We determined the magnetic structures of the antiferroquadrupolar (AFQ) ordering material with a tetragonal crystal structure, Ho11B2C2, by neutron diffraction technique. The ground state of Ho11B2C2 below TQ shows a characteristic magnetic structure due to competitive coexistence of AFQ and antiferromagnetic (AFM) interactions, which is basically the same as that of the other tetragonal AFQ material DyB2C2. An angle of 70° between adjacent magnetic moments along [0 0 1] indicates that the main component 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 TQ indicates that the AFQ ordering in HoB2C2 at TQ is a rare case which is realised in a magnetic ordered state. The magnetic structure in phase IV is a long period one caused by a short range magnetic ordering with a characteristic energy of about 9 K which is higher than TN. The suppressed magnetic entropy at TN is, therefore, attributed to the short range magnetic ordering.
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