Structure,Magnetic And Dielectric Properties Of B-Cr/Co-doped And A-site Zr-doped YMnO₃

Multiferroic material is a kind of multifunctional material which integrates the physical properties of ferromagnetism,ferromagnetism and ferroelastic.It has great application prospects in spintronic devices,magnetic sensors,information storage and cryogenic magnetic refrigeration systems due to the coupling effect between ferroelectricity and ferromagnetism.However,the multiferroic materials are far from the practical applications because of the deficiencies of single-phase multiferroic materials at room temperature and the weak coupling between magnetic orders and ferroelectric orders.Therefore,the exploration of multiferroic materials has been a key subject in condensed matter physics and materials science.The hexagonal YMnO₃ has become one of the most popular multiferroic materials due to its novel physical properties.In this paper,we focus on hexagonal YMnO₃ to discuss the influence of Cr,Co and Zr doping on the crystal structure,dielectric and magnetic properties.The main research results are as follows:?1?Cr doping at B site of a series of polycrystalline Y_1-xCr_xMnO₃?x=0.00,0.03,0.05,0.07,0.10?compounds were synthesized in air by solid-state method.Powder X-ray diffraction gives evidence that all the samples are single phase and exhibit hexagonal structure with P6₃cm space group as observed for YMnO₃,but the grain size decreases with increasing doping amount.The origins of dielectric relaxations of Cr doped YMnO₃ were investigated by the influences of temperature and ac bias voltage on the dielectric property.The results show that the inverse S shape curve in the temperature range of 70-90 K region which is related to the magnetoelectric coupling.The obvious dielectric relaxations were observed in the temperature range of 160-280 K.This dielectric relaxation follows the Arrhenius law,which is the thermal activated process.It is found that the magnetization increases significantly with increasing doping content of Cr³⁺ions,and the antiferromagnetic ordered transition temperature T_N increases to 86 K in the sample of x=0.10.Cr³⁺ions doping weakens the long-range antiferromagnetic exchange interaction between the Mn³⁺ions in YMnO₃,and it also introduces the short-range ferromagnetic exchange intercction between Mn³⁺and Cr³⁺/Mn⁴⁺,simultaneously.The competition between the ferromagnetic exchange intercction and antiferromagnetic exchange interaction results in the decrease of magnetoresistance frustration effect of YMnO₃.The hysteresis loops of Y_1-xCr_xMnO₃ were measured at 5 K.The exchange bias field accompany horizontal magnetization shift was observsed and the exchange bias field increases with the increase of Cr³⁺ion doping amount.The trend of exchange bias effect with the doping concentration shows that antiferromagnet plays an important role on the pinning of ferromagnetic or uncompensated spins.Meanwhile,the YMn_0.9Cr_0.1O₃ ceramic is processed in oxygen and nitrogen atmosphere and the effect of annealing treatment of different atmospheres on the magnetic properties of YMn_0.9Cr_0.1O₃was studied.The results show that the increase of Mn⁴⁺ion leads to the increase of YMn_0.9Cr_0.1O₃ ferromagnetic composition in O₂ atmosphere annealing,and the significant increase of oxygen vacancy concentration is the main factor leading to the weakening of YMn_0.9Cr_0.1O₃ ferromagnetism in the annealing of N₂ atmosphere.?2?Cr doped YMnO₃ single crystal with a diameter of 4-5 mm and a length of 60-80 mm was produced by high pressured floating zone technique.The primary crystal defects sub-angle grain boundaries and inclusions in the crystal have been investigated by X ray diffraction and scanning electron microscopy.The formation mechanisms of these defects were discussed and the relevant approaches of restraining these defects have also been presented,which essential to optimize the growth parameters and improve crystal quality.?3?B site substitution of Y_1-xCo_xMnO₃?x=0.00,0.03,0.05,0.07,0.10?samples are prepared by standard solid state sintering method.The X-ray diffraction and Raman scattering spectra show that the samples are of hexagonal phase with group P6₃cm when x?0.07.While x>0.07,the hexagonal phase with group P6₃cm and the orthorhombic phase with group Pbnm coexist in those samples.It is shown that the ferroic magnetization and the peak of-?S_M-T curve increase with the increasing of Co doping and the maximum magnetic entropy change and the ferroic magnetization follow a non-linear relationship.The introduction of ferromagnetic exchange between Mn²⁺and Mn³⁺in addition to the ferromagnetic exchange between Mn³⁺and Mn⁴⁺cuases the increasing magnetization.The dielectric test results show that there is an intrinsic dielectric relaxation of the thermal activation process at the temperature range of350-650 K.?4?Zr doping at A-site of Y_1-xZr_xMnO₃?x=0.00,0.05,0.10?polycrystalline samples were prepared by solid-state method under air atmosphere.The Curie temperature,Curie constant and the effective magnetic moment were systematically analyzed.The results suggest that the ferromagnetic composition increases with the increase of doping amount and the samples enter the spin glass state below 15 K.This is related to the competition between ferromagnetic double exchange and antiferromagnetic super-exchange,and the antiferromagnetic tilt is also an important factor.