Magnetic anisotropy and magnetic phase transitions in RFe(10)Mo(2) (R=Pr,Nd,Sm,Dy,Ho,Er,Tm)

RFe(10)Mo(2) (R=Pr, Sm, Nd, Dy, Ho, Er, Tm) intermetallics were investigated by studying the temperature or field-induced spin-reorientation transitions (SRT's). The temperature dependence of the magnetic anisotropy field was determined by means of the singular point-detection technique for the polycrystalline samples of YFe10Mo2, NdFe10Mo2, DyFe10Mo2, and ErFe10Mo2. Main emphasis was given to the theoretical analysis of the magnetic anisotropy constants and the magnetic phase transitions. The temperature dependences of the rare-earth anisotropy constants were calculated using the single-ion model within linear theory. The applicability of the linear theory of the R anisotropy is discussed. It is shown that the accuracy of this theory increases considerably with increasing temperature. Fitting the experimental data, a set of the crystal field and exchange field parameters for the rare-earth R(3+) ions was deduced. The observed SRT's and first-order magnetization processes (FOMP's) were explained and classified. FOMP-like transitions in PrFe10Mo2, HoFe10Mo2, and ErFe10Mo2 were identified. The temperature dependence of the FOMP fields was calculated for HoFe(10)MO(2) and ErFe10Mo2. The physical origin of a low-temperature anomaly in the magnetization process is discussed for SmFe10Mo2. The spin-reorientation transitions:in ErFe10Mo2 and TmFe10Mo2 are determined to be of first order with a discontinuous jump of the magnetization. The SRT's detected in NdFe10Mo2 and DyFe10Mo2 are of second order. The calculated temperature dependences of the anisotropy fields for DyFe10Mo2 and NdFe10Mo2 are in good agreement with the experimental data over a wide temperature range. FOMP's are predicted at low temperatures for NdFe10Mo2, DyFe10Mo2, and TmFe10Mo2.55138038

Description of magnetic anisotropy and spin-reorientation transitions in NdFe12-xMox and NdFe12-xMoxN (x=1,0, 2.0, 3.0)

NdFe12-xMox and NdFe12-xMoxN (x = 1,2,3) were investigated by studying the anisotropy field and the temperature or field-induced spin-reorientation transitions. The temperature dependence of the magnetic anisotropy field was determined by means of the singular-point-detection technique for polycrystalline aligned samples. A theoretical explanation of the magnetic anisotropy and the magnetic phase transitions is given. The temperature dependencies of the rare-earth anisotropy constants were calculated using the single-ion model within linear theory. Fitting the experimental data, a set of crystal-field and exchange-field parameters for Nd3+ ions was deduced. A first-order spin-reorientation transition from uniaxial to conical phase and a type-2 first-order magnetization process in the perpendicular field are calculated for NdFe11Mo. A canted magnetic structure and a type-1 first-order magnetization process in the axial field are predicted for NdFe12-xMoxN. A change of rare-earth anisotropy after nitrogenation was explained by a bonding charge and a superposition model. The calculated temperature dependence of the anisotropy fields in NdFe12-xMoxN is in good agreement with the experimental data over a wide temperature range. (C) 1996 American Institute of Physics.8031659166

Nano-structured magnetic metamaterial with enhanced nonlinear properties

Nano-structuring can significantly modify the properties of materials. We demonstrate that size-dependent modification of the spin-wave spectra in magnetic nano-particles can affect not only linear, but also nonlinear magnetic response. The discretization of the spectrum removes the frequency degeneracy between the main excitation mode of a nano-particle and the higher spin-wave modes, having the lowest magnetic damping, and reduces the strength of multi-magnon relaxation processes. This reduction of magnon-magnon relaxation for the main excitation mode leads to a dramatic increase of its lifetime and amplitude, resulting in the intensification of all the nonlinear processes involving this mode. We demonstrate this experimentally on a two-dimensional array of permalloy nano-dots for the example of parametric generation of a sub-harmonic of an external microwave signal. The characteristic lifetime of this sub-harmonic is increased by two orders of magnitude compared to the case of a continuous magnetic film, where magnon-magnon relaxation limits the lifetime

Walker-like domain wall breakdown in layered antiferromagnets driven by staggered spin–orbit fields

Funder: STSM grant: COST Action CA17123AbstractWithin linear continuum theory, no magnetic texture can propagate faster than the maximum group velocity of the spin waves. Here, by atomistic spin dynamics simulations and supported by analytical theory, we report that a strongly non-linear transient regime due to the appearance of additional magnetic textures results in the breaking of the Lorentz translational invariance. This dynamical regime is akin to domain wall Walker-breakdown in ferromagnets and involves the nucleation of an antiferromagnetic domain wall pair. While one of the nucleated domain walls is accelerated beyond the magnonic limit, the remaining pair remains static. Under large spin–orbit fields, a cascade of multiple generation and recombination of domain walls are obtained. This result may clarify recent experiments on current pulse induced shattering of large domain structures into small fragmented domains and the subsequent slow recreation of large-scale domains.</jats:p

Large microwave generation from d.c. driven magnetic vortex oscillators in magnetic tunnel junctions

Spin polarized current can excite the magnetization of a ferromagnet through the transfer of spin angular momentum to the local spin system. This pure spin-related transport phenomena leads to alluring possibilities for the achievement of a nanometer scale, CMOS compatible and tunable microwave generator operating at low bias for future wireless communications. Microwave emission generated by the persitent motion of magnetic vortices induced by spin transfer effect seems to be a unique manner to reach appropriate spectral linewidth. However, in metallic systems, where such vortex oscillations have been observed, the resulting microwave power is much too small. Here we present experimental evidences of spin-transfer induced core vortex precessions in MgO-based magnetic tunnel junctions with similar good spectral quality but an emitted power at least one order of magnitude stronger. More importantly, unlike to others spin transfer excitations, the thorough comparison between experimental results and models provide a clear textbook illustration of the mechanisms of vortex precessions induced by spin transfer

Magnetic Vortex Core Reversal by Excitation of Spin Waves

Micron-sized magnetic platelets in the flux closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin wave excitation the concept of a critical velocity as the switching condition has to be modified.Comment: Minor corrections and polishing of previous versio

Optimal control of vortex core polarity by resonant microwave pulses

In a vortex-state magnetic nano-disk, the static magnetization is curling in the plane, except in the core region where it is pointing out-of-plane, either up or down leading to two possible stable states of opposite core polarity p. Dynamical reversal of p by large amplitude motion of the vortex core has recently been demonstrated experimentally,raising fundamental interest for potential application in magnetic storage devices. Here we demonstrate coherent control of p by single and double microwave pulse sequences, taking advantage of the resonant vortex dynamics in a perpendicular bias magnetic field. Optimization of the microwave pulse duration required to switch p allows to experimentally infer the characteristic decay time of the vortex core in the large oscillation regime. It is found to be more than twice shorter than in the small oscillation regime, raising the fundamental question of the non-linear behaviour of magnetic dissipation

Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors

Tunable energy transfer between dipolar-coupled magnetic disks by stimulated vortex gyration

A wide variety of coupled harmonic oscillators exist in nature1. Coupling between different oscillators allows for the possibility of mutual energy transfer between them2-4 and the information-signal propagation5,6. Low-energy input signals and their transport with low-energy dissipation are the key technical factors in the design of information processing devices7. Here, utilizing the concept of coupled oscillators, we experimentally demonstrated a robust new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks - stimulated vortex gyration. Direct experimental evidence was obtained by time-resolved soft X-ray microscopy. The rate of energy transfer from one disk to the other was deduced from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides the advantages of tunable energy transfer rate, low-power input signal, and low-energy dissipation for magnetic elements with negligible damping. Coupled vortex-state disks are promising candidates for information-signal processing devices that operate above room temperature

Origin of temperature and field dependence of magnetic skyrmion size in ultrathin nanodots

Understanding the physical properties of magnetic skyrmions is important for fundamental research with the aim to develop new spintronic device paradigms where both logic and memory can be integrated at the same level. Here, we show a universal model based on the micromagnetic formalism that can be used to study skyrmion stability as a function of magnetic field and temperature. We consider ultrathin, circular ferromagnetic magnetic dots. Our results show that magnetic skyrmions with a small radius—compared to the dot radius—are always metastable, while large radius skyrmions form a stable ground state. The change of energy profile determines the weak (strong) size dependence of the metastable (stable) skyrmion as a function of temperature and/or field