Observation of ion beam induced magnetic patterning using off-specular polarized neutron reflectometry

The long-range magnetic structure in Co/Pt multilayers magnetically patterned by ion irradiation is observed by off-specular polarized neutron reflectivity. While both specular and off-specular measurements indicate the formation of an artificial domain structure when the sample is in its remanent state, resonant peaks seen in the diffuse scatter reveal long-range magnetic ordering with periodicity in agreement with the design value. These peaks are completely suppressed when the sample is saturated in plane, confirming their origin in the magnetic patterning of the multilayer

Non-collinear long-range magnetic ordering in HgCr2S4

The low-temperature magnetic structure of \HG has been studied by high-resolution powder neutron diffraction. Long-range incommensurate magnetic order sets in at T$_N\sim$22K with propagation vector \textbf{k}=(0,0,$\sim$0.18). On cooling below T$_N$, the propagation vector increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The magnetic structure below T$_N$ consists of ferromagnetic layers in the \textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis. Symmetry analysis using corepresentations theory reveals a point group symmetry in the ordered magnetic phase of 422 (D$_4$), which is incompatible with macroscopic ferroelectricity. This finding indicates that the spontaneous electric polarization observed experimentally cannot be coupled to the magnetic order parameter

Spin and orbital moments of ultra-thin Fe films on various semiconductor surfaces

The magnetic moments of ultrathin Fe films on three different III-V semiconductor substrates, namely GaAs, InAs and In0.2Ga0.8As have been measured with X-ray magnetic circular dichroism at room temperature to assess their relative merits as combinations suitable for next-generation spintronic devices. The results revealed rather similar spin moments and orbital moments for the three systems, suggesting the relationship between film and semiconductor lattice parameters to be less critical to magnetic moments than magnetic anisotropy

Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, γ\gamma-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

The frustrated magnet clinoatacamite, $\gamma$-Cu$_2$(OH)$_3$Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagom\'e planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagom\'e planes. Recent work pointed out that this exchange model is inappropriate for $\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a $\mu_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure ($T<6$ K) was canted and involved significant spin ordering on all the Cu$^{2+}$ spins, which is incompatible with the interpretation of simultaneous VBS and N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated Magnetism 2008 (HFM2008

Spectroscopic Study of the Effects of Bioprotectant Systems on the Protein Stability

In the present article the effect of kosmotrope compounds, i.e. systems having the capability to stabilize biological macromolecules, is investigated by using complementary techniques. The attention is focused on the kosmotrope character of trehalose, a glucose disaccharide, compared to its homologous maltose and sucrose. Complementary techniques of neutron scattering, such as Inelastic Neutron Scattering (INS) and Quasi Elastic Neutron Scattering (QENS) allow to point out the capability of trehalose to strongly affect both the structural and dynamical properties of water. Finally the stabilization effect of trehalose on a well know protein, lysozyme, is studied as a function of temperature by Small Angle Neutron Scattering (SANS)

Spin polarization and barrier oxidation effects at the Co/alumina interface in magnetic tunnel junctions

Copyright © 2004 American Institute of PhysicsThe electronic structure and polarization in magnetic tunnel junctions prepared with varying degrees of barrier-layer oxidation have been studied using x-ray absorption spectroscopy across the Co L2,3 absorption edges. It was found that the Co electronic structure near the Co∕alumina interface tended to that of cobalt oxide as the barrier oxidation time was increased. However, the net Co 3d spin polarization, determined from x-ray magnetic circular dichroism, increased for moderate oxidation times compared to that obtained for an under-oxidized Co∕Al interface. It is proposed that the expected dilution of the measured polarization due to the formation of (room temperature) paramagnetic cobalt oxide, is offset by an increase in the Co 3d spin-polarization of the interface layer as the interface bonding changes from Co–Al to Co–O with increasing oxidation times

The cellular magnetic response and biocompatibility of biogenic zinc- and cobalt-doped magnetite nanoparticles.

The magnetic moment and anisotropy of magnetite nanoparticles can be optimised by doping with transition metal cations, enabling their properties to be tuned for different biomedical applications. In this study, we assessed the suitability of bacterially synthesized zinc- and cobalt-doped magnetite nanoparticles for biomedical applications. To do this we measured cellular viability and activity in primary human bone marrow-derived mesenchymal stem cells and human osteosarcoma-derived cells. Using AC susceptibility we studied doping induced changes in the magnetic response of the nanoparticles both as stable aqueous suspensions and when associated with cells. Our findings show that the magnetic response of the particles was altered after cellular interaction with a reduction in their mobility. In particular, the strongest AC susceptibility signal measured in vitro was from cells containing high-moment zinc-doped particles, whilst no signal was observed in cells containing the high-anisotropy cobalt-doped particles. For both particle types we found that the moderate dopant levels required for optimum magnetic properties did not alter their cytotoxicity or affect osteogenic differentiation of the stem cells. Thus, despite the known cytotoxicity of cobalt and zinc ions, these results suggest that iron oxide nanoparticles can be doped to sufficiently tailor their magnetic properties without compromising cellular biocompatibility

Probing the strongly correlated magnetic state of Co2_2C nanoparticles at low temperatures using μ\muSR

Co$_2$C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. A recent study by Nirmal Roy et al. [1] showed that a collection of Co$_2$C NPs exhibit an exchange bias (EB) effect below T$_{EB}$ = 50 K and also a spin glass (SG) state below T$_{SG}$ = 5 K. We use magnetic, electrical transport, specific heat, and muon spin rotation ($\mu$SR) measurements to explore further the magnetic properties of these NPs. We uncover the onset of Kondo localization at Kondo temperature T$_K$ (= 40.1 K), near the onset of EB effect. A crossover from the Kondo-screened scenario to an RKKY interaction-dominated regime is also observed for T < T$_K$. Specific heat measurements confirm Kondo localization and heavy fermionic nature in Co$_2$C at low T. At low T, zero field $\mu$SR spectra reveal a dominant magnetically disordered fraction with slow relaxation and a smaller fraction with short-range order exhibiting fast relaxation, with no evidence of long-range magnetic order. We observe an increase in this fast relaxation rate between T$_{EB}$ and T$_{SG}$, suggesting a slowing down of the fluctuating local magnetic environment around muons. Transverse field $\mu$SR spectra show the emergence of a stable, multi-peaked local magnetic field distribution below T$_{EB}$. Longitudinal field $\mu$SR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Our results suggest that below T$_{EB}$, Co$_2$C NPs pellet develops a magnetic interface, separating disordered and short-range order fractions. The Exchange interaction that sets in below T$_{EB}$ at the interface couples them and suppresses the fluctuations. With the suppression of magnetic fluctuations below T$_{EB}$, strong correlation effects in the electronic state of Co$_2$C lead to Kondo localization.Comment: 37 Pages, 11 Figure