npsm 새물리 New Physics : Sae Mulli

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New Phys.: Sae Mulli 2023; 73: 1135-1139

Published online December 31, 2023 https://doi.org/10.3938/NPSM.73.1135

Copyright © New Physics: Sae Mulli.

Physical Properties of a New Ternary Compound RPt3Al5 (R = rare earth)

Hiroto Fukuda1, Takatsugu Koizumi1, Yoshiki J. Sato1†, Yusei Shimizu1, Ai Nakamura1, Dexin Li1, Yoshiya Homma1, Atsushi Miyake1, Dai Aoki1, Masashi Tokunaga2, Ryoma Kato3, Masanobu Shiga3, Tatsuya Kawae3, Fuminori Honda4‡

1Institute for Materials Research, Tohoku University, Oarai, Ibaraki, 311-1313, Japan
2Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan
3Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan
4Central Institute of Radioisotope Science and Safety, Kyusyu University, Fukuoka 819-0395, Japan

Correspondence to:*H. Fukuda also belongs to the Graduate School of Engineering, Tohoku University, Japan
Present address: Faculty of Science and Technology, Tokyo Uni-versity of Science
honda.fuminori.790@m.kyushu-u.ac.jp

Received: September 26, 2023; Revised: October 21, 2023; Accepted: October 23, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Electronic properties of a new ternary f-electron system RPt3Al5 (R: rare earth elements) have been investigated. RPt3Al5 crystalizes in the CePt3Al5-type orthorhombic structure where R atoms form 1-dimensional chains along the b-axis. LaPt3Al5 shows superconductivity below 0.4 K, while others show magnetic ordering. CePt3Al5 orders antiferromagnetically below TN = 1.2 K with spontaneous moment and shows successive magnetic transition at Tm = 0.9 K. The nearly divalent antiferromagnet EuPt3Al5 indicates an unusual valence cross-over behavior into the nearly trivalent state under high pressure of 9 GPa. It is also found that most of RPt3Al5 orders antiferromagnetically and, in some cases, shows characteristic features in magnetic susceptibility, where the broad maximum appears slightly above TN, reflecting the low-dimensional nature.

Keywords: Rare-earth compounds, High pressure, High field magnetization, Magnetism, RPt3Al5

Rare-earth compounds with 4f-electrons exhibit a variety of attractive phenomena such as magnetic orderings, superconductivity, heavy fermion state, valence instability, etc. Among these compounds, most of their electronic properties at the ground state are well explained regarding the competition between two conflicting effects, the RKKY interaction and the Kondo effect. Recently, characteristic crystal structures such as global/local inversion symmetry breaking, chirality, low dimensionality, and geometrical frustration are also considered the sources of exotic electronic states, and it is recognized that a strong spin-orbit coupling often enhances these phenomena[1-3].

Here, we present the physical properties of a new ternary compound RPt3Al5 (R = rare earth elements). RPt3Al5 crystallizes in an orthorhombic CePt3Al5-type structure (space group: Pnma)[4]. Figures 1(a) and 1(b) show the crystal structure of RPt3Al5. One of the characteristics of the crystal structure is one-dimensional arrangements of the R-ions along the orthorhombic b-axis. The intra-atomic distance of R ions along the b-axis is the shortest among the other R-R distances and about 4 Å, as shown by the solid green lines in Fig. 1(b), which forms a one-dimensional chain. The interchain distance shown by the black dashed line in Fig. 1(a) is 6–7 Å. Thus, the intrachain interactions are expected to be stronger than the interchain interactions, which may be regarded as quasi-one-dimensional magnetic systems. Another characteristic is the lack of inversion symmetry on the R-ion site, i.e., local inversion symmetry breaking.

Figure 1. (Color online) (a) The conventional unit cell, (b) arrangement of R-ions in RPt3Al5, and (c) unit cell volumes with respect to the rare-earth elements.

The existence of the RPt3Al5 family has been known since the discovery of CePt3Al5[4], but physical properties were not well investigated. We succeeded in growing single crystals of EuPt3Al5 and revealed details of physical properties with a special focus on the valence instability[5]. Another ternary Eu-Pt-Al compound, Eu2Pt6Al15 is known to show the temperature-induced valence transition[6]. We also succeeded in growing other RPt3Al5 with polycrystal forms and investigated their physical properties in detail.

Single crystals of CePt3Al5 were grown by the Czochralski method from the stoichiometric starting composition. While EuPt3Al5 crystals were synthesized by the Bridgman method using an excess Eu and pre-reacted Pt3Al5 alloy[5]. The other RPt3Al5 compounds were polycrystals made by the conventional arc-melting method, where the ingots were melted several times with turned over. The crystal structure was confirmed with the single crystal x-ray diffraction technique for R= Ce and Eu, and with a conventional powder x-ray diffraction technique for the others. The obtained XRD patterns of RPt3Al5 (R = La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, and Tm) were well explained with the CePt3Al5-type of structure, though there are little peaks from impurity phase were observed in some cases. Figure 1(c) shows the determined unit cell volumes of RPt3Al5 with respect to the rare-earth elements. Note that the unit cell volume of EuPt3Al5 differs from the tendency of the so-called lanthanide contraction because the valence state of Eu is nearly 2+ with a magnetic moment. The electrical resistivity was measured using a conventional four-terminal method. Specific heat was measured by the relaxation method with the Physical Property Measurement System (Quantum Design Inc.) using 3He option. Magnetization down to 2 K was measured using a SQUID magnetometer, MPMS, (Quantum Design Inc.). Magnetization below 2 K was measured using a capacitive Faraday-force magnetometer with 3He refrigerator for R = Ce[7, 8] and using a homemade 3He insert for MPMS for R = Pr.

Figure 2(a) shows the temperature dependence of the electrical resistivity ρ(T) of LaPt3Al5 polycrystal. ρ(T) decreases monotonically with decreasing temperature. ρ(T) reaches residual resistivity below about 10 K, and suddenly drops at 0.4 K and shows superconductivity. Here we defined the superconducting temperature TSC = 0.3 K as a midpoint of resistivity drop. Figures 2(b) and 2(c) show the temperature dependence of the magnetization of CePt3Al5 for the magnetic field along the a- and b-axes, respectively. For Ha, a small but significant spontaneous magnetization has been observed, implying a ferromagnetic (FM) component of the magnetic structure. For Hb, a kink appears at 1.2 K in 0.1 T and 1.1 K in 1 T, indicating antiferromagnetic (AFM) ordering. It is also found that there is another magnetic transition at 0.9 K in 0.1 T, which infers a first-order phase transition with large hysteresis. Both transitions were clearly reflected as the anomalies of the specific heat (not shown here). From these data, it is recognized that the magnetic structure is most probably a kind of canted AFM state. From the crystal structure, it is suggested that the magnetic interaction along the b-axis, inter-chain, is dominant, but non-negligible intra-chain interaction might play an important role in the magnetic ordering. Since the structure has no local inversion symmetry between Ce atoms in a plane, the DM interaction may affect to tilt of the magnetic moment. To clarify the magnetic structure, a neutron diffraction experiment is highly desirable.

Figure 2. (Color online) (a) Temperature dependence of the electrical resistivity of LaPt3Al5 polycrystal, (b) magnetic moment and (c) magnetic susceptibility of CePt3Al5 for the magnetic field along the a- and b-, respectively.

It is known that Eu compounds often exhibit mixed valent states of Eu2+ and Eu3+. When Eu is divalent (the total angular moment J = 7/2), it orders magnetically, whereas a trivalent electronic state exhibits no magnetic ordering because J = 0. The valence state of Eu can be tuned with temperature, magnetic field, pressure, etc. Thus, in Eu compounds, not only magnetic quantum critical point (behavior) but also valence critical behavior, and its competitive and cooperative effects have attracted much attention and have been studied intensively[9, 10]. EuPt3Al5 orders antiferromagnetically at 12.4 K as shown in Fig. 3(a). When the magnetic field is applied along the antiferromagnetic easy axis (c-axis), there appears 2 metamagnetic transitions at Hm1=4 T and Hc = 12.3 T followed by a slight slope change at Hm2 = 11.4 T at 1.4 K as shown in Fig. 3(b). A clear concave shape of M(H) below Hm1 is observed. The spin-flop state can be realized for Hm1<H<Hm2. Magnetization keeps increasing and approaches the saturation moment above 45 T. The tremendous observation in this compound is a pressure effect on the electronic state. Figure 3(c) shows the temperature dependence of ρ(T) under high pressure up to 12 GPa. TN gradually increases with increasing pressure up to 8 GPa and starts to decrease rapidly with further increasing pressure. TN suddenly disappears and residual resistivity indicates a sharp peak around Pc = 9 GPa, implying a dramatic change in the electronic states or Fermi surfaces in the critical region. A more detailed discussion can be found in our previous article[5]. Further investigation on the magnetic structure and the effect of pressure on the valence states are in progress.

Figure 3. (Color online) (a) Magnetic susceptibility and (b) high field magnetization of EuPt3Al5 at 0 GPa. (c) Temperature dependence of the electrical resistivity under pressure and (d) the constructed pressure-temperature phase diagram of EuPt3Al5. Here (a) and (d) are cited from Ref. 5.

To overview the electronic properties of the RPt3Al5 series, we prepared RPt3Al5 polycrystals with other rare earth elements. Note that those with R = Sm and Yb have not been tried yet because of their high vapor pressure. Figure 4(a) shows the temperature dependence of the specific heat in the form of C/T in the logarithmic T-scale of these materials. Most of them show clear λ-type anomalies, indicating the second-order phase transition except for R = Tm. The low-temperature upturn in C/T in PrPt3Al5 is suggested to contribute to the nuclear-specific heat of Pr nuclei. In DyPt3Al5, two successive phase transitions are detected. A hump and broad peak observed in TmPt3Al5 around 1 K and 0.55 K can be attributed to the Schottky anomaly originating due to the crystalline electric field (CEF) splitting and magnetic ordering, respectively. Temperature dependence of the magnetic susceptibility of RPt3Al5 (R = Pr, Nd, Gd, Tb, Dy, Ho, Er, and Tm) down to 2 K and that of PrPt3Al5 down to 0.5 K are shown in Fig. 4(b) and 4(c), respectively. The compounds with R = Pr, Nd, Gd, Tb, and Dy are found to order antiferromagnetically, which are bulk in nature as observed in the specific heat anomalies. M/H shows a broad maximum above TN and a relatively steep drop of magnetization with magnetic ordering. The broad maximum in M/H might reflect the low-dimensional nature of the spin system. These behaviors are reminiscent of the Bonner-Fisher model often seen in low-dimensional magnetic systems[11]. It is intriguing to study the detailed magnetic properties and their relation with low-dimensional crystal structure in RPt3Al5. The study using single crystals is strongly desirable.

Figure 4. (Color online) Temperature dependence of (a) the specific heat in the form of C/T, (b) magnetic susceptibility of RPt3Al5, and (c) low-temperature magnetic susceptibility of PrPt3Al5. Note that M/H of R = Dy is plotted according to the right axis.

We prepared and investigated the physical properties of the new ternary rare-earth system RPt3Al5 (R = rare earth), which crystalizes in the CePt3Al5-type orthorhombic structure consisting of a 1-dimensional chain of magnetic atoms along the crystallographic b-axis. RPt3Al5 reveals various types of electronic states such as superconductivity (R = La), canted antiferromagnetism (R = Ce), successive magnetic orderings (R = Ce and Dy), and unique critical valence behavior from nearly divalent to the nearly trivalent state under high pressure (R = Eu). We believe that the RPt3Al5 system opens a new playground for electronic structure study for low-dimensional magnetism in f-electron systems.

This work was supported by Grants-in-Aid for Challenging Research (Exploratory) (JP19K21840), Scientific Research (A) (JP19H00646, JP19H00648), and (C) (JP20K03827) from the Japan Society for the Promotion of Science. Part of this work was carried out under the Visiting Researcher’s Program of the Institute for Solid State Physics, the University of Tokyo, the GIMRT Program of the Institute for Materials Research, Tohoku University (Proposal No. 202012-IRKAC-0056, 202112-IRKAC-0029, and 202212-IRKAC-0503), and equipment share program at the Low-Temperature Center, Kyushu University.

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