DOMAIN WALL AND BLOCH LINE PHENOMENA IN (111) - AND (110) - ORIENTED GARNETS AND ITS APPLICATION TO BLOCH LINE MEMORY

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DOMAIN WALL AND BLOCH LINE PHENOMENA IN (111) - AND (110) - ORIENTED GARNETS AND

ITS APPLICATION TO BLOCH LINE MEMORY

J. Engemann, J. Heidmann, D. Klein

To cite this version:

J. Engemann, J. Heidmann, D. Klein. DOMAIN WALL AND BLOCH LINE PHENOMENA IN (111) - AND (110) - ORIENTED GARNETS AND ITS APPLICATION TO BLOCH LINE MEMORY.

Journal de Physique Colloques, 1985, 46 (C6), pp.C6-119-C6-126. �10.1051/jphyscol:1985621�. �jpa- 00224868�

JOURNAL DE PHYSIQUE

Colloque C6, suppl6ment a u n09, Tome 46, s e p t e m b r e 1985 p a g e C6-119

DOMAIN W A L L A N D BLOCH LINE PHENOMENA IN (111) - A N D (110) - O R I E N T E D GARNETS AND ITS APPLICATION T O B L O C H LINE MEMORY

J. Engemann, J. Heidmann and D. Klein

Univ. of WuppertnZ, Dept. EZectrical Engineering, 5600 WuppertaZ I , F.R.G.

RESUME :

Les lignes de Bloch Verticales (VBL) peuvent &re uti- lisses comme 616ment de m6moire magnstique. Leur existence est intimement liGe au comportement dynamique des parois dans des couches minces de grenats epitaxiss. Jusqu'a prgsent, les travaux ont surtout port6 sur des couches d'orientation 111.

Dans ce travail, l'influence de l'existence d'une direction d'aimantation prsf6rentielle dans le plan est considsrge.

Trois effets importants en r6sultent : 1) L'anisotropie pla- naire stabilise une direction de ruban et renforce le confi- nement imposs par les gorges usinges dans le matsriau. 2) Une variete de parois avec ou sans lignes de Bloch ont 6t6 obser- v&es ainsi que les transitions d'un stat de paroi vers un autre &tat. 3) Le seuil de nucl6ation de lignes de Bloch ho- rizontales ainsi que leur conversion en lignes verticales sont modelables par variation de la constante d'anisotropie pla- naire Si. ces r6sultats sont 6valu6s 2 la lumi6re de ceu& con- nus pour les couches 111.

ABSTRACT - Vertical Bloch Lines are used as memory elements in the VBLM-concept. The existence of these Bloch lines is closely coupled to domain wall phenomena in thin ferrimagnetic garnet films. Most of the work done so far concentrates on (111)- oriented garnets. In this paper the influence of an additional preferential axis of magnetization in the film plane is consi- dered. Three main effects have been found: 1. The inplane anise tropy helps to stabilize stripe domains locally and therefore supports the so-called grooving, 2. There exists a variety of magnetic domains withandwithout Bloch lines which can easily be converted into each other, 3. With varying inplane anisotro- py constant Ki the onset of horizontal Bloch line nucleation and subsequent HBL-punch through can be manipulated. These fin- dings are discussed with respect to the VBLM and critically com- pared to those obtained for (111)-garnets.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985621

C6-120 JOURNAL DE PHYSIQUE I - INTRODUCTION

The increase in storage density of conventional bubble devices is li- mited by physical and technological constraints to maximum values around 16 to 64 ~bit/cm'. However when considering micromagnetic sub- structures such as vertical gloch lines (VBL) as information carriers the cell size needed to accomodate 1 bit in a 0,5 um garnet film is reduced by a factor of approximately 100. This principle has been proposed recently by KONISHI et a1./1//2/ who also specified the ne- cessary fundamental device functions for a major-minor loop organiza- tion similar to the magnetic bubble memory. These functions include the controlled generation of vertical Bloch lines in a thin (111)- oriented garnet film with uniaxial anisotropy,the propagation of VBL- pairs having the same rotational sense,the nondestructive readout of vertical loch lines including their annihilation and also the local stabilization of VBL and stripe domains which are used as "minor loops" /3//4/.

To date all basic functions have been demonstrated in principle either experimentally /3/ or using computer simulations /5/. However, the overall picture is far from being complete. This holds, for in- stance, even for one of the most "simple" basic device functions such as the propagation of a VBL-pair along a stripe domain wall and around a stripe domain head.

In this paper we describe the current research status of domain wall and Bloch line phenomena in (111)- and (110)-oriented garnets in Wup- pertal and report on some new findings on orthorhombic materials which could be beneficial to the Bloch line memory concept especial- ly with respect to the high speed properties of these new devices.

I1 - HIGH SPEED PROPERTIES

In thin ferrimagnetic garnet films with uniaxial anisotropy the line- ar velocity regime is limited either by the Walker-breakdown /6/ or the nucleation and subsequent punch through of Bloch loops /7/.

Although undesired in conventional bubble devices the velocity indu- ced generation of Bloch loops followed by a punch through /7/ is used to generate a pair of vertical Bloch lines (VBL). These VBL then ser- ve as information carriers in the Bloch line memory concept / I / . Usu- ally VBL are generated at stripe domain heads by pushing these heavi- ly via a current carrying conductor and propagated using gyrotropic interactions between a moving stripe domain wall and the VBL /3/.

Clearly the wall velocity during VBL-propagation has to be below the critical limits mentioned above. A more serious constraint occurs when one tries to read-out the VBL, a process which is based on the wall-wall interaction during the stripe domain chopping /I/. If, du- ring the chopping process the domain walls move toofast, then additi- onal Bloch loops may be generated which in turn make a discrimination whether the moving domain walls have a parallel (VBL) or antiparallel (no VBL) center wall magnetization very difficult. This then leads to an insufficient discrimination margin as is evident from KONISH18s data /I/. Therefore it is desirable to control the onset of Bloch loop generation such that the VBL-nucleation at the stripe domain head is guaranteed and at the same time the Bloch loop generation during chopping is supressed. This may be done using a (110)-orien- ted garnet film with orthorhombic anisotropy (i.e. inplane anisotro- py constant Ki

+

Fig. 1 exemplifies this effect for LPE garnet films grown on a (111)- and (110)-substrate1 having the same nominal composition ( Y L u B ~ ) ~

(FeGa)5012. The relevant material parameters are listed in Tab.1.

These films have been kindly supplied by Laboratoire d9Electro- nique et de Technologie de L81nformatique L.E.T.I.,Grenoble/

France

Garnet Film Orientation h (vm) 4rMS (G) A (erg/cm) Q1= KU/2rMS 2 Q2= Ki/2nMS 2

BILY 77 (111) 4.7 191

1.14.10-~

12.1

BILY 86

BILY 8 6

---

(1101

to,o/o, P

/'

BILY 77

-

M

-6 ^-;;;;-

0 0.5 1 1.5 2 2.5

BUBBLE DRIVE (Oe)

Tab.1 Material parameters of (111)- and (110)-oriented garnet films having the same nominal composition (YLuBiI3

(FeGa) 2

Fig. 1

Translational velocities of S=l and (O,O/O)-wall coded magnetic bubbles in (111)- and (110)- oriented garnet films having the same nominal composition (YLuBil3

(FeGa) 501 2.

-Bubble Rocking Experiment- The (O,O/O) -bubble is driven pa- rallel to the medium axis of magnetization.

For material parameters see Tab. 1

.

In this example we have vma,(llO) /vmax(lll)= 44, and for the li- near velocity regime pw ( 1 10) /

~w(lll)= 1,s.

What optimum values of Ki have to be chosen with respect to Bloch Line Memory is still to be answered. Detailed investigations are un- derway.

I11

-

The BLM as it is known today is still based on the major-minor loop principle. Therefore the local stabilization of stripe domains inde- pendent of the actual load with VBL and compatible with film orien- tation, whether (111)- or (110), is essential. One possible solution to this problem is to use ion-milled, grooves with a fixed aspect ra- tio (i .e. depth Ah/width w)

.

1. <1112=or&ested_sacneta

As a guideline for (111)-oriented garnet films one can state / 8 , 9 / that stripe domains are stabilized inside the grooves if 0 -AIl1>o.

o0 describes the biaxial tension in the film plane with ⁰

cso < 0 : compression

oo > 0 : expansion

and relates to the lattice mismatch between the GGG-substrate and the LPE-film having different magnetostriction coefficients X l l l and XIo0. However this situation is overruled if the shape anisotropy

(-MS ) is larger than the stress induced anisotropy (- ooX 1 )

.

this case the stripe domain always stabilizes in the thinned region, i.e. groove. Table 2 summarizes the effect of the different contri- butions to the stable domain position.

C6-122 JOURNAL DE PHYSIQUE

If ooXlll 2 0 then the magnetostatic and magnetoelastic interactions lead to an effective uniaxial anisotropy Ku(xl which varies across the milled groove such that there are pronounced minima located at

XI 2 close to both flanks of the groove (Fig. 2, Technological and reievant material data see Tab. 3). In consequence the domain walls tend to be pinned at X I and x2 if a stripe domain is stabilized with- in the thinned region. When using expanding biasfield pulses neces- sary to propagate vertical Bloch lines the domain walls are driven towards the groove flanks.

oOhl 0

I

I

1

I

the preferred stripedomain position in (111)- oriented gar- net films.

Fig. 2 Local stabilization of stripe domains in a ferrimagnetic gar- net film (BILY 77).

A. SEM-wicture of the actual device structure showins ionmil- led-grooves , writing (W) and chopping (C) conductors. The

[112]-crystalline axis is perpendicular to the groove flanks.

B. Variation of the uniaxial anisotropy KU(x) normal to an ion-milled groove.

(xi1 11121).

The calculations are based on the technological and mate- rial data given in Tab. 1 and 3.

Groove depth Ah(vm)

Biaxial tension oo (dyn/cm 2 )

1

1

Magnetostriction coefficients X I 1 1

X1oo

Poisson number v

BILY 86 3 Garnet Film

Groove width w(um)

Tab. 3 Technological and material data underlying Figs.2 and 3. The magnetostriction coefficients A l l and X as well as the Poisson number v have been assumed to beloothe same as for YIG.

BILY 77 3

At a critical amplitude which correlates to the condition aKu(x)/ax=

max. the walls can no longer be confined to the thinned region and are swept out of the groove. Additionally, since the local variation of Ku(x) is unsymmetric with respect to the groove midplane we ex- pect the domain wall experiencing the smallest local change of Ku(x) to be swept out first (Fig.3A). Therefore for practical applications one has to orient the groove with respect to the major crystalline axes such that aKu(x)/ax is maximized.

! of magnetization

B.

Fig. 3 Azimuthal dependence of the relative local anisotropy change KU1 (x) /KUt (max) in a (1 1 1 )

-

(BILY 77 and BILY 86)

In the (Ill)-garnet the angular dependence of aKu(x)/ax shows a threefold symmetry, whereas in the (110)-film this reduces to the twofold case. At a given groove orientation within the

(111)-plane the domain wall subjected to the smallest aKu(x)/ax is swept out first. Using a (110)-oriented ortho- rhombic garnet film both domain walls leave the thinned region simultaneously.

2 - IllOk~rLCltCd-95~letS

In (110)- oriented garnets with orthorhombic anisotropy we have to consider the uniaxial and inplane anisotropy constants

KU = - 3/4 oo (X1ll + X1oo) (1

and Ki = 3/4 oo(X1

-

,respectively. For an easy axis perpendicular to the film surface it is necessary that (KU + ^{Ki)> 0} which leads to (-3/2)aoXIo0 > 0. Since within the groove we usually have a. < 0 a positive XiOO is required to stabilize a stripe domain there.

Furthermore the inplane anisotropy constant Ki can either be positive or negative referring to the fact that for Ki < 0 the medium axis of magnetization aligns parallel to the [110]- crystalline axis while

for Ki > 0 this is [OOI]. In consequence a stripe domain having the structure (0,0/0)+ (see below and / 6 / ) spontaneously aligns parallel to the [0011-axis when A l l l > Xl0o while for X l l l < XIOO the [ 1 1 0 1 - axis is favoured. For the stray-field dominated type (0,4/2) (see al- so below and /6/)we expect the corresponding stripe domains to align spontaneously perpendicular to the medium axis of magnetization. The- refore only the stray-field dominated stripe domains spontaneously align such that the equilibrium position corresponds to the groove

C6-124 JOURNAL DE PHYSIQUE

orientation where the local stabilization by technological means, i.e. grooving is maximised.

IV - WALL CODING

Wall coding in (Ill)-oriented garnets and its applications to Bloch Line Memory have been described in detail in several publications al- ready /I-5/. Therefore, in this paper we will concentrate on those effects in (110)-oriented garnets with orthorhombic anisotropy which have been newly found /lo/ and which possibly will be of benefit to the high speed properties of BLM.

Three principal bubble types are known in (110)-oriented garnet films the structure of which can be described by the general expression

(Sf v/h). S is the winding number, h and v correspond to the number of horizontal and vertical Bloch lines (or portions thereof), respec- tively. Using this notation we have to consider (0,0/0)+, (0,2/1)+

and (0,4/2)

.

spontaneously align either parallel (+ sign) or antiparallel ( - sign) to the [0011

-

Fig. 4

Typical wall configurations of

M

(0,4/2), (0,2/1)+ and (O,O/O)t magnetic bubble domains in ( 1 10)

-

oriented garnet films with ortho- rhombic anisotropy.

The unsymmetrical (0,2/l)f-wall structure derives from (0,4/2) by omitting one of the two Bloch loops, e.g. the dotted one.

In all cases the main wall magne-

tizations align parallel to the ^(0,412)

OR tO,O/O)+

\

medium axis of magnetization M.A.

(M.A.) (0,2/1)2

Although the actual bubble types (0,0/0) +, ^(0,0/0) - and (0,2/1 )+,

(0,2/1)- have identical wall energies they can be easily generated and discriminated statically and/or dynamically /11,12/. This holds also for the symmetrical stray-field oriented type (0,4/2). An ex- ample which demonstrates the identification of (0,2/1) and (0,4/2) via the static bubble collapse and, at the same time, the accuracy of our domain wall models is given in Fig. 5. From earlier experi- ments it is well known that (0,2/1)+ and (0,4/2)- bubbles having one or two Bloch loops are statically stable [Ill. When driving such a stray-field dominated bubble very hard these Bloch loops should punch through resulting in a pair of vertical Bloch lines with opposite chirality. However, such bubbles with vertical Bloch lines of even number have not been observed so far in orthorhombic garnets. Indeed one only finds the dynamic transitions (0,4/2) + (0,0/0)+ or (0,2/1)+

+ (0,0/0)+ where (0,0/0)+ represent wall states having no VBL / 1 3 / .

One reason for this situation may be that along the bubble axis the strayfield and inplane anisotropy iqteract in such a way that eventu- ally existinq VBL of different chirality are distorted and are no longer stable (12/. This situation changes for a planar domain wall where at least numerical simulations indicate that a dynamically in- duced punch through is possible after nucleation of a horizontal Bloch line (Fig. 6)

.

Since a stripe domain used as a minor loop in the VBL-memory closely approaches the concept of a planar domain wall one might expect that VBL are stable there and could be propagated by means of gyrotropic

forces. Problem areas are the stripe domain heads where the wall cur- vature and also the inplane anisotropy could be harmful to the VBL.

The extent to which this depends on the actual value of the inplane quality facor Q2= ~ ~ / is unknown at this time. Again no experi- 2 n ~ ~ ~ mental data are available yet.

Fig. 5

Inplane-field dependent static collapse field of (0,2/1)+ and (0,4/2)- wall coded magnetic bubbles.

0 , A Experimental data

---

Sample : $3 81 # 1

Composition : (YGd) 3 (FeGaMn) 2 h = 4.3 pm Q 1 = 9.52 4rMS = 166.4 G Q2=-7. 53 A = 1 .92

-

BlLY 77

-100 -50 0 50 100

INPLANE FIELD H ,p (Oe)

A. (1 1 1 )-Oriented Garnet Film B. ( 1 10)-oriented Garnet Film

Fig. 6 Nucleation of horizontal Bloch lines in (111)- and (110)- oriented garnet films having the nominal composition

(YLuBi) (FeGa) 0 (Planar domain wall)

For material pgra&eters see Tab.1. Additionally the gyromagne- tic ratio Y = 1.79. lo7 rad/sOe and the Gilbert damping con- stant a = 0.00206 have been included in the computation.

C6-126 JOURNAL DE PHYSIQUE V - SUMMARY AND CONCLUSION

The idea of the vertical Bloch Line Memory as it is known today is based on the interaction of domain wall and Bloch line phenomena.

Both have to be understood and controlled such that real device

structures can be derived. One of these basic structures is an arran- gement which locally stabilizes stripe domains and is prerequisite for stable bit positions along the stripe domain periphery. Calculations and experiments show that it is possible to stabilize such stripe do- mains inside or outside etched grooves in (111)- or (110)-oriented garnet Groove depths in the order of 10% of the total film thickness are sufficient. If the domains are located inside grooves properly aligned with respect to the major crystalline axes expanding bias field pulse amplitudes in excess of several 10 Oe can be applied with- out sweeping the domains out of the grooves

[a].

From computer simulations we know that horizontal Bloch lines punch through in (110)-oriented garnet films with orthorhombic anisotropy eventually yielding VBL, However, no experiments are known to date

ACKNOWLEDGEMENT

This work was supported in part by the Stiftung Volkswagenwerk and the German Ministry of Research and Technology BMFT under Grant No.

423-7291-NT 2576.

REFERENCES

/ I/ S.Konishi, IEEE Trans.Mag.rMAG-19(5)1183811983

/ 2/ S.Konishi, K.Matsuyama, I,Chida, S-Kubota, H-Kawahara and M.Ohbo, IEEE Trans.Mag.,MAG-20(5),1129,1984

/ 3/ Y.Hidaka, K.Matsuyama and S.Konishi, INTERMAG 1983 Phildadelphia/USA, Paper GA-5

/ 4/ Y.Hidaka, H.Matsutera, IEEE Trans.Mag.,MAG-20(5),1135,1984

/ 5 / K.Matsuyama, S.Konishi, IEEE Trans.Magn.,MAG-20(5),1141,1984

/ 6/ A.P.Malozemoff, J.C.Slonczewski, Magnetic Domain Walls in Bubble Materials, p.137, Academic Press, New York,London,l979 / 7/ R.A.Kosinski, J.Engemann, Numerical simulation of wall dyna-

mics in (Ill)-oriented garnet films in the presence of an in-plane magnetic field, accepted for publication in J-Magn.

Magn .Mat.

/ 8/ J.Engemann, VW-Foundation, Research Report #2, Contract No.

83128

/ 9/ D.Klein, J.Engemann, 30th Annual C0nf.Magnetis.m & Magn-Mat., San Diego/USA, Nov.27-30, 1984, Paper BD-04

/lo/ D.Krurnbholz, J.Heidmann, J.Engemann, R.A.Kosinski, IEEE Trans.

Magn., MAG-20(5) ,1138,1984

/11/ S.Konishi, J-Engemann, J.Heidmann, T.Hibiya, Appl.Phys.Lett., 38(6) ,467,1981

/12/ x ~ n g e m a n n , BMFT-Research Report # 8 Contract No. 423-7291-NT 2576

/13/ J.Engemann, J.Nyenhuis, F.J.Friedlaender, D.Musselman, Dynamic studies of (110)-oriented garnets in the rotating gradient ex- periment, to be presented at the International Conference on Magnetism, San Francisco/USA,1985

/14/ D.Klein, J.Engemann, Stress-Related Phenomena at Grooving Edges in Heteroepitaxial Garnet Films, Magnetic Materials For Application-Conference M.M.A.85, Grenoble/France1l985,Paper E3-C