The equilibrium phase space of magnetic textures in thin-films of cubic chiral ferromagnets, including skyrmions, is explored as a function of in-plane magnetic field strength, film thickness and uniaxial anisotropy. The interplay between these system parameters is found to give rise to a phase space with a rich structure, distinct from that of the nano-stripes that have been previously studied. For certain values of the anisotropy, the range of thicknesses supporting in-plane skyrmions and/or helicoids with an out-of-plane propagation vector is found to be disconnected, suggesting a possible direction for future experiments. We explain how this interesting phase space topology arises due to the geometric confinement of the thin-film system, and identify the optimal parameter ranges for future explorations of novel magnetic textures such as the oblique spiral phase.

We perform a symmetry analysis of the ABC-stacked kagome planes of Mn atoms common to the L12 Mn3X and antiperovskite Mn3AB alloys. In addition to the single-ion-anisotropy and Kitaev-type anisotropic exchange known to stabilize 120 degrees spin structures in these materials, our analysis results in a staggered Dzyaloshinskii-Moriya interaction and further gamma-type anisotropic exchange between nearest-neighbor spins. The presence of these new terms is shown not to affect the energetics of the 120 degrees ground states, which explains their absence in prior minimal magnetic models. We go on to show, however, that their influence becomes apparent when spin-wave excitations are considered. We highlight these effects by calculating inelastic neutron-scattering cross sections to illustrate experimental means of probing the existence and relative strengths of these cloaked interactions.

We study a Heisenberg-Dzyaloshinskii-Moriya Hamiltonian on AB-stacked kagome bilayers at finite temperature. In a large portion of the parameter space, we observe three transitions upon cooling the system: a crossover from Heisenberg to the XY chiral paramagnet, Kosterlitz-Thouless transition to a chiral nematic phase, and a fluctuation-induced first-order transition to an Ising-like phase. We characterize the properties of phases numerically using Monte Carlo finite-size analysis. To further explain the nature of the observed phase transitions, we develop an analytical coarse-graining procedure that maps the Hamiltonian onto a generalized XY model on a triangular lattice. To leading order, this effective model includes both bilinear and biquadratic interactions and is able to correctly predict the two phase transitions. Lastly, we study the Ising fluctuations at low temperatures and establish that the origin of the first-order transition stems from the quasi-degenerate ring manifold in the momentum space.

Micromagnetic simulation results on dynamic hysteresis loops of clusters of iron oxide nanoparticles (NPs) with internal structure composed of nanorods are compared with the widely used macrospin approximation. Such calculations allowing for nanorod-composed NPs is facilitated by a previously developed coarse-graining method based on the renormalization group approach. With a focus on applications to magnetic hyperthermia, we show that magnetostatic interactions improve the heating performance of NPs in chains and triangles, and reduce heating performance in fcc arrangements. Hysteresis loops of triangular and fcc systems of complex NPs are not recovered within the macrospin approximation, especially at smaller interparticle distances. For triangular arrangements, the macrospin approximation predicts that magnetostatic interactions reduce loop area, in contrast to the complex NP case. An investigation of the local hysteresis loops of individual NPs and macrospins in clusters reveals the impact of the geometry of their neighbours on individual versus collective magnetic response, inhomogenous heating within clusters, and further differences between simulating NPs with internal structure and the use of the macrospin approximation. Capturing the internal physical and magnetic structure of NPs is thus important for some applications.

We present the results of an extensive study of the phase diagram and spin wave excitations for a general spin model on a hexagonal AB-stacked kagome system. The boundaries of the magnetic phases are determined via a combination of numerical (Monte Carlo) and analytical (Luttinger-Tisza) methods. We also determine the phase coexistence regions by considering the instabilities in the spin wave spectra. Depending on the strength of the spin-orbit coupling (SOC), some spin and lattice rotations become decoupled, leading to considerably larger symmetry groups than typical magnetic groups. Thus, we provide a detailed symmetry description of the magnetic Hamiltonian with negligible, weak, and intermediate strength of SOC. The spin symmetry in these three cases has a strong effect on the splittings observed in the spin excitation spectra. We further identify a number of self-duality transformations that map the Hamiltonian onto itself. These transformations describe the symmetry of the parameter space and provide an exact mapping between the properties of different magnetic orders and lead to accidental degeneracies. Finally, we discuss the physical relevance of our findings in the context of Mn 3 X compounds.

We demonstrate using micromagnetic simulations and a theoretical model that a gradient in the saturation magnetization ($M_s$) of a perpendicularly magnetized ferromagnetic film induces a non-reciprocal spin wave propagation and, consequently an asymmetric dispersion relation. The $M_s$ gradient adds a linear potential to the spin wave equation of motion consistent with the presence of a force. We consider a transformation from an inertial reference frame in which the $M_s$ is constant to an accelerated reference frame where the resulting inertial force corresponds to the force from the $M_s$ gradient. As in the Doppler effect, the frequency shift leads to an asymmetric dispersion relation. Additionally, we show that under certain circumstances, unidirectional propagation of spin waves can be achieved which is essential for the design of magnonic circuits. Our results become more relevant in light of recent experimental works in which a suitable thermal landscape is used to dynamically modulate the saturation magnetization.

We extend a renormalization group-based (RG) coarse-graining method for micromagnetic simulations to include properly scaled magnetostatic interactions. We apply the method in simulations of dynamic hysteresis loops at clinically relevant sweep rates and at 310 K of iron oxide nanoparticles (NPs) of the kind that have been used in preclinical studies of magnetic hyperthermia. The coarse-graining method, along with a time scaling involving sweep rate and Gilbert damping parameter, allow us to span length scales from the unit cell to NPs approximately 50 nm in diameter with reasonable simulation times. For both NPs and the nanorods composing them, we report effective uniaxial anisotropy strengths and saturation magnetizations, which differ from those of the bulk materials magnetite and maghemite of which they are made, on account of the combined non-trivial effects of temperature, inter-rod exchange, magnetostatic interactions and the degree of orientational order within the nanorod composites. The effective parameters allow treating the NPs as single macrospins, and we find for the test case of calculating loops for two aligned NPs that using the dipole approximation is sufficient for distances beyond 1.5 times the NP diameter. We also present a study on relating integration time step to micromagnetic cell size, finding that the optimal time step size scales approximately linearly with cell volume.

This work investigates the effects of further neighbor exchange and cubic anisotropy on the magnetic excitations of the fcc kagome antiferromagnet IrMn3. Spin wave frequencies are calculated using the torque equation and the dynamical structure factor S(Q, omega) is determined by a Green's function method as an extension of our previous work [LeBlanc et al. Phys. Rev. B 90, 144403 (2014)]. These spin wave calculations are compared with inelastic neutron scattering data of polycrystalline IrMn3, where magnetic Mn ions occupy lattice sites of ABC-stacked kagome planes. The data are best explained by a spin wave model including exchange interactions up to fourth nearest neighbor and strong cubic anisotropy, as previously predicted by density functional theory calculations.

Mn3X compounds in which the magnetic Mn atoms form AB-stacked kagome lattices have received a tremendous amount of attention since the observation of the anomalous Hall effect in Mn3Ge and Mn3Sn. Although the magnetic ground state has been known for some time to be an inverse triangular structure with an induced in-plane magnetic moment, there have been several controversies about the minimal magnetic Hamiltonian. We present a general symmetry-based model for these compounds that includes a previously unreported interplane Dzyaloshinskii-Moriya interaction, as well as anisotropic exchange interactions. The latter are shown to compete with the single-ion anisotropy which strongly affects the ground state configurations and elementary spin-wave excitations. Finally, we present the calculated elastic and inelastic neutron scattering intensities and point to experimental assessment of the types of magnetic anisotropy in these compounds that may be important.

The magnetic field evolution of ground spin states of the stacked planar triangular antiferromagnet with antiferromagnetic interlayer interaction Jc is explored using a minimal 3D classical Heisenberg model. A bi-quadratic coupling is also used to mimic the effect of spin fluctuations (Zhitomirsky 2015 J. Phys.: Conf. Ser. 592 012110) which are known to stabilize the magnetization plateau. A single ion anisotropy is included and states with a magnetic field applied in the ab plane and along the c axis are determined. For -plane, an additional new state, in contrast to 2D model (Zhitomirsky 2015 J. Phys.: Conf. Ser. 592 012110), is obtained with weak interlayer interaction, while the magnetization plateau vanishes at large Jc and other new states with z components of spins emerge. For -axis, an extra state, compared with 2D model, is obtained with a weak interlayer interaction. When Jc is large enough, only the state corresponding to the Umbrella phase in 2D model exits.

We use micromagnetic simulations based on the stochastic Landau-Lifshitz-Gilbert equation to calculate dynamic magnetic hysteresis loops at finite temperature that are invariant with simulation cell size. As a test case, we simulate a magnetite nanorod, the building block of magnetic nanoparticles that have been employed in preclinical studies of hyperthermia. With the goal to effectively simulate loops for large iron-oxide-based systems at relatively slow sweep rates on the order of 1 Oe ns-1or less, we modify and employ a previously derived renormalization group approach for coarse-graining (Grinstein and Koch 2003Phys. Rev. Lett.20207201). The scaling algorithm is shown to produce nearly identical loops over several decades in the model cell volume. We also demonstrate sweep-rate scaling involving the Gilbert damping parameter that allows orders of magnitude speed-up of the loop calculations.

High-precision ultrasonic measurements are used to study magnetoelastic coupling as a function of the inplane magnetic field orientation in the spin-1/2 triangular lattice antiferromagnet Ba3CoSb2O9. The relevance of this coupling in stabilizing the 1/3 magnetization plateau is explored. The analysis indicates that, while the magnetoelastic coupling in Ba3CoSb2O9 is large in comparison to other triangular lattice antiferromagnets, the strength of this coupling is still too small to fully account for the magnetization plateau width in Ba3CoSb2O9. Spin fluctuations are therefore the dominant mechanism inducing and stabilizing the magnetization plateau. Our results also show that the amplitude of the spin fluctuations suddenly drop as the V phase is induced at higher field.

M. Li,1,2 A. Zelenskiy,1 J. A. Quilliam,2 Z. L. Dun,3 H. D. Zhou,3 M. L. Plumer,1 and G. Quirion1,2 1Department of Physics and Physical Oceanography, Memorial University, St. John’s, Newfoundland, Canada A1B 3X7 2Institut Quantique and Département de Physique, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 3Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1200, USA

Results are presented on analytic and computational analyses of the spin states associated with a three-dimensional (3D) fcc lattice composed of ABC stacked kagome planes of magnetic ions with only long-range dipole-dipole interactions. Extending previous work on the 2D kagome system, where a discrete six-fold degeneracy of the ground state was revealed [Holden et al. Phys. Rev. B 91, 224425 (2015)], we show that the 3D lattice exhibits a continuous degeneracy characterized by just two spherical angles involving six sublattice spin vectors. Application of a heat bath Monte Carlo algorithm shows that thermal fluctuations reduce this degeneracy at very low temperature in an order-by-disorder process. A magnetic field applied along directions of high symmetry also results in lifting the continuous degeneracy to a subset of states from the original set of ground states. Metropolis Monte Carlo simulation results are also presented on the temperature and system size dependence of the energy, specific heat, and magnetization, providing evidence for a phase transition at T similar or equal to 0.38 (in units of the dipole strength). The results can be relevant to a class of magnetic compounds having the AuCu3 crystal structure.

A hybrid Landau Lifshitz Gilbert/kinetic Monte Carlo algorithm is used to simulate experimental magnetic hysteresis loops for dual layer exchange coupled composite media. The calculation of the rate coefficients and difficulties arising from low energy barriers, a fundamental problem of the kinetic Monte Carlo method, are discussed and the methodology used to treat them in the present work is described. The results from simulations are compared with experimental vibrating sample magnetometer measurements on dual layer CoPtCrB/CoPtCrSiO media and a quantitative relationship between the thickness of the exchange control layer separating the layers and the effective exchange constant between the layers is obtained. Estimates of the energy barriers separating magnetically reversed states of the individual grains in zero applied field as well as the saturation field at sweep rates relevant to the bit write speeds in magnetic recording are also presented. The significance of this comparison between simulations and experiment and the estimates of the material parameters obtained from it are discussed in relation to optimizing the performance of magnetic storage media. (C) 2018 Elsevier B.V. All rights reserved.

Properties of films of geometrically frustrated ABC stacked antiferromagnetic kagome layers are examined using Metropolis Monte Carlo simulations. The impact of having an easy-axis anisotropy on the surface layers and cubic anisotropy in the interior layers is explored. The spin structure at the surface is shown to be different from that of the bulk 3D fcc system, where surface axial anisotropy tends to align spins along the surface [1 1 1] normal axis. This alignment then propagates only weakly to the interior layers through exchange coupling. Results are shown for the specific heat, magnetization and sub-lattice order parameters for both surface and interior spins in three and six layer films as a function of increasing axial surface anisotropy. Relevance to the exchange bias phenomenon in IrMn3 films is discussed.