Stephen L. Lee

University of St Andrews · School of Physics and Astronomy

Manipulating the spin state of thin layers of superconducting material is a promising route to generate dissipationless spin currents in spintronic devices. Approaches typically focus on using thin ferromagnetic elements to perturb the spin state of the superconducting condensate to create spin-triplet correlations. We have investigated simple stru...

Ba2CuTeO6 has attracted significant attention as it contains a two-leg spin ladder of Cu2+ cations that lies in close proximity to a quantum critical point. Recently, Ba2CuTeO6 has been shown to accommodate chemical substitutions, which can significantly tune its magnetic behavior. Here, we investigate the effects of substitution for non-magnetic Z...

Vacancy-ordered halide perovskites have received great interest in optoelectronic applications. In this work, we report the novel inorganic halide Cs10MnSb6Cl30 with a distinctive 10H (10-layer hexagonal) perovskite polytype structure with (hcccc)2 stacking. Cs10MnSb6Cl30 has 30% B-site vacancies ordered at both corner- and face-sharing sites, resu...

Two-dimensional (2D) organic-inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium...

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A2CuCl4 by incorporating ortho-, meta-, and para-fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)2CuCl4 (3-FbaH+ = 3-fluorobenzylammonium) is successfully obtained, whic...

We present experimental results on the observed flux screening in proximity coupled superconductor-ferromagnet thin film structures using Nb and Co as the superconductor and ferromagnet respectively. Using the low-energy muon-spin rotation technique to locally probe the magnetic flux density, we find that the addition of the ferromagnet (F) increas...

Muonium is a combination of first- and second-generation matter formed by the electrostatic interaction between an electron and an antimuon (μ+). Although a well-known physical system, their ability to form collective excitations in molecules had not been observed. Here, we give evidence for the detection of a muonium state that propagates in a mol...

Combining magnetic and superconducting functionalities enables lower energy spin transfer and magnetic switching in quantum computing and information storage, owing to the dissipationless nature of quasi-particle mediated supercurrents. Here, we put forward a system where emergent spin-ordering and diffusion of Cooper pairs are achieved at a non-in...

The new iron-based mixed polyanionic material Na 2 Fe(C 2 O 4 )(HPO 4 ) has been synthesized by a hydrothermal technique. The compound contains four Na sites with a three-dimensional crystal structure. This compound shows promising reversible Li and Na insertion properties as a cathode material. The redox potentials observed were ∼3.2 V (vs Li ⁺ /L...

We show that hybrid MnO x /C 60 heterojunctions can be used to design a storage device for spin-polarized charge: a spin capacitor. Hybridization at the carbon-metal oxide interface leads to spin-polarized charge trapping after an applied voltage or photocurrent. Strong electronic structure changes, including a 1-eV energy shift and spin polarizati...

A family of hybrid Perovskite-oxalates (“Perovzalates”) of general composition AILi3MII(C2O4)3 (A = K+, Rb+, Cs+; M = Fe2+, Co2+, Ni2+) are presented. All eight new compounds are isostructural with the previously reported examples NH4Li3Fe(C2O4)3 and KLi3Fe(C2O4)3, crystallising in the rhombohedral space group R3 @#x0305;c, with a ~11.3 - 11.6 Å, c...

Using the newly emerged theory model of an electromagnetic proximity effect, we demonstrate that it provides a good description of our previously reported anomalous Meissner screening observed in thin film superconductor-ferromagnet proximity structures. Using the low energy muon spin rotation measurement technique, we further investigate this new...

We present low-energy muon-spin rotation measurements on Cu/Nb/AlOx/Co thin films that probe the newly described electromagnetic (EM) proximity effect. By varying the thickness of the insulating AlOx layer we control the degree of coupling between the superconductor and ferromagnet and thus the EM proximity effect. For barrier thicknesses up to 4 n...

The title compound is the first example of a layered fluoroperovskite containing an interlayer organic cation. Preliminary magnetic characterisation is reported, and structural relationships to related layered perovskites are discussed.

Highly ordered arrays of nanoscale magnetic structures form the basis of artificial spin ices, uniform particles for bio-medical applications, and data storage as Bit Patterned Media. We demonstrate that small-angle X-ray scattering (SAXS) allows the size distribution and the positional alignment of highly ordered arrays to be determined with high...

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-tem...

We have observed the spatial distribution of magnetic flux in Nb, Cu/Nb and Cu/Nb/Co thin films using muon-spin rotation. In an isolated 50 nm thick Nb film we find a weak flux expulsion (Meissner effect) which becomes significantly enhanced when adding an adjacent 40 nm layer of Cu. The added Cu layer exhibits a Meissner effect (due to induced sup...

There have been a large number of papers on bismuth ferrite (BiFeO3) over the past few years, trying to exploit its room-temperature magnetoelectric multiferroic properties. Although these are attractive, BiFeO3 is not the ideal multiferroic due to weak magnetization and the difficulty in limiting leakage currents. Thus there is an ongoing search f...

An extended family of iron oxalates, featuring two-dimensional, geometrically frustrated networks of kagome or trian-gular magnetic motifs with a range of structurally distinct inter-layer bridges, is obtained through the selective engi-neering of chemical bonding. The present work succinctly illustrates how both crystallographic and magnetic dimen...

We demonstrate that the critical current of superconducting Nb/Ni multilayers can be continuously tuned by up to a factor of three during magnetization reversal of the Nifilms under an applied in-plane magnetic field. Our observations are in reasonably good agreement with a model of vortex pinning by Bloch domain walls that proliferate in the sampl...

There have been a large number of papers on bismuth ferrite (BiFeO3) over the past few years, trying to exploit its room-temperature magnetoelectric multiferroic properties. Although these are attractive, BiFeO3 is not the ideal multiferroic, due to weak magnetization and the difficulty in limiting leakage currents. Thus there is an ongoing search...

Significance Interfaces are critical in quantum physics, and therefore we must explore the potential for designer hybrid materials that profit from promising combinatory effects. In particular, the fine-tuning of spin polarization at metallo–organic interfaces opens a realm of possibilities, from the direct applications in molecular spintronics and...

Superconducting spintronics in hybrid superconductor/ferromagnet (S-F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, $T_c$, of the S-layer can be controlled by the relative orientation of two (or more) F-layers. Here, w...

For spintronic devices excited by a sudden magnetic or optical perturbation, the torque acting on the magnetization plays a key role in its precession and damping. However the torque itself can be a dynamical quantity via the time dependent anisotropies of the system. A challenging problem for applications is then to disentangle the relative import...

Magnetic nanoparticles (MnPs) are relevant to a wide range of applications including high density information storage and magnetic resonance imaging to name but a few. Among the materials available to prepare MnPs, FePt is attracting growing attention. However, to harvest the strongest magnetic properties of FePt MnPs, a thermal annealing is often...

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case switching is driven by changes in the magnetostatic energy du...

In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux-the diamagnetic Meissner effect-from the interior of superconducting lead. This discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a...

Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we cre...

Superconducting spintronics has emerged in the last decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by p...

The study of thin film magnetic systems that are structured on the nanoscale is an area of intense interest. Small-angle neutron scattering is an extremely powerful probe of nanomagnetism in the bulk, but in thin-film systems the experiments are challenging due both to the small scattering volume available and also to scattering from other sources...

Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled non-collinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferro...

Scanning Hall probe microscopy has been used to search for spontaneous fields at the well-defined edges of large mesoscopic disks, etched into the ab surface of very high quality single crystal Sr 2 RuO 4 . Such fields are predicted to exist at locations of broken translational symmetry as a consequence of the proposed two-component spin-triplet ch...

Scanning Hall probe microscopy has been used to search for spontaneous fields at the well-defined edges of large mesoscopic disks, etched into the ab surface of very high quality single crystal Sr 2 RuO 4 . Such fields are predicted to exist at locations of broken translational symmetry as a consequence of the proposed two-component spin-triplet ch...

Scanning Hall probe microscopy has been used to search for spontaneous fields at the well-defined edges of large mesoscopic disks, etched into the ab surface of very high quality single crystal Sr 2 RuO 4 . Such fields are predicted to exist at locations of broken translational symmetry as a consequence of the proposed two-component spin-triplet ch...

Muon-spin rotation has been used to probe vortex state in Sr$_2$RuO$_4$. At moderate fields and temperatures a lattice of triangular symmetry is observed, crossing over to a lattice of square symmetry with increasing field and temperature. At lower fields it is found that there are large regions of the sample that are completely free from vortices...

Muon-spin rotation has been used to observe directly the spatial variation of the magnetic flux density near the ferromagnetic-superconducting interface in a permalloy-niobium trilayer. Above the superconducting transition temperature Tc the profile of the induced magnetic flux density within the niobium layer has been determined. Below Tc there is...

Muon-spin rotation measurements, performed on the mixed state of the classic anisotropic superconductor Bi_{2.15}Sr_{1.85}CaCu_{2}O_{8+δ}, obtain quantities directly related to two- and three-body correlations of vortices in space. A novel phase diagram emerges from such local probe measurements of the bulk, revealing an unusual glassy state at int...

Muon-spin rotation measurements, performed on the mixed state of the classic anisotropic superconductor Bi2.15Sr1.85CaCu2O8+δ, obtain quantities directly related to two- and three-body correlations of vortices in space. A novel phase diagram emerges from such local probe measurements of the bulk, revealing an unusual glassy state at intermediate fi...

Scanning Hall probe microscopy has been used to study vortex structures in very-high-quality single crystals of the unconventional superconductor Sr2RuO4 (Tc≅1.5 K). In none of our samples do we find credible evidence for the existence of the spontaneous fields or chiral domains predicted for the expected time-reversal symmetry-breaking order param...

Perpendicular magnetic recording media based on CoCrPt-SiO(x), thin-films have been studied with polarised small angle neutron scattering (SANSPOL) to probe the local (sub-10 nm) granular structure and its magnetisation distribution. SANSPOL is sensitive to the direction of the magnetisation vector in the grains over a size range of 1 100 nm. This...

Aqueous low temperature pathways are attractive for synthesizing colloidal iron–platinum nanoparticles, which are promising candidates for applications ranging from data storage to biomedicine. Identifying the inhomogeneous composition of the products as the major hurdle of such syntheses, we present and discuss data including synthesis time, iron...

Small-angle neutron studies have been performed on samples of continuous inhomogeneous magnetic recording media. This has allowed the local magnetic structure to be probed at a sub-nanometre resolution, revealing some interesting information about the size and shape of the magnetic grains and their relation to the physical grains. How to Cite This...

Polarized small-angle neutron scattering has been used to measure the magnetic structure of a CoCrPt – SiO _x thin-film data storage layer, contained within a writable perpendicular recording media, at granular (<10 nm ) length scales. The magnetic contribution to the scattering is measured as the magnetization is reversed by an externa...

Iron-platinum alloy nanoparticles (FePt NPs) are extremely promising candidates for the next generation of contrast agents for magnetic resonance (MR) diagnostic imaging and MR-guided interventions, including hyperthermic ablation of solid cancers. FePt has high Curie temperature, saturation magnetic moment, magneto-crystalline anisotropy, and chem...

The superfluid density ρs in underdoped (Tc≃23 K), optimally doped (Tc≃35 K), and overdoped (Tc≃29 K) single-crystalline (Bi,Pb)2(Sr,La)2CuO6+δ samples was studied by means of muon spin rotation (μSR). By combining the μSR data with the results of angle-resolved photoemission spectroscopy measurements on similar samples [T. Kondo et al., Nature (Lo...

The superfluid density \rho_s in underdoped (T_c\simeq23K), optimally doped (T_c\simeq35K) and overdoped (T_c\simeq29K) single crystalline (BiPb)_2(SrLa)_2CuO_{6+\delta} samples was studied by means of muon-spin rotation (\muSR). By combining the \muSR data with the results of ARPES measurements on similar samples [Nature 457, 296 (2009)] good self...

Polarized small-angle neutron scattering has been used to measure the local magnetic structure of writable thin-film perpendicular media with a granular CoCrPt-SiO _x recording layer. By exploiting the cross terms between the nuclear and magnetic scattering, we are able to probe simultaneously both the grain structure and the subgranular...

Spin-polarized neutron-reflectivity measurements have been performed on superconducting Pb films. For fields applied in the plane of the film we are able to determine the magnetic field profile across the thickness of the film in the superconducting state. This allows the direct observation of distinct mesoscopic ground states in this geometry, fro...

We present an approach that allows the reconstruction of the zero-field magnetic penetration depth lambda0 based on the results of muon-spin-rotation (muSR) experiments conducted in a superconductor in the vortex state. It was successfully applied to describe the muSR experiments in optimally doped (BiPb)2(SrLa)2CuO6+delta (OP Bi2201). We found tha...

We present a muon-spin rotation study of the optimally doped cuprate superconductor (BiPb)2(SrLa)2 CuO6+δ . The measured magnetic field dependence of the in-plane magnetic penetration λ ab suggests superconductivity with a dominant d-wave order parameter. The comparison of the temperature dependence of λ ab with calculations, assuming the angular...

The in-plane magnetic penetration depth lambda ab in optimally doped (BiPb)2(SrLa)2CuO6+delta (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda ab(-2)(T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming...

We report on measurements of the in-plane magnetic penetration \lambda_{ab} in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+\delta (OP Bi2201) by means of muon-spin rotation (\muSR). We show that in unconventional $d-$wave superconductors (like OP Bi2201), \muSR experiments conducted in various magnetic fields allow to evaluate...

We describe how the ferromagnetic spin echo technique may be used to make a direct and microscopic measurement of the motion of flux lines in a superconductor under the influence of an applied current. Our experimental results agree well with theory and indicate that both the value and distribution of velocities of flux lines may be measured.

We demonstrate that the thermal response of uncalibrated atomic force microscope cantilevers can be used to extract the density and the viscosity of viscous liquids with good accuracy. Temperature dependent thermal noise spectra were measured in water/poly(ethylene glycol) mixtures. Empirical parameters characteristic of the resonance behavior of t...

In principle, a complex assembly of strongly interacting electrons can self-organize into a wide variety of collective states, but relatively few such states have been identified in practice. We report that, in the close vicinity of a metamagnetic quantum critical point, high-purity strontium ruthenate Sr3Ru2O7 possesses a large magnetoresistive an...

We describe the recent uses of the technique of small-angle neutron diffraction to investigate flux-line structures within the bulk of superconductors in the mixed state. Despite the small signal in superconductors with a long penetration depth, useful results have been obtained in both High-Tc and heavy-fermion superconductors. These can give info...

The response of magnetically driven noncalibrated atomic force microscope cantilevers was measured in water/ethanol mixtures. Data recorded in pure water and in pure ethanol were used independently to extract parameters characteristic of the resonance behavior of the system. These parameters were then employed to determine the viscosity and density...

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical...