Terahertz Magneto-Optical Excitations of the sd-Hybrid States of Lithium Nitridocobaltate Li2(Li1-xCox)N.

We report the results of the experimental and theoretical study of the magnetic anisotropy of single crystals of the Co-doped lithium nitride Li2(Li1-xCox)N with x = 0.005, 0.01, and 0.02. It was shown recently that doping of the Li3N crystalline matrix with 3d transition metal (TM) ions yields superior magnetic properties comparable with the strongly anisotropic single-molecule magnetism of rare-earth complexes. Our combined electron spin resonance (ESR) and THz spectroscopic investigations of Li2(Li1-xCox)N in a very broad frequency range up to 1.7 THz and in magnetic fields up to 16 T enable an accurate determination of the energies of the spin levels of the ground state multiplet Ŝ = 1 of the paramagnetic Co(I) ion. In particular, we find a very large zero field splitting (ZFS) of almost 1 THz (∼4 meV or 33 cm-1) between the ground-state singlet and the first excited doublet state. On the computational side, ab initio many-body quantum chemistry calculations reveal a ZFS gap consistent with the experimental value. Such a large ZFS energy yields a very strong single-ion magnetic anisotropy of easy-plane type resembling that of rare-earth ions. Its microscopic origin is the unusual linear coordination of the Co(I) ions in Li2(Li1-xCox)N with two nitrogen ligands. Our calculations also evidence a strong 3d-4s hybridization of the electronic shells resulting in significant electron spin density at the 59Co nuclei, which may be responsible for the experimentally observed extraordinary large hyperfine structure of the ESR signals. Altogether, our experimental spectroscopic and computational results enable comprehensive insights into the remarkable properties of the Li2[Li1-x(TM)x]N magnets on the microscopic level.

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.

Interplay of magnetic exchange interactions and Ni-S-Ni bond angles in polynuclear nickel(II) complexes.

The ability of bridging thiophenolate groups (RS(-)) to transmit magnetic exchange interactions between paramagnetic Ni(II) ions is examined. Specific attention is paid to complexes with large Ni-SR-Ni angles. For this purpose, dinuclear [Ni(2)L(1)(mu-OAc)I(2)][I(5)] (2) and trinuclear [Ni(3)L(2)(OAc)(2)][BPh(4)](2) (3), where H(2)L(1) and H(2)L(2) represent 24-membered macrocyclic amino-thiophenol ligands, are prepared and fully characterized by IR- and UV/Vis spectroscopy, X-ray crystallography, static magnetization M measurements and high-field electron spin resonance (HF-ESR). The dinuclear complex 2 has a central N(3)Ni(2)(mu-S)(2)(mu-OAc)Ni(2)N(3) core with a mean Ni-S-Ni angle of 92 degrees . The macrocycle L(2) supports a trinuclear complex 3, with distorted octahedral N(2)O(2)S(2) and N(2)O(3)S coordination environments for one central and two terminal Ni(II) ions, respectively. The Ni-S-Ni angles are at 132.8 degrees and 133.5 degrees . We find that the variation of the bond angles has a very strong impact on the magnetic properties of the Ni complexes. In the case of the Ni(2)-complex, temperature T and magnetic field B dependencies of M reveal a ferromagnetic coupling J=-29 cm(-1) between two Ni(II) ions (H=JS(1)S(2)). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0) which implies a bistable (easy axis) magnetic ground state. In contrast, for the Ni(3)-complex we find an appreciable antiferromagnetic coupling J'=97 cm(-1) between the Ni(II) ions and a positive axial magnetic anisotropy (D>0) which implies an easy plane situation.

Microwave Radiation Detection with an Ultrathin Free-Standing Superconducting Niobium Nanohelix.

We present a superconducting bolometer fabricated by a rolled-up technology that allows one to combine the two-dimensionality (2D) of the superconducting layer with a helical spiral curvature. The bolometer is formed as a free-standing Nb nanohelix acting as an ultrathin transition-edge sensor (TES) and having a negligible thermal contact to the substrate. We demonstrate the functionality of the thin-film TES by examining its microwave-detection performance in comparison with a commercial cryogenic bolometer from QMC Instruments. The nanohelix has been revealed to feature a noise equivalent power (NEP) of about 2 × 10-10 W Hz-1/2 at a microwave radiation power of 9 W m-2, which is 4 orders of magnitude smaller than the NEP of the QMC sensor at a similar radiation power. Furthermore, the forecast for the nanohelix is a 1 to 2 orders of magnitude shorter response time as compared to sensors based on commonly used 1 µm thick Si3N4 membranes. The reason is the extremely low heat capacity of the 50 nm thick supporting material and the few contact points between the TES and the substrate. Our findings indicate that microwave radiation detection can be substantially improved by extending 2D superconducting structures into the 3D space.

Cantilever detected ferromagnetic resonance in thin Fe50Ni50, Co2FeAl0.5Si0.5 and Sr2FeMoO6 films using a double modulation technique.

In this work we introduce a new method, which employs commercial piezo-cantilevers, for a ferromagnetic resonance (FMR) detection from thin, nm-size, films. Our setup has an option to rotate the sample in the magnetic field and it operates up to the high microwave frequencies of 160GHz. Using our cantilever based FMR spectrometer we have investigated a set of samples, namely quasi-bulk and 84nm film Co2FeAl0.5Si0.5 samples, 16nm Fe50Ni50 film and 150nm Sr2FeMoO6 film. Low frequency and room temperature test of our setup using 84nm Co2FeAl0.5Si0.5 film yielded a result identical to a standard X-Band spectrometer, namely a single line with quite small linewidth. Our measurements at low temperatures and high frequencies revealed a quite strong FMR response detected in all samples. The FMR spectra share common features, such as the emergence of the second line with an opposite angular dependence, and a drastic increase of the linewidths with increasing microwave frequency. We believe that these findings are results of the complicated dynamics of the magnetization at low temperatures and high frequencies, which we were able to probe using our cantilever based FMR setup.