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Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young's modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for ...

We present a characterization through magnetization measurements as a function of field and temperature of the magnetic superconductor RuSr 2 RCu 2 O 8 (RϭEu or Gd͒. The effective magnetic moment was carefully determined from measurements up to 900 K. For both compounds we have obtained almost the same value, e f f ϳ3.15(4) ␤ . This value confirms recent x-ray absorption experiments where a mixed valence state (40% Ru 4ϩ -60% Ru 5ϩ ) was proposed. We have found that the predominant interaction among Ru ions is ferromagnetic ͓⌰ϭ100(3) K and ⌰ϭ87(3) K for the Eu and Gd compounds, respectively͔, but the high-field susceptibility curve displays the characteristics of an antiferromagnetic transition. The possible type of order of the Ru moments is discussed in view of these and previous experiments. We propose a type-I ordering ͑antiferromagnetically coupled ferromagnetic RuO 2 planes͒ and give reasons supporting this choice.

By means of X-and Q-band magnetic-resonance measurements we have investigated the magnetic interactions and derived the magnon gap in the ferromagnetic superconductor RuSr 2 GdCu 2 O 8 . Two microwave absorptions are observed: first, a paramagnetic resonance of the Gd 3ϩ ions, and second, appearing below the Curie temperature (T Curie ), a ferromagnetic resonance due to the ruthenium lattice. Located between the superconducting CuO 2 planes, the Gd ions serve as intrinsic probes of the internal magnetic fields near the Cu sites. Below T Curie the Gd 3ϩ signal shifts, evidencing the appearance of a homogeneous internal field, H Ru-Gd ϳ600 Oe. We show that H Ru-Gd is due to a Ru-Gd ferromagnetic exchange interaction. The Ru ferromagnetic resonance indicates the existence of a magnon anisotropy gap /␥տ4600 Oe. We estimate the out-of-plane and in-plane anisotropy fields to be, respectively, H z ϳ110 kOe and H x ϳ200 Oe.

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Magnetic dc susceptibility (χ) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for χ(T) with a ferromagnetic Θ = 446(5) K and µ ef f = 4.72(9)µ B /f.u., this being lower than that expected for either F e 3+ (5.9µ B ) or F e 2+ (4.9µ B ) ions. The ESR g-factor g = 2.01(2), is associated with F e 3+ .We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized F e 3+ (3d 5 ) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, 1 assuming for the itinerant electrons a bare susceptibility independent on T .We obtained χ 0 e = 3.7 10 −4 emu/mol. We show that the reduction of µ ef f for F e 3+ arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that Θ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a g-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in Sr 2 F eM oO 6 does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites.PACS: 75.10. 76.60.Es, 75.30.Vn