Condensed Matter Seminars

Layered cobalt oxides have attracted much attention due to their wide variety of magnetic, electrical and structural properties. The parent confound La ₂ CoO ₄ is an antiferromagnetic insulator with quite high T _N =275K. Upon Sr or Ca doping, a checkerboard Co ²⁺ -Co ³⁺ charge order is realized in La _1.5 Ca _0.5 CoO ₄ and Sr ₂ CoO ₄ shows a ferromagnetic metal with T _c =250K. The background of these various magnetic states is proposed the relationship of spin state transition from high spin state of Co ³⁺ (S=3/2) to intermediate spin state (S=1) [1]. However, the spin state of Co ³⁺ is now controversial because there is no direct evidence to observe the magnetic scatterings of Co ³⁺ by using neutron experiment. One of the big problems in this system is searching for magnetic scattering in an elastic condition. In this study, we measured global spin dynamics (inelastic condition) in La _1.5 Ca _0.5 CoO ₄ using the Fermi chopper spectrometer 4SEASONS in J-PARC [2]. Figure 1 shows the magnetic excitation of La _1.5 Ca _0.5 CoO ₄ below and above T _N (=50K). Spin-wave due to Co ²⁺ spins is seen up to 16meV below T _N . Another feature of the excitation spectrum is flat mode around 27meV. On the other hand, two magnetic modes seem to be merged into single mode above T _N . These results can not be explained by simple spin wave theory, therefore we need to consider another parameters such as Co ³⁺ spin or angular moment L. In this presentation, I will discuss the origin of this novel magnetic behavior based on the Co ³⁺ spins and orbital angular moment L. In order to discuss whether there is a significant orbital angular moment or not in this system, I will show you the recent research of soft x-ray scattering experiment. [1] Y. Moritomo, K. Migashi, K. Matsuda and A. Nakamura, Phys. Rev. B 55, 14725 (1997) [2] M. Nakamura, R. Kajimoto, Y. Inamura, F. Mizuno, M. Fujita, T. Yokoo and M. Arai, JPSJ 78, 093002 (2009)