Magnetic and transport properties of a new ferromagnetic orthorhombic compound CePtAl2

We have synthesized polycrystalline samples of CePtAl2 by arc melting method and examined their magnetic, transport and thermal properties by measuring the magnetization, the electrical resistivity, and the specific heat down to 0.4 K. As a result of these measurements, we found that CePtAl2 is a ferromagnetic Ce-based compound with the Curie-temperature Tc = 2.7 K.


Introduction
It has been reported that ferromagnetism and superconductivity, which are generally contradictory properties, coexist in some U-based compounds [1]. In order to elucidate the coexistence mechanism, it is necessary to investigate similar phenomena in a larger number of compounds. However, no coexistence phenomena have been found except in a few U-based compounds thus far. Ce-based compounds are the promising candidate showing coexistence since they have the same strong correlation between f-electrons and conduction electrons as U-based compounds. We have focused on new compounds RPtAl2 (R = Sc, Y, La-Nd, Sm, Gd-Tm, Lu) [2]. As shown in the figure 1, RPtAl2 crystallizes in the orthorhombic MgCuAl2-type structure (space group Cmcm, D2h 17 , No. 63) [2]. The magnetization measurements of CePtAl2 down to 2.5 K have revealed that CePtAl2 did not show any phase transition and the effective magnetic moment eff and the paramagnetic Curie temperature θP were estimated to be 2.52 B/Ce and -20.6 K, respectively. In this study, we have grown polycrystalline samples of CePtAl2 and examined their magnetic and transport properties by measuring the magnetization, the electrical resistivity, and the specific heat down to 0.4 K.

Experimental Methods
Polycrystalline samples of CePtAl2 are synthesized as the following procedure. First, Ce (99.9%), Pt (99.99%), and Al (99.999%) were weighed in the ideal 1:1:2 atomic ratio and melted using an arc furnace under an Ar gas atmosphere. Next, the samples wrapped in Ta foils were sealed in an evacuated quartz tube to prevent oxidation and annealed in a muffle furnace at 1320 K for one week. The annealed samples were characterized by a powder X-ray diffraction experiment. Almost all of the Bragg peaks in the diffraction pattern can be indexed on the basis of the MgCuAl2-type structure, although small unidentified impurity peaks are discernible. The lattice parameters were estimated to be a = 4.218 Å, b = 11.062 Å, and c = 7.032 Å, which agree with the reported values within the experimental precision.
The magnetization was measured in the temperature and magnetic-field ranges of 1.8 ≤ T ≤ 300 K and 0 ≤ 0H ≤ 5 T, respectively, using a superconducting quantum interference device magnetometer (Quantum Design, MPMS). The electrical resistivity was measured with a dc four-probe method in a temperature range of 0.4 ≤ T ≤ 300 K using a 3 He cryostat. The specific heat was measured with a thermal relaxation method in the temperature range of 0.4 ≤ T ≤ 10 K using a 3 He cryostat.  Figure 3 shows the magnetic-field dependences of the magnetization M of CePtAl2 measured at 1.8 and 5 K. In contrast to the paramagnetic behavior of M measured at 5 K, M measured at 1.8 K shows ferromagnetic behavior, i.e., M increases steeply by applying magnetic field and saturates above 2 T.    Figure 4 shows the temperature dependence of the electrical resistivity ρ of CePtAl2. The lowtemperature part of ρ is shown in the inset. The ρ decreases almost linearly with decreasing temperature in the high temperature region. However, ρ decreases rapidly below 2.7 K as indicated by an arrow in the inset. The decrease temperature corresponds to the temperature at which the magnetic susceptibility M/H increases steeply. Figure 5 shows the temperature dependence of the specific heat C. The C has a clear λ-type jump at TC = 2.7 K. This means that the increase in M/H and the decrease in  are due to the second-order ferromagnetic transition at TC. The negative P (antiferromagnetic) obtained in this study may be explained by the magnetic anisotropy described above: if we measure the M/H with the different sample angle to the magnetic field, positive θP (ferromagnetic) may be obtained. The temperature dependence of the total entropy S deduced by integrating C/T in temperature is also shown in the figure 5. Since the S value at TC is 3.9 J/mol K, which corresponds to 68% of Rln2 (R : gas constant), we consider that the ground doublet of Ce 3+ is responsible for the ferromagnetic transition. Here, the 32% reduction of S is not explained by the Kondo effect since the  does not show any signature of the Kondo effect and the localized nature of 4f electrons are kept down to low temperatures. By considering the fact that the C increases with decreasing temperature below 9 K, the reduced S value at TC can be ascribable to the development of short-range ferromagnetic correlation above TC. In fact, the S value reaches 98% at 9 K. Note that ferromagnetic Ce compounds are rather rare compared with antiferromagnetic ones. Therefore, further studies of CePtAl2 such as pressure effect measurements are worth performing to search for the coexistence of ferromagnetism and superconductivity.

Conclusion
In this study, we have grown polycrystalline samples of CePtAl2 and measured their magnetic, transport, and thermal properties. We have found that CePtAl2 is a ferromagnetic Ce-based compound with the Curie temperature TC = 2.7 K. Since the electrical resistivity does not show any signature of the Kondo effect, we consider that the localized nature of 4f electrons is kept down to the low temperatures. The development of the ferromagnetic short-range correlation is responsible for the reduced entropy at TC.

Acknowledgment
This work was supported partly by JPJS KAKENHI Grant Number JP15H05882 (J-Physics).