Magnetic and transport properties of diluted granular multilayers
Silva, H. G × Gomes, H. L Pogorelov, Y. G Pereira, L. M. C Kakazei, G. N Sousa, J. B Araujo, J. P Mariano, J. F. L Cardoso, S Freitas, P. P #
American Institute of Physics
Journal of Applied Physics vol:106 issue:11 pages:-
The magnetic and transport properties of Co80Fe20(t)/Al2O3(4 nm) multilayers with low nominal thicknesses t=0.7 and 0.9 nm of Co80Fe20 granular layers are studied. Magnetic studies find a superparamagnetic state above the blocking temperature T-b (of field-cooled/zero-field-cooled splitting) that grows with t and decreases with H. The low-voltage Ohmic tunnel transport passes to non-Ohmic I proportional to V-3/2 law for applied fields above similar to 500 V/cm. At fixed V, the temperature dependence of conductance reveals an anomalous dip around similar to 220 K, which can be attributed to the effect of surface contamination by supercooled water. Current-in-plane tunnel magnetoresistance (MR) ratio tends, at lower t, to higher maximum values (similar to 8% at room temperature) but to lower field sensitivity. This may indicate growing discorrelation effect (e.g., between shrinking areas of correlated moments) in this regime and corroborates the deficit of granule magnetization estimated from the Inoue-Maekawa MR fit, compared to that from direct magnetization measurements. MR displays a mean-field-like critical behavior when t approaches the point of superparamagnetic/superferromagnetic transition (t(c)similar to 1.3 nm at room temperature) from below, different from the formerly reported percolationlike behavior at approaching it from above. With growing temperature, MR reveals, beyond the common decrease, an anomalous plateau from T-b similar to 30-50 K up to some higher value T-*similar to 150-200 K, not seen at higher t.