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We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical Nb1-yFe2+y using muon spin rotation and relaxation (μSR). By means of an angular dependent study of the muon spin rotation signal in applied magnetic fields on a single crystal in the paramagnetic state we establish the muon stopping site in the crystallographic lattice of NbFe2. With this knowledge we develop models to describe the muon spin rotation and relaxation signals in the weakly ferromagnetic, spin density wave, and quantum critical phases of Nb1-yFe2+y and fit the corresponding experimental data. In particular, we quantify the μSR response for quantum critical behavior in Nb1.0117Fe1.9883 and extract the influence of residual weak structural disorder. From our analysis, Nb1-yFe2+y emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order.

In frustrated magnetic systems, the competition amongst interactions can introduce extremely high degeneracy and prevent the system from readily selecting a unique ground state. In such cases, the magnetic order is often exquisitely sensitive to the balance among the interactions, allowing tuning among novel magnetically ordered phases. In antlerite, Cu3SO4(OH)4, Cu2+ (S=1/2) quantum spins populate three-leg zigzag ladders in a highly frustrated quasi-one-dimensional structural motif. We demonstrate that at zero applied field, in addition to its recently reported low-temperature phase of coupled ferromagnetic and antiferromagnetic spin chains, this mineral hosts an incommensurate helical+cycloidal state, an idle-spin state, and a multiple-q phase which is the magnetic analog of misfit crystal structures. The antiferromagnetic order on the central leg is reentrant. The high tunability of the magnetism in antlerite makes it a particularly promising platform for pursuing exotic magnetic order., This project was funded by the German Research Foundation (DFG) via the projects B01, B03, C03, and C06 of the Collaborative Research Center SFB 1143 (project-id 247310070); Research Training Group GRK 1621 (project-id 129760637); the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter – ct.qmat (EXC 2147, project-id 390858490); and through grants No. IN 209/9-1, PE 3318/2-1, and LI 244/12-2, and Project No. 422219907.

Made available in DSpace on 2020-12-10T20:06:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-07-22 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) Fundacao de Amparo a Pesquisa do Estado de Goias (FAPEG) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) The delicate balance between spin-orbit coupling, Coulomb repulsion, and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in La2CoIrO6. A great advantage of the use of Ca2+ as a replacement for La3+ is the similarity of their ionic radii. Thus, the observed magnetic changes can more easily be associated with electronic variations. A thorough investigation of the structural, electronic, and magnetic properties of the La2-xCaxCoIrO6 system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation (mu SR), AC and DC magnetization, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism, Raman spectroscopy, electrical resistivity, and dielectric permittivity. Our XAS results show that up to 25% Ca substitution at the La site results in the emergence of Co3+, possibly in a high-spin state, while the introduction of a larger amount of Ca leads to an increase in the Ir valence. The competing magnetic interactions resulting from the mixed valences lead to the coexistence of a magnetically ordered and an emerging spin-glass (SG) state for the doped samples. Our mu SR results indicate that for La2CoIrO6 a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperatures, possibly related to the presence of a small amount of Ir3+ and to the antisite disorder at Co/Ir sites. For doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% Ca doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the transition-metal ions, the antisite disorder at Co/Ir sites, and the distorted crystalline structure. Univ Fed Goias, Inst Fis, BR-74001970 Goiania, Go, Brazil Tech Univ Carolo Wilhelmina Braunschweig, Inst Phys Kondensierten Materie, D-38106 Braunschweig, Germany Tech Univ Dresden, Inst Festkorper & Mat Phys, D-01069 Dresden, Germany Ctr Brasileiro Pesquisas Fis, BR-22290180 Rio De Janeiro, RJ, Brazil Univ Estadual Campinas, Inst Fis Gleb Wataghin, BR-13083859 Campinas, SP, Brazil Ctr Nacl Pesquisa Energia & Mat, Lab Nacl Luz Sincrotron, BR-13083970 Campinas, SP, Brazil Sao Paulo State Univ, Dept Phys & Chem, BR-15385000 Ilha Solteira, SP, Brazil Sao Paulo State Univ, Dept Phys & Chem, BR-15385000 Ilha Solteira, SP, Brazil CNPq: 400134/2016-0 FAPERJ: Li 244/12

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq ��\leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn., 13 pages, 9 figures + supplementary material

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor J1 and next-nearest-neighbor J2 interactions (J2/J1≈0.5). The double perovskites Sr2CuTeO6 and Sr2CuWO6 are isostructural spin-1/2 square-lattice antiferromagnets with Néel (J1 dominates) and columnar (J2 dominates) magnetic order, respectively. Here we characterize the full isostructural solid-solution series Sr2Cu(Te1-xWx)O6(0≤x≤1) tunable from Néel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the x=0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of x≈0.1-0.6. This coincides with significant T-linear terms in the low-temperature specific heat. However, density-functional theory calculations predict most of the materials are not in the highly frustrated J2/J1≈0.5 region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in x=0.5.