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Phase diagram of CeRuPO under pressure investigated by $^{31}$P-NMR - Comparison between CeRuPO under pressure and the Ce(Ru$_{1-x}$Fe$_{x}$)PO system -

We have performed $^{31}$P-NMR measurements on single-crystalline CeRuPO under pressure in order to understand the variation in magnetic character against pressure. The NMR spectra for $H \perp c$ and $H \parallel c$ at 2.15GPa split below the ordered temperature, which is a microscopic evidence of the change in the magnetic ground state from the ferromagnetic (FM) state at ambient pressure to the antiferromagnetic (AFM) state under pressure. The analysis of NMR spectra suggests that the magnetic structure in AFM state is the stripe-type AFM state with the AFM moment $m_{\rm AFM} \perp c$-axis and changes by magnetic field perpendicular to $c$-axis. In addition, the dimensionality of magnetic correlations in the spin and the $k$ space is estimated. We reveal that three-dimensional magnetic correlations in CeRuPO are robust against pressure, which is quite different from the suppression of the magnetic correlations along the $c$-axis by Fe substitution in Ce(Ru$_{1-x}$Fe$_{x}$)PO.