Equilibrium and nonequilibrium properties of synthetic metamagnetic films: A Monte Carlo study

Abstract

Synthetic antiferromagnets with strong perpendicular anisotropy can be modeled by layered Ising antiferromagnets. Accounting for the fact that in the experimental systems the ferromagnetic layers, coupled antiferromagnetically via spacers, are multilayers, we propose a description through Ising films where ferromagnetic stacks composed of multiple layers are coupled antiferromagnetically. We study the equilibrium and non-equilibrium properties of these systems where we vary the number of layers in each stack. Using numerical simulations, we construct equilibrium temperature−magnetic field phase diagrams for a variety of cases. We find the same dominant features (three stable phases, where one phase boundary ends in a critical end point, whereas the other phase boundary shows a tricritical point at which the transition changes from first to second order) for all studied cases. Using time-dependent quantities, we also study the ordering processes that take place after a temperature quench. The nature of long-lived metastable states are discussed for thin films, whereas for thick films we compute the surface autocorrelation exponent.PACS numbers: 75.10. Hk,75.30.Kz,75.40.Mg Heterostructures formed by ferromagnetic multilayers that are coupled antiferromagnetically via spacers have a wide range of possible applications, ranging from high-density storage technology to spintronic devices. 1-4 Cases with strong perpendicular anisotropy (as encountered, for example, in [Co/Pt]/Ru or [Co/Pt]/NiO with ferromagnetic [Co/Pt] multilayers)have been the objects of intensive studies, both experimentally 5-10 and theoretically. 9,[11][12][13][14][15][16] Due to the overall structure of these antiferromagnetically coupled multilayers, they are sometimes referred to as synthetic or artificial metamagnets. In phenomenological approaches, these systems are commonly modeled by layered antiferromagnets where a ferromagnetic multilayer is described by a single variable, namely the total magnetization of the multilayer. For strong perpendicular anisotropy, this naturally leads to the description via an Ising metamagnet where layers with ferromagnetic in-layer interactions are coupled antiferromagnetically. Ising bulk metamagnets have been the subject of many studies in the past and their properties are well understood by now. 17-30 Surprises show up, however, when studying Ising metamagnets in thin film geometry. In systems with an even number of layers and a magnetic field applied perpendicular to the surfaces, the temperature-field phase diagram exhibits a new phase in addition to the two phases of the bulk system (the low field antiferromagnetic and the high field paramagnetic phases): for intermediate field strengths, the surface layer, which is magnetized oppositely to the applied magnetic field in the ground state, aligns with the magnetic field. The phase transition between the antiferromagnetic phase and this intermediate phase is discontinuous and ends in a critical end point. 31The characterization of the ferromagnetic multil...

Ising metamagnet driven by propagating magnetic field wave: Nonequilibrium phases and transitions

“…The comprehensive phase boundary obtained here may be realised qualitatively and physically as follows [14]: for lower values of field amplitude the system undergoes a transition from paramagnetic to antiferromagnetic (layered) phase as the temperetured is lowered, since the intra layer antiferromagnetic interaction strength wins over the competition against the field strength. As the strength of the field increases, it becomes relatively stronger enough to create the clusters of up spins within the plane of down spins.…”

“…The nonequilibruim phase transition is studied and a phase boundary is drawn in the plane formed by the temperature and amplitude of the oscillating magnetic field. Very recently, the nonequilibrium behaviours of synthetic metamagnetic (ising type) film are studied [14] by Monte Carlo simulation. The time dependent response, growth of domains inside the material and surface autocorrelation (for thick films) are studied in details.…”

Monte Carlo study of the phase diagram of layered XY antiferromagnet

“…When the strength of the field is increased starting from zero, its phase transition point separating the ordered and disordered phases from each other gets to shift to the lower temperature region. From the theoretical point of view, thermal and magnetic phase transition properties of different kinds of metamagnetic systems have been studied by a wide variety of techniques such as Mean-Field Theory [1][2][3][4][5][6][7][8], Effective-Field Theory [9][10][11][12][13], Monte-Carlo simulation method [14][15][16][17][18][19][20][21][22][23][24][25], and High Temperature Series Expansion method [26,27]. The studies done so far show us that metamagnetic systems can include multicritical points such as tricritical point, bicritical end point and also critical end point depending on the ratio between these ferromagnetic and antiferromagnetic interactions.…”

Influences of Crystal-Field and Interlayer Coupling Interactions on Dynamic Phase Diagrams of a Mixed-Spin (3/2,2) Bilayer System

“…Theoretically, the spin-1/2 (see Refs. [18][19][20][21][22][23][24][25][26][27][28][29][30] and references therein), spin-1 (see Refs. [31][32][33][34][35] and references there in) and also mixed Ising systems, namely spin (2, 5/2), [36−37] spin (1, 3/2), [38] spin (3/2, 2) [39] and spin (3/2, 5/2), [40] have been used to study magnetic behaviors of the bilayers and multilayers thin films within the methods of the equilibrium statistical physics.…”