A Giant Magneto-Superelasticity of 5% Enabled by Introducing Ordered Dislocations in Ni34Co8Cu8Mn36Ga14 Single Crystal.

Elasticity, featured by a recoverable strain, refers to the ability that materials can return to their original shapes after deformation. Typically, the elastic strains of most metals are well-known 0.2%. In shape memory alloys and high entropy alloys, the elastic strains can be several percent, as called superelasticity, which are all triggered by external stresses. A superelasticity induced by magnetic field, termed as magneto-superelasticity, is extremely important for contactless work of materials and for developing brand-new large stroke actuators and high efficiency energy transducers. In magnetic shape memory alloys, the twin boundary motion driven by magnetic field can output a strain of several percent. However, this strain is unrecoverable when removing the magnetic field and hence it is not magneto-superelasticity. Here, a giant magneto-superelasticity of 5% in a Ni34Co8Cu8Mn36Ga14 single crystal is reported by introducing arrays of ordered dislocations to form preferentially oriented martensitic variants during the magnetically induced reverse martensitic transformation. This work provides an opportunity to achieve high performance in functional materials by defect engineering.

Influence of topology on the phase transition of a ferromagnetic metal.

The topological ferromagnet CoS2 exhibits an anhysteretic, weakly first-order transition at the Curie temperature of 119.8 K with a tricritical point µ0Htcp at 0.034 T. Magnetic symmetry and the mixing of majority and minority spin eg bands at a subband crossing just above the Fermi level produce a topological component of the magnetization that leads to a negative M3 term in the Landau free energy. The position of the Fermi level relative to the subband crossing is critical for controlling the order of the transition. Hole doping in Co0.89Fe0.11S2 drains the minority-spin eg pocket and results in a normal second-order phase transition. Electron doping in Co0.94Ni0.06S2 raises the Fermi level toward the subband gap, producing a strongly first-order transition with 15 K hysteresis. Our results demonstrate a relation between topological electronic structure and thermal hysteresis at the Curie point, which may help in the search for magnetocaloric materials.

Ruthenium-Alloyed Iron Phosphide Single Crystal with Increased Fermi Level for Efficient Hydrogen Evolution.

Transition metal phosphide alloying is an effective approach for optimizing the electronic structure and improving the intrinsic performance of the hydrogen evolution reaction (HER). However, obtaining 3d transition metal phosphides alloyed with noble metals is still a challenge owing to their difference in electronegativity, and the influence of their electronic structure modulated by noble metals on the HER reaction also remains unclear. In this study, we successfully incorporated Ru into an Fe2P single crystal via the Bridgeman method and used it as a model catalyst, which effectively promoted HER. Hall transport measurements combined with first-principles calculations revealed that Ru acted as an electron dopant in the structure and increased the Fermi level, leading to a decreased water dissociation barrier and an improved electron-transfer Volmer step at low overpotentials. Additionally, the (21̅1) facet of Ru-Fe2P was found to be more active than its (001) facet, mainly due to the lower H desorption barrier at high overpotentials. The synergistic effect of Ru and Fe sites was also revealed to facilitate H* and OH* desorption compared with Fe2P. Therefore, this study elucidates the boosting effect of Ru-alloyed iron phosphides and offers new understanding about the relationship between their electronic structure and HER performance.

Identification of Interface Structure for a Topological CoS2 Single Crystal in Oxygen Evolution Reaction with High Intrinsic Reactivity.

Transition metal chalcogenides such as CoS2 have been reported as competitive catalysts for oxygen evolution reaction. It has been well confirmed that surface modification is inevitable in such a process, with the formation of different re-constructed oxide layers. However, which oxide species should be responsible for the optimized catalytic efficiencies and the detailed interface structure between the modified layer and precatalyst remain controversial. Here, a topological CoS2 single crystal with a well-defined exposed surface is used as a model catalyst, which makes the direct investigation of the interface structure possible. Cross-sectional transmission electron microscopy of the sample reveals the formation of a 2 nm thickness Co3O4 layer that grows epitaxially on the CoS2 surface. Thick CoO pieces are also observed and are loosely attached to the bulk crystal. The compact Co3O4 interface structure can result in the fast electron transfer from adsorbed O species to the bulk crystal compared with CoO pieces as evidenced by the electrochemical impedance measurements. This leads to the competitive apparent and intrinsic reactivity of the crystal despite the low surface geometric area. These findings are helpful for the understanding of catalytic origins of transition metal chalcogenides and the designing of high-performance catalysts with interface-phase engineering.

Large linear non-saturating magnetoresistance and high mobility in ferromagnetic MnBi.

A large non-saturating magnetoresistance has been observed in several nonmagnetic topological Weyl semi-metals with high mobility of charge carriers at the Fermi energy. However, ferromagnetic systems rarely display a large magnetoresistance because of localized electrons in heavy d bands with a low Fermi velocity. Here, we report a large linear non-saturating magnetoresistance and high mobility in ferromagnetic MnBi. MnBi, unlike conventional ferromagnets, exhibits a large linear non-saturating magnetoresistance of 5000% under a pulsed field of 70 T. The electrons and holes' mobilities are both 5000 cm2V-1s-1 at 2 K, which are one of the highest for ferromagnetic materials. These phenomena are due to the spin-polarised Bi 6p band's sharp dispersion with a small effective mass. Our study provides an approach to achieve high mobility in ferromagnetic systems with a high Curie temperature, which is advantageous for topological spintronics.

Thermoelectric Properties of Novel Semimetals: A Case Study of YbMnSb2.

The emerging class of topological materials provides a platform to engineer exotic electronic structures for a variety of applications. As complex band structures and Fermi surfaces can directly benefit thermoelectric performance it is important to identify the role of featured topological bands in thermoelectrics particularly when there are coexisting classic regular bands. In this work, the contribution of Dirac bands to thermoelectric performance and their ability to concurrently achieve large thermopower and low resistivity in novel semimetals is investigated. By examining the YbMnSb2 nodal line semimetal as an example, the Dirac bands appear to provide a low resistivity along the direction in which they are highly dispersive. Moreover, because of the regular-band-provided density of states, a large Seebeck coefficient over 160 µV K-1 at 300 K is achieved in both directions, which is very high for a semimetal with high carrier concentration. The combined highly dispersive Dirac and regular bands lead to ten times increase in power factor, reaching a value of 2.1 mW m-1 K-2 at 300 K. The present work highlights the potential of such novel semimetals for unusual electronic transport properties and guides strategies towards high thermoelectric performance.

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording.

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc , coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2 CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m-3 is combined with a saturation magnetization of µ0 Ms  = 0.52 T at 2 K (2.2 MJ m-3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 µB on Co, which is hybridized with neighboring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m-1 K-1 , make Rh2 CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.-2 .