0000000000266678
AUTHOR
Peter Adler
Intersystem crossing in Fe(II) coordination compounds
Fe(II) spin-crossover systems can be quantitatively converted from the low-spin (LS) to the high-spin (HS) state well below the thermal transition temperature by irradiating either into the metal-ligand charge transfer or d-d absorption bands, and even in low-spin systems a transient population of the HS state can be achieved. This fact can be made use of to determine HS → LS relaxation rate constants for a wide variety of Fe(II) spin-crossover and low-spin systems. The HS → LS relaxation shows strong deviations from an Arrhenius behaviour, with nearly temperature-independent tunnelling below ∼70 K and a thermally activated process above ∼100 K. The range of more than 12 orders of magnitude…
Dynamics of spin state conversion processes in the solid state
High spin (HS) ⇌ low spin (LS) conversions in transition metal complexes are nonradiative transitions between spin states. In this contribution, we present a study of the temperature and pressure dependence of the HS ⇌ LS intersystem crossing dynamics. For some iron(II) spin-crossover complexes, the rate constants were determined by line shape analysis of57Fe Mossbauer spectra. Their temperature dependence is described by an Arrhenius equation, their pressure dependence is interpreted within absolute rate theory. HS → LS conversion rates at low temperatures were determined from the relaxation of light-induced formation of HS states, monitored by optical spectroscopy. Deviations from a simpl…
Ultrafine MnWO4 nanoparticles and their magnetic properties
Abstract Ultrafine nanoparticles of MnWO4, a compound showing low-temperature multiferroicity in the bulk, were synthesized by the polyol method. Studies using powder X-ray diffraction, scanning and transmission electron microscopy, dynamic light scattering, differential sedimentation and sorption techniques show the formation of a single-phase material, which is composed of MnWO4 nanoparticles with a prolate ellipsoidal shape (short axis of 4–5 nm, long axis of 11–12 nm) and an unprecedented high specific surface area of 166 m2 g−1. The as-prepared MnWO4 nanoparticles are readily crystalline after the liquid-phase synthesis. Temperature and field dependent magnetization measurements indica…
Mössbauer relaxation spectra in arbitrarily ordered absorbers—Line shape analysis for an iron(II) spincrossover complex in the presence of texture
The stochastic theory of Mossbauer line shapes is formulated in a fashion which allows the evaluation of the spectral shapes for absorbers of arbitrary thickness, texture, and an anisotropic Lamb—Mossbauer factor. The results are specialized to a two-state-relaxation model of fluctuating electric hyperfine interaction in the case of an absorber of axially symmetric texture. The formalism is applied to the line shape analysis of Mossbauer spectra of a textured sample of the spin-crossover complex [Fe(mtz)6] (PF6)2 (mtz=1-methyltetrazole). It is found that between 185 and 240 K the rate constants for the HS→LS conversion are temperature independent, whereas an Arrhenius behaviour is found for…
Iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties
The present work reports on the new soft ferromagnetic Heusler phases Fe${}_{2}$NiGe, Fe${}_{2}$CuGa, and Fe${}_{2}$CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal Heusler structure. Together with the known phases Fe${}_{2}$CoGe and Fe${}_{2}$NiGa these materials have been synthesized and characterized by powder x-ray diffraction, ${}^{57}$Fe M\"ossbauer spectroscopy, superconducting quantum interference device, and energy-dispersive x-ray measurements. In particular M\"ossbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in con…
Lattice Instability and Competing Spin Structures in the Double Perovskite InsulatorSr2FeOsO6
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron-osmium layers along the tetragonal c axis. Neutron powder diffraction experiments, 57Fe Mossbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.
Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO
From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca[FeSe2/2O2/2]∞2 can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [FeSe2/2O2/2]2−∞2 layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mossbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data…
Mössbauer effect study of the temperature and pressure dependence of the singlet-quintet intersystem crossing dynamics in an iron(II) spin crossover complex
The lineshapes of Mossbauer spectra of the iron(II) spin crossover complex [Fe(6-mepy)3 tren] (PF6)2 are affected by the dynamics of the HS⇌LS equilibrium. The lineshapes are reproduced with a stochastic two-state-relaxation-model yielding rate constants similar to those determined for related complexes in solution. Application of an external pressure of 150 MPa increases the relaxation rate.
Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron - osmium layers along the tetragonal c-axis. Neutron powder diffraction experiments, 57Fe M\"ossbauer spectra, and density-functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example for a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.
Influence of nanoscale order–disorder transitions on the magnetic properties of Heusler compounds for spintronics
Modifications in nanoscale chemical order are used to tune the magnetic properties, namely T-C, of Co2FeSixAl1-x (0 < x < 1). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast reveals nanoscale regions of L2(1) order within a B2 matrix in the off-stoichiometry samples. Perhaps surprisingly, the latter, more chemically disordered structure, exhibits a higher T-C. Upon annealing, the off-stoichiometry samples become more homogeneous with the fraction of L2(1) order decreasing. The short-range order was also investigated using X-ray absorption fine structure (XAFS) measurements at the Co and Fe K edges. Since the local atomic environments of C…