CoGa Alloys Research Articles

Fine tuning of Mn/Co vacancies for optimized magnetocaloric performance in MnCoGe alloys

As a family of rare-earth-free magnetocaloric materials, MnCoGe-based alloys have attracted intensive attention in the past few years. In this study, we reveal that the deficiency of either Mn or Co reduces the valence electron concentration of MnCoGe system, thereby lowering its structural transition temperature (Ttr) by pushing the Fermi level away from the Mn-Mn antibonding electronic states. Moreover, Ttr is found to be more sensitive to Co content. By systematically analyzing the temperature dependent magnetization plots and Arrott curves, it is proven that the magnetic and structural degrees of freedom are coupled in the Mn0.97CoGe alloy, resulting in a first-order magnetic phase transition. On the contrary, MnCo0.98Ge undergoes a second-order transition. As a consequence, the Mn0.97CoGe alloy exhibits a maximum magnetic entropy change of 27.6 J kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 265 K, corresponding to a refrigeration capacity of 212.1 J kg−1. Our work demonstrates Mn/Co vacant MnCoGe alloy as a robust candidate of magnetic substance for room temperature magnetocaloric refrigeration.

The Effect of Substitution of Mn by Pd on the Structure and Thermomagnetic Properties of the Mn1-xPdxCoGe Alloys (Where x = 0.03, 0.05, 0.07 and 0.1).

In the present paper, the influence of partial substitution of Mn by Pd on structure, thermomagnetic properties, and phase transitions in the MnCoGe alloys was investigated. The studies of phase constitution revealed an occurrence of the orthorhombic TiNiSi-type and hexagonal Ni2Ti- type phases. Deep analysis of the XRD pattern supported by the Rietveld analysis allowed us to notice the changes in lattice parameters and quantity of recognized phases depending on the Pd content. An increase of palladium in alloy composition at the expense of manganese induced a rise in the Curie temperature. The values of ΔSM measured for the variation of external magnetic field ~5 T equaled 8.88, 23.99, 15.63, and 11.09 for Mn0.97Pd0.03CoGe, Mn0.95Pd0.05CoGe, Mn0.93Pd0.07CoGe, and Mn0.9Pd0.1CoGe alloy, respectively. The highest magnetic entropy change ΔSM was observed for samples with Pd content x = 0.05 induced by magnetostructural transformation. The analysis of the n vs. T curves allowed confirmation of the XRD and DSC results of an occurrence of the first-order magnetostructural transition in Mn0.95Pd0.05CoGe and Mn0.93Pd0.07CoGe alloys samples.

Martensitic transformation and magnetocaloric effect of Mn<sub>1−</sub><sub><italic>x</italic></sub>Bi<sub><italic>x</italic></sub>CoGe alloys

<p indent="0mm">Quaternary alloys of MnCoGe with the partial addition of other elements can realize coupling between structural and magnetic transformations, and it can present apparent magnetocaloric effects during martensitic transformation. In this paper, we have prepared Mn₁₋_{<italic>x</italic>}Bi_{<italic>x</italic>}CoGe (<italic>x</italic>=0, 0.03, 0.04, 0.05, 0.055, 0.06) series alloys using arc melting in a protective argon atmosphere. Experimental results indicate that the structural transformation temperature (<italic>T</italic>_M) of the samples shifts to low temperatures with increasing elemental Bi doping. The Mn_0.945Bi_0.055CoGe alloy (valence electron concentration<italic>e</italic>/<italic>a</italic>=6.63) undergoes a first-order magnetostructural transition (FOMST) near room temperature, and a large magnetic entropy change of <sc>36.8 J kg⁻¹ K⁻¹</sc> is obtained under an external magnetic field of <sc>0–5 T,</sc> which proves that elemental Bi doping in MnCoGe alloys can effectively tune the martensitic transformation temperature and further improve the magnetic entropy change.

Tuning structural-transformation temperature toward giant magnetocaloric effect in MnCoGe alloy: A theoretical study

We propose a new model and scheme to consider the effect of structural transformation on magnetocaloric properties of Mn1-xCuxCoGe. The magnetic exchange coupling constants are estimated from first-principles calculations based on density functional theory. The magnetic phase diagram is semi-quantitatively reproduced by using mean-field approximation. The giant isothermal magnetic entropy change with the magnetostructural coupling of Mn0.89Cu0.11CoGe alloy is well reproduced by using the modified Potts model. The enhancement of magnetostructural coupling on the isothermal magnetic entropy change strongly depends on the condition of the magnetic phase transition temperature of orthorhombic and hexagonal phases.

Structure, magnetocaloric properties and thermodynamic modeling of enthalpies of formation of (Mn,X)-Co-Ge (X = Zr, Pd) alloys

Enthalpies of formation of (Mn,X)-Co-Ge (X = Zr, Pd) system in different structural states were analyzed utilizing the semi-empirical Miedema's model combined with the geometric one. Substitution of Mn by Zr significantly improves glass forming ability of Mn-Co-Ge, with parent equiatomic alloy among them. Chosen half-Heusler alloys, which are thermodynamically stable for the mentioned elemental compositions, were obtained by arc-melting. The x-ray diffraction studies confirmed coexistence of orthorhombic NiTiSi-type and hexagonal Ni2In-type structures for parent MnCoGe alloy and (Zr, Pd)-substituted samples. Thermomagnetic curves and magnetic isotherms allowed to reveal the Curie temperature TC and a magnetic entropy change ΔSM and allowed to analyze the influence of both substitutive elements. The Curie temperatures are equal to 293 K for MnCoGe, 278 K for Mn0.9Zr0.1CoGe alloy and 318 K for Mn0.9Pd0.1CoGe. The maximum magnetic entropy change ΔSM values, calculated for the change of external magnetic field of about 5 T, amount to 6.17, 2.94, and 11.11 J (kg K)−1 for MnCoGe, Mn0.9Zr0.1CoGe and Mn0.9Pd0.1CoGe, respectively. Differences in magnetic characteristics of the analyzed alloys are mainly caused by the changes of extent of undergoing first order magnetostructural transition with Pd and Zr atoms substitution. The presence of Pd d moments is also beneficial for the enhancement of total magnetic moment and maximum value of magnetic entropy changes in Mn0.9Pd0.1CoGe alloy.

Thermodynamic description and solidified microstructure of the Co-Ge system

The Co–Ge binary system was reassessed by CALPHAD method in view of new phase diagram data and drawbacks of the previous modeling. A three-sublattice model (Co,Va)1(Co)1(Ge)1 was used to describe the B82-type βCo5Ge3-phase based on its crystal structure. In order to describe the transformation between ordered L12-type phase (Co3Ge) and the disordered fcc_A1 phase (αCo), one single Gibbs energy function was used for both ordered and disordered phases. In almost all the previous thermodynamic calculations, the ordered phase with a negligible homogeneity range, such as Co3Ge, is treated as a stoichiometric compound. Such a treatment is not physically sound. For (αCo) and (εCo), the magnetic contribution to Gibbs energy is taken into account. Both substitutional solution model and associated model were applied to describe the liquid phase, and thus two sets of self-consistent thermodynamic parameters for this system were obtained. It was found that the associated model can account for the experimental data more satisfactorily than the substitutional solution model, especially for the partial enthalpy of mixing data for the liquid phase. Five representative as-cast Co-Ge alloys were prepared to compare the solidified microstructure with that predicted according to thermodynamic calculations. According to the calculated Scheil solidification curves of two key alloys, the solidified microstructure for the alloys was analyzed. Also, the calculated amounts of the solidified phases in five as-cast alloys are compared with the experimental data resulting from automatic image analysis of the BSE images, showing a good agreement between the calculation and experiment, in particular for the case in which the associated model is used to describe the properties of the liquid phase. It is demonstrated that the combined use of the thermodynamic calculation and decisive experiment is an efficient strategy to obtain the desired microstructure.

CoGe surface oxidation studied using X-ray photoelectron spectroscopy

Cobalt germanides have been widely studied as semiconductor contact materials, but recent theoretical studies suggest that they may also be excellent catalysts for methane steam reforming with stabilities and activities comparable to more expensive noble metal catalysts. We have sputter deposited CoGe alloy films and characterized their structure and morphology after post-deposition annealing in high vacuum up to 1000 °C. We used X-ray photoelectron spectroscopy to study the initial oxidation of amorphous and crystalline CoGe alloy surfaces under low pressures of O2 and H2O. The oxidation rate in O2 was found to be faster for an amorphous CoGe surface compared to a crystalline surface. We also found that there was little difference in the oxidation rate in H2O for either amorphous or crystalline surfaces. During O2 oxidation, the crystalline surface preferentially forms GeO and the amorphous surface preferentially forms GeO2. We have also observed preferential oxidation of Ge in the CoGe thin films. During temperature programmed desorption studies, we found that GeO desorption begins near 350 °C and that GeO2 decomposes to GeO and desorbs near 700 °C. More studies of CoGe catalysts are warranted, however GeO desorption may be a concern under reaction conditions when the film is subjected to an oxidizing environment.

High pressure synchrotron x-ray diffraction study of the Mn0.94Ti0.06CoGe alloy

High pressure x-ray diffraction studies up to 10.4 GPa were performed on the Mn0.94Ti0.06CoGe alloy using synchrotron radiation with a diamond anvil cell. No structural phase transitions occurred in the entire range of our measurements. Unit cell parameters were determined up to 10.4 GPa and the calculated unit cell volumes were found to be well represented by a third order Birch-Murnaghan equation of state. The bulk modulus determined from the pressure – volume data was found to be, B0 = 231.72 ± 7.79 GPa. This study, employing high resolution synchrotron x-rays has helped clarify the behaviour of the Mn0.94Ti0.06CoGe alloy under high pressure.

Tuning the magnetostructural transformation in slightly Ni-substituted MnCoGe ferromagnet

In this work, the Mn1-xNixCoGe (0.01 ≤ x ≤ 0.075) alloys are prepared. By slight substitution of Mn upon Ni element, the magnetic transition and structural transformation of stoichiometric MnCoGe ferromagnet are controlled to coincide. Accordingly, the coupled magnetostructural transformation occurs during the wide temperature window of ∼91.5 K covering room temperature from the ferromagnetic TiNiSi-type to paramagnetic Ni2In-type state. Strikingly, a magnetic-field-driven metamagnetic martensitic transformation is observed in Mn0.975Ni0.025CoGe alloy. As a result, large room temperature magnetocaloric effects are obtained including large magnetic entropy change of ∼30.3 Jkg−1K−1 for sample with x = 0.025 and large refrigeration capacity of ∼276.8 Jkg−1 for x = 0.035 under the applied field change of ΔH = 0–7 T. The influence of slight Ni-substitution for Mn on the magnetic and magnetocaloric properties of MnCoGe alloy is studied and the origins are discussed.

Evidence for reentrant spin glass behavior in transition metal substituted Co-Ga alloys near critical concentration

In the present study magnetic and electrical transport properties of transition metal substituted Co-Ga alloys (near critical cobalt concentration) have been investigated. Analysis of temperature and field dependence of dc magnetization and ac susceptibility (ACS) data suggests an evidence of reentrant spin glass (RSG) phase in Co55.5TM3Ga41.5 (TM = Co, Cr, Fe, Cu). The magnetic transition temperatures (TC and Tf) are found to depend on the nature of TM element substitution with the exchange coupling strength Co-Fe > Co-Co > Co-Cu > Co-Cr. From magnetization dynamics precise transition temperatures for the glassy phases are estimated. It is found that characteristic relaxation times are higher than that of spin glasses with minimal spin-cluster formation. The RSG behavior has been further supported by the temperature dependence of magnetotransport studies. From the magnetic field and substitution effects it has been established that the magnetic and electrical transport properties are correlated in this system.

Assessment of Co-Ga alloys magnetostriction

The magnetostriction of Co-Ga alloys: Co-14Ga, Co-24Ga and Co-33Ga (atomic), is studied at room temperature for the first time. All three alloys are ferromagnetic. Co-14Ga microstructure is biphasic, a mixture of ε (hcp) and α (fcc) phases. Co-24Ga presents three phases ε, α and β (ordered bcc) and Co-33Ga is single β phase alloy. It was found that all three alloys present negative saturation magnetostriction λs. The most interesting material for applications would be the alloy Co-33Ga since the magnetostriction saturates for a small field.

Enhancement of Curie temperature and transition temperature range induced by Al doping in Mn1-xAlxCoGe

Abstract 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.

Giant magnetoresistance in the cluster glass regime of Co-Ga alloys

A detailed study of low temperature electrical transport properties of CoxGa100−x (x = 54, 55.5, 57) alloy has been carried out. The origin of the resistivity anomalies and correlation between magnetic and electrical transport properties are identified through an elaborate analysis. The weak localization and enhanced electron-electron interaction effects partially support the electrical transport properties of the system. Further, the observed magnetoresistance can be well represented by localized model along with quantum corrections. The low temperature magnetoresistance value near critical composition is comparable to that reported in giant magnetoresistance materials.

The Martensitic Transition and Magnetocaloric Effect of Mn-Poor MnCoGe Melt-Spun Ribbons

We investigated the martensitic transition and the magnetic properties of Mn1− x CoGe melt-spun ribbons. The as-prepared Mn1− x CoGe ribbons crystallize in austenite hexagonal phase with a textured structure. The postannealing process promotes the formation of the martensitic phase and homogenization of the alloy, resulting in a first-order magnetostructural transition in the annealed ribbons, and thus a giant magnetocaloric effect. The magnetic entropy change around the transition reaches 19 J/kgK for a magnetic field change of 0–5 T. Furthermore, it is found that the hysteresis loss around the magnetostructural transition is negligible in present annealed ribbons, which would facilitate the application of Mn1− x CoGe alloys.

Spin correlations and magnetic order in Co-Ga alloys: A comprehensive study

Low temperature magnetic properties of binary CoxGa100−x alloy with Co concentration in the range 54 ≤ x ≤ 61.5 at% have been investigated. From the temperature and magnetic field dependent magnetization measurements magnetic phase diagram has been identified. Cluster spin glass like features are noticed in x = 54, 55 compositions, while the compositions x > 57 exhibit double magnetic transition i.e., at higher temperatures paramagnetic (PM) – ferromagnetic (FM) and at lower temperatures FM-SG like transition. The critical concentration is identified to be near x = 57 composition where discernible spontaneous magnetization emerges and the long range ferromagnetic order develops above this composition in addition to the spin glass transition (or mixed magnetic phase). Analysis of temperature dependence magnetization data in the different temperature ranges for the compositions x = 60 and 61.5 indicate that the mean field models are not suitable to understand the phase transition. Magnetic isotherms in the critical region were analyzed using non-mean-field approach and the critical exponents (γ = 1.31 and β = 0.337) found to be close to 3D Heisenberg model suggesting the importance of short range magnetic order. The data satisfies magnetic equation of state characteristic of a second order phase transition. The results obtained from the present study corroborate well with the phenomenological interacting spin cluster model.

Co thin film with metastable bcc structure formed on GaAs(111) substrate

Co thin films are prepared on GaAs(111) substrates at temperatures ranging from room temperature to 600 oC by radio-frequency magnetron sputtering. The growth behavior and the detailed resulting film structure are investigated by in-situ reflection high-energy electron diffraction and X-ray diffraction. In early stages of film growth at temperatures lower than 200 oC, Co crystals with metastable A 2 (bcc) structure are formed, where the crystal structure is stabilized through hetero-epitaxial growth. With increasing the film thickness beyond 2 nm, the metastable structure starts to transform into more stable A 1 (fcc) structure through atomic displacements parallel to the A 2{110} close-packed planes. The crystallographic orientation relationship between the A 2 and the transformed A 1 crystals is A 1{111} || A 2{110} . When the substrate temperature is higher than 400 oC, Ga atoms of substrate diffuse into the Co films and a Co-Ga alloy with bcc-based ordered structure of B 2 is formed.

NMR studies of the magnetic state of interfacial cobalt in (Co/Ge) n films

The results of NMR studies of film structures in a cobalt-germanium system as a function of the cobalt layer thickness are presented. Two phases of cobalt, one is a face-centered cubic phase and the other is presumably a Co-Ge alloy with a weakly ferromagnetic order, have been found to exist. A “dead” layer no more than 2 nm in thickness is formed at the interface.

Critical phenomena and estimation of the spontaneous magnetization by a magnetic entropy analysis in Mn0.96Nb0.04CoGe alloy

Magnetic and magnetocaloric properties of the alloy Mn0.96Nb0.04CoGe have been investigated. According to the mean-field theory prediction, the relationship between ΔSM ∝ (H/TC)2/3 has been confirmed in the temperature region near TC for that system. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical components, γ, β, and δ determined using the Kouvel-Fisher method, the modified Arrott plot, as well as the critical isotherm analysis agree well. Moreover, these critical exponents are confirmed by the Widom scaling law and the validity of the calculated critical exponents was also confirmed by the scaling theory. The values deduced for the critical exponents are close to the theoretical prediction of the mean-field model values, thus indicating that long range interactions dominate the critical behavior in the Mn0.96Nb0.04CoGe system. It is also speculated that the competition between the localized Mn-Mn magnetic interactions should be responsible for the critical behavior in this system. Moreover, an excellent agreement is found between the spontaneous magnetization determined from the entropy change (−ΔSM vs. M2) and the classical extrapolation from the Arrott curves (H/M vs. M2), thus confirming that the magnetic entropy change is a valid approach to estimate the spontaneous magnetization in this system.

Synthesis and characterization of surface oxide films on CoGa(100)

It has been shown that a Ga2O3 film forms on the surface of CoGa alloy crystals when exposed to oxygen (Pan, 2001 and Vlad, 2010). In this work we report the results of the characterization of surface oxides on CoGa(100) using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). The oxides were synthesized using either O2 or NO2 as the oxidant at 300K or in excess of 700K. ISS scans showed that cobalt was always present in the top surface layer regardless of oxidation conditions. XPS showed that depending on the oxidant and the temperature, the composition of the oxide films vary depending on oxidation treatment, with some oxides being nearly all Ga2O3 and ordered with a sharp LEED pattern consisting of (2×1) domains rotated by 90º and others being Co–Ga mixed oxides that gave no diffraction pattern.

Investigation of the critical behavior in Mn0.94Nb0.06CoGe alloy by using the field dependence of magnetic entropy change

The critical behaviour of Mn0.94Nb0.06CoGe alloy around the paramagnetic-ferromagnetic phase transition was studied based on the field dependence on magnetic entropy change. By using the obtained exponents, the modified Arrott plot is consistent with that by using conventional method. These critical exponents are confirmed by the Widom scaling relation. Based on these critical exponents, the magnetization, field and temperature data around Tc collapse into two curves obeying the single scaling equation M(H, ε) = εβf ± (H/εβ+γ). The calculated critical exponents not only obey the scaling theory but also anastomose the deduced results from the Kouvel-Fisher method [J. S. Kouvel and M. E. Fisher, Phys. Rev. 136, A1626 (1964)]. The values deduced for the critical exponents in the Mn0.94Nb0.06CoGe alloy are close to the theoretical prediction of the mean-field model, indicating that the magnetic interactions are long range. This method eliminates the drawback due to utilization of multistep nonlinear fitting in a conventional manner. So it provides an alternative method to investigate the critical behaviour.