M. Kovylina

Controlling exchange bias in Co?CoOx nanoparticles by oxygen content

We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in an amorphous zirconia matrix. The deposition method allows one to control the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2 x 10(-5) to 10(-1) mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoO(x), with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. For an oxygen pressure of about 10(-3) mbar the ratio between the FM and AFM phases is optimum with an exchange bias field of about 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also at the origin of the induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.

Tuning exchange bias in Ni/FeF2 heterostructures using antidot arrays

The transition from positive to negative exchange bias can be systematically tuned with antidot arrays artificially introduced into Ni/FeF2 ferromagnetic/antiferromagnetic heterostructures. These results are a consequence of the energy balance and suggest that the nanostructure plays a key role in the formation of pinned uncompensated spin regions in the antiferromagnetic FeF2 layer. These noninterfacial magnetic moments created at the antidot faces favor the onset of positive exchange bias at lower cooling fields.

Antiferromagnetic/ferromagnetic nanostructures for multidigit storage units

The pursuit of higher densities in binary storage media is facing serious operating limitations. In order to overcome these constraints, several multistate techniques have been investigated as alternatives. Here, we report on an approach to define multistate switching memory units based on magnetic nanostructures exhibiting exchange bias. Writing and reading conditions were studied in patterned antiferromagnetic/ferromagnetic thin films. We establish the necessary and sufficient requirements for this multidigit memory concept that might open up new possibilities for the exploration and design of suitable room temperature spintronic devices.

The fabrication of ordered arrays of exchange biased Ni/FeF2 nanostructures

The fabrication of ordered arrays of exchange biased Ni/FeF(2) nanostructures by focused ion beam lithography is reported. High quality nano-elements, with controlled removal depth and no significant re-deposition, were carved using small ion beam currents (30 pA), moderate dwell times (1 micros) and repeated passages over the same area. Two types of nanostructures were fabricated: square arrays of circular dots with diameters from 125 +/- 8 to 500 +/- 12 nm and periodicities ranging from 200 +/- 8 to 1000 +/- 12 nm, and square arrays of square antidots (207 +/- 8 nm in edge length) with periodicities ranging from 300 +/- 8 to 1200 +/- 12 nm. The arrays were characterized using scanning ion and electron microscopy, and atomic force microscopy. The effect of the patterning on the exchange bias field (i.e., the shift in the hysteresis loop of ferromagnetic Ni due to proximity to antiferromagnetic FeF(2)) was studied using magneto-transport measurements. These high quality nanostructures offer a unique method to address some of the open questions regarding the microscopic origin of exchange bias. This is not only of major relevance in the fabrication and miniaturization of magnetic devices but it is also one of the important proximity phenomena in nanoscience and materials science.

Mirror symmetry in magnetization reversal and coexistence of positive and negative exchange bias in Ni/FeF2

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.

ac conductance in granular insulating Co-ZrO2 thin films: A universal response

The ac conductance in granular insulating Co-ZrO2 thin films prepared by pulsed laser deposition is systematically studied as a function of the Co volume content x. An absorption phenomenon at low frequencies that mimics the universal response of disordered dielectric materials is observed in the range of metal content below the Co percolation threshold xp 0.35 in the so-called dielectric regime. The temperature and frequency dependences of this absorption phenomenon are successfully analyzed in terms of random competing conduction channels between Co particles through thermally assisted tunneling and capacitive conductance. The ac conductance is well correlated with the nanostructure of the samples obtained by the transmission electron microscopy and perfectly matches the calculated ac response for a random resistor-capacitor network. We also show the occurrence of fractional power-law dependences on the frequency of the ac conductance taking place at very low frequencies as compared to the typical ranges at which dispersive behavior is observed in classicaldisordered dielectric materials.