Magnetic Properties Of Mg-doped MnV₂O₄ Single Crystal

There are many kinds of complex couplings between spin,orbit,and lattice in vanadate spinel oxides AV₂O₄ system,which become famous materials of the research in strong relation system.MnV₂O₄ has additional magnetic Mn2+ ions,so it has more complex intercoupling and interesting physical properties.In the AV₂O₄ system,whether a bit ion has magnetic moment determines its physical nature.It is an effective means to study the interaction of the spin orbit with a bit non-magnetic ion doped by MnV₂O₄.Currently,there is no research about Mg doped MnV₂O₄ single crystal.Therefore,growing high quality Mg doped MnV₂O₄ single crystal and studies its properties is helpful in studying the interaction between spin and orbit in vanadium oxide system.In this study,MnV₂O₄ and four kinds of proportion of Mg doped MnV₂O₄ single crystals were grown by Ir floating zone method.We researched the magnetic properties of all single crystals,the effects of Mg doping on MnV₂O₄ crystal were analyzed in this paper.Vanadium ion of MnV₂O₄ is at + 3 state,it is easy to be oxidized by air,so we synthesized the rods in a vacuum sealed quartz tube.we found that V2O3 evaporation during the growth,so extra 1 percent V2O3 was added in the starting material.,the growth atmosphere influences the chemical composition of the crystal,because the MnV₂O₄ will break down and decompose into V2O3 and mno.the oxygen in the growing atmosphere will oxidize V2O3,making V2O3 more volatile.We have added vacuum pumps and oxygen levels to the growth system to clean and monitor oxygen levels.Bubbles overflow during the growth process,causing the floating area to be unstable.We adopted a high rotate speed to solve the problem,the bubble was expelled before the bubble was enlarged.moreover,the viscosity of MnV₂O₄ is very small and can easily lead to the excess of melt in the floating zone,we have solved the problem by using fast growth speed and different speed of the feed and seed.For the first time,we have produced a variety of mg doped MnV₂O₄.The powder x-ray diffraction test results showed that our samples were accurate and the Mg ions were incorporated into the system.the polarizing microscope and laue photos show that our single crystal structure is complete and the quality is perfect.We tested the magnetism of pure single crystal at 100 Oe,the results wereconsistent with the previous study.the results show that the magnetic field will inhibit the collinear ferrimagnetic order and show a magnetic phase transition in the high field.10% doped samples shows obvious anisotropy,through the analysis we think adding Mg ion change the direction of the magnetic moment of V ions in ferrimagnetic ordering,which changes the magnetism of two transformation point.we could find a metamagnetic transition in low magnetic field from the hysteresis loop,which is caused by the anti-ferromagnetic composition in the V ion.In high fields of magnetic susceptibility curve there is a "Steps" Type change,this is caused by the coexistence of multiphase,system is in a metastable state,increasing the rate of magnetization of the magnetic field will make a jump.Finally,we tested the properties of different doped proportion of samples in the magnetic field,the results showed that the temperature of both phase-transition points decreased as the doping ratio increased.It is shown that the addition of the Mg ion introduce the disorder which has inhibitory effect on the two phase transitions,and the inhibition of the structure phase transition is stronger than the magnetic phase transition.when the doped ratio achieves 20%,higher temperature phase transition disappear,only the structure transformation left over.The reason is that the doping ratio of Mg is too large which introduce strong spin frustration.The system could not keep the collinear ferromagnetic order,therefore,the second phase transition takes place directly.