  We present Monte Carlo simulations of the reversible transverse
susceptibility (RTS) for a hexagonal array of dipolar interacting magnetic
nanoparticles with random anisotropy. RTS curves with the bias-field in-plane
and out-of-plane are compared. With increasing temperature the RTS curves
evolve from a three-peak structure to a double-peak and eventually a
single-peak at the blocking temperature of the system. This trend is preserved
for weak interactions. Dipolar interactions at low temperature are responsible
for the suppression of the coercivity peak in the out-of-plane geometry and its
progressive merge to the anisotropy peak with decreasing interparticle
separation in the in-plane geometry. The anisotropy peaks are located at higher
field values in the out-of-plane geometry relative to the in-plane one. When
the bias field lies in-plane (out-of-plane) the anisotropy peaks are shown to
shift to lower (higher) field values with decreasing interparticle separation.
The coercivity peak shifts to lower field values in both geometries. Our
results are compared with recent experimental findings in self-assembled arrays
of Fe nanoparticles.
