Soft x-ray Spectroscopic Investigation of Strongly Correlated Magnetic Materials

Abstract

We investigated strongly correlated magnetic materials by using the soft x-ray spectroscopy tools. X-ray absorption spectroscopy enables us to investigate the nature of the orbital anisotropy, the magnetic moments and the magnetic easy axes. In addition, angle resolved photoemission and soft x-ray resonant magnetic diffraction were also conducted to investigate the electronic structure of conduction electron and the magnetic ordering respectively. To perform the comprehensive investigation of exotic magnetic materials, we found the microscopic origin and information about the materials - LuFe2O4, Fe1/4TaS2, BiFeO3 thin film and La1.4Sr1.6Mn2O7. Then we shows the capabilities of soft x-ray spectroscopy to investigate strongly correlatedmagnetic materials.LuFe2O4 is a pure electronic multiferroic where the polar charge ordering and the ferrimagnetic ordering coexist. The magnetic property shows the gigantic magnetocrystalline anisotropy (H_A > 70 T) and the large coersivity (Hc ∼ 10 T). By using x-ray magnetic circular/linear dichroism, we found that the unquenched large orbital moment (m_o ∼ 0.8μB), which is the origin of magneticrystalline anisotropy combining with the orbital anisotropy. In addition, we suggested the probable ferrimagnetic ordering by considering the antiferroelectric polar charge ordering.Fe1=4TaS2 is an intercalated dichalcogenide compound, in which the Fe ions are ordered as 2 × 2 super structure and separated spatially. The material shows the highest ferromagnetic transition temperature (TC = 160 K) and the huge magnetocrystalline anisotropy (H_A > 59 T). X-ray absorption spectroscopy revealed that the elongated trigonal structure stabilizes the e_g^pi states, then the orbital anisotropy derives the large orbital magnetic moment (m_o ∼ 1.0μB). The covalency of chalcogenides were investigated by using ab initio calculation which enabled us to explain the saturated magnetic moment (Msat = 4μB) including the orbital magnetic moment. In addition, the Fermi surface enabled us to found that the inter-site RKKY interaction is maximized at 2a distance which is the origin of the highest transition temperature.The BiFeO3 thin film is a hopeful system to apply on the future magnetoelectric devices. In this system, the antiferromagnetic axes are on the AFM plane which is perpendicular to the electric polarization. We performed x-ray magnetic linear dichroism on BiFeO3/SrRuO3/SrTiO3(001) thin films, and found that the AFM axes are not degenerated and the spin axis prefers to lie on the plane of thin film with the compressive strain. The finding was confirmed with BiFeO3/SrRuO3/GdScO3(110) where the strain is relaxed and the spin does not show the preferred spin axis. Meanwhile, the antiferromagnetism of the super-tetragonal thin film was investigated, which had been introduced as the piezoelectric response at the morphotropic phase boundary.We found that the large tetragonal distortion changes the electronic structure of Fe ion and suppresses Neel temperature.La1.4Sr1.6Mn2O7 was investigated by using soft x-ray magnetic diffraction and linear dichroism. In this material, the A-type antiferromagnetic order and the ferromagnetic order are coexisting and competing. By using soft x-ray magnetic diffraction, we found that the A-type antiferromagnetic axis are flipped from c to b by applying the in-plane weak magnetic field. In addition we found that the electronic structure between the AFM and FM phase is different and the external magnetic field can modify the electronic structure by changing the inter bilayer magnetic correlation.In summary, we have investigated the microscopic feature of the strongly correlated magnetic materials by using the soft x-ray spectroscopy tools. We found that the orbital and spin degree of freedom are essential to describe the exotic properties of these materials. The spectroscopic approaches provided the key information to complete the comprehensive description, and we anticipating that the spectroscopic approach will be encouraged to unveil the microscopic origins of the strongly correlated exotic phenomena.