Thermodynamics of Heisenberg ferromagnets with arbitrary spin in a magnetic field

Abstract

The thermodynamic properties (magnetization, magnetic susceptibility, transverse and longitudinal correlation lengths, specific heat) of one-and two-dimensional ferromagnets with arbitrary spin S in a magnetic field are investigated by a second-order Green-function theory. In addition, quantum Monte Carlo simulations for S = 1/2 and S = 1 are performed using the stochastic series expansion method. A good agreement between the results of both approaches is found. The field dependence of the position of the maximum in the temperature dependence of the susceptibility fits well to a power law at low fields and to a linear increase at high fields. The maximum height decreases according to a power law in the whole field region. The longitudinal correlation length may show an anomalous temperature dependence: a minimum followed by a maximum with increasing temperature. Considering the specific heat in one dimension and at low magnetic fields, two maxima in its temperature dependence for both the S = 1/2 and S = 1 ferromagnets are found. For S > 1 only one maximum occurs, as in the two-dimensional ferromagnets. Relating the theory to experiments on the S = 1/2 quasi-one-dimensional copper salt TMCuC [(CH3)4NCuCl3], a fit to the magnetization as a function of the magnetic field yields the value of the exchange energy which is used to make predictions for the occurrence of two maxima in the temperature dependence of the specific heat.

AAg2M[VO4]2Möller1, Amuneke2, Daniel3 et al. 2012Phys. Rev. B381480View full textAdd to dashboardCite

“…An interesting aspect is that for a 2D-FM spin system the ground state remains degenerate by symmetry whereas frustration phenomena will arise, e.g., from one of the three equal and effectively FM exchange coupling constants tuned into the AFM regime. Recently, frustrated FM lattices have emerged as an intriguing topic in condensed matter theory, [22][23][24][25][26][27] and we believe that our series of compounds may prove suitable for further development of new insights into the competition of various degrees of freedom.…”

Spin-flop and antiferromagnetic phases of the ferromagnetic half-twist ladder compounds Ba3Cu3In4O12and Ba3Cu3Sc4O12

“…We return to C P (T, H) in the context of Heisenberg FM chains. C P (T, 0) goes [35] as T 1/2 and has a broad maximum [28,36] around k B T ∼ 0.3|J 1 | that is of no concern here for |J 1 | > 100 K aside from noting the extended range of spin contributions. Since there is a finite energy gap for H > 0, C P (T, H) is exponentially small as T → 0.…”

“…Since there is a finite energy gap for H > 0, C P (T, H) is exponentially small as T → 0. The field-induced gap leads [36] to C P (T, H)/T ∼ a(H) at low T and H, and a(H) decreases with increasing field. In this regime, C P (T, H) has two maxima as a function of T that merge at large H as expected on general grounds.…”