Common effect of chemical and external pressures on the magnetic properties of $R$CoPO ($R$ = La, Pr, Nd, Sm). II

TL;DR

This study demonstrates that chemical and external pressures similarly influence the magnetic properties of $R$CoPO compounds, with structural changes primarily affecting ferromagnetism, as shown through muon-spin spectroscopy and ab-initio calculations.

Contribution

It reveals the equivalence of chemical and external pressures in tuning ferromagnetism in $R$CoPO and clarifies the role of rare-earth ions via structural effects rather than electronic degrees of freedom.

Findings

Linear correlation between $T_{C}$ and unit cell volume $V$.

Chemical and external pressures produce equivalent effects.

Rare-earth ions influence ferromagnetism mainly through structural shrinkage.

Abstract

The direct correspondence between Co band ferromagnetism and structural parameters is investigated in the pnictide oxides $R$CoPO for different rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and {\it ab-initio} calculations, complementing our results published previously [G. Prando {\it et al.}, {\it Phys. Rev. B} {\bf 87}, 064401 (2013)]. Both the transition temperature to the ferromagnetic phase $T_{_{\textrm{C}}}$ and the volume of the crystallographic unit cell $V$ are found to be conveniently tuned by the $R$ ionic radius and/or external pressure. A linear correlation between $T_{_{\textrm{C}}}$ and $V$ is reported and {\it ab-initio} calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, $R$ ions are shown to be influencing the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizeable magnetic contribution at much lower temperatures.