arXiv subjects

C. H. Marrows

Publications and source records attributed to C. H. Marrows.

At least 37 records · Page 2Linked to original sources

arXiv2013

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 ε-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 ε-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 $x$ and metallic-like for x \gtrsim 0.3. The films exhibit the positive linear magnetoresistance that is characteristic of ε-FeCoSi below their magnetic ordering temperatures $T_\mathrm{ord}$, as well as the huge anomalous Hall effect of order several μΩcm. 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 μ_{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.

cond-mat.mtrl-sci↗

arXiv2012

Hall effect characterization of electronic transition behind the metamagnetic transition in FeRh

The antiferromagnetic ground state and the metamagnetic transition to the ferromagnetic state of CsCl-ordered FeRh epilayers have been characterized using Hall and magnetoresistance measure- ments. On cooling into the ground state, the metamagnetic transition is found to coincide with a suppression in carrier density of at least an order of magnitude below the typical metallic level shown by the ferromagnetic state. The data reveal that this sub-metallic density of electron-like majority carriers in the antiferromagnetic phase are attributable to intrinsic doping from Fe/Rh substitution defects, with approximately two electrons per pair of atoms swapped. Based on these observations it is suggested that an orbital selective Mott transition, selective to the Fe 3d electrons drives the metamagnetic transition.

cond-mat.str-el↗

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

Interfacial coupling across a modified interface studied with ferromagnetic resonance

Using spin waves we directly probe the interface of an exchange biased Ni$_{80}$Fe$_{20}$/Ir$_{25}$Mn$_{75}$ film which has been modified by the presence of an Au dusting layer. Combining this experimental data with a discretised simulation model, parameters relating to interface exchange coupling and modification of interface magnetisation are determined. Exchange coupling is found to be relatively uniform as gold thickness is increased, and undergoes a sudden drop at 1.5$\textrmÅ$ of gold. Interface magnetisation decreases as a function of the gold dusting thickness. Antiparallel alignment of the ferromagnet and antiferromagnet supress the interface magnetisation compared to when they are in parallel alignment. These findings imply that the interface region has specific magnetisation states which depend on the ferromagnet orientation.

cond-mat.mes-hall↗

arXiv2011

Origin of in-plane uniaxial magnetic anisotropy in CoFeB amorphous ferromagnetic thin-films

Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechanism may describe the origin of in-plane UMAs in amorphous ferromagnetic films.

cond-mat.mtrl-sci↗

arXiv2010

Superconducting transition in Nb nanowires fabricated using focused ion beam

Making use of focused Ga-ion beam (FIB) fabrication technology, the evolution with device dimension of the low-temperature electrical properties of Nb nanowires has been examined in a regime where crossover from Josephson-like to insulating behaviour is evident. Resistance-temperature data for devices with a physical width of order 100 nm demonstrate suppression of superconductivity, leading to dissipative behaviour that is shown to be consistent with the activation of phase-slip below Tc. This study suggests that by exploiting the Ga-impurity poisoning introduced by the FIB into the periphery of the nanowire, a central superconducting phase-slip nanowire with sub-10 nm dimensions may be engineered within the core of the nanowire.

cond-mat.mes-hall↗

arXiv2010

Reply on the comment on the paper "Superconducting transition in Nb nanowires fabricated using focused ion beam"

In this communication we present our response to the recent comment of A. Engel regarding our paper on FIB- fabricated Nb nanowires (see Vol. 20 (2009) Pag. 465302). After further analysis and additional experimental evidence, we conclude that our interpretation of the experimental results in light of QPS theory is still valid when compared with the alternative proximity-based model as proposed by A. Engel.

cond-mat.mes-hall↗

arXiv2010

Spin-orbit strength driven crossover between intrinsic and extrinsic mechanisms of the anomalous Hall effect in epitaxial L1o FePd and FePt

We determine the composition of intrinsic as well as extrinsic contributions to the anomalous Hall effect (AHE) in the isoelectronic L1o FePd and FePt alloys. We show that the AHE signal in our 30 nm thick epitaxially deposited films of FePd is mainly due to extrinsic side-jump, while in the epitaxial FePt films of the same thickness and degree of order the intrinsic contribution is dominating over the extrinsic mechanisms of the AHE. We relate this crossover to the difference in spin-orbit strength of Pt and Pd atoms and suggest that this phenomenon can be used for tuning the origins of the AHE in complex alloys.

cond-mat.mes-hall↗

arXiv2008

Non-equilibrium magnetism in dual spin valves

The field of spin electronics (spintronics) was initiated by the discovery of giant magnetoresistance (GMR) for which Fert[1] and Grunberg[2] were awarded the 2007 Nobel Prize for Physics. GMR arises from differential scattering of the majority and minority spin electrons by a ferromagnet (FM) so that the resistance when the FM layers separated by non-magnetic (NM) spacers are aligned by an applied field is different to when they are antiparallel. In 1996 Slonczewski[3] and Berger[4] predicted that a large spin-polarised current could transfer spin-angular momentum and so exert a spin transfer torque (STT) sufficient to switch thin FM layers between stable magnetisation states[5] and, for even higher current densities, drive continuous precession which emits microwaves[6]. Thus, while GMR is a purely passive phenomenon which ultimately depends on the intrinsic band structure of the FM, STT adds an active element to spintronics by which the direction of the magnetisation may be manipulated. Here we show that highly non-equilibrium spin injection can modify the scattering asymmetry and, by extension, the intrinsic magnetism of a FM. This phenomenon is completely different to STT and provides a third ingredient which should further expand the range of opportunities for the application of spintronics.

cond-mat.mtrl-sci↗

arXiv2008

Investigation of artificial domains realised by local gallium focused ion beam (FIB) modification of Pt/Co/Pt trilayer structures

We present the results of experimental investigations of magnetic switching and magnetotransport in a new generation of magnetic devices containing artificially patterned domains. Our devices are realised by locally reducing the coercive field of a perpendicularly magnetised Pt (3.5 nm)/Co (0.5 nm)/Pt (1.6 nm) trilayer structure using a gallium focused ion beam (FIB). Artificial domain walls are created at the interfaces between dosed and undosed regions when an external magnetic field switches the former but not the latter. We have exploited this property to create stripe-like domains with widths down to sub-micron lengthscales, separated by undosed regions. Using the extraordinary Hall effect to monitor the local magnetisation we have investigated the reversal dynamics of these artificial domains by measuring major and minor hysteresis loops. The coercive field of regions irradiated with identical doses systematically increases as their size decreases. In the lower branch of minor loops, reversal is seen to occur via a few large Barkhausen events. Preliminary measurements of transport across domain walls reveal a positive domain wall resistance, that does not change sign from 4.2 K to 300 K.

cond-mat.mtrl-sci↗

arXiv2007

Controlled enhancement or suppression of exchange biasing using impurity $δ$-layers

The effects of inserting impurity $δ$-layers of various elements into a Co/IrMn exchange biased bilayer, at both the interface, and at given points within the IrMn layer a distance from the interface, has been investigated. Depending on the chemical species of dopant, and its position, we found that the exchange biasing can be either strongly enhanced or suppressed. We show that biasing is enhanced with a dusting of certain magnetic impurities, present at either at the interface or sufficiently far away from the Co/IrMn interface. This illustrates that the final spin structure at the Co/IrMn interface is not only governed by interface structure/roughness but is also mediated by local exchange or anisotropy variations within the bulk of the IrMn.

cond-mat.other↗

arXiv2007

Diffusive and ballistic current spin-polarization in magnetron-sputtered L1o-ordered epitaxial FePt

We report on the structural, magnetic, and electron transport properties of a L1o-ordered epitaxial iron-platinum alloy layer fabricated by magnetron-sputtering on a MgO(001) substrate. The film studied displayed a long range chemical order parameter of S~0.90, and hence has a very strong perpendicular magnetic anisotropy. In the diffusive electron transport regime, for temperatures ranging from 2 K to 258 K, we found hysteresis in the magnetoresistance mainly due to electron scattering from magnetic domain walls. At 2 K, we observed an overall domain wall magnetoresistance of about 0.5 %. By evaluating the spin current asymmetry alpha = sigma_up / sigma_down, we were able to estimate the diffusive spin current polarization. At all temperatures ranging from 2 K to 258 K, we found a diffusive spin current polarization of > 80%. To study the ballistic transport regime, we have performed point-contact Andreev-reflection measurements at 4.2 K. We obtained a value for the ballistic current spin polarization of ~42% (which compares very well with that of a polycrystalline thin film of elemental Fe). We attribute the discrepancy to a difference in the characteristic scattering times for oppositely spin-polarized electrons, such scattering times influencing the diffusive but not the ballistic current spin polarization.

cond-mat.mtrl-sci↗

arXiv2006

Angular dependence of domain wall resistivity in artificial magnetic domain structures

We exploit the ability to precisely control the magnetic domain structure of perpendicularly magnetized Pt/Co/Pt trilayers to fabricate artificial domain wall arrays and study their transport properties. The scaling behaviour of this model system confirms the intrinsic domain wall origin of the magnetoresistance, and systematic studies using domains patterned at various angles to the current flow are excellently described by an angular-dependent resistivity tensor containing perpendicular and parallel domain wall resistivities. We find that the latter are fully consistent with Levy-Zhang theory, which allows us to estimate the ratio of minority to majority spin carrier resistivities, rho-down/rho-up~5.5, in good agreement with thin film band structure calculations.

cond-mat.mtrl-sci↗

arXiv2005

Nb/CuNi Sandwiches as Superconducting Spin-Valve Core Structures

We have investigated CuNi/Nb/CuNi trilayers, as have been recently used as the core structure of a spin-valve like device [J. Y. Gu et al., Phys. Rev. Lett. 89, 267001 (2002)] to study the effect of magnetic configurations of the CuNi layers on the critical temperature, Tc, of the superconducting Nb. After reproducing a Tc shift of a few mK, we have gone on to explore the performance limits of the structure. The results showed the Tc shift we found to be quite close to the basic limits of this particular materials system. The ratio between the thickness and the coherence length of the superconductor and the interfacial transparency were the main features limiting the Tc shift.

cond-mat.supr-con↗

arXiv2003

Anisotropic magnetoresistance in a 2DEG in a quasi-random magnetic field

We present magnetotransport results for a 2D electron gas (2DEG) subject to the quasi-random magnetic field produced by randomly positioned sub-micron Co dots deposited onto the surface of a GaAs/AlGaAs heterostructure. We observe strong local and non-local anisotropic magnetoresistance for external magnetic fields in the plane of the 2DEG. Monte-Carlo calculations confirm that this is due to the changing topology of the quasi-random magnetic field in which electrons are guided predominantly along contours of zero magnetic field.

cond-mat.mes-hall↗

arXiv2000

Impurity Scattering from $δ$-layers in Giant Magnetoresistance Systems

The properties of the archetypal Co/Cu giant magnetoresistance (GMR) spin-valve structure have been modified by the insertion of very thin (sub-monolayer) $δ$-layers of various elements at different points within the Co layers, and at the Co/Cu interface. Different effects are observed depending on the nature of the impurity, its position within the periodic table, and its location within the spin-valve. The GMR can be strongly enhanced or suppressed for various specific combinations of these parameters, giving insight into the microscopic mechanisms giving rise to the GMR.

cond-mat↗

arXiv1999

Nanoconstriction Microscopy of the Giant Magnetoresistance in Cobalt/Copper Spin Valves

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

cond-mat.mes-hall↗