  Experiments in the antiferromagnetic phase of a quasi 2D $S=1/2$ quasi-square
lattice antiferromagnet Cu(pz)2(ClO4)2 reveal a biaxial type of the anisotropy,
instead of the easy-plane one, considered before. The weak in-plane anisotropy,
found by means of electron spin resonance spectroscopy and magnetization
measurements, is about an order of magnitude weaker, than the off-plane
anisotropy. The weak in-plane anisotropy results in a spin-flop phase
transition for the magnetic field aligned along easy axis, and, thereby, in a
bicritical point on the phase diagram. A remarkable feature of the weak
in-plane anisotropy is the abrupt change of its sign at the spin-flop point.
This anisotropy switching disappears at the tilting of magnetic field to the
easy axis by the angle of 10$^\circ$ within the plane. The nature of the abrupt
anisotropy reversal remains unclear. The phase diagram is characterized by the
increase of the ordering temperature in the magnetic field used, except for a
dip near the bicritical point.
