Title: GdRh$_2$Si$_2$: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy,
Abstract: The anisotropy of magnetic properties commonly is introduced in textbooks
using the case of an antiferromagnetic system with Ising type anisotropy. This
model presents huge anisotropic magnetization and a pronounced metamagnetic
transition and is well-known and well-documented both, in experiments and
theory. In contrast, the case of an antiferromagnetic $X$-$Y$ system with weak
in-plane anisotropy is only poorly documented. We studied the anisotropic
magnetization of the compound GdRh$_2$Si$_2$ and found that it is a perfect
model system for such a weak-anisotropy setting because the Gd$^{3+}$ ions in
GdRh$_2$Si$_2$ have a pure spin moment of S=7/2 which orders in a simple AFM
structure with ${\bf Q} = (001)$. We observed experimentally in $M(B)$ a
continuous spin-flop transition and domain effects for field applied along the
$[100]$- and the $[110]$-direction, respectively. We applied a mean field model
for the free energy to describe our data and combine it with an Ising chain
model to account for domain effects. Our calculations reproduce the
experimental data very well. In addition, we performed magnetic X-ray
scattering and X-ray magnetic circular dichroism measurements, which confirm
the AFM propagation vector to be ${\bf Q} = (001)$ and indicate the absence of
polarization on the rhodium atoms.