MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

78 Chapter 3
3.3.1.1 Apparatus
The apparatus, method of operation, data reduction and analysis used at
Stanford is described in detail in [102] and references therein. The sample is
attached to a bolometer by means of Wakefield thermal compound. The
bolometer is a patterned sapphire substrate containing two phosphorus doped
silicon thermometers (used for different temperature ranges) and a heater
with aluminum contact pads (6). Gold-7%Cu wire of 0.001” diameter is
spotwelded to these contacts to mechanically, electrically, and thermally
connect the sample containing bolometer to the surrounding sample holder.
The gold wire provides the weak thermal link to the surroundings and has a
thermal conductivity approximately linear in temperature. The sample
chamber is evacuated before the measurement and care must be taken to keep
out He since it adds to the thermal conductivity and even the heat capacity if
it condenses on the bolometer. The heat capacity due to the sapphire
bolometer, thermal compound, gold wire and aluminum contacts (in order of
decreasing importance) must be subtracted from the raw heat capacity data.
The precision of the measurement is typically 0.5% with or without an
applied magnetic field.
3.3.2 Analysis
The specific heat at low temperatures is usually of the form c = γT + βT 3 .
Because of this, the data are usually plotted as c/T vs. T 2 giving a straight line
with slope β and intercept γ.
The specific heat of spin waves are discussed in section 3.2.2.2.8.
3. 3. 2. 1 Electronic specific heat
The linear term comes from the specific heat of itinerant electrons and is
proportional to the density of electronic states at the Fermi level, n(E F ).
2 2
γ = n(EF )kB π /3. Insulators and semiconductors have no free electrons and
therefore no linear term. Good metals tend to have broad bands with low