R. Evans and R. W. ChantrellDepartment of Physics, University of York, Heslington, York YO10 5DD, UKMagnetic nanoparticles with Néel surface anisotropy, different internal structures, surface arrangements and elongation are modelled as many-spin systems. The results suggest that the energy of many-spin nanoparticles cut from cubic lattices can be represented by an effective one-spin potential containing uniaxial and cubic anisotropies. It is shown that the values and signs of the corresponding constants depend strongly on the particle's surface arrangement, internal structure and elongation. Particles cut from a simple cubic lattice have the opposite sign of the effective cubic term, as compared to particles cut from the face-centered cubic lattice. Furthermore, other remarkable phenomena are observed in nanoparticles with relatively strong surface effects: (i) In elongated particles surface effects can change the sign of the uniaxial anisotropy. (ii) The competition between the core and surface anisotropies leads to a new energy that contributes to both the 2 nd − and 4 th −order effective anisotropies. We also evaluate energy barriers ∆E as functions of the strength of the surface anisotropy and the particle size. The results are analyzed with the help of the effective one-spin potential, which allows us to assess the consistency of the widely used formula ∆E/V = K∞ + 6Ks/D, where K∞ is the core anisotropy constant, Ks is a phenomenological constant related to surface anisotropy, and D is the particle's diameter. We show that the energy barriers are consistent with this formula only for elongated particles for which the surface contribution to the effective uniaxial anisotropy scales with the surface and is linear in the constant of the Néel surface anisotropy.
We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M .We also calculate the temperature scaling of the 2-ion anisotropy in L1 0 FePt, and recover the experimentally observed M 2.1 scaling. The method is newly applied to evaluate the temperaturedependent effective anisotropy in the presence of the Néel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures. PACS numbers: 75.30.Gw, 75.70.Rf, 75.10.Hk, 75.70.Ak Recently, the high temperature behavior of magnetic anisotropy has become important due to the applications in heat-assisted magnetic recording (HAMR) 5-7 . The idea of HAMR is based on the heating of the recording media to decrease the writing field of the high anisotropy media (such as FePt) to values compatible with the writing fields provided by conventional recording heads. Since the writing field is proportional to the anisotropy field H k = 2K eff u (T )/M(T ), the knowledge of the scaling behavior of the anisotropy K u with the magnetization M has become a paramount consideration for HAMR 8 . It should be noted that even in relatively simple systems, a simple scaling behavior predicted by the Callen-Callen theory is only valid at temperatures far from the Curie temperature. The systems proposed for HAMR applications can also include more complex composite media such as soft/hard bilayers 9 , FePt/FeRh with metamagnetic phase transition 10,11 , or exchange-bias systems 12 .The evaluation of the temperature dependence of magnetic anisotropy is also important for the modeling of the laser-induced demagnetization processes. The thermal decrease of the anisotropy during the laser-induced demagnetization has been shown to be responsi-
We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111) surface with a Pt1−xIrx alloy overlayer from ab initio calculations. While the ground state of the system is ferromagnetic for x = 0.00, it becomes a spin spiral state as Ir is intermixed into the overlayer. Although the Dzyaloshinsky-Moriya interaction is present in the system, we will demonstrate that the frustrated isotropic exchange interactions play a prominent role in creating the spin spiral state, and these frustrated couplings lead to an attractive interaction between skyrmions at short distances. Using spin dynamics simulations, we show that under these conditions the individual skyrmions form clusters, and that these clusters remain stable at finite temperature.The magnetic skyrmion corresponds to a configuration where the directions of the spin magnetic moments at different lattice sites span the whole sphere[1, 2], in contrast to collinear ferromagnetic or antiferromagnetic systems and spin spiral states. Several years after the theoretical prediction [3, 4] In agreement with the original theoretical description [4, 13], the appearance of skyrmions in the above systems was attributed to the DzyaloshinskyMoriya interaction [14, 15] present in noncentrosymmetric magnets. This chiral interaction competes with the ferromagnetic exchange and easy-axis anisotropy, and may lead to a planar spin spiral ground state in the system [16, 17], which can in turn transform into a skyrmion lattice at finite external magnetic field.Since frustrated isotropic exchange interactions may also stabilize a spin spiral phase, skyrmions could also be present in such systems at finite external magnetic field, even if the Dzyaloshinsky-Moriya interaction is absent due to symmetry reasons. It was shown in Ref. [18] for a model Hamiltonian with competing ferromagnetic and antiferromagnetic interactions on a triangular lattice that at least at finite temperature, this is indeed the case. It was demonstrated later [19][20][21] that the presence of an easy-axis on-site anisotropy extends the stability range of the skyrmion lattice to zero temperature. If only isotropic exchange interactions are present, Blochtype and Néel-type skyrmions with different helicities, as well as skyrmions and antiskyrmions with opposite topological charges [19], are energetically degenerate. Furthermore, the magnetization profile of skyrmions with frustrated exchange interactions is different from that of skyrmions stabilized by the Dzyaloshinsky-Moriya interaction. This leads to an interaction potential between skyrmions with several local energy minima, while the interaction between Dzyaloshinsky-Moriya skyrmions is repulsive at all distances at low temperature [22].Magnetic skyrmions have also been explored in ultrathin film systems such as PdFe bilayer [23] or Fe triplelayer[24] on Ir(111) surface, and Pt|Co|Ir multilayers [25]. Since bulk inversion symmetry is broken at the surface, the Dzyaloshinsky-Moriya interaction is present in such systems;...
We observe metastable localized spin configurations with topological charges ranging from Q = −3 to Q = 2 in a (Pt 0.95 Ir 0.05 )/Fe bilayer on a Pd(111) surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The DzyaloshinskyMoriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with Q = −1, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.
Magnetic vortex dynamics in lithographically prepared nanodots is currently a subject of intensive research, particularly after recent demonstration that the vortex polarity can be controlled by in-plane magnetic field. This has stimulated the proposals of nonvolatile vortex magnetic random access memories. In this work, we demonstrate that triangular nanodots offer a real alternative where vortex chirality, in addition to polarity, can be controlled. In the static regime, we show that vortex chirality can be tailored by applying in-plane magnetic field, which is experimentally imaged by means of variable-field magnetic force microscopy. In addition, the polarity can be also controlled by applying a suitable out-of-plane magnetic field component. The experiment and simulations show that to control the vortex polarity, the out-of-plane field component, in this particular case, should be higher than the in-plane nucleation field. Micromagnetic simulations in the dynamical regime show that the magnetic vortex polarity can be changed with short-duration magnetic field pulses, while longer pulses change the vortex chirality.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.