The effects of substituting Ag for In on the magnetoresistance and magnetocaloric properties of Ni-Mn-In Heusler alloys

The effect of substituting Ag for In on the structural, magnetocaloric, and thermomagnetic properties of Ni50Mn35In15−xAgx (x = 0.1, 0.2, 0.5, and 1) Heusler alloys was studied. The magnitude of the magnetization change at the martensitic transition temperature (TM) decreased with increasing Ag concentration. Smaller magnetic entropy changes (ΔSM) were observed for the alloys with larger Ag concentrations and the martensitic transition shifted to higher temperature. A shift of TM by about 25 K to higher temperature was observed for an applied hydrostatic pressure of P = 6.6 kbar with respect to ambient pressure. A large drop in resistivity was observed for large Ag concentration. The magnetoresistance was dramatically suppressed due to an increase in the disorder of the system with increasing Ag concentration. Possible mechanisms responsible for the observed behavior are discussed.


I. INTRODUCTION
Magnetic materials with large magnetic entropy changes (∆S M ) induced by relatively small changes in magnetic field are potential candidates for applications in new refrigeration technology. 1 In the vicinity of the phase transition temperatures, ∆S M shows a maximum value due to the sharp change in magnetization.Magnetic materials undergoing first order transitions (FOT) exhibit a jump-like change in their magnetization and magnetization-dependent properties.Some of the off-stoichiometric Ni-Mn-In based Heusler alloys undergo a temperature induced magnetostructural (martensitic) transition below the magnetic ordering temperature (T C ).3][4][5][6][7][8][9] These properties are a consequence of a magnetostructural phase transition and therefore are related to each other.It has been shown that the properties of these alloys are extremely sensitive to changes in intrinsic parameters such as chemical composition and the type of crystal structure, and also on extrinsic parameters such as fabrication techniques and conditions, annealing temperature, applied magnetic field, pressure, rate of heating and cooling, sequence of measurements, and cycling [see Refs .10 and 11].Therefore, systematic studies of the magnetic and related properties of the new In-based system are important from a basic science and applications point of view.
The magnetic behavior of Ni 50 Mn 35 In 15 can be characterized by the transitions from a ferromagnetic martensitic to a low magnetization state (LMS) at T CM = (190-200) K, a first order magnetostructural transition from a LMS to a ferromagnetic austenitic state (FAS) T A = (270-310K), and from a FAS to paramagnetic austenitic state at T C ∼ 325 K. 6,7,12 The effects of compositional variations and applied pressure on the phase transitions and phenomena related to the magnetostructural transition in Ni 50 Mn 35 In 15 based Heusler alloys have been reported in Refs.has been shown that small variations in atomic compositions induced through "local" inhomogeneity can be considered as one of the key factors in the temperature stabilization of the martensitic and austenitic phases. 11][15] In our work, we describe the evolution of the magnetic and structural properties of Ni 50 Mn 35 In 15−x Ag x induced by small substitutions of Ag in the In sites and the effects of applied hydrostatic pressure.

II. EXPERIMENTAL TECHNIQUES
Bulk polycrystalline samples of Ni 50 Mn 35 In 15−x Ag x (x = 0.1, 0.2, 0.5, and 1) with a total mass of 4 g were prepared by arc-melting under the constant flow of "ultra-high" purity argon using a water-cooled bronze crucible and tungsten electrode.The samples were melted four times to establish homogeneity.Total losses of about 1% or less of the total mass has been detected for all samples after arc-melting.It is assumed (and is a justified assumption) that the 1% loss is due to lost Mn.The samples were wrapped in tantalum foil and annealed in high vacuum (∼10 −5 torr) at 850 • C for 48 hrs.X-ray diffraction (XRD) measurements were done at room temperature with an X-ray diffractometer using Cu-Kα radiation to determine the phase purity and crystal structures.The magnetic properties were measured at temperature ranging from 5-400 K and in magnetic fields up to 5 T using a Quantum Design superconducting quantum interference device magnetometer (SQUID).The resistance and magnetoresistance (MR) of the samples were studied using the four probe method in the same temperature and magnetic field intervals.The ∆S M (T, H) was estimated from the isothermal magnetization curves near the martensitic/magnetic transition temperature using the Maxwell relation. 13

∆S
In the measurements under hydrostatic pressure, Daphne (7373) oil was used as the pressuretransmitting medium and the value of the applied pressure was calibrated by measuring the shift of the superconducting transition temperature of Pb used as a reference manometer (T C ∼7.2 K at ambient pressure).Thermal behavior was studied using a differential scanning calorimetry (DSC) by employing a Perkin-Elmer DSC 8000 instrument (with a ramp rate of 20 K/min during heating and cooling) in the temperature range of 123-473K.

III. RESULTS AND DISCUSSION
The  2).The saturation magnetization (at T = 10 K and H = 5T) decreases with Ag concentration (see inset of Fig. 2).This behavior can be attributed to an increase in the antiferromagnetic interaction or in the magneto-crystalline anisotropy in the alloys.The M(H) curves obtained under applied pressure demonstrate a behavior characteristic of a ferrimagnetic non-collinear magnetic structure.Thus the applied hydrostatic pressure induces a change in magnetic structure to a non-collinear ferrimagnetic type.A similar behavior has been observed for Ni 50 Mn 35 In 15 based compounds in Ref. 15.
The magnetization (M) as a function of temperature (T) curves for various concentrations are shown in Fig. 3.All magnetization curves were measured in an applied field of 0.1 T. The thermal hysteresis observed in the M(T) data obtained while cooling and heating signifies the first order nature of the martensitic transition.The M(T) curves for Ni 50 Mn 35 In 14.9 Ag 0. shows that the hysteresis increases with the application of pressure.A shift of T M by about 25 K to a higher temperature was observed for P = 6.6 kbar.A suppression of the magnetization with the application of pressure was observed.This is due to a decrease in the Mn-Mn separation which is most likely related to an increase in antiferromagnetic interaction induced by a reduction of crystal cell volume with the applied pressure.
DSC heat flow curves as a function of temperature for Ni 50 Mn 35 In 15-x Ag x obtained for heating and cooling cycles are shown in Fig. 5.The large endothermic/exothermic peaks, observed during heating/cooling cycles are related to the latent heat of the first-order magnetostructural transition from the low magnetization to the ferromagnetic structure.The temperature hysteresis of the heat behavior as a function of temperature.For a field variation of 5 T, the maximum values of -∆S M were found to be 6 J/(kg K) for x=0.1 in the vicinity of T M .This value of ∆S M is smaller than that of the parent alloy Ni 50 Mn 35 In 15 . 4The small value of ∆S M could be due the fact that martensitic transition is not as sharp as in the parent alloy.The magnitude of the magnetization change decreases dramatically with increasing Ag concentration.Since the magnetocaloric effects depends on the magnitude of the magnetization change for Mn -rich Heusler alloys, smaller ∆S M are observed for the alloys with higher Ag concentration (see Fig. temperature until it approaches the martensitic transition temperature (T M ), and then decreases abruptly in the vicinity of T M .After that, the resistivity increases almost linearly as the temperature increases further.A large jump in the resistivity was observed near the magnetostructural transitions due to a metamagnetic transition where the system changes from a low magnetization state with strong antiferromagnetic correlations to a ferromagnetic austenitic phase. 11,16The large value of the resistivity observed for the martensitic phase is most likely related to a change in the electronic band structure, which alters the density of states near the Fermi surface and therefore strongly affects the electronic transport properties of the system.Fig. 7 shows the abruptly drop in resistivity with increasing Ag concentration.The application of magnetic field results in a shift of T M to lower temperature and therefore induces a large magnitoresistance (MR) (see Fig. 8).Maximum value of MR of about −16% was observed in the vicinity of T M for x=0.1, and the peak MR decreases significantly with increasing Ag concentration.The decrease in MR is due to the increase in disorder of the systems with increasing Ag concentration.This result is consistent with the magnetization data shown in Fig. 2, 3, and 4.

IV. CONCLUSION
In summary, we have investigated the magnetic, structural, and transport properties of Ni 50 Mn 35 In 15−x Ag x Heusler alloys.A shift in the martensitic transition temperature (T M ) by about 25 K was observed under an applied hydrostatic pressure for Ni 50 Mn 35 In 14.9 Ag 0.1 .The magnitude of the magnetization change decreases dramatically with increasing Ag concentration, which results in a smaller ∆S M for the alloys with higher Ag concentrations.A splitting of the ∆S M (H, T) curves into two maxima was observed for Ni 50 Mn 35 In 14.9 Ag 0.1 under the application of pressure.The magnetoresistance is dramatically suppressed with increasing Ag concentration.The application of external pressure results in a decrease in magnetization and an increase in the hysteresis of the martensitic transition.
FIG. 1. Room temperature XRD patterns of Ni 50 Mn 35 In 15−x Ag x for different Ag concentrations.The subscripts 'M' and 'A' denote the martensitic and austenitic phases, respectively.The cell parameters of the austenitic phase have been calculated using the (220) XRD peaks.
1 and Ni 50 Mn 35 In 14.8 Ag 0.2 alloys at H = 5 T and 0.1 T, obtained for cooling (open symbols) and heating (closed symbols) cycles without and with applied pressure, are shown in Fig. 4(a) and Fig. 4(b), respectively.As shown in Fig 4(a), the magnitude of the magnetization change at T M decreases dramatically with increasing Ag concentration.Fig. 4(b)

056213- 4 Pandey
FIG. 3. M(T) curves for Ni 50 Mn 35 In 15-x Ag x obtained at H = 0.01 T. Open and closed symbols show the data collected during heating and cooling, respectively.