Research On The Novel Electromagnetic Properties In Fe_1+xV_2-xO₄（0≤x≤1）

The ferroelectricity in single-phase multiferroics is mainly caused by thenoncollinear spin structure. Due to the complex interactions among lattice, spin, charge,and orbital degrees of freedom spinel ferrites exhibit rich and exotic magnetic structures,which may induce strong ferroelectricity. In this thesis we studied the magnetical andelectrical properties as well as their evolutions with the composition x inFe_1+xV_2-xO₄（0≤x≤1） ferrites, for better understanding of the interactions among differentdegrees of freedom and the mechanism of the multiferroic effects. Our investigationswould provide new clues for searching new multiferroicis with higher transitiontemperatures.We firstly studied the structural, magnetical, and electrical properties of theFe_1+xV_2-xO₄（0≤x≤0.4） system systematically. When0≤x<0.35, there are three successivephase transitions from high to low temperatures: from cubic and paramagnetic phase（C-PM） to high temperature tetrahedral （c<a） and paramagnetic phase （HT-PM） at TS;from HT-PM to orthorhombic and collinear ferrimagnetic phase （O-CFIM） at TN1; fromO-CFIM to low temperature tetrahedral （c>a） and noncollinear ferrimagnetic phase（LT-NFIM） at TN2. We found that the noncollinear spin structure led to strongferroelectricity at the temperatures below TN2. The saturation polarization of x=0sampleis as high as142μC/m²at10K under the poling electric field of40kV/m, which ismuch higher than that of other single-phase multiferroicis reported so far. An appliedmagnetic field of1T suppressed the spontaneous polarization at10K by22.7%. Withincreasing x, the Jahn-Teller effect of Fe2+at A-sites becomes weaker, and theantiferromagnetic coupling between spins at A-and B-sublattice becomes stronger. Asa result TSand TN2are decreased with the ferroelectricity supressed, while TN1isincreased. The O-CFIM to LT-NFIM transition together with the inducedferroelectricity at TN2disappears around x≈0.35which is proved to be a criticalcomposition in Fe_1+xV_2-xO₄system. We explained the evolution of the ferroelectricitywith x by the spin-current model.Secondly, we studied the electric transport properties in Fe_1+xV_2-xO₄（0≤x≤0.5）system. We found that in the range of0≤x<0.35the high-temperature resistivity of the samples obeys the variable range hopping model, while the high temperature resistivityof the samples with x≥0.35exhibits a simple thermally excited behavior. A significantmagnetoresistance appears at the critical concentration x≈0.35with themagnetoresistance as high as12.7%at TN1under1T magnetic field.We then studied the ac and dc magnetic properties of Fe_1+xV_2-xO₄（0.5≤x<1）. Amagnetization compensation point was discovered in the sample around x≈0.8. Wediscovered a reverse magnetization phenomenon in x=0.8sample. Taking account of thetemperature dependence of the coercivity and the magnetic moments on both A-and B-sublattices we qualitatively explained this phenonmenon.Finally, we studied the dielectric properties in Fe_1+xV_2-xO₄, and found afrequency-independent peak in the real part of the permittivity. Carefull research showsthat this is not caused by the ferroelectric phase transition as previously claimed in theliterature. but due to the water absorption during the measurement in the air. Thisphenomenon should be common in other porous polycrystalline samples.