Characteristic magnetic structure due to antiferroquadrupolar ordering in (HoB2C2)-B-11

We determined the magnetic structures of the antiferroquadrupolar (AFQ) ord ering material with a tetragonal crystal structure, (HoB2C2)-B-11, by neutr on diffraction technique. The ground state of (HoB2C2)-B-11 below T-Q shows a characteristic magnetic structure due to competitive coexistence of AFQ and antiferromagnetic (AFM) interactions, which is basically the same as th at of the other tetragonal AFQ material DyB2C2. An angle of 70 degrees betw een adjacent magnetic moments along [0 0 1] indicates that the main compone nt of magnetic couplings along [0 0 1] in the ground state is ferromagnetic one, which is strongly affected by AFQ interactions. The existence of the intermediate magnetic ordered phase, phase IV, above T-Q indicates that the AFQ ordering in (HoB2C2)-B-11 at T-Q is a rare case which is realised in a magnetic ordered state. The magnetic structure in phase IV is a long perio d one caused by a short range magnetic ordering with a characteristic energ y of about 9K which is higher than T-N. The suppressed magnetic entropy at T-N is, therefore, attributed to the short range magnetic ordering.