Inhomogeneous magnetic ordered state and evolution of magnetic fluctuations in Sr(Co1-xNix)2P2 revealed by 31P NMR

TL;DR

This study uses $^{31}$P NMR to investigate how Ni substitution affects magnetic states and fluctuations in Sr(Co$_{1-x}$Ni$_x$)$_2$P$_2$, revealing the evolution from paramagnetic to ferromagnetic and antiferromagnetic orderings.

Contribution

It provides a microscopic understanding of magnetic fluctuation evolution and ordered states in Sr(Co$_{1-x}$Ni$_x$)$_2$P$_2$ through NMR analysis, highlighting the role of ferromagnetic fluctuations.

Findings

Ferromagnetic spin fluctuations dominate in ferromagnetic samples.

Distribution of Co moments in ordered states shows a rectangular NMR spectrum shape.

Temperature dependence of $^{31}$P $1/T_1T$ and $K$ explained by a double-peak density of states model.

Abstract

SrCo$_2$P$_2$ with a tetragonal structure is known to be a Stoner-enhanced Pauli paramagnetic metal being nearly ferromagnetic. Recently Schmidt et al. [Phys. Rev. B 108, 174415 (2023)] reported that a ferromagnetic ordered state is actually induced by a small Ni substitution for Co of $x$ = 0.02 in Sr(Co$_{1-x}$Ni$_x$)$_2$P$_2$ where antiferromagnetic ordered phase also appears by further Ni-substitution with $x = 0.06-0.35$. Here, using nuclear magnetic resonance (NMR) measurements on $^{31}$P nuclei, we have investigated how the magnetic properties change by the Ni substitution in Sr(Co$_{1-x}$Ni$_x$)$_2$P$_2$ from a microscopic point of view, especially focusing on the evolution of magnetic fluctuations with the Ni substitution and the characterization of the magnetically ordered states. The temperature dependences of $^{31}$P spin-lattice relaxation rate divided by temperature ($1/T_1T$) and Knight shift ($K$) for SrCo$_2$P$_2$ are reasonably explained by a model where a double-peak structure for the density of states near the Fermi energy is assumed. Based on a Korringa ratio analysis using the $T_1$ and $K$ data, ferromagnetic spin fluctuations are found to dominate in the ferromagnetic Sr(Co$_{1-x}$Ni$_x$)$_2$P$_2$ as well as the antiferromagnets where no clear antiferromagnetic fluctuations are observed. We also found the distribution of the ordered Co moments in the magnetically ordered states from the analysis of the $^{31}$P NMR spectra exhibiting a characteristic rectangular-like shape.