MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 59
Examples of nearly ferro- or antiferromagnetic metals which show a large
susceptibility are CoSe 2 (Curie Weiss) Pd or YCo 2 [84], and V 2 O 3 [89]. The
nearly ferro- or antiferromagnetic semiconductor FeSi (Curie Weiss) shows a
Curie-Weiss susceptibility above 700K with a broad transition at 500K to a low
susceptibility state [84].
3.2.2.2.4 Critical region
As the temperature approaches the critical temperature T C , the
spontaneous magnetization vanishes and the susceptibility diverges. This
makes these properteis not analytic near T C . The observed properties are well
described by a power law |T - T C | n where n is a scaling exponent. For
example, the spontaneous magnetization varies as (T C -T) β for T < T C , while
the susceptibility varies as (T C - T) γ for T > T C , and for T = T C (critical isotherm)
M δ = H. The scaling exponents in the critical region are called critical
exponents.
The critical exponents are very similar for a wide range of second order
phase transitions. They tend to depend only on the general form of the
interaction causing the phase transition, such as the dimensionality, and not
on the particular material. Thus most ferromagnets fall into the universality
class of 3-dimentional Heisenberg or Ising ferromagnets. The theoretical
critical exponents calculated for these models is given in Table 3-1. There are
only two unique critical exponents in each universality class. The others are
related via the scaling relations such as γ = β(δ -1).
According to the scaling hypothesis [90], the magnetic equation of state in
the critical region depends only on the scaled variables H/|T C /T - 1| β+γ and
M/|T C /T - 1| β . For example, a plot of the scaled M 2 vs. the scaled H/M, (scaled