PHASE DIAGRAM AND MAGNETOCALORIC EFFECTS IN Ni 1-x Cr x MnGe 1.05

Transcription

1 PHASE DIAGRAM AND MAGNETOCALORIC EFFECTS IN Ni 1-x Cr x MnGe 1.05 Anil Aryal 1, Abdiel Quetz 1, Sudip Pandey 1, Michael Eubank 1, Tapas Samanta 2, Igor Dubenko 1, Shane Stadler 2, and Naushad Ali 1 1 Department of Physics, Southern Illinois University, Carbondale, IL USA 2 Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA USA I. ABSTRACT: The structural and magnetic properties of the Ni 1-x Cr x MnGe 1.05 system (for x = 0, 0.035, 0.070, 0.105) have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. A change in crystal structure from orthorhombic to hexagonal was observed in the XRD data at chromium concentrations of x = 0.035, 0.070, and The values of the cell parameters and volume of the unit cell for orthorhombic and hexagonal phase were determined. It was found that the partial substitution of Ni for Cr in Ni 1-x Cr x MnGe 1.05 results in a first order magnetostructural transition from antiferromagnetic to ferromagnetic (FM) at T M = 132 K for x = A FM to paramagnetic second order transition has been observed at T C = 204 K. A magnetic entropy change of S M = 4.5 J/kg K for ΔH = 5T was observed in the vicinity of T C and T M for x = The values of the latent heat (L = 2.2 J/g) and corresponding total entropy changes (ΔS T = 16 J/kg K) have been determined from DSC measurements. A concentration-dependent phase diagram of transition temperatures has been constructed using the magnetic and DSC data.

2 II. INTRODUCTION Materials that undergo magnetostructural transitions have been actively studied in recent years. Some of the MnNiGe-based intermetallic compounds undergo temperature-induced magnetostructural phase transitions (MST). The MST is responsible for many observed multifunctional properties in these materials such as magnetocaloric effects (MCE), magnetoresistance, magnetostriction, and magnetic shape memory [1-5]. By changing intrinsic and extrinsic parameters (such as chemical composition, crystal structure, applied magnetic field, temperature, and pressure), the MST can be controlled, which in turn influences the material s multifunctional properties. Such materials are of great importance for their possible practical application. The stoichiometric alloy MnNiGe has a helical antiferromagnetic (AF) structure below the Neel temperature (T N ~ 370 K) with a TiNiSi-type orthorhombic structure (Pnma space group). Upon increasing the temperature it undergoes a first order structural transition (FOT) at T = 470 K from an orthorhombic TiNiSi-type crystal structure to a hexagonal Ni 2 In type (P6 3 /mmc space group), with both phases being paramagnetic [6]. Recently it has been observed that, by changing the stoichiometry and chemical composition of the parent MnNiGe compound, the first order structural transition temperature (FOT) can be altered. In some MnNiGe-based off-stoichiometric compounds, shifting the FOT below the Curie temperature results in a magnetostructural transition [7-11]. The concentration of valence electrons per atom (e/a), interatomic distance, and degree of hexagonal distortion (c/a ratio) are the major factors that control the phase transition. It has been reported that the structural stability of these compounds at low temperature are greatly affected by Mn-Mn separation (i.e., by the volume expansion of unit cell) [7, 8, 12]. In this work, we present the results of our studies on the fractional substitution of Ni by Cr in the Ni 1-

3 xcr x MnGe 1.05 system (for x = 0, 0.035, 0.070, and 0.105) and investigate the magnetostructural phase transition and magnetocaloric effects (MCEs), and construct a phase diagram of transition temperature as a function of concentration (x) and e/a ratio. III. EXPERIMENTAL TECHNIQUES The off-stoichiometric Ni 1-x Cr x MnGe 1.05 samples with 0 < x < were arc-melted in an ultra-high purity argon atmosphere using 99.99% purity elements. The compounds were annealed in high vacuum ( 10 5 torr) for 48 hours at 850 o C. An X-ray diffractometer with Cu Kα radiation was implemented to measure the room temperature X-ray diffraction (XRD) patterns, and the Rietveld profile refinement method was employed within the FULLPROF program to obtain the lattice constants of the samples. The magnetization measurements were obtained in a temperature interval of 5 to 380 K in magnetic fields up to 5 T using a superconducting quantum interference device magnetometer (SQUID by Quantum Design). The differential scanning calorimetry (DSC) measurements were obtained using a DSC 8000 instrument (with a ramp rate of 20 K/min during heating and cooling) in the temperature range of K. The latent heat (L) was estimated from the endothermic peak of the heat flow curves using L = T f dq T s dt dt, where dq is the change of heat flow with respect to temperature, dt and T s and T f are the initial and final temperatures of the magnetostructural phase transitions on heating, respectively. IV. RESULTS AND DISCUSSION The room temperature XRD patterns of Ni 1-x Cr x MnGe 1.05 (x= 0, 0.035, 0.070, 0.105) are shown in Figure 1. The parent compound, NiMnGe, has an orthorhombic TiNiSi- type structure at room temperature. Samples with higher chromium content (x = 0.035, 0.070, and 0.105) exhibit a hexagonal Ni 2- In type structure. It has been reported that doping the stoichiometric compound with foreign atoms of different size can induce a chemical

4 pressure with a similar effect as the application of an external mechanical pressure [13]. It was observed that an increase in Cr concentration results in an increase of lattice parameters of the hexagonal phase, resulting in the expansion of cell volume (see table 1). Thus the increase in lattice parameters and cell volume is due to the substitution of the larger ionic radius Cr (R = Å) for smaller Ni (1.246 Å), acting like negative chemical pressure, increasing the Mn Mn distance. It has been found from band structure calculations that the free energy of the low temperature phase depends on the degree of hexagonal distortion (c/a) [14]. In the case of x = 0.035, 0.070,and compounds, as c/a ratio decreases, the free energy is minimized, and the high temperature phase becomes stable at low temperature. Thus, the increase in cell volume of high temperature hexagonal phase results in stability loss of low temperature phase. The temperature dependence of the magnetization M(T) of Ni 1-x Cr x MnGe 1.05 (for x= 0.105) on heating and cooling under a field of 100 Oe is shown in Figure 2. The sample with higher Cr concentration (x = 0.105) shows two successive magnetic phase transitions. In the low temperature region, weak magnetization is observed that corresponds to the non-collinear AF state with a TiNiSi-type orthorhombic structure. The observation is consistent with stoichiometric MnNiGe [6]. On increasing the temperature, a jump-like increase in magnetization is observed at T M = 132 K with a thermal hysteresis of about 14 K. This lowtemperature transition corresponds to a first order magnetostructure transition (MST) from non-collinear AF orthorhombic phase to non-collinear ferromagnetic (FM) hexagonal phase. Further heating results a second order transition (SOT) from non-collinear FM to paramagnetic (PM) Ni 2 In-type structure at T C = 204 K. Figure 3 shows the DSC heat flow curves for Ni 1-x Cr x MnGe 1.05 (x= 0, 0.035, 0.070, 0.105) on heating and cooling cycles. Large endothermic and exothermic peaks were observed during heating and cooling cycles. For the compound with x = 0.105, two peaks

5 were observed. The first peak at 142 K corresponds to the crystallographic transition and second peak at 204 K is expected to be related to the magnetic phase transition. The observed thermal hysteresis of 17 K confirms the first order nature of the transition. The values of latent heat (L) and the corresponding total entropy changes (ΔS T ) were found to be 2.2 J/g and 16 J/KgK respectively in the vicinity of MST. The result from DSC experiment is consistent with the first order magnetostructural transition from AF orthorhombic phase to FM hexagonal phase seen in the magnetization measurements. The isothermal magnetization M(H) curves for the sample x = were measured around the transition temperature (T M ) in a magnetic fields up to 50 koe (Figure 4). At temperatures near T M, a change in magnetization was observed which resembles the noncollinear ferromagnetic nature. Considering the magnetization curve M(H), it can be inferred that the low-temperature transitions are from noncollinear AF to noncollinear FM. The magnetic entropy changes (ΔS M ) in the vicinity of the magnetostructural transition temperature for different magnetic fields (ΔH) are shown in Figure 5. The magnetic entropy changes (ΔS M ) were calculated using the Maxwell relation, ( S/ H) T = ( M/ T) H, from the magnetization isotherms measured at different temperatures [15]. An inverse MCE has been observed in the vicinity of T M (see Figure 5), which is due to the jump-like change of magnetization (see Figure 2) corresponding to the first-order magnetostructural transition from orthorhombic AF to hexagonal FM phases. Using DSC and magnetic data, a concentration-dependent phase diagram as a function of concentration (x) and e/a ratio has been constructed (Figure 6). With the chromium concentration x = 0.070, a triple-point was observed near the critical temperature of 210 K. A first order structural transformation and the second order magnetic phase transitions coincide to produce a first order magnetostructural transformation around the critical temperature. For x < 0.070, magnetic transitions from AF to PM states were observed with negligible change in

6 magnetization. For x > 0.070, magnetostructural transitions between an AF, orthorhombic TiNiSi-type state to a FM hexagonal Ni 2 In-type state were observed, followed by a magnetic transition from a FM to PM state. From Figure 6 it is concluded that with the increase in chromium concentration, the e/a ratio decreases, resulting in a decrease in the phase transition temperature. V. CONCLUSION The magnetostructural phase transitions and MCEs in off-stoichiometric NiMnGe 1.05 alloys have been studied, and a concentration-dependent phase diagram has been constructed for this system. The experimental results show that increasing the Cr concentration results a decrease of T M. As T M decreases below T C, a magnetostructural transition from an AF state to FM state was observed, followed by a jump in magnetization. Large positive and negative values of magnetic entropy changes were observed near T M and T C, respectively. Acknowledgements: Work at Southern Illinois University was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award No. DE-FG02-06ER Work at Louisiana State University was supported by DOE, Office of Science, BES under Award No. DE-FG02-13ER46946

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8 Tables Table1: Cell parameters of Ni 1-x Cr x MnGe 1.05 at room temperature. Chromium concentration (x) a (Å) c (Å) Volume of unit cell (Å) 3 c/a Figure captions Figure 1. The room temperature XRD patterns of Ni 1-x Cr x MnGe 1.05 (x = 0, 0.035,0.070, and 0.105). Figure 2. The temperature dependence of the magnetization M(T) of Ni 1-x Cr x MnGe 1.05 (x = 0.105) on heating and cooling under the field of 100 Oe. Figure 3. DSC heat flow curves for Ni 1-x Cr x MnGe 1.05 on heating and cooling cycles. Figure 4. The isothermal magnetization M(H) curves for the sample with x = Figure 5. The magnetic entropy changes as a function of temperature for Ni 1-x Cr x MnGe 1.05 (x=0.105) at different applied fields around T M and T C Figure 6. The phase diagram of transition temperatures as a function of concentration (x) and e/a ratio.

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