R. Sarkar, M. Baenitz, A. Jesche, C. Geibel, F. Steglich
A detailed $^{31}$P (I=1/2) and $^{75}$As (I=3/2) NMR study on
polycrystalline CeFeAs$_{1-x}$P$_{x}$O alloys is presented. The magnetism of
CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a
heavy fermion system without long range order whereas CeFeAsO exhibits a
Fe-3$d$ SDW type of ordering accompanied by a structural transition from
tetragonal ($TT$) to orthorhombic ($OT$) structure. Furthermore Ce-4$f^{1}$
orders antiferromagnetically (AFM) at low temperature. At the critical
concentration where the Fe-magnetism is diminished the Ce-Ce interaction
changes to a ferromagnetic (FM) type of ordering. Three representative samples
of the CeFeAs$_{1-x}$P$_{x}$O ($x$=0.05, 0.3 and 0.9) series are systematically
investigated. 1) For the $x$=0.05 alloy a drastic change of the line width at
130 K indicates the AFM-SDW type of ordering of Fe and the structural change
from $TT$ to $OT$ phase. The line width roughly measures the internal field in
the ordered state and the transition is most likely first order. The small and
nearly constant shift for $^{31}$P and $^{75}$As suggests the presence of
competing hyperfine interactions between the nuclear spins and the 4$f$ and
3$d$ ions of Ce and Fe. 2) For the $x$=0.3 alloy, evolution of the Fe-SDW type
order takes place at around 70 K corroborating the results of bulk measurement
and $\mu SR$. Here we found strong evidence for phase separation of
paramagnetic and magnetic SDW phases. 3) In contrast to heavy fermion CeFePO
for the $x$=0.9 alloy a phase transition is found at 2 K. The field-dependent
NMR shift gives strong evidence of FM ordering. Above the ordering the spin
lattice relaxation rate $^{31}$(1/T$_{1}$) shows unconventional, non-Korringa
like behaviour which indicates a complex interplay of Kondo and FM
fluctuations.
View original:
http://arxiv.org/abs/1106.0652
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