CoGe Compound Research Articles

Reduction of thermal hysteresis and improvement of magnetocaloric properties in ball milled Mn0.98CoGe

The Mn0.98CoGe compound can be as alternative refrigerants due to its low cost and large magentocaloric effect near room temperature. However, the narrow work temperature range and the large thermal hysteresis restrict its application. In this work, we investigate the influence of ball milling (BM) time on the thermal hysteresis and magnetocaloric properties of Mn0.98CoGe compound. Results show that the thermal hysteresis of Mn0.98CoGe reduced markedly after BM 5 h, and disappeared completely after BM 12 h. Furthermore, the magnetic entropy change (ΔSM) over a wider temperature span are observed, which leads to the relative cooling power (RCP) of BM-12 h sample as high as those typical refrigerants.

Effect of doping with rare-earth element La in MnCoGe system

<p indent=0mm>MnCoGe is one of the most representative materials that undergoes martensitic transformation in the MM′X system. Because the phase transition temperature of MnCoGe is higher than its magnetic ordering temperature, various methods have been used to decrease the phase transition temperature and realize magneto-structural coupling. However, there are rare reports about the effect of doping with rare-earth elements. In this work, the structures and physical properties of Mn_{1−<italic>x</italic>}La<italic>_x</italic>CoGe compounds with <italic>x</italic>=0, 0.01, 0.02, 0.03, and 0.05 were studied. The introduction of La causes a deficiency of both Mn and Ge atoms in the main phase, which leads to a decrease in the martensitic transformation temperature and the realization of magneto-structural coupling. However, due to its limited ability of deficiency formation, a further increase in the La content leads to large amount of secondary phase instead of a continuous decrease of the phase transition temperature. Additionally, the MnCoGe matrix is found to be divided into small “domains” by the needle-like secondary phase, which is the reason for the large amount of retained austenite phase in the sample with a high La content.

FP-(L)APW + lo study of mechanical stability and electronic behavior of CoGe in B20 structure

AbstractThe aim of this work is a theoretical study of structural, elastic, electronic and thermal properties of CoGe compound in B20 structure using All-electron self-consistent Full Potential Augmented Plane Waves plus local orbital “FP(L)APW + lo” within the framework of Density Functional Theory DFT. GGA-PBEsol is the exchange-correlation potential selected in this work. This choice is motivated by the success of this functional in predicting structural and mechanical properties of solids. The values obtained by the study of structural properties are in very good agreement with those found previously. In this work, the elastic constants have been predicted for the first time and the obtained values confirm the mechanical stability of the CoGe compound in its B20 structure. The electronic part of this work shows that CoGe has metallic behavior with a mixed bonding between cobalt and germanium of covalent-metallic type. The effect of temperature and hydrostatic pressure on the lattice parameter - a0, heat capacity at constant volume - CV, thermal expansion coefficient - α and entropy - S of the CoGe have been studied using Debye model.

Large magnetostrain in magnetic-field-aligned Mn0.965CoGe compound

By applying external stimulus (temperature or magnetic field), MnCoGe-based compounds undergo a martensitic transformation from hexagonal Ni2In-type to orthorhombic TiNiSi-type structure accompanied with a giant negative thermal expansion, which suggests a large magnetic-field-induced strain. However, these compounds naturally collapse into powders and are difficult to be oriented, which hinder their applications for magnetostrain. In this paper, a magnetic-field-aligned Mn0.965CoGe compound was prepared by bonding with epoxy resin and orientating in a magnetic field. The XRD patterns revealed the texture in this sample. By introducing vacancies of Mn element, the magnetostructural transformation temperature of Mn0.965CoGe compound was shifted down to 278 K. The magnetostrain was measured at some selected temperatures and the maximal strain could reach up to 925 ppm at 270 K.

Giant magnetocaloric effect induced by reemergence of magnetostructural coupling in Si-doped Mn0.95CoGe compounds

It has been reported that the martensitic structural transformation temperature (Tstr) can be lowered and even depressed by Mn vacancy in MnCoGe-based alloys. In the present work, the structural transformation is tuned to reappear and the Tstr increases significantly by substituting Si for Ge in Mn0.95CoGe system, and thus the magnetostructural transition from paramagnetic Ni2In-type phase to ferromagnetic TiNiSi-type phase is again realized around room temperature. Meanwhile, the working temperature window is enhanced to be 22% wider than that of Mn1−xCoGe system. The history-dependent phase transitions have been discussed by measuring the M-H curves in different processes. Mn0.95CoGe0.9Si0.1 experiences a forward martensitic transition in loop process, and then a field-induced metamagnetic transition (FIMT) with large magnetic hysteresis can be observed around Tstrcooling. On the other hand, a reverse martensitic transition is observed in standard process, and the FIMT is depressed by thermal disturbance at higher temperatures. Large MCE is obtained due to the reemergence of magnetostructural transition, e.g., the maximum −ΔSM is 15.0J/kgK for a relatively low field change of 2T, which is larger than those of Mn1−xCoGe system and some other typical magnetocaloric materials in similar temperature range. The comprehensive advantages including giant low-field MCE, wide working temperature range, and lower cost of Si than Ge, suggest that Mn0.95CoGe1−xSix compounds could be promising materials for room temperature magnetic refrigeration.

Magnetocaloric effect and magnetostructural coupling in Mn0.92Fe0.08CoGe compound

The structural properties of Mn0.92Fe0.08CoGe have been investigated in detail using synchrotron x-ray diffraction in zero and applied pressure (p = 0–10 GPa). A ferromagnetic transition occurs around TC = 300 K and a large magnetic-entropy change −ΔSM = 17.3 J/kg K detected at TC for a field change of ΔB = 5 T. The field dependence of −ΔSMmax can be expressed as −ΔSMmax ∝ B. At ambient temperature and pressure, Mn0.92Fe0.08CoGe exhibits a co-existence of the orthorhombic TiNiSi-type structure (space group Pnma) and hexagonal Ni2In-type structure (space group P63/mmc). Application of external pressure drives a structure change from the orthorhombic TiNiSi-type structure to the hexagonal Ni2In-type structure. A large anomaly in heat capacity around TC is detected and the Debye temperature θD (=319(±10) K) has been derived from analyses of the low temperature heat capacity, T ≲ 10 K.

Reconstructed Structures of Nanosized Co Islands on Ag/Ge(111) √3 × √3 Surfaces

Structural evolution of Co/Ag/Ge(111) at high temperatures was studied by using scanning tunneling microscopy and low energy electron diffraction. The mean square root of 3 x mean square root of 3-Ag layer between the substrate Ge( 11) and Co adatoms can avoid the formation of Co-Ge compounds below 800 K. The Co atoms nucleate to form islands where mean square root of 13 x mean square root of 13 or 2 x 2 reconstructions were observed after annealing between 373 K and 737 K. The mean square root of 13 x mean square root of 13 structure with mirror symmetry relative to [-211], [11-2], and [1-21] axes was observed for 1-2 layer Co islands. Co islands with over 2 layers appear 2 x 2 structure. All reconstruction structures of the nano-sized Co islands and substrate Ag/Ge(111) mean square root of 3 x mean square root of 3 surface were analyzed using the atomic hard sphere model. The bright protrusions of these reconstructions all sit in the centers of Ag or Ge trimers, which were predicted to have maximum binding energy.

Effect of annealing on the magnetic properties of ultrathin Co/Ge(1 1 1) films

Effect of annealing on the magnetic properties of ultrathin Co/Ge(1 1 1) films has been investigated using the surface magneto-optic Kerr effect technique. As the annealing temperature increases from 200 to 450 K, the Kerr intensity decreases to zero for all the samples. The temperature, where ferromagnetism vanishes, increases from below 200 to 375 K as the coverage of the cobalt film increases from 5 to 28 monolayers (ML). Corresponding compositions obtained from the Auger electron spectroscopy measurements show an intermixing of the cobalt adlayer and the Ge(1 1 1) substrate. For a 20-ML-thick Co/Ge(1 1 1) film, the intensity ratio of Co to Ge Auger signals drops to roughly half of the original value as the annealing temperature increases to 325 K, at which the hysteresis disappears. This indicates the formation of a CoGe compound at the surface layers. Cobalt films possess an in-plane anisotropy as deposited at a substrate temperature of 200 K. The magnetic easy axis retains in the surface plane at an elevated temperature.