Publications
450 results found
Boldrin D, Mihai A, Zou B, et al., 2018, Giant Piezomagnetism in Mn3NiN., ACS Appl Mater Interfaces, Vol: 10, Pages: 18863-18868
Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn3NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn3NiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60K and magnetisation changes consistent with theory. Films grown on BaTiO3 display a striking magnetisation jump in response to uniaxial strain from the intrinsic BaTiO3 structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α=dB/dE) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over Cr2O3 predicted by theory. Overall our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.
Matsui T, Li Y, Hsu M-HM, et al., 2018, Highly Stable Plasmon Induced Hot Hole Transfer into Silicon via a SrTiO3 Passivation Interface, ADVANCED FUNCTIONAL MATERIALS, Vol: 28, ISSN: 1616-301X
Extracting plasmon‐induced hot carriers over a metal–semiconductor Schottky barrier enables photodetection below the semiconductor bandgap energy. However, interfacial carrier recombination hinders the efficiency and stability of this process, severely limiting its implementation in telecommunication. This study proposes and demonstrates the use of epitaxially grown lattice‐matched SrTiO3 for interfacial passivation of silicon‐based plasmonic Schottky devices. The devices are activated by an electrical soft‐breakdown of the interfacial SrTiO3 layer, resulting in reproducible rectified Schottky characteristics. The transition to a low resistance state of the SrTiO3 layer boosts the extraction efficiency of hot holes upon resonant plasmonic excitation, giving rise to a two orders of magnitude higher photocurrent compared to devices with a native oxide layer. Photoresponse, tunability, and barrier height studies under reverse biases as high as 100 V present superior stability with the incorporation of the SrTiO3 layer. The investigation paves the way toward plasmon‐induced photodetection for practical applications including those under challenging operating conditions.
Jeon K-R, Ciccarelli C, Ferguson AJ, et al., 2018, Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents, Nature Materials, Vol: 17, Pages: 499-503, ISSN: 1476-1122
Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin.1,2 Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet (FM) spacers, where spin transport can only occur within the FM and in conjunction with a charge current. Ferromagnetic resonance (FMR) injects a pure spin current from a precessing FM into adjacent non-magnetic materials.3,4 For spin-singlet pairing, FMR spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor (SC).5,6 Here we present FMR experiments in which spin sink layers with strong spin-orbit coupling are added to the SC. Our results show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state; although further work is required to establish the details of the spin transport process we show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent.
Tomov RI, Mitchel-Williams TB, Maher R, et al., 2018, The synergistic effect of cobalt oxide and Gd-CeO₂ dual infiltration in LSCF/CGO cathodes, Journal of Materials Chemistry A, Vol: 6, Pages: 5071-5081, ISSN: 2050-7496
La0.6Sr0.4Co0.2Fe0.8O3 d/Ce0.9Gd0.1O1.9 composite cathodes were nano-engineered via “dual” inkjetprinting infiltration of nitrate salt solutions in a single step procedure. After calcination in air at 700 Cthe cathodes were decorated with Ce0.9Gd0.1O1.9 and CoxOy nanoparticles ( 20 nm in size). The effectsof the as-created nano-decoration on the electrochemical activity and the performance stability in theintermediate temperature range (500–700 C) were investigated. The nano-engineered microstructurewas found to extend the active three-phase boundary and to promote adsorption–dissociation–surfaceexchange reactions. Electrochemical impedance tests conducted on symmetric cells showeda reduction in the polarisation resistance of between 1.5 and 7.0 times depending on temperature (500–700 C). High-resolution X-ray photoelectron spectroscopy and in situ high temperature Ramanspectroscopy were used to study aging and thermal cycling effects on the cathodes' surface chemistry.Aging tests of the infiltrated electrodes up to 100 hours in air revealed an enhanced stability of thedecorated electrodes ascribed to the suppression of SrO surface segregation. This work demonstratedthat the sequence of infiltration of both inks introduces noticeable differences in the oxygen reductionreaction.
Gartside JC, Arroo DM, Burn DM, et al., 2018, Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing, Nature Nanotechnology, Vol: 13, Pages: 53-58, ISSN: 1748-3387
Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing-a scanning probe technique-provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy 'monopole-chain' states that exhibit negative effective temperatures.
München, Matsui T, Li Y, et al., 2018, Plasmonic hot carrier detection via SrTiO<inf>3</inf> interfacial layer
Energetic hot carrier injection from metal plasmonic structure to adjacent semiconductor lies at the center of the application of non-radiative decays, photochemical reactions and energy harvesting. Plasmonic hot-electron devices, till now, have been highly focused on a Schottky barrier structures to separate the energetic carriers before the thermalization. For instance, much efforts were putted in exploring high absorption structure with enough thin metal; enough thin compared to mean free path. However, interfacial engineering of the device; exploring a new device structure, have not been fully investigated.
Mendonca AA, Jurado JF, Stuard SJ, et al., 2017, Giant magnetic-field-induced strain in Ni2MnGa-based polycrystal, Journal of Alloys and Compounds, Vol: 738, Pages: 509-514, ISSN: 0925-8388
Ferromagnetic Ni2MnGa-based alloys play an important role in technological fields, such as smart actuators, magnetic refrigeration and robotics. The possibility of obtaining large non-contact deformation induced by an external perturbation is one of its key strengths for applications. However, the search for materials with low cost, practical fabrication procedures and large signal output under small perturbing fields still poses challenges. In the present study we demonstrate that by judicial choice of substitution on the Mn site, an abrupt magnetostructural transition from a paramagnetic austenite phase to a ferromagnetic martensite one can be tuned to close to room temperature achieving large and reproducible strains. The required magnetic field to induce the strain varies from small values, as low as 0.25 T for 297.4 K and 1.6% of strain, to 8 T for 305 K and 2.6% of strain. Our findings point to encouraging possibilities for application of shape memory alloys in relatively inexpensive, scalable polycrystalline materials.
Rogdakis K, Alfert N, Srivastava A, et al., 2017, Electric power transfer in spin-pumping experiments, Applied Physics Express, Vol: 11, ISSN: 1882-0778
Spin pumping has become an established method for generating voltages using magnetic dynamics. The standard detection method of spin pumping is based on open-circuit voltage measurement. In this study, we demonstrate that it is also possible to measure the associated electric current by using macroscopic closed circuitry. Using variable load resistors connected in series to the sample, we quantified the charge currents and associated electric power dissipation with respect to the load resistance. Through basic circuit analysis, we can describe spin-pumping cells as a non-ideal voltage source or an equivalent current source with an internal resistor.
Lovell E, Bratko M, Caplin AD, et al., 2017, Nucleation and dynamics of the metamagnetic transition in magnetocaloric La(Fe,Mn,Si)(13), Journal of Physics D: Applied Physics, Vol: 50, ISSN: 0022-3727
Refrigeration cycle rates of the order of 15 Hz are desirable for efficient solid state based magnetocaloric cooling, placing an upper bound on the combined magnetic transition and the heat transfer times of the order of tens of msecs. We use microcalorimetry and magnetometry to probe the transition dynamics as a function of magnetic field sweep-rate, sample size, thermal environment, temperature and hydrostatic pressure in LaFe11.74Mn0.06Si1.20. Although second order caloric materials follow the magnetisation or demagnetisation driving field without lag, here we show that the field driven evolution of the first-order phase transition in La(Fe,Si)13-based compounds show temporal dynamics on timescales that are significantly longer than tens of msecs, associated with the thermal linkage within the sample and the linkage to the external bath. We observe that features associated with the first nucleation of the transition are field sweep rate independent, and from measurements of the latent heat we infer that the barriers to magnetisation and demagnetisation are of different magnitude. Increasing the temperature or applying hydrostatic pressure reduces the dynamic effects, suggestive of diminishing first-order character of the transition under these conditions.
Wells MP, Zou B, Doiron BG, et al., 2017, Tunable, Low Optical Loss Strontium Molybdate Thin Films for Plasmonic Applications, Advanced Optical Materials, Vol: 5, ISSN: 2195-1071
Strontium molybdate (SrMoO3) thin films are grown epitaxially on strontium titanate (SrTiO3), magnesium oxide (MgO), and lanthanum aluminate (LaAlO3) substrates by pulsed laser deposition and possess electrical resistivity as low as 100 µΩ cm at room temperature. SrMoO3 is shown to have optical losses, characterized by the product of the Drude broadening, ΓD, and the square of the plasma frequency, ωpu2, significantly lower than TiN, though generally higher than Au. Also, it is demonstrated that there is a zero-crossover wavelength of the real part of the dielectric permittivity, which is between 600 and 950 nm (2.05 and 1.31 eV), as measured by spectroscopic ellipsometry. Moreover, the epsilon near zero (ENZ) wavelength can be controlled by engineering the residual strain in the films, which arises from a strain dependence of the charge carrier concentration, as confirmed by density of states calculations. The relatively broad tunability of ENZ behavior observed in SrMoO3 demonstrates its potential suitability for transformation optics along with plasmonic applications in the visible to near infrared spectral range.
Cohen LF, 2017, Contributions to hysteresis in magnetocaloric materials, Physica Status Solidi B: Basic Solid State Physics, Vol: 255, ISSN: 0370-1972
Hysteresis is detrimental to the refrigeration cooling cycle efficiency and yet many of the magnetocaloric materials under consideration as solid state refrigerants possess this property. This article discusses some aspects related to the factors leading to hysteresis in real materials. In the absence of high quality single crystals, determining the intrinsic energy cost associated with the transformation between metastable phases is an experimental challenge. We describe a micro‐calorimetric method that provides valuable insight into intrinsic behavior with the sensitivity to measure micro‐crystallites. We show that there is no correlation between the strength of first order character and magnitude of hysteresis between material families. We review some of the extrinsic factors which contribute to the hysteresis in real materials particularly those that can be accounted for using local imaging techniques such as scanning Hall probe microscopy. We discuss a number of mechanisms by which the extrinsic hysteretic properties of a material can be modified.
Black NCG, Liu CG, Pearce R, et al., 2017, Graphene gas sensing using a non-contact microwave method, NANOTECHNOLOGY, Vol: 28, ISSN: 0957-4484
We report a non-contact CVD graphene gas sensing method that utilises a high Q microwave dielectric resonator perturbation technique. A graphene sample is coupled to the evanescent field of a dielectric resonator whereupon nitrogen dioxide (NO2), a p-doping gas, is detected by monitoring the change in the linewidth and frequency of the resonant mode. The resonant peak shape is dependent on the number of carriers in the graphene sheet. Therefore, the linewidth perturbation can be converted to a measurement of the graphene sheet resistance. To demonstrate the strength of this technique, sensor response curves for NO2 at different concentrations and temperatures are measured showing sub ppm sensitivity. This technique eliminates interactions between the trace gas and metal contacts that otherwise effect the sensor response of the graphene device.
Doiron B, Li Y, Mihai AP, et al., 2017, Comparison of the ultrafast hot electron dynamics of titanium nitride and gold for plasmonic applications, SPIE Optics + Photonics Conference on Plasmonics - Design, Materials, Fabrication, Characterization, and Applications XV, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X
With similar optical properties to gold and high thermal stability, titanium nitride continues to prove itself as a promising plasmonic material for high-temperature applications in the visible and near-infrared. In this work, we use transient pump probe differential reflection measurements to compare the electron energy decay channels in titanium nitride and gold thin films. Using an extended two temperature model to incorporate the photoexcited electrons, it is possible to separate the electron-electron and electron-phonon scattering contributions immediately following the arrival of the pump pulse. This model allows for incredibly accurate determination of the internal electronic properties using only optical measurements. As the electronic properties are key in hot electron applications, we show that titanium nitide has substantially longer electron thermalization and electron-phonon scattering times. With this, we were also able to resolve electron thermal conduction in the film using purely optical measurements.
Lovell E, Bez HN, Boldrin DC, et al., 2017, The La(Fe,Mn,Si)(13)H-z magnetic phase transition under pressure, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, Vol: 11, ISSN: 1862-6254
We study the magnetocaloric metamagnetic transition in LaFe11.74Mn0.06Si1.20 and LaFe11.76Mn0.06Si1.18H1.65 under hydrostatic pressure up to 1.2 GPa. For both compounds, hydrostatic pressure depresses the zero field critical temperature. However, in detail, pressure influences the magnetic properties in different ways in the two compounds. In the dehydrogenated case the transition broadens under pressure whereas in the hydrogenated case the transition sharpens. In both cases thermal hysteresis increases under pressure, although with different trends. These observations suggest both intrinsic and extrinsic hysteresis loss brought about by the use of hydrostatic pressure. We explore the multicaloric field-pressure cycle, demonstrating that although the gain introduced by overcoming the magnetic hysteresis loss is closely countered by the loss introduced in the pressure cycle, there are significant advantages in that the temperature range of operation can be finely tuned and extended, and the magnetocaloric transition can operate in lower absolute applied fields (<0.5 T), potentially overcoming one of the most significant bottlenecks to the commercialization of this technology.
Hayes DG, Allford CP, Smith GV, et al., 2017, Electron transport lifetimes in InSb/Al1-xInxSb quantum well 2DEGs, Semiconductor Science and Technology, Vol: 32, ISSN: 0268-1242
We report magnetotransport measurements of InSb/Al1-xInxSb modulation doped quantum well(QW) structures and the extracted transport ( ) tt and quantum (tq) lifetime of carriers at lowtemperature (<2K.) We consider conventional transport lifetimes over a range of samples withdifferent doping levels and carrier densities, and deduce different transport regimes dependent onQW state filling calculated from self-consistent Schrödinger–Poisson modelling. For sampleswhere only the lowest QW subband is occupied at electron densities of 2.13 10 ´ 11 cm−2 and2.54 10 ´ 11 cm−2 quantum lifetimes of tq » 0.107 ps, and tq » 0.103 ps are extracted fromShubnikov–de Haas oscillations below a magnetic field of 0.8 T. The extracted ratios of transportto quantum lifetimes, t t t q » 17 and t t t q » 20 are similar to values reported in other binaryQW two-dimensional electron gas systems, but are inconsistent with predictions from transportmodelling which assumes that remote ionized donors are the dominant scattering mechanism.We find the low t t t q ratio and the variation in transport mobility with carrier density cannot beexplained by reasonable levels of background impurities or well width fluctuations. Thus, thereis at least one additional scattering mechanism unaccounted for, most likely arising fromstructural defects.
Boldrin D, Boldrin P, Ruiz-Trejo E, et al., 2017, Recovery of the intrinsic thermoelectric properties of CaMn0.98Nb0.02O3 in 2-terminal geometry using Ag infiltration, Acta Materialia, Vol: 133, Pages: 68-72, ISSN: 1359-6454
Oxide based thermoelectric (TE) materials offer several advantages over currently used intermetallic alloys due to their chemical and thermal stability at high temperatures, non-toxic elements, low cost and ease of manufacture. However, incorporation of oxides into thermoelectric generators (TEGs) is hindered by factors such as the requirement for polycrystalline materials over single crystals and the large electrode/ceramic contact resistances. The latter significantly limits the performance efficiency of a working TEG. Here we report the TE properties of Ag infiltrated polycrystalline CaMn0.98Nb0.02O3 ceramics. We demonstrate that by using this route the intrinsic TE properties of this material are fully recovered in 2-terminal geometry through Ag infiltration, thereby overcoming the electrode TEG contact problem. This synthetic route provides opportunities for bridging the performance gap between the intrinsic TE and TEG device properties of oxides.
Sundar S, Salem-Sugui S, Amorim HS, et al., 2017, Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba0.75K0.25Fe2As2, Physical Review B, Vol: 95, ISSN: 1550-235X
We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with H∥c on an underdoped Ba0.75K0.25Fe2As2 pnictide single crystal, with superconducting transition temperature Tc=28 K. The second magnetization peak (SMP) has been observed at temperatures below Tc/2 and vanished at higher temperatures. The observed behavior of the SMP has been studied by measuring the magnetic field dependence of relaxation rate R(H) and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting H−T phase diagram is obtained. The critical current density (Jc) was estimated using Bean's model and found to be ∼3.4×109 A/m2 at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe2As2 superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual δl and δTc pinning models, which suggest the intrinsic nature of pinning in the compound.
Boldrin D, Cohen LF, 2017, The role of competing magnetic interactions on the abnormal expansion properties in manganese antiperovskites, Mn(3+x)A(1-x)N (A = Ni, Sn), Journal of Alloys and Compounds, Vol: 699, Pages: 887-891, ISSN: 0925-8388
The role of competing magnetic interactions in Mn-doped manganese antiperovskites Mn3+xA1−xN (A = Ni and Sn) has been explored using magnetometry and dilatometry, with particular focus on their relation to abnormal expansion properties. Rare negative magnetisation phenomena are found in zero-field-cooled conditions for both A = Sn and Ni when Mn is doped on the A site. Such behaviour is evidence of both competing magnetic sublattices and magnetocrystalline anisotropy. By comparison with other compounds with the antiferromagnetic magnetic structure, we suggest that competing interactions along with magnetovolume coupling influence the near-zero thermal expansion behaviour at intermediate temperatures below in the A = Ni family.
Yates KA, Olde Olthof LAB, Vickers ME, et al., 2017, Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra, Physical Review B, Vol: 95, ISSN: 2469-9950
As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7−Pb junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.
Bratko M, Lovell E, Caplin AD, et al., 2017, Determining the first-order character of La(Fe,Mn,Si)(13), PHYSICAL REVIEW B, Vol: 95, ISSN: 2469-9950
Definitive determination of first-order character of the magnetocaloric magnetic transition remains elusive. Here we use a microcalorimetry technique in two modes of operation to determine the contributions to entropy change from latent heat and heat capacity separately in an engineered set of La(Fe,Mn,Si)13 samples. We compare the properties extracted by this method with those determined using magnetometry and propose a model-independent parameter that would allow the degree of first-order character to be defined across different families of materials. The microcalorimetry method is sufficiently sensitive to allow observation at temperatures just above the main magnetic transition of an additional peak feature in the low field heat capacity associated with the presence of Mn in these samples. The feature is of magnetic origin but is insensitive to magnetic field, explicable in terms of inhomogeneous occupancy of Mn within the lattice resulting in antiferromagnetic ordered Mn clusters.
Kaeswurm B, Barcza A, Voegler M, et al., 2016, Behaviour of the Young's modulus at the magnetocaloric transition in La(Fe,Co,Si)(13), Journal of Alloys and Compounds, Vol: 697, Pages: 427-433, ISSN: 1873-4669
Magnetic solid state cooling applications require families of samples where the magnetic transition is cascaded across the working range of the fridge. Although magnetic properties are widely studied, information relating to the mechanical properties of such systems is less prevalent. Here we study the mechanical properties of a series of magnetocaloric La(Co,Fe,Si)13 samples where the Co content is varied to produce a range of transition temperatures. It was found that at room temperature the flexural strength decreases and the Young's modulus increases with increasing Co content. Interestingly we find a significant reduction of Young's modulus at temperature around the magnetic transition temperature. This reduction was less pronounced with increasing Co content. We associate the softening with the magnetovolume coupling known to exist in these materials.
Salem-Sugui S, Moseley D, Stuard SJ, et al., 2016, Effects of proton irradiation on flux-pinning properties of underdoped Ba(Fe0.96Co0.04)(2)As-2 pnictide superconductor, Journal of Alloys and Compounds, Vol: 694, Pages: 1371-1375, ISSN: 1873-4669
We study the effect of proton irradiation on Ba(Fe0.96Co0.04)2As2 superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy U0 of the samples prior to and after irradiation, as well as comparison of the values of U0 obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces Tc modestly, but significantly reduces U0 in both regimes: the critical current density Jc is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of Jc for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By considering a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in U0 and Jc.
Liu X, Jervis R, Maher RC, et al., 2016, 3D-Printed Structural Pseudocapacitors, Advanced Materials Technologies, Vol: 1, ISSN: 2365-709X
Direct metal laser sintering is used to create 3D hierarchical porous metallic scaffolds which are then functionalized with a co-electrodeposition of MnO2, Mn2O3, and doped conducting polymer. This approach of functionalizing metal 3D printed scaffolds thus opens new possibilities for structural energy storage devices with enhanced performance and lifetime characteristics.
Lovell E, Ghivelder L, Nicotina A, et al., 2016, Low-temperature specific heat in hydrogenated and Mn-doped La(Fe, Si)(13), Physical Review B, Vol: 94, ISSN: 2469-9950
It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric La(FeSi)13 is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of LaFexMnySiz samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced d-band electronic interactions at play. The Sommerfeld coefficient is found to be 90–210mJmol−1K−2, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development.
Waske A, Lovell E, Funk A, et al., 2016, The impact of surface morphology on the magnetovolume transition in magnetocaloric LaFe11.8Si1.2, APL Materials, Vol: 4, ISSN: 2166-532X
Home > Publishers > AIP Publishing > APL Materials > Volume 4 Number 10 > Article banner imageOAThe impact of surface morphology on the magnetovolume transition in magnetocaloric LaFe11.8Si1.2 A. Waske1,2,a), E. Lovell3, A. Funk1,2, K. Sellschopp1,2, A. Rack4, L. Giebeler1, P. F. Gostin1,b), S. Fähler1 and L. F. Cohen3+ VIEW AFFILIATIONSa) Author to whom correspondence should be addressed. Electronic mail: a.waske@ifw-dresden.deb) Current address: School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.APL Mater. 4, 106101 (2016); http://dx.doi.org/10.1063/1.4963840 Download PDF PREVIOUS ARTICLE TABLE OF CONTENTSNEXT ARTICLE facebook twitter Share this page separator email print this pageAbstractFull TextReferences (19)Cited By (1)Data & MediaMetricsRelatedFirst order magnetocaloric materials reach high entropy changes but at the same time exhibit hysteresis losses which depend on the sample’s microstructure. We use non-destructive 3D X-ray microtomography to understand the role of surface morphology for the magnetovolume transition of LaFe11.8Si1.2. The technique provides unique information on the spatial distribution of the volume change at the transition and its relationship with the surface morphology. Complementary Hall probe imaging confirms that on a morphologically complex surface minimization of strain energy dominates. Our findings sketch the way for a tailored surface morphology with low hysteresis without changing the underlying phase transition.
Carter-Gartside J, Burn DM, Cohen LF, et al., 2016, A Novel Method for the Injection and Manipulation of Magnetic Charge States in Nanostructures, Scientific Reports, Vol: 6, ISSN: 2045-2322
Realising the promise of next-generation magnetic nanotechnologies is contingent on the development of novel methods for controlling magnetic states at the nanoscale. There is currently demand for simple and flexible techniques to access exotic magnetisation states without convoluted fabrication and application processes. 360° domain walls (metastable twists in magnetisation separating two domains with parallel magnetisation) are one such state, which is currently of great interest in data storage and magnonics. Here, we demonstrate a straightforward and powerful process whereby a moving magnetic charge, provided experimentally by a magnetic force microscope tip, can write and manipulate magnetic charge states in ferromagnetic nanowires. The method is applicable to a wide range of nanowire architectures with considerable benefits over existing techniques. We confirm the method’s efficacy via the injection and spatial manipulation of 360° domain walls in Py and Co nanowires. Experimental results are supported by micromagnetic simulations of the tip-nanowire interaction.
Shautsova V, Gilbertson AM, Black N, et al., 2016, Hexagonal Boron Nitride assisted transfer and encapsulation of large area CVD graphene, Scientific Reports, Vol: 6, ISSN: 2045-2322
We report a CVD hexagonal boron nitride (hBN-) assisted transfer method that enables a polymer-impurity free transfer process and subsequent top encapsulation of large-area CVD-grown graphene. We demonstrate that the CVD hBN layer that is utilized in this transfer technique acts as a buffer layer between the graphene film and supporting polymer layer. We show that the resulting graphene layers possess lower doping concentration, and improved carrier mobilities compared to graphene films produced by conventional transfer methods onto untreated SiO2/Si, SAM-modified and hBN covered SiO2/Si substrates. Moreover, we show that the top hBN layer used in the transfer process acts as an effective top encapsulation resulting in improved stability to ambient exposure. The transfer method is applicable to other CVD-grown 2D materials on copper foils, thereby facilitating the preparation of van der Waals heterostructures with controlled doping.
Zeissler K, Chadha M, Lovell E, et al., 2016, Low temperature and high field regimes of connected kagome artificial spin ice: the role of domain wall topology, Scientific Reports, Vol: 6, ISSN: 2045-2322
Artificial spin ices are frustrated magnetic nanostructures where single domain nanobars act as macrosized spins. In connected kagome artificial spin ice arrays, reversal occurs along one-dimensional chains by propagation of ferromagnetic domain walls through Y-shaped vertices. Both the vertices and the walls are complex chiral objects with well-defined topological edge-charges. At room temperature, it is established that the topological edge-charges determine the exact switching reversal path taken. However, magnetic reversal at low temperatures has received much less attention and how these chiral objects interact at reduced temperature is unknown. In this study we use magnetic force microscopy to image the magnetic reversal process at low temperatures revealing the formation of quite remarkable high energy remanence states and a change in the dynamics of the reversal process. The implication is the breakdown of the artificial spin ice regime in these connected structures at low temperatures.
Lovell E, Bratko M, Caplin AD, et al., 2016, Magnetic relaxation dynamics driven by the first-order character of magnetocaloric La(Fe,Mn,Si)13., Philosophical Transactions of the Royal Society A: Math Phys Eng Sci, Vol: 374, ISSN: 1364-503X
Here, we study the temporal evolution of the magnetic field-driven paramagnetic to ferromagnetic transition in the La(Fe,Mn,Si)13 material family. Three compositions are chosen that show varying strengths of the first-order character of the transition, as determined by the relative magnitude of their magnetic hysteresis and temperature separation between the zero-field transition temperature Tc and the temperature Tcrit, where the transition becomes continuous. Systematic variations in the fixed field, isothermal rate of relaxation are observed as a function of temperature and as a function of the degree of first-order character. The relaxation rate is reduced in more weakly first-order compositions and is also reduced as the temperature is increased towards Tcrit At temperatures above Tcrit, the metastability of the transition vanishes along with its associated temporal dynamics.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.
Eslava GG, Parisi F, Bernardo PL, et al., 2016, Coupled magnetic and elastic properties in LaPr(CaSr)MnO manganites, Physics Letters A, Vol: 380, Pages: 3107-3110, ISSN: 0375-9601
We investigate a series of manganese oxides, the La0.225Pr0.4(Ca1−xSrx)0.375MnO3 system. The x=0 sample is a prototype compound for the study of phase separation in manganites, where ferromagnetic and charge ordered antiferromagnetic phases coexist. Replacing Ca2+ by Sr2+ gradually turns the system into a homogeneous ferromagnet. Our results show that the material structure plays a major role in the observed magnetic properties. On cooling, at temperatures below ∼100 K, a strong contraction of the lattice is followed by an increase in the magnetization. This is observed both through thermal expansion and magnetostriction measurements, providing distinct evidence of magneto-elastic coupling in these phase separated compounds.
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