Switching of anisotropy and phase diagram of the Heisenberg square-latticeS=12antiferromagnet Cu(pz)2(ClO

Abstract

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• within the plane. The nature of the abrupt anisotropy reversal remains unclear. The phase diagram is characterized by the increase of the ordering temperature TN in the magnetic field used, except for a dip near the bicritical point.

Interplay of Spin and Spatial Anisotropy in Low-Dimensional Quantum Magnets with Spin 1/2

“…However, as was shown in Cu(pz)2(ClO4)2, in this orientation, the fields above 2 T introduced the XY regime with the effective anisotropy ( Figure 16 in reference [94]). Apparently, alike in the case of the aforementioned easy-plane XXZ model, the theoretical studies of the S = 1/2 2D XYZ model on the square lattice with a weak spin anisotropy are necessary, to verify the persistence of the Ising-like ground state as well as the Ising-XY crossover, induced by the field applied along the middle axis.…”

“…Previous studies of Cu(pz) 2 (ClO 4 ) 2 , the 2D square-lattice magnet with J"/J ≈ 8.8 × 10 −4 , T N = 4.2 K and B sat ≈ 49 T, identified the presence of the easy-plane anisotropy ∆ λ ≈ 4.6 × 10 −3 [59]. Subsequent antiferromagnetic resonance experiments and magnetization measurements in the ordered phase refined the character of the spin anisotropy comprising of the out-of-plane (easy-plane) anisotropy ∆ λ ≈ 3.1 × 10 −3 and the in-plane anisotropy ∆ in ≈ 3.1 × 10 −4 , the latter breaking a continuous symmetry within the easy xy plane [94,110]. Thus, the description of Cu(pz) 2 (ClO 4 ) 2 within the XYZ model with extremely weak anisotropy is more realistic.…”

“…Subsequent antiferromagnetic resonance experiments and magnetization measurements in the ordered phase refined the character of the spin anisotropy comprising of the out-of-plane (easyplane) anisotropy , the latter breaking a continuous symmetry within the easy xy plane [94,110]. Thus, the description of Cu(pz)2(ClO4)2 within the XYZ model with extremely weak anisotropy is more realistic.…”

“…In the presence of magnetic interactions between the paramagnetic ions, the exchange anisotropy (symmetric and/or antisymmetric) appears with the relative strength comparable with the order of the g-factor anisotropy [93]. Accordingly, experimental studies of quasi-2D spin 1/2 magnets revealed that the spin anisotropy is very weak, ranging from 10 −4 to 10 −2 times the intra-layer exchange coupling J [59,80,94].…”

“…The kinks in the curves were associated with the onset of the 3D LRO. The application of the field along the hard axis z, leads to the behavior of M(T)/B typical for the XY regime, since the magnetic field enforces the effect of the intrinsic easy-plane anisotropy, resulting in the effective easy-plane anisotropy ∆ e f f = ∆ λ + a(gµ B B/J) 2 [94]. Concerning the middle axis y, authors of reference [16] expected enforcing of the in-plane easy-axis anisotropy, ∆ e f f = ∆ in + a(gµ B B/J) 2 .…”