polarized neutron reflectometry studies of magnetic oxidic fe3o4/nio and fe3o4/coo multilayers

ELSEVIER Physica B 221 (1996) 388-392

PHYSICA[

Polarized neutron reflectometry studies of magnetic oxidic Fe304/NiO and Fe304/CoO multilayers

A.R. Ball a, b, H. Fredrikze b, D.M. Lind ~, R.M. Wol f a, P.J.H. Bloemen d, M.Th. Rekveldt b, P.J. van der Zaag ~' *

a Philips Research Laboratories. Prq[; Holstlaan 4, 5656 AA Eindhoven, The Netherlands" b htterjacultair Reactor Instituut, Deljt Universi O' o f Technology, 2629 JB DelJ?, The Netherlands

c Deparmwnt oJ Physics, 315 Keen Building, Florida State University, Tallahassee. FL 32306-3016, USA d Deparmwnt q[' Physics, Eindhoven University o[' Technolo~ty (EUT), 5600 MB Eindhoven, The Netherlands

Abstract

The magnetic properties of [1 0 0] oriented Fe3Oa/NiO and Fe304/CoO multilayers, MBE-grown on MgO(0 0 l ) substrates, have been studied by polarized neutron reflectometry. In both samples, the Fe304 layer exhibits a depth-dependent magnetic profile characterized by a reduction in the magnetization near the interfaces. The possible origins of this behaviour, such as domain wall formation in the ferrimagnetic layer and deviations in stoichiometry, are discussed.

1. ln~oducfion

Interest in transition metal oxidic thin films has greatly increased recently, due to progress in the synthesis of high- quality single-crystalline thin films [1 3]. Furthermore, due to their chemical stability, such oxides have potential for industrial applications, making the study of their properties interesting. Among the properties studied is that of exchange biasing. When a ferromagnetic or ferrimagnetic (F) layer in contact with an antiferromagnetic (AF) layer is field-cooled through the AF Neel temperature, a shift in the magnetic hysteresis loop may be observed along the field axis. This shift is known as the exchange biasing field, HCb, and was discovered nearly 40 years ago in CoO-coated Co particles [4]. In the model of Meiklejohn and Bean [4], HCb can be qualitatively understood as the field value nec- essary to break the exchange coupling between interface spins on either side of a perfect, uncompensated F/AF interface.

When applied to examples such as Nis0Fe20/FeMn [5] or Fe304/CoO [6] bilayers, the above simple argument leads to larger H~b values than are actually observed. The

* Corresponding author.

0921-4526/96/S15.00 @ 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4526(95)00954-X

experimental H~b is 35 times weaker than predicted in the case of [1 1 1] oriented Fe304/CoO bilayers [6]. The main problem is the lack of understanding of the interfacial mi- crostructure of a real system and its effects on the interlayer coupling mechanisms. This has led to much more sophis- ticated models where parallel [7] or random perpendicular [8] domain walls are expected to form near the interface in the AF layer. These domains are invoked to reduce the effective interfacial exchange coupling and therefore the predicted mcb values.

In this context, we have investigated the magnetic properties of Fe304/CoO and Fe304/NiO systems which exhibit exchange biasing. These oxides have simple collinear magnetic spin structures (ferrimagnetic for Fe304, AF for CoO and NiO), in contrast with the disordered metallic alloys previously studied. This aspect should make the analysis of the properties of these systems easier, In this paper, we present the results of a room-temperature polarized neutron reflectometry (PNR) study of the depth-dependent magnetism of a Fe304/CoO and a Fe304/NiO multilayer. The first sample exhibits a weak interlayer exchange coupling and no exchange biasing, due to the proximity of the ordering temperature of CoO (Tn - 291 K) to room tempera- tare. On the contrary, NiO has a high ordering temperature

A.R. Ball el al./ Physica B 221 (1996) 388~92 389

(Ty - 521 K). The second sample is exchange biased and therefore exhibits a strong interlayer exchange coupling. It is hoped that the comparison of these two oxidic samples will help to understand the magnetic mechanisms in systems exhibiting exchange biasing. Previous PNR studies on systems with NisoFe20 did not reveal any deviations from uniform magnetization within the Ni~0Fe20 layer [9-11].

2. Polarized neutron reflectometry

Neutron reflectometry allows the study of nuclear and magnetic structure as a function of the depth from a flat surface or interface [12]. A collimated beam of polarized neutrons impinges on a flat sample at grazing incidence 0. The reflected intensity is then measured versus the component of the incident neutron wave vector normal to the sample surface q0 = 2~ sin 0/)., where 2 is the neutron wavelength.

The reflectivity is proportional to the square of the modu- lus of the reflection amplitude of the neutron wave function perpendicular to the surface. The neutron wave function, tP(z), satisfies the Schr6dinger equation

~2 7~(z) ~z ~ + (q2 _ F±(z))Tj(z ) = 0. (1)

The reflected intensity therefore depends on the scattering length density (SLD)

r ±(z ) = 4~(n(z )( b,(z ) :k Ctt(z ) )), (2)

where n(z) is the in-plane averaged atomic density, bn(z) the nuclear scattering length and #(z), the in-plane average magnetization. All are a function of the depth z from the sample surface. C is a constant of value 2.699 fm//xB.

From this expression, it can be seen that F:L(z) is in fact the sum of two contributions F±(z) = Fn(z) =t: F,,(z). The first term, Fn(z), is the nuclear SLD and depends on the average in-plane atomic composition and density of the sample. The second term, F,,(z), is the magnetic SLD, proportional to the in-plane average magnetization. The + / - in F:~(z) corresponds to the neutron spin parallel or antiparal- lel to it(z). Thus comparison of the reflectivities R±(qo) for the two neutron spin states determines the in-plane magnetization. In principle, structures such as domain walls should therefore be detectable by PNR if they influence the sample in-plane magnetization.

An important point is that the neutrons are insensitive to magnetic structure perpendicular to the sample plane. There- fore, by saturating the sample perpendicularly to the surface, we can obtain a reflectivity measurement dependent solely on the nuclear density profile Fn(z) and independent of the neutron spin state. Thus, the magnetic and nuclear properties of the sample can be deduced from separate measurements.

Interpretation of the measured reflectivity requires numer- ical simulation. The sample is divided into N slabs and the

SLD F±(z) is supposed constant within each of these slabs. A least-squares algorithm is then used to fit the calculated reflectivity to the measured reflectivity by adjusting the N F±(z) values and N slab thicknesses. The reflectivity is calculated using a standard optical matrix method [13] and convoluted with the experimental resolution. The choice of N is made according to the complexity of the problem. For the analysis of the experiments reported in this paper, the individual F and AF layers were divided into N = 3 slabs. This was found to be the minimum requirement to account for the measured reflectivity in a satisfactory way.

All PNR measurements were performed on the time-of- flight (TOF) reflectometer ROG of the IRI in Delft [14]. In the TOF method, variation of q0 is obtained by reflecting neutrons of all wavelengths, 2, determined by measuring the neutron velocity which is deduced from the time taken by the neutrons to cover the distance from a chopper to the detector. All measurements were performed at room temperature and an electromagnet was used to generate fields up to 710 kA/m at the sample position.

3. Sample preparation

The samples were prepared by molecular beam epitaxy [2, 3]. A [NiO(16nm)/Fe304(16nm)]xl5 and a CoO(15nm)/ [Fe304(15 nm)/CoO(15 nm)]×10 superlattice were de- posited on (00 1) MgO substrates. Structural ordering was characterized by RHEED and X-ray diffraction, and confirmed the crystalline quality and alignment with the substrate lattice. In the case of the Fe30/NiO superlattice, the sample was cooled from above the NiO ordering temperature of 521 K in a field of 8 kA/m, applied along the [100] in-plane crystalline axis to create exchange biasing.

X-ray reflectometry was performed on both samples. The superlattice periodicity was found to be 32 -t-0.5 nm for the Fe304/NiO sample and 30 i 0 . 5 nm for the Fe304/CoO sample. The root-mean-square-roughness for the samples was 0.8 and 0.3 nm on either side of the interfaces for Fe304/NiO and Fe304/CoO, respectively.

Magnetometry studies were performed in a SQUID mag- netometer at fields up to 4400 kA/m. For Fe3Oa/NiO, the saturation magnetization at 300 K was 350 :t: 17 kA//m, 73% of the bulk value (Ms = 480 kA/m). For Fe304/CoO, a value of 420 + 20 kA/m was found, or 87% of the bulk value at 300 K. Similar lower than bulk magnetizations have previously been reported in the literature for Fe304/NiO [15] and Fe304/CoO [1, 6].

4. Experimental results

Magnetic reflectivity measurements on the samples were performed at room temperature in the saturated part of the

390 A.R. Ball et at / Phvsica B 221 (1096) 388 392

hysteresis loops. The applied in-plane field was 640 kA/m. The samples were also magnetized perpendicularly to the plane in a field of 710 kA/m, in order to obtain the purely structural-dependent rettectivity. From the hysteresis loops measured normal to the sample plane, we had found that the samples are saturated at this field value. Consequently, the structural SLD profiles, F,(z), deduced from this measurement were coherent with the results from X-ray reflectometry. After the structural analysis, the magnetic sample properties were studied.

4.1. FesOa/CoO

The measured nuclear and magnetic reflectivities for the Fe304/CoO multilayer are presented in Fig. I. The peaks correspond to first, second- and third-order Bragg reflections from the periodic superlattice. The relative intensities of these peaks give information about the reflective potential barrier seen by the neutrons at each interface, which is linked to the difference between the F±(z) values of each layer. Also, deviations from a perfectly periodic superlattice give rise to a broadening of these Bragg peaks. In addition, the F=(z) profile determines the rate of decay of the specular reflectivity versus qo onto which the Bragg reflections are superimposed.

From fits to the measured reflectivities, the nuclear and magnetic profiles of Fig. 2 were derived. With a few ex- ceptions, throughout the sample, the profiles exhibit the same global trends. In the case of the nuclear scattering length density, F,(z), a sharp contrast is observed between the Fe304 and the CoO layers, due to the very different scattering lengths of iron and cobalt, Within the individual

MgO(001 ) / [COO( 15nm)/Fe304( 15nm)] l0 / CoO( 15rim )

Nuc 10° S + ~ £ T = 2 9 0 K

l04

10-" . . . . . . . . 0.2 0 4 0 6

q0 (hill ~ )

Fig. I. Polarized neutron reflectivity R ± (qo) versus the perpendicular incident neutron wave vector component qo for the Fe3 O4/COO multilayer when magnetized ( I ) perpendicularly to the film plane (top curve (Nuc)), and (2) in-plane (bottom curves, corresponding to +/- (S+ /S ) neutron spin states). The solid lines are the calculated fits. All curves have been offset for clarity,

i

00100r

MgO(001 ) ? [COO( 15nm)/Fe O4115nlrl)]~o / CoOl 15am)

"1000 Nuclear T = 290 K

25 50 75 100 125 150

Deplh from ~ample snrtace Him]

Fig. 2. Nuclear and magnetic profiles deduced from reflectivity fits for the Fe304/CoO multilayer, ['or the topmost 170 nm of the sample. The topmost curve is the nuclear scattering length density. The bottom curve is the magnetization profile of the in-plane saturated sample.

layers, a depth-dependent variation is seen. For the Fe304 layers, F,(z) is maximum in the centre and close to the bulk value, but decreases towards the interfaces. The CoO layers exhibit the opposite trend, with a minimum F,,(z) at the centre. These variations probably arise from the effects of the interface roughness, variable stoichiometry and strain due to the 1.3% lattice mismatch between Fe304 (a = 8.398 &) and CoO (2a = 8.508 A).

The magnetic study shows no visible contribution from the CoO layer. However, the Fe304 layers exhibit a non- uniform magnetization profile. The value of the magnetization near the layer centre is close to that of the bulk material (455 i 23 kA/m or 95% of bulk value). But towards the interfaces, a noticeable decrease is observed. This profile accounts for the reduced average magnetization of 420 ± 20 kA/m measured by SQUID magnetometry.

4.2. ~'£~304/~¥i0

The Fe304/NiO multilayer was studied in detail in both exchange biased and non-exchange biased states. In particu- lar, differences were observed in the magnetic profiles corresponding to the left and right saturated states of the exchange biased F hysteresis loop. These results will be reported in detail elsewhere [16]. Fig. 3 shows the nuclear profile and magnetization for just the right-hand saturated state of the exchange biased sample. Contrary to the Fe304/CoO behaviour, an increase of F,,(z) is observed in the Fe304 layers near the interfaces, with the opposite behaviour in the NiO. The same reasons as above apply here. However, it must be noted that in Fe304/NiO, the strain is weaker because of the smaller lattice mismatch of 0.6% (2a - 8.352 & for NiO). Furthennore, the Fe304 layers are compressively strained in this case. As for Fe304/CoO, the magnetization of the Fe304 layers decreases near the interfaces. However, the effect is

A.R. Ball el aL/ Physica B 221 (1996) 388 392 391

E

MgO[O01 b/[FetOa( 16nm)/NiO116nrm]~15

T = 290 K Nuclear

J ~ 3 - - j ",_ 0 0 1 0 0

_ --L~ [-- " ~ L- 75o

00075

0 0050 S • -

00025 O t 4 i NiO NiO i

(I ' ! . . . . . . . . . . . t l 0 25 "~0 7~ I 0() 125 150

500

4OO Ms(kA /m}

3OO

2OO

100

0

[1001 ( F e 3 O j C o O ) x ~ . . . . o - -

i Ill /i

d i /

/ /

p I //i

/ . o " T = 300K

5 10 15 210

tF%041 n m)

Depth trorn sample surface/nml

Fig. 3. Nuclear and magnetic profiles deduced from reflectivity fits for the Fe304/NiO multilayer, for the topmost 170 nm of the sample. The topmost curve is the nuclear scattering length density. The bottom curve is the magnetization profile of the in-plane saturated and exchange biased sample.

much larger in the case of Fe304/NiO. The profile leads to an average magnetization value of 340 + 17 kA/m or 73% of the bulk value.

5. Discussion

In both superlattices studied, the magnetization within the Fe304 layer exhibits a deviation from the expected uniform behaviour, even though the sample is in the saturated state of the magnetic hysteresis loop. The magnetization, maximum at the centre of the Fe304 layer, decreases towards the interfaces. Furthermore, the CoO and NiO layers do not ap- pear to contribute to the magnetic profile. This behaviour is accompanied by density variations in the layers. Further experimental confirmation of this Fe304 layer magnetization profile has been obtained by SQUID magnetometry studies of Fe304/CoO multilayers for different Fe304 layer thicknesses. The result of this study is shown in Fig. 4. For Fe304 layers thicker than 20 rim, the magnetization is that of the bulk material. However, below 20 nm, the magnetization gradually decreases. Similar behaviour is seen in Fe304/NiO samples varied over the same thickness range [15]. This behaviour can be understood when compared with the profile determined by PNR.

To explain this profile, several reasons can be found. To begin with, in the case of the Fe304/NiO sample, the observed reduction in the Fe304 magnetization at the interfaces can be linked to exchange coupling with the anisotropic NiO layers. If the magnetic field is not strong enough to wrench the NiO spins free, it can be expected that the Fe304 spins near the interface will only partially rotate towards the applied field, due to competition between the Zeeman and exchange energies. The result is a parallel domain wall near the interface that penetrates into the F layer. This would

Fig. 4. Saturation magnetization M.~ of Fe304 in l1 00] oriented Fe304/CoO multilayers versus Fe304 layer thickness tFe~O4. Data were taken from multilayers grown on MgO (o), on NaC1 (0), according to Ref. [1]) and from bilayers grown on SrTiO3 (*'). The dashed curve is a guide for the eye.

weaken the observed magnetization near the interface and, more importantly, the global interfacial exchange coupling responsable for exchange biasing.

For Fe304/CoO, due to the proximity of the AF ordering temperature (/'-~ 291 K), the exchange coupling between the CoO and Fe304 layers can be expected to be much weaker than is the case for NiO. This would explain the less pronounced magnetic profile observed in this sample. It must be noted that our hypothesis of a wall in the F layer is in strong contrast with the assumption on to which theo- retical models for exchange biasing [7, 8] have been built, that the domains are confined to the AF layer alone.

Other effects can also be expected to contribute to the observed magnetization reduction at the interfaces. The main one is the Fe304 layer stoichiometry with possible vacancies in the Fe sites and variations in the oxygen/metal ratio. Non- equilibrium cation distributions over the tetrahedral and oc- tahedral sites also have to be considered. However, in-depth M6ssbauer studies have been performed on the local magnetic properties of similar Fe304 layers [17]. These studies have shown that the Fe304 magnetic behaviour is that of the bulk material except for the outermost atomic planes, where surface restructuring is thought to take place. Furthermore, X-ray analysis of the samples shows no evidence of seri- ous structural anomalies, although a modified magnetic layer may exist at the interfaces, linked to the interface roughness (between 0.3 and 0.8 nm according to X-ray reflectometry).

Although these structural effects must play a role in any interface magnetism and exchange coupling processes, it is improbable that they alone could account for the magnetization reduction to 73% observed in the Fe304/NiO sample. For example, if we consider that the Fe304 layer has zero magnetization to a depth of 0.8 nm from the interface due to roughness, and the bulk magnetization value elsewhere, then this would lead to an average layer magnetization of 90% of the bulk value. However, in the case of Fe304/CoO,

392 A.R. Ballet al. / Physica B221 (1996)388 392

where the interface exchange coupling at room temperature is expected to be much weaker, the contribution of stuctural disorder to the interface magnetism may be the dominant factor. The observed magnetization profile is therefore most likely attributable to a combination of the above structural and magnetic processes.

6. Conclusion

We have established the existence of a non-uniform magnetization in thin Fe304 layers exchange coupled to AF NiO and CoO layers. This is characterized by a reduction in the magnetization in the region close to the F/AF interface, the magnitude of which appears to be linked to the strength of the exchange coupling between the F and AF layers. These results suggest that a domain wall forms at the Fe304 layer side of the interfaces in Fe304/NiO, due to exchange coupling to the anisotropic AF layers. This domain wall formation, combined with structural de- fects such as stoichiometry variations, would explain the observed low values of the exchange biasing field in such systems.

Acknowledgements

The research of A.R. Ball is supported by the Euro- pean Union Human Capital and Mobility programme, and that of D.M. Lind by the US National Science Foun- dation. The research of P.J.H. Bloemen has been made possible by a fellowship of the Royal Dutch Academy of Arts and Sciences.

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polarized neutron reflectometry studies of magnetic oxidic fe3o4/nio and fe3o4/coo multilayers

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