E L S E V I E R
Journal of Magnetism and Magnetic Materials 148 (1995) 285-286
,•
Journal of amnadO netlsm magnotlg materialsMagnetic and magneto-optical polar Kerr spectra studies in N i / A u multilayers
R. Krishnan a H. Lassri a, M. N3~vlt b, V. Prosser b, D. Rafaja b, V. Valvoda b, S. Vi~fiovsk~, b,*
a Laboratoire de Magn~tisme et Matdriaux Magndtiques, CNRS, F-92195 Meudon, France b Charles University, College of Mathenaatics and Physics, CZ-12116 Prague 2, Czech Republic
A b s t r a c t
Magnetic, m a g n e t o - o p t i c a l ( M O ) and X - r a y diffraction studies on N i / A u m u l t i l a y e r s deposited b y e - b e a m e v a p o r a t i o n onto glass substrates are reported. T h e i n d i v i d u a l l a y e r thickness tNi v a r i e d b e t w e e n 0.6 and 2.7 nm a n d tAu was f i x e d at 1.5 nm. The m a g n e t i c r o e a s u r e m e n t s at 5 K s h o w negative surface a n i s o t r o p y and the a b s e n c e o f a d e a d layer. T h e most p r o m i n e n t feature in the M O p o l a r K e r r s p e c t r a is a negative p e a k at 2.7 eV. T h e M O spectra are w e l l e x p l a i n e d b y the t h e o r y using t h e bulk Ni a n d A u optical and M O data. The m o d e l l i n g indicates that the optical constants o f A u c h a n g e w h e n the A u layer b e c o m e s ultrathin.
R e c e n t l y w e have reported on s o m e m a g n e t i c p r o p e r - ties o f N i / A u m u l t i l a y e r s [1]. In this p a p e r w e d e s c r i b e our results of o t h e r studies such a s detailed X-ray d i f f r a c - tion and c o m p l e x p o l a r K e r r s p e c t r o s c o p y at photon ener- g i e s between 1.5 and 5.2 eV. T h e s a m p l e s w e r e d e p o s i t e d b y e - b e a m e v a p o r a t i o n o n t o glass substrates. The i n d i v i d - ual l a y e r thicknesses, tr~i and tAu w e r e c o n t r o l l e d by p r e c a l i b r a t e d q u a r t z oscillators, tNi w a s v a r i e d b e t w e e n 0.6 and 2.7 n m and tAu was f i x e d at 1.5 nm. T h e first a n d the last A u layers w e r e 5 nm t h i c k a n d the n u m b e r of p e r i o d s ranged between 10 and 25 ( T a b l e 1). W e first d e s c r i b e our structural studies, recall b r i e f l y the m a g n e t i c properties, and then present the m a g n e t o - o p t i c a l ( M O ) spectra.
X - r a y d i f f r a c t i o n ( X R D ) studies w e r e c a r r i e d out with Cu K,~ radiation. Both the l o w - a n g l e r e g i o n ( 2 0 < 15 °) and the high-angle region ( 2 0 > 15 °) were evaluated (Fig.
1). A t l o w 20, ]Bragg r e f l e c t i o n s c o r r e s p o n d i n g to A = tNi + tAu w e r e o b s e r v e d . A t h i g h 2 0 the a n a l y s i s provides an a v e r a g e interplanar spacing, d, and A ( T a b l e 1). X R D data on the b i l a y e r thickness a g r e e with c a l i b r a t e d oscillator data to within 0.1 rim.
T h e m a g n e t i z a t i o n d e c r e a s e s w i t h tr~ i and the a n a l y s i s o f the d a t a at 5 K indicates that there is no Ni dead layer present in this s~ystem. T h i s result s h o w s that the interfaces are quite sharp with no s i g n i f i c a n t intermixing. The study o f the effective a n i s o t r o p y o f these m u l t i l a y e r s s h o w s a negative contribution to the surface a n i s o t r o p y , with K s =
• Corresponding author. Fax: +42-2-29 92 72; e-mail: vis- [email protected].
- 0 . 1 erg c m - 2 , i n d i c a t i n g that the m a g n e t i z a t i o n is l y i n g in the plane. W e b e l i e v e that this is the first such o b s e r v a - tion o f a negative K s.
T h e M O p o l a r K e r r rotation ( P K R ) and ellipticity ( P K E ) e x p e r i m e n t s w e r e c a r r i e d out in a field o f 1.1 T. Fig. 2 g r o u p s the P K R s p e c t r a for three different values o f /Ni- T h e y s h o w a negative, p e a k centred near 2.75 eV, slightly a b o v e the o n s e t o f i n t e r b a n d transitions in A u at 2.45 e V [2]. T h i s feature is also t y p i c a l o f other A u - c o n t a i n i n g ultrathin film s a n d w i c h e s and multila3/ers like F e / A u and C o / A u [ 3 - 6 ] . But the a m p l i t u d e s are smaller, b e c a u s e t h e y d e p e n d on b o t h tNi and tAu. Furthermore, since the Curie t e m p e r a t u r e s for these thicknesses are also close to
1 0 6
1 0 ~
O 1 0 ~"
m 1 0 't t~
2 t~
- - 1 0 2
,tO 1 ~
(3 1 0 2 D 3 0 4.0 5 0
Angle ('2e)
Fig. 1. X.RD pattern of a N i / A u multilayer with tr~i = 1.0 rim.
The inset shows Kiessig's interferences. The positions of reflec- tions diffracted by the crystallographic planes (111) of Au and Ni are identified by arrows.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 3 9 - 1
286 R. Krishnan et aL /Journal o3f Magnetism and Magnetic Materials 148 (1995) 285-286 Table 1
Some XRD data on Ni/Au multilayers
tNi IAu No. Quartz, Low-angle XRD, High-angle region
(nm) (nm) of bilayers A (nnl) A (nm) d (nm) A (nm)
0.64 1.5 25 2.14 2.11 5:0.01 " 0.248 5:0.001 2.10 5:0.01
1.0 1.5 25 2.5 2.49 5:0.01 0.240 5:0.001 2.48 + 0.O1
2.7 1.5 10 4.2 4.26 5:0.01
1 0 i i , i i a ,
'-~,, . tNj [nnn]: ' F "
" o • 0.6<. . / / ".
E 5 " - - - - 1 . 0 S F = 0 . . . 3 7 6 / / . _ "
. - . - - 2 . 7 S F = 0 . 2 3 2 ~'~..~. .
°
. o b u l k N i S F = O .
2
o " ~ ' ~ " - - ' ~ ' ~ - - ~ / t A = = 1 . 5 n m
Q .
- - 3 ~ i i I I I I t
2 3 dl. 5
P h o t o n e n e r g y [ e V ]
Fig. 2. Polar Kerr rotation spectra for three different l N i , The PKR spectrum of Ni is included for comparison. SF are the reduction factors used.
roto¢. .
o
"" "; , - • GD
~ - 1 0
! i I I | •
2 3 4 5
Photon energy l e V I
Fig. 3. Polar Kerr effect spectra in Ni/Au sample with I- m = 0.64 nm. Solid lines are calculated from the optical data of bulk metals;
the dashed lines show the effect of Au plasma edge shift
i i
O
.=_ 4
"~ 2
E
- 2
2 3 4. 5
Photon energy leVI
Fig, 4. Real ( c 1) and imaginary (62) part of permittivity in gold.
Full curves, original data; broken curves, modified data.
the r o o m telnperature, the amplitudes are further reduced.
The peak is better pronounced at lower tNi/IA, ratios. In this case the shapes o f P K R and PKE approach those o f the reflectivity phase and amplitude in gold, respectively [7].
T h e spectra are explained by the theory [8] using the optical [7,9] and M O [10] constants o f the balk metals. Fig.
3 shows the P K R and P K E spectra f o r the sample with tNi ---- 0.64 rum compared with the theory. The 0ff-diagonal permittivity tensor element o f Ni was reduced to 15.5% to account for the reduced magnetization [1]. Although the agreement between the experiment and theory is satisfac- tory in high-quality A u ( 5 n m ) / C o / A u ( 2 5 nmt) the experi- ment can be better reproduced [7]. This indicates that the optical constants o f A u change when tA, approaches a few atomic mon¢layers. The same conclusion can be drawn from the results on M B E - g r o w n A n / C o multila.yers [6].
To illustrate this effect w e have changed the A u permit- tivity near 2.45 e V without affecting the Kram~rs-Kronig compatibility (Fig. 4-). T h e computed P K R and P K E spec- tra for tNi --- 0.64 n m based on the modified A.-u data are included in Fig. 3. A n explanation o f the changes in the A u spectra ~ h e n the A u thickness is reduced d o w n to a few atomic monolayers should consider the effects o f electronic interactions at N i - A u interfaces, the strains due N i - A u lattice mismatch, and defects on an atoxaic scale.
Ack~owledgeraents: This work was partially sponsored by G A C R 2 0 2 / 9 3 / 2 4 2 7 . The parti~,| support f r o m B R I T E E U R A M (BREff 0153) and Konstruktis Praha Ltd is grate- fully aekno~,,ledged. The authors thank R. Lopu~n[k and R.
Urban for their skillful experimental assistance.
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[8]
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[lO]
