PHYSICAL REVIEW B 85, 104418 (2012) Three-dimensional spin orientation in antiferromagnetic domain walls of NiO studied by x-ray magnetic linear dichroism photoemission electron microscopy Kuniaki Arai, 1 Taichi Okuda, 2 Arata Tanaka, 3 Masato Kotsugi, 4,5 Keiki Fukumoto, 4,* Takuo Ohkochi, 4 Tetsuya Nakamura, 4 Tomohiro Matsushita, 4 Takayuki Muro, 4 Masaki Oura, 6 Yasunori Senba, 4 Haruhiko Ohashi, 4 Akito Kakizaki, 1 Chiharu Mitsumata, 7 and Toyohiko Kinoshita 4,5,† 1 Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277–8581, Japan 2 Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739–0046, Japan 3 Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima, Hiroshima 739–8530, Japan 4 Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679–5198, Japan 5 JST-CREST, Kawaguchi, Saitama 332–0012, Japan 6 RIKEN-SPring-8 Center, Sayo, Hyogo 679–5148, Japan 7 Graduate School of Engineering, Tohoku University, Sendai 980–8579, Japan (Received 29 November 2011; published 29 March 2012) A determination of the three-dimensional spin directions in all types of domain walls (DWs) of antiferromag- netic NiO has been successfully performed by photoemission electron microscopy combined with x-ray magnetic linear dichroism (XMLD), both for s - and p-polarized light. By comparing the azimuthal angle dependence of the XMLD contrast in the DWs with cluster model calculations which include the crystal symmetry and full-multiplet splitting, we determine the spin structures in the {001} T walls, {011} T walls, 120 ◦ S walls, and 180 ◦ S walls. In some cases, distinct S walls are not formed between two adjacent S domains, and the spin direction changes gradually over a wide range of the S domain structures. In the S walls, the spin direction is parallel to the magnetic easy {111} plane. These spin configurations arise from the large difference in anisotropy energy between the in-plane and out-of-plane directions. Unexpectedly large widths in the several hundred nanometer range were observed for all the DWs. This also shows that NiO has a small magnetocrystalline anisotropy energy. Together with Monte Carlo simulation results, the qualitative phenomena concerning the wall energies are discussed. We further investigated the difference in wall energy between the {001} T wall and the {011} T wall. From the Monte Carlo simulation and an experimental study of heating effects, it is revealed that the {001} T wall energy is smaller than the {011} T wall energy. DOI: 10.1103/PhysRevB.85.104418 PACS number(s): 75.25.−j, 75.60.Ch, 78.20.Bh, 78.70.Dm I. INTRODUCTION It is very important to understand domain and domain wall (DW) structures of both ferromagnetic (FM) and of antifer- romagnetic (AFM) materials, because macroscopic magnetic phenomena are related to these microscopic structures. 1 From a technological point of view, FM/AFM interface and multilayer systems are now widely utilized for magnetic recording techniques. 2 In order to design devices based on fundamental knowledge, information on such microscopic magnetic structures is crucial. One of the most interesting phenomena is known as the exchange bias, which is combined with the magnetic resistance (MR) effect and utilized in spin-valve recording devices. It is considered that the exchange coupling through AFM substrates and FM thin films is the origin of “pinning” of spins at the interface; therefore, determining the spin structures is essential. Mauri and co- workers suggested how spins are coupled in FM/AFM systems from a calculation, 3 and predicted spin rotation from the substrate to the outermost surface through the interface. Such model calculations rely on knowledge of parameters such as anisotropy, exchange energies, and magnetostatic energies. Therefore, detailed investigation of domain and DW structures of realistic materials is very important. Here, we focus on the detailed domain and DW structures of NiO, which is a fundamental and typical AFM material with a N´ eel temperature of T N = 523 K and a collinear spin structure. The AFM superexchange interaction of the Ni-O-Ni bonds along the 〈100〉 directions leads to the formation of FM-ordered {111} planes, where spins in the adjacent {111} planes align in the antiparallel direction, as shown in Fig. 1. Owing to the magnetostriction caused by AFM ordering, the NiO crystal consists of many twinned crystals for temperatures below T N . This crystallographic twinning leads to four different domains, i.e., the so-called twin domains (T domains), with four different contractions along the 〈111〉 axes. In a single T domain, there are three possible spin easy axes along the 〈112〉 directions. Thus, in total, there are 12 types of spin domains (S domains). This makes the domain structure in NiO very complicated. It has been suggested 4 that domain walls are formed at the boundaries between adjacent domains, in the same way as in FM materials. Figure 2 shows the possible DWs between the T domains. This type of DW is called a T wall. As shown in the figures, two types of DWs exist, i.e., the {001} and {011} T walls. In the same way, S walls are formed between two different S domains. Schematic illustrations of the possible types of S wall are shown in Fig. 3. Because the easy axes of the spin are along the 〈112〉 directions, the angle of two S domains adjacent to the S wall should be 60 ◦ or 120 ◦ , as shown in Figs. 3(a) and 3(b). Usually these angles cannot be distinguished because of the AFM nature of spin vectors. However, there is a possibility of the existence of these two types of S walls as described below. As shown in Fig. 3(c), an 180 ◦ S wall is formed. In this case, the two S 104418-1 1098-0121/2012/85(10)/104418(12) ©2012 American Physical Society