Journal of Magnetism and Magnetic Materials 72 (1988) 181-186 North-Holland, Amsterdam

181

SATURATION MAGNETOSTRICTION AND INDUCED MAGNETIC ANISOTROPY FOR (CoTm),Zr,, (Tm = Fe, Cr, MO) AMORPHOUS ALLOYS

H. TANGE, K. INOUE *, Y. TANAKA

Faculty of Science, Ehime University, Matsuyama 790, Japan

and

K. SHIRAKAWA

Research Institute of Electric and Magnetic Alloys, Sendai 982, Japan

Received 15 October 1987; in revised form 2 December 1987

Temperature dependences of saturation magnetostriction A, for (CoTm),Zr,, (Tm = Fe, Cr, MO) amorphous alloys were measured by the 3-terminal capacitance method, and show almost two-ion character for Tm = Fe and one-ion character for Tm = Cr and MO. The concentration dependence of AS at room temperature for Tm = Fe has the same tendency as that of

the induced magnetic anisotropy K, by magnetic annealing.

1. Introduction

Co-Zr based amorphous alloys have been in- vestigated intensively from the viewpoint of their applications and researches. In the applications, materials with zero saturation magnetostriction, thermal stability and high permeability are de- sired. For this purpose, the saturation magneto- striction has been investigated briefly [l]. In the interest of researches, the induced magnetic ani- sotropy by magnetic annealing has been investi- gated [2-41. However, there are few papers related to the temperature and the concentration depen- dences of the saturation magnetostriction, which are considered to be related to the strain deriva- tive of the anisotropy energy [5,6].

The effective spin Hamiltonian containing two- ion exchange and one-ion anisotropy (crystal-field) terms [5,6],

H = Si l qj l Sj + Si l D l Si + Helastic, 0)

* Present address: Mitsubishi Electric Co., Amagasaki 661, Japan.

where Si is the spin operator at site i, may be generalized by expanding 4j and D to first order in the strain cc:

47 is isotropic and large compared to Do. The two-ion term aJ/& can have both isotropic and anisotropic parts but only the anisotropic part of the one-ion term aD/ac is physically significant. Magnetic anisotropy may arise from either of the resulting anisotropic terms (SfS;) and ((S~)2) with appropriate two-ion or one-ion coefficients; magnetostriction may arise from either of the an- isotropic terms with the appropriate strain-deriva- tive coefficients [7]. The anisotropic spin-correla- tion functions (Sfy) and ((St)2), show char- acteristic temperature dependences. Therefore, the temperature dependence of reduced magnetostric-

0304-8853/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

182 H. Tange et al. / h, and K, for (CoTm)Zr (Tm = Fe, Cr, MO)

tion would reveal the one-ion or two-ion nature of anisotropic spin correlation.

In this paper, the results on the temperature dependence of the saturation magnetostriction X,(T, H) for (CoTm),Zr,, (Tm = Fe, Cr, MO) amorphous alloys are reported in comparison with the theory of Callen [5,6]. Furthermore, the results on the concentration dependence of A,(T, H) at room temperature are reported in comparison with the results on the induced magnetic anisotropy K, by magnetic annealing [2-41.

2. Experimental procedures

Specimens were prepared by the single-roller quenching technique in the form of ribbons l-2 mm wide, and were used in the state of as- quenched. Specimens made are (Co,_,Tm,),Zr,, (Tm = Fe[O Q x Q 11, Cr[O Q x < 0.191, Mo[O Q x G 0.18]), which were checked by X-rays to be amorphous.

For measurements of A,(T, H), the 3-terminal capacitance method [8] was employed using rib- bons 22 mm long in fields up to 18 kOe and at temperatures 77 K to the Curie temperature T, or the crystallization temperature TCry. The saturation magnetostriction h,(T, 0) was obtained from

(2/3)]%(T, H) - UT, WI, where WY W and A ,(T, H) are the longitudinal and the transverse magnetostrictions, respectively, and by extrapo- lating it to H = 0. The saturation magnetostriction A,(O, 0) at 0 K was determined by extrapolating A,(T, 0) to 0 K.

For measurements of the spontaneous magneti- zation u,(T, H), T, and TCry [9,10], the vibrating sample magnetometer and the magnetic balance were used in fields up to 18 kOe in the tempera- ture range from 77 K to room temperature and above it, respectively. The spontaneous magnetiza- tion u,(T, 0) extrapolated to H = 0 and T, were determined by Arrott plots. The spontaneous magnetization ~~(0, 0) at 0 K was determined by extrapolating u,(T, 0) to 0 K. The determination of T, in the range of T, > TCry was done from the temperature dependence of u,(T, 0) using Bril- lioun functions.

3. Results and discussion

3.1. Temperature dependence of saturation magneto- striction

Figs. la, b and c show the temperature depen- dence of the saturation magnetostriction h,(T, 0) for (Co,_,Tm,),,Zr,, (Tm = Fe, Cr, MO) amorphous alloys. Temperature dependences of A,(T, 0) for all amorphous alloys examined show the monotonical decrease to zero at T/T, = 1 with increasing temperature. Concentration depen- dences of A,(O, 0) for Tm = Fe show a maximum around x = 0.4, but for Tm = Cr and MO show a decrease to negative and then coming back to zero. This sign change may be interpreted by the virtual-bound-states conception and the band splitting model [7,11,12].

Figs. 2a, b and c show the reduced saturation magnetostriction A,(T, O)/h,(O, 0) as a function of the reduced spontaneous magnetization m(T) = I,,,(X) = u,(T, O)/u,(O, 0) for (Co,_,- Tm,),Zr,, (Tm = Fe, Cr, MO) amorphous alloys. The two solid lines are the theoretical ones [6,7] fpr the one-ion anisotropy of _uniaxial symmetry I5,2(X) and cubic symmetry I9,2(X), which are the reduced hyperbolic Bessel functions of order

(1+ I/2), jr+i,AX), wiih degree of spin oper- ators I= 2 and 4. Here. I,, ifl ( X) is expressed as

i ,+1,2(X) = I - I/(/+ 1)/4 [I- m(T)1

= m (T)‘(‘+ ‘)/‘_ (3)

The other solid line is also a theoretical one [7] for the anisotropic two-ion exchange of uniaxial sym- metry WZ(T)~.

For (CoTm),Zr,, (Tm = Cr, MO) amorphous alloys, the plots lie almost on the line I = 2. For

(Co,-,Fe&Zri,, however, the plots for 0.1 < x < 0.9 lie on the line m(T)’ except for Co,Zr,, and Fe,Zr,,. Therefore, figs. 2a, b and c suggest that the one-ion mechanism dominates A,(T, 0) for Tm = Cr or MO and for CoNZr,, and Fe,,Zr,,, but that two-ion terms may be significant for (Co,_,Fe,),,Zr,, (0.1 < x < 0.9).

H. Tange et d. / A, and K, for (CoTm)Zr (Tm = Fe, Cr, MO) 183

b (a) (Co,_xFex)gOZr,o

8 c (b) (Co,_xCrx)goZr,o

r

8 (cl Ko,_xMox)gOZr,o

x= 0 0.0 l 0.022 - 0 0.056

‘p n 0.089 0 A 0.133

..’ \

.\ ’ . . \ 0

-A-A-

I 0 0.5 1.0

TITc

Fig. 1. Temperature dependence of saturation magnetostriction X,(T, 0) for (Co,_,Tm,),Zr,, (Tm = Fe(a), Cr(b), Ma(c)) amorphous alloys.

3.2. Saturation magnetostriction and induced mag- netic anisotropy

Figs. 3a and b show concentration dependences of the saturation magnetostriction h,(T, 0) at room temperature and the induced magnetic ani-

sotropy K, by magnetic annealing for (Co,_,- Tm,),Zr,, (Tm = Fe, Cr, MO) amorphous alloys. The results of KU are quoted from those after Fukunaga et al. [2-41, who carried out heat treat- ments and measurements using a torque magne- tometer as follows; disks were heated to 650 K,

184 H. Tange et al. / A, and K, for (CoTm)Zr (Tm = Fe, Cr, MO)

(a)

1

_ (b) (Co,_,Cr,)gOZr,o

j

x= L 0 00

%

0 02 0 04

. . 0.6

‘t;

A 0.8

* . A 1.0

. .

?

. \

n

A

k . \

1=4 k2 A \ m2

i 912 i

Y---d

512 A

0.5 1 c-m

1.0 b k) (Co,_xMox)goZr,o

I\

x: 0 0.0 0 0.022 0 0.056 n 0.009 A 0.133 . 0.156

0.5 0 +-m

x= 0 0.0 l 0.05 q 0.06 n 0.07 A 0.1 A O.lhL

L

1 0.5 tm

Fig. 2. Reduced saturation magnetostriction X,(T, O)/h,(O, 0) as a function of reduced spontaneous magnetization m(T) = q(T, O)/q(O, 0) for (Co, _xTm,),Zr,, (Tm = Fe(a), Cr(b), Ma(c)) amorphous alloys, where the meaning of theoretical solid lines is

seen in text.

kept for 25 mm, and then cooled under applica- due to the same origin between X,(T, 0) and K,. tion of a magnetic field of 3.6 kOe. Subsequently, That is, the magnetostriction h,(T, 0) is caused a magneto-torque curve was measured at room mainly by the anisotropic two-ion exchange of temperature in an applied field of 6 kOe. uniaxial symmetry m(T)2. Also, the induced mag-

Fig. 3a shows that h,(T, 0) has the same con- netic anisotropy K, is considered to be caused by centration dependence as that of K, for the directional order in the pseudodipolar interac- (Co,_,Fe,),Zr,, amorphous alloys. This may be tion model [2-4,131, i.e. by the two-ion mecha-

H. Tange et al. / h, and K, for (CoTm)Zr (Tm = Fe, Cr, MO)

t

(a) (Col-,Fe&oZrlo

I

15

t

(b) (Co,_xTmx)SOZr,, (Tm=Cr,Mo 1

185

0 0.5 1 0 0.1 0.2 X X

Fig. 3. Concentration dependences of the saturation magnetostriction h,(T, 0) at room temperature and the induced magnetic anisotropy K, by magnetic annealing [2-41 for (Co,_,Tm,),Zr,, (Tm = Fe(a), Cr(b), h&(b)) amorphous alloys.

nism. Briefly, fig. 3a then means that K, and h,(T, 0) have the same compositional dependence because of the two-ion term.

Fig. 3b shows a difference between K, and h,(T, 0), because two-ion terms are absent from X,(T, 0) for Tm = Cr and MO. This suggests that K, is determined by a two-ion mechanism only (one-ion, i.e. crystal field, certainly plays a major role in many systems) and A,( T, 0) will only show the same compositional dependence as K, if X,(T, 0) contains appreciable two-ion contribu- tions.

The relation between the saturation magne- tostriction and the induced magnetic anisotropy by magnetic annealing, needs to be investigated in more detail for other amorphous alloys, for exam- ple (FeCoNi)B [14-161 and (FeCoNi)SiB [17,18].

Acknowledgement

The authors wish to express their thanks to Mr. Y. Kobayashi of the Research Institute of Electric and Magnetic Alloys for technical assistance in the preparation of the samples.

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