013906-2 Ingale et al. J.Appl.Phys.102.013906(2007 balance the contribution from the sample holder. The data were collected during the heating as well as the cooling cycles at a given 20 K/min rate. A hysteresis of the thermo- gram occurs in the two cycles due to thermochemistry of the aPec ample of MC effects of interest in this work s|a=06320mm 2|b=0.553 Orthorhombic(7M) c=0.5369 ca <1 I. RESULTS AND DISCUSSION A. Phase transformation and crystal structure a=0.546nr Tetragonal(NM Analysis of XRD patterns in Fig. I reveals that all three c=0.647nm c/a >1 alloys are nearly single phase with no detectable secondary phases. The Nis4. 8Mn203 Ga24.9 alloy has a nonmodulated (NM)tetragonal martensite structure with lattice parameters Diffraction angle 20(degree) a=0.5460 nm and c=0.6471 nm Fig. 1(a)]. The lissMn1&oGa261 alloy [Fig. 1(b)] resumes a seven-layer iGM -ray dittranon patterns of (a) Nis4 Mn 20 Ga24. (b) modulated(7M) structure of orthorhombic symmetry: a =0. 6320 nm. b=0.5573 nm. and c=0.5369 nm was ob served. The NM or 7M structures are analyzed by splitting of by confirming the alloys'chemistry. The as-cast alloy ingots the (202) peak into three peaks:(220), (202), and(022) were cut into small species and used for the proposed studies in this work Similar XRD patterns were reported in NissMn18 Ga26 B Measurements and analyses Mn- ga alloys in terms of xrd can be considered onlv as a The crystalline structure of the alloys was analyzed by preliminary observation rather than the confirmation. Using X-ray diffraction(XRD). The XRD patterns of the samples XRD data, Martynov et ad 2i reported c/a>l(a were measured (after cutting, crushing, and pulverizing the 0.5520 nm and c=0.6440 nm) for the NM martensite alloy ingots as a course powder)using a Philips diffracto- phase. The crystal structures of the five-layer modulated meter with 0.154 058 nm Cu Ka radiation. The microstruc (5M)and seven-layer modulated(7M) martensite phases are ture characterization was done using a Leo model 440i scan- still complex. Such structure can form in the martensite ning electron microscope(SEM) Magnetic proprieties were phase from the parent (p)austenite phase by a periodic shuf- measured with a DMS-1600 model vibrating sample magne- fling of the lattice along(110)[110]p or with a long-period tometer(VSM) with a magnetic field H up to 12 kOe. Ther- stacking of (110)p close-packed planes. 19.21 The basic unit momagnetic measurements were carried out at a fixed H cell of the 5M phase is approximated to a tetragonal or =500 Oe value in order to determine Ty and Tc values. monoclinic cell,cla<l, while that of the 7M phase is de Magnetization as a function of temperature was measured termined to be orthorhombic or monoclinic(a=0.6140 nm using a variable temperature cryostat attached to the VSM. b=0.5780 nm, and c=0.5510 nm) The data were collected during a heating cycle at a rate of 10 ence, in such a complex modulated structure of the K/min. The temperature was controlled within an accuracy martensite phase in Ni-Mn-Ga alloy, the c/a ratio is taken as +l K. For different regions, the temperature ste the measure to predict roughly the kind of the modulation; in this study we have adopted a similar approach, taking into A modulated differential scanning calorimeter (TA in- account the (202) splitting as we discussed above. It is evi- struments DSC model Q100)was used to monitor the heat dent from XRD in Fig. 1(c)that in the Niss. 2 Mn8 Ga26 How during magnetostructural transformations. The sample, alloy the austenite phase has tuned at the expense of the sealed in a standard aluminum cup with a lid, was measured martensite phase at room temperature. A small change in the against a similar cup with a lid under identical conditions to Mn/Ga ratio tunes the phase formation sensitively FIG. 2. Typical SEM images NisssMnxoaGaz49; Niss MniggGa26by confirming the alloys’ chemistry. The as-cast alloy ingots were cut into small species and used for the proposed studies in this work. B. Measurements and analyses The crystalline structure of the alloys was analyzed by x-ray diffraction
XRD. The XRD patterns of the samples were measured
after cutting, crushing, and pulverizing the alloy ingots as a course powder using a Philips diffracto￾meter with 0.154 058 nm Cu K radiation. The microstruc￾ture characterization was done using a Leo model 440i scan￾ning electron microscope
SEM Magnetic proprieties were measured with a DMS-1600 model vibrating sample magne￾tometer
VSM with a magnetic field H up to 12 kOe. Ther￾momagnetic measurements were carried out at a fixed H =500 Oe value in order to determine TM and TC values. Magnetization as a function of temperature was measured using a variable temperature cryostat attached to the VSM. The data were collected during a heating cycle at a rate of 10 K/min. The temperature was controlled within an accuracy ±1 K. For different regions, the temperature steps were var￾ied from 1 to 2.5 K. A modulated differential scanning calorimeter
TA in￾struments DSC model Q100 was used to monitor the heat flow during magnetostructural transformations. The sample, sealed in a standard aluminum cup with a lid, was measured against a similar cup with a lid under identical conditions to balance the contribution from the sample holder. The data were collected during the heating as well as the cooling cycles at a given 20 K/min rate. A hysteresis of the thermo￾gram occurs in the two cycles due to thermochemistry of the sample of MC effects of interest in this work. III. RESULTS AND DISCUSSION A. Phase transformation and crystal structure Analysis of XRD patterns in Fig. 1 reveals that all three alloys are nearly single phase with no detectable secondary phases. The Ni54.8Mn20.3Ga24.9 alloy has a nonmodulated
NM tetragonal martensite structure with lattice parameters a=0.5460 nm and c=0.6471 nm Fig. 1
a. The Ni55Mn18.9Ga26.1 alloy Fig. 1
b resumes a seven-layer modulated
7M structure of orthorhombic symmetry; a =0.6320 nm, b=0.5573 nm, and c=0.5369 nm was ob￾served. The NM or 7M structures are analyzed by splitting of the 202 peak into three peaks:
220,
202, and
022. Similar XRD patterns were reported in Ni55Mn18Ga26 alloy.19,20 The analysis of the modulated structure in these Ni￾Mn-Ga alloys in terms of XRD can be considered only as a preliminary observation rather than the confirmation. Using XRD data, Martynov et al.21 reported c/a1
a =0.5520 nm and c=0.6440 nm for the NM martensite phase. The crystal structures of the five-layer modulated
5M and seven-layer modulated
7M martensite phases are still complex. Such structure can form in the martensite phase from the parent
p austenite phase by a periodic shuf- fling of the lattice along
110110P or with a long-period stacking of 110P close-packed planes.19,21 The basic unit cell of the 5M phase is approximated to a tetragonal or monoclinic cell, c/a1, while that of the 7M phase is de￾termined to be orthorhombic or monoclinic
a=0.6140 nm, b=0.5780 nm, and c=0.5510 nm. 21–24 Hence, in such a complex modulated structure of the martensite phase in Ni-Mn-Ga alloy, the c/a ratio is taken as the measure to predict roughly the kind of the modulation; in this study we have adopted a similar approach, taking into account the 202 splitting as we discussed above. It is evi￾dent from XRD in Fig. 1
c that in the Ni55.2Mn18.1Ga26.7 alloy the austenite phase has tuned at the expense of the martensite phase at room temperature. A small change in the Mn/Ga ratio tunes the phase formation sensitively. FIG. 1. X-ray diffraction patterns of
a Ni54.8Mn20.3Ga24.9;
b Ni55Mn18.9Ga26.1; and
c Ni55.2Mn18.1Ga26.7 alloys. FIG. 2. Typical SEM images in
a Ni54.8Mn20.3Ga24.9;
b Ni55Mn18.9Ga26.1; and
c Ni55.2Mn18.1Ga26.7 alloys. 013906-2 Ingale et al. J. Appl. Phys. 102, 013906 2007