0428同步年報-2021-全

076 ACTIVITY REPORT 2021 M agnetoresistance (MR) is such a technologically important phenomenon that the resistivity of a material changes with an applied magnetic field. Such a property can be observed in many conducting materials, but the changes of resistance are typically about a few percent and with rather limited sensitivity. Nowadays MR materials are widely used in our daily life, as reader heads of hard drives in computer or consumer electronics. More functional materials have been further explored for potential technological application; for instance, in 1990s the colossal magnetoresistance (CMR) was demonstrated in the manganite perovskites. The archetypal LaMnO 3 based system was intensively investigated, in which a colossal change in resistance against magnetic field can be realized on tuning the electronic configuration of the system. An optimal MR response was soon established with a specific doped region, but the phase diagrams such as La 1− x Ca x MnO 3 (LCMO) remain controversial. The CMR was believed to result from electronic phase segregation between ferromagnetic metallic and antiferromagnetic insulating states. 1 It is, however, a long-standing conundrum that such a precise doping value leads to a maximized MR effect; the phase segregation has prevented an atomistic-level understanding of the orbital ordered state at this doping level. Wei-Tin Chen (National Taiwan University), Mark Senn (University of Warwick, UK), Yu-Chun Chuang and Chin- Wei Wang (NSRRC) recently demonstrated a successful collaboration to provide structural insight for a mechanistic understanding of such intriguing physical phenomena. These properties are exhibited in strongly correlated electron systems that tend to have dense and strongly bound structures. The application of high-pressure high- temperature (HPHT) synthesis techniques is therefore particularly useful to explore novel functional materials as unstable structural distortions and metastable magneto- electric phases might be stabilized under extreme conditions. Chen’s research group is dedicated in HPHT techniques and utilizes AA’ 3 B 4 O 12 quadruple perovskites as a model framework to investigate target functional properties, such as CMR and multiferroicity (MF) ( Fig. 1 ). Detailed crystallographic information was extracted from high-resolution synchrotron X-ray powder diffraction (SXRD) data collected from the TPS 19A beamline at the NSRRC, and high-resolution neutron powder diffraction (NPD) data collected from beamline ECHIDNA at Australian Nuclear Science and Technology Organisation (ANSTO); furthermore, it is particularly beneficial to investigate a tiny sample obtained from a HPHT synthesis with high-intensity neutron powder diffraction instrument WOMBAT , at ANSTO. With assistance of thorough systematic symmetry element examination, Chen and Senn provided crucial structural evidence to understand the intrinsic properties. The archetypal CMR systems are the doped manganite Colossal Magnetoresistance and Improper Ferroelectric Polarization Explicated in Quadruple Perovskites From various designed high-pressure synthesized AMn 7 O 12 series, the specific electron stripes arrangement provides mechanistic understanding of how magnetoresistance might be enhanced, and how the novel charge ordered state reveals novel coupling of electronic and ferroelectric order parameters. Fig. 1 : (a) Schematic crystal structure of AMn 7 O 12 quadruple perovskites, showing B-site octahedra Mn (left) and A’-site square-planar Mn (right). (b) DIA-type cubic anvil high-pressure apparatus employed in the related studies. The pressure cell is placed at the center of six tungsten carbide anvils for HPHT synthesis. [Image courtesy of Wei-Tin Chen]