轨道序和巡游性对尖晶石结构钒氧化物AV2O4（A=Mn,Fe,Co）物性影响的研究

The orbital-ordering and itinerancy effects on the physical properties in AV2O4（A = Mn,Fe,Co）

在尖晶石钒基氧化物AV2O4中,因为自旋阻挫、轨道序、巡游性等独特的内禀属性间存在着相互合作和竞争,所以该体系常常表现出复杂而有趣的物理现象。通过对A位磁性离子的调控,我们详细研究该体系中不同物性的起源,比如磁相变与结构相变的不同起因,局域和巡游电子的交叉行为等等。我们以Mn1-xCoxV2O4和Fe1-xCoxV2O4体系为研究对象,通过变温X射线衍射、磁化率、比热和中子散射等测试手段,结合第一性原理计算,对其物性起源进行了详细的研究。我们发现:(i)在低Co^2+离子掺杂时,体系受到局域V^3+离子以及A位Fe^2+离子的轨道序作用,往往会在磁有序附近伴随着结构相变的出现,以至于弱化体系中钒离子独立形成的四面体造成的几何阻挫。这也说明Co^2+离子低掺杂下,该体系有着强的自旋-晶格耦合;(ii)在高Co^2+离子掺杂时,由于巡游性的增强,轨道序的弱化,JAB交换相互作用增强,体系也表现出明显的磁各向同性。因此磁相变温度向更高的温度移动,而结构相变温度向低温移动甚至消失。

Due to the complicated interplays of spin-frustration,orbital-ordering,and electron-itinerancy,the spinel vanadate,AV2O4,exhibits the interesting physical phenomena and attracts a lot of attention in both scientific and industrial areas.With the method of adjusting the A-site ions,we studied the mechanisms of those specific physical properties,such as the magnetic-structural phase transition,the effects of the local and itinerant electrons.Applying the techniques of magnetic susceptibility,specific heat,variable temperature X-ray diffraction and neutron scattering,and first-principles calculations,we studied the systems of Mn1-xCoxV2O4 and Fe1-xCoxV2O4,and found that:(i)For the low Co-doping compounds,the system was subject to local V^3+and A-site Fe^2+.The geometrical frustration from the V-tetrahedra was weak,and the orbital sequence action tended to accompany the structural phase transitions with the magnetic ordering.Hence,the spin-lattice coupling was strong.(ii)For the high Co-doping compounds,the strong electronic itinerance decreased the orbital effects of the V-ion,and the exchange energy,JAB,enhanced,which induced the system isotropic.Therefore,the magnetic phase transition temperature increased,while the structural phase transition temperature shifted to a lower temperature or even finally disappears.