(anti-)ferromagnetic coupling in fe/si multilayers from polarized neutron reflectometry

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ELSEVIER Physica B 234-236 (1997) 498-499 (Anti-)ferromagnetic coupling in Fe/Si multilayers from polarized neutron reflectometry H. Fredrikze a'*, A. van der Graaf a, M. Valkier a, J. Kohlhepp b'c, F.J.A. den Broeder c alnterfacultair Reactor lnstituut, TU Delft, 2629 JB Delft, The Netherlands bDepartment of Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands ePhilips Research Laboratories, 5655 AA Eindhoven, The Netherlands Abstract Polarized neutron reflectometry data on Fe/Si multilayers are interpreted using strongly depth-dependent magnetization in the Fe layers. This behaviour is ascribed to a depth-dependent mixture of ferromagnetic and anti-ferromagnetic coupled regions in the sample. Keywords: Magnetic multilayers; Polarized neutrons; Reflectometry; Thin films 1. Introduction Magneto-optical Kerr effect measurements show very different hysteresis curves on the top and the bottom side of Fe/Si multilayers sputtered on a glass substrate [1]. To obtain information on the magnetization as a function of depth we did neutron reflection experiments with incident neutron spin parallel and anti-parallel to the applied magnetic field on two Fe/Si multilayer samples (20 bilayers with nominal Fe thickness 3.0 nm and Si thicknesses 1.1 or 1.4 nm, sputtered in argon of 1 Pa on oxidized Si) in mag- netic fields up to ~0.8 T on ROG at IRI [2]. Specular reflection of polarized neutrons [3,4] contains infor- mation on the (magnetic) neutron scattering length density F(z) as a function of depth, defined as F±(z) = 4n(n(z)bn(z)) 4- CMII(Z), * Corresponding author. where n(z) is the atomic number density, bn(z) the nuclear scattering length, C = 3.657 × 109 A -1 m -1 and (+) or (-) applies for neutron spin of the incident beam (anti)-parallel to the in-plane magneti- zation MII(z) of the sample. 2. Results and discussion All data show an 'AFM' (half-order) Bragg-peak (much broader than corresponding to a fully coherent AFM order), that decreases in intensity with increas- ing applied field, which indicates a decrease of AFM alignment of the Fe layers with applied field. Taking into account instrumental resolution and polarization of the beam, we made fits to the data con- sistently (Z2 ~ 1.2) using the matrix method [5] for all magnetic states simultaneously, i.e. for each sample we used one nuclear profile (Fn) and a strongly depth- dependent magnetization, MII(z), shown in Fig. 1. The bilayer thickness obtained (3.4 and 3.6 nm for the 1.1 and 1.4 nm Si sample) is, within the accuracy 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S0921-4526(96)01023-X

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Page 1: (Anti-)ferromagnetic coupling in Fe/Si multilayers from polarized neutron reflectometry

ELSEVIER Physica B 234-236 (1997) 498-499

(Anti-)ferromagnetic coupling in Fe/Si multilayers from polarized neutron reflectometry

H. Fredr ikze a'*, A. van der G r a a f a, M. Va lk ie r a, J. K o h l h e p p b'c, F.J .A. den Broeder c

alnterfacultair Reactor lnstituut, TU Delft, 2629 JB Delft, The Netherlands bDepartment of Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

ePhilips Research Laboratories, 5655 AA Eindhoven, The Netherlands

Abstract

Polarized neutron reflectometry data on Fe/Si multilayers are interpreted using strongly depth-dependent magnetization in the Fe layers. This behaviour is ascribed to a depth-dependent mixture of ferromagnetic and anti-ferromagnetic coupled regions in the sample.

Keywords: Magnetic multilayers; Polarized neutrons; Reflectometry; Thin films

1. Introduction

Magneto-optical Kerr effect measurements show very different hysteresis curves on the top and the bottom side of Fe/Si multilayers sputtered on a glass substrate [1].

To obtain information on the magnetization as a function of depth we did neutron reflection experiments with incident neutron spin parallel and anti-parallel to the applied magnetic field on two Fe/Si multilayer samples (20 bilayers with nominal Fe thickness 3.0 nm and Si thicknesses 1.1 or 1.4 nm, sputtered in argon of 1 Pa on oxidized Si) in mag- netic fields up to ~0.8 T on ROG at IRI [2]. Specular reflection of polarized neutrons [3,4] contains infor- mation on the (magnetic) neutron scattering length density F(z) as a function of depth, defined as

F±(z) = 4n(n(z)bn(z)) 4- CMII(Z),

* Corresponding author.

where n(z) is the atomic number density, bn(z) the nuclear scattering length, C = 3.657 × 109 A -1 m -1 and (+) or ( - ) applies for neutron spin of the incident beam (anti)-parallel to the in-plane magneti- zation MII (z) of the sample.

2. Results and discussion

All data show an 'AFM' (half-order) Bragg-peak (much broader than corresponding to a fully coherent AFM order), that decreases in intensity with increas- ing applied field, which indicates a decrease of AFM alignment of the Fe layers with applied field.

Taking into account instrumental resolution and polarization of the beam, we made fits to the data con- sistently (Z 2 ~ 1.2) using the matrix method [5] for all magnetic states simultaneously, i.e. for each sample we used one nuclear profile (Fn) and a strongly depth- dependent magnetization, MII (z), shown in Fig. 1.

The bilayer thickness obtained (3.4 and 3.6 nm for the 1.1 and 1.4 nm Si sample) is, within the accuracy

0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S0921-4526(96)01023-X

Page 2: (Anti-)ferromagnetic coupling in Fe/Si multilayers from polarized neutron reflectometry

H. Fredrikze et al./ Physica B 234~36 (1997) 498-499 499

2O0O

I 1.1 nm Si/0.10 T -2000 0 20 40 =-- - - - - ' ~ 80

-1000 :~ [ _ _ 1.1 n~__ssi/o.79 r

-2000 0 20 40 60 80

~ " -1000

-2000 0 20 40 60

2OOO

-2000 0 20 40 60

depth ( n m )

Fig. 1. Magnetization from fits to polarized neutron reflection data on 20 x (3.0 nmFe and 1.1/1.4 nmSi) multilayers at the indicated applied magnetic fields.

of about 5%, in agreement with SXRD results (3.51 -4- 0.05 and 3.72 + 0.05 nm). The reduction compared to the nominal thickness (4.1 and 4.4 nm) is due to dif- fusion of Fe into Si. The coherence length perpendic- ular to the surface (~ 20 nm from high-angle XRD) suggests a CsCl-structured (metallic) FeSi spacer, matching the BCC lattice spacing of iron. This implies that for the spacer Fn = 7.3 x 10 -3 nm -2 is expected and for the Fe layers 1.0 x 10 -2. The values we ob- tained from the fits (7.0 x 10 -3 nm -2 for the spacer and 9.6 x 10 -3 nm -2 for Fe) agree well with the ex- pectation. Also the integrated magnetization from the fits agrees with the hysteresis curves. The magnetiza- tion in the Fe layers anti-parallel to the applied field is larger for the thicker sample. In the thinner sample the negative component of F,n is smaller.

Instead of assuming a (strongly temperature- dependent) biquadratic coupling (used in Ref. [6] to explain the temperature dependence of hysteresis loops of Fe/Si multilayers), we suggest that these re- suits can be explained by a depth-dependent concen- tration of magnetic bridges or 'pinholes' consisting of chains of Si or Fe atoms through the spacer layer, which results in FM- and AFM coupled regions. Ac- cording to Kohlhepp et al. [1] the fraction of AFM coupling increases with the number of bilayers, prob- ably because the spacer layers more to the top of the sample contain fewer 'pinholes', which yields a cou- pling mechanism that varies with depth. Obviously, the thinner spacer has relatively more 'pinholes' than the thicker sample. This is the origin of the differences observed in the magnetization in the Fe layers as a function nf field for the two samples: a larger fraction of FM-coupling and a smaller negative component of Fm in the thinner sample than in the thicker one. The temperature dependence of the hysteresis curves is due to paramagnetic 'pinholes' in the spacer that are ferromagnetic below To.

More sophisticated PNR experiments than the present ones, including also polarization analysis of the reflected neutrons, might shed more light on the question whether or not biquadratic coupling exists in Fe/Si multilayers.

References

[1] J. Kohlhepp, M. Valkier, A. van der Graaf and F.J.A. den Broeder, J. Magn. Magn. Mater. (1996), in press.

[2] V.-O. de Haan, J. de Blois, P. van der Ende, H. Fredrikze, A. van der Graaf, M.N. Schipper, A.A. van Well and J. van der Zanden, Nucl. Instr. Meth. Phys. Res. A 362 (1995) 434.

[3] J. Penfold and R.K. Thomas, J. Phys.: Condens. Matter 2 (1990) 1369.

[4] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb and G. Ostrowski, Rev. Sci. Instr. 58 (1987) 609.

[5] J. Lekner, Theory of Reflection of Electromagnetic and Particle Waves (Martinus Nijhoff, Dordrecht, 1987).

[6] E.E. Fullerton and S.D. Bader, Phys. Rev. B 53 (1996) 5112.