Domain patterns and reversals by wall movements of thin films of iron and nickel iron

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Domain patterns and reversals by wall movements of thin films of iron and nickel iron

C.E. Fuller

To cite this version:

C.E. Fuller. Domain patterns and reversals by wall movements of thin films of iron and nickel iron. J.

Phys. Radium, 1959, 20 (2-3), pp.310-318. �10.1051/jphysrad:01959002002-3031000�. �jpa-00236040�

DOMAIN PATTERNS AND REVERSALS BY WALL MOVEMENTS OF THIN FILMS OF IRON AND NICKEL IRON

By ^{C. E.} FULLER,

Mullard Research Laboratories, Salfords, Surrey, England.

Résumé.

L’inversion de la direction de l’aimantation par les déplacements ^des parois ^de Bloch, ^a été étudiée _pour deux couches minces (Fe ^et Ni-Fe) par la méthode colloïdale magnétique.

Pour chaque couche, l’inversion de l’aimantation a été observée dans les directions parallèles ^et

et normales au champ magnétique appliqué pendant l’évaporation. ^Pour la couche de Fe, l’inver- sion se produit ^de façon semblable dans ces deux directions. Dans le cas de la couche Ni-Fe, l’inver-

sion de l’aimantation dans la direction normale se fait surtout par rotation.

Abstract.

The reversal of the direction of magnetization by domain wall movements has been studied on two thin evaporated ferromagnetic ^films (Fe ^and Ni-Fe) by the Bitter colloid

technique. For each film the magnetization ^{reversal was} observed both parallel ^and perpendi-

cular to the direction in which a magnetic ^{field was} applied during evaporation. The behaviour of the iron film is similar in both directions but in the case of the nickel-iron film the reversal of

magnetization ^{in the} perpendicular direction takes place mainly by ^rotation.

20, 1959,

In recent years several papers have been publish-

ed showing experimental observations of domain

configurations ⁱⁿ evaporated polycrystalline ^thin

films. The paper of Fowler, Fryer ^{and Stevens} [1]

describes domain reversals in nickel-iron films observed by ^{the Kerr} effect, and that of Williams and Sherwood [2] gives ^a general survey of dema-

gnetised, ^and magnetisation reversal, patterns ⁱⁿ

thin . films of various alloys. ^These papers show that a wide variety of domain wall processes ^can

occur in thin films but little correlation has been established betweenthese processes and the control- lable factors affecting ^the deposition ^{of the} film, although it is well established that evaporation ⁱⁿ

a magnetic ^{field can} produce ^uniaxial anisotropy.

In the present work, ^{it has been} found that iron and nickel-iron films evaporated ^{under the same}

conditions exhibit uniaxial anisotropy ⁱⁿ varying degrees, although nickel-iron is less variable than iron. Domain patterns ^are shown for an iron film (400 ^A thick) ^{with exhibits little} anisotropy

and a nickel-iron (80 % nickel) ^film (900 A thick)

which has a well defined anisotropy. ^{The course} adopted, ^{was to} observe the domain reversal both in the direction in which the field was applied during evaporation (referred ^{to as "} parallel ^direc-

tion " in the text and marked E. D. ^on the figures)

and perpendicular* ^to this direction (transverse direction). ^{It was} hoped that the additional infor- mation gained would allow a better understanding

of the factors affecting the reversal _process.

The films were evaporated ^{under the same condi-}

tions onto thin glass substrates at 330 °C in a

magnetic ^{field of 120} oersted, ^{at a} pressure of 3 X 10-5 mm Hg. An aluminium mask was used to give ^{a circular film about 0. 6 cm diameter. The}

domain structures were observed by the Bitter colloid method.

The Iron Film.

PARALLEL MAGNETISATION REVERSAL. - Figures ^{1 to 4 show} stages ^{in the} magnetisation ^{reversal in the} parallel ^direction.

The first of the composite photographs (Fig. 1)

FIG. 1.

shows that island domains are nucleated at faults in the film well away from the edge ^and also, ^{that a}

narrow domain has run round part ^{of the circum-}

ference of the film but is actually isolated from the extreme edge. ^{The inner wall} of this latter domain is zig-zag ^{where it runs} roughly perpendi-

cular to the field direction, ^{and is} fairly straight

where its general ^{direction is in the field direction.}

The structure of this film seems to prevent ^the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3031000

311 nucleation of domains at the extreme edge ^and

this may be ’due either ^to thinning ^{of the film at}

the edge or, possibly, ^to contamination by ^the

aluminium mask used in the evaporation ^process.

The two narrow domains across the bottom of the

picture ^{take little} part ^{in the reversal} process.

The initial growth ^and development ^{of the} largest

island domain into a parallel ^{walled domain runn-} ing ^{into the} edge domain is shown in figure ^2.

FIG. 2.

The magnetisation ^reversal developes mainly

around this and the small island domains (Fig. 3

and 4). ^The growth ^{of these reversed} magneti-

sation domains takes place by ^{localized move-}

ments of sections of the walls ; ^{where the walls lie}

in thé field dir,ection, small sections of the wall make _very small sideways movements, ^différent sections of the wall moving ^{so as to} keep ^the wall,

as a whole, aligned in the field direction.. Large parts ^{of the} points of the island domains ^{move for-} ward as a whole, mostly by ^small amounts, ^but sometimes making larger ^{movements of which the} extreme example ^is the island domain becoming ^a parallel wall domain to the edge ^{of the film.}

Since the magnetisation ^{of the} domains is pro-

bably parallel ^{to the field direction the film would} give ^{a square} hysteresis loop ^{with a} coercive ^field

of about 35 oersted.

FIG. 3.

FiG. 4.

TRANSVERSE MAGNETISATION REVERSAL. - The

initial nucleation process (Fig. 5) ^is very similar

to the parallel ^{case ; as} before, ^a circumferential

domain is nucleated and small island domains grow

round faults ; ^the main island domain growing

round the same faults as in the parallel case,

although now, growth ^takes place ^{in the transverse}

direction in the film. The inset picture ^{shows a} magnified portion ^of the circumferential zig-zag

FIG. 5.

FIG. 6.

domain wall. _{A narrow,} parallel walled, ^domain

has grown ^out from the main wall in a direction

making ^an angle ^{of about 60° with} the field. This

type ^{of domain is} characteristic ^of magnetisation

processes in thin films where the magnetising ^field

makes a large angle with the direction of the field

applied during evaporation.

FIG. 7.

Figures ^{6 and 7 show two further} stages ^{in the} reversal process. The ^{actual movement of} the domain walls occurred in the ^same way ^as that described for the parallel ^reversal.

There is a marked similarity between the paral-

lel and transverse domain reversal patterns ; ^rever-

sal in the 450 direction also occurs at 35 oersted and

again ^the patterns ^{are similar.} It appears that the general outline of the domain patterns ^is

determined by ^{the field direction rather than} by

any fixed direction in the film.

The Nickel-Iron Flm.

PARALLEL MAGNE-

TISATION REVERSAL. - In preliminary experi-

ments ^some evidence of a remanent domain ^struc- ture was found and ^so the field was reduced from 100 oersted, through zero, ^to fields in the reverse

direction to show the complete reversal mechanism.

The first well defined domain structure (Fig. 8) ^is

formed before the field has been reduced to 4 oers-

ted, ^{that is while the field is still in} the direction of the overall magnetisation ^{of the} film. As the field is decreased to zero and is reversed the structure

undergoes ^distinct changes (Fig. ^{8 and} 9) to become

a reversed magnetisation spike ^{with the} type ^of

walls which Goodenough [3] ^has called " cross-tie walls ". The nucleation process ^{occurs at} the

edges ^{of the film which are} perpendicular, ^or

313

nearly ^{so, to the easy} direction and applied field,

and similar domains at the opposite edge ^{of the}

FIG. 8.

FIG. 9.

film are also shown ⁱⁿ figure 9. The behaviour of the nucleation domains is shown in more detail

later.

Figures ^{10 and 11 are} composite photographs ^of

two stages ^{in the} growth of these edge ^{domains to}

FIG: 10.

FIG. ^11.

complète saturation of the ^{film. The first of these}

shows a very simple ^domain configuration ^with

two* 1800 ^cross-tie walls which are held ^{on small} faults in the film. Between these faults the walls

are slightly ^{bowed out into} the unreversed ^do-

mains ; indicating ^that they ^{have been held while} travelling perpendicularly ^{to their} length. ^{At the} edges of the film in the central reversed domain there are several small, unreversed domains. The other photograph (Fig. 11) ^shows only ^one long wall, which is held for the greater part ^{of its} length by ^a straight ^{scratch on the} film, ^the wall is noti-

ceably ^{curved at} its ends and it seems that imper-

fections at the edges ^{hold the wall as} it travels

across the film. At the top ^{of the} photograph

small sections of the other wall remain between indentations in the edge of the film.

Although ^{all the} photographs ⁱⁿ figures ^{8 to 11}

were taken during ^{the same} reversal, ^{it has been}

found on other occasions that the domain configu-

ration went from the stage ^where small cross-tie

spikes have formed ^on opposite ^{sides of the} film,

to the state shown ⁱⁿ the second composite photo- graph. This latter state was quite repeatable, photographs ^taken during ^{different reversal} expe- riments show that the wall is held in almost exactly

the same position ^{even at the} edges of the film.

A further small increase of field is sufficient ^to release the wall and saturate the film with the

exception ^{of the} small domains at the edges ^{of the}

film perpendicular ^to the field. These domains

finally disappear when the field exceeds 10 oersted.

REMANENT MINOR HYSTERESIS Loop.

Figu-

res 12 to 15 show the formation of a reverse magne- tisation domain to the stage ^{where it becomes a}

cross-tie spike ^{and the} collapse of this domain when the field is reversed to oppose its magnetisation.

The process corresponds ^{to a minor} hysteresis loop

as shown in diagram ^1.

DIAGRAM 1.

Minor hysteresis loop.

In the nucleating stage, ^{above remanence} (AB).

the domain has clear walls and a complicated

structure. As the field is decreased and revers-

ed (BC) the semi-circular wall becomes straighter

to form part ^{of one of the two} diverging points ^at

the head of the growing ^domain (Fig.12, ^{.H = G.} 7).

At the edge ^{of the film is a} complex ^but recogni-

sable closure structure (the slight ^colloid deposit

on these walls is characteristic of small angle

walls (- 90°) ^{in this} film. This domain _grows

FIG. 12.

FIG. 13.

(Fig. 12) until the sudden transition to the ^cross- tie wall domain (Fig. 13, H ^{= -} 0.2).

Decreasing the field from this stage (CD) pro-

duces only ^small changes ^{in the head} of the domain

315

until 1.6 oersted ^is reached, ^{when a} sudden decrease in size occurs (Fig, 14) ; ^the divergent spikes ^{at the}

FIG. 14.

FIG. 15.

head have disappeared, together with ^the compli-

cated closure pattern ^{at the} edge ^{of the film - the}

domain ^{is now} uniformly ^marked by colloid along

its entire length. The domain shrinks ^as the field is increased and reverts to the clear-wall structure at about 3.7 oersted (Fig. 15, ^{77 = +} 3.7), ^and finallydisappears ^{at about 20} oersted. On decreas-

ing ^{the field a remanence structure} appeared ^at

about 3 oersted ^{at a} different place ^{on the} edge ^of

the film.

TRANSVERSE MAGNETISATION REVERSAL. -The

transverse behaviour is quite ^{different to that in} the parallel ^{case. It is thought that the main}

reversal takes place by ^{rotation of the} magneti-

sation in the plane of the film with the formation of a subsidiary ^domain structure at the edge ^{of the}

film. This structure is due to the demagnetising

effect of the film edge. ^{In order to} obtain a clear

picture ^{of the} reversal process the field was devia-

ted slightly ( 50) ^{out of} the transverse direction

(Diagram 2).

.

FIG. 16.

In this _case, as the field is decreased and revers-

ed to, ^{take the} magnetisation from saturation in

the + ^{H direction to} saturation in the opposite direction, ^{similar domain} structures ^occur along

the edges ^of the film in the first and third quadrant (Diagram 2). Figures ^{16 to} 18 show the various

stages ^{in the} appearance and disappearance ^{of the}

colloid pattern only ^{in the third} quadrant.

Firstly, ^{as the field is} reduced from the + ^H

direction very faint walls appear and with further reduction of the field more colloid is deposited ^on

the walls which, ^{at 2} oersteds, ^are quite ^distinct (Fig. ^{16). The} clearwalled, long spike ^domains point in the easy direction from the edge ^{of the}

film. Photograph ^{16 b shows a} magnified portion

of the edge ^{of the} film ; in this series all the addi-

tional, high magnification single photographs ^fol-

low the development of the domain structure in the ^area covered by ^this photograph, only photo- graph ^{16c shows a structure over an area} wall away from the edge.

FIG. 17.

As the field is further reduced there is some slight

movement of the walls, but the overall rough pat-

tern is retained, ^{and short} cross-ties appear ^on the walls- these cross-ties increase in length ^and

number as the field is reduced ^to zero (Fig. 17).

In the early stages ^{of the} appearance of the cross-

ties, ^{their number and} lengtb may be increased by

the application ^{of more colloid.}

Reversing ^and increasing ^{the field in the}

^H

direction causes a reversal in the events taking place ⁱⁿ the decrease from saturation, ^{until the}

walls have entirely ^faded (Fig. 18). Reducing ^the

field at this stage ^{causes colloid to be} deposited again ^{on the} walls, which have suff ered no appre- ciable movement. But, ^{after a field of about}

-

5 oersted has been exceeded a new structure appears ^on decreasing ^{the field.}

FIG. 18..

This behaviour _may be explained by considering

the rotation of the magnetisation. As the field is reduced, ^the magnetisation ^{of the film turns}

from the field direction to the nearest easy direc-

DIAGRAM 3.

tion, ^i.e. anti-clockwise. In the first and third

quadrants ^the magnetisation ^{is thus} turning ^into

a direction perpendicular ^{to the} edge ^{and in the}

second and fourth quadrants ^{it is} becoming paral-

lel ^to the edge. ^{Domains are nucleated at the} edges ^{in the} first and third quadrants ^{so as to}

decrease the magnetostatic energy of the film (Dia-

gram 3).

317 The domain walls are small-angle ^walls and, ^as

with the faint closure structure at the base of the nucleated domain in the parallel case, do not attract dense colloid deposits. As thé field is decreased fur- ther the magnetisation ^{in the film} is rotated anti- clockwise and that of the spike ^{domains clockwise}

thus making the angle ^{across the walls} greater ^and hence attracting more colloid. Atremanencethema- gnetisation ^{of the film is} relaxed into the easy direc- tion (ED) ^{and the domain} magnetisation ^{is towards}

the opposite direction. ^The cross-ties appear ^at this stage (Fig. 17) ^{and are thus} probably ^charac-

teristic of 1800 walls. Also at this stage, ^closure domains form in gaps between thé spikes ^towards

the bottom of the film, again reducing the magne- tostatic energy and the spike magnetisation ^rotates

clockwise through the easy direction. Increasing

the field in the

H direction now rotates the

magnetisation ^{of the film and the} spikes ^towards

the field direction. As the angle ^of magnetisation

across the walls decreases the walls lose their cross-ties and fade. However, ^{in the} region ^of

the film edge whiçh ^is perpendicular ^{to the field}

the magnétisation ^does not rotate so far because of the demagnetising field, ^{and the} walls in this

region ^{are the last to fade.}

Conclusion.

The domain reversal processes in tbese films are markedly different. In the case

of the iron film the coercive fields are the same in the mutually perpendicular directions and the walls tend to form in the field direction rathpr ^than

in a fixed direction in the film. On the other hand,

the nickel-iron film shows a well defined anisotropy

and the magnetisation ^{reversal domain} structures, parallel ^and perpendicular ^{to the easy} direction,

may be qualitatively explained by considering ^this anisotropy, ^the shape ^{of the film and} the effect at the edge. The behaviour of the walls in this latter film is interesting ; ^the gradual appearance of colloid deposits ^{in the} transverse reversal expe- riment suggest tbat ^{the walls are} initially ^formed

with little or no component ^of magnetisation per-

pendicular ^to the film and this component ^increases

as the angle between the magnetisation ^{of the} adjacent ^{domains increases until} the cross-tie walls

are formed.

Aeknowledgements.

^{This work was carried}

out at the Mullard Research Laboratories under the direction of Dr K. Hoselitz whose helpful ^advice

is readilyacknowledged. The author is alsoindebt- ed to Miss D. Sturgess ^{and Mr. J. L.} Page ^{for their}

assistance in making the films and in the photo- graphie ^work.

REFERENCES

FOWLER (C. A.), ^FRYER (E. M.) and STEVENS (J. R.), Phys. Rev., 1956, 104, 645-649.

WILLIAMS (H. J.) and SHERWOOD (R. C.), ^J. Appl. Physics,

1957, 28, ^548-555.

HUBER (E. E.), ^SMITH (D. O.) and GOODENOUGH (J. B.),

J. Appl. Physics, 1958, 29, ^294-295.

DISCUSSION

Prof. ^Bates.

^{I admired} your beautiful pictures

and 1 was surprised ^{to see such} complete ^absence

of stray deposits ; ^{was any} specially dilute colloid used ?

Mr. Fuller.

No. The colloid ^was prepared according ^{to the} original ^Elmore recipe.

Mr. Moore.

By applying ^a reversing ^field gradient ⁱⁿ the " easy " ^{direction to a} single

domain region ^{of a thin} film, ^{it is} possible ^to

obtain a domain boundary ^whose position ^{can be}

chosen to lie in a region of interest. From its posi-

tion in relation to the field, ^{values can be obtained}

for the field required ^{to overcome the} opposition

to domain wall movement in the chosen region.

Using ^{a similar} technique ^to Mr. Fuller we’have

observed such domain boundaries ^{on an} 80/20

nickel-iron film 1 000 A thick. They ^{take the}

form of a sawtooth edge. ^The points ^{in the} higher

field region ^{for a} particular ^film corresponded ^{to a} component ^{of the} reversing ^{field in the} plane ^{of the}

film lying ⁱⁿ the range 3.2 ^to 4.0 oersteds which

included the observed coercive force ^{of this} region

of the specimen. ^The points in the lower field

region lay between ^{1.2 and} 2.0 oersteds. On remov-

ing the applied field, ^no macroscopic ^movement

of the domain wall took place. ^{If a field of great-}

er gradient ^was applied ^the points of the sawteeth remained in the same range of field values, ^the angle ^{of the teeth was unaltered but the} pitch

became finer.

The simplest ^{method of} obtaining ^{such field gra-}

dients is ^to lay ^{a current} carrying ^{wire across the}

film. A picture ^{with two} domain boundaries is then obtained as shown in the figure.

Mr. Goodenough.

^{I would like to} report ^the observation by Mr. Huber of our laboratory ^of

FIG. 1.

another domain-wall structure which is closely

related to that of the " cross-tie " walls illustrated

by ^{Mr. Fuller. In a} single ^{film of 80-20} Ni-Fe,

Mr. Huber observed the _presence of both cross-tie walls and the ^new wall structures. This structure is called a " chain " wall and illustrated by ^the figures

FIG. 2.

The interprétation ^{of the "} chain " wall follows from that of the " cross-tie " wall. In the region

of the " cross-tie ", the 1800 domain wall splits

into two 900 domain walls. The 900 walls have a

sinusoidal shape, ^{not a linear} zig-zag shape,

because of the surface tension of the wall. This

means that magnetic poles ^{exist as shown in the} figure ^{so that the "} cross-tie " extends beyond ^the edge ^{of the} wall. This extension is completely analogous ^{to the case of the "} cross-tie " wall.

Mr. Fuller (remark).

Single ^{links of such}

" chain " walls have been observed in the ^cross-

tie walls in the ni-fe film used in the present expe-

riments.