In-plane uniaxial magnetic anisotropy induced by anisotropic strain relaxation in high lattice-mismatched Dy/Sc superlattices

We report on the magnetic and structural characterization of high lattice-mismatched [Dy2nm/SctSc] superlattices, with variable Sc thickness tSc= 2-6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K66,ef reverses sign and undergoes a dramatic reduction, attaining values of ≈13-24 kJm-3, when compared to K66=-0.76 MJm-3 in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to ≈175-142 kJm-3. We attribute the large downsizing in K66,ef to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant Mγ,22≈1 GPa and a morphic orthorhombiclike strain γ,1≈10-4, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers. © 2014 American Physical Society

In-plane uniaxial magnetic anisotropy induced by anisotropic strain relaxation in high lattice-mismatched Dy/Sc superlattices

We report on the magnetic and structural characterization of high lattice-mismatched [Dy2nm/SctSc] superlattices, with variable Sc thickness tSc= 2-6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K66,ef reverses sign and undergoes a dramatic reduction, attaining values of ≈13-24 kJm-3, when compared to K66=-0.76 MJm-3 in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to ≈175-142 kJm-3. We attribute the large downsizing in K66,ef to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant Mγ,22≈1 GPa and a morphic orthorhombiclike strain γ,1≈10-4, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers. © 2014 American Physical Society

Positive magnetoresistance induced by fan-type phases in a spin-spiral magnet

We report on the positive magnetoresistance (MR) that accompanies the onset of fan-type magnetic structures in an epitaxially grown Dy/Y superlattice (SL). We find that MR ratios for current perpendicular to plane (CPP) and in-plane (CIP) geometries yield comparable values and a similar nonmonotonic dependence on the applied magnetic field H and temperature, which result in a maximum MR∼0.45% at T=110 K. We demonstrate that the rise in resistance is due to the increase in the number of superzone band gaps that accompanies the magnetization process of the anisotropic spin-spiral magnet, which reflects on the increasing complexity of the field-induced modulated magnetic structures. Furthermore, we find that the suppression of the giant MR in Dy/Y SL is due mainly to the combined action of chemical modulation and epitaxial strain effects. Finally, MR presents a linear (negative slope) and unsaturated scaling with H in the magnetized state, which is a clear fingerprint of magnon MR in highly anisotropic magnetic materials. © 2013 American Physical Society

New surprises "down below": Recent successes in the synthesis of actinide materials

Recent discoveries of novel electronic states, including relatively high-temperature superconductivity, in the actinides point to exciting prospects for future discoveries at the bottom of the periodic table. A key ingredient in all of the successes discussed here is the role of high-quality synthesis in enabling advances. Results on PuCoGa5, NpPd5Al 2, and single crystal uranium are discussed

Positive magnetoresistance induced by fan-type phases in a spin-spiral magnet

We report on the positive magnetoresistance (MR) that accompanies the onset of fan-type magnetic structures in an epitaxially grown Dy/Y superlattice (SL). We find that MR ratios for current perpendicular to plane (CPP) and in-plane (CIP) geometries yield comparable values and a similar nonmonotonic dependence on the applied magnetic field H and temperature, which result in a maximum MR∼0.45% at T=110 K. We demonstrate that the rise in resistance is due to the increase in the number of superzone band gaps that accompanies the magnetization process of the anisotropic spin-spiral magnet, which reflects on the increasing complexity of the field-induced modulated magnetic structures. Furthermore, we find that the suppression of the giant MR in Dy/Y SL is due mainly to the combined action of chemical modulation and epitaxial strain effects. Finally, MR presents a linear (negative slope) and unsaturated scaling with H in the magnetized state, which is a clear fingerprint of magnon MR in highly anisotropic magnetic materials. © 2013 American Physical Society

Direct evidence of the anisotropy of magnetization in rare-earth metals and rare-earth/Fe-2 alloys

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceWe report on the genuine origin of the anisotropy of the magnetization M in rare-earth (RE) metals and RE-based alloys. Taking Ho-based layered nanostructures as testing ground, we prove that the anisotropy of M is substantial despite that the sixfold magnetic anisotropy constant K-6(6) vanishes, which contradicts the established wisdom [E. R. Callen and H. B. Callen, J. Phys. Chem. Solids 16, 310 (1960)]. Furthermore, we show that the symmetric anisotropic contributions to M and K-6(6) vary with temperature distinctively from one another, which indicates that both anisotropic effects are unrelated and stem from dissimilar microscopic sources. Our findings are discussed according to the theory [R. J. Elliott and M. F. Thorpe, J. Appl. Phys. 39, 802 (1968)] that predicts the emergence of symmetric anisotropic indirect-exchange terms under the presence of orbital moments. We show evidence that the anisotropy of M is caused by the indirect-exchange coupling among localized 4f magnetic moments mediated by spin-orbit coupled conduction electrons, which ultimately generates a spatially nonuniform spin polarization that replicates the lattice symmetry

New surprises "down below": Recent successes in the synthesis of actinide materials

Recent discoveries of novel electronic states, including relatively high-temperature superconductivity, in the actinides point to exciting prospects for future discoveries at the bottom of the periodic table. A key ingredient in all of the successes discussed here is the role of high-quality synthesis in enabling advances. Results on PuCoGa5, NpPd5Al 2, and single crystal uranium are discussed

Epitaxial PtMn/NiFe exchange-biased bilayers containing directly deposited ordered PtMn

PtMnNiFe exchange-biased bilayers have been grown epitaxially on Si (001) using molecular-beam epitaxy. Spontaneous formation of the chemically ordered face-centered-tetragonal phase of PtMn layer was confirmed without postgrowth magnetic-field annealing, whose Ńel axis is perpendicular to the PtMnNiFe interface. The exchange anisotropy field stabilizes above a PtMn thickness of 15 nm which is much lower than that for polycrystalline PtMn-based exchange-biased systems. For comparison, PtMnNiFe exchange-biased bilayers have been prepared epitaxially on MgO (001) substrate. Spontaneous formation of the chemically ordered PtMn layer was also confirmed with Ńel axis parallel to the PtMnNiFe interface. The exchange anisotropy field of the bilayer on MgO stabilizes beyond a PtMn thickness of 15 nm as well. © 2005 American Institute of Physics

Growth and properties of epitaxial PtMn/NiFe bilayers on Si (001) substrate containing directly deposited ordered PtMn

Equiatomic PtMn layers have been grown epitaxially on Si (001), with the face-centered tetragonal L1 0-ordered antiferromagnetic phase forming without post-growth magnetic field annealing. The thickness dependence of the exchange anisotropy field and coercivity of the NiFe ferromagnetic layer in the epitaxial PtMn/NiFe exchange-coupled bilayers showed the critical thickness of the PtMn to be 10 nm. The exchange biasing properties are stabilised above a PtMn thickness of 15 nm which is much lower than that for polycrystalline PtMn-based exchange-biased systems. The highest value of exchange-bias is observed for a NiFe ferromagnetic layer thickness of 6 nm. The temperature dependence of the magnetic properties in the range 50-400 K shows that H ex and H c increase monotonically in a quasi-linear manner as temperature decreases. Non-saturation of the in-plane magnetisation component of the PtMn, due to the Néel axis lying normal to the interface, is suggested to be responsible for the temperature dependence. © 2005 Elsevier B.V. All rights reserved

The structural and magnetic characterization of molecular-beam-epitaxy-grown FeMn-NiFe exchange-biased bilayers

Structural and magnetic characterizations have been performed on NiFe-FeMn exchange-biased bilayers grown by molecular beam epitaxy. The growth of the films showed high-quality epitaxy with well-defined flat interface between the layers. Magnetic measurements confirmed that the films showed not only a comparable exchange anisotropy field (Hex) to that of sputtered films but also a lowered threshold thickness of 2 nm at which Hex vanishes for the FeMn