VOLUME 86, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 26 MARCH 2001 Spin Reorientation at the Antiferromagnetic NiO(001) Surface in Response to an Adjacent Ferromagnet H. Ohldag,1,2,3 A. Scholl,2 F. Nolting,1,2 S. Anders,2 F. U. Hillebrecht,4 and J. Stöhr1 1Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, California 94309 2Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 3Institut für Angewandte Physik, Universität Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany 4Max Planck Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany (Received 22 September 2000) Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin struc- ture near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antifer- romagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet. DOI: 10.1103/PhysRevLett.86.2878 PACS numbers: 75.70.­i, 75.70.Rf, 75.50.Ee, 78.20.Ls The macroscopic exchange bias effect, i.e., the unidi- antiferromagnetic spins near the NiO(001) surface upon rectional exchange coupling between an antiferromagnet deposition of the ferromagnetic film. More importantly, (AFM) and a ferromagnet (FM), that appears when an they provide clear experimental evidence that symmetry AFM-FM sandwich is grown or heated in a magnetic field, breaking at surfaces and interfaces will in general lead to has been the subject of extensive studies because of its spin reorientation effects in antiferromagnets. technological relevance and lack of scientific understand- The x-ray photoemission electron microscopy ing [1,2]. In light of the difficulty of determining the AFM (XPEEM) experiments were performed on beam line structure close to the FM interface most models have as- 7.3.1.1 at the Advanced Light Source using the PEEM2 sumed that the antiferromagnetic spin structure in the bulk, microscope [5]. The microscope has a lateral resolution which in most cases is known from neutron diffraction, of about 50 nm when used for magnetic imaging. The extends all the way to the interface. An example is the depth sensitivity is determined by the escape depth of the recent study of the Fe NiO(001) system [3] where a bulk- secondary electrons. Because of the elemental specificity like NiO(001) domain structure was assumed to model the of the x-ray absorption process we can distinguish the AFM-FM coupling. signal from layers with different elemental composition During the last year, progress made with imaging with a 1 e probing depth of about 2 nm for each layer methods based on variable polarization x rays has allowed [6]. The bending magnet x-ray source and the spherical the observation of the microscopic spin structure on grating monochromator provide monochromatic x rays both sides of an AFM-FM interface [4]. Studies of the with an energy resolution of 0.6 eV at 850 eV and ad- Co LaFeO justable polarization ranging from linear ( 90%) to left- 3(100) system convincingly showed the direct correlation of the antiferromagnetic and ferromagnetic or right-handed circular ( 80%). Figure 1 shows the spin structure near the interface and the existence of local experimental geometry. X rays are incident on the sample exchange bias, domain by domain. However, because at an angle u 30± from the surface. of the complex antiferromagnetic structure of twinned For x-ray magnetic linear dichroism (XMLD) measure- epitaxial LaFeO ments we typically use linearly polarized x rays with the 3 100 the three-dimensional correlation between the ferromagnetic and antiferromagnetic spin electric field vector E parallel to the sample surface. In directions could not be determined. Using the well-studied all XMLD measurements we use the experimentally ob- antiferromagnet NiO, coupled to either a thin Fe or Co served fact that the intensity of the higher energy peak layer, we are able to experimentally determine here the at the Ni L2-edge is at a maximum when the E vector is relative alignment of the antiferromagnetic and ferromag- aligned parallel to the antiferromagnetic axis [7,8]. By ro- netic spin systems. We show that the magnetic spins near tating the sample about the surface normal we can change the surface of a NiO(001) single crystal and those in a thin the azimuthal angle f between the in-plane Ek vector and film of Co or Fe deposited on top align perfectly parallel the 100 direction of the NiO crystal. This allows us to to each other. Our finding is in complete contrast to the determine the in-plane orientation of the antiferromagnetic recent conclusions of Matsuyama et al. [3] for the same axis. We cannot rotate the direction of the linearly polar- system, who assumed a bulklike NiO spin structure at the ized x rays but we can obtain out-of-plane XMLD sen- interface. Rather, our results reveal a reorientation of the sitivity by using circularly polarized x rays. In this case 2878 0031-9007 01 86(13) 2878(4)$15.00 © 2001 The American Physical Society VOLUME 86, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 26 MARCH 2001 the crystallographic and magnetic mirror planes 100 and 110 . The magnetic axis in the wall is the average of those in the two adjacent T domains [10]. Examples of bulk T walls are shown in the lower panel of Fig. 2 as a green plane that separates two T domains, whose spin directions are also shown. The wall planes are mirror planes for both the magnetic and the crystallographic structures and the magnetic axes in the walls lie along 010 and 110 , respectively. Figure 2 shows an XPEEM image of the antiferromag- netic structure near the surface of cleaved NiO(001) using plane polarized x rays. Four different gray scales are ob- FIG. 1. Experimental geometry. The x rays are incident at an served, corresponding to antiferromagnetic domains with angle u 30± from the surface and the elliptical polarization different spin orientations. A 100 wall separates domains may in general be divided into an in-plane Ek and out-of-plane exhibiting large contrast. They are further divided by E component. E is inclined by u 30± from the surface 110 and 110 walls into domains with weaker contrast. normal and Ek is oriented at an angle f from the in-plane [100] Local absorption spectra taken with either plane or linear axis. polarization in each domain show that the magnetic axes in these domains are parallel to the 61 6 2 6 1 directions. The in-plane orientations of the antiferromagnetic axes are the handedness of the x rays is irrelevant since only the indicated by the arrows in the image. Domains with the relative sizes of the in-plane ( Ek) and out-of-plane ( E ) same in-plane orientation but different out-of-plane com- electric field vector components matter (see Fig. 1), and ponents can be distinguished because they have different we shall simply speak of plane polarized x rays. They can projections on the out-of-plane component ( E ). In our be used to determine the direction of the out-of-plane mag- analysis of the detailed spin orientation in the various do- netic axis. The NiO domain XMLD images are obtained mains we used a variety of images and spectra taken at by dividing two images taken at the two multiplet peaks different azimuthal orientations f that will be published comprising the Ni L2-edge absorption spectra [8]. elsewhere [11]. We use right or left circular polarization for x-ray mag- At the bottom of Fig. 2 we illustrate a particularly im- netic circular dichroism (XMCD) experiments and the sign portant result which clearly demonstrates that the domain of the contrast depends on the handedness of the x rays [6]. structure at the surface deviates from that in the bulk. Here The ferromagnetic Co XMCD images are obtained by di- we compare the observed spin directions for the case of viding images taken at the L2 and L3 edges [4]. 100 and 6110 walls in bulk NiO to those observed at The NiO crystal was cleaved ex situ and immediately the NiO surface. Although the surface 6110 walls ob- introduced into the ultrahigh vacuum PEEM2 instrument. served by XPEEM are still crystallographic mirror planes The crystal was then annealed at 380 K for several hours they are no longer magnetic mirror planes, in contrast to to desorb contamination resulting from the short exposure 6110 walls in the bulk. The observed surface 6110 to air. The low energy electron diffraction pattern of such walls minimize the dipole and stray field energies at the crystals was characteristic of a well-ordered 001 surface. surface by a compensated spin orientation perpendicular In a second step thin films (,2 nm) of Co or Fe were to the wall. In contrast, bulklike 6110 walls would re- deposited onto the crystal. The base pressure in the UHV sult in uncompensated surface spins, oriented parallel to system was 5 3 10210 torr. The deposition rate was about the wall. 0.04 nm per minute. Figure 3 shows images taken after deposition of eight Above the Néel temperature (TN 523 K) the NiO lat- monolayers of Co onto the NiO surface. The left column tice has a perfect fcc rocksalt structure. After cooling shows antiferromagnetic and the right column ferromag- below TN magnetoelastic forces cause a rhombohedral netic domain patterns. The upper two images were taken contraction of the crystal along different 111 axes and for the very same sample position as for Fig. 2, with the the crystallographic twinning leads to so-called T(win) do- NiO image on the left taken with plane polarized x rays. mains. Within each T domain the spins lie in ferromag- The lower two images correspond to a different azimuthal netic 111 planes, perpendicular to the contraction axes, orientation and the NiO image was taken with linear po- with adjacent planes exhibiting antiferromagnetic align- larized x rays to enhance the contrast. Comparison of the ment. Each T domain may furthermore split into three top left image in Fig. 3 with that of the bare NiO surface different S(pin) domains with spins along three possible in Fig. 2 reveals that after Co deposition the 6110 walls directions, e.g., 211 , 121 , and 112 [9]. Within each disappear and only the 100 walls remain. S domain the crystal exhibits a triclinic distortion. Low The ferromagnetic domains in the Co layer split up into energy domain walls between T domains correspond to two subgroups with each subgroup spatially following the 2879 VOLUME 86, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 26 MARCH 2001 FIG. 3 (color). Comparison of antiferromagnetic (left column) and ferromagnetic (right column) domain patterns for eight monolayers of Co on NiO(001) and two different azimuthal geometries. Arrows and wavy lines indicate the directions of the magnetic axes and photon wave vectors, respectively. Linear polarization is labeled p, circular and plane polarization s. Top panel: Same geometry as for Fig. 2 showing only half of the NiO domains. Each NiO domain has two corresponding Co domains with antiparallel spin alignment. Bottom panel: The sample is rotated so that Ek k 110 (f 45±). The contrast from one subgroup of ferromagnetic domains vanishes. The local spectra show the origin of the magnetic domain contrast. antiferromagnetic domains. The observed spatial align- ment of antiferromagnetic and ferromagnetic domains is caused by exchange coupling and it breaks up upon heating the system above the Néel temperature of NiO. In order to determine the orientation of the antifer- romagnetic and ferromagnetic axes, the sample was FIG. 2 (color). Antiferromagnetic domain structure of cleaved azimuthally rotated and images and local spectra were NiO(001) observed by XPEEM using plane polarization and an taken for different geometries. For simplicity we show orientation f 230±. Four different gray scales, representing four different antiferromagnetic domains are observed. The ar- only the spectra taken with Ek parallel to 110 and rows indicate the in-plane projections of the antiferromagnetic the in-plane photon spin projection, parallel to 110 axes which are 120 (red and blue) and 120 (white and yel- (f 45±) in the lower row of Fig. 3. In this geometry low). Each in-plane axis splits up into two different out-of-plane the contrast from one subgroup of ferromagnetic domain axes. These are 121 (red), 121 (blue), 121 (yellow), and vanishes, indicating that they are oriented perpendicular 121 (white). The inset in the upper right corner shows a sketch of the domain structure as a guide to the eye. Three type of do- to 110 . The dichroism contrast of the other subgroup main walls 110 , 110 , and 100 can be identified. Models for of ferromagnetic domains (black and white in lower typical domains and domain walls (green) for bulk NiO and for right image) is about 30%, while the antiferromagnetic those observed by us near the cleaved surface are shown below. contrast in the lower left image is 14%. Both dichroism 2880 VOLUME 86, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 26 MARCH 2001 Co deposition the 110 walls in the image vanish. Now all spins are parallel to 110 . The spins in the original T domains rotate by 630± in the 111 -like planes and align parallel to the surface, along 110 . All spins now lie in the 001 plane which is the spin configuration of a 001 wall parallel to the interface. In summary, for the cleaved NiO(001) surface we ob- serve only a subset of the bulk antiferromagnetic domains with formations of novel 110 domain walls. After Co de- position the antiferromagnetic spins reorient and align, do- main by domain, parallel to the Co spin direction which is in plane. The antiferromagnetic NiO(001) surface resem- bles a NiO 001 wall parallel to the surface with fourfold domain symmetry about the surface normal. The Co layer itself assembles into ferromagnetic domains indicating a strong uniaxial anisotropy of the Co parallel to the antifer- romagnetic axis. We observe the same behavior for Co and Fe deposition on the surface in the thickness range from 0.5­2 nm. The results are completely reproducible under the specified preparation conditions. Our results clearly demonstrate the sensitivity of the antiferromagnetic spin orientation to surface and interface effects. More gener- ally, they point out that any realistic model for the magnetic FIG. 4 (color). Illustration of the reorientation process of the exchange coupling at ferromagnetic-antiferromagnetic in- antiferromagnetic axes at the surface of NiO(001) after deposi- terfaces has to be based on the actual spin structure near tion of Co. The observed domain structure for the bare crystal the interface which may significantly deviate from that ex- (top) and the coupled system (bottom) is compared to three- dimensional models on the right. The colors in the PEEM im- pected from the bulk. ages correspond to those of the spin planes and axes in the models. The blue 111 and the red 111 planes are separated by a 110 wall. After Co deposition all spins align along 110 . [1] J. Nogués and I. K. Schuller, J. Magn. Magn. Mater. 192, values are completely consistent with the maximum bulk 203 (1999). values determined by Chen et al. [12] for Co and Alders [2] A. E. Berkowitz and K. Takano, J. Magn. Magn. Mater. et al. [7] for NiO, taking into account the different energy 200, 552 (1999). resolution in our experiment which affects the peak [3] H. Matsuyama, C. Haginoya, and K. Koike, Phys. Rev. intensity. Because of the 1 e electron yield probing depth Lett. 85, 646 (2000). of about 2.5 nm [6] for both the Co and NiO layers, the [4] F. Nolting, A. Scholl, J. Stöhr, J. W. Seo, J. Fompeyrine, observed domain structures arise from the entire Co layer, H. Siegwart, J.-P. Locquet, S. Anders, J. Lüning, E. E. on one hand, and from more than the top ten layers of Fullerton, M. F. Toney, M. R. Scheinfein, and H. A. Pad- the NiO. Within this near-interface region of NiO the more, Nature (London) 405, 767 (2000). antiferromagnetic spin directions are completely in plane, [5] S. Anders, H. A. Padmore, R. M. Duarte, T. Renner, parallel to 6110 and parallel to those in Co. Our ori- T. Stammler, A. Scholl, M. R. Scheinfein, J. Stöhr, L. Séve, and B. Sinkovic, Rev. Sci. Instrum. 70, 3973 entational precision for the determination of the in-plane (1999). components is 65± and for the out-of-plane compo- [6] J. Stöhr, H. A. Padmore, S. Anders, T. Stammler, and M. R. nent 67±. Scheinfein, Surf. Rev. Lett. 5, 1297 (1998). Figure 4 illustrates the reorientation process in more de- [7] D. Alders, L. H. Tjeng, F. C. Voogt, T. Hibma, G. A. tail. In the left column we show domain images taken be- Sawatzky, C. T. Chen, J. Vogel, M. Sacchi, and S. Ia- fore and after Co deposition at the same sample position. cobucci, Phys. Rev. B 57, 11 623 (1998). On the right we show the corresponding three-dimensional [8] J. Stöhr, A. Scholl, T. J. Regan, S. Anders, J. Lüning, M. R. models of the structure and the spin orientation. The spin Scheinfein, H. A. Padmore, and R. L. White, Phys. Rev. planes and axes are marked in corresponding colors. The Lett. 83, 1862 (1999). top panel describes the results for the cleaved surface, al- [9] S. Saito, M. Miura, and K. Kurosawa, J. Phys. C 13, 1513 ready discussed in conjunction with Fig. 2. T domains (1980). [10] W. L. Roth, J. Appl. Phys. 31, 2000 (1960). with spin axes parallel to 121 (dark blue) and 121 (dark [11] H. Ohldag et al. (to be published). red) are separated by a 110 wall. The spin direction in [12] C. T. Chen, Y. U. Idzerda, H.-J. Lin, N. V. Smith, G. Meigs, the wall (green) is perpendicular to the wall, parallel to E. Chaban, G. H. Ho, E. Pellegrin, and F. Sette, Phys. Rev. the common 110 projection of both T domains. Upon Lett. 75, 152 (1995). 2881