Anisotropic transport and magnetic properties and magnetic-polaron-like behavior in CeTe_2-x

Highly anisotropic properties of CeTe_2-x with the layered Cu₂Sb-type structure have been found by the measurements of electrical resistivity _ρ(T), magnetic susceptibility _χ(T) and magnetization M(B) on single crystals for x = 0.15-0.18. Both M(B ∥ c) and M(B ⊥ c) at 2 K exhibit a metamagnetic transition at 0.04T. M(B ∥ c) saturates rapidly to a value of 1.06 μ_B/f.u. at around B = 0.2 T, whereas M(B ⊥ c) gradually increases to a value of 0.75 μ_B/f.u. at 5.5T. The absence of hysteresis and remanence in M(B) indicates an antiferromagnetic ground state. The resistivity ratio _ρ∥c/_ρ⊥c increases from 3 to 150 on cooling from 300 K to 1.5 K. Both _ρ∥c and _ρ⊥c exhibit a sharp peak at 6 K before the onset of long-range magnetic order at T_N = 4.3 K, where _χ(T) exhibits a sharp peak. Measurements of the Hall coefficient, R_H(T), specific heat C(T), magnetoresistance _ρ(B) and electron tunneling have been done for polycrystals with x = 0.13 and 0.18. Electron-tunneling spectra at 77K revealed the presence of a charge density wave with a pseudogap of 0.35eV. It is found that C(T) exhibits a peak at T_N with a significant tail up to 10 K, whereas R_H(T) continuously increases. The magnetic entropy at TN is only 0.7Rln 2, being indicative of a short-range order above T_N. At 1.5 K, _ρ(B) shows a peak at 0.075 T and then decreases rapidly for B ≤ 1 T, resulting in a negative magnetoresistance of 20%. These results are analogous to those found in EuTe_0.9, GdAs_0.95 and CeSb, where low-density carriers form a pseudo-localized state and align the 4f spins within a magnetic polaron.