H.M. van Noort

Properties of metastable ternary compounds and amorphous alloys in the Nd-Fe-B system

Abstract We have studied the crystallization behaviour of numerous amorphous alloys of iron-rich composition in the Nd-Fe-B system. Increasing the boron(iron) concentration leads to an increase (decrease) of the thermal stability. Increasing the neodymium concentration gives rise to a strong initial increase in the thermal stability but leads to a decrease at higher neodymium concentrations. Two novel ternary compounds were observed in the iron-rich corner of the Nd-Fe-B system. The compound Nd2Fe23B3 is cubic and orders magnetically at 655 K. The compound NdFe12B6 is hexagonal and its Curie temperature is 230 K. Both compounds are metastable and transform to equilibrium phases at temperatures near or above 1000 K.

Optical, magneto-optical and mössbauer spectroscopy on Co3+ substituted cobalt ferrite Co2+Fe2−xCo3+xO4(0 ⩽ x ⩽ 2)

Abstract The magneto-optical spectra of Co 1+ x Fe 2− x O 4 show with increasing Co 3+ content an increasing intensity of the 4 A 2 − 4 T 1 ( F ) and 4 A 2 − 4 T 1 ( P ) transition of Co 2+ at 0.8 and 2.0 eV. A decrease in the Co 2+ -Fe 3+ charge transfer transitions on octahedral sites is found. In the optical spectra a strong increase in optical absorption is found with dominant transitions at 0.8, 1.6 and 2.6 eV due to Co 3+ crystal field transitions on octahedral sites and a Co 2+ -Co 3+ charge transfer. Conversion Electron Mossbauer Spectroscopy has been used to determine the cation distribution in the surface layer of the samples. The results indicate a shift of Co 2+ from octahedral to tetrahedral sites when Co 3+ is substituted in CoFe 2 O 4 . This results in enhanced optical absorption, enhanced magneto-optical effects and a lower Curie temperature.

Phase relationships, magnetic and crystallographic properties of NdFeB alloys

Abstract From a study of the isothermal sections of the Nd-Fe-B phase diagram at 900 °C (neodymium-poor alloys) and 700 °C (neodymium-rich alloys) it was found that the phase Nd2Fe14B has no or at best only a very small homogeneity range. Results of a preliminary structure determination are given for Nd2FeB3. Magnetic properties and lattice constants were determined for the compounds NdFe4B4 and Nd2FeB3 and their gadolinium counterparts.

57Fe Mössbauer spectroscopy study of the magnetic properties of R2Fe14B compounds (R=Ce, Nd, Gd, Y)

57Fe Mossbauer spectra were taken at 25 and 293 K on four representative specimens of compounds of the series R2Fe14B (R=Ce, Nd, Gd, and Y). In all these cases it proved possible to analyze the spectra in terms of six subspectra associated with the six crystallographically nonequivalent Fe sites in the tetragonal R2Fe14B unit cell. Assuming a linear relation between the observed hyperfine fields and magnetic moments it follows from our analysis that, although the average Fe moment in R2Fe14B is equal to that of α‐Fe, there are Fe sites that give rise to moments much higher and much lower than the average value of 2.2 μB/Fe.

Magnetic and structural properties of Nd2Fe14B, Th2Fe14B, Nd2Co14B and related materials

Abstract Several alloys close in composition to Nd 2 Fe 14 B and Nd 2 Co 14 B were investigated by standard metallography. It was found that the tetragonal Nd 2 Fe 14 B-type structure applies also to Nd 2 Co 14 B. This structure does not tolerate significant deviations from stoichiometric composition. Annealed off-stoichiometric alloys are composed of impurity phases, the presence of which can have an unfavourable effect on the magnetic properties. The Curie temperature of Nd 2 (Fe 1− x Co x ) 14 B was studied as a function of composition in the range 0 ⩽ x ⩽ 0.6. The compound Th 2 Fe 14 B also crystallizes in the tetragonal Nd 2 Fe 14 B-type structure. Its Curie temperature is 480 K and the room temperature anisotropy field is of the order of 3 T. From a study of the 57 Fe Mossbauer spectra of Nd 2 Fe 14 B and Th 2 Fe 14 B it was deduced that different crystallographic iron sites give rise to different sizes of the iron moments, the average iron moment in Th 2 Fe 14 B being only slightly lower than that in Nd 2 Fe 14 B.

57Fe Mössbauer investigation of ternary compounds of the R2Fe14B type

Abstract The magnetic properties of compounds of the R2Fe14B type (R  Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Lu) have been systematically investigated by means of 57Fe Mossbauer spectroscopy. It is shown that relatively large differences in effective hyperfine fields are present at the siv crystallographically non-evuivalent iron sites and that these differences persist through the whole series. All siv hyperfine fields show a slight tendency to increase towards the middle of the lanthanide series. The magnetic phase transition in Er2Fe14B at about 330 K is shown to be due to a change in the direction of easy magnetivation from perpendicular to the c avis (T ⩽ 330 K) to parallel to the c avis (T ⩾ 300 K).

Magnetic properties and 57Fe Mössbauer effect of ErFe4B, TmFe4B and LuFe4B☆

Abstract Using X-ray diffraction the compounds ErFe 4 B, TmFe 4 B and LuFe 4 B were found to crystallize in the hexagonal CeCo 4 B-type structure which, in the case of ideal atomic ordering, can be characterized by two different 3d sites and one B site. The lattice constants of all compounds were determined. From 57 Fe Mossbauer spectroscopy it was derived that there are a fair number of Fe atoms occupying the B site and vice versa. Magnetic measurements showed that these materials have comparatively high Curie temperatures and high magnetocrystalline anisotropies, the easy magnetization direction being parallel to the hexagonal c axis. The antiparallel coupling between the Fe moments and the R moments in ErFe 4 B and TmFe 4 B leads to a temperature dependence of the magnetization exhibiting a compensation temperature.

Mössbauer study of Cu-Fe composition-modulated thin films

Abstract Ion beam sputtered composition-modulated Cu-Fe thin films of equiatomic composition with wavelengths from 1.2 to 8 nm were studied by 57 Fe conversion electron Mossbauer spectroscopy (CEMS) at room temperature. The spectra are characterized by those from pure α-iron, on which are superposed spectra due to Fe atoms with Cu atoms as nearest neighbours. These latter spectra increase in relative intensity with smaller modulation wavelengths. The experimental spectra can be fitted with a model in which the Cu-Fe interfaces have a composition profile extending over only three atomic planes.

Investigation of luminescent Eu-doped sesquioxides Ln2O3 (Ln = In, Sc, Y, La, Gd, Lu) and some mixed oxides by 151Eu Mössbauer spectroscopy

Abstract Luminescent Eu-doped sesquioxides Ln 2 O 3 (Ln = In, Sc, Y, La, Gd, Lu) and some mixed oxides have been investigated with 151 Eu Mossbauer spectroscopy. The spectra are interpreted by using crystallographic and luminescence data. Effects of Eu concentration, substitution and crystal structure on the isomer shift are discussed in terms of Eu-O distance, coordination number and covalency. For the compounds with the cubic modification, no preferential site occupation by Eu 3+ ions on the two available crystallographic sites can be concluded.

The cation distribution of CoFe2−xAlxO4 as determined by conversion electron mössbauer spectroscopy

Abstract The conversion electron Mossbauer spectra of CoFe2−xAlxO4 (x = 0.1, 0.3, 0.6, 0.8 and 1.0) were measured at room temperature. Cation distributions for all samples were derived and compared with the cation distributions as obtained for powdered samples from normal transmission measurements. It is shown that the fraction of Co2+ ions at the tetrahedral lattice sites increases linearly with the aluminium content, although a difference between powders and bulk samples is observed.