12. STRUCTURE AND MAGNETIC PROPERTIES OF RFe2HX NANOHYDRIDES OBTAINED BY HYDROGEN-INDUCED AMORPHIZATION


A.Ye. Yermakov, N.K. Zajkov, N.V. Mushnikov, V.S. Gaviko, V.V. Serikov, N.M. Kleinerman
Institute for Metal Physics
Ural Branch of the Russia Academy of Sciences
18 S.Kovalevskaya Str., GSP-170
Ekaterinburg, 620219, Russia
e-mail: yermakov@hotmail.com or strictor@ifm.e-burg.su

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.

Table 12.1
Hydrogen-induced Amorphization Regimes of Obtained RFe2Hx Hydrides

Compound

T, ° C

P, MPa

t, hours

am-YFe2H3.6

200

1.1

4

Am-GdFe2H3.4

240

1.1

6

am-TbFe2H2.5

330

0.2

1

Am-DyFe2H3.6

250

1.6

6

Am-HoFe2H4.1

250

1.35

5

am-ErFe2H4.1

240

1.3

7

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 GdFe2Hx (Fig. 12.5). Hence, it is possible to assume an increase of Fe in the process of hydrogen-induced amorphization from 1.6 to 2.2 B. The distribution function of hyperfine fields for the amorphous GdFe2Hx (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 alpha-Fe are formed (Figures 12.3 and 12.8).


Fig. 12.1. Differential thermal analysis-curve of TbFe2H4.2 sample in hydrogen atmosphere at the heating rate of 20 K min-1 (Yermakov 1993).


Fig. 12.2. X-ray diffraction patterns of HoFe2Hx 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 (TbFe2H4.2), 600 K (am-TbFe2H2.5), 800 K (TbH2 + alpha-Fe) and amorphous phase obtained by mechanical grinding (am-TbFe2) (Yermakov 1993).

Table 12.2
Magnetic Properties of Crystalline and Amorphous Hydrides

Compound

Tcomp (K)*

Tc (K)**

m S (B)***

c-YFe2H4

am-YFe2H3.6

-

-

306

398

3.66

3.90

c-GdFe2H4

am-GdFe2H3.4

110

50

388

420

4.10

1.64

c-TbFe2H4.2

am-TbFe2H2.5

160

50

303

400

4.18

0.18

c-DyFe2H4

am-DyFe2H3.6

150

30

385

395

4.9

0.46

c-HoFe2H4.5

am-HoFe2H4.1

60

30

387

343

2.35

0.93

c-ErFe2H3.7

am-ErFe2H4.1

42

15

280

380

5.60

0.5



*Tcomp - compensation temperature
**Tc - Curie temperature
***µS - spontaneous magnetic moment


Fig. 12.4. Magnetization curve of am-RFe2Hx 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.

Table 12.3
The Intersublattice Exchange Coupling Constant nGdFe and nDyFe Obtained From µR(T) Using One and Two Constants and From the Slope of the Magnetization Curve in Field 1-2 T

 

nRFe (B)

 

From m R(T)

From m (H)

Compound

One Parameter

Two Parameters

 
   

n1

n2

 

am-GdFe2H3.4

11.2

2.40

23.5

3.4

am-DyFe2H3.6

8.9

3.85

16.0

2.1


Fig. 12.7. Temperature dependence of Gd magnetic moment: experimental data (open circles; best fit using the following equations: µR(T)/ µR(0) = BJ (gJBHm/kBT), Hm(T) = nGdFeµFe(T) (where nGdFe is the intersublattice exchange coupling constant, BJ is the Brillouin function) (dotted line); best fit using a rectangular distribution of nGdFe (dashed line); best fit using two nGdFe 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

In the process of hydrogen-induced amorphization, the structure of RFe2 compounds decomposes into areas enriched in R, not participating in the exchange interaction with Fe, and areas enriched in Fe. Two exchange coupling constants differing by an order of magnitude are proposed. The following are several characteristics of hydrogen-induced amorphization:

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).



Published: August 1997; WTEC Hyper-Librarian