Fen multilayers

H. Takahashi et al. / Magnetic properties of PdNi alloys and muhilayers 0.2,

,

i

471

fact, the magnetostriction of P d / N i (3.5 A/1.7.~l) MLs at room temperature is about one third of a P d / C o ML with a similar layer structure.

i

31 a t % Ni 3, 0.1

4. Conclusion )<

I

0

•

5

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an...

15

20

tNi ( R )

Fig. 3. Ni layer thickness tNi dependence of effective anisotropy times bilayer period K~jIA of Pd/Ni MLs with various average composition.

anisotropy is much smaller in P d / N i MLs than in P d / C o . A possible reason for this difference is as follows: With decreasing bilayer period, the magnetic properties of P d / N i MLs become similar to those of PdNi alloys, and the Curie temperature becomes lower. Then, in general, magnetostriction and magnetic anisotropy become smaller. In -0.20 o

-'-Pd(A)/Ni(A)=5.1/2.5 - - - 10.I/5.1 -o-19.3/9.4

-0.15

RF sputtered PdNi alloy films have uniaxial anisotropy with the easy axis perpendicular to the film plane. At high Ar pressure of 25reTort, a large K u up to 1 0 6 e r g / c m 3 is induced. The origin of the anisotropy is ascribed to the stress-magnetostriction mechanism. In P d / N i MLs with a short bilayer period of about 15 A, the easy axis becomes perpendicular to the film plane, and the estimated surface anisotropy is around 0.05 e r g / c m 2.

Acknowledgements This work was supported by Grant-in-Aid for Special Project Research from the Ministory of Education, Science and Culture of Japan. The authors are grateful to Mr. Adachi for EPMA measurement. The authors would like to thank Dr. S. Iwata, Dr. M. Nawate and Mr. K. Nakamura for valuable discussion.

--Pd63Ni37 ALLOY

v -0. I0

References -0.05 °

+0.05

e

200

400

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i

600

800

WAVELENGTH (nm ) Fig. 4. Kerr rotation spectra of Pd/Ni MLs with various bilayer period, where the average composition is about 37 at% Ni. Dotted curve is the spectrum of Pd~3Ni37 alloy film.

[1] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [2] H.J.G. Draaisma and W.J.M. de ,longe, J. Appl. Phys. 62 (1987) 3318. [3] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer and W. Hoving, Phys. Rev. Lett. 60 (1988) .2769. [4] S. Tsumashima, K. Nagase, K. Nakamura and S. Uchiyama, IEEE Trans. Magn. MAG-25 (1989) 3761. [5] S. Tsunashima, H. Takagi, K. Kamegaki, T. Fujii and S. Uchiyama, IEEE Trans. Magn. MAG-14 (1978) 844. [6] T. Tokunaga, M. Kohri, H. Kadomatsu and H. Fujiwara, J. Phys. Soc. Japan 50 (1981) 1411.

472

Journal of Magnetism and Magnetic Materials 93 (1991) 472 476 North-Holland

Structure and magnetism of NiJFe,, multilayers Nigel M. J e n n e t t a n d D.J. D i n g l e y tt.tt. Wills Physics Laboratory, 7~vndallAre., Bristol BS8 ITL, England

Ni,,/Fe,, multilayers have been grown, in Ultra High Vacuum (UHV) conditions, on 150nm C u ( l l l ) single crystals grown, in-situ, on mica(0001 ) substrates. Modulation wavelength ranged from 2n = 311 monolayers (ML) to 2n - 6 ML. X-ray diffractometry of samples showed satellite peaks corresponding to a range of modulations implying that the layering was incommensurate. Cross-sectional Transmission Electron Microscopy (TEM) confirmed layering persisted even at modulations of n = 3 ML. Plan view TEM electron diffraction showed a transition from fcc(l 11 )/fcc(l 11 ) to fcc(111 )/bcc(110) layering as n increased. The transition was not sharp but progressive with a midpoint at about n = 10ML. Room temperature measurements of saturated magnetic moment per w)lume, using a vibrating sample magnetometer, varied with modulation layer thickness. The moment per volume for the fcc material with a [111] plane spacing of 0.2059 nm (equivalent to a lattice parameter of 0.357 nm) was a factor of three less than for an homogeneous NiFe alloy. It is noted that this lattice parameter coincides with the range predicted by theorists fl)r a change in fcc Fe Fe magnetic coupling.

1. Introduction The increasing availability of U H V conditions has rekindled interest in the epitaxial deposition of metals and the fabrication of metal multilayers. Striking variations in elastic, electrical and magnetic properties have been observed to occur as a function of modulation wavelength [1-4]. Models of the mechanisms driving epitaxy have become more refined [5, 6] (reviews) and the growth of metals in metastable phases has become possible. Examples of this are bcc Ni on Fe(001) [7-9], fcc Fe on Cu(001) [11t-12] and fcc Fe on C u ( l l l ) and on C u A u ( l l l ) [13-15]. Our interest in the magnetic properties of Fe in an fcc lattice is that a low spin, weakly ferromagnetic (FM) state for fcc Fe with a lattice p a r a m e t e r less than about 11.358 nm [16] has been proposed. More recent theoretical work has suggested that fcc Fe with a lattice parameter around 6.6 atomic units is antiferromagnetically (AFM) coupled [17]. FM/AFM multilayers are of interest as a means of studying the exchange anisotropy effect which changes coercivity and shifts the B - H loop. Recent studies have used FeMn as the A F M component [18].

In this paper the structure of Ni,,/Fe,, multilayers is examined as a function of modulation layer thickness (n = number of monolayers per layer) and related to saturated magnetic moment per volume (M,,).

2. Experiment Mica sheets, precleaned by boiling in soap solution and then distilled water, were used as substrates. The substrate temperature (T~) was monitored by two c h r o m e l - a l u m e l thermocouples to a precision of 0.5°C. The substrate assembly was heated to 480°C for 2h prior to evaporation. A 150nm single crystal C u ( l l l ) backing layer was thermally evaporated onto the mica with Tg=313°C and annealed for 2h at 425°C. The Ni and Fe layers were evaporated, from electrostatically focussed electron beam evaporators, onto the Cu at 285°C. The base pressure, after baking the whole U H V system for 24h at 200°C, was in all cases better than 10 '~ mbar. During growth, the residual gas pressure was measured by mass spectrometer and was

(B04-8853/91/$113.50 .~c: 1991 -Elsevier Science Publishers B.V. (North-Holland)

N.M. Jennett, D.J. Dingle)' / Structure and magnetism o[ Ni ,, / Fe,, multilayers

less than 5 × 10 - s m b a r for the Cu evaporation and less than 5 × 10 '~ mbar for the evaporation of the Fe and Ni layers. Partial pressures of oxygen were not detected ( < 0.5 × 10 m m b a r ) and partial pressures of mass 28 species were an order of magnitude less than the total pressure. Evaporation rate and layer thickness were monitored in-situ by 6 M H z quartz crystal oscillators interfaced to a microcomputer. T h e c o m p u t e r recorded each layer thickness on floppy disc and controlled a m o t o r driven shutter. G r o w t h rates were 0.2 n m / s for the Cu and 0.1 n m / s or less for the Ni and Fe layers. T h e shuttering time was about 0.02 s and overall layer thickness precision was easily better than 0.01 nm. Pure evaporant materials were used ( < 20 p p m total metal impurity). T h e Fe used was ultra pure ( < 10 ppm total impurity including non-metals). The last layer was always Ni to protect the Fe from exposure to air. T h e multilayers were characterised by T E M in a Philips 3 0 0 k V EM430 and by X-ray diffraction in a double crystal Rigaku powder diffractometer interfaced to a microcomputer. For plan view T E M the foils were stripped from the mica and m o u n t e d in 100mesh Cu folding grids. For edge-on T E M , stripped foils were e m b e d d e d in electrodeposited Cu and spark cut into cross sectional slices. The slices were thinned until elec-

473

tron transparent by mechanical lapping, dimple polishing, A r ion milling and precision ion milling. Some X-ray m e a s u r e m e n t s were m a d e on samples stripped from the mica and glued onto glass formers and some were m a d e on material still on the mica. A Mo X-ray source was used which emitted a KA doublet plus KB lines. A 0.075 m m zirconium filter reduced the KB lines to 2% of the KA~ intensity. The angle step size was 0.01 ° (0.02 ° in 2theta). T h e bent graphite crystal m o n o c h r o m a t o r was m o u n t e d as the second crystal in the diffractometer. Magnetic m e a s u r e m e n t s were made using an LDJ9500 Vibrating Sample M a g n e t o m e t e r (VSM) interfaced to an I B M XT. Precision was range d e p e n d e n t and was 0 . 5 O e in H for the range usually employed. Uncertainty in absolute sample m o m e n t m e a s u r e m e n t was determined by the combined magnetic m o m e n t of the mica substrate, Cu backing layer and VSM sample holder. This was m e a s u r e d to be 7 × 1 0 - S e m u at 1 kOe applied field ( H ) .

3. Results The short modulation wavelength F e / N i multilayers grew as a totally fcc structure with the [111]

Ll11/1107

a Fig, 1. Plan view TEM diffraction micrographs imaged down the Cu and Ni[lll] axis. (a) From n = 15ML Ni,/Fe,, multilayer. Note the Fe bcc[110] axis is parallel to Ni[lll] with two orientations in plane; (b) from n - 3 ML Ni,,/Fe,, multilayer. Note the absence of bcc(110)Fe diffracted intensity.

N.M. Jennett. D.J. Dingier / Structure and magnetism 0[" Ni ,, / Fe ,, multilayers

474

CU KA1

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Fig. 2. X-ray rocking curve a b o u t the m u l t i l a y e r [222] axis for the n = 3 M L m u l t i l a y e r (125 layers) on mica. First o r d e r satellites are shown c o r r e s p o n d i n g to several n values, the most intense being from the n = 3 and 4 periodicities.

Fig. 3. T E M m i c r o g r a p h of e d g e - o n n = 5 M E m u l t i l a y e r i m a g e d in a (111) two b e a m condition.

IV.M. Jennett, D.J. Dingley / Structure and magnetism of Ni, / Fe, multilayers

direction perpendicular to the substrate plane. Large modulation wavelength multilayers grew as an f c c / b c c structure with the bcc Fe growing in the Kurdjumov-Sachs and N i s h y a m a Wassermann orientations with [110] perpendicular to the substrate plane (fig. 1). The transition was progressive with a midpoint at a modulation of n = 10 ML. X-ray scans showed a transition in average lattice p a r a m e t e r of the multilayer coinciding with the structural transition observed by plan view TEM. The width of the multilayer Bragg peak also changed with the transition. The f c c / b c c material had a broader peak and thus a range of planar spacings. The pure fcc material had a well defined planar spacing of 0.2059nm corresponding to a lattice p a r a m e t e r of 0.357nm. Modulation X-ray diffraction satellites were not observed for samples removed from the mica substrate. Samples had to be left on the mica to observe them. X-ray patterns for the n = 3 M L sample had modulation satellites corresponding to several n values (fig. 2). The layering was thus not

475

perfectly commensurate. Cross sectional T E M using a (111) two b e a m condition confirmed that the n = 3 and 5 M L samples were layered (fig. 3), so interracial mixing is limited to, at most, two monolayers. R o o m temperature magnetic measurements made on samples of approximately the same total multilayer thickness showed a progressive decrease in saturated magnetic moment per volume (My) with decreasing n. When the magnetic moment contribution of the Ni (calculated by assuming bulk values) was subtracted from the total sample M v, the bulk value for bcc Fe was obtained for samples with n = 15 ML. Applying this procedure to shorter modulation samples, the apparent contribution of the Fe to the sample M v falls until, for samples with n < 5ML, it is a factor of seven less than that of bulk bcc Fe (fig. 4). Samples of 5 0 - 1 0 0 n m pure Fe evaporated directly onto the Cu(111) backing layer had an M v of bulk bcc Fe.

4. Discussion and conclusions

EMU • Mtotal/vol 0 M t o t - M N i / V o l Fe

0

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--NiFe alloy

0 0

•

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100

S 4 s

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Fig. 4. G r a p h of sample M v vs. layer thickness. Overlaid by graph of Fe M v calculated by subtraction of Ni contribution to sample m o m e n t (assuming the bulk value for Ni moment).

The dependence of both structure and M v of the N i / F e multilayers on modulation wavelength is pronounced. There is a clear structural transition with increasing layer thickness concurrent with a progressive change in M v. An homogeneous 50:50 NiFe alloy (the approximate average composition of the multilayers) would have an M v greater than any of the multilayers reported here and a factor of three greater than M v for those with n < 5 ML. Cross sectional T E M shows that these samples are layered with little interracial mixing. The mixed success in observing X-ray satellites was probably due to the difficulty in maintaining very long scale flatness of the samples after removing the mica substrate. However, the X-ray scan of the n = 3 ML sample on mica does have satellite peaks, but they correspond to several modulation periods. Such a lack of commensurate layering has been shown by other workers [19] to have a large effect on satellite intensity.

476

N.M. Jennett, D.J. Dingh, y / Structure and magnetism ()f Ni ,, / Fe ,, multilayers

It is interesting to note that these remarkable changes in magnetic properties occur in the range of lattice parameter where theoretical calculations of Fe magnetic moment in an fcc lattice have predicted a collapse of the Fe moment and a change from FM to A F M coupling [17]. The measurements reported here do not reveal the nature of the magnetic coupling in the multilayers. Nevertheless, the observed variation in M v with modulation wavelength for Nin/Fe,, multilayers is extraordinary.

Acknowledgements The Science and Engineering Research Council provided for the part purchase of the U H V system and the subsistence of N.M. Jennett. We wish to thank IBM Hursley (UK) for the use of their VSM and G E C Research Ltd. for the use of their ion thinner.

References [1] B.J. Thaler, J.B. Ketterson and J.E. Hilliard, Phys. Rev. Lett. 41 (1978) 336.

[2] J.O. Zheng, J.B. Ketterson, C.M. Falco and I.K. Schuller, J. App. Phys. 53 (1982) 3150. [3] T. Tsakatakos and J.E. Hilliard, J. Appl. Phys. 54 (1983) 734. [4] P.J. Orozco, PhD Thesis, Bristol University UK (1988). [5] D.W. Pashley, MRS Proc. 37 (1985) 67. [6] U. Gradmanm Nato ASI Series B 163 (1987) 261. [7] B. Heinrich, A.S. Arrott, J.F. Cochran, C. Liu and K. Myrtle, J. Vac. Sci. Tech. A 4 (1986) 1376. [8] B. Heinrich, S.T. Purcell, J.R. Dutcher, K.B. Urquhart, J.F. Cochran and A.S. Arrott, Phys. Rev. B 38 (1988) 12879. [9] Z.Q. Wang, Y.S. Li, F. Jona and P.M. Marcus, Solid State Commun. 61 (1987) 623. [10] W.A. Jesser and J.W. Matthews, Phil. Mag. 15 (19¢~7) 1097, 17 (1968) 595. Also G.H. Olsen and W.A. Jesser, Acta Met. 19 (1971) 1009. [11] D.A. Steigerwald, I. Jacob and W.F. Egelhofl" Jr., Surface Sci. 202 (1988) 472. [12] O. Haase, Naturf. AI4 (1959) 92(I. [13] U. Gradmann, W. Kiimmerle and P. Tillmanns, Thin Solid Films 34 (1976) 249. [14] U. Gradmann and H.O. Isbert, J. Magn. Magn. Mat. 15-18 (1980) 1109. [15] R. Halbauer and U. Gonser, J. Magn. Magn. Mat. 35 (1983) 55. [16] R.J. Weiss, Proc. Phys. Soc. 82 (1963) 281. [17] F.J. Pinski, J. Staunton, B.L. Gyorffy, D.D. Johnson and G.M. Stocks, Phys. Rev. Lett. 56 (1986) 2096. [18] J.K. Howard and T.C. Huang, J. App. Phys. 64 (1988) 6118. [19] N.K. Flevaris, D. Baral, J.E. Hilliard and J.B. Ketterson, Appl. Phys. Lett. 38 (1981) 992.