Thermodynamic, Electromagnetic, and Lattice Properties of Antiperovskite Mn3SbN

Title Thermodynamic, Electromagnetic, and Lattice Properties of Antiperovskite Mn3SbN Author(s) Sun, Ying; Guo, Yan-Feng; Tsujimoto, Yoshihiro; Wang, Xia; Li, Jun; Sathish, Clastin I.; Wang, Cong; Yamaura, Kazunari Citation Advances in Condensed Matter Physics, 2013: 286325 Issue Date 2013 Doc URL http://hdl.handle.net/2115/52006 Rights(URL) http://creativecommons.org/licenses/by/3.0/ Type article File Information ACMP2013_286325.pdf


Introduction
Antiperovskite compounds with the formula Mn 3 XN or Mn 3 XC (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][3][4] such as the giant magnetoresistance of Mn 3 GaC [5], negative thermal expansion (NTE) of Mn 3 Cu(Ge)N [6] and Mn 3 Zn(Ge)N [7], magnetostriction of Mn 3 CuN [8] and Mn 3 SbN [9], and nearzero temperature coefficient of the resistivity of Mn 3 CuN [10] and Mn 3 NiN [11]. Speci�cally, Takenaka and Takagi found that Ge-doped Mn 3 CuN 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 T structure [6]. Asano et al. discovered large magnetostriction in tetragonal Mn 3 CuN; it expands 0.2% and shrinks 0.1% in the directions parallel and perpendicular to an external 90 kOe magnetic �eld, respectively [8]. In previous studies, we found a peculiar phase separation and accompanying anomaly in the electronic transport properties of Mn 3 ZnN [13,14], while further study indicated that the thermal expansion properties of Mn 3 ZnN can be controlled by introducing Zn vacancies [15]. In addition, Song et al. observed a canonical spin-glass state in Mn 3 GaN 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].
e 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 Mn 3 SbN are investigated. In particular, we focused on the notable transition entropy that accompanies the magnetic and crystal structure transition above room temperature.

Experimental Details
A polycrystalline Mn 3 SbN sample was prepared via the solid-state reaction of �ne powders of Sb (99.99%, Rare Metallic Co.) and Mn 2 N, which was synthesized by �ring 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. e 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. e crystal structure of Mn 3 SbN 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. e SXRD data were collected for 2 ranging from 2 ∘ to 60 ∘ at intervals of 0.003 ∘ . e incident beam was monochromatized at 2 Å. e evolution of the Mn 3 SbN crystal structure with temperature was also determined via the measurement of the SXRD patterns.
e temperature dependence of magnetization was measured between 2 and 400 K with applied magnetic �elds of 0.1 and 5 kOe using a Magnetic Property Measurements System (Quantum Design). e measurements were conducted on loosely gathered powder under both zero-�eld cooling (ZFC) and �eld cooling (FC) conditions. e isothermal magnetization curve was recorded at 10 K between −50 and 50 kOe.
Speci�c heat ( ) values were measured between 2 and 400 K with cooling using a Physical Properties Measurement System (Quantum Design). e sample was �xed on a stage using a small amount of grease; the heat capacity of the grease was measured �rst and subtracted from the total . e electrical resistivity ( ) was measured between 2 and 400 K with cooling and heating using a conventional fourprobe techniques with the same apparatus. e AC gauge current and frequency were 10 mA and 30 Hz, respectively. e electrical contacts were prepared on the surface of a barshaped piece of the pellet using silver paste and Pt wires.

Results and Discussion
As shown in Figure 1, the synchrotron XRD pattern at room temperature �t well with a model pattern of the proposed structure (space group: P4/mmm). e structural parameters of Mn 3 SbN were re�ned by the Rietveld method using the RIETAN-FP program [18]. e occupancy factors of Sb, N, Mn1, and Mn2 were re�ned to be 1 (�xed), 1 (�xed), 0.97(1), and 0.99(1), respectively, while the isotropic atomic displacement parameters were 0.42(1), 0.84(5), 0.86(1), and 0.78(1) Å 2 , respectively. e lattice constants were calculated to be a = b = 4.17994(4) Å and c = 4.27718(5) Å. e �nal wp and reliability indexes were below 5.56% and 4.09%, respectively. e 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 signi�cantly impact the measurements of Mn 3 SbN in this study. Figure 2 displays the temperature dependence of magnetization of polycrystalline Mn 3 SbN. e 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 �rst-order character of the magnetic transition. e 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 signi�cant at a higher magnetic �eld of 5 kOe, which supports the domain picture. 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 −1 -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. e value of was determined to be 354 K, which suggests that ferromagnetic correlation is dominant in the spin system. e effective Bohr magneton ( eff ) was estimated to be 1.28 /Mn from eff 2.83(C/ ) 0.5 , where is the number of magnetic atoms in the molecular formula ( 3 in the present case). e value of eff is much lower than that of other antiperovskite manganese nitrides (e.g., 2.87 for Mn 3 ZnN [14]) and even lower than the expected moment for localized 1 2 spins, suggesting an itinerant character of the 3d electrons in Mn 3 SbN.
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. e gap suggests that the spins of the Mn atoms are possibly Ferrimagnetically ordered. is Ferrimagnetic interaction is also suggested by the magnetization characteristics above 10 kOe, that is, the magnetization continuously increases with increasing magnetic �eld without approaching saturation. e Ferrimagnetic order of a related Mn-based antiperovskite compound was explained by a Γ 4 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 Mn 3 SbN below 353 K.
To further characterize the magnetic transition, the spe-ci�c heat was measured from 400 to 2 K. As shown in Figure  3, the temperature dependence of features a sharp and narrow peak around (Δ /R = 87 and ΔT = 3 K, where is the ideal gas constant). is is indicative of a �rst-orderlike transition, as discussed in [20]. An estimation of entropy change is essential to understanding the nature of the transition of Mn 3 SbN. e peak was roughly separated from the baseline using a polynomial function. Analysis indicates that the total transition entropy (Δ ) 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 (Δ ) 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]: e magnetic entropy change, Δ ( ), can be calculated by e temperature dependence of Δ calculated from (2) with �elds of 10, 20, 30, 40, and 50 kOe is shown in Figure 4. e Δ 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.
To investigate the electronic contribution (i.e., the Sommerfeld coefficient or ), the versus 2 plot below 10 K was analyzed by applying the approximate Debye model, as follows: , where denotes the number of atoms per formula unit, is the Boltzmann constant, 0 is the Avogadro constant, and Θ is the Debye temperature. Fitting to the linear part of the versus 2 plot using the least-squares method yielded and Θ values of ∼7.03 1 mJ mol −1 K −2 and 326 2 K, respectively. Compared with the parameters determined for other antiperovskite nitrides, Mn 3 SbN has a much lower , which indicates that the electronic correlation is somewhat weakened [20]. us, 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. e variation of the synchrotron XRD pattern with temperature was measured. As shown in Figure 5(b). It can be seen that some typical re�ections disappear with temperature, for example, the two re�ections (002) and (200) for the P4/mmm lattice merge to one re�ection. By the Rietveld analysis of the synchrotron XRD patterns, the structural change from tetragonal to cubic was de�ned, 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 unidenti�ed cubic structure. �ence, the lattice distortion must contribute to the total entropy change.
According to the thermodynamic relation, the magnetization ( ) is equal to the �rst derivative of the magnetic free energy by the magnetic �eld, that is, df ( )/dH [22]. erefore, 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 o�en sharply re�ected in acurve, the electronic transport properties of Mn 3 SbN 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 �rst-order transition.

Conclusions
In conclusion, the thermodynamic, electromagnetic, and transport properties of antiperovskite Mn 3 SbN were studied. e phase crystallizes in a tetragonal structure with a = b = 4.17994(4) Å and c = 4.27718(5) Å at room temperature. e 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 ). e 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.
�on��ct of �nterests e authors declare that they have no con�ict of interests.