Method Of Producing A Single Crystal Of Orthoferrite And Thin Platelets Thereof By Means Of The Floating Zone Method

Abstract

An orthoferrite single crystal is grown by the floating zone method with the growth direction of the crystal perpendicular to the easy axis of magnetic anisotropy by using a starting seed crystal whose easy axis is disposed perpendicular to the growth direction. The thus produced crystal is then cut into thin platelets in which the plane surfaces thereof are perpendicular to the easy axis of magnetic anisotropy.

Description

This invention relates to a method of producing a single crystal of orthoferrite by the floating zone method and thin platelets thereof.

BACKGROUND OF THE INVENTION

It is known that single crystals of spinel ferrites or other oxides are obtainable by the floating zone method. Similarly, it is possible to produce by the floating zone method a single crystal of orthoferrite characterized by uniaxial magnetic anisotropy. The single crystal of orthoferrite so produced is cut into thin platelets which have plane surfaces perpendicular to the easy axis of magnetic anisotropy suitable for use as bubble domain devices. Each thin platelet, however, contains many dislocations at the four corners of the platelet. The dislocations impede smooth domain wall motions and moreover restrict the size of the effective area of the device.

OBJECTS OF THE INVENTION

It is therefore an object of this invention to provide a method of producing an orthoferrite single crystal from which improved bubble domain devices can be producted.

Another object is to provide a method of manufacturing wide-area thin platelets of orthoferrite.

It is still another object to provide a method of manufacturing bubble domain devices having a substantially wide or large area and substantially without defects.

SUMMARY OF THE INVENTION

According to this invention, a method is provided of producing an orthoferrite single crystal by floating zone method, said crystal having a uniaxial magnetic anisotropy, wherein the improvement resides in the step of growing the crystal along an axis perpendicular to the easy axis of magnetic anisotropy.

According to one embodiment of the invention, there is provided a method of manufacturing thin platelets out of an orthoferrite single crystal produced by floating zone method, the crystal having a uniaxial magnetic anisotropy, wherein the improvement comprises the steps of growing the crystal along an axis perpendicular to the easy axis of magnetization, the single crystal thus produced is cut into thin platelets having parallel surfaces perpendicular to the easy axis of magnetization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the essential portion of an apparatus for growing a single crystal by floating zone method, which is used in carrying out the method according to this invention;

FIG. 2 is a schematic perspective view of a single crystal of orthoferrite grown by a conventional method;

FIG. 3 is a like view of a single crystal of orthoferrite cut into thin platelets according to a conventional method;

FIG. 4 is a schematic perspective view of a single crystal of orthoferrite grown in accordance with the invention; and

FIG. 5 is a similar view of a single crystal of orthoferrite cut into platelets in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the essential portion of an apparatus for growing a single crystal by floating zone method comprises a lower axle 1 having a lower chuck 2 for holding a seed 3 of crystal on which a single crystal 4 is to be grown. The apparatus further comprises an upper axle 5 having an upper chuck 6 for supporting a polycrystalline rod 7 to be subjected to the floating zone method. By axial movement of the axles 1 and 5, the seed 3 and the rod 7 are brought into contact at a predetermined position where the heat from a heat source (not shown) is caused to melt the interface portions of the seed 3 and the rod 7. With the relative position unchanged, the axles 1 and 5 are lowered in the direction shown by arrows D. The single crystal 4 grows on the seed 3 from molten zone 8 which moves upwards relative to the axles 1 and 5.

With the apparatus of this type, it is already known that a single crystal 4 can be grown along a particular crystallographic axis when that axis of the seed 3 is placed parallel to the direction D. It has now been found that a so-grown single crystal 4 of orthoferrite or a substance, such as an oxide, having a large ionization tendency has a specific crystal habit and a characteristic shape.

It should be noted here that the floating zone 8 is subjected to localized heat to a temperature above the melting point. As a result, the single crystal 4 grown on the seed 3 passes through a steep temperature gradient whereby thermal stresses are imposed on the crystal. When the single crystal 4 has a habit of growing into a generally rectangular parallelepiped configuration (an elongated crystal), the stress is marked at the four corners transverse to the longitudinal axis. More particularly, the four corners are characterized by many dislocations which probably result from crystallographic slip due to the imposed high thermal stresses.

Referring to FIGS. 2 and 3, an elongated single crystal 9 of the orthoferrite having the easy axis of magnetization in the direction of the <001> axis is grown along the <001> axis, that is, along longitudinal axis X--X. Cutting the single crystal 9 perpendicular to the axis of growth, thin platelets 10, 11, 12, etc., are obtained. When, for example, the platelet 10 is etched by hot phosphoric acid after polishing, it is observed by a metallurgical microscope that the etch pits are abundant at the four corners 13, 14, 15 and 16 and scarce at the other surface portion between the corners. The etch pits show the dislocations caused by high thermal stresses set upon the single crystal 9 at the four corners 13, 14, 15 and 16 formed in accordance with the habit.

Since the single crystal is imperfect because of the abundant dislocations at the four corners, the performance of the bubble domain devices formed of thin platelets 10 is adversely affected where the magnetic domains are driven two-dimensionally within platelet 10. This is because the dislocations impede the movement of the magnetic domains. A bubble domain device is usually manufactured by cutting the four corners 13, 14, 15 and 16 away. However, it is necessary that the device have the largest or widest possible area and also that the magnetic domains should be movable throughout the area. Thus, the conventional method of manufacturing bubble domain devices has certain inherent disadvantages in that the serviceable area of the device is markedly reduced.

Referring to FIGS. 4 and 5, an elongated single crystal 17 of orthoferrite having the easy axis of magnetization in the direction of the <001> axis is grown with the <010> axis of the seed 3 held parallel to or coaxial with the axis of the apparatus, the longitudinal axis Y--Y of the crystal being perpendicular to the <001> axis. In accordance with the habit, a cross section 18, namely, the (010) surface taken perpendicular to the axis of growth has a generally rectangular shape having the longer sides parallel to the <001> axis. The etch pits observed as mentioned with reference to FIG. 3 are abundant at the four corners 19, 20, 21 and 22 of the rectangle. Even with this invention, it is impossible to obviate the dislocations. It is, however, possible to cut the single crystal 17 into a multiplicity of thin platelets, such as 23, 24, 25 and 26. Thin platelets 24, 25, with a substantial decrease in or having no dislocations are quite useful in bubble domain devices and have larger areas despite the fact that they are cut from a single crystal 17 having almost the same volume as the conventional single crystal 9 shown in FIGS. 2 and 3. It will be noted, however, that the area of working faces of platelets 24 and 25 without the dislocations are much larger than the area of working faces of platelets 10, 11 and 12 of FIG. 3.

EXAMPLE 1

A single crystal of yttrium orthoferrite (YFeO.sub.3) was grown on the seed of the same material placed with the <010> axis parallel to the axis of the apparatus for growing the single crystal by floating zone method. The single crystal had the dimensions shown in FIG. 4. With the initial and the last grown portions of the crystal of a length of about 10 mm each cut away, the 30 mm long single crystal remaining is then cut with a slicing machine into eight sheets of thin platelets with the planes thereof being the (001) planes. Four of the platelets had no portions abundant with the dislocations. With each of the thin platelets 30 mm long and 5.5 mm wide, it was possible to drive the bubble domains over the whole area without any impediment.

As a reference, a single crystal of the same material was grown along the <001> axis in accordance with the conventional method. The single crystal had the dimensions shown in FIG. 2. With the initial and the last grown portions of a length of 10 mm each cut away, the 30 mm long single crystal remaining is then cut into thirty-eight sheets of thin platelets having the (001) planes. Each thin platelet of the generally square shape of 6 mm by 6 mm had only an effective area of 4 mm by 4 mm as a bubble domain device on account of the four-corner portions in which the dislocations were abundant.

The effective area attained in accordance with this invention is more than ten times as wide or larger as is obtained with the conventional method.

EXAMPLE 2

A single crystal of yttrium orthoferrite (YFeO.sub.3) was grown on the seed of the same material placed with the <100> axis parallel to the axis of the apparatus for growing the single crystal by floating zone melting. The single crystal had the dimensions illustrated in FIG. 4 except the axes <100> and <010> are interchanged. The crystal was cut with a slicing machine into eight sheets of thin platelets in the manner shown in FIG. 5. Four of them were free from the portions in which the dislocations were abundant. With each of the thin platelets 30 mm long and 5.5 mm wide, it was possible to drive the bubble domains over the whole area as was the case with Example 1.

EXAMPLE 3

The same results as described in conjunction with Example 1 were achieved for a single crystal of terbium orthoferrite (TbFeO.sub.3) except the single crystal was grown on the seed of terbium orthoferrite.

EXAMPLE 4

The same results as described in conjunction with Example 2 were attained for a single crystal of terbium orthoferrite (TbFeO.sub.3) except the single crystal was grown on the seed of terbium orthoferrite. pg,9

EXAMPLE 5

The same results as described in connection with Examples 1 and 2 were attained for a single crystal of erbium orthoferrite (ErFeO.sub.3), ytterbium orthoferrite (YbFeO.sub.3), or a mixed ortherferrite including an optional ratio of any two of yttrium, terbium, erbium and ytterbium.

EXAMPLE 6

The same results as described in conjunction with Examples 1 and 2 were achieved for a single crystal of a cobalt-substituted yttrium orthoferrite (YFe.sub.1.sub.-x Co.sub.x O.sub.3) wherein Co.sup.3.sup.+ ions were substituted for a portion of Fe.sup.3.sup.+ ions.

EXAMPLE 7

The same results as described in connection with Examples 1 and 2 were achieved for a single crystal of a cobalt-titanium substituted yttrium orthoferrite (YFe.sub.1.sub.-x Co.sub.x/2 Ti.sub.x/2 O.sub.3) wherein an equal number of Co.sup.2.sup.+ ions and Ti.sup.4.sup.+ ions were substituted for a portion of Fe.sup.3.sup.+ ions.

From the Examples described above, this invention is apparently applicable to single crystals of all kinds of orthoferrite, wherein each has a uniaxial magnetic anisotropy and is manufacturable by the floating zone method. Furthermore, it is possible to manufacture those thin platelets of a single crystal of orthoferrite which have (001) surfaces, by growing the single crystal along an axis, such as <110> axis, that is perpendicular to the <001> axis. It should be noted here that the notations of the crystallographic axes and planes mentioned above are not limitative. For example, thin platelets having (100) surfaces are manufacturable in accordance with this invention from a single crystal of orthoferrite, such as samarium orthoferrite, wherein the easy axis of magnetic anisotropy is the <100> axis.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

Claims

We claim:

1. In a method of manufacturing thin platelets of orthoferrite single crystals produced by the floating zone method, wherein said crystal has uniaxial magnetic anisotropy, the improvement which comprises,

growing said single crystal along an axis which is perpendicular to the easy axis of magnetization, thereby forming an elongated generally rectangular parallelepiped whose longitudinal axis is the axis of growth,

and then cutting said single crystal into thin platelets with their plane surfaces parallel to said axis of growth and perpendicular to said easy axis of magnetization.

2. A method as claimed in claim 1 wherein said crystal cutting step is carried out at that portion of the single crystal in which the dislocations are substantially reduced.

3. A method as claimed in claim 1 wherein the easy axis of magnetization is the axis <001> and said parallel surfaces are (001) planes.

4. A method as claimed in claim 1 wherein the easy axis of magnetization is the axis <100> and said parallel surfaces are (100) planes.