Advances in Condensed Matter Physics

Volume 2013 (2013), Article ID 286325, 5 pages

http://dx.doi.org/10.1155/2013/286325

Research Article

Thermodynamic, Electromagnetic, and Lattice Properties of Antiperovskite Mn₃SbN

Ying Sun,¹ Yan-Feng Guo,² Yoshihiro Tsujimoto,³ Xia Wang,² Jun Li,² Clastin I. Sathish,2,4 Cong Wang,⁵ and Kazunari Yamaura2,4

¹International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Ibaraki, Tsukuba 305-0044, Japan

²Superconducting Properties Unit, National Institute for Materials Science, 1-1 Namiki, Ibaraki, Tsukuba 305-0044, Japan

³Materials Processing Unit, National Institute for Materials Science, 1-1 Namiki, Ibaraki, Tsukuba 305-0044, Japan

⁴Graduate School of Chemical Sciences and Engineering, Hokkaido University, Hokkaido, Sapporo 060-0810, Japan

⁵Department of Physics, Center for Condensed Matter and Materials Physics, Beihang University, Beijing, Haidian 100191, China

Received 1 October 2012; Revised 4 December 2012; Accepted 5 December 2012

Academic Editor: Y. Sun

Copyright © 2013 Ying Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The physical properties of polycrystalline Mn₃SbN were investigated using measurements of the magnetic, calorimetric, and electronic transport properties. At room temperature, the phase crystallizes in a tetragonal structure with symmetry. A remarkably sharp peak in the heat capacity versus temperature curve was found near 353 K. The peak reaches 723 J mol⁻¹ K⁻¹ at its highest, which corresponds to a transition entropy of 10.2 J mol⁻¹ K⁻¹. The majority of the large entropy change appears to be due to lattice distortion from the high-temperature cubic structure to the room-temperature tetragonal structure and the accompanying Ferrimagnetic transition.

1. Introduction

Antiperovskite compounds with the formula Mn₃XN or Mn₃XC (X = Cu, Zn, Ga, Cu, In, or Sn) were discovered in the middle of the last century [1]. Recently, interest in these materials has intensively renewed owing to discoveries of new, potentially useful properties [2–4] such as the giant magnetoresistance of Mn₃GaC [5], negative thermal expansion (NTE) of Mn₃Cu(Ge)N [6] and Mn₃Zn(Ge)N [7], magnetostriction of Mn₃CuN [8] and Mn₃SbN [9], and near-zero temperature coefficient of the resistivity of Mn₃CuN [10] and Mn₃NiN [11]. Specifically, Takenaka and Takagi found that Ge-doped Mn₃CuN compound has a large NTE (NTE parameter = −25 × 10⁻⁶ K⁻¹) [12]; using neutron diffraction, the broad NTE was determined to be associated with the local T₄ structure [6]. Asano et al. discovered large magnetostriction in tetragonal Mn₃CuN; it expands 0.2% and shrinks 0.1% in the directions parallel and perpendicular to an external 90 kOe magnetic field, respectively [8]. In previous studies, we found a peculiar phase separation and accompanying anomaly in the electronic transport properties of Mn₃ZnN [13, 14], while further study indicated that the thermal expansion properties of Mn₃ZnN can be controlled by introducing Zn vacancies [15]. In addition, Song et al. observed a canonical spin-glass state in Mn₃GaN below the spin-freezing temperature of 133 K [16]. Lukashev et al. systematically studied the spin density of the spin-frustrated state of a Mn-based antiperovskite under mechanical stress [17].

The above-mentioned properties enable a variety of potential applications for this type of material. Although the prospective industrial markets are expected to be large and much effort has already been devoted to studying their structural, electromagnetic, and transport properties, further investigations on antiperovskite materials are still required. In this study, the thermodynamic, electromagnetic, and electronic transport properties of Mn₃SbN are investigated. In particular, we focused on the notable transition entropy that accompanies the magnetic and crystal structure transition above room temperature.

2. Experimental Details

A polycrystalline Mn₃SbN sample was prepared via the solid-state reaction of fine powders of Sb (99.99%, Rare Metallic Co.) and Mn₂N, which was synthesized by firing Mn powder (99.99%, Sigma Aldrich Co.) in nitrogen at 800°C for 60 h. Stoichiometric amounts of the starting materials were thoroughly mixed, and the mixture was pressed into a pellet. The pellet was sealed in an evacuated quartz tube, heated in a box furnace at 800°C for three days, and then slowly cooled to room temperature in the furnace.

The crystal structure of Mn₃SbN was analyzed by synchrotron X-ray diffraction (SXRD) using a large Debye-Scherrer camera at the BL15XU NIMS beam line of the SPring-8 facility in Hyogo, Japan. The SXRD data were collected for 2θ ranging from 2° to 60° at intervals of 0.003°. The incident beam was monochromatized at  Å. The evolution of the Mn₃SbN crystal structure with temperature was also determined via the measurement of the SXRD patterns.

The temperature dependence of magnetization was measured between 2 and 400 K with applied magnetic fields of 0.1 and 5 kOe using a Magnetic Property Measurements System (Quantum Design). The measurements were conducted on loosely gathered powder under both zero-field cooling (ZFC) and field cooling (FC) conditions. The isothermal magnetization curve was recorded at 10 K between −50 and 50 kOe.

Specific heat () values were measured between 2 and 400 K with cooling using a Physical Properties Measurement System (Quantum Design). The sample was fixed on a stage using a small amount of grease; the heat capacity of the grease was measured first and subtracted from the total .

The electrical resistivity () was measured between 2 and 400 K with cooling and heating using a conventional four-probe techniques with the same apparatus. The AC gauge current and frequency were 10 mA and 30 Hz, respectively. The electrical contacts were prepared on the surface of a bar-shaped piece of the pellet using silver paste and Pt wires.

3. Results and Discussion

As shown in Figure 1, the synchrotron XRD pattern at room temperature fit well with a model pattern of the proposed structure (space group: P4/mmm). The structural parameters of Mn₃SbN were refined by the Rietveld method using the RIETAN-FP program [18]. The occupancy factors of Sb, N, Mn1, and Mn2 were refined to be 1 (fixed), 1 (fixed), 0.97, and 0.99, respectively, while the isotropic atomic displacement parameters were 0.42, 0.84(5), 0.86, and 0.78 Å², respectively. The lattice constants were calculated to be a = b = 4.17994(4) Å and c = 4.27718(5) Å. The final and reliability indexes were below 5.56% and 4.09%, respectively. The analysis revealed 1.91 mass% MnO in the sample as an impurity; as shown later, the magnetic, , and measurements suggest that the impurity does not significantly impact the measurements of Mn₃SbN in this study.

Figure 1: Rietveld refinement of the synchrotron XRD pattern of Mn₃SbN. The crosses and solid lines represent the observed and calculated profiles, respectively, and the difference between the curves is shown at the bottom. Bragg positions are marked by small ticks (upper: Mn3ZnN, lower: MnO).

Figure 2 displays the temperature dependence of magnetization of polycrystalline Mn₃SbN. The magnetization steeply increases upon cooling to around 353 K, which suggests the establishment of long-range magnetic order at the magnetic transition temperature (). In addition, a small hysteresis can be observed between the heating and cooling process, implying the first-order character of the magnetic transition. The remarkable bifurcation between the ZFC and FC curves may originate from the spontaneous alignment of random magnetic Mn moments in domain boundaries. It is worth noting that the hysteresis is less significant at a higher magnetic field of 5 kOe, which supports the domain picture.

Figure 2: Temperature dependence of magnetization in magnetic fields of 0.1 and 5 kOe. The left and right insets show the isothermal magnetization curve at 10 K and versus T plot of the FC data at 0.1 kOe, respectively.

To further study the magnetic properties, we applied the Curie-Weiss law to the paramagnetic portion. As shown in the right inset of Figure 2, the plot is well represented by the Curie-Weiss law, that is, the spin-only expression for magnetic susceptibility: , where is the Curie constant and is the Weiss temperature. The value of was determined to be 354 K, which suggests that ferromagnetic correlation is dominant in the spin system. The effective Bohr magneton () was estimated to be 1.28 /Mn from (C/η)0.5, where is the number of magnetic atoms in the molecular formula ( in the present case). The value of is much lower than that of other antiperovskite manganese nitrides (e.g., 2.87  for Mn₃ZnN [14]) and even lower than the expected moment for localized spins, suggesting an itinerant character of the 3d electrons in Mn₃SbN.

From the isothermal magnetization curve (see the inset of Figure 2), it was found that the magnetization at 50 kOe is ~0.35 /Mn, which is too small to be caused by full ferromagnetic order. The gap suggests that the spins of the Mn atoms are possibly Ferrimagnetically ordered. This Ferrimagnetic interaction is also suggested by the magnetization characteristics above 10 kOe, that is, the magnetization continuously increases with increasing magnetic field without approaching saturation. The Ferrimagnetic order of a related Mn-based antiperovskite compound was explained by a spin structure, where two of the three Mn magnetic moments are antiferromagnetically coupled and the third exhibits FM behavior [19]. It is possible that a similar magnetic structure is established in Mn₃SbN below 353 K.

To further characterize the magnetic transition, the specific heat was measured from 400 to 2 K. As shown in Figure 3, the temperature dependence of features a sharp and narrow peak around (/R = 87 and ΔT = 3 K, where is the ideal gas constant). This is indicative of a first-order-like transition, as discussed in [20].

Figure 3: Temperature dependence of the of Mn₃SbN. The left and right insets show a linear fit to the /T versus curve and an estimation of , respectively.

An estimation of entropy change is essential to understanding the nature of the transition of Mn₃SbN. The peak was roughly separated from the baseline using a polynomial function. Analysis indicates that the total transition entropy () is ~1.23R (10.2 J/mol K). Since the total entropy change comprises all contributions, including the lattice, electronic, and magnetic changes [20], we evaluated each contribution independently.

For the present compound, the abrupt change of the magnetization at may induce a large ∂ M/∂ T; therefore, a large magnetic entropy change () is expected. A series of magnetization curves with small temperature steps were measured; the data allow for a rough estimation of the magnetic entropy change via the thermodynamic Maxwell relation, as follows [21]:

The magnetic entropy change, (), can be calculated by

The temperature dependence of calculated from (2) with fields of 10, 20, 30, 40, and 50 kOe is shown in Figure 4. The is maximized around , and the maximum is estimated to be ~2.1 J mol⁻¹ K⁻¹, which implies that the lattice and electronic changes provide a fairly large contribution to the total entropy change.

Figure 4: Temperature dependence of the magnetic entropy change when the magnetic field changes from 0 to 10, 20, 30, 40, and 50 kOe, respectively.

To investigate the electronic contribution (i.e., the Sommerfeld coefficient or ), the versus plot below 10 K was analyzed by applying the approximate Debye model, as follows: = + 2.4n (1/) (,), where denotes the number of atoms per formula unit, is the Boltzmann constant, is the Avogadro constant, and is the Debye temperature. Fitting to the linear part of the versus plot using the least-squares method yielded and values of ~7.03 mJ mol⁻¹ K⁻² and 326 K, respectively. Compared with the parameters determined for other antiperovskite nitrides, Mn₃SbN has a much lower , which indicates that the electronic correlation is somewhat weakened [20]. Thus, the electronic contribution might not be a dominant contributor to the total transition entropy.

In addition to the magnetic and electronic contributions, a possible lattice change may need to be investigated to analyze the total transition entropy. The variation of the synchrotron XRD pattern with temperature was measured. As shown in Figure 5(b). It can be seen that some typical reflections disappear with temperature, for example, the two reflections (002) and (200) for the P4/mmm lattice merge to one reflection. By the Rietveld analysis of the synchrotron XRD patterns, the structural change from tetragonal to cubic was defined, and the lattice constants were determined as a function of temperature, as shown in Figure 5(a). It is obvious that lattice parameter increases slightly with increasing temperature, whereas gradually decreases. When the temperature crosses , the tetragonal structure completely transforms to an unidentified cubic structure. Hence, the lattice distortion must contribute to the total entropy change.

(a)

(b)

Figure 5: Variation in (a) lattice parameters and (b) synchrotron XRD patterns with temperature for Mn₃SbN.

According to the thermodynamic relation, the magnetization () is equal to the first derivative of the magnetic free energy by the magnetic field, that is, df()/dH [22]. Therefore, the sharp transition indicates that the energy barrier in the free energy that separates the paramagnetic and ferromagnetic states is large. Accordingly, and the energy barrier height probably correlate with the electronic density of states, which exhibits a sharp peak near the Fermi level [23]; therefore, the large entropy change is possibly related to the reconstruction of the electronic structure, which could induce the magnetic and structural transition. Since such an electronic reconstruction is often sharply reflected in a curve, the electronic transport properties of Mn₃SbN were carefully measured (shown in Figure 6). It is evident that an abnormal drop appears at in the curve, which is indicative of an electronic structure reconstruction. In addition, as shown in the inset of Figure 6, a small hysteresis was observed between the warming and cooling curves; this is in agreement with a first-order transition.

Figure 6: Variation of with temperature for Mn₃SbN upon cooling and warming. The inset shows an enlarged view of the variation around the magnetic transition.

4. Conclusions

In conclusion, the thermodynamic, electromagnetic, and transport properties of antiperovskite Mn₃SbN were studied. The phase crystallizes in a tetragonal structure with a = b = 4.17994(4) Å and c = 4.27718(5) Å at room temperature. The measurements revealed a sharp endothermic peak in the curve at 353 K, which corresponds to a large entropy change (~10.2 J mol⁻¹ K⁻¹). The present study clearly indicates that the entropy change is accompanied with a Ferrimagnetic transition and lattice distortion as well as a possible electronic structure reconstruction.

Conflict of Interests

The authors declare that they have no conflict of interests.

Acknowledgments

The authors would like to thank the staff members at BL15XU, the National Institute for Materials Science (NIMS), and SPring-8 for their help in the use of the beamline. The SXRD measurements were performed with the approval of the NIMS Beamline Station (Proposal no. 2011A4502). This study was supported in part by the World Premier International Research Center Initiative of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT); a Grant-in-Aid for Scientific Research grant (no. 22246083) from the Japan Society for the Promotion of Science (JSPS); the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program) of the JSPS; the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).

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