Timothy R. Charlton

Magnetic ordering in Cr-doped Bi2Se3 thin films

We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. High-resolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of . The ferromagnetic transition temperature was determined to be analyzing the magnetization-temperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.

Soft x-ray resonant magnetic scattering from an imprinted magnetic domain pattern

The authors report on the use of a Co∕Pt multilayer, which exhibits strong perpendicular magnetic anisotropy, to magnetostatically imprint a domain pattern onto a 50A thick Permalloy layer. Element specific soft x-ray magnetic scattering experiments were then performed so as to be sensitive to the magnetic structure of the Permalloy only. Off-specular magnetic satellite peaks, corresponding to a periodic domain stripe width of 270nm, were observed, confirmed by magnetic force microscopy and micromagnetic modeling. Thus the authors have exploited the element specificity of soft x-ray scattering to discern the purely magnetic correlations in a structurally flat Permalloy film.

Thickness-dependent magnetic properties of oxygen-deficient EuO

We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu.

High coverage fluid-phase floating lipid bilayers supported by ω-thiolipid self-assembled monolayers

Large area lipid bilayers, on solid surfaces, are useful in physical studies of biological membranes. It is advantageous to minimize the interactions of these bilayers with the substrate and this can be achieved via the formation of a floating supported bilayer (FSB) upon either a surface bound phospholipid bilayer or monolayer. The FSBs independence is enabled by the continuous water layer (greater than 15 Å) that remains between the two. However, previous FSBs have had limited stability and low density. Here, we demonstrate by surface plasmon resonance and neutron reflectivity, the formation of a complete self-assembled monolayer (SAM) on gold surfaces by a synthetic phosphatidylcholine bearing a thiol group at the end of one fatty acyl chain. Furthermore, a very dense FSB (more than 96%) of saturated phosphatidylcholine can be formed on this SAM by sequential Langmuir–Blodgett and Langmuir–Schaefer procedures. Neutron reflectivity used both isotopic and magnetic contrast to enhance the accuracy of the data fits. This system offers the means to study transmembrane proteins, membrane potential effects (using the gold as an electrode) and even model bacterial outer membranes. Using unsaturated phosphatidylcholines, which have previously failed to form stable FSBs, we achieved a coverage of 73%.

Effect of MgO barriers on ferromagnetic metallic layers studied by polarized neutron reflectivity

The effect of MgO tunnel barriers on the magnetic moment of ultrathin magnetic Fe and Co layers is studied by polarized neutron reflectivity. The MgO barriers were grown by molecular beam epitaxy using either direct evaporation from a source crystal or evaporation of Mg in an oxygen background atmosphere. A decrease of the magnetic moment is observed for both Fe and Co, and atomic intermixing of 1.5–3.5 ML at interfaces between the Fe, Co, and MgO is determined. Evaporation from a MgO source crystal yields a tunnel layer which displays better stoichiometry and epitaxy than is obtained using the other growth method.

Manipulation of the spin helix in FeGe thin films and FeGe/Fe multilayers

Magnetic materials without structural inversion symmetry can display the Dzyaloshinskii-Moriya interaction, which manifests itself as chiral magnetic ground states. These chiral states can interact in complex ways with applied fields and boundary conditions provided by finite sample sizes that are of the order of the lengthscale of the chiral states. Here we study epitaxial thin films of FeGe with a thickness close to the helix pitch of the helimagnetic ground state, which is about 70 nm, by conventional magnetometry and polarized neutron reflectometry. We show that the helix in an FeGe film reverses under the application of a field by deforming into a helicoidal form, with twists in the helicoid being forced out of the film surfaces on the way to saturation. An additional boundary condition was imposed by exchange coupling a ferromagnetic Fe layer to one of the interfaces of an FeGe layer. This forces the FeGe spins at the interface to point in the same direction as the Fe, preventing node expulsion and giving a handle by which the reversal of the helical magnet may be controlled.

Evidence for nonmonotonic magnetic field penetration in a type-I superconductor

The ability of the polarized neutron reflectivity (PNR) technique to reveal the non-local electrodynamics effect in the profile of magnetic field penetrated into a superconductor in the Meissner state is explored with an extreme low-? type-I superconductor. The sample is a thick film of pure indium deposited on a silicon oxide substrate having a neutron refraction index smaller than that for indium. It is shown that the PNR technique allows one to distinguish between exponential and non-exponential shape of the field profile. The data obtained are consistent with the magnetic field distribution following from the non-local theory.

Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown CrxSb2−xTe3 thin films

We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of (in Cr x Sb2−x Te3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from to almost per Cr ion. The transition temperature depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase-pure films of , a of 125 K was determined. Electric transport measurements find surface-dominated transport below ~10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.

Probing the spiral magnetic phase in 6 nm textured erbium using polarised neutron reflectometry

We characterise the magnetic state of highly-textured, sputter deposited erbium for a film of thickness 6 nm. Using polarised neutron reflectometry it is found that the film has a high degree of magnetic disorder, and we present some evidence that the films local magnetic state is consistent with bulk-like spiral magnetism. This, combined with complementary characterisation techniques, show that thin film erbium is a strong candidate material for incorporation into device structures.

The spin polarization of Mn atoms in paramagnetic CuMn alloys induced by a Co layer

Using the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the polarization, and hence the spin accumulation in Mn high susceptibility material in close proximity to a ferromagnetic layer. The magnetic polarization of Mn and Cu 3d electrons in paramagnetic CuMn layers is detected in a Co/Cu(x)/CuMn structure for varying copper layer thicknesses (x). The size of the Mn and Cu L2–3-edge dichroism shows a decrease in the polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization appears to be much higher than that of Cu.