Magnetic properties of Nd2Fe14C and some related pseudoternary compounds

Journal of Magnetism and Magnetic Materials 73 (1988) 263-266 North-Holland, Amsterdam

263

MAGNETIC PROPERTIES OF Nd2Fe14C AND S O M E RELATED P S E U D O T E R N A R Y C O M P O U N D S F.R. de BOER, R. V E R H O E F and Z H A N G Zhi-dong Natuurkundig Laboratoriurn, University of Amsterdam, 1018 XE Amsterdam, The Netherlands

D.B. de MOOIJ and K.H.J. BUSCHOW Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands Received 2 March 1988

We have investigated the magnetic properties of Nd2F%4C and of several pseudoternary compounds of the type

(Nd1_xRx)2Fe14 C, where R represents Dy or Lu. For all compounds investigated we have determined the Curie temperatures and the saturation magnetization at 4.2 K, the latter being derived from magnetic isotherms measured in fields up to 35 T. The compounds Nd2F%4C and Ho2F%4C show spin reorientations at about 120 and 35 K, respectively, below which temperature the easy magnetization direction deviates from the c-axis. At 4.2 K we found indications for a FOMP transition near Her = 23 T in the Nd rich Nd I _xDyxFe>~C compounds.

1. Introduction In a previous investigation we reported on the magnetic properties of ternary carbides of the type R2Fe14C [1]. The experimental data presented for Nd2Fe14C were incomplete because we had some difficulties at that time in preparing single-phase samples of this compound. These difficulties have now been solved and details regarding the formation of Nd2Fe14C are given elsewhere [2]. In the present investigation we have focussed our attention on Nd2FexaC and one some related pseudoternary compounds of the same structure type. It will be shown that the compound Nd2Fe14C gives rise to magnetic phase transitions of a similar nature as observed in Nd2Fe14B.

2. Experimental Various samples (Nd1_xRx)2Fe14C with R = Dy or Lu were prepared from 99.9% pure starting materials by means of arc melting in purified argon gas. After arc melting the samples were

wrapped in Ta foil and sealed into evacuated quartz tubes. They were vacuum annealed for several weeks at temperatures between 850 and 900°C. X-ray diffraction was performed on powdered samples with a standard powder diffractometer equipped with a graphite crystal monochromator using CrK~ radiation. The temperature dependence of the magnetization o ( T ) was measured on polycrystalline samples by means of an automatic o - T recorder, based on the Faraday method. The Curie temperatures (Tc) were determined from low-field a ( T ) measurements. The spin reorientation temperatures were determined by means of measurements of the temperature dependence of the ac-susceptibility. The high-field magnetization measurements with B up to 35 T were performed at 4.2 K in the high-field magnet at the University of Amsterdam [3]. For the high-field measurements powder samples of cylindrical shape were used, prepared by aligning powder particles parallel and perpendicular to the cylinder axis in a magnetic field of 1 T and fixing their direction with epoxy resin. The high-field isotherms were recorded with

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FR. de Boer et al. / Properties of Nd2Fel4C

264

the external field either parallel (Oll) o r perpendicular (a±) to the alignment field. The values of the saturation magnetization (as) were derived from the high-field part of the a(B) curves, by extrapolation to zero field.

3. Results and discussion

The X-ray diagrams obtained from the annealed samples showed that all of the compounds investigated had crystallized in the tetragonal Nd2Fe14B structure. The lattice constants derived from the X-ray diagrams are included in table 1. All samples were approximately single phase, the amount of impurity phases being about 5%. The temperature dependence of the magnetization of the various (Nd l_xRx)2Fe14 C compounds had the shape expected for ferromagnetic materials. The corresponding Curie temperatures are listed in table 1. In most cases we found indications for the presence of a spin-reorientation temperature, which leads to a small shoulder in the low-temperature range of the a ( T ) curves and to a step-like increase upon coding in the temperature dependence of the ac-susceptibility. Results for Nd2Fe14C are shown in fig. 1. More detailed experimental information regarding the occurrence of a spin reorientation at about TSR = 120 K in this material was obtained by means of 57Fe M~Sssbauer spectroscopy and will be reported elsewhere [4]. Representative examples of high-field magnetic measurements made at 4.2 K are shown in figs.

2-4. The values of the saturation magnetization derived from the Oil c u r v e s are listed in table 1. In the case of Nd2Fe14C it is seen (fig. 2) that the o± curve increases somewhat more steeply at about 23 T. We attribute this additional increase to the occurrence of a first-order magnetic phase transition (FOMP), analogous to the magnetic behaviour of the isotypic compound Nd2Fe14B [5,6]. In single crystal data of the latter compound there is a strong steplike increase in the o(B) curve (at Her = 17 T) when the field is applied at 4.2 K in the [100] direction. In the present study the magnetic isotherms at 4.2 K were measured on aligned powder particles. The o± (B) curve has therefore to be considered as an average over the various directions perpendicular to the [001] direction so that the steplike increase is considerably less pronounced than in the case of single crystals. In the series (Ndl_xDyx)zFea4 C we found this steplike increase to be almost concentration independent for x < 0.4. In Nda.sLu0.zFea4C the value of Bcr was found to be slightly lower (18 T) than in Nd 2Fea4C (see fig. 4). N o such steplike increase was observed in the o±(B) curve of Ndl.2Dy0.sFea4B and Dy2Fe14B. It may be seen, however, from the results shown in fig. 3 that there is a small steplike increase in the o,,(B) curve of Ndl.zDy0.8Fe14 B. This is quite unexpected and calls for more detailed investigations in the near future. The os values listed in table 1 have been used to calculate the magnetic moment per formula unit (Ms). These values may be compared with the values M c calculated on the basis of an Fe sub-

Table 1 Magnetic and crystallographic properties of several tetragonal N d 2 Fe 14B-type compounds: os saturation magnetization at 4.2 K; M~ saturation moment per formula unit at 4.2 K; T~ Curie temperature; TSR spin reorientation temperature; a and c are the lattice constants. For M c see main text Compound

as (Am2/kg)

Ms (#B/F.U.)

Mc (/.tB/F.U.)

TSR (K)

Tc (K)

a (nm)

c (nm)

N d 2 Fe14C Ndl.sDy0.2 Fe14C Ndl.6Dy0.4 Fe14C Ndl.2Dy0.8Fe14C Dy2Fe14C Ndl.sLu02Fe14C Ho 2 Fe14C

175.0 165.6 142.5 129.4 52.5 175.0 54.0

32.5 30.9 26.9 24.8 10.5 34.1 10.9

36.6 33.9 31.3 26.0 10.0 35.9 10.0

120 108 98 98

535 535 537 540 555 530 525

0.8827 0.8810 0.8804 0.8786 0.8771 0.8788 0.8739

1.2022 1.1998 1.1969 1.1925 1.1901 1.1964 1.1797

117 35

F.R. de Boer et aL / Properties of NdzFeHC i

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Fig. 1. Temperature dependence of the magnetization in N d 2 F e ] 4 C measured with B = 1.25 T on powder particles that were allowed to rotate freely in the sample holder. The inset shows the temperature dependence of the ac-susceptibility (in arbitrary units).

lattice moment of 30/~B/F.U. [1] and rare earth sublattice magnetizations equal to the free ion values. The fact that the M s values are significantly lower than the M c values for high N d concentrations may be regarded as further proof that the [001] direction is not the easy magnetization direction at 4.2 K. The values of M s and M c can be used to obtain an impression on the tilt angle 0 between the

Fig. 3. Field dependence of the magnetization at 4.2 K in Nda.2Dy0.aFel4C. The symbols Oll and o± apply to measurements made with the external field in a direction parallel and perpendicular to the alignment field of the powder particles.

c-axis and the easy magnetization axis at 4.2 K. Using the fact that the powder particles are aligned along the c-direction and using M s = M c cos 0 one finds that 0 in Nd2Fe14C is equal to about 26 ° Although this value is subject to considerable experimental error owing to the possibility of some misalignment of the powder particles, it is interesting to not that it is almost equal to the tilt angle (30 °) found by various authors for the isotypic compound Nd2Fe14B at this temperature [7]. The c o m p o u n d Dy2Fel+C, like Dy2Fe14B , favours an easy c-axis magnetization at all temper-

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Fig. 2. Field dependence of the magnetization at 4.2 K in Nd2Fez4C. The symbols Oll and a± apply to measurements m a d e with the external fields in a direction parallel and perpendicular to the alignment field of the powder particles.

O0

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B (T) Fig. 4. Field dependence of the magnetization at 4.2 K in Ndt.sLu0.2Fez4C. The symbols Oll and oj. apply to measurements with the external field in a direction parallel and perpendicular to the alignment field of the powder particles.

266

F.R. de Boer et al. / Properties of Nd2Fel4C

atures below T~. A c c o r d i n g l y one m a y expect that the tilt angle will g r a d u a l l y close with x in ( N d l _ ~ D y x ) 2 F e a 4 C. I n s p e c t i o n of the values of M s a n d M c in table 1 suggests that 0 b e c o m e s zero at a b o u t e q u i a t o m i c p r o p o r t i o n s of D y a n d Nd. Closely c o n n e c t e d with this result is the syst e m a t i c b e h a v i o u r of ( N d a _ x R ~ ) 2 F % 4 B c o m p o u n d s to show values of TsR decreasing (increasing) with x when R is a rare e a r t h e l e m e n t for which the second o r d e r Steven factor a j is negative (positive) [9]. This b e h a v i o u r is also f o u n d for the TsR values of the c a r b i d e s investigated in the course of the present investigation (see table 1). Concluding, we have shown that the c o m p o u n d N d z F e 1 4 C exhibits the same interesting m a g n e t i c p h e n o m e n a as N d 2 F e a 4 B . The spin r e o r i e n t a t i o n t e m p e r a t u r e a n d the critical field a s s o c i a t e d with the F O M P t r a n s i t i o n are n e a r l y the same in b o t h c o m p o u n d s . These results are in a g r e e m e n t with the analysis of m a g n e t i c d a t a [1,8] a n d M/3ssbauer d a t a [10] of R2Fea4C c o m p o u n d s r e p o r t e d previously, showing that the m a g n e t i c c o u p l i n g constants a n d crystal field p a r a m e t e r s in R z F e 1 4 C are very similar to those in R2Fea4 B.

Acknowledgements T h e present investigation has b e e n carried out within the scientific exchange p r o g r a m b e t w e e n C h i n a a n d T h e N e t h e r l a n d s a n d forms p a r t of the

research p r o g r a m of the " S t i c h t i n g v o o r F u n d a m e n t e e l O n d e r z o e k d e r M a t e r i e ( F O M ) " , which is f i n a n c i a l l y s u p p o r t e d b y the " N e d e r l a n d s e Organisatie voor Zuiver-Wetenschappelijk Onderz o e k " a n d b y the C E A M ( C o n c e r t e d E u r o p e a n A c t i o n on M a g n e t s ) p r o g r a m m e of the C o m m i s sion of the E u r o p e a n C o m m u n i t i e s .

References [1] F.R. de Boer, Huang Ying-kai, Zhang Zhi-dong, D.B. de Mooij and K.H.J. Buschow, J. Magn. Magn. Mat. 72 (1988) 167. [2] K.H.J. Buschow, D.B. de Mooij and C.J.M. Denissen, J. Less-Common Metals 140 (1980) in press. [3] R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Muller and R. Roeland, in: High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983) p. 127. [4] C.J.M. Denissen, D.B. de Mooij and K.H.J. Buschow, J. Less-Common Metals (to be published). [5] S. Sinnema, R. Verhoef, J.J.M. Franse and F.R. de Boer, Proc. Intern. Workshop on Magnetic Anisotropy and Coercivity, Bad Soden 1987, eds. C. Herget and R. Poerschke (Publ. by Deutsche Physikalische Gesellschaft) p. 69. [6] R.J. Radwanski and J.J.M. Franse, Phys. Rev. B in press. [7] K.H.J. Buschow, Mater. Sci. Rep. 1 (1987) 1. [8] S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij and K.H.J. Buschow, J. Magn. Magn. Mat. 44 (1984) 333. [9] K.H.J. Buschow, Proc. Ninth Intern. Workshop on Rare Earth Magnets, Bad Soden 1987, eds. C. Herget and R. Poerschke (Published by Deutsche Physikalische Gesellschaft, Bad Honnef FRG) p. 453. [10] K.H.J. Buschow, D.B. de Mooij, M. Brouha, H.H.A. Smit and R.C. Thiel, IEEE Trans. Magn. MAG (March 1988).