Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study

Two stable nitronyl nitroxide free radicals {R ¹ = 4'-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R ² = 2-(2'-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac)₂ (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal-organic complexes Co(hfac)₂(NNPhOMe)₂, 1; Co(hfac)₂(NNT)₂·(H₂O), 2; Mn(hfac)₂(NNPhOMe)·x(C₇H₁₆), 3; and Mn(hfac)₂(NNT)₂, 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P1̅, space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P2₁/c space group in which the N-O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/K _B) are derived using isotropic spin Hamiltonian H = -2J∑(S _metal S _radical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the 1, 2, 3, and 4 metal-organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal-radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.

Revealing magnetic ordering and spin-phonon coupling in Y1-x Tb x MnO3 (0.1 ⩽ x ⩽ 0.3) compounds

The structural and magnetic properties of the Y_1-x Tb _x MnO₃ (0.1  ⩽  x  ⩽  0.3) compounds were investigated. Neutron diffraction patterns for all three samples, recorded at room temperature (RT), were fitted to the nuclear structure confirming the paramagnetic nature of the compounds. At 2.8 K, for the x  =  0.1 sample magnetic moments of the Tb³⁺ ionic as well as Mn³⁺ ionic were ordered. At 5 K for the x  =  0.2 sample only the Mn³⁺ ionic magnetic moments were ordered. There were six sites for Mn atoms. Three on the z  =  0 plane and three on the z  =  0.5 plane (where z corresponds to  +c axis).The Mn³⁺ionic moments were confined to the a-b plane with a net magnitude of 2.78(3) µ _B, and 2.90(3) µ _B for the x  =  0.1 and the x  =  0.2 samples. The Tb³⁺ionic moments had a magnitude of 1.36(4) µ _B at 2.8 K and were aligned antiferromagnetically along the crystallographic c-axis for the x  =  0.1 sample. The low moment in comparison with Mn³⁺ free ions has been attributed to crystalline electric fields similar to that found in the parent compound YMnO₃ and also in another rare earth manganite viz HoMnO₃. The x  =  0.3 sample was found to be a canonical spin glass. To investigate the role of the above spin ordering in Y_1-x Tb _x MnO₃ in governing the phonon dynamics, temperature dependent Raman measurements were carried out. We observed the deviation of the phonon frequency near 685 cm^-1 and its line-width from the expected anharmonic behaviour around magnetic ordering temperature for Tb substituted compounds with x  =  0.1 and 0.2. This was attributed to the spin-phonon coupling in these systems. The anomalous behaviour of this phonon mode in the canonical spin glass compound with x  =  0.3, indicated that the coupling sustained even in the presence of only local magnetic ordering.

Synthesis, Characterization and Exploration of Multiferroic Properties in Nano-Crystalline Tb1-xYxMnO3 (0 ≤ x ≤ 0.4)

We report the synthesis and electric properties of nano-crystalline Tb1-xYxMnO3 (x = 0, 0.1, 0.2, 0.3 and 0.4) compounds prepared by gel-combustion method. These samples were characterized by a number of techniques including X-ray diffraction (XRD), Raman spectroscopy, specific-heat measurement, neutron diffraction, and magnetic field dependent pyrocurrent measurement. All the samples crystallize in the orthorhombic structure with space group Pnma at room temperature. Anomalies were observed in low temperature specific-heat measurement corresponding to magnetic and electric phase transitions. The magnetic phase transitions occurred at ~35, ~22-28 and ~7 K for all the samples. Signatures of coupling between magnetic and electric order parameters were revealed by pyrocurrent measurements carried out in presence of magnetic fields.

Core-shell Prussian blue analogue molecular magnet Mn(1.5)[Cr(CN)6]·mH2O@Ni(1.5)[Cr(CN)6]·nH2O for hydrogen storage

Core-shell Prussian blue analogue molecular magnet Mn1.5[Cr(CN)6]·mH2O@Ni1.5[Cr(CN)6]·nH2O has been synthesized using a core of Mn1.5[Cr(CN)6]·7.5H2O, surrounded by a shell of Ni1.5[Cr(CN)6]·7.5H2O compound. A transmission electron microscopy (TEM) study confirms the core-shell nature of the nanoparticles with an average size of ∼25 nm. The core-shell nanoparticles are investigated by using x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy. The Rietveld refinement of the XRD pattern reveals that the core-shell compound has a face-centered cubic crystal structure with space group Fm3m. The observation of characteristic absorption bands in the range of 2000-2300 cm(-1) in IR spectra corresponds to the CN stretching frequency of Mn(II)/Ni(II)-N≡C-Cr(III) sequence, confirming the formation of Prussian blue analogues. Hydrogen absorption isotherm measurements have been used to investigate the kinetics of molecular hydrogen adsorption into core-shell compounds of the Prussian blue analogue at low temperature conditions. Interestingly, the core-shell compound shows an enhancement in the hydrogen capacity (2.0 wt % at 123 K) as compared to bare-core and bare-shell compounds. The hydrogen adsorption capacity has been correlated with the specific surface area and TGA analysis of the core-shell compound. To the best of our knowledge, this is the first report on the hydrogen storage properties of core-shell Prussian blue analogue molecular magnet that could be useful for hydrogen storage applications.

Structural, magnetic, and dielectric studies on Gd0.7Y0.3MnO3

Structural, magnetic, specific heat, and dielectric studies were carried out on Y substituted (30 at. %) GdMnO(3) compound as a function of temperature. Anomalies occur at ~41 and 18 K, in the specific heat measurements and are ascribed to paramagnetic, to sinusoidal incommensurate antiferromagnetic transition (ICAFM) and ICAFM to commensurate antiferromagnetic transitions, respectively. Changes in the lattice parameters across these temperatures indicate magneto-elastic coupling present in the compound. However, in the dielectric measurements, an anomaly at 18 K alone is observed and is ascribed to a ferroelectric transition, giving rise to spontaneous ferroelectric ordering at low temperatures. This observation is supported by an anomaly in lattice parameters, across the transition temperature. From the frequency dependent dielectric studies, a strong coupling between Gd(3+) and Mn(3+) magnetic sublattices is inferred and Y substitution results in substantial changes in the relaxation process compared to that of GdMnO(3).