Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4 and CoFe(2-x)SmxO4 silver coated-PDMS elastomers at room temperature.

J. L. MIETTA; M. RUIZ; P. S. ANTONEL; O. E. PÉREZ; A. BUTERA; G. A. JORGE; R. M. NEGRI

Buenos Aires

Congreso; New frontiers in the physics of two dimensional electron systems; 2011

ICAM

Elastic organic matrices were prepared displaying anisotropic magnetoresistance and piezoresistivity. The materials are dispersions of magnetic nanoparticles coated with silver in an elastomeric polymer (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 (needles) aligned in the direction of the field (magnetorheological elastomers, MRE, Antonel et al, 2011). Since the agglomerated of magnetic particles are coated with silver, the MRE are not only magnetic but also electrical conductors. The structure induces elastic, magnetic and electrical anisotropic properties. For example, by decreasing the concentration of particles in the elastic matrix it is possible to obtain resistances of few ohms () when measured parallel to the needles or of several M in the perpendicular direction. Different nanoparticles (NP) were synthesized: magnetite (Fe3O4) and ferrites substituted with samarium cobalt in various proportions (CoFe(2-x)SmxO4 , x = 0,1, 2,3,4). Then agglomerates of the NP were coated with silver. While the sizes of the NP are between 10 and 20 nm, the silver coated corresponds to micrometer size aggregates, referred as Fe3O4@Ag and CoFe(2-x)SmxO4@Ag, respectively. Both the nanoparticles and the MRE were characterized by XRD, TEM, SEM, EDS, VSM, SQUID and FMR. At room temperature the synthesized magnetites (13 nm) are superparamagnetic (TB = 205 K to 0.01 T) while the CoFe(2-x)SmxO4 are ferromagnetic. The electrical conductivity () increases exponentially for these materials in the presence of an external magnetic field (H). Excellent fits of the experimental data  vs. H was achieved by using a model that considers the inter-grains electron transport as a thermally activated process where a barrier dependent on H must be considered in addition to the intrinsic inter-grain barrier. The H-dependent barrier decreases with the applied field, which is attributed to increasing matching of spin-polarization in the silver coating 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 Fe3O4@ Ag and when H and J are parallel to the needles in the PDMS matrix, we obtain changes of  up to 50% for fields of 4000 G and with resistances of the order of ´s. The conductance is also dependent on the applied pressure (piezoresistivity).The elastic properties and Young's modulus were measured as function of the orientation using a texture analysis device (Negri et al, 2010). Magnetic anisotropy in the MRE composite was investigated by FMR. The anisotropic magnetoresistive and piezoresistive properties make these materials highly interesting for applications in flexible electronics, electronic skins, and anisotropic pressure and magnetic field sensors.