Long-time variation of magnetic structure in a geometrically frustrated magnet Ca3Co2O6

Magnetization and neutron scattering measurements have been made on a single crystal of geometrically frustrated compound Ca3Co2O6 which shows successive magnetic transitions at Tc1~25 K and Tc2~13 K and multi-step metamagnetic transitions in low magnetic field. In this compound without random magnetic interactions, a long-time variation of magnetic structure has been detected. The magnetic Bragg peaks corresponding to an incommensurate antiferromagnetic structure observed at (1 0 ± 6) reciprocal lattice positions after cooled below Tc2 move toward (1 0 0) with time. The characteristic time for these variations follows the Arrhenius law with an activation energy Ea/kB=57 K.