Fe1-xCox multilayers studied by Mössbauer spectroscopy

Journal of Magnetism and Magnetic Materials 126 (1993) 248-250 North-Holland

Local magnetic properties of Pt/Fel_xCO x multilayers studied by M6ssbauer spectroscopy R.A. Brand, Th. von Schwartzenberg, O. Bohn6 and W. Keune Laboratorium fiir Angewandte Physik, Universitiit Duisburg, 47048 Duisburg, Germany Pt(deO/Fel_xCo~(dF~ce) multilayers were prepared by UHV evaporation for thicknesses dpt = 10 and 20 A, and dwc o = 2 A (x = 0) 2.5 A (x = 0.3-0.9) 5 ,~ (x = 0-0.99) and 10 A. (x = 0.3) and studied by MSssbauer spectroscopy between T = 4.2 and room temperature. Perpendicular magnetic anisotropy was found to increase for increasing x and decreasing dF~co. The 5VFe hyperfine field and isomer lineshift show strong deviations from bulk behaviour, depending on x and dFeCo. 1. Introduction The P t / C o multilayer (ML) system [1] has been shown to exhibit a magnetic easy axis perpendicular to the film surface. Many studies on this and similar M L systems [1-7] report conventional bulk magnetic and surface magneto-optical measurements on MLs prepared by sputtering. Only very few [8,9] studies have been reported so far on P t / C o ML grown by evaporation. We have applied the 5VFe MSssbauer effect (ME) as a local method to study the properties of the multilayer system P t / F e l _ x C o x for first time. 57Fe M E results [10-12] on bulk alloys indicate that the hyperfine field Bhf as a function of x behaves similarly to the Slater-Pauling curve for the spontaneous magnetization, showing a maximum at x = 0.3 [13]. Together with the perpendicular magnetic anisotropy (PMA), this could make these multilayer alloy films interesting for potential magneto-optical applications. We have also used U H V evaporation techniques to achieve a cleaner, defect-free and simpler film structure [6,7]. 2. Experimental We have prepared a series of P t / F e I xCox M L alloy systems on polyimid foil for M E spectroscopy and on Si wafers for small and large-angle X-ray diffraction. Some samples were prepared on NaCI single crystals for transmission electron microscopy (TEM). They were prepared in ultrahigh vacuum ( U H V ) by alternating evaporation of Pt and simultaneous evaporation of 57Fe and Co. The base pressure during evapo-

Correspondence to." Professor W. Keune, Universitht Duisburg, Laboratorium fiir Angewandte Physik, Lotharstrasse 1-21, 47048 Duisburg, Germany. Fax: + 49-203-379-3163.

ration was kept well below 10 s mbar. To reduce intermixing, the substrate temperature was held below 180 K during evaporation. The F e - C o layers were deposited at 0.1 , ~ / s and the Pt layers at 1.0 A,/s.oThe Pt layer thickness det was held at 10 or 20 A. A complete series from pure 57Fe (x = 0) to 57Fe-dopcd Co (x = 0.99) was made at the F e - C o layer thickness d w c o = 5 ,~. Other samples were prepared from 2.5 to 20 A.

3. Results The X-ray patterns show Pt Bragg peaks with very strong (111) texture with the correct lattice constant. No peaks of the FeCo alloy could be found. The T E M images show a very fine-grained Pt with (111) texture, and no other phases. These MLs have been studied from T = 4.2 K to room temperature using 57Fe-ME spectroscopy. The spectra were fitted using a histogram distribution of hyperfine fields Bhf. We have calculated the field (Bhf), averaged over the distribution P(Bhf), the standard deviation o-B, the averagc isomer line shift ( 6 ) (relative to a-Fe at RT), the average angle ( O ) (see below), where 6) is the angle between the sample normal and the hyperfine field. It was found necessary to include a linear change in isomer lineshift with hyperfine field given as ~6/~)Bhf. The temperature dependence of (Bhf) (T 3/z) is given in fig. 1 for the samples of the x = 0.3 series. (Bhf) decreases faster than would be expected for bulk bcc Fe. The low temperature behaviour shows a three-dimensional spin-wave-type dependence of (Bhf)(0)(l - - b T 3/2) with values of b much larger than the value b = 5.5 × 10 6 K 3/2 [14] for bulk bcc Fc. For x = 0 (dye = 2 ,~), the spectra for T above about 100 K are reminiscent of superparamagnetic relaxation, making conclusions on both b and the critical

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)

R.A. Brand et al. / Local magnetic properties of Pt / Fe l xCO~ multilayers

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H a m d e h et al. [12] for bulk samples at T = 77 K. W e observe a very large d e c r e a s e in ( B h f ) for the thin films c o m p a r e d to bulk behavior. Note that the results n e a r x = 0 and x = 1 are much closer to bulk values. In fact, the x = 0, dvcco = 5 ,~ sample shows Bhf = 35.5 T, about 1.5 T above the bulk value at T = 4.2 K. This is an e n h a n c e m e n t of Bhf at t h e P t / F e interface. The M L data for d w c o = 2.5 and 5 A show a roughly linear behaviour b e t w e e n t h e s e two limits, with no pron o u n c e d maximum n e a r x = 0.3, as was the case for the bulk data. The spectra show incomplete p e r p e n d i c u l a r magnetic texture which increases strongly for decreasing dveco, and increasing x, shown in fig. 3. F r o m the line intensities we can calculate a typical angle ( O ) = arcsin((sinZfg)l/2). F r o m the results p r e s e n t e d in fig. 3, we see that (~9) decreases, and so the P M A increases, with increasing x. For most samples, the average isomer line shift 6 ( T ) has b e e n fitted to a D e b y e model for the secondo r d e r D o p p l e r shift t~ts°D)(T), yielding the D e b y e tern-

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t e m p e r a t u r e difficult for this sample. The o t h e r samples do not show this behavior. The values of b and ( B h f ) (0) are given in table 1. ( B h f ) at T = 4 . 2 K for various x and d w c o are s u m m a r i z e d in fig. 2, and c o m p a r e d to the data of

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Table 1 Results for the fit to the spin wave expression for Bhf(T) at low temperature, and to the second-order Doppler shift for ~(T) x or metal

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250

R.A. Brand et al. / Local magnetic properties of Pt / Fe I x Co ~ multilayers

p e r a t u r e O D a n d the chemical lineshifl 60. T h e results are given in table 1 (the bulk results are from ref. [15]). Thus as the Fe layer b e c o m e s thinner, 0 0 of the Fe layer evolves from that of bulk Fe, to that of bulk Pt. T h e values of O6/OBhf f o u n d for all samples were roughly of the o r d e r of - 0 . 0 2 m m s 1 T 1. W e can estimate this quantity which would be due to pressure changes in the film from the pressure data in the literature [16]: we o b t a i n a b o u t 0.14 m m s 1 T ~, showing that the b r o a d hyperfine distributions P ( B h f ) are not due to pressure gradients in the film. 4. Conclusions T h e main results of this work are as follows. (1) T h e m a g n e t i c transition in Pt(20 , ~ ) / F e ( 2 ,~) is r e d u c e d to below RT. T h e second o r d e r D o p p l e r shift yields a D e b y e t e m p e r a t u r e n e a r to t h a t of bulk Pt, r a t h e r t h a n bulk Fe. P M A is seen at low t e m p e r a t u r e in this pure P t / F e ML. (2) T h e Pt(20 ~ , ) / F e ( 5 ,~) M L shows conventional in-plane anisotropy at all t e m p e r a t u r e s , a n d a Tc above RT. Bhf shows a b r o a d distribution, not seen in the 2 ,~ ML. (3) T h e P t / ( F e C o ) alloy MLs show strongly r e d u c e d ( B h f ) at low t e m p e r a t u r e s c o m p a r e d with bulk values, deviating strongly from S l a t e r - P a u l i n g - t y p e behavior. P M A is seen to increase with X a n d decreasing dF~co. P ( B h f ) b e c o m e s n a r r o w e r with increasing Co. W e show that the isomer line shift and ( B h f ) is not explained by internal stresses in the Fe or (Fe, Co) films. T h u s the observed P M A is d u e to the c o m b i n e d effects of the b r o k e n symmetry at the P t / F e C o interface a n d the orbital m a g n e t i s m of Co, similar to the model of D a a l d e r o p [17]. It is known that in P t / C o multilayers, the Pt is polarized by the Co m o m e n t s [18], a n d it seems t h a t the orbital m o m e n t of the Co is also e n h a n c e d [19]. T h u s the strong s p i n - o r b i t scattering of the Pt plays an i m p o r t a n t role in the P M A of this system. This is also seen in the c o n t r i b u t i o n s by Kyuno a n d Dreyss6 at this meeting.

Acknowledgements: This work was p e r f o r m e d with the support of the D F G SFB 166 and the E U R A M project M A 1E-0081-C.

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