Pt multilayers

Journal of Magnetism and Magnetic Materials 132 (1994) 219-222

ELSEVIER

Ferromagnetic

resonance study of Co/Pt

iournal ot magnetism and magnetic materials

multilayers

Shl-Ming Zhou al*, Llang-Yao Chen a, Yl Su a, You-Hua Qlan a, Zhl-Hong Jlang b, De-Fang Shen b 0T D Lee Physrcs Laboratory and Department of Physics, Fudan Unrversrty, Shanghac, 200433, Chrna b Shanghar Instltute of Metallurgy, Chmese Academy of Sciences, Shanghai, 200050, China

(Received 19 August 1993, m revised form 20 October 1993)

Abstract In this paper the results of ferromagnetic resonance study of sputtered Co/Pt multllayers, with a fured Co layer thickness of 15 A, are presented For these multdayers with small Pt layer thickness, m addltlon to a uniform resonance mode, a spm wave resonance mode 1s found to exist, which confirms the existence of an mterlayer coupling between the nelghbormg Co layers Simultaneously, the effective magnetlzatlon ~ITM,, and the resonance hnewldth AH are found to change correlatively with varymg Pt layer thickness This phenomenon can be explained as a result of an interplay between the mterlayer coupling and the low-dlmenaonal effect

1. Introduction

The mterlayer coupling between ferromagnetic layers m multilayered and layered films contammg transition metals has been shown to have an Important mfluence on the magnetic and electronic properties m these systems [l-31 The alternative ferromagnetic and antiferromagnetic mterlayer couplmg with varying nonmagnetic layers, for instance, can induce an oscillatory and giant magneto-resistance On the other hand, the field and the temperature dependence of the magnetization 1s strongly dependent on the mterlayer couplmg [4] Ferromagnetic resonance (FMR) has become a powerful technique to study the magnetization and its dlstnbutlon, relaxation and long range couplmg, especially, mterlayer

* Correspondmg

in multilayers [5-91 Co/Pt multilayers have a potential to become the next generation magneto-optical storage medium In this paper the magnetic properties of Co/Pt multllayers are presented, as studied by FMR

coupling

author

2. Experimental

All samples were deposited on substrates of glass by dc sputtering with two targets of Co and Pt The background pressure was 10 X 10P6 Torr, Ar pressure 5 0 x low3 Torr The deposltlon rates $f Co and Pt were variable m the region of 2-3 A/s The layer thickness was controlled by sputtering ttme and power The Co layer thickness was 15 A and tbe Pt layer thickness d,, was 5, 10, 20, 25 and 40 A respectively The bllayer number of the first four and the last two samples 1s 100 and 50, respectively

0304~8853/94/$07 00 0 1994 Elsevler Science B V All rights reserved SSDI 0304-8853(93)E0622-J

results and discussions

220

S -M Zhou et al /Journal of Magnetwn and Magnetic Materials 132 (1994) 219-222

in

Fig 1 The ?bsorptlon derlvatlon d, /dH spectra of multllayers Co(15 A)/Pt, m the perpendicular and parallel geometries, a-d refer to dpt of 5, 10, 15 and 20 A

X-ray 8-28 diffraction was used to analyze the perlodlclty and the crystal structures It 1s found that the perlodlclty calculated from the low angle diffraction peaks agrees with the value of the designed value The platinum layer has fcc(ll1) texture structure and the cobalt layer has hcp(002) texture structure Fu-st order satellite peaks around the (111) diffraction peak are observed An electronic paramagnetic resonance spectrometer, model ER-200D-SRC, was employed to measure the FMR spectra with a frequency of 9 78 GHz at room temperature The field was first applied perpendicular to the film plane and then changed until It was parallel to the film plane In FMR experiments with perpendicular geometry, there 1s an intense resonance peak (main mode) for al10Co(15 &/Pt multllayers With dpt less than 20 A a weak peak 1s located m the low field side with the perpendicular geometry, m addition to the main mode, as shown from the typical FMR spectra m Fig 1 When the angle between the applied field and the film normal direction, OH increases from zero to 90” the mtense peak shifts towards the low field side but Its intensity does not change slgmflcantly It 1s obvlous that the mam mode 1s the usual uniform resonance mode In this process, however, the

weak peak also shifts towards the low field side and becomes small rapidly At a critical angle e HC less than 12” these weak peaks overlap with the intense resonance peak and disappears for 8, larger than 8,, These weak peaks are lmposslble to originate from the magnetization mhomogenelty, otherwise these weak ones will not dlsappear for all 8, between 0 to 90” These addltlonal ones are spm wave modes obviously Because of that only when an mterlayer coupling occurs to make the multllayer behave as a single layer film, spm wave resonances can be excited by microwave of X-band [lo] So the spm wave modes confirm the existence of the mterlayer couplmg between neighboring Co layers m multllayers It can be concluded that an mterlayer couglmg does exist in samples with dpt less than 20 A The Pt atoms m Co/Pt multllayers were suggested to be spin-polarized, as previously pointed by Suglmoto et al and us [l&12] By mtultlon, the spm-polanzatlon of nonmagnetic layers m multllayers should exist simultaneously when the mterlayer couphng occurs and the spin-polarized free electrons are responsible for the mterlayer couplmg m these multllayers In Fig 1 it can be found that with increasing dpt the resonance fields H, m the perpendicular geometry decreases and H,, m the parallel geometry increases respectively The Landau-Llfshltz equation of motion 1s used for the magnetic system 1 ah4 - - x y&- =MXH,,, Y

(1)

where H,, includes an applied magnetic field, a demagnetization field Hd = -47r1i4, and the amsotropy fields Since the Co layer has a hcp structure, we can assume a umaxlal amsotropy, the dispersion relation of the uniform mode for Co/Pt multilayers with the first and the second amsotropy constants K, and K, can be shown as follows [13] (w/y)2=

4rM,,

[ff,_ c0s(e-e,)+

H,,

x [H,,,

+ 4H,,(

c0s2e

s&20] COS(~

- e,)

-

4rM,,

sln22e - sm2e)]

cos

28

(2)

S -M Zhou et al /Journal

of Magnetwn and Magnetic Materials 132 (1994) 219-222

According to the eqmhbrmm equations of the magnetlzatlon MS we have the followmg equation

221

7oo------12

M,H,,, sm( 8 - 0”) + (K, - 2~M,2) sin 28 + 4K, sin28 cos 8 = 0,

(3)

where the effective magnetization 4rM,, = 4~rM, - 2K,/M,, the second order amsotropy field H, = K,/M,, 8 1s the angle between the magnetization vector MS of Co layer and the film normal direction The microwave angular frequency w = 2rf (f = 9 78 GHz), y =ge/2mc 1s gyromagnetic factor As 0,=8=0, Hres=HI, w/y = H,

-4~rM,,

.

I? \f

‘%. ‘-0

-8

./

sOoowd&-W Fig 3 The d,, dependence of the effective magnetlzatlon 4aM,, and the hnewldth AH m the parallel geometry

(4)

and as 8, = 8 = 90”, H,,, = H,,, (w/~)*=H,,x(H,,+~~M,~~-~HK~)

(5)

Knowmg the resonant fields H, and H,, the 8, dependence of the resonance field can be obtained by using Eqs (2-5) Ftr the samples with dpt of 15, 20, 25 and 40 A the effective magnetization 4~rM,, and H, can be evaluated to fit the calculated dependence with the expenmental results, as shown m Fig 2 The good agreement between the calculated and the experimental results may suggest that the assumption

8H(dW) Fig 2 The calculated and the measured results about the 0, fependence of the resonanct field for multdayers Co (15 A)/Pt with dpt of 20 and 25 A

of the umaxlal amsotropy m Co/Pt multilayers 1s reasonable It 1s found that for the four samples H, 1s at least one order smaller than 4rM,, and can be neglected For multilayers with dpt of 5 and 10 A, H, 1s beyond the top hmlt of our applied field, 14 kOe In order to model the calculated dependence with the measured results for these two samples, an approxlmatlon 1s made to let H, be zero In this case the calculated OH dependence of the resonance field still fits well with the experimental results and the 4nM,, can also be evaluated As displayed m Fig 3, the 4rM,, and the hnewldth AH m the parallel geometry of Co/Pt multilayers change correlatively With increasing d,, the effective magnetization decreases and the lmewldth increases respectively This phenomenon can be explained as a result of an interplay between the mterlayer couplmg and a low-dimensional effect In general, when the nonmagnetic layer becomes thick the mterlayer coupling between the magnetic layers will become weak The magnetic properties will change from three-dimensional to two-dimensional magnetic behavior with increasing nonmagnetic layer thlckness In this case both the magnetic moment per atom and the Curie temperature decrease, as predicted previously [4], which leads to the decrease of the room temperature saturation magnetization and the effective saturation magnetlzatlon accordingly with Increasing Pt layer thlck-

222

S -M Zhou et al /Journal

of Magnetwn and Magnetic Materials 132 (1994) 219-222

ness On the other hand, the lmewldth reflects the relaxation, which is related to the mhomogene@, defects, the interface diffusions, and especially, the mterlayer couphng m multllayers Because with increasing Pt layer thickness the mterlayer coupling decreases, the process of the magnetic moment m each magnetic layer ~111 become less uniform and the hnewldth increases accordmgly The field difference between the spin wave mode and the uniform one 1s found to increase, which can be explained by the fact that, as pointed by Stapele et al [lo], when the mterlayer couplmg between magnetic layers 1s very weak, the first and even the second order spm wave modes may merge with the uniform mode, as displayed by the curves b-c m Fig 1 The spin wave resonance peaks are also found to become weak, compared with the uniform resonance one with increasing dpt

Acknowledgement

This work was supported by State Education Commlsslon, National Science Foundation of China and the State Key Laboratory of Mag-

netism, Institute of Physics, Chinese Academy of Sciences References

[II

S S P Parkm, N More and K P Roche, Phys Rev Lett 64 (1990) 2304 Dl R Coehoorn, Phys Rev B44 (1991) 9331 t31 Z Celmskl and B Hemrlch, J Magn Magn Mater 99 (1991) L25 141Y H Lm, X D Mao and L M Mel, Phys Rev B45 (1992) 10459, 151 Z Zhang, P E Wlgen and S S P Parkm, J Appl Phys 69 (1991) 452 [61 S N Kaul and V Slrugurl, J Phys Conden Matter 4 (1992) 505 [71 G Venugopal Rao, C S Sunandana and A K Bhatnagar, J Phys Conden Matter 4 (1992) 1373 60 Bl R Krlshnan and M Tessler, Solid State Commun (1986) 637 [91 R Krlshnan, M Tarhoum, i%j Tessler, A Ganguler, J Appl Phys 53 (1982) 2243 9 and J W Smites, [lOI R P van Stapele, F J A M Gremdanus J Appl Phys 57 (1985) 1280 1111T Suglmoto, T Katayama, Y Suzuki and Y Nlshlhara, Jpn J Appl Phys 28 (1989) L2333 1121 S M Zhou, H R Zhal, J T Song and H Y Zhang, J Appl Phys 73 (1993) 986 1131C Chappert, K Le Dang, P Beauvdlam, H Hurdequmt and D Renard, Phys Rev B34 (1986) 3192