We investigate the effect of disorder on the heat transport properties of the S = 1 2 Heisenberg chain compound Sr2CuO3 upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu-O-Cu spin chains, bond disorder, i.e. a spatial variation of the exchange interaction J, is expected to be realized in these chains. We observe that the magnetic heat conductivity (κmag) due to spinons propagating in the chains is gradually but strongly suppressed with increasing amount of Ca, where the doping dependence can be understood in terms of increased scattering of spinons due to Ca-induced disorder. This is also reflected in the spinon mean free path which can be separated in a doping independent but temperature dependent scattering length due to spinon-phonon scattering, and a temperature independent but doping dependent spinon-defect scattering length. The latter spans from very large (> 1300 lattice spacings) to very short (∼ 12 lattice spacings) and scales with the average distance between two neighboring Ca atoms. Thus, the Ca-induced disorder acts as an effective defect within the spin chain, and the doping scheme allows to cover the whole doping regime between the clean and the dirty limits. Interestingly, at maximum impurity level we observe, in Ca-doped Sr2CuO3, an almost linear increase of κmag at temperatures above 100 K which reflects the intrinsic low temperature behavior of heat transport in a Heisenberg spin chain. These findings are quite different from that observed for the Ca-doped double spin chain compound, SrCuO2, where the effect of Ca seems to saturate already at intermediate doping levels.

Muon spin rotation technique is used to study magnetic ordering in ultra-pure samples of SrCu1−xNixO2, an archetypical S = 1/2 antiferromagnetic Heisenberg chain system with a small amount of S = 1 defects. The ordered state in the parent compound is shown to be highly homogeneous, contrary to previous report [M. Matsuda et al., Phys. Rev. B 55, R11953 (1997)]. Even minute amount of Ni impurities result in inhomogeneous order and a decrease of the transition temperature. At as little as 0.5 % Ni concentration, magnetic ordering is entirely suppressed. The results are compared to previous theoretical studies of weakly coupled spin chains with site-defects.

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 Néel temperature which was observed by recent µSR and susceptibility measurements, 1 and is most likely an effect of the impurity spin S = 0.

Magnetic properties and magnetic structure of the Ba2Mn(PO4)2 antiferromagnet featuring frustrated zigzag chains of S = 5 2 Mn 2+ ions are reported based on neutron diffraction, density-functional band-structure calculations, as well as temperature-and field-dependent measurements of the magnetization and specific heat. A magnetic transition at TN 5 K marks the onset of the antiferromagnetic order with the propagation vector k = ( 1 2 0 1 2 ) and ordered moment of 4.33 ± 0.08 µB/Mn 2+ at 1.5 K, pointing along the c direction. Direction of the magnetic moment is chosen by the single-ion anisotropy, which is relatively weak compared to the isostructural Ni 2+ compound. Geometrical frustration has strong impact on thermodynamic properties of Ba2Mn(PO4)2, but manifestations of the frustration are different from those in Ba2Ni(PO4)2, where frustration by isotropic exchange couplings is minor, yet strong and competing single-ion anisotropies are present. A spin-flop transition is observed around 2.5 T. The evaluation of the magnetic structure from the ground state via the spin-flop state to the field-polarized ferromagnetic state has been revealed by a comprehensive neutron diffraction study as a function of magnetic field below TN . Finally, a magnetic phase diagram in the H − T plane is obtained. FIG. 2. (Color online) (a) Projection of the crystal structure of Ba2Mn(PO4)2 along the a axis, with the honeycomb arrangement of the Mn 2+ ions highlighted by the honeycomb-unit. (b) Schematic representation of the exchange interactions J1, J2, J3 and J4 in Ba2Mn(PO4)2. The honeycomb units are formed by two J1 and four J4 exchange interactions, whereas J2 and J3 connect the honeycomb planes.(c) An alternative view of the spin lattice in terms of zigzag spin chains along the crystallographic a-axis. The chains are built by nearest-neighbor couplings J1, J2 and the second-neighbor coupling J3. The coupling J4 connects the zigzag chains.