Method For Polishing Magnetic Oxide Materials

Abstract

This invention relates to a method of polishing magnetic oxides or bubble crystal surfaces to a featureless and strain-free condition. The method comprises pre-polishing or lapping with a suspension of polycrystalline garnets to a conchoidal condition followed by final polishing with a zirconium oxide slurry under polishing pressure between 2 and 40 psi.

Description

BACKGROUND OF THE INVENTION

The surface treatment and polishing of magnetic host materials commonly referred to as orthoferrites, garnets, and magnetic oxides or bubbles requires methods and techniques different from the procedures heretofore followed in the polishing of metals, ceramics and other crystalline materials. Application Ser. No. 110,779, filed Jan. 28, 1971, and entitled Method For Polishing Magnetic Oxide Materials discloses a silicon dioxide method of polishing magnetic oxides. The orthoferrites are a class of rare earth (RE), iron oxides having the general formula REFeO.sub.3, and they have a perovskite related orthorombic structure. The iron in these compounds is trivalent in contrast to the spinel ferrites where both divalent and trivalent iron exists. These magnetic materials are melt-grown and growth techniques can be grouped into three classes; namely, pure melts, self-fluxes melts where an excess of one crystal constituent serves as a solvent, and molten solutions employing an added solvent. Similarly, the Czachrasski molten pool single crystal pulling method is applicable to the production of magnetic oxide materials. These techniques produce a monocrystalline structure capable of containing magnetic domains or bubbles. It is also known to grow homogeneous uniaxial magnetic garnet films using liquid phase epitaxial techniques. These techniques permit fabrication of devices with an excess of 1 million bubbles per square inch for use in computer and digital communications applications. In order to accomplish this type film growth, the substrate surface must be polished perfectly to a featureless state free of cracks and strains. Otherwise, the film growth will be imperfect. Before bubbles can be created in a crystal, a source or host crystal for the bubbles must first be grown. A rare earth such as thulium or terbium is placed in a crucible and a ferrite is added. This combination is then heated to well above the molten state and then cooled slowly to room temperature. The resultant single crystal magnetic oxide is called an orthoferrite. The crystal is then sliced and polished to make a platelet several mils thick. The condition of the surface of single crystal magnetic materials is important, because substantial narrowing of the ferromagnetic resonance line width may be accomplished if the surface of a specimen is prepared in a featureless, highly polished, and strain-free state. Strain-free domain patterns are more readily made if the strained surface layers caused by mechanical polishing or other treatments can be removed.

DESCRIPTION OF THE PRIOR ART

It has been found that certain ferromagnetic oxides become optically transparent when their thickness is reduced to several tens of microns. These materials exhibit Faraday rotation and magnetic birefringence in transmitted polarized light. These effects have been utilized to study the magnetic domain structure in the certain iron garnet oxides. The surfaces of these materials have been mechanically polished or chemically treated during the polishing operation to produce polished surfaces on both sides of a specimen.

The prior art methods used in an attempt to prepare highly polished featureless surfaces on the subject material specimens have their origin in either the metallographic or petrographic art. The metallographic practices generally entail sampling of the material by sawing or "cutting off" a representative section, rough-grinding, fine-grinding, polishing, and removal of the damaged surface layers by etching, chemical or electro-chemical polishing. The rough-grinding and fine-grinding steps mentioned above utilize successively finer grades of silicon carbide or emery abrasive papers. Polishing is accomplished with successively finer grades of diamond grit, aluminum oxide, or magnesium oxide abrasive powders.

The petrographic methods have been reasonably standardized and are generally applied in the study and utilization of refractories, ceramics, and gem stones. Often, refractory and ceramic specimens are prepared as thin sections. The preparation of these materials follow the same procedure and sequence of steps as those mentioned above and applicable to metallographic specimen preparation, except that there is a greater tendency to use silicon carbide and diamond in lapping and polishing down, because these specimens are harder than metals. Similarly, the lap and polish surfaces differ greatly as compared to "cloths" used in metallographic polishing. The final thickness of these specimens is often important and the mounting of the specimen is therefore more critical than the degree of sophistication used in the surface treatment operations. Flatness and thickness of the specimen is important in order to produce a substrate of uniform thickness and parallel surfaces. Chemical mechanical methods have also proved inadequate and impractical. The use of hot phosphoric acid at temperatures of approaching 500.degree. C to chemically polish the subject materials by rotating a specimen in the hot H.sub.3 PO.sub.4 bath.

These methods have failed to produce the required specular and damage-free surfaces on flux-grown or other type single crystal garnets, orthoferrites and similar materials.

Cracks, holes, surface-scratches, twinning planes and inclusions or other planes are believed to be caused by the prior art polishing and surface treatment methods all of which impede domain wall motion and prevent control of the establishment of domains into single configurations. These conditions also restrict narrowing of the ferromagnetic resonance line width which can be accomplished if the surface of the specimen is prepared in a highly polished and strain-free state. Elimination of mechanical polishing defects and strains makes observations of strain-free domain patterns more readily possible. Damaged surface conditions on thin platelets of the basic materials of this invention have been removed to some extent by chemical polishing and etching. However, these chemical dissolution techniques cannot be applied to all materials. Attempts to eliminate strains and internal crystal dislocations by high temperature annealing in purified oxygen and argon atmospheres have been reported, where the orthoferrite thin platelets are positioned in an orthoferrite holder within an appropriate annealing furnace and atmosphere.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for the surface treatment of single crystal magnetic materials whereby the polished surface is featureless and specular and damage free.

It is a further object of this invention to provide a method for polishing single crystal garnets and orthoferrites having featureless surfaces and void of internal sub-surface strains.

A still further object of this invention is to provide a method for the surface treatment of magnetic monocrystalline materials to produce completely featureless surfaces void of cracks, holes, scratches, preferential topography and the like.

It is still a further object of this invention to provide a surface treatment or polishing method for accomplishing the aforesaid results within practical time limits and suitable for large-scale commercial manufacturing procedures.

A still further object of this invention is to provide a method for polishing and surface treatment of monocrystalline magnetic materials so as to produce a slice or platelet free of internal strains and strained surface layers.

These and other objects are accomplished in accordance with the broad aspects of the present invention by providing a pre-polishing or lapping step using a suspended mineral silicate and which produces a shallow dish-like flaked conchoidal fractured surface, followed by a polishing step utilizing materials and conditions which produce a completely polished featureless strain-free specimen.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to avoid and minimize the penetration of damage into the crystal during the polishing steps of this invention, an abrasive material which breaks down rather rapidly and removes stock as a result of conchoidal fracturing is contemplated, for example, in the use of natural polycrystalline garnet abrasive powders which is believed to remove material chips or flakes from the surface of a specimen through the exertion of lateral rather than vertical forces. Garnets are a group of silicate minerals with the general formula R.sub.3.sup.II R.sub.2.sup.III (SiO.sub.4).sub.3, where R.sup.II is calcium, magnesium, iron or manganese and R.sup.III is aluminum, iron titanium or chromium. Conchoidal fracture lapping produces a very thin layer of damaged material on the surface of the workpiece by removal or relatively uniform shallow-dished or flaked pieces. In contrast, lapped surfaces can be obtained where the abrasive grains produce a series of deep penetrating flaws that extend into the bulk of the workpiece in a non-uniform manner.

When the damaged (lapped) layer is very thin and uniform in thickness, as results in conchoidal flaking, two conditions occur separately or act simultaneously. The abrasive grains break down rapidly because they are friable and not harder than the workpiece on the forces acting to remove material from the surface are primarily tangential to the surface of the material and act to flake out shallow conchoidal stock or shear away existing protuberances. It is essential and critical that this type of surface is accomplished in the lapping or pre-polishing step.

The gentlest or least severe practical lapping system where stock removal occurs by conchoidal fracturing is the use of polycrystalline garnet powder having a a particle size between 7 and 30 microns with a pearlitically cast-iron wheel system. Frosted-plate glass can be substituted for the cast-iron wheel. Yttrium iron garnet, Y.sub.3 Fe.sub.5 O.sub.12 crystal was sliced into platelets 20 mils thick. These platelets were then cemented onto a steel plug with glycol phthalate. A conventional aqueous lapping slurry was prepared with 12 micron-sized garnet powders and water. The distribution pattern of the 12 micron-sized garnet powder should be between 5 and 14 microns. The garnet slurry was applied to frosted-glass plate and sliced specimens of garnet affixed to the steel plugs were manually moved over the glass plate in a random figure-eight type motion. The stock removal was extremely rapid and a uniform surface with two to three mil conchoidal stock removal was accomplished after ten minutes of pre-polishing. A comparable diamond pre-polished sequence requires two or three hours or more polishing time and produces a surface having fissures and cracks and internal strains which makes final featureless polishing unobtainable. The aforesaid lapping or pre-polishing step was followed by final polishing steps. Although a garnet-type abrasive, suspended in water, is adequate for accomplishing the pre-polishing or lapping step, the water-based slurry tends to separate and settle upon standing. A wide variety of commercial suspending agents are available for slurry preparation to maintain the abrasive material in constant suspension.

The pre-polishing surface preparation described above reduces process polishing time from 1 week to not more than 4 hours for a specimen polished on both sides to a thickness of 1 to 2 mils and possessing a featureless damage-free surface.

Garnet grit ranging in particle size from three to thirty microinches is useful. Particle sizes of about 12 microinches are believed to be optimum.

Final polishing is accomplished by using a conventional water polishing slurry of zirconium oxide powders. An aqueous suspension equivalent to between 150 to 700 grams SiO.sub.2 in 3 to 4 liters of water produces a smooth specular and featureless surface. Surface conditions are measured and judged by visual naked eye and microscopic observations, as well as standard examination of control specimens after hot H.sub.3 PO.sub.4 etching procedure.

Final polishing is accomplished by using conventional polishing wheel equipment and a suitable polishing cloth in conjunction with the polishing slurry. Any convenient polishing wheel speed for manual or mechanical manipulation can be used. A wheel speed between 60 and 120 revolutions per minute with a presence of from 10 to 40 psi is illustrative of contemplated process conditions.

Upon complete polishing of the first surface, the process is repeated for the second or opposing surface. Simultaneous mechanical double surface polishing is contemplated within the scope of this method using appropriate handling techniques and thickness measurement methods such as the air gauge of microscopic differential focusing. Final featureless inspection and control is accomplished by the use of Nomanski illumination, which is a form of interference contrast lighting to detect and see a defective structure.

While the invention has been particularly described and shown with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A method for polishing magnetic crystalline material comprising:

lapping the surface with a suspension of polycrystalline silicate mineral, and

finish polishing with a suspension of zirconium oxide.

2. A method in accordance with claim 1 wherein said silicate material is garnet.

3. A method in accordance with claim 1 wherein the particle size of said polycrystalline silicate is between 7 and 30 microns.

4. A method in accordance with claim 1 wherein said zirconium oxide suspension is aqueous.

5. A method in accordance with claim 1 wherein said finish polishing is done under pressure between 10 and 40 psi.

6. A method for polishing magnetic crystalline material comprising:

lapping the surface with an aqueous suspension of polycrystalline silicate mineral to produce a surface characterized by shallow flaked conchoidal fracture patterns, and

finish polishing with an aqueous suspension of zirconium oxide at a pressure between 10 and 40 psi.

7. A method in accordance with claim 6 wherein said polycrystalline silicate material is garnet.

8. A method in accordance with claim 6 wherein said polycrystalline silicate material has a predominant particle size of 12 microns.

9. A method for polishing magnetic crystalline material which comprises:

lapping the surface with an aqueous suspension of garnet having a predominant particle size of 12 microns to produce a surface characterized by shallow flaked conchoidal fracture patterns, and

finish polishing said surfaces with an aqueous slurry of zirconium oxide under a pressure between 2 and 40 psi.