MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 29
attached to this surface with double-sided sticky tape to measure the Hall
effect and transverse magnetoresistance. A simple copper plate attached to
the tip of the transport probe provides a surface parallel to the magnetic field
for longitudinal and transverse magnetoresistance measurements. One
limitation of the SQUID magnetometer is its relatively small sample chamber
(8mm diameter).
High temperature (300K < T < 1200K) measurements were performed in a
furnace (in air) with a slowly varying temperature (≈ 1 K/min.). The
temperature was recorded using a Type-K thermocouple positioned a few
millimeters from the sample. The sample holder was made from an 8 bore
alumina rod. The 2 thermocouple wires and 4 platinum wires which contact
the sample were fed through the bores of the alumina rod. The wires are
positioned to facilitate changing the sample and attaching new contacts. The
furnace was programmed to heat and cool using a standard Eurotherm
(model 818) power regulator and temperature controller. Data were
continuously recorded by a computer using LabView software as the furnace
heated and cooled. Lower temperature measurements can be similarly made
by dipping the probe into a dewar of liquid nitrogen or liquid helium.
3.1.3 Analysis
Careful transport measurements can help identify transport mechanisms.
Knowledge of transport mechanisms not only helps us to understand the
microscopic nature of the material but also allows us to predict other
properties using previous theoretical or empirical knowledge of other
materials with the same transport mechanism. For example a few percent
impurity due to processing may not affect the room temperature resistivity of
a metal while greatly decreasing that of a band semiconductor.
Some of the common transport mechanisms and their predicted transport
properties are given below. Since there is not much overlap of mechanisms
or properties between metals and insulators, it is convenient to categorize