Spin reorientation in Nd2Fe14C

Journal

of the Less-Common

SPIN REORIENTATION C. J. M. DENISSEN, Philips Research (Received

195

- 202

195

IN Nd*Fe,&

B. D. DE MOOIJ

Laboratories,

January

142 (1988)

Metals,

5600

and K. H. J. BUSCHOW

JA Eindhoven

(The

Netherlands)

15, 1988)

Summary

The temperature dependences of the magnetization and the “Fe hyperfine interaction were studied in the tetragonal compound Nd,Fe,&, which is isotypic with Nd,Fe,,B. The compound Nd,Fe,,C is ferromagnetic with T, = 535 K. Below T,, at about 140 K, a spin reorientation occurs, the corresponding easy magnetization direction deviating from that found at room temperature.

1. Introduction

Ternary intermetallic compounds of the type R,Fe,,B have been shown to harbour a wealth of interesting properties. These comprise hard magnetic properties suitable for industrial applications, huge anomalies in their thermal expansion behaviour, several types of magnetic phase transition and reversible absorption of hydrogen. Less familiar are compounds of the type R2Fe,&. Recently it has been shown that these compounds are isotopic with R,Fe,,B [l]. Curie temperatures have been reported for most of the R,Fe,& compounds and these are only slightly lower than those of the corresponding borides [ 2 - 41. In several of the R2Fei4B compounds a spin reorientation occurs at low temperatures (R = Nd, Ho, Er). In view of the fact that the unit cell dimensions and the magnetic interactions in the carbides are in very close agreement with those of the corresponding borides it is of interest to investigate whether or not a spin reorientation is also present in R,Fe,J. In this investigation we have therefore focused our attention on the low temperature behaviour of the magnetization and the “‘Fe hyperfine interaction in Nd,Fe ,&.

2. Experimental

details

The 57Fe Mossbauer spectra of Nd,Fe,,C were measured at various temperatures between 20 and 300 K using finely ground powders. The 0022-5088/88/$3.50

@ Elsevier Sequoia/Printed

in The Netherlands

196

Miissbauer spectrometer was a constant-acceleration type, equipped with a 10 mCi 57Co in palladium source that was moved whilst the absorber remained in a fixed position. Line broadening as a result of self-absorption was avoided as far as possible by employing relatively thin absorbers. For the analysis of the spectra a standard fitting procedure with lorentzian profiles was used. The sample Nd*Fe,,C was prepared by arc melting and vacuum annealing. Details regarding the formation of the tetragonal phase (Nd,Fe,,B structure) have been reported elsewhere [ 51. Magnetic measurements were made using the Faraday method on a powdered sample, after aligning the powder particles in a magnetic field and fixing their direction with epoxy resin.

3. Magnetic measurements The compound Nd2Fe,,C is a strongly anisotropic compound with large differences between the magnetizations measured parallel (al,) and perpendicular (uI) to the alignment field. The temperature dependence of uIl has a normal ferromagnetic behaviour; the Curie temperature T, = 535 K. There is no clear indication of the spin reorientation phenomenon at any temperature. However, small deviations of the magnetization vector from the c direction may not show up easily as discontinuities in the 011curves of powder samples. In this respect ol curves may be more relevant. Results obtained for Nd,Fe,& are shown in Fig. 1. It can be seen that there is a strong increase in ul around 140 K; this temperature may be identified as a spin reorientation temperature. Owing to the fact that the magnitude of ul is strongly affected by any misalignment of powder particles, we did not

40 Nd,Fe,,C

T (K)

Fig. 1. Temperature dependence of the increase in magnetization ically aligned powders in a field (40 kA m-l) applied perpendicular

measured on magnetto the alignment field.

197

attempt to derive values of the temperature-dependent magnetization vector and the c axis.

tilt angle between

the

4. 57Fe Mijssbauer spectra and their interpretation We have shown in ref. 1 that a close correspondence exists between the structures of R,Fe14C and R,Fe,4B compounds. The crystal structure of these materials comprises six different iron sites, denoted in Wyckoff notation by the symbols 8j?, 16k,, 16k2, Sj i, 4e and 4c. ,4t room temperature the easy magnetization direction in Nd,Fe,,C is along the c axis, as found in NdzFelaB. It was shown in the preceding section that no spin reorientation takes place above 140 K. Therefore, we fitted the Mijssbauer spectra of Nd,Fe,,C measured above 140 K with six different subspectra with relative intensity ratios 8:16:16:8:4:4, assuming that the recoil-free fractions for the various sites are the same. In this way we were able to obtain good fits for each of the spectra recorded above 150 K. To decide between the various possibilities for making such fits, we applied the same constraints regarding quadrupole and hyperfine field splitting as described in detail elsewhere [6]. In addition, we made the reasonable assumption that the temperature dependences of the isomer shifts are the same for the six subspectra. In fact, the temperature dependence of the isomer shift in “Fe is attributable to the second-order Doppler effect, which is expected to be the same for the various sites [ 71. Further comment is made on this later in the discussion. Some representative spectra are shown in Fig. 2. The full lines represent the fits to the experimental data points. The resulting parameters from the various spectra are shown in Figs. 3 - 5. Since the direction of the hyperfine field deviates from the c axis below TSR (for instance, with decreasing temperature it may tend towards the [ 1101 direction), crystallographically equivalent sites will become subdirection of the EFG (electric field gradient) principal axis. As shown in ref. 8, the sites 16j,, 16j,, Sj, and Sj, become subdivided, and consequently the spectra below T,, should be analysed with as many as 12 Zeeman sextuplets, However, this would mean an unreliable and complex fitting procedure. Therefore we fitted the low temperature spectra with only six subspectra, assuming that the extra subdivision can be incorporated in the linewidth. It is obvious from our fits, indicated by full lines in Fig. 2, that a good description of the Mossbauer spectra of NdzFe,,C can also be obtained at low temperatures. The corresponding parameters, which, in fact, have to be considered as averaged values for each of the six non-equivalent sites, are shown in Figs. 2 - 5. In Fig. 6 the linewidths of the lorentzian lines are shown as a function of temperature. From the temperature dependences of the various hyperfine parameters shown in Figs. 3 - 6, it may be deduced that a change in the easy magnetization direction takes place at low temperatures, commencing at 120 - 140 K. This is close to the temperature below which the cl value starts to increase (Fig. 1).

198

1

1

-5

0

1

5

MM

‘SEC

VELOCITY

Fig. 2. Miissbauer spectra of NdzFe14C at various temperatures. fits to the experimental data points.

The full lines represent

199

200

Fig. 5. Values of the quadrupole splitting as a function of the temperature. The curves are guides to the eye. The symbols used for the various sites are the same as in Fig. 3. 0.4

<

I 100

200

300

T (K)

Fig. 6. The linewidths of temperature.

of the lorentzian

lines of the subspectra

of NdzFel&

as a function

5. Discussion The results obtained from the Nd2Fe,4C Mijssbauer spectra at the various temperatures (Figs. 3 - 5) closely resemble the results obtained for Nd,Fe14B. This was expected on the basis of the results obtained by us for R#‘e,& (R - Gd, Lu) [l]. The assignment of the various subspectra to the six crystallographically non-equivalent sites was carried out in a similar manner to that in Nd2FeiaB [6,8]. However, there are some small differences. For instance, the hyperfine fields of the 16k,, Sj, and 4e sites are more similar than the corresponding hyperfine fields in the boride compounds. The full line in Fig. 4 represents the temperature dependence of the relative isomer shift due to the second-order Doppler effect obtained from the high temperature approximation [ 7 1. It is obvious that the experimental temperature dependence is only slightly weaker than the result expected on the basis of this approximation. However, below 300 K this high tempera-

201

ture approximation has to be corrected for the temperature dependence of the recoil-free fraction. As is shown in ref. 7, this correction results in a temperature-dependent second-order Doppler effect, that tends to saturate the isomer shift towards lower temperatures; this is observed experimentally. The temperature dependence of the recoil-free fraction depends only on the Debye temperature. We therefore expect that the temperature dependences of the isomer shifts of the six subspectra will be approximately the same. In fact, it appears that for the four most intense subspectra this behaviour is found without using a common temperature dependence as an additional constraint. We therefore used this constraint only for the 4e and 4c sites. In ref. 8 an anomalous behaviour of the isomer shift in Nd,Fe,,B at the spin reorientation temperature TSR = 148 K was found. However, in our analysis we did not find any indication of such an anomaly. It is possible that the anomaly of ref. 8 is an artefact of the very complex fitting procedure, which involved as many as 12 subspectra. With the conversion factor 14.8 T/ps [ 91 the values of the iron moments can be estimated from the hyperfine field values. Using the low temperature data a moment equal to 30.9 pg per formula unit (Nd,Fe&) is found. A comparison of this moment with those of Gd,Fe,,C and Lu,Fe,,C (which are equal to 31.9 pg and 27.5 PB respectively [ 11) shows that their relative magnitudes behave in the same way as in the series R,Fe,4B [6]. Furthermore, for Nd,Fe,& the average iron moment at low temperatures is equal to 2.2 ps(Fe atom))‘; the corresponding value for Nd2Fe14B is 2.26 p,(Fe atom))’ [8]. This indicates that the iron moments in the carbides are slightly lower (3%) than in the borides, which agrees well with the results of the lutetium compound [ 11. Finally, we will briefly mention that the analysis of the Mossbauer spectra reveals the presence of small amounts of impurity phases. About 5% of the area fraction of the spectra can be assigned to the o-Fe phase. Another 2% of the iron (area fraction) is present at all temperatures in the form of a so far unidentified paramagnetic compound. In conclusion, we have shown that the thermal variation of the s7Fe Miissbauer spectra of Nd,Fe,,C is very similar to that found in Nd2Fei4B. The spin reorientation phenomenon is also found in Nd,Fe,& and occurs at about the same temperature as in Nd,Fe14B. We have confirmed earlier results by showing that the magnetic moments of the iron atoms in R*Fe,& are only a few percent lower than the iron moments in R,Fe,,B. References 1 C. J. M. Den&en, B. D. de Mooij and K. H. J. Buschow, J. Less-Common Met., 139 (1988) 291. 2 K. H. J. Buschow, in C. Herget and R. Pijrschke (eds.), Proc. 9th Int. Worhshop on Rare Earth Magnets, Bad Soden, August 1987, Deutsche Physicalische Gesellschaft, Bad Honnef, F.R.G., 1987, p. 459. 3 F. R. de Boer, Huang Yink-kai, Zhang-Zhi done, D. B. de Mooij and K. H. J. Buschow, J. Magn. Magn. Mater., 72 (1988) 167.

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A. Bezinge, K. Yvon and J. Muller, Solid State Commun., 64 (1987) 639. K. H. J. Boschow, D, B. de Mooij and C. J. M. Denissen, J. Less-Common Met., 141 (1988) L15. H. M. van Noort, D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 115 (1986) 155;5. Appl. Phys., 57 (1985) 5414. G. K. Shenoy, F. E. Wagner and G. M. Kalvius, in G. K. Shenoy and F. E. Wagner (eds.), Mhsbauer Isomer Shifts, North-Holland, Amsterdam, 1978, p. 49. H. Onodera, A. Fujita, H. Yamamoto, M. Sagawa and S. Hirosawa, J. Mugn. Magn.

Mater., 68 (1987) 6; 68 (1987) 15. 9 P. C. M. Gubbens, J. H. F. van Apeldoorn, J. Phys. F, 4 (1974) 921.

A. M. van der Kraan and K. H. J. Buschow,