Magnetic after-effects in ultrathin cobalt films with perpendicular anisotropy

Magnetic after-effects in ultrathin cobalt films with perpendicular anisotropy

Thin Solid Films, 175 (1989) 341-346 341 MAGNETIC AFTER-EFFECTS IN ULTRATHIN COBALT FILMS WITH PERPENDICULAR ANISOTROPY* G. BAYREUTHER Institut fiir...

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Thin Solid Films, 175 (1989) 341-346

341

MAGNETIC AFTER-EFFECTS IN ULTRATHIN COBALT FILMS WITH PERPENDICULAR ANISOTROPY* G. BAYREUTHER Institut fiir Angewandte Physik, Universit~itRegensburg, 8400 Regensburg (F.R.G.)

P. BRUNO lnstitut d'Electronique Fondamentale, 91405 Orsay (France)

G. LUGERT Institut fiir Angewandte Physik, Universitiit Regensburg, 8400 Regensburg (F.R.G.)

D. RENARD lnstitut d'Optique, 91405 Orsay (France)

C. TURTUR Institut ffir Angewandte Physik, UniversiRit Regensburg, 8400 Regensburg (F.R.G.)

Using an alternating gradient magnetometer (AGM) magnetic after-effects have been observed in ultrathin ferromagnetic films for the first time. The relaxation time of magnetic reversal in epitaxial cobalt films on Au(111) in a constant applied field varies over several orders of magnitude depending on field and film thickness. The extrapolated relaxation time in zero field increases from 14 days to 11 years if the film thickness is reduced from 9.5 to 5.4/1,. This should be an adequate stability for most practical applications of such films.

1. INTRODUCTION Magnetic after-effects, i.e. changes of magnetic state in a constant applied field, are a c o m m o n phenomenon in ferromagnetic materials. In bulk material very often the relaxation time is so large that the aftereffects cannot be observed. In contrast, systems containing small ferromagnetic particles can show pronounced aftereffects. The presence of such effects in magnetic films m a y be important for applications like information storage. Ultrathin epitaxial cobalt films on Au(111) have been shown to exhibit a strong perpendicular anisotropy 1'2 which makes them interesting for future applications. However, very little is known about the magnetization reversal process in ultrathin films, e.g. no direct observation of domains and wall motion has been reported until now. An investigation of relaxation phenomena might shed some light on this question and allow a distinction between magnetization reversal by rotation or by wall motion. In order to check the existence of magnetic aftereffects in such films by measuring time dependent hysteresis loops we need a magnetometer which combines a very high sensitivity with a sufficiently high measuring speed. The * Paper presented at the 2nd International Symposiumon Trends and New Applications in Thin Films, Regensburg, F.R.G., February 27 March 3, 1989. 0040-6090/89/$3.50

© ElsevierSequoia/Printed in The Netherlands

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G. BAYREUTHERet at.

recently developed alternating gradient magnetometer (AGM) 3 allows us to measure magnetization curves of films in the monolayer range with unprecedented precision in sufficiently short time to detect time dependent processes with time constants from seconds to hours. This technique has allowed us to observe important aftereffects in ultrathin films for the first time. 2. EXPERIMENT Ultrathin films of cobalt have been prepared by evaporation in U H V on Au(111) films previously grown on glass substrates. The preparation and structural characterization of the samples have been described elsewhere4. The gold substrate films and, hence, the cobalt films are polycrystatline with a strong texture. After annealing the gold films in UHV, the crystallites have an average size of 2000/~; their surface is constituted of atomically fiat terraces 200-300/~ wide and separated by monoatomic steps. The h.c.p, structure of the cobalt films with (0001) orientation has been verified in thicker films. An additional gold layer is finally deposited in order to prevent corrosion. The complete sandwich is removed from the glass substrate for all further investigations. Ferromagnetic resonance and S Q U I D magnetometry have already shown 1.2 that (i) down to one monotayer all cobalt films are ferromagnetic at 10 K with a high remanence, (ii) at 300 K only films of two monolayers or more show remanence and hysteresis and (iii) a strong interface anisotropy is present acting opposite to the shape anisotropy and forcing the magnetization to lie perpendicular to the film plane in films thinner than 12/~ (about 6 ML). In the present study magnetization curves of the films have been measured at T = 294 K by means of an A G M with the field applied perpendicular and parallel to the film plane. 3.

RESULTS A N D D I S C U S S I O N

Figure 1 shows magnetization loops of a 8.1/~ cobalt film with the field perpendicular to the film plane. Due to the strong perpendicular anisotropy the loops are nearly square shaped if we subtract the diamagnetic contribution of the substrate and sample holder. The coercive field H c decreases from 650 to 560 Oe if we increase the sweeping time from 2 to 45 min. This is a clear indication of a magnetic aftereffect. Next, the time dependence of the magnetization is studied in the following way. The sample is first saturated in the easy axis. Then a field in the opposite direction (which is negative by definition) is applied with continuously increasing magnitude and stopped at a given value. The total magnetic moment of the sample, re(t), is then m e a s u r e d as a function of time in this constant field. Figure 2 shows the time variation of the magnetic moment for a 5.4/~ cobalt film at different values of the applied field. A pronounced aftereffect is observed which even leads to a reversal of the total magnetic moment within a few tens of seconds. The relaxation rate increases strongly with the magnitude of the reversing field. This behaviour suggests that the relaxation is caused by thermal activation of the spin system against local energy barriers.

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MAGNETIC AFTER-EFFECTS IN C o FILMS

8.1 A T=294 K

I

m

[a.u.]

Hj. [ kOe ]

2.'5

2rnin: H c : 6 5 0 0 e ~45

rain : Hc =560 Oe

J Fig. 1. Perpendicular hysteresis loops of an 8.1 A cobalt film grown on Au(111) (about 5 mm 2) measured with an alternating gradient magnetometer (AGM); linear field sweeps with 2 and 45 rain cycle time,

I-.--~

H±[Oe] -3oo

~

-310 -320

-330 -340 -350

--.0

-360

E

-370 -380 -390

s.4 A T= 294 K

o

I

8o

I

loo

t [s ]

Fig. 2. Relaxation of the magnetic moment for a 5.4/~ cobalt film in constant applied field for different field values.

The time law of a thermal aftereffect is expected to be exponential according to Am(t) = Am(0)e -'/~ Am(t): =

(1)

re(t)-- m(~)

if the distribution of barrier heights E. is narrow (AE. << kT)S; if the energy barriers show a wide distribution (AEa >>kT), a quasi-logarithmic time law should be

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observed: re(t) = m o + S o l n t / t

for t ~> t o

o

(2)

The e q u i l i b r i u m value of the m a g n e t i c m o m e n t rn(oo) will d e p e n d on the applied field a n d is not k n o w n in general. A s s u m i n g a square m a g n e t i z a t i o n loop, however, we expect m(oo) = t ms or - m s for the m a g n e t i z a t i o n reversal in either easy direction. I n o r d e r to test the validity of the relations (1) a n d (2) we p l o t the r e l a x a t i o n for the 8.1/~ c o b a l t film in a reversing field of - 500 O e in two different w a y s (Fig. 3/. T h e result is that the r e l a x a t i o n is neither e x p o n e n t i a l n o r logarithmic. H o w e v e r , Fig. 3 can be i n t e r p r e t e d as an e x p o n e n t i a l r e l a x a t i o n with a steadily increasing r e l a x a t i o n time. This can be u n d e r s t o o d if we t a k e into a c c o u n t the p a r t i c u l a r g e o m e t r y of o u r e x p e r i m e n t : the o r i e n t a t i o n of the m a g n e t i z a t i o n p e r p e n d i c u l a r to the film p l a n e creates a s t r o n g d e m a g n e t i z i n g field which acts in the same direction as the a p p l i e d reversing field before the reversal starts. D u r i n g the reversal of the m a g n e t i z a t i o n the d e m a g n e t i z i n g field a n d therefore the local field acting o n the spins will g r a d u a l l y decrease which a c c o r d i n g to Fig. 2 leads to an increase of the r e l a x a t i o n time.

50

=~

100

150

200

250

t [sI

0.5 8 0

c

(a)

7

-02

8.1 /~, T = 294 K

6 c;

H L: -500 Oe 1 3

2

3~"~A 10

30

5 100

6 "~

7

5 8

I0~3000

(b)

\

In t t [sl

L,

300 -I-

\ s6o

600

-H, [Oe]

Fig. 3. Time dependence of the magnetic moment of a 8.1/~ cobalt film in constant field H rr,s, rnlo0) = - ms); (a) ln[(m(0 - m(oo))/m(O)] vs. t; (b) m(tJ/m(O) vs. In t. Fig. 4. Logarithmic plot ofintitial relaxation time z vs. applied field for a 8.1 • cobalt film.

500 Oe

(m(O~ =

I n o r d e r to e l i m i n a t e the effect of the v a r i a t i o n of the d e m a g n e t i z i n g field we d e t e r m i n e the r e l a x a t i o n time at the b e g i n n i n g of the m a g n e t i z a t i o n reversal w h e n the s a m p l e is still close to saturation, I n Fig. 4 the r e l a x a t i o n time o b t a i n e d in this w a y is p l o t t e d in a l o g a r i t h m i c scale as a function of the a p p l i e d field for the 8.1/~ c o b a l t film. T h e d a t a c a n well be fitted b y a straight line. This b e h a v i o u r is o b s e r v e d for all films of different thickness. It allows us to e x t r a p o l a t e the r e l a x a t i o n time d o w n to zero a p p l i e d field a n d to o b t a i n a stability time ~st, i.e, r e l a x a t i o n time for r e m a n e n t m a g n e t i z a t i o n of the films in the s a t u r a t e d state. T h e results for different

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M A G N E T I C AFTER-EFFECTS I N C O FILMS

TABLE I RELAXATION TIME OF THE MAGNETIC MOMENT EXTRAPOLATED TO ZERO APPLIED FIELD FOR DIFFERENT COBALT~LAYERTHICKNESS WITH AN EASY AXIS PERPENDICULAR OR PARALLEL TO THE FILM PLANE

Thickness (]~)

Easy axis

z(H = O)

9.5 8.1 5.4

.k .1_ _1_

14 d a y s 1.3 years 11.4 years

4.1 22

L ]l

5.5h 9.3 months

values of film thickness are shown in Table I. The variation of the relaxation time with thickness can be understood as follows. (i) For films thinner than 12 A the total anisotropy field perpendicular to the film plane increases with decreasing thickness t because the contribution of the perpendicular interface anisotropy is proportional to lit (ref. 3). The stabilization of the spin direction by the anisotropy field causes the relaxation time to increase by several orders of magnitude with decreasing film thickness. (ii) Below 5 ~ the cobalt films are not completely continuous and contain holes. This makes the saturated state less stable due to lower activation energies for magnetization fluctuations. (iii) Above 12 ~ the easy axis of magnetization is in the film plane. In this case the demagnetizing field is practically zero leading to an enhanced stability of the remanent state. Finally, we may ask if the experiments described above allow us to distinguish between two possible mechanisms of magnetization reversal, i.e. by wall motion or by successive switching of mostly uncoupled parts of the film. Assuming an Arrhenius law for the relaxation time r = z o exp

(3)

Figure 4 suggests a linear dependence of the activation energy on the applied field according to E~ =- V*Ms(Ho + H )

forH ~ 0

(4)

where V* is the Barkhausen or particle volume respectively. It has been shown elsewhere 6 that this behaviour is compatible both with a wall motion mechanism and with a switching of independent particles. Using relation (4) and the data of Fig. 4 we obtain V* and an average extension of this volume d* = 300-400 ~ independent of film thickness. This value is considerably smaller than the crystallite size dcryst i> 1000 ~ found in previous investigations 2,4. Also, structural investigations and the magnetic behaviour of the films showed that they are essentially flat and continuous except the thinnest films 1,2. We therefore conclude that the experimental data Can only be explained consistently if we assume a magnetization reversal by domain wall motion. This is an important result because the presence of domain walls in ultrathin ferromagnetic films has not been verified

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by direct observation until now. Hopefully, such experiments will be possible in the near future. 4. CONCLUSION For the first time magnetic aftereffects have been observed in ultrathin ferromagnetic films. Epitaxial cobalt films o n Au(l i1) a few monolayers thick show a strong anisotropy with the easy axis perpendicular to the film planeiThe magnetic aftereffect leads to a pronounced dependence of the coercive field on the rate of change of the applied field and a spontaneous magnetization reversal in a constant applied field. The relaxation time varies over several orders of magnitude depending on the applied field and film thickness. The stability time in the remanent state was determined by extrapolation to zero appliedfield. While the magnetic aftereffect in general:is rather detrimental for technical applications of magnetic materials, by a p r o p e r choice of the cobalt layer thickness, the ,relaxation time in zero field can be greater than 11 years as a lower limit which should be adequate for most practical purposes. REFERENCES 1 C. Chappert, K Le Dang, P. Beauvillain. H. Hurdequint and D. Renard. Phys. Rev. B, 34 11986) 3192. 2 C. Chappert and P. Bruno, J. Appk Phys., 64 (1988) 5736. 3 P.J. Flanders. J. Appl. Phys., 63 (1988) 3940. 4 D. Renard and G. Nihoul. Phil. Mag., B55 (1987) 75. 5 M. Lambeck. ,Barkhausen--Effekt und Naehwirkung in Ferromagnetika, Walter de Gruyter. Berlin. 1971. ~6~ G. Bayreuther. P. Bruno. G. Lugert and C. Turtur. submitted for publication.