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 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.

The magnetic properties of rare earth rich intermetallic compound, Tb3Co, were studied under external pressures up to  ∼1.21 GPa. The application of external pressure results in a decrease of the transition temperatures, (paramagnetic to modulated antiferromagnetic) by about 6 K, and the order to order transition that coincides with a glass transition at 72 K by about 15 K, respectively. The low temperature drop in the zero-field cooled magnetisation signifying the strengthening of spin–orbit coupling remains more or less unaffected (shifts only by 3 K) by the external pressures but significant changes in the magnetic behaviour were observed above 40 K. The overall long range non-collinear magnetic order coexisting with glassy behaviour (below 72 K) is sustained even at  ∼1.07 GPa pressure. The rate of decay of (d) is found to be linear with  −7.2 K GPa−1 up to  ∼0.9 GPa and then it deviates from linearity. The magnetic relaxations and memory effects studied under different measurement protocols confirm the presence of glassiness right up to the highest pressure, although the glassy behaviour is weakened to some extent with increasing pressure as reflected by faster relaxations.

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The magnetocaloric effect of intermetallic compounds of Tb3Co and Ho3Co is studied under high pressures up to ∼1 GPa using pressure dependent dc magnetisation and specific heat measurements at ambient conditions. The magnetic entropy change (−ΔSM) obtained from magnetisation data and adiabatic change in temperature (ΔTad) determined from zero-field specific heat and magnetisation data are found to be nearly identical within error limits with those deduced from purely field dependent specific heat experiments. With increasing hydrostatic pressure to ∼1 GPa, the −ΔSM and ΔTad, both show a significant enhancement of about 37% and 13%, respectively for 9 T field change in case of Tb3Co. On the other hand, Ho3Co exhibits a decrease of about 8% in both −ΔSM and ΔTad with increasing pressure. The refrigerant capacity (RC) also increases from 650 J kg−1 to 847 J kg−1 in the case of Tb3Co and it goes down from 665 J kg−1 to 615 J kg−1 for Ho3Co for an increase of pressure to 1 GPa. With increasing pressure, the peak widths of both −ΔSM and ΔTad increase in case of Tb3Co, although the increase is more in −ΔSM. However, such noticeable changes in peak widths with pressure were not observed in Ho3Co. At ambient pressure, peak of −ΔSM () scales with for both the compounds, consistent with the prediction of mean field theory (MFT) for second order magnetic transition. However, deviation from MFT was noticed at high pressures as was found to scale with instead of for both the alloys. Further, normalised −ΔSM curves for different ΔH and pressures collapse on a single universal curve in both the compounds thereby indicating that the second order magnetic transition persists even up to ∼1 GPa pressure.

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