Hybrid quantum-classical Monte Carlo study of a molecule-based magnet

Abstract

Using a Monte Carlo (MC) method, we study an effective model for the Fe(II)Fe(III) bimetallic oxalates. Within a hybrid quantum-classical MC algorithm, the Heisenberg S = 2 and S ′ = 5/2 spins on the Fe(II) and Fe(III) sites are updated using a quantum MC loop while the Ising-like orbital angular momenta on the Fe(II) sites are updated using a single-spin classical MC flip. The effective field acting on the orbital angular momenta depends on the quantum state of the system. We find that the mean-field phase diagram for the model is surprisingly robust with respect to fluctuations. In particular, the region displaying two compensation points shifts and shrinks but remains finite.

Monte Carlo simulation of magnetic properties of a mixed spin-2 and spin-5/2 ferrimagnetic Ising system in a longitudinal magnetic field

“…As observed in Fig. 4(b), the M curve labeled D B /|J AB | ¼ À1.7 displays L-type in the Néel classification [48], which has not been observed in the previous researches for the molecular-based magnets [13][14][15][16][17][18][19][20][21][22]46]. It is why the coercivity of the system drops sharply to zero for D B /|J AB | ¼ À1.7 at low temperature.…”

“…Thus, the molecular-based magnets AFe II Fe III (C 2 O 4 ) 3 can be properly represented by a mixed spin-2 and spin-5/2 ferrimagnetic Ising model [13]. In theory, many works have been carried out to investigate the magnetic properties of the mixed spin-2 and spin-5/2 ferrimagntic system with different numerical methods, such as Monte Carlo (MC) simulations [14][15][16][17], effective-field theory (EFT) [18][19][20][21][22], the exact recursion equations [23], mean field theory (MFT) [24][25][26] and the Green's function approach [27][28][29]. These studies have focused mainly on magnetic and thermodynamic properties of the mixed spin ferrimagnetic system, including magnetization, the initial susceptibility, internal energy, specific heat and the ground phase diagram.…”

Spin waves in antiferromagnetically coupled bimetallic oxalates

“…Results in the next section also employ the estimate J c = 0.5 meV obtained from MF theory [9]. Although a recent Monte Carlo study [12] suggests that J c is about twice as large, our results are insensitive to the precise value of J c , provided that it is small compared to the spin-orbit coupling.…”

“…While the square-root behavior is an artifact of MF theory [12], the proportionality factor is a function of L 2 and L 3 . As T → 0, the ground-state magnetization depends on the signs of the spin-orbit coupling on the M(II) and M (III) sites.…”

Magnetic Compensation and Ordering in the Bimetallic Oxalates: Why Are the 2D and 3D Series so Different?

“…Hence, the transition temperature of 2D compounds must vanish in the absence of spin−orbit anisotropy. A recent Monte-Carlo analysis of the 2D Fe(II)Fe(III) bimetallic oxalates confirms that T c →0 as L cf →0 and that MF theory overestimates T c /| J | by about 40% for L cf |λ/ J | ≫ 1. Nevertheless, Monte-Carlo simulations qualitatively confirmed the most important predictions of MF theory: the appearance of magnetic compensation for L cf above a threshold close to 0.25 and below 1, and the increase of the transition temperature with L cf .…”