Spin and orbital magnetic moments of Fe in the n -type ferromagnetic semiconductor (In,Fe)As
M. Kobayashi, L. D. Anh, P. N. Hai, Y. Takeda, S. Sakamoto, T. Kadono, T. Okane, Y. Saitoh, H. Yamagami, Y. Harada, M. Oshima, M. Tanaka, A. Fujimori
aa r X i v : . [ c ond - m a t . m t r l - s c i ] M a y Spin and orbital magnetic moments of Fe in the n -type ferromagnetic semiconductor(In,Fe)As M. Kobayashi,
1, 2, ∗ L. D. Anh, P. N. Hai, Y. Takeda, S. Sakamoto, T. Kadono, T. Okane, Y. Saitoh, H. Yamagami, Y. Harada,
2, 6
M. Oshima,
1, 2
M. Tanaka, and A. Fujimori Department of Applied Chemistry, School of Engineering,University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Synchrotron Radiation Research Organization, University of Tokyo,1-490-2 Kouto, Sayo-cho, Tatsuno, Hyogo 679-5165, Japan Department of Electrical Engineering and Information Systems,University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Sayo-gun, Hyogo 679-5148, Japan Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Institute for Solid State Physics, The University of Tokyo, 1-1-1 Koto, Sayo, Hyogo 679-5198, Japan (Dated: September 15, 2018)The electronic and magnetic properties of Fe atoms in the ferromagnetic semiconductor (In,Fe)Ascodoped with Be have been studied by x-ray absorption spectroscopy (XAS) and x-ray magneticcircular dichroism (XMCD) at the Fe L , edge. The XAS and XMCD spectra showed simplespectral line shapes similar to Fe metal, but the ratio of the orbital and spin magnetic moments( M orb / M spin ) estimated using the XMCD sum rules was significantly larger than that of Fe metal,indicating a significant orbital moment of Fe 3 d electrons in (In,Fe)As:Be. The positive value of M orb / M spin implies that the Fe 3 d shell is more than half-filled, which arises from the hybridizationof the Fe ( d ) state with the charge-transfer d L states, where L is a ligand hole in the hostvalence band. The XMCD intensity as a function of magnetic field indicated hysteretic behavior ofthe superparamagnetic-like component due to discrete ferromagnetic domains. PACS numbers: 75.50.Pp, 71.55.-i, 78.70.Ck, 78.70.En
Ferromagnetic semiconductors (FMSs) are key materi-als for semiconductor spintronics [1] and have attractedmuch attention both from the application and fundamen-tal physics points of view. Charge carriers doped into thehost semiconductors are considered to mediate ferromag-netic interaction between the magnetic ions, that is, so-called carrier-induced ferromagnetism [2], which enablesus to utilize both the charge and spin degrees of freedomof the electron for functional devices. Indeed, the pro-totypical p -type III-V-based FMS Ga − x Mn x As showsferromagnetic properties depending on carrier concentra-tion [3, 4], and novel spintronic functional devices usingGa − x Mn x As have been fabricated [5, 6]. However, itsCurie temperature ( T C ) still remains below room tem-perature. Furthermore, in addition to the p -type FMSs, n -type FMSs are necessary for the applications of FMSsto spintronics.Recently, Hai et al . [7–9] have succeeded in fabricat-ing a new n -type FMS In − x Fe x As:Be (InFeAs). Here,Be atoms are located at the interstitial site under the low-temperature growth condition and act as double donors.One can, therefore, independently control the concentra-tions of magnetic dopants and electron carriers by chang-ing the Fe and Be contents. When the electron carrierconcentration is larger than 10 cm − , InFeAs shows fer- ∗ Present address: Photon Factory, Institute of Materials StructureScience, High Energy Accelerator Research Organization, Tsukuba,Ibaraki 305-0801, Japan; Electronic address: [email protected] romagnetic properties. The magnetization curves showhysteresis in agreement with the magnetic field depen-dence of the anomalous Hall effect and magnetic circu-lar dichroism (MCD) intensity in the visible-ultravioletphoton-energy region [8]. These results indicate that theferromagnetic property of InFeAs is intrinsic and mostlikely carrier-induced. The observation of a light electroneffective mass comparable to that of the conduction elec-tron in InAs implies that electron carriers are doped intothe conduction band and that the Fermi level is locatedabove the conduction-band minimum [9]. The knowledgeof the electronic structure related with the Fe ion in theInAs host is, therefore, indispensable to understand themechanism of the ferromagnetism in InFeAs.X-ray magnetic circular dichroism (XMCD) is a power-ful tool to investigate the electronic structure of magneticdopants in FMSs [10–13]. XMCD is defined as the differ-ence between the x-ray absorption spectroscopy (XAS)spectra taken with circularly polarized x rays with thephoton helicity parallel ( µ + ) and antiparallel ( µ − ) tothe spin polarization; ∆ µ ≡ µ + − µ − . Since XMCDis element-specific and sensitive only to magnetically ac-tive (ferromagnetic and paramagnetic) species, XMCDenables us to extract the electronic and magnetic proper-ties of doped magnetic ions. Compared with macroscopicmagnetization measurements, XMCD detects no diamag-netic signal from the substrate and has the capability todistinguish between the paramagnetic and ferromagneticcomponents [13]. In the present work, we report on theresults of XMCD measurements of InFeAs, and study therole of the Fe ions in the ferromagnetism.An In − x Fe x As:Be ( x = 0 .
05) thin film with the thick-ness of 20 nm was grown on a InAs(001) substrate at240 ◦ C in an ultra-high vacuum by the molecular beamepitaxy method. The Be concentration was 2 . × cm − . In order to avoid surface oxidation, the sampleswere covered by an As capping layer ( ∼ − x Fe x As:Be layer. The Curie tem-perature T C of the sample was ∼
13 K as determined bythe Arrott plot of MCD.X-ray absorption spectroscopy (XAS) and x-ray mag-netic circular dichroism (XMCD) measurements wereperformed at the helical undulator beam line BL23-SUof SPring-8 [14–16]. The monochromator resolution was E/ ∆ E> , µ + and µ − for circu-larly polarized x rays were obtained by reversing photonhelicity at each photon energy and were recorded in thetotal-electron-yield mode. The µ + and µ − spectra weretaken both for positive and negative applied magneticfields and were averaged in order to eliminate spuriousdichroic signals arising from the slightly different opti-cal paths for the two circular polarizations. Externalmagnetic fields were applied perpendicular to the samplesurface. Backgrounds of the XAS spectra at the Fe L , edge were assumed to be hyperbolic tangent functions.Figures 1(a) and 1(b) show the Fe L , XAS andXMCD spectra of the InFeAs thin film, respectively,taken at H = 10 T and T = 10 K. The XAS and XMCDspectra are simple spin-orbit doublets without fine struc-tures similar to these of Fe metal [17] and Fe pnictides[18], but unlike Fe oxides [19]. The absence of clear Feoxide signals in the spectra demonstrates that the amor-phous As passivation layer protected the InFeAs surfacefrom oxidation [20]. The pre-edge structure around 705eV in the XAS spectrum comes from the In M edge.Figure 1(c) shows the Fe L XMCD spectra for different H ’s and T ’s normalized to the main peak height. Thealmost unchanged XMCD line shapes indicate that thelocal electronic structure and the magnetic state of theFe 3 d electrons do not change with H and T . However,the small shoulder structure around hν = 710 eV slightlyincreases with H , indicating that the structure may orig-inate from a small amount of extrinsic paramagnetic ox-idized Fe atoms.The spin ( M spin ) and orbital ( M orb ) magnetic mo-ments of Fe in (In,Fe)As in units of µ B /atom are esti-mated by applying the XMCD sum rules [21, 22]: M orb = − R L , dω ∆ µ R L , dω ¯ µ n h , (1) M spin + 7 M T = − R L dω ∆ µ − R L , dω ∆ µ R L , dω ¯ µ n h , (2)to the Fe L , XAS and XMCD spectra and are listed inTable I. Here, n h is the number of empty 3 d states andis assumed to be 5 because of the formal valence of Fe XA S I n t e g r a l -30-20-100 X M C D Photon Energy (eV) -60-40-200 X M C D I n t e g r a l -1.0-0.8-0.6-0.4-0.20 N o r m a li ze d X M C D Photon Energy (eV) XA S (a) ( m + + m - )/2 Background Integral Fe L XASFe L XMCD (c) In M m + − m - Integral (b) H =10 T, T =10 K In Fe As:Be Fe L XMCD
FIG. 1: Fe L , -edge XAS and XMCD spectra ofIn . Fe . As:Be thin film. (a) XAS spectra [¯ µ = ( µ + + µ − ) / ≡ µ = µ + − µ − ) at H = 10T and T = 10 K. The scale of the vertical axis is the sameas panel (a). (c) Scaled XMCD spectra for different appliedmagnetic fields and temperatures. (3 d ). M T is the expectation value of the magnetic dipoleoperator and is assumed to be negligibly small because ofthe high symmetry of the Fe site. The correction factorfor M spin of the Fe ( d ) configuration, 0.685 [23], hasbeen applied. By dividing Eq. (1) to Eq. (2), M orb /M spin is given by: M orb /M spin ≈ R L , dω ∆ µ R L dω ∆ µ − R L , dω ∆ µ . (3)Notably, the value of M orb /M spin is independent of n h and the XAS integral. The finite positive value of M orb /M spin reflects the finite orbital moment of a morethan half-filled Fe 3 d shell. Because the ratio M orb /M spin of InFeAs is positive and larger than that of Fe metal[17], the electronic structure of Fe in InFeAs should bedifferent from that of Fe in Fe metal. We consider thatthe ground state of Fe in InFeAs is basically Fe ( d )(because Fe substitutes for In ) but that the charge-transfer d L configuration, where L denote a hole in theAs 4 p ligand band, is mixed into the predominant d con-figuration through the Fe 3 d -As 4 p hybridization, whichindeed makes the number of d electron n d (= 10 − n h ) isa little larger than 5.Figure 2(a) shows the H dependence of the XMCD in-tensity at hν = 708 . L XMCD at various temperatures. Since the line shapeof the XMCD spectrum remains nearly unchanged withvarying H and T as shown in Fig. 1(c), the XMCD peakintensity should be proportional to the total magneticmoment of Fe M (= M spin + M orb ) or the magnetization.In Fig. 2(a), therefore, we have converted the vertical TABLE I: Spin and orbital magnetic moments of Fe in unitsof Bohr magnetron in the In . Fe . As:Be thin film at H =10 T and T = 10 K compared with those of Fe metal [17].Here, the number of d electrons has been assumed to be 5.The correction factor for the Fe ion (0.685) [23] has beenemployed. M spin M orb M orb /M spin In . Fe . As:Be 1 . ± .
02 0 . ± .
02 0 . ± . . ± .
08 0 . ± .
004 0 . ± . axis from the XMCD peak intensity to M . At several H values, the XMCD sum rules have been applied tothe entire spectra and the deduced M values are plot-ted by filled symbols. With increasing T , M decreasesand the susceptibility, i.e., the slope of the M - H curvesaround H = 0, diminishes. The M - H curves are concave,suggesting globally superparamagnetic-like behavior. Inthe previous studies [8], such superparamagnetic-like hys-teresis is attributed to the existence of discrete ferromag-netic domains due to the spatial fluctuations of electronconcentration n e ; areas with high n e become ferromag-netic, while areas with low n e remain paramagnetic. Ata zero magnetic field, the magnetization is minimized toreduce the total static magnetic energy due to dipolarinteractions between ferromagnetic domains, which ex-plains the observed small remanent magnetization. Notethat this behavior is significantly different from the su-perparamagnetism due to thermal fluctuations in con-ventional nanocluster systems. Indeed, the coexistenceof ferromagnetic and paramagnetic domains on a 10 µ mscale in InFeAs has been confirmed by magneto-opticalimaging [8].To further confirm this picture, we measured an M - H curve at T = 10 K for magnetic fields ranging frompositive to negative directions, as shown in Figure 2(b).Since the line shape of the XMCD spectra is independentof temperature, the electronic structure of the intrinsicparamagnetic Fe components is indistinguishable fromthat of the ferromagnetic one. The coexistence of ferro-magnetic and paramagnetic regions is evidenced by twocharacteristics. First, the saturated magnetic momentper Fe atom of the ferromagnetic component is only 1.2 µ B , which is much smaller than the expected ∼ µ B if allFe atoms contributed to ferromagnetism. This indicatesthat there must be regions that remain paramagnetic.Second, the M - H curve is not saturated and has a linearcomponent even at 7 Tesla, which indicates the contri-bution of paramagnetic areas. Nevertheless, the sampleis not superparamagnetic, since clear coercivity of ∼ T C = 13 K.In conclusion, we have performed an XMCD studyon InFeAs thin film to investigate the electronic struc- M a gn e ti za ti on ( m Β / F e ) Magnetic Field (T) -0.4-0.200.20.4 -0.1 0 0.11.21.00.80.60.40.20 M a gn e ti za ti on ( m Β / F e ) Magnetic Field (T) M - H curve T = 10 K
10 K 20 K 30 K 60 K (b)(a) In Fe As:Be
FIG. 2: M - H curves deduced from the magnetic-field de-pendence of the Fe L , XMCD intensity. (a) M - H curvestaken at various temperatures. The XMCD peak intensity at hν = 708 . L ) is plotted by open symbols. The totalmagnetic moment M = M spin + M orb estimated by applyingthe XMCD sum rules for several H values is plotted by filledsymbols. (b) M - H curve taken at T = 10 K for both positiveand negative magnetic fields. The inset shows an enlargedplot around the zero magnetic fields. ture of the Fe ions related to the magnetism. The lineshape of the XMCD is unchanged with magnetic fieldsand temperatures, the latter of which indicates that theXMCD signal from doped Fe atoms is intrinsic. The ratio M orb /M spin is positive and larger than that of Fe metal,indicating that the electronic structure of the Fe ions inInFeAs is different from that of Fe metal. The positive M orb /M spin is explained by the charge transfer from theligand to Fe 3 d orbitals through the hybridization be-tween the Fe 3 d and As 4 p ligand states. The XMCDintensity as a function of magnetic field shows hystereticbehavior of the superparamagnetic-like component dueto discrete magnetic domains in InFeAs:Be. We suggestthat, in order to improve the magnetic characteristics ofInFeAs, homogenous electron doping is necessary, whichmay be obtained if group VI atoms are used as donors inInFeAs.This work was supported by Grants-in-Aid for Scien-tific Research (S22224005 and 23000010) from the JapanSociety for the Promotion of Science (JSPS), Japan. The experiment at SPring-8 was approved by the Japan Syn-chrotron Radiation Research Institute (JASRI) ProposalReview Committee (Proposal No.2012B3823). MK ac-knowledges financial support from JSPS. [1] A. Dietl, Nature Mater. , 950 (2010).[2] T. Jungwirth, J. Sinova, J. Maˇsek, J. Kuˇcera, and A. H.MacDonald, Rev. Mod. Phys. , 809 (2006).[3] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S.Katsumoto, and Y. Iye, Appl. Phys. Lett. , 363 (1996).[4] F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara,Phys. Rev. B , R2037 (1998).[5] H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T.Dietl, Y. Ohno, and K. Ohtani, Nature , 944 (2000).[6] M. Sawicki, D. Chiba, A. Korbecka, Y. Nishitani, J. A.Majewski, F. Matsukura, T. Dietl, and H. Ohno, NaturePhys. , 22 (2010).[7] P. N. Hai, D. Sasaki, L. D. Anh, and M. Tanaka, Appl.Phys. Lett. , 262409 (2012).[8] P. N. Hai, L. D. Anh, S. Mohan, T. Tamegai, M.Kodzuka, T. Ohkubo, K. Hono, and M. Tanaka, Appl.Phys. Lett. , 182403 (2012).[9] P. N. Hai, L. D. Anh, and M. Tanaka, Appl. Phys. Lett. , 252410 (2012).[10] H. Ohldag, V. Solinus, F. U. Hillebrecht, J. B. Goedkoop,M. Finazzi, F. Matsukura, and H. Ohno, Appl. Phys.Lett. , 2928 (2000).[11] K. W. Edmonds, N. R. S. Farley, T. K. Johal, G. van derLaan, R. P. Campion, B. L. Gallagher, and C. T. Foxon,Phys. Rev. B , 064418 (2005).[12] M. Kobayashi, Y. Ishida, J. I. Hwang, T. Mizokawa,A. Fujimori, K. Mamiya, J. Okamoto, Y. Takeda, T.Okane, Y. Saitoh, Y. Muramatsu, A. Tanaka, H. Saeki,H. Tabata, and T. Kawai, Phys. Rev. B , 201201(2005).[13] Y. Takeda, M. Kobayashi, T. Okane, T. Ohkochi, J.Okamoto, Y. Saitoh, K. Kobayashi, H. Yamagami, A. Fu-jimori, A. Tanaka, J. Okabayashi, M. Oshima, S. Ohya,P. N. Hai, and M. Tanaka, Phys. Rev. Lett. , 247202 (2008).[14] A. Yokoya, T. Sekiguchi, Y. Saitoh, T. Okane, T.Nakatani, T. Shimada, H. Kobayashi, M. Takao, Y.Teraoka, Y. Hayashi, S. Sasaki, Y. Miyahira, T. Harami,and T. A. Sasaki, J. Synchrotron Rad. , 10 (1998).[15] J. Okamoto, K. Mamiya, S. I. Fujimori, T. Okane, Y.Saitoh, Y. Muramatsu, A. Fujimori, S. Ishikawa, and M.Takano, AIP Conf. Proc. , 1110 (2004).[16] Y. Saitoh, Y. Fukuda, Y. Takeda, H. Yamagami, S. Taka-hashi, Y. Asano, T. Hara, K. Shirasawa, M. Takeuchi, T.Tanaka, and H. Kitamura, J. Synchrotron Rad. , 388(2012).[17] C. T. Chen, Y. U. Idzerda, H.-J. Lin, N. V. Smith, G.Meigs, E. Chaban, G. H. Ho, E. Pellegrin, and F. Sette,Phys. Rev. Lett. , 152 (1995).[18] W. L. Yang, A. P. Sorini, C.-C. Chen, B. Moritz, W.-S. Lee, F. Vernay, P. Olalde-Velasco, J. D. Denlinger,B. Delley, J.-H. Chu, J. G. Analytis, I. R. Fisher, Z. A.Ren, J. Yang, W. Lu, Z. X. Zhao, J. van den Brink, Z.Hussain, Z.-X. Shen, and T. P. Devereaux, Phys. Rev. B , 014508 (2009).[19] T. Kataoka, M. Kobayashi, G. S. Song, Y. Sakamoto,A. Fujimori, F.-H. Chang, H.-J. Lin, D. J. Huang, C. T.Chen, S. K. Mandal, T. K. Nath, D. Karmakar, and I.Dasgupta, Jpn. J. Appl. Phys. , 04C200 (2009).[20] N. J. Kawai, T. Nakagawa, T. Kojima, K. Ohta, and M.Kawashima, Eelectro. Lett. , 47 (1984).[21] B. T. Thole, P. Carra, F. Sette, and G. van der Laan,Phys. Rev. Lett. , 1943 (1992).[22] P. Carra, B. T. Thole, M. Altarelli, and X. Wang, Phys.Rev. Lett. , 694 (1993).[23] C. Piamonteze, P. Miedema, and F. M. F. de Groot,Phys. Rev. B80