Journal of Magnetism and Magnetic Materials 240 (2002) 514­516 Observation of the bulk spin-flop in an Fe/Cr superlattice L. Botty!ana,*, L. De!aka, J. Dekosterb, E. Kunnenc, G. Langoucheb, J. Meersschautb, M. Majora,b, D.L. Nagya, H.D. R.uterd, E. Szil!agyia, K. Temstc a KFKI Research Institute for Particle and Nuclear Physics, P.O. Box 49, 1525 Budapest, Hungary b Instituut voor Kern-en Stralingsfysica, K.U. Leuven, Celestijnenlaan 200D B-3001, Leuven, Belgium c Laboratorium voor Vaste-Stoysica en Magnetisme, K.U. Leuven, Celestijnenlaan 200C B-3001, Leuven, Belgium d II. Institut f.ur Experimentalphysik Universit.at Luruper Chaussee 149 D-22761, Hamburg, Germany Abstract The layer magnetisation reorientation transition (spin-flop, SF) was studied in an artificial layer antiferromagnet (AF), namely in MgO(0 0 1)/[57Fe(2.6 nm)/Cr(1.3 nm)]20 epitaxial superlattice (SL) by synchrotron M.ossbauer reflectometry and Kerr effect (SMOKE). The SF occurs simultaneously in the entire SL stack (bulk SF) in an increasing field of HSF ¼ 13 mT along the easy direction parallel to the layer magnetisations. It is recognised by the kink in the SMOKE loop and by the sharp up-rise of the AF Bragg peak in the delayed M.ossbauer reflectivity. The moderate value of observed HSF is compared with estimations from a spin-chain model and interpreted as due to intraplane domain-wall motion during SF. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Artificial superlattices; Interlayer coupling; Kerr measurements; M.ossbauer spectroscopy; Synchrotron radiation An interesting model system of an `artificial layer records the total number of delayed photons as a antiferromagnet' is a periodic Fe/Cr antiferromagnetic function of the angle of grazing incidence Y: Structural (AF) superlattice (SL) with even number of Fe layers. Bragg peaks due to the electronic SL periodicity are When the external magnetic field is aligned along the observed in the prompt and in the delayed signal, but the easy axis of the Fe layers parallel/antiparallel to the magnetic (hyperfine) super-cell doubling in an AF SL magnetizations Mk ðk ¼ 1; 2; : : : ; 2n; 2n being the num- appears only in the delayed TISMR. The AF Bragg- ber of bilayers), the anisotropy-stabilised configuration peak intensity in TISMR is at maximum for the becomes energetically unfavourable at a certain critical photon's wave vector k; parallel/antiparallel to Mk; in-plane field strength and a sudden magnetisation and zero for k>Mk [3]. Therefore, SMR is especially reorientation is expected in a finite multilayer stack [1­ suitable for studying the spin-flop (SF) phenomena. 4] with surface spin-flop [5,6] or bulk spin-flop (BSF) [7] Here, we report on TISMR of the (bulk) SF in a Fe/Cr scenarios, in the cases of uniaxial and four-fold in-plane AF SL with a four-fold in-plane anisotropy. The anisotropy, respectively. observed HSF is compared with a spin-chain calculation Synchrotron M.ossbauer Reflectometry (SMR, [8­11]) with the aim of elucidating the magnetisation reorienta- is sensitive to the alignment of local hyperfine fields in tion mechanism. the film. Consequently, in an 57Fe-containing magnetic The [57Fe(2.6 nm)/Cr(1.3 nm)]n (n ¼ 20) periodic mul- SL, the Fe-layer magnetisation directions can be tilayer was grown on a MgO(0 0 1) substrate at 450 K by determined relative to the photon's propagation and MBE using an electron beam gun (Cr) and a Knudsen polarisation vectors [8,9]. Time integral (TI) SMR cell (57Fe) at a base pressure of 1 10 9 mbar following a degassing of the substrate at 873 K for 30 min. *Corresponding author. Tel.: +36-1-392-2761; fax: +36-1- RHEED patterns and high-angle X-ray diffractograms 395-9151. confirmed the epitaxial quality and excellent layering E-mail address: bottyan@rmki.kfki.hu (L. Botty!an). of the SL film. Low-angle X-ray diffraction at 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 9 4 - 0 L. Botty!an et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 514­516 515 The presence or absence of the 12 peak reveals if k8Mk or k>Mk: In the SL, Mk points parallel or anti-parallel to either of the Fe[0 1 0] or Fe[1 0 0] easy axes in the film plane, with AF domains oriented at random, parallel/antipar- allel with either of those (inset (1) in Fig. 1). The initial magnetic state was carefully prepared by aligning Mk along a single easy axis: the SL film was magnetised to 46 mT in order to induce an SF (inset (2)). Then, the field was decreased to 3 mT and the sample was rotated in plane by p=4: At this point, a TISMR scan was recorded, which is shown in Fig. 1(b). Since in this state (inset (3)), k>Mk; no AF superreflections were observed (Fig. 1b, inset (4)). Having increased the field to 35.3 mT, the 12-order AF Bragg peak appeared (Fig. 1c) as a direct evidence of the BSF. Due to the four-fold anisotropy, this state was preserved when the magnetic field was decreased again to 3 mT (inset (5)). Accord- ingly, the 12-order AF Bragg-peak intensity did not change (Fig. 1d). The BSF transition was also confirmed by SMOKE (Fig. 2). High-field loops (see inset in Fig. 2) were indicative of AF coupling and a saturation field of HSE0:9 T. First, the Mk were prepared in an easy direction of Fe (by exerting and releasing a saturating field). Afterwards, the sample was rotated in plane by p=4: A kink was observed in the first looparound HSF ¼ 13 mT, which did not re-occur until the sample Fig. 1. Prompt (a) and TISMR (b­d) scans of MgO(0 0 1)/ was turned to the perpendicular direction. This is in full [57Fe(2.6 nm)/Cr(1.3 nm)]20 superlattice taken in (b): 3 mT, (c): agreement with the TISMR scans. A difference between 35.3 mT and (d): repeated 3 mT magnetic fields, respectively. SMOKE and TISMR is that the latter probes the entire The appearance of the 12-order AF Bragg peak shows the multilayer stack at the AF Bragg angle, while the former reorientation of the layer magnetisations at a bulk SF transition remains more sensitive to the upper layers. The field HSF: The system of co-ordinates in the insets is fixed to the agreement indicates that the SF reorientation occurs substrate. simultaneously in the entire SL stack (bulk SF). In order to relate the layer parameters to the measured HSF; for simplicity, an infinite `two-sublattice' l ¼ 0:086 nm (Fig. 1a) showed extended Kiessig-fringes and structural SL reflections upto the third order (not shown) with a bilayer period of 3.9 nm and root-mean- 3 square interface roughness of 0.43 nm. The Fe/Cr 10 2 5 thickness ratio was determined by Rutherford back- 0 scattering. Conversion electron M.ossbauer spectroscopy 1 -5 revealed an in-plane orientation of the Fe moments, an -10 0 -1.0 -0.5 0.0 0.5 1.0 expected consequence of the shape anisotropy. H (T) Hsf 57Fe SMR experiments were performed on the BW4 -1 nuclear resonance beamline in HASYLAB, Hamburg, at up Kerr angle (mdeg) down room temperature in vertical scattering geometry. -2 up again Motorized permanent magnets provided horizontal -3 fields between 3 and 46 mT perpendicular to k: TISMR -30 -20 -10 0 10 20 30 scans were recorded at grazing angles between 0 and H (mT) 20 mrad. The SMR results are shown in Fig. 1. The solid Fig. 2. Surface magneto-optical Kerr loops of a superlattice lines in (a)­(d) are simulations [9,12]. Peaks labelled `0', MgO(0 0 1)/[57Fe(2.6 nm)/Cr(1.3 nm)]20. SF occurs around `12' and `1' are the total reflection peak [13,14], the AF HSF ¼ 13 mT only once following a 901 rotation of the Bragg peak and the structural Bragg peak, respectively. substrate relative to the field direction. 516 L. Botty!an et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 514­516 spin-chain scheme is invoked. The energy E per Support by the IHP Programme `Access to Research unit area of a SL with quadratic anisotropy (experi- Infrastructures' of the European Commission (Contract mentally found for Fe/Cr on MgO(0 0 1)) in an external HPRI-CT-1999-00040), the Flemish-Hungarian bilateral field H is Project No. BIL98/20 and Project No. T 029409 of the EðH; W Hungarian Scientific Research Fund (OTKA) is grate- 1; W2Þ ¼ J1 cosðW1 W2Þ J2 cos2ðW1 W2Þ fully acknowledged. J.M. and K.T. are Post-Doctoral þ Aðsin2 2W1 þ sin2 2W2Þ Fellows of the Flemish FWO. HMðcos W1 þ cos W2Þ; ð1Þ where J1; J2 and A are the bilinear and biquadratic References coupling coefficients between the two sublattices and the magneto-crystalline energy (J1; J2o0; A > 0). The bulk [1] F.C. N.otermann, R.L. Stamps, A.S. Carri-co, R.E. anisotropy energy K1 ¼ 4A=ttot Fe ; with ttot Fe and M being Camley, Phys. Rev. B 46 (1992) 10847. the Fe sublattice layer thickness (i.e. 26 nm in the present [2] M. Major, L. Botty!an, L. De!ak, D.L. Nagy, in: E.A. case), and moment per unit area, respectively, and G.orlich, A. Pedziwiatr (Eds.), Proceedings of the XXXIV, M ¼ njMkj; k ¼ 1; 2: M Zakopane School of Physics, Jagellonian University, 1 and M2 decline by W1 and W2; respectively, from the field (the field pointing along an Cracow, 1999, p. 165. easy direction of Fe). The SF and saturation occur in [3] M. Major, Master's Thesis, E.otv.os Lor!and University, increasing and decreasing fields, at which the energy Budapest, 1999 (in Hungarian). given by Eq. (1) is no longer positive-definite. The res- [4] A.L. Dantas, A.S. Carri-co, Phys. Rev. B 59 (1999) 1223. ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p ffi [5] R.W. Wang, D.L. Mills, E.E. Fullerton, J.E. Mattson, pective field values are H0SF ¼ 4 Að4A J1 þ 2J2Þ=M S.D. Bader, Phys. Rev. Lett. 72 (1994) 920. and HS ¼ 2ðJ1 þ 2J2 þ 4AÞ=M: (As shown by Dantas [6] N.S. Almeida, D.L. Mills, Phys. Rev. B 52 (1995) 13504. and Carri-co [4], HS remains unaffected, while the SF [7] K. Temst, E. Kunnen, V.V. Moshchalkov, H. Maletta, field H0SF is lowered in a finite SL stack due to the H. Fritzsche, Y. Bruynseraede, Physica B 276­278 (2000) dangling surface layers. For a strongly AF-coupled finite 684. SL ( J1 þ 2J2bA), this lower value of the SF field is ffiffi p ffi [8] D.L. Nagy, L. Botty!an, L. De!ak, E. Szil!agyi, H. Spiering, H J. Dekoster, G. Langouche, Hyperfine Interactions 126 SFEH0SF= 2: This latter HSF value is considered in the following estimations.) From the SMOKE loops, (2000) 349. H [9] L. De!ak, L. Botty!an, D.L. Nagy, H. Spiering, Phys. Rev. SE0:9 T. Using this and literature value of K1 ¼ 47 kJ/ m3 [15], H B 53 (1996) 6158. SF was calculated. Assuming a pure bilinear [10] R. R.ohlsberger, Hyperfine Interactions 123/124 (1999) coupling (J2=J1 ¼ 0), this gives Hcalc SF ¼ 260 mT: Allow- 455. ing for a variation of 0oJ2=J1o0:45; a range of Hcalc SF [11] A.I. Chumakov, D.L. Nagy, L. Niesen, E.E. Alp, was estimated. For K1 ¼ 47 kJ/m3, Hcalc SF >130 mT. Hyperfine Interactions 123/124 (1999) 427. Varying K1 in a range as broad as 24 kJ/m3o [12] H. Spiering, L. De!ak, L. Botty!an, Hyperfine Interactions K1o47 kJ/m3, Hcalc 125 (2000) 197. SF remains by a factor of 5 above the measured value. These facts imply that, as expected, [13] A.Q.R. Baron, J. Arthur, S.L. Ruby, A.I. Chumakov, rather than by coherent rotation of the sublattice G.V. Smirnov, G.S. Brown, Phys. Rev. B 50 (1994) 10354. magnetisations, the SF is likely to occur by intralayer [14] L. De!ak, L. Botty!an, D.L. Nagy, Hyperfine Interactions 92 domain wall motion in this artificial layer antiferro- (1994) 1083. magnet. The latter requires much lower field to over- [15] H.-P. Klein, E. Keller, Phys. Rev. 144 (1966) 372. [16] D.L. Nagy, L. Botty!an, B. Croonenborghs, L. De!ak, come the anisotropy barrier. The balance between the B. Degroote, J. Dekoster, H.J. Lauter, V. Lauter­Pasyuk, Zeeman energy and the anisotropy energy at the SF field O. Leupold, M. Major, J. Meersschaut, O. Nikonov, was found to be essential in shaping the AF domain A. Petrenko, R. R.uffer, H. Spiering, E. Szil!agyi, Phys. Rev. structure [16]. Lett, submitted for publication.