Liquid crystal nondestructive inspection of magnetization and variations in magnetization of high energy magnets

A method for coupling liquid crystals and a magnetic field to monitor changes in local magnetic properties (e.g., direction of magnetization) is provided. A magneto-optical sensor is used to detect and observe inhomogenieties, impurity phases, of a high energy magnet by monitoring the response of liquid crystals to a magnetic and/or electric field applied perpendicular or antiparallel to the magnetic field supplied by the magnet. The intensity of field necessary to cause a rotation of the liquid crystals in the direction of the applied field, and hence a modification of the optical properties of the liquid crystals, can be used as a rapid, inexpensive quality control test for batch magnet fabrication. The method can be applied to both thin films and bulk specimens. The magnetically weak spots of the sample can be mapped out by noting the appearance of dark and light regions as the liquid crystals are rotated into positions which obscure the illuminating light. Pleochroic dyes, which are long, cylindrically-shaped molecules containing chromophoric groups, can be used to increase the contrast and thus sensitivity of the method since the angle formed between the path of illuminating light and the axis of the pleochroic material determines the color reflected.

DESCRIPTION 
BACKGROUND OF THE INVENTION 
Conventional methods used to study fine details of magnetic materials 
include the Bitter Method (as first suggested by Bitter, F. in Phys. Rev. 
38, 1903 (1931) and Von Hamos, L. and Thiessen, P. A., Z. Physik 77, 442 
(1931)) and Lorentz microscopy (Cullity, B. D., "Introduction to Magnetic 
Materials"; Addison-Wesley Publishing Company, Reading, Mass., 1972, p. 
292). They can only be used to observe domain walls. Individual domains, 
which are magnetic regions in which the direction of magnetization is 
uniform, look similar using these methods. A major disadvantage of the 
Bitter method is the need of a lengthy specimen preparation, which 
involves mechanical polishing and electropolishing of the surface before 
application of a colloidal Fe.sub.3 O.sub.4 suspension (recipes for the 
preparation of a colloidal suspension of magnetite are given by Elmore, W. 
C., Phys. Rev. 54, 309 (1938) and Kittel, C. and Galt, J. K., Solid State 
Phys. 3, 439 (1956)). A limitation of Lorentz microscopy is the 
requirement that the sample must be very thin, approximately 1000 .ANG. or 
less, to transmit electrons. Additionally, an electron microscope equipped 
for Lorentz microscopy represents a very substantial capital investment. 
The Kerr and Faraday Effects are common techniques used in the study of 
magnetic materials (Morrish, A. H., "The Physical Principles of 
Magnetism," Robert E. Krieger Publishing Company, Malabar, Fla., 1983, p. 
374). They can distinguish one domain from another but supply no 
information about the domain walls. The Kerr Effect involves the rotation 
of the plane of polarization of a light beam during reflection from a 
magnetized sample. Since the amount of rotation is much less than one 
degree, the method is not easy to apply. In the Faraday Effect, the plane 
of polarization of a light beam is rotated as it is transmitted through 
the magnetized sample. This technique is therefore limited to very thin 
samples which can transmit light. In many materials including those in 
this disclosure, the Faraday Effect is not applicable because the sample 
does not transmit light. In both of these techniques (i.e., Kerr and 
Faraday Effects), the magnet interacts directly with the light beam to 
generate an observable effect. 
While the above techniques are useful, none is a fast, inexpensive 
technique suitable for studying essential details of the process of 
magnetization reversal, i.e., change in direction of magnetization of 
domains, which is essential in characterizing magnetic materials in both 
bulk and thin film form. 
Coercivity is a very important magnetic property of a permanent magnet. It 
describes the magnet's resistance to magnetization reversal, i.e., the 
change in direction of magnetization by 180.degree.. Coercivity is often a 
significant factor in determining the commercial value of a permanent 
magnet. In many cases coercivity of the material of which the magnet is 
constructed is controlled by impurities which are magnetically soft and 
not desired. Magnetization reversal occurs preferentially at these soft 
spots and subsequently may spread throughout the entire material. Thus the 
existence of these impurity soft spots is linked to the utility of the 
magnet. A rapid, inexpensive, non-destructive method for observing these 
soft spots would be very desirable. No existing techniques have all these 
attributes. 
It is known that a liquid crystal (or a liquid crystal/pleochroic dye 
combination) can be oriented by a magnetic field. Priestley, E. B.; 
Wojtowicz, P. J.; Sheng, P. in "Introduction to Liquid Crystals"; Plenum 
Press, N.J., 1975, p. 115 report that about 1 oe of magnetic field is the 
equivalent of about 1 volt/cm of electric field in orienting liquid 
crystals. Further, it is known that orientation of liquid crystals can be 
detected optically. This has been shown by many people such as: Creagh, L. 
T.; Kmetz, A. R.; Reynolds, R. A., "Performance Characteristics of Nematic 
Liquid Crystal Display Devices," IEEE Trans. Electron Devices, p. 672 
(1971) and Heilmeier, G. H.; Zanoni Applied Physics Letters, Vol. 13, No. 
3, 1968, p. 91. 
Permanent magnets are usually fabricated by batch processes. The yield is 
less than 100%, i.e., not all batches lead to a successful product. A 
rapid, inexpensive technique to assay a sample magnet from a batch would 
enable one during processing to make compositional adjustments, modify 
processing parameters, etc., and thus increase yield. The present 
invention is a method for rapid, inexpensive assaying of permanent magnet 
materials. 
A method is provided to use (liquid crystals or liquid crystals/pleochroic 
dyes) and polarized light to detect and quantify the soft spots in the 
magnetic material as it is processed. To applicant's knowledge, there are 
no existing methods which use the interaction between polarized light and 
(liquid crystals or liquid crystals/pleochroic dyes) to study magnetic 
properties, i.e., coercivity, saturation magnetization and magnetization 
reversal of magnets. 
BRIEF DESCRIPTION OF THE INVENTION 
The invention is directed to a method for detecting magnetically weak or 
soft spots in a high energy magnet. More particularly, the method 
comprises subjecting a magnet plus a liquid crystal array to an electric 
and/or magnetic field applied either perpendicular or antiparallel to the 
field supplied by said magnet. The strong region of the magnet is observed 
to orient liquid crystals, said orientation being detected optically. When 
an opposing or antiparallel magnetic and/or electric field is applied, 
magnetization of the sample changes by 180.degree. at the soft spots but 
generally not in the hard bulk material because of its strong magnetic 
field. This 180.degree. change in magnetization is detected optically. If 
the external fields are applied perpendicular to the field of the magnet, 
magnetization rotates at the soft spots by 90.degree.. Either 
reorientation of the liquid crystals gives a detectable optical effect 
antiparallel means running parallel but in a contrary direction.

DETAILED DESCRIPTION OF THE INVENTION 
Liquid crystals are comprised of molecules containing multiple double bonds 
along their long axis, one or more benzene rings, and polar or long chain 
terminal groups. The dimensions of the liquid crystals can vary depending 
on side groups attached to the major axis of the molecule. Liquid crystals 
are diamagnetic and prefer to align with the plane of their benzene rings 
parallel to a magnetic field. (Dorfman, Y. G. "Diamagnetism and Chemical 
Bond"; American Elsevier Publishing Company Inc.: New York, 1965, p. 12). 
This occurs because this is a lower energy state. 
Typical liquid crystals which can be employed include: 
______________________________________ 
Type Name (Common Abbreviation) 
______________________________________ 
Smectic Cyanobenzylidene octyl oxyanilin (CBOOA) 
Terephthal-bis(-p-butylaniline) (TBBA) 
p-decycloxy benzylidene 
p'amino 2-methylbutyl cinnamate 
Nematic p-azoxyanisole (PAA) 
N--(p-methoxybenzylidene)-p-butylaniline (MBBA) 
4-ethoxybenzylidene-4'cyano aniline (PEBAB) 
p-n-butoxy benzoic acid 
phenyl-cyclohexane type 
biphenyl type 
Schiff's base type 
Cholesteric 
Cholesterol 
Cholesterol acetate 
Poly(.gamma.-benzyl glutamate) 
Cholesteryl oleate 
Cholesteryl oleyl carbonate 
______________________________________ 
Pleochroic dyes, which are long, cylindrically shaped molecules containing 
chromophoric groups, can be added to the liquid crystals. They will move 
in conjunction with the liquid crystals. An important property of these 
dyes is they reflect light differently depending on the angle of the 
radiating light in respect to their major axes. If the electric vector of 
a polarized light source is parallel to the long axis of the molecule, the 
color characteristic of the dye is seen. When the long axes of these dyes 
are perpendicular to the electric vector of the incident light a resulting 
decrease in the optical density occurs which is accompanied by the 
disappearance of color. Thus the direction of alignment of the liquid 
crystal/pleochroic dye combination can be detected optically through the 
observation of a color change. This alignment can be modified 
magnetically. Hence the liquid crystal/pleochroic dye combination can be 
used to sense direction of magnetization and magnetization reversal. 
Typical pleochroic dyes which can be employed include: Indophenol Blue, 
N-N'-Dipalmitoylindigo, Isolar Green M, 4-Nitrobenzylidenephenylhydrazone, 
N-N'-Dimethylindigo and methyl red. 
Typical magnets include SmCO.sub.5, PrCO.sub.5, Nd.sub.2 Fe.sub.14 B and 
Pr.sub.2 Co.sub.17. 
The basic components of this method include: a liquid crystal sandwich type 
cell, sample magnet, source of external magnetic and/or electric field, 
and an optical microscope. The liquid crystal sandwich type cell is 
constructed in a conventional way. It consists (see FIG. 1) of two glass 
sides with polarizers which are separated by non-conductive spacers made 
of Teflon, Mylar, glass frits, or the like. The liquid crystals or liquid 
crystal/pleochroic dye medium will fill the volume created by the glass 
slides, spacers and appropriate sealing material. The polarizers are not 
shown because the number and positioning of them depends on the type of 
liquid crystals being used. In the case of twisted nematic liquid 
crystals, one polarizer is placed on the top glass slide while a second 
polarizer is cross-aligned at 90.degree. on the bottom glass plate. In the 
case of the liquid crystal/pleochroic dye combination only one polarizer 
is required. It is positioned on top of the top glass slide. Typical 
sealing materials are solder glasses, polymeric materials or glass frits. 
A reflective surface such as a mirror will be placed below the bottom 
glass slide. The liquid crystals will be uniformly oriented by aligning 
solutions such as tertiary amines (Petrie, S. E.; Bucher, H. K.; 
Klingbiel, R. T. and Rose, P. I., Organic Chemical Bulletin 45, No. 2 
(1973), Eastman Kodak Co., Rochester, N.Y.), 
hexadecyltrimethylammoniumbromide (Haller, I.; Huggins, H. A., U.S. Pat. 
No. 3,656,834, Apr. 18, 1972), and silane coupling agents (Kahn, F. J., 
App. Phys. Lett. 22, p. 386 (1973)) or general aligning procedure such as 
described in Zocher, H., and Coper, K., Z. Phys. Chem., 132, p. 195 (1928) 
before the sample is tested. 
The liquid crystal sandwich cell can be placed on or near the test magnet. 
The distance will be optimized using spacers as shown in FIG. 2. Some of 
the liquid crystals will be reoriented by the sample's magnetic field. The 
sandwich cell can be observed with an optical microscope as an external 
field applied perpendicular or antiparallel to the field of the sample 
(FIG. 3) is gradually increased. Soft spots will be detected by the 
alteration in color of the pleochroic dyes. The rate of magnetization 
reversal with increasing external field will readily be observed by 
observing with the microscope the changes in optical properties of the 
sandwich cell. 
An example of a suitable liquid crystal/pleochroic dye pair is 
p-azoxyanisole and methyl red. Methyl red displays a gradual color change 
from red to colorless with various degrees of color from orange to yellow 
as the electric vector of the polarized light source changes from a 
parallel to a perpendicular position in relation to its long axis. One can 
therefore see the magnetization reversal take place through the same 
degrees of movement. 
The above procedure can also be done using liquid crystals without 
pleochroic dyes. In that case, black, white and gray regions will be 
visible depending on the position of the liquid crystals in the sandwich 
cell. Changes in the color of the regions with the application of an 
external electric and/or magnetic field is the important festive, enabling 
magnetic inhomogeneities to be detected. 
FIG. 4 is a side view of a twisted nematic liquid crystal display as taught 
by Bylander, E. G. "Electronic Displays--Texas Instruments Electronica 
Series"; McGraw-Hill: New York, 1979, p. 138. In this type of device, the 
mesophase material is preferentially aligned on the top plate and then 
cross-aligned at 90.degree. with respect to the bottom plate. The 
polarized light will be rotated 90 degrees in passing normally through the 
cell. Application of a field destroys the rotation by tipping the 
molecules. 
It is important to align the liquid crystals 90.degree. from one another at 
each plate so that the effect of the magnetic field may be detected. When 
light passes through the first polarizer, it becomes plane polarized light 
as shown by the top arrow in FIG. 4. If the twisted nematic is 
undisturbed, it will rotate the light by 90.degree. so it can pass through 
the second polarizer. The light will then impinge on a mirror which will 
reflect it back through the polarizers and a bright spot will appear on 
the display. If the liquid crystals lose their ability to rotate the plane 
of the light, the light passing through the first polarizer will be 
blocked by the second polarizer. These regions will appear dark on the 
display. A strong magnetic field can unwind the twisted nematic to form 
the rod shape pure nematic. They will align parallel with the field 
without rotating the light. 
The weight ratio of the dye to liquid crystal will depend on the materials 
employed but generally will be between about 0.001 and about 1.0. 
The nematic type liquid crystals are preferred because they are generally 
more chemically and thermally stable. Their length/width ratio will 
generally be between about 10 and about 10,000. 
The field applied will depend upon the materials employed but generally 
will be between about 500 and 1300 Oe for a magnetic field and between 
about 0.5 and 10.0 V/cm for an electric field. 
While standard temperature and pressure conditions will be generally 
sufficient, higher temperatures up to about 200.degree. C. may be used to 
induce faster response of the liquid crystals.