by K. H. J. BUSCHOW Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands

Transcription

1 Philips J. Res. 40, , 1985 RI114 RARE EARTH BASED INVAR ALLOYS by K. H. J. BUSCHOW Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands Abstract New types of Invar alloys on the basis of iron can be obtained by forming intermetallic compounds with cubic crystal structure of the NaZnlS type having a nominal composition La(Fe,Co,X)is, where X is Si or AI. PAC~ numbers: y. 1. Introduction Although no intermetallic compounds are known to exist in the La-Fe system, cubic NaZn13 type compounds based on Fe and La can be stabilized by combining these elements with either Si or Al. Cubic compounds of the type La(FexSh-xh3 exist in the range 0.81 ~ x ~ 0.88, cubic compounds ofthe type La(FexAh-xh3 exist in the range 0.46 ~ x < ). The La(FexAh-xh3 compounds in particular were found to exhibit a unique magnetic phase diagram. At the Fe-rich phase boundaries the structure is not stable with respect to LaFe4AIs and pure a-fe. In spite of its limited composition range, the system may be found 2) in three different magnetic structures: (i) at low iron concentration a mictomagnetic state was found, originating from a competition between antiferromagnetic Fe-AI-Fe superexchange and ferromagnetic Fe-Fe direct exchange; (ii) at higher iron concentrations ferromagnetism occurs and finally (iii) at the highest iron concentrations an antiferromagnetic state was found. In this antiferromagnetic region distinct metamagnetic transitions were observed with large hysteresis effects in relatively low fields (H:::::: 5T) with respect to the transition temperature (T:::::: 200 K) 3). The metamagnetic transitions are accompanied by large magnetostrictive effects (~V/V:::::: 1070). Also the magnetic ordering in the ferromagnetic regime is accompanied by strong Invar effects. Results for one of these compounds are shown as an example in fig. 1, where the thermal expansion behaviour expected on the basis of the Phlllps Journalof Research Vol. 40 No

2 K. H. J. Buschow 80r ~----~ 70 A!.10-3 I 60 La Fe11.2 A " " """ _ TIK) Fig. 1. Temperature dependence of the linear thermal expansion (!:i lil) for the cubic compound LaFell.2Ah.8. The broken line represents the Grüneisen function calculated with a (300 K) = and a Debije temperature equal to BD = 300 K (after Palstra et al. 1). Grüneisen function is represented by a broken line. The relatively huge magnetic contribution to the thermal expansion was explained with a combined band and local moment model.'). Magnetovolume effects like those described above form the basis of the large class of materials commonly referred to as Invar alloys 4). When the magnetic contribution to the thermal expansion is still sufficiently strong and negative at temperatures above room temperature it can counterbalance the equally strong but positive normal thermal expansion, the result being materials with the favourable property that their specific volume is to a certain extent independent of temperature changes. In the present investigation we have tried to utilize the strongly negative magnetic contributions to the thermal expansion in alloys of the type La(FexAh-xhs and La(FexSh-xhs to obtain suitable Invar type alloys. For this purpose Co was substituted for part of the Fe in these.materials, shifting the magnetic ordering temperature into ranges of practical interest. 306 Phlllps Journalof Researe,! Vol.40 No

3 Rare- earth based Invar alloys 2. Expertmental Various compounds of the type La(Fe,Co,AI)13 and La(Fe,Co,Si)Is were prepared by means of are melting in an atmosphere of purified argon gas. Starting materials of at least 99.90/0 purity were used. The arc-melted alloy buttons were wrapped in tantalum foil and were sealed in a quartz tube for vacuum annealing (2 weeks, 900 C). Powder diffraction patterns of the samples were obtained using a Philips X-ray diffractometer (type PW 1050/25) with CuKa radiation in conjunction with a graphite crystal monochromator. The X-ray diagrams of the annealed sample were indexed on the basis of a cubic NaZnlS type unit cell, the amount of second phases being negligibly small. The thermal expansion 111/1 of the various alloys was measured in the temperature range from 20 C to 700 C on a standard dilatometer under purified argon gas. 3. Experimental results Results of dilatometric measurements obtained on several La(Fe,Co)uSb compounds are shown in fig. 2. The Co content of these compounds increases from A to. C, and the same holds for the Curie temperature. For the compound LaFesCosSb (C) the Curie temperature is close to 200 C. The mag- 40 LolFe,Co)nSi2 20 o o Fig. 2. Temperature dependence of the linear thermal expansion àtl! for the compounds: (A) LaFeIOCo,Si2; (B) LaFegCo2Si2 and (C) LaFeaCosSb. Philips Journol of Research Vol. 40 No

4 K. H. J. Buschow netostriction leads to an anomaly in the thermal expansion behaviour close to the temperature where magnetic ordering sets in, making the thermal expansion coefficient almost ternperature-independent in the range from room temperature to about 200 C. The Curie temperature is much lower in the other two La(Fe,Co)l1Sb alloy's shown in fig. 2. It is close to room temperature in LaFelOCoSb (A). Since the magnetostrictive effects are limited mainly to temperatures below Tc the thermal expansion in LaFe lo CoSi2 does not show much of an Invar effect in the temperature range of interest. This is also the case for the two alloys LaFell.2Sh.a and LaFelO.6Sb.6,for which Tc is below room temperature (fig. 3) L- ~ ~L ~ ~ L ~ o T (Del Fig. 3. Temperature dependence of the linear thermal expansion /),./// for the compounds: (A) LaFel1.2Sh.s; (B) LaFelO.5Si2.5;(C) LaFesCo5Sb and (D) LaFe 7 Co 4 Si 2 The temperature dependence of the linear thermal expansion shows the same behaviour in intermetallic compounds of the type La(Fe,Coh1.6AI1.6. The results are shown in fig. 4. The Co concentration in these compounds increases in the direction from A to E. The temperature range where the linear thermal expansion shows an anomaly extends from approximately 20 C to approximately 300 C in the case of the intermetallic compounds B to E. In the case ofcompound A the Invar anomaly has moved to lower temperatures. 308 Phlllps Journalof Research Vol. 40 ~o

5 Rare earth based Invar alloys ~ ~----~ 25 C Fig. 4. Temperature dependence of the linear thermal expansion àtl! for the compounds: (A) LaFellCoO.6Ah.6; (B) LaFelO.6Co1Ah.6; (C) LaFe9.6Co2AI1.6; (D) LaFes.9Co2.sAI1.6and (E) LaFe8.6CosAh.6. Lo(Fe,Co)11A12 25 Fig. 5. Temperature dependence of the linear thermal expansion for the compounds: (A) LaFelO.6Coo.6Ab;(B) LaFelOColAb and (C) LaFe9Co2Ab. Phlllps Journalof Research Vol. 40 No

6 K. H. J. Buschow A third group of intermetallic compounds 'that has been examined can be represented as La(Fe,Co)llAb. Results of the dilatometric measurements are given in fig. 5. The temperature range where the linear thermal expansion shows an anomaly extends from approximately 0 C to approximately 200 C in the case of the intermetallic compounds Band C. In the case of compound A, the Co concentration is too low and the magnetic ordering temperature and the Invar anomaly have moved again to temperatures below room temperature. 4. Discussion The results described in the previous section can be summarized as follows: The intermetallic compounds considered in this report can be represented by the general formula LaFelS-x-yCO X Xy, where X is Si or AI. All these compounds crystallize in the cubic NaZnlS structure, and have predominantly a ferromagnetic coupling between the magnetic 3d moments. If y becomes too large (larger than 2.5 when X = Si and larger than 3 when X = AI) the magnetic ordering temperature is located too far below room temperature for practical applications. If y becomes too small (y < 1.5 when X = Si and y < 1 when X = AI) the cubic NaZn13 type structure is not formed. With respect to the Co content, one may state that for practical applications, it should preferably be larger than x = 1.5. The temperature range where the anomaly in the linear thermal expansion occurs decreases with smaller x values to temperatures too far below room temperature. On the other hand the Co concentration must not become too high because then the magnetostrictive effect becomes too low to be able to outweigh the normal thermal expansion (see fig. 3, examples C and D). The materials described here differ from the class of materials usually referred to as Invar alloys in that they are brittle whereas the latter are ductile. This offers the possibility of using powder metallurgy, including sintering on suitable mixtures of powders, to obtain materials in which the thermal expansion virtually vanishes in a limited temperature range or materials in which the Invar behaviour extends over a fairly large temperature range. An example of the first class might be obtained by mixing equal amounts of powders of the compounds LaFellCoO.5AI1.5(A) and LaFe9.5C02Ah.5 (C), leading to a nearly vanishing thermal expansion in the range from 20 C to 200 C (see fig. 6). An example of the second class is shown in fig. 7, where equal amounts of powders of the compounds LaFe9.2C02.3Ah.5(C) and LaFesCos.5Ah.5 (E) are expected to lead to a material in which the thermal expansion remains low over a temperature range of more than 300 C. 310 Phllips Joumal'of Research Vol.40 No

7 Rare earth based Invar alloys T(Oe) Fig. 6. Temperature dependence of the linear thermal expansion expected for a material obtained by combining powders of LaFeIlCog.5AI1.5(A) and Lag.5Co2A11.5(C). ~r ~ e -25'--_-'--_-'--_--'-_--"-_---L_---' o 200 T (Oe) Fig. 7. Temperature dependence of the linear thermal expansion expected for a material obtained by combining powders of LaFe9.5Co2.sAI1.5(C) and LaFesCo3.5AI1.5(E). Phillps Journalof Research Vol.40 No

8 K. H. J. Buschow Acknowledgement The author wishes to express his gratitude performing the dilatometric measurements. to C. A. M. van Tienhoven for REFERENCES 1) T. T. M. Palstra, J. A. Mydosh, G. J. Nieuwenhuys, A. M. van der Kraan and K. H. J. Buschow, J. Magn. Magn. Mater. 36, 290 (1983). 3) T. T. M. Palstra, G. J. Nieuwenhuys, J. A. Mydosh and K. H. J. Buschow, Phys. Rev. B 31, 4622 (1985). 3) T. T. M. Palstra, H. G. C. Werij, G. J. Nieuwenhuys, J. A. Mydosh, F. R. de Boer and K. H. J. Buschow, J. Phys. F 14, 1961 (1984). 4) 'Physics and Application of Invar Alloys', Honda Mem!. Ser. Mater. Sci. 3 (1978). 312 Philip. Journalof Research Vol.40 No