Magnetocaloric materials: the search for new systems

The prospect of efficient solid state refrigeration at room temperature is driving research into magnetic cooling engine design and magnetic phase transition-based refrigerants. In this Viewpoint an Ashby-style map of magnetic refrigerant properties is constructed, comparing popular materials with limits derived from an idealised first order transition model. This comparison demonstrates the potential for new magnetocaloric material systems to be established through structural control and optimisation at the atomic-, nano- and micro-scale.Comment: 6 pages, 2 figures (both in colour). Section 2.2 on cooling power is an online Appendix in published version in Scripta Materialia. Version 5: corrected typo in the Figure 2 captio

Designed Metamagnetism in CoMnGe_{1-x}P_{x}

We extend our previous theoretical study of Mn-based orthorhombic metamagnets to those that possess large nearest neighbour Mn-Mn separations (d1>3.22A). Based on our calculations, we design and synthesize a series of alloys, CoMnGe_{1-x}P_{x}, to experimentally demonstrate the validity of the model. Unusually, we predict and prepare several metamagnets from two ferromagnetic end-members, thus demonstrating a new example of how to vary crystal structure, within the Pnma symmetry group, to provide highly tunable metamagnetism

Structurally driven metamagnetism in MnP and related Pnma compounds

We investigate the structural conditions for metamagnetism in MnP and related materials using Density Functional Theory. A magnetic stability plot is constructed taking into account the two shortest Mn-Mn distances. We find that a particular Mn-Mn separation plays the dominant role in determining the change from antiferromagnetic to ferromagnetic order in such systems. We establish a good correlation between our calculations and structural and magnetic data from the literature. Based on our approach it should be possible to find new Mn-containing alloys that possess field-induced metamagnetism and associated magnetocaloric effects.Comment: 8 pages, 9 figure

Capsulata edenensis gen. et sp. nov. a New Cestode with an Unusual Type of Growth, from Limosa lapponica (L.); with Systematic Notes on the Genera Southwellia Moghe, 1925 and Malika Woodland, 1929

Description: The specimens were collected from three Bartailed Godwits shot in the winters of 1956 and 1957. The description below applies mainly to the first infestation found. There are certain differences between the infestations which are noted later. There were up to 150 individuals in each infestation, varying from immature worms consisting of a scolex and a few proglottides to mature strobilas containing some 300 segments and reaching a length of 75 mm. The maximum breadth of the strobila is 1·8 mm. The onset of maturity is accompanied by a marked increase in the breadth of the strobila and the mature proglottides are broader than lon

Magneto-elastic coupling and competing entropy changes in substituted CoMnSi metamagnets

We use neutron diffraction, magnetometry and low temperature heat capacity to probe giant magneto-elastic coupling in CoMnSi-based antiferromagnets and to establish the origin of the entropy change that occurs at the metamagnetic transition in such compounds. We find a large difference between the electronic density of states of the antiferromagnetic and high magnetisation states. The magnetic field-induced entropy change is composed of this contribution and a significant counteracting lattice component, deduced from the presence of negative magnetostriction. In calculating the electronic entropy change, we note the importance of using an accurate model of the electronic density of states, which here varies rapidly close to the Fermi energy.Comment: 11 pages, 9 figures. Figures 4 and 6 were updated in v2 of this preprint. In v3, figures 1 and 2 have been updated, while Table II and the abstract have been extended. In v4, Table I has updated with relevant neutron diffraction dat

Hydroxyapatite promotes superior adhesion and proliferation of telomerase transformed keratocytes in comparison with inert plastic skirt materials used in leading contemporary keratoprostheses

Aim: Published clinical series suggest the osteoodontokeratoprosthesis (OOKP) may have a lower extrusion rate than current synthetic keratoprostheses. The OOKP is anchored in the eye wall by autologous tooth. The authors’ aim was to compare adhesion, proliferation, and morphology for telomerase transformed keratocytes seeded on calcium hydroxyapatite (the principal mineral constituent of tooth) and materials used in the anchoring elements of commercially available synthetic keratoprostheses. Methods: Test materials were hydroxyapatite, polytetrafluoroethylene (PTFE), polyhydroxyethyl methacrylate (HEMA), and glass (control). Cell adhesion and viability were quantified at 4 hours, 24 hours, and 1 week using a calcein-AM/EthD-1 viability/cytotoxicity assay. Focal contact expression and cytoskeletal organisation were studied at 24 hours by confocal microscopy with immunoflourescent labelling. Further studies of cell morphology were performed using light and scanning electron microscopy. Results: Live cell counts were significantly greater on hydroxyapatite surfaces at each time point (p<0.04). Dead cell counts were significantly higher for PTFE at 7 days (p<0.002). Β1 integrin expression was highest on hydroxyapatite. Adhesion structures were well expressed in flat, spread out keratocytes on both HA and glass. Keratocytes tended to be thinner and spindle shaped on PTFE. The relatively few keratocytes visible on HEMA test surfaces were rounded and poorly adherent. Conclusions: Keratocyte adhesion, spreading, and viability on hydroxyapatite test surfaces is superior to that seen on PTFE and HEMA. Improving the initial cell adhesion environment in the skirt element of keratoprostheses may enhance tissue integration and reduce device failure rates

Phase diagram and magnetocaloric effect of CoMnGe_{1-x}Sn_{x} alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_{1-x}Sn_{x}, in the range x less than or equal to 0.1. Our phase diagram is a result of magnetic and calometric measurements. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04 to x=0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as 4.5 J/(K kg) in a field of 1 Tesla. However, the large thermal hysteresis in this transition (~20 K) will limit its straightforward use in a magnetocaloric device.Comment: 12 pages, 5 figure

Piezomagnetic effect as a counterpart of negative thermal expansion in magnetically frustrated Mn-based antiperovskite nitrides

The interplay of magnetic and elastic properties due to geometrical frustration in antiferromagnetic Mn-aniperovskite nitrides manifests itself in a range of phenomena such as the barocaloric (BCE), piezomagnetic (PME), magnetovolume effect (MVE), and the related negative thermal expansion (NTE). This systematic computational study uses density functional theory across a wide range of cubic antiperovskites Mn$_3$AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) in order to account for variations in the magnetic frustration based on features of the electronic structure. It focuses on PME - the linear dependence of magnetisation on applied biaxial strain. The PME in Mn$_3$SnN predicted here is an order of magnitude larger than PME modelled so far in Mn$_3$GaN,cite{lukashev2008theory} which opens the way to composite magnetoelectric effect in piezomagnetic/piezoelectric heterostructures. Moreover, the simulated PME as a zero temperature property is shown to be inversely proportional to the measured spontaneous volume expansion at a phase transition from paramagnetic (PM) to antiferromagnetic (AFM) state.cite{takenaka2014magnetovolume} On the fundamental level, such relation implies a significant suppression of spin fluctuations by the strong frustration in these systems. At the same time it can be used as a tool in search for materials with large negative thermal expansion and barocaloric effect

Epizonal I- and A-type granites and associated ash-flow tuffs, Fogo Island, northeast Newfoundland

Magmatic activity of Silurian-Devonian age is widespread in the Appalachian-Caledonian Orogen. A marked characteristic of this magmatism is the composite nature of the igneous suites, which range from peridotite to granodiorite in single plutonic bodies. Such a suite of intrusive rocks, ranging in composition from minor peridotite to granodiorite, intrudes an openly folded sequence of Silurian volcanogenic sandstones and ash-flow tuffs on Fogo Island, northeast Newfoundland. Two units, the Rogers Cove and Hare Bay microgranites, consist of fine-grained hastingsite granites with spherulitic and flow-banded textures. These rocks have an A-type granitoid affinity. A third and the most voluminous granitic unit, the Shoal Bay granite exhibits mineral parageneses similar to the microgranites, but chemical characteristics more typical of calc-alkaline, I-type granitoids. Volcanic-sedimentary sequences spatially associated with the granitic rocks include dense, welded, high-silica, hastingsite-bearing ash-flow tuffs with compositions that suggest they represent erupted equivalents of fractionated end members of the Shoal Bay granite. -from Authorspublished_or_final_versio