Magnetic anisotropy of CeCoSi under high magnetic field

We have investigated the high magnetic field properties of the Ce-based Kondo lattice system CeCoSi by means of magnetization, magnetocaloric effect, contactless resistivity, specific heat, and magnetic torque measurements under pulsed high magnetic fields up to 55 T. CeCoSi is proposed to show an unconventional ordered phase called Phase II, which appears between paramagnetic Phase I and antiferromagnetic Phase III. The magnetization, magnetocaloric effect, and contactless resistivity measurements determine the field-angle dependence of Phase II-III boundary. Magnetic torque measurements detect a fourfold symmetry of the magnetic anisotropy under high magnetic fields in all phases, implying that there are two magnetic hard axes in the (100) or (110) plane of the tetragonal structure. Through comparison between the mean-field calculation and magnetization data, we determine the crystal electric field energy scheme, which agrees with the previous inelastic neutron scattering experiment. We successfully reproduce the fourfold magnetic anisotropy through our calculation, which is attributed to hybridization of the crystal electric field wave functions. The hybridization increases multipole degrees of freedom and is the origin of the unconventional order in Phase II. Moreover, the specific heat below 4 K exhibits a nonphononic ${T}³$ term in Phase II, which indicates that Phase II is not a simple polarized paramagnetic state and can be a quadrupolar ordered state.