Field cooled induced magnetic anisotropy in pure Co metal

917

Journal of Magnetism and Magnetic Materials 54-57 (1986) 917-919 FIELD COOLED Jin H A N M I N

INDUCED

MAGNETIC

*, S. K A D O W A K I

ANISOTROPY

IN PURE Co METAL

t a n d M. T A K A H A S H I

Department of Applied Physi(~, Tohoku University, Sendai, Japan

A development of crystalline texture in Co metal cooled in a magnetic field is detected by applying a statistical method on Schulz's method of pole figure analysis. The field cooled induced uniaxial magnetic anisotropy originates from the texture.

A uniaxial magnetic anisotropy is induced in polycrystalline Co metal field cooled through the fcc ~ hcp phase transition temperature [1]. This anisotropy is higher when the density of lattice defects is lower [2] and is not related with stacking faults [3]. It will be shown in this paper that the anisotropy originates from the crystallographic texture which is developed during the field cooling treatment. The pure Co specimen of ~ 11 × 1 mm 2 cooled from 1090°C in vacuum in a zero magnetic field, in a field of 20 kOe perpendicular to and parallel to the specimen disk are denoted as (H0), ( H + ) and (HII), respectively. the texture is examined measuring the intensity of the * Present address: Dep. Phys. Jilin Univ. Changchun, China. t The Res. Inst. of Electric and Magnetic Alloys, Sendai, Japan.

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X-ray reflection from (10-3) at the diffraction angle, l(a, fl), as a function of a and fl, where a is the angle made by the normal of the specimen with the reflection plane and fl the rotating angle of the disk around the normal (fig. 1). Reflection (00- 2) is overlapped by that from (111) of the residual fcc phase and is not suited for the purpose. The reflection intensity is highly scattering and it is very difficult to judge if there the texture exists (fig. 1). This may be the reason that attempts to detect the texture have been unsuccessful [4,5]. Since the regular component of the reflection intensity l ( a , 13) of (H0) and ( H . ) specimens should be independent of fl, l(a, fl), after background subtraction which is measured by setting the reflection angle some degrees away from the diffraction angle, is averaged over fl with histograms like fig. 1. Two to three specimens of the

C O I I l"l "~ ~ r

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Fig. 1. Essential arrangement in Schulz's reflection method, and the reflection intensity histograms from (10.3) of specimen ~ 8(HII) for a = 3 0 ° and 35 ° . 0304-8853/86/$03.50

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Jin Hanmin et al. / Anisotropy in pure Co metal

918

200 0 L)

~- 15o 7 I,I bZ 7

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Fig. 2. The dependence of normalized reflection intensity i(~). A: (H±), I : (H0)and O: (HII) specimens.

same kind are used to get a fairly smooth dependence of the intensity on a, I ( a ) . The increment of ,8 was 2 ° or 5 °. Fig. 2 shows a dependences of the normalized reflection intensities, /x(cQ = l~(a)32jo(a ) sin c~/Y'.~Ix(a ) sin a (x = 0, ± and II), of specimens of ( H o), ( H ± ) and (HII), respectively. The texture can be characterized by the relative solid angle density of (10 • 3) of ( H . ) specimens, Pro. 3(a), the ratio of the density of ( H a ) to that of (H0), as a function of the angle between the normal of ( 1 0 . 3 ) and the field direction of H t applied during the cooling treatment, ~. Thus P m 3(~) = I± (a)/io(~) assuming the random grain distribution in (H0) specimens. Pm 3(c0 varies with ~ apparently

showing the existence of the texture (10 - 3) prefering to orient normal to H t (fig. 3). The data of /tl(a) in fig. 2, while not offering quantitative information about the texture, is consistent with this conclusion. P10 3(a) can be transformed to the similar quantity for (00-1), P00.1(~'), also as a function of the angle between the c axis and Ht, T. P00.1(~') is approximated by an analytical expression Pooq(Y) and plotted in fig. 3 (see appendix). The density of ( 0 0 . 1 ) for the field cooled specimens is about 30% more than that for the (H0) specimens in the angle region perpendicular t o n t. The induced magnetic anisotropy constant K u originating from the texture is estimated to be K u = 0.111K 1 + 0.09K 2 by evaluating the energy difference between the states magnetized along and normal to the H t direction respectively.Using the values of Ka(T ) and K2(T ) [6], the value of K u at room temperature is estimated to be 6.0 × 105 e r g / c m 3 which is in good agreement with the value of (6.1 + 0.5) × 105 e r g / c m 3 obtained directly from torque measurements on (hll) specimens. In addition, the temperature dependence of K u evaluated from the texture is proportional to earlier experimental results [4,5,7]. The "background K u" measured on ( H 0) specimens is one order smaller justifying the assumption of random grain distribution in these specimens. From above, it is concluded that the field cooled induced magnetic anisotropy originates from the texture. It is also reasonable that the same mechanism plays a major role in Co rich alloys. Appendix Let n and r denote the unit normal vector of (10 - 3) and ( 0 0 . 1 ) of a grain, respectively. Then n. r = cos 8, 8 = 32 °, and r must be directed somewhere along the circular cone surface from the top of it (fig. 4). P]0.3(n, r ) d % being the component of P]0.3(n) with the pair (00-1) direction in the range of r and r + (d r/dcp)d~v,

P,o.3(,~)=Vlo.3(n)=fo2"~Pm.3(n, rldcp.

(1)

In a similar way with Poo,l(r, n) independent of n,

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Poo.](y)= Poo.,(r)= fo Poo.l(r, n)d~O: Poo.l(r, n)

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Fig. 3. The relataive density of (10-3) and (00.1) versus the angle between the normal of the planes and H t.

Fig. 4. Geometrical meaning of n, r, 8, a, ~', q0 and +.

(2)

Jin Hanmin et al. / Anisotropy in pure Co metal

F r o m eqs. (1), (2) we get the relationship between P10.3(a) and P00-1(Y) as PlO.3 (o~) = (1/2"~)fo2"~Poo .1 ( Y )dqo,

(3)

where the coefficient 1/2a, can be obtained from the case of Plo.3(ot)= Poo.l(y) = 1. Poo.l(Y) in eq. (2) is evaluated by using a least square method, approximating it by Poo.1(3') = R + Q cos(27), where R and Q are variable parameters. Putting this relation with relation, cosy=cos6cosa-sin6cosp s i n a , into eq. (3), Plo .3(a) is formulated as Plo. 3(a) = R + Q(3 cos28 - 1) cos2c~ - Q cos26. The values of R = 0.85 and Q = - 45 are deduced from the normalization condition fo

~(R+ Q cos 2 y ) sin 3, d y 2 = f0"sin ~,dy,

(4)

and minimizing the quantity Y ~ ( P , o . s ( a ) - Pi0.3(a)) 2.

919

The angle dependences of Plo.3(c0 and Poo.l(Y) are plotted in fig. 3. A more detailed description of this work is reported in ref. [8] in Chinese. [1] M. Takahaski et al., J. Phys. Soc. Japan, 15 (1960) 936. [2] Chen Ensheng, Jin Hanmin, Huang Maorong et al., Acta Scientiarum Naturalium Universitatis Jilinensis (ASNUJ) 1 (1985) 52. [3] Jin Hanmin, Jia Hongcheng, Zhu Cheng et al., ASNUJ 3 (1983) 61. [4] C.D. Graham, Jr., J. Phys. Soc. Japan 17 (1962) suppl. B-I 321. [5] M. Takahashi et al., J. Phys. Soc. Japan 47 (1979) 1110. [6] M. Takahashi et al., J. Phys. Soc. Japan 44 (1978) 825. [7] T. Sambongi and T. Mitsui, J. Phys. Soc. Japan 18 (1963) 1253. [8] Jin Hanmin, ASNUJ, 1 (1985) 55.