(H, T) PHASE DIAGRAM OF A UNIAXIAL DIPOLAR FERROMAGNET : LiHoF₄

Abstract

We report on the first determination of the (H, T) magnetic phase diagram of a uniaxial dipolar ferromagnet : LiHoF4 below Tc=1.53 K. It is deduced from the evolution of the magnetic domain pattern (branched stripes structure, bubbles), visualized by Faraday rotation down to 1.3 K

Size and fluctuation effects on the dynamics of linear domain walls in an Ising ferromagnet

“…1 (b)] by a technique described in a previous paper. 5 there is a tendency towards bubble formation rather than to stripes, which we associate with two facts: (i) the absence of a preferred magnetic direction perpendicular to the easy axis, i.e., in the tetragonal plane; and (ii) the energies of stripe and bubble structures are very close to each other. 19 Hence, we may discuss the results in terms of this stripe model modified by branching near the surface.…”

“…Since the deviation parameter a, traditionally being attributed to a distribution of relaxation rates, 23 remains small in the present case, a =0.10(5), we can restrict our discussion to the (mean) domain relaxation rate IV As an intrinsic dynamical quantity, we consider the so-called Onsager coefficient of wall motion, Lj^thd/Ah, determining the magnetization change as linear response to a small nonequilibrium field Ah, and related to r d by the following form: 12 (3) The temperature dependence of L d depicted in Fig. 2(b) reveals a speeding up which can be fitted to a power law, (4) with z =0.75 (5). This fit covering almost three decades of the reduced temperature provides the strongest evidence on the effect of critical fluctuations on domainwall motion.…”

“…First, to discuss the critical effect on L d we start with the prediction obtained from a relaxational Ansatz for the kinetics of the local magnetization in the linear wall: 12 L d -3L\te-ld), (5) where L\\ and the ratio §-A/ represent the microscopic flip rate and the relative number of wall spins, respectively. Inserting d from Eq.…”

“…x Recently, the occurrence of stripe, bubble, and branched domains in the phase diagram of strongly uniaxial ("Ising") ferromagnets depending on temperature, magnetic field, and sample (slab) thickness has been evaluated 2 " 4 in agreement with experimental results. 5 Current theoretical work on the dynamics, based on phenomenological coefficients of wall kinetics, predicts exponential 6 and logarithmic 7 decays of the magnetization for striped and branched domains, respectively. Existing data on Ising ferromagnets 8 " 11 clearly indicate exponential behavior, and within the limited ranges of temperatures under examination all relaxation rates have been fitted by an Arrhenius law.…”

Crossover from first- to second-order transition in frustrated Ising antiferromagnetic films

“…18 The phase transition of the bulk frustrated FCC Ising antiferromagnet has been found to be of the first order. 19,20,21,22,23 Note that for the Heisenberg model, the transition is also found to be of the first order as in the Ising case. 17,24 Other similar frustrated antiferromagnets such as the HCP XY and Heisenberg antiferromagnets 25 and stacked triangular XY and Heisenberg antiferromagnets 26,27 show the same behavior.…”