  Finite-temperature diamagnetic properties of doped antiferromagnets as
modeled by the two-dimensional t-J model were investigated by numerical studies
of small model systems. Two numerical methods were used: the worldline quantum
Monte Carlo method with a loop cluster algorithm (QMC) and the
finite-temperature Lanczos method (FTLM), yielding consistent results. The
diamagnetic susceptibility introduced by coupling of the magnetic field to the
orbital current reveals an anomalous temperature dependence, changing character
from diamagnetic to paramagnetic at intermediate temperatures.
