Magnetocaloric effect in two-dimensional spin-1/2 antiferromagnets

Abstract

The magnetocaloric effect is studied at the transition to saturation in the antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional lattices, namely the square and the triangular lattice. Numerical results are presented for the entropy which are consistent with identical universal properties. However, the absolute values of the entropy are bigger on the geometrically frustrated triangular lattice than on the non-frustrated square lattice, indicating that frustration improves the magnetocaloric properties. Recent theoretical investigations predict an enhanced magnetocaloric effect in geometrically frustrated spin systems at low temperatures [1,2]. Indeed, experiments on the pyrochlore magnet Gd2Ti2O7 found substantial drops of temperature around the saturation field during adiabatic demagnetization [3]. These results suggest applications for efficient lowtemperature magnetic refrigeration.Investigations of quantum spin systems on twodimensional lattices have revealed a plethora of semiclassically ordered and unusual quantum ground states (see [4,5] for recent reviews). Rich behavior is also found in a magnetic field, including plateaux in the zero-temperature magnetization curve and fieldinduced quantum phase transitions (compare [5,6] and references therein).However, the magneto-thermodynamics on such lattices has received suprisingly little attention. As a first step we study the quantum phase transition at the saturation field on the square and triangular lattice. The Hamiltonian for a Heisenberg antiferromagnet in an external magnetic field h reads * Corresponding Author. Tel: +49-531-391 5190, fax: +49-531-391 5833Email address: a.honecker@tu-bs.de (A. Honecker).The Si are spin s = 1/2 operators at site i, J > 0 is the exchange constant, and i, j are nearest neighbor pairs on the given lattice. The magnetocaloric properties are determined by the entropy S(h, T ) at temperature T . One approach is to compute S(h, T ) from the eigenvalues of the Hamiltonian (1) [2] which are obtained by exact diagonalization (ED) on a finite lattice with N sites. Complete spectra are obtained for N ≤ 21 while for bigger N we use a truncation procedure valid at low energies [7]. Another approach is to obtain the entropy from a stochastic evaluation of the partition function using an extended ensemble quantum Monte Carlo (QMC) method, based on a flat-histogram formulation [8]. The latter method is restricted to unfrustrated lattices, and we employ it for the square lattice. Fig. 1 shows the entropy per site S/N as a function of temperature at the saturation field, given by hsat. = 4J and 9J/2 for the square and triangular lattice, respectively (see e.g. [6]). This corresponds to a quantum critical point in dimension d = 2 with a dynamical critical exponent z = 2. According to a scaling theory for the low-temperature physics at such a quantum critical point [9], the entropy should follow a power law

Magnetic field influence on the Néel, dimer, and spin-liquid states of the low-dimensional antiferromagnetsNiTa2O6andCoSb2Christian1, Masunaga2, Schye3 et al. 2014Phys. Rev. B

“…While anisotropy in the MCE of conventional antiferromagnets has been reported in a few cases [31,32], it has not been widely studied [40] and we are unaware of previous investigations of the MCE's anisotropy in quasi-1D antiferromagnets. The MCE reported herein differs from that observed [33] and theoretically studied [34][35][36][37] for low-dimensional quantum-spin systems, where saturation to a fully-polarized magnetic system is responsible. In the case of NiTa 2 O 6 and CoSb 2 O 6 , the MCE is associated with the unusual antiferromagnetic ordering of the quasi-1D spin system, and the disruption of that ordering through the application of magnetic field.…”

“…The absence of a MCE in CuSb 2 O 6 is attributed to its different antiferromagnetic structure, more robust magnetic exchange, and its propensity to exhibit a spin-flop transition [9]. The MCE reported herein differs from that observed [33] and theoretically studied [34][35][36][37] for low-dimensional quantum-spin systems, where saturation to a fully polarized magnetic system is responsible.…”

Exact solution of the mixed spin-1/2 and spin-S Ising-Heisenberg diamond chain

“…Note that this particular value of the frustration parameter was chosen so that the highly-degenerate FRU phase will constitute the ground state in the limit of vanishing external magnetic field. Namely, it has been recently proved that one achieves a substantial enhancement of the cooling rate during the adiabatic demagnetization of highly frustrated spin systems in comparison with the adiabatic demagnetization of unfrustrated spin systems [51][52][53][54][55][56]. As one can readily see from figures 5a-d, the mixed-spin Ising-Heisenberg diamond chains exhibit a pronounced valley-peak structure in the field dependence of the temperature at a fixed value of the entropy, which differ mainly in the total number of peaks (valleys) that gradually increases with the Heisenberg spin S. The most obvious drop (grow) of the temperature can always be found in the vicinity of zero field and transition fields at which the system undergoes zero-temperature phase transitions (see figure 2 for the ground-state phase diagram).…”

Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements

“…As far as we are aware, such multiple peak behaviour has not been observed previously. However, secondary minima have been predicted theoretically for ideal frustrated 2D lattices as a function of decreasing size57, and also for very high-symmetry (cuboctahedral, icosidodecahedral) frustrated clusters10111213, that is, they arise as a function of finite-size effects.…”