Fumagalli, Paul

Karoutsos, V.

Pappas, S. D.

Politis, C.

Poulopoulos, Panos

Velgakis, M. J.

Institutional Repository of the Freie Universität Berlin

 Observation of irregular labyrinthine magnetic domains with dendritic edges in a Co-rich CoCu alloyed filmS. D. Pappas1, P. Poulopoulos2,1,3,*, V. Karoutsos3,P. Fumagalli2,  M.J. Velgakis1 and C. Politis1,41Laboratory of High-Tech Materials, School of Engineering, University of Patras, 26504 Patras, Greece2Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany3Materials Science Department, University of Patras, 26504 Patras, Greece4Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019, USA Submitted: 30 May 2012  We report on the observation of irregular labyrinthine magnetic domains with dendritic edges in a 1500 nm thick Co96Cu4 alloyed film grown via radio-frequency magnetron sputtering on a polyimide substrate. The film shows strong texture along the  c-axis  of  the  hexagonal  closed  packed  system. As  a  result,  its  magnetic properties  resemble  those  of  single-crystalline  hexagonal  (001)  Co films.  Indeed, magnetization hysteresis loops recorded under fields up to 16 kOe via a home-made magneto-optic Kerr-effect magnetometer reveal strong tendency for perpendicular magnetic anisotropy. Hysteresis loops with the field parallel or perpendicular to the film plane present zero remanence. Magnetic force microscopy after perpendicular magnetization  of  the  film  shows  the  formation  of  regular  bubble  ferromagnetic domains  in  a  metastable  state  with  the  local  spins  oriented  up  and  down, perpendicular to the film plane. After perpendicular demagnetization, however, the  labyrinthine ferromagnetic domains show irregularly distributed dendritic edges. This effect is attributed to Cu precipitates in a Co matrix, which act as pinning sites in the free motion of domain walls.             Keywords: Magnetic thin films, magnetic force microscopy, bubble, labyrinthine, dendritic domains, Cobalt, pinning sites* Corresponding author: P. Poulopoulos, Tel. (+49) 30 838 56078, Fax (+49) 30 838 56299, email: poulopoulos@zedat.fu-berlin.de 1Progress in magnetic domain imaging by Scanning Probe Microscopy techniques has allowed for high-resolution study of ferromagnetic domains formed on magnetic thin films and nanostructures and its correlation with structure and magnetic anisotropy.1-6 Films with perpendicular  magnetic  anisotropy (PMA)  are  important  for  magnetic  and  magneto-optic recording  applications,  see  e.g.  Ref.  [7].  When  PMA  is  weak,  the  films  show  canted magnetization configurations and a mixture of parallel and perpendicular magnetic domains.8,9 On the other hand, films with a significant  PMA component  show usually three types of ferromagnetic domains depending on the magnetic history of the sample: Bubble magnetic domains for field slightly smaller than the nucleation field or at remanence after perpendicular magnetization  of  the  film,  stripe  magnetic  domains  after  parallel  magnetization  or demagnetization of the sample and labyrinthine domains after perpendicular demagnetization of the samples.5 In all such cases, the spins point up or down with respect to the film plane and the domain edges (boarders) are usually smooth.In this communication, we report the formation of irregular labyrinthine magnetic do-mains with dendritic edges in a thick Co-rich CoCu film with very strong c-axis hexagonal closed packed (hcp) texture. Domain images showing some similarity to our results have been recorded on different materials and with different physical properties compared to our system; namely, either on thermomechanically processed (die-upset) melt-spun NdFeB bulk hard per-manent  magnets  with  very large coercivity (interaction  domains),10 or  on magnetostrictive FeGa bulk alloys,11,12 related to inhomogeneous stress. However, to our knowledge, such do-main patterns have not been reported previously for magnetic thin films. Our film has a signi-ficant component of PMA, as magneto-optic Kerr effect (MOKE) hysteresis loop reveal, al-lowing for the formation of magnetic domains (bubbles, labyrinth or stripes) with spins ori-ented up or down with respect to the film plane. The particular and unusual decoration of the 2labyrinth domain edges is attributed to  Cu precipitates in a Co matrix, which act as pinning sites in the free motion of domain walls. A Co96Cu4 film, 1500 nm thick,  was deposited on polyimide (Kapton) substrate by radio-frequency (r.f.) magnetron sputtering at about 400 K. The thickness was selected to be in the range of 1 micron, as we have shown recently that in such a case films with strong texture are favored.13 The base pressure of the vacuum chamber was 5 x 10-8 mbar. The Ar pressure during deposition had been kept constant with the help of a fine valve at a pressure of about 6x10-3 mbar. The r.f. power had been kept constant at 50 W with the help of a feedback power supply and a matching circuit. This power resulted in a deposition rate of 0.4 nm/s. Deposition rate was monitored with the help of a precalibrated quartz balance. The calibration was performed as follows: In the inset of Fig. 1, we see the appearance of Kiessig fringes in the  small-angle XRD spectrum of  a thin  CoCu film.  These are  present  for  homogeneous samples  with  small  surface roughness;  with  the help of  them,  the  film thickness  may be accurately determined.7 In order to form the alloy, a Cu foil (99.9 at.% purity) was sandwiched between a Co foil (99.99 at.% purity), having small holes randomly distributed on its surface, and  the  water-cooled  magnetron  head  with  the  help  of  attractive  magnetic  forces. The composition of the sample has been checked by  energy-dispersive X-ray analysis (EDAX) measurements  carried  out  in  a  Scanning  Electron  Microscope  (SEM)  Zeiss  Supra  35VP model. In our film, the Co concentration was found to be 96 at.%. The structural characterization was mainly performed with the help of x-ray diffraction (XRD) patterns recorded using a standard powder diffractometer (Berthold) with Ni- and Co- filtered CuKa1 radiation (λ = 0.154059 nm).  The magnetic anisotropy of the samples was studied by recording hysteresis loops with the applied field parallel and perpendicular to the film  plane  at  room  temperature  via  a  home-made  magneto-optic  Kerr  effect  (MOKE) magnetometer.14 The  magnetic domain morphology in our films has been observed at room 3temperature   by  using  the  Magnetic  Force  Microscopy  (MFM)  mode  of  a  Multimode Microscope with a Nanoscope IIIa controller and a 120 μm x 120 μm magnet-free scanner (Model AS-130VMF) developed by Digital Instruments. We used a Co/Cr coated Si probe tip whose magnetization is parallel to its axis. The images were recorded at magnetic remanence by using the amplitude detection mode.1In Fig. 1, the high-angle XRD pattern of the 1500 nm Co96Cu4 film grown on Kapton is shown. The reference data for polycrystalline hcp and face centered cubic (fcc) Co powders have also been also introduced in Fig. 1.15,16 The amount of fcc grains is obviously negligible as (i) the fcc(200) diffraction peak is very small and (ii) the largest diffraction peak is much closer to the hcp(002) than the fcc(111). The slight difference of 2θ = 0.070 in the (002) peak of our sample with respect to Co-reference may be attributed to the presence of some residual strain of about 0.15% due to growth conditions and the diferrence of the thermal expansion coefficient between the metallic film and the polymer substrate. Similar shifts in the (002) peak have been also observed in pure Co films grown under similar experimental conditions.9 The presence of Cu should not have any effect due to the immiscibility between Co and Cu. Moreover, the fact that the hcp(002) peak is much larger than all the other hcp ones, unlike the ratios of the vertical lines which correspond to a reference random polycrystalline powder,15 reveals that there is a strong texture with the c-axis perpendicular to the film plane. We have shown recently that ~ 1  µm thick  CoPd alloyed films prepared under similar experimental conditions  show perfect  fcc {111} texture and this  has a large influence on the magnetic properties.13 Texture  is  influenced  mainly by the  total  film  thickness  due  to  competition between  residual  strain,  which  may  vary  with  film  thickness  and  surface  energy.17-19 Application of the Scherrer formula to the  hcp(002) peak taking the corrected full width at half-maximum (FWHM),13 estimates a relatively large average grain size of about 85 nm. 4In  Fig.  2  we  show  hysteresis  loops  of  the  Co96Cu4 film  recorded  via  MOKE magnetometry with the external magnetic field H applied parallel and perpendicular to the film plane, as indicated. MOKE is not an absolute magnetometry technique as, for example, Superconducting Quantum Interference Device (SQUID).20 For this reason, we have calibrated the y axis in magnetization units taking the magnetization for bulk CoCu alloys of the same composition as ours from literature data.21  The sample seems to be easier magnetized in the film plane,  however,  zero remanence  is  observed for both field directions.  From the area between the anhysteretic hard- and the easy-magnetization axes,  one can calculate  the so-called  effective  magnetic  anisotropy energy Keff to  be  equal  to  6  x  106 erg/cm3.  If  one considers Keff to be the sum of the shape anisotropy Kd  = 2πM2 and a first  order uniaxial anisotropy Ku1, then Ku1 = -5.3 x 106 erg/cm3. (The sign ‘-’ in our case reveals tendency for PMA. This tendency increases with the absolute value of the quantity Q = Ku1/Kd  ;6 for our film Q = 0.47) The absolute value of Ku1 is  even by about 20% larger than the value of magnetocrystalline anisotropy of single crystalline  hcp Co along the c axis. As discussed in Ref. [13], error bars at least as large as 10% in the determination of the anisotropy constants  by the hysteresis loops may be expected for films with Q values like ours due to appearance of perpendicular magnetic domains. The excess of the uniaxial anisotropy with PMA tendency in our film compared to epitaxial Co should be attributed to the magnetoelastic anisotropy due to the presence of residual anisotropic strain in our film, for example due to the large difference between  the  thermal  expansion  coefficient  between  the  metallic  film  and  the  polymer substrate.   In Fig. 3(a), we present the topography of our film. One may see the presence of small crystallites with a mean diameter of about 85 nm, as aforementioned. In Fig. 3(b), a MFM image  of  our  film  is  presented  after  perpendicular  magnetization.  This  process  includes magnetization  to  the  perpendicular  saturation  field  and  sudden  removal  of  this  field.5 5Magnetic domains are formed at the nucleation point (which in our case is practically equal to the saturation field), and have the configuration of magnetic bubbles. As the field is removed fast,  the system does not have the time to reach equilibrium at remanence.  Therefore, the bubble configuration is clearly maintained. This is one of the few times that many clearly circular  bubbles  are  observed  as  usually  they  are  elongated  and  merge  tending  to  be transformed  into  labyrinths.5 Even  more  spectacular  is  the  image  of  Fig.  3(c)  after perpendicular  demagnetization.  This  means  that  after  perpendicular  saturation  the  film  is demagnetized by periodic decrease of the field amplitude.5 One may observe at remanence the appearance of irregular labyrinthine domains with dendritic edges. In the present work, the deposition process and film thickness are similar to the ones of fully textured Co58Pd42 alloyed thin film.13 Moreover, the magnetic Q value of our Co96Cu4 film is almost equal to the one of a fully textured Co58Pd42 film. The latter film showed, however, regular labyrinthine magnetic domains after perpendicular demagnetization without any decoration of the edges. A typical image of this kind is presented in Fig. 3(d). The only difference between the two films is that, according to the binary alloy phase diagrams and verified experimentally, Co and Pd form solid solutions and therefore the CoPd film is structurally and magnetically homogeneous,13 while Co and Cu do not show any miscibility.22 Therefore, presence of Cu precipitates in a Co matrix  is  expected.  The Cu nanograins  should act  as  obstacles  (pinning sites)  in  the free motion  of  domain  walls  during  the  slow  pseudo-static  demagnetization  process,  and  the labyrinthine domains present irregular dendritic edges.          In conclusion, we have investigated the magnetic properties of a  Co96Cu4 film with a thickness  of  1500  nm,  deposited  on  polyimide  by  r.f.  magnetron  sputtering.  The  film presented a  hcp structure with very strong c-axis texture. As a result,  it  showed enhanced perpendicular  magnetic  anisotropy  as  compared  to  epitaxial  single-crystalline  Co  films.5 Bubble magnetic domains were observed at remanence after perpendicular magnetization of 6the film; some of them retained their original circular shape as they did not merge with others. Finally, we showed the unusual formation of irregular labyrinthine magnetic domains with dendritic edges after perpendicular demagnetization of the film. This effect was attributed to structural  and local  magnetic  inhomogeneity of  the film due to  separation  of  Co and Cu phases.  Acknowledgements: The authors would like to acknowledge the Grant Karatheodori 2009, number C.905 of the Research Committee of the University of Patras. Dr. V. Drakopoulos ICEHT-FORTH, Greece, is acknowledged for the SEM-EDS analysis. One of us (P.P.) thanks the Center for International Cooperation (C.I.C.) of the Freie Universität  Berlin for partial financial support and the Institute for Experimental Physics, Freie Universität Berlin for the great hospitality. 7References1. V. Karoutsos, J. Nanosci. Nanotechnol. 9, 6783 (2009).2. N. Amos, R. Ikkawi, R. Haddon, D. Litvinov, and S. Khizroev, Appl. Phys. Lett. 10, 5692 (2008).3. M. R. Loblischka and U. Hartmann, Ultramicroscopy 97, 103 (2003).4. M. Pratzer, H.J. Elmers, M. Bode, O. Pietzsch, A. Kubetzka and R. Wiesendanger, Phys. Rev. Lett. 87, 127201 (2001).5. M. Hehn, S. Padovani, K. Ounadjela, and J.P. Bucher, Phys. Rev. B54, 3428 (1996).6. A.  Hubert,  and  R.  Schäfer:  Magnetic  Domains:  The  Analysis  of  Magnetic Microstructures, Springer – Verlag, Berlin Heidelberg (1998), 3rd Printing (2009).7. P. Poulopoulos, M. Angelakeris, E.Th. Papaioannou, N.K. Flevaris, D. Niarchos, M. Nyvlt,  V.  Prosser,  S.  Visnovsky,  Ch.  Mueller,  P.  Fumagalli,  F.  Wilhelm,  and  A. Rogalev, J. Appl. Phys. 94, 7662 (2003).8. R. Froemter,  H. Stillrich,  C.  Menk and H.P.  Oepen,  Phys.  Rev.  Lett. 100, 207202 (2008).9. V. Karoutsos, P. Poulopoulos, V. Kapaklis, S.D. Pappas, D. Trachylis, and C. Politis, J. Nanosci. Nanotechnol. 10, 6120 (2010).10. K. Khlopkov, O. Gutfleisch, D. Hinz, K.-H. Mueller and L. Schultz,  J. Appl. Phys. 102, 023912 (2007).11. Feiming Bai, Jiefang Li, D. Viehland, D. Wu and T. A. Lograsso,  J. Appl. Phys. 98, 023904 (2005).12. H.Z. Song, Y.X. Li, K.Y. Zhao, H.R. Zeng and S.X. Hui, G.R. Li, Q.R. Yin and G.H. Wu, J. Appl. Phys. 105, 013913 (2009).813. A. Vlachos, S.D. Pappas, V. Kapaklis,  V. Karoutsos,  A. Kordatos, F. Wilhelm,  A. Rogalev,  P.  Fumagalli,  P.  Poulopoulos,  M.J.  Velgakis  and  C.  Politis,  J.  Nanosci.  Nanotechnol. xx, xxxx (2012).14. V. Kapaklis, S. D. Pappas, P. Poulopoulos, D. Trachylis, P. Schweiss and C. Politis, J. Nanosci. Nanotechnol. 10, 6024 (2010).15. JCPDS Card No. 050727 (unpublished).16. JCPDS Card No. 150806 (unpublished).17. R. Daniel, K.J. Martinschitz, J. Keckes, and C. Mitterer,  Acta Materialia 58, 2621  (2010).18. A.R. Shetty and A. Karimi, Appl. Surf. Sci. 258, 1630 (2011).19. B. Viswanath,  Changhyun Ko,  and Shriram Ramanathan,  Philos.  Magaz. 91,  4311 (2011).20. A. Ney, P. Poulopoulos, M. Farle, and K. Baberschke, Phys. Rev. B62, 11336 (2000).21. J.R. Childress and C.L. Chien, Phys. Rev. B43, 8089 (1991).22. A. Hütten and G. Thomas, Ultramicroscopy 52, 581 (1993).9Figure CaptionsFigure 1 (coloured on-line)High-angle XRD pattern of the 1500 nm Co96Cu4 film grown on Kapton. The reference data for random polycrystalline hcp and fcc Co powders have been also introduced by vertical lines ended in squares or circles, respectively. The inset shows Kiessig fringes for a thin sample. With the help of them, we determine the film thickness and calibrate our quartz balance.Figure 2 (coloured on-line)Hysteresis loops of the Co96Cu4 film recorded via MOKE magnetometry with the external magnetic field H applied parallel and perpendicular to the film plane, as indicated. The Y-axis has been calibrated with the help of Ref. [21]. Figure 3 (coloured on-line please use double column width)Topography (a) and magnetic domain configurations after (b) perpendicular magnetization and (c) perpendicular demagnetization of the Co96Cu4 film. In (b) we may observe bubble magnetic  domains  and in  (c)  irregular  labyrinthine  ones.  In (d)  a  MFM image of  regular labyrinthine  domains  for  a  CoPd  film  is  presented  for  comparison.  Light  (dark)  color corresponds to up (down) oriented domains with respect to the film plane. The images size is 10x10 μm2.10Figure 1, Pappas et al.Observation of irregular labyrinthine magnetic domains with dendritic edges in a Co-rich CoCu alloyed film11Figure 2, Pappas et al.Observation of irregular labyrinthine magnetic domains with dendritic edges in a Co-rich CoCu alloyed film12Figure 3, Pappas et al.Observation of irregular labyrinthine magnetic domains with dendritic edges in a Co-rich CoCu alloyed film13

Observation of Irregular Labyrinthine Magnetic Domains with Dendritic Edges in
a Co-Rich CoCu Alloyed Film

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Observation of Irregular Labyrinthine Magnetic Domains with Dendritic Edges in a Co-Rich CoCu Alloyed Film

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