Magnetic cooling could be a radically different energy solution that could replace conventional vapour compression refrigeration in the future. It is now shown that a Heusler-type magnetocaloric alloy exhibits a remarkable cooling capability due to the effect of a sharp structural transformation at a specific temperature. The finding may be of relevance beyond Heusler alloys and represents an important step towards the implementation of cooling systems based on magnetocaloric materials.

Giant magnetocaloric effect driven by structural transitions

In this paper, results of ab initio band structure calculations for A2BC Heusler compounds that have A and B sites occupied by transition metals and C by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap, the peculiar transport properties and magnetic behaviour found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both A and B atoms to the total magnetic moment. The magnitude of the total magnetic moment shows a trend consistent with the Slater–Pauling type behaviour in several classes of these compounds. The total magnetic moment also depends on the kind of C atoms although they do not directly contribute to it. In Co2 compounds, a change of the C element changes the contribution of the t2g states to the moment at the Co sites. The localized moment in these magnetic compounds resides at the B site. Other than in the classical Cu2-based Heusler compounds, the A atoms in Co2, Fe2 and Mn2-based compounds may contribute significantly to the total magnetic moment. It is shown that the inclusion of electron–electron correlation in the form of LDA + U calculations helps to understand the magnetic properties of those compounds that already exhibit a minority gap in calculations where it is neglected. Besides the large group of Co2 compounds, half-metallic ferromagnetism was here found only in such compounds that contain Mn.

https://iopscience.iop.org/article/10.1088/0022-3727/40/6/S01/pdf

Calculated electronic and magnetic properties of the half-metallic, transition metal based Heusler compounds

The Curie temperatures for the Heusler compounds Co{sub 2}TiAl, Co{sub 2}VGa, Co{sub 2}VSn, Co{sub 2}CrGa, Co{sub 2}CrAl, Co{sub 2}MnAl, Co{sub 2}MnSn, Co{sub 2}MnSi, and Co{sub 2}FeSi are determined ab initio from the electronic structure obtained with the local-density functional approximation and/or the generalized gradient approximation. Frozen spin spirals are used to model the excited states needed to evaluate the spherical approximation for the Curie temperature. The spherical approximation is found to describe the experimental Curie temperatures very well which, for the compounds selected, extend over the range from 95 to 1100 K; as a function of the valence electron count, they show an approximately linear trend which finds an explanation by our calculations.

Understanding the trend in the Curie temperatures of Co2-based Heusler compounds: Ab initio calculations

Martensitic and magnetic transformation behaviors of Ni50MnIn, Ni45Co5MnIn, and Ni42.5Co7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M
 s
 ) decreases with increasing In composition, while the Curie temperatures (T
 c
 ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M
 s
 and an increase of the T
 c
 , and the degree of the decline of M
 s
 was accelerated by magnetic transformation of the parent phase. The M
 s
 temperature change induced by the magnetic field was also confirmed. It was found that the degree of M
 s
 change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T
 c
 -M
 s
 . These behaviors can be qualitatively explained on the basis of thermodynamic considerations.

Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnIn and NiCoMnIn Metamagnetic Shape Memory Alloys

Cobalt-based full-Heusler compounds with composition Co2M'Z (where M' is a transition metal and Z is a main group element) are attracting attention due to their predicted half-metallic behaviour, a greatly desired property for spin-dependent electron transport devices. Knowledge of the basic magnetic properties of these materials, in particular in the form of thin films, is required both to fully exploit these promising materials and to understand their underlying electronic structure and establish structure–property relationships. In this topical review, we present a survey of the magnetic anisotropy, exchange and damping of Co2M'Z compounds. These properties are directly related to spin–spin and spin–orbit interactions.

/pdf/magnetic-anisotropy-exchange-and-damping-in-cobalt-based-5dw51r0qpc.pdf

Magnetic anisotropy, exchange and damping in cobalt-based full-Heusler compounds: an experimental review

The effects of chemical order on the phase stability and magnetic properties of the metamagnetic shape memory alloy Ni45Co5Mn36.7In13.3 were investigated. Alloys quenched from the B2 and L21 phase regions were found to transform to the L10 and 6M martensite phases, respectively. For alloys quenched from the B2 region the martensitic transformation starting temperature is about 80 K higher than that for alloys quenched from the L21 region. The Curie temperature of the parent phase and the magnetization of the martensite phase were both lower for the alloy quenched from the B2 region than those for the alloy quenched from L21 region.

Atomic ordering and magnetic properties in the Ni45Co5Mn36.7In13.3 metamagnetic shape memory alloy

Magnetic and half-metallic properties of ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Ga}$ Heusler alloys have been investigated. In the entire concentration range, the $L{2}_{1}$-type ordered phase is obtained. The saturation magnetic moment ${M}_{s}$ at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.0$ $(\ensuremath{\equiv}{\mathrm{Co}}_{2}\mathrm{Cr}\mathrm{Ga})$ is $3.01{\ensuremath{\mu}}_{B}∕\mathrm{f.u.}$, in agreement with the generalized Slater-Pauling line and consistent with the theoretical value. The experimental value of ${M}_{s}$ increases with increasing $x$, slightly larger than that of the theoretical value above $x=0.60$. The Curie temperature ${T}_{C}$ also increases with $x$, and qualitatively accords with the theoretical values estimated from the effective exchange constant ${J}_{0}$. The present theoretical results disclose that not only the $L{2}_{1}$-type ordered phase but also the $B2$-type phase exhibits high values of the spin polarization ratio $P$ in a low concentration range of $x$. The ratio $P$ of the $L{2}_{1}$- and $B2$-type phases decreases with increasing $x$, because the large peak of the density of states at the Fermi energy for Cr in the majority spin band becomes smaller and a strong hybridization between $\mathrm{Co}\text{\ensuremath{-}}3d$ and $\mathrm{Fe}\text{\ensuremath{-}}3d$ makes a gap narrower in the minority spin band.

Half-metallic properties ofCo2(Cr1−xFex)GaHeusler alloys

A cobalt-base alloy which has a basic composition comprising, in terms of mass proportion, 0.1-10% Al, 3.0-45% W, and Co as the remainder and has an intermetallic compound of the L12 type [Co3(Al,W)] dispersed and precipitated therein. Part of the Co may be replaced with Ni, Ir, Fe, Cr, Re, or Ru, while part of the Al and W may be replaced with Ni, Ti, Nb, Zr, V, Ta, or Hf. The intermetallic compound [Co3(Al,W)] has a high melting point, and this compound and the matrix are mismatched little with respect to lattice constant. Thus, the cobalt-base alloy can have high-temperature strength equal to that of nickel-base alloys and excellent structure stability.

Cobalt-base alloy with high heat resistance and high strength and process for producing the same

Magnetic properties of quaternary Heusler alloysNi2−xCoxMnGa

A guide wire includes a distal core member made of a ferrous alloy which has shape memory properties and superelasticity. The ferrous alloy preferably includes substantially two phases, and has a difference of 100° C. or less between an Af point and an Ms point in a thermal hysteresis of martensitic transformation and reverse transformation. The guide wire may include a proximal core member made of an iron-containing alloy and having a higher modulus of elasticity than the distal core member. The two core members may be joined together by welding to form a core of the guide wire.

Ryosuke Kainuma

Papers

Atomic ordering and magnetic properties in the Ni45Co5Mn36.7In13.3 metamagnetic shape memory alloy

Half-metallic properties ofCo2(Cr1−xFex)GaHeusler alloys

Cobalt-base alloy with high heat resistance and high strength and process for producing the same

Magnetic properties of quaternary Heusler alloysNi2−xCoxMnGa

Guide wire and stent