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We performed first-principles calculations to investigate the effect of Cu or Fe partial substitution in MnCoGe on the stabilization of the hexagonal structure. In the case of Cu partial substitution at x = 0.125, a substitution for one site is more effective than that for both sites. In the case of Fe partial substitution at x = 0.125, there is no difference between two types of substitution for both sites and the Mn site. The result of Fe partial substitution at x = 0.25 indicates that Fe partial substitution for both sites is more effective than that for the Mn or the Co site. [ABSTRACT FROM AUTHOR]

This work investigates the substitution of main-group element Al for Ge in the MnCoGe system to study the effect of magnetic phase transition and the magnetocaloric effect of the compounds. The Room temperature X-ray diffraction indicates clearly that most of the reflections can be identified with the orthorhombic structure (TiNiSi-type, space group Pnma) for MnCoGe and the hexagonal structure (Ni2In-type, space group P63/mmc) for MnCoAl. The substitution of Al for Ge in MnCoAl transmutes the crystal structure from TiNiSi-type to Ni2In-type structure. The magnetic measurement revealed the curie temperatures, Tc decreases from 340 K to 285 K with the substitution of Al with Ge. Besides, the Arrott plots (M2 against B/M) for the MnCoGe and MnCoAl, demonstrated characteristics of second-order transition for both samples. The calculated magnetic entropy change, -∆SM values shows no difference for increasing and decreasing fields, which confirms that there is no hysteresis loss occurred near the magnetic transition. The - ∆SM maximal values is about 4.41 J·kg-1K-1 for MnCoGe and 3.73 J·kg-1K-1 for MnCoAl, for a field change of 0-5 T. [ABSTRACT FROM AUTHOR]

Abstract The behavior of magnetic and structural transitions in Y substituted MnCoGe system is investigated in this study. Both Mn and Ge deficiencies in MnCoGe phase are induced by a single-site Y doping. The structural transition temperature thus shifts to lower temperatures and creates a Curie temperature window as wide as 86 K. The largest magnetic entropy change of about 15.6 J/kg∙K and refrigerant capacity of about 132.8 J/kg are obtained in these compounds. The samples at the boundary of the temperature window show a critical phenomenon, which is characterized by a magnetostructural transition during the cooling process and separate magnetic plus structural transitions during the heating process. This makes it possible to determine the contribution from the magnetic transition to the total entropy change in magnetostructural transition, which is 23–26% in present system. Highlights • Y replacing Mn causes both Mn and Ge deficiencies. • Y substitution leads to a Curie temperature window as wide as 86 K. • Samples at the boundary of CTW show different phase transitions upon heating and cooling, i.e. a critical phenomenon. [ABSTRACT FROM AUTHOR]

MnCoGe-based compounds have been increasingly studied due to their possible large magnetocaloric effect correlated to the magnetostructural coupling. In this research, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−xSix is reported. Room temperature X-ray diffraction indicates that the MnCoGe1−xSix (x = 0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of an orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreasing others (a and c) with increasing Si concentration. Along with M-T and DSC measurements, it is indicated that the Tc value increased with higher Si concentration and decreased for structural transition temperature Tstr. The temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis, and it is shown that the transition is the second-order type. It is also shown that the decreased magnetization on the replacement of Si for Ge decreases the value of −ΔSM from −ΔSM~8.36 J kg−1 K−1 at x = 0 to −ΔSM~5.49 J kg−1 K−1 at x = 0.2 with 5 T applied field. The performed Landau theory has confirmed the second-order transition in this study, which is consistent with the Banerjee criterion. The magnetic measurement and thermomechanical properties revealed the structural transition that takes place with Si substitution of Ge.

MnCoGe thin films were produced using simultaneous magnetron sputtering of Mn, Co, and Ge on SiO 2 , followed by non-diffusive reaction. The MnCoGe compound begins to form at ∼588 K, and structural characterizations show that the obtained MnCoGe film is polycrystalline with the hexagonal Ni 2 In-type structure. This structure is found to be stable from 873 K down to room temperature, the expected hexagonal/orthorhombic structural transition being prevented. The film exhibits a lower average Mn composition than the standard MnCoGe stoichiometry. Furthermore, small clusters (<3 nm) forming planar distributions parallel to the sample surface are observed. They are regularly located every ∼11 nm in the specimen depth. They mainly contain Mn and O atoms. Magnetic characterizations show very good magnetic properties, allowing the perpendicular and parallel magnetocrystalline anisotropy constants to be measured down to 100 K, using the Chappert model to fit ferromagnetic resonance measurements. The film magnetic properties match the properties of bulk stoichiometric MnCoGe in the hexagonal structure, with a Curie temperature of ∼269 K and a negligible coercive field at room temperature. The only difference between the magnetic properties of bulk and thin film specimens appears to be the film shape anisotropy, forcing the internal magnetic field to be contained in the film plane. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the critical behavior and the magnetocaloric effect of the polycrystalline MnCo 1- x Ti x Ge samples with x = 0.02, 0.04, 0.06 and 0.08. For such materials, we show that the maximum entropy change S M depends on applied magnetic field H 2/3 . In order to present a detailed study on the critical behaviors, various techniques such as modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis are used to determine the critical exponent β , γ and δ . The exponents calculated from the critical magnetization isotherm are found to obey the Widom scaling relation remarkably well. Moreover, we also report the accuracy of the three methods using the so-called relative slopes (RS) approach. These critical exponents are found similarly to the theoretical values of the mean-field model, consistent with the existence of long-range ferromagnetic interactions. Suitable magnetocaloric effect and good tunable T C responses are good candidates for magnetic refrigeration materials room temperature. [ABSTRACT FROM AUTHOR]

MnCoGe-based alloys have recently attracted much attention due to their possible large magnetocaloric effect related to the magnetostructural coupling. In this paper, we report a comprehensive study of the spontaneous magnetization and magnetic entropy change of MnCo 1- x Cd x Ge ( x = 0.04 and x = 0.06) alloys for magnetization and demagnetization modes. Landau theory and the dispersion width of the vertical spread of normalized magnetic entropy curves are performed to confirm the second-order transition in this series, consistent with the Banerjee criterion. The maximum isothermal entropy change is presented to depend on magnetic field as follows: |Δ S Max | = A ( H + H 0 ) 2/3 – AH 0 2/3 + BH 4/3 . Here A and B are the intrinsic parameters, and H 0 is a measure of homogeneity. Moreover, we verify the validity and usefulness of magnetic entropy change to estimate the spontaneous magnetization in this system. Finally, these new findings and related discussions with negligible hysteretic losses contribute to the research on the magnetic properties and magnetocaloric potentials in thermomagnetic applications. [ABSTRACT FROM AUTHOR]

Here we study the critical behavior, magnetocaloric properties, and universal curve of the MnCo 1 − x Sn x Ge system with x = 0.02 , 0.04 , 0.06 and 0.08 . To model the magnetic transition and magnetocaloric effect, Kouvel–Fisher (KF) method, Landau theory, and phenomenological universal curve are performed. By using the KF method, the critical exponents β , γ and δ are close to the theoretical prediction from the mean-field model, consistent with the long-range ferromagnetic interactions in this series. Based on the Landau theory and Maxwell relation, we find an excellent agreement between theoretical and experimental magnetic entropy changes in this system. Moreover, a corresponding monotonous decrease between the maximum magnetic entropy change and magnetic moment is presented, and a recently proposed master curve behavior has been confirmed. [ABSTRACT FROM AUTHOR]

Magnetic phase transitions and the magnetocaloric effect of the polycrystalline Mn 1 − x Sn x CoGe samples with x = 0.02 , 0.04, and 0.06 have been investigated. The transformation temperature, room temperature magnetocaloric effect and magnetic orders of these alloys could be effectively tuned by Sn substitution. A detailed study on the critical behavior was performed by various techniques, such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis, to determine the critical exponents ( β , γ , and δ ). Those exponents calculated from the critical magnetization isotherm were found to obey the Widom scaling relation remarkably well and the validity of the obtained critical exponents was also verified by the prediction of the scaling theory in the critical region. All the results are similar to the theoretical values of the mean-field model, indicating the presence of long-range ferromagnetic coupling in this system. Moreover, we made an attempt to contrast the universal curve for magnetocaloric effect in the present system, finding that rescaled entropy data for all alloys collapse into a universal curve. [ABSTRACT FROM AUTHOR]

Mn 1-x Al x CoGe alloys with a second order transition were produced by arc-melting method. The substitution of Mn by Al increased the Curie temperature ( T C ) from 260.5 K to 300.8 K, the magnetic entropy change (|Δ S M |) decreased from 3.78 J·Kg −1 K −1 to 2.35 J·Kg −1 K −1 under a field change of Δµ 0 H=5 T. In addition, the |Δ S M | well linearly depends on the H 2/3 around T C . Furthermore, the relative cooling power (RCP) can reach 242.3 J·Kg −1 with a large full width at half maximum of |Δ S M | (75.5 K) for x=0.02. The decrease of |Δ S M | is explained by the corresponding monotonical decrease of magnetic moment per formula unit. [ABSTRACT FROM AUTHOR]

MnCoGe-based compounds have been increasingly studied due to their possible large magnetocaloric effect correlated to the magnetostructural coupling. In this research, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−xSix is reported. Room temperature X-ray diffraction indicates that the MnCoGe1−xSix (x = 0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of an orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreasing others (a and c) with increasing Si concentration. Along with M-T and DSC measurements, it is indicated that the Tc value increased with higher Si concentration and decreased for structural transition temperature Tstr. The temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis, and it is shown that the transition is the second-order type. It is also shown that the decreased magnetization on the replacement of Si for Ge decreases the value of −ΔSM from −ΔSM~8.36 J kg−1 K−1 at x = 0 to −ΔSM~5.49 J kg−1 K−1 at x = 0.2 with 5 T applied field. The performed Landau theory has confirmed the second-order transition in this study, which is consistent with the Banerjee criterion. The magnetic measurement and thermomechanical properties revealed the structural transition that takes place with Si substitution of Ge. [ABSTRACT FROM AUTHOR]

• Aging treatment can be controlled transformation temperature of MnCoGe. • After aging treatment, first-order magnetic transition exhibits around 297–307 K. • Volume of parent phase and transformation temperature have positive correlation. X-ray powder diffraction, differential scanning calorimetry, and magnetization measurements were performed to investigate the effects of heat treatment on the magnetic and structural properties of as-cast and aging treated MnCoGe. The martensitic transformation temperature of the as-cast sample was 526 K for its first heating cycle, which decreased with repeating thermal cycles of 300–1273 K. MnCoGe prepared by aging treatment of 873 K for 7 days showed a first-order magnetic transition, which was accompanied by a martensitic transformation at 307 K. However, the transformation was not detected when the aging temperature was increased up to 973 K. The martensitic transformation temperature and volume of the parent phase was found to have a positive correlation. [ABSTRACT FROM AUTHOR]