The Effect Of Strain And Mg/Zn Doping At Mn Site On The Magnetic Properties Of MnV₂O₄

In the past decade, vanadate spinel oxides had attracted much attention from the strongly correlated electronic system in condensed matter physics. There are complex couplings between spin, orbit and lattice degrees of freedom in v anadate spinel oxides, so they exhibit many intriguing physical behaviors, such as colossal magnetoresistance effect, magnetothermal effect and multiferroic behavior. MnV₂O₄ is typical vanadate spinel oxide, and it is also an ideal platform to discuss the couplings between spin, orbit and lattice. In addition, MnV₂O₄ has much application value in magnetic refrigeration materials and magnetic switch fields because it exhibits large magnetoresistance effect and magnetic-field-induced structural phase transition. There have been many researches about MnV₂O₄ before and they are all focused on bulk MnV₂O₄. In this research, we deposited epitaxial MnV₂O₄ films by pulsed laser deposition successfully and characterized the growth orientations, strain states and magnetic properties of MnV₂O₄ films. The effect of compressed strain on magnetic properties was discussed. We also prepared MnV₂O₄ polycrystalline samples with Mg or Zn doping and discussed the effect of Mg or Zn doping on magnetic phase transition temperature.Vanadium is easily oxidized in air, so we synthesized MnV₂O₄ target and polycrystalline samples with Mg or Zn doping in an evacuated silica tube. There is no impurity phase in the XRD patterns of polycrystalline samples. The diffraction peak of polycrystalline samples with Mg or Zn doping move to higher diffraction angle when the doping amount increasing. The moving of diffraction peak means a constriction of MnV₂O₄ lattice, which agrees with theoretical calculation.The ideal growth condition of MnV₂O₄ films is setting substrate temperature as 650 oC, pressure as 2×10-4Pa, repetition rate of laser as 5Hz, energy of laser 150 m J. MnV₂O₄ films with different thickness were grown on La Al O3（001）、Sr Ti O3（001）、Mg O（001） substrates. The XRD patterns of MnV₂O₄ films indicate the（004） oriented epitaxial growth. The diffraction peak of films move to lower diffraction angle, which means there is compressed strain in films. When the mismatch between film and substrate is bigger or the film is thinner, the compressed strain will be stronger. MnV₂O₄ films grown on three kinds of substrates are all ferrimagnetic under low temperature and their ferromagnetic phase transition temperature TN are all lower than that of MnV₂O₄ bulk. The TN will be lower when the film gets thinner. It is the compressed strain reducing the TN by inducing a lattice distortion to extend the V-V distance and weaken the spin-spin interaction in MnV₂O₄. In addition, the biaxial compressed strain also impedes the tetragonal transition in MnV₂O₄ films, so only 80-min-growth films grown on La Al O3 and Mg O present the thermal hysteresis.The ordering of magnetic structure and lattice in MnV₂O₄ is disturbed by Mg or Zn doping, so the ferrimagnetic phase transition temperature TN and structural transition temperature of polycrystalline samples get lower when the doping amount increasing. The structural transition temperature decreases faster than TN. When the doping amount x ≥ 0.2, the structural transition in polycrystalline samples will vanish. The TN of Mg doping samples is higher when the doping amount is the same because there is stronger magnetic interaction in Mg doping samples.