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The subject of metal-insulator transition (MIT) has been a research focus over the last two decades, following the discovery of high-Tc superconductivity. Systems that undergo MIT show a dramatic change in their physical properties when the carrier concentration, temperature, external pressure, or external magnetic field is varied. Magnetic anisotropy is another subject of interest in several scientific and industrial fields. From a scientific viewpoint, at the microscopic level, the occurrence of magnetic anisotropy is mostly governed by the complex interchange between spin-orbit interaction, microscopic-level electronic states such as orbital and spin magnetic moments. In this study, MIT in NiS2-xSex (0 ≤ x ≤ 0.5) systems was investigated and analyzed based on the electrical and magnetic properties under chemical pressure, hydrostatic pressure, and uniaxial pressure. Furthermore, magnetic anisotropy in NiS2 and CoS2 was studied.
Hydrostatic pressure was applied to single crystals of NiS2-xSex (0 ≤ x ≤ 0.5). Resistance-temperature (RT) measurements of different samples of Mott insulator NiS2 showed clear insulating behavior of the compound, even at high pressures. Derivatives of RT curves revealed that the compound showed weak ferromagnetic (WFM) ordering at low temperatures, while it was an antiferromagnetic insulator at slightly high temperatures. For x = 0.1, NiS2-xSex showed insulating behavior at low pressures and MIT at pressure (P)= 7.5 kbar. Magnetic transitions from paramagnetic to antiferromagnetic (AFM) ordering and from AFM to WFM ordering were determined from magnetic measurements. MIT occurred in compounds with x = 0.3 and 0.4 at a high pressure (P) of 10.6 kbar and a low pressure of 1.3 kbar, respectively. Under chemical pressure (doping), NiS2-xSex with x = 0.5 showed MIT at ambient pressure, and when external pressure was applied, the MIT temperature shifted toward room temperature.
When uniaxial pressure was applied to NiS2-xSex (0 ≤ x ≤ 0.5), NiS2 showed insulating behavior, even at a maximum pressure of 15.1 kbar. For x = 0.3, MIT was observed at a pressure of 8.1 kbar and compound with x = 0.4 showed insulating behavior at ambient and lower pressure values, and it showed MIT at 2.4 kbar. For x = 0.5, MIT occurred at ambient pressure because of the chemical pressure (i.e., high Se content); however, the transition temperature shifted toward room temperature with an increase in the pressure. While comparing the hydrostatic and uniaxial pressures, the phase transition temperature remains almost unchanged. The difference in the effect of pressures can be observed in the change in resistance values at room temperature (RRT). The RRT decreases under both pressures which correspond to the band overlap. The difference in resistance for composition x = 0, 0.3 is much greater at 1 bar is due to the extra stress given by the stycast used as a medium for uniaxial pressure. The RRT under uniaxial pressure is higher than hydrostatic pressure resistance values in low Se content compounds (0 ≤ x ≤ 0.4) and this pattern reversed when the Se content is high for x = 0.5 composition as Se-Se bonds are more flexible and expand under pressure. The normalized resistance data R0/RRT (R0: residual resistance) decreases under pressure indicates that the band gap closes with increase in pressure and system is heading towards metallic state. The trend for pure NiS2 and composition of x = 0.3 under hydrostatic and uniaxial pressure are almost same. The uniaxial pressure is more effective as it shows smooth decreasing trend as compared to hydrostatic pressure.
The magnetic anisotropy of CoS2 was precisely determined by using torque magnetometery. The angle-dependent torque τ(θ) along (100) plane was measured at different temperatures (T) and magnetic fields (H) where torque signal increases with decreasing T and increasing H. The torque curves at different crystal orientations (ψ) and τ(θ) fitting revealed that the magnetic anisotropy in CoS2 is two-fold and intrinsic in nature. The τ(θ) measurements at different ψ confirmed the presence of in-plane magnetic anisotropy.
Fourfold anisotropy, namely, cubic anisotropy, was observed in single-crystal NiS2. The torque signal at T = 10 K was considerably large, and it decreased to almost zero above Tc, at T = 30 K. This decrease could have resulted from increased thermal fluctuations at elevated temperatures, which disturbs the Ni spins. The hysteresis behavior was stronger at low fields, and the hysteretic effect weakened at higher fields. This indicated the ordering of the domain magnetization in a specific direction and the weakening of magnetic canting. Another reason for hysteresis behavior is the existence of net magnetization, which refers to the vectoral sum of WFM moments in different domains of the NiS2 samples. Field-dependent torque measurements for single-crystal NiS2 were obtained at T = 10 K. The measurement curve in the region from 0 to 0.14 T was steeper than that from 0.36 to 1 T. The slope change was 38%, which indicated that magnetic canting was strong in the low- magnetic-field region and weak in the higher-magnetic-field region.
Thesis Advisor: Prof. Younjung Jo