Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn 1 x CoGe alloys

Transcription

1 July 2010 EPL, 91 (2010) doi: / /91/ Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn 1 x CoGe alloys E. K. Liu 1,W.Zhu 1,L.Feng 1,J.L.Chen 1,W.H.Wang 1,G.H.Wu 1(a),H.Y.Liu 2, F. B. Meng 2,H.Z.Luo 2 and Y. X. Li 2 1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences Beijing , China 2 School of Material Science and Engineering, Hebei University of Technology - Tianjin , China received 25 March 2010; accepted in final form 16 June 2010 published online 20 July Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.) Kf Martensitic transformations Sg Magnetocaloric effect, magnetic cooling Abstract It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn 1 xcoge alloys (0 x 0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field induced martensitic transformation and a giant magnetocaloric effect in the Mn 1 xcoge system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration. Copyright c EPLA, 2010 The magnetic-field induced martensitic transformation and the associated magnetocaloric effect in various kinds of systems have been increasingly studied for sensing [1 4] and magnetocaloric applications [5,6]. Accompanying the martensitic structural transformation, there always exists a pronounced magnetic transition between different magnetic states with a large magnetization difference ( M), which is of great importance for the various magnetostructural transitions. As a typical MM X (M, M = transition metals, X = Si, Ge, Sn) compound, stoichiometric MnCoGe crystallizes in a hexagonal Ni 2 Intype austenite (A) phase and, at about 420 K, in the paramagnetic state, it undergoes a martensitic structural transformation to the, also paramagnetic, orthorhombic TiNiSi-type martensite (M) phase [7 10]. Experiments and calculations have shown that both phases are collinear ferromagnets [10,11] and the M phase carries a larger magnetic moment than the A phase [12,13]. As NiAs-Ni 2 In-type compound, off-stoichiometric hexagonal MnCoGe is prone to form transition-metal Co vacancies at the 2d sites [14] and the vacancies notably decrease the martensitic-transformation temperature (T m ) [15,16]. (a) ghwu@iphy.ac.cn In Co-poor Mn 1.07 Co 0.92 Ge, therefore, a field-induced martensitic transformation from ferromagnetic austenite to ferromagnetic martensite (FM/FM-type) has been reported [15]. Meanwhile, various studies [15,17 19] have shown that the Curie temperatures of both martensite and austenite depend intensively on the Co content in Co-poor off-stoichiometric alloys. According to the aforementioned, one may expect that, if the T m could be appreciably lowered, while keeping A and T C M approximately unchanged, a martensitic transformation inside a temperature window limited by A and T C M from paramagnetic austenite to ferromagnetic martensite (PM/FM-type transformation), similar to the situation in Fe 2 MnGa [20,21], might be obtained. This may lead to a larger magnetization difference ( M) between austenite and martensite. Simultaneously, this PM/FM-type transition may also lead to an enhanced caloric effect because of the equivalent exothermic/endothermic behavior at the magnetostructural transition. Very recently, Hamer et al. [22] have substituted Sn for Ge in MnCoGe and found a fixed temperature window, in which a PM/FM-type martensitic transformation takes place. This has made the MnCoGe 1 x Sn x system potentially interesting as p1

2 E. K. Liu et al. field-induced shape memory alloys and as magnetocaloric material. In this letter, we report that it is possible to realize a quite similar temperature window around room temperature by tuning the Co-vacancy concentration in Mn-poor Mn 1 x CoGe alloys. It will be shown that the transformation temperature T m can be tuned in the temperature window between A and T C M and that the desired PM/FM-type martensitic transformation is accompanied by a considerable M. From a high-temperature X-ray diffraction (XRD) study on Mn 0.98 CoGe [23], it is known that Co atoms partially occupy the Mn-deficient sublattice and that vacancies at Co sublattices are formed instead. In the present study of the Mn 1 x CoGe system, we consider the Mn-vacancy and Co-vacancy structures and calculate their total energies in the Ni 2 In-type parent phase using an ab initio method. The results confirm that the Co-vacancy structure (Mn 1 x Co x )(Co 1 x x )Ge, is energetically favourable. We may thus attempt to take advantage of this Co-vacancy structure by properly reducing the Mn content to tune the phase transition in Mn 1 x CoGe alloys. One can see that the Co-vacancy content is equal to the deficiency level (x) ofmninthe Mn 1 x CoGe alloys. Polycrystalline ingots of Mn 1 x CoGe (x = 0, 0.010, 0.020, 0.030, 0.035, and 0.050) were prepared by arcmelting pure metals in high-purity argon atmosphere. The ingots were annealed at 1123 K for five days and quenched in cool water. The crystal structures of the specimens were identified by powder XRD analysis with Cu-K α radiation. Magnetization measurements were carried out in a superconducting quantum interference device (SQUID). Also, differential thermal analysis (DTA) with heating and cooling rates of 2.5 K/min was used to investigate the martensitic transformation. Figure 1 shows the XRD patterns obtained at different temperatures for the alloy with x = At 320 K, the Ni 2 In-type structure is detected with lattice parameters a = and c = Å. At 220 K, the Bragg reflections of the TiNiSi-type structure are well indexed with the lattice parameters a =5.9323, b = and c = Å. The unit-cell volume has increased by 4.0%, which means a huge lattice distortion during the transformation. The crystal structures of both phases are also shown in fig. 1. XRD examinations at room temperature (not shown) confirm that the alloys with the compositions x = all exhibit a martensitic transformation with T m monotonously decreasing in a temperature region around room temperature. This shows that the value of T m in Mn 1 x CoGe is sensitive enough to the Mn-deficiency level (x), i.e. to the Co-vacancy concentration, to be tailored to a desired temperature. Figure 2 shows the M(T) curves of Mn 1 x CoGe (x = 0.010, 0.030, and 0.045) alloys in a low magnetic field of 0.01 T; the characteristic parameters T m, M, T C A, and the thermal hysteresis ( T ) are listed in table 1. Fig. 1: (Color online) Power XRD patterns of the Mn 1 xcoge alloy with x =0.035 at 320 and 220 K upon cooling. The Miller indices hkl denote the Ni 2In-type hexagonal and TiNiSi-type orthorhombic structures, respectively. The unit cells of both structures are also shown. Fig. 2: (Color online) Temperature dependence of the magnetization of Mn 1 xcoge (x = 0.010, 0.030, and 0.045) alloys in a field of 0.01 T. M s(t m)andm f denote the starting and finishing temperatures of the martensitic transformation, A s and A f the starting and finishing temperatures of the reverse transformation, while the A and M denote Curie temperatures of A and M phases, respectively. The inset shows the phase diagram for the Mn 1 xcoge system. The solid pentacles denote the measured T C values and the open pentacles extrapolated T C values. In fig. 2, M = 353 K at the high-temperature side corresponds to the Curie temperature of the M phase for the alloy with vacancy content x =0.010, while A = 261 K at the low-temperature side corresponds to that of the A phase for the alloy with vacancy content x = These values of M and T C A are consistent with those of stoichiometric MnCoGe ( A = 273 and T C M = 355 K) [8,9] and determine a temperature interval here. It can be seen p2

3 Temperature window in Mn 1 x CoGe alloys Table 1: Composition dependence of T m, T M C, T A C, M and T for Mn 1 xcoge alloys. x T m (K) T M C (K) T A C (K) M (A m2 /kg) T (K) a a b c c d 260 a The T m and T are determined using DTA as the transformation occurs in the temperature range where the system is in paramagnetic state. The M values are absent due to the same reason. b The data of this alloy are determined using DTA and magnetic measurements as its transformation is situated at the M. c The M and T C A are immeasurable due to the PM/FM-type martensitic transformation behaviors of these two alloys. d The T m, M and T are absent as the martensitic transformation behavior of this alloy vanishes after annealing. T A C that the martensitic transformations of these two alloys, occurring at 365 K and 242 K (see table 1) respectively, are both outside the temperature interval. Further measurements of the alloys with x = 0 and show that their M values lie, with slight variation, at the highest border temperature, the M value for x = Similarly, the A value of the alloy with x =0.050 also coincides with the lowest border temperature, the A value for x = This clearly shows that the T C values of martensite and austenite are both insensitive to the Mn-deficiecy level (x) in Mn 1 x CoGe alloys with x between 0 and Thus, in the Mn-poor Mn 1 x CoGe system, there exists a temperature window as wide as about 100 K between Mand in which martensitic transformations occur. This situation is very similar to that in MnCoGe 1 x Sn x alloys with a similar window [22]. It implies that this intrinsic temperature window can be uncovered by different tailoring methods. Collecting these results, a phase diagram for the Mn 1 x CoGe system (0 x 0.050) is proposed in the inset of fig. 2. The T m of all alloys regularly decreases through the window. In the vacancy content range of <x<0.045, the alloys exhibit a PM/FMtype martensitic transformation. On cooling, the paramagnetic A phase transforms to the ferromagnetic M phase above the hypothetical ordering temperature A so that the A becomes irrelevant. Likewise, on heating, the ferromagnetic M phase inversely transforms to the paramagnetic A phase below M and the T C M value is not experimentally observable anymore. For clarity, we have extrapolated the A and T C M values and indicated them in the inset of fig. 2 by open pentacles ( ). Here, one should note that this coincidence of the structural and the magnetic transitions is quite different from the situation in the Ni 2 MnGa system, in which T m merges with A at the same temperature [24]. According to the results above, it is clearly observed that T m in this Mn-poor Mn 1 x CoGe system strongly correlates with the Mn deficiency-level (x), while with a weak Mn deficiency-level dependence of Curie temperatures of both austenite and martensite. In the MnCoGe crystal structure, the Co and Ge atoms form threedimensional chain networks via covalent bonds owing to the sp and sd orbital hybridization [9,25]. The existence of Co vacancies in the Co-Ge-Co chains inevitably destroys a part of the chemical bonds, which may result in destabilization of the M phase at lower temperature and in weakening of the elastic modulus of the A phase [26,27]. The phase transformation thus occurs at lower temperature and needs a smaller thermodynamic driving force to overcome the energy barrier of M nucleation [28,29]. As shown in table 1, T of the martensitic transformation indeed exhibits a decreasing trend, from 22 to 12 K, as a function of increasing vacancy content. The decrease of both T m and T may profit the field-induced martensitic transformation and magnetocaloric effect. In the Mn-poor Mn 1 x CoGe system, a notable difference with respect to the Co-poor MnCo 1 x Ge system is that the Curie temperatures of martensite and austenite depend only weakly on the Mn deficiency level (x). If the Mn deficiency changes, the valence-electron concentration (e/a) determined as the concentration-weighted sum of s, d, and p electrons, will also change. We here propose e/a as a tentative parameter to examine the change of Curie temperatures, as shown in fig. 3. It can be seen that the Curie temperatures of both phases remain almost unchanged, whereas T m, exhibits a strong dependence on e/a in the present e/a range. The weak e/a dependence of the Curie temperature suggests there is only a weak change of the ferromagnetic exchange in both the martensite and the austenite phases which, according to the spin-fluctuation theory [30], may be ascribed to a slight alteration of the electronic states at the Fermi level. Similar features have been reported in Heusler-type NiMnbased alloys with phase transitions [31,32]. From the inset p3

4 E. K. Liu et al. Fig. 3: (Color online) Valence-electron concentration (e/a) dependence of the martensitic transition temperature (T m), Curie temperatures of austenite and martensite (T A C and T M C ) of Mn 1 xcoge alloys. The inset shows the vacancy-content (x) dependence of e/a. The value of e/a has been determined as the concentration-weighted sum of s, d, and p electrons. Fig. 5: (Color online) Isothermal magnetization curves of the alloy with x =0.035 at various temperatures across the martensitic transformation. Fig. 6: (Color online) Isothermal magnetic-entropy changes derived from magnetic isotherms of the alloy with x =0.035, measured in fields up to 5 T. Fig. 4: (Color online) Temperature dependence of the magnetization of Mn 1 xcoge (x =0.010, 0.030, and 0.045) alloys in a field of 5 T. of fig. 3, one can also see that, in Mn 1 x CoGe alloys, the e/a change with decreasing Mn content is very small, whereas, in sharp contrast, the e/a change with the same decrease of the Co content is much larger in MnCo 1 x Ge alloys. This may account for the stable values of Mand A in the Mn 1 xcoge system and this deserves further study. In fig. 4, we present the thermomagnetization of the Mn 1 x CoGe alloys in a high field of 5 T. The M associated with the martensitic transformation are listed in table 1. The alloys in the window, exhibit quite abrupt increments of the magnetization due to the PM/FM-type transformation with M values up to about 60 A m 2 /kg. This value is about twice as large as the value of about 32 A m 2 /kg of Co-poor MnCoGe at the FM/FM-type martensitic transformation [15]. For the alloy with x = 0.045, along with the transformation temperature decreasing out of the window, i.e. T m <T A C, a FM/FM-type transformation also occurs and the M reduces to only 34 A m 2 /kg. Concerning the large value of M at the transformation in the window, we present in fig. 5 isothermal M(H) curves for the alloy with x =0.035, measured upon cooling in fields up to 13 T. From 300 down to 285 K, a metamagnetic transition with hysteresis indicates that a field-induced martensitic transformation occurs in this temperature region, due to the large difference in Zeeman energy between the PM A and the FM M phases. At 5 K, the curve shows typical ferromagnetic magnetization behavior of the M phase with a high saturation magnetization of 109 A m 2 /kg p4

5 Temperature window in Mn 1 x CoGe alloys The magnetocaloric effect for the alloy with x =0.035 has been derived from magnetization curves measured in fields up to 5 T, as shown in fig. 6. Around T m,the magnetic entropy S m shows giant and negative values. Maximum values of 10 J/kg K for B = 0 2T and 26 J/kg K for B = 0 5 T are observed, which are comparable with values for other materials in a similar temperature range, such as Gd 5 Si 2 Ge 2 ( 14 J/kg K for B = 0 2 T) [33], Ni 50 Mn In ( 28.6J/kg K for B = 0 5 T) [34] and MnCoGe 0.95 Sn 0.05 alloys ( 4.5 J/kg K for B = 0 1 T) [22]. As mentioned previously, in this PM/FM-type field-induced martensitic transformations, the exothermic/endothermic effect corresponding to the magnetic-entropy change has the same sign as that of the martensitic structural transformation, which enhances the caloric effect. 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