Single crystal growth of PrNiGe 2 compounds

We report on the magnetic characterization of a praseodym intermetallic PrNiGe 2 . Several single crystals are grown by the Czochralski method from Ni-deficient sample as the initial one. X-ray analysis of PrNi 0.8 Ge 2 indicated the CeNiSi 2 -type structure as the only phase. The magnetic measurements clearly indicate that PrNi x Ge 2 exhibits a ferro-magnetic orderings at 12 K, which is independent with of composition of Ni. A strong anisotropy along the three principal crystallographic directions was observed, reflecting the orthorhombic symmetry of the crystal structure. The b -axis was found to be the easy axis of magnetization


Introduction
The ternary compounds CeTX 2 (T = transition metal and X = Si, Ge, Sn) form a large family having the orthorhombic CeNiSi 2 -type layered structure, which is constructed from deformed fragments of the CeGa 2 Al 2 and α-ThSi 2 structures [1][2][3]. The lattice parameter along b-axis is extremely large compared to those along aand caxes, and it is expected that highly anisotropic magnetic property exists. Indeed, these compounds have received considerable interest of a great variety of magnetic behaviors [4][5][6]. There are not so many reports on single crystal growth due to the difficulty in obtaining single phase samples. On the other hand, the magnetic properties of polycrystals of RT x Ge 2 (T: Fe, Co, Ni, Cu, x < 1) have been investigated in a previous study, and it seems to be easier to grow polycrystals when the amount of T is reduced [7][8][9][10]. PrNiGe 2 is known to order ferromagnetically below 13 K [11]. In this study, we have grown a single crystal of PrNi x Ge 2 and report the anisotropic magnetic properties here.

Experimental
The polycrystalline samples of PrNi x Ge 2 (x = 0.6, 0.8, and 1.0) were prepared as starting materials by arc-melting praseodym (99.9% ), nickel (99.99% ) and germanium (99.999% ) in a tetra-arc furnace under argon atmosphere. To improve homogenity, the ingot was turned over and remelted several times. The weight loss is negligible. Single crystals were grown by a Czochralski pulling method. The pulling parameter was kept constant during the growth (pulling rate: 10 mm/h; seed rotation speed: 10 rpm; crucible-rotation speed: 5 rpm). An ingot was 2-3 mm in diameter and 30 mm in length as shown in Figure 1. Its homogeneity and chemical composition were checked by microprobe analysis, which was made using JEOL SEM and Oxford Instrument EDX at Venture Business Laboratory Kanazawa University, based on the measurement of the Ce L α1 , Ni K α1 and Ge K α1 X-ray emission lines. The experimental atomic percentages of PrNi x Ge 2 are obtained to be Pr 25.3%, Ni 23.7% and Ge 50.9% for x = 1.0, Pr 30.0%, Ni 26.6% and Ge 43.3% for x = 0.8, and Pr 21.5%, Ni 16.4% and Ge 61.9% for x = 0.6, respectively.
The samples were checked by conventional x-ray powder diffraction experiments using Cu-K α radiation. The single crystalline state was confirmed using back-scattering Laue technique. The dc magnetic susceptibility was measured in the temperature range 2.0-300 K using a Quantum Design MPMS-5 superconducting quantum interference device magnetometer. The specific heat was measured by utilizing the Heat Capacity option on a Physical Properties Measurement System. Figure 2 shows X-ray powder diffraction pattern of PrNi x Ge 2 . All diffraction peaks can be indexed in the orthorhombic CeNiSi 2 type layered structure (space group Cmcm), as shown in the calculated spectrum. The lattice parameters a, b, and c are determined by a least-squares approximation from the d-values corresponding to the reflections, and are gathered in Table 1. They are consistent with those of previous reports [7,11].

Results and Discussion
The specific heat of PrNi x Ge 2 single crystal is shown in Figure 3. All samples show an anomaly reaching a maximum of ∼14 J mol −1 K −1 at 12 K. It is consistent with the previous result of PrNiGe 2 polycrystalline sample [11], which is associated with the onset of ferromagnetic order. A small anomaly is observed only in PrNi 0.6 Ge 2 at ∼17 K, indicating some magnetic impurity may be included because the sample deviate from stoichiometry. Figure 4 shows the temperature dependence of M /H in a magnetic field of 1 kOe and the 002.2   inverse magnetic susceptibility of PrNi x Ge 2 . A strong decrease in magnetization is observed at around 13 K for both x=1.0 and 0.8 samples, which is consistent with the result of specific heat measurement. The magnetic susceptibilities of those compounds in the paramagnetic state are fitted using the following formula: where N is the Avogadro number, k B is Boltzmann's constant, Θ is the paramagnetic Curie temperature, and χ 0 is a temperature-independent part of the magnetism including the diamagnetic core correction, the Pauli susceptibility of the electron gas, or the Van Vleck temperature independent paramagnetism. Fitting was carried out in the temperature range 100 K < T < 180 K. Θ, χ 0 , and the effective magnetic moment µ eff are given in Table 2. The χ 0 values are relatively large for both x=1.0 and 0.8 compounds while the µ eff values are in good agreement with the Hund's rule ground state for Pr 3+ ion (3.58 µ B ). The isothermal magnetization of PrNi x Ge 2 measured at T = 5.0 K along the three principal crystallographic directions is shown in figure 5 (a). For H//b-axis the magnetization increases 002.3  more rapidly with field than along the other two directions, thus indicating the b-axis as the easy axis of magnetization. Hysteretic behaviour is observed especially along b-axis, confirming the ferromagnetic ground state. At 5.0 K the magnetization saturates to 2.55 µ B /Pr and 2.89 µ B /Pr for PrNiGe 2 and PrNi 0.8 Ge 2 , respectively, which is larger than that of a PrNiGe 2 porycrystalline sample [12]. The saturation moment of the free Pr 3+ ion (g J Jµ B = 3.20µ B ) is relatively close to the saturation magnetization of PrNi 0.8 Ge 2 . Figure 5 (b) show the temperature dependences of the inverse magnetic susceptibility of PrNi 0.8 Ge 2 . The anisotropy in the magnetic susceptibility along the three principal directions is clearly evident. The magnetic susceptibilities in the paramagnetic state are fitted using the formula (1). Fitting was carried out in the temperature range 100 K < T < 180 K. The Weiss temperature Θ is positive for b-axis, as expected for a ferromagnetic ordering compound. On the other hand, Θ is negative for aand c-axes.

Summary
In this study PrNiGe 2 single crystals are grown by the Czochralski method from Ni-deficient sample as the initial one. X-ray analysis of the sample indicated the CeNiSi 2 -type structure as the only phase. The specific heat and the magnetic measurements clearly indicate that PrNi x Ge 2 exhibits a ferromagnetic orderings at 12 K, which is independent with of composition of Ni. A strong anisotropy along the three principal crystallographic directions was observed, reflecting the orthorhombic symmetry of the crystal structure. The b-axis was found to be the easy axis of magnetization. From the result of the magnetic susceptibility along the bplane, the effective magnetic moment is close to the value expected for the free Pr 3+ ions, while a temperature-independent part of the magnetism remains.