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John Z Zhang - One of the best experts on this subject based on the ideXlab platform.

  • controlled synthesis and magnetic properties of bimagnetic spinel ferrite cofe2o4 and mnfe2o4 nanocrystals with core shell architecture
    Journal of the American Chemical Society, 2012
    Co-Authors: Qing Song, John Z Zhang
    Abstract:

    A combination of hard phase CoFe(2)O(4) and soft phase MnFe(2)O(4) as the bimagnetic nanocrystals in a core-shell architecture has been synthesized, and their magnetic properties have been systematically studied. Both HRTEM and EDS results confirmed the formation of bimagnetic core-shell structured nanocrystals. On the basis of the systematic and comparative studies of the magnetic properties of a mechanical mixture of pure CoFe(2)O(4) and MnFe(2)O(4) nanocrystals, chemically mixed Co(1-x)Mn(x)Fe(2)O(4) nanocrystals, and bimagnetic core-shell CoFe(2)O(4)@MnFe(2)O(4) and MnFe(2)O(4)@CoFe(2)O(4) nanocrystals, the bimagnetic core-shell nanocrystals show very unique magnetic properties, such as the Blocking Temperature and coercivity. Our results show that the coercivity correlates with the volume fraction of the soft phase as the theoretical hard-soft phase model has suggested. Furthermore, switching the hard phase CoFe(2)O(4) from the core to the shell shows great changes in the coercivity of the nanocrystals. The bimagnetic core-shell nanocrystals evidently demonstrate the rational design capability to separately control the Blocking Temperature and the coercivity in magnetic nanocrystals by varying the materials, their combination, and the volume ratio between the core and the shell and by switching hard or soft phase materials between the core and shell. Such controls via a bimagnetic core-shell architecture are highly desirable for magnetic nanocrystals in various applications.

  • size dependent superparamagnetic properties of mgfe2o4 spinel ferrite nanocrystallites
    Applied Physics Letters, 1998
    Co-Authors: Qi Chen, John Z Zhang
    Abstract:

    Superparamagnetism is a unique and important aspect of magnetism in nanoparticles. The superparamagnetic properties of the MgFe2O4 spinel ferrite nanoparticles with the particle size from about 6 to 18 nm are studied. The Blocking Temperature is a function of the particle size and increases with increasing particle size. The coercivity of MgFe2O4 nanoparticles also is a function of the particle size below the Blocking Temperature. When the Temperature rises above the Blocking Temperature, the nanoparticles show nonhysteresis magnetization behaviors. With these interesting superparamagnetic properties, MgFe2O4 nanoparticles have potentials for applications such as ferrofluids, magnetocaloric refrigeration, and the contrast agents for magnetic resonance imaging.

V Baltz - One of the best experts on this subject based on the ideXlab platform.

  • monte carlo investigation of how interfacial magnetic couplings affect Blocking Temperature distributions in exchange bias bilayers
    Journal of Applied Physics, 2016
    Co-Authors: G Lhoutellier, D Ledue, R Patte, V Baltz
    Abstract:

    Exchange bias in ferromagnetic (F)/antiferromagnetic (AF) bilayers is a function of both the bulk properties of the AF layer and the interfacial properties determining the effective interfacial couplings between the F and AF layers. The distinction between bulk and interface can be clearly revealed in Blocking Temperature distributions, where AF grain volume distribution results in a high-Temperature peak while disordered interfacial magnetic phases produce a low-Temperature contribution. However, the coupling conditions producing such bimodal Blocking Temperature distributions remain to be specified. In this article, we use a granular model which accounts for the disordered interfacial phases by considering small magnetic grains (SGs) with weaker anisotropy and coupling with the F grains at the F/AF interface. The SG are included in the AF material. The coupling conditions producing bimodal Blocking Temperature distributions were determined. Then, using Monte Carlo simulations, these conditions were vali...

  • bimodal distribution of Blocking Temperature for exchange bias ferromagnetic antiferromagnetic bilayers a granular monte carlo study with less stable magnetic regions spread over the interface
    Journal of Physics D, 2015
    Co-Authors: G Lhoutellier, D Ledue, R Patte, Fabrice Barbe, B Dieny, V Baltz
    Abstract:

    In exchange bias based devices, ferromagnetic/antiferromagnetic disordered interfacial spins which exhibit low freezing Temperatures contribute to the alteration of the exchange bias properties. In particular, the Blocking Temperature distributions earlier observed experimentally show two contributions: the common high-Temperature peak due to the antiferromagnetic grain volume distribution and a less usual low-Temperature contribution presumably attributed to disordered interfacial spins. Here, in order to test this assumption Monte Carlo simulations based on a granular level model are used to calculate Blocking Temperature distribution. Small magnetic grains with weaker anisotropy and interfacial coupling are introduced at the ferromagnetic/antiferromagnetic interface to account for the disordered interfacial spins. As a result, the bimodal character of Blocking Temperature distributions is reproduced and varying the amount of these smaller magnetic grains tunes the low-Temperature contribution of the distribution. These simulations therefore validate the assumption that less stable magnetic regions spread over the ferromagnetic/antiferromagnetic interface may originate the bimodal character of Blocking Temperature distributions, as earlier brought forward by experimentalists to explain their results. In addition, because of the heterogeneities of the interface in the presence of less stable small grains, our simulations show that such bilayers cannot be described using a simple model of uniform rotation of the ferromagnetic layer interacting with an average interfacial coupling.

  • bimodal distribution of Blocking Temperature in exchange biased ferromagnetic antiferromagnetic bilayers
    Physical Review B, 2010
    Co-Authors: V Baltz, Bernard Rodmacq, A Zarefy, L Lechevallier, B Dieny
    Abstract:

    The Blocking Temperature $({T}_{\text{B}})$ distribution of various polycrystalline ferromagnetic (F)/antiferromagnetic (AF) bilayers exhibits two distinct peaks. This is ascribed to the existence of two families of magnetic entities determining the Temperature dependence of exchange bias. One is formed by the AF grains which undergo thermally activated magnetic reversal. The other is formed by F/AF disordered interfacial spins which exhibit spin-glass-like behavior. The bulk versus interfacial nature of these two families is indicated by the dependence of the ${T}_{\text{B}}$ distribution on the AF thickness.

  • Tailoring Size Effects on the Exchange Bias in Ferromagnetic-Antiferromagnetic < 100 nm Nanostructures
    Physical Review Letters, 2005
    Co-Authors: V Baltz, Bernard Rodmacq, J Sort, S. Landis, B Dieny
    Abstract:

    The hysteresis loop shift in sub-100 nm ferromagnetic- (FM-)antiferromagnetic (AFM) nanostructures can be either enhanced or reduced with respect to continuous films with the same composition, with varying the AFM layer thickness. An enhancement of the coercivity and a reduction of the Blocking Temperature are also observed. These effects are mainly ascribed to the physical limitations that the dot sizes impose on the AFM domain size and the concomitant weakening of the pinning strength exerted by the AFM during magnetization reversal of the FM.

Talal Mallah - One of the best experts on this subject based on the ideXlab platform.

B Dieny - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic bimodal distribution of Blocking Temperature with multiferroic bifeo3 epitaxial thin films
    Applied Physics Letters, 2012
    Co-Authors: C K Safeer, M Chamfrault, Julie Allibe, C Carretero, C Deranlot, E Jacquet, J F Jacquot, M Bibes, A Barthelemy, B Dieny
    Abstract:

    Controlling BiFeO3 (BFO)/ferromagnet (FM) interfacial coupling appears crucial for electrical control of spintronic devices using this multiferroic. Here, we analyse the magnetic behaviour of exchange-biased epitaxial-BiFeO3/FM bilayers with in-plane or out-of-plane magnetic anisotropies. We report bimodal distributions of Blocking Temperatures similar to those of polycrystalline-antiferromagnet (AF)/FM bilayers. The high-Temperature contribution depends on the FM anisotropy direction and is likely related to thermally activated depinning of domain walls in the BiFeO3 single crystal film as opposed to thermally activated reversal of spins in AF grains for polycrystalline AF. In contrast, the low-Temperature contribution weakly depends on the anisotropy direction, consistent with a spin-glass origin.

  • Tailoring Size Effects on the Exchange Bias in Ferromagnetic-Antiferromagnetic < 100 nm Nanostructures
    Physical Review Letters, 2005
    Co-Authors: V Baltz, Bernard Rodmacq, J Sort, S. Landis, B Dieny
    Abstract:

    The hysteresis loop shift in sub-100 nm ferromagnetic- (FM-)antiferromagnetic (AFM) nanostructures can be either enhanced or reduced with respect to continuous films with the same composition, with varying the AFM layer thickness. An enhancement of the coercivity and a reduction of the Blocking Temperature are also observed. These effects are mainly ascribed to the physical limitations that the dot sizes impose on the AFM domain size and the concomitant weakening of the pinning strength exerted by the AFM during magnetization reversal of the FM.

  • perpendicular exchange bias in antiferromagnetic ferromagnetic nanostructures
    Applied Physics Letters, 2004
    Co-Authors: Jordi Sort, B Dieny, Michael Fraune, C Koenig, F Lunnebach, B Beschoten, G Guntherodt
    Abstract:

    Exchange bias effects have been induced along the perpendicular-to-film direction in nanostructures prepared by electron beam lithography, consisting of a ferromagnetic [Pt/Co] multilayer exchange coupled to an antiferromagnet (FeMn). As a general trend, the exchange bias field and the Blocking Temperature decrease, whereas the coercivity increases, as the size of the nanostructures is reduced.

Mamoun Muhammed - One of the best experts on this subject based on the ideXlab platform.

  • cubic versus spherical magnetic nanoparticles the role of surface anisotropy
    Journal of the American Chemical Society, 2008
    Co-Authors: German Salazaralvarez, Jian Qin, Vladimir Sepelak, I Bergmann, Marianna Vasilakaki, K N Trohidou, Jose D Ardisson, W A A Macedo, Maria Mikhaylova, Mamoun Muhammed
    Abstract:

    The magnetic properties of maghemite (γ-Fe2O3) cubic and spherical nanoparticles of similar sizes have been experimentally and theoretically studied. The Blocking Temperature, TB, of the nanoparticles depends on their shape, with the spherical ones exhibiting larger TB. Other low Temperature properties such as saturation magnetization, coercivity, loop shift or spin canting are rather similar. The experimental effective anisotropy and the Monte Carlo simulations indicate that the different random surface anisotropy of the two morphologies combined with the low magnetocrystalline anisotropy of γ-Fe2O3 is the origin of these effects.

  • synthesis and characterization of surfactant coated superparamagnetic monodispersed iron oxide nanoparticles
    Journal of Magnetism and Magnetic Materials, 2001
    Co-Authors: Yu Zhang, W Voit, Mamoun Muhammed
    Abstract:

    Synthesis and coating of superparamagnetic monodispersed iron oxide nanoparticles was carried out by chemical solution method. Controlled co-precipitation technique was used to prevent undesirable critical oxidation of Fe. The obtained Fe O nanoparticles were coated with sodium oleate. Low-"eld AC susceptibilityand SQUID measurement show superparamagnetism with a Blocking Temperature around 150 K, and almost immeasurable remanence and coercivity. 2001 Elsevier Science B.V. All rights reserved.