F.K. Lotgering

Magnetic Properties and Electrical Conduction of Copper‐Containing Sulfo‐ and Selenospinels

CuCr1+xRh1−xSe4 with −1 < x ≦ 1 and CuCr1+xTi1−xS4 with 0 < x ≦ 1 show p‐type metallic conduction and are ferromagnetic, while CuCr1+xTi1−xS4 with x<0 show n‐type metallic conduction and no magnetic ordering at 4°K. From the magnetic and electrical properties, which indicate that Cu is monovalent in these compounds, a model for the electronic structure of copper‐containing sulfospinels is deduced. Ferromagnetism and p‐type conduction are attributed to holes in a broad valence band, n‐type conduction to electrons in a broad conduction band. Susceptibility measurements on CuCo2S4, CuTi2S4, CuCoTiS4, CuRh2S4, and CuRh2Se4 are reported. The main contribution to the temperature‐independent susceptibility observed is attributed to the Van Vleck susceptibility of Co3+ or Rh3 ions in the low spin state t26.

Magnetic anisotropy and saturation of LaFe12O19 and some related compounds

The saturation magnetization σs and uniaxial anisotropy constants K1 and K2 are measured on a polycrystalline, crystal-oriented sample of LaFe12O19 contaminated with known amounts of Fe3O4 and LaFeO3. K1 and K2 increase strongly with decreasing temperature and the value of K1 = (19–24) × 105 erg/g at T = 0 shows that the substance is considerably more anisotropic than BaFe12O19 (K1 ≏ 8·5 × 105 erg/g) at low temperatures. The σs-T curve is more convex than that of BaFe12O19, so that σs is 11 per cent higher at room temperature but lower at T = 0. The value σs(T = 0) = 96·2 G cm3/g (19·2 μB/molecule) and the anisotropic behaviour are attributed to the presence of 1 Fe2+/molecule occupying the octahedral 2a sites in the magnetoplumbite lattice and having a uniaxial anisotropy of 10–15 cm−1/ion. From measurements on polycrystalline, crystal-oriented samples of BaFe10·8Fe2+0·6Ti4+0·6O19 and BaFe10·5Fe2+1·0Sb5+0·5O19 it was found that, in comparison with LaFe11Fe2+O19, σs (T = 0) is smaller and K1 is much smaller and much less temperature-dependent. The difference in anisotropic behaviour is attributed to a different distribution of the Fe2+ ions among the lattice sites due to the effective positive charge of the Ti4+ and Sb5+ ions.

Magnetic properties, conductivity and ionic ordering in Fe1−xCuxCr2S4

The magnetic and electrical behaviour of Fe1−xCuxCr2S4 recently measured by Haacke and Beegle is interpreted on the basis of a model for sulphospinels given by Lotgering and Van Stapele, with the additional assumption that the Fe2+ levels are situated in the energy gap between the valence and conduction band. Experimental data supporting this interpretation are given. Ionic ordering on A-sites in Fe12Cu12Cr2S4 and ln12Cu12Cr2X4 (X = S or Se) is found by means of Mossbauer spectroscopy and X-ray diffraction.

Permanent‐magnet material obtained by sintering the hexagonal ferrite W=BaFe18O27

The hexagonal ferrite W=BaFe2+2Fe3+16O27 has a saturation magnetization about 10% higher than the widely used permanent‐magnet material M=BaFe3+12O19 and the anisotropy field is about equal. This opens up the possibility of using W as a permanent‐magnet material with a higher remanence Br and energy product BHm. This paper describes the sintering of W having a slight deviation from the stoichiometric composition and containing a trace of SiO2 at, e.g., 1220 °C in a reducing atmosphere to 92–95% of the x‐ray density and containing well‐oriented grains of 2–8 μm. Samples with coercive forces Hc=1.5–2.1, Br=4.4–4.7, and BHm=3.7–4.3 (B in kG, H in kOe) were obtained, which may be compared with Hc=2.1, Br=4.0, and BHm=3.4 found from our own measurements on high‐quality commercial M. We found surprisingly that the pickup of a trace iron during grinding increases the sintering density considerably, and this is therefore essential for the preparation of useful material.

Mössbauer effect in LaFe12O19

Abstract Mossbauer spectra of 57 Fe in LaFe 12 O 19 show that the substitution of La 3+ for Ba 2+ or Sr 2+ in X Fe 12 O 19 is associated with a valency change of Fe 3+ to Fe 2+ at the 2 a or 4 f 2 site. Temperature dependences of the hyperfine fields at the various sublattices are given.

Anisotropy of hexagonal ferrites with M, W and Y structures containing Fe3+ and Fe2+ as magnetic ions

The ferrimagnetic saturation moment and hexagonal anisotropy constant K1 have been measured at 4K on a Zn2Y single crystal and on polycrystalline BaFe2+2W and SrFe2+2W samples. The moment of Fe2W is in agreement with a collinear spin coupling and with the known site occupation for the Fe2+ ions. The moment of Zn2Y is 9% lower than the value for a collinear configuration. The uniaxial anisotropy of Fe2+ in hexagonal ferrites is discussed and compared with that of Co2+. No noticeable Fe2+ anisotropy is found in Fe2W in contrast to LaM = LaFe2+Fe3+11O19, in which the Fe2+ anisotropy is strong. The difference is attributed to the symmetry difference of the sites occupied by the Fe2+ ions in both compounds. The current theory does not satisfactorily explain the anisotropy and quadrupole splitting of Fe2+ in LaM. From this it is concluded that admixing of 5E states and (or) the influence of a dynamical Jahn-Teller effect cannot be neglected. The dipole-dipole anisotropy is computed for the M, W and Y structure and some deviation from the literature data is found. Using these results, a mean anisotropy of 1.3 to 2.3 cm−1 per Fe3+ ion is found for the three structures.

Electron exchange between Fe2+ and Fe3+ ions on octahedral sites in spinels studied by means of paramagnetic Mössbauer spectra and susceptibility measurements

Abstract Mossbauer spectra were obtained of the paramagnetic spinels Zn 2+ | Zn 2+ (1−x) 2 Ti 4+ (1+x) 2 Fe 3+ (1− x) Fe 2+ x | O 4 and susceptibilities were measured. The strong difference between the paramagnetic Fe 2+ and Fe 3+ spectrum, due to the different quadrupole splitting, is used for the distinction between the two species. At 300 K a superposition of the Fe 3+ and the Fe 2+ spectra is found for most of the iron and, in addition, some continuous absorption. The latter is strongest for equal Fe 3+ and Fe 2+ concentration (x = 1 2 ) while it disappears towards the end members (Fe 3+ only or Fe 2+ only) as well as with decreasing temperature (between 78 and 200 K). From this it is concluded that it arises from thermally activated electron exchange, the frequency of which passes a “critical” value of ∼10 8 sec −1 for increasing temperature. Paramagnetic susceptibilities are found to obey a Curie-Weiss law down to low temperatures. From the dependence of the asymptotic Curie temperature on the composition the magnetic interaction parameters J 11 = −1.4 K, J 22 = −3.3 K and J 12 = + 1.6 K for the Fe 3+ Fe 3+ , Fe 2+ Fe 2+ and Fe 3+ Fe 2+ interactions are derived. The experimental results are discussed in terms of a hopping model with an activation energy q ∼- 0.12eV and a non-equivalence of the octahedral sites expressed by a varying potential energy difference U 0 between neighbouring sites. The continuous absorption at 300 K for x = 1 2 is attributed to about 17% of the iron on sites with U 0 running from 0 to ≅−0.06 eV. The ferromagnetic Fe 3+ , Fe 2+ interaction ( J 12 ) is attributed to electron transfer from localized Fe 2+ ions to Fe 3+ neighbours via a transfer integral b of the order of 0.05 eV. The magnitudes of J 12 and b are tentatively explained.

Valencies of manganese and iron ions in cubic ferrites as observed in paramagnetic Mössbauer spectra

Abstract The ground state of the ionic system Mn2++Fe3+Mn3++Fe2+ on octahedral sites in a spinel lattice is investigated on paramagnetic materials with specially chosen compositions. The valencies are established by means of the very different Mossbauer spectra of divalent and trivalent iron. Divalent iron is found to be unstable in the presence of trivalent manganese contrary to Simsas widely accepted model. A sample of ZnFe1·75Ti0·25O4 shows at room temperature an Fe3+, Fe2+ spectrum, in which divalent and trivalent iron are indistinguishable, and at 78°K the superposition of a similar spectrum and a ferrous spectrum.

Trivalent and tetravalent chromium in the seleno-spinel series CuCr0.3Rh1.7−xSnxSe4

Abstract The Curie constant per gramatom chromium varies from 1.03 for x = 0 to 1·86 for x = 1 in the mixed crystal series CuCr 0.3 Rh 1.7− x Sn x Se 4 . This is interpreted as a transition from tetravalent to trivalent chromium. The ferromagnetic ordering occurring for x = 0 is found to be destroyed rapidly by Sn substitution.

Mössbauer spectra of iron-chromium sulphospinels with varying metal ratio

Abstract Samples of the sulphospinel FeCr 2 S 4 with varying Fe/Cr ratio around the nominal composition have been investigated with Mossbauer spectroscopy. The spectra with the narrowest lines, as well as a γ-type peak in the specific heat-temperature curve at the low temperature transition previously described, were obtained for an overall composition with a small Fe deficit. The broadening often observed in Mossbauer spectra of FeCr 2 S 4 samples is attributed to Fe 2+ on octahedral sites in the spinel phase, resulting in an electric field gradient at the Fe 2+ ions on the tetrahedral sites. The paramagnetic Mossbauer spectrum of Fe 1.06 Cr 1.94 S 4 is in accordance with this interpretation.