Abstract:
Inelastic neutron scattering (INS) is a powerful and unique tool for probing low energy excitations, such as phonon and magnon, which is the crucial parameters for new energy and spintronic materials. However, because of the inelastic scattering exhibits an extremely low scattering crosssection, a huge amount of sample, several grams, is the fundamental requirement to provide a clear signal. Take the ferromagnetic SrRuO3 as an example; its magnon excitations were detected by using 73 g or 6 g of powder or single crystal, repectively. Uptodate, electronic or energy generation elements are made of thin films that has properties may quite different to their bulks. To be precise measuring the basic property, to explore the INS into thin film format is significant for material research. How to achieve a clear detection of the INS signals for the ultra small amount of material for a thin film is a great challenge for the neutron technique development.
SrRuO3 in powder [1] and single crystal [2] forms were found to exhibit a magnon gap due to the Weyl fermion node nearby its Fermi surface. To understand the defect effect on the magnon gap and Weyl fermion, we follow the fact that the massive Ru defects and some oxygen vacancies can be easily created when SrRuO3 is grown in a thin film format. In this study, two single crystalline ferromagnetic SrRuO3 films were prepared by the PLD method and examined and their magnon spectra at low temperature were taken at SIKA, ANSTO. We found that both films show clear magnon dispersion curves following quadratic relation along [002] direction. The film with fewer defects was found to have a dispersion curve with a small magnon gap of around 0.32meV [3], as shown in Fig. 1, which is much smaller than Itoh’s and Jenni’s observations. We proposed a model, the impurity level near Fermi level, due to Ru and O vacancies, could weaken the spin-orbit coupling and the anticrossings, and eventually destroy the Weyl Fermion node. In addition, the electrons captured by defects and vacancies induce on-site Coulomb interactions that open a small magnon gap. Meanwhile, the magnon gap for the defect-rich film shows a zero magnon gap, Fig. 2, strongly indicating the existence of Weyl fermion node is very sensitive to the presence of defects.
Keywords – Inelastic Neutron Scattering, ferromagnetism, magnon