First-principles calculation of magnetoelastic coefficients and magnetostriction in the spinel ferrites CoFe2O4 and NiFe2O4

We present calculations of magnetostriction constants for the spinel ferrites CoFe2O4 and NiFe2O4 using density functional theory within the GGA+U approach. Special emphasis is devoted to the influence of different possible cation distributions on the B site sublattice of the inverse spinel structure on the calculated elastic and magnetoelastic constants. We show that the resulting symmetry-lowering has only a negligible effect on the elastic constants of both systems as well as on the magnetoelastic response of NiFe2O4, whereas the magnetoelastic response of CoFe2O4 depends more strongly on the specific cation arrangement. In all cases our calculated magnetostriction constants are in good agreement with available experimental data. Our work thus paves the way for more detailed first-principles studies regarding the effect of stoichiometry and cation inversion on the magnetostrictive properties of spinel ferrites.Comment: 11 pages, 6 figure

Size and polydispersity effect on the magnetization of densely packed magnetic nanoparticles

The magnetic properties of densely packed magnetic nanoparticles (MNP) assemblies are investigated from Monte Carlo simulations. The case of iron oxide nanoparticles is considered as a typical example of MNP. The main focus is put on particle size and size polydispersity influences on the magnetization curve. The particles are modeled as uniformly magnetized spheres isolated one from each other by a non magnetic layer representing the organic coating. A comparison with recent experimental results on $\gamma-$Fe$_2$O$_3$ powder samples differing by their size is given.Comment: To be published in the Journal of Applied Physics, to be found at http://jap.aip.org

Magnetization of densely packed interacting magnetic nanoparticles with cubic and uniaxial anisotropies: A Monte Carlo study

International audienceThe magnetization curves of densely packed single domain magnetic nanoparticles (MNP) are investigated by Monte Carlo simulations in the framework of an effective one spin model. The particles whose size polydispersity is taken into account are arranged in spherical clusters and both dipole dipole interactions (DDI) and magnetic anisotropy energy (MAE) are included in the total energy. Having in mind the special case of spinel ferrites of intrinsic cubic symmetry, combined cubic and uniaxial magnetocrystalline anisotropies are considered with different configurations for the orientations of the cubic and uniaxial axes. It is found that the DDI, together with a marked reduction of the linear susceptibility are responsible for a damping of the peculiarities due to the MAE cubic component on the magnetization. As an application, we show that the simulated magnetization curves compare well to experimental results for $\gamma$--Fe$_2$O$_3$ MNP for small to moderate values of the field

Magnetoelectric coupling in polycrystalline FeVO4

We report coupling between magnetic and electric orders for antiferromagnetic polycrystalline FeVO4 in which magnetism-induced polarization has been recently found in noncollinear antiferromagnetic state below the second antiferromagnetic phase transition at TN2=15.7K. In this low symmetry phase space group P-1, the magnetic field dependence of electric polarization evidences a clear magnetoelectric coupling in the noncollinear spin-configured antiferromagnetic phase. The discontinuity of magnetodielectric effect observed at the vicinity of the polar to nonpolar transition evidences competition between different magnetodielectric couplings in the two different antiferromagnetic states. The existence of thermal expansion anomaly near TN2 and magnetostriction effect support magnetoelastically mediated scenario of the observed magnetoelectric effect.Comment: 4 pages, 6 figures, Phys. Rev. B 80, 172103 (2009

Giant Magnetoelastic Effects in BaTiO3-based Extrinsic Multiferroic Hybrids

Extrinsic multiferroic hybrid structures consisting of ferromagnetic and ferroelectric layers elastically coupled to each other are promising due to their robust magnetoelectric effects even at room temperature. For a quantitative analysis of these magnetoelectric effects, a detailed knowledge of the piezoelectric and magnetoelastic behavior of both constituents as well as their mutual elastic coupling is mandatory. We here report on a theoretical and experimental study of the magnetic behavior of BaTiO3-based extrinsic multiferroic structures. An excellent agreement between molecular dynamics simulations and the experiments was found for Fe50Co50/BaTiO3 and Ni/BaTiO3 hybrid structures. This demonstrates that the magnetic behavior of extrinsic multiferroic hybrid structures can be determined by means of ab-initio calculations, allowing for the design of novel multiferroic hybrids

Tetragonal magnetostriction and magnetoelastic coupling in Fe-Al, Fe-Ga, Fe-Ge, Fe-Si, Fe-Ga-Al, and Fe-Ga-Ge alloys

This paper presents a comparative study on the tetragonal magnetostriction constant,λγ,2, [ = (3/2)λ100] and magnetoelastic coupling, b1, of binary Fe100-xZx (0 \u3c x \u3c 35, Z = Al, Ga, Ge, and Si) and ternary Fe-Ga-Al and Fe-Ga-Ge alloys. The quantities are corrected for magnetostrains due to sample geometry (the magnetostrictive form effect). Recently published elastic constant data along with magnetization measurements at both room temperature and 77 K make these corrections possible. The form effect correction lowers the magnetostriction by ∼10 ppm for high-modulus alloys and by as much as 30 ppm for low-modulus alloys. The elastic constants are also used to determine the values of the magnetoelastic coupling constant, b1. With the new magnetostriction data on the Fe-Al-Ga alloy, it is possible to show how the double peak magnetostriction feature of the binary Fe-Ga alloy flows into the single peak binary Fe-Al alloy. The corrected magnetostriction and magnetoelastic coupling data for the various alloys are also compared using the electron-per-atom ratio, e/a, as the common variable. The Hume-Rothery rules link thee/a ratio to the regions of phase stability, which appear to be intimately related to the magnetostriction versus the solute concentration curve in these alloys. Using e/a as the abscissa tends to align the peaks in the magnetostriction and magnetoelastic coupling for the Fe-Ga, Fe-Ge, Fe-Al, Fe-Ga-Al, and Fe-Ga-Ge alloys, but not for the Fe-Si alloys for which the larger atomic size difference may play a greater role in phase stabilization. Corrections for the form effect are also presented for the rhombohedral magnetostriction,λɛ,2, and the magnetoelastic coupling, b2, of Fe100-xGax (0 \u3c x \u3c 35) alloys

Dilatomètre linéaire capacitif à usages multiples

Ce dilatomètre permet de mesurer avec précision (1 %) tout allongement compris entre 0,5 Å et 100 μ que peut subir un échantillon solide entre 4,2 et 1 000 K. Quelques exemples d'utilisation de ce dilatomètre sont présentés : mesures de magnétostriction, de piézoélectricité, de dilatation thermique. La possibilité d'appliquer aux échantillons une contrainte uniaxiale en cours d'expérience ou un traitement thermique sous champ in situ, permet d'aborder les problèmes d'ordre directionnel dans d'excellentes conditions expérimentales

An improved capacitance method of measuring thermal expansion and magnetostriction of amorphous ribbons : application to FeNiCr metallic glasses

Magnetostriction and thermal expansion can be accurately measured on amorphous ribbons wound in order to get rigid hollow cylinders : the strains are measured along the cylinder axis by means of a capacitance dilatometer. This method, easier and more reliable than the conventional ones, has been tested by studying the magnetoelastic properties of amorphous Fe40-x /2Ni40-x/2CrxMo 2B8S10 alloys. The one-ion character of the magnetostriction has been verified.On peut mesurer avec précision la magnétostriction et la dilatation thermique de rubans amorphes enroulés de façon à obtenir des cylindres creux rigides : les déformations sont mesurées selon l'axe du cylindre à l'aide d'un dilatomètre capacitif. Cette méthode, plus facile et fiable que celles qui ont été employées à ce jour, a été testée en étudiant les propriétés magnétoélastiques des alliages amorphes : Fe40-x/2Ni 40-x/2CrxMo2B 8Si10. Nous avons vérifié le caractère à un ion de la magnétostriction

An improved capacitance method of measuring thermal expansion and magnetostriction of amorphous ribbons : application to FeNiCr metallic glasses

Magnetostriction and thermal expansion can be accurately measured on amorphous ribbons wound in order to get rigid hollow cylinders : the strains are measured along the cylinder axis by means of a capacitance dilatometer. This method, easier and more reliable than the conventional ones, has been tested by studying the magnetoelastic properties of amorphous Fe40-x /2Ni40-x/2CrxMo 2B8S10 alloys. The one-ion character of the magnetostriction has been verified