Nicholas A. Porter

Linear magnetoresistance in n -type silicon due to doping density fluctuations

Free electron theory tells us that resistivity is independent of magnetic field. In fact, most observations match the semiclassical prediction of a magnetoresistance that is quadratic at low fields before saturating. However, a nonsaturating linear magnetoresistance has been observed in exotic semiconductors such as silver chalcogenides, lightly-doped InSb, N-doped InAs, MnAs-GaAs composites, PrFeAsO, and epitaxial graphene. Here we report the observation of a large linear magnetoresistance in the ohmic regime in commonplace commercial n-type silicon wafer. It is well-described by a classical model of spatially fluctuating donor densities, and may be amplified by altering the aspect ratio of the sample to enhance current-jetting: increasing the width tenfold increased the magnetoresistance at 8 T from 445 % to 4707 % at 35 K. This physical picture may well offer insights into the large magnetoresistances recently observed in ntype and p-type Si in the non-ohmic regime. The conventional theory of magnetoresistance (MR) in metals and semiconductors relies upon a distribution of scattering times amongst the conducting carriers that cannot be compensated by a unique Hall field. For a material with a closed free electron Fermi surface (FS) and a principal charge carrier this leads to positive MR that is quadratic in weak magnetic fields, B, and saturates in strong magnetic fields according to:We report the observation of a large linear magnetoresistance in the ohmic regime in commonplace commercial n-type silicon wafer with a P dopant density of (1.4±0.1) ×1015 cm–3, and report measurements of it in the temperature range 30–200 K. It arises from the deformation of current paths, which causes a part of the Hall field to be detected at the voltage probes. In short, wide samples we found linear magnetoresistance as large as 4707% in an 8 tesla field at 35 K. Sample geometry effects like these are commonplace in commercial Hall sensors. However, we found that the effect persisted in long, thin samples where the macroscopic current flow should be uniform between the voltage probes: we observed a magnetoresistance of 445% under the same conditions as above. We interpret this result as arising due to spatial fluctuations in the donor density, in the spirit of the Herring model.

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.

Dependence of magnetoresistance on dopant density in phosphorous doped silicon

The transverse magnetoresistance at 280 K is reported as a function of phosphorous dopant density in silicon ranging from 1011 to 4×1018 cm−3. A reduction in magnetoresistance from 55% to 1.8% in an 8 T magnetic field occurred for dopant densities greater than 1016 cm−3. This reduction is related to the decrease in mobility due to increased scattering from ionized donors. A quadratic relationship of magnetoresistance with mobility is proposed in agreement with classical theory.

Strain-induced effects on the magnetic and electronic properties of epitaxial Fe 1 − x Co x Si thin films

We have investigated the Co-doping dependence of the structural, transport, and magnetic properties of \epsilon-FeCoSi epilayers grown by molecular beam epitaxy on silicon (111) substrates. Low energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy studies have confirmed the growth of phase-pure, defect-free \epsilon-FeCoSi epitaxial films with a surface roughness of ~1 nm. These epilayers are strained due to lattice mismatch with the substrate, deforming the cubic B20 lattice so that it becomes rhombohedral. The temperature dependence of the resistivity changes as the Co concentration is increased, being semiconducting-like for low

Changes in the layer roughness and crystallography during the annealing of CoFeB/MgO/CoFeB magnetic tunnel junctions

x

Finite size suppression of the weak field magnetoresistance of lightly phosphorous-doped silicon

and metallic-like for x \gtrsim 0.3. The films exhibit the positive linear magnetoresistance that is characteristic of \epsilon-FeCoSi below their magnetic ordering temperatures

Stochastic switching asymmetry in magnetoresistive stacks due to adjacent nanowire stray field

T_\mathrm{ord}

Structural and magnetic roughness in a Co∕Ru multilayer patterned into a large scale hexagonal array

, as well as the huge anomalous Hall effect of order several \mu\Omega cm. The ordering temperatures are higher than those observed in bulk, up to 77 K for x = 0.4. The saturation magnetic moment of the films varies as a function of Co doping, with a contribution of ~1 \mu_{B}/ Co atom for x \lesssim 0.25. When taken in combination with the carrier density derived from the ordinary Hall effect, this signifies a highly spin-polarised electron gas in the low x, semiconducting regime.

Spin-polarized tunneling with Au impurity layers

Annealing is necessary to achieve giant tunneling magnetoresistance (TMR) values in sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). In this study three complementary techniques were used to study the resulting changes in junction microstructure. The as-deposited TMR was modest, 5%, but rose to 101% after annealing at 325 °C for 1 h, corresponding to the tunneling spin polarization rising from 16% to 58%. Soft x-ray resonant magnetic scattering showed a roughening of the magnetic interfaces of the MTJ free layer, confirmed by transmission electron microscopy, which also showed the changes in the CoFeB and MgO to a lattice-matched polycrystalline form.

Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films

We report magnetoresistance measurements of lightly phosphorous doped silicon in samples that are fabricated from silicon-on-insulator wafers and so confined in one dimension. All three principal magnetic field orientations were studied at 50 and 270 K for thicknesses between 1.5−530 μm, and as thin as 150 nm at 270 K. The weak field magnetoresistance was suppressed in the orientations with the field in the sample plane when the sample is thinner than ∼1 μm at 270 K (∼10 μm at 50 K). This suppression occurred for samples that are much thicker than the carrier mean free path and the Debye screening length, and the relevant lengthscale is instead the energy relaxation length.