Effect of different in-chain impurities on the magnetic properties of the spin chain compound ${\mathrm{SrCuO}}_2$ probed by NMR

The $S=1/2$ Heisenberg spin chain compound ${\mathrm{SrCuO}}_2$ 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 strongly decaying spin-lattice relaxation rates $T_1^{-1}$ towards low temperatures indicate the opening of spin gaps. A distribution of gap magnitudes is implied 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 gaplike decrease, and the low-temperature spectra get very broad. This could be related to an increase of the Néel temperature and is most likely an effect of the impurity spin $S\ne 0$.

Figure 1

Complete Cu NMR spectra of ${\mathrm{SrCu}}_{0.99}{\mathrm{Ni}}_{0.01}{\mathrm{O}}_2$ at 300 K and 50 K as examples. They have been obtained with a fixed frequency of 80 MHz by varying the magnetic field $H$ while keeping it parallel to the crystallographic $b$ axis (perpendicular to the chains). The normal set of Cu NMR lines and the additional set due to a local lattice distortion around the nickel impurities are marked.

Figure 2

${}^{63}\mathrm{Cu}$ high-field satellite of ${\mathrm{SrCuO}}_2, {\mathrm{SrCu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{O}}_2, {\mathrm{SrCu}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_2$, and ${\mathrm{SrCu}}_{0.99}{\mathrm{Co}}_{0.01}{\mathrm{O}}_2$ measured with a fixed frequency of $80\text{MHz}$ by varying the magnetic field $H\left|\right|b$. The black dashed lines (more precise: their intersections with the base lines of the spectra) indicate the $1/\sqrt{(}T)$ behavior, which is expected for the shoulder feature from the model of semi-infinite chains with $\sqrt{(}T)\mathrm{\Delta }H/2H_0=0.0326$. The plots of the pure, Zn-doped, and Ni-doped samples are arranged such that they show increasing impurity effects from the left to the right (see text). All spectra are normalized to the maximal intensity.

Figure 3

Spectra simulated based on the model of finite-sized chain segments (see text). The spectra are each normalized to their maximum. The last panel shows the contribution of chain segments with even and odd number of sites separately.

Figure 4

XAS at the Ni ${\mathrm{L}}_{2/3}$ edge for a sample with 1% Ni doping compared to multiplet calculations for a high spin [left panel: (a), (c), and (e)] and low spin [right panel: (b), (d), and (f)] ground state. (a) and (b) correspond to a polarization of the incident light parallel to the ${\mathrm{CuO}}_2$ plane, whereas (c) and (d) show the XAS for incident light polarization perpendicular to the ${\mathrm{CuO}}_2$ plane. In (e) and (f) the linear dichroism is shown. Blue thick lines represent the experimental data (equal in left and right panel) and red thin lines correspond to the result from multiplet ligand-field theory.

Figure 5

Spin-lattice relaxation rates and stretching exponents $\lambda$ of ${\mathrm{SrCu}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_2$ for varying $x$ measured on the center of the ${}^{63}\mathrm{Cu}$ high-field satellite and $T_1^{-1}$ of pure ${\mathrm{SrCuO}}_2$ measured on the central transition for comparison. While all doped samples were measured with the magnetic field ${\mu }_{0}H\approx 7T$ parallel to the crystallographic $c$ axis (parallel to the chains), the values of the pure compound were obtained with the magnetic field parallel to the $a$ axis (perpendicular to the chains). $T_1^{-1}$ values measured with $a\left|\right|H$ are scaled. The black arrows indicate the crossover temperatures $T^{*}$.

Figure 6

Frequency-dependent spin-lattice relaxation measurement on the ${}^{63}\mathrm{Cu}$ high-field satellite of ${\mathrm{SrCu}}_{0.99}{\mathrm{Ni}}_{0.01}{\mathrm{O}}_2$ (a) and of ${\mathrm{SrCu}}_{0.99}{\mathrm{Zn}}_{0.02}{\mathrm{O}}_2$ (b) at different temperatures. $T_1^{-1}$ (filled black squares), stretching exponents $\lambda$ (open cyan circles), and spectrum (dark gold line) were measured with the $b$ axis parallel to the field ${\mu }_{0}H=6.9981\mathrm{T}$ (a) and ${\mu }_{0}H=7.0488\mathrm{T}$ (b). The spectral intensity is normalized to the maximum of the high-field satellite and its scale is not shown in the graph.

Figure 7

NMR spectra of ${\mathrm{SrCu}}_{0.99}{\mathrm{Pd}}_{0.01}{\mathrm{O}}_2$ at different temperatures measured with a fixed frequency of $80\text{MHz}$ by varying the magnetic field $H\left|\right|b$. The spectra of ${\mathrm{SrCu}}_{0.99}{\mathrm{Ni}}_{0.01}{\mathrm{O}}_2$ are plotted as black lines for comparison.

Figure 8

$T_1^{-1}$ and stretching exponents $\lambda$ of ${\mathrm{SrCu}}_{0.99}{\mathrm{Pd}}_{0.01}{\mathrm{O}}_2$ and ${\mathrm{SrCu}}_{0.99}{\mathrm{Co}}_{0.01}{\mathrm{O}}_2$ and of ${\mathrm{SrCu}}_{0.99}{\mathrm{Ni}}_{0.01}{\mathrm{O}}_2$ and pure ${\mathrm{SrCuO}}_2$ for comparison, each measured on the center of the ${}^{63}\mathrm{Cu}$ high-field satellite. All samples were measured with the magnetic field ${\mu }_{0}H\approx 7\text{T}$ parallel to the crystallographic $b$ axis, except of the pure sample which was measured with the magnetic field parallel to the $a$ axis. Both directions are perpendicular to the chains.

Figure 9

$T_1^{-1}$ and stretching exponents $\lambda$ of ${\mathrm{SrCu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{O}}_2$ for varying $x$ and of ${\mathrm{SrCu}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_2$ for comparison, each measured on the center of the ${}^{63}\mathrm{Cu}$ high-field satellite. All samples were measured with the magnetic field ${\mu }_{0}H\approx 7\text{T}$ parallel to the crystallographic $b$ axis. The black arrows indicate the crossover temperatures $T^{*}$.