Thin soft magnetic film and method of manufacturing the same

Abstract: Disclosed is a soft magnetic film comprising a thin film of magnetic materiai or cubic symmetry, characterized in that crystal face (111) of the thin film is oriented substan­ tially parallel to the surface of the thin film.

Description: Field of the Invention The present invention relates to a thin soft magnetic film used, for example, in a magnetic head, and more specifically, to a thin soft magnetic film having a crystal face of a magnetic material of cubic system oriented to a particular direction and a method of manufacturing the same. Related Art In general, a method of making a magnetostriction constant small can be employed as one of the conditions for forming a thin soft magnetic film. A magnetostriction constant is usually determined depending on kinds of magnetic substances. In the case of alloy, the magnetostriction constant thereof can be made to a very small value by selecting a composition of the alloy, but in many cases, since magnetic substances are composed of crystals and the magnetostriction constant thereof has different values depending on the crystallographic directions, it is impossible to make the magnetostriction constant zero in all the directions. Polycrystals are often used as a soft magnetic material, and in this case the effect of magnetostriction is avoided in such a manner that an average value of magnetostriction constants in respective directions is caused to approach zero. This is also applicable to a polycrystal thin film. However, it is difficult to perfectly remove the effect that a partial magnetostriction suppresses magnetization rotation. Earlier EP-A-0 360 055 (published on 28/03/90) discloses a thin soft magnetic film made of an alloy containing mainly Fe, Co and Ni and having a structure in which [110] or [111] planes of a face-centered cubic-lattice structure are given to its thin film surface as a preferred crystallographic orientation, in order to improve the saturation magnetic-flux density thereof. Object of and Summary of the Invention It is an object of the present invention to overcome the above-mentioned drawback and to provide a thin soft magnetic film not adversely affected by magnetostriction and a method of manufacturing the same. Said object is achieved, according to the present invention, by a thin soft magnetic film as claimed in claim 1 and a method of manufacturing the same as claimed in claim 5. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram of an X-ray diffraction pattern of a Fe-Si thin soft magnetic film formed on a Zn-Se underlayer; Figure 2 is a schematic diagram showing the arrangement of crystals when a Fe-Si thin soft magnetic film is formed on a Zn-Se underlayer; and Figure 3 is a characteristic diagram of coercive force of a thin soft magnetic film obtained by an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As described above, when a thin film composed of a magnetic material of cubic system is formed and crystal face [111] thereof is oriented substantially parallel to the surface of the thin film formed on an underlayer, a so-called isotropic magnetostriction is exhibited wherein magnetostriction does not depend on the magnetization directions in the plane. Therefore, a thin soft magnetic film of high magnetic permeability can be obtained wherein magnetization is directed to the film face except at the portion of a magnetic wall unless vertical magnetic anisotropy liable to direct to a vertical direction with respect to the film face is not specially given, no distortion is produced in the grain boundaries, if any as in the case of polycrystalline films, due to the magnetostriction difference between the crystallites which will otherwise exist, and thus no adverse effect by magnetostriction exists. Further, if the value of (λ₁₀₀ + 2λ₁₁₁), where λ₁₀₀ and λ₁₁₁ stand for the magnetostriction coefficients in 〈100〉 and 〈111〉 directions, respectively, is small, and then the following equation is established, |λ₁₀₀ + 2λ₁₁₁|< 2/3{|λ₁₀₀| + 2|λ₁₁₁|} more preferably, |λ₁₀₀ + 2λ₁₁₁|< 1/3{|λ₁₀₀| + 2|λ₁₁₁|}, or in other words, if the composition of the film is selected so as to make the saturation magnetostriction coefficient negligible, a thin soft magnetic film which is not affected at all by magnetostriction can be obtained. The present invention will be described below with reference to an embodiment in which iron is used. The present invention, however, is not limited to iron, but, for example, Ni, Ni-Fe alloy, or ferrite having a spinel structure such as Mn-Zn ferrite and Ni-Zn ferrite, and the like can be used. In this case, however, it is also needed that an environment in which an underlayer corresponding to a magnetic material of cubic system, or the like is provided so that crystal face [111] of the magnetic material of cubic system is oriented substantially parallel to the surface of a thin film. Although a thin film was formed using sputtering in the following examples, vapor deposition and the like are also applicable. A thin soft magnetic film obtained by the present invention can be used as various magnetic materials such as, for example, a magnetic head, a high frequency transformer, and the like. EMBODIMENT A magnetic material of cubic system used in the present invention includes Fe, Ni, Fe-Ni alloy, or ferrite having a spinel structure such as Mn-Zn ferrite and Ni-Zn ferrite, and the like. Iron containing 6.9 wt% of Si was formed on substrates of MgO, ZnO and Zn-Se by sputtering (substrate temperature: about 300 o C) and Fe-Si thin films having [100], [110] and [111] orientation, respectively were obtained. As a result of measurement of coercive force of the respective specimens thus fabricated, both the specimens having a [100] orientation film and a [110] orientation film had a coercive force of about 0.32 kA/m (4 [Oe]), but the specimen having a [111] orientation film had a coercive force reduced to 0.16 kA/m (2 [Oe]) which was a half of that of the above two specimens, and thus a magnetic film of high magnetic permeability was obtained. Figure 1 is a diagram showing an X-ray diffraction pattern of the Fe-Si thin magnetic film having the [111] orientation formed on the Zn-Se film, as described above. As shown in Figure 1, diffraction peaks corresponding to the crystal faces [211] and [222] are observed and it was found that there is a tendency that as the diffraction intensity of the crystal face [222] is increased, coercive force is made smaller. The rate of the change [δℓ/ℓ] of the linear dimension in the crystallographic planes [100], [110] and [111] of a single crystal due to magnetostriction is expressed as follows: [100] plane: δℓ/ℓ = 3a + (3λ₁₀₀cos²χ/2) + (-λ₁₀₀ +λ₁₁₁) cos (ϑ + χ) sin (ϑ + χ) cosϑ sinϑ, 1 [110] plane: δℓ/ℓ = 3a + (3λ₁₀₀cos²χ/2) + (-λ₁₀₀ +λ₁₁₁) cos²χ (sin⁴ϑ/4 + Sin²ϑ cos²ϑ) - 3 sin²χ sin²ϑ cos²ϑ/4 + sinχ cosχ (sin³ϑ cosϑ/2 - sinϑ cos³ϑ), 2 [111] plane: δℓ/ℓ = 3a + (λ₁₀₀ -λ₁₁₁)/12 + (3λ₁₀₀ + 6λ₁₁₁)/6 x cos²χ, 3 In the above equations, ϑ represents an angle between a particular crystallographic axis and a direction in which elongation is measured, χ represents an angle between magnetization and the direction in which elongation is measured, ϑ + χ represents an angle between the particular crystallographic axis and the magnetization, λ₁₀₀ represents a magnetostriction coefficient in 〈100〉 direction, λ₁₁₀ represents a magnetostriction coefficient in 〈110〉 direction, and λ₁₁₁ represents a magnetostriction coefficient in 〈111〉 direction. Further, saturation magnetostriction (λs) of a polycrystalline film of each specimen mentioned earlier is shown as follows: [100] oriented film: λs = (λ₁₀₀ +λ₁₁₁)/2, 4 [110] oriented film: λs = (3λ₁₀₀ + 5λ₁₁₁)/8, 5 [111] oriented film: λs = (3λ₁₀₀ + 6λ₁₁₁)/9. 6 As apparent from these equations, since functional terms with respect to both ϑ and χ exist in the equations in the case of the [100] oriented film (Equation 1) and the [110] oriented film (Equation 2), when the magnetization is directed in one direction in the specimen, each crystallite in the film tends to elongate or contract in a different direction or by a different amount from each other depending upon the direction of a crystallographic axis of each crystallite. On the other hand, in the case of the [111] oriented film (Equation 3), the direction and amount of elongation and contraction are determined only by the magnetizing directions χ in respective crystals, and thus when magnetizing directions coincide each other, the respective crystals simultaneously elongate and contract by the same amount. Therefore, the [111] orientation film has an isotropic magnetostriction property regardless of magnetizing direction. From the above-mentioned, it is found that in the [100] oriented film and the [110] oriented film, even if a saturation magnetostriction (λs) is zero, a difference in elongation and contraction is caused in each crystallite when a magnetizing direction changes, whereas in the [111] oriented film, a difference of elongation and contraction is not caused in each crystallite, that is, it is found to be isotropic with respect to magnetostriction. Further, in this case, assuming that λs is ∼ 0, magnetostriction is not changed at all by the change of magnetizing direction, which is preferable to obtain a thin soft magnetic film. Further, a magnetic anisotropic energy Ea of a single crystalline specimen in a particular face thereof is expressed as follows. [100] plane: Ea = - (K₁cos4)/8 + const. 7 specifically in the case of iron; - K₁/8 = 5.9 x 10⁴ [110] plane: Ea = (- K₁/8 + K₂/128)cos2 + (- 3K₁/32 - K₂/64cos4 + const. 8 specifically in the case of iron; - K₁/8 + K₂/128 = - 5.9 x 10⁴ - 3K₁/32 - K₂/64 = - 4.4 x 10⁴ [111] plane: Ea = K₂cos6 /128 + const. 9 specifically in the case of iron; K₂/128 = - 69 In the above equations,  means the above (ϑ + χ) which is an angle between a particular crystallographic axis and magnetization. As apparent from Equations 7 to 9, the [111] oriented film has a magnetic anisotropic energy which is approximately one-hundredth of that of the other [110] oriented film and [110] oriented film. Therefore, a superior thin soft magnetic film can be obtained from a [111] oriented Fe-Si film λs of which is negligible. Figure 2 is a schematic diagram showing the arrangement of crystallite obtained by sputtering a Zn-Se film (zinc sulfide structure of cubic symmetry fcc, a = 0.565 nm (5.65Å)) on a glass substrate and further sputtering iron (bcc, a = 0.287 nm (2.87Å)) thereon. As apparent from Figure 2, both of Zn-Se and Fe has substantially the same lattice constant. Therefore, Fe is grown on the crystals of Zn-Se heteroepitaxially, and thus it is easy to get [111] orientation. In this example, Fe was used as a soft magnetic material and a Zn-Se film was used as an underlayer. For Fe, however, an underlayer of a crystallographic structure of fcc the lattice constant a of which is nearly equal to 0.572 nm (0.286 x 2 = 0.572 nm) (5.72Å (2.86 x 2 = 5.72Å)) can be used and the following materials are included therein. Material a Cd-S compound 0.582nm (5.82Å) Cu-Br compound 0.568nm (5.68Å) Mn-Se compound 0.582nm (5.82Å) Hg-S compound 0.584nm (5.84Å) Al-As compound 0.562nm (5.62Å) Ga-As compound 0.564nm (5.64Å) Figure 3 shows the results of the measurement of coercive force (Hc), when a Zn-Se underlayer of 10nm (100Å) thick was formed on glass substrates (by high speed sputtering, film forming speed: 6 - 8nm (60 - 80Å)) and iron containing 6.9 wt% of silicon was further formed thereon to a thickness of 96nm (960Å) and the glass substrates were kept at 100 o C, 200 o C, 300 o C, and 400 o C, respectively. In Figure 3, marks ○ show coercive force (Hc //) measured in a direction parallel to that of the in-plane magnetic field applied during sputtering and marks  show coercive force (Hc ⊥) measured in the direction perpendicular thereto. According to the experiment effected by the inventors, when a Fe-Si film was directly formed on the same glass substrate as that used in the above test which was heated to 100 o C, Hc // was 1.528 kA/m (19.1 [Oe]) and Hc ⊥ was 1.296 kA/m (16.2 [Oe]). On the other hand, the samples prepared according to the present invention in which a film was formed at 100 o C and 200 o C had a Hc // and Hc ⊥ of about 0.8 kA/m (10 [Oe]), exhibiting an about 50 % reduction in Hc // and an about 38 % reduction in Hc ⊥ and thus the specimens had high magnetic permeability. Further, when the substrate was heated to 300 o C or more, the coercive force thereof was lowered to about 0.24 kA/m (3 [Oe]), exhibiting a 84 % reduction as compared with the above specimen having a Hc // of 1.528 kA/m (19.1 [Oe]) and a 82 % reduction as compared with the above specimen having Hc ⊥ of 1.296 kA/m (16.2 [Oe]), and thus a thin soft magnetic film having much higher magnetic permeability was obtained.

Claims: A thin soft magnetic film of high magnetic permeability exhibiting isotropic magnetostriction, which comprises a thin film of magnetic material of cubic crystallographic symmetry, formed on an underlayer with corresponding crystallographic symmetry to the magnetic material, the [111] plane of the thin film of magnetic material being oriented substantially in parallel to the surface of the thin film. A thin soft magnetic film according to claim 1, wherein said magnetic material of cubic symmetry is composed of iron containing a small amount of silicon, and the underlayer is composed of a material selected from the group of a Zn-Se compound, Cd-S compound, Cu-Br compound, Mn-Se compound, Hg-S compound, Al-As compound, and Ga-As compound. A thin soft magnetic film according to claim 1, wherein assuming that a magnetostriction coefficient in the 〈100〉 direction of said magnetic material of cubic symmetry is λ₁₀₀ and a magnetostriction coefficient in the 〈111〉 direction of the same is λ₁₁₁, the following equation is established: |λ₁₀₀ + 2λ₁₁₁|< 2/3{|λ₁₀₀| + 2|λ₁₁₁|} A thin soft magnetic film according to claim 1, wherein assuming that a magnetostriction coefficient in the 〈100〉 direction of said magnetic material of cubic symmetry is λ₁₀₀ and a magnetostriction coefficient in the 〈111〉 direction of the same is λ₁₁₁, the following equation is established: |λ₁₀₀ + 2λ₁₁₁|< 1/3{|λ₁₀₀| + 2|λ₁₁₁|}, A method of manufacturing the thin soft magnetic film according to claim 1, comprising forming the thin film of a magnetic material of cubic symmetry on the underlayer heated to 300°C or more and composing the crystal surface of said thin film substantially of the [111] plane. A method of manufacturing the thin soft magnetic film according to claim 5, wherein said underlayer is composed of a material selected from the group of a Zn-Se compound, Cd-S compound, Cu-Br compound, Mn-Se compound, Hg-S compound, Al-As compound, and Ga-As compound, and said magnetic material of cubic symmetry is composed of iron containing a small amount of silicon.

Inventors: FUJIWARA HIDEO, HOSONO AKIHIKO, SHIMADA YUTAKA, FUJIWARA, HIDEO, HOSONO, AKIHIKO, SHIMADA, YUTAKA
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1990
Main classification: H01F10
IPC codes: H01F10