Magnetic glassy behaviour coupled with long range non-collinear magnetic order in Tb3Co

Detailed and systematic study of rare earth rich intermetallic compound Tb3Co, using dc magnetisation, neutron powder diffraction, and linear and non-linear ac-susceptibilities, shows the presence of an unexpected magnetic glassy state along with complex non-collinear or modulated antiferromagnetic (AFM) order. Our neutron diffraction study shows that the magnetic structure remains more or less the same except for a decrease in moment values in the temperature range of 2 K-70 K and rules out any phase transition around 30 K. However, it reveals sharp changes in structural parameters around 30 K, which indicates strong spin-lattice coupling and change in strength. It appears to be mainly responsible for the observed increase in ZFC magnetisation on warming around 30 K. Another important unexpected result of this study is the strong frequency dispersion in linear and non-linear (higher order harmonics) ac-susceptibilities below T-f similar to 72 K. The analysis in terms of various spin glass theoretical formulisms and even stronger frequency dispersion in non-linear susceptibilities provides evidence for the presence of a spin glass like state in Tb3Co. The final picture that emerges out of this study is that a spin glass like state coexists with the long range modulated AFM order below 72 K.