Vector magnetometry with polarized neutron reflectometry with spin analysis

Physica B 241 243 (1998) 1055 1059

ELSEVIER

Vector magnetometry with polarized neutron reflectometry with spin analysis C. Fermon a'*, S. Gray a, G. Legoff a, V. Mathet b, S. Mathieu a, F. Ott a, M. Viret ~, P.

Warin

a

~SerHce de Physique de I'Etat Condense, CEA Saclav, 91191 Gi[=sur-Yvette Cedex, France b lnstitut d'Elecownique l~ndamentale, URA CNRS 022, Unirersitk Paris-sud, 91405 Orsav Cede~, France

Abstract

The possibility of selective vector magnetometry by polarized neutron reflectometry is investigated. Experimental studies of samples with one and two cobalt layers are presented. Magnetisations of individual layers are measured independently and individual hysteresis cycles are inferred. ( 1998 Elsevier Science B.V. All rights reserved. Kevwords: Polarised neutron reflectometry; Magnetic thin films

1. Introduction

Magnetic thin films are now largely produced for fundamental studies and technological applications. Following the discovery of giant magnetoresistance in antiferromagnetically coupled multilayers [1], there has been extensive interest in the precise measurement of the magnetic moment directions in each layer and at the interface between the layers. Owing to the large magnetic coupling between the neutron and the magnetic moment, neutron diffraction is a powerful tool for obtaining information about magnetic configurations. Polarized neutron reflectometry (PNR) has been used for several years [2,3] to investigate such problems. PNR with polarization analysis (PNRPA) has pro* Corresponding author.

ved to be a unique tool to probe in-depth vectorial magnetic profiles [-4-6]. Here, it is shown that it is also possible to use PNR to measure hysteresis loops on magnetic thin films. Compared to other more classical techniques for measuring magnetic moments, PNR can be useful in several particular cases. The first point is its sensitivity which compares easily with the most sensitive techniques (SQUID or optical magnetometry). For example, it is possible to measure the magnetic moment of one atomic layer. It is self-calibrating since the measurements give an absolute value for the moments in laB per atom irrespectively of the size of the sample. Among other qualities, it is possible to study buried layers in complex systems. The technique is also insensitive to the magnetic state of the substrate since only thin layers are probed.

0921-4526/98/$19.00 ~: 1998 Elsevier Science B.V. All rights reserved Pl! S092 1-4526(97)008521

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('. k'ernlon e! al. , Ptn,,sica B 241 243 (199~) 1055

Here, we investigate the possibility of doing selective hysteresis loops for a sample containing different magnetic layers.

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1.E+06 I.E~5 I.FA-04 I.E~3

2. G e n e r a l

principles l.E~2

The theory of P N R P A can be found in Ref. [4] and only the basic principles are given here. A reflectivity curve consists of the measurement of the intensity reflected by a film as a function of the neutron diffusion vector q = (4~/2) sin(0) where 0 is the incident and reflected angle (0, 20 geometry). In PNRPA, the intensities (called R ++, R ,R+ and R -) are measured for the different states of polarization for the incident and reflected neutrons. For q smaller than a critical value, the beam is totally reflected while for larger q the intensity decreases as the 4th power of q but presents oscillations due to the difference of indices which are related to the composition and the magnetism of the different layers. Roughly R ~ ~ and R depends on the chemical and the magnetic profile component along the magnetic field. At reasonably small fields R + = R ~ and depends mainly on the inplane magnetism perpendicular to the external magnetic field. The neutron is sensitive only to the in-plane components of magnetisation. From the measurement of reflectivity curves for the different polarizations of incident and reflected beams, it is possible to obtain the chemical profile and the in-plane magnetization vector as a function of the distance from the surface. Reflectivity curves cannot be inverted to give real space profiles because the phase of the reflection is not known. Chemical and magnetic profiles are obtained by simulation and fitting of the experimental curves. The accuracy of the profile is roughly given by d = 2rc/q .... where q .... is the maximum of the diffusion vector. As the reflected signal decreases as 1/q 4. the lack of intensity usually limits the experiments to q .... ~0.2/~ ~. However, by choosing correctly the different thicknesses of the layers of the thin film, the magnetization of a single monolayer can be measured. For example, Fig. 1 shows the reflectivity of a tri-layel: Au(3.5 nm)/ Co(5nm)/Au(6.5nm) deposited on float glass (sample 1) measured in a saturated state. Fig. 2

I.E~I I 0.05

1.E+(~)

I 0.1

t 0.15

q(,~-l)

0.2

Fig. I. Rcflectivity curves for s a m p l e I: R ' " whilc squares. R black squares. The c o n t i n u o u s lines are the fits.

I.E-R)6TI

Intensity

1 .E+05

1.E+04

1.E+03 1.E+02 -

q(A-1) I.E+01 0

I

I

I

I

I

I

I

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Vig. 2. Reflectivity curves for s a m p l e 2: R ~ " white squarcs. R black squares. The c o n t i n u o u s lines are the fits.

shows the reflectivity curves obtained for a more complicated system (called sample 2) Pt/Co/ Pt/Co/Pt on silicon oxide grown at IEF, Orsay. Fig. 2 has been obtained with a 1 tesla in plane applied field. The measurements have been carried out on the reflectometer PADA of the reactor Orph6e, Laboratoire Leon Brillouin, Saclay, France.

3. H y s t e r e s i s

loops

A complete set of measurement of reflectivity curves (R ++,R and R + ) takes about 12h. Studying the magnetization of different layers as a function of field or temperature would take far too long considering the allocated beam time on

C F~,rmon et al. / Physica B 241 243 (1998) 1055 1059

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a neutron reflectometer (typically one week per year). So the idea is the following: from the fitting of the reflectivity curves in the saturated state we get the different thicknesses, compositions and roughness. X-ray reflectometry can also be performed to increase the accuracy for small thicknesses. We are then able to calculate the reflectivity curves for different module and orientation of the magnetic layers. It is then not necessary to perform complete reflectivity measurements for each value of the field but it is sufficient to measure only the reflectivity for 01 + 1) well chosen q values where n is the number of different magnetic layers. For example, a set of 3 measurements on sample 2 takes only 20 min at each field. Comparison between the experimental values and the calculation using the parameters obtained from the saturated state allows us to deduce the magnetic evolution as a function of the applied external field.

rotation from reduction of the moments. Furthermore, even in the absence of moment rotation, the use of spin analysis gives much more precise results. Matricial corrections of the reflectivities are performed on each reflectivity curve.

4. Experimental setup

M- ~ \ /; R ~ + -

In order to perform hysteresis loops with an in-plane field, it is necessary to take care of neutron spin depolarization as the field changes. The best configuration is given in Fig. 3. The beam is polarized along the z axis. The magnetic field is determined in order to present an almost zero field on the sample. A coil produces a main longitudinal field on the sample, so that the neutron spin adiabatically in the field, even for very large fields. This configuration allows for the continuous measurement of the reflectivity for either negative or positive fields. During the different experiments, flipping ratios (I ~ +1/1 + and I + +1/1 + in the direct beaml reach about 30. The continuous use of spin analysis allows us to separate the effect of in-plane

5. Case of a single magnetic layer We treat first the case of a single homogeneous magnetic layer aligned with the external magnetic field chosen along an in-plane z axis (case of a film without in-plane anisotropy). The difference between R ++ and R is directly linked to the moment of the film. Usually, quantities such as the flipping ratio ( R + + / R -) or spin assymetry (R++-R -)/(R +~ + R ) are used but these two functions are not proportional to the moment. For very high incident angles, the moment obtained in the Born approximation ,,// R

(1)

gives a more meaningful value. In the case of very thin layers or layers with a very weak moment, the magnetization is just a perturbation of the non magnetic reflectivity and relation (1) is almost correct for all the angles. For high moments near the reflectivity plateau, that assumption is clearly wrong. Fig. 4 gives the value of M, obtained from Eq. (1) as a function of the moment

0.6 0.4 0.2 0 -0.2 -0.4

-0.6 -0.8 0 Fig. 3 Setup of the experiment, a: the polarizer, the a r r o w s give the direction of the m a g n e t i c field, b: the a n a l y s e r and the counter. In the center, s u p e r c o n d u c t i n g coils can create a large h o r i z o n t a l field on the sample. The vertical c o m p o n e n t of the m a g n e t i c field is adjusted to vanish on the sample.

Fig. 4. Theoretical

0.5 values

1 of

x:R + '

1.5 vR

2 , "R' ' +

x/'R for an Au(5 nm)/Co/Au(5 nm) as a function of the Co m o m e n t given in l~u per atom. Black squares: Co(5 nm) at q = ( 0 . 3 l, white squares C o ( 5 0 n m ) at q = ( 0 . 3 ) and stars: Co(50 nm) at q - 1.3 .

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C. Fermon et al. ,, P&'sica B 241 243 (1998) 1055 1059

I.E+~

1,5

0 -0,5

+ u.a

.

.

.

.

.

.

.

.

.

.

.

.

:

1.E+06 ~-

1

0,5

7-

I.EMI7

~

~eidl(mT)

1.FM)5 I I.E+04 1.E+03

-1

1.E+02

-1,5

1.E+O1

-2

l.E+00

-2,5 -3 Fig. 5. Hysteresis loop obtained from R + ' and R . Black squares deduced from measurements at 0.3 , white squares at 0.45. The continuous line represents the Kerr loop measured for nearly the same orientation of the sample.

[ 0.5

0

I 1

I 1.5

I 2

I

2.5Nmd

I 3

Fig. 6. X-ray reflectivity curve for sample 2: upper curve measured at 1.54 A. lower curve: theoretical fit.

2 1.5

M set for the simulation in different cases. For high values ofq (q larger than 3 times the critical q valuel Eq. (1) becomes more and more valid. As an example, hysteresis loops obtained from the reflectivity at 0.3 and 0.45 are plotted in Fig. 5 together with Kerr loop. The Eq. (11 is correct for this sample at these angles.

6. Case of multiple magnetic layers Spin valves can be easily studied by this method but we present here the case of a complex situation. Sample 2 contains two different magnetic layers of cobalt, the first one, is 8 ~, thick and the second is only 4 ~, thick. This is a rather limiting case regarding the present flux intensities available. The corresponding thicknesses are too small to be precisely determined by neutron measurements. In order to measure them, X-rays reflectivity has been performed and lead to the curve of Fig. 6. The thicknesses found are: substrate/Pt(98 At/Co(7.6 *)/ Pt(33 k,i/Co(3.8 ~,I/'Pt(32 A) with a roughness of about 4 k,. The 3.8/~ layer appears to be made of 80% of cobalt and 20% of platinum. Fig. 2 shows the curves obtained by neutron reflectivity. The thicknesses given by the best fit for neutrons agree very well with the data obtained from X-rays except for the top layer which is 3 8 ' thick for neutrons.

1 0.5 t

0

0

5

Kgauss 10

Vig. 7. R " R at 0.3 {white squaresk 0.6 (black Iozengesl and 11.9 (black squares) as a function of in-plane external field.

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

5

10

Fig. 8. Moments in lq~ per atom deduced from the data of Fig. 7. Black squares correspond to the thick layer and white squares are for the thin one.

The moments obtained for an in-plane field of 1 Tesla are 1.57t~R for the thick cobalt layer and 1.3p, for the thin one. We have measured the reflected intensity at 3 different angles 0.3", 0.6 and 0.9. Fig. 7 shows

C. Fermon et al. / Phvsica B 241 243 (1998) 1055 1050 - as a function of the magnetic field. In this case, simulations show that this ratio is slightly more sensitive to the magnetic moments than given in Eq. (1). For each field, we have then fitted these three values with the parameters obtained from the result at 1 Tesla. The only parameters left free are the moments of each layer. The result is given in Fig. 8. R + +/R-

7. Conclusions We have demonstrated that selective hysteresis loops can be inferred from spin polarised neutron reflectivity measurements. Individual magnetic layers as thin as 4 A have been measured. Measurements at fixed angles coupled with theoretical simulations allow to plot the complete magnetic evolution of single layers within one day. Use of the spin flip reflectivities to describe vectorial magnetic evolution was not relevant here, but other measure-

1059

ments on different systems which present in-plane rotations will be published elsewhere.

Acknowledgements We wish to acknowledge funding from the European RTD contract XENNI (No. ERBFMGECT950005).

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