Low-temperature physical properties of new orthorhombic compounds R4Pt9Al13 (R = Ce, Pr)

Magnetic, transport, and thermal properties of new orthorhombic compounds Ce4Pt9Al13 and Ce4Pt9Al13 have been investigated by the magnetization, the electrical resistivity, and the specific-heat measurements. Ce4Pt9Al13 is a Kondo-lattice compound and shows a ferromagnetic or ferrimagnetic transition at T C = 0.88 K. Pr4Pt9Al13 is an antiferromagnetic compound with the transition temperature at T N = 2.6 K.


Introduction
Recently, coexistence of ferromagnetism and superconductivity in some U-based compounds have attracted much attention [1]. In contrast to the discovery of the U-based ferromagnetic superconductors, no other systems such as Ce-based or Yb-based ones have been discovered thus far. In order to examine the mechanism of the coexistence more precisely, it is important to examine physical properties of the other systems showing the coexistence. From this view point, we have focused on a new orthorhombic compound Ce4Pt9Al13 (space group: Pmmn, D2h 13 , No. 59) [2]. There are three independent Ce sites in this structure as shown in the figure 1. The physical property measurements down to 1.72 K have revealed that this compound does not show any magnetic transition above 1.72 K, although the positive paramagnetic Curie temperature (41.6 K) implies ferromagnetic interaction between Ce moments [2]. In this study, magnetic and transport properties of Ce4Pt9Al13 have been examined down to 0.4 K. Additionally, we have tried to prepare samples of Pr4Pt9Al13 and examined their physical properties since

Experimental methods
Polycrystalline samples of Ce4Pt9Al13 and Pr4Pt9Al13 were synthesized by arc melting the stoichiometric constituent elements in an Ar atmosphere. The X-ray powder diffraction experiments for these compounds revealed that all of the Bragg peaks in the diffraction patterns can be indexed on the basis of the reported structure. No impurity peaks were detected in the diffraction patterns. The lattice constants were determined to be a = 4.1739, b = 11.4373, and c = 19.8125 Å for Ce4Pt9Al13, and a = 4.1615, b = 11.4162, and c = 19.7620 Å for Pr4Pt9Al13. The lattice constants of Ce4Pt9Al13 agree with the previously reported ones [2] within the experimental precision. The magnetization M was measured using a superconducting quantum interference device magnetometer (Quantum Design, MPMS) as functions of the temperature T and magnetic field H between 1.8 and 300 K up to 5 T. The electrical resistivity ρ was measured with a dc four-probe method between 0.4 and 300 K in a laboratory-built 3 He cryostat. The specific heat C was measured with a thermal relaxation method between 0.5 and 9 K in a 3 He cryostat.

Ce4Pt9Al13
The inverse magnetic susceptibility H/M of Ce4Pt9Al13 (not shown) above 100 K obeys the Curie-Weiss law H/M = (T -θP)/CCurie, where CCurie = 0.894 emu K/Ce-mol and θP = -189.8 K show the Curie constant and the paramagnetic Curie temperature, respectively. The effective magnetic moment µeff calculated from CCurie is 2.67 µB/Ce (µB: Bohr magneton). Since the calculated µeff is close to the theoretical one for the free Ce 3+ ion (2.54 µB/Ce), we can consider that the Ce ions in this compound are trivalent. The negative θP indicates that the antiferromagnetic interaction acts between the Ce magnetic moments, although the positive θP (ferromagnetic interaction) was reported in the previous report. No anomaly of H/M has been detected above 1.8 K as already reported previously. Figure 2 shows the temperature dependence of the electrical resistivity ρ of Ce4Pt9Al13. ρ decreases   3 with decreasing temperature, followed by a minimum at 18 K, and shows -lnT dependence between 18 K and 7 K. These ρ(T) behavior indicates that Ce4Pt9Al13 is a Kondo lattice compound. In addition, ρ shows a steep decrease at TC = 0.88 K, indicating that some phase transition occurs at this temperature.
The low-temperature part of ρ measured in various magnetic fields are shown in the inset of the figure 2. TC shifts to higher temperatures with increasing magnetic field, indicating that the decrease in the ρ at 0.88 K corresponds to the ferromagnetic transition temperature. No superconductivity has been detected above 0.4 K. Figure 3 shows the temperature dependence of the specific heat C of Ce4Pt9Al13. The λ-type anomaly at TC indicates that the second-order ferromagnetic transition occurs at TC. Figure 3 also shows the temperature dependence of the total entropy S derived by integrating C/T in T. Since the symmetry of the crystalline electric field is monoclinic for Ce1 site, and orthorhombic for Ce2 and Ce3 sites, the sixfold ground multiplet of Ce 3+ splits into three doublets for these Ce sites. In this case, it is expected that the entropy released at TC reaches Rln2 (R: gas constant). However, the S value at TC is only 45% of Rln2 (2.6 J/Ce-mol K). Such a large reduction of S from Rln2 cannot be explained only by the Kondo effect. The reduced S is mainly ascribable to the short-range magnetic correlation below 4 K. In fact, the C increases gradually below 4 K and the S value reaches 99% of Rln2 at 4 K.
There are three independent Ce sites in the crystal structure of Ce4Pt9Al13 as described above. In such structure, the magnitude of Ce magnetic moments and/or the magnetic interaction are not necessarily identical among these Ce sites. We therefore cannot decide from the present study if the magnetic structure is simple ferromagnetic or ferrimagnetic. As the next step, we have to determine magnetic structure by neutron diffraction experiment. Figure 4 shows the temperature dependence of the inverse magnetic susceptibility H/M of Pr4Pt9Al13 measured at 0.1 T. H/M above 100 K follows the Curie-Weiss law (see the solid line in the figure. 4).

Pr4Pt9Al13
The µeff and the θP derived by the Curie-Weiss fitting are 3.53 µB/Ce and -91.4 K, respectively. Since the derived µeff is close to the theoretical one for the free Pr 3+ ion (3.58 µB/Ce), we can consider that the Pr ions in Pr4Pt9Al13 are trivalent. The negative θP indicates that the antiferromagnetic interaction acts between the Pr magnetic moments. The magnetic susceptibility M/H shows a cusp at TN = 2.6 K as  The electrical resistivity ρ of Pr4Pt9Al13 (not shown) decreases with decreasing temperature and shows an increase below TN. The increase below TN is probably due to the formation of a superzone gap accompanied by the antiferromagnetic transition. Figure 5 represents the temperature dependence of the specific heat C of Pr4Pt9Al13. The λ-type anomaly at TN indicates that the second-order antiferromagnetic transition occurs at TN. The total entropy S calculated by the same method for Ce4Pt9Al13 is also shown in the figure. 5. The S value at TN is 78% of Rln2 and exceeds Rln2 above 3.8 K. The nine-fold ground multiplet of Pr 3+ splits into nine singlets under the crystalline electric field with orthorhombic or monoclinic symmetry. The abovementioned S value suggests that the first excited and/or the second excited singlets are located at the vicinity of the ground singlet. We therefore consider that the pseudo doublet ground state is responsible for the antiferromagnetic transition. The increase in S below 0.8 K is probably attributed to the nuclear specific heat of Pr 3+ ions.

Conclusion
In this study, polycrystalline samples of new orthorhombic compounds R4Pt9Al13 (R = Ce, La) have been synthesized and their physical properties down to 0.4 K have been examined.
Magnetic-susceptibility measurement has revealed that the Ce ions in Ce4Pt9Al13 are trivalent. Ce4Pt9Al13 is a ferromagnetic or ferrimagnetic Kondo-lattice compound with the transition temperature at TC = 0.88 K. No superconductivity has been detected above 0.4 K.
We have found that Pr4Pt9Al13 exists as the isomorphous compound of Ce4Pt9Al13. Magneticsusceptibility measurement has revealed that the Pr ions in Pr4Pt9Al13 are trivalent. The pseudo doublet ground state of Pr 3+ is responsible for the antiferromagnetic transition at 2.6 K.