Ca~(2+)掺杂对SmFeO_3的介电、铁磁特性及磁相变温度的影响

采用固相反应法制备Sm_(1-x)Ca_xFeO_3(x=0,0.1,0.2,0.3)样品,研究Ca~(2+)掺杂对SmFeO_3介电性能、铁磁性及磁相变温度的影响.X射线衍射图谱分析表明:所有样品的主衍射峰与SmFe03相符合且具有良好的晶体结构.随着x的增加,SmFeO_3样品的晶粒尺寸由原来的0.5μm逐渐增大到2μm.当f=1 kHz时,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的ε_r分别是SmFe03的5倍、3倍和2.6倍,而tgσ增大一个数量级.在3T磁场作用下,SmFe03样品的M-H呈线性,随着x的增加,M-H逐渐趋向饱和,Sm_(1-x)Ca_xFeO_3(x=0.1,0.2,0.3)样品的M_r分别是SmFeO_3的20倍、31倍和68倍.X射线光电子能谱分析表明:Fe~(2+)和Fe3+共存于Sm_(1-x)Ca_xFeO_3样品中,Fe~(2+)/Fe~(3+)比例随着x的增加而增大,证明Ca~(2+)掺杂增加了Fe~(2+)的含量,形成Fe~(2+)—O~(2-)—Fe~(3+)超交换作用,增强SmFe03的铁磁特性.测量了Sm_(1-x)Ca_xFeO_3样品在外加磁场为1000 Oe(1 Oe=79.5775 A/m)的M-T变化关系,观测到其自旋重组温度(T_(SR))和尼尔温度(T_N)分别为438 K和687 K,发现SmFe03样品的T_(SR)和T_N均随着x的增加向低温方向移动,当x=0.3时,自旋重组现象消失.这主要是SmFeO_3样品磁结构的稳定性和Fe~(3+)—O~(2-)—Fe~(3+)及Sm~(3+)—O~(2-)—Fe~(3+)超交换三者共同作用的结果.

Effects of Ca2+ doping on dielectric,ferromagnetic properties and magnetic phase transition of SmFeO3 ceramics

In this paper we deal with the preparation of Sm_1-xCa_xFeO₃(x=0-0.3) ceramics by the solid stat reaction and study the influences of Ca²⁺ doping on the dielectric,ferromagnetic properties and magnetic phase transition of SmFeO₃.The crystalline structures of the Sm_1-xCa_xFeO₃(x=0-0.3) samples are characterized by X-ray diffraction.The dielectric property is measured by a precisive impedance analyzer (HP4294A) in a frequency range from 40 to 110 MHz.The microstructures of Sm_1-xCa_xFeO₃ are imaged with scanning electron microscope under an operating voltage of 20 kV.The coexistence of Fe^3+/2+ ions in Sm_1-xCa_xFeO₃ samples is investigated with X-ray photoelectron spectroscopy (XPS).The magnetic properties of Sm_1-xCa_xFeO₃ are measured with the physical property measurement system.The result shows that all the peaks for Sm_1-xCa_xFeO₃ samples can be indexed according to the crystal structure of pure SmFeO₃ and their fine crystal structures are obtained by XRD.The lattice parameter a value of SmFeO₃ gradually increases,while the values of b and c decrease,and the unit cell volume (V) shrinks slightly with the increase of x.The scan electron microscope images indicate that Ca²⁺ doping significantly increases the grain size of SmFeO₃ ceramic.The average grain sizes of Sm_1-xCa_xFeO₃ samples range from 0.5 to 2μm with Ca²⁺ doping.The ε_r values of Sm_1-xCa_xFeO₃(x=0.1,0.2,0.3) measured at 1 kHz are about 5,3 and 2.6 times greater than that of SmFeO₃,respectively,and dielectric loss increases by an order of magnitude.The increase of ε_r is mainly caused by the interaction between the dipole and the space charge orientation polarization.Both the conductance current and the space charge limiting current are the main factors to increase the dielectric loss.The magnetic measurements show that the M-H curves of Sm_1-xCa_xFeO₃(x=0-0.3) samples exhibit saturated magnetic hysteresis loops with the increase of Ca²⁺,and the M_r values of Sm_1-xCa_xFeO₃(x=0.1,0.2,0.3) are 20,31,and 68 times that of SmFeO₃,respectively,indicating the weakly ferromagnetic behavior.The XPS spectrum indicates that the Fe²⁺ and Fe³⁺ co-exist in each of Sm_1-xCa_xFeO₃ samples.The ratio of Fe²⁺/Fe³⁺ increases with doping Ca²⁺ increasing,and the magnetic preparation of SmFeO₃ is enhanced.It can be attributed to the structural distortion and the formation of Fe²⁺–O^2-–Fe³⁺ super-exchange.The spin recombination temperature (T_SR) and the Neel temperature (T_N) are obtained,respectively,to be 438 K and 687 K by measuring the M-T curves.It is noted that both T_SR and T_N of SmFeO₃ samples move toward low temperature with the increase of x,and the spin recombination disappears when x=0.3.This is mainly due to the stability of the magnetic structure of SmFeO₃ sample and the interactions of Fe³⁺–O^2-–Fe³⁺ and Sm³⁺–O^2-–Fe³⁺ super-exchange.