T.H. Jacobs

Magnetic and crystallographic characteristics of rare-earth ternary carbides derived from R2Fe17 compounds

Abstract We have studied the magnetic properties of the rhombohedral R 2 Fe 17 C compounds with R = Ce, Pr, Sm, Gd, Tb, Dy, Ho or Y and the hexagonal R 2 Fe 17 C compounds with R = Er, Tm or Lu. For all compounds the lattice parameters were determined. The Curie temperatures were found to be considerably enhanced with respect to the C-free counterparts. The magnetic anisotropy of the R 2 Fe 17 C compounds was studied on magnetically aligned powders in field strengths up to 35 T. The rare-earth sublattice anisotropy is much stronger in R 2 Fe 17 C than in R 2 Fe 17 , leading to an easy c -axis anisotropy in Sm 2 Fe 17 C even at room temperature. The Curie temperatures and the high field data obtained at 4.2 K were analysed in terms of a mean field model.

Magnetic interactions in R2Fe17−xAlx compounds ( R = Ho, Y)

Abstract The temperature and field dependence at 4.2 K of the magnetization of Y2Fe17−xAlx and Ho2Fe17−xAlx ( x ≤ 10) compounds has been studied. Coupling constants have been derived by analyzing both the temperature dependence and the field dependence of the magnetization with a mean-field model. The results are compared to each other. The influence of the Ho-Ho interaction is considered. There is no pronounced temperature dependence of the interaction constant between the rare-earth and transition-metal sublattice.

Magnetic properties of the ternary carbides Tm2Fe17Cx

Abstract We have investigated the crystallographic and magnetic properties of the ternary carbides Tm2Fe17Cx by means of X-ray diffraction, 57Fe Mossbauer spectroscopy, 169Tm Mossbauer spectroscopy and magnetic measurements. It is shown that small amounts of carbon raise the Curie temperature in Tm2Fe17Cx from below room temperature to about 500 K, at the same time increasing the average Fe moment. Important conclusions regarding the rare-earth sublattice anisotropy were derived from the quadrupole splitting of the 169Tm Mossbauer spectra and from the strong concentration dependence of the spin reorientation temperature in Tm2Fe17Cx.

MAGNETOCRYSTALLINE ANISOTROPY AND MAGNETIC PHASE-TRANSITION IN R2FE17CX-BASED ALLOYS

Abstract Investigations into the magnetic and crystallographic properties of several ternary R 2 Fe 17 C x compounds (R = Sm, Tm) are reported. The following experiments were performed: measurements of the anisotropy field using the SPD (Singular Point Detection) technique, determination of the easy axis and magnetic phase transitions by X-ray diffraction on magnetically aligned samples and measurements of the ac initial susceptibility. A large uniaxial magnetocystalline anisotropy was found in several of the Sm 2 Fe 17 C x compounds. Spin reorientations were detected in Tm 2 Fe 17 C x and will be discussed on the basis of the CEF theory.

Intrinsic magnetic properties of R2Fe17CyNx compounds: (R=Y, Sm, Er, and Tm)

Samples of R2Fe17C(y)N(x) (R = Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17C(y)N(x) irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17C(y)N(x) are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17C(y)N(x) compounds at room temperature because some N was lost during the heating to T(c). In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17C(y)N(x) when y < 1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17C(y)N(x) are higher than that of Sm2Fe17N(x). Indications of a magnetic phase transition were found also in Sm2Fe17C0.7N(x) and Sm2Fe17C0.9N(x).

57Fe and 169Tm Mössbauer effect and magnetic properties of Tm2Fe15M2 (M ≡ Al, Ga, Si)

Abstract The Curie temperatures Tc and spin reorientation temperatures TSR were found to increase strongly when aluminium, gallium or silicon was substituted for iron in Tm2Fe17. By means of 57Fe Mossbauer spectroscopy we were able to show that there is no preferential substitution of these elements into the 4f or dumbbell site. The 169Tm Mossbauer effect was studied in Tm2Fe15Si2. We found that silicon substitution has led to a marked shift of the second-order crystal field parameter A02 to more negative values, the corresponding increase in K1 explaining the shift of TSR towards higher temperatures.

Thermal expansion anomalies and spontaneous magnetostriction in R2Fe17Cx intermetallic compounds

Abstract The influence of interstitial solution of carbon on the spontaneous magnetostriction of R 2 Fe 17 ( R = Y , Tb , Tm ) has been investigated by means of X-ray dilatometry in the temperature range 5–800 K. The basal-plane linear deformation λ a at 5 K increases with increasing C content for R = Y and Tb, whereas the uniaxial deformation λ c remains practically the same. In the Tm compounds both deformations increase with increasing C content. The volume effect ω s increases from 1.1 × 10 -2 to 2.0 × 10 -2 in the Y series, reaching similar values as in R 2 Fe 14 B compounds, despite the fact that the average magnetic moment of Fe atoms μ Fe does not change with increasing C content. Similar behaviour is found in the Tb and Tm compounds. For all compounds studied, the temperature dependence of ω s is in a good agreement with μ 2 Fe ( T ) in Y 2 Fe 17 , which points to a dominating contribution of the Fe sublattice to the spontaneous magnetostriction.

Magnetic properties of Er2Fe14−xMnxC

Abstract The magnetic properties of the compounds Er 2 Fe 14− x Mn x have been determined by means of X-ray diffraction, magnetic measurement and 57 Fe Mossbauer spectroscopy. Magnetization measurements made in fields up to 38 T showed that the saturation moment exhibits a minimum at a Mn concentration corresponding to about x =2, due to the antiparallel coupling between the R and 3d sublattice magnetizations the latter becoming strongly reduced with increasing Mn concentration. The reduction of the 3d-sublattice magnetization is accompnied by a corresponding reduction in the 57 FE hyperfine fields and by a lowering of the Curie temperature and the spin orientation temperature. Most of the compounds investigated have a break in the magnetization curves measured at 4.2 K which is interpreted as the onset of decoupling between the antiparallel rare earth and 3d sublattices. A mean field analysis was used to derive the intersublattice molecular field coefficient n RT .

Magnetic behaviour of Al and Mn substituted Gd2Fe17 compounds

Abstract We investigated the magnetic properties of Gd 2 Fe 17− x M x compounds with M=Al or Mn. We performed low-field magnetization measurements to obtain values for the Curie temperature and high-field magnetization measurements ( B ≤38T) on powdered samples that were free to orient themselves in the field applied to obtain values for the saturation magnetization. From the high-field slope of the magnetization we derived values for the R–T coupling constants n RT using the mean-field approximation. We find that substitution of Al or Mn, although it reduces the Fe magnetization in different ways, has only little influence on the intersublattice coupling strength.

Magnetic and crystallographic structural changes in ternaries of the type Er2Fe17Cx

The compounds Er2Fe17C(x) with x = 0, 0.4, 0.6, 0.8, 1.0 and 1.5 were investigated by means of AC initial susceptibility measurements and X-ray diffraction. Spin reorientation transitions, marking the changes in easy magnetization direction (EMD) with increasing temperature from parallel to the c-axis to perpendicular to the c-axis, were observed in Er2Fe17C(x) for x greater-than-or-equal-to 0.8. From X-ray diffraction measurements it was found that the crystal structure of Er2Fe17C(x) changes with increasing carbon concentration from hexagonal (Th2Ni17-type) to rhombohedral (Th2Zn17-type). In samples (x > 0.8) in which both crystallographic forms were found we detected two different spin reorientation temperatures. The difference in spin reorientation temperatures between the two crystallographic forms was explained on the basis of differences in the crystal structure and concomitant differences in crystal field effects.