source: A second source for the Molspin VSM MK 2 is listed
below:
Website: www.compressionmoulding.co.uk
e-mail: lindabiggins@fairgrieve87.freeserve.co.uk
Molspin
Ltd
