Jean-Claude Bissey

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Abstract Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x )(0.63B 2 O 3 · 0.37Li 2 O) · x Fe 2 O 3 , with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe 3+ concentrations (Fe 2 O 3 molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g ef = 4.3 peak accompanied by a shoulder continuing down to g ef = 9.7 , are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe 2 O 3 contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe 3+ ions. It is concluded that the spectrum is due to diluted Fe 3+ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe 3+ ions is estimated as 4.9 A. A diluted state of Fe 3+ ions in the glass network in this range is also confirmed by the temperature dependence of the g ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe 3+ ions (Fe 2 O 3 molar contents from 0.01 to 0.1), the width of the g ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g ef ≈ 2 line, which coexists at these concentrations with the g ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe 3+ ions. The temperature dependence of the g ef ≈ 2 line intensity in 0.1 mol Fe 2 O 3 glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe 2 O 3 glass, which is attributed to an appearance, at higher Fe 2 O 3 contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g ef ≈ 2 line in the low-temperature EPR spectra.

Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in t...

Lineshapes in magnetic resonance spectra

In magnetic resonance, and in particular, in superparamagnetic resonance studies at variable temperatures, a correlation between the apparent resonance magnetic field and the apparent linewidth is often observed. In order to account for this correlation, we consider the resonance lineshapes resulting from different phenomenological equations of damped motion of the magnetic moments in the cases of a linear paramagnet and of a perfect soft ferromagnet. The Bloch-Bloembergen, modified Bloch, Gilbert, Landau-Lifshitz and Callen equations are analysed. In most cases we obtain analytical expressions for the apparent resonance-field shift. Finally, we report an experimental variable-temperature study of the superparamagnetic resonance of ultrafine Fe2O3 particles in sol-gel glass. Computer simulations using the Landau-Lifshitz lineshape provide good fits of the resonance spectra at different temperatures for the same magnetic and morphological parameters of the particles.

Superparamagnetic resonance of annealed iron-containing borate glass

A lithium borate glass containing a small amount of iron oxide is studied by electron magnetic resonance at room temperature after repeated annealing steps between 460 and . As the anneal temperature increases, the sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow line emerges at , superposed with a broader one, the narrow and the broader components predominating respectively after annealing at lower and at higher temperatures. Computer simulations of spectra have been carried out, based on a model of resonance of ferromagnetic single-domain nanoparticles randomly dispersed in the devitrified glass (superparamagnetic resonance). As the anneal temperature increases, the most probable particle diameter obtained assuming a log-normal distribution of diameters increases from 2.9 to 4.7 nm showing a saturation at higher anneal temperatures, whereas the relative number of larger particles grows continuously.

Structures and thermal behavior in the series of two-dimensional molecular composites NH3(CH2)4NH3MCl4 related to the nature of the metal M. Part 1: Crystal structures and phase transitions in the case M = Cu and Pd

Abstract X-ray diffraction and differential scanning calorimetry studies have been undertaken on the layered NH3(CH2)4NH3 MCl4 complexes with M = Cu and Pd. Both complexes are structurally very similar at room temperature (monoclinic P2 1 c :a = 0.9270(3) nm , b = 0.7600(3) nm, c = 0.7592(3) nm, β = 103.14(4) ° for M = Cu; a = 0.9087(2) nm, b = 0.7699(2) nm, c = 0.7792(2) nm, β = 103.82(2) ° for M = Pd). The organic layers are composed of [NH3(CH2)4NH3]2+ cations with a left-handed conformation at both ends; the mineral layers are composed of [MCl4]2− square planar anions. The structure cohesion is achieved via NH---Cl hydrogen bonds. The copper complex exhibits a structural phase transition at T = 328 K, characterized by an increase of the interlayer distance (+0.137 nm) from powder diffraction results and which can be interpreted as due to a change in the molecular conformations, from left-handed to all-trans. This assumption is confirmed by single crystal structure determination. (High temperature phase M = Cu, monoclinic P2 1 c :a = 1.0420(3) nm , b = 0.7442(1) nm, c = 0.7225(5) nm, β = 93.46(4) °.) The palladium complex is stable up to its decomposition temperature (490 K). However, a detailed thermal expansion analysis shows that a virtual structural phase transition is expected above 490 K. The correlation between the nature of the metal, the strength of the hydrogen bonds and the occurrence of left-handed conformations and related phase transitions, is discussed.

Differentiated g-values and exchange interaction between dissimilar copper ions as studied by EPR in the linear chain system CuSO4.5H2O

Abstract We have studied the room temperature EPR of a single crystal CuSO 4 .5H 2 O, when rotated around its β and γ magnetic axes at X- and Q-band. A careful analysis of experimental data permits one to obtain precise values of the differentiated principal g factors of the dissimilar copper ions ( g ⊥ = 2.080, g ∥1 = 2.4286, g ∥2 = 2.4046). When the magnetic field is parallel to magnetic axes α, β and γ, the two Cu 2+ ions are magnetically equivalent and the EPR spectra reduces to a Lorentzian singlet at X- and Q-band. When the magnetic field is parallel to a tetragonal axis, the EPR spectra are respectively a broadened singlet and a resolved doublet at X- and Q-band, due to the two dissimilar copper ions. For the compound under study, the simulation of these spectra with a method recently proposed by Hoffmann, in good agreement with the experimental results, permits one to determine an accurate value of the exchange interaction between dissimilar Cu 2+ ions: J ′ = (0.0374±0.0003)cm −1 .

X-band electron paramagnetic resonance of a single crystal Cu(NH4)2(SO4)2.6H2O. Exchange interaction between dissimilar copper ions

Abstract We have studied the EPR of a single crystal of the Tutton salt Cu(NH4)2(SO4)2.6H2O in rotations around the a and c ∗ axes, and around the magnetic β(≡b) and γ axes at X-band and room temperature. When the magnetic field lies in the (a,c) plane, the two Cu2+ ions are magnetically equivalent and the EPR spectrum reduces to a quasi-Lorentzian single line. Due to the two dissimilar copper ions, the EPR spectrum is a more or less resolved doublet, according to the orientation of the magnetic field. When the magnetic field is parallel to one tetragonal axis, the EPR spectrum is a perfectly resolved doublet, characteristic of a very weak exchange interaction J′ between the dissimilar ions. The simulation of the EPR spectrum for the compound under study reveals that the exchange coupling between dissimilar ions is practically equal to zero at room temperature at the accuracy of 0.0002 cm−1.

An X-band and Q-band single crystal electron paramagnetic resonance study of CuCl2·2H2O

Abstract Room temperature electron paramagnetic resonance (EPR) measurements were carried out on single crystals of CuCl 2 ·2H 2 O around their three crystallographic axes at X-band (9.5 GHz) and Q-band (35 GHz). Owing to the interchain exchange interaction which is stronger than the difference in Zeeman energies of the dissimilar ions, the EPR spectrum is always reduced to a Lorentzian singlet, not only at X-band, but even at Q-band. There is no frequency dependence of the g factors from X-band to Q-band. The sinusoidal angular dependence of g 2 in any plane is related to the fact that the relative anisotropy Δ g / g is moderate in the a , c plane. The angular dependence of the linewidth is analyzed, namely in the a , c plane, where the interchain contribution Δ H 12 is extracted from the total linewidth Δ H . This permits us to determine the following value of the interchain exchange interaction: J ′/ k = (0.40 ± 0.04)K, from which we conclude that the single crystal EPR study at Q-band provides a direct and precise method to determine the exchange interaction J ′ between inequivalent copper ions.

Determination of the exchange interaction between dissimilar copper ions by X- and Q-band electron paramagnetic resonance (EPR) in 3D Heisenberg ferromagnetic systems M2CuCl4 · 2H2O (M = Rb, K, NH4)

Abstract In the case of rather strong exchange, we have used a method already published by two of us in order to determine for the Rb compound J ′/ k = (0.28±0.03) K. In the case of rather weak exchange, we have used a method proposed by Hoffmann in order to determine J ′/ k = (0.050±0.002) K for the K compound and J ′/ k = (0.050±0.002) K for the NH 4 compound

X-band ESR faraday and cotton-mouton-voigt effects of CuCl2 · 2H20 and Cu(NH4SO4)2 · 6H20 in polycrystalline powder

Abstract Magneto-microwave Faraday and Cotton-Mouton-Voigt effects related to anisotropic ESR powder spectra were studied at X-band and room temperature on polycrystalline samples of CuCl 2 ·2H 2 O and Cu(NH 4 SO 4 ) 2 ·6H 2 O. For both compounds, computer simulations of the powder ESR spectra were carried out with a rather simplified model accounting for a crystallite Lorentzian lineshape and complete anisotropies of the g factor, transition probability, and linewidth. One observes a good agreement between simulated and experimental spectra, and an automatic fitting procedure permits one to obtain for each compound the principal values of the g factor and linewidth, in close agreement with results from previous single-crystal ESR studies. The same model is then transposed to the magnetomicrowave effects, by substituting for the ESR lineshape function the various crystallite lineshape functions corresponding to the rotation and ellipticity of elementary Faraday and Cotton-Mouton-Voigt effects. Computer simulations of the magneto-microwave powder effects are in close agreement with experimental data.