US3498836A

US3498836A - Method for obtaining single crystal ferrite films

Info

Publication number: US3498836A
Application number: US544788A
Authority: US
Inventors: Richard J Gambino
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1966-04-25
Filing date: 1966-04-25
Publication date: 1970-03-03
Anticipated expiration: 1987-03-03
Also published as: DE1619968A1; DE1619968B2; FR1513081A; GB1135168A; DE1619968C3; JPS4944239B1

Description

March 3, 1970 R. J. GAMBINO 3,

METHOD FOR OBTAINING SINGLE CRYSTAL FERRITE FILMS Filed April 25, 1966 FIG. 1

FERRITE 2 SINGLE CRYSTAL MAGNESIUM OXIDE INVENTOR. RICHARD J. GAMBINO United States Patent 3,498,836 METHOD FOR OBTAINING SINGLE CRYSTAL FERRITE FILMS Richard J. Gambino, Yorktown Heights, N.Y., assignor to International Business Machines Corporation, Armonk, N .Y., a corporation of New York Filed Apr. 25, 1966, Ser. No. 544,788 Int. Cl. G03g 19/00; H01f 1/34 US. Cl. 117-237 14 Claims ABSTRACT OF THE DISCLOSURE A method for epitaxially depositing a single crystal ferrite film on a single crystal substrate is disclosed. Epitaxial deposition of a ferrite having a spinel structure is accomplished by heating a solution of a polymetallic ferrite and a flux on the surface of a single crystal substrate by heating the solution to a temperature sufficient to volatilize the flux completely thereby leaving a single crystal ferrite film residue.

Transparent films of magnetic materials are of current interest because of their utility as magneto-optical memory devices. In the usual case, the variation of polarization of an incident light beam with variation in magnetic field or with localized changes in the magnetic state of the films is used to obtain outputs indicative of stored information.

Magneto-optical films and methods of manufacture are well known in the prior art. 'Methods of manufacture of these films include such techniques as vacuum deposition and the chemical reduction of appropriate materials. Most of the known techniques do not provide epitaxial single crystal films which are particularly desirable in that better optical transmission characteristics can be attained. Prior art vapor deposition techniques, for example, usually produce polycrystalline films in which grain boundaries affect both the optical and magnetic properties of the resulting films. Prior art techniques are also limited in the size of films obtainable which possess the desired single crystal characteristic. Where single crystal films of desired surface area and of sufficient thinness to transmit light are desired, they must be cut and ground from bulk single crystals prior to use. The strains, cracks and other defects introduced in such thinning operation usually degrade the crystals magneto-optical properties.

It is, therefore, an object of this invention to provide transparent epitaxial thin films of spinel ferrites.

Another object is to provide transparent epitaxial thin films of spinel ferrites which have improved optical transmission characteristics.

Another object is to provide a method of growing single crystal epitaxial films by a volatile flux growth process.

Another object is to provide a method of growing single crystal epitaxial ferrite films in which the size of the resulting films is limited only by the surface area of the seed upon which growth occurs.

Still another object is to provide a method of growing single crystal epitaxial ferrite films in which the thickness of the resulting films is controllable.

Still another object is to provide a method of growing single crystal ferrite films which is both inexpensive and reproducible.

Yet another object is to provide single crystal ferrite films which are transparent, magnetic in nature and suitable for use in magneto-optical memory devices.

In accordance with the present invention, a method for preparing epitaxial single crystal films of spinel ferrites selected from the group consisting of lithium, lithium chromium, nickel zinc, nickel, magnesium and cobalt ferrites is provided. Epitaxial deposition is accomplished from a solution of a polymetallic ferrite having a spinel structure and flux by heating the solution at a temperature sufficient to melt the flux. Briefly, the method includes the steps of providing a seed crystal of magnesium oxide or other suitable material which is transparent and noncontaminating to the resulting ferrite film. One of the ferrites from the above mentioned group is mixed with a flux such :as anhydrous sodium or lithium carbonate and with isopropyl alcohol to form a slurry. The slurry is applied to the surface of the seed crystal and air dried to remove the alcohol solvent. The seed crystal or substrate is then placed in a platinum dish and introduced into a pre-heated furnace for firing. The substrate and mixture are then heated for 20 hours in air or oxygen at a temperature Slll'fiClCl'lt to dissolve the ferrite in the flux and to cause crystallization of the ferrite by volatilization of the flux. The seed, after heating, is covered with a single crystal epitaxial film of ferrite 5 to 50 microns thick.

The foregoing and other objects and features of the present invention will become more apparent when taken in conjunction with the accompanying specification and drawings in which FIG. 1 is a perspective view of a single crystal epitaxial film of spinel ferrites disposed on the surface of a single crystal substrate.

FIG. 2 is a diagrammatic drawing showing a magnetooptical system utilizing a single crystal epitaxial film of a spinel ferrite.

Referring now to FIG. 1, there is shown a perspective view of a substrate 1 having an epitaxial single crystal film 2 of a spinel ferrite disposed on a surface thereof. Substrate 1 has the characteristic that it is transparent to the wave lengths of light usually used in conjunction with magneto-optical devices. Substrate 1 also has the characteristic that it is a single crystal material which, in effect, forms the pattern for subsequently applied layers of material. By providing the single crystal substrate, subsequently formed films will be epitaxial i.e., they will conform to the single crystal pattern of the substrate. In addition to the above mentioned characteristics, the substrate should be of such a nature that it is noncontaminating to the finally formed epitaxial film. One substrate which fulfills all the above criteria is magnesium oxide. Crystals of aluminum oxide and nickel oxide while not as satisfactory as crystals of magnesium oxide, have been successfully used.

In forming film 2, a slurry is first formed by mixing equal parts by weight of a flux such as sodium or lithium carbonate together with a spinel ferrite such as lithium, lithium chromium, nickel zinc, nickel magnesium or cobalt ferrite in isopropyl alcohol. The mixture is then ground in a mortar until homogenous and air dried until a powdered mixture forms. Alcohol is then added to the mixture which is stirred to form a slurry having the consistency of light cream.

Prior to the actual preparation of the film, the surface area of the best large face of the magnesium oxide substrate is measured, rinsed with alcohol, dried and weighed. To form the epitaxial film, the substrate is placed on a fiat surface with the best face upward. The slurry of ferrite and flux is applied to the substrate crystal face from a medicine dropper, three or four drops at a time and then air dried for at least 15 minutes after which the substrate is placed on a hot plate for 5 to 10 minutes. A thin homogeneous coating of ferrite and flux covers a face of the substrate using the above steps. The ferrite and coating are then weighted to determine if sufficient ferrite has been applied to the surface to give a film of the desired thickness. If thicker coatings are desired, the application and heat treating of the slurry should be repeated until a coating of desired weight is obtained.

With a coating of desired Weight on the substrate, the substrate is placed in a platinum boat and introduced into a furnace which has been pre-heated to approximately 1200 C. The coated substrate is then heated in air or oxygen for approximately 20 hours.

After firing for the desired time, the boat is withdrawn and the substrate face is coated With an epitaxial single crystal of a spinel ferrite.

It should be appreciated that the constituents making up any of the ferrites used may be obtained separately and fired to provide the desired ferrite rather than starting with the reacted ferrites as has been suggested above.

The mechanism which produces the resulting epitaxial film is one of volatilized flux growth. When the substrate and coating are introduced into the furnace, the flux first becomes molten at a temperature of about 700 C. As the substrate heats up to the furnace temperature, the ferrite dissolves in the flux forming a flux-ferrite solution which wets the surface of the substrate to form a liquid layer of uniform thickness. In the molten state, a convective stirring occurs within the material such that the flux, which must be less dense than the ferrite, is displaced to the topmost portion of the molten material. The flux then gradually volatilizes leaving behind a supersaturated solution of ferrite in the molten flux. As the flux volatilizes further, the super-saturated solution sinks to the bottom of the molten material and deposits ferrite on the face of the substrate which conforms to the crystalline structure of the substrate and results in an epitaxial film of spinel ferrite having a thickness of to 50 microns depending on the initial Weight of ferrite per unit area of substrate.

The following are specific examples of the method of preparing epitaxial thin films of spinel ferrites using different ferrites and fluxes.

EXAMPLE 1 A substrate of magnesium oxide which is transparent to desired light wavelengths is provided. The substrate has a face which is oriented in one of the major crystallographic planes i.e. (111) (100). A (100) plane can be obtained by cleaving the crystal. The substrate is prepared by rinsing with isopropyl alcohol and drying. A mixture containing equal parts by Weight of anhydrous sodium carbonate and a lithium chromium ferrite having the formula 0.5 2.5-x x 4 where:

is mixed in alcohol by grinding in a mortar to form an homogenous slurry. Typical weights of flux and ferrite to form the mixture are 0.2 gm. of each. The slurry is air dried to form a powder and re-mixed with sufficient alcohol to form a slurry having the consistency of light cream. Droplets of the slurry are then applied to the surface of the substrate and air dried for minutes. Further drying is accomplished by heating on a hot plate for 5 to 10 minutes. The slurry may be advantageously applied by spraying, painting or dipping. Depending on the ultimate film thickness desired, the step of applying and drying the slurry may be repeated several times.

The weight of ferrite and flux mixture which must be applied to the substrate to obtain any desired thickness maybe calculated knowing the density of the ferrite, the surface area of the substrate and the Weight percentage of ferrite in the mixture. If a (20 10 cm.) thick film is required on a substrate 1 cm.- in area of a ferrite with a density of 6 gm./cm.- then .0120 gm. of ferrite are required which would be obtained by applying .024 gm. of a 50 wt. percent ferrite-flux mixture.

The weight ratio of flux to ferrite is not critical and may vary considerably without affecting the utility of the resulting ferrite film. The ratio of fiux to ferrite may be varied over the range of 1 part of flux to 3 parts of 4 ferrite by weight to 9 parts of flux to 1 part of ferrite by weight.

The substrate is then placed in a fiat-bottomed shallow platnum boat and introduced into a furnace pre-heated to a preferred temperature of 1200 C. Firing temperatures in the range of l050 C. to 1300 C. have been used successively without affecting the quality of the resulting epitaxial films. The substrate is heated at the firing temperature (1200 C.) for 20 hours in air or oxygen. Periods of 1 hour to 48 hours have also been used to successively produced epitaxial ferrite films. The thicker the resulting film required, the longer the firing time necessary to volatilize the flux. After heating, the substrate is cooled to room temperature and used with the substrate attached (MgO is transparent) or the substrate may be removed by grinding or cleaving. The resulting ferrite film is from 5 to 50 microns in thickness depending on the weight of ferrite per unit area of substrate of the initially applied coatings.

EXAMPLE II The steps used are the same as described in connection with Example I With the exception that anhydrous lithium carbonate is used as a flux instead of anhydrous sodium carbonate.

EXAMPLE III The steps used are the same as in Example I with the exception that the ferrite used has the formula:

The steps used are the same as in Example I with the exception that the ferrite used is:

NiFe O EXAMPLE VI The steps used' are the same as in Example II with the exception that the ferrite used is:

NiF'e O.

EXAMPLE VII The steps used are the same as in Example I with the exception that the ferrite used is:

Ni Zn Fe O Where:

EXAMPLE VIII The steps used are the same as in Example II with the exception that the ferrite used is:

Ni ZII F62O4 where:

EXAMPLE IX The steps used are the same as used in Example I with the exception that the ferrite used is:

EXAMPLE X The steps used are the same as used in Example II with the exception that the ferrite used is:

EXAMPLE XI The steps used are the same as used in Example I with the exception that the ferrite used is:

EXAMPLE XII The steps used are the same as used in Example II with the exception that the ferrite used is:

CoFe O In the magneto-optical device shown diagrammatically in FIG. 2 of the drawing, a single crystal film of a spinel ferrite 2 on a substrate 1 is mounted between spaced cross polarizing filters (i.e., polarizer 3 and analyzer 4). The crystal is placed in a magnetic field (e.g., that produced by an electromagnet 5 or by Helmoholtz coils). A light source 6 and a photocell 7 are placed such that light to which the cell 7 is exposed is that which originated at light source 6 and passes successively through polarizer 3, single crystal film 2, substrate 1 and analyzer 4. Since the degree of rotation of the plane of polarized light passing through the single crystal film of spinel ferrite is dependent upon the magnetic field, the amount of rotation of light originating in the light source and passing through the polarizer, analyzer and the single crystal film to the photocell can be varied by varying the strength of the magnetic field of the magnet. In the ferromagnetic region at magnet saturation, the rotation is independent of the applied magnetic field and the maximum rotation can be obtained.

In FIG. 2, a crystal of Li Fe O the manufacture of which has been described in connection with Examples III and IV above, five microns in thickness transmits 34% of the incident light at a wavelength of 0.8 microns. The magneto-optical rotation at saturation for this material is 1000 of rotation per centimeter of thickness. Thus, a film five microns thick rotates the plane of polarization O.5. Spinel ferrite films such as described in the other examples provide similar rotations under the same conditions.

When spinel ferrite, single crystal films of suflicient thickness are grown, the supporting substrate can be removed by either cleaving or grinding it away. While the films of the present invention have been described in conjunction with devices Which utilize Faraday rotation, it should be appreciated that other uses for such films can be made. For instance, a plurality of films laminated together could be utilized to form a magnetic recording head which utilizes only the magnetic properties of the ferromagnetic films.

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

What is claimed is:

1. A method of making single crystal ferrite films comprising the step of epitaxially depositing from a solution of a polymetallic ferrite and a molten salt solvent a poly metallic ferrite having a spinel structure on a single crystal substrate by heating said solution at a temperature sufficient to volatilize said molten salt solvent.

2. A method of making single crystal ferrite films according to claim 1 wherein the polymetallic ferrite includes at least one ferrite selected from the group consisting of:

where:

NIFC2O4 3. A method of making single crystal ferrite films according to claim 1 wherein said molten salt solvent is one selected from the group consisting of anhydrous sodium and lithium carbonate.

4. A method of making single crystal ferrite films according to claim 1 wherein said substrate is a single crystal of magnesium oxide.

5. A method of making single crystal ferrite films according to claim 1 wherein said temperature sufficient to volatilize said molten salt solvent includes the temperature range of 1050" C. to 1300 C.

6. A method of making single crystal ferrite films according to claim 1 wherein said temperature sufiicient to volatilize said molten salt solvent is 1200 C.

7. A method of making single crystal ferrites comprising heating a mixture of a ferrite selected from the group consisting of:

and a flux selected from the group consisting of anhydrous sodium and lithium carbonate on a face of a single crystal of magnesium oxide for a time and temperature sufficient to dissolve said ferrite in said flux and completely volatilize said flux such that a residue of epitaxially deposited film remains on said crystal face.

8. A method of making single crystal ferrites according to claim 7 wherein said time includes the range of 1 hr. to 48 hrs.

9. A method of making single crystal ferrites according to claim 7 wherein said time is 20 hrs.

10. A method according to claim 7 wherein said temperature includes the temperature range of 1050 C. to 1300 C.

11. A method according to claim 7 wherein said temperature is 1200 C.

12. A method of making single crystal ferrites comprising the steps of:

preparing a slurry consisting of a homogeneous mixture of ferrites selected from the group consisting of:

where and a flux selected from the group consisting of anhydrous sodium and lithium carbonates and isopropyl alcohol;

providing a single crystal substrate of magnesium oxide, applying at least one coating of said slurry to a crystal face of said substrate, firing said coated crystal in a furnace in an oxidizing atmosphere in a temperature range of 1050 C. to 1300 C. for 1 to 48 hours to dissolve said ferrite in said flux and to volatilize said flux such that a where 7 8 ferrite film 5 to 50 microns thick is epitaxially de- 3,332,796 7/1967 Kooy 252--62.56 posited on said substrate crystal face. 3,404,026 10/1968 Skudera et a1 117169 13. A method accordlng to clalm 12 wherein the step OTHER REFERENCES of preparing a slurry includes the step of mixing flux and ferrite over a range of weight ratios of one part flux to Cech et Pfepafatloll of and C00 y three parts of ferrite by weight to nine parts flux to one tals by Halide Decomposition, TTaTlS- Metals, part of ferrite by weight. vol. 51, pp. 150 to 161.

14. A method according to claim 12 wherein the step of preparing a slurry includes the step of mixing equal WILLIAM MARTIN Pnmary Exammer parts by weight of said ferrite and said flux. 10 B PIANALTO, Assistant Examiner References Cited U.S. C1. X.R.

UNITED STATES PATENTS 117-236; 25262.56, 62.61, 62.64

3,150,925 9/1964 Gambino 252--62.56

3,305,301 2/1967 Remeika 25262.61 15