Voltage and temperature dependence of high-field magnetoresistance in arrays of magnetic nanoparticles

Huge values of high-field magnetoresistance have been recently reported in large arrays of ferromagnetic CoFe nanoparticles embedded in an organic insulating matrix in the Coulomb blockade regime. The magnetoresistance displays two characteristic behaviors: a scaling with the magnetic field/temperature ratio and an unusual exponential decrease with increasing voltage. To describe all these features, we propose a model where the electronic charges tunnel from one nanoparticle to another through a paramagnetic impurity. It is assumed that the noncollinearity between the magnetic moment of the ferromagnetic nanoparticles and the paramagnetic moment induces an effective tunnel barrier, the height of which depends on the relative angle of the paramagnetic moment with respect to the ferromagnetic one. A systematic study of the magnetoresistance behavior as a function of the effective tunnel barrier parameters and applied bias voltage is carried out. Finally, we show that by using Fowler-Nordheim current expressions, i.e., in the hypothesis of small energy barriers, the main features of the magnetoresistance are well reproduced with realistic parameters. (c) 2008 American Institute of Physics.