Polymorphous Transformations And Property Research On Ga_2-xFe_xO₃ Ceramics

Spintronics using spin and magnetic moment of an electron make the solid state devices in addition to charge transport, also cotained the spin and magntic moment. Magnetic semiconductor is a kind of materials which contain ferromangnetic and semiconductor at the same time, provides a new way for conductive materials. Ferrite belongs to semiconductor category in electrically and occurs spontaneous magnetization at room temperature, is one of the high-profile magnetic semiconductor.The different Fe content Ga2-xFexO3 samples synthesized by using solid-phase sinter method, and the stability structure at room temperature are rhombohedral(α, 1.6≤x≤2.0), orthogonal(ε, 0.8≤x≤1.2) and monoclinic(β, 0.0≤x≤0.4) phase. For the ε phase samples, with the increase of Fe content, the XRD patterns refinement reveal the lattice constant increases monotonically, while the Raman peaks position toward to the lower wavenumber, which are related to the increased bond length caused by the macroion radius Fe3+ ions instead of small ionic radius Ga3+ ions. The low temperature DC magnetic susceptibility measurement indicates that the Curie temperature Tc, corresponding to the temperature sample magnetic properties changed from ferromagnetic to paramagnetic, following monotone increasing with the increase of Fe content. Bellow the Tc, samples exist obvious spin frustration phenomenon, and the AC magnetic susceptibility measurement also proved this point. For the β phase samples, the UV-Vis spectras reveal that the electronic transiton of Fe3+ ions leads to reduction of the band gap Eg. The DC and AC magnetic susceptibility measurement show that the sample exhibited paramagnetic behavior at low temperature, and the adjacent Fe3+ ions through the O2- ions formed antiferromagnetic super-exchange interaction.Use ε and β phase as the precursor, single α pahse(0.0≤x≤2.0) synthesized through the high temperature and high pressure method. The Rietveld refinement reveal the lattice constant increases monotonically, while the Raman peaks position toward to the lower wavenumber as the increased Fe content. The DSC measurement shows the increase of Fe content in favor of α pahse stability, while the Eg obtained from UV-Vis spectra is monotonically decreased. The results of DC and AC susceptibility measurement at low temperature for the low Fe content(0.1≤x≤0.4) samples show that antiferromagnetic super-exchange interaction was formed between Fe3+ ions. With continue increasing Fe content(0.5≤x≤1.9), the measurement of DC magnetic susceptibility indicates that the sample’s Tc monotone increasing, while the imaginary part of AC susceptibility found a peak Tf moving to high temperature with increasing frequency at low temperature. In the measurement protocol of memory effect, zero field cooling magenetization MZFC cureves exist trough phenomenon and field cooling magnetization MFC cureves show a step rising behavior near the stay temperature, which prove the sample has self recovery effect. The α pahse samples show spin glass behavior at low temperature may related to the frustration effect caused by Fe3+ ions replacement and random effect resulting from the disorder distribution of cations.Through the method of high temperature quenching in liquid nitrogen, the cubic pahse of Ga3-1.5xFe1.5xO4(1.4≤x≤2.0) samples were successfully prepared, and vacuum annealing treatment enlarged the synthesized range to 1.4≤x≤2.0. The DSC measurement result in air atmosphere shows that a complex structural phase transition occurred at high temperature with decreasing Fe content and monoclinic structure was found as an intermediate product, and the high temperature in situ XRD results also confirmed this point. The result of low temperature DC magnetic susceptibility shows that both queneched and annealed samples not containing Ga3+ ions occur Verywey phase transition at ~125 K, while distinct magnetic phenomenon were found as the temperature continue to decrease. Similar magnetic behavior are observed for the samples which with the same composition but synthesized by different method. AC susceptibility measurements found that all samples present a spin frozen peak Tf which is moveing to high temperature as the measuring frequency increased. The results of temporary cooling, ZFC and FC measurement method with variable magnetic field show that the Ga3-1.5xFe1.5xO4 samples have relaxation phenomenon and significant memory effect at low temporary.By using alcohol as process control agent, through the wet milling successfully get the nano crystal size of cubic(1.4≤x≤2.0), orthogonal(0.8≤x≤1.2) and rhombohedral(1.6≤x≤2.0) and monoclinic phase samples. In dry grinding condition, the phase transition from orthogonal and monoclinic to rhombohedral phase was found. By using a reverse microemulsion method synthesize Ga0.4Fe1.6O3 samples, a continuous phase transition behavior γâ†'εâ†'α was found as the increasing of sintering temperature, which is related to the changes of surface energy, chemical potential caused by increase of grain size. The temperature dependence of magenetization curves M-T of nanosized samples synthesized by ball milling are similar to the bulk samples, while the remanence Mr and coercivity Hc obtained from low temperature hysteresis loop measurement are significantly improved. For the reverse microemulsion samples, γ phase sample with sintered temperature 800℃ shows super paramagnetic behavior, while the ε and α phase behave obvious ferromagnetic phenomenon at room temperature which are sintered at 1050℃and 1300℃, respectively. The hysteresis loop measurements show that the Hc and Mr for three different samples are monotonically decreased with the increasing of measured temperature.