| The coupling among spin, orbital, lattice degrees of freedom attracted much attention worldwide. And through the deep investigation, the mechanism could be employed to tune the physical properties and is important in the realm of potential application. The complex coupling mechanism exists in the materials RVO3(R3+)、RTiO3、RV2O4、RTi2O4and so forth.1. The structural, transport and magnetic properties of MnV2O4have been studied. The magnetic hysteresis is caused by the phase separation which also results in strains and thus leads to the variations of the intensity of the electron-spin-resonance (ESR) spectra. We successfully explain the magnetic properties by means of micromagnetic mechanism at low temperatures through a theoretical analysis within mean field level and get the three exchange energies JA,JB,JAB.2. We have studied the structural, transport and magnetic properties of Mn1-xMgxV2O4(0≤x≤8). Both the magnetic transition temperature and the structural transition temperature decrease gradually with increasing Mg2+concentration, together with the shrinkage of the lattices and the decrease of the activation energy for the electrical conductivity. The two transition temperatures become closer to each other and even-tually become equal at x=0.2. For x>0.2, the magnetic ground state switches from a long-range ordered ferrimagnetic state to a glassy state. The transition temperature of the glassy state continues to decrease with further increase of Mg content. On the other hand, the saturated moment at low temperature shows a non-monotonous change with Mg-doping, which is ascribed to the competing impact induced by Mg-doping. We successfully explain these phenomena through a theoretical analysis within mean field level. Our analysis supports that the orbital ordering in MnV2O4should be dominated by the lattice-orbital interaction. 3. In summary; investigations of the electric, magnetic, structural, and ther-mal properties of spinel COV2O4have been made. The experimental results about the magnetism and heat capacity indicate the presence of a paramagnetic-to-fer-rimagnetic transition at TC=142K and another phase transition at T1=59K. At59and100K, the thermal conductivity exhibits two valleys. Below T1, the ac magnetization shows that the phase transition leads to spin glass behavior, but the phase transition cannot be ascribed to the structural transition as revealed by the x-ray diffraction patterns. The relation between the series of phenomena around T1and the orbital state is discussed.4. In summary, the investigations on structural, magnetic and thermal prop-erties for Co1-xZnxV2O4(0≤x≤0.2) is carried out. The rise△M in magnetiza-tion, the frequency dependence of AC susceptibility, the emergence of a valley in thermal conductance and the specific heat peak around T1indicate the existence of the transition at T1for the parent compounds. As the Zn2+-doped content increases, these phenomena are successively suppressed. The series of consistent changes is attributed to the gradual suppression of the transition at T1, which cause partial melting or melting of the new-spin-cluster glassy (NSCG) behavior in V3+sublattices. Through the analysis of the energy bands and the fourth-order perturbation theory, the suppression of the transition at T1and partial melting of the NSCG behavior may originate from the uplifting of the oxygen2p bands in energy with the increase of the Zn2+-doped content. The shift of the oxygen band is supported by the redshift of the infrared absorption spectrum.5. The sample MnTi2O4is conducted by the small polarons with differen-t activation energies across the temperature T1and is subject to the deviation from the Curie-Weiss law for the magnetic susceptibility at T1. At T1, the spe-cific heat display a crossover. The series of phenomena near the characteristic temperature T1are ascribed to the micro-structural distortion as indicated by the X-ray diffraction peaks. The structural distortion implies the emergence of the short-range orbital ordering state, which is evidenced by the tiny upturn of thermal conductance at T1. This is on contrary to the previous report that M- nTi2O4experience the cubic-to-tetragonal structural transition at180K. As the temperature decreases, the systems MnTi2O4enter the long-range ferrimagnetic Neel state at TN-For the systems, the positive thermal-electric power decreases with the decrease of temperature and exhibits a p-type to n-type transition at Tpn, which are assigned to the cooperative effects of relativistic spin-orbit (SO) cou-pling and the Jahn-Teller (JT) effect. For the Zn2+-doped polycrystalline samples Mn1-xMgxTi2O4, the series of phenomena around T1still exist. But it is different that the ESR signals become broadening below25K, and this may be induced by the non-uniformity of the effective micro-magnetic fields. The Seebeck coef-ficients lie in positive value across the whole temperature zone, which indicates that the hole polarons play the dominant role in the transport properties. The Seebeck coefficients decreases as the temperature decreases, this may be due to the micro-structural distortion which suppresses the spin-orbit coupling and the JT effect.We also discuss the effects of the spin-phonon coupling on the related phe-nomena and the possible mechanism for the orbital ordering.1. In conclusion, we have incorporated the s-p interaction and investigated the effects of s-p interaction on transport, magnetic properties and the oxygen isotope effect in R1-xAxMnO3. The result shows that (i) the ferromagnetic ex-change energy between spins is strengthened by external magnetic field, which accounts for the increases of Tc, and a magnetic gap for spin waves at the center of Brillouin zone appears due to the s-p interaction,(ii) it. is predicted that s-p would lead to the oxygen isotope effect in CdCr2O4, and s-p together with e-p determine the oxygen isotope effect in some colossal magneto-resistance systems,(iii) consequently, the s-p could also cause the hardening of phonon frequency below Tc,(iv) it could induce the increase of the resistance in ferromag-netic metallic materials. The resistivity is proportional to T at high temperature below Tc and is proportional to T3at low temperature2. In conclusion, the regime of the orbital ordering (00) caused by the direct coulomb interaction is proposed the first time for the ions carrying the eg orbital freedoms. The ongoing investigation indicates that the super-exchange interaction governs the behaviors of the orbital occupancy accompanied by the weak spin-orbital coupling in the180-degree configuration, and the direct coulomb repulsion plays the crucial role in the00for the case of the90-degree configuration. The ground state for the one-dimensional chain in the case of90-degree configuration is the stable ferro-OO.3. In summarized, the single-ion orbital ordering is investigated quantitative-ly for the spinel systems RV2O4in terms of exact diagonalization. If the relativistic spin-orbit coupling and Jahn-Teller effect are much larger than electron hopping, they can lead to the orbital ordering. However, this is not the case for RV2O4(R=Mg,Zn,Cd). since the electron hopping is implied to be notable according to the energy gap in conductivity. And the spin-orbit coupling and Jahn-Teller effect are not the main physical reasons for the orbital ordering of the spinel systems RV2O4. |
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