Experimental study of magnetization reversal processes in nonsymmetric spin valve V. S. Gornakov and V. I. Nikitenko Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow 142432, Russia L. H. Bennett,a) H. J. Brown, M. J. Donahue, W. F. Egelhoff, R. D. McMichael, and A. J. Shapiro National Institute of Standards and Technology, Gaithersburg, Maryland 20899 We have investigated a nonsymmetric bottom giant magnetoresistance spin valve with the structure Si/NiO/Co/Cu/Co/Ta, as well as single ferromagnetic Co layers on antiferromagnetic NiO, with or without a nonmagnetic Cu spacer. Magnetic hysteresis loops have been measured by SQUID magnetometry, and magnetic domain structures have been imaged using an advanced magneto-optical indicator film MOIF technique. The MOIF technique demonstrated that the first stage of magnetization reversal is characterized by nucleation of many microdomains. With increasing reversed field, the domain walls move over small distances 5­20 m until annihilation. The domain size was observed to increase with the thickness of the Co layer. When an alternating magnetic field was applied, the domain structure was dramatically changed. © 1997 American Institute of Physics. S0021-8979 97 73908-2 I. INTRODUCTION balt layer, occurring over the range 50­80 mT. These tran- The discovery of the giant magnetoresistance GMR ef- sition fields correspond to the results of the MOIF images. fect has evoked increased interest in magnetic multilayers, The MOIF technique is based on the Faraday rotation of and especially in spin valves. Determining the domain struc- linearly polarized light in an indicator film, a Bi-substituted ture and its dynamics in magnetic multilayers is important, iron garnet film with in-plane anisotropy, placed on the and there are a number of techniques1 which will reveal the sample. The polarized light passes through the indicator film domain structure: transmission electron microscopy,2 scan- and is reflected by an Al underlayer covering the bottom ning electron microscopy with polarization analysis surface of the film, adjacent to the sample surface. While the SEMPA ,3 Bitter pattern,4 Kerr, magnetic bire- light is passing through the indicator film its polarization fringence, and magneto-optical gradient effects,5 and mag- experiences a Faraday rotation through an angle proportional netic force microscopy.6,7 Recently, we have used the to the component of the local magnetic field parallel to the magneto-optical indicator film MOIF technique to demon- light propagation direction. The transmitted intensity of the strate that the reversal of the free center layer proceeds by reflected beam through an analyzing polarizer varies with the nonuniform magnetization rotation.8 In this article we apply local field in the light path. The bright and dark variations of the MOIF technique to a nonsymmetric spin valve, and dem- the image formed by an optical system represent the varia- onstrate the ability to reveal not only the static domain struc- tions of the stray fields in the indicator film, which are asso- ture, but also its dynamics upon application of an ac mag- ciated with the magnetizations not only near the sample sur- netic field. face but also inside it. The MOIF technique9,10 uses a garnet film placed on the Three types of specimens were investigated in this study. specimen to be studied. A domain structure of the specimen Their structures are schematically profiled in Figs. 2­4. The is directly imaged in real time through the magneto-optical NiO substrates were 50 nm thick polycrystalline films, de- Faraday effect in the indicator film. The resulting Faraday portrait of the sample's stray magnetic fields presents de- tailed information about the static and dynamical domain structure, as well as the defects of crystal structure that affect the spin distribution in the sample. II. EXPERIMENTAL DETAILS AND DISCUSSION The hysteresis loop of the spin valve was measured with a superconducting quantum interference device SQUID magnetometer Fig. 1 . It is evident that there are two critical fields, the first corresponding to the switching of the top, nonpinned cobalt layer, occurring over the range of 6­9 mT, and the second the switching of the bottom, pinned co- FIG. 1. The room-temperature hysteresis loop of a nonsymmetric bottom a Also at Institute for Magnetic Research, The George Washington Univer- Si/NiO/Co/Cu/Co/Ta GMR spin valve, measured with a SQUID magneto- sity, Ashburn, VA 22011. meter. J. Appl. Phys. 81 (8), 15 April 1997 0021-8979/97/81(8)/5215/3/$10.00 © 1997 American Institute of Physics 5215 Downloaded¬07¬Jun¬2002¬to¬148.6.178.13.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp FIG. 2. Schematic profile of the structure of a nonsymmetric spin-valve FIG. 3. Schematic profile of the structure of a single cobalt layer sample, sample. The MOIF images represent the process of remagnetization. otherwise similar to the nonsymmetric spin-valve sample. The three MOIF images on the left and the top right image represent the process of remag- netization. The image on the bottom right is the result of a 30 Hz ac field. posited on a Si wafer. The metal films were deposited at room temperature by dc-magnetron sputtering in an Ar pres- sure of 2 mTorr in a system with a background pressure of of magnetization. When the field direction and value are 1 10 8 Torr. The first specimen investigated, shown in Fig. changed to approximately 6.5 mT middle picture in left 2, is a nonsymmetric bottom GMR spin valve with the struc- row , this ripple acquires more contrast, and intensive micro- ture Si/NiO/Co/Cu/Co/Ta. The analysis of the sample mag- domain nucleation starts throughout the sample. At further netization was evident from the magneto-optical image of increase of field H 8 mT, bottom picture in left row , the the leakage field on the sample edges. The details of the microdomains become larger in size, adjacent microdomains domain structure were studied using computer subtraction of merge, and the domain structure seen appears. Simulta- the background image, which reduces the contrast from non- neously with the development of the magnetic structure, re- magnetic defects. The background image was obtained by duction in the black contrast is observed at the sample edge. averaging two image fields saturated at large magnetic fields Its contrast is practically extinguished after the micro- of opposite polarities and then reduced to zero field. Using domains disappear at the field near 10 mT. The appearance this technique, the leakage fields compensated each other of a new white contrast at the sample edge occurs gradually. the same saturated regions of the sample was light in one of The set of images in Fig. 3 represents the process of the images and dark in the other . remagnetization of one Co layer, which was prepared at the The set of images in Fig. 2 represents the process of same conditions as the upper low-coercive layer in the two demagnetization in the nonsymmetric spin valve. The sample Co-layer spin-valve structure of Fig. 2. This structure was was first magnetized to saturation in a 80 mT field. After prepared to be similar to the spin valve without the pinned reducing the field to zero, the sample had the magneto- Co layer. As in the previous figure, the sample was first optical portrait displayed in the top left picture. As is evident magnetized to saturation in a 80 mT field, then the field is from the picture, the sample is magnetized in its own plane, reduced to zero, left top picture . There is no magnetic along the plane parallel to the top edge. Magnetic charges structure visible in the magneto-optical portrait except from concentrated along the vertical edge of the sample produce scratches mainly in the right part of the picture . Positive leakage fields which form the dark band in the left region of field increase produces a large amount of nucleation of a new the image. Light regions correspond to the leakage field from magnetic phase, as can be seen in the middle left picture at magnetic charges of opposite sign. Black and white images H 3.2 mT. The density is not even across the sample. With of scratches are visible in the picture, as are incipient ripples the given orientation of the field and sample, the most in- 5216 J. Appl. Phys., Vol. 81, No. 8, 15 April 1997 Gornakov et al. Downloaded¬07¬Jun¬2002¬to¬148.6.178.13.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp domain boundaries begin to be oriented along some direc- tion. Since the coil that produces this field has a large in- ductance, its amplitude decreases with increased frequency. Upon achieving the critical frequency 30 Hz, 4.4 mT , the stabilized domain structure abruptly stopped changing and settled upon a pattern similar to that presented in the bottom right picture. This dissipative structure appears only in a small range of orientations of the external magnetic field. The set of images in Fig. 4 demonstrates the behavior of the domain structure in Co layers of different thickness dur- ing remagnetization. The field is oriented horizontally in these images. The Co layers were deposited onto NiO sub- strates, under the same conditions as in the two Co-layer spin-valve structure in Fig. 2. The Co was 20 nm thick in the top picture, 5 nm in the middle picture. The sample remag- netization was obtained from the negative saturated state. During remagnetization of the thicker sample 20 nm, top picture large-scale domain structure was formed at low valve field. In thinner films the domain structure appeared at larger fields. For the 5 nm film bottom picture , the structure FIG. 4. Schematic profile of the structure of two different thickness single appeared at 15 mT. The characteristic dimensions of the do- cobalt layer adjoining NIO, as in the bottom layer of a nonsymmetric spin- main structure in such films are substantially smaller, and the valve sample. The image contrasts the domain structure in Co of different thickness during remagnetization. The field is oriented horizontally in these field range where remagnetization occurs expands. For a 2.5 images. The structures shown were capped with a protective Au layer. nm film, we were unable to detect domain structure by the MOIF technique, although the domains were resolved in the spin-valve structure of Fig. 2, with layers of similar thick- tense domain nucleation appears at the lower left region of ness. After magnetization of the film in large fields, the mag- the sample. With further field increase 3.8 mT, left bottom netostatic field is visible on the sample edge. The magnetic picture , this nucleation process moves to the top right and image intensity from the charges on the sample edge practi- generates new domain nucleations in the top right of the cally disappeared after the field was decreased, inverted and sample as well as the expansion due to boundary movement then increased to 49 mT. This indicates that there was a and merging of already existing domains in the left bottom remagnetization process on a smaller scale. region. With subsequent field increases not shown , the re- ACKNOWLEDGMENTS magnetization process is practically completed at the field of The work of V.S.G. and V.I.N. was partially supported 4.5­5.0 mT. The sample edge at the top becomes white by the Russian Fundamental Research Foundation by Grant and the magnetic structure is limited to a few defects in the No. 94-02-03815. L.H.B. acknowledges useful discussions film. This situation continues as the field is further increased with the other members of the GWU Institute for Magnetics to 80 mT not shown , and then reduced to zero the top Research. right picture . On the whole, the magnetization parameters of the Co layer coercive field, characteristic dimensions of do- 1 K. M. Krishan, ed., MRS Bull. 20, 24 1995 . 2 main structure, etc. are similar to the characteristics of the J. Urguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 69, 1125 1992 . spin valve structure. Some observed differences and behav- 3 L. J. Heyderman, J. N. Chapman, and S. S. P. Parkin, J. Magn. Magn. ior peculiarities can be explained by differences in the cobalt Mater. 138, 344 1994 . thickness, and particularly by the absence of the exchange 4 H. D. Chopra, B. J. Hockey, L. J. Swartzendruber, S. Z. Hua, P. J. Chen, interaction with the other Co layer, as distinct from the spin- K. Raj, M. Wuttig, and W. F. Egelhoff, Jr., NanoStruct. Mater. in press . 5 M. Ru¨hrig, R. Scha¨fer, A. Hubert, R. Mosler, J. A. Wolf, S. Demokritov, valve where such exchange is present. and P. Grundberg, Phys. Status Solidi A 125, 635 1991 . In the same sample with an alternating magnetic field, an 6 U. Hartmann, J. Magn. Magn. Mater. 157/158, 545 1996 . effect of specific domain structure formation was discovered, 7 M. S. Valera, A. N. Farley, S. R. Hoon, L. Zhou, S. McVitie, and J. N. similar to dissipative structures observed in diverse nonlinear Chapman, Appl. Phys. Lett. 67, 2566 1995 . 8 V. I. Nikitenko, V. S. Gornakov, L. M. Dedukh, Yu. P. Kabanov, A. F. systems.1 The magneto-optical portrait of such a structure for Khapikov, L. H. Bennett, P. J. Chen, R. D. McMichael, M. J. Donahue, L. an ac frequency of 30 Hz is presented in the bottom right J. Swartzendruber, A. J. Shapiro, H. J. Brown, and W. F. Egelhoff, Jr., picture. When an 1 Hz, 6 mT ac field was applied, micro- IEEE Trans. Magn. 32, 4639 1996 . 9 domains formed similar to those shown in the dc presented L. A. Dorosinskii, M. V. Indenbom, V. I. Nikitenko, Yu. A. Ossip'yan, A. A. Polyanski, and V. K. Vlasko-Vlasov, Physica C 203, 149 1992 . pictures in the middle and bottom left. The sample was re- 10 L. H. Bennett, R. D. McMichael, L. J. Swartzendruber, S. Hua, D. S. magnetized in each cycle. As the frequency increased, some Lashmore, A. J. Shapiro, V. S. Gornakov, L. M. Dedukh, and V. I. Ni- of the microdomains merged and formed domains whose di- kitenko, Appl. Phys. Lett. 66, 1 1995 . 11 mensions were comparable with the sample dimension. The G. Nicolis and I. Prigogine, Self-organization in Nonequilibrium Systems: from Dissipative Structures to order through Fluctuations Wiley, New number of new domains nucleating declined, and the macro- York, 1977 . J. Appl. Phys., Vol. 81, No. 8, 15 April 1997 Gornakov et al. 5217 Downloaded¬07¬Jun¬2002¬to¬148.6.178.13.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp