For submicron particles shaped as any axisymmetric body and made with standard canted antiferromagnet like hematite or iron borate, the ground state may comprise of a magnetic vortex with topologically non-trivial distribution of the sublattice magnetization $\vec{l}$ and planar coreless vortex-like structure for the net magnetization $\vec{M}$. For antiferromagnetic particles in the vortex state, in addition to low-frequency modes, there are high frequency modes with frequencies over the range of hundreds gigahertz, including a mode localized in the region of the radius 30-40 nm near a vortex core.
Here we consider micron-sized samples with any axisymmetric body shape and made with a canted antiferromagnet, like hematite or iron borate. We find that its ground state can be a magnetic vortex with a topologically non-trivial distribution of the sublattice magnetization $\vec{l}$ and planar coreless vortex-like structure for the net magnetization $\vec{M}$. For antiferromagnetic samples in the vortex state, in addition to low-frequency modes, we find high-frequency modes with frequencies over the range of hundreds of gigahertz, including a mode localized in a region of radius $\sim$ 30--40 …
We propose Raman-induced collinear difference-frequency generation (DFG) as a method to manipulate dynamical magnetization. When a fundamental beam propagates along a threefold rotational axis, this coherent second-order optical process is permitted by angular momentum conservation through the rotational analogue of the Umklapp process. As a demonstration, we experimentally obtained polarization properties of collinear magnetic DFG along a [111] axis of a single crystal of antiferromagnetic NiO with micro multidomain structure, which excellently agreed with the theoretical prediction.
Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. Additionally, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. Contrary to the previously known or proposed reversal modes, the vortex is distorted close to the interface an…
Direct observation of vortex states in an antiferromagnetic layer have been recently reported [Wu, et al, Nature Phys. 7, 303 (2011)]. In contrast to their analogues in ferromagnetic systems, namely in nanomagnets, the vortex core of antiferromagnets are not expected (and have not been observed) to present gyrotropic or any other remarkable dynamics, even when external fields are applied. Using simulated annealing and spin dynamics techniques we have been able to describe a number of properties of such a vortex state. Besides of being in agreement with reported results, our results also indica…
Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.
We report on the switching of a magnetic vortex core in a sub-micron Permalloy disk, induced by a short current pulse applied in the film plane. Micromagnetic simulations including the adiabatic and non-adiabatic spin-torque terms are used to investigate the current-driven magnetization dynamics. We predict that a core reversal can be triggered by current bursts a tenth of a nanosecond long. The vortex core reversal process is found to be the same as when an external field pulse is applied. The control of a vortex core's orientation using current pulses introduces the technologically relevant …
We have studied magnetic vortex oscillations in soft sub-micron cylindrical dots with variable thickness and diameter by an analytical approach and micromagnetic simulations. We have considered two kinds of modes of the vortex magnetization oscillations: 1) low-frequency translation mode, corresponding to the movement of the vortex as a whole near its equilibrium position; 2) high-frequency vortex modes, which correspond to radially symmetric oscillations of the vortex magnetization, mainly outside the vortex core. The vortex translational eigenmode was calculated numerically in frequency and …
We observe the dynamics of a single magnetic vortex in the presence of a random array of pinning sites. At low excitation amplitudes, the vortex core gyrates about its equilibrium position with a frequency that is characteristic of a single pinning site. At high amplitudes, the frequency of gyration is determined by the magnetostatic energy of the entire vortex, which is confined in a micron-scale disk. We observe a sharp transition between these two amplitude regimes that is due to depinning of the vortex core from a local defect. The distribution of pinning sites is determined by mapping flu…
We present a Monte Carlo study of the magnetic properties of thin cylindrical nanowires composed of a ferromagnetic core and an antiferromagnetic shell implementing a classical spin Hamiltonian. We address systematically the impact of interface exchange coupling on the loop characteristics and the magnetization reversal mechanism. We study the effect of shell polycrystallinity on the characteristic fields of the isothermal hysteresis loop (coercivity, exchange-bias). We demonstrate that coupling to a polycrystalline antiferromagnetic shell increases the critical core diameter for transition fr…
We study the magnetic properties of small Mn ring shaped clusters on a Pt(111) surface in the framework of density functional theory. We find that the Mn atoms possess large magnetic moments, of the order of 4uB/atom, and have dominating antiferromagnetic interatomic exchange interactions. A quantum confinement effect within the ring like clusters was found, indicating that even very small clusters can be seen as quantum corrals. The antiferromagnetic exchange couplings lead to collinear magnetic arrangements in simple corrals, as well as complex non-collinear ordering, as vortex-like structur…
Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.
The induced motion of a magnetic vortex in a micron-sized ferromagnetic disk due to the DC current injection is studied by measuring planar Hall effect. The DC current injection is found to induce the spin torque that sweeps the vortex out of the disk at the critical current while bias magnetic field are applied. The current-induced vortex core displacement deduced from the change in planar Hall resistance is quantitatively consistent with theoretical prediction. Peak structures similar to those originated from spin wave excitations are observed in the differential planar Hall resistance curve…
Dipole-dipole interactions in a square planar array of sub-micron magnetic disks (magnetic dots) have been studied theoretically. Under a normal magnetic field the ground-state of the array undergoes many structural transitions between the limiting chessboard antiferromagnetic state at zero field and the ferromagnet at a threshold field. At intermediate fields, numerous ferrimagnetic states having mean magnetic moments between zero and that of the ferromagnetic state are favorable energetically. The structures and energies of a selection of states are calculated and plotted, as are the fields …
We theoretically study the interplay between bulk Weyl electrons and magnetic topological defects, including magnetic domains, domain walls, and $\mathbb{Z}_6$ vortex lines, in the antiferromagnetic Weyl semimetals \mnsn\ and Mn$_3$Ge with negative vector chirality. We argue that these materials possess a hierarchy of energies scales which allows a description of the spin structure and spin dynamics using a XY model with $\mathbb{Z}_6$ anisotropy. We propose a dynamical equation of motion for the XY order parameter, which implies the presence of $\mathbb{Z}_6$ vortex lines, the double-domain p…
We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-size circular Permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10-ns timescale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio (thickness/radius) of the dots are in agreement with theoretical calculations presented for the same geometry.
Time-resolved Kerr microscopy is used to study the excitations of individual micron- scale ferromagnetic thin film elements in their remnant state. Thin (18 nm) square elements with edge dimensions between 1 and 10 $μ$m form closure domain structures with 90 degree Neel walls between domains. We identify two classes of excitations in these systems. The first corresponds to precession of the magnetization about the local demagnetizing field in each quadrant, while the second excitation is localized in the domain walls. Two modes are also identified in ferromagnetic disks with thicknesses of 60 …
We theoretically study the oscillatory dynamics of a vortex core in a ferrimagnetic disk near its angular momentum compensation point, where the spin density vanishes but the magnetization is finite. Due to the finite magnetostatic energy, a ferrimagnetic disk of suitable geometry can support a vortex as a ground state similar to a ferromagnetic disk. In the vicinity of the angular momentum compensation point, the dynamics of the vortex resemble those of an antiferromagnetic vortex, which is described by equations of motion analogous to Newton's second law for the motion of particles. Owing to…
We report an interesting case where magnetic phenomena can transcend mesoscopic scales. Our system consists of tubes created by assembly of dipolar spheres. The cylindrical topology results in the breakup of degeneracy observed in the planar square and triangular packings. As far as the ground state is concerned, tubes switch from circular to axial magnetization with increasing tube length. All magnetostatic properties found in magnetic nanotubes, in which the dipolar interaction is comparable or dominate over the exchange interaction, are reproduced by the dipolar tubes including an intermedi…
We have prepared crystalline nanowires (diameter ~ 50 nm, length ~ a few microns) of the charge ordering manganite Pr_0.5Ca_0.5Mn_O3 using a low reaction temperature hydrothermal method and characterized them using X-ray diffraction, transmission electron microscopy, SQUID magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ~ 105 K. Further, t…
In small volumes, sample dimensions are known to strongly influence mechanical behavior, especially strength and crystal plasticity. This correlation fades away at the so-called mesoscale, loosely defined at several micrometers in both experiments and simulations. However, this picture depends on the entanglement of the initial defect configuration. In this paper, we study the effect of sample dimensions with a full control on dislocation topology, through the use of a novel observable for dislocation ensembles, the $Λ$-invariant, that depends only on mutual dislocation linking: It is built on…
Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external …
Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to…
Combining magnetism with band topology provides various novel phases that are otherwise impossible. Among several cases, noncollinear metallic antiferromagnets can reveal particularly rich topological physics due to their diverse magnetic ground states. However, there are only a few experimental studies due to the lack of suitable materials, especially with triangular lattice antiferromagnets. Here, we report that metallic triangular antiferromagnet Co1/3TaS2 exhibits a substantial anomalous Hall effect (AHE) related to its noncollinear magnetic order. Our first-principles calculations found t…
Measurements of magnetic noise emanating from ferromagnets due to domain motion were first carried out nearly 100 years ago and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise, but this must be sampled at spatial wavelengths of order several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present the first direct measurement of the fluctuations in the na…
