J. D. Moore

Reducing extrinsic hysteresis in first-order La(Fe,Co,Si)13 magnetocaloric systems

Simultaneous magnetization and sample temperature measurements were performed as a function of magnetic field and magnetic field sweep-rates to study the influence of these conditions on the hysteresis of the magnetocaloric transition in La(Fe1−x−yCoxSiy)13 samples. The large magnetocaloric effect in the compounds that show a first-order transition cause a significant departure from isothermal conditions leading to dynamic sweep-rate dependent magnetic hysteresis. Here we show how this deleterious effect can be greatly reduced by changing the sample geometry or by use of materials which show a second-order transition only. The key signatures of nonisothermal conditions in the magnetization data are highlighted.

Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

We studied polycrystalline samples of the prototype magnetocaloric system Gd5Ge4 using scanning Hall imaging and global magnetometry. The magnetic field required to complete the phase transition that is critical for magnetocaloric application is reduced by up to 20% when small fragments, each consisting of several randomly oriented crystallites, are removed from the bulk. The removal of competing strain fields from neighboring grains when the fragment is embedded in the bulk underlies the observation. We support the results by showing local Hall imaging of phase separation in the bulk. When the bulk is ground into a fine powder, the effect vanishes.

Magnetocaloric materials with first-order phase transition: thermal and magnetic hysteresis in LaFe11.8Si1.2 and Ni2.21Mn0.77Ga1.02 (invited)

This is a report on a comparative study of thermal and magnetic hysteresis in LaFe11.8Si1.2 and Ni2.21Mn0.77Ga1.02 polycrystalline samples near their first-order phase transitions by measurements of magnetization and direct measurements of the adiabatic temperature change, ΔTad (H). Based on the obtained M(T) and M(H) dependencies, H-T diagrams were constructed for both alloys and used to explain the ΔTad (H) dependencies. For low thermal hysteresis and a sharp transition in the temperature dependence of magnetization (LaFe11.8Si1.2), the field dependencies of ΔTad (H) are governed by the phase boundary between ferromagnetic and paramagnetic states. For large thermal hysteresis and a broad transition width in temperature(Ni2.21Mn0.77Ga1.02), the phase boundary has little impact on ΔTad (H). It is also shown that, in the phase transition region, ΔTad measured upon the first application of magnetic field (virgin ΔTad (H) leg) can differ from that measured upon subsequent runs of the field. This can lead to an overestimation of the magnetocaloric effect, if only the virgin leg of ΔTad (H) is considered.

Magnetic field dependence of the maximum adiabatic temperature change

The field dependence of the maximum magnetocaloric ΔT-effect in ferromagnets with second-order phase transitions is studied by way of direct measurements. All studied materials are found to follow the formula ΔTmax=A(H+H0)2/3-AH02/3, where A and H0 are constants and H is the internal magnetic field. It is essential to distinguish the latter from external field Hext. The dependence of ΔTmax on Hext is qualitatively distinct, the difference being particularly pronounced in the low-field region. In the field range relevant to applications (0.1–2 T), ΔTmax follows a linear dependence on H2/3. It is proposed to use the slope of this dependence as a figure of merit of magnetic refrigerants.

Evidence for supercurrent connectivity in conglomerate particles in NdFeAsO1−δ

Here we report the use of 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 ~5?nm amorphous oxide around individual particles, and second phase neodymium oxide which may contribute to 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?cm?2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120??m it is clear that, on smaller scales, there is better current connectivity with a critical current density of the order of 5 ? 104?A?cm?2. We find enhanced flux creep around the second peak anomaly in the magnetization curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry.

Contributions to the entropy change in melt-spun LaFe11.6Si1.4

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.

Heat capacity and latent heat measurements of CoMnSi using a microcalorimeter

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.

Superconducting critical fields and anisotropy of a MgB2 single crystal

Using a double axis vibrating sample magnetometer, we have made detailed magnetic measurements of the lower critical field Hc1 for fields parallel to the two crystallographic directions of MgB2 single crystals. Additionally, using a novel Hall probe magnetometer we have measured high precision magnetization loops, from which we directly determine the upper critical field Hc2 for both field orientations. Our results suggest that Hc1 is much larger than most previous estimates and that consequently the Ginzburg–Landau parameter κ is very low (less than 5). We find the anisotropy parameter γ ~ 2, independent of temperature over the measured range.

The effect of columnar defects on the pinning properties of NdFeAsO0.85 conglomerate particles

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.

Magnetic refrigeration: phase transitions, itinerant magnetism and spin fluctuations

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.