Journal of Magnetism and Magnetic Materials 156 (1996) 55-57

~ i ! Journal of magnetism and magnetic

J R materials ELSEVIER

Optimisation of neutron reflectometry in the measurement of magnetic multilayers by means of 'invisible' spacer layers

J.K. Hong 1, D. Spilsbury, N. Cowlam *, H.J. Hatton, M.R.J. Gibbs

Department t~f Physics and the Sheffield Centre for Adt'anced Magnetic Materials and Del'ices, Unil'ersity +~/' She~f'ield. She]field. $3 7RH, UK

Abstract The physical and magnetic structures of magnetic, metallic multilayer samples can be measured directly by neutron

reflectometry. The optimum choice of samples for neutron reflectometry is discussed and the concept of 'invisible' non-magnetic spacer layers is introduced.

Neutron reflectometry can be used to determine the physical and magnetic structures of multilayer samples from the periodic variation of refractive index normal to their surface. The refractive indices n+, for neutrons polarised parallel ( + ) and antiparallel ( - ) to a transverse magnetic field applied to a ferromagnetic sample are,

A ~ b + p n~ = 1

2~r V

where b / V and p ~ V are the nuclear and magnetic scat- tering length densities and A the neutron wavelength.

The neutron reflectivity profile R(Q) versus Q of a multilayer sample, contains Bragg peaks from whose posi- tions.

Q,, = 2rcn/d ,

its bilayer period d can be found. Q is the scattering vector, Q = 4~- sin 0/A. The neutron reflectivity profile of a multilayer consisting of the repetition of a ferromagnetic layer plus a non-magnetic layer [1], contains Bragg peaks of enhanced intensi O, if there is a parallel alignment between the ferromagnetic layers and additional Bragg peaks whose positions correspond to the new, double periodicity if there is an antiparallel alignment. The physi- cal and magnetic periodicities of multilayer samples, the total film thickness, the overall composition, the air-f i lm roughness, the interface roughness and the film-substrate roughness can all be obtained from the analysis of R(Q) versus Q from a single neutron experiment. However, the

Corresponding author. Fax: +44-114-272-8079; email: n.cowlam @ sheffield.ac.uk.

Present address: SAIT, P.O. Box I 11, Suwon 440-600, South Korea.

10 0

10-1

. - - 10 -4 .~_

~ ' to- '

10 - + ;

1 0 - 0

1 0 - 1

s

2

C

-?

_4

' ' ' ' I . . . . I . . . . I . . . . I . . . . I r

a)

i i I t i i • i . . . . i . . . . i . . . . t ,

2 3 4 5 6

!i!i~i~! ~ b )

! !il I Wavelength in ,~

c'

bilayer thickness in

Fig. 1. The simulated neutron reflectivity profiles of a Co:Ru multilayer are given for the CRISP instrument in (a). Histograms showing the variation of the total scattering length density through the bilayer of this sample are given for sharp (b) and relaxed (c) magnetisation densities.

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56 J.K. Hong et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 55-57

Table 1 Values of magnetic moment in /x B and scattering length density in l0 -6 ,~-2 for cobalt, iron and nickel

Element /z b~ V ( b + p ) / V ( b - p ) / V

Co 1.72 2.30 6.49 - 1.89 Fe 2.22 8.17 13.13 3.04 Ni 0.606 9.45 10.94 7.96

high cost of neutron facilities and the competition from X-ray reflectometry, provide strong incentives to optimise neutron reflectometry experiments.

The ferromagnetic layer in an optimum multilayer sam- ple can be iron, nickel or cobalt, which have quite different 'optical' characteristics in a neutron beam, see Table 1. Nickel and iron both have large values of b / V, but that of cobalt is much smaller, while iron and cobalt have large magnetic moments and p / V values, while that for nickel is much smaller. The non-magnetic spacer layer can be an element like copper, with a large value of b~ V = 6.55 x 10-6 ~ - 2 ; one with an average value like aluminium b / V = 2.98 X 10 -6 ~ - 2 ; a very small value like vana- dium b / V = - 0 . 2 7 x 10 -6 ~ - 2 ; or even an element like titanium b / V = - 1.99 X 10 -6 ~ - 2 or manganese b / V = - 3 . 0 4 x 10 -6 ,~-2, which scatter neutrons by a reso- nant process and have a negative value of b / V.

An optimised multilayer sample for neutron reflectome- try experiments is the one whose reflectivity profile is most sensitive to the details of its magnetic and physical structures. Theory, e.g. Ref. [2], predicts that the bilayer combination with the greatest discontinuity in the scatter- ing length density at the interfaces will be most suitable. However, a weakly scattering spacer layer may enhance the relative visibility of the magnetic layer, but reduce the

1.0 "6"

10-1

o 10-2 -

..~ 10-3 -

*~ 10 -4 _

10-5-

10-6

E I * I I

precision with which the reflectivity profile as a whole can be measured. In practice, optimum samples for neutron reflectometry can be identified by simulating their reflec- tivity profiles [3] using the computer codes based on optical matrix methods [4]. The upper curve in Fig. la gives the simulated neutron reflectivity profile of a Co:Ru multilayer sample consisting of 20 repeats of 20 ,~ of cobalt and 6 A of ruthenium, having an antiferromagnetic alignment of the cobalt layers [1]. It was calculated with an angle of glancing incidence of 4 °, to place the first Bragg peak at the centre of the profile and a 3% resolution of the CRISP reflectometer at ISIS. The variation of the total scattering length density through the bilayers of this sam- ple is given in the lower curve of Fig. lb. The reflectivity profile expected for a 'relaxation' of the magnetisation density at the magnetic:non-magnetic interfaces is also shown in Fig. la. Here an exponential variation of mag- netisation, M ( z ) = Moexp( - z / A ) has been approximated by a single, discrete, step shown in Fig. lc - although any number of steps can be specified in the optical matrix program. This small variation in the magnetic structure leads to significant changes throughout the reflectivity profile, which confirm the sensitivity of the neutron reflec- tometry method.

Simulations have also been made [3] using the so-called 'null-matrix' or 'zero-alloy' materials as the non-magnetic spacer layers. Such alloys are composed of an element with a positive value of b / V and one with a negative value of b / V, in the proportion to give (b / V ) = 0 and therefore have n---1 - the free space value. Multilayer samples incorporating these alloys are clearly optimum candidates for the study of magnetism in the low-dimen- sional limit by the reflectometry method. The magnetic material can be a small fraction of the sample, without the

I I ~ h t I I I

\

o o

o o

o o

o • *

i i ~ i i i , , i , ,

0.02 Scattering vector Q in/~-1 0.2

Fig. 2. The composite neutron reflectivity profile R(Q) versus Q (log scale) of a Fe:TiZr multilayer sample with 'invisible' spacer layers, obtained on the CRISP instrument is given.

J.K. Hong et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 55-57 57

usual penalty of having both the magnetic and the non- magnetic layers contributing to the reflectivity profile since the non-magnetic spacer layers are essentially 'invisible' to the neutron beam.

Neutron experiments have been made by us on CRISP with Fe:TiZr ( 'null-matrix') multilayers produced by UHV sputter deposition, in order to confirm the findings of these simulations. Fig. 2 shows a composite reflectivity profile R(Q) versus Q (obtained using three different values of glancing angle) for the first ever specimen examined by us. Five orders of well-defined Bragg peaks are observed down to values of R ( Q ) = 10 -5, from which it is con-

firmed that the sample consists of 25 repeats of 97 ~, of iron and 6 A of TiZr alloy. A detailed study of the physical and magnetic structures of a wider series of Fe:TiZr multilayers is currently under way.

References

[1] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304.

[2] J. Als-Neilsen, Current Topics in Physics 43 (1987) 181. [3] J.K. Hong, PhD Thesis, University of Sheffield, 1993. [4] J. Penfold, Rutherford Appleton Laboratory Report, RAL-88-

088, 1988.