MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 69
Figure 3-13; and constant H, Figure 3-12) give an irreversibility line, much
like that in superconductors. For polycrystalline SrRuO 3 the reversible field is
approximately proportional to (1 - T/T C ) 1.5 .
Like many glassy systems, the magnetic properties in the irreversible
region are time dependent. In a ferromagnet the domain walls are pinned
and so must overcome some barrier energy to move. Thus the
magnetization increases in a time dependent manner. Figure 3-15 shows the
magnetization of a SrRuO 3 pellet at 5 K after increasing the magnetic field
from 0 Gauss (where the magnetization was 0.0076 µ B ) to 100 Gauss. The
magnetization increases with a Log(time) dependence, with 98% of the
increase happening before the first data point (about 1 minute). Such a
logarithmic time dependence is also characteristic of a spin glass [99].
SrRuO 3 has all the properties of a ferromagnet, and because of this long
range order can not be a spin glass. The domain structure of a ferromagnet
provides magnetic disorder on a larger length scale than a spin glass, but to be
a spin glass, the disorder must be on the atomic scale.
3.2.2.3 Antiferromagnetism
In an antiferromagnetic substance, the neighboring magnetic moments
align antiparallel. Such a material is called an antiferromagnet. In the
ordered state, there is often no net moment in zero field. Thus magnetization
measurements cannot easily measure the magnetic order. In neutron
diffraction, the magnetic unit cell multiplies giving extra diffraction peaks.
The intensity of these peaks is a direct measure of the ordering.
In a magnetic field, an antiferromagnet has a small positive susceptibility.
Above the ordering temperature, or Néel temperature T N , the susceptibility
can be well described by the Curie-Weiss law used for ferromagnets. Since the
interactions are antiparallel, the coupling is of the opposite sign as that for a