ELSEVIER Journal of Magnetism and Magnetic Materials 166 (1997) 249-252

~ l ~ Journal of magnetism and magnetic materials

Structure and magnetostriction of R( Fe l_xMn x) 1.85 alloys (R = DYo.6Tbo.3Pro. l)

C.H. Wu a,*, C.P. Yang b, Y.C. Chuang b,c, X.M. Jin b, j .y. Lid a International Centre for Materials Physics, Academia Sinica, Shenyang 110015, China

b Institute of Metal Research, Academia Sinica, Shenyang 110015, China c South China University of Technology, Guangzhou 510641, China

d Institute of Physics, Academia Sinica, Beijing 100080, China

Received 21 January 1996; revised 29 March 1996

Abstract

The structure, Curie temperature and magnetostriction of R(Fe 1 _xMnx)~.s5 alloys (x = 0-0.3) were investigated. These alloys are essentially single phase with a cubic Laves structure. The lattice constant increases steadily and the Curie temperature decreases linearly with increasing Mn content. The largest magnetostriction occurs at about x = 0.05.

Keywords: Magnetostriction; Cubic Laves phase

1. Introduction

For most industrial applications of magnetostric- rive materials, high strain at low magnetic field is necessary. In this case a low magnetocrystalline anisotropy while still maintaining a large magne- tostriction is immensely important. The cubic Laves phase compounds TbFe 2 and DyFe 2 have high mag- netostriction at ambient temperature but also have high anisotropy, which necessitates a high field strength to saturate the magnetostriction. Therefore, they are unsuitable for technological applications. However, pseudobinary compounds Dyl_xTbxFe 2 have high magnetostriction and relatively low anisot- ropy [1]. The pseudobinary alloy Dyo.7Tbo.3Fe 2 was first suggested by Clark [2] as a candidate material.

* Corresponding author. Fax: +86-24-3891-1320.

Numerous investigations have been performed on the magnetostrictive and magnetic properties of Dy l_ xTbxFe2 alloys.

Modifications to the properties via the addition of other elements such as Mn to replace some iron have been studied. A number of investigations on the effect of substituting Fe with Mn on the magne- tostrictive properties of Dy I _ xTbxFe2 have been car- ried out [3-7]. The replacement of Fe by Mn in polycrystalline Dyl_xTbxFe~ has been reported to shift the magnetocrystalline anisotropy compensation composition to higher Tb content and improve the magnetostriction [3-5]. However, for alloy samples prepared by a free-standing zone method, low Tb concentration samples of Dyl xTbxFek95 with Mn substitution appear to have a lower ratio of hysteresis field to magnetostriction ( A H / A ) than unsubstituted samples [6]. Substitution of Mn for Fe for fixed x may decrease the temperature of the anisotropy com-

0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00419-2

250 C.H. Wu et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 249-252

pensation (T m ) in Dy, _ ~Th~(Fe i - y Mn y)1.9 alloys [7]. These results indicate that a small amount of Mn substitution could play a relatively important role in improving the magnetostrictive properties of unsub- stituted samples.

In the search for a novel practical material for magnetostrictive applications, the magnetostrictive and magnetic properties of some multicomponent alloys have recently been investigated by our group [8-10]. It was found that the magnetostriction and magnetic properties are markedly influenced by the addition of Pr [8,9]. The addition of P r to D y o . 9 xTbxPro.lFel.85 a n d DYo.9_xTbxPro.l(Feo.9Mno.1)l. 8 alloys [10] shifts the magnetocrystalline anisotropy compensation compo- sition to lower Tb content as compared with that of DYt_xTbxFe2 [2] and Dy l_xTbx(Fe0.9Mn0,1) 2 [3] al- loys, respectively.

In the present investigation the magnetostriction, Curie temperature and structure of R(Fe ~ _ ~ Mn x)1.85 alloys (R = Dy0.6Tbo.3Pr0. ~) were determined in or- der to reveal the effect of substitution.

2. Experimental techniques

The alloys were prepared by melting the compo- nent metals in a magneto-controlled arc furnace un- der a high purity Ar atmosphere. The purity of the rare earths (Dy, Tb and Pr), Fe and Mn was 99.9, 99.8 and 99.7%, respectively. R(Fe I xMn~)185 al- loys (R=Dy0.6Tb0.3Pr0. l) with x = 0, 0.05, 0.10, 0.15, 0.20 and 0.30 were prepared. Weight losses during melting due to evaporation of Dy and Mn were compensated for by starting with a surplus of 3 wt% Dy and 5 wt% Mn, respectively. The weight loss of each sample during melting was less than 0.7 wt%.

Cylindrical samples (Q 10 mm × 15 mm) for magnetostriction measurements were prepared by arc casting and homogenized at 950°C for 50 h. The magnetostriction was measured using a strain gauge. For conveniently fixing the strain gauge, a narrow flat surface parallel to the longitudinal axis of the cylindrical specimen was prepared. The magne- tostriction of the specimens parallel and perpendicu- lar to the applied field was measured at room tem- perature up to 10 kOe.

X-ray diffraction analysis and lattice parameter measurements were carried out using a D/max-rA X-ray diffractometer with a pyrolytic graphite mono- chromator. CuK~ radiation was used. The Curie temperature was determined by measuring the tem- perature dependence of the ac susceptibility. For metallographic examination, the specimens were etched with 2% Nital.

3. Results and discussion

X-ray diffraction analysis indicated that all ho- mogenized samples of R(Fel_xMnx)l.85 alloys are essentially single phase with a cubic Laves structure. The lattice parameter of these cubic Laves phases increases steadily from 0.7338 nm for x = 0 to 0.7381 nm for x = 0.30 (Fig. 1). However, metallographic examination showed that there was a small amount of dark rare-earth-rich phase in all samples. This dark second phase increases slightly with increasing Mn substitution. It seems that the substitution of Fe by Mn shifts the homogeneity range of the Laves phase towards the stoichiometric composition. This rare-earth-rich second phase appeared both as intra- granular and intergranular precipitates.

Fig. 2 shows the Mn dependence of the Curie temperature for R(Fe l_xMnx)l.85 alloys. It is appar- ent that the Curie temperature decreases linearly with increasing Mn content. The interatomic distance be- tween the nearest transition metal atoms in the cubic Laves phase is (x/2/4)a. Because all of the values of (x/2/4)a calculated for the R(Fe t_~Mnx)l.85 system (x < 0.3) do not exceed 0.261 nm the Mn atom will

0.739

0.738

8.737

0.736

~. 735

8.734,

0 . 7 3 3 8

Fig . 1. La t t i c e p a r a m e t e r v e r s u s

(Dyo.6Tbo 3 Pro i )(Fel _ , Mn~)Ls5 alloys.

' ' ' 8.'2 ' 8.85 O.I 0.15 8.25 838

x

M n c o n t e n t for

C.H. Wu et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 249-252 251

458

0,05 011 0.115 fl,2 8,125 0.30

×

Fig. 2. Cu r i e t e m p e r a t u r e v e r s u s M n c o n t e n t

(Dyo.6Tbo.3Pro.I XFel -xMn~)l.85 alloys.

for

1888 1 6 0 0 . t-,.~. ~,, o., [

= H--~e. ek Oe 14110 - . H-4.eko.,, . H ~ . 5 . O1,: O,~,

.~4 88e

f~e

488

i i i i i

0.05 8. l 8. ,5 8.2 8,25 838

×

Fig , 4. M a g n e t o s t r i c t i o n c u r v e s o f p o l y c r y s t a l l i n e (Dyo.6Tbo.3Proi)(Fe I_xMnx)l.85 alloys at various applied mag- netic fields.

experience antiferromagnetic exchange with itself and with the Fe atoms [11]. The reduction in the magnetic ordering temperature (T c) of this system presumably results from antiferromagnetic Fe-Mn interactions, which is similar for Er(Fe)_xMnx) 2 [11] and (Y0.1Th0.9)(Fel_xMnx)2 [12].

From the magnetic field dependencies of the mag- netostriction for polycrystalline R(Fe~_~Mnx)].s5 al- loys at room temperature (Fig. 3), it is obvious that even in an applied field of 10 kOe the magnetostric- tion of some low Mn content alloys has not reached saturation, but for high Mn content samples it has reached saturation. The magnetostriction in various applied magnetic fields for the sample with a very small amount of Mn substitution is superior to that of other alloys in the system studied (Fig. 4). When the Mn content is in the 0.05 < x < 0.15 range the magnetostriction decreases sharply in high applied fields ( H > 2 kOe), whereas it only decreases slightly in low applied fields (H < 1 kOe). However, when

the Mn content is increased to x = 0.30 the magne- tostriction decreases drastically even in low applied magnetic fields. The optimal Mn content for this system is approximately x = 0.05. This result is similar to that for (Y0.1Tbo.9)(Fe]_~Mnx)2 reported in Ref. [12]. This interesting magnetostrictive be- haviour may be ascribed to the complex coupling between R (rare earth) and Mn moments. The contri- bution of Mn atoms to the magnetocrystalline anisot- ropy of rare earth sites takes place through R-Mn coupling. As stated above, because the Mn-Mn in- teratomic spacing in the R-F e -Mn alloy series stud- ied is just below the critical value d c (0.266 nm) the itinerant 3d moment of the Mn atom becomes unsta- ble. The Mn magnetism in these alloys is close to instability and is very sensitive to external parame- ters such as temperature, magnetic field, pressure and alloying [13-16]. The effect of the substitution of Fe by Mn on the magnetostrictive behaviour is complex and requires further investigation.

2888

×=8.85 ×=~ ~ -

n A o ,~ A o

A o o W 1200 -o , ," ~" , , ,: x=e.10 X=8, 15

888 .o • o ~

400 ×=0.30 ; - - ? . : . . . . . . . i i i i

88 2 4- G 8 18 12 14 H(k0e)

F ig . 3. M a g n e t o s t r i c t i o n cu rves f o r p o | y c r y s t a l l i n e

(Dy0.6Tb0.3Pr0.j)(Fe I _xMnx)l.85 alloys at room temperature.

4. Concluding remarks

Dy0.6Tb0.3Pr0A(Fel_xMnx)l.85 alloys with x < 0.3 are essentially single phase with a cubic Laves struc- ture. The lattice constant increases monotonically with increasing Mn substitution. On the contrary, the Curie temperature decreases linearly with increasing Mn content. The magnetostriction is influenced markedly by the substitution of Mn for Fe, especially for samples with a small amount of Mn. The optimal Mn content for the system studied is approximately x = 0.05.

252 C.H. Wu et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 249-252

Acknowledgements

This work was suppor ted by the Na t iona l Na tura l

Sc ience F o u n d a t i o n of China .

References

[1] A.E. Clark, in: Ferromagnetic Materials, Vol. 1, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980) p. 531.

[2] A.E. Clark, in: AIP Conf. Proc., Vol. 18 (American Institute of Physics, New York, 1974) p. 1015.

[3] M. Sahashi, T. Kobayashi and T. Funayama, in: Proc. 10th Int. Workshop on Rare-Earth Magnets and Their Applica- tions, Part 1, Kyoto, Japan, May 1989 (Society of Nontradi- tional Technology, Tokyo, 1989) p. 347.

[4] T. Funayama, T. Kobayashi, I. Sakai and M. Sahashi, Appl. Phys. Lett. 61 (1992) 114.

[5] W.D. Zhong and J.H. Wang, in: Proc. 2nd Int. Symp. on Physics of Magnetic Materials, Beijing, China, July 1992, ed. S.G. Zhang (International Academic Publishers, Beijing, 1992) p. 93.

[6] J.P. Teter, A.E. Clark, M. Wun-Fogle and O.D. McMaster, IEEE Trans. Magn. 26 (1990) 1748.

[7] A.E. Clark, J.P. Teter and M. Wun-Fogle, J. Appl. Phys. 69 (1991) 5771.

[8] C.H. Wu, W.Q. Ge, X.M. Jin, Y.C. Chuang and X.P. Zhong, J. Alloys Compounds 191 (1992) 169.

[9] Y.J. Tang, C.H. Wu, X.P. Zhong and H.L. Luo, Phys. Status Solidi (a) 140 (1993) 231.

[10] C.H. Wu, X.M, Jin, W.Q. Ge, Y.C. Chuang, X.P. Zhong, R.Q. Li and J.Y. Li, J. Magn. Magn. Mater. 163 (1996) 360.

[11] A.S. Ilyushin and W.E. Wallace, J. Solid State Chem. 17 (1976) 131.

[12] Hong-Ying Yang, Hui-Qun Guo, Bao-Gen Shen, Lin-Yuan Yang, Jian Yuan and Jian-Gan Zhao, Phys. Status Solidi (a) 129 (1992) KI07.

[13] H. Wada, K. Yoshimura, M. Shiga, Y. Nakamura, J. Sakurai and Y. Komura, J. Phys. F 18 (1988) 981.

[14] K. Yoshimura, M. Shiga and Y. Nakamura, J. Phys. Soc. Jpn. 55 (1986) 3585.

[15] C. Ritter, S.H. Kilcoyne and R. Cywinski, J. Phys. Condens. Matter 3 (1991) 727.

[16] J.P. Brown, B. Ouladdiaf, R. Ballou, J, Deportes and A.S. Markosyan, J. Phys. Condens. Matter 4 (1992) 1103.