Anomalous evolution of the magnetocaloric effect in dilute triangular Ising antiferromagnets Tb$_{1–x}$Y$_x$(HC O$_2$)$_3$

We investigate the effects of diamagnetic doping in the solid-solution series Tb$_{1–x}$Y$_x$(HC O$_2$)$_3$, in which the parent Tb(HC O$_2$)$_3$ phase has previously been shown to host a combination of frustrated and quasi-1D physics, giving rise to a triangular Ising antiferromagnetic ground state that lacks long range 3D order. Heat capacity measurements show three key features: (i) a low temperature Schottky anomaly is observed, which is constant as a function of x; (ii) the transition temperature and associated entropy change are both surprisingly robust to diamagnetic doping; and (iii) an additional contribution at T < 0.4 K appears with increasing x. The origin of this unusual behaviour is rationalised in terms of the fragmentation of quasi-1D spin chains by the diamagnetic Y$^{3+}$ dopant. Magnetocaloric measurements show a nonlinear dependence on x. The mass-weighted magnetocaloric entropy decreases across the series from the promising values in (HC O$_2$)$_3$; however, the magnetocaloric entropy per magnetic Tb$^{3+}$ ion first decreases then increases with increasing x. Our results establish Tb$_{1–x}$Y$_x$(HC O$_2$)$_3$ as a model system in which to explore the functional ramifications of dilution in a low dimensional magnet.