EUROPHYSICS LETTERS 20 May 1997

Europhys. Lett., 38 (6), pp. 459-464 (1997)

Magnetism of Mn ultra-thin films grown on (001) bcc Festudied by X-ray magnetic circular dichroism

S. Andrieu1, M. Finazzi

2, F. Yubero

2, H. M. Fischer

1, Ph. Arcade

1

F. Chevrier2, L. Hennet

1, K. Hricovini

3, G. Krill

2 and M. Piecuch1

1 Laboratoire de Metallurgie Physique et Science des MateriauxCNRS-Universite Henri Poincare, BP239, 54506 Vandoeuvre, France2 Laboratoire pour l’Utilisation du Rayonnement ElectromagnetiqueCNRS-CEA-Universite Paris-sud, 91405 Orsay, France3 Laboratoire de Physique des Materiaux de Surface Universite Cergy-Pontoise95011 Cergy-Pontoise, France

(received 12 February 1997; accepted in final form 18 April 1997)

PACS. 75.70Ak – Magnetic properties of monolayers and thin films.PACS. 68.55−a – Thin film structure and morphology.PACS. 61.10Ht – X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.

Abstract. – The magnetic properties of Mn ultra-thin films epitaxially grown on (001) bcc Feare studied by soft–X-ray magnetic circular dichroism. At room temperature, the Mn growthis two-dimensional without interdiffusion in Fe, as checked by RHEED and Auger spectroscopy.A net Mn magnetic moment is observed in the Mn thickness range up to 2 monolayers. Wefind that the Mn/Fe interfacial coupling is definitely ferromagnetic. In this range, the magneticmoment carried per Mn atom is found to be about 1.7 µB, which is much smaller than the valueof 3 µB predicted for ferromagnetic Mn ultra-thin films.

By using Molecular-Beam-Epitaxy techniques, it becomes possible to grow metals in stronglystrained or metastable structures. Indeed, the correlation existing between the structural andthe magnetic properties of the transition metals can now be studied on real systems. This is ofparticular interest in the case of 3d-transition metals, since a large variety of magnetic statesare predicted depending on the structure and atomic volume [1]. Among the 3d-transitionmetals, Mn is probably the most exciting element to study since the atomic Hund’s rule limitleads to a magnetic moment of 5 µB. Second, in the standard structures of bulk Mn (as Mnα,Mnβ and fcc structures) Mn was always found to be in an antiferromagnetic (AF) state, and aMn high-spin ferromagnetic state was never observed. That is why in the last decade, severalexperimental works were devoted to the study of the magnetic properties of thin Mn filmsgrown in unusual structures where the Mn atomic volume is significantly expanded [2], but noferromagnetic behavior was found in thin Mn films up to now.

Recently, this topic received renewed interest since several theoretical works predicted thatthe magnetic properties of ultra-thin films (from the submonolayer range to 2 monolayers)

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can be much different from the magnetic properties of films of few monolayers thickness [3].However, in the case of a Mn monolayer (ml) on (001) bcc Fe, the situation is confused. Someauthors predict that Mn can be stabilized in a high-spin ferromagnetic state with a momentaround 3 µB [4]-[6], but more recent calculations [7]-[9] show that an AF c(2 × 2) surfacearrangement is the most stable configuration. Both parallel or anti-parallel interfacial couplingsare predicted. From an experimental point of view, the situation is almost the same: a Mnferromagnetic state has been evidenced by Soft–X-Ray Magnetic Circular Dichroism (XMCD)measurements performed on a monolayer Mn film deposited on Co [10], or on MnNi surfacealloys [11]. In the case of Mn grown on Fe, an AF Mn state was observed for thicknesses largerthan 3 ml [12]-[14]. In the submonolayer range, from spin-resolved core level photoelectronspectroscopy, Roth et al. [14] came to the conclusion that the magnetic order of the Mn filmis ferromagnetic, whereas the coupling between the Mn and Fe moment is clearly antiparallel.These authors recently confirmed their conclusions by an XMCD study [15]. However, werecently demonstrated that their method of preparation of the Mn film does not lead to puretwo-dimensional Mn layers, but, on the contrary, to the formation of a diluted Mn/Ag alloy atthe Fe surface [16]. Obviously, the magnetic properties of ultra-thin Mn films grown on (001)Fe are still an open question which must be reconsidered, at least from an experimental pointof view. In this paper, we present an investigation of the magnetic properties of ultra-thinMn/Fe(001) films (up to 2 ml) performed with XMCD spectroscopy. Indeed, in recent years,XMCD asserted itself as a powerful technique to determine the magnetic properties of ultra-thinfilms [17]. A particular care has been dedicated to the characterization, both structural andchemical, of our samples, since the magnetic behavior of thin Mn films can be significantlyaffected by the presence of imperfections or contaminants.

The samples are prepared in situ in a MBE chamber at a pressure of about 10−10 Torr.A 200 A Fe(001) buffer layer is evaporated from a Knudsen cell on (001) single-crystal MgOsubstrates at room temperature (RT), and annealed at 700 K in order to eliminate the dynamicalroughness. After annealing, the Fe film is in its relaxed bcc structure. No O contamination isdetected by Auger spectroscopy, while small amounts of C, never exceeding the equivalent of4% of a monolayer, are sometimes observed. Mn is sublimated from a Knudsen cell on the Febuffer kept at RT. Growth rates of the order of 1 A/min are obtained. The Mn layers surfacestructure, growth mode and in-plane distance variation are determined by RHEED, while thesurface contamination and Mn/Fe intermixing are checked by Auger and ESCA spectroscopy.No O and C are detected after the deposition of the Mn overlayer.

The RHEED patterns of the annealed Fe buffer layer surface (fig. 1) are typical of anatomically flat surface. The surface structure of the Mn films deposited on this Fe surfaceat room temperature is observed to be pseudomorphic to the square Fe lattice, as shown byRHEED (fig. 1). It is important to underline that the AF c(2 × 2) superstructure of the Mnmonolayer on Fe(001) predicted by several authors [7]-[9] cannot be observed by unpolarizedelectron diffraction. However, this magnetic c(2 × 2) structure should be accompanied by abuckling c(2×2) reconstruction [7]-[9]. Such a surface superstructure is observable by RHEED,but we never detect it, not even for submonolayer Mn coverages.

RHEED oscillations are clearly observed during the evaporation of Mn on Fe(001) (fig. 1).This proves that the Mn growth mode is layer by layer, and that a monolayer Mn/Fe(001)film is two-dimensional. Moreover, this allows us to control the thickness θ of the Mn film ina very accurate way. The in-plane parameter is measured continuously during the depositionof Mn (fig. 1). Note that the Mn in-plane parameter is not significantly different from the oneof Fe up to 5 atomic layers, the difference between them being less than 0.5%. The absenceof interdiffusion between Mn and Fe at room temperature was checked by the analysis of therelative intensities of the Mn and Fe Auger LMM peaks as a function of θ. This conclusion is

s. andrieu et al.: magnetism of Mn ultra-thin films etc. 461

Fig. 1. – Top: RHEED pattern observed on the initial Fe surface and during Mn growth on Fe along(100) azimuth, and bottom: RHEED oscillations and in-plane parameter variation observed during thegrowth of Mn on Fe at room temperature.

in agreement with the results by Walker and Hopster [12].

In order to test the influence of roughness on the magnetic properties of Mn/Fe films, we havealso prepared some samples evaporating Mn on an Fe buffer layer grown at room temperaturewithout annealing. The RHEED performed on this Fe surface shows a three-dimensionaldiffraction pattern typical of a rough surface. By using the technique described in ref. [18], inthis last situation, the roughness is found to be around 20 A.

The XMCD experiments were performed on the grating beamline SU23 inserted in theSUPERACO storage ring at LURE. The degree of circular polarization is estimated around70%. A magnetic field equal to 250 Oe is applied in order to align the magnetic domains of theFe substrate along its (100) crystallographic axis (easy magnetization axis of bulk Fe). Sincethe magnetization of the Fe buffer is in-plane, the XMCD measurements are performed with offnormal light incidence. The XMCD spectra are collected flipping the direction of the magneticfield and measuring the total electron yield for each photon energy. The measurements wereperformed at 100 K and 300 K.

The Fe and Mn-LII-LIII absorption edges and XMCD spectra are shown in fig. 2 for oneatomic Mn layer on Fe (001). Note that we always get “metallic” shape for the edges, withoutfine structure, in contradiction with the results reported in ref. [15]. This lineshape is observedin all the thickness range we have investigated, from θ = 30 ml down to the submonolayerregion (θ = 0.3 ml). This means that, even for a 0.3 ml thick film, an atomic description forthe Mn atoms is not correct in our case.

The existence of a net Mn-3d magnetic moment is clearly indicated by the presence of aXMCD signal at the Mn-LII-LIII absorption edge (see fig. 2). The dichroism has the samesign (negative at the LIII and positive at the LII) for both Fe and Mn. This means that,at the interface, the coupling between the average 3d magnetic moments of Fe and Mn is

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Fig. 2. – Top: Fe and Mn L2-L3 absorption spectra, σ− solid line, and σ+ dashed line, observed atroom temperature for 1 ml Mn / 200 A Fe(001) / MgO (001). Bottom: as-measured Mn and Feσ− − σ+ dichroic spectra showing the parallel alignement of Mn and Fe moment.

parallel, in contradiction with the most recent theoretical predictions [7]-[9] and experimentalresults [14], [15].

In fig. 3 we compare the Mn-LII-LIII dichroic signals obtained for θ = 0.3, 1 and 2 ml. Thespectra have been normalized to the maximum of the Mn-LII-LIII absorption edge intensity (thismaximum is obtained by substracting the two-step–like function for edge-jump removal [19])and are corrected for the incomplete circular light polarization and non-parallel alignement ofsample magnetization and photon propagation direction. Note that for θ = 2 ml the XMCDrelative intensity is only slightly weaker than the ones for θ = 0.3 and 1 ml, but the shape andsign of the signals remain the same. This is the evidence that the second Mn layer contributesto the total moment of the film and that the average moments of the surface and interface layersare ferromagnetically coupled.

By applying the sum rules of ref. [20] to the XMCD spectra obtained on the Mn monolayer,and assuming that the Mn-3d occupation number is close to 5, the site- and shell-projectedspin and orbital contributions to the total Mn-3d moment M3d

Mn can be estimated equal to1.0± 0.15 µB and 0.1± 0.05 µB, respectively. Unfortunately, the applicability of the spin sumrule to Mn has not been established yet. Actually, it is valid only if the spin-orbit interactionis sufficiently large with respect to correlation effects, which might not be the case for Mn. Forinstance, using an atomic model, we have verified that the application of the spin sum rule tothe theoretical XMCD and isotropic spectra of the isolated Mn2+ ion underestimated the Mnmoment of about 60%. Thus, in order to have a better estimation of M3d

Mn, we compare ourresults to the local density approximation calculations performed by Wu and Freeman [7]. Theydemonstrated that their model gives the correct LII-LIII isotropic and dichroic signals, bothin shape and magnitude, in the case of metallic Fe and Co [21]. For a Mn magnetic momentof 3.26 µB, they obtain a dichroic spectrum which corresponds at the Mn-LIII to about 30%of the intensity of the isotropic edge [7]. Since there is a good qualitative agreement betweenthe shape of their theoretical Mn dichroic and absorption spectra and our experimental results,

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Fig. 3. – XMCD spectra (normalized to the maximum of the Mn L3 aborption edge) observed at roomtemperature for 0.3, 1 and 2 ml of pure Mn on Fe(001).

we are confident that the total moment of the Mn monolayer can be obtained comparing therelative intensities of the theoretical and experimental XMCD. For 0.3 ≤ θ ≤ 1 ml, we observea Mn-LIII dichroic signal whose intensity corresponds to 16± 1% of the LIII absorption edge,which means that 〈M3d

Mn〉 = 1.75 µB/at, while for θ = 2 ml the dichroic relative intensity is13± 1%, giving 〈M3d

Mn〉 = 1.4 µB/at. These values are 60% larger than the values obtained byusing the sum rules, which is consistent with the error obtained when applying the sum rulesto the theoretical XMCD of isolated Mn2+.

We would like to stress that, even considering large error bars, the values we obtain for M3dMn

are significantly smaller than the ones (> 3 µB) found by the theoretical works that predicta ferromagnetic order of the Mn monolayer [4]-[6] or by the XMCD experiments of ref. [15].This is true even for submonolayer coverages (θ = 0.3 ml). However, the relatively small Mnmoment we observe could be explained if the Curie temperature (TC) of the Mn film is closeto 300 K. In order to reject this hypothesis, we have studied the temperature dependence ofthe Mn-LII-LIII XMCD for one monolayer. We have found that the intensity of the dichroicsignal measured at 100 K does not change significantly, meaning that TC is well above roomtemperature. We have also checked that Fe and Mn moments are collinear verifying that thedependence of the dichroic intensity on the angle α between the substrate magnetization andthe photon helicity follows a cosα law. The role played by the surface roughness in determiningthe magnetic properties of Mn thin films has also been investigated. The XMCD measurementsperformed on one Mn monolayer deposited on a rough Fe buffer layer (prepared as describedabove) show that the film is always magnetic, but with a slightly decreased (≈ 25%) XMCDsignal.

A recent calculation [22] shows that a ferrimagnetic p(2× 2) state is degenerated to the AFc(2× 2) state proposed by several authors to describe the magnetic order of one Mn monolayerdeposited on Fe(001). In this ferrimagnetic state, the four Mn atoms in the squared unit cellcarry a magnetic moment of about 3 µB each. For three of them this moment is directedparallel to the moment of the Fe buffer, while the coupling is antiparallel for the forth. The

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resulting average magnetic moment is found to be equal to 1.6 µB. This spin configuration canthus explain our results in the monolayer range. However, a buckling p(2× 2) superstructureis predicted, which has not been observed by RHEED in this study.

In summary, we have shown that pure two-dimensional Mn layers pseudomorphic to thebcc (001) Fe lattice are ferromagnetic up to 2 ml. We have clearly demonstrated that theinterfacial Fe/Mn coupling is ferromagnetic. The average Mn magnetic moment for 0.3 and1 ml is observed to decrease for 2 ml. Even if it is difficult to give the exact Mn magneticmoment values since the use of the sum rules is not appropriate in the case of Mn, these magneticmoments are significantly smaller, at least by a factor of around 2, than the theoretical value of∼ 3 µB for Mn in a high spin state. This analysis finally shows that an appropriate modeling ofthe magnetic properties of 1 and 2 ml Mn on (001) bcc Fe should lead to a ferro- or ferrimagneticstates with a dominant ferromagnetic coupling between Mn and Fe. A recent calculation [22]showed that a ferrimagnetic state consisting of a squared unit cell with 3 moments orientedparallel and 1 antiparallel to the Fe moment is a degenerated state of the AF c(2 × 2) stateproposed by several authors. Such a ferrimagnetic state could explain the interfacial couplingand the low magnetic moment observed on the Mn monolayer deposited on Fe(001). However,there is no direct experimental evidence of such a spin configuration in the monolayer range,which is at least no more valid for 2 Mn ml.

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