A. Alfonsov

Tuning the magnetic ground state of a tetranuclear nickel(II) molecular complex by high magnetic fields

Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a \mu_4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ~25 T.

Signatures of a magnetic field-induced unconventional nematic liquid in the frustrated and anisotropic spin-chain cuprate LiCuSbO 4

Modern theories of quantum magnetism predict exotic multipolar states in weakly interacting strongly frustrated spin-1/2 Heisenberg chains with ferromagnetic nearest neighbor (NN) inchain exchange in high magnetic fields. Experimentally these states remained elusive so far. Here we report strong indications of a magnetic field-induced nematic liquid arising above a field of ~13 T in the edge-sharing chain cuprate LiSbCuO4 ≡ LiCuSbO4. This interpretation is based on the observation of a field induced spin-gap in the measurements of the 7Li NMR spin relaxation rate T1−1 as well as a contrasting field-dependent power-law behavior of T1−1 vs. T and is further supported by static magnetization and ESR data. An underlying theoretical microscopic approach favoring a nematic scenario is based essentially on the NN XYZ exchange anisotropy within a model for frustrated spin-1/2 chains and is investigated by the DMRG technique. The employed exchange parameters are justified qualitatively by electronic structure calculations for LiCuSbO4.

Nuclear magnetic resonance study of thin Co2FeAl0.5Si0.5 Heusler films with varying thickness

Type and degree of structural order is an important aspect for understanding and controlling the properties of highly spin polarized Heusler compounds. In this work, we compare the structural order and the local magnetic properties revealed by nuclear magnetic resonance (NMR) with the physical properties of thin Co2FeAl0.5Si0.5 Heusler films with varying thickness. We find a very high degree of L21 type ordering up to 81% concomitantly with excess Fe of 8 to 13% in the expense of Al and Si. The very good quality as demonstrated by our NMR study suggest that the novel technique of off-axis sputtering technique used to grow the films sets stage for the optimized performance of Co2FeAl0.5Si0.5 in spintronic devices.

Toward Full Zigzag-Edged Nanographenes: peri-Tetracene and Its Corresponding Circumanthracene

Zigzag-edged nanographene with two rows of fused linear acenes, called as n- peri-acene (n-PA), is considered as a potential building unit in the arena of organic electronics. n-PAs with four ( peri-tetracene, 4-PA), five ( peri-pentacene, 5-PA) or more benzene rings in a row have been predicted to show open-shell character, which would be attractive for the development of unprecedented molecular spintronics. However, solution-based synthesis of open-shell n-PA has thus far not been successful because of the poor chemical stability. Herein we demonstrated the synthesis and characterization of the hitherto unknown 4-PA by a rational strategy in which steric protection of the zigzag edges playing a pivotal role. The obtained 4-PA possesses a singlet biradical character ( y0 = 72%) and exhibits remarkable persistent stability with a half-life time ( t1/2) of ∼3 h under ambient conditions. UV-vis-NIR and electrochemical measurements reveal a narrow optical/electrochemical energy gap (1.11 eV) for 4-PA. Moreover, the bay regions of 4-PA enable the efficient 2-fold Diels-Alder reaction, yielding a novel full zigzag-edged circumanthracene.

Origin of a spin-state polaron in lightly hole doped LaCoO3

We performed electron spin and nuclear magnetic resonance and inelastic neutron scattering measurements of a single crystal of lightly hole-doped La1-xSrxCoO3, x ~ 0.002 in order to establish the origin of a surprisingly strong magnetization due to a very small Sr doping. The data provide experimental evidence for the creation at low temperatures of extended spin clusters with a large spin multiplicity. We argue that the doped hole couples ferromagnetically seven magnetic Co ions yielding a spin-state polaron with a huge local magnetic moment with a strong orbital contribution.

Tuning the magnetocrystalline anisotropy in RCoPO by means of R substitution: A ferromagnetic resonance study

We report on broad-band electron-spin resonance measurements performed within the itinerant ferromagnetic phase of RCoPO (R = La, Pr, Nd, and Sm). We reveal that the R substitution is highly effective in gradually introducing a sizable easy-plane magnetocrystalline anisotropy within the Co sublattice. We explain our results in terms of a subtle interplay of structural effects and of indirect interactions between the f and d orbitals from R and Co, respectively. Our observations provide interesting information on the crucial role of R ions in affecting the electronic properties of the CoP layers, of possible interest also for the isostructural FeAs-based family of high-T-c superconductors.