Magnetic properties of the UNiGe2 at low temperature

We report on the magnetic characterization of a novel ternary uranium intermetallic UNiGe2. When we assume that UNiGe2 has the orthorhombic structure of CeNiGe2-type which is same as that of UNiSi2, the lattice constants were obtained to be a = 3.97 Å, b = 16.48 Å, and c = 4.08 Å. The unit cell volume of UNiGe2 is larger than that of UNiSi2. It comes from the fact that the atomic radius of Ge is larger than that of Si. The temperature dependence of the magnetic susceptibility shows two peaks at TN=45 K and TN′=65 K. Taking an account that UNi2Ge2 secondary phase exists in the compound, UNiGe2 is an antiferromagnet below TN while TN′ may come from the antiferromagnetic order of UNi2Ge2. At 5 K, the slope of the magnetization curve increases as increasing the magnetic field up to 5 T, indicating the presence of a metamagnetic transition. The residual magnetization remains on the magnetization curve at 5 K, which may come from a week ferromagnetism of UNi2Ge2 at low temperature.


Introduction
Intermetallic compounds including Ce or U atoms have been investigated extensively because these compounds give important information for studying the role of strong electron correlations in metallic systems [1,2,3,4]. In these compounds, the ferromagnetic/antiferromagnetic interaction and Kondo effect compete each other.
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 [5]. The lattice parameter along b-axis is extremely large compared to those along a-and c-axes, and it is expected that highly anisotropic magnetic property exists. Indeed, these compounds have received considerable interest of a great variety of magnetic behaviors [6,7,8].
UNiSi 2 also crystallizes in orthorhombic CeNiSi 2 -type layered structure, and is a ferromagnet at T C = 95 K [9,10]. Single crystals can be brown by Czochralski pulling method because UNiSi 2 melts congruently. The large anisotropic behavior is observed in the measurement of the magnetization of UNiSi 2 single crystal at low temperature ferromagnetic phase. The easy magnetization direction is in the ac plane. From the result of the magnetic susceptibility along the ac plane, the effective magnetic moment is obtained to be µ eff ∼ 2.47µ B , which is smaller than the value expected for the free U 4+ or U 3+ ions. It may come from highly anisotropic magnetic property of UNiSi 2 [11]. Recently a new compounds UNiGe 2 are also synthesized, but the crystal structure is unknown yet [12]. In the present work, we report on the synthesis of a ternary uranium compound UNiGe 2 and the magnetic property.

Experimental
Polycrystalline sample of UNiGe 2 was synthesized by arc melting with a stoichiometric composition in an Ar gas atmosphere. Weight losses were less than 0.3%. To improve homogeneity, the sample was turned over and re-melted several times. The sample was characterized by X-ray powder diffraction experiments using a Rigaku MiniFlex II diffractometer with Cu-K α radiation. The dc magnetization was measured by using a Quantum Design MPMS-5 superconducting quantum interference device magnetometer. Figure 1 shows the X-ray diffraction pattern of UNiGe 2 at room temperature. Here we assumed that UNiGe 2 has the same crystal structure as UNiSi 2 and indexed the Bragg peaks as the orthorhombic CeNiSi 2 type structure with the space group of Cmcm. The lattice constants were obtained to be a = 3.97Å, b = 16.48Å, and c = 4.08Å for UNiGe 2 . The unit cell volume is calculated to be 266Å 3 , which is lager than that of UNiSi 2 , 258.83Å 3 [11]. It comes from the fact that the atomic radius of Ge is lager than that of Si. On the other hand, several unknown peaks are also observed in the X-ray diffraction pattern, and may correspond to the diffraction of UNi 2 Ge 2 as a secondary phase [13]. Such behavior has been also observed in the previous report, in which it is suggested that a new dominant phase and UNi 2 Ge 2 secondary phase exist in the compound [12].  Figure 2 shows the temperature dependence of M/H of UNiGe 2 between 2 and 300 K in a magnetic field of 1 kOe. The ZFC M/H curve is obtained by cooling in zero field from a high temperature while the FC one by cooling at small applied field. M/H increases as temperature decreases and two peaks are visible on the M/H(T ) curve at T N = 45 K and T N ′ = 65 K. At low temperature below T N , on the other hand, M/H in the FC curve tends to increase as decreasing temperature while that in the ZFC one approaches to zero. It suggests that a dominant phase of UNiGe 2 is an antiferromagnet below T N . Taking account that UNi 2 Ge 2 is an antiferromagnet below 74 K and that a weak ferromagnet moment is also present at low temperature [13,14], T N ′ and the residual magnetization in the FC curve may also come from a secondary phase in the sample. Figure 3 displays the behavior of the magnetization M of UNiGe 2 as a function of magnetic field H up to 50 kOe at 5 K. M increases as increasing magnetic field, and reaches 0.139 µ B , which is much smaller than the saturation magnetization M s = 1.12 µ B of UNiSi 2 [9]. Since the slope of magnetization ∂M/∂H increases up to 50 kOe, a magnetic phase transition such as metamagnetism is expected at a high magnetic field below T N . Moreover, hysteresis loop is also observed in the magnetization curve at 5 K, suggesting the the presence of some magnetic ordering phase such as weak ferromagnetism or canted magnetism. Taking account that UNi 2 Ge 2 a weak ferromagnet moment is also present at low temperature [13], pure samples are needed to discuss the ground state of UNiGe 2 .

Summary
In this study, we prepared a novel ternary uranium intermetallic UNiGe 2 during our investigation of UNiX 2 with the CeNiSi 2 -type orthorhombic structure. The unit cell volume of UNiGe 2 is lager than that of UNiSi 2 . Our preliminary result of the magnetization indicates that UNiGe 2 is an antiferromagnet of 45 K. Further experiments are in progress to synthesize UNiSn 2 and UNiC 2 to investigate magnetic properties of UNiX 2 (X = C, Si, Ge, Sn) series.