Koshi Takenaka

Negative thermal expansion in Ge-free antiperovskite manganese nitrides: Tin-doping effect

Giant negative thermal expansion (NTE) recently discovered in antiperovskite manganese nitrides Mn3AN (A=Zn,Ga, etc.) is achieved by doping Ge on A as “relaxant” of the sharp volume change at the magnetic transition. To promote wider applications, we synthesized NTE antiperovskites without expensive Ge. We discovered that Sn broadens the volume change, though less effective than Ge. Simultaneous substitution of Sn for A and C for N expands the operation-temperature window of NTE almost as broad as that of the Ge-doped counterpart. We discuss relation between the broadening and the phase instability caused by Ge or Sn.

Magnetostriction in Mn3CuN

Discovery of large magnetostriction in an antiperovskite Mn3CuN is reported. Mn3CuN undergoes the first-order transition from high-temperature (high-T) paramagnetic to low-temperature ferromagnetic (FM) phase at the Curie temperature TC=143K, accompanied by cubic-to-tetragonal structural deformation. In the tetragonally distorted FM phase, Mn3CuN, even in a polycrystalline form, expands 0.2% and shrinks 0.1% in the direction parallel and perpendicular to the external field of 90kOe, respectively. This magnetostriction is possibly due to rearrangement of thermoelastic martensite variants by magnetic field, similar to FM Heusler alloys such as Ni2MnGa.

Zero thermal expansion in a pure-form antiperovskite manganese nitride

A zero thermal expansion material in a pure form is fabricated using an antiperovskite manganese nitride. The isotropic zero thermal expansion is achieved by optimizing the heat treatment and the chemical composition. The present study suggests that the heat treatment affects the thermal expansion mainly via the nitrogen content of the material. The obtained materials exhibit a low expansion of |α|<0.5×10−6 K−1 (α is the coefficient of linear thermal expansion) over a broad temperature range, which includes room temperature. They are desirable for many fields of industry as reliable, mechanically hard, and low-cost zero thermal expansion materials.

Giant barocaloric effect enhanced by the frustration of the antiferromagnetic phase in Mn3GaN

First-order phase transitions are accompanied by a latent heat. Consequently, manipulating them by means of an external field causes a caloric effect. Although transitions from antiferromagnetic to paramagnetic states are not controlled by a magnetic field, a large barocaloric effect is expected when strong cross-correlations between the volume and magnetic order occur. Here we examine how geometric frustration in itinerant antiferromagnetic compounds can enhance the barocaloric effect. We study the thermodynamic behaviour of the frustrated antiferromagnet Mn3GaN, and report an entropy change of 22.3 J kg(-1) K(-1) that is concomitant with a hydrostatic pressure change of 139 MPa. Furthermore, the calculated value of the adiabatic temperature change reaches 5 K by depressurization of 93 MPa. The giant barocaloric effect in Mn3GaN is caused by a frustration-driven enhancement of the ratio of volume change against the pressure coefficient of the Néel temperature. This mechanism for enhancing the barocaloric effect can form the basis for a new class of materials for solid-state refrigerants.

Universality of the Mott–Ioffe–Regel limit in metals

The absence of resistivity saturation in many strongly correlated metals, including the high-temperature superconductors, is critically examined from the viewpoint of optical conductivity measurements. Coherent quasiparticle conductivity, in the form of a Drude peak centred at zero frequency, is found to disappear as the mean free path (at ω = 0) becomes comparable with the interatomic spacing. This basic loss of coherence at the so-called Mott–Ioffe–Regel (MIR) limit suggests that the universality of the MIR criterion is preserved even in the presence of strong electron correlations. We argue that the shedding of spectral weight at low frequencies, induced by strong correlation effects, is the primary origin of the extended positive slope of the resistivity to high temperatures observed in all so-called ‘bad metals’. Moreover, in common with those metals which exhibit resistivity saturation at high temperatures, the scattering rate itself, as extracted from optical spectra, saturates at a value consistent with the MIR limit. We consider possible implications that this ceiling in the scattering rate may have for our understanding of transport within a wide variety of bad metals and suggest a better method for analysing their optical response.

Extremely low temperature coefficient of resistance in antiperovskite Mn3Ag1−xCuxN

Electrical resistivity is systematically investigated in Mn3AgN and related compounds with an antiperovskite structure. Despite its overall metallic character, Mn3AgN features a broad maximum in the temperature-resistivity curve in the paramagnetic state and the temperature coefficient of resistance (TCR) is negative at higher temperatures. The resistivity-peak temperature was tuned to just room temperature by the partial substitution of Cu for Ag, and a TCR as low as 10−6 K−1 was achieved over a wide temperature window including room temperature. These peculiar behaviors are possibly due to collapse of coherent quasiparticle states by strong magnetic scattering.

Magnetovolume effects in manganese nitrides with antiperovskite structure

Abstract Magnetostructural correlations in antiperovskite manganese nitrides were investigated systematically for stoichiometric and solid solution Mn3Cu1−xAxN (A = Co, Ni, Zn, Ga, Ge, Rh, Pd, Ag, In, Sn or Sb). This class of nitrides is attracting great attention because of their giant negative thermal expansion, which is achieved by doping Ge or Sn into the A site as a relaxant of the sharp volume contraction on heating (spontaneous volume magnetostriction ωs) because of the magnetovolume effects. The physical background of large ωs and mechanism of how the volume contraction becomes gradual with temperature are central concerns for the physics and applications of these nitrides. An entire dataset of thermal expansion, crystal structure and magnetization demonstrates that the cubic triangular antiferromagnetic state is crucial for large ωs. The intimate relationship between ωs and the magnetic structure is discussed in terms of geometrical frustration related to the Mn6N octahedron and magnetic stress concept. The results presented herein also show that ωs depends on the number of d electrons in the A atom, suggesting the important role of the d orbitals of the A atom. Not all the dopants in the A site, but the elements that disturb the cubic triangular antiferromagnetic state, are effective in broadening the volume change. This fact suggests that instability neighboring the phase boundary is related to the broadening. The relation between the gradual volume change and the local structure anomaly is suggested by recent microprobe studies.

Optical reflectivity spectra measured on cleaved surfaces of La1-xSrxMnO3 : Evidence against extremely small Drude weight

Optical reflectivity spectra were measured on cleaved surfaces of La 1- x Sr x MnO 3 single crystals (0≤ x ≤0.30). The optical conductivity σ(ω) shows, keeping single-component nature, an incoheren...

In situ growth of superconducting NdFeAs(O,F) thin films by molecular beam epitaxy

Superconducting NdFeAs(O,F) thin films were grown on GaAs substrates by molecular beam epitaxy. Films grown with a sufficiently long growth time exhibited a clear superconducting transition with an onset temperature up to 48 K and zero resistance temperature up to 42 K without the need of an ex situ annealing process. Electron probe microanalysis and Hall coefficient measurements indicated that the superconducting films are doped with fluorine, and depth-profile analysis by Auger electron spectroscopy revealed the formation of a NdOF layer near the surface, which is probably connected with the fluorine doping.

Giant negative thermal expansion in antiperovskite manganese nitrides

Giant negative thermal expansion (NTE), over α = −30 × 10−6 K−1 (α: coefficient of linear thermal expansion), at room temperature can be achieved in Mn3ZnN-based antiperovskite manganese nitrides by simultaneous substitution of C and B for N as well as Sn for Zn. The developed NTE nitrides show larger negative α, although the width of the operating-temperature window is comparable to that of Mn3CuN-based materials developed to date. Such a large, isotropic, and high-stiffness NTE material can compensate for the large positive thermal expansion of, for example, even aluminum or plastic materials.