The effects of high-energy milling on GdFe2

~ ELSEVIER

Journal of Magnetism and Magnetic Materials 176 (1997) 272-278

,~

Journalof na;netlsm magnetic materials

The effects of high-energy milling on GdFe2 A. B i o n d o a, C. L a r i c a a'*, K . M . B . A l v e s a, A . P . G u i m a r S . e s b, E. B a g g i o - S a i t o v i t c h b a Universidade Federal do Espirito Santo, Departamento de Fisica, 29060-900 Vit6ria, ES Brazil b Centro Brasileiro de Pesquisas F~sicas, 22290-180 Rio de Janeiro, RJ Brazil

Received 26 December 1996; received in revised form 1 April 1997

Abstract With the aim of better understanding, the effects of severe milling on the magnetic and structural properties of crystalline compounds, samples of the intermetallic compound GdFe2 were milled up to 276 h under inert atmosphere and characterized by X-ray diffractometry (XRD) and MiSssbauer spectroscopy (MS). On milling GdFez a strong segregation of the elements Gd and Fe initially occurred, and the characteristic peaks of the initial compound GdFez in the XRD pattern are absent after 1 h of milling. MS and XRD results also show that the milled samples contain a mixture of three main components: the segregated elements, the initial compound and an amorphous alloy, with relative proportions that vary with milling time. At the longer milling times the process induces a solid-state reaction between these components leading to the formation of an amorphous solid solution of Gd and Fe. In these cases the M6ssbauer spectra taken at 300, 84 and 4.2 K were fitted with a combination of a set of peaks typical of metallic e-Fe and a continuous distribution of hyperfine magnetic fields that are ascribed to the amorphous Gd Fe phase. © 1997 Elsevier Science B.V. All rights reserved.

Keywords: Rare earth-intermetallic compounds; High-energy milling; Laves phase compounds; Hyperfine field

I. Introduction The high-energy milling method has been increasingly applied in the recent years to prepare amorphous, micro- and nano-crystalline materials [1, 2]. The milling may overcome some of the difficulties found in other methods when preparing materials in the metastable states, such as melt spinning, condensed vapor and sputtering, because

*Corresponding author. Fax: +55 27 335 2460; e-mail: [email protected].

it may start from elements insoluble in the liquid state or just off a compound stoichiometry, to get bulk powder disordered materials. Solid-state reaction induced by severe milling has proved to be a convenient method to prepare metastable powder samples starting from powders of pure chemical elements. Previous work done with YFe2 as the initial compound has produced, according to the X-ray diffraction patterns, an amorphous Y - F e phase after a 50 h milling process [1], without any signs of segregation of the elements as had been found by other authors [3]. In addition, our results with M6ssbauer spectroscopy (MS) and magnetic susceptibility measurements have indicated the

0304-8853/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 4 9 7-6

A. Biondo et al. / Journal of Magnetism and Magnetic Materials 176 (1997) 272-278

persistence of ferromagnetic ordering with a substantial reduction in the magnetic transition temperature, from 542 K to about 100 K. In the present work we have milled the intermetallic compound GdFe2 up to 276 h with the aim of obtaining metastable disordered alloys that were analyzed in terms of their structural and hyperfine field properties. GdFe2 has the same structure as YFe2, a C15-type Laves phase with lattice parameter ao = 7.396 A; it has a magnetic ordering temperature of 796 K and a ferrimagnetic alignment between Gd and Fe moments with a saturation moment of 3.55 laB/f.u. [4].

2. Experimental details Four buttons of the compound GdFe2 were prepared in an initial series of samples - here designated as series (a) by arc melting the components under argon atmosphere; a second series (b) was later prepared, also with four buttons. The alloy buttons weighted around 2.5 g each and were annealed in evacuated quartz tubes for 166h at 950°C. For each series, small pieces taken from the buttons were put together and then powdered under acetone for X-ray and M6ssbauer analysis, in order to confirm their structural and hyperfine field properties. To mill the compound, about 9 g of it were sealed in a hardened steel vial (hardness of HR~ = 65 __ 1), under argon atmosphere. The vial and the milling tool are made of the same material and have cylindrical shapes; the massive cylinder occupies 57% of the vial internal volume. The sealed vial was placed in a commercial vibrating-frame machine and the details of the milling procedures have been described previously [1]. Samples were collected always under an argon atmosphere at pre-established times of milling and they have been labelled according to their actual milling times plus alphabetic letters (a and b) for the different batches. As a precaution, they have been stored in a liquid-nitrogen container in order to avoid contamination and the possibility of a room-temperature annealing of the existing phases. Transmission MS was performed in all samples at room temperature; MS measurements were also

273

done at 84 K with a 12 mCi Rh/Co 57 radioactive source, and at 4.2 K in a liquid He bath cryostat with a 40 mCi source. The M6ssbauer data are referred to cx-Fe standard for isomer shift comparisons. The spectra were fitted using a computer program written by Brand [5]: the 0 h sample spectrum was fitted with the usual two magnetic sextets related to the two magnetic Fe-sites in the lattice [6], while for the spectra of the milled samples the fittings were performed with a combination of a distribution of hyperfine magnetic fields (Bh~) plus a crystalline sextet. This crystalline component is found to have hyperfine parameters values close to those of metallic iron. X-ray diffraction results were obtained with a conventional diffractometer using a Co tube. Some sample compositions were also checked by scanning electron microscopy (SEM) with a J E O L JSM-U3 working at 25 kV coupled to a Tracor Northern, model 5500 energy dispersive X-ray spectrometer (EDS). The second batch (b) of crystalline GdFe2 samples was prepared and milled up to 100 h with the same steps as before, except that more care was taken to keep the vial temperature closer to room temperature during the milling procedure, by applying forced air to it; there was also an improvement in the purity (99.999%) of the argon gas used in the vial and in the glove box for the manipulation of the samples.

3. Results and discussion Fig. la and Fig. lb show the X-ray diffraction results at several milling times for the two batches. At the initial time, the diffraction lines agree with those of the cubic structure of GdFe2 but, starting at the a01h sample, other lines appear indicating a drastic segregation effect. It is possible to identify in these results the formation of lines typical of metallic iron plus well-defined peaks of another crystalline phase that persist up to the longer milling times. The broad metallic iron peaks are clearly observed at 20 values of 52 and 77 °, corresponding to a 2.865 lattice parameter. The peaks at values of 20 about 36, 42, 61 and 73 ° of the other crystalline phase do not coincide with the peaks found in known G d - F e compound compositions, such as 1:3, 6 : 2 3 or 2:17; furthermore, they are not

274

A. Biondo et al. ,/'Journal of Magnetism and Magnetic Materials 176 (1997) 272 278 i

i

i

i

1

I a276h

~_=.~.aTOh

E

aOlh

(a)

I

20

610

I

4hO

810

I

20 (degrees)

,.a

=

I

20 (b)

I

I

I

I

40 60 20 (degrees)

I

I

80

Fig. 1. X-ray diffraction pattern of Gd/Fe milled samples for different milling times of batch (a) and (b) samples, respectively.

identified with any of the known Fe or Gd oxides that eventually could appear as impurities due to an improper manipulation of the samples. The angular positions of these peaks fit to a FCC structure with lattice parameter of 4.987 A, which may be attributed to a modified Gd structure of a

Gd-rich phase. This lattice parameter ao gives a closest plane distance in the FCC stacking sequence of 2.879 A, corresponding to ao, a value that agrees quite well with the value of 2.890 A for the closest plane distance (=c/2) in the H C P ~-Gd structure [7]. The FCC-Gd lattice parameter ao obtained in the present work is 8% smaller than the values reported for that structure observed in Gd thin film (150 thick) [8] and in very small particles (2 0 0) of Gd [9] but it is only about 0.8% larger than the FCC lattice parameter of milled Gd recently reported [10, 11]. Rare-earth (RE) metals are formed in the well-known sequence of related crystal structures, H C P - S m - t y p e - D H C P - F C C . It has also been observed that a RE metal can have its particular structure transformed to the subsequent one along the crystal structure sequence by adequate applied pressure. In the case of Gd (HCPtype), a FCC structure could only be achieved in the pure metal at pressures above 29 G P a [12]. On the other hand, a particular crystal structure in the RE crystalline structure sequence can be obtained by suitably alloying a RE metal with an element of different atomic size. Furthermore, the occurrence of modifications in the structure of Gd from H C P to FCC has been reported as a function of the reduction in the size of Gd particles, with a simultaneous decrease in its magnetic ordering temperature [9], the FCC structure occurring for particles sizes of about 200 A. This high-pressure cubic crystal structure has also been observed in several rare-earth metals as a consequence of the milling process; this effect was initially attributed by the authors to the high pressures developed during the milling process but, more recently, they considered the effect as due to a Gd reaction with undesired nitrogen, oxygen and hydrogen atoms, present in the vial atmosphere [10, 11]. In our case, where no contamination in the samples was detected within the resolution of the analysis employed, the observed FCC structure could be stabilized by the formation of very small clusters of segregated Gd. The presence of impurities (Fe, N and O) trapped in the FCC lattice as well as the high pressures developed in the milling process cannot be discharged as additional mechanisms of structural stabilization. The presence of Fe atoms in the FCC Gdrich phase may be associated to the small hyperfine

A. Biondo et al. / Journal of Magnetism and Magnetic Materials 176 (1997) 272 278

magnetic field components observed in the MS spectra, as will be discussed below. With the increase in the milling time the X-ray diffraction results show a systematic broadening of the crystalline lines of those two identified crystalline components, i.e., metallic iron and the Gd-rich phase. This effect of broadening is represented in Fig. 2 by the reduction of the grain size, as calculated by the Scherrer formula on the (1 1 1) FCC Gd-rich phase and (1 1 0) 0t-Fe reflection lines. No correction of systematic broadening was considered here, which may underestimate the grain-size values, however, the grain-size reduction undoubtedly dominates the curve behaviour in Fig. 2. Two spurious lines at 20 about 37.6 and 49.4 °, not observed in any other sample, are apparent in the sample a01 h XRD diffraction pattern. They may be attributed, respectively, to the compounds Gd3FesOlz and GdFeO3, whose presence could be related to the fact that this sample was accidentally exposed to air during its manipulation, after its collection from the vial. The X-ray results for the samples of the second

I

I

I

I

275

batch (b) have generally reproduced the pattern observed for batch (a) samples and do not include the oxide peaks observed in the a01 h XRD pattern. The results obtained from the fittings of the M6ssbauer spectra are presented in Table 1. Figs. 3 and 4 show the M6ssbauer spectra measured at various milling times, respectively, at 300 and 4.2 K, and the hyperfine magnetic field distribution (BhfD) obtained with the fitting model. The metallic-iron lines start to appear after 1 h of milling time in both series of samples. The relative absorption area of metallic-Fe lines varies along the series: at 4.2 K, it initially increases, reaching a maximum of 69% in the a25 h sample, and then decreases as the milling proceeds (Fig. 5). By subtracting the metallic-iron lines from each spectrum, a broad hyperfine magnetic field distribution (BhfD) is obtained for all samples, with a relative absorption area that increases after 25 h of milling as the metallic-iron component is reduced. In this distribution there is a contribution from low hyperfine magnetic field components (Bhr < 5 T) that shows up after 1 h of

I

I

~

l

I

I

o

I

e

(I 10) of ~-Fe

•

(I11)of Gd- fcc

i oQ-°

*

N

E--

I

o2! o

,\'\,

4 2

I

o

i

~

50

i

I

100

i

I

150

,

I

200

L

I

250

0

,

0

303

Milling time ( h ) Fig. 2. Scherrer grain size versus milling time of the segregated Gd rich and ~-Fe phases of milled GdFe2. No correction for systematic broadening was considered here.

I

-10

I

-5

I

I

I

I

0 5 Velocity (mm/s)

20

40

~r(T)

i

10

Fig. 3. M6ssbauer spectra and hyperfine magnetic field distributions at T = 300 K for samples of milled GdFe2 (batch (a)).

276

A. Biondo et al. / Journal of Magnetism and Magnetic Materials 176 (1997) 272 278

'

I

'

I

'

6

~=

Y'a 2

,

6

of'/, 6 25

-10

-5

0 5 Velocity (mm/s)

10

50

Bhf(T )

Fig. 4. M6ssbauer spectra and hyperfine magnetic field distributions at T = 4.2 K for samples of milled GdFe2 (batch (a)).

80

"-" 60 "~"

~

•

low fields (< 5T) etallic iron

CD

< 40

•

O;C)

I

50

i

I

i

100 Milling

i

150

[

n

200 Time

250

i

300

(h)

Fig. 5. Relative absorption area of metallic iron and low field (5 T) components at 4.2 K versus milling time as obtained from the fittings for samples of milled GdFe2 (batch (a)).

milling, reaches a maximum at about 7 h, and then decreases smoothly with further milling. These small-field components observed at 4.2 K (Fig. 4) may come from isolated iron atoms trapped in the segregated Gd-rich phase: theoretical predictions and experimental reports for hyperfine magnetic field on Fe impurities in Gd metal indicate field values within 2 and 6 T [13-15]. In addition, M6ssbauer measurements made at 300, 84 and 4.2 K in the samples of intermediate milling times have also shown that, as the temperature is lowered, there is an increase in the metallic iron contribution to the spectra and a reduction in the relative intensity of the low hyperfine magnetic field components. This behaviour indicates that in this case the low-field components include a contribution due to superparamagnetism of small metallic-iron particles. An analysis combining results from the X-ray diffraction (Fig. 1) and the M6ssbauer spectra (Figs. 3 and 4), suggests that the segregation process has formed metallic iron at the Fe-rich side of the G d - F e composition range, with the possibility of some Gd impurities trapped in it. On the other side of the range, the X-ray patterns indicate the presence of a Gd-rich FCC crystalline structure• The broad distribution of hyperfine magnetic fields in the M6ssbauer spectra is related to the amorphous Gd/Fe alloy phase which is not observed in the X-ray results (Table 1). The average iron percentage in this latter alloy is estimated to vary from about 21% in the a25 h sample to about 62% in the a276 h sample, while the initial crystalline compound has 66.7% of Fe. The composition of some samples were checked by EDS analysis, and this reveals an addition of iron impurities in the last members of the series (5% in the a200 h sample and 12% in the a276 h), that has been attributed to the wear of the milling tools. The increase in rate of growth of iron content at large milling times is associated with an increase in the number of collisions between the tools themselves due to the reduction of the total powder volume in the vial, that is gradually taken out for the samples preparation. Fig. 5 reproduces the dependence with milling time of the relative absorption area, obtained from the 4.2 K spectra fittings, of the metallic iron and the low fields (Bhf ~< 5 T) components. It should be noted in the figure the simultaneous reduction

A. Biondo et al. / Journal of Magnetism and Magnetic Materials 176 (1997) 272 278

277

Table 1 M6ssbauer relative absorption area and hyperfine parameters of some Gd/Fe milled samples Milling time (h)

Temp. (K)

0.0

300

-- 0.208

20.36

0.188

21.87

-- 0.251

20.74

-

-- 0.173

-- 0.188

23.70

-

0.272

55

- 0.183

21.61

-

-- 0.162

45

300

0.108

32.85

-- 0.173

20.35

0.088

38

84

-- 0.037

32.66

-- 0.085

20.90

0.044

42 49

300

- 0.194

11

-

0.295

49

-

-- 0 . 2 1 4

51

0.108 -

30.93

- 0.180

24.00

0.175

-- 0.104

32.78

- 0.070

18.49

0.063

36

84

0.037

32.79

-- 0.328

21.20

-- 0.043

45

- 0.120

34.06

-- 0.110

26.39

0.028

49

300

-- 0 . 1 1 0

32.62

-- 0.042

16.54

0.106

40

84

-- 0.036

32.90

- 0.012

18.52

0.075

55

-- 0.115

34.22

- 0.032

18.07

0.004

60

300

-- 0.109

32.79

-- 0.042

21.23

-- 0.007

43

84

-- 0 . 0 4 4

32.37

-- 0.074

19.40

0.044

39

-- 0.118

34.09

-- 0.029

20.89

-- 0.085

69

300

-- 0.135

32.50

0.010

18.89

0.094

20

84

-- 0 . 0 5 0

32.35

0.069

17.58

0.194

31

- 0.117

34.29

-- 0.076

20.77

- 0.023

38

- 0.91

32.63

-- 0.011

16.67

0.091

20

-- 0.028

34.39

0.109

17.51

-- 0 . 0 6 2

29

-- 0.131

34.27

0.062

21.07

0.092

31

-- 0.125

32.61

0.084

18.69

0.085

12

- 0.125

34.54

0.072

25.66

0.040

18

- 0.140

32.28

0.139

18.45

0.209

22

-- 0.122

34.37

0.051

26.25

-- 0.003

19

300 4.2 300 4.2

276

-

89

-

-- 0.122

84 200

-

300

4.2 100

0.094

Area (%)

21.53

4.2 70

QS (mm/s)

- 0.144

4.2 25

( B h f ) (T)

20.67

4.2 07

(mm/s)

21.81

4.2 03

(ISO)

- 0.185

4.2 01

Bhex (T)

- 0.253 4.2 30 min

ISx (mm/s)

300 4.2

of the intensities of these components for larger milling times, an effect that is attributed to the occurrence of a solid-state reaction between the initially segregated components as the milling time increases. The continuous broadening of the X-ray diffraction lines related to metallic iron and to the Gd-rich phase indicates a reduction in the grain size of these components. As the milling proceeds from medium times to longer times, the Bhf distribution at 4.2 K also becomes narrower and more symmetric, in comparison to the distribution found at lower times, suggesting that Gd and Fe atoms are more randomly distributed within this phase (see Fig. 4). The average magnetic hyperfine field at 4.2 K for the a276 h sample is 26.3 T, a value larger

than the hyperfine fields found in the initial GdFe2 compound (21.5 and 20.4 T). 4. Conclusions The severe milling of the Laves-phase intermetallic compounds RFe2 (R = Gd and Y) gives different results concerning their structural and magnetic properties. In our previous study, YFe2 reached an amorphous state within 30 h of milling and presented no signs of segregation or formation of any other crystalline state, according to the X-ray and M6ssbauer results [1], while other authors have obtained a partial segregation of metallic iron with the milling of YFe2 [3]. For the Gd/Fe system

278

A. Biondo et al. / Journal o f Magnetism and Magnetic Materials 176 (1997) 272-278

in the initial ratio 1 : 2 , the chemical elements have partially segregated after a few hours of milling. These observations show that the results obtained with the milling m e t h o d can be dependent on several conditions, such as the tools, the details of the experimental procedures and the materials initially employed. As the milling time increases, there is indication of a solid-state reaction between the segregated c o m p o n e n t s that results in the formation of an a m o r p h o u s G d / F e alloy phase, with an estimated composition of 62% of Fe. It should be stressed here that the formation of a F C C crystalline structure associated to a Gd-rich phase, is clearly observed in X-ray patterns of the samples, but with no distinct absorption lines in the corresponding M6ssbauer spectra. It is possible that the observed F C C structure results from the stabilization of a high-pressure G d phase due to several undistinguished factors, such as the small G d clusters size, lattice defects and also the presence of Fe, O or N impurities. The M6ssbauer spectra at 4.2 K show a distribution of magnetic hyperfine fields related to the formation of the G d / F e a m o r p h o u s phase that becomes more symmetrical with milling time. F o r longer times, the average Bhf in the distribution presents a value that is lower than in metallic iron but higher than the value in the initial GdFe2 c o m p o u n d .

Acknowledgements The authors are indebted to the C N P q for partial financial support, to Dr. A.A. Fernandes (Instituto

Militar de Engenharia, Rio de Janeiro) for the E D S analysis, Dr. R o m e u A. Pereira (Centro Brasileiro de Pesquisas Fisicas) for the X-ray diffraction measurements and E.P. C a e t a n o for helpful discussions.

References [1] C. Larica, K.M.B. Alves, E. Baggio-Saitovitch, A.P. Guimar~es, J. Magn. Magn. Mater. 145 (1995) 306. I-2] A.W. Weeber, H. Bakker, Physica B 153 (1988) 93. [3] H. Waba, M. Shiba, J. Nucl. Instr. Meth. 76 (1993) 301. 1-4] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179. I-5] R.A. Brand, Duisburg University, version of 1988. 1-6] G.J. Bowden, D.St.P. Bunbury, A.P. Guimar~es, R.E. Snyder, J. Phys. C I (1968) 1376. 1-7] R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1990. 1-8] A.E. Curzon, H.G. Chlebek, J. Phys. F 3 (1973) 1. 1-9] Yu.G. Morozov, A.N. Kostygov, V.I. Petinov, P.E. Chizhov, Phys. Stat. Sol. A 32 (1975) Kl19. [10] T. Alonso, Yinong Liu, T.C. Parks, P.G. McCormick, Scr. Metall. Mater. 25 (1991) 1607. [11] T. Alonso, Yinong Liu, T.C. Parks, P.G. McCormick, Scr. Metall. Mater. 26 (1992) 1931. [12] J. Akella, G.S. Smith, A.P. Jephcoat, J. Phys. Chem. Solids 49 (5) (1988) 573. [13] I.A. Campbell, W.D. Brewer, J. Flouquet, A. Benoit, B.W. Marsden, N.J. Stone, Solid. State. Commun. 15 (1974) 711. [14] W.D. Brewer, S. Hauf, D. Jones, S. Frota-Pessoa, J. Kapoor, Yi Li, A. Metz, D. Riegel, Phys. Rev. B 51 (18) (1995) 12595. [15] M. Forker, R. Trzcinski, T. Merzh~iuser, Hyperfine Interactions 15/16 (1983) 273.