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ABSTRACT: Microcalorimetry has proven to be a versatile tool to investigate first order magnetic phase transitions as it can be used in different experimental modes to separate the latent heat from heat capacity. However, the methodology fails if the latent heat contribution is below instrumental resolution of 10 nJ. If the nucleation size of the new phase is much less than 100 μm, the typical size of the fragment measured, the latent heat could appear to be too distributed in temperature or magnetic field to be detected. Here, we show that for certain classes of magnetic transition, our microcalorimetry technique can be extended to enable an estimate of the latent heat to be obtained from a combination of heat capacity and magnetic measurements. This technique is best suited for material systems with weakly first order phase transitions, or highly distributed due to inhomogeneity.
The Review of scientific instruments 03/2012; 83(3):033901. · 1.52 Impact Factor
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ABSTRACT: Magnetic refrigeration at around ambient temperatures has become of considerable technical and commercial interest over the last few years. It depends upon the magnetocaloric effect, and suitable working materials are those that undergo a phase transition which can be driven by a modest magnetic field. We focus here on one attractive family of intermetallics based on the compound La(Fe,Si)13. Its metamagnetic phase transition is accompanied by a peak in the heat capacity that can be several times larger than the background and, for certain compositions and fields, also a well-defined first order transition with associated latent heat. It seems that some key aspects drawn from the bestiary of magnetism are particularly helpful in optimising magnetocaloric performance, namely itinerant electron magnetism and spin fluctuations. They appear to assist in maximising the entropy change at the phase transition without incurring the penalty of unduly large hysteresis. Many of these features are shared by other groups of compounds that have attractive performance.
Philosophical Magazine 01/2012; 92(1-3):292-303. · 1.51 Impact Factor
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ABSTRACT: We have made a detailed study of the microstructure of (Tl0.78Bi0.22) (Sr1.6Ba0.4)Ca2Cu3Ox/Ag (Tl(Bi)-1223/Ag) powder-in-tube superconducting tapes using transmission electron microscopy. Melt processing led to a markedly different microstructure compared to that obtained from more conventional primarily solid state reaction routes. Clusters of well connected grains were observed resulting in a denser and more uniform microstructure. This is consistent with an improvement in the magnetic field dependence of the critical current density (Jc) for the melt processed tapes with reduced degradation of Jc in field and an increase in the high-field Jc plateau. In addition, the magnetic screening currents flow on a scale of the same order of magnitude as the grain clusters. The Ag/Tl(Bi)-1223 interface was also studied and Tl(Bi)-1223 was shown to align preferentially on a flat Ag surface with the (001) Tl-1223 planes parallel to the interface. The morphology of the Ag surface is often too rough to maintain the alignment along the interface and, as a result, no long-range texture is observed.
Philosophical Magazine A. 12/2010; July 1(1998):57-84.
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ABSTRACT: Here we study the influence of sample preparation on the magnetocaloric properties of CoMnSi. Slow cooling from the high temperature hexagonal phase of the melt to the room temperature orthorhombic phase encourages the formation of a homogeneous material with large entropy changes when the system undergoes a coincident first order structural and (meta) magnetic transition. Samples that were quenched directly after annealing show a compressed a axis lattice parameter. Hall probe imaging indicates that the quenched sample has spatially inhomogeneous magnetic properties, which we attribute to strain because within error neither x-ray diffraction nor energy dispersive x-ray analysis indicates a second compositional phase. Calorimetric methods and global magnetization are used to examine the entropy changes of the pure and mixed magnetic phase compounds and we make a direct comparison of these materials in terms of their refrigerant capacity.
Journal of Physics D Applied Physics 04/2010; 43(19):195001. · 2.54 Impact Factor
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ABSTRACT: Here we study the calorimetric and magnetic behaviour of melt-spun LaFe11.6Si1.4, a potential magnetic refrigerant material system that exhibits the rare combination of a large entropy change and low thermal and magnetic field hysteresis. We are able to separate the calorimetric contribution from latent heat and changes in equilibrium heat capacity explicitly by using two separate calorimetric probes. The heat capacity of this sample exhibits significant changes of the order of 500–1000 J K−1 kg−1 in response to magnetic field that results in large changes in entropy. The different contributions to entropy change from latent heat and heat capacity are shown to evolve as the material is field driven through its itinerant metamagnetic transition. We demonstrate explicitly that in the melt-spun sample studied here, the majority of the total entropy change comes from the equilibrium change of heat capacity.
Journal of Physics D Applied Physics 03/2010; 43(13):132001. · 2.54 Impact Factor
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ABSTRACT: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7 T at 2 K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter, framework which would be consistent with the recent theoretical model where the height of the P in the Fe–P–Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
Superconductor Science and Technology 12/2009; 23(2):022001. · 2.66 Impact Factor
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ABSTRACT: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains. Comment: Published version
08/2009;
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J D Moore,
L. F. Cohen,
Y. Yeshurun, A. D. Caplin,
K Morrison,
K. A. Yates,
C.M. McGilvery,
J. M. Perkins,
D.W. McComb,
C. Trautmann,
Z. A. Ren,
J Yang,
W Lu,
X. L. Dong,
Z. X. Zhao
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ABSTRACT: Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions. Comment: 10 pages, 4 figures
07/2009;
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ABSTRACT: 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 Tcrit 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 Tcrit. 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.
Physical Review B 04/2009; 79(13). · 3.69 Impact Factor
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ABSTRACT: CoMnSi and CoMnSi1−xGex have recently been shown to exhibit negative magnetocaloric properties at a first-order magnetic-field-driven metamagnetic phase transition between a low-temperature antiferromagnetic phase and a high-temperature ferromagnetic phase. We find that doping with Fe changes the magnetocaloric properties drastically by (a) lowering the metamagnetic transition temperature and field, (b) yielding a higher magnetocaloric effect at lower fields than in CoMnSi, and (c) producing an unusual downturn in the entropy change in the highest fields measured. We also find that fragmentation of the sample lowers the transition field reflecting the role of strain in these samples but makes no significant change to the hysteresis loss or the entropy change.
Physical Review B 10/2008; 78(13). · 3.69 Impact Factor
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ABSTRACT: A new method of utilizing a commercial silicon nitride membrane calorimeter to measure the latent heat at a first order phase transition is presented. The method is a direct measurement of the thermoelectric voltage jump induced by the latent heat, in a thermally isolated system ideally suited for single crystal and small microgram samples. We show that when combined with the ac calorimetry technique previously developed, the resultant thermal measurement capabilities are extremely powerful. We demonstrate the applicability of the combined method with measurements on a 100 microm size fragment of CoMnSi exhibiting a sizable magnetocaloric effect near room temperature, and obtain good agreement with previously reported values on bulk samples.
Review of Scientific Instruments 08/2008; 79(7):074901. · 1.37 Impact Factor
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J D Moore,
K Morrison,
K. A. Yates, A. D. Caplin,
Y. Yeshurun,
L. F. Cohen,
J. M. Perkins,
C.M. McGilvery,
D.W. McComb,
Z. A. Ren,
J Yang,
W Lu,
X. L. Dong,
Z. X. Zhao
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ABSTRACT: Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry. Comment: 11 pages, 4 figures
06/2008;
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ABSTRACT: Here we use highly sensitive magnetisation measurements performed using a Hall probe sensor on single crystals of highly anisotropic high temperature superconductors $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ to study the dynamic interactions between the two species of vortices that exist in such superconductors. We observe a remarkable and clearly delineated high temperature regime that mirrors the underlying vortex phase diagram. Our results map out the parameter space over which these dynamic interaction processes can be used to create vortex ratchets, pumps and other fluxonic devices.
05/2008;
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ABSTRACT: The commonly-used NiW substrate for RaBiTS-process 2G conductors is weakly-ferromagnetic, so that the interaction between the superconductor and the magnetic material is important, particularly for AC applications. We have studied the AC losses in a model system of a high-current 2G YBCO tape that has been fabricated on a non-ferromagnetic substrate (Hastelloy), with and without adjacent weakly-ferromagnetic Ni-5at%W tape(s). This approach allows us to explore fully the interactions between substrate and superconductor. The sample architectures examined were YBCO, NiW-YBCO, NiW-YBCO-NiW, YBCO-NiW and NiW in magnetic fields applied perpendicular, parallel and at 45° to the tape normal. The power losses were measured in applied ac-magnetic fields of up to 100 mT rms at close to power line frequencies and at liquid nitrogen temperature. In addition, complementary magnetisation measurements were made on some architectures. Below the penetration field of the superconducting tape, the overall AC losses in the perpendicular field orientation are reduced significantly by a single adjacent NiW layer, with a further slight decrease with the addition of a second NiW layer. Above the penetration field the losses are not altered greatly. At very low fields the dominant loss mechanism appears to be the ferromagnetic hysteresis loss.
Journal of Physics Conference Series 03/2008; 97(1):012078.
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ABSTRACT: In this study we compare the critical current density, the irreversibility line and the upper critical field of four MgB2 polycrystalline samples, which are either undoped or have 5% carbon or 5% carbon plus either 1% aluminium or 2% zirconium. We discuss how care must be taken for the extraction of the irreversibility line in such samples. We also show how ac susceptibility and Hall probe imaging can be used to examine whether the samples remain fully connected to the highest available fields. Compared to simple 5% carbon doping we find that co-doping provides modest improvement in the pinning properties at intermediate fields in the carbon plus zirconium doped sample.
Superconductor Science and Technology 08/2007; 20(9):S278. · 2.66 Impact Factor
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ABSTRACT: We present a detailed study of the dynamic interactions between Josephson vortices and stacks of pancake vortices in a Bi2Sr2CaCu2O8 single crystal, obtained by measuring the effect of applied inplane magnetic field pulses on the c-axis magnetisation. The predominant interaction is to relax the system towards equilibrium. However, using a highly sensitive AC technique we are able to measure also the forces acting to drive the system away from equilibrium, consistent with the existence of dragging interactions between the Josephson and pancake systems. Such forces were discussed recently as the basis of possible flux ratchet devices. Comment: 4 pages, 4 figures
08/2005;
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ABSTRACT: We present magnetization results on a proton irradiated MgB2 single crystal that displays a peak in magnetization for the field applied parallel to the c axis. Magnetic history effects are observed, which are ascribed to the occurrence of a disorder driven phase transition close to an inflection point in the magnetization-field curve. We demonstrate that the angular and temperature dependence of this feature is significantly different to that of the lower and upper critical fields.
Phys. Rev. B. 06/2005; 71(22).
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ABSTRACT: We demonstrate that the variation of the Andreev reflection with applied magnetic field provides a direct means of comparing the properties of MgB2 with the theory for a dirty two-band superconductor, and we find good agreement between the two. The ratio of electron diffusivities in the s and p bands can be inferred from this experiment. We find that the field dependence of the density of states at the Fermi level in the p band is independent of the field direction, and that the anisotropic upper critical field is determined by the anisotropic diffusivity in the s band.
02/2005;
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ABSTRACT: We have performed a comparative study of two thin films of magnesium diboride (MgB2) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetisation curves, the structure of superconducting order parameter was obtained from point-contact spectroscopy, and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that the film with higher Hc2 also had stronger scattering in the sigma band and smaller value of the superconducting gap in this band. As the scattering in sigma band is primarily due to the defects in boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced Hc2 and better pinning properties in magnetic field. Comment: Paper presented at EUCAS'03
Superconductor Science and Technology, 17, (issue 5), S350 (2004). 09/2004; 17(5):S350.
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ABSTRACT: First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.
Physical Review Letters 05/2004; 92(14):147203. · 7.37 Impact Factor
