J.C. Ousset

Nearest neighbor exchange constants in Hg1−xMnxTe, Hg1−xMnxSe and other semimagnetic semiconductors

Abstract High field magnetization experiments (B⩽35 T) have been performed at 1.8 K in Hg 1− x Mn x Te ( x = 0.0056) and Hg 1− x Mn x Se ( x = 0.065). At B > 6 T the magnetization exhibits step-like increases due to the magnetic field alignment of the antiferromagnetically coupled nearest neighbor Mn pairs and triangles. A theoretical fit of the magnetization curves provide a determination of NN exchange constants J NN k B = (-5.1 ±0.5) K for Hg 1−x Mn x Te and J NN K B = (-6±0.5)K for Hg 1− x Mn x Se. Analysis of all the presently available J NN values for semimagnetic semiconductors is presented and the observed trends and discrepancies are discussed.

Structural, magnetic, transport, and magneto-optical properties of single crystal La2/3Sr1/3MnO3 thin films

We have grown La0.7Sr0.3MnO3 thin films by reactive facing target radio frequency sputtering on SrTiO3 (001) substrates with the nominal stoichiometry and the perovskite structure down to nanometer-scale film thickness. The films are found to be perfectly expitaxied on SrTiO3, without misfit dislocations. The surfaces of the layers were found to be highly flat with terraces of ≈4 A height corresponding to the manganite unit cell. Magnetic measurements evidence a decrease of the Curie temperature for decreasing thickness. Low temperature magnetoresistance is very small, confirming the absence of grain boundaries in the films but it reaches ≈−50% at room temperature. Finally, magneto-optical Kerr effect measurements between 20 and 400 K reveal the onset of ferromagnetic transition via the coercive field increase and the Kerr rotation and ellipticity measurements.

Growth and stress relaxation of Co/NiO bilayers on MgO(001)

Co/NiO bilayers have been grown on a MgO(001) surface in an ultrahigh vacuum sputtering chamber under various deposition conditions. Investigations by x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy give evidence for a close relationship between structure and deposition conditions for the two different layers. The role of the deposition rate on the oxide layer surface roughness is particularly emphasized. At high deposition rates, surface roughness increases with temperature, whereas the surface remains flat for low deposition rates. Whatever the growth conditions, the NiO layer grows epitaxially on the MgO substrate with the orientation relationship NiO(001)[100]//MgO(001)[100]. The misfit strain (about 1%), elastically accommodated in the thinner layers (3 nm), is fully relaxed in 50-nm-thick layers. A temperature dependence of the cobalt layer structure is observed: at room temperature it grows in its high temperature face-centered cubic structure, where...

Magnetic easy-axis switching in Pt/Co/Pt sandwiches induced by nitrogen ion beam irradiation

The effect of N+ ion irradiation on the magnetic anisotropy of ultrathin Co films in the Pt/Co(1 nm)/Pt (111) system is investigated. The magneto-optical measurements indicate that the as-deposited trilayers display a perpendicular magnetic anisotropy whose properties drastically change with the fluence in the range of 1012 to 1016 ions/cm2. The coercive field progressively decreases as the fluence increases. The superconducting quantum interference device measurements evidence that, for the highest fluences, the paramagnetic limit is not reached moreover an in-plane switching of the anisotropy is observed. Simulations are performed to evaluate damage energy induced by N+ irradiation.

Correlations between coercivity and exchange bias in epitaxial NiO–Co(110) bilayers

We have sputtered epitaxial NiO–Co samples on MgO (110) substrates. NiO epitaxially grows on the isostructural fcc MgO substrate but, due to surface energy minimization, its surface morphology is saw-tooth-like with terraces aligned along [001] direction and either (100) or (010) termination planes. The obtained nanostructures are 80–200 A wide facets with micron-size length. Subsequently deposited Co layers adopt a fcc structure conformal with the NiO nanofacets. It consists thus in a set of connected nanostripes as evidenced by complementary structural characterizations. Shape anisotropy induces a strong easy axis along the stripe edges [001] and a hard axis along the [-110] MgO direction. Magnetization loops recorded along [001] have a total squareness. Thermal treatments were done in zero field for investigating the NiO/Co exchange thermal stability and activation. We observed thermally assisted exchange bias field (HE) variations on 1000 Oe field treated samples for various temperatures between 300 K...

Influence of atomic saw and residual strain on the perpendicular magnetic anisotropy of Au/Co/Au sandwiches

We have grown Au/Co/Au trilayers on MgO substrates exhibiting perpendicular anisotropy. Then the samples were plastically compressed with the atomic saw technique to cut the films in parallel stripes. The plastic deformation of the substrates caused a bending of the film, inducing compressive in-plane residual strain in the Co layers which tends to reduce the perpendicular magnetic anisotropy. To confirm this position-dependent behavior, we have used for the first time polar Kerr effect with a focussed laser beam.

Anisotropy in metallic thin films patterned by the atomic saw method

The use of atomic saw method applied to thin metallic epitaxial systems with in-plane magnetization has allowed us to exhibit strong magnetic anisotropy. The structuration of thin 40 A Co thin films into 0.5 μm large stripes have induced a uniaxial anisotropy field whose direction and magnitude 500 Oe is consistent with shape energy consideration. On the other hand, the same structuration applied to 20 A Fe thin films have induced a uniaxial magnetic anisotropy explained by the observation of a uniaxial relaxation of the strain field after process. EXAFS experiments has permitted to quantify this lattice relaxation which is seen to distort the initial BCC Fe cell uniaxially inducing strong magnetoelastic effects.

Exchange-Biased Nio-Co Nanofaceted Bilayers Grown on MgO (110)

Abstract : We have sputter-grown self-organised faceted NiO-Co epitaxial bilayers on MgO(110). Due to very close lattice parameters, NiO adopts the same NaCl crystallographic structure as the substrate but it minimises its surface energy growing in a stripe-shaped morphology elongated along 001 MgO direction. The Co layers then deposited on NiO adopt a fcc structure. They consist of a set of connected nanowires whose height is about 50 A, length is near to 1 micrometer and lateral periodicity 100 A. Magnetic properties of the Co layers were investigated by magneto-optical Kerr effect from 10 K to room temperature. They are dominated by a strong shape-induced uniaxial anisotropy and exchange coupling with the antiferromagnetic underlayer. Magnetisation loops recorded along the easy axis exhibit a perfect squareness and switch in a field range smaller than 10 Oe. Transverse measurements indicate that switching occurs by domain nucleation and/or domain wall propagation. On the contrary, close to the 110 hard axis, magnetic switching occurs by coherent rotation. The bi-stable Co magnetisation state along its easy axis has been used for ordering the NiO spins configuration from room temperature to 10 K. Sign and value of exchange bias induced by such a thermal treatment can be modulated thanks to a wide magnetocrystalline or local exchange path energies distributions.

Oscillatory Angular Dependence of Exchange Bias for Epitaxial NiO-Co (001) Bilayers

Abstract : We have sputter-deposited NiO-Co bilayers on MgO (001) substrates. NiO grows epitaxially on MgO at 900 deg C and subsequently the room deposited 80 A thick Co films have a fcc crystal structure in epitaxy with the oxide underlayer. These samples were warmed up to 300 deg C and then zero-field or field cooled through the NiO Neel temperature (a 300 Oe magnetic field was applied along the 100 or the 110 MgO axis). Magnetic hysteresis loops were obtained by magneto-optical Kerr effect, the magnetic field being oriented in the plane of the substrate for various angles alpha with respect to the 100 direction. The usually expected behavior for such experiments is a smooth angular a dependence of the exchange bias H(sub E) close to a cosine with only one sign change over 180 deg. The high crystallographic coherence of our NiO/Co bilayers induces a very unusual oscillatory HE (alpha) dependence with several sign changes according to the NiO axis field application. Despite of the Co magnetization switching mechanism which is not a pure coherent rotation. we propose a Stoner-Wohlfhart model including four fold anisotropy and unidirectional exchange anisotropy giving a realistic description of these typical magnetic properties.