Yves Servant

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe3+ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe2+ ions in an octahedral ferrous complex [Fe(trz)(Htrz)2](BF4). The temperature dependence of the intensity of the Fe3+ EPR line atg=4.3 reveals a spin transition which occurs for the Fe2+ ions, with hysteresis. The transition temperatures areTc↑=374 K in the warming mode andTc↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.

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.

Spin transition polymer with a large hysteresis around room temperature: optical response and electron paramagnetic resonance

A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.

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.

Variable temperature EPR study of Fe2+ spin transition in Cu2+ doped [Fe(trz)(Htrz)2](BF4)

The temperature dependence of the EPR spectrum of Cu2+ in the range 293–393 K exhibits a low-spin (S=0) to high-spin (S=2) transition of the Fe2+ ions, with hysteresis (Tc↑=363 K,Tc↓=343 K). At 103 K, the principal values of theg and hyperfine tensors of Cu2+ ions are revealed by hyperfine structure.

Vanishing exchange coupling between dissimilar copper ions in CuK2(SO4)2.6H2O as evidenced by X-band electron paramagnetic resonance

Abstract We have studied the electron paramagnetic resonance (EPR) of a single crystal of the CuK 2 (SO 4 ) 2 ·6H 2 O Tutton salt when rotated around a and c ∗ axes, and around β(≡ b ) and γ axes at X-band and room temperature. When the static magnetic field lies in the ( a , c ) plane, both Cu 2+ ions are magnetically equivalent and EPR spectrum reduces to a singlet whose shape varies considerably, from a quasi-Lorentzian lineshape following the c axis, to an intermediate (between Gaussian and Lorentzian) lineshape following the a axis. For other orientations of the static magnetic field, the two Cu 2+ ions are magnetically unequivalent and the EPR spectrum is a moreorless resolved doublet. When the static magnetic field is parallel to a tetragonal axis, the EPR spectrum is a perfectly resolved doublet, revealing a very weak exchange interaction ( J ′) between dissimilar ions. For the compound under study, the computer simulation of the EPR spectrum was carried out by adopting a convolution for the shape of the elementary line. This reveals that the exchange interaction between dissimilar ions is vanishingly small at room temperature at an accuracy of 0.0002 cm −1 .

Electron paramagnetic resonance and spin-lattice relaxation in two-dimensional systems

The anisotropies of the EPR linewidth andg-factor were investigated in two-dimensional molecular composites of the type [NH3−R−NH3]MX4. Measurements were performed both in single crystals and powders over the temperature range 4.2–290 K. The spin-lattice relaxation timeT1 was measured using the modulation method, as a function of temperature. The samples exhibit different structures and coupling interactions, according to the nature of the halogen X, the metal M and the organic radical R. We have analysed the influence of these parameters on spin behavior by studying the samples [NH3−(CH)n−NH3]MX4 with M=Mn, Cu; X=Cl, Br, andn=2, 3, 4, 5. When R is constituted by molecules with unsaturated bonds, these materials can be considered as excellent matrices for selective polymerisation reactions by irradiation or thermal processing. We have performed EPR measurements on the heated complex of propargylamine and cadmium chloride [HC∈C−CH2−NH3]2CdCl4. The obtained data are interpreted taking into account the strong exchange interaction and the various coupling interactions in the samples. The thermal dependences ofT1 are explained by means of the Bloembergen and Wang three-reservoir model. The data whow spin diffusion when the metal is manganese, and an antisymmetric exchange interaction modulated by phonons in the case of copper. The nature of the halogen plays an important role in spin dynamics and namely in spin-lattice relaxation. The results obtained on [HC∈C−CH2−NH3]2CdCl4 after heating under vaccum show the creation of many paramagnetic centers due to the vanishing of triple bonds and the occurrence of a strong exchange interaction.