arXiv subjects

S. Langridge

Publications and source records attributed to S. Langridge.

27 records · Page 2Linked to original sources

arXiv2014

The malleability of uranium: manipulating the charge-density wave in epitaxial films

We report x-ray synchrotron experiments on epitaxial films of uranium, deposited on niobium and tungsten seed layers. Despite similar lattice parameters for these refractory metals, the uranium epitaxial arrangements are different and the strains propagated along the a-axis of the uranium layers are of opposite sign. At low temperatures these changes in epitaxy result in dramatic modifications to the behavior of the charge-density wave in uranium. The differences are explained with the current theory for the electron-phonon coupling in the uranium lattice. Our results emphasize the intriguing possibilities of producing epitaxial films of elements that have complex structures like the light actinides uranium to plutonium.

cond-mat.str-el↗

arXiv2014

Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Neél temperature. Combined with our demonstration of room-temperature exchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2012

Disorder strength and field-driven ground state domain formation in artificial spin ice: experiment, simulation and theory

Quenched disorder affects how non-equilibrium systems respond to driving. In the context of artificial spin ice, an athermal system comprised of geometrically frustrated classical Ising spins with a two-fold degenerate ground state, we give experimental and numerical evidence of how such disorder washes out edge effects, and provide an estimate of disorder strength in the experimental system. We prove analytically that a sequence of applied fields with fixed amplitude is unable to drive the system to its ground state from a saturated state. These results should be relevant for other systems where disorder does not change the nature of the ground state.

cond-mat.mes-hall↗

arXiv2011

Domain dynamics and fluctuations in artificial square ice at finite temperatures

The thermally-driven formation and evolution of vertex domains is studied for square artificial spin ice. A self consistent mean field theory is used to show how domains of ground state ordering form spontaneously, and how these evolve in the presence of disorder. The role of fluctuations is studied, using Monte Carlo simulations and analytical modelling. Domain wall dynamics are shown to be driven by a biasing of random fluctuations towards processes that shrink closed domains, and fluctuations within domains are shown to generate isolated small excitations, which may stabilise as the effective temperature is lowered. Domain dynamics and fluctuations are determined by interaction strengths, which are controlled by inter-element spacing. The role of interaction strength is studied via experiments and Monte Carlo simulations. Our mean field model is applicable to ferroelectric `spin' ice, and we show that features similar to that of magnetic spin ice can be expected, but with different characteristic temperatures and rates.

cond-mat.stat-mech↗

arXiv2011

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 the reduced number of nearest neighbors, band bending and the partial depopulation of the electronic states that carry the spins associated with the 4f orbitals of Eu are all contributing factors in the surface-induced change of the magnetic properties of EuO$_{1-x}$.

cond-mat.mtrl-sci↗

arXiv2008

Neutron Reflection from the Surface of Liquid 4He with and without a Layer of 3He

We report and discuss the first neutron reflection measurements from the free surface of normal and superfluid 4He and of liquid 3He-4He mixture. In case of liquid 4He the surface roughness is different above and below the lambda transition, being smoother in the superfluid state. For the superfluid, we also observe the formation of a surface layer ~200 A thick which has a subtly different neutron scattering cross-section. The results can be interpreted as an enhancement of Bose-Einstein condensate fraction close to the helium surface. We find that the addition of 3He isotopic impurities leads to the formation of Andreev levels at low temperatures.

cond-mat.other↗

arXiv2007

Neutron reflection from the surface of normal and superfluid 4He

The reflection of neutrons from a helium surface has been observed for the first time. The 4He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Angstroms thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface.

cond-mat.soft↗

arXiv2007

Profile of the U 5f magnetization in U/Fe multilayers

Recent calculations, concerning the magnetism of uranium in the U/Fe multilayer system have described the spatial dependence of the 5f polarization that might be expected. We have used the x-ray resonant magnetic reflectivity technique to obtain the profile of the induced uranium magnetic moment for selected U/Fe multilayer samples. This study extends the use of x-ray magnetic scattering for induced moment systems to the 5f actinide metals. The spatial dependence of the U magnetization shows that the predominant fraction of the polarization is present at the interfacial boundaries, decaying rapidly towards the center of the uranium layer, in good agreement with predictions.

cond-mat.other↗

arXiv2005

Observation of spin-wave characteristics in the two-dimensional ferromagnetic ordering of in-plane spins

The role of dipolar interactions and anisotropy are important to obtain, otherwise forbidden, ferromagnetic ordering at finite temperature for ions arranged in two-dimensional (2D) arrays (monolayers). Here we demonstrate that conventional low temperature magnetometry and polarized neutron scattering measurements can be performed to study ferromagnetic ordering of in-plane spins in 2D systems using a multilayer stack of non-interacting monolayers of gadolinium ions. The spontaneous magnetization is absent in the heterogenous magnetic phase observed here and the saturation value of the net magnetization was found to depend on the applied magnetic field. The net magnetization rises exponentially with lowering temperature and then reaches saturation following a $T\ln(βT)$ dependence. These findings verify predictions of the spin-wave theory of 2D in-plane spin system with ferromagnetic interaction and will initiate further theoretical development.

cond-mat.mes-hall↗