Journal of Magnetism and Magnetic Materials 198}199 (1999) 477}479 Direct experimental study of the microscopic remagnetization mechanism in Co/Cu magnetic superlattices V.I. Nikitenko *, V.S. Gornakov , L.M. Dedukh , A.F. Khapikov , T.P. Mo!at , A.J. Shapiro , R.D. Shull , M. Shima , L. Salamanca-Riba Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432, Russia National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742, USA Abstract Using the magneto-optical indicator "lm technique, the correlation between the magnitude of the giant magnetoresis- tance (GMR) and the micromechanism of the magnetization reversal of electrodeposited Co/Cu superlattices are investigated for a range of Cu spacer thicknesses. The multilayers showing vanishing GMR exhibit a cooperative spin behaviour, which is similar to that exhibited by thin ferromagnetic "lms with in-plane fourfold anisotropy. In contrast, superlattices with a substantial GMR demonstrate partially coupled noncollinear spin con"gurations, which are probably responsible for the observed giant magnetoresistance phenomenon. 1999 Elsevier Science B.V. All rights reserved. Keywords: Metallic multilayers; Giant magnetoresistance; Magnetization reversal; Domian structure The correlation between magnetic and transport prop- gonal spin alignment in adjacent FM layers leading to erties of thin-"lm metallic multilayers has been the sub- noncollinear spin con"gurations. This coupling orig- ject of many studies during the past decade. Most of this inates most likely from the multilayer defects such as activity has focused on the oscillatory exchange coupling interface roughness and pinholes [3}5]. Since in the between ferromagnetic (FM) layers separated by a non- simplest GMR model, the magnetoresistance depends on magnetic spacer layer and on the giant magnetoresis- the angle between magnetic moments in successive tance (GMR) associated with such layered structures. layers, the study of the occurrence of noncollinear spin In particular, the Co/Cu superlattices are of potential structures in magnetic multilayers may give some clues to interest as elements in magnetic sensors because of the increasing the GMR e!ect. substantial GMR value, up to&65% measured for sput- In this paper, we present a study of the micromechan- tered "lms at room temperature [1,2]. The increase in isms of the magnetization reversal of electrodeposited resistance occurs with decreasing external magnetic "eld Co/Cu multilayers in relation to the GMR magnitude. as the interlayer bilinear exchange coupling produces [Co(16 As)/Cu(d! )] multilayers used in our study were antiferromagnetic (AF) spin alignment of neighboring Co electrochemically deposited onto Si(0 0 1) substrates layers. The situation seems to be even more intriguing which had a 200 As copper seed layer [6]. The thickness of due to a biquadratic interaction, which favors ortho- the Cu layers, d! , was varied in a range from 5 to 40 As. The magnetoresistance was measured using a conven- tional four-point probe technique. Magnetic hysteresis * Corresponding author. Tel.: #7-96-576-411; fax: #7-95- loops were obtained by vibrating sample magnetometry. 524-5063. The magneto-optical indicator "lm (MOIF) technique E-mail address: nikiten@issp.ac.ru (V.I. Nikitenko) was used to image magnetic domain structures during 0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 5 1 - 2 478 V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 477}479 Fig. 1. Dependence of the magnetoresistance ratio on the thick- ness of Cu spacer for Co/Cu multilayers electrodeposited on Si(1 0 0) substrates. R/R"(R !R )/R . Inset: Magneto- resistance curves for the Co/Cu multilayer with d! "30 As (a) and d! "8 As (b). Fig. 2. MOIF images of the [1 1 0] easy-axis magnetization reversal in Co/Cu multilayer with d! "8 As. MH"17.1 mT (a),!1.68 (b),!1.96 (c), and ! 2.37 (d). White arrows indicate the direction of the applied "led, black arrows show the magnet- ization in domains. the remagnetization of the multilayers. The stray "eld around domain walls, edges and defects of the multilayer were imaged due to variations in the Faraday e!ect in an overlying indicator "lm with in-plane anisotropy [7]. All tion occurred at spike heads, due probably to the stray measurements were performed at room temperature. "eld concentration. The domain wall motion was fol- The &longitudinal' magnetoresistance R/R of multi- lowed by the spin rotation. layers as a function of the Cu layer thickness is shown in In contrast, specimens, which showed measurable Fig. 1. In contrast to sputter deposited Co/Cu superlatti- GMR, exhibited completely di!erent domain reversal ces, we observed no peaks in the GMR at Cu thicknesses behaviour. Fig. 3 shows MOIF patterns of the corresponding to expected maxima for interlayer AF [Co(16 As)/Cu(30 As)] sample during magnetization coupling. The dependence of R/R upon d! was a stead- along the easy axes of magnetization. The set of patterns ily increasing function up to d! +40 As. The inset in in Fig. 3 is associated with a minor hysteresis loop. In Fig. 1 shows a typical GMR curve for the specimen with general, similar to the above case, the easy-axis reversal d! "30 As. proceeds by the nucleation and motion of domain walls. All our specimens were found to exhibit in-plane four- However, it starts by the nucleation of a zigzag domain fold magnetic anisotropy with easy axes along [1 1 0] wall (further referred to as DW-I) at the [1 0 0] specimen and [1 1 0] directions [6]. However, multilayers with edge (Fig. 3a). The teeth of the wall are directed along the thin and thick Cu spacers demonstrated di!erent hyster- [0 1 0] axis. With increasing "eld, a new domain wall esis properties. For example, the hysteresis loop of the with teeth parallel to the [1 0 0] axis (we denote this wall sample with the Cu layer thickness d! "8 As is perfect as DW-II) nucleates at the [0 1 0] edge and translates square with a coercive force of about 2.7 mT. The sample into the sample interior (Fig. 3b). having the thicker Cu spacer, d! "30 As, possesses Fig. 3c}f illustrate a surprising phenomenon of the a slightly diminished remanet magnetization and its &interpenetration' of those domain walls. Fig. 3c shows coercive force is about 20 mT. Note the GMR scales the domain con"guration after stopping the magnetic inversely with the reduction of the remanent magnetiz- "eld sweep at the H"!18 mT and applying the "eld ation. This same relationship was also found earlier for of 11.3 mT in the opposite direction. This results in the sputter deposited multilayers [8]. nucleation and propagation of a new domain wall (bright Fig. 2 shows the MOIF easy-axis reversal patterns of contrast) which seems to be similar to the DW-I. This the [Co(16 As)/Cu(8 As)] multilayer. This sample dem- new domain wall passes through the DW-II (Fig. 3d) onstrated almost no GMR. Sample edges are parallel to to annihilate with the DW-I (Fig. 3f). The DW-II annihi- the [1 0 0] and [0 1 0] directions, while the easy axes lie lates with its counterpart as shown in Fig. 3e. along 11 1 02. First, the specimen was magnetized to This unexpected domain-wall behaviour can be under- saturation along the easy axis and then the applied "eld stood in terms of DW-I and of DW-II being associated was reversed (Fig. 2b}d). The spike-like domains nu- with di!erent layers of the compositional superlattice. cleated at "lm edges due to edge magnetostatic "elds and There are several possibilities for the origin of a wall quickly propagated over the specimen. Curiously, sec- through the multilayer thickness. First, both DWs-I ondary domains with the perpendicular tooth orienta- and II could consist of many domain walls localized in V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 477}479 479 magnetic moments in a large number of successive layers could cooperate to produce an inhomogeneous spin be- haviour, similar to what has been observed in a bilayer, trilayer, etc. Here, we do not discuss a reason for that behavior. Note, however, that the orientation of teeth of DWs-I and II as well as the change in the magneto-optic contrast at the "lm edges and defects allows one to roughly determine the orientation of the magnetization in the di!erent layers. A weak change in contrast at the "lm edges indicates that the magnetization process de- scribed above is related to the partial reversal as com- pared to the multilayer thickness. It is necessary to stress that noncollinear spin con"gurations appear in this su- perlattice, and the domain wall behavior is governed by spin reorientation phase transitions in a superlattice. We believe that noncollinear spins are responsible for the GMR phenomenon observed in the superlattices having thick Cu spacers. This work was partially supported by Grant 97-02- 16879 from the Russian Foundation for Basic Research and a NIST Visiting Researcher's Program. Fig. 3. MOIF images of the [1 1 0] easy-axis magnetization reversal in Co/Cu multilayer with d! "30 As. Arrows with black and white heads indicate magnetization direction in di!er- References ent Co layers. White arrows indicate the direction of an applied "eld. H"!11.2 mT (a), !16.0 (b), 11.3 (c), 12.4 (d), 13.9 [1] S.S.P. Parkin et al., Phys. Rev. Lett. 66 (1991) 2152. (e), 15.7 (f). All images are of the same area in the same sample [2] D. Mosca et al., J. Magn. Magn. Mater. 94 (1991) L1. orientation. [3] J.C. Slonczewski, Phys. Rev. Lett. 67 (1991) 3172. [4] J. Bobo et al., J. Magn. Magn. Mater. 126 (1993) 440. [5] S. Demokritov et al., Phys. Rev. B 49 (1994) 720. alternating layers. For example, the walls lying in odd [6] M. Shima et al., J. Appl. Phys. (1998) in press. layers could form the DW-I, while the walls occupying [7] V. Nikitenko et al., IEEE Trans. Magn. 33 (1997) 3661. even layers condense into the DW-II. Alternatively, the [8] S.K.J. Lenczowski et al., Phys. Rev. B 50 (1994) 9982.