GB2034297A

GB2034297A - Garnet Film for a Magnetic Bubble Device

Info

Publication number: GB2034297A
Application number: GB7935577A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1978-10-13
Filing date: 1979-10-12
Publication date: 1980-06-04
Also published as: JPS5552205A; DE2941442C2; NL7907616A; JPS6057210B2; DE2941442A1; NL178920C; NL178920B; GB2034297B

Abstract

A garnet film for a magnetic bubble device has the formula R<I>3-xRx<II>Fe5-yAlyO12 where R<I> is at least one of Y, Gd, Yb, Tm, Lu and La, R<II> is at least one of Sm and Eu, 0.5<x<2.0 and 0.2<y<0.9, with the proviso that if Gd is present as some or all of R<I> then the amount of Gd is not greater than 0.5 mol per molecular formula. Such a film can produce a device with bubbles having a diameter of not greater than 1.5 mu m because the saturation induction may be kept at a low level without excessive reduction in the Curie point.

Description

SPECIFICATION Garnet Film for a Magnetic Bubble Device This invention relates to a garnet film -for a magnetic bubble device and more particularly to a single crystal garnet film suitable for a high density memory, magnetic bubble memory device having a bubble diameter of not greater than about 1.5,um.

As is well known in the art, magnetic bubble memory devices have attracted increasing attention as promising information processing devices, especially as promising memory devices, and active research and development is being carried out at present.

One of the most important characteristics of such a device when used as a memory device is memory density, which is determined by the diameter of the magnetic bubble. The devices employed generally at present have a bubble diameter of about 3 to 5 ym, and it is expected that memory density can be drastically increased by further reducing the diameter of the magnetic bubble.

In order to extend the range of practical applications of magnetic bubble memory devices, which would then replace other devices generally used at present such as disc memory, semiconductor memory and the like, it is necessary drastically to increase the memory density of the bubble device by reducing the bubble diameter to not greater than 1.5 m.

Hence, it is imperative to find a material for the magnetic bubble medium which is able stably to retain and operate with a magnetic bubble having such a fine diameter.

The following references are cited to show the state of the art.

(1) F. B. Hagedorn; AIP Conf. Proc., vol. 5, pages 72-90, (1972) This reference discloses Yo gGd1,1 LaO,4Fe4,4Al0Sso12 and other garnet films.

(2) D. H. Smith et al., AIP Conf. Proc. vol. 5, pages 120-123(1972) This reference discloses Er2GdAI04Fe4 N0,2, Er,5Eu,5AI04Fe460,2, and other garnet films.

(3) W. A. Bonner et al., J. Appl. Phys. vol. 43, No. 7 pages 3226-3228, July 1972 This discloses a garnet film having the formula Y,~,Eu,AI,Fe,,O12(x=1.5-2.0, y=0.7-1.2).

(4) L. G. Van Uitert et al. Mat. Res. Bull., vol. 6, pages 1185--1200 (1971) Here are disclosed EU,5Gd15Al0*5Fe4S5O12 and other garnet films.

The garnet disclosed in each of these references is not for the fine bubble and, though the amount of Gd present is up to 0.5 in the film of the present invention to be described in detail below, it exceeds 1 in each reference. Though Gd is added to adjust 4rrMs by magnetization of Gd ions in each of the references, the addition of Gd results in increased temperature variation of various properties such as 4#Ms and Ho (bubble collapse field).

The amount of Eu present is up to 1.4 in the film of the present invention whereas it is at least 1.5 in the cited references so that in the latter cases the gyromagnetic ratio and magnetic wall mobility are smaller and high speed transfer of the bubbles becomes difficult.

The amount of Al added falls within a specified range in the present invention but none of the references disclose the addition of Al in order to adjust the exchange stiffness constant A to a desired value as in the present invention. The cited references add Al in order to adjust the mismatch between the lattice constant of a substrate crystal and that of a bubble film arising from the addition of Gd and the like.

According to the present invention there is provided a garnet film for a magnetic bubble memory device having the composition R3~xRx'Fe5-yAlyo12 wherein: R' is at least one of Y, Gd, Yb, Tm, Lu, and La: R" is at least one of Sm and Eu; and 0.5 < x < 2.0, 0.2 < y < 0.9; with the proviso that if Gd is present as some or all of R', the amount of Gd is not greater than 0.5 mol per molecular formula.

In the present invention the saturation induction [47rMs] is kept at a low level by substituting a prescribed amount of iron with aluminium without excessively lowering the Curie point, and other property values are maintained at desired levels by adding prescribed amounts of other elements such as yttrium and gadolinium, thereby enabling stable retention of fine bubbles.

Based on the theory of Thiele (Bell, Syst. Tech.

J 50, ('71) 725), explanation will initially be given of the properties of a material which allow the stable presence of fine magnetic bubbles.

If a magnetic garnet film is selected so that its thickness h is substantially equal to the diameter d of the magnetic bubble, d is substantially eight times the characteristic length I: d=8l (1) In this instance, I can be expressed by the equation below in terms of the saturation induction (47tMs), anisotropy field (Hk) and exchange stiffness constant (A): I=2(87rA Hk)1/5(47rMs)3/2 (2) Since Hk can be defined as HK=q-(47rMs) with q being the factor representing stability of the magnetic bubbles, d can be expressed by: d=1 6(87GAq)"2/4nMs (3) Hence, in order to make d small, it is necessary to make both A and q as small as possible and 47toms as large as possible.

From the practical point of view, however, the following two restrictions are imposed on the magnetic bubble device.

(1) In order to prevent generation of bubbles at positions other than the magnetic bubble generator, it is preferred that q exceeds about 3.

To assure easy generation of the bubble by the magnetic bubble generator, on the other hand, it is preferred that q is up to 10.

(2) Transfer of the magnetic bubbles is effected by means of a rotating field. Experiment shows that the intensity of the rotating field required for this transfer is substantially proportional to 47toms.

Hence, it is necessary to make 4zMs as small as possible so as to reduce the power required for the generation of the rotating field and to minimize heat generation in a rotating field generating coil.

These restrictions mean that the only free factor in equation (3) above is A. To achieve small d, A must be made small.

As materials for garnet films for fine bubbles (bubbles having a diameter of about 1 ,us), the Ga-substituted type garnets (EuTm)(FeGa)6012, (EuYb)3(FeGa)sO2, (EuLu)3(FeGa)sO,2 and the like have already been proposed. Other materials proposed are (SmLu)3(FeFaisO12, which incorporates Sm in place of Eu, and (f;mTm)3FesO,2 in which Fe is not substituted.

When compared with ordinary garnets, these garnets have the following specific features.

(1) The saturation induction (47rMs) is larger by at least about 800G than that of the ordinary garnet. (In a material in which no Fe-substitution is made, 4Ms > =1 ,200G but in a 2 ymsX material (YSmLuCa),(FeGe),O,, 4nMs-430G and in (YSmLu)3(FeGa) > 0,2,47rMs~380G, for example).

(2) The Curie point is as high as at least 2000C and the exchange stiffness constant A is at least 3x 10-7 erg/cm.

(3) The anisotropy energy Ku is greater by at least twice than that of a material used for a magnetic bubble having a diameter 2 ym.

In using these magnetic films in a bubble device, one of the most critical problems is that the power consumption for the transfer of the bubbles is inordinately high and extreme heat production occurs in the coil for the bubble transfer. This is because the power consumption of the coil increases substantially proportionally to the square of the value Ms of the garnet film, as will be understood from the foregoing explanation. It can be said fairly, therefore, that the first requirement for the fine magnetic bubble garnet is that its Ms value is as small as possible.

To achieve this, it is necessary to make A small, because of the aforementioned two restrictions.

Since A is primarily determined by the mutual interaction between Fe ions, some of the Fe ions may be substituted by other ions thereby to reduce the amount of Fe and thus make A small.

In other words, the greater amount of Fe which is substituted, the smaller the A value.

However, if A is below 1 .5x 10-7 erg/cm, the Curie temperature Tc becomes below 1 500C so that the temperature dependence of various properties of the magnetic bubble device is extremely great and the temperature range in which the device can be used is narrow. In practical use it is necessary that the device is operated at 1 000C. If Tc is lower than 1 500C however, the device cannot be used at 1 000C and is impractical.

Ions that can be used to make A small while keeping Tc at least 1 500C include Al3+, Ga3+, Si4+, Ge4+ and V5+. Among these, Al3+ provides the most desirable results. In other words, since Al3+ has the maximum A-reducing effect of these ions, it is possible to minimize 47rMs when using fine bubbles Al3±substitution. For this reason, the present invention uses Al3+ as a substitute for some Fe. The degree of substitution y must be in the range 0.2 to 0.9 in practical application.

When the magnetic bubble diameter is not greater than about 1.5 ym, the value 4rims of the garnet film must be from 550 to 1,300 Gauss. If 47tMS is below 550G, it is impossible stably to form such fine bubbles unless the film is extremely thin. When 47toms exceeds 1 ,300G, on the other hand, an anisotropy energy Ku of at least 2x 105 erg/cm3 is necessary in order to achieve stable transfer the bubbles. With present techniques however, a Ku value of 2x106 erg/cm3 is substantially the upper limit and beyond this limit, it is impossible to obtain a stable bubble film.

If y is less than 0.2, 47toms becomes more than 1 ,300G and when Y exceeds 0.9, 47tMs becomes less than 550G. Hence, y should be within the range 0.2 to 0.9.

Since Y, Gd, Yb, Tm Lu and La all have small magnetic loss, they are added (as R') so as to bring the lattice constant of the garnet film into conformity with that of the substrate.

Eu and Sm are added (as R") because they have not only great anisotropy exhibiting effect but also relatively small magnetic loss.

The amountxof Eu and Sm used must be within a predetermined range. As 4'rims must be within the range 550-1,300 Gauss, it is necessary that x be in the range of about 0.5 to about 2.0 in order to achieve the stable presence of the magnetic bubbles. If x is smaller than 0.5, the anisotropy energy is insufficient so that the bubbles are unstable and their stable transfer is impossible. If x is greater than 2.0, on the other hand, the bubbles can be stable but their transfer becomes difficult and high speed transfer is impossible.

Because Gd, Yb and Tm also have an anisotropy exhibiting effect, they are effective for obtaining a desired anisotropy energy in conjunction with Eu and Sm. When the amount of Gd exceeds 0.5 mol per molecular formula, however, the temperature dependence of 47toms becomes great and the temperature characteristics deteriorate. It is therefore necessary to avoid the addition of Gd in an amount exceeding 0.5 mol.

If the amount of Eu exceeds 1.4 mols per molecular formula, on the other hand, the bubble saturation speed becomes remarkably slow; hence, the Eu amount is preferably up to 1.4. As Sm does not cause this adverse effect, its amount may exceed 1.4 mols.

Example 1 Raw material oxides viz. 0.56 g of Y203, 0.87 g of Sm2O3, 16g of Fe2O3 and 0.54 g of Al2O3, and flux, viz. 230 g of PbO and 4.6 g of B203, are heated and homogenized at 1 ,2000C for 10 hours in a platinum crucible, and are then grown epitaxially in the liquid phase on a Gd3Ga50,2 single crystal at 9200C for 3 minutes. The resulting garnet film has the following properties: film thickness (h)=bubble diameter (d)0.8 ym, characteristic length (I)=0.09 m, Curie temperature (Tc)=1 800C A=2.0x 10-7 erg/cm,4xMs=700 G, magnetic wall mobiiity uw=250 cm/s.0e, Hk=1,500 Oe.

In the bubble device produced using this material, high speed transfer of the bubble is possible and the bubbles are sufficiently stable.

The coercive force Hc also is as small as 0.80 Oe.

The properties of the device are excellent.

Example 2 Liquid phase growth is carried out in the same way as in Example 1 using, as the raw oxide materials, 1.2 g of Sm203, 0.6 g of Tm2O3, 16 g of Fe203 and 0.3 g of Al203 and as the flux, 250 g of PbO and 5.0 g of B203. The resulting garnet film has the following properties: h=d=0.5 m, l=0.056 ym, Curie point, Tc= 2200C, A=2.5x 1 0-7 erg/cm, 4#Ms=950 G, w=1 80 cm/s.0e Hk=1,700 Oe.

It is found that an ultra-high density magnetic bubble device having a bit period of 4 ym can be produced using this garnet film.

Example 3 Liquid phase growth is carried out in the same way as in Example 1 using, as the raw oxide materials, 0.85 g of Eu203, 0.80 g of Sm2O3, 1.2 g of Tm203, 15.5 g of Fe203 and 0.28 g of Al203 and as the flux, 235 g of PbO and 4.7 g of B203. The resulting garnet film has the following properties: h=0.6 ym, d=0.4 m, 1 1=0.05 ym, Tc=210 C, A=2.4x 10-7 erg/cm, 4#Ms=900G.

It is thus possible to reduce 4#Ms by not less than 200G in comparison with the previously proposed film having the same bubble diameter.

Since the film uses Eu, Sm and Tm each of which has a large anisotropy-exhibiting effect, it is confirmed that Hk#2,000 Oe and the bubble is sufficiently stable.

Example 4 A garnet film having the molten liquid composition listed below and grown epitaxially on a Gd3Ga5Or2 single crystal has practically the composition Y1 76smo 92Gdo 32Fe438Alode2o12- The film thickness is 1.0 m and the bubble diameter is 1.0 m at room temperature. The saturation induction (4xMs) of the garnet film is 665G, which is an extremely low value for a garnet film for fine bubbles. Since this garnet film incorporates a small amount of Gd, the temperature variation of 4#Ms is reduced.For example, the temperature change of the bubble collapse field is as small as from -0.19 to -0.23%/0C over a wide range of 0 to 1000C. The film has excellent temperature characteristics.

Garnet composition: Y203 0.738g Sm203 0.592g Gd203 0.213g Fe203 1 5.97 g Al203 0.537 g Flux components: PbO 222 g B203 4.44 g As with conventional films, the single crystal garnet film of the present invention can be formed by epitaxial growth onto a Gd3Ga5O,2 single crystal.

In these examples it will be noted that the film composition does not correspond directly to the composition of the oxide mixture or of the melt.

The film composition is determined both by the melt composition and by the distribution coefficient. In each of the above examples a film composition falling within the scope of claim 1 below is obtained.

The film thickness (h) is substantially equal to the diameter (d) of the magnetic bubble to be formed and can be chosen accordingly. The range of d/h is from about 0.5 to about 2.0.

The film of the present invention is especially suitable for forming fine bubbles having a diameter of not greater than 1.5 ym and its maximum thickness is generally about 1.5 m. As to a film having a thickness of not greater than 0.3 ssm, it is difficult to obtain a uniform film having such a thickness by liquid phase epitaxial growth. Since the properties of the magnetic bubble memory element vary when such a film is employed, it is preferred that the film has a thickness of not less than about 0.3 ym.

Claims

1. A garnet film for a magnetic bubble memory device having the composition R3~xRx'Fe5-yAlyo12 wherein R' is at least one of Y, Gd, Yb, Tm, Lu and La; R" is at least one of Sm and Eu; and 0.5 x < 2.0, 0.2 < y < O.9; with the proviso that if Gd is present as some or all of R', the amount of Gd is not greater than 0.5 mol per molecular formula.

2. A garnet film according to claim 1 having a thickness in the range from about 0.3 to about 1.5,us.

3. A garnet film according to claim 1 or claim 2 which has been formed onto a Gd3Ga5O,2 single crystal.

4. A garnet film according to any one of claims 1 to 3 containing Eu as some or all of R" in an amount up to 1.4 moles per molecular formula.

5. A garnet film for a magnetic bubble memory device substantially as herein described in any one of the Examples.

6. A magnetic bubble memory device having a garnet film according to any one of the preceding claims.