The role of the exchange and dipolar interactions in the determination of the magnetic ordering of a two-dimensional lattice with random vacancies

• Published in: Journal of physics. Condensed matter Vol. 19; no. 19; pp. 196207 - 196207 (10)
• Main Authors: Patchedjiev, S M, Whitehead, J P, De’Bell, K
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 16.05.2007; Institute of Physics
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Magnetic properties and materials; Physics; Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.); Fluctuations; Monte Carlo methods; Vacancy density; Ground states; Anisotropy; Magnetic ordering; Long-range order; Magnetic-phase diagrams; Square lattices; Dipole interactions; Exchange interactions
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/19/19/196207

The phase diagram of a system of interacting spins on a square lattice with a small percentage of random vacancies is determined from Monte Carlo simulations. The spins are confined to rotate in the plane and interact through the exchange and dipolar interactions. Despite the anisotropic nature of the dipolar interaction, for the parameter set considered in the present work, the ground states of the system are continuously degenerate in the absence of vacancies. In the case J = 0, it has been shown that the disorder arising from both vacancies and thermal fluctuations lifts the degeneracy of the ground state and generates long-range antiferromagnetic order. This phenomenon is referred to as 'order from disorder'. Because of the competing nature between the disorder created by vacancies and thermal fluctuations, for J = 0 such a system has been shown to undergo a transition from a collinear to microvortex phase with decreasing temperature. In this paper we extend the results from these earlier simulations to include a finite exchange interaction. A number of new and interesting results emerge from these simulations. For J/g = -0.3 and -2.5 (where J and g denote the exchange and dipolar interaction constants respectively), we obtain results that are qualitatively similar to those obtained for J = 0, with a transition from the collinear to the microvortex phase with decreasing temperature. However, for J/g = -1.5 our simulations show that the sequence is reversed and we instead observe a transition from the microvortex phase to the collinear phase with decreasing temperature. Furthermore, preliminary results for larger values of |J|/g indicate that the competition between the disorder created by the vacancies and the thermal fluctuations extends to both the simple antiferromagnetic case (J/g = -4.0) and the ferromagnetic case (J/g = 1.5), giving rise to an analogous in-plane reorientation transition. In all the cases we consider, the temperature at which the transition occurs increases with increasing vacancy concentration.