12. STRUCTURE AND MAGNETIC PROPERTIES OF RFe₂H_X NANOHYDRIDES OBTAINED BY HYDROGEN-INDUCED AMORPHIZATION

Amorphous hydrides obtained by alloy treatment in hydrogen under special conditions (hydrogen-induced amorphization) belong to a new and scarcely studied class. Amorphous hydrides, based upon RFe₂ (R is a rare earth element), which have a MgCu₂-type crystal structure and consist of two-sublattice ferrimagnets, have been synthesized (Table 12.1) and are discussed in this presentation.

The RFe₂ compounds under hydrogen treatment had a few exothermic peaks corresponding to structural exchange (Fig. 12.1). Thermally activated deterioration of the long-range crystallographic order and variation of the amorphous matrix parameters (width and position of the amorphous diffraction halo, Fig. 12.2) are shown to take place under a change of hydrogen treatment regimes. X-ray diffraction patterns of these alloys differ strongly from amorphous RFe₂ compounds obtained by usual methods (Fig. 12.3) and represent, as a rule, one broad halo placed in the vicinity of the most intensive line of the RH₂ phase.

During the amorphization process, both a weakening of the intersublattice exchange interaction (decreasing of compensation temperature) and a strengthening of the Fe-Fe exchange interaction (increasing of Curie temperature) occur (Table 12.2). Due to the crystal field symmetry distortion, a local uniaxial anisotropy is induced, and a canted magnetic structure is realized in a rare-earth sublattice, which results in a considerable magnetic structure deformation in the field, which, in turn, influences the magnetization curves (no ferrimagnetic saturation, Fig. 12.4, curve 3) and a great volume magnetostriction appears. It is also possible that a canted magnetic structure in the Fe-sublattice due to a non-uniform exchange interaction takes place. Mossbauer measurements show that the mean hyperfine field grows from 22.5 T for the initial compound to 33.4 T for the amorphous GdFe₂H_x (Fig. 12.5). Hence, it is possible to assume an increase of µ_Fe in the process of hydrogen-induced amorphization from 1.6 to 2.2 µ_B. The distribution function of hyperfine fields for the amorphous GdFe₂H_x (Fig. 12.6) is very extended owing to the fluctuation of the local interatomic distances and the nearest atomic distribution of Fe. The amorphous samples are characterized by two parameters of the intersublattice exchange R-Fe interaction differing by an order of magnitude (Table 12.3 and Fig. 12.7) and, most likely, consist of structural and chemically heterogeneous regions of nanocrystalline size (<10 nm). One of these parameters (the larger in value) characterizes the exchange inside the particles; the other (the smaller) is determined by the exchange at the interface. Upon hydrogen treatment, the diffusion of the metal atoms between the cluster and the interlayer takes place, and then a rare-earth hydride and -Fe are formed (Figures 12.3 and 12.8).

Fig. 12.1. Differential thermal analysis-curve of TbFe₂H_4.2 sample in hydrogen atmosphere at the heating rate of 20 K min^-1 (Yermakov 1993).

Fig. 12.2. X-ray diffraction patterns of HoFe₂H_x compounds, obtained at various treatment regimes. Temperature (°C): 1— 200, 2— 230, 3— 250, 4— 310, 5— 350 (P = 1.35 MPa, t = 5hs) (Zajkov 1997).

Fig. 12.3. X-ray diffraction patterns of crystalline TbFe₂ compounds; the phases obtained by hydrogenation at room temperature (TbFe₂H_4.2), 600 K (am-TbFe₂H_2.5), 800 K (TbH₂ + -Fe) and amorphous phase obtained by mechanical grinding (am-TbFe₂) (Yermakov 1993).

Fig. 12.4. Magnetization curve of am-RFe₂H_x samples with R = (1) Y, (2) Gd, and (3) Dy at 4.2 K (Zajkov 1997).

Fig. 12.5. Mossbauer spectra (a) at 88 K of parent GdFe₂, (b) hydrogenated at 240°C, and (c) hydrogenated at 300°C.

Fig. 12.6. Hyperfine field distribution functions of (a) parent GdFe₂, (b) hydrogenated at 240°C, and (c) hydrogenated at 300°C.

Fig. 12.7. Temperature dependence of Gd magnetic moment: experimental data (open circles; best fit using the following equations: µ_R(T)/ µ_R(0) = B_J (g_JJµ_BH_m/k_BT), H_m(T) = n_GdFeµ_Fe(T) (where n_GdFe is the intersublattice exchange coupling constant, B_J is the Brillouin function) (dotted line); best fit using a rectangular distribution of n_GdFe (dashed line); best fit using two n_GdFe values (solid line) (Mushnikov 1997).

Fig. 12.8. X-ray diffraction patterns of (a) parent GdFe₂, (b) hydrogenated at 240°C, and (c) hydrogenated at 300°C (Mushnikov 1997).

Conclusion

• Hydrogen-induced amorphization causes a noticeable increase of Fe-magnetic moment and Fe-Fe exchange interaction, and a decrease of the R-Fe exchange interaction.
• The magnetic state of amorphous hydrides is non-uniform and presumably represents canted structure in both R- and Fe-sublattices owing to a local uniaxial anistropy and/or a non-uniform exchange interaction.
• As the contributions of the areas with two different interactions R-Fe are nearly the same, it is possible to expect the characteristic size of the structural components to be of the order of a few nanometers (1-10 nm).

References

Mushnikov, N.V., N.K. Zajkov, V.V. Serikov, N.M. Kleinerman, V.S. Gaviko, and A.Ye. Yermakov. 1997. J.Magn. Magn. Mater. 167:93-98.

Yermakov, A.Ye., N.V. Mushnikov, N.K. Zajkov, V.S. Gaviko, and V.A. Barinov. 1993. Phil.Mag.B 68:883-890.

Zajkov, N.K., N.V. Mushnikov, V.S. Gaviko, and A.Ye. Yermakov. 1997. Phys. Solid State 39:908-912 (in Russian).