W(110)

Journal of Magnetism and Magnetic Materials 93 (1991) 345-348 North-Holland

345

Ferromagnetic resonance in ultrathin Ni(111) /W(110) Yi Li, M. F a r l e a n d K. B a b e r s c h k e lnstitut fiir Experimentalphysik, Freie Unicersitiit Berlin, Arnimallee 14, W-I O00 Berlin 33, Germany

20, 25 and 3 0 A thin N i ( l l l ) films have been prepared on W(II0) in U H V and characterized by low energy electron diffraction and A u g e r spectroscopy. F M R measurements at 9 GHz have been used to study the magnetic properties between 300 and 600 K. The effective in and out of plane anisotropies increase with decreasing temperature and film thickness. These anisotropies are decomposed into surface ( K s) and volume (K v) contributions. K , is 30 times larger than in bulk Ni, due to internal stress caused by the lattice mismatch between Ni and W.

The investigation of the spontaneous magnetization the Curie temperature T¢, and the magnetic anisotropy is of great interest in order to characterize the magnetic properties of very thin films. Recently [1] we have reported critical spin fluctuations on ultrathin N i ( l l l ) / W ( l l 0 ) films characterized using magnetic resonance near To. In this paper we focus on the magnetic anisotropy of ultrathin Ni(111)/W(110) films near and far below Tc. The Ni films were epitaxially grown on W(110), which was clean within the detection limit of our Auger system [1]. During deposition of Ni the pressure was below 2 × 1 0 - 1 ° m b a r (base pressure < 5 × 10 -I1 mbar) and the substrate temperature was maintained at room temperature. The Ni films were characterized by L E E D and Auger spectroscopies. The ferromagnetic resonance was performed at 9.0GHz. The external field H could be rotated in a plane normal to the Ni film, the microwave magnetic field was always parallel to the film plane and perpendicular to H. The t e m p e r a t u r e was varied from 300 to 600 K. The F M R resonance conditions for a thin film are [2]: for H perpendicular to the film plane

and for H in the plane

M~p,

(~}

= H ± - 4"rrM +

2K~ni M

(1)

(~)2=HII(HII+4~M

2Kerr II),

M

(2)

where w/21v is the microwave frequency, Hll and H l are the resonance fields (fig. 1), and KeffU' Keffi the effective anisotropy energies. From eqs. (1) and (2), Kert ll and Keffz may be determined if M is known. The resonance fields H r obtained with the external field parallel ( H u) and perpendicular ( H l ) to the film plane are plotted in fig. 1. Since Tc varies as a function of the film thickness [1, 3], a plot of H r as a function of the absolute temperature (fig. la) is less meaningful, whereas a plot as a function of the reduced temperature shows a systematic trend (fig. lb). For the perpendicular orientation the resonance field of the thinner films is larger than of the thick films. For the parallel orientation the resonance field shifts to zero external field between 400 and 470K depending on the thickness of the film and is not observable for lower temperatures. As we will show in the following the thin films under investigation show strong stress effects. Under those conditions K e f f II and K e f t, i obtained by F M R are not equal, as has been shown previ-

0304-8853/91/$03.50 © 1991- Elsevier Science Publishers B.V. (North-Holland)

T/Te

Y. lJ et aL / FMR in ultrathin Ni( l l l ) / W( I l O)

346

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Keff~(106erg/cm 3)

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0.6 Fig. 2. The effective anisotropy e n e r g y o b t a i n e d from the H z data as a function of the reciprocal film thickness l / d 1or two r e d u c e d t e m p e r a t u r e s , t = {).54 (e) and t = 0.8 (•). In absolute t e m p e r a t u r e s the f o r m e r r a n g e s from 300 to 32[)K, the latter from 445 to 475 K.

0./, : g:2.2

.5

.6

.7

.8

.9

1.0 TITc

Fig. 1. (a) The r e s o n a n c e field as a function of t e m p e r a t u r e for the 20, 25 and 30,~ thin Ni(111) films o r i e n t e d parallel and p e r p e n d i c u l a r with respect to the e x t e r n a l field. The e r r o r c o r r e s p o n d s to the symbol size. The solid lines are guide to the eyes; (b) same as in (a) as a function of r e d u c e d t e m p e r a lure T/T~(d!. The T~'s are t a k e n from ref. [1]: T~.(20,~) 557K, T~.(25 A) = 583K, T~.(30,~)- 591K.

ously [4]. Only for the perpendicular orientation we do have the full temperature d e p e n d e n c e of K~,rr, which will allow to analyse K~, r in terms of volume (K,,) and surface ( K , ) effects [5] at the same reduced temperature: Kef r = K v + 2K./d.

(3)

Using eq. (1) and taking the magnetization M from ref. [3] we calculate the Ke,,l, . . The Ken ± of 20, 25 and 3 0 A Ni films in two reduced temperatures are plotted in fig. 2 as a function of l / d . It is known that the magnetization and the anisotropy energy in the Ni films are d e p e n d e n t upon the t e m p e r a t u r e [3]. In addition, the Curie temperatures in the Ni films are d e p e n d e n t upon the thickness [1, 3]. T h e r e f o r e the analysis and

the comparison of the effective anisotropy energy should be p e r f o r m e d for Ni films of different thickness at the same reduced temperature. Most of the published experiments so far were measured at fixed absolute, namely room temperature only. It is seen in fig. 2 that our data for each fixed reduced t e m p e r a t u r e obey the 1 / d linear relation (3). At T / T c = 0.54 (300K for the 20,~ film) one obtains K, = - 0.23 e r g / c m 2 a factor of two smaller than for the Re substrate, and K , = 1.59 × 106 e r g / c m 3. Since a microscopic theory of magnetic surface anisotropy is missing and the phenomenological NOel theory gives the wrong sign prediction for the Ni(111) surface anisotropy energy, we only c o m p a r e our results with other measurements. The results for different temperatures and the available data from refs. [5, 6] are listed in table 1. For temperatures above 300 K and close to T~, no other experiments exist where K~ and K v have been determined. Let us now examine the volume anisotropy energy K,. To first order the magnetic cubic anisotropy energy of a single Ni crystal is K~ = - 5 . 7 x 1 0 4 e r g / c m 3 at 3 0 0 K [7]. The observed

347

Y. Li et al. / FMR in ultrathin Ni( l l l ) / W( I lO) Table 1 Experimental data for magnetic surface anisotropy of Ni. For our data the temperatures are the absolute temperatures of the 20 A Ni film

System

T (K)

Ni(111 )/Re(0001 ) Cu,Pd/Ni(111)/Re Re/Ni(111)/Re Ni(111)/W(110) Ni(l 11)/W(110) Ni(111 )/W(110) Ni(111)/W(110)

300 300 300 300 390 450 500

T/T c

K~ ( e r g / c m 2)

K,, (106 e r g / c m 3)

Ref.

0.54 0.7 0.8 0.9

-

1.59(20) 1.39(20) 1.03(10) 0.37(10)

[5] [5, 6] [5] this work this work this work this work

anisotropy energy at T / T c = 0.54 (corresponding to 3 0 0 - 3 1 8 K for the 20, 25 and 30,~ films) equals Kv = 1.59 × 106 e r g / c m 3, which is almost 30 times larger than the anisotropy energy of Ni bulk. From a simple qualitative consideration the physical origin for the increased volume anisotropy energy K v is clear. The lattice constants of Ni and W are not the same. When the N i ( l l l ) films grow on the W ( l l 0 ) substrate, the lattice mismatch between the Ni layers and the W substrate causes stress in the Ni films. Through the magnetostrictive effect the internal stress in the films results in the increase of K v. According to this we calculate the stress in the films using our experimental K~. The volume anisotropy energy can be expressed as follows [8]:

Kv = K -

Ao',

(4)

where the first term is the magnetocrystalline anisotropy energy and the second term is the anisotropy energy clue to stress (o-) via magnetostriction (h). Because K is very small compared with Kv, eq. (4) can be written as follows:

Kv=

0.48 0.22 0.19 0.23(3) 0.20(3) 0.15(2) 0.05(2)

changes in the magnetic anisotropy energy of the Ni films. From o--- 10 ~ d y n / c m 2 we calculate a lattice mismatch of 4.2%. This is in reasonable agreement with the experimental value of 3.6% between Ni(111) and W(ll0). The observed coercive force measured by F M R with the field in the film plane is 150G, which is 20 times larger than in Ni bulk [10]. The increased coercive force is also considered to be due to the stress in the films. In conclusion, we have determined the temperature dependence of the surface anisotropy energy K S and the volume anisotropy energy K v of the ultrathin Ni films from FMR. We find that K S at 300K is of the same order of magnitude as from other experiments. The enhancement of the K v is attributed to the large tensile stress in the Ni films, which comes from the lattice mismatch between the N i ( l l l ) layers and the W substrate.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 6.

(5)

For the Ni(111) films we have A = - 2 . 5 × 10 -5 [9]. Using eq. (5) and K v at t = 0.54 one obtains = 9.9 × 10~° d y n / c m 2. This large tensile stress comes from the lattice mismatch between the Ni layers and the W substrate and results in the

References [1] Yi Li, M. Farle and K. Baberschke, Phys. Rev. B 41 (1990) 9596. [2] S.V. Vonsovskii, ed., Ferromagnetic Resonance (Per-

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Y Li et al. / FMR in ultrathin Ni( l l l ) / W( I I O)

gamon Press, Oxford, 1966). [3] R. Bergholz and U. Gradmann, J. Magn. Magn, Mat. 45 (1984) 389. [4] M.J. Pechhan and I.K. Schuller, Phys. Rev. Lett. 59 (1987) 132. [5] U. Gradmann, J. Magn. Magn. Mat. 54-57 (1986) 7303. [6] W. Robl and G. Bayreuther, 12th ICMS Conf., Le Creusot (1988) to be published.

[7] H.P.J. Wijn, ed., Landolt-B6rnstein, New Series, vol. 19, sub a (Springer-Verlag, Berlin, in press). [8] F.J.A. den Broeder, D. Kuiper, H.C. Donkersloot and W. Hoving, Appl. Phys. A 49 (1989) 507. [9] A.H. Morrish, The Physical Principles of Magnetism (John Wiley, New York, 1965). [10] S.V. Vonsovskii, ed., Magnetism, vol. 2 (John Wiley, New York, 1974).