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Metallic cobalt nanoparticles offer attractive magnetic properties but are vulnerable to oxidation, which suppresses their magnetization. In this article, we report the use of ion beam synthesis to produce ultra-small, oxidation-resistant, cobalt nanoparticles embedded within substoichiometric TiO2-δ thin films. Using high fluence implantation of cobalt at 20–60 keV, the particles were assembled with an average size of 1.5 ± 1 nm. The geometry and structure of the nanoparticles were studied using scanning transmission electron microscopy. Near-edge X-ray fluorescence spectroscopy on the L2,3 Co edges confirms that the majority of the particles beneath the surface are metallic, unoxidised cobalt. Further evidence of the metallic nature of the small particles is provided via their high magnetization and superparamagnetic response between 3 and 300 K with a low blocking temperature of 4.5 K. The magnetic properties were studied using a combination of vibrating sample magnetometry, element-resolved X-ray magnetic circular dichroism, and depth-resolved polarised neutron reflectometry. These techniques provide a unified picture of the magnetic metallic Co particles. We argue, based on these experimental observations and thermodynamic calculations, that the cobalt is protected against oxidation beneath the surface of titania owing to the enthalpic stability of TiO2 over CoO which inhibits solid state reactions.

Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface Bi$_2$Se$_3$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface Bi$_2$Se$_3$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of Bi$_2$Se$_3$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by Eu3+, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of mz,spin = 7 $\mu$B/Eu$^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of Eu2+. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K., Comment: 11 figures, 35 references

Charge-transfer-induced interfacial ferromagnetism and its impact on the exchange bias effect in La0.7Sr0.3MnO3/NdNiO3 correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni3+ cations which are naturally part of the NdNiO3 layer, Ni2+ ions are formed at the interface due to a charge-transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn4+ ions in the La0.7Sr0.3MnO3 layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line-shape changes of the x-ray absorption at the Ni L2,3 edges, the metal-insulator transition of the NdNiO3 layer is resolved in an element-specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of the NdNiO3 layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Néel temperature of the NdNiO3 layer. Different from La0.7Sr0.3MnO3/LaNiO3, the exchange bias observed in La0.7Sr0.3MnO3/NdNiO3 is due to the antiferromagnetism of NdNiO3 and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge-transfer mechanism.

Evidence for a spin reorientation in antiferromagnetic (AFM) Mn2Au thin films induced by high magnetic fields as well as by the application of in-plane mechanical stress is provided. The AFM domain population in the samples was investigated by resonant X-ray Magnetic Linear Dichroism (XMLD) measurements at the L3 edge of Mn using a variable linear polarization of the incident photon beam. As grown samples show no XMLD signal due to averaging over a random AFM domain distribution. After the exposure to a 70 T in-plane magnetic field a clear XMLD signal indicating the generation of a preferential AFM domain orientation is obtained. The same type of XMLD signal is observed when the thin films are strained, demonstrating the feasibility of AFM Domain manipulation by magnetic fields and stress in Mn2Au.

Item does not contain fulltext We employ time-resolved X-ray magnetic circular dichroism with fs time resolution to investigate the ultrafast, laser-driven dynamics of multi-sublattice materials, with both ferromagnetic and antiferromagnetic coupling. These measurements provide evidence for a demagnetization time that scales with the elemental magnetic moment and varies with the exchange interaction symmetry. Ultrafast Magnetism I: International Conference UMC 2013 Strasbourg, France, October 28th - November 1st, 2013

X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(+5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomic-like 4f1 states (Pr4+) and f- symmetry states on the O 2p valence band (Pr3+L states) indicative of a strong 4f- 2p interaction., 19 pages (.doc), 4 figures, Phys. Rev. B, in press