The Experts below are selected from a list of 279 Experts worldwide ranked by ideXlab platform
N.a. Usov - One of the best experts on this subject based on the ideXlab platform.
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Ferromagnetic resonance in thin ferromagnetic film with Surface Anisotropy
Journal of Magnetism and Magnetic Materials, 2019Co-Authors: N.a. UsovAbstract:Abstract The ferromagnetic resonance frequencies are obtained for a thin ferromagnetic film with Surface Anisotropy for the cases when the external magnetic field is applied perpendicularly or parallel to the film Surface, and for various combinations of boundary conditions on the film Surface. It is shown that in the presence of Surface Anisotropy the ferromagnetic resonance frequency essentially depends both on the film thickness and on the value of the Surface Anisotropy constant. The results obtained provide a basis for the correct interpretation of experimental data obtained by means of broadband ferromagnetic resonance in thin film structures.
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Magnetization reversal of thin ferromagnetic elements with Surface Anisotropy
Journal of Magnetism and Magnetic Materials, 2018Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Abstract The magnetization reversal process in thin-film ferromagnetic elements with Surface Anisotropy of various shapes and sizes is investigated by means of numerical simulation. The dependencies of the perpendicular and in-plane hysteresis loops on the element thickness, and the value of the Surface Anisotropy constant are obtained. For sufficiently large values of the Surface Anisotropy constant the magnetization reversal of thin-film elements is shown to occur due to the nucleation of the buckling mode. For an elongated rectangular element the nucleation field of the buckling mode is proportional to the absolute value of the Surface Anisotropy constant, and inversely proportional to the element thickness.
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Influence of Surface Anisotropy on magnetization distribution in thin magnetic films
Journal of Applied Physics, 2017Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Three-dimensional numerical simulation of equilibrium micromagnetic configurations existing in thin ferromagnetic films with Surface Anisotropy is carried out taking into account the strong demagnetization field acting on the film magnetization and the true micromagnetic boundary condition on the film Surface. The numerical results are obtained in the simplest Neel approximation for Surface Anisotropy energy, a Surface Anisotropy constant Ks being a single phenomenological parameter. It is found that the spin canted state has the lowest total energy as compared to various multi-domain configurations in the intermediate range of thickness, Lz,min Lz,max, nearly uniform in-plane magnetization, or the vortex has been obtained depending on the film in-plane aspect ratio. On the other hand, different labyrin...
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Influence of Surface Anisotropy on domain wall dynamics in magnetic nanotube
Applied Surface Science, 2017Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Abstract It is shown that Surface domain structure arises in magnetic nanotube with uniaxial Anisotropy if Surface Anisotropy constant is negative and sufficiently high in absolute value. The Surface magnetic Anisotropy affects also the structure and dynamics of a head-to-head domain wall propagating along the nanotube axis in applied magnetic field. The hopping mode is observed for stationary movement of a head-to-head domain wall. The average speed of the domain wall in the hopping mode is found to be several times less than the stationary velocity of the wall in the absence of Surface Anisotropy. This effect is important for various applications where fast propagation of the domain wall along the sample is essential.
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Influence of Surface Anisotropy on magnetization distribution in a single-domain particle
Journal of Applied Physics, 2008Co-Authors: N.a. Usov, Yu. B. GrebenshchikovAbstract:The magnetization distribution in a single domain particle with appreciable Surface Anisotropy energy contribution is investigated for particles of cylindrical, spherical, and rectangular shapes. It is shown that the behavior of the particle in applied magnetic field can be described using effective energy functional. The latter determines the direction of average particle magnetization in external magnetic field, as well as the particle coercive force, at least for the case when the uniform rotation mode is the easiest one. The nonuniform correction to average particle magnetization is found to be proportional to a small parameter KsL/C⪡1, where Ks is the Surface Anisotropy constant, L is the characteristic particle size, and C is the exchange constant.
Hamid Kachkachi - One of the best experts on this subject based on the ideXlab platform.
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ac susceptibility of an assembly of nanomagnets: Combined effects of Surface Anisotropy and dipolar interactions
Physical Review B, 2014Co-Authors: Francois Vernay, Z. Sabsabi, Hamid KachkachiAbstract:We compute the ac susceptibility of a weakly dipolar-interacting monodisperse assembly of magnetic nanoclusters with oriented Anisotropy. For this purpose, we first compute the relaxation rate in a longitudinal magnetic field of a single nanomagnet taking account of both dipolar interactions in the case of dilute assemblies and Surface Anisotropy. We then study the behavior of the real and imaginary components of the ac susceptibility as functions of temperature, frequency, Surface Anisotropy, and interparticle interactions. We find that the Surface Anisotropy induces an upward shift of the temperature at the maximum of the ac susceptibility components and that its effects may be tuned so as to screen out the effects of interactions. The phenomenological Vogel-Fulcher law for the effect of dipolar interaction on the relaxation rate is revisited within our formalism and a semianalytical expression is given for the effective temperature in terms of inter alia the applied field, Surface Anisotropy, and dipolar interaction.
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Thermal and Surface Anisotropy effects on the magnetization reversal of a nanocluster
Journal of Physics D: Applied Physics, 2008Co-Authors: Pierre-michel Déjardin, Hamid Kachkachi, Yu. P. KalmykovAbstract:The relaxation rate and temperature-dependent switching field curve of a spherical magnetic nanocluster are calculated by taking into account the effect of Surface Anisotropy via an effective Anisotropy model. In particular, it is shown that Surface Anisotropy may change the thermally activated magnetization reversal by more than an order of magnitude, and that temperature-dependent switching field curves noticeably deviate from the Stoner-Wohlfarth astroid. With recent and future $\mu$-SQUID measurements in mind, we indicate how comparison of our results with experimental data on isolated clusters may allow one to obtain valuable information on Surface Anisotropy.
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Effective anisotropies and energy barriers of magnetic nanoparticles with Néel Surface Anisotropy
Physical Review B, 2007Co-Authors: R. Yanes, Oksana Chubykalo-fesenko, Richard F. L. Evans, Hamid Kachkachi, Dmitry A. Garanin, Roy W. ChantrellAbstract:Magnetic nanoparticles with Neel Surface Anisotropy, different internal structures, Surface arrangements, and elongation are modeled 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 shape. 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) In symmetric particles (spherical and truncated octahedral) with cubic core Anisotropy Surface effects can change the sing of the latter. We also show that the competition between the core and Surface anisotropies leads to a new energy that contributes to both the second- and fourth-order effective anisotropies. We evaluate energy barriers ΔE 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 ΔE/V= K∞ +6 Ks /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 Neel Surface Anisotropy. © 2007 The American Physical Society.
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Effective anisotropies and energy barriers of magnetic nanoparticles with Néel Surface Anisotropy
Physical Review B, 2007Co-Authors: R. Yanes, Oksana Chubykalo-fesenko, Richard F. L. Evans, Hamid Kachkachi, Dmitry A. Garanin, Roy W. ChantrellAbstract:Magnetic nanoparticles with N\'eel Surface Anisotropy, different internal structures, Surface arrangements, and elongation are modeled 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 shape. 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) In symmetric particles (spherical and truncated octahedral) with cubic core Anisotropy Surface effects can change the sing of the latter. We also show that the competition between the core and Surface anisotropies leads to a new energy that contributes to both the second- and fourth-order effective anisotropies. We evaluate energy barriers $\ensuremath{\Delta}E$ 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 $\ensuremath{\Delta}E∕V={\mathcal{K}}_{\ensuremath{\infty}}+6{\mathcal{K}}_{s}∕D$, where ${\mathcal{K}}_{\ensuremath{\infty}}$ is the core Anisotropy constant, ${\mathcal{K}}_{s}$ 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 N\'eel Surface Anisotropy.
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Surface Anisotropy in nanomagnets: transverse or Néel?
Journal of Magnetism and Magnetic Materials, 2004Co-Authors: Hamid Kachkachi, H MahboubAbstract:Through the hysteresis loop and magnetization spatial distribution we study and compare two models for Surface Anisotropy in nanomagnets: a model with transverse Anisotropy axes and Neel's model. While Surface Anisotropy in the transverse model induces several jumps in the hysteresis loop because of the cluster-wise switching of spins, in the Neel model the jumps correspond to successive coherent partial rotations of the whole bunch of spins. These calculations together with some hints from available experimental results, suggest that Neel's model for Surface Anisotropy is more appropriate.
O. N. Serebryakova - One of the best experts on this subject based on the ideXlab platform.
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Magnetization reversal of thin ferromagnetic elements with Surface Anisotropy
Journal of Magnetism and Magnetic Materials, 2018Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Abstract The magnetization reversal process in thin-film ferromagnetic elements with Surface Anisotropy of various shapes and sizes is investigated by means of numerical simulation. The dependencies of the perpendicular and in-plane hysteresis loops on the element thickness, and the value of the Surface Anisotropy constant are obtained. For sufficiently large values of the Surface Anisotropy constant the magnetization reversal of thin-film elements is shown to occur due to the nucleation of the buckling mode. For an elongated rectangular element the nucleation field of the buckling mode is proportional to the absolute value of the Surface Anisotropy constant, and inversely proportional to the element thickness.
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Influence of Surface Anisotropy on magnetization distribution in thin magnetic films
Journal of Applied Physics, 2017Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Three-dimensional numerical simulation of equilibrium micromagnetic configurations existing in thin ferromagnetic films with Surface Anisotropy is carried out taking into account the strong demagnetization field acting on the film magnetization and the true micromagnetic boundary condition on the film Surface. The numerical results are obtained in the simplest Neel approximation for Surface Anisotropy energy, a Surface Anisotropy constant Ks being a single phenomenological parameter. It is found that the spin canted state has the lowest total energy as compared to various multi-domain configurations in the intermediate range of thickness, Lz,min Lz,max, nearly uniform in-plane magnetization, or the vortex has been obtained depending on the film in-plane aspect ratio. On the other hand, different labyrin...
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Influence of Surface Anisotropy on domain wall dynamics in magnetic nanotube
Applied Surface Science, 2017Co-Authors: N.a. Usov, O. N. SerebryakovaAbstract:Abstract It is shown that Surface domain structure arises in magnetic nanotube with uniaxial Anisotropy if Surface Anisotropy constant is negative and sufficiently high in absolute value. The Surface magnetic Anisotropy affects also the structure and dynamics of a head-to-head domain wall propagating along the nanotube axis in applied magnetic field. The hopping mode is observed for stationary movement of a head-to-head domain wall. The average speed of the domain wall in the hopping mode is found to be several times less than the stationary velocity of the wall in the absence of Surface Anisotropy. This effect is important for various applications where fast propagation of the domain wall along the sample is essential.
J. M. D. Coey - One of the best experts on this subject based on the ideXlab platform.
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Ferromagnetic nanoparticles with strong Surface Anisotropy: Spin structures and magnetization processes
Physical Review B, 2008Co-Authors: L. Berger, Y. Labaye, M. Tamine, J. M. D. CoeyAbstract:Monte Carlo simulations are used to investigate the effect of Surface Anisotropy on the spin configurations and hysteresis loops of ferromagnetic nanoparticles. Spherical particles of radius a are composed of N atoms located on a simple cubic lattice with interatomic spacing a. The particles have 213. A classical Heisenberg model is assumed, with Surface and bulk Anisotropy. When Surface Anisotropy is positive there are two types of ground states separated by a large energy barrier: a “throttled” configuration with reduced magnetization for intermediate values of Surface Anisotropy and a “hedgehog” configuration with zero magnetization in the strong Surface Anisotropy limit. Beyond a threshold, Surface Anisotropy of either sign induces 111 easy axes for the net magnetization. Easy-axis hysteresis loops are then square, with a continuous approach to saturation, and the effective Anisotropy is deduced either from the switching field or from the initial slope of the perpendicular magnetization curve. The hedgehog state shows a stepwise magnetization curve involving discrete configurations, and it passes to a throttled configuration before saturating. The hysteresis loop has the unusual feature that it involves a state in the first quadrant, which lies on the reversible initial magnetization curve; it is possible to recover the zero-field cooled state after saturation. A survey of the exchange and Anisotropy parameters for a range of ferromagnetic materials indicates that the effects of Surface Anisotropy on the spin configuration should be most evident in nanoparticles of ferromagnetic actinide compounds such as US, and rare-earth metals and alloys with Curie points below room temperature; the effects in nanoparticles of 3d ferromagnets and their alloys are usually insignificant, with the possible exception of FePt.
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Surface Anisotropy in ferromagnetic nanoparticles
Journal of Applied Physics, 2002Co-Authors: Y. Labaye, Jean-marc Greneche, O. Crisan, L. Berger, J. M. D. CoeyAbstract:The effect of Surface Anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between Surface and bulk magnetocrystalline Anisotropy imposes a “throttled” spin structure where the spins of outer shells tend to orient normal to the Surface while the core spins remain parallel to each other. For large values of Surface Anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a “hedgehog” configuration with no net magnetization. Implications for FePt nanoparticles are discussed.
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Charge freezing and Surface Anisotropy on magnetite (100)
Journal of Applied Physics, 1993Co-Authors: J. M. D. Coey, Igor V. Shvets, Roland Wiesendanger, H.-j. GüntherodtAbstract:Scanning tunneling microscope images of the (100) Surface of slightly nonstoichiometric magnetite taken at room temperature show static arrays of pairs of Fe2+ ions with short‐range order, and a charge fluctuation time greater than 103 s. The Surface appears to be a Wigner glass with electron pairs localized on adjacent ions as the basic unit. The explanation of Wigner localization at room temperature on the Surface only is that the spin‐polarized minority‐spin band derived from dyz orbitals is stabilized and narrowed by the absence of an apicial oxygen from the B‐site octahedron. This leads to Surface Anisotropy where the Fe2+ spins are pinned normal to the {100} Surfaces. Surface Anisotropy is expected to outweigh bulk Anisotropy in submicron particles.
Amílcar Labarta - One of the best experts on this subject based on the ideXlab platform.
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Surface Anisotropy broadening of the energy barrier distribution in magnetic nanoparticles
Nanotechnology, 2008Co-Authors: Nicolas Perez, Òscar Iglesias, Pablo Guardia, Alejandro G. Roca, Carlos J. Serna, Fernando Bartolomé, Luis Miguel García, Xavier Batlle, M P Morales, Amílcar LabartaAbstract:The effect of Surface Anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the Tln(t/?0) scaling approach. The comparison between the distributions of the Anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core Anisotropy, and of the total Anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable Tln(t/?0), enables the determination of the Surface Anisotropy as a function of the particle size. We show that the contribution of the particle Surface to the total Anisotropy energy can be well described by a size-independent value of the Surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective Anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe3?xO4 particles of 4.9?nm average size and x about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective Surface Anisotropy constant Ks = 2.9 ? 10?2?erg?cm?2 gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective Anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.
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Surface Anisotropy broadening of the energy barrier distribution in magnetic nanoparticles
Nanotechnology, 2008Co-Authors: Nicolas Perez, Òscar Iglesias, Pablo Guardia, Alejandro G. Roca, María Del Puerto Morales, Carlos J. Serna, Fernando Bartolomé, Luis Miguel García, Xavier Batlle, Amílcar LabartaAbstract:The effect of Surface Anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the Tln(t/?0) scaling approach. The comparison between the distributions of the Anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core Anisotropy, and of the total Anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable Tln(t/?0), enables the determination of the Surface Anisotropy as a function of the particle size. We show that the contribution of the particle Surface to the total Anisotropy energy can be well described by a size-independent value of the Surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective Anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe3?xO4 particles of 4.9?nm average size and x about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective Surface Anisotropy constant Ks = 2.9 ? 10?2?erg?cm?2 gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective Anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.
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Surface Anisotropy broadening of the energy barrier distribution in magnetic nanoparticles
arXiv: Materials Science, 2008Co-Authors: Nicolas Perez, Òscar Iglesias, Pablo Guardia, Alejandro G. Roca, Carlos J. Serna, Fernando Bartolomé, Luis Miguel García, Xavier Batlle, M P Morales, Amílcar LabartaAbstract:The effect of Surface Anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the $T\ln(t/\tau_0)$ scaling approach. The comparison between the distributions of the Anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core Anisotropy, and of the total Anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable $T\ln(t/\tau_0)$, enables the determination of the Surface Anisotropy as a function of the particle size. We show that the contribution of the particle Surface to the total Anisotropy energy can be well described by a size--independent value of the Surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective Anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe$_{3-x}$O$_{4}$ particles of 4.9 nm in average size and $x$ about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective Surface Anisotropy constant $K_{s}=2.9\times 10^{-2}$ erg cm$^{-2}$ gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective Anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.
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Influence of Surface Anisotropy on the hysteresis of magnetic nanoparticles
Journal of Magnetism and Magnetic Materials, 2004Co-Authors: Òscar Iglesias, Amílcar LabartaAbstract:Abstract We present the results of Monte Carlo simulations of the magnetic properties of individual spherical nanoparticles with the aim of explaining the role played by Surface Anisotropy on their low-temperature magnetization processes. Phase diagrams for the equilibrium configurations have been obtained, showing a change from quasi-uniform magnetization state to a state with hedgehog-like structures at the Surface as k S , the Surface Anisotropy constant, increases. Through the simulated hysteresis loops and the analysis of spin configurations along them, we have identified a change in the magnetization reversal mechanism from quasi-uniform rotation at low k S values, to a non-uniform switching process at high k S . Results for the dependence of the coercive field and remanence on k S and particle size are also reported.
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Influence of Surface Anisotropy on the magnetization reversal of nanoparticles
physica status solidi (c), 2004Co-Authors: Òscar Iglesias, Amílcar LabartaAbstract:The influence of Surface Anisotropy on the magnetization processes of maghemite nanoparticles with ellipsoidal shape is studied by means of Monte Carlo simulations. Radial Surface Anisotropy is found to favor the formation of hedgehog-like spin structures that become more stable as the Surface Anisotropy constant at the Surface kS is increased form the value at the core. We have studied the change in the low temperature hysteresis loops with the particle aspect ratio and with kS, finding a change in the magnetization reversal mode as kS or the particle elongation is increased. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
