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Abstract: High-temperature superconducting (HTS) cuprate materials, with the ability to carry large electrical currents with no resistance at easily reachable temperatures, have stimulated enormous scientific and industrial interest since their discovery in the 1980's. However, technological applications of these promising compounds have been limited by their chemical and microstructural complexity and the challenging processing strategies required for the exploitation of their extraordinary properties. The lack of theoretical understanding of the mechanism for superconductivity in these HTS materials has also hindered the search for new superconducting systems with enhanced performance. The unexpected discovery in 2008 of HTS iron-based compounds has provided an entirely new family of materials for studying the crucial interplay between superconductivity and magnetism in unconventional superconductors. Alkali-metal-doped iron selenide (AxFe(2−y)Se2, A=alkali metal) compounds are of particular interest owing to the coexistence of superconductivity at relatively high temperatures with antiferromagnetism. Intrinsic phase separation on the mesoscopic scale is also known to occur in what were intended to be single crystals of these compounds, making it difficult to interpret bulk property measurements. Here, we use a combination of two advanced microscopy techniques to provide direct evidence of the magnetic properties of the individual phases. First, x-ray linear dichroism studies in a photoelectron emission microscope, and supporting multiplet calculations, indicate that the matrix (majority) phase is antiferromagnetic whereas the minority phase is nonmagnetic at room temperature. Second, cryogenic magnetic force microscopy demonstrates unambiguously that superconductivity occurs only in the minority phase. The correlation of these findings with previous microstructural studies and bulk measurements paves the way for understanding the intriguing electronic and magnetic properties of these compounds.

Open Access

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Abstract: Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.

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Abstract: We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic TbMnO3 by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. The use of orthogonal linear x-ray polarizations provides information on the contribution from the different magnetic moment directions, which can be interpreted as signatures from multiferroic cycloidal spin order and sinusoidal spin order. Tracking these signatures in the time domain enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond time scale. The transient phase is shown to exhibit mostly spin density wave character, as in the adiabatic case, while nevertheless retaining the wave vector of the cycloidal long-range order. Two different pump photon energies, 1.55 and 3.1 eV, lead to population of the conduction band predominantly via intersite d−d or intrasite p−d transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a time scale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic order and orbital reconstruction. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such theoretical studies.

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Abstract: Spin pumping has been studied within Ta / Ag / Ni81Fe19 (0–5 nm) / Ag (6 nm) / Co2MnGe (5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of Ni81Fe19 sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the Co2MnGe source layer damping in vector network analyzer ferromagnetic resonance (VNAFMR) experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and super-paramagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR line width that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.

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Abstract: For over half a century, the technological promise of spins manipulable by a small voltage has captivated the interest of experimental and theoretical researchers alike. However, if thin-film multiferroics are to be incorporated into future data storage devices, a much greater understanding of their behaviour and how they differ from their bulk counterparts is required. In this thesis, we probe the fundamental multiferroic properties of BiFeO3 films through a combination of state-of-the-art diffraction and microscopy techniques. We investigate the coupling between magnetic, ferroelectric, and structural order, with a focus on domains, and how the domain structure may be manipulated in order to tailor the multiferroic properties of the material. Using non-resonant magnetic x-ray scattering (NXMS) and neutron diffraction, we study the magnetic and structural properties of (111)pc-oriented BiFeO3 films. Contrary to the general belief that to they grow as a rhombohedral monodomain, we find that they comprise a sub-micron texture of monoclinic domains. The magnetic structure is found to be intimately coupled to the structure, resulting in the propagation vector being locked to the monoclinic b-axis. This magnetoelastic coupling opens up a route to strain-engineer the magnetic domains via epitaxial strain. By growing BiFeO3 films on a lower-symmetry, TbScO3 substrate, we are able to engineer a magnetic, structural and ferroelectric monodomain, coherent over the entire film, constituting an increase in the domain size by over five orders of magnitude. We directly demonstrate the coupling between ferroelectric and magnetic order parameters of the cycloidal magnetic structure. Using NXMS polarimetry to measure directly the magnetic polarity, we show that upon switching the ferroelectric polarisation, the magnetic polarity switches accordingly---a major rearrangement of the magnetic structure, with each spin rotating by 90 degrees on average. This goes counter to idea that magnetic and ferroelectric order parameters are only weakly coupled in type-I multiferroics. Finally, using photoemission electron microscopy we are able to directly image the sub-micron magnetostructural domain structure. We further show that there is a strong interfacial coupling between the magnetostructural domains of BiFeO3 with a ferromagnetic overlayer. The BiFeO3 domains are found to impose a uniaxial anisotropy in the overlayer, opening up a route to control ferromagnetic domains.