294 results found
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.
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.
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.
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'.
Lovell E, Pereira AM, Caplin AD, et al., 2015, Dynamics of the first-order metamagnetic transition in magnetocaloric La(Fe,Si)(13): Reducing hysteresis, Advanced Energy Materials, Vol: 5, ISSN: 1614-6840
The influence of dynamics and sample shape on the magnetic hysteresis in first‐order magnetocaloric metamagnetic LaFe13–xSix with x = 1.4 is studied. In solid‐state magnetic cooling, reducing magnetic and thermal hysteresis is critical for refrigeration cycle efficiency. From magnetization measurements, it is found that the fast field‐rate dependence of the hysteresis can be attributed to extrinsic heating directly related to the thickness of the sample and the thermal contact with the bath. If the field is paused partway through the transition, the subsequent relaxation is strongly dependent on shape due to both demagnetizing fields and thermal equilibration; magnetic coupling between adjacent sample fragments can also be significant. Judicious shaping of the sample can both increase the onset field of the ferromagnetic–paramagnetic (FM–PM) transition but have little effect on the PM–FM onset, suggesting a route to engineer the hysteresis width by appropriate design. In the field‐paused state, the relaxation from one phase to the other slows with increasing temperature, implying that the process is neither thermally activated or athermal; comparison with the temperature dependence of the latent heat strongly suggests that the dynamics reflect the intrinsic free energy difference between the two phases.
Morrison K, Dupas A, Mudryk Y, et al., 2013, Identifying the critical point of the weakly first-order itinerant magnet DyCo2 with complementary magnetization and calorimetric measurements, PHYSICAL REVIEW B, Vol: 87, ISSN: 2469-9950
Jiang Z, Long NJ, Staines M, et al., 2012, Transport AC Loss Measurements in Single- and Two-Layer Parallel Coated Conductor Arrays With Low Turn Numbers, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol: 22, ISSN: 1051-8223
Morrison K, Lyubina J, Moore JD, et al., 2012, Contributions to the entropy change in melt-spun LaFe11.6Si1.4 (vol 43, 132001, 2010), JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 45, ISSN: 0022-3727
Morrison K, Bratko M, Turcaud J, et al., 2012, A calorimetric method to detect a weak or distributed latent heat contribution at first order magnetic transitions, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 83, ISSN: 0034-6748
Jiang Z, Long NJ, Badcock RA, et al., 2012, AC loss measurements in pancake coils wound with 2G tapes and Roebel cable: dependence on spacing between turns/strands, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, Vol: 25, ISSN: 0953-2048
Morrison K, Lyubina J, Moore JD, et al., 2012, Magnetic refrigeration: phase transitions, itinerant magnetism and spin fluctuations, PHILOSOPHICAL MAGAZINE, Vol: 92, Pages: 292-303, ISSN: 1478-6435
Hoffmann C, Pooke D, Caplin AD, 2011, Flux Pump for HTS Magnets, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol: 21, Pages: 1628-1631, ISSN: 1051-8223
Jiang Z, Thakur KP, Staines M, et al., 2011, The dependence of AC loss characteristics on the spacing between strands in YBCO Roebel cables, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, Vol: 24, ISSN: 0953-2048
Jiang Z, Thakur KP, Staines M, et al., 2011, Transport AC Loss Characteristics of a Five Strand YBCO Roebel Cable With Magnetic Substrate, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Vol: 21, Pages: 3289-3292, ISSN: 1051-8223
Morrison K, Podgornykh SM, Shcherbakova YV, et al., 2011, Effect of Al substitution on the magnetocaloric properties of La(Fe0.88Si0.12-xAlx)(13), PHYSICAL REVIEW B, Vol: 83, ISSN: 2469-9950
Morrison K, Barcza A, Moore JD, et al., 2010, The magnetocaloric performance in pure and mixed magnetic phase CoMnSi, JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 43, ISSN: 0022-3727
Morrison K, Lyubina J, Moore JD, et al., 2010, Contributions to the entropy change in melt-spun LaFe11.6Si1.4, JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 43, ISSN: 0022-3727
Jiang Z, Badcock RA, Long NJ, et al., 2010, Transport AC loss characteristics of a nine strand YBCO Roebel cable, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, Vol: 23, ISSN: 0953-2048
Yates KA, Usman ITM, Morrison K, et al., 2010, Evidence for nodal superconductivity in Sr2ScFePO3, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, Vol: 23, ISSN: 0953-2048
Moore JD, Cohen LF, Yeshurun Y, et al., 2009, The effect of columnar defects on the pinning properties of NdFeAsO0.85 conglomerate particles, Superconductor Science & Technology, Vol: 22, ISSN: 1361-6668
Morrison K, Moore JD, Sandeman KG, et al., 2009, Capturing first- and second-order behavior in magnetocaloric CoMnSi0.92Ge0.08, PHYS REV B, Vol: 79, ISSN: 1098-0121
Here we examine the constituent components that make a magnetocaloric material attractive for application. The field-temperature phase diagram is studied and using calorimetry, the first-order and second-order components of the magnetic field-driven magneto-structural phase transition in CoMnSi0.92Ge0.08 are extracted. It is demonstrated that below 262 K the transition shows a latent heat component associated with first-order behavior when the material changes from antiferromagnetic to ferromagnetic order. Such a transition is known as a metamagnetic transition. We identify 262 K as a tricritical point and above this temperature T-crit the transition shows only continuous, second-order characteristics. Hall-probe imaging that has a five micron pixel resolution is then used to study the striking differences in the spatial evolution of the transition above and below T-crit. We demonstrate that the hysteresis of the transition is linearly related to the magnitude of the latent heat; an observation that has important implications for the use of this and other first-order systems for application as magnetic refrigerants.
Morrison K, Miyoshi Y, Moore JD, et al., 2008, Measurement of the magnetocaloric properties of CoMn(0.95)Fe(0.05)Si: Large change with Fe substitution, PHYSICAL REVIEW B, Vol: 78, ISSN: 1098-0121
Moore JD, Morrison K, Yates KA, et al., 2008, Evidence for supercurrent connectivity in conglomerate particles in NdFeAsO(1-delta), Superconductor Science & Technology, Vol: 21, ISSN: 1361-6668
Miyoshi Y, Morrison K, Moore JD, et al., 2008, Heat capacity and latent heat measurements of CoMnSi using a microcalorimeter, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 79, ISSN: 0034-6748
Tesei M, Perkins GK, Caplin AD, et al., 2008, Mapping the dynamic interactions between vortex species in highly anisotropic superconductors, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, Vol: 21, ISSN: 0953-2048
Rupp SH, Caplin AD, Staines MP, 2008, Magnetisation ac-loss measurements on YBa2Cu3O7 tapes with weakly-ferromagnetic NiW substrates., 8th European Conference on Applied Superconductivity, Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Rupp SH, Caplin AD, 2008, Magnetisation ac-loss measurements on YBa2Cu3O7 tapes with weakly-ferromagnetic NiW substrates, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY
Moore JD, Perkins GK, Branford W, et al., 2007, The superconducting properties of co-doped polycrystalline MgB2, Conference on Flux Pinning and Other Related Phenomena in Type-II Superconductors, Publisher: IOP PUBLISHING LTD, Pages: S278-S281, ISSN: 0953-2048
Bugoslavsky Y, Miyoshi Y, Perkins GK, et al., 2005, Electron diffusivities in MgB2 from point contact spectroscopy, PHYSICAL REVIEW B, Vol: 72, ISSN: 2469-9950
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