Magnetoresistance of Ni-Fe-Co ternary alloy films

Journal of Magnetism North-Holland

and Magnetic

Materials

Magnetoresistance T. Miyazaki Department Received

171

97 (1991) 171-177

of Ni-Fe-Co

ternary alloy films

and M. Oikawa

of Applied Physics, Tohoku University, Sendai 980, Japan

19 September

1990; in revised form 27 November

1990

An investigation has been made into the dependence of magnetoresistance on both the composition and annealing temperature for Ni-Fe-Co’temary alloy films. The anisotropic magnetoresistivity ratio Ap/pa exhibits a maximum in the composition range of 76-86 wt% for Ni, O-7 wt% for Fe and lo-20 wt% for Co. The anisotropic magnetoresistivity Ap coupling is the most likely exhibits a broad maximum in the neighborhood of the tra = 0.9 pa, suggesting that the spin-orbit mechanism to explain the magnetoresistance effect. The resistivity decreased about 15% for annealing temperatures above 300 o C, while the anisotropic magnetoresistivity remained unchanged. Annealing at temperatures above 300°C increases the dispersion of the induced magnetic anisotropy due to the growth of crystallites. Consequently, the hysteresis in the p vs. H curve becomes enhanced. The dependence of resistivity on the size of the crystallites is discussed based on the gram-boundary scattering model proposed by Mayadas and Shatzkes.

1. Introduction Recently, there has been a renewed interest in the anisotropic magnetoresistance effect of magnetic films [l] due to the practical application of this effect to magnetic and electronic devices. In spite of a large number of alloy magnetoresistance data reported in the past [2-91, little effort has been directed to find an alloy film having a large magnetoresistance effect. In order to seek new alloys or materials, a systematic experimental study is required to find a milestone for fabricating large magnetoresistance materials. In addition to the value of the magnetoresistance ratio Ap/p,, the thermal stability of the film and the shape of p vs. H curve are also important factors in the application of the film. A few studies have been reported [lo-121 of the magnetoresistance in an Ni-Fe-Co ternary alloy system. However, no systematic study has been conducted of the dependence of magnetoresistance on the composition of the system. In two recent papers [13,14], a description has been given of the preliminary data of magnetoresistance in a 82Ni-Fe film and the effect of impurities on the anisotropic magnetoresistivity of the alloy film. 0304-8853/91/$03.50

0 1991 - Elsevier Science Publishers

Following these investigations, the study of NiFe-Co ternary alloy films was extended in order to seek a film with a higher magnetoresistivity ratio Ap/pO. The present paper describes the dependence of the magnetoresistance and magnetic properties on the composition of Ni-Fe-Co ternary alloy films. Also, the annealing effect on an 80Ni-5 Fe-15 Co alloy film, which exhibits a maximum value of Ap/p,,, will be described.

2. Experiment Electric iron, nickel and cobalt having a 99.9 wt% purity were melted in a vacuum of approximately 5 X 10e2 Torr. Films were prepared on a slide-glass substrate by means of an electron beam evaporation method in the presence of a magnetic field of 25 Oe, applied parallel to the substrate. The pressure during the evaporation was (4-8) X lop6 Torr, with the evaporation rate being roughly 10 A/s. The film thickness was 1000-1700 A. Based on preliminary data previously reported [lo], the substrate temperature was set to a constant temperature of 350” C and then cooled to

B.V. (North-Holland)

T. Miyazaki, M. Oikawa / Magnetoresistance of Ni-Fe-Co

172

room temperature at a rate of 100 o C/h under the influence of a magnetic field. The annealing was conducted at various temperatures, from 150450 o C, in a 1 X lop6 Torr vacuum under a magnetic field of 330 Oe, applied parallel to the easy direction of magnetization. The magnetoresistance was measured using a

four-probe method in magnetic fields up to 50 Oe, with the current set at 1 mA. The saturation magnetization was measured by use of a torque method (the Neugebauer method [15]) in magnetic fields up to 20 kOe. The structure analysis was conducted by X-ray diffraction using CuK, radiation. The average size of the crystallites was

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95

A

5

Fe (wt%i-

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wt % l-

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20

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Fig. 1. Electrical resistivity p0 (a), anisotropic resistivity Ap (b) and the anisotropic resistivity ratio Ap/pO in percent (c) of Ni-Fe-Co ternary alloy films prepared at a substrate temperature of 350 o C. The dashed and dashed-dotted lines (c) show the composition for K, = 0 and h, = 0, respectively.

T. Miyazaki,

M. Oikawa / Magnetoresistance

173

of Ni-Fe-Co

estimated from the half width of (111) diffraction patterns making use of Scherrer’s equation.

3. Experimental results and discussion 3.1. Dependence of the magnetoresistance and magnetic anisotropy field on the composition Fig. 1 shows the dependence of resistivity p,, (a), anisotropic resistivity Ap (b) and anisotropic resistivity ratio Ap/p, (c) on the composition of Ni-Fe-Co ternary alloy films. The lines for K, = 0 and X, = 0 shown in fig. lc are taken from the literature [16]. It is seen that p0 strongly depends on the content of iron and increases with increasing amounts of iron. The absolute value of p,, in a thin film is much larger than that found in the bulk. This is mainly due to the difference in the crystalline size between the film and bulk samples, which will be described in section 3.2. However, the dependence of pO on the composition of thin film samples is very similar to that of the bulk [17]. On the other hand, Ap exhibits a maximum in the neighborhood of lo-20 wt% for Co and 75-80 wt% for Ni. Also, Ap/pO strongly depends on the composition of the films and exhibits a

Fe (wt

Fig. 2. Magnetic

anisotropy

%)-

field H, of the Ni-Fe-Co alloy films.

ternary

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Fig. 3. Anisotropic resistivity ratio Ap/p,, (b) as a function

I

I

20

25

1 I

30

(Oe)

Ap (a) and anisotropic resistivity of the magnetic anisotropy field.

maximum in the composition range of 78-86 wt% for Ni, O-7 wt% for Fe and lo-20 wt% for Co. Fig. 2 shows the dependence of the magnetic anisotropy field H, on the composition of the films. It is clearly seen that H, exhibits a remarkable increase with the increase in the Co content. Usually, anisotropic magnetoresistivity is explained in terms of the spin-orbit coupling [18]. On the other hand, the induced magnetic anisotropy of binary and/or ternary magnetic films can be explained by the pair model of constituent atoms [19]. Furthermore, spin-orbit coupling contributes to the anisotropic energy of atom pairs [20]. If these explanations are applicable to the present data, a correlation between Ap and H, and between Ap/p, and H, can be expected. In order to confirm this, both Ap, and Ap/p, are plotted as functions of Hk in fig. 3: As seen in the figure, both Ap and Ap/p,, are proportional to the magnetic anisotropy field. These results support the above consideration and suggest that the spin-orbit coupling is the promising mechanism for as the origin of the magnetoresistance of these films.

T. Miyazaki,

174

M. Oikawa / Magneioresistance

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tion of the g factor shows a maximum at an electron concentration of 27.7 electrons per atom [16], which approximately corresponds to the concentration at the n a = 0.9~~. Therefore, spin-orbit coupling is the most promising mechanism for the anisotropic magnetoresistance of the Ni-Fe-Co ternary alloy films. Recently, Berger [21] has calculated the dependence of magnetoresistance ratio on composition for A,_,B, binary alloys. The result suggests that the magnetoresistance depends much on the ratio of magnetic moment, pA/pr,. The central idea of his calculation is based upon that the scattering potential associated with alloy disorder is so strong enough to cause a variation of 3d wave-function amplitude between chemically different atoms. Further discussion concerning the origin of magnetoresistance in ternary alloy films needs a theoretical consideration taking into account the idea of Berger. 3.2. Annealing

effect on the magnetoresistance

I

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I

IO

I5

“B (I%) Fig. 4. Electrical resistivity pO (a), anisotropic resistivity and anisotropic magnetoresistivity ratio Ap/p,, (c) as tion of the mean number of the Bohr magneton na Ni-Fe-Co ternary alloy films. Dashed lines are a guide eye.

of Ni-Fe-Co

Ap (b) a funcfor the for the

Furthermore, to clarify the magnetic effect on the magnetoresistance, resistivity was compared with the intensity of the saturation magnetization in fig. 4, p,,, Ap and Ap/p, are plotted as functions of the magnetic moment per 3d transition metals. It should be noted that the values of Ap and Ap/p, for evaporated films exhibit maxima in the neighbourhood of the na = 0.9~~. This behavior is very similar to that found in data of bulk binary and ternary alloy systems [8,18]. The appearance of a maximum around the nB = 0.9~~ has been considered as due to the spin-orbit interaction which influences the scattering of conduction electrons. Experimentally, the magnitude of the spin-orbit interaction can be regarded as the deviation of the g factor from 2.0. The devia-

In order to verify whether or not the magnetoresistance anisotropy is dependent on the annealing temperature and to examine the effect of this anisotropy, magnetic annealing was carried out for the 80Ni-5Fe-15Co composition film. This film exhibits a maximum value of Ap/p, in NiFe-Co alloy films in the prepared state as described above. Fig. 5 shows the p vs. H curves measured after annealing at the various temperatures indicated in the figure. No remarkable change is found in the curves annealed at temperatures below 300°C. However, above 350 o C, the hysteresis of p vs. H curve increases rapidly. The curve observed in samples annealed above 450°C is very similar to that of magnetically isotropic film. Fig. 6 displays pO, Ap and Ap/p, as a function of the annealing temperature. It is noted that pO displays a remarkable decrease above 300 o C, while Ap shows a slight increase. As a consequence, Ap/p,, increases about 1.5% for annealing above 350 “C. Here, it should be noted that the increase of Ap/p, follows the enlargement of the hysteresis in p vs. H curves. This may be due to the increase in the dispersion of magnetic anisotropy.

175

T. Miyazaki, M. Oikawa / Magnetoresistance of Ni-Fe-Co

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Fig. 6. Electrical resistivity no (a), anisotropic resistivity Ap (b) and anisotropic magnetoresistivity ratio Ap/po (c) as a function of the annealing temperature for the 80Ni-5Fe-15Co alloy film.

Fig. 5. Magnetoresistance curves (p vs. H) after annealing was performed at the various temperatures indicated in the figure for a IONiFe-15Co alloy film. The values of p are given in arbitrary units. 30

Fig. 7 shows the magnetic anisotropy field H, as a function of the annealing temperature T,,,. In this figure, it can be seen that the anisotropy field is nearly constant below about 25O’C and displays a slight increase above 300°C. As the annealing temperature increases above 425” C, Hk exhibits a very sharp increase and the hysteresis curve becomes isotropic. In order to examine the relationship between the structure of the film and the magnetoresistame, a structure analysis using X-ray diffraction patterns was carried out for these samples which were annealed at various temperatures. The average size of the crystallites D was evaluated from the half width of the (111) diffraction patterns. Fig. 8 shows the value of D as a function of the annealing temperature. As can be seen in the

25

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100

200

300 T,,,

400

500

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Fig. 7. Magnetic anisotropy field H, as a function of annealing temperature for the 80 Ni-5 Fe-15 Co alloy film.

T. Miyazaki, M. Oikawa / Magnetoresistance of Ni-Fe-Co

176 I

I

I

I

I

and (Y= I,R/d(l

400

T a”“( “Cl

Fig. 8. Mean diameter annealing temperature

of the crystallites as a function of for the 80Ni-5 Fe-15 Co alloy film.

figure, the sizesOof the crystallites are in the range of 200 to 275 A, in good agreement with previously reported values. The size remains nearly constant up to approximately 250° C and increases remarkably above 300” C. By comparing the data of fig. 6a with fig. 8, it can be seen that the resistivity of the films is strongly dependent on the size of the crystallites. Therefore, it is concluded that annealing above 300 o C increases the magnetoresistance ratio due to the decrease in the resistivity. However, annealing increases the average crystallites growth which is accompanied the increase in the dispersion of magnetic anisotropy. Mayadas and Shatzkes [22] proposed a model for computing the resistivity due to both the grain boundary and isotropic background scattering, and scattering by phonons and defects. In the model, the grain boundaries are represented by H parallel planes, oriented perpendicular to the direction of current, with an average separation d. The potential SS(x - x,,) at the position x, of the n th plane, with S being the strength of the potential, are assumed. Further, a Gaussian distribution of x, is assumed. By solving the Boltzmann equation, the resistivity for grain boundary scattering can be obtained: ~s/~r = I + 3a/2, = 4a/3,

(a K I), (a X=-1)

-R),

(2)

where pi, is the resistivity due to phonons and defects, while I, and R are the background mean free path and reflection coefficient, respectively. It is assumed that the parameter d in their model corresponds to the average diameter size D of the crystallite in the present experiment. Then, the resistivity should be inversely proportional average diameter size D of the crystallite. Fig. 9 shows the graph of p0 vs. l/D. The experimental results obey eq. (1) and p is about 12 I&? cm, which is in good agreement with the measured value of bulk resistivity. If it is assumed that R = 0.3-0.5 [23], the mean free path I, is between 110 and 250 A. These values do not contradict those previously reported [13]. As discussed above the main feature of Ap/p, with annealing can be explained by the decrease of resistivity arising out of the grain boundary scattering. However, the grain boundary scattering alone cannot explain the slight increase of Ap caused by annealing and also by increasing film thickness [13,24]. A more complicated behavior, such as a scattering due to vacancies, dislocations and/or stacking faults [24] may play an important role in real films.

(I) (I’)

Fig. 9. Electrical resistivity as a function of the reciprocal the mean diameter of the crystallites.

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T. Miyazaki,

M. Oikawa / Magnetoresistance

4. Summary and conclusions

PI E.N. Mitchell, H.B. Haukaas,

The magnetoresistance of Ni-Fe-Co ternary alloy films having a composition of more than 30 wt% of Ni has been investigated. The resistivity of the films in their as-prepared state is much larger than that of the bulk samples. The dependence of pa on the composition of the film samples is roughly the same as that of the bulk. The anisotropic magnetoresistivity Ap and its ratio Ahp/p, exhibit maxima in the composition range of 76-86 wt% for Ni, O-7 wt% for Fe and lo-20 wt% for Co. After annealing the film samples at temperatures above 300°C the resistivity approaches that of the bulk. Also, the hysteresis of the p vs. H curves increases as the resistivity decreases. The hysteresis in the curve is due to the increase in the dispersion of magnetic anisotropy, in accordance with the increase of crystalline growth. The dependence of resistivity on the size of the crystallites can be explained by the grain boundary scattering model proposed by Mayadas and Shatzkes.

Acknowledgements The authors gratefully acknowledge Dr. S. Kadowaki for the bulk alloy sample preparation. We also acknowledge the referee for pertinent initial manuscript. This work comments on the part by a grant from Izumi was supported in Research Grant.

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Magn.

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