Magnetic Frustration Related Physical Properties Of Chalcogenide Spinels

Frustrated magnets have been at the focus of renewed attention because of their underlying physics that might be relevant to high-TC superconductivity, topological quantum state and quantum computing e.g. Magnetic frustration refers to all the competing interactions in a system that cannot be satisfied simul-taneously. In geometrically frustrated system, the geometric topology restricts the long-range order and leads to a large ground-state degeneracy, which means no energy scale of its own and hence any perturbation is strong. As a result, the system shows some novel ground state, such as spin glass, spin ice. spin liquid, et. al.The chalcogenide spinels, as a model frustrated magnetic material, show many fascinating physical effects, such as negative thermal expansion, large mag-netostriction and magnctostructural transition e.g. In addition, these materials belong to electron-strong-correlation system. Under this circumstance. complex magnetic ground states can be built up in the presence of external perturbations accompanied by strong coupling between the spin. lattice, charge and orbit de-grees of freedom. It greatly augments the difficulty of understanding the physical nature in this system. For example. the polycrystalline sample of FeCr2S4 displays orbital ordering, while single crystal sample shows orbital glass due to disorder: both negative thermal expansion and large magnetostriction were reported in Zn-Cr2Se4, whereas no evidence shows ZnCr2S4 exhibits similar effects although it displays more strong frustration. In this research background, we have selected FeCr2S4 with orbital-frustrated effect and strong bond-frustrated ZnCr2Se4 as ob-ject, detailedly investigated the evolution of orbital order, spin fluctuation and spin-lattice coupling. The main contents are summarized as follows:1. we prepared a set of polycrystalline(PC) FeCr2-xAlxS4(0≤ x<≤0.2) sam- ples to investigate the evolution of the orbital state with increasing disorder. For PC FeCr2S4, the onset of long-range orbital order at Too～9K is evidenced by the appearance of a step-like transition in M(T). a small kink at about 5.5 T below 9 K in the isotherms M(H) curves as well as a A-typc anomaly in specific heat. With increasing Al content, the Too decreases gradually. For the samples with x≥ 0.1. the orbital ordering is replaced by orbital glass, where the specific heat obeys a T2-dependence. Consistently, the calculated residual orbital entropy increases with x, implying the progressive frozen-in of the orbital moments and coexistence of orbital ordering and orbital glass in the middle doping level.2. We trace the possible spin fluctuation for bond frustrated ZnCr2Se4 down to a temperature as low as 0.3 K from the dc and ac susceptibilities. specific heat and thermal conductivity under different magnetic fields. They reveal that for bond-frustrated ZnCr2Se4 with large spins, spin fluctuation counter-intuitively shows no sign of wane with decreasing temperature down to 0.3 K, implying the coexistence of spin liquid and magnetic order. Moreover with increasing field to 6.5 T at the base temperature, a quantum phase transition from tetragonal anti-ferromagnetic to cubic ferromagnetic structure is revealed. The field induced phase transition is discussed with a H-T phase diagram by taking account of the competition between thermal and quantum fluctuation.3, We report systematical investigations on the magnetization, heat capaci-ty, thermal expansion and magnetostriction for single crystalline ZnCr2(Se1-xSx)4 (0≤ x≤ 0.1) to study the evolution of spin-lattice coupling with sulfur substitu-tion. We show that with increasing the sulfur content, the antiferromagnetic tran-sition is gradually replaced by a spin-glass transformation. At the same time, the temperature region of the negative thermal expansion expands. while the magne-tostriction is suppressed gradually. These phenomena are explained qualitatively by taking account of the enhancement of the antiferromagnetic interaction and bond disorder introduced by sulfur substitution.