JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002 Anomalous switching behavior of antiparallel-coupled Co layers separated by a super thin Ru spacer V. S. Gornakova) and V. I. Nikitenko Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 W. F. Egelhoff, Jr., R. D. McMichael, A. J. Shapiro, and R. D. Shull National Institute of Standards and Technology, Gaithersburg, Maryland 20899 The details of the magnetization reversal in coupled ferromagnetic Co/Ru/Co trilayers deposited on obliquely sputtered Ta underlayers were studied using the magneto-optical indicator film technique. The ground states of the sandwich are characterized by noncollinear magnetization orientations in the two Co layers, which can be remagnetized by the motion of 180° and non-180° domain walls. In the latter case, the angle between the magnetization vectors in the adjacent domains was revealed to be about 100°, and an anomalous magnetization reversal was observed. The canted magnetization states and their mutual transformations are discussed in terms of the competition between ferromagnetic coupling through pinholes and antiferromagnetic coupling across the Ru layer. DOI: 10.1063/1.1452706 Modern methods of epitaxial heterophase structure magnetic field component perpendicular to the indicator film. growth allow synthesis of quasi-two-dimensional magnetic The intensity at each image point is determined by the total materials with unique physical properties and exciting poten- magnetostatic charge at that point, due to the divergence of tial practical applications.1 For trilayers composed of two M either at domain walls or at the sample edges. Figure 1 ferromagnets separated by a nonferromagnet, the bilinear ex- shows the MO image of the corner of a uniformly magne- change interaction between ferromagnetic layers oscillates tized sample in zero field. The magnetization angle was cal- from ferromagnetic to antiferromagnetic2 as the nonmagnetic culated from the maximal MO intensities II and III measured spacer thickness is varied. along vertical and horizontal lines, respectively indicated by If the exchange interaction is antiferromagnetic, antipar- the long axes of the small rectangles shown in Fig. 1 a at allel spin orientation in the adjacent ferromagnetic layers the corner of the sample. In Fig. 1 a the intensities indicate will occur. However, that orientation relationship may be that the magnetic moment of the free Co layer is rotated changed if pinholes exist in the nonmagnetic layer,3­5 be- clockwise almost 100° from the direction of the pinned Co cause direct local contact between the ferromagnetic layers layer as described below. have been created. Such pinholes are inevitably present in The topological relief formed during oblique sputtering ultrathin in the order of 1 ML nonmagnetic spacers. Real of the Ta underlayer induces a very high anisotropy in time magnetic domain studies of the remagnetization charac- the adjacent Co layer.6 From that earlier study, it is known teristics of such materials have not yet been performed. In that the easy axis of the bottom pinned Co layer will bep- this work an investigation of the remagnetization processes erpendicular to the direction of oblique sputtering, and there- in the Co/Ru/Co system is presented. The Co layers having fore lies nearly perpendicular to the horizontal edge of the thicknesses of 2.6 and 2.1 nm, respectively and Ru 0.5 nm sample. In this article the magnetization directions of the thick were deposited onto a Si substrate covered with Ta pinned and free Co layers are indicated in the figures by 10.6 nm thick that was sputtered at an oblique angle of 60° white and black arrows, respectively. Since we know the with respect to the Si substrate. The oblique sputtering pro- direction of M in the pinned Co layer, we can use Fig. 1 a to vided an unusually high value for the uniaxial anisotropy of determine the direction of the free Co layer as follows. If the the Co layer deposited directly on top of the Ta underlayer.6 magnetization in both Co layers were collinear the magnetic The domain structure was studied by means of the charges on the vertical sample edge would be nearly absent magneto-optical indicator film MOIF technique.7 The leak- because no charge is produced when M lies parallel to an age field from the sample causes local deflection of the mag- edge. Magneto-optical contrast would reveal only the hori- netization of the indicator film from the in-plane direction. zontal sample edge. These deflections are revealed by the Faraday effect in the The existence of contrast on the mutually perpendicular garnet of the light that is reflected from the bottom aluminum sample edges points unambiguously to noncollinearity be- layer. When the polarizer and analyzer are slightly un- tween the magnetization directions in the Co layers due to crossed, these deviations are responsible for the black and direct coupling through the ultrathin Ru layer.3­5 So, the white contrast formation in the magneto-optical MO im- sample under investigation is not, in the strict sense, a syn- ages, corresponding to the opposite directions of the leakage thetic antiferromagnet, but may be thought of as a ``synthetic weak ferromagnet.'' The remagnetization mechanism of the a Electronic mail: gornakov@nist.gov synthetic ``weak ferromagnet'' was shown to be analogous to 0021-8979/2002/91(10)/8272/3/$19.00 8272 Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Gornakov et al. 8273 FIG. 2. Color MOIF images of the domain structure of the sample after it was remagnetized in an alternating field applied at an angle of 90° clockwise relative to the field orientation of Fig. 1. FIG. 1. MOIF image of the remagnetization process of a Co/Ru/Co homo- geneous structure in a field oriented along the total magnetization vector: a system, the remagnetization takes place in three stages. In oH 0 mT after magnetization in a field of 60 mT; b oH 9.4 mT; c the first stage at a small, 8.8 mT field , remagnetization of oH 9.4 mT after a 5 s wait; d oH 13.0 mT. a and d show the intensity change of the MO signal. the newly created pink domains occurs first via the nucle- ation and growth of 180° domains marked with dark-pink color . The nucleation of such ordinary 180° domains, how- the remagnetization mechanism of the synthetic antiferro- ever, takes place as a rule on the non-180° domain walls magnet when the external magnetic field is oriented either Fig. 3 b . The newly formed domain walls subsequently along the total magnetization axis or along the underlayer- move through the previously created new domain area only, induced easy axis. In each of the Co layers, at some value of and during a short period of time that depends on the field the external magnetic field, heterogeneous spin-flip processes value Figs. 3 b ­3 e , sweep up nearly the complete pink begin in a correlated fashion that leads to the nucleation and motion of 180° domain walls Figs. 1 b and 1 c . Compari- son of the images of Figs. 1 a and 1 d reveals inversion of the contrast on the sample edges, indicating that the magne- tization has reversed in both Co layers. The remagnetization process for fields applied perpen- dicular to the net magnetization direction takes place in a different manner that includes switching of the magnetiza- tion one layer at a time. Under field cycling, the sample exhibits domains that correspond to the original ground state and to a second state that is not an inversion of the original ground state. Figure 2 contains a MO image of the sample remagnetized from the original ground state Fig. 1 d after approximately 80 cycles of field reversal at an amplitude of 60 mT perpendicular to the original magnetization direc- tion. The newly formed domains which increase in size with each cycle are emphasized by means of computer process- ing by being colored pink, while the original domain areas are indicated in green. One can clearly see in Fig. 2 that both vertical and horizontal sample edges incorporated in the new domain areas are colored in black, indicating that the total magnetization has changed its orientation from Fig. 1 d by 90°. The magnetization directions of the Co layers in the new domains were determined in the same manner as shown in Fig. 1 from the MO signal intensity analysis at the mutu- ally perpendicular sample edges after the sample was com- pletely remagnetized into the new ground state. In that new ground state, the magnetic moment of the free Co layer is turned 100° counterclockwise relative to the magnetization of the pinned layer. Figure 3 shows the details of the anomalous remagneti- zation process of the sample having the two different ground FIG. 3. Color MOIF images of the remagnetization processes of the Co/ Ru/Co two-phase structure: a states as described in the above paragraph separated by oH 0 mT after magnetization in field 60 mT; b non-180° walls. In such a two-ground state e.g., two-phase oH 8.4 mT; c 8.4 mT after 45 s; d 8.4 mT and 90 s; e 12 mT; f 29.4 mT; g 30 mT; h 34.8 mT. Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp 8274 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Gornakov et al. ropy layer. Figure 4 b illustrates the spin distribution corre- sponding to the MO image in Fig. 3 a . As mentioned above, from the analysis of the MO signal intensity from Figs. 2 and 3 one can derive that the magnetic moment in one of the domains of the ``free'' Co layer lies in a mirror position compared to its orientation in the domain of the other ground state. The free layer magnetization is turned counterclock- wise from the magnetization direction of the pinned layer. This domain wall in Fig. 4 a between adjacent domains in the free layer is a 20°-domain wall and it is bound to a 180°-domain wall in the high coercivity pinned layer. Figure 4 c correlates with Figs. 3 b ­3 d and shows the inversion of the spin directions in a new magnetic phase generated by the simultaneous motion of 180° walls in the top and bottom layers. If the sense of rotation within the walls is the same for the two layers, the top and bottom films FIG. 4. Color Schematics of the magnetic moment distribution in Co/ Ru/Co at different stages of its remagnetization process. can maintain the same relative orientation as the coupled walls move. Figure 4 d demonstrates the spin distributions that are formed at the end of the first stage Fig. 3 e . The area. During this process the non-180° walls do not move. In pinned layer became a monodomain and in the free layer, a the second remagnetization stage from 9 to 29 mT , 160° domain is left. Figure 4 e shows the inversion of spin there was no directly visible change in the domain structure. directions in the initial magnetic phase Figs. 3 f and 3 g The third stage Figs. 3 f ­3 h starts at higher fields during the third stage of remagnetization accompanying a 29.4 mT with the nucleation of new 180° domains field reversal. Figure 4 f illustrates the spin structure after marked with dark-green color in the regions comprised of completion of the third stage that ultimately leads to a shift the initial ground state phase. in the original domain boundary by a small value x. Note, By computer substraction of each MO image from the also, that on opposite sides of the 160° domain wall in the preceding image, it is possible to track changes in position of free Co layer shown in Fig. 4 d the magnetization twists in the non-180° walls. Analysis of such changes showed that opposite directions. Because of the competing forces acting the non-180° walls did not move during stage 1. In the sec- on the magnetic moments on either side of the wall, includ- ond stage, the non-180° walls moved only a very small dis- ing the antiparallel coupling through the Ru, the inhomoge- tance on the order of the MO image wall width. The largest neous pinhole coupling, and the demagnetizing field of the shift took place during stage 3. It should be noted, however, film, such a domain wall would be expected to require a that non-180° domain wall movements occurred only during large field to move. Field cycling allows this domain wall to the passage of an adjacent 180° domain wall. move by the easier process of adding and subtracting 180° Figure 3 illustrates the magnetization changes in both Co walls from alternate sides. layers during a single field reversal. During field reversal The observations in this study show that pinholes in the back to its original direction, the magnetization changes oc- nonmagnetic Ru layer lead to noncollinearity of the spins in cur in a similar three stage process and similarly concluded the exchange biased ferromagnetic layers, leading to a non- with further growth of the new-phase region pink colored . zero magnetization at H 0 and to magnetization reversal Thus, during each half-cycle of a hysteresis loop, there oc- processes which involve the cooperative behavior of both Co curs a one-directional movement of the slow-moving i.e., layers simultaneously. If H is applied perpendicular to the non-180° boundaries, which separate the regions of the total magnetization vector in the case of the present study, M sample with different ground states. After creating the do- reverses via a multiple-stage process which involves the main structure shown in Figs. 2 and 3 using a perpendicular nucleation and motion of non-180° domain walls. field, external magnetic fields with an orientation different from that shown in Figs. 2 and 3 did not visibly shift the non-180° domain walls, even when applying our maximum 1 B. Dieny et al., Phys. Rev. B 43, 1297 1991 . achievable 250 mT magnetic field. 2 S. S. P. Parkin et al., Phys. Rev. Lett. 64, 2304 1990 . 3 Spin reorientations in the Co/Ru/Co trilayer that occur D. B. Fulghum and R. E. Camley, Phys. Rev. B 52, 13436 1995 . 4 V. M. Uzdin and C. Demangeat, J. Magn. Magn. Mater. 165, 458 1997 . under application of an alternating external magnetic field 5 J. F. Bobo, H. Kikuchi, O. Redon, E. Snoeck, M. Piecuch, and R. L. are schematically summarized in Fig. 4. Figure 4 a corre- White, Phys. Rev. B 60, 4131 1999 . sponds to the ground state of the sample as shown in Fig. 6 R. D. McMichael, C. G. Lee, J. E. Bonevich, P. J. Chen, W. Miller, and W. 1 d . In this state, the free layer spins are turned 100° clock- F. Egelhoff, J. Appl. Phys. 88, 5296 2000 . 7 V. I. Nikitenko, V. S. Gornakov, A. J. Shapiro, R. D. Shull, K. Liu, S. M. wise relative to the spin direction in the bottom, high anisot- Zhou, and C. L. Chien, Phys. Rev. Lett. 84, 765 2000 . Downloaded 07 Jun 2002 to 148.6.178.13. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp