Magnetization Of Single-domain Particles Research Articles

Micromagnetic analysis of spin-reorientation transitions. The role of magnetic domain structure

A method for calculating micromagnetic state of ferro- or ferrimagnetic single-crystals based on the Néel's method of phases is proposed. The standard Néel technique requires different approaches to calculation of micromagnetic state of samples with different anisotropy types. Furthermore, this technique cannot be used to calculate magnetization curves of materials with a complex anisotropy type, in which the first-order magnetization process (FOMP) occurs. On the contrary, the technique proposed in the present work makes it possible to calculate micromagnetic state of a sample within one unified approach. This technique has no limitations in terms of the anisotropy type as well. In case of the FOMP, the simulation methods that we used show results different from conventional calculation methods. The reason is that the conventional methods imply coherent rotation of magnetization in single domain particle (so-called Stoner–Wohlfarth model). We explain this discrepancy by the fact that a magnetic domain structure appears in the region of the FOMP. In the present work we show that magnetization processes do not occur in a jump under the FOMP but gradually pass though nucleation and new high-field phase growing, which substitutes for the low-field phase.

Dynamics of the magnetization of single domain particles having triaxial anisotropy subjected to a uniform dc magnetic field

Thermally induced relaxation of the magnetization of single domain ferromagnetic particles with triaxial (orthorhombic) anisotropy in the presence of a uniform external magnetic field H0 is considered in the context of Brown’s continuous diffusion model. Simple analytic equations, which allow one to describe qualitatively the field effects in the relaxation behavior of the system for wide ranges of the field strength and damping parameters are derived. It is shown that these formulas are in complete agreement with the exact matrix continued fraction solution of the infinite hierarchy of linear differential-recurrence equations for the statistical moments, which governs the magnetization dynamics of an individual particle (this hierarchy is derived by averaging the underlying stochastic Landau-Lifshitz-Gilbert equation over its realizations). It is also demonstrated that in strong fields the longitudinal relaxation of the magnetization is essentially modified by the contribution of the high-frequency “intrawell” modes to the relaxation process. This effect discovered for uniaxial particles by Coffey et al. [Phys. Rev. B 51, 15947 (1995)] is the natural consequence of the depletion of population of the shallow potential well. However, in contrast to uniaxial anisotropy, for orthorhombic crystals there is an inherent geometric dependence of the complex magnetic susceptibility and the relaxation time on the damping parameter α arising from the coupling of longitudinal and transverse relaxation modes.

Numerical study for ballistic switching of magnetization in single domain particle triggered by a ferromagnetic resonance within a relaxation time limit

Magnetization reversal via successive ballistic rotation caused by an application of alternative field temporally varying within a relaxation time limit has been numerically investigated. As the Gilbert damping constant is smaller than 0.03, a pronounced beat appears in the temporal evolution of rotating angle of magnetization. The amplitude of the rotating angle at the first peak of the beat is larger than that achieved in the stable ferromagnetic resonance, so that the magnetization reversal occurred at the first peak of the beat. Consequently, subnanosecond switching of magnetization is realized even though a ferromagnetic resonant phenomenon is utilized to enhance the rotating angle of magnetization. In such a switching scheme, it is most important to optimize the frequency of the alternating field to ensure a practical margin of the operation current creating the switching field.

Magnetite-based magnetoreception in birds: the effect of a biasing field and a pulse on migratory behavior.

To test the hypothesis that single domain magnetite is involved in magnetoreception, we treated Australian silvereyes Zosterops l. lateralis with a strong, brief pulse designed to alter the magnetization of single domain particles. This pulse was administered in the presence of a 1 mT biasing field, either parallel to the direction of the biasing field (PAR group) or antiparallel (ANTI group). In the case of magnetoreceptors based on freely moving single domain particles, the PAR treatment should have little effect, whereas the ANTI treatment should cause remagnetization of the magnetite particles involved in a receptor and could produce a maximum change in that receptor's output for some receptor configurations. Migratory orientation was used as a criterion to assess the effect on the receptor. Before treatment, both groups preferred their normal northerly migratory direction. Exposure to the biasing field alone did not affect their behavior. Treatment with the pulse in the presence of the biasing field caused both the PAR and the ANTI birds to show an axial preference for the east-west axis, with no difference between the two groups. Although these results are in accordance with magnetite-based magnetoreceptors playing a role in migratory orientation, they do not support the hypothesis that single domains in polarity-sensitive receptors are free to move through all solid angles. Possible interpretations, including other arrangements of single domains and superparamagnetic crystals, are discussed.

Measurement of thermal switching of the magnetization of single domain particles (invited)

We present an experimental study of the thermally activated switching of the magnetization of individual isolated γ-Fe2O3 particles. These particles are prolate ellipsoids ∼3000 Å long and 650 Å wide. The measured angular dependence of the switching field, Hs(θ), is consistent at large angles with a uniform rotation, but as θ approaches zero, other modes of reversal appear possible, and most likely the mode of reversal is curling. By measuring the probability of reversal of the moment as a function of time and applied magnetic field at T∼300 K, we found that the switching was thermally assisted, but couldn’t be described by hopping over a single energy barrier. Our results indicate that the dynamics of switching are described by a complex path in the energy landscape.

Effect of interaction on the magnetization of single-domain particles

A model is proposed to permit the calculation of the remanence in a magnetic tape that is subject to various magnetizing processes. A combination of a regenerative local field and the Preisach diagram is proposed to account for particle interaction. It is shown that, although the Preisach diagram is not stable, its shape does not have to change as it moves, due to changes in the bulk magnetization I. In fact, the Preisach density function J(H+, H-) could be replaced by J(H+ + αI, H- -αI). The regenerative action is necessary to reduce the calculated spread of the distribution, which is many times the observed value. The best agreement between calculation and experiments is obtained by assuming that the fundamental magnetic particle is spherical, which is consistent with the chain of spheres model for interaction-free magnetization.