US3182195A - Method of and device for magnetizing a specimen in an electron microscope - Google Patents

Description

May -4, 1965 l IN AN ELECTRON MICROSCOPE Filed April 25, 1962 l4 Sheefcs-Sheet l E. FucHs ETAI. 3,182,195 METHOD OF AND DEVICE FOR MAGNETIZING A SPECIMEN 4 Sheets-Sheet 2 3,182,195 ING A sPEcIMEN OPE ETIZ CROSC CHS ETAL FOR MAGN E. FU DEVICE AN ELECTRON MI AND IN ETHOD OF May 4, 1965 Filed April 25, 1962 FigA 135g "e" al May 4, 1965 E. FucHs ETAL 3,182,195 METHOD OF AND DEVICE FOR MAGNETIZING A SPECIMEN IN AN ELECTRON MICROSCOPE FiledApril 25, 1962 4 Sheets-Sheet 3 M a@ ZM/.

May 4,1965 E. FucHs ETAL 3,182,195

METHOD OF AND DEVICE FOR MAGNETIZING A SRECIMEN 1N AN ELECTRON MICROSCOPE Filed April 23, 1962 4 Sheets-Sheet 4 Fig.9

" J x Jy o i i V: i o o,s 1,0 n

United States Patent O 11 Claims. (Ci. 25o-49.5)

The invention disclosed herein is concerned with a method of and a device for magnetizing a specimen in an electron microscope.

The method of defocused imaging is successfully applied in the investigation of magnetic structures in thin magnetizable layers, such method making it possible to obtain information concerning submicroscopic details, which cannot be obtained with other known methods. The papers by Boersch, H., H. Raith and D. Wohlleben, Z. Phys. 159, 388 (1960) and by Fuller, H. W., and M. E. Hale, I. Appl. Phys. 31, 238 (1960) may be consulted on the subject.

The various objects and features of the invention will appear in the course of the description thereof which is rendered below with reference to the accompanying drawings.

FIG. 1 shows the defiection of an image from the viewing plane due to the action of a magnetic field;

FIG. 2 illustrates the principle applied according to the invention;

FIG. 3 shows the arrangement of parts of a device according to the invention;

FIG. 4 indicates details with respect to the dimensions of two E-shaped cores;

FIG. 5 represents the arrangement of the corrective;

FIG. 6 illustrates the manner in which the electron beam is guided through the device shown in FIG. 5;

FIG. 7 shows separately a part of the course of the electron beam;

FIG. 8 is intended to aid in explaining the compensation of scattering fields; and

FIG. 9 is a graph obtained `by plotting the values of certain equations.

Upon providing, in an investigation of magnetic structures by the use of the method of defocused imaging, with the use of an optical corpuscular radiation microscope, an arrangement for magnetizing the specimen under observation, including coil means disposed in the specimen plane, the image will, with the operatively connected magnetic field, migrate out of the viewing plane, as indicated in FIG. 1 in dash lines, such migration being due to the aperture error of the electron lenses (ct=amgn -l0*2). Y

The electron optical image is indicated in FIG. l by E, and the migrated or displaced image as well as the beam course are indicated in dash lines. 'I'he beam course and image shown in full lines are obtained only in the absence of the magnetic field B, standing perpendicularly to the plane of the drawing and provided for magnetizing the specimen P. It will be seen that the two beam courses converge underneath the lens or objective O at different points F0 and FB only the point Fo lying in the absence of the magnetic field B upon the optical axis s.

This defiection from the optical axis of the arrangement can be nullified by tipping the entire irradiation system by a given angle, whereby the beam is again admitted to the lens centrally thereof. However, this procedure is cumbersome and time consuming since each magnetic field has a'given defiectionangle a. It is practically impossible to utilize the arrangement according to FIG. 1, in case the alteration of the magnetic structure is to be held or retained, for example, with the aid of a film 3,182,195 Patented May 4, 1965 camera, as function of the magnitude and direction of the magnetic field.

The invention is concerned with a method of and a device for the electron microscopic representation of magnetizable objects or specimens, employing for the magnetization of a specimen a magnetic field which is preferably adjustable as to magnitude and/or direction, and which is oriented transverse to the electron beam. The invention is particularly intended for the defocused imaging or representation of subrnicroscopic details of magnetic structures.

In order to avoid the above indicated difficulties, the electron beam is, according to the invention, caused to enter into the magnetic field serving for the magnetization of the specimen, in a direction deviating from the optical axis, and such beam is by the action of this magnetic field guided in the direction of the optical axis, thereby being admitted into the objective substantially in parallel with the axis.

The direction of the deflection must be so selected that the electron beam is always admitted centrally of the objective, that is, in the direction of the optical axis. The deiiection angle az must always be equal to the oppositely oriented angle aB of the deflection which is caused by the magnetic field.

It will be seen from FIG. 2, showing the principle of the device according to the invention, that the electron beam does not fall perpendicularlyupon the specimen, as in FIG. l, but under an angle az. However, the beam is admitted to the lens always perpendicularly, that is, the imaging errors are always held at a minimum. The device must be constructed so as to always fulfill the requirement lZ]=|aB]. The direction must follow the course indicated in FIG. 2. This means, however, that the terminal image E remains at the same place for any magnetic field strength and magnetic field direction.

The device employed for this purpose can operate with mechanical electrical and/or magnetic means. In a preferred embodiment of the invention, the desired defiection is effected with the aid of a magnetic field.

FIG. 3 shows the arrangement of parts of a corresponding device in the principal features thereof. There are provided three magnetic fields of identical strength and different directions, arranged one above the other and equally spaced by distances a. The extent of the respective magnetic fields is in the direction of the optical axis different. vThe magnetic field II has the extent 2l and is oriented anti-parallel to the fields I and III. The magnetic fields I and III have the extent l. The electron beam is in the field I deflected about the angle az and enters at such angle into the field II. Since this field II is of the same magnitude but oriented in a direction displaced by and twice the length of the field I, the electron beam is displaced in the direction yof the optical axis by 2,2. Accordingly, the beam is admitted to the objective at the angle az. The strength of the magnetic fields is regulated by a current common thereto, so that the requirement a3: az is always met.

The magnetic elds are arranged so that they can be rotated with respect to the specimen, thus permitting continuous variation of the strength as well as of the direction of the fields and therewith investigation of the infiuence ofthe magnetic field on the specimen.

The remaining figures show a preferred embodiment of the invention.

FIG. 4 shows two E-shaped cores, preferably made of Mu Metal, having the indicated dimensions. These cores are energized by a pair of coils disposed upon the central arms of the cores. The cross-sectional areas Q of the magnet are selected so that nach;

and the magnetic induction B of the magnetic fields I, Il and III will therefore be correspondingly similar:

(2) lBrl=lBrrl=lBrrrl Upon changing the current fiowing in the coil, the Equation Z will remain unaltered, that is, the condition shown in FIG. 3 is satisfied for each B.

The Equation 2 lBI|=lBHl=|Bml is valid only when the flux 11 is not partially lost due to a magnetic shunt.

The correction magnet (corrective) is often disposed with its lower part, facing the objective, near the pole pieces of such magnetic objective, resulting in a weakening of the field Bm at the object place, and there would consequently apply:

However, in accordance with another feature of the invention, this influence may be mechanically equalized with the aid of suitable magnetic shunts and/ or electrically compensated with the aid of further pairs of coils at the field I and III, until the Equation 2 is again satisfied. In the latter case (compensation by means of further coils), one further coil will not suffice at the field III, since the fields I and II are likewise influenced. The compensation with the aid of further coils is to be preferred since it entails a considerably reduced mechanical expenditure. As will be presently shown, the further coil for the field I must be polarized in field-strengthening sense when it is desired to compensate the above mentioned weakening Of the field BH1.

FIG. 5 shows the arrangement of the corrective. The pair of E-shaped magnets 3a and 3b, their coil pairs '7a to 7], are ixedly disposed within an insert 1. Spacing members (not shown) are provided for holding the pair of magnets laterally in exactly defined position. The insert 1 is rotatably journalled within a cross table 8. The insert 1 and therewith the magnet field can be rotated by means of the drive 9 with respect to the specimen cartridge 13, that is, with reference to the specimen which is disposed centrally of the lower pole pieces of the magnet. In addition, the cross table 8 together with the insert 1 can be moved in a manner such as is customary with arrangements which are not provided with a magnetic insert.

Instead of a pair of E-shaped magnets, which produce the two initial deflections and also the magnetic field in the specimen plane, there may be used a pair of U-shaped magnets, the poles of which merely produce the two fields for the initial deiiections of the electron beam. However, such an arrangement results in a more difiicult mechanical construction than the one obtained with the use of E- shaped magnets, the field strength requirements being easier met with the latter structure, once the compensation of these fields is in the described manner achieved by the further or auxiliary windings disposed upon the outer legs of the E-shaped magnets.

The field directions or orientations which influence the electron beam must in any case act in the same direction and the deflection must be centrally oppositely directed. For this purpose, the pairs of coils 7c and 7d, shown in FIG. 5, form the main winding which determines these associations of the field directions, while the pairs of coils 7a, 7b and 7e, 7i form the auxiliary correction coils for the two outer fields of the E-shaped magnet pair.

The manner in which the electron beam is guided through the device of the invention, will now be explained more in detail with reference to FIG. 6.

The electron beam enters at A1 into the magnetic field I which is of the magnitude BI. The curvature radius amounts to wherein e is the charge and m the mass of the electrons, and VI the electron velocity in the field BI.

For 40-kv. electrons,

viokv.: l 185.1077113.l

=1.75s.1011 as iig-1 nl cos a1=0.9982.

Since lr=lrn=lg=l=4 mmthe deflection angle in the various fields The spacing from the optical axis at A2 amounts t0 x=(lcos az)=0.122 mm.

The electron beam enters at B1 into the field II. Along its path, the beam is additionally deflected from the axis by the amount of Y. There applies wherein tg is the rotation function tangent.

FIG. 7 shows on a larger scale the course of the electron beam in the field II. From this figure may be 0btained wherein ctg is an abbreviation for the rotation function cotangent.

Entering numerical values, there will be obtained z=0.5 mm.

The total deflection amounts to x-l-y|z=l.04 mm.

The defiection angles are identical since the field arrangement lies symmetrically about the center of the field II, and the beam enters the optical axis again at C2. The total deflection of about 1 mm. is the maximum possible deflection since the calculations involved the lowest acceleration voltage of UB=40 kv. and the largest magnetic field of B== gauss.

It has been mentioned above that the scattering fields at the eld III can be equalized with the aid of an auxiliary coil pair at the field I and at `the field III. This will bc shown with the aid of the following calculation, referring to FIG. 8. Y

The calculation involves the magnetic flux In The requirement that the resultant magnetic field strength shall be BI=BH=BIH leads to the resultant magnet iuxes With corresponding dimensions f The original magnetic tension SZ at the point of the field III will be at a maximum when the total ux, which proceeds from @y (at the point of the field II) is shunted, that is, when the portion based on @y at the field III is Zero. The total flux required at the field III must then be produced by QZ. There will apply:

@s maxFQZ! (e: w.J with w=turns, J :electric current) Rmagn being the magnetic resistance in the flux of the respective magnetic circuit.

Since all coils are traversed by the same current, the number of turns will be in the relation v wir wr 10111:?

follows from (2) and (4) as function of the scattering factor n. There applies The partial fluxes are for practical purposes better eX- pressed by partial currents.

In general l q 9 J'w :MM Rmagn Illing l whereby J are partial currents, w the numbers of turns and Rmargn the magnetic resistance. In the described arrangement:

Upon entering these values and forming the relations, there will be obtained l@ JZ. wrrr-oqrr1lrr=i l @y Jy-w1r-lweq11lur 2Jy There follows analogously-z 1I J x ry-2J, and from (5.1) and (5.2) follows: L: .3* Jy (3i-7l) is: 'LL Jy (3"170 The graph shown in FIG. 9 is obtained upon plotting the Equations 6.1 and 6.2. The values JZ JY and Jy which have the same n are always joined. The requirement is met for this case. The valve Sx must have a negative direction, as shown in the Equation 6.2 by the minus symbol. The relation cannot be satised for @x positive.

All scattering fields in the eld III can be corrected with three coil pairs so that there will always apply BII=BH=IBIII| Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

We claim:

1. A method of electron microscopically picturing magnetizable objects which are permeable by the electron beam, wherein the object is, for the magnetization thereof subjected to the influence of a magnetic eld which is oriented transverse to the direction of the electron beam, especially for the defocused picturing of sub-microscopic details of magnetic structure, comprising the steps of shifting the direction of the electron beam prior toits entry into the magnetic field, serving for the magnetization of the object, in a direction deviating from the optical axis, and utilizing said magnetic field to guide the beam in the direction of the optical axis.

2. A method according to claim l, comprising the additional steps of deilecting the electron beam from the optical axis by a lirst pre-deflection and then deilecting the beam back in opposite direction by a second predeection, thus causing the electron beam to enter into magnetic eld serving for the magnetization of the 0bject, in a direction deviating from the optical axis.

3. A method according to claim 2, wherein the beam is by the second pre-deiiection deflected back by an angle which is twice as great as the angle by which it was deected from the optical axis by the first pre-deection.

4. An arrangement for controlling action of the electron beam of an electron microscope during permeation of a magnetizable object while subjected to the inuence of a magnetic field oriented transverse to the direction of the electron beam, comprising a pair of magnets constructed to form at least two poles disposed adjacent such electron beam, at a point prior to the entry of said beam into said magnetic eld, to produce a magnetic field through which said beam passes, operative to effect a predeilection of said beam in a direction dev iatng from the optical axis prior to the entry of said beam into said first mentioned magnetic iield, whereby the latter ield is operable to deect the beam in the direction of the optical axis.

5. An arrangement according to claim 4, wherein the strength of the magnetic elds are identical.

6. An arrangement for controlling the action of the electron beam of an electron microscope during permeation of a magnetizable object while subjected to the inuence of a magnetic iield oriented transverse to the direction of the electron beam, comprising a pair of E- shaped magnet cores constructed to form three pairs of poles disposed adjacent such electron beam to produce a magnetic eld through which said beam passes, operative to effect a predeection of said beam in a direction deviating from the optical axis prior to the entry of said beam into said first mentioned magnetic eld, whereby the latter eld is operable to deflect the beam in the direction of the optical axis.

7. An arrangement according to claim 6, wherein the cross-sectional area of the field formed by the centrally disposed pair of poles is twice as great as the cross-sectional area of either one of the elds formed by each of the other pairs of poles of said magnets.

8. An arrangement according to claim 7, comprising a pair of coils, each of which is disposed upon one pole of the centrally disposed pair, and which determine the direction of the three fields, and further pairs of coils disposed upon the two outer pole pairs, serving for the compensation of ux scattering.

9. An arrangement according to claim 8, wherein the coils upon the poles of t-he magnet cores are serially connected to a regulatable current source.

10. An arrangement according to claim 9, wherein the windings of said further pairs of coils upon the outer poles, are identical, the number of turns of these windings being smaller than the number of turns of the respective windings disposed upon the central poles.

1l. An arrangement according to claim l0, wherein the magnetic ield 4serving for the first pre-deection is amplified by the magnetic ield produced by said further coil pairs.

References Cited by the Examiner UNITED STATES PATENTS 2,460,609 2/49 Torsch 313-77 2,581,487 1/52 Jenny 313-77 2,614,223 10/52 Ramberg Z50-49.5 2,824,987 2/58 Weissenberg et al. 250-49.5 2,926,254 2/60 Haine et al. Z50-49.5

RALPH G. NILSON, Primary Examiner.

Claims (1)

1. 4. AN ARRANGEMENT FOR CONTROLLING ACTION OF THE ELECTRON BEAM OF AN ELECTRON MICROSCOPE DURING PERMEATION OF A MAGNETIZABLE OBJECT WHILE SUBJECTED TO THE INFLUENCE OF A MAGNETIC FIELD ORIENTED TRANSVERSE TO THE DIRECTION OF THE ELECTRON BEAM, COMPRISING A PAIR OF MAGNETS CONSTRUCTED TO FORM AT LEAST TWO POLES DISPOSED ADJACENT SUCH ELECTRON BEAM, AT A POINT PRIOR TO THE ENTRY OF SAID BEAM INTO SAID MAGNETIC FIELD, TO PRODUCE A MAGNETIC FIELD THROUGH WHICH SAID BEAM PASSES, OPERATIVE TO EFFECT A PREDEFLECTION OF SAID BEAM PASSES, OPERATIVE DEVIATING FROM THE OPTICAL AXIS PRIOR TO THE ENTRY OF SAID BEAM INTO SAID FIRST MENTIONED MAGNETIC FIELD, WHEREBY THE LATTER FIELD IS OPERABLE TO DEFLECT THE BEAM IN THE DIRECTION OF THE OPTRICAL AXIS.

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