B. Kalska

Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mossbauer spectroscopy study

Abstract The extraction and insertion of lithium in solid-state synthesized LiFePO 4 has been followed by in situ X-ray diffraction and Mossbauer spectroscopy in ‘coffee-bag’ cells of type 〈Li-metal | liq. el. | LiFePO 4 〉 during the first cycle. Two-phase Rietveld refinement of the X-ray diffractograms gives the triphylite (LiFePO 4 ) to heterosite (FePO 4 ) phase-ratios as charging and discharging of the cell proceeds. The Fe 3+ /Fe 2+ ratios at each step, as measured by Mossbauer spectroscopy and X-ray diffraction, were in good general agreement with the amount of lithium calculated from the charge passed through the cell; there was, however, a slight tendency for the Mossbauer technique to record a higher concentration of the oxidized phase. The possible existence of a thin interface region at the phase boundary is discussed.

Thermal Stability of LiFePO4 ‐ Based Cathodes

The capacity and cyclability of solid-state synthesized LiFePO4-based laminate cells of type have been studied at 23, 40, and 60 degrees C. Larger capacities were obtained for cells cycled at the elevated temperatures. No evidenc

Lithium insertion into rhombohedral Li3Fe2(PO4)3

Abstract The lithium insertion process has been studied in rhombohedral Li 3 Fe 2 (PO 4 ) 3 (NASICON-type structure) by electrochemical and Mossbauer spectroscopic methods. The form of the discharge curve and the effective discharge capacity is found to depend on the mode of cathode preparation: two plateaus (one clear at ∼2.80 V and one less distinct at ∼2.65 V vs. Li/Li + ), corresponding to ca. 1.5–1.6 inserted lithium ions during the first cycle, are seen after more extreme grinding; milder treatment gave only the 2.8 V plateau and ca. 1.1 inserted lithium ions. Mossbauer spectra for the more extensively ground material show the Fe environments in R-Li 3 Fe 2 (PO 4 ) 3 to be highly symmetric; only a very narrow doublet with small quadrupolar splitting is observed, and the two crystallographically independent Fe-atoms cannot be distinguished. As lithium insertion proceeds, two doublets (average intensity ratio 1.5:1) appear, which can be assigned to two Fe 2+ sites. The average intensity ratio of 1.5:1 suggests that the extra lithium ions occupy sites closer to one of the Fe-atoms.

Interface roughness/intermixing and magnetic moments in a Fe/Co(001) superlattice

The superiority of a local probe method over diffraction methods in determining the interface details is demonstrated by a comparison between Fe/Co and Fe/V superlattice data. From Mossbauer spectra, the higher interface quality in the Fe/Co superlattice was evident although the x-ray diffraction data were similar in the two cases. In fact, by comparison of the details of the Mossbauer spectrum for the Fe/Co case with published values of the magnetic hyperfine field for iron in cobalt no detectable roughness/intermixing was found. From the variation of the iron magnetic hyperfine field as a function of location of the iron atoms, with respect to the interface, the individual iron magnetic moments could be derived. The magnitudes of the magnetic moments thus obtained correlate well with recent calculated values.

Conversion electron Mössbauer spectroscopy studies of the magnetic moment distribution in Fe/V multilayers

The Fe hyperfine field distribution in Fe(x ML)V(y ML) as function of x and y has been determined. The samples were prepared in a ultra-high vacuum sputtering system with 57 Fe as a probe. Low- and high-angle x-ray diffraction and Mossbauer spectroscopy were used to determine the monolayer structure of the samples. The multilayer growth is not layer-by-layer, as found from the magnetic hyperfine field distributions. No Fe magnetic ordering is found above 133 K for multilayers with x 3 and y 14 in contrast to recently reported antiferromagnetic ordering. The average Fe magnetic moments as deduced from the average magnetic hyperfine fields changes in the sequence 1.2, 1.5, 1.7 to 2.0 µB layerwise in going towards the centre of a 10 ML film of Fe.

Magnetic anisotropy and magnetic fields in bcc Fe/Co (001) superlattices

Abstract Alternating layers of iron and cobalt were grown in the (001) direction on MgO substrates. The resulting superlattices, 15–120 repetitions of x ( x =4–8) monolayers (MLs) of iron combined with y ( y =3–8) MLs of cobalt were investigated using X-ray diffraction and Mossbauer spectroscopy at room temperature and at 10 K. The magnetic field direction was found to be in the basal plane (001) and along the [110] direction. The observed Fe hyperfine fields converted to magnetic moments gave the values 2.48, 2.39 and 2.24 μ B , respectively, for Fe at the interface layer, close to the interface and in the centre of the Fe sublayer (totally 5–6 MLs).

The antiferromagnetism of (Fe1−xMnx)3P, x⩾0.67, compounds

Abstract The structural and magnetic properties of (Fe 1− x Mn x ) 3 P compounds, in the concentration range 0.67⩽ x ⩽1, have been investigated by means of X-ray and neutron diffraction experiments, magnetisation measurements, Mossbauer spectroscopy and first principles electronic structure calculations. The magnetic ordering temperatures obtained from the different techniques were consistent with each other but different from previously published results. The magnetisation curves and neutron diffraction intensities can be explained by an antiferromagnetic ordering with a magnetic unit cell four times larger than the crystallographic unit cell. The magnetic moments are small as judged from neutron diffraction and Mossbauer spectroscopy results. The band calculations on the other hand suggest a competition between Pauli paramagnetism and magnetic solutions with large magnetic moments.

Magnetic ordering in a [Fe/Co]35 BCC film studied by nuclear resonant reflectometry

An analysis of the angular dependences of nuclear resonant reflectivity time spectra for different models of magnetic ordering in films reveals an ambiguity in the magnetization direction determined from spectra measured at one orientation of the sample. This analysis explains features in the spectra of the nuclear resonant reflectivity from a MgO/[Fe(6 ML)/Co(3 ML)]35/V (1 nm) film measured before and after sample rotation by 90° about the normal to the surface. It is shown that the spectrum measured only at one orientation of the sample determines only the effective azimuth angle of magnetization γeff. This does not exclude the occurrence of a domain structure, while the angle γeff does not correspond to the true direction of the preferred orientation of magnetization. The results of measurements at two orientations of the sample can be satisfactorily matched using a model that considers a coherent mixture of states with magnetization directed along the 〈110〉 axis (77%) and with a chaotic orientation of the magnetic hyperfine field Bhf in the film plane for the other nuclei.

Layer-resolved magnetic moments for Fe in a Co/Fe (0 0 1) superlattice

From the Mossbauer spectrum of a high-quality Fe/Co superlattice, the interface quality was ascertained by comparison of the details of the spectrum with the published value of the magnetic hyperfine field for iron in cobalt and no detectable roughness/intermixing was found. From the variation of the iron magnetic hyperfine field as a function of location of the iron atoms, with respect to the interface, the individual iron magnetic moments could be derived. The magnitude of the magnetic moments thus obtained correlate well with recently calculated values.

The influence of V and Fe thicknesses on the Fe magnetic exchange in Fe/V multilayers

Abstract The Fe hyperfine field distribution in Fe( x ML)/V( y ML) with x=5 and 10 and y=5,10 and 14 has been determined. A strong dependence in Fe magnetic field distribution as a function of thickness V as well as Fe layers has been found.