K. Ounadjela

Thermally assisted switching in exchange-biased storage layer magnetic tunnel junctions

A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field.

The defining length scales of mesomagnetism: a review

This review is intended as an introduction to mesomagnetism, with an emphasis on what the defining length scales and their origins are. It includes a brief introduction to the mathematics of domains and domain walls before examining the domain patterns and their stability in 1D and 2D confined magnetic structures. This is followed by an investigation of the effects of size and temperature on confined magnetic structures. Then, the relationship between mesomagnetism and the developing field of spin electronics is discussed. In particular, the various types of magnetoresistance, with an emphasis on the theory and applications of giant magnetoresistance and tunnelling magnetoresistance, are studied. Single electronics are briefly examined before concluding with an outlook on future developments in mesomagnetism.

Magnetization reversal in cobalt and nickel electrodeposited nanowires

We have investigated the magnetization reversal processes in arrays of sub-micron Ni and Co wires by means of magnetization and torque experiments together with micromagnetic calculations. The wires were produced by electrodeposition in the cylindrical pores of track-etched polymer membranes. Diameters in the range 35–400 nm have been studied. The arrays of Co and Ni nanowires display different magnetic behaviors. Particular emphasis is given to the competing shape and crystal magnetic anisotropies that exist in the Co nanowire system. In both systems, explaining the experimental results requires domain formation, except for the smallest diameters where single domain behavior occurs.

Anisotropic transport and magnetic properties of arrays of sub-micron wires

We report a comparative study of anisotropic magnetoresistance and magnetic properties in arrays of sub-micron Ni; and Co wires electro-deposited in the cylindrical pores of track-etched polymer membranes. The variation of coercivity and squareness as a function of wire diameter for arrays of almost isolated sub-micron wires is reported in the range 30-500 nm. The Wi and Go-based systems demonstrate different magnetic and magnetoresistive behaviors except for the smallest diameter. It is believed that the observed difference originates from the presence of competing crystal anisotropy in the Co-based system.

Arrays of nanowires of magnetic metals and multilayers: Perpendicular GMR and magnetic properties

The template strategy combined with electrodeposition techniques have been used to fabricate arrays of nanowires of magnetic metals and multilayers in the cylindrical pores of track-etched polymer membranes, The giant magnetoresistance effects have been investigated in two different types of multilayered nanowires systems: Co/Cu and Ni80Fe20/Cu. In addition, a comparative study of the magnetic properties of sub-micron Ni, Co, Fe and Ni80Fe20 wires is made by means of anisotropic magnetoresistance and magnetization experiments.

Anisotropies, cone states, and stripe domains in Co/Pt multilayers

We present results for perpendicular uniaxial anisotropies obtained using torque magnetometry and magnetic force microscopy on epitaxially grown Co/Pt multilayers. A strong dependence of the two lowest order uniaxial anisotropies on Pt film thickness and temperature were observed. The possibility of an interfacial strain origin for the anisotropies due to lattice mismatch between the Co and Pt is discussed. A surprising result is the appearance of magnetic orientations at oblique angles with respect to the film plane and the surface normal. These orientations are identified as “cone states” and found to exist in limited ranges of Pt film thickness and temperatures. The temperature dependence is very unusual and controlled through the behavior of the lowest order perpendicular uniaxial anisotropies. The magnetization process was also examined in terms of saturation fields and domain formation. Narrow stripe domains with periods less than 100 nm were observed in the demagnetized state using magnetic force m...

Artificial antiferromagnetic tunnel junction sensors based on Co/Ru/Co sandwiches

A novel method is used for pinning the magnetization of the magnetically hard subsystem in micron-size magnetic tunnel junctions: the so-called artificial antiferromagnetic structure. The latter uses the strong antiparallel exchange coupling between two Co layers through a Ru spacer layer to ensure a high rigidity of the hard subsystem magnetization. The tunnel barriers were formed by sputter etching previously deposited Al layers in a rf Ar/O2 plasma. Wafers, 3 in. in diameter, were patterned into arrays of square junctions with lateral sizes of 20 and 50 μm. All junctions of a given size show resistances reproducible within several percents. The tunnel magnetoresistance (TMR) is found to be independent of the junction size and TMR ratios of 14%–16% are achieved at room temperature.

Effects of swift heavy ion bombardment on magnetic tunnel junction functional properties

In this article, we report on recent irradiation experiments on magnetic tunnel junctions (MTJ) using either light (C, O) or heavy (Ni) ion beams with energies in the range of 10 MeV/A. For all ions, albeit with different fluence thresholds, the tunnel magnetoresistance decreases irreversibly with increasing ion fluence, with conversely little or no impact on overall resistance. This can only be explained by a modification of the alumina barrier stoechiometry, rather than by interdiffusion. Measurements on irradiated so-called “spin-mirrors” sheet films show that AlOx/metal interface is altered by radiations. Finally, it is shown that MTJs are not a fortiori insensible to swift heavy ion bombardment.

Magnetic domain structures in arrays of submicron Co dots studied with magnetic force microscopy

The magnetic properties of submicron circular epitaxial Co dot arrays fabricated by electron beam lithography have been studied as a function of thickness using high resolution magnetic force microscopy. As a function of thickness, in-plane single domain, vortex, or perpendicular multidomain phases have been identified. The evolution of the domain structures with dot thickness agrees well with theory.

Domain structures in epitaxial (101~0) Co wires

A systematic investigation of the magnetic domain structure is presented for epitaxial sub-micron (101~0) Co wires characterized by a strong in-plane uniaxial magneto-crystalline anisotropy whose easy axis is oriented perpendicular to the long wire axis. Wires of varying width (100 to 1000 nm) and thickness (30 to 80 nm) were patterned by electron beam lithography and lift-off process. We establish experimentally the boundaries between the ground state transverse single domain state and the open stripe structure as a function of wire thickness and width. Moreover, the stability of the transverse single domain state is investigated as a function of the magnetization history.