Pressure driven magnetic order in Sr$_{1-x}$Ca$_x$Co$_2$P$_2$

The magnetic phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ($\mu^+$SR). The weak pressure dependence for the $x\neq1$ compounds suggests that the rich phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of $x$ at ambient pressure may not only be attributed to solely chemical pressure effects. The $x=1$ compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at $p_{\rm c2}\approx9.8$~kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of dilute ferromagnetic islands (FMI) is formed above $p_{\rm c1}\approx8$~kbar where they co-exist with a magnetically disordered state. Moreover, such FMI phase seems to consist of an high- (FMI-\textcircled{\small{1}}) and low-temperature (FMI-\textcircled{\small{2}}) region, respectively, separated by a phase boundary at $T_{\rm i}\approx20$~K.