3B2v7:51c                                                                                 ED:chanakshi bg
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                                          Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]]
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7          Observation of the bulk spin-flop in an Fe/Cr superlattice
9
                   L. Botty!ana,*, L. De!aka, J. Dekosterb, E. Kunnenc, G. Langoucheb,
11    J. Meersschautb, M. Majora,b, D.L. Nagya, H.D. R.uterd, E. Szil!agyia, K. Temstc
13                           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
15                      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
17

19    Abstract
21       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
23    reflectometry and Kerr effect (SMOKE). The SF occurs simultaneously in the entire SLstack (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
25    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
27    domain-wall motion during SF. r 2001 Published by Elsevier Science B.V.
29    Keywords: Artificial superlattices; Interlayer coupling; Kerr measurements; M.ossbauer spectroscopy; Synchrotron radiation

31

33
         An interesting model system of an `artificial layer                     function of the angle of grazing incidence Y: Structural     57
35    antiferromagnet' is a periodic Fe/Cr antiferromagnetic                     Bragg peaks due to the electronic SLperiodicity are
      (AF) superlattice (SL) with even number of Fe layers.                      observed in the prompt and in the delayed signal, but the    59
37    When the external magnetic field is aligned along the                      magnetic (hyperfine) super-cell doubling in an AF SL
      easy axis of the Fe layers parallel/antiparallel to the                    appears only in the delayed TISMR. The AF Bragg-             61
39    magnetizations Mkšk ¼ 1; 2; 2nŽ; the anisotropy-stabi-                     peak intensity in TISMR is at maximum for the
      lised configuration becomes energetically unfavourable                     photon's wave vector k; parallel/antiparallel to Mk;         63
41    at a certain critical in-plane field strength and a sudden                 and zero for k>Mk [3]. Therefore, SMR is especially
      magnetisation reorientation is expected in a finite                        suitable for studying the spin-flop (SF) phenomena.          65
43    multilayer stack [1­4] with surface spin-flop [5,6] or bulk                Here, we report on TISMR of the (bulk) SF in a Fe/Cr
      spin-flop (BSF) [7] scenarios, in the cases of uniaxial and                AF SLwith a four-fold in-plane anisotropy. The               67
45    four-fold in-plane 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-     69
47    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-     71
49    SL, the
                                                                                                                                              73
51
                                                                                                                                              75
53
        *Correspo           UNCORRECTED PROOF
                        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
      records the total number of delayed photons as a                           a degassing of the substrate at 873 K for 30 min.
                                                                                 RHEED patterns and high-angle X-ray diffractograms
                                                                                 confirmed the epitaxial quality and excellent layering
                         nding author. Tel.: +36-1-392-2761; fax: +36-1-                                                                      77
55                                                                               of the SLfilm. L
                                                                                                           ow-angle X-ray diffraction at
      395-9151.
          E-mail address: battyan@rmki.kfki.hu (L. Botty!an).                    l ¼ 0:086 nm (Fig. 1a) showed extended Kiessig-fringes

      0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V.
      PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 9 4 - 0



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      2                          L. Botty!an et al. / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]]

1                                                                              In the SL, Mk points parallel or anti-parallel to either                                              57
                                                                             of the Fe[0 1 0] or Fe[1 0 0] easy axes in the film plane,
3                                                                            with AF domains oriented at random, parallel/antipar-                                                   59
                                                                             allel with either of those (inset (1) in Fig. 1). The initial
5                                                                            magnetic state was carefully prepared by aligning Mk                                                    61
                                                                             along a single easy axis: the SLfilm was magnetised to
7                                                                            46 mT in order to induce an SF (inset (2)). Then, the                                                   63
                                                                             field was decreased to 3 mT and the sample was rotated
9                                                                            through p=4: At this point, TISMR scan 1=b was                                                          65
                                                                             recorded. Since in this state (inset (3)), k>Mk; no AF
11                                                                           superreflections were observed (Fig. 1b, inset (4)).                                                    67
                                                                             Having increased the field to 35.3 mT, the 12-order AF
13                                                                           Bragg peak appeared (Fig. 1c) as a direct evidence of the                                               69
                                                                             BSF. Due to the four-fold anisotropy, this state was
15                                                                           preserved when the magnetic field was decreased again to                                                71
                                                                             3 mT (inset (5)). Accordingly, the 12-order AF Bragg-
17                                                                           peak intensity did not change (Fig. 1d).                                                                73
                                                                               The BSF transition was also confirmed by SMOKE
19                                                                           (Fig. 2). High-field loops (see inset in Fig. 2) were                                                   75
                                                                             indicative of AF coupling and a saturation field of
21                                                                           HSE0:9 T. First, the Mk were prepared in an easy                                                        77
                                                                             direction of Fe (by exerting and releasing a saturating
23                                                                           field), afterwards, the sample was rotated through p=4:                                                 79
                                                                             A kink was observed in the first loop around
25                                                                           HSF ¼ 13 mT, which did not re-occur until the sample                                                    81
                                                                             was turned to the perpendicular direction. This is in full
27                                                                           agreement with the TISMR scans. A difference between                                                    83
                                                                             SMOKE and TISMR is that the latter probes the entire
29    Fig. 1. Prompt (a) and TISMR (b­d) scans of MgO(0 0 1)/                multilayer stack at the AF Bragg angle, while the former                                                85
      [57Fe(2.6 nm)/Cr(1.3 nm)]20 superlattice taken in (b): 3 mT, (c):      remains more sensitive to the upper layers. The
31    35.3 mT and (d): repeated 3 mT magnetic fields, respectively.          agreement indicates that the SF reorientation occurs                                                    87
      The appearance of the 12-order AF Bragg peak shows the                 simultaneously in the entire SLstack (bulk SF).
33    reorientation of the layer magnetisations at a bulk SF transition        In order to relate the layer parameters to the                                                        89
      field HSF: The system of co-ordinates in the insets is fixed to the    measured HSF; for simplicity, an infinite `two-sublattice'
35    substrate.                                                             spin-chain scheme is invoked. The energy E per                                                          91
                                                                             unit area of a SLwith quadratic anisotropy (experi-
37                                                                           mentally found for Fe/Cr on MgO(0 0 1)) in an external                                                  93
      and structural SLreflections up to the third order (not
39    shown) with a bilayer period of 3.9 nm and root-mean-                                                                                                                          95
      square interface roughness of 0.43 nm. The Fe/Cr
41    thickness ratio was determined by Rutherford back-                                               3                                                                             97
      scattering. Conversion electron M.ossbauer spectroscopy                                                10
                                                                                                       2      5
43    revealed an in-plane orientation of the Fe moments, an                                                  0                                                                      99
      expected consequence of the shape anisotropy.                                                    1      -5
           57
45               Fe SMR experiments were performed on the BW4                                                -10
                                                                                                       0       -1.0 -0.5     0.0     0.5    1.0                                      101
      nuclear resonance beamline in HASYLAB, Hamburg, at                                                                    H (T)                             Hsf
47    room temperature in vertical scattering geometry.                                                -1                                                                            103
      Motorized permanent magnets provided horizontal                                                                                                                 up
                                                                                  Kerr angle (mdeg)                                                                   down 
49    fields between                                                                                   -2
                                                                                                                                                                      up again       105
                                                                                                       -3
51                                                                                                           -30      -20            -10           0    10           20        30    107
                                                                                                                                             H (mT)
53                       UNCORRECTED PROOF
                          3 and 46 mT perpendicular to k: TISMR
      scans were recorded at grazing angles between 0 and
      20 mrad. The SMR results are shown in Fig. 1. The solid
      lines in (a)­(d) are simulations [9,12]. Peaks labelled `0',
      `12' and `1' are the total reflection peak [13,14], the AF             Fig. 2. Surface magneto-optical Kerr loops of a superlattice                                            109
      Bragg peak and the structural Bragg peak, respectively.                MgO(0 0 1)/[57Fe(2.6 nm)/Cr(1.3 nm)]20. SF occurs around
55    The presence or absence of the 12 peak reveals if k8Mk or              HSF ¼ 13 mT only once following a 901 rotation of the                                                   111
      k>Mk:                                                                  substrate relative to the field direction.



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                                                                                                      L. Botty!an et al. / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]]                                                              3

1     field H is                                                                                                                                                                     HPRI-CT-1999-00040), the Flemish-Hungarian bilateral                  43
      EšH; W                                                                                                                                                                         Project No. BIL98/20 and Project No. T 029409 of the
                1; W2Ž ¼   J1 cosšW1   W2Ž   J2 cos2šW1   W2Ž
3                                                                                                                                                                                    Hungarian Scientific Research Fund (OTKA) is grate-                   45
                                               ž Ašsin2 2W1 ž sin2 2W2Ž                                                                                                              fully acknowledged. J.M. and K.T. are Post-Doctoral
5                                                HMšcos W1 ž cos W2Ž;                                                                                                         š1Ž    Fellows of the Flemish FWO.                                           47
      where J1; J2 and A are the bilinear and biquadratic
7     coupling coefficients between the two sublattices and the                                                                                                                                                                                            49
      magneto-crystalline energy (J1; J2o0; A > 0). The bulk
9                                                                                                                                                                                    References
      anisotropy energy K                                                                                                                                                                                                                                  51
                                                                                    1 ¼ 4A=ttot
                                                                                                                                        Fe ; with ttot
                                                                                                                                                    Fe and M being
      the Fe sublattice layer thickness (i.e. 26 nm in the present                                                                                                                    [1] F.C. N.otermann, R.L. Stamps, A.S. Carri-co, R.E.
11    case), and moment per unit area M ¼ njM                                                                                                                                                                                                              53
                                                                                                                                                          kj=2; k ¼ 1; 2:                Camley, Phys. Rev. B 46 (1992) 10847.
      M1 and M2 decline by W1 and W2; respectively, from the                                                                                                                          [2] M. Major, L. Botty!an, L. De!ak, D.L. Nagy, in: E.A.
13    field (the latter pointing along an easy direction of Fe).                                                                                                                         G.orlich, A. Pedziwiatr (Eds.), Proceedings of the XXXIV,         55
      The SF and saturation occur in increasing and decreas-                                                                                                                             Zakopane School of Physics, Jagellonian University,
15    ing fields, at which the energy given by Eq. (1) is no                                                                                                                             Cracow, 1999, p. 165.                                             57
      longer positive-definite. The respective field values are                                                                                                                       [3] M. Major, Master's Thesis, E.otv.os Lor!and University,
                    ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
                  p                                                                             ffiffiffiffiffiffiffiffiffiffiffiffi
17    H0                                                                                                                                                                                 Budapest, 1999 (in Hungarian).                                    59
        SF ¼ 4      Aš4A   J1 ž 2J2Ž=M                                                                                                       and     HS ¼  2šJ1 ž
      2J                                                                                                                                                                              [4] A.L. Dantas, A.S. Carri-co, Phys. Rev. B 59 (1999) 1223.
            2 ž 4AŽ=M: (As shown by Dantas and Carri-co [4],
19    H                                                                                                                                                                               [5] R.W. Wang, D.L. Mills, E.E. Fullerton, J.E. Mattson,             61
        S remains unaffected, while the SF field H0SF is lowered                                                                                                                         S.D. Bader, Phys. Rev. Lett. 72 (1994) 920.
      in a finite SLstack due to the dangling surface layers.                                                                                                                         [6] N.S. Almeida, D.L. Mills, Phys. Rev. B 52 (1995) 13504.
21    For a strongly AF-coupled finite SL( J                                                                                                                                                                                                               63
                                                                                                                                                           1 ž 2J
                                                                                                                                                              p 2bA),
                                                                                                                                                                 ffiffiffi            [7] K. Temst, E. Kunnen, V.V. Moshchalkov, H. Maletta,
      this lower value of the SF field is HSFEH0SF= 2: This                                                                                                                              H. Fritzsche, Y. Bruynseraede, Physica B 276­278 (2000)
23    latter H                                                                                                                                                                                                                                             65
                  SF value is considered in the following estima-                                                                                                                        684.
      tions.) From the SMOKE loops, HSE0:9 T. Using                                                                                                                                   [8] D.L. Nagy, L. Botty!an, L. De!ak, E. Szil!agyi, H. Spiering,
25    this and literature value of K                                                                                                                                                                                                                       67
                                                                                                                                          1 ¼ 4:5 kJ/m3 [15], HSF                        J. Dekoster, G. Langouche, Hyperfine Interactions 126
      was calculated. Assuming a pure bilinear coupling                                                                                                                                  (2000) 349.
27    (J                                                                                                                                                                              [9] L. De!ak, L. Botty!an, D.L. Nagy, H. Spiering, Phys. Rev.        69
            2=J1 ¼ 0), this gives Hcalc
                                                                                                                    SF ¼ 260 mT: Allowing for a
      variation of 0oJ                                                                                                                                                                   B 53 (1996) 6158.
                                                                       2=J1o0:45; a range of Hcalc
                                                                                                                                                               SF             was
29    estimated. For K                                                                                                                                                               [10] R. R.ohlsberger, Hyperfine Interactions 123/124 (1999)           71
                                                            1 ¼ 4:7 kJ/m3, Hcalc
                                                                                                                                            SF >130 mT. Varying                          455.
      K1 in a range as broad as 2.4 kJ/m3oK1o4.7 kJ/m3,                                                                                                                              [11] A.I. Chumakov, D.L. Nagy, L. Niesen, E.E. Alp,
31    Hcalc                                                                                                                                                                                                                                                73
        SF remains by a factor of 5 above the measured value.                                                                                                                            Hyperfine Interactions 123/124 (1999) 427.
      These facts imply that, as expected, rather than by                                                                                                                            [12] H. Spiering, L. De!ak, L. Botty!an, Hyperfine Interactions
33    coherent rotation of the sublattice magnetisations, the                                                                                                                            125 (2000) 197.                                                   75
      SF is likely to occur by intralayer domain wall motion in                                                                                                                      [13] A.Q.R. Baron, J. Arthur, S.L. Ruby, A.I. Chumakov,
35    this artificial layer antiferromagnet. The latter requires                                                                                                                         G.V. Smirnov, G.S. Brown, Phys. Rev. B 50 (1994) 10354.           77
      much lower field to overcome the anisotropy barrier.                                                                                                                           [14] L. De!ak, L. Botty!an, D.L. Nagy, Hyperfine Interactions 92
37    The balance between the Zeeman energy and the                                                                                                                                      (1994) 1083.                                                      79
      anisotropy energy at the SF field was found to be                                                                                                                              [15] H.-P. Klein, E. Keller, Phys. Rev. 144 (1966) 372.
39    essential in shaping the AF domain structure [16].                                                                                                                             [16] D.L. Nagy, L. Botty!an, B. Croonenborghs, L. De!ak,              81
                                                                                                                                                                                         B. Degroote, J. Dekoster, H.J. Lauter, V. Lauter­Pasyuk,
                                                                                                                                                                                         O. Leupold, M. Major, J. Meersschaut, O. Nikonov,
41          Support by the IHP Programme `Access to Research                                                                                                                                                                                               83
                                                                                                                                                                                         A. Petrenko, R. R.uffer, H. Spiering, E. Szil!agyi, Phys. Rev.
      Infrastructures' of the European Commission (Contract                                                                                                                              Lett, submitted for publication.






                                          UNCORRECTED PROOF