Domain nucleation and wall movement in TbFeCo

Thin Solid Films, 175 (1989) 265-271

265

D O M A I N N U C L E A T I O N A N D W A L L M O V E M E N T IN TbFeCo* S. WINKLER Institut f. Werkstoffwissenschaften 6 der Universitdt EHangen, 8520 Erlangen (F.R.G.) and Siemans AG, Zentrale Forschung und Entwicklung, 8520 Erlangen (F.R.G.)

W. REIM AND K. SCHUSTER Siemens AG, Zentrale Forschung und Entwicklung, 8520 Erlangen (F.R.G.)

The magnetic after-effect in Tb~(FessCol 5)1-2 with 0.•9 < x < 0.33 measured at different applied fields H a and domain observations using the polar Kerr effect show that the distinct differences of the after-effect between the samples investigated correspond to differences in domain behaviour. Samples exhibiting growth of nonequilibrium domains and simultaneous formation of new nuclei with time are distinguished from samples showing instantaneous formation of all nuclei running out as stripes into an equilibrium maze-like pattern with time. The evaluation of the after-effect measurement as a function of time for one sample yields a thermally activated wall motion with a potential barrier of ~ 100 eV and an activation volume of ~ 1.4 x 10 -~ gm 3.

1. INTRODUCTION Amorphous rare earth-transition metal alloys can be used as thin film media for magneto-optical (MO) data storage applications. The information unit in M O storage is a single domain which is thermomagnetically written and read out using the polar Kerr effect. To be able to optimize M O storage, it is important to understand the processes of domain nucleation and domain wall movement in these materials. We report on quantitative investigations of the magnetic after-effect in amorphous TbFeCo films as well as qualitative direct domain observations of nucleation and wall movement using the M O Kerr effect1'2. 2. EXPERIMENTS

Amorphous Tb~(Fe s 5Co t 5)1-2 films sandwiched between two AIN layers were prepared on quartz substrates by r.f. sputtering 3. Compositions x between 0.21 and 0.31 have been determined by Rutherford back-scattering spectrometry (RBS). The

*Paper presentedat the 2rid International Symposiumon Trends and New Applicationsin Thin Films, Regensburg, F.R.G., February 27-March 3, 1989. 0040-6090/89/$3.50

© ElsevierSequoia/Printedin The Netherlands

266

S. W I N K L E R ,

W. REIM, K. SCHUSTER

compensation composition at room temperature Xcompis near 0.24. Table I provides some characterization results of the samples investigated. TABLE I CHARACTERIZATION RESULTS OF THE INVESTIGATED SAMPLES; J s WAS MEASURED IN A VIBRATING SAMPLE MAGNETOMETER (VSM), H e IN A KERR MAGNETOMETER. THE THICKNESS WITH A SURFACE PROFILER. AND THE COMPOSITION BY RUTHERFORD BACKSCATTERING SPECTROMETRY (RBS)

Sample 1 2 3 4

Composition

Thickness

Saturation magnetization

Coerciveforce

(at.%Tb)

D (nm)

L (T)

Hc (kArn- ~)

32,8 + 0.15 30.6±0.15 20.7±0,15 19.2±0.15

114 t13 115 113

0.458 0.299 0.276 0.443

65.4 91.9 180.8 114.7

The magnetic after-effect was measured quantitatively by detecting the polar M O Kerr rotation with a sensitive detection system. In this measurement, the sample is located in a magnetic field perpendicular to the film surface and illuminated by linearlY polarized light of a H e - N e laser. The reflected light passes a half-wave retardation plate, which rotates the plane of polarization, and a Wollaston prism, Which splits the polarization into two mutually perpendicular components. The half-wave plate is adjusted so that the intensities of these two components 11 and 12 measured with silicon photodetectors are equal at zero magnetization. The electrical signals of the detectors are electronically processed to obtain the normalized difference ( I 1 - I 2 ) / ( I 1 + I 2 ) as the output signal which is directly proportional to the Kerr rotation 4, Thus the output is symmetrical with respect to zero for positive and negative saturation. The experimental procedure described below was chosen with the intention of distinguishing between domain nucleation and domain walt motion. The sample was first saturated in the negative field direction. The external field was then changed in steps of < 80 Am- x until the detector output signal decreased from the level at saturation by a defined fraction of'the saturation signal, which was I 0 ~ in these experiments. The corresponding field is the "threshold field" H~, which is lower than Hc as measured in the hysteresis,loop. The decrease in signal level corresponds to an area of 5~o of changed Kerr rotation of the film, which in turn represents the respective volume of opposite magnetization in the film. Then, the field was immediately lowered to H < Ht to freeze the domain structure. Finally, a field Ha with a predefined ratio Ha/H t was applied to start the after-effect detection of the frozen-in domain structure. The change of magnetization J with time was .recorded. Fig~ures 1 and 3 show these curves for samples 3 and 1 {Table I), respectively. Direct domain observations at various times during these after-effect measurements were performed in a polarizing microscope (numerical aperture, 0.9) using the polar M O Kerr effect. The after-effect m( temperatures between 306 and 310 K in Ha = 180, 181, 182 and 1 8 3 k A m - 1 respectively, in order to identify any thermal activation of the after-effect.

DOMAIN NUCLEATION AND WALL MOVEMENT IN TbFeCo

Figure -2a

Fimtre 2b o

i

,

,

~

267

Figure u t~2c ,

,

,

0.8 0.6

. ~ '.,

~'~

'..

",,.%

0,4

'~\ 0.2

~ 9 4

kA/m

\

\

~.~

196

kA/m

0 -0.2 elOs

-0.4 -0.6

.... Lo ,!o !Lo ,f ; ,~o ,~! ~!."',~7'

-0.8

....~

2oo ~ / m |

-1 0

20

40

0 1,4 [

20 '

40

(a)

60

80

Time (s) '

6O

80

lO0

/

1,3 ~

.~--'--

(b)

Fig. 1. (a) Kerr optical hysteresis loop and normalized Kerr rotation as a function of time at different applied fields H, for sample 3(Xxb < X~o,~). (b) Normalized areal density of domains as a function of time.

(a)

(b)

(c) t

500 Ixm

Fig. 2. Typical domain structures for sample 3 (Fig. 1) (a) after 15 s, (b) after 50s, (c) after 100s.

268

S. WINKLER, W. REtM, K. SCHUSTER

3. RESULTS AND DISCUSSION

Figure l(a) shows the almost square hysteresis loop and the curves of the aftereffect measurement at different applied fields H a for sample 3. All curves exhibit the same characteristic shape and approach a finite magnetization asymptotically at long measurement times. The domain pictures taken at 15, 50 and 100 s respectively represent the typical behaviour and are shown in Fig. 2. The number of domains per unit area was determined from these and from some additional pictures taken at different times t but not included here. The obtained values were normalized to the initial areal density of domains, and the results are plotted as a function of time t in Fig. l(b'). It is observed that the initial domain nuclei grow as irregular nonequilibrium domains, and at the same time new nuclei are formed and grow during the whole measurement time. The reversal rate decreases as the domains approach each other, thereby reducing the demagnetizing field. As the volume fraction of reversed magnetization by the newly formed nuclei is much smaller than the volume fraction reversed by wall motion of the initially formed nuclei, we conclude that the time-dependent magnetization reversal as measured (Fig. l(a)) is mainly determined by a wall motion process. The hysteresis loop and the after-effect results of sample 1 are shown in Fig. 3(a). The characteristic shape of the after-effect curves differs from that of sample 3. At different applied fields Ha, the magnetization decreases rapidly, and reaches a magnetization value after a time t corresponding to H , in the sheared hysteresis loop The domain pictures taken along the solid curve at H , = - 2 1 k A m - 1 at 1, 2 and 20 s, respectively, are shown in Fig. 4. The round nuclei formed initially run out in stripes and grow partly together with increasing time, finally forming an equilibrium maze-like pattern. The areal density of domains relative to the initial areal density plotted as a function of time in Fig. 3(b) decreases with increasing time. Analysis of these results reveals that no more new nuclei are formed during the measurement period and that the magnetization reversal with time is a wall motion process. No time dependence of the nucleation process itself could be detected because of limitations in the rise time of the applied field and the nine resolution of the measurement. Similar investigations were conducted on samples presented here. They show similar behaviour of the nucleation and growth as samples 1 and 3 respectively. The after-effect for sample 3 mea;ured evaluated using a simple model suggested by Stacey 5, neglecting the contribution of the activated magnetization after-effect can be expressed as ~J 2Js with E = Eo -- 2 V/-/aJs E is the activation energy for the thermally activated process; E o the potential

DOMAIN NUCLEATION AND W A L L MOVEMENT IN Figure 4a Figure 4b

TbFeCo

269

Fi~m~re4c

: , .............. ~/~ ~ ~ ~(kA)~ ........... + ..............

0 6~

~

" " " " " ...............

,/Ha .................... - 21 kA/m .......... "..................

0.4 ".+

0.2 0

""'"

/'Ha = 42 kA/m

""

7Ha = 63 kA/m

-0.2

~/osr

-0.4 -0.6

-,~oo - 3 o o =20o

~oo

20O

300

400

-0.8 -1

[

0

5

(a) 5

10 lr5 Time (s) 10 15 i

20 20

25

f

1,0 ~ ' x

=?

0,9 0,8

"~. ~---.

0,7

(b) Fig. 3. (a) Kerr optical hysteresis loop and normalized Kerr rotation as a function of time at different applied fields H a for sample 1 (XTb >> Xcomp). (b) Normalized areal density of domains as a function of time.

(a)

(b)

(c)

I

20 tam

I

Fig. 4. D o m a i n structures for sample 1 (Fig. 3) at H, = -- 21 kA m - 1: (a) after 1 s; (b) after 2 s, (c) after 20 s.

270

S. WINKLER, W. REIM, K. SCHUSTER

barrier which is lowered by the magnetic energy of the activated Volume Vin the applied field H a a n d which is assumed t o be constant over the Considered temperature range within the measurement accuracy; J, is the saturation magnetization (Table I). The slope of each measured after-effect curve was determined a t the time to.5 when the magnetization of half of the volume detected by the laser spot is reversed, and its logarithm was plotted as a function of the inverse temperature (Fig, 5), The slopes of the resulting straight lines at the four different applied fields were determined. Together with Jo = 0.28T and the film thickness D = 115 nm, the potential barrier E o, the activation v o l u m e V and the corresponding area S were obtained: Eo=16xl0-1aJ+_10xl0-18J~

100 eV +_63 eV

V = L 4 x 1 0 - 2 2 m 3 _ 1 × 10-22 m a = 1.4× 10-4 (Ixm)3 S = 1.2 × 10-15 m 2 +__0.9 × 10-15 m 2 = 1.2 × 10- a (txm)2

310 0 -1 •~

t J. ~t]to, -4"3:

T (K) 308 i

309 ,

307 i

306

Ha = 183 kA/m

Ha='181kA/m

/""-~.,~.~__.. Ha- 180 kA/m

0.00323

0.00324

0.00325 1/T

0.00326

0.00327

K -~

Fig. 5. Logarithmic rate of magnetization change at time to. 5 as a function of inverse temperature at four different applied fields Ha for sample 3 (Fig. 1).

According to the domain analysis above, these results describe a thermally activated wall motion for this sample, The value obtained seems very high. The value for the calculated "activation optical resolution limit. It should be noted that a small chosen for a reasonable experimental evaluation of the of the strong influence of temperature T on to.5. 4. CONCLUSIONSAND OUTLOOK

and shor before wall motion starts to produce an equilibrium maze-like domain pattern. In the former case, nucleation and wall motion occur at the same time during the whole

DOMAIN NUCLEATIONAND WALL MOVEMENTIN T b F e C o

271

magnetization reversal. In both cases, the domain structure reached after sufficiently long measurement times which corresponds to a finite value of the magnetization is determined by an equilibrium between the wall energy, the demagnetizing energy and the magnetic energy in the applied field 6'7. Further analysis in terms of the characteristic length 1 = V#o/ds 2 = V/(2Kd), where 7 is the wall energy and K d is the demagnetizing energy, will be necessary to improve our understanding of the domain structures 8. On the basis of a simple model a thermal activation of the wall motion is apparent. The high value for the potential barrier obtained for the sample investigated within the limits of this simple model, neglecting demagnetizing fields, promises stability of written domains at r o o m temperature with no applied field. The size of the activation volume m a y give a measure of a domain wall irregularity, which in turn m a y provide some understanding of writing noise in M O storage 9. A more thorough analysis, which considers a temperature dependence of the potential barrier, is necessary for the large temperature range occurring during the thermomagnetic writing process. ACKNOWLEDGMENTS We would like to thank Dr. G. Rupp and W. M a r k o for the magnetic characterizations and Prof. A. Hubert for helpful discussions. The RBS analysis was carried out by Prof. J.A. Leavitt and Prof. L.C. McIntyre at the O D S C at the University of Arizona, Tucson. REFERENCES 1 K. Ohashi, H. Tsuji, S. Tsunashima and S. Uchiyama,Jpn, J. Appl. Phys., 19 (1980) 1333. 2 S. Andrieu, P. Bernstein, M. Labrune and F. Rio, presented at the 12th lnt. Coil. on Magnetic Films and Surfaces, Le Creusot, 1988.

3 A. Sch6ne-Warnefeld,D. Weller, W. Reim, G. Rupp, W. Marko, H. Hiilsing, K. SchUster,M. Vieth, V. Weissenbergerand B. Hillenbrand, IEEE Trans. Mag., MAG-25 (1989) 2778. 4 D. Rugar, C. J. Lin and R. H. Geiss, 1EEE Trans. Mag., MAG-23 (1987) 2263. 5 F.D. Stacey,Aust. J. Phys., 13 (1960) 599. 6 P. Hansen, J. AppL Phys., 62 (1987) 216. 7 P. Hansen, J. AppL Phys., 63 (1988) 2364. 8 A.H. Eschenfelder, Magnetic Bubble Technology, Springer, Berlin, 1980, p. 19. 9 F. Tanaka, S. Tanaka and N. Imamura, Jpn. J. Appl. Phys., 26 (1987) 231.