Time and place: 2018.12.4, 9:30 am, Wuhan National High Magnetic Field Center B206

Presenter: Masayuki Hagiwara

Title: High Magnetic Field Sciences at AHMF in Osaka University

Abstract：

I will talk about two topics studied in high magnetic fields at the Center for Advanced High Magnetic Field Science (AHMF) after brief introduction of our high magnetic field facilities and experimental apparatus.

The first topic is “the spin multipolar excitations observed in åkermanite compounds Sr2CoB2O7 (B=Ge and Si)”. In these compounds, Co2+ ions have magnetic moments (S=3/2). These are magneto-electric multiferroic materials in which spontaneous electric polarization appears due to the d-p hybridization mechanism. The Sr2CoGe2O7, abbreviated as SCGO, has small magnetic anisotropy, while the Sr2CoSi2O7, abbreviated as SCSO, has large magnetic anisotropy. In SCGO, we observed a two-magnon spin excitation appearing above the saturation magnetic field (Hsat) in the electron spin resonance spectra. Owing to small anisotropy, this excitation has purely spin-quadrupolar nature, and hence its observation is possible by electric component of the light (electro-magnon) due to the spin nematic interaction in SCGO. On the other hand, we observed not only two-magnon but also three-magnon spin excitations above Hsat in SCSO. The three-magnon (spin-octupolar) excitation was observed due to large magnetic anisotropy which mixes magnetic dipole and quadrupole terms, resulting in making this excitation possible by magnetic component of the light. We discuss the similarity and difference in spin excitations between SCGO and SCSO.

The second topic is “the high-field magnetization of the S=1/2 honeycomb-lattice antiferromagnet Cu2(pymca)3(ClO4) where pymca stands for pyrimidine-2-carboxylate”. Recently, a honeycomb-lattice magnet with bond-dependent exchange interactions has been extensively studied as the Kitaev model [3], but in this talk, I will present the studies on a simple Heisenberg honeycomb-lattice antiferromagnet. The magnetic susceptibility shows a broad peak near 25 K, and no long range order is observed down to 0.6 K in the specific heat measurements. The magnetization curve up to 70 T at 1.4 K shows three step-like increases. Assuming three different exchange bonds based on the structure, the calculated magnetization curve reproduces the observed one except for the magnetization near 70 T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between the magnetization plateaus and exchange bonds is discussed based on the numerical calculations.