2016 Volume 63 Issue 7 Pages 652-656
We synthesized polycrystalline samples of ternary compounds Ln2Co12P7 (Ln = Y, Pr, Nd, Sm, Gd and Dy) with the Curie temperature being about 150 K and measured magnetization to clarify magnetism of Co sublattice and also to clarify behavior of magnetic moments of Ln3+ in ferromagnetic phase. As a result, magnetism of Y2Co12P7 was found to be a possible weakly itinerant electronic ferromagnetism and found to be similar to that of Lu2Co12P7, indicating that magnetism of Co sublattice does not change with Ln. From results for Ln2Co12P7, we found that spin components of magnetic moments of Ln3+ ferromagnetically couple with exchange field of ferromagnetic moments of Co sublattice. This result means that behavior of magnetic moments of Ln3+ can be described as the single ion model applied to the permanent magnets (LnCo5, Ln2Fe14B etc.) though the sign of coupling constant is opposite of the cases of permanent magnets. This is because magnetic moments of Co and Ln couple through P, not a direct coupling in cases of permanent magnets.
Ternary lanthanoid transition metal pnictides have much variety of compounds with different crystal structure and thus have much variety of physical properties. For example, in the LnM4Pn12 system which is well known as the filled Skutterudite, physical property is ranging from unconventional superconductivity to anomalous thermal conductivity because of rattling motions of Ln. Here, Ln is lanthanoids, M is transition metal elements and Pn is pnictogen elements. Ln2M12Pn7 system is one of such ternary systems and has Zr2Fe12P7-type crystal structure (hexagonal, space group: P-6) as shown in Fig. 1. The crystal structure of Ln2M12Pn7 can be seen as a filled Cr12P7 structure1,2), reminding us of the case of the filled Skutterudite. Actually, all the lanthanoid elements except La, Y, Sc and tetravalent ions of Zr and Ti are reported to occupy the Ln site3–5), indicating that the Ln site is in a robust cage of phosphorous atoms as the case of the Skutterudite. For this reason, the Ln2M12Pn7 system has aroused interest for a long time and intensive studies have been devoted.
Crystal structure of Ln2Co12P7.
Magnetic properties of the M sublattice of Ln2M12Pn7 have been studied, and it is revealed that the M sublattice shows the Pauli paramagnetism in the cases of M = Fe and Ni while it shows ferromagnetism with the Curie temperature of TC ~ 150 K in the case of M = Co6,7). Therefore, from a view point of itinerant electronic magnetism Ln2Co12Pn7 are the most important compounds in the Ln2M12Pn7 system. For now, magnetism of several compounds have been studied and anomalous magnetic properties have been reported8,9). For example, Nd2Co12P7 shows antiferromagnetic ordering at low temperature without any phase transition from ferromagnetic state with decreasing temperature. For understanding magnetic properties of the Ln2Co12P7 system, systematic study on Ln dependence of magnetism is necessary. However, such a systematic study has been done only on paramagnetic state6), and thus anomalous magnetism in ordered state has not been fully understood.
In this paper, we synthesized polycrystalline samples of Ln2Co12P7 with Ln = Y, Pr, Nd, Sm, Gd and Dy and measured magnetization of the samples to systematically study magnetism of the Ln2Co12P7 system in ordered state.
Polycrystalline samples of Ln2Co12P7 were synthesized by a solid state reaction method. Powder of Ln (Ln = Y, Ce, Pr, Nd, Sm, Gd or Dy) (purity: 99.9 %), Co (99.9 %) and P (99.9999 %) were mixed and then heated at 1,273 K for 12 hs in evacuated silica tubes. Obtained samples were well ground and fired at 1,473 K for 12 hs in evacuated silica tubes. Samples were characterized by using powder X-ray diffraction (XRD) measurements with Cu Kα radiation. Magnetization (M) of samples were measured by using vibrating sample magnetometer (MagLabVSM, Oxford) and also measured by using a non-destructive pulse magnet installed in Institute for Solid State Physics, the University of Tokyo up to 56 T.
Fig. 2 (a) shows results of powder XRD measurements of Ln2Co12P7 for Ln = Y, Pr, Nd, Sm, Gd and Dy. Red and blue data are experimentally obtained patterns and simulated patterns, respectively. These simulated patterns are obtained from calculation by using lattice parameters estimated from the experimental patterns. Good agreement of the experimental pattern with the simulated one for each sample indicates that samples are in a single phase of Ln2Co12P7. Fig. 2 (b), 2 (c) and 2 (d) show lattice parameters a, c and cell volume V of samples. For cases of Ln = Pr, Nd, Sm, Gd and Dy, these parameters are in agreement with those reported in Ref.3). For the case of Ln = Y, the estimated lattice parameters are a = 9.055 Å, c = 3.609 Å, and V = 256.3 Å3.
(a) XRD patterns of Ln2Co12P7. Red and blue data are obtained and simulated patterns, respectively. (b)–(d) Lattice parameters a, c and V of Ln2Co12P7, respectively.
By choosing nonmagnetic lanthanoids as Ln, we can study magnetic property of Co sublattice in detail. For understanding of magnetism of Co sublattice in Ln2Co12P7 system, it is also important to study magnetic property of Y2Co12P7. Fig. 3 (a) shows temperature (T) dependence of M measured after field cooling (FC) and zero field cooling (ZFC) under magnetic field (H) = 0.1 T and T dependence of H/M measured under H = 1 T. In the paramagnetic state, H/M shows a linear behavior, indicating that magnetic moments of Co obey the Curie Weiss law. As the result of fitting, we estimated magnetic parameters, i.e. the effective Bohr magneton number Peff and the Weiss temperature θ, of magnetic moments of Co as Peff = 1.18 per Co and θ = 158 K. Fig. 3 (b) shows isothermal magnetizations of Y2Co12P7 measured at various temperatures. At T = 2 K, there is observed hysteresis loop below H = 1.7 T. This is consistent with divergence between ZFC and FC observed in T dependence of M below 140 K. Such hysteresis loop in magnetization curve was also reported in the case of Ln = Lu9). We estimated spontaneous magnetization at T = 2 K as Ps = 0.14 μB/Co. Spontaneous magnetization becomes zero at T = 156 K, so the Curie temperature is determined as TC = 156 K. The estimated values of magnetic parameters are listed in Table 1. The parameters for Y2Co12P7 are almost the same with those for Lu2Co12P7 reported in Refs.6,9), showing that magnetism of Co sublattice in Ln2Co12P7 is almost independent of Ln.
(a) T dependence of M of Y2Co12P7 at H = 0.1 T and H/M at H = 1 T. Solid line is a result of the Curie Weiss fitting. (b) Isothermal magnetizations of Y2Co12P7 at various temperatures.
| Ln | Ps (μB) | Peff | θ (K) | TC (K) | Peff/Ps |
|---|---|---|---|---|---|
| Y | 0.14 | 1.18 | 158 | 156 | 8.43 |
| Lu | 0.15 | 1.14 | 158 | 150 | 7.60 |
It is noteworthy that the ratio of Peff and Ps for Co sublattice of Ln2Co12P7 system is about 8, indicating shrinkage of ordered moments due to spin fluctuations. In the theory for itinerant electronic ferromagnetism by Takahashi, Peff/Ps is related to TC/T0 as Peff/Ps = 1.4 (TC/T0)−2/3 for the three-dimensional case10). Here, T0 characterizes the energy width of the dynamical spin fluctuation spectrum. From this relation, TC/T0 is estimated as 0.068 and then T0 is estimated as 2300 K. Since this value is reasonable compared with intermetallic ferromagnets11), ferromagnetism of Co sublattice is categorized as weakly itinerant ferromagnetism though the value of TC is rather high compared with typical compounds. In fact, it is difficult to estimate spontaneous magnetization of weakly itinerant electronic ferromagnets from magnetic measurements in usual. We also estimated T0 as the lower limit value by using saturation magnetization per Co (Psat) instead of Ps. Since Psat was estimated as 0.31 μB per Co from isothermal magnetization measurement using a pulse magnet as shown later, TC/T0 and T0 are estimated as 0.22 and 710 K, respectively. These values indicate that ferromagnetism of Co sublattice is rather strong compared with typical weakly itinerant electronic ferromagnets. The value of T0 can be estimated from the slope of M4 versus H/M at T = TC if M4 shows a linear relation with H/M. For the case of Y2Co12P7, unfortunately, M4 does not show such a linear relation with H/M, which is thought to be due to magnetic anisotropy. For further precise analysis, study using single crystals is needed.
Fig. 4 shows temperature dependence of magnetization measured at H = 0.1 T: Fig. 4 (a) is for compounds with Ln = Y, Pr, Nd and Sm and Fig. 4 (b) for compounds with Ln = Y, Gd and Dy. The data of Ho2Co12P7 are after Reehuis9) which were measured at H = 0.2 T. Obviously, for the case of light rare earth elements, the values of M are smaller than that for Ln = Y, and for the case of heavy rare earth elements, the values of M are larger than that for Ln = Y. These indicate that magnetic moments of Ln3+ couple with ferromagnetic moments of Co, and as a result magnetic moments of Ln3+ align antiparallel to ferromagnetic moments for the case of light rare earth while align parallel to those of Co for the case of heavy rare earth. Such a coupling has been reported in the cases of permanent magnets LnCo5, Ln2Co7 and Ln2Fe14B, though the sign of coupling constant is opposite of the case for Ln2Co12P7: for the case of light rare earth elements, the values of M are larger, and for the case of heavy rare earth elements, the values of M are smaller for the cases of those permanent magnets. In the model for these permanent magnets12), localized magnetic moments of Ln3+ couple with ferromagnetic moments of transition metals through the term of Hamiltonian H = 2 Hex·S, where Hex is an exchange field from 3d electrons of transition metals and S is spin component of 4f electrons. In this model, since spin components of 4f electrons align antiparallel to ferromagnetic moments of transition metals, magnetic moments of 4f electrons couple with ferromagnetic moments as shown above. Ln2Co12P7 can be understood as similar model but sign of the term of Hamiltonian above is opposite. The difference of sign of the term is thought to originate from whether lanthanoids directly form chemical bonds with transition metals or not. In the case of Ln2Co12P7, a possible scenario is that ferromagnetic split of 3d band of Co induces magnetic moments on 3p band of P in antiparallel with ferromagnetic moments of Co and then these magnetic moments on 3p band induces magnetic moments on 5d band of Ln in antiparallel with magnetic moments on P. This is just a speculation, and theoretical approaches are expected for further understanding.
T dependence of M of Ln2Co12P7 (Ln = Y, Pr, Nd, Sm, Gd and Dy) measured after field cooling at H = 0.1 T. The data of Ho2Co12P7 are reproduced from Ref.13) which were measured at H = 0.2 T.
As shown in Fig. 4, divergence between M of Ln2Co12P7 and that of Y2Co12P7 enhances with decreasing temperature. Taking into account the fact that ferromagnetic magnetization of Co sublattice, or Hex, does not develop so much below 100 K, thermal average of magnetic moments of Ln3+ is thought to develop with decreasing temperature as if magnetization which weakly induced by external magnetic field in paramagnetic states develops by obeying the Curie Weiss law with decreasing temperature. This is more evident in anomalous temperature dependence of intensity of peaks of neutron diffraction measured for the cases of Ln = Nd, Ho and Tb8, 9).
Fig. 5 shows isothermal magnetization curves measured at T = 1.4 K. For the case of Ln = Y, saturation magnetization is 3.7 μB per formula unit, showing that saturation magnetization of Co sublattice can be regarded as 3.7 μB for all the cases of Ln2Co12P7. Therefore, if magnetic moments of Ln3+ are forced to align parallel to magnetic moments of Co sublattice by external magnetic field, saturation magnetization is expected to reach the value of Msat = (3.7 + 2 gJJ) μB, where gJ is the Lande g-factor and J is total angular momentum. The factor “2” of the second term is number of Ln atoms in formula unit. The expected values of gJJ and Msat for isolated Ln3+ ions are listed in Table 2. As seen in Fig. 5 (b), M of Gd2Co12P7 easily reached the value of Msat for isolated ions, and for the case of Dy2Co12P7, M seemed to approach the value of Msat for the isolated ion with increasing H. In addition, for the case of Ho2Co12P7, there has been reported that M reaches 14 μB per formula unit at H = 5 T and T = 5 K9). If Ho3+ ion is isolated from environment, gJJ = 10 and thus Msat = 23.7 μB as the case of Dy. Therefore, M of Ho2Co12P7 also seems to approach the value of Msat for isolated ion as Dy2Co12P7. For the cases of Ln = Dy, M rather gradually increases with H compared with the case of Ln = Gd, which is possibly due to anisotropic nature of these ions.
Isothermal magnetization curves of Ln2Co12P7 (Ln = Y, Pr, Nd, Sm, Gd and Dy) measured after field cooling at T = 1.4 K.
| Ln | Pr | Nd | Sm | Gd | Dy |
|---|---|---|---|---|---|
| gJJ | 16/5 | 36/11 | 5/7 | 7 | 10 |
| Msat | 10.1 | 10.2 | 5.1 | 17.7 | 23.7 |
On the other hand, the cases of light rare earth elements Ln dependence of magnetic curves are a little bit complicated compared with the cases of heavy ones. For the cases of Ln = Pr and Nd, as shown in Fig. 5 (a), M is larger than that of Ln = Y in the range of H > about 15 T. Since the cases of light rare earth elements, magnetic moments of Ln3+ couple in antiparallel with those of Co sublattice and as a result M of Ln2Co12P7 is smaller than that of Y2Co12P7, the larger value of M observed above 15 T indicates that magnetic moments of Ln3+ flip and align parallel to those of Co sublattice in some degree. Both the cases of Ln = Pr and Nd, M almost saturates around H = 60 T, where the values of M are 5 μB for Pr and 7.3 μB for Nd. From these values, magnetic moments of Pr3+ and Nd3+ are estimated as 0.6 μB for Pr and 1.8 μB for Nd. These values are smaller than the values of gJJ μB for isolated ions. The shrinkage of magnetic moments possibly originates in anisotropy of 4f electrons. It is also possible that such shrinkage is related with the fact that in the low-H region magnetic moments of Ln3+ are induced by the exchange field. For the case of Ln = Sm, there is no sign of spin-flip up to H = 56 T but is just a gradual increase of M against H in contrast to the cases of Ln = Pr and Nd. Energy of field induced magnetic transition like spin-flip or metamagnetic ones is related to both anisotropy and Zeeman energies, and the latter is closely related with size of magnetic moments. Therefore, character of magnetization curve of Sm2Co12P7 is possibly understood by large anisotropy energy and/or smaller size of magnetic moments of Sm3+.
From the results of magnetic measurements, behavior of magnetic moments of Ln3+ in Ln2Co12P7 is found to be roughly understood by the single ion model applied to the permanent magnets. However, our discussion above lacks consideration of anisotropy of magnetic moments of Ln3+ ions. Usually, the ground state of 4f electrons for the case of Ln3+ ions well isolated from environments splits by the effect of crystal field and thus size of magnetic moments and their anisotropy change depending on the true ground state. Therefore, information about the ground state of Ln3+ with taking crystal field effects into account is necessary for further understanding.
In summary, we synthesized polycrystalline samples of Ln2Co12P7 with Ln = Y, Pr, Nd, Sm, Gd and Dy, and measured magnetization of these compounds. We confirmed that magnetic property of Y2Co12P7 is quite similar to that of Lu2Co12P7, indicating that magnetic property of Co sublattice is common in all the Ln2Co12P7. We also pointed out that magnetism of Co sublattice is possibly categorized as weakly itinerant ferromagnetism. Magnetism of Ln sublattice can be understood by the single ion model applied to permanent magnets, though the sing of coupling constant between magnetic moments of Ln3+ and exchange field derived from Co sublattice is opposite.
This work was supported by Grant-in-Aid for Young Scientists (B) from Japan Society for the Promotion of Science (Grant No. 24760534).
