MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 23
when characterizing new materials. The danger is finding a new effect or
result that is reproducible but none the less spurious. Since measurements of
new materials have usually not been previously performed, it is often
impossible to compare with previous results. Some considerations and
solutions are outlined below.
3.1.2.1 Linearity
Most transport measurements assume some sort of linear response.
Conductivity measurements assume I is linearly proportional to V.
Experimentally this is never exactly true. A test for linearity should always be
made before measuring a new sample. The resistance over several orders of
magnitude should be constant within a few percent. Even if I vs. V is linear,
thermal or contact voltages usually add a nonzero offset voltage V 0 so that V
= IR + V 0 . Using wires from the same spool, pure copper contacts and
shielding may help but will not totally eliminate V 0 . This offset voltage is
easily subtracted. The easiest method is by measuring the voltage at I and -I.
The resistance is then R(I) = (V(I) - V(-I))/2. The best method is to take a full I
vs. V curve and measure the slope. This may take more time and therefore
introduce other errors such as those caused by temperature drift. The current
range used for measurement should be well within the Ohmic regime. A
current too small may have V 0 comparable to V; it is best not to rely heavily
on the subtraction of V 0 . Large currents may produce I 2 R heating of the
sample, in which case the measured resistance should look like it has an
additive term proportional to I 2 . AC measurements are ideal for extracting
only the linear response; in this case, the frequency dependence should also
be checked, and compared to the DC value.
Thermopower S measurements similarly assume the linearity of the
voltage with the temperature difference: V = SΔT. These measurements also,
however, have an offset voltage V 0 that should be subtracted.