Magnetic and electrical properties of R₅Ir₃ (R = Tb, Er)

Rare earth compounds R₅Ir₃ (R = Tb, Er) crystallize in the Mn₅Si₃ type hexagonal structure with the space group P6₃/mcm in which R occupies two non-equivalent sites. In the R₅Ir₃ system, it has been reported that the light rare earth compounds (R = La to Gd) have tetragonal Pu₅Rh₃ type structure;¹ heavy rare earth ones (R = Tb to Lu) show the hexagonal Mn₅Si₃ type structure.² Among the Tb₅M₃ or Er₅M₃ family with Mn₅Si₃ type structure, Tb₅Si₃ possesses antiferromagnetic state with incommensurate helical structure below T_N = 50 K.³ Tb₅Ge₃ is also an antiferromagnet with T_N = 85 K;⁴ complex magnetic structure has been revealed.⁵ Magnetic and neutron diffraction measurements have been carried out for Er₅Sb₃,⁶ Er₅Ge₃;⁷ antiferromagnetic properties were reported. However, for the R₅Ir₃ compounds, the magnetic and electrical properties as well as magnetic phase transitions have not been reported so far, as far as we know. In this study, we have prepared single phase polycrystalline samples of R₅Ir₃ (R = Tb, Er) and investigated magnetic and electrical properties by the measurement of magnetization, magnetic susceptibility and electrical resistivity.

Polycrystalline ingots of R₅Ir₃ (R = Tb, Er) were prepared by arc-melting the constituent elements of 99.9% rare earth and 99.9% Ir in high purity argon atmosphere using a penta-arc furnace. In this process, ingots were turned over several times to ensure homogeneity. Powder X-ray diffraction (XRD) profiles of Er₅Ir₃ and Tb₅Ir₃ are shown in Fig. 1. Observed diffraction peaks can be indexed by the Mn₅Si₃ type structure;² it was confirmed that the samples were in the single phase. Magnetization and dc magnetic susceptibility were measured by a SQUID magnetometer (QD MPMS) from 2 K to 300 K. Electrical resistivity measurements were performed using conventional four-terminal method in the temperature range from 2 K to 300 K.

Magnetization (M – H) curve of Tb₅Ir₃ at 2 K is shown in Fig. 2(a). As is shown in the inset, ferromagnetic like hysteresis curve was observed after the initial magnetizing process up to 50 kOe. However, small magnetization jumps were recognized at H_C1 and H_C2 in the initial magnetizing process; as shown in Fig. 2(a), remanent magnetization of 2.0 μ_B/Tb was observed after decreasing the field. This indicates that Tb₅Ir₃ show an irreversible antiferromagnetic (AFM) to ferromagnetic (FM) transition which has been reported in several compounds, Dy₃Co⁸ or Nd₅Ge₃.⁹ Interestingly, a relatively large magnetization jump was observed in the second magnetizing process in the induced ferromagnetic state, too. This implies that the antiferromagnetic structure exists in the induced ferromagnetic state, too. Since the examined sample is polycrystal, it is difficult to discuss the detailed metamagnetic transition process; further measurements with single crystals are required. Magnetization curve of Tb₅Ir₃ at various temperatures from 6 K to 100 K is shown in Fig. 2(b). The curves are plotted with offset to make clear the temperature dependence. Magnetic field induced AFM to FM transition can be seen at 6.0 K as well; the ferromagnetic character was observed up to about 60 K. Hence, it is considered that the ground state of Tb₅Ir₃ is antiferromagnetic. The field-induced transition results in a ferromagnetic state that exists in competition with some remnant antiferromagnetic phase; such competition could give rise to magnetic-glass-like interactions.¹⁰ 

Magnetization (M – H) curve of Er₅Ir₃ at 2 K is shown in Fig. 3(a). In the initial magnetizing process, magnetization rapidly increases with external field indicating the ferromagnetic property. However, a small magnetization jump can be seen at about 22 kOe; a first order metamagnetic transition is observed at 32 kOe. After the second jump, magnetization is almost saturated at 50 kOe; a small remanent magnetization was observed after decreasing magnetic field as shown in Fig. 3(a). This result indicates that the mixed magnetic state with AFM and FM can also exist in Er₅Ir₃. Magnetization curve at various temperatures for Er₅Ir₃ is shown in Fig. 3(b). The curves are plotted with offset to make clear the temperature dependence as well as those in Fig. 2(b). At 3.0 K, no hysteresis was observed in the magnetization curve; this indicates a simple ferromagnetic property. Ferromagnetic like M - H curve can be seen up to 14 K.

The dc magnetic susceptibility χ of Tb₅Ir₃ and Er₅Ir₃ under the external magnetic field of 1.0 kOe is shown in Fig. 4 as a function of temperature T. Reciprocal susceptibility χ^-1 is also shown in each figure. The χ of Tb₅Ir₃ increases with decreasing temperature and shows a large increase from about 60 K indicating the transition from paramagnetic (PM) to FM state. A large decrease of χ was observed below about 16 K. This indicates the FM to AFM transition; the ground state of Tb₅Ir₃ can be antiferromagnetic. Curie temperature and the FM to AFM transition temperature were determined to be T_C = 49 K and T_t = 6.5 K, respectively, from the inflection point of the χ - T curve. As shown in Fig. 2(b), although the M – H curve at 6.0 K also shows a field induced AFM to FM transition, this irreversibility cannot be observed at 10 K.

The χ of Er₅Ir₃ also increases with decreasing T; PM to FM transition was observed below about 20 K. Curie temperature T_C was determined to be 10 K from the inflection point of the χ - T curve as well. Meanwhile, the decreases of χ was observed below 2.2 K; another magnetic transition was implied. As shown in Fig. 3, the M – H curve at 2.0 K indicates an irreversible magnetic field-induced AFM to FM transition; magnetic property at 2.0 K is different from that at 3.0 K. Hence, another FM to AFM transition can be considered to take place at T_t = 2.1 K.

Paramagnetic susceptibility χ of Tb₅Ir₃ and Er₅Ir₃ shows the Curie-Weiss behavior; the reciprocal susceptibility χ^-1 indicates linear temperature variation in the paramagnetic state as shown in Fig. 4. The small deviation from the linear variation of χ^-1 in the low temperature part implies the small crystal electric field (CEF) effect on these compounds. The effective magnetic moment μ_eff which was obtained from the χ^-1 vs. T curves are 9.82 μ_B/Tb and 9.60 μ_B/Er for Tb₅Ir₃ and Er₅Ir₃, respectively. These values are in good agreement with the theoretical μ_eff values of 9.72 μ_B and 9.59 μ_B for Tb³⁺ and Er³⁺ ions. The asymptotic Curie temperature θ_p was also determined to be 59.0 K and 11.7 K for Tb₅Ir₃ and Er₅Ir₃, respectively.

Electrical resistivity ρ of Tb₅Ir₃ and Er₅Ir₃ is shown in Fig. 5 as a function of temperature T. The inset shows the low temperature part. The ρ of both compounds decreases with decreasing temperature indicating the metallic property; distinct anomalies were observed at T_C and T_t for both samples. In the Er₅Ir₃, the ρ shows a hump at T_C and slight increase with decreasing temperature to T_t. Though the origin of the small increase of ρ below 5 K is not clear, an impurity effect might be considered.

We have studied the magnetic and electrical properties of Tb₅Ir₃ and Er₅Ir₃ using polycrystalline samples. Tb₅Ir₃ shows reentrant magnetic property; the two magnetic transitions from PM to FM and FM to AFM were observed at T_C = 49 K and T_t = 6.5 K, respectively. Meanwhile, Er₅Ir₃ possesses ferromagnetic state below T_C = 10 K; another FM to AFM transition takes place at T_t = 2.1 K. Almost the same magnetic field-induced AFM to FM transition exists in the low temperature AFM state. Paramagnetic susceptibility shows Curie-Weiss behavior; the effective magnetic moment obtained from the reciprocal susceptibility is in good agreement with the theoretical values of Tb³⁺ and Er³⁺ ions. Electrical resistivity measurements revealed that Tb₅Ir₃ and Er₅Ir₃ shows metallic property; distinct anomalies were observed at the transition temperatures. For the more detailed study, single crystals of these compounds are required for the magnetic and electrical measurements as well as the neutron diffraction experiments.

A part of this work was supported by the grant-in-aid for scientific research (A) (No. 17H00820) from Japan Society for the Promotion of Science. This work was also supported by the Center for Chiral Science in Hiroshima University (the MEXT program for promoting the enhancement of research universities, Japan) and JSPS Core-to-Core Program, A. Advanced Research Networks.