New Type Diluted Magnetic Semicondctors Studied With Advanced Spectroscopies

Diluted magnetic semiconductors?DMS?materials combining the tunability of charge and spin degrees of freedom offer a rich variety of remarkable properties which attracts extensive interest in materials science.In the Mn-doped III-V semiconductors,substitution of trivalent cation Ga for Mn simultaneously introduces an acceptor and a source of magnetic moments.This dual role of Mn complicates theoretical understanding.The recently discovered Mn-doped I-II-V semiconductor Li_1+x?Zn1-y Mny?As and Mn-doped II-II-V semiconductor(Ba_1-xK_x)?Zn1-y Mny?₂As₂,belonging to a new generation of bulk ferromagnets,in which holes are induced independent of spin doping.Much progress has been made in the control of the exchange interactions,however,a microscopic understanding of the physical properties of DMS is still limited.By virtue of high energy resolution,element-and orbital-resolved synchrotron spectroscopy?emission,absorption and dichroism?is suitable for detecting the specific electronic states,while X-ray Diffraction?XRD?has the advantage of distinguishing subtle lattice changes.Here,(Ba_1-xK_x)?Zn1-y Mny?₂As₂ materials are chosen as the target samples,and the mechanism of indirect exchange interactions leading to high ferromagnetic ordering temperature Tc by hole doping was studied.Besides the x-ray spectroscopy techniques,pressure is also employed to draw out the complicated interplay between structural,magnetic and electronic degrees of freedom in these novel DMSs.The As K-edge x-ray magnetic circular dichroism?XMCD?signal scales with the sample magnetization?dominated by Mn?,and scales with Tc,hence representative of the bulk magnetization.The X-ray Emission Spectroscopy?XES?,X-ray Absorption Spectroscopy?XAS?and XMCD measurements,taken together,indicate that hole doping increases the p-d hybridization strength?electron redistribution within MnAs4 ligands?at the cost of reduction in Mn local spin density,resulting in enhanced exchange interactions between Mn dopants.Upon compression,in both hole optimally doped and under-doped compounds,the bandgap is reduced and hole carriers mediating the long-rang magnetic order become delocalized,suppressing the magnetic order.Both effects are caused by the strong response of As p states?band broadening?at applied pressures.We revealed that changes in the geometry of MnAs4 tetrahedra and interlayer As-As distance with applied pressure are intimately connected to the evolution of electronic structure in anion p and cation d states.These results provide valuable information for us to understand the relationship between structure and magnetism.The structure evolution of Li_1+x(Zn_1-yMn_y)As is studied by synchrotron XRD measurements under high pressure.A pressure-induced phase transition is observed around 11.6 GPa and the new phase with space group of Pmca is proposed.The structural phase transition leads to a contraction of the unit cell volume of about 8% after the structural phase transition.These results on the low-pressure phase would be valuable and instructive for the future study of the ferromagnetism in Li?Zn,Mn?As.Employing the Synchrotron Mossbauer Spectroscopy?SMS?,the magnetic transition of Fe has been studied in the 1111-type pnictide superconductor SmFeAsO under high pressure and low temperature conditions.Pressure decreases the antiferromagnetic phase transition temperature and the antiferromagnetic state of Fe is lost at about 25 GPa.Combined with the transport results which provide the superconductivity temperature,P-T phase diagram of SmFeAsO is obtained.These results could supplement important information to the understanding of the relationship between superconductivity and antiferromagnetism.