Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: distortion-induced lifting of geometrical frustration

摘要

The crystal structure and the magnetism of BaMn$_2$O$_3$ have been studied bythermodynamic and by diffraction techniques using large single crystals andpowders. BaMn$_2$O$_3$ is a realization of a $S = 5/2$ spin ladder as themagnetic interaction is dominant along 180$^\circ$ Mn-O-Mn bonds forming thelegs and the rungs of a ladder. The temperature dependence of the magneticsusceptibility exhibits well-defined maxima for all directions proving thelow-dimensional magnetic character in BaMn$_2$O$_3$. The susceptibility andpowder neutron diffraction data, however, show that BaMn$_2$O$_3$ exhibits atransition to antiferromagnetic order at 184 K, in spite of a full frustrationof the nearest-neighbor inter-ladder coupling in the orthorhombichigh-temperature phase. This frustration is lifted by a remarkably strongmonoclinic distortion which accompanies the magnetic transition.