A 57Fe Mössbauer study of Sm2Fe17

Journal of the Less-Common

Metals, 171(1991)

33

33-39

A 57Fe Mijssbauer study of Sm,Fe, 7 Bo-Ping Hu and J. M. D. Coey Department of Pure and Applied Physics, Trinity College, Dublin 2 (Ireland)

(Received December 17.1990)

Abstract s7Fe M&batter spectra of an oriented absorber of Sm,Fe,, were studied in the temperature range 15-290 K. The isomer shifts decrease with temperature and the 6c site has a shift which is 0.2 mm s ’ greater than the other three iron sites. The quadrupole splitting hardly changes with temperature for all four sites. The hyperhne fields for each Fe site decrease smoothly with temperature without any anomaly. Average values are 30.1 T and 22.1 T at 15 K and 290 K respectively. The data indicate that the easy magnetization direction is along a b-axis over the whole temperature range. There is no spin reorientation of the magnetization to the c-axis.

1. Introduction The rare-earth iron-rich intermetallic compounds RZFe,, have always been an attractive subject for researchers in magnetism. Drawbacks of these compounds from a practical point of view are their low Curie temperatures and their easyplane anisotropy [l]. Recent work has shown that the magnetic properties of R2Fe,, intermetallics can be greatly improved by the interstitial atoms carbon 12, 31 and nitrogen [4, 51 which raise the Curie temperature T, by approximately 200 K and 400 K respectively. At room temperature, the R,Fe, ,X3 c)compounds (X = C and N) usually have the same easy direction, with the magnetization lying in the basal plane as it does in RzFe,,. The exceptions are Sm,Fe,,X, _ d (X = C and N, 6 < 2). SmzFe,,C shows a uniaxial anisotropy field of 5.3 T at room temperature [6], whereas the room temperature anisotropy field of Sm,Fe,,N, has been found to be 14 T [7]. The combination of a high Curie temperature of 749 K [5], high saturation polarization of about 1.5 T [4, 61 and strong uniaxial anisotropy make Sm>Fe,,N, _ b a most promising material for permanent magnets [S, 91. In order to improve our understanding of the magnetic properties of the carbide Sm,Fe,,C, _ 6 and nitride Sm,Fe,,N,- c), it is important to clarify the magnetic behaviour of their parent compound Sm,Fe, ,. Recently G&singer et al. [lo] observed an anomaly in the a.c. susceptibility of Sm,Fe,, and several carbides range 150-250 K. This Sm,Fe, ,C, - hJ with 6 > 2.1 in the temperature phenomenon might be associated with one of two possible magnetic transitions [ 111: a spin reorientation transition (SRT ) or a first order magnetization process (FOMP). The temperature dependence of the anisotropy field, by singular point detection measurement, shows an easy c-axis for Sm,Fe,,C,_, (2.1 < 6 < 2.5) below the magnetic ordering temperature [lo], which suggests that no spin reorien0022-5088/91/$3.50

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34

tation exists in these carbides. There is also no FOMP, although a kink was observed in the temperature dependence of the anisotropy field at about 240 K for Sm,Fe, 7Co.sYalmost the same temperature as the peak in the a.c. susceptibility curve [lo]. The question arises of whether there is an SRT or an FOMP for the Sm,Fe,, compound. 57Fe Miissbauer spectroscopy can be effectively used for investigating SRTs. Since the iron atomic magnetic moment is usually not isotropic in rare-earth iron intermetallics owing to the orbital contribution, there is a discontinuity of hyperfine field when the iron moments change direction. Examples are Tm*Fe,, [12], R2Fe,7N2,7 (R=Er and Tm) [13] and R(Fe,,Ti) (R=Nd, Tb and Dy) [14]. There may also be an effect on the quadrupole splitting (QS) when the angular relationship between the magnetic moment and the electric field gradient (EFG) changes. For example, in R2Fe,7Nz,7 (R= Er and Tm) the QS changes sign at the 12k site with a change in QS of approximately 0.25 mm s- ’ at the spin reorientation [ 131. Furthermore, by using a single-crystal absorber [ 151 or an oriented absorber [ 141, the relative intensity of the Am = 0 transitions (lines 2 and 5) of the spectrum will be seen to change when a spin reorientation out of the c-plane occurs. The aim of this work is to examine the hyperfine interactions of Sm,Fe,, as a function of temperature by 57Fe Miissbauer spectroscopy on an oriented absorber, in order to investigate in an independent way if there exists any spin reorientation transition in SmzFe, ,.

2. Experimental

methods

The Sm,Fe,, compound was obtained from Rare Earth Products Ltd and the ingot was annealed in vacuum at 980 “C for 3 days. X-ray diffraction showed the compound had the Th,Zn,, structure, with a minor amount of a-Fe. The absorber was prepared by mixing well ground fine powder (about 15 mg cm-‘) with epoxy resin and then setting it in an applied field of 1.5 T until dry. “Fe Miissbauer spectra were collected using a conventional constant acceleration spectrometer with a 20 mCi source of 57Co in rhodium. The y-ray was along the alignment direction. The velocity scale was calibrated using an a-Fe absorber at room temperature. Low temperature data were obtained using a closed-cycle two-stage helium refrigerator (Air Products, model, HC-2).

3. Results and discussion

The Sm,Fe,, compound has a rhombohedral Th,Zn,, structure. Under the combined effects of dipolar fields and quadrupole interactions, the spectrum of Sm,Fe,, is expected to be made up of seven independent lorentzian sextets, with an overall intensity ratio of 1:2:2:4:2:4:2 for the 9d,, 9d,, 18f,, lSf,, 18h,, 18h, and 6c subspectra [ 161. This model was used in the least-squares fits of the data. Differ-

3.5

ent linewidths were permitted for inner, middle and outer pairs of lines (IX,4 9d > 18f > 18 h. Listed in Table 1 are the average hyperfine fields B,, at the inequivalent crystallographic

-10

-5

0

5

V ( mm/s Fig. 1. The s7Fe MGssbauer fits shown as full lines.

10

)

spectra

of Sm>Fe,,

at different

temperatures

from

15 to 290 K, with the

36 TABLE I Hyperhne fields for each crystallographic inequivalent Fe site and the values of the angle between the magnetization and the y-rays at different temperatures, deduced from j7Fe Mossbauer spectra on an oriented absorber of Sm,Fe, ,. The error for the values in each column is the same as given for each first entry.

15 50 100 I50 170 200 250 290

36.0( 5) 35.8 34.9 33.7 33.1 32.1 29.7 26.8

BTd)

l8f (T)

18h (T)

@,,J (T)

Pdeg)

32.6(5) 32.3 31.5 30.3 29.8 28.7 25.9 23.4

29.4(5) 29.0 28.3 27.1 26.6 25.7 23.9 21.6

27.7( 5) 27.8 27.2 26.1 25.6 24.8 22.8 20.3

30.1(5) 29.9 29.3 28.1 27.6 26.7 24.6 22.1

43(3) 43 44 44 44 45 46 46

sites, the overall average hyperfine fields (Bhf) and the fitted values of the angle 0 between the magnetization direction and the y-rays. The hyperfine fields for each Fe site and the overall average hyperfine field as a function of temperature are shown in Figs. 2 and 3 respectively. At 15 K, the 6c dumbell sites have the largest hyperfine field I?,, of 36.0 T, the 18h sites have the smallest of 27.7 T and the average value over all sites (&) is 30.1 T. Corresponding values at 290 K are 26.8 T, 20.3 T and 22.1 T respectively. The reduction of (&) from 15 K to 290 K is 26.6%. Supposing the atomic iron magnetic moment is proportional to the hyperfine field acting on the nucleus, the average atomic iron magnetic moments are 2.00 ,uB at 15 K and 1.47 ,uB at 290 K respectively, assum-

35-

,30I-.

E

m

25-

v

6c 9d A 16f . 16h l

20

I 0

50

100

150

200

250

300

T(K)

Fig. 2. The hyperfine fields for each crystallographic temperature.

inequivalent Fe site of SmzFe,, as a function of

37

I

I.

I.

I

I

/

2at

-

90

-

60 s

I-

%26-

'ti

d:

24-

-

30

-

0

e

.

W

22t 0

100

60

160

200

250

300

T(K)

Fig. 3. The temperature dependence of overall average hyperfine field and the fitted value of the angle f3between the magnetization direction and y-rays for Sm?Fe, ,.

ing a factor of 15 T,L+- ‘. There is no anomaly in the temperature variation of the hyperfine field (Figs. 2 and 3), even on the 6c sites which are the sites most sensitive to spin reorientation [ 10, 111. The fitted values of the angle 8 between the magnetization and y-rays are plotted in Fig. 2. They are within 2” of 44” from 15 K to 290 K (see Table 1). In view of the error in fitting angle of 3”, the angle 8 may be considered to be a constant. This indicates that no effect on the intensities of 57Fe Miissbauer lines of an oriented absorber can be seen, as would be expected from any change in magnetic configuration of Sm,Fe,,. The isomer shifts as a function of temperature are shown in Fig. 4. The 9d,

L

0.2P I

O.l-

F n k E d

o.o-

-O.l-

I

I

0

I

60

I

100

I

150

I

200

I

250

I

300

T(K)

Fig. 4. The average isomer shifts for each crystallographic inequivalent function of temperature. The values are relative to that of a-Fe at 290 K.

Fe site of Sm,Fe,,

as a

38

6C .

.

.

TV

.

9%

r

I

I

0

100

I

200

.

.

1

I1 300

T(K)

Fig. 5. The quadrupole splittings for each Fe site of Sm*Fe,, as a function of temperature. 18f,, 18h, correspond to the component with two thirds intensity of each site.

The 9d,,

18f and 18 h sites have very similar values and the 6c site has a value distinct from those of the other three. The temperature dependence of the QS values of the components with a large intensity are shown in Fig. 5. The 9d,, lSf, and 18h, values correspond to the component with two thirds intensity at each site. There is no discontinuity in any of the data. The components of the smallest hyperfine field 18 h can be used to distinguish the easy magnetization direction in the c-plane [16]. For SmzFel,, the subspectrum 18 h, with one third and 18 h, with two thirds of the intensity have positive and negative QS values respectively, which indicates that the easy magnetization direction for Sm,Fe,, is along the b-axis [ 161.

4. Conclusion A detailed study of 57Fe ~~ssbauer spectra on an oriented absorber of Sm,Fe,, was completed in the temperature range 15-290 K. From the temperature dependence of the hyperfine fields, isomer shifts, quadrupole splitting and the angle between the magnetization and y-rays, we can assert that there is no spin reorientation transition in Sm,Fe,, from the c-plane to the c-axis. On the contrary, there is evidence that the magnetization remains along a b-axis throughout the temperature range.

Acknowledgments This work forms part of the Concerted European Action on Magnets. It was supported as part of the B~TE~EW~~ programme of the European Commis-

39

sion. We are grateful to Sun Hong for help in sample annealing and Dr. Li HongShuo for helpful discussions. References 1 K. H. J. Buschow, Rep. f’rog. Phys., 40 (1977) 1179. 2 X.-P. Zhong, R. J. Radwanski, F. R. de Boer, T. H. Jacobs and K. H. J. Buschow, J. Mugn. Magn. Maler., Nh (1990) 333. 3 H. Sun, B.-P. Hu, H.-S. Li and J. M. D. Coey, Solid Snare Cotnmun., 74 (1990) 727. 4 J. M. D. Coey and H. Sun, J. Magn. Magn. Mater., 87( 1990) L251. 5 H. Sun, J. M. D. Coey, Y. Otani and D. P. F. Hurley, J. Whys. Condens. Matter, 2 ( 1990) 6465. 6 X. C. Kou, R. G&singer, T. H. Jacobs and K. H. J. Buschow, J. Mugn. Magn. Mater., &Y (1990) I. 7 M. Katter, J. Wecker, L. Schultz and R. G&singer, J. Magn. Magn. Marer., 92 (1990) L14. 8 J. M. D. Coey, H. Sun and Y. Otani, in S. G. Sanker (ed.), Proc. Eleventh fnt. Workshop on RareEarth Magnets and Sixth Int. Symp. on Magnetic Anisotropy and Coercivity in Rare-Earth Transition Melal Alloys, f’i~~sburgh, f’A , Vol. 2, 1990, p. 36. 9 Y. Otani, D. P. F. Hurley, H. Sun and J. M. D. Coey, J. Appl. Whys., 69 ( 199 1), in the press. 10 R. &&singer, X. C. Kou, T. H. Jacobs and K. H. J. Buschow, J. Appl. Phys., 69 (1991), in the

press. 1 1 X. C. Kou and R. GrGssinger, J. Mugn. Magn. Muter., in the press. 12 P. C. M. Gubbens and K. H. J. Buschow, J. Appf. Phys., 44 (1973) 3739. 13 B.-P. Hu, H.-S. Li, H. Sun, J. F. Lawler and J. M. D. Coey, So/id State (hmmun., 76 (1990) 587. 14 B.-P. Hu, H.-S. Li and J. M. D. Coey, Hyperfine Interact., 4.5 (1989) 233. 15 N. C. Koon, M. Abe, E. Callen. B. N. Das. S. H. Liou, A. Martinez and R. Segnan. J. Magn. Mugn. Mater., 54-57

( 1986) 593.

16 B.-P. Hu, H.-S. Li, H. Sun and J. M. D. Coey, J. f’hys. Condens. Matter, 3 (1991) in the

press.