Magnetism and Magnetocaloric Properties of Mn 3 Zn 1 x Sn x C and Mn 3 x Cr x ZnC Compounds

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1 J. Mater. Sci. Technol., 212, 28(1), Magnetism and Magnetocaloric Properties of Mn Zn 1 x Sn x C and Mn x Cr x ZnC Compounds Naikun Sun 1), Yaobiao Li 2), Feng Liu 1) and Tongbo Ji 2) 1) School of Science, Shenyang Ligong University, Shenyang 11159, China 2) Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 1, China [Manuscript received September 1, 211, in revised form December 12, 211] Upon substitution of Sn for Zn, the Curie temperature of Mn ZnC is lowered from 8 to 75 K for Mn Zn.95 Sn.5 C and to 5 K for Mn Zn.75 Sn.25 C. In accordance with the second-order ferromagneticparamagnetic transition, a room-temperature magnetocaloric effect without thermal and magnetic hysteresis is observed over a wide temperature range. The maximum value of the magnetic-entropy change S M for a magnetic-field change from to 5 T is 2.42 J kg 1 K 1 1 at 86 K for Mn Zn.95 Sn.5 C and 1.7 J kg 1 K at 8 K for Mn Zn.75 Sn.25 C. Meanwhile, substitution of Cr for Mn lowers the temperature of ferromagneticferrimagnetic transition from 2 K for Mn ZnC to 2 K for Mn 2.9 Cr.1 ZnC and to 175 K for Mn 2.1 Cr.9 ZnC. An inverse magnetocaloric effect with S M equal to.28 J kg 1 K 1 at 22 K for a field change from to 1.47 T is observed for Mn 2.9 Cr.1 ZnC. KEY WORDS: Magnetocaloric effect; Hysteresis; Second-order magnetic transition 1. Introduction In recent years, it has been reported that M XC(N) (M, transition metal, X main group element) compounds with the antiperovskite structure exhibit a variety of interesting physical properties such as giant magnetoresistance [1], superconductivity [2], and close correlation among lattice, spin and charge around the Curie temperature []. In particular, a large magnetocaloric effect (MCE) has been found for Mn GaC [4] and Mn SnC [5]. However, the MCE reported so far for compounds with the antipervoskite structure is based on a first-order magnetic transition (FOMT) and inevitably accompanied by large thermal and magnetic hysteresis, which limits practical application of these materials. Recently, the systems have been found to exhibit a reversible MCE at room temperature based on a second-order magnetic transition (SOMT), such as (Gd 1 x RE x ) 5 Si 4 (RE=Dy,Ho) [6], MnCo 1 x Al x Ge [7], Corresponding author. Assoc. Prof., Ph.D.; Tel./Fax: ; address: liyaobiao@yahoo.cn (Y.B. Li). Mn 5 PB [8] 2 and (Co.5 Mn.65 ) 2 P [9]. On the other hand, generally, the cost of MCE materials based on d elements is much lower than that for materials based on rare-earth elements. Moreover, the M XC(N) system does not contain a poisonous element such as As in the MnAs and FeMnPAs systems. All this has motivated us to investigate the reversible room-temperature MCE based on a SOMT in the M XC(N) family. Mn ZnC is a ferromagnet with a Curie temperature of 8 K and another magnetic transition at 2 K from a ferromagnetic (FM) state to a ferrimagnetic (FI) one with a noncollinear magnetic structure. The latter transition is accompanied by a structural change from cubic to tetragonal [1]. In the present study, through the substitution of Sn for Zn, the Curie temperature of Mn ZnC is lowered from 8 K to around room temperature, 5 K, for Mn Zn.75 Sn.25 C and a reversible MCE with a maximum value of the magnetic-entropy change S M of 2.42 J kg 1 K 1 is obtained at 86 K for a field change from to 5 T for Mn Zn.95 Sn.5 C. The effect of substitution of Cr for Mn on the FM FI transition in

2 942 N.K. Sun et al.: J. Mater. Sci. Technol., 212, 28(1), Mn ZnC has been investigated as well. 2. Experimental Mn Zn 1 x Sn x C (x=.5 and.25) and Mn x Cr x ZnC compounds (x=.1 and.9) were synthesized from Mn, Cr, Zn, Sn and carbon powders with purity higher than 99.9%. The starting materials were mixed in stoichiometric proportion and an excess (5%) of carbon powder was added to compensate for mass loss during sintering. Then, the mixtures were pressed into pellets and first slowly heated to 6 C and kept at this temperature for 24 h in an evacuated silica tube. Then, the samples were heated at 8 C for 7 days. After cooling down to room temperature, the samples were pulverized, pressed into pellets again and heated at 8 C for another 7 days and then gradually cooled to room temperature. X-ray diffraction (XRD) was performed using CuKα radiation with a Rigaku d/max-γa diffractometer equipped with a graphite-crystal monochromater. The temperature dependence of the ac initial susceptibility was measured to determine the Curie temperature of Mn Zn.95 Sn.5 C. The magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer and a vibratingsample magnetometer (Lakeshore 747) (1 K T 4 K, T B 5 T). The change of the magnetic entropy was derived from M B plots at temperatures close to the phase transition.. Results and Discussion.1 Magnetocaloric properties of Mn Zn 1 x Sn x C compounds XRD results show that the Mn Zn 1 x Sn x C compounds are single phase, only displaying reflections that are characteristic for the anti pervoskite cubic structure. The lattice parameters derived from the XRD patterns are.9 and.95 nm for x=.5 and.25, respectively. Since the lattice constant of Mn ZnC is.924 nm, the results indicate that the lattice parameter increases upon substitution of Sn for Zn. The temperature dependence of the magnetization of Mn Zn.75 Sn.25 C was recorded in a field of.1 T, in both cooling and heating processes between 28 and 8 K (Fig. 1). The temperature dependence of the ac susceptibility was measured to determine the Curie temperature of Mn Zn.95 Sn.5 C (inset of Fig. 1). It can be clearly seen that, with increasing temperature, a second-order FM to paramagnetic (PM) transition occurs at Curie temperatures of 5 and 75 K for x=.25 and.5, respectively. The ordering temperatures are in good agreement with published reports [1,11]. The Curie temperature (T C ) is defined as the temperature corresponding to the minimum in the temperature dependence of the first derivative of the magnetization (dm/dt ). There is x=.25 a.u. x= Fig. 1 Temperature dependence of the magnetization at.1 T of Mn Zn.75Sn.25C compounds during the cooling and heating processes. The inset shows the temperature dependence of the ac initial susceptibility of Mn Zn.95Sn.5C no clear thermal hysteresis, which shows the secondorder nature of the FM PM transition. To investigate the MCE of the Mn Zn 1 x Sn x C compounds, the magnetic field dependence of magnetization was measured near Curie temperature in applied fields up to 5 T (Fig. 2(a) and (b)). The magnetic isotherms clearly show the second-order nature of the transition. Moreover, as shown in the inset of Fig. 2(a), the M(B) curves for Mn Zn.75 Sn.25 C at 1 K measured with increasing and decreasing field completely overlap, which also confirms the secondorder nature of the FM PM transition. It is worth noting that the magnetization of Mn Zn.75 Sn.25 C at 5 T and T C =5 K, is.1 A m 2 kg 1, while the magnetization of Mn Zn.95 Sn.5 C at 5 T and T C =75 K is 41.1 A m 2 kg 1. As stated above, upon substitution of Sn for Zn, the lattice parameter increases and the Mn Mn distance increases accordingly. Therefore, the FM coupling between the nearest Mn moments decreases, leading to a decrease of both Curie temperature and saturation magnetization [11,12]. A similar decrease of the Curie temperature has been observed for Ge-doped Mn ZnC, where T C was lowered to 275 K for Mn Zn.5 Ge.5 C [1]. In general, the value of the isothermal magneticentropy change S M (T,H) is given by the following expression associated with the Maxwell relationship: H S M (T, H)=S(T, H) S(T, )= ( M ) dh T H (1) It has been reported that the Maxwell relationship is inadequate to calculate the magneticentropy change of some compounds in the case of a FOMT [14,15], because of the coexistence of PM

3 N.K. Sun et al.: J. Mater. Sci. Technol., 212, 28(1), (a) T=5 K 285 K x= (b) 15 1 K K - S M / J kg -1 K x=.25 x= Fig. Magnetic-entropy changes of Mn Zn 1 xsn xc compounds at an external-field change from to 5 T 68 K 7 K 2 75 K 77 K 79 K 81 K 1 x=.5 8 K 88 K 9 K 4 K Fig. 2 Field dependence of the magnetization of Mn Zn.75Sn.25C (a) and Mn Zn.95Sn.5C (b) at temperatures indicated and FM phases. As analyzed above, the magnetic transition in the present compounds is a SOMT, so that the Maxwell relationship can safely be used to derive S M. The temperature dependence of the magnetic-entropy change S M of Mn Zn 1 x Sn x C with a magnetic field change from to 5 T is shown in Fig.. The maximum S M for this field change is 2.42 J kg 1 K 1 at 86 K for Mn Zn.95 Sn.5 C and 1.7 J kg 1 K 1 at 8 K for Mn Zn.75 Sn.25 C. Furthermore, S M values larger than 1.8 and 1.44 J kg 1 K 1 are obtained in the temperature ranges of 7 97 K and K for Mn Zn.95 Sn.5 C and Mn Zn.75 Sn.25 C, respectively. Moreover, it should be emphasized that the MCE in Mn Zn 1 x Sn x C compounds is completely reversible with no thermal and magnetic hysteresis, which is very practically meaningful. Finally, the Mn Zn 1 x Sn x C compounds are based on d elements. Compared with the systems based on rare earth elements, such as FC ZFC FC ZFC x=.1 x=.9 Fig. 4 Temperature dependence of the magnetization at.15 T of Mn xcr xznc compounds after ZFC and FC Gd 5 (Ge 1 x Si x ) 4, the lower cost of raw materials is another advantage of the present compounds..2 Magnetism and magnetocaloric properties of Mn x Cr x ZnC compounds XRD patterns certify that the prepared Mn x Cr x ZnC compounds are single phase with the cubic antiperovskite structure. The derived lattice-parameter values are.921 and.914 nm for Mn 2.9 Cr.1 ZnC and Mn 2.1 Cr.9 ZnC, respectively, smaller than that for Mn ZnC. Fig. 4 shows the temperature dependence of the magnetization of Mn x Cr x ZnC compounds measured in a field of.15 T after zero-field cooling (ZFC) and field cooling (FC) processes from 1 to K. Quite clearly, with

4 944 N.K. Sun et al.: J. Mater. Sci. Technol., 212, 28(1), K K; T=5 K K K; T=5 K 18 K 2 x= K 1 x= Fig. 5 Field dependence of the magnetization of Mn 2.9Cr.1ZnC and Mn 2.1Cr.9ZnC at temperatures indicated S M / J kg -1 K Fig. 6 Magnetic-entropy changes of Mn 2.9Cr.1ZnC at an external-field change from to 1.47 T increasing temperature, a FOMT from the FI to the FM state takes place at phase-transition temperature T FM FI equal to 2 and 175 K for the compounds with x=.1 and.9, respectively, showing that the substitution of Cr for Mn shifts the FI FM transition in Mn ZnC (at 2 K) to lower temperatures. The phase-transition temperatures, T FM FI, and T C, are defined as the temperatures where the first derivative of the magnetization (dm/dt ) with respect to temperature has a maximum and minimum, respectively. The thermal hysteresis of the both FM FI transitions is about K, which confirms that the transition is first order. It has been reported by Antonov et al. [1] that the transition is accompanied by a structural change from cubic to tetragonal. The magnetic isotherms of Mn 2.9 Cr.1 ZnC around T FM FI are shown in Fig. 5. The M(B) curves at temperatures from 225 to 2 K show a rapid increase of magnetization in low magnetic fields and the magnetization is almost saturated when the field reaches.5 T, exhibiting typical FM behaviour. Although the magnetization at temperatures below T FM FI, such as 195 K, increases rapidly at small magnetic fields, it is not saturated even at a field of 1.47 T, indicating a FI state at these temperatures. As shown in the inset 1 of Fig. 5, the magnetization is only.5 A m2 kg at T FM FI =175 K for Mn 2.1 Cr.9 ZnC, while the 1 2 kg at magnetization is 55 A m T FM FI =2 K for Mn 2.9 Cr.1 ZnC. This result shows that both the Curie temperature and the saturation magnetization decrease upon substitution of Cr for Mn. The magnetic isotherms for Mn 2.1 Cr.9 ZnC, measured with steps of 5 K in the range of K, almost coincide and do not exhibit a tendency to saturate below 1.47 T. Similar to the FM FI transition, an inverse MCE is observed 1 for Mn 2.9 Cr.1 ZnC, with S M =.28 J kg 1 K at 22 K for a field change from to 1.47 T (Fig. 6). The values of the isothermal magnetic-entropy change S M (T,H) of Mn 2.9 Cr.1 ZnC have been calculated by means of the Maxwell relationship and the sign of S M is determined by the sign of M/ T, as shown in formula (1). No clear MCE is observed for Mn 2.1 Cr.9 ZnC. 4. Conclusion A room-temperature magnetocaloric effect without clear thermal and magnetic hysteresis is observed over a wide temperature range in the Mn Zn 1 x Sn x C system. The maximum value of the magnetic entropy change S M for a magnetic field change from to 5 T is 2.42 J kg 1 K 1 at 86 K for Mn Zn.95 Sn.5 C and 1.7 J kg 1 K 1 at 8 K for Mn Zn.75 Sn.25 C. Substitution of Cr for Mn lowers the temperature of the FM FI transition in Mn ZnC. An inverse magnetocaloric effect with S M =.28 J kg 1 K 1 at 22 K at a field change from to 1.47 T is observed for Mn 2.9 Cr.1 ZnC. Acknowledgements This work was supported by the Dr Research Startup Fund of Shenyang Ligong University (No. 28 (2)), the National Natural Science Foundation of China (No ) and the Natural Science Foundation of Jilin Province, China (No ). REFERENCES [1 ] Y.B. Li, W.F. Li, W.J. Feng, Y.Q. Zhang and Z.D. Zhang: Phys. Rev. B, 25, 72, [2 ] T. He, Q. Huang, A.P. Ramirez, Y. Wang, K.A. Reran, N. Rogado, M.A. Hayward, M.K. Haas, J.S. Slusky, K.

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