2020同步年報

078 ACTIVITY REPORT 2020 A bulk crystal or multiple crystals of mass of order gram are necessary for a successful detection of extremely weak signals. No successful inelastic neutron scattering (INS) measurement on films has been reported, but most materials are difficult to grow into large high-quality crystals, and some of them can be grown as high-quality single-crystal films. Pushing the INS measurement to the limit of a film is now extremely crucial. Hsiung Chou of National Sun Yat-sen University and his collaborators have used SrRuO 3 single-crystal epitaxial thin films ( Fig. 1 ) to investigate the magnon dispersion curve in this ferromagnetic system to extend the limitation of INS measurements and to understand better the underlying mechanisms. This limitation imposes a strong constraint on the applicability of neutron inelastic-scattering instruments. Even though many materials can be grown as single-crystal films, their small mass makes this limitation a significant barrier. If the material has, however, a specific property that has a strong correlation with the low excitation phase along a specific direction and is measured with a highly sensitive and low-noise neutron inelastic-scattering instrument, this limitation might be overcome. Taiwan has built a state-of- the-art neutron inelastic-scattering instrument, SIKA , in Australian Nuclear Science and Technology Organisation (ANSTO), which has the potential to undertake neutron inelastic-scattering measurement on thin films. Chou’s group has publised an example 1 on choosing SrRuO 3 (SRO), which is one of few itinerant ferromagnetic materials from the 4 d -transition-metal group widely used as a conducting electrode in multilayer device appli- cations, magnetic tunnel junctions, electronic transport tuning, dynamic random-access memory applications, switchable acoustic-wave resonators and spintronic devices, among others. Unexplained phenomena such as the non-saturated magnetization at a large magnetic field, changing of the sign Fig. 1 : Four 1 × 1 cm 2 SrRuO 3 films, stacked with the film’s surface in contact with each other and placed side by side on an aluminium sample holder. [Courtesy of Chun-Ming Wu] of Hall coefficients, an unbelievable high upper-limit-temperature (T ≤ 30 K) for Fermi liquid conduction and non-Fermi-liquid behavior within 90 ≤ T/K ≤ 150, are, however, waiting to be explored. Chou’s team has hence a great motivation to challenge the unbreakable limitation for inelastic neutron scattering on a thin film as well as to understand the exotic mag- non dispersion profile of SrRuO 3 films. To confirm that their observed signal is really contributed by a magnon and not by phonons or instrumental er- rors, it was necessary to compare the result at high temperature at which the material is in a paramagnetic state. First Inelastic Neutron Scattering from a SrRuO 3 Thin Film SIKA makes it possible to investigate the magnetic property of single-crystal epitaxial thin film with inelastic neutron scattering Fig. 2 : Direction [002] shows the largest count difference, which indicates that the magnon disper- sion curve that extends along this direction should the most readily detectable. INS spectra of L = 1.90 at 5 K and 200 K. [Reproduced from Ref. 1]