Vortex ground state for small arrays of magnetic particles with dipole coupling

Abstract

We show that a magnetic vortex is the ground state of an array of magnetic particles shaped as a hexagonal fragment of a triangular lattice, even for an small number of particles in the array N ≤ 100. The vortex core appears and the symmetry of the vortex state changes with the increase of the intrinsic magnetic anisotropy of the particle β; the further increase of β leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as dozen of nanometers.

Out-of-Plane and In-Plane Magnetization Behavior of Dipolar Interacting FeNi Nanoislands around the Percolation Threshold

“…With the increase of the particle magnetic anisotropy, the vortex core starts to protrude out-of-plane. And with the further increase, the symmetry of the vortex ground state increases, where the planar magnetization component vanishes (featuring zero-net magnetic moment), but the perpendicular component changes significantly [7]. This may be well consistent with the magnetization properties found in the present study for the nanoisland FeNi films with the nominal film thickness 1.1 nm ≲ d ≲ 1.8 nm.…”

“…For small magnetic anisotropy, a purely planar vortex can exist on close-packed hexagonal fragments of a triangular lattice, with in-plane distribution of FM NP's magnetic moments. Here, the total out-of-plane projection of the magnetic moment vanishes, but the in-plane component of the total magnetization is nonzero, as the magnetic moment of the vortex core remains not compensated [7]. This may be relevant to the in-plane magnetization properties found in the present study for the nanoisland FeNi film with the nominal film thickness above the percolation threshold at ≃ 1.8 nm.…”

“…We would like to note that such system with an effective perpendicular magnetic anisotropy has also a tendency to inhomogeneous distribution of magnetic moments in the form of supervortices [7]. We infer that the studied nanoisland FeNi films, composed of inhomogeneously distributed FM single-domain nanoislands, represent a unique playground to challenge a supervortex state and its magnetic properties.…”

“…For square lattices with in-plane anisotropy, the magnetic structure has four-sublattice AF order, whereas, for the triangular lattices, a FM order is implemented [5]. Here, for finite fragments of triangular lattices, the ground state can be a vortex state formed by the magnetic moments lying in the plane (supervortex) [6,7]. Similarly, for soft magnetic dots [8][9][10][11], supervortex represents a topologically nontrivial ordered state, more intricate than the standard domain structure.…”

Self-organization of topological defects for a triangular-lattice magnetic dots array subject to a perpendicular magnetic field

“…It is worth mentioning here that the behavior of a dot array with in-plane anisotropy is essentially different from our case. In this case, the dipolar interaction is not clearly AFM; and for a model of infinite unbounded array, the ferromagnetic state is stable [21], whereas for finite array, there appears a mesoscopic non-uniform state of a form of either domain wall [48] or magnetic vortex [48,49].…”