We report on the magnetic properties of thoroughly characterized ${\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x\mathrm{O}$ epitaxial thin films, with low Co concentration, $x=0.003–0.005$. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated ${\mathrm{Co}}^{2+}$ ions in ZnO possess a strong single ion anisotropy which leads to an “easy plane” ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in $\mathrm{ZnO}:\mathrm{Co}$ materials.

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ and ${\mathrm{CuGeO}}_{3}$. By tuning the incoming photon energy to the oxygen $K$ edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring ${\mathrm{CuO}}_{4}$ plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.

We present an inelastic neutron scattering investigation of Li2CuO2 detecting the long sought quasi-1D magnetic excitations with a large dispersion along the CuO2-chains studied up to 25 meV. The total dispersion is governed by a surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral J1=-228 K. An anomalous quartic dispersion near the zone center and a pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral excitation with a pitch angle about 41 degree point to the vicinity of a 3D FM-spiral critical point. The leading exchange couplings are obtained applying standard linear spin-wave theory. The 2nd neighbor inter-chain interaction suppresses a spiral state and drives the FM in-chain ordering below the Ne'el temperature. The obtained exchange parameters are in agreement with the results for a realistic five-band extended Hubbard Cu 3d O 2p model and L(S)DA+U predictions.

On the basis of extensive experimental studies of Pb${}_{1-x}$Ba${}_x$(Fe${}_{1/2}$Nb${}_{1/2}$)O${}_3$ (PFN-BFN) and Pb(Fe${}_{1/2}$Nb${}_{1/2}$)${}_{1-x}$Ti${}_x$O${}_3$ (PFN-PT) single crystals by several experimental methods, we have proposed phase diagrams describing the magnetic properties of these solid solutions. The comprehensive consideration of the magnetic properties of the PFN-based solid solutions has shown that these phase diagrams can be explained on the basis of a model suggested earlier for pure PFN [ Phys. Rev. Lett. 105, 257202 (2010)]. This model assumes the coexistence in the crystal lattice of the long-range antiferromagnetic (AFM) cluster, which defines the Néel order parameter, with the finite-size mixed ferromagnetic-AFM clusters, responsible for the spin-glass order parameter. We state that one of these parameters, the Néel temperature, linearly decreases with the increasing dopant concentration and eventually disappears at some critical concentration as a result of the percolation phase transition. The other parameter survives until the maximal concentrations studied. We have also found a phase which can be related to the super-AFM order. These data can have important implications and provide the basis for the development of novel fundamental theory of multiferroics with the site, charge, and spin disorder.

The two-magnon problem for the frustrated XXZ spin-$1∕2$ Heisenberg Hamiltonian and external magnetic fields exceeding the saturation field $B_s$ is considered. We show that the problem can be exactly mapped onto an effective tight-binding impurity problem. It allows to obtain explicit exact expressions for the two-magnon Green’s functions for arbitrary dimension and number of interactions. We apply this theory to a quasi-one-dimensional helimagnet with ferromagnetic nearest-neighbor $J_1<0$ and antiferromagnetic next-nearest neighbor $J_2>0$ interactions. An outstanding feature of the excitation spectrum is the existence of two-magnon bound states. This leads to deviations of the saturation field $B_s$ from its classical value $B_s^{\mathrm{cl}}$ which coincides with the one-magnon instability. For the refined frustration ratio $\mid J_2∕J_1\mid >0.374661$ the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin zone. Based on the two-magnon approach, we propose general analytic expressions for the saturation field $B_s$, confirming known previous results for one-dimensional isotropic systems, but explore also the role of interchain and long-ranged intrachain interactions as well as of the exchange anisotropy.

We quantify the stability of the formation of multipolar states against always present interchain couplings in quasi-one-dimensional spin-$\frac{1}{2}$ chain systems with a frustrating in-chain $J_1-J_2$ exchange, including parameter regimes that are of direct relevance to many edge-shared cuprate spin-chain compounds. Three representative types of antiferromagnetic interchain coupling and the presence of uniaxial exchange anisotropy are considered. The magnetic phase diagrams are determined by density matrix renormalization group calculations and completed by very accurate analytic and numerical results for the nematic and the dipolar phases employing the hard-core-boson approach. We establish that a sizable interchain coupling has a strong influence on the possible instability of multipolar phases at high magnetic fields in the vicinity of the saturation fields in favor of the usual dipolar one-magnon phase. Moreover, skew interchain couplings strongly affect the pitch of spiral states. Our theoretical results bring to the fore candidate materials close to quantum nematic/triatic ordering.