Elastic Instabilities within Antiferromagnetically Ordered Phase in the   Orbitally-Frustrated Spinel GeCo$_2$O$_4$
  Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase. 
Tadataka Watanabe, Shigeo Hara, Shin-Ichi Ikeda, and Keisuke Tomiyasu
cond-mat.str-el cond-mat.mtrl-sci