APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 7 13 AUGUST 2001 Application of off-specular polarized neutron reflectometry to measurements on an array of mesoscopic ferromagnetic disks K. Temsta) and M. J. Van Bael Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium H. Fritzsche Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany Received 12 February 2001; accepted for publication 4 June 2001 Using off-specular polarized neutron reflectometry with neutron spin analysis, we determined the magnetic properties of a large array of in-plane magnetized ferromagnetic Co disks. Resonant peaks are clearly observed in the off-specular reflectivity, due to the lateral periodicity of the disk array. Using polarized neutrons, the intensity of the resonant peak in the off-specular reflectivity is studied as a function of the magnetic field applied in the sample plane. Spin analysis of the reflected neutrons reveals the magnetization reversal and saturation within the disks. © 2001 American Institute of Physics. DOI: 10.1063/1.1389764 Recent advances in lithography and deposition tech- while Au is evaporated from a Knudsen cell at a rate of 0.25 niques have created the possibility to produce high-quality Å/s, both at a working pressure of 10 10 mbar. X-ray diffrac- micron and nanometer sized magnetic structures.1 The moti- tion indicates that the disks are polycrystalline. The total vation is twofold: first, new physical effects emerge when the patterned area of the sample is 2 2 cm2. Figure 1 shows an mesoscopic regime is explored in which the size of the mag- optical microscope picture of the sample. The disks have a netic structures becomes comparable to some relevant physi- diameter of 4 m and are placed in a square pattern with cal length2 or when the magnetic entities interact with each sides of 10 m. As indices for the directions on the sample other,3 or with a semiconducting layer,4 or a superconducting surface lower right inset of Fig. 1 , 10 and 01 are used layer.5,6 The second reason is that the newly gained insight for the directions along the rows of disks, while 11 indi- into the physics of mesoscopic magnetic structures is applied cates the diagonal direction of the array. The upper left inset to large-scale industrial applications in, e.g., magnetic stor- of Fig. 1 shows a more detailed picture of a single disk, age media, computer memories, and sensors. The most com- obtained by atomic force microscopy AFM . The AFM re- mon experimental tools which are used in the study of these sults show that the surface roughness of the disks equals 7 magnetic structures are typically magnetization measure- 1 Å on an area of 1 m2, which is comparable to that of ments, electrical transport magnetoresistance , and ever an unpatterned film. The magnetic domain structure within more frequently imaging methods like magneto-optical Kerr the disks was imaged by MFM, revealing that the disks are microscopy or magnetic force microscopy MFM . typically in a multidomain state. MFM images of the rema- The structural properties of arrays of dots have been in- nent state after saturation in a magnetic field parallel to the vestigated by means of specular and off-specular x-ray re- substrate plane reveal that all disks have a very similar do- flectivity measurements.7 By means of the magneto-optical main state. This implies that the magnetic behavior measured Kerr effect MOKE , different reflection orders can be probed to study the magnetic properties of magnetic arrays.8,9 It has remained an interesting challenge, however, to exploit the huge potential of polarized neutron reflecto- metry PNR 10­14 in the study of the structural and magnetic properties of patterned magnetic elements.15,16 In this letter, we report on the observation of resonant peaks in off- specular reflectivity using polarized neutrons with spin analysis to monitor the magnetization reversal in a huge ar- ray of ferromagnetic disks. Periodic arrays of magnetic disks were produced by UV lithography and molecular beam deposition. A Si/SiO2 wafer was covered with a resist mask, into which the film was deposited. A lift-off step removes the photoresist. Each disk consists of a Au 75 Å /Co 200 Å /Au 75 Å trilayer. Co is FIG. 1. Optical microscope picture of the sample, showing the arrangement evaporated from an electron beam gun at a rate of 0.45 Å/s, of the Co disks in a square lattice with a 10 m period. The length of the white bar corresponds to 20 m. The inset upper left corner shows an AFM image of a single disk. In the lower right corner the in-plane directions a Electronic mail: kristiaan.temst@fys.kuleuven.ac.be are defined. 0003-6951/2001/79(7)/991/3/$18.00 991 © 2001 American Institute of Physics Downloaded 29 Nov 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp 992 Appl. Phys. Lett., Vol. 79, No. 7, 13 August 2001 Temst, Van Bael, and Fritzsche ( 0) direction.19 A patterned sample presents a special type of roughness: periodic roughness with a well- determined amplitude and lateral length scale. The off- specular reflectivity then shows a clear set of satellites, which can be attributed to the lateral periodicity.7 These sat- ellites are visible at the left of the specular peak Fig. 2 . Satellites at 0 can also be expected, but with a lower intensity, because of the larger magnitude of the scattering vector and hence a lower reflectivity. In this particular ex- periment, no satellites at 0 were observed. They were visible, though, in other scans with smaller . The position of the satellites scales with according to the formula qx (2 / ) cos( ) cos( ) with qx the in-plane compo- nent of the scattering vector, which is related to the real space periodicity d 2 /qx . The position of the satellites FIG. 2. Specular and off-specular reflectivity for a neutron beam along the ( 0.36 for 10 and 0.28 for 11 reflects the pe- 10 direction 0.57°; closed symbols and 11 direction 0.55°; riodicity d 2 /q open symbols . The inset shows the scattering geometry. For clarity, the x 10 and 14 m along 10 and 11 , re- curves are shifted vertically. In both cases, the peak at 0 corresponds to spectively. the specular reflection, while the peak at 0 is due to the lateral period- In order to extract magnetic information about the disk icity of the disk array. lattice, experiments with polarized neutrons were carried out with spin analysis of the reflected neutrons. was fixed at in the PNR experiments may be related to the properties of 0.5° i.e., qz 0.024 Å 1 , while H was increased from 0 to an individual disk. The in-plane saturation field H about 200 G. The magnetic state of the disks was studied by s of the disks was determined from superconducting quantum inter- monitoring the integrated intensity of the off-specular satel- ference device SQUID magnetization measurements to be lite. The intensity of the four polarized beam cross sections about 400 Oe. the four possible combinations of incident and reflected The PNR experiments were performed at the V6 reflec- neutron spin was measured as a function of H. The nonspin tometer at the Hahn-Meitner-Institut, Berlin, using cold neu- flip NSF intensities I and I are generated by mag- trons wavelength 0.466 nm . This reflectometer uses a netization components anti parallel to the neutron spin, graphite monochromator, a liquid-nitrogen cooled Be filter, a while the spin flip SF intensities I and I are gener- set of two diaphragms, and a polarizing supermirror, creating ated by magnetization components perpendicular to the neu- a polarized, monochromated, well-collimated neutron tron spin. Figure 3 shows the intensity of the off-specular beam.17,18 The experiments were carried out at room tem- satellite as function of H for two different initial configura- perature and in ambient conditions. A magnetic field H par- tions obtained after saturating the disks in a field Hprior along allel to the film plane and perpendicular to the path of the different directions. In Fig. 3 a the neutron beam was in the neutron beam, i.e., perpendicular to the scattering plane was 11 direction, and H was in the sample plane, perpendicular applied using an electromagnet. The neutron spin was to the neutron beam. The sample was initially in the rema- aligned parallel or antiparallel to the applied field by a nent state after saturation with Hprior opposite to H. The SF Mezei-type flipping coil. The neutron intensities were mea- contribution is almost zero at all H, indicating that there are sured using a position-sensitive detector, specifically, a mul- negligible magnetization components perpendicular to the tiwire proportional counter with an active area of 180 neutron spin. At the smallest H, I I , indicating that a 180 mm2 and a spatial resolution of about 1.5 mm. certain fraction of the disks has a remanent magnetization Specular reflectivity measurements with a polarized neu- component parallel to Hprior . As H is increased and magne- tron beam did not provide information about the magnetic tization reversal within the disks sets in, I increases while state of the disks, i.e., no splitting between the spin-up and I decreases. This splitting between I and I saturates spin-down reflectivities was observed. The specular reflec- when the disk magnetization is saturated in the direction of tivity is dominated by the nonmagnetic substrate since only H. Note that the off-specular intensities do show clear mag- 12% of the sample surface is covered by magnetic material. netic splitting, in contrast to our observations of the specular We can obtain magnetic information, however, from the off- reflectivity. Due to the small film thickness, the disk magne- specular reflectivity which shows resonant intensity peaks tization is confined to the sample plane and can be reversed resulting from the in-plane periodicity of the disk array, by domain wall motion and/or coherent rotation. No increase analogous to the interference pattern of a laser-illuminated of the SF intensities is observed, indicating that there is no optical grating. Figure 2 shows the reflected intensity for a coherent rotation of the magnetization during which magne- neutron beam incident along the 10 and 11 directions. tization components perpendicular to the neutron spin would The angle of incidence was 0.57° and 0.55°, respectively. arise . Rather, it can be inferred that the reversal takes place The inset schematically shows the scattering geometry. by domain wall motion, starting at domains that are already These data have been corrected for the background intensity favorably oriented. Figure 3 b shows the magnetization re- from the directly transmitted beam. The peak at 0 origi- versal in the disks after saturation with Hprior perpendicular nates from the specular reflection. Due to sample roughness, to H and to the neutron spin. At H 0, there is now also a SF some intensity will also be scattered into the off-specular contribution, confirming that there is a remanent fraction of Downloaded 29 Nov 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp Appl. Phys. Lett., Vol. 79, No. 7, 13 August 2001 Temst, Van Bael, and Fritzsche 993 to complete ``mapping'' of the reciprocal space near the ori- gin, and could provide information about magnetic order in the plane of the disk array as well as along the growth direc- tion of the magnetic islands. The success of this experiment implies that the specific advantages of neutron reflectometry noninvasiveness, depth sensitivity, complete vectorial deter- mination of the magnetization directions are applicable to investigate the manifestation of important magnetic effects such as anisotropy, antiferromagnetic coupling in layered nanostructures, dipolar coupling between nearby entities, ex- change bias, etc., in patterned magnetic structures. The authors thank Y. Bruynseraede, H. Maletta, V. V. Moshchalkov, and C. Van Haesendonck for valuable discus- sion and suggestions and J. Bekaert and M. Cannaerts for the microscopy pictures. This work was supported by the Fund for Scientific Research­Flanders FWO , the Belgian Inter- universitaire Attractiepolen IUAP , the Flemish Geconcer- teerde Onderzoekoacties GOA programs, and the European Commission within the framework of the Training and Mo- bility of Researchers TMR Program, Large-Scale Facilities LSF Contract No. ERBFMGE CT950060. Two of the au- thors K.T. and M.J.V.B. are postdoctoral research fellows FIG. 3. Intensity of the resonant peak in the off-specular scattering as a of the FWO. function of the field H applied along the 11 direction. The four possible combinations of incident and reflected neutron spin are shown: I and I 1 F. J. Himpsel, J. E. Ortega, G. J. Mankey, and R. F. Willis, Adv. Phys. 47, correspond to nonspin-flip scattering while I and I correspond to spin- 511 1998 . flip scattering. In a the sample was initially in the remanent state after 2 Ultrathin Magnetic Structures I and II, edited by J. A. C. Bland and B. being saturated in a field Hprior antiparallel to H. In b the sample was Heinrich Springer, Berlin, 1994 . initially in the remanent state after being saturated in a field Hprior perpen- 3 C. Mathieu, C. Hartmann, M. Bauer, O. Buettner, S. Riedling, B. Roos, S. dicular to H. The insets show the scattering geometry and the direction of O. Demokritov, B. Hillebrands, B. Bartenlian, C. Chappert, D. Decanini, Hprior. F. Rousseaux, E. Cambril, A. Mu¨ller, B. Hoffmann, and U. Hartmann, Appl. Phys. Lett. 70, 2912 1997 . 4 magnetization oriented perpendicular to the neutron spin. As P. D. Ye, D. Weiss, R. R. Gerhardt, and H. Nickel, J. Appl. Phys. 81, 5444 1997 . the field is increased, the magnetization of the disks is forced 5 M. J. Van Bael, K. Temst, V. V. Moshchalkov, and Y. Bruynseraede, Phys. into the direction of the external field, reducing the magne- Rev. B 59, 14674 1999 . 6 tization component perpendicular to the field and, hence, J. I. Marti´n, M. Ve´lez, A. Hoffmann, I. K. Schuller, and J. L. Vicent, Phys. Rev. Lett. 83, 1022 1999 . also the SF scattering, as can be seen most clearly in the I 7 K. Temst, M. J. Van Bael, V. V. Moshchalkov, and Y. Bruynseraede, J. and I signals. At the same time, the NSF intensity I Appl. Phys. 87, 4216 2000 . increases and maximal splitting between I and I is 8 P. Vavassori, V. Metlushko, M. Grimsditch, B. Ilic, P. Neuzil, and R. achieved when the disks are saturated in the direction of H. Kumar, Phys. Rev. B 61, 5895 2000 . 9 T. Schmitte, T. Schemberg, K. Westerholt, H. Zabel, K. Scha¨dler, and U. In conclusion, we have demonstrated that off-specular Kunze, J. Appl. Phys. 87, 5630 2000 . reflection experiments of polarized neutrons combined with 10 G. P. Felcher, Physica B 192, 137 1993 . spin analysis can be carried out on patterned magnetic struc- 11 S. J. Blundell and J. A. C. Bland, Phys. Rev. B 46, 3391 1992 . 12 tures. Both, off-specular reflectivity and the four polarized H. Zabel, Physica B 198, 156 1994 . 13 J. F. Ankner and G. P. Felcher, J. Magn. Magn. Mater. 200, 741 1999 . beam cross sections are used to determine the magnetization 14 G. P. Felcher, J. Appl. Phys. 87, 5431 2000 . reversal process and the approach toward saturation within 15 C. Fermon, F. Ott, B. Gilles, A. Marty, A. Menelle, Y. Samson, G. Legoff, the magnetic disks. An important advantage of this technique and G. Francinet, Physica B 267­268, 162 1999 . 16 is that simultaneously both in-plane components of the mag- B. P. Toperverg, G. P. Felcher, V. V. Metlushko, V. Leiner, R. Siebrecht, and O. Nikonov, Physica B 283, 149 2000 . netization vector are probed by measuring the SF and NSF 17 F. Mezei, R. Golub, F. Klose, and H. Toews, Physica B 213­214, 895 intensities. In order to evaluate the data more quantitatively, 1995 . a theoretical framework for the calculation of off-specular 18 T. Krist, K. Pappas, T. Keller, and F. Mezei, Physica B 213­214, 939 polarized neutron reflectivity from magnetic islands would 1995 . 19 S. K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley, Phys. Rev. B 38, be very desirable. This type of measurement can be extended 2297 1988 . Downloaded 29 Nov 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp