journal of ~ magnetism

and • magnetic materials

ELSEVIER Journal of Magnetism and Magnetic Materials 163 (1996) 360-364

Magnetostriction and anisotropy compensation composition of DYo.9-xTbxPro.lFel.85, Dyl- Tbx(Feo.9Mno.1)1.8 and

Dyo.9-xTb Pro.l(Feo.9Mno.1) 1.8 alloys C.H. Wu a,b,*, X.M. Jin b W.Q. Ge b, Y.C. Chuang b,c, X.P. Zhong d, R.Q. Li d,

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 Intitute of Physics, Academia Sinica, Beijing 100080, China

Received 26 June 1995; revised 21 March 1996

Abstract

The structure, Curie temperatures and magnetostriction of the alloys Dyo.9_xTbxPro.lFe1.85, Dy l_xTbx(Feo.9Mno.1)l.8 and Dyo.9_xTbxPro.l(Feo.9Mno.1)l.8 have been investigated. These alloys are essentially single phase with cubic Laves structure. The lattice parameters and Curie temperatures increase steadily with increasing Tb content. The Tb dependence of the magnetostriction in various applied fields for polycrystalline Dyo.9_xTbxPro.lFel.85 and Dyo.9_xTbxPro.l(Feo.9Mno.l)l.8 alloys exhibits peaks at x = 0.25 and 0.3, respectively. From a comparison of Dy I _xTbxFe2 and Dy 1 _xTb~(Feo.9Mno.1) 2, it appears that the addition of Pr shifts the magnetocrystalline anisotropy compensation composition to lower Tb contents.

Keywords: Cubic Laves phase pseudobinary compound; Magnetostriction; Anisotropy compensation composition

1. Introduction

For the development of rare earth-iron alloys with MgCuz-type cubic structure as magnetostrictive materials, the magnetic and magnetostrictive proper- ties of pseudobinary compounds Dyl_xTbxFe2 [1,2] and D y l _ x T b x ( F e l _ y M y ) 2 (M = A1, Co, or Mn) [3 - 8] have been studied extensively by many investiga- tors. Sahashi et al. [3] reported that the Tb dependence of the magnetostr ict ion for Dyl - xTbx(Fe i - y Mn y)2 compounds is markedly in-

* Corresponding author.

fluenced by the addition of Mn. This system pos- sesses low magnetocrystalline anisotropy energy, while the magne tos t r i c t ion remains large. The magnetostr ict ion for the polycrystal l ine Dy0.sTbo.s(Fe0.9Mno.1)2 sample is superior to those for both Dy0.sTb0.sFe2 and Dy0.TTb0.3Fe 2. However, few investigations of the magnetostriction and mag- netic properties of the multicomponent pseudo- b inary c o m p o u n d s D y i - x - y T b x P r y F e 2 and D y l _ x _ y T b x P r y ( F e l _ z M n z ) 2 have been reported.

Our recent investigations indicate that the magnetostr ict ion and magnet ic properties of Dy0.9_xPrxTb0.1Fe2 compounds are remarkably in-

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C.H. Wu et al. /Journal of Magnetism and Magnetic Materials 163 (1996) 360-364 361

fluenced by the addition of Pr. The magnetostriction of the compounds exhibits a peak near x = 0.1 [9], and the calculated anisotropy constants decrease with increasing Pr content [10]. These preliminary results stimulated our interest in investigating further the magnetostriction of the Dyl_~_yTb~PryFe 2 and Dyl_~_yTb~Pry(Fe l_zMnz)2 compounds with the Pr content maintained at y = 0.1. In our previous inves- tigation, we found that the stoichiometric pseudobi- nary compounds Dy~_xTb~Fe 2, Dy l_xTb~ - (Fel_yMny) 2 and Dyl_x_yTb~Pry(Fe1_zMnz)e al- ways contain small amounts of 1:3 phase. In order to obtain samples that are essentially single phase with cubic Laves structure, the ratio of the rare earths to iron and other metal elements was kept slightly different from 1:2.

In the present work the structure, Curie tempera- tures and magnetostriction of Dyo.9_xTbxPro.lFel.85, Dyl_xTbx(Feo.9Mno.1)l. 8 and DYo.9_xTb~Pro. 1- (Feo.9Mno.i)Ls alloys have been investigated to test the effects of the addition of Pr on the relevant properties of the original pseudobinary compounds.

2. Experimental techniques

The alloys for this investigation were prepared from materials of the following purities: rare earths (Dy, Tb and Pr) 99.9%, Fe 99.8% and Mn 99.7%. The constituent metals were melted in a magneto- controlled arc furnace under high-purity argon. We produced the alloys DY0.9_xTbxPr0.1Fel.85 with x = 0.1, 0.2, 0.25 and 0.3; Dyl_~Th~(Fe0.9Mn0.1)l.8 wi th x = 0.2, 0.3, 0.4 and 0.5; and Dy0.9_xTbxPr0.1(Fe0.9Mn0.1)l.8. with x =0.25, 0.30 and 0.35.

For the metallographic examinations, the speci- mens were etched with 2% Nital. The temperature dependence of the magnetization, o-, was determined using a vibrating sample magnetometer in an applied field of 3 kOe. The Curie temperatures were assessed by extrapolating to o-= 0 in the o- versus T curve. X-ray diffraction analysis and lattice parameter mea- surements were carded out in a D / m a x - r A diffrac- tometer with a pyrolytic graphite monochromator. Cu Ke~ radiation was used.

For the magnetostriction measurements, cylindri- cal samples ( ~ 10 × 15 mm) were prepared by

arc-casting and homogenized at 1000°C for 50 h under a purified Ar atmosphere. A narrow flat sur- face was fabricated by cutting along the longitudinal axis of each cylindrical specimen, onto which a strain gauge was fixed. The magnetostriction parallel and perpendicular to the applied field were measured at room temperature up to 20 kOe.

3. Results and discussion

X-ray diffraction analysis indicated that all these samples were almost cubic Laves single phase. How- ever, metallographic examination revealed that in the DY0.9_xTbxPr0.1Fea.s5 alloys, there were also some fine bright (Dy, Tb, Pr)Fe 3 second phase, whereas in the alloys DYt_xTbx(Feo.9Mno.1)l. 8 and DYo.9_xTbxPr0.1(Feo.9Mno.l)l.8 some dark rare earth-rich phase appeared at the grain boundaries.

Fig. 1 shows the Tb dependence of the lattice parameters and Curie temperatures for the Dyo.9_xTbxPro.tFeL85 alloys. The lattice parameters increase appreciably with increasing x in the ranges 0.1 < x < 0.2 and 0.25 < x < 0.3, and remain little changed when 0.20 < x < 0.25. The Curie tempera- tures increase steadily with increasing Tb content.

The magnetostriction was expressed as the differ- ence between the strain parallel and perpendicular to the applied field. The magnetic field dependence of the magnetostriction at room temperature for the polycrystalline Dyo.9_xTb~Pro.lFe1.85 alloys is shown in Fig. 2. It is evident that the magnetostriction of the Dyo.65Tbo.25Pro.lFel.85 alloy ( x = 0.25) is supe- rior to those of the other alloys in the Dy0.9_xTbxPr0.1Fel.85 system. The Tb dependence of

400

• 0.7340

-.?_.

"6

0.7330 o 3 5 , . i 0 2 0.3

x

Fig. 1. Lattice parameter a and Curie temperature versus Tb

content of Dyo.9_xTbxPro.lFet.85 alloys.

362 C.H, Wu et a l . / Journal of Magnetism'and Magnetic Materials 163 (1996) 360-364

1 , 5 0 0 . ,~ . . . . . . . . . . . . . . . . .

,:, x = 0 . 3 0

~. x = 0 . 2 5

• x ~ 0 . 2 0

• x = O . I 0 I 0 0 0 !

..<

5 0 0

z~ tx zx zx ~ zx z~ ~o o o o o o o

~o o • • • z~ o • o •

o o ~

OA • A • • • • • A •

~o • • A •

• A

z t r , , , i , , , , r I I ~ I I r I

0 5 I 0 1 5 2 0

, H IkOe}

Fig. 2. Magnet ic ~ield dependence of the magnetostrict ion (A -

A ) for polycrystal l ine Dy0.9LJb~Pr0jFel .85 alloys at room temperature ( x ~ 0.10, 0.20, 0.25 and 0.30).

the magnetostriction ofthese alloys at room tempera- ture for various applied magnetic fields is shown in Fig. 3. The magnetostriction increases markedly with increasing Tb content and exhibits a peak at x - 0.25, whereas the polycrystalline Dy~_~TbxFe 2 samples exhibited peaks near x - 0.3 [1]. This indicates that the addition of Pr shifts the magnetocrystalline ani- sotropy compensation composition to lower Tb con- tents. In general, the magnetostriction for the poly- crystalline pseudobinary compounds RI_~R'xF % or Rl_x_yRrxR"yFe2 in various applied fields exhibits a peak at certain values of x or y, reflecting the near-zero magnetic anisotropy at this concentration

1 5 0 0 . . . . . . . . ' . . . . . . .

• 2 kOe o 4 k O e

• 6 k O e ~ 8 k O e

• I O k O e ,7 ] 4 k O e

X 5oo°

0 , i i i i i ~ i i , r i i i , i I

0 .1 0 . 2 0 , 3 ×

Fig. 3. Tb dependence of the magnetostrict ion (A I - A ± ) for

polycrystaUine Dyo.9_xTbxPro.lFel.s5 alloys at room temperature ( x - 0.10, 0.20, 0.25 and 0.30).

4 0 0

- 0.7560

.c,- -- ~ 0 . 7 3 5 0

0 . 7 3 3 0

2 0 £ . . . . . . . . . . . . . . 0 . 7 5 2 0

0 _ 2 0 . 3 0 - 4 0 . 5 x

Fig. 4. Lattice parameter a and Curie temperature versus Tb

content of Dy 1 _ xTbx(Feo.9Mno 1)1.8 alloys.

[11]. The coupling between the rare earth and 3d moments is parallel for the light rare earths such as Pr, and antiparallel for the heavy rare earths such as Tb and Dy. Since the moment of Pr is parallel to that of the 3d sublattice, and thus antiparallel to that of Tb and Dy, the addition of Pr to the Dyl_xTbxF% compounds would lead to a decrease in the rare earth sublattice moment. Clark [11] suggested that PrF% and DyFe 2 compounds possess anisotropy constants with signs opposite to that of the TbFe 2 compound. The Tbl_xDyxFe2, Tbt_xPrxFe 2 and Tbl_x_yDyxPryF % are acceptable anisotropy com- pensating systems. Due to these conditions, the ef- fect of the addition of Pr on the magnetic anisotropy compensation becomes more complicated. The addi- tion of Pr shifts the anisotropy compensation compo- sition of the Dy0.9_xTbxPr0.1Fe 2 system to lower Tb content.. It seems likely that this effect results from the anisotropy constants K1 and K 2 of PrFe 2 being different from that of DyFe 2 compound, and the rare earth sublattice moment reduced by the addition of Pr probably also plays a certain role.

Fig. 4 shows the lattice parameters and Curie temperatures of the Dyl_~Tbx(Fe0.9Mn0.1)l. 8 alloys as function of the Tb content. The lattice parameters increase gradually in the range 0.2 < x < 0.4, and decrease appreciably with further increase of x. The Curie temperatures increase slightly in the range 0.2 < x _< 0.4, and then increase appreciably with further increases of x. It is believed that changes in the lattice parameters and Curie temperatures of the Pr-bearing alloys studied with respect to changes in the Tb content will follow similar trend.

Figs. 5 and 6 show the magnetic field dependence of the magnetostriction at room temperature for the

C.H. Wu et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 360-364 363

1600

1200

"9 o

2 ..~.~ 800

400

, , , , , , , , . . . . . , , , , , , !

o x = O . 5 o o o o o o o o

• x=0-3 o ~ #" l • x=0.2 o , ~ • • • • * * • • •

o i i o

& • • • • • • •

.o • • z x •

a

1 0 r i i , i ~ ~ r ~ f ~ r I i , T r i r

0 5 10 I5 20 H{kOe)

Fig. 5. Magnetic field dependence of the magnetostriction (Al l - A± ) for polycrystalline Dy 1 _xTb~(Feo.9Mno.~)L8 alloys at room temperature (x = 0.2, 0.3, 0.4 and 0.5).

1600 •2kOe o4kOe '

.6kOe ~8kOe ~ ' 1 " " ~ . . ~ " ~ v IOkOe '~ 14kOe ~ . . . . j . l ~

, 00

2 ~ 80C

400

T i i i , , i i , i i ~ t i r p i

0.2 0.3 0.4 0.5 ×

Fig. 7. Tb dependence of the magnetostriction (All- A m ) for polycrystalline Dy I _ ~Tb~(Fe0.9Mn0.1 )1.8 alloys at room tempera- ture (x = 0.2, 0.3, 0.4 and 0.5).

polycrystal l ine Dy t_xTb~(Feo.9Mno.1)l.8 and DYo. 9 xTb~Pro.t(Feo.gMno.1)l. 8 samples, respec- tively. It can be seen that the magnetostrictions of the alloys Dyo.5Tbo.5(Feo.gMno.1)l. s ( x = 0 . 5 ) and DYo.6Tbo.3Pro.l(Feo.9Mno.1)li8 (x = 0.3) are superior to their fe l low a l loys . The resul t for Dyo.fTbo.5(Feo.9Mno.1)l.8 is in good agreement with that reported by Sahashi et al. [3] for the Dy 1_ xTbx(Feo.9Mno.1) 2 system.

Figs. 7 and 8 s h o w the Tb dependence of the magnetostriction at room temperature in var ious applied magnet ic fields for four

, ,p

O

1500

I000

500

~ ^ o o o o o o o o

o

z~ zx o o o

o

o •

o

• x =0.25 z~ o ~ x=O.30

" o x-~3-35

0 5 I0 15 20 HIkQe)

Fig. 6. Magnetic field dependence of the magnetostricdon (All - A l ) for polycrystalline Dy0.9_xTbxPr0.1(Feo.9Mn0.1)l.8 alloys at room temperature (x = 0.25, 0.30 and 0.35).

D y l _ x T b x ( F e o . 9 M n o . 1 ) l . 8 and fo r th ree Dyo.9_xTbxPro.l(Feo.9Mno.~)l. 8 polycrystalline speci- mens, respectively. Fig. 7 shows that the magne- tostriction for the Dyl_xTbx(Feo.9Mno.1)l.8 speci- mens in higher applied fields first increases dramati- cally near x = 0.3, and then more slowly up to x = 0.5. However, in low applied fields, e.g. 2 kOe, the magnetostriction increases appreciably with in- creasing x up to x = 0.3, and only slightly beyond x > 0.3. Irrespective of the applied magnetic field, the largest magnetostriction appears at x = 0 . 5 . This situation is very similar to that of Dyl_xTb~(Fe0.9Mn0.1) 2 compounds reported by Sa- hashi et al. [3]. It reflects the magnetic anisotropy is nearly zero due to the compensation at this concen- tration. Compared with the DYl_xTb~F % system [1], the substitution of Fe by Mn shifts the magnetocrys- talline anisotropy compensation composition to higher Tb contents. This probably arises from the combined effect of a magnetic-nonmagnetic instabil- ity and a high magnetic frustration of the occurrence of Mn band antiferromagnetism. Because of the Mn instability, the magnetic properties of RMn 2 (R = rare earth) are very sensitive to external parameters such as the applied pressure, the applied magnetic field or the chemical pressure induced by alloying [12-15]. Recent studies have shown that the onset of the Mn moment is strongly sensitive to the Mn-Mn interatomic distances in the RMng compounds. There is a critical distance (d c = 2.66 A) below which Mn

364 C.H. Wu et aL / Journal of Magnetism and Magnetic Materials 163 (1996) 360-364

1500

500 ~ " ' ~ • 2kOe o 4 kOe A6kOe ~. 8 kOe v IOkOe v 14kOe * 20k0e

0 ' , , L , 0.25 0.3 035

X

Fig. 8. Tb dependence o f the magnetostriction ( A l l - A± ) for

polycrystal l ine Dyo.9_xTbxPro.l(Feo.gMno.l)Ls alloys at room temperature ( x = 0.25, 0.30 and 0.35).

is not magnetic and above which a large Mn moment is stabilized [16,17]. The magnetostructure and mag- netic anisotropy compensation of the original pseu- dobinary compound will be markedly influenced by the contribution of Mn atom to the magnetocrys- talline anisotropy of rare earth site.

Fig. 8 shows that the magnetostriction for the

Dy0.9_xTb~Pro.l(Fe0.9Mn0.1)l. 8 alloys increases abruptly in the range 0.25 _< x _< 0.3, and beyond x = 0.3 it decreases appreciably. At x = 0.3, the magnetostriction in different applied fields ex- h ib i t s a peak. In c o m p a r i s o n wi th the Dyl_xTb~(Fe0.9Mn0.i)i.8 system, the addition of Pr shifts the magnetocrystalline anisotropy compensa- tion composition back to low Tb content. This effect

is very similar to that in the Dy0.9_xTbxPr0.1Fel.85 system. However, it does not completely counteract the influence of the substitution of Mn for Fe on the anisotropy compensation.

4. Concluding remarks

T h e D y o . 9 _ x T b x P r o . l F e l . 8 5 , D y i _ x T b ~- (Feo.9Mno.1)l. 8 and Dyo.9_xTbxPro.l(Feo.9Mno.1)l. 8 alloys are essentially single phase with cubic MgCu2-type structure. The lattice parameters and Curie temperatures increase gradually with increas- ing Tb concentration. The magnetostriction and ani- sotropy compensation are strongly affected by the

addition of Pr, as well as by substitution of Mn for Fe. The substitution of Fe by Mn shifts the magne- tocrystalline anisotropy compensation composition to higher Tb contents, and the addition of Pr shifts it back to lower Tb contents. Nevertheless, it does not entirely offset the contribution of the Mn atom to the magnetocrystalline anisotropy of the rare earth site.

Acknowledgements

This work was supported by the National Natural Science Foundation of China.

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