H. Kachkachi

Université de Perpignan, Perpinyà, Languedoc-Roussillon, France

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Publications (45)91.17 Total impact

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    A. F. Franco, J. L. Déjardin, H. Kachkachi
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    ABSTRACT: We develop a general formalism for analyzing the ferromagnetic resonance characteristics of a magnetic dimer consisting of two magnetic elements (in a horizontal or vertical configuration) coupled by dipolar interaction, taking account of their finite-size and aspect ratio. We study the effect on the resonance frequency and resonance field of the applied magnetic field (in amplitude and direction), the inter-element coupling, and the uniaxial anisotropy in various configurations. We obtain analytical expressions for the resonance frequency in various regimes of the interlayer coupling. We (numerically) investigate the behavior of the resonance field in the corresponding regimes. The critical value of the applied magnetic field at which the resonance frequency vanishes may be an increasing or a decreasing function of the dimer's coupling, depending on the anisotropy configuration. It is also a function of the nanomagnets aspect ratio in the case of in-plane anisotropy. This and several other results of this work, when compared with experiments using the standard ferromagnetic resonance with fixed frequency, or the network analyzer with varying frequency and applied magnetic field, provide a useful means for characterizing the effective anisotropy and coupling within systems of stacked or assembled nanomagnets.
    08/2014;
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    F. Vernay, Z. Sabsabi, H. Kachkachi
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    ABSTRACT: 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 inter-particle 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 semi-analytical expression is given for the effective temperature is given in terms of inter alia the applied field, surface anisotropy and dipolar interaction.
    07/2014;
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    Z. Sabsabi, F. Vernay, O. Iglesias, H. Kachkachi
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    ABSTRACT: We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization taking account of temperature, applied field, core and surface anisotropy, and dipolar inter-particle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of $\gamma-Fe_{2}O_{3}$ nanoparticles assemblies in the low concentration limit.
    Physical Review B 09/2013; 88:104424. · 3.66 Impact Factor
  • A F Franco, H Kachkachi
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    ABSTRACT: We investigate the effect of coupling (intensity and nature), applied field, and anisotropy on the spin dynamics of a multi-layer system composed of a hard magnetic layer coupled to a soft magnetic layer through a nonmagnetic spacer. The soft layer is modeled as a stack of several atomic planes while the hard layer, of a different material, is either considered as a pinned macroscopic magnetic moment or again as a stack of atomic planes. We compute the magnetization profile and hysteresis loop of the whole multi-layer system by solving the Landau-Lifshitz equations for the net magnetic moment of each (atomic) plane. We study the competition between the intra-layer anisotropy and exchange interaction, applied magnetic field, and the interface exchange, dipolar or Dzyalozhinski-Moriya interaction. Compared with the exchange coupling, the latter two couplings present peculiar features in the magnetization profile and hysteresis loop that may help identify the nature of the interface coupling in multi-layer magnetic systems.
    Journal of Physics Condensed Matter 07/2013; 25(31):316003. · 2.22 Impact Factor
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    N. Barros, M. Rassam, H. Kachkachi
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    ABSTRACT: We analytically determine the optimal microwave field that allows for the magnetization reversal of a nanomagnet modeled as a macrospin. This is done by minimizing the total injected energy. The results are in good agreement with the fields obtained numerically using the optimal control theory. For typical values of the damping parameter, a weak microwave field is sufficient to induce switching through a resonant process. The optimal field is orthogonal to the magnetization direction at any time and modulated both in amplitude and frequency. The dependence of the pulse shape on the applied field and damping parameter is interpreted. The total injected energy is found to be proportionnal to the energy barrier between the initial state and the saddle point and to the damping parameter. This result may be used as a means for probing the damping parameter in real nanoparticles.
    Physical review. B, Condensed matter 03/2013; 88(1). · 3.77 Impact Factor
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    Andres Felipe Franco, Hamid Kachkachi
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    ABSTRACT: We investigate the effect of coupling (intensity and nature), applied field, and anisotropy on the spin dynamics of a multi-layer system composed of a hard magnetic slab coupled to a soft magnetic slab through a nonmagnetic spacer. The soft slab is modeled as a stack of several atomic layers while the hard layer, of a different material, is either considered as a pinned macroscopic magnetic moment or as an atomic multi-layer system. We compute the magnetization profile and hysteresis loop of the multi-layer system by solving the Landau-Lifshitz equations for the net magnetic moment of each (atomic) layer. We study the competition between the intra-layer anisotropy and exchange interaction, applied magnetic field, and the inter-slab exchange, dipolar or Dzyaloshinski-Moriya interaction. Comparing the effects on the magnetization profile of the three couplings shows that despite the strong effect of the exchange coupling, the dipolar and Dzyaloshinski-Moriya interactions induce a slight (but non negligible) deviation in either the polar or azimuthal direction thus providing more degrees of freedom for adjusting the spin configuration in the multi-layer system.
    11/2012;
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    ABSTRACT: We comment on some misleading and biased statements appearing in the manuscript arXiv:1209.0298 ("Thermal fluctuations of magnetic nanoparticles") about the use of the damped Landau-Lifshitz equation and the kinetic Langer theory for the calculation of the relaxation rate of magnetic nanoclusters. We reiterate simple scientific arguments, part of which is well known to the whole community, demonstrating that the authors' criticisms are unfounded and that they overstate the issue of damping in the Landau-Lifshitz equation with no unanimous experimental evidence.
    10/2012;
  • Article: Comment on
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    ABSTRACT: We comment on some misleading and biased statements appearing in the manuscript arXiv:1209.0298 ("Thermal fluctuations of magnetic nanoparticles") about the use of the damped Landau-Lifshitz equation and the kinetic Langer theory for the calculation of the relaxation rate of magnetic nanoclusters. We reiterate simple scientific arguments, part of which is well known to the whole community, demonstrating that the authors' criticisms are unfounded and that they overstate the issue of damping in the Landau-Lifshitz equation with no unanimous experimental evidence.
    10/2012;
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    ABSTRACT: We compute the temperature-dependent spin-wave spectrum and the magnetization for a spin system using the unified decoupling procedure for the high-order Green's functions for the exchange coupling and anisotropy, both in the classical and quantum case. Our approach allows us to establish a clear crossover between quantum-mechanical and classical methods by developing the classical analog of the quantum Green's function technique. The results are compared with the classical spectral density method and numerical modeling based on the stochastic Landau-Lifshitz equation and the Monte Carlo technique. As far as the critical temperature is concerned, there is a full agreement between the classical Green's functions technique and the classical spectral density method. However, the former method turns out to be more straightforward and more convenient than the latter because it avoids any \emph{a priori} assumptions about the system's spectral density. The temperature-dependent exchange stiffness as a function of magnetization is investigated within different approaches.
    Physical review. B, Condensed matter 05/2012; 86(9). · 3.77 Impact Factor
  • G. Margaris, K. Trohidou, H. Kachkachi
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    ABSTRACT: We study the interplay between intrinsic properties and collective effects in assemblies of magnetic nanoparticles. Analytical expressions for the magnetization are obtained for weak dipolar interactions in dilute assemblies. Our study is based on thermodynamic perturbation theory for the effective macrospin model where a nanoparticle is represented by its macroscopic magnetic moment, taking into account surface effects of each nanoparticle. The approximate analytical expressions for the magnetization are compared to Monte Carlo simulations and their range of validity is established. Our calculations show that the magnetization is influenced by the nanoparticle anisotropy and the shape of the assembly and that in all cases the effect of the second-order dipolar interaction term of the perturbation theory is the reduction of the magnetization.
    Physical review. B, Condensed matter 01/2012; · 3.77 Impact Factor
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    ABSTRACT: We investigate the dynamics of a magnetic system consisting of two magnetic moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya interaction. We compare the switching mechanisms and switching rates as induced by the three couplings. For each coupling and each configuration of the two anisotropy axes, we describe the switching modes and, using the kinetic theory of Langer, we provide (semi-)analytical expressions for the switching rate. We then compare the three interactions with regard to their efficiency in the reversal of the net magnetic moment of the dimer. We also investigate how the energy barriers vary with the coupling. For the dipole-dipole interaction we find that the energy barrier may either increase or decrease with the coupling depending on whether the latter is weak or strong. Finally, upon comparing the various switching rates, we find that the dipole-dipole coupling leads to the slowest magnetic dimer, as far as the switching of its net magnetic moment is concerned.
    Physical review. B, Condensed matter 06/2011; 84. · 3.77 Impact Factor
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    N. Barros, M. Rassam, H. Jirari, H. Kachkachi
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    ABSTRACT: We develop an efficient and general method for optimizing the microwave field that achieves magnetization switching with a smaller static field. This method is based on optimal control and renders an exact solution for the 3D microwave field that triggers the switching of a nanomagnet with a given anisotropy and in an oblique static field. Applying this technique to the particular case of uniaxial anisotropy, we show that the optimal microwave field, that achieves switching with minimal absorbed energy, is modulated both in frequency and in magnitude. Its role is to drive the magnetization from the metastable equilibrium position towards the saddle point and then damping induces the relaxation to the stable equilibrium position. For the pumping to be efficient, the microwave field frequency must match at the early stage of the switching process the proper precession frequency of the magnetization, which depends on the magnitude and direction of the static field. We investigate the effect of the static field (in amplitude and direction) and of damping on the characteristics of the microwave field. We have computed the switching curves in the presence of the optimal microwave field. The results are in qualitative agreement with micro-SQUID experiments on isolated nanoclusters. The strong dependence of the microwave field and that of the switching curve on the damping parameter may be useful in probing damping in various nanoclusters. Comment: 9 pages, 8 figures
    Physical review. B, Condensed matter 12/2010; · 3.77 Impact Factor
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    ABSTRACT: We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M(z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M(z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field. Comment: 10 pages, 9 figures, PRB submitted (12-07-2010)
    Physical Review B 09/2010; 82(10):104433. · 3.66 Impact Factor
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    ABSTRACT: For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m1.66 for systems on a simple cubic lattice and as m1.76 for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A~ m2), comes from the nonlinear spin-wave effects.
    Physical review. B, Condensed matter 01/2010; · 3.77 Impact Factor
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    D. A. Garanin, H. Kachkachi
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    ABSTRACT: By numerically solving the equations of motion for atomic spins we show that internal spin-wave processes in large enough magnetic particles, initially in unstable states, lead to complete magnetization reversal and thermalization. The particle's magnetization strongly decreases in the middle of reversal and then recovers. We identify two main scenarios, exponential and linear spin-wave instabilities. For the latter, the longitudinal and transverse relaxation rates have been obtained analytically. Orientation dependence of these rates leads to a nonexponential relaxation of the particle's magnetization at long times.
    Physical review. B, Condensed matter 02/2009; 80(1). · 3.77 Impact Factor
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    ABSTRACT: 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. Comment: 8 pages, 3 eps figures (Oral communication at ICFPM, Rome 9-12/10/07, by H. Kachkachi)
    Journal of Physics D Applied Physics 06/2008; 41:134004. · 2.53 Impact Factor
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    ABSTRACT: Magnetic nanoparticles with Neel surface anisotropy, different internal structures, surface arrangements and elongation are modelled 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 elongation. 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 its sign. We also show that the competition between the core and surface anisotropies leads to a new energy that contributes to both the 2nd- and 4th-order effective anisotropies.
    Physical review. B, Condensed matter 06/2007; · 3.77 Impact Factor
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    H. Kachkachi
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    ABSTRACT: In a many-spin approach that takes account of the internal structure, microscopic interactions and single-site anisotropies, we investigate the effect of spin non-collinearities induced by the boundary and surface anisotropy on the behaviour of individual magnetic nanoparticles. Through analytical and numerical calculations, we show that there are mainly two regimes separated by some critical value of the surface-anisotropy constant Ks which controls the intensity of spin non-collinearities: (i) the so-called Stoner–Wohlfarth or Néel–Brown regime of a macrospin undergoing a coherent switching, and (ii) the many-spin regime where the strong spin non-collinearities invalidate the coherent mechanism, and where the particle's magnetic state and switching mechanisms can no longer be modelled by a macrospin. For small-to-intermediate values of Ks, and within two models of surface anisotropy (transverse and Néel), the behaviour of the nanoparticle can be modelled by that of a modified macrospin with an effective potential energy containing a uniaxial and cubic-anisotropy terms. This effective spin model provides a crossover between the two regimes above.
    Journal of Magnetism and Magnetic Materials 01/2007; · 1.83 Impact Factor
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    H. Kachkachi
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    ABSTRACT: In this paper we review some results of our works on the magnetization processes in: i) Isolated nanomagnets, both in the one-spin approximation and as many-spin systems. Here, we focus on the intrinsic properties, e.g., those induced by finite-size, boundary and surface effects. We also investigate the crossover between the two regimes. ii) Assemblies of nanomagnets, also in the two situations. We focus on their behavior mainly due to dipole-dipole interactions. Then, we will comment on the interplay between these intrinsic and collective effects.
    12/2006;
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    D. A. Garanin, H. Kachkachi, L. Reynaud
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    ABSTRACT: A magnetic particle with atomic spins ordered in an unstable direction is an example of a false vacuum that decays via excitation of internal spin waves. Coupled evolution of the particle's magnetization (or the vacuum state) and spin waves, considered in the time-dependent vacuum frame, leads to a peculiar relaxation that is very fast at the beginning but slows down to a nonexponential long tail at the end. The two main scenarios are linear and exponential spin-wave instabilities. For the former, the longitudinal and transverse relaxation rates have been obtained analytically. Numerical simulations show that the particle's magnetization strongly decreases in the middle of reversal and then recovers.
    EPL (Europhysics Letters) 10/2006; · 2.26 Impact Factor

Publication Stats

527 Citations
91.17 Total Impact Points

Institutions

  • 2009–2013
    • Université de Perpignan
      Perpinyà, Languedoc-Roussillon, France
  • 1999–2008
    • Université de Versailles Saint-Quentin
      • Groupe d'Etude de la Matière Condensée (GEMaC)
      Versailles, Île-de-France, France
    • Pierre and Marie Curie University - Paris 6
      • Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP)
      Lutetia Parisorum, Île-de-France, France
  • 2003
    • Johannes Gutenberg-Universität Mainz
      • Institute of Physics
      Mainz, Rhineland-Palatinate, Germany
  • 1999–2000
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France