The Institute of Physics has made progress in the study of interface ferromagnetism induced by symmetry mismatch. The results are published in Advanced Functional Materials

As an important family in complex oxide systems, 4d ruthenate (ARuO3) exhibits a series of colorful physical properties, such as traveling ferromagnetism, magnetic Weyl fermions, magnetic monopole, unconventional superconductivity, non-fermi liquids, etc. As the only perovskite oxide with natural ferromagnetism and strong spin orbit coupling (SOC), SrRuO3 has become a star material in the study of this system. The ferromagnetic Curie temperature of SrRuO3 up to 160K and good metal conductivity make it have great potential in the research of spinner devices, and the novel physical properties such as the huge anomalous Hall effect, topological Hall effect and even quantum anomalous Hall effect caused by the coexistence of ferromagnetism and strong SOC have also attracted much attention. However, the cruise ferromagnetism of SrRuO3 appears to be a special case among various 4d and 5d transition metal oxides, which brings limitations to the design of novel spin/orbit devices based on it. Although 4d and 5d oxides have strong SOC, it is usually difficult to form long-range magnetic sequence due to the decrease of electron correlation caused by the expansion of d orbital band. To design more new material systems that integrate strong SOC and time-inversion symmetry breaking (i.e., ferromagnetism) by hand is a highly concerned problem in spintronics research.

The bulk material of CaRuO3 has the same GdFeO3 type orthogonal crystal structure and electronic configuration as SrRuO3. However, due to the small radius of Ca ion, the Ru-O-Ru bond Angle of CaRuO3 is only 148°, much lower than the 163° of SrRuO3. Therefore, CaRuO3 material or thin film material exhibits paramagnetic gold property in the whole temperature region. In recent years, the research team of the Institute of Physics of the Chinese Academy of Sciences has been committed to the design and regulation of the physical properties of oxide heterogeneous interfaces, hoping to use the effects of the heterogeneous interface crystal field, stress field, charge recombination, orbital reconstruction, etc., to induce new interfacial physical states that do not exist in the bulk phase materials. Recently, the team has successfully induced a long-range ferromagnetic sequence in the CaRuO3 system by using the structural neighbor effect. By pulsed laser deposition, two kinds of symmetric mismatching films, diamagnetic SrTiO3 (a0a0a0) and paramagnetic CaRuO3 (a-a-c+), were grown alternately on the substrate, and high quality epitaxial superlattice samples were obtained. The tilt/rotation of RuO6 octahedron in CaRuO3 layer is inhibited by coupling distortion of interfacial oxygen octahedron. The results of scanning transmission electron microscopy (TEM) show that the torsion of RuO6 octahedron in CaRuO3 layer with the thickness of 3 cells at the interface is greatly regulated, and the Angle of Ruo6 octahedron increases from ~ 150° to ~ 165°, which is similar to the Angle of Ru-O-Ru bond in SrRuO3 thin film. The regulation of interface structure coupling will inevitably bring about the change of electronic structure. The first-principles calculation shows that the inhibition of tilt/rotation of RuO6 octahedron will greatly increase the density of states at the Fermi surface of CaRuO3 [N (EF)], and finally make the three cell layers of the interface CaRuO3 layer meet the Stoner criterion of cruise ferromagnetism [IN (EF) > 1, where I is the Stoner coefficient]. From the paramagnetic state of the block to the ferromagnetic ordered state. Hall transport measurements and macroscopic magnetic measurements give sufficient evidence for the presence of interboundary ferromagnetic phase in the system. The maximum Curie temperature is about 120K and the maximum saturation magnetization is ~ 0.7 μB/f.u. . Anisotropic magnetoresistance measurements further surface CaRuO3 interface of the magnetic phase of the magnetic axis in plane < 110> The direction. In this work, we report an example of interfacial ferromagnetism generated solely based on interfacial oxygen octahedral coupling distortion design. In particular, the two oxides that constitute the heterogeneous interface do not have long-range magnetic order respectively. Some of the principles of this work will also be applicable to other oxide systems with similar symmetric mismatching, providing a new idea for exploring multifunctional oxide materials and devices.

Relevant results were published in Advanced Functional Materials under the title Symmetry-mismatch-induced ferromagnetism in the interfacial layers of CaRuO3 / SrTiO3 superlattic. (Advanced Functional Materials). The relevant research work is supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of Chinese Academy of Sciences and the key project of Chinese Academy of Sciences.

Source: Institute of Microelectronics, Chinese Academy of Sciences