# Magnetic order and spin dynamics across a ferromagnetic quantum critical point: $\mu$SR investigations of YbNi$_4$(P$_{1-x}$As$_x$)$_2$

## TL;DR

This study uses muon-spin relaxation to investigate how magnetic order and spin dynamics evolve across a ferromagnetic quantum critical point in a heavy-fermion system, revealing persistent slow fluctuations and reduced magnetic moments.

## Contribution

It provides detailed experimental insights into the magnetic order and spin dynamics across the ferromagnetic quantum critical point in YbNi$_4$(P$_{1-x}$As$_x$)$_2$, highlighting the homogeneous nature of magnetism and critical slowing down.

## Key findings

- Ordered Yb moments are strongly reduced with As substitution.
- Magnetism remains homogeneous without disorder effects.
- Critical fluctuations slow down approaching the QCP.

## Abstract

In the quasi-1D heavy-fermion system YbNi$_4$(P$_{1-x}$As$_x$)$_2$ the presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_c$ $\approx 0.1$ with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation ($\mu$SR) experiments on polycrystals of this series to study the magnetic order and the low energy 4$f$-electronic spin dynamics across the FM QCP. The zero field $\mu$SR measurements on pure YbNi$_4$(P$_{2}$ proved static long range magnetic order and suggested a strongly reduced ordered Yb moment of about 0.04$\mu_B$. With increasing As substitution the ordered moment is reduced by half at $x = 0.04$ and to less than 0.005 $\mu_B$ at $x=0.08$. The dynamic behavior in the $\mu$SR response show that magnetism remains homogeneous upon As substitution, without evidence for disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows 1/$T_{1}T\propto T^{-n}$ with $1.01 \pm 0.04 \leq n \leq 1.13 \pm 0.06$. The critical fluctuations are very slow and are even becoming slower when approaching the QCP.