Title: The effect of different in-chain impurities on the magnetic properties of the spin chain compound SrCuO$_2$ probed by NMR,
Abstract: The S=1/2 Heisenberg spin chain compound SrCuO2 doped with different amounts
of nickel (Ni), palladium (Pd), zinc (Zn) and cobalt (Co) has been studied by
means of Cu nuclear magnetic resonance (NMR). Replacing only a few of the S=1/2
Cu ions with Ni, Pd, Zn or Co has a major impact on the magnetic properties of
the spin chain system. In the case of Ni, Pd and Zn an unusual line broadening
in the low temperature NMR spectra reveals the existence of an impurity-induced
local alternating magnetization (LAM), while exponentially decaying
spin-lattice relaxation rates $T_1^{-1}$ towards low temperatures indicate the
opening of spin gaps. A distribution of gap magnitudes is proven by a stretched
spin-lattice relaxation and a variation of $T_1^{-1}$ within the broad
resonance lines. These observations depend strongly on the impurity
concentration and therefore can be understood using the model of finite
segments of the spin 1/2 antiferromagnetic Heisenberg chain, i.e. pure chain
segmentation due to S = 0 impurities. This is surprising for Ni as it was
previously assumed to be a magnetic impurity with S = 1 which is screened by
the neighboring copper spins. In order to confirm the S = 0 state of the Ni, we
performed x-ray absorption spectroscopy (XAS) and compared the measurements to
simulated XAS spectra based on multiplet ligand-field theory. Furthermore, Zn
doping leads to much smaller effects on both the NMR spectra and the
spin-lattice relaxation rates, indicating that Zn avoids occupying Cu sites.
For magnetic Co impurities, $T_1^{-1}$ does not obey the gap like decrease, and
the low-temperature spectra get very broad. This could be related to the
increase of the Neel temperature which was observed by recent muSR and
susceptibility measurements, and is most likely an effect of the impurity spin
$S\neq0$.