The magnetic moments of ultrathin Fe films on three different III-V semiconductor substrates, namely GaAs, InAs and In0.2Ga0.8As have been measured with X-ray magnetic circular dichroism at room temperature to assess their relative merits as combinations suitable for next-generation spintronic devices. The results revealed rather similar spin moments and orbital moments for the three systems, suggesting the relationship between film and semiconductor lattice parameters to be less critical to magnetic moments than magnetic anisotropy

Claydon, J S

Kirkman, I W

Niu, D X

Telling, N D

van der Laan, G

Xu, Y B

English

White Rose Research Online

This is a repository copy of Spin and orbital moments of ultra-thin Fe films on various semiconductor surfaces.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/1845/Article:Claydon, J S, Niu, D X, Xu, Y B orcid.org/0000-0002-7823-0725 et al. (3 more authors) (2005) Spin and orbital moments of ultra-thin Fe films on various semiconductor surfaces. IEEE Transactions on Magnetics. pp. 3325-3327. ISSN 1941-0069 https://doi.org/10.1109/TMAG.2005.855201eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 3325Spin and Orbital Moments of Ultra-Thin Fe Filmson Various Semiconductor SurfacesJill S. Claydon1, Daxin Niu1, Yongbing Xu2, Neil D. Telling3, Ian W. Kirkman4, and Gerrit van der Laan3The Spintronics Laboratory, Department of Electronics, University of York, York YO10 5DD, U.K.The Department of Electronics, University of York, York YO10 5DD, U.K.Magnetic Spectroscopy Group of Daresbury Laboratory, Warrington WA4 4AD, U.K.Station Scientist of Station 1.1 Daresbury Laboratory, Warrington WA4 4AD, U.K.The magnetic moments of ultrathin Fe films on three different III–V semiconductor substrates, namely GaAs, InAs and In0 2Ga0 8Ashave been measured with X-ray magnetic circular dichroism at room temperature to assess their relative merits as combinations suitablefor next-generation spintronic devices. The results revealed rather similar spin moments and orbital moments for the three systems,suggesting the relationship between film and semiconductor lattice parameters to be less critical to magnetic moments than magneticanisotropy.Index Terms—Fe/InGaAs, interface magnetism, spin electronics, ultrathin films.I. INTRODUCTIONNEXT-GENERATION spintronic devices are expected torevolutionize conventional electronic circuitry by utilizingboth the charge and spin of the electron while retaining linkswith present day semiconductor technologies [1]–[3]. However,the development of such devices is at present limited by our abil-ities in transporting spin into nonmagnetic media [4], [5]. Andso research into breaching the gap in characteristics betweenthe magnetic materials and semiconductor technologies has be-come a hot topic of debate. Several technologies have been putforward to overcome this barrier such as magnetic semiconduc-tors [5], where great advances have brought Curie temperaturesup to 173 K [6] and methods based on ferromagnet/semicon-ductor contacts [5], including epitaxially grown ferromagneticmetal contacts, which appear to offer the promise of room tem-perature spin transport.After many years of investigation [7]–[11], it has recentlybeen shown that ferromagnetic films have the capability to retainmagnetic properties to interface thicknesses [12]. However, withpredicted levels of spin transport still to be realized [4], manyquestions still remain unanswered about the degree to which in-terface magnetism and hence spin transport relates to the sta-bility of growth and hence to the lattice match and surface ofthe materials involved.In order to determine whether the property of lattice matchand related film strain might be important for magnetic behaviorclose to the interface we have undertaken room temperatureX-ray magnetic circular dichroism (XMCD) investigations intothe properties of ultrathin films grown on substrates of variedlattice constant. The substrates chosen for study, GaAs, InAs,and In Ga As were chosen for their lattice constants which,in a 2:1 relationship with Fe give mismatches of 1.56%,5.32%, and 0%, respectively.Digital Object Identifier 10.1109/TMAG.2005.855201II. SAMPLE FABRICATION AND MEASUREMENTThe specialist substrates used to establish the different latticematches were constructed by the EPSRC’s UK National Centrefor III–V Semiconductor Technologies in Sheffield and took theform of GaAs (100) wafers topped with a 0.3 m epilayer ofGaAs, InAs or InGaAs followed by an As capping layer. TheAs capping layer allowed for safe transit between systems andremoved the requirement for pre-growth wet etching usually re-quired to achieve the reconstructed surfaces.Once in our own MBE system all substrates were initiallyheated to 350 C to desorb the As capping layer and then furtherannealed for 1 h to 560 C for GaAs, 525 C for InGaAs and514 C for InAs RHEED analysis of the surface indicated anAs rich 2 4 reconstruction was achieved for GaAs and InAswhile a 1 1 surface was observed for In Ga As. Followingcooling to room temperature 7 ML Fe, chosen for the largeuniaxial anisotropy displayed at this thickness for the Fe/GaAssystem [15], was deposited at a rate of 0.66 ML per minute, fol-lowed by the 15 ML Cr cap to protect against oxidation. Allstages of growth were monitored by RHEED measurement andthe samples’ uniaxial magnetic qualities were confirmed ex situwith longitudinal MOKE measurements.XMCD measurements were conducted at station 1.1 of theSynchrotron Radiation Source at Daresbury Laboratory (U.K.)which was equipped with a flipper magnet with a maximumfield of 0.6 T. This configuration allows illumination of the sam-ples with 85% circularly polarized X-rays parallel/antiparallelto an applied magnetic field and attainment of XAS spectra mea-sured in these opposite applied field conditions, and . Theflipper magnet offers the ability to reverse the magnetizationat each measurement point and thus reduces anomalies in theXMCD or difference spectra. Measurements were made withthe samples at an angle of 45 to the incident X-rays to maxi-mize the effect of both the incident light and the applied field.III. RESULTS AND DISCUSSIONDuring growth RHEED patterns were obtained both prior andsubsequent to the deposition of Fe on the reconstructed semi-0018-9464/$20.00 © 2005 IEEE3326 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005Fig. 1. RHEED images collected before (i, ii, iii) and after (iv, v, vi) depositionof 7 ML Fe onto the respective reconstructed GaAs, InAs, and In Ga Assurfaces.Fig. 2. MOKE loops taken along, the easy (i, iv, and v) and hard (ii, iii, andvi) axes of the Fe/GaAs, Fe/InAs, and Fe/In Ga As samples respectively,show uniaxial anisotropy in all samples. Note the 90 rotation of hard and easyaxes in the case of Fe/InAs.conductor surfaces. The images, which are shown in Fig. 1,confirm the epitaxial growth of bcc Fe on the three differentsurfaces with the same epitaxial relationship of Fe(100)<001>In Ga As , where .Ex situ longitudinal MOKE measurements were also made,shown in Fig. 2. Although the hard axis of the Fe/GaAs andthe Fe/In Ga As films could not be saturated, the hysteresisloops clearly show the films to be uniaxial in nature. An in-teresting aspect of these results is the confirmation of the re-versal of easy axis and hard axis directions between the GaAsand InAs substrates. This result is in good agreement with pre-vious studies [14] and is thought to be indicative of the influenceof the magneto-elastic effect in the establishment of the uni-axial anisotropy. The magneto-elastic effect occurs as a directFig. 3. XMCD spectra for Fe L edge studies of 7 ML Fe on GaAs (i), InAs(ii) and In Ga As (iii). Solid lines correspond to XMCD spectra, whiledashed and dotted lines represent the XAS spectra taken for opposite appliedfields I and I spectra, respectively.result of the compression or expansion of the Fe film in orderto fit the GaAs or InAs lattice respectively. However, in theseresults we can also see that the perfectly lattice matched systemFe/In Ga As also displays uniaxial characteristics. Whilethis can not be explained by magneto-elastically induced uni-axial anisotropy the result would seem to support the “chemical”effect proposed by Kneedler et al. [9] in which the uniaxial mag-netic anisotropy (UMA) is derived from the unidirectional na-ture of Fe-As or Fe-Ga bonds. The results show that both chem-ical and magneto-elastic effects are in some part responsible forthe observed UMA in Fe/III-V semiconductor heterostructures.The obtained XMCD spectra, shown in Fig. 3, were analyzedthrough application of the sum rules, see (1)–(2), with the valuesof the parameters , and derived from the integrals of theXMCD and XAS spectra in the manner of Chen et al. [15]. Thenumber of holes, , was taken as 3.39 for Fe [16] and the degreeof polarization, , was equal to 0.85(1)(2)The values of the spin and orbital moments found from sumrule analysis and those of the bulk values are described inTable I. By far the most striking aspect of the XMCD resultsis, within error, the similarity of observed magnetic momentsbetween the three different substrates.The highest spin moment of all the samples tested is seenfor Fe/GaAs, however, this represents only 72% of the bulkvalue. This reduction compared with previous results on Ga richCLAYDON et al.: SPIN AND ORBITAL MOMENTS OF ULTRA-THIN FE FILMS 3327TABLE ISPIN AND ORBITAL MOMENTS OF BULK Fe IN COMPARISON TO 7 ML FeFILM ON GaAs, InAs, AND In Ga As4 6 surface [17], [18] may originate from the different na-ture of the interface for these samples, which were grown on Asrich reconstructions in this study, thought to give rise to reactedphases and hence reduced Fe moments [19]. It is also possiblethat the clustered growth phase that occurs in Fe at this thick-ness just prior to the establishment of a continuous film [12],[20] could require a higher external field, than was availableto fully saturate these films. The Fe/GaAs result is commensu-rate with the similarly reduced values of spin moment derivedfrom Fe/InAs and Fe/In Ga As, which are in close agree-ment with the Fe/GaAs result representing 68% and also 72%of the bulk value, respectively.In contrast to the reduction in spin moment the orbital mo-ment, and hence the ratio / , for all samples shows amarked enhancement, with an orbital increase of around 200%,with respect to the bulk value, as witnessed in earlier studiesof Fe/GaAs and Fe/InAs [17], [18]. This orbital enhancementis thought to derive from the brake-down in symmetry close tothe interface as a result of qualities of the substrate surface in-cluding steps or terraces and surface roughness. It is expectedthat these features cause localization of the 3d Fe wave func-tions close to the interface [21], our evidence would appear tosupport this since such surface issues would not be overcome bya perfect lattice match.IV. CONCLUSIONThe spin and orbital moments and hysteresis loops of 7ML Fe films grown on specially fabricated GaAs, InAs andIn Ga As epilayers have been measured using XMCD andMOKE, respectively. The results show that magneto-elasticeffects provoked by the difference in substrate lattice constanthave little effect on the spin and orbital moments which arefound to be within error of one another for all substrates. Thetolerance of the Fe films to withstand such magneto-elasticeffects might further suggest Fe/III–V semiconductor het-erostructures could, in future provide suitable solutions forspin-transport devices and hints that the flexibility of thesesystems may offer practical manufacturing solutions in the fieldof future spintronics device design.ACKNOWLEDGMENTThis work was supported in part by an EPSRC AdvancedFellowship and a CASE studentship awarded by DaresburyLaboratory.REFERENCES[1] G. A. 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Spin and orbital moments of ultra-thin Fe films on various semiconductor surfaces

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