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

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 $1.01\pm 0.04\le n\le 1.13\pm 0.06$. The critical fluctuations are very slow and are even becoming slower when approaching the QCP.

6 pages, 4 figures

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