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FORCED VOLUME MAGNETOSTRICTION IN
RAPIDLY QUENCHED AMORPHOUS R-Fe (R = Pr,
Nd, Gd, Dy AND Y) ALLOYS
S. Ishio
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, dkembre 1988
FORCED VOLUME MAGNETOSTRICTION IN RAPIDLY QUENCHED AMORPHOUS R-Fe
(R
= Pr, Nd, Gd, Dy ANDY)
ALLOYSDept. of Appl. Phys., Tohoka University, Sendai 980, Japan
Abstract.
-
The forced volume magnetostriction (Ow/OH) of R1-,Fez (R = Pr, Nd, Gd, Dy and Y: 0.4<
z<
0.9) alloys has been measured at temperatures from 77 K to 400 K. The contribution from the molecular field of Fe and R subnetworks to (dw/aH) is discussed.1. Introduction
Rare earth metal(R)-transition metal(TM) com- pounds exhibit a large magnetovolume effect. While the magnetovolume effect of RCo2 originates from the strong molecular field of the rare earth [I], that of RMn2 may be attributed to the depefldence of Mn 7:
0
magnetic moments on the volume [2]. On the other hand, in amorphous Fe-TM and Fe-metalloid alloys, '0
-
dense-packed Fe atoms in the amorphous states cause
-
the instability of ferromagnetism and the large magne- tovolume effect [3]. Although it is known that amor- phous R-Fe alloys show a magnetovolume effect, only a few experimental works were carried out so far. Present paper describes the experimental results of the 3rn
forced volume magnetostriction in amorphous R-Fe al-
-
0loys.
2. Experimental
Amorphous Rl-=Fe, alloys were prepared by a rapid quenching in an argon atmosphere. They were formed into ribbon, about 1 mm and 10-20 pm thick, and the
amorphous structure was confirmed by X-ray Debye- 100 2 0 0 300 4 00
Scherrer analysis. The magnetostriction was measured T ( K ) by a three terminal capacitance method in magnetic
fields up to 20 k o e and in temperatures from 77 K Fig. phous Ndl-,Fez alloys. 1. - Temperature dependence of (dw/dH) for amor- to 400 K [3]. The forced volume magnetostriction
( a w / a ~ ) is defined by ( a ~ l a ~ ) =
a
(A,,+
2 ~/ a ~ ,
~ ) where All+
2Al is volume expansion due to appliedfields. this investigation, the of ( a w / a ~ ) was nomenon relates t o an extraordinary coercive force evaluated by using data between 10 kOe and 20 kOe, found at low temperatures for Pr, ~ d , D Y - F ~ alloys
[41. 3. Experimental results
Figure 1 shows the temperature dependence of (dw/aH) for Ndl-,Fez. Curie temperatures indicated by arrows were determined by Arrott plots. The (aw/aH) value for
x
= 0.8 is about 55x
lo-''
0e-l at 77 K and increases to about 295 xlo-''
0e-' a tT,. Hysteretic A-H curves were observed at low tem-
peratures for x 0.7 and, below
T H ~ ,
A-H curves became irreversible even at more than 10 kOe. In this case, (8wlaH) was evaluated by using A-H curves in decreasing H and this causes the sudden decrease of (aw/aH) as can be seen in the figure. This phe-The temperature dependence of (aw/aH) fof Gd- Fe is shown in figure 2. While (aw/aH) for x = 0.7 is negative in temperatures from 77 K t o 400 K, (aw/dH) for x = 0.6 changes a sign at about 220 K.
The composition dependence of (aw/aH) at 77 K is shown in figure 3. The data points having error bars were obtained by an extrapolation from temper- atures above
T H ~ .
The (aw/aH) values for the amor- phous R-Fe alloys decrease rapidly with decreasing z, which is the same trend as the result for Fe-B [3]. It should be noted here that the sign of (aw/SH) for R = Gd and Dy changes from positive t o nega- tive around x = 0.8 with decreasingx.
MagnetizationArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888615
C8 - 1346 JOURNAL DE PHYSIQUE
8 . 0
4. Discussion
Fig. 2. - Temperature dependence of ( a w / a H ) for amor- phous Gdl-,Fez alloys.
Fig. 3. - Composition dependence of ( d w / d H ) at 77 K for
amorphous R1-,Fez alloys. The results of Fe-B, Zr, Hf alloys are also shown.
In rare earth metal(R)-transitio metal (TM) alloys and compounds, the magnetic mom.ent of TM elements is induced by the molecular field [I, 2, 51. The Fc magnetic moment at low temperatures can be written [51
where mFe is the Fe magnetic moment, mFe (0) the Fe magnetic moment in a zero magnetic field,
x
the sus- ceptibility, ( J ) the thermal average of the R moments, and JFeFe and J F ~ R are the molecular field coefficientsfrom Fe and R subnetworks. The sign for H in the parenthesis corresponds to whether H is parallel or anti-parallel to mw in ferrimagnets (or sperimagnets). The forced volume magnemtostriction is attributed to the volume expansion due to the increase of Fe mag- netic moments in applied fields, given by (dw/dH) = 2nCmpe ( d m ~ ~ / a H )
,
and is rewritten as(2)
where KC is the magnetovolume coupling constant.
The first term in the parenthesis gives to a change of a sign of (dw/dH) in ferrimagnets (or sperimagnets) and the second one which comes from molecular fields of R-subnetworks (HR) makes a positive contribution to (dw/dH). As can be seen in the figure 3, the sign of (dw/dH) of Dy-Fe and Gd-Fe changes around .,,,z, This suggests that the second tenn in equation (2) (hence the induced Fe moment due to HR in Eq. (1)) is negligible.
From these results it could be considered that the large forced volume magnetostricticln found in amor- phous R-Fe alloys, especially in the Fe rich composi- tion range, originates from only a nature of Fe atoms; namely the R-dependence of (dw/dhT) must be consid- ered in terms of a modification of thfe electronic struc- ture of Fe atoms. Thermal expansion measurements as well as Mijssbauer spectroscopy are now in progress to obtain more informations about the origin of the magnetovolume effect in amorphous R-Fe alloys.
[I] Beille, J., Bloch, D. and Voiron, J., AIP Conf. Proc. No 29 (1976) p. 123.
[2] Yoshimura, K., Shiga, M. and Nakarnura, Y., J. Phys. Soc. Jpn 55 (1986) 3585.
measurements revealed that a magnetic compensation [3] Ishio, S. and Takahashi, M., .I. Magn. M a p . composition x,,,, at 77 K is 0.78 for Dyl-,Fez and Mater. 50 (1985) 93.
0.75 for Gdl-,Fe,. This x,,,,-value is very close to [4] Rhyne, J. J., Schelleng, J. H. amd Koon, N. C., the composition at which the sign of (dw/dH) changes. Phys. Rev. B 10 (1974) 4672.
