  Complex magneto structural behaviour of rare rich intermetallic Tb$_3$Co is
reported in this study. Below the 84K (T$_N$), it undergoes a first order
magnetic transition 72K confirmed from specific heat and magnetization
measurements. Detailed study using magnetization, specific heat, neuron
diffraction and ac-susceptibility measurements suggests that the compound in
the question possesses canted magnetic structure. Neutron diffraction reveals
that Tb moments are aligned antiferromagnetically (AFM) in ab plane and
ferromagnetically (FM) along c direction. This study focusses on the
temperature evolution of magnetic order at low temperatures. Neutron
Diffraction clearly shows that magnetic structure remains more or less same up
to 70K although low field magnetization exhibits a transition like feature
around 30K. Field dependent neutron diffraction suggests that a strong
spin-lattice coupling is present at lower temperature region (below 20K)
compared to that for T above 40K. This is responsible for the observed drop in
ZFC magnetization around 30K and is not due to any change in magnetic
structure. Another important but surprising result that observed is the
signature of magnetic glass below 72K in frequency dependent ac susceptibility
measurements. Real part of ac-susceptibility data shows dispersion with
frequency which on detailed analysis provides evidence for spin glass behaviour
in the compound riding on top of non-collinear AFM order. Higher order
harmonics in ac non-linear) susceptibilities, which are expected to show a
well-defined behaviour for pure FM or AFM or spin glass or cluster glass
systems, were also measured. However, the non-linear susceptibilities do not
exhibit any well-known variation but show complex behaviour thus indicating
Tb$_3$Co is neither a pure AFM or FM or spin glass system but provides another
evidence for non-collinear magnetic structure.
