Frustrated spin-12J1−J2Heisenberg ferromagnet on th

Abstract

We investigate the ground-state magnetic order of the spin-1/2 J 1 -J 2 Heisenberg model on the square lattice with ferromagnetic nearest-neighbor exchange J 1 < 0 and frustrating antiferromagnetic next-nearest neighbor exchange J 2 > 0. We use the coupled-cluster method to high orders of approximation and Lanczos exact diagonalization of finite lattices of up to N = 40 sites in order to calculate the ground-state energy, the spin-spin correlation functions, and the magnetic order parameter. We find that the transition point at which the ferromagnetic ground state disappears is given by J c1 2 = 0.393|J 1 | (exact diagonalization) and J c1 2 = 0.394|J 1 | (coupled-cluster method). We compare our results for ferromagnetic J 1 with established results for the spin-1/2 J 1 -J 2 Heisenberg model with antiferromagnetic J 1 . We find that both models (i.e., ferro-and antiferromagnetic J 1 ) behave similarly for large J 2 , although significant differences between them are observed for J 2 /|J 1 | 0.6. Although the semiclassical collinear magnetic long-range order breaks down at J c2 2 ≈ 0.6J 1 for antiferromagnetic J 1 , we do not find a similar breakdown of this kind of long-range order until J 2 ∼ 0.4|J 1 | for the model with ferromagnetic J 1 . Unlike the case for antiferromagnetic J 1 , if an intermediate disordered phase does occur between the phases exhibiting semiclassical collinear stripe order and ferromagnetic order for ferromagnetic J 1 then it is likely to be over a very small range below J 2 ∼ 0.4|J 1 |.PACS codes: 75.10.Jm Quantized spin models 75.45.+j Macroscopic quantum phenomena in magnetic systems 75.10.Kt Quantum spin liquids, valence bond phases and related phenomena

Magnetic correlation in the square-lattice spin system (CuBr)Sr2Nb3O10: A neutron diffraction study

“…The helical chains are coupled ferromagnetically along the a axis. The ordered moment is found to be 0.79(7) μ B /Cu 2+ -ion at 2 K. The observed helical AFM structure for the present square-lattice system (CuBr)Sr 2 Nb 3 O 10 is in contrast to the reported collinear AFM structure for other square-lattice systems, such as layered vanadium phosphates AA VO(PO 4 ) 2 with AA = Pb 2 , 27 BaZn, 28 SrZn, 29 PbZn, 16 and BaCd 30 with the FM J 1 and AFM J 2 . A helical AFM structure is also in contrast to the theoretically predicted magnetic states for the FM J 1 -J 2 model, namely, a collinear AFM structure for values of α (=|J 2 /J 1 |) 0.6, and a FM structure for α 0.4.…”

“…A helical AFM structure is also in contrast to the theoretically predicted magnetic states for the FM J 1 -J 2 model, namely, a collinear AFM structure for values of α (=|J 2 /J 1 |) 0.6, and a FM structure for α 0.4. [15][16][17][18][19] The role of α in (CuBr)Sr 2 Nb 3 O 10 as well as in other square-lattice compounds may be described as follows. The α value for the present compound was found (from the earlier magnetization measurement 32 ) to be 0.59 [J 2 /J 1 = 30.6 K/(−51.5 K)] with a FM J 1 .…”

“…From the value of α, it is evident that this system is at the phase boundary between ordered collinear AFM (with a dominating AFM J 2 ) and disordered states in the theoretical phase diagram. [15][16][17][18][19] A collinear AFM structure [Q = (π 0) or (0 π )] was found for (CuBr)LaNb 2 O 7 , 43 an n = 2 member of the present Dion-Jacobson series [(CuX)A n−1 B n O 3n+1 ] with FM J 1 and an α value of (J 2 /J 1 = 41.3 K/(−35.6 K)) = 1.1, which is well inside the range for a collinear AFM state (α 0.6). The other reported square-lattice compounds with FM J 1 having a collinear AFM structure, such as Pb 2 VO(PO 4 ) 2 (α ∼ 1.6) 27 and BaCdVO(PO 4 ) 2 (α ∼ 0.9), 30 are as well far above the lower limit of α 0.6.…”

“…While the model with the AFM J 1 has a spin-liquid ground state for a narrow range of α, 14 the intermediate state of the model with the FM J 1 is either a spin nematic 15 or presumably does not exist at all. 16 Extensive experimental investigations, carried out on a number of vanadium-based square-lattice compounds Li 2 VOSiO 4 and Li 2 VOGeO 4 , reveal AFM J 1 and J 2 . 20 The AFM J 1 and J 2 have also been found for other systems, such as the two-dimensional (2D) square-lattice antiferromagnet Cu(pz) 2 (ClO 4 ) 2 (where pz denotes pyrazine), 21 VOMoO4, 22 and the layered perovskite PbVO 3 .…”

“…However, for the model with FM J 1 and AFM J 2 , so far only a few studies exist. [15][16][17][18][19] The FM J 1 -J 2 model yields a FM ground state [Q = (0 0) for lower values of α 0.4. However, the FM state breaks down at α ∼ 0.4.…”