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Abstract:
A two-dimensional magnetic particle in the presence of an
external magnetic field is studied. Equilibrium thermodynamical
properties are derived by evaluating analytically the partition
function. When the external field is applied perpendicular to
the anisotropy axis the system exhibits a second-order phase
transition with order parameter being the magnetization
parallel to the field. In this case the system is isomorph to a
mechanical system consisting of a particle moving without
friction in a circle rotating about its vertical diameter.
Contrary to a paramagnetic particle, equilibrium magnetization
shows a maximum at finite temperature. It is also shown that
uniaxial anisotropy in a system of noninteracting particles can
be misinterpreted as a ferromagnetic or antiferromagnetic
coupling among magnetic particles, depending on the angle
between the anisotropy axes and magnetic field.
