A. Bollero

Magnetoresistance switch effect in a multiferroic Fe3O4∕BaTiO3 bilayer

Multiferroic bilayers composed of a magnetite (Fe3O4) and a BaTiO3 layer show nonlinear current-voltage characteristics in current perpendicular to plane configuration. The magnetoresistance of the bilayers is strongly bias dependent and can be switched from negative at low bias to positive at large bias. It is shown that these effects do not arise from charge-carrier modulation in the magnetite layer by an electric field effect. Therefore both the nonlinear transport characteristics and the switchable magnetoresistance are attributed to interfacial transport phenomena.

Balancing interlayer dipolar interactions in multilevel patterned media with out-of-plane magnetic anisotropy

Nanostructures consisting of a stack of two magnetic multilayers with out-of-plane anisotropy and distinct coercivities hold great promises in spintronics devices. Yet their miniaturization leads to interlayer dipolar coupling, which results in detrimental asymmetrical behaviors of magnetization reversal or even in the loss of intermediate antiparallel configuration. In this letter, we take advantage of Ruderman–Kittel–Kasuya–Yosida interactions in order to compensate for this coupling and restore symmetry. The study has been performed on an array of square dots of 200 nm lateral size, each dot consisting of two [Co/Pt] multilayers antiferromagnetically coupled through a thin Ru spacer.

Enhanced exchange bias effects in a nanopatterned system consisting of two perpendicularly coupled ferromagnets

The effect of patterning on the shift of the hysteresis loop HE and coercivity HC in a system composed of two perpendicularly exchange-coupled ferromagnets (NiFe sputtered onto a [Pt∕Co] multilayer) is investigated in long stripes and square dots. Setting the exchange bias coupling along the stripes results in a threefold increase of HE compared to the continuous films. HC increases dramatically when the coupling is set perpendicular to the stripes and also in the dots. Magnetic force microscopy studies and micromagnetic simulations suggest that differences in the number and orientation of the magnetic domains can account for the observed effects.

Out-of-plane exchange bias in [Pt∕Co]–IrMn bilayers sputtered on prepatterned nanostructures

Exchange bias effects along the out-of-plane direction have been investigated in arrays of 100nm nanostructures prepared on top of prepatterned substrates, consisting of a ferromagnetic [Pt∕Co] multilayer with out-of-plane anisotropy exchange coupled to an antiferromagnetic IrMn layer. A significant loop shift is observed in these nanostructures (dots and trenches). The relative evolutions of the bias fields with the IrMn thickness in the nanostructures and in the continuous film are ascribed to both the effects of the IrMn domain size and thermal activation. Lower coordinated spins in the trenches and at the dot edges are assumed to play a key role on the bias properties. A reduction of the blocking temperature is observed for both the dots and the trenches with respect to the continuous film.

Exchange Bias in Annealed [Pt/Co]/NiFe Systems

Exchange bias without antiferromagnetic layer was observed in systems consisting of two exchange coupled ferromagnetic materials: (Pt/Co)n multilayer (with out-of-plane anisotropy) and permalloy, NiFe, (with in-plane anisotropy). We investigated the influence of various parameters on this phenomenon: Pt buffer layer thickness, number of repeats in the multilayer, thickness of the NiFe layer. Annealing yields a doubling in the magnitude of the loop shift as a result of an improved texture and mosaicity of the films

Exchange bias effect in virgin and annealed[Pt/Co]/NiFe systems

The main role played by the Pt buffer layer on the magnetic configuration of the [Pt/Co] multilayer, i.e. on the pattern consisting of closure domains, and the possibility of doubling HE by annealing the[Pt/Co]/NiFe system is shown.

Finite size effects on the exchange bias properties of ferromagnetic/antiferromagnetic bilayers

Exchange bias (EB) refers to the shift of the hysteresis loop, typically observed in exchange interacting ferromagnetic (FM)-antiferromagnetic (AFM) materials. During the last decades, EB properties have been extensively investigated, mainly in thin films, due to their technological applications in magnetic random access memories and magnetoresistive read heads based on spin valves or tunnel junctions. Recently, the drastic increase in the areal density of magnetic recording media has motivated the study of EB properties in systems of reduced lateral dimensions. The reduction of the lateral dimensions of an EB system down to length scales comparable to the FM or to the AFM magnetic domain sizes is also interesting from a fundamental point of view since this results in a confinement and subsequent alteration of the FM and AFM domain structures, hence allowing to probe the role of domains on EB.