Lake Shore Cryotronics Measure Ready M91 FastHall, User Manual

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10 CHAPTER 1: Introduction
MeasureReady™ M91 FastHall™ Measurement Controller
Since this voltage is independent of current, when a magnetic field is present the
voltage is eliminated by current reversal.
1.5.5.2.2 Righi-Leduc Voltage (V
R
)
The Nernst (diffusion) electrons also experience an Ettingshausen-type effect since
their spread of velocities result in hot and cold sides on the sample; consequently,
they set up a transverse thermoelectric voltage, known as the Righi-Leduc voltage,
VR. The Righi-Leduc voltage is also proportional to magnetic field, but not to external
current. Since this voltage is independent of current, the voltage is eliminated by
current reversal when a magnetic field is present.
1.5.5.3 Field Magnitude
The field magnitude must be entered to run a Hall measurement. The Hall voltage is
proportional to the magnetic field. This would seem to imply that the larger the
magnetic field, the larger the Hall voltage, and therefore, the easier the
measurement. However, this is not always the case. Some materials show a nonlinear
Hall voltage. For example, a magnetic material produces a part of the Hall voltage
proportional to the magnetic field and another part of the Hall voltage proportional
to the magnetization of the material. There are other instances of nonlinear Hall
effect; therefore, it is good practice to measure the Hall voltage using at least two
magnetic fields to confirm the linearity of the Hall measurement. If these
measurements confirm linearity, then the higher field measurement is typically the
better measurement.
1.5.5.4 Field Reversal
DC Hall measurements require field reversal. Hall voltage is proportional to the
magnetic field magnitude and polarity. Some unwanted signals are not proportional
to field. This allows a straight forward method for separating the desired signals from
those undesired signals—field reversal.
The misalignment voltage V
M
is the largest of these unwanted voltages. V
M
is caused
by the misalignment of the contacts. Its magnitude is proportional to the physical
asymmetry of the sample contacts, the sample resistance, and the excitation current.
In the ideal case, V
H
is expected to be zero when field (B) is zero.
When misalignment voltage is accounted for in a positive field measurement, the
measured voltage (V
+
) is:
If field is reversed the measured voltage (V
-
) is:
The desired voltage changes sign, but the unwanted voltage does not change sign.
The field reversed voltage VFR is calculated by subtracting the negative field voltage
(V-) from the positive field voltage (V+) and dividing by 2.