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Abstract: The orbital contribution to the magnetic moment of the transition-metal ion in the isostructural weak ferromagnets ACO3 (A= Mn,Co,Ni) and FeBO3 was investigated by a combination of first-principles calculations, nonresonant x-ray magnetic scattering, and x-ray magnetic circular dichroism. A nontrivial evolution of the orbital moment as a function of the 3d orbitals filling is revealed, with a particularly large value found in the Co member of the family. Here, the coupling between magnetic and lattice degrees of freedom produced by the spin-orbit interaction results in a large single-ion anisotropy and a peculiar magnetic-moment-induced electron cloud distortion, evidenced by the appearance of a subtle scattering amplitude at space-group-forbidden reflections and significant magnetostrictive effects. Our results, which complement a previous investigation on the sign of the Dzyaloshinskii-Moriya interaction across the series, highlight the importance of spin-orbit coupling in the physics of weak ferromagnets and prove the ability of modern first-principles calculations to predict the properties of materials where the Dzyaloshinskii-Moriya interaction is a fundamental ingredient of the magnetic Hamiltonian.

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Abstract: A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micro-magnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [Co/Ni]x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at −33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (∼100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0–50 mT), where it is expected to observe regimes of the unstable droplet formation.

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Abstract: Spin crossover complexes are one of the most studied classes of molecular switches and have attracted considerable attention for their potential technological use as active units in new multifunctional devices. A fundamental step towards their practical implementation is their effective processability into thin films. Crucially, the physical property of technological interest shown by these materials in the bulk phase has to be retained once they are deposited on a solid surface. These conditions are not easily satisfied by most of the intrinsically fragile coordination compounds, either because the material processing methods can compromise their molecular structure, or the interaction between the molecule and the surface can induce drastic changes into the resulting properties. Herein, we report the identification of a novel high-vacuum processable spin-crossover complex, [Fe(qnal)2] (qnal = quinoline-naphthaldehyde), and the preparation of a 50 nm sublimated film of this molecular switch on gold. X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) were used to investigate the composition and the temperature- and light-induced spin-crossover of the deposited material, providing full evidence on the capability of this molecular system to be efficiently processed into nanometric films with retention of its switchable magnetic properties.

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Abstract: This dissertation investigates the growth and magnetic properties of magnetic thin films deposited on semiconductor GaAs and the insulator MgO, which could be useful for devices such as Spin-FET and MRAM. CoFeB amorphous films were grown on both GaAs and MgO. We have studied the origin of the uniaxial magnetic anisotropy (UMA) and perpendicular magnetic anisotropy (PMA) with TEM, VSM and XMCD. Our results demonstrated that the orbital moment of Co atoms play an important role to both UMA and PMA. The origin of UMA in Fe/GaAs (100) system with Cr interlayers is explored. The values of UMA in the Fe/GaAs systems were found to be dependent on the thickness of Cr interlayer by the SQUID-VSM measurements. RHEED patterns and TEM images offered the morphology and crystalline structure information of different layers in the samples. Our results show that the UMA disappears when the interlayer Cr forms continuous film of around 5 ML. This offers direct evidence for the first time that the origin of UMA is from the interface bonding rather than the lattice mismatch related film stress. Finally, Fe films were deposited onto GaAs (100) substrate with a heavy metal element Au interface layer. The enhancement of UMA was found in the Fe/Au/GaAs system by the VSM measurements. The XMCD results show that the orbital moment of Fe is enhanced when the Au interlayer is under 0.5 ML, which leads to the enhancement of the UMA in the Fe/Au/GaAs system. Results from the three different systems provide an important understanding of the research into the interface magnetic properties of ferromagnetic metal/semiconductors. These interesting discoveries are very useful for the development of next generation electronic devices like MRAM and SpinFET.

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Abstract: The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped three-dimensional topological insulator (TI). The pioneering transition-metal doped (Sb,Bi)2(Se,Te)3 thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth-doped (Sb,Bi)2(Se,Te)3 thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allowfor the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:Bi2Te3/Cr:Sb2Te3 thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:Sb2Te3 can introduce long-range magnetic order in high-moment Dy:Bi2Te3—upto a temperature of 17 K—in excellent agreement with first-principles calculations,which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.