U.S. patent number 3,662,500 [Application Number 05/110,779] was granted by the patent office on 1972-05-16 for method for polishing magnetic oxide materials.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Eric Mendel.
United States Patent |
3,662,500 |
Mendel |
May 16, 1972 |
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 silicon dioxide alkaline slurry
under polishing pressure not in excess of about 20 psi.
Inventors: |
Mendel; Eric (Poughkeepsie,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
22334879 |
Appl.
No.: |
05/110,779 |
Filed: |
January 28, 1971 |
Current U.S.
Class: |
451/37;
51/308 |
Current CPC
Class: |
B24B
1/00 (20130101); C09K 3/14 (20130101) |
Current International
Class: |
B24B
1/00 (20060101); C09K 3/14 (20060101); B24b
001/00 () |
Field of
Search: |
;51/281R,281SF,283,326,328,323 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Bell Switches Bubble Material" by Martin Gold, Electronic News,
Apr. 27, 1970. .
"Chemical Polish for Rare Earth Orthoferrites," by L. K. Shick,
Electrochemical Society, Vol. 118, No. 1, January, 1971, pages
179-181. .
"Properties and Device Applications of Magnetic Domains in
Orthoferrites" by A. H. Bobeck, The Bell System Technical Journal,
October, 1967, pages 1901-1925..
|
Primary Examiner: Swingle; Lester M.
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 silicon dioxide.
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 silicon dioxide
suspension is aqueous.
5. A method in accordance with claim 1 wherein said finish
polishing is done under pressure between 2 and 20 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 silicon dioxide
having a pH between 9 and 12 at a pressure up to 20 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 silicon
dioxide under a pressure of up to 20 psi.
10. A method in accordance with claim 9 wherein said silicon
dioxide slurry pH is between 11 and 12.
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. The orthoferrites are a class of
rare earth (RE), iron oxides having the general formula
REFeO.sub.3, and they have a perovskite relative 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 of 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 silicon dioxide powders having an adjusted pH
between 9 and 12. At pH conditions below this range, reduced stock
removal rates and orange peal type surface conditions, which may
prevent ultimate featureless polished pitting, occurs. The slurry
pH can be adjusted with sodium or potassium hydroxide or any
convenient suitable alkali. 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 under pressure of
from 2 to 20 psi. Polishing pressures in excess of 20 psi are
detrimental and prevent accomplishment of the ultimate polished
condition contemplated by this invention.
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.
* * * * *