A. D. Beath

Structure and magnetic properties of La(Fe0.88Al0.12)13Cx interstitial compounds

Structure and magnetic properties of La(Fe0.88Al0.12)13Cx interstitial compounds with x=0–0.8, were studied by x-ray diffraction and magnetic measurements. The interstitial compounds retain the NaZn13-type structure and the lattice constant increases with carbon content. The Wigner–Seitz cell volume and void calculations show there are seven different voids in the NaZn13 structure with the void at the 24d site (0, 0.25, 0.25) having the largest volume and the largest number of La neighbors. It is highly likely that the carbon atoms enter the 24d site. The small carbon doping collapses the antiferromagnetic state in La(Fe0.88Al0.12)13 and forms a ferromagnetic state. The magnetic transition temperature increases from 198 K to 280 K and the spontaneous magnetization (5 K) decreases from 22.9 μB to 21.3 μB per formula unit with increasing carbon content from x=0 to 0.8. The change in magnetic properties on carbon addition is attributed to cell volume expansion and Fe–Fe distance variations.

Field dependence of the transverse spin glass phase transition: Quantitative agreement between Monte Carlo simulations and experiments

The field dependence of the transverse spin glass phase transition temperature, Txy(B), has been determined both for a – FexZr100−x (x = 90, 91, 92, and 93) using muon spin relaxation and for the bond-frustrated nearest neighbor ±J Heisenberg spin glass using Monte Carlo simulations. In both cases, we find Txy(B) ∝ 1/B, providing direct, quantitative agreement between experiments and simulations.

Ferromagnetic phase boundary in the bond frustrated heisenberg model

We determine the ferromagnetic phase boundary for the short range ±J bond frustrated Heisenberg model in three dimensions using a very efficient Monte Carlo algorithm which eliminates the critical slowing down usually experienced at a second order phase transition. The phase boundary is identified by measuring the correlation length directly, a method which we show to be superior to more conventional methods such as the crossing of the Binder cumulant. The critical concentration of antiferromagnetic bonds beyond which ferromagnetism is lost is xc=0.208(2).

Structure and magnetocaloric effect in the pseudobinary system LaFe11Si2–LaFe11Al2

Structure and magnetocaloric effect (MCE) were investigated by x-ray diffraction and magnetic measurement in the pseudobinary system LaFe11Si2–LaFe11Al2. The compounds have the cubic NaZn13 structure and the lattice constant increases with increasing Al content. The Curie temperature TC decreases from 236 to 186 K with increasing Al content. All compounds exhibited large MCE and the values of magnetic entropy changes ΔS ranged from 10.8 to 8.5 J/kg K for LaFe11Si2–LaFe11Al2 system under an external field change of 7 T. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization M(μ0H) using the Landau expansion of the magnetic free energy. The results indicate the replacement of Si by Al changes the ferromagnetic to paramagnetic transition near TC from first order to second order.

Finite temperature phase transition in the three-dimensional Heisenberg ±J spin glass model

The three-dimensional Heisenberg spin glass model with ±J interactions is studied using an over-relaxed Monte Carlo algorithm. We have measured the correlation length ξ, and using the crossing of ξ∕L for different L find a finite temperature spin glass transition, with TSG=0.220(5). In addition, we have varied the number of Monte Carlo steps used prior to, and during, thermal averaging to control the effects of finite time averaging. We find that the over-relaxation algorithm allows for an accurate measurements using as few as 300 Monte Carlo steps.

Latent heat of the fcc Ising antiferromagnet

To obtain critical parameters of the fcc Ising antiferromagnet from Monte Carlo data, a numerical estimate of the latent heat Δ is required. The precision of current estimates is about 3%, and ultimately limits the precision achieved for the disordered state at the Neel temperature TN. Here we make several different estimates of the latent heat, using finite size scaling of Monte Carlo data, and show that a superior method yields a 25-fold improvement [Δ=0.4559(6)] in comparison with our older result [Δ=0.455(15)].

Order Parameter Profiles in a Twisted Heisenberg Model

We present the results of Monte Carlo simulations of the 3-D Heisenberg model with twisted boundary conditions. The boundaries are chosen to have a saturated magnetization in equal and opposite directions which imposes a twist on the order parameter profile centered at the midpoint of the sample. The magnetization profiles are distinct from mean field theory. We present a detailed examination of the magnetization profile and suggest experiments which can verify the finite size scaling forms we observe

Universal scaling functions and multi-critical points in the site frustrated Heisenberg model

Recently, the ordering scenario of three-dimensional site frustrated Heisenberg models has been challenged. The existence of a decoupled tetracritical point (DTP) where ferromagnetic (FM) and antiferromagnetic (AF) order occur simultaneously has been modified to include a line of multicritical points where both FM and AF occur simultaneously. In addition, numerous studies claim that, within the mixed phase, critical exponents are altered from Heisenberg values. Here we show that within the narrow region of proposed multicritical points, the transition temperatures away from the tetracritical point are indeed distinct, confirming the occurrence of a DTP. In addition, we show a universal scaling collapse of the magnetization both away from, and within, the mixed phase using Heisenberg exponents, demonstrating that the stable fixed points are within the Heisenberg universality class.

Intercluster coupling in site-frustrated random magnets

The effects of intercluster coupling strength (|JFA|) on the ordering behavior of the site-frustrated Heisenberg model have been investigated. We find that following a weak initial increase, the lower temperature transition is driven to T=0 for |JFA|⋆∼5.0(5), while the upper transition temperature appears to increase without limit.

Ordering in the site frustrated Heisenberg ferromagnet revisited

Monte Carlo simulations of frustrated and nonfrustrated versions of the three-dimensional site disordered Heisenberg model have been used to determine the sequence and nature of magnetic ordering. At sufficiently high concentrations, x, of antiferromagnetic sites for an infinite percolating network to form (0.31<x<0.69), two ordering events occur: ferromagnetic and antiferromagnetic order that coexist. Coupling the two networks introduces frustration but does not destroy either transition. However, it appears to change the universality class of the antiferromagnetic transition.