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Advances in Condensed Matter Physics

Volume 2013 (2013), Article ID 729458, 9 pages

http://dx.doi.org/10.1155/2013/729458

## The Magnetic/Electrical Phase Diagram of Cr-Doped Antiperovskite Compounds ()

^{1}Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei 230031, China^{2}High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China

Received 19 September 2012; Revised 13 November 2012; Accepted 20 November 2012

Academic Editor: Koshi Takenaka

Copyright © 2013 S. Lin 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

We report the effect of Cr doping on the structural, magnetic, and electrical transport properties in Cr-doped antiperovskite compounds (). With increasing the Cr content , the lattice constant increases while both the Curie temperature and the saturated magnetization decrease gradually. The electrical resistivity shows different behaviors as a function of . For , there exists a semiconductor-like behavior below a certain temperature which decreases with increasing . In contrast, for , the resistivity shows a metallic behavior in the whole temperature measured (2–350 K). In particular, the Fermi-liquid behavior is obtained below 70 K. Finally, based on the magnetic and electrical properties of () a magnetic/electrical phase diagram was plotted.

#### 1. Introduction

Since MgCNi_{3} was firstly reported as a new superconductor with the antiperovskite structure [1], the antiperovskite compounds AXM_{3} (A = Ga, Al, Sn, Zn, Cu, In, Ge, Ag; X = C, N; M = Mn, Fe, Ni) have been extensively investigated [2–13]. Particularly, plenty of theoretical and experimental studies were performed for the Ni-based and Mn-based antiperovskite compounds [2–12]. Based on previous theoretical investigations [14, 15], the antiperovskite compounds AXM_{3} have similar electrical band structure which determines the basical physical properties, indicating that all these antiperovskite compounds (e.g., Mn-, Fe-, or Ni-based) may exhibit similar physical properties.

Recently, considerable attention has been paid to the Fe-based antiperovskite compounds due to their interesting physical and mechanical properties, such as low temperature coefficient of resistance (LTCR) [16], magnetocaloric effect (MCE) [13, 17, 18], and good corrosion resistance [19, 20]. GaCFe_{3} has been investigated for several decades as a typical Fe-based antiperovskite compound. It is a ferromagnetic (FM) material with the FM Curie temperature () of 510 K as reported previously [15]. Very recently, the studies of chemical doping at Ga site in GaCFe_{3} have been reported, and some interesting physical properties such as extremely LTCR and large room-temperature MCE have been observed [13, 16, 17]. Based on the previous investigations of Mn- or Ni-based antiperovskite compounds, the chemical doping has been proved to be an effectual method to manipulate the basic physical properties [6, 10, 21–24]. As reported previously in antiperovskite compounds [25], [26, 27], and [28], the chemical doping at Mn site can affect the magnetic and electrical transport properties significantly. Similarly, the chemical doping at Fe site for GaCFe_{3} may be performable. However, no report was found on the chemical substitutions for the Fe site of Fe-based antiperovskite compound GaCFe_{3}. As reported previously [14, 15], the density of states at the Fermi level [()] is mainly from Fe 3d electrons in AXFe_{3}. As we know, the Cr (3d^{5}4s^{1}) has one less 3d electron than Fe (3d^{6}4s^{2}) atom. Meanwhile, the Cr atom is bigger in size than that of Fe. The influences of hole doping and the change of lattice constant on the physical properties in will be thus interesting.

In this paper, the structural, magnetic, and electrical evolutions with doping level in () were investigated. It is found that the lattice expands while both the Curie temperature and the saturated magnetization decrease gradually as increases. For with , the electrical resistivity exhibits a semiconductor-like behavior at low temperature while displaying a metallic behavior at elevated temperature. As the doping level is above 0.7, the resistivity is metallic with a temperature dependence below 70 K. Finally, a magnetic/electrical phase diagram of () was presented.

#### 2. Experimental Details

Polycrystalline samples () were prepared as described in our previous paper [13]. Powder X-ray diffraction (XRD) data was collected at room temperature using an X-ray diffractometer with Cu radiation (PHILIPS, nm). The data of XRD was refined using a standard Rietveld technique to determine the crystal structure parameter and the purity of phase. Magnetic measurements were performed on a superconducting quantum interference device magnetometer (SQUID 5T, Quantum Design) and on a vibrating sample magnetometer (VSM, Quantum Design). The electrical resistivity and specific heat were carried out on a physical property measurement system (PPMS 9T, Quantum Design). The electrical transport properties were measured by the standard four-probe method to eliminate the contact resistivity.

#### 3. Results and Discussion

Figure 1(a) shows the room-temperature XRD patterns for the samples (, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0). Obviously, all are single phase with a cubic antiperovskite structure (space group: *Pm*3*m*) except for a small amount of chromium (marked by open triangle) for . Moreover, according to the vertical bars the central position of peak (200) of shifts to lower angles with an increase of Cr content,suggesting that the lattice constant expands with increasing . The Rietveld refinements of XRD patterns were performed for all samples to determine the lattice parameter. As shown in Figure 1(b), the refined lattice parameter increases with , which is consistent with the shift of peak (200) in Figure 1(a). The expansion of lattice can be attributed to the larger atom radius of Cr than that of Fe.

Figure 2(a) presents temperature-dependent magnetization K) curves for () at 100 Oe under zero-field-cooled (ZFC) process. Obviously, each curve exhibits an FM-PM transition as reported in GaCFe_{3} [15]. The Curie temperature (determined by the minimal value of , shown in Figure 2(b)) is presented in Figure 2(d). It displays that the value of decreases with increasing Cr content . In order to investigate the magnetic ground state of , the isothermal curves at 5 K are measured and plotted in Figure 2(c). Apparently, all the curves display similar -dependent behavior: with increasing the magnetic field, the magnetization increases initially and then tends to saturation above 10 kOe. Here, the value of saturated magnetization is obtained by the extrapolation of the high field (10–45 kOe) curve to -axis, which is shown in Figure 2(c) (the green dash line). As shown in Figure 2(d), the decreases with increasing .

According to the previous theoretical studies [14, 15, 29], the density of states at the Fermi level [()] is mainly from Fe 3d electrons in AXFe_{3}. Generally, the change of can be caused by many factors such as expansion/shrinkage of lattice, Fe/Cr 3d-C 2p hybridization effect, and band filling effect. In a conventional scheme, the expansion of lattice constant may give rise to the increasing of () due to the decrease of bandwidth [13, 16]. However, another important factor is associated with the detailed electronic structure at Fermi level. In GaCFe_{3}, the calculation of band structure suggests that the position of is located at the left hand of a sharp peak [14]. Considering the electronic structure of Cr (3d^{5}4s^{1}) and Fe (3d^{6}4s^{2}), the Cr substitution for Fe can be considered as hole doping. Assuming a rigid band model, it will shift the to lower energy side. As a result, the value of() in is reduced with increasing . Based on the Stoner model of itinerant ferromagnetism, the reduced () will lead to a decrease in the magnetization [30]. Therefore, -dependent could be understood based on the above discussions. Now, how can the Cr-doping reduce the ? In itinerant ferromagnetism the main contribution to the magnetism is the itinerant electrons. Here, in GaCFe_{3} the itinerant electrons are mainly 3d electrons of transition element. With the Cr doping level increasing, the 3d electrons decrease, which leads to a decrease in the exchange integral . Correspondingly, the value of decreases with increasing . In a word, the evolutions of and are mainly attributed to the cooperation of the lattice expansion and changes of electronic structure by chemical doping.

In order to investigate the type of magnetic transition for , we measured the curves around the corresponding for the selected samples. Figure 3(a) displays the isothermal magnetization curves measured between 180 and 390 K with the magnetic fields up to 45 kOe for GaCFe_{2.6}Cr_{0.4}. The curves were measured under the increasing/decreasing field processes around (305 K). As shown in Figure 3(a), all the curves around are reversible during the increasing/decreasing field processes without any hysteresis, indicating a second-order magnetic transition [9]. In addition, the Arrott plots derived from around are presented in Figure 3(b). For GaCFe_{2.6}Cr_{0.4}, it is evident that the slope of versus curve at high magnetic fields is positive for each temperature measured, confirming a second-order magnetic transition again [9, 23]. Analogously, for GaCFe_{2.2}Cr_{0.8} ( K), the curves and the Arrott plots in a temperature range of 100–250 K were shown in Figures 3(c) and 3(d), respectively, which confirm a second-order magnetic transition too. It is reasonable to conclude that the FM-PM transition is of second order for the left compositions in serial .

Figures 4(a) and 4(b) illustrate the temperature dependence of resistivity () at zero magnetic field (2–350 K) for . All the magnitude of resistivity is comparable with other isostructural compounds [9, 22, 31, 32]. Figure 4(c) presents the temperature-dependent resistivity at both zero magnetic field and 5 T. Obviously, both curves are almost overlapped. The MR value (defined as ) is positive and very small (≤1%) even at 5 T, suggesting that the magnetic field has little impact on the resistivity in . Figure 5(a) shows the normalized resistivity for between 2 and 350 K. There exists a minimal resistivity () and the corresponding temperature decreases with increasing (to see Figure 5(b)) for low-doping samples. In high-doping samples (), the disappears, which can be seen clearly in Figure 5(c). As shown in Figure 5(d), the low- resistivity was well fitted by the formula ( and represent the residual resistivity and -term coefficient of the resistivity, resp.) up to about 70 K, indicating a Fermi-liquid behavior for () at lower temperatures. That is to say, the electron-electron scatterings are dominant at lower temperature (5–70 K) [5, 9]. Furthermore, the fitting parameter decreases with increasing , which is shown in the left inset of Figure 5(d). However, at the elevated temperatures (100–270 K), increases almost linearly with temperature for the samples with according to the fitting results (see the inset of Figure 5(c)), meaning that the contributions of electron-phonon scatterings exceed those of the electron-electron scatterings [9, 16]. In addition, the slope of linear part decreases with increasing as displayed in the right inset of Figure 5(d). The above results can be simply understood as follows: with increasing the temperature, the number of excited phonons increases quickly, resulting in the enhancement of the phonon scatterings [9].

Figures 6(a)–6(c) show the specific heat for () measured between 7 and 70 K at zero field. As shown in the inset of Figures 6(a)–6(c), the data below 20 K is plotted as versus which can be well expressed by using the following equation [5]:
where (Sommerfeld constant) is the electronic contribution, is the phonon contribution, and is deviations from the linear dispersion of the acoustic modes in extended temperature range. The fitted parameters , *, *and as a function of are shown in Figure 6(d) and the insets of Figure 6(d), respectively. With increasing, the and increase while the decreases. Accordingly, the Debye temperature (where is the number of atoms in a unit cell and equal to 5 for ()) is derived from the value of coefficient . The detailed results of , , , *, **, *and for () are summarized in Table 1. Generally, the value of Kadowaki-Woods (KW) ratio is the well-known measure of electron-electron correlation [3, 16]. By taking the values of and listed in Table 1, the KW ratio was calculated for (). For , 0.8, and 0.9 the values of KW ratio are 2.70 × 10^{−7} *μ*Ω cm/(mJ/mol K)^{2}, 3.46 × 10^{−7} *μ*Ω cm/(mJ/mol K)^{2}, and 2.38 × 10^{−7} *μ*Ω cm/(mJ/mol K)^{2}, respectively, which are much less than the universal value *μ*Ω cm/(mJ/mol K)^{2} of for strong electron-electron system [3], meaning a weak electron-electron correlation.

Finally, based on the data of magnetic and electrical transport properties, the magnetic/electrical phase diagram for () is plotted in Figure 7. As for the magnetic phase diagram there are FM and PM regions; and the metallic, semiconducting, and Fermi-liquid areas are shown in electrical phase diagram.

#### 4. Conclusion

In summary, we present the magnetic/electrical phase diagram of (). The influences of the Cr doping on the structure, magnetic, and electrical transport properties have been investigated systematically. With increasing the Cr content , the lattice parameter increases while both the and decrease gradually. For the samples with , the resistivity curves show a semiconductor-like behavior below a certain temperature, which decreases as increases. On the contrary, for the semiconductor-like behavior disappears, and the metallic behavior is dominant in the whole temperature range (2–350 K). Particularly, the samples with show a Fermi-liquid behavior at low temperatures. In addition, we confirm that the FM-PM transition in the serial is of second order.

#### Acknowledgments

This work was supported by the National Key Basic Research under Contract no. 2011CBA00111 and the National Natural Science Foundation of China under Contracts nos. 51001094, 51171177, 11174295, and 91222109.

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