Strain-induced nonequilibrium magnetoelastic domain structure and spin reorientation of NiO(100) Suman Mandal and Krishnakumar S. R. Menon* Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Francesco Maccherozzi and Rachid Belkhou Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France and Sincrotrone ELETTRA, 4-km 163,5 in AREA Science Park, 34012 Basovizza, Trieste, Italy Received 14 July 2009; revised manuscript received 24 August 2009; published 9 November 2009 We report the observation of strain-induced antiferromagnetic domain structure on cleaved surface of NiO single crystal. This nonequilibrium domain structure undergoes various spin reorientations from in plane to different in plane, out of plane to in plane after mild annealing, indicating a direct correlation between the surface strain field and domain morphology. These reorientations are found to be driven by structural modifi- cation on the surface generated by cleaving process and buried dislocations, altering the surface magnetic anisotropy and their relaxation through mild annealing. These observations establish that the magnetoelastic effect plays a dominant role in determining antiferromagnetic domain structure. DOI: 10.1103/PhysRevB.80.184408 PACS numbers: 75.50.Ee, 75.60.Ch, 68.37.Xy, 68.47.Gh I. INTRODUCTION Magnetic domains occur naturally in all magnetic materi- als and play a vital role in determining their magnetic properties. 1 Applications in the area of magnetic data stor- age, magnetoelectronics and spintronic devices have en- hanced the significance of surface magnetism and a detailed understanding of the magnetic microdomain structures is es- sential for their practical applications. 2,3 In general, there is a lack of understanding of the surface antiferromagnetic AF domains in comparison to their ferromagnetic counterparts, contributed largely by the limited experimental probes sensi- tive to surface antiferromagnetic order. The antiferromag- netic domains have different origin compared to those in ferromagnets due to their magnetically compensated nature of the spin structure and higher-order contributions such as magnetoelastic anisotropy have to be considered. Thus, anti- ferromagnetic domains are more susceptible to external or internal stress field 4 for their stabilization which needs to be addressed to understand their properties. There are other im- portant effects such as the presence of perpendicular magne- tocrystalline anisotropy PMA where surface magnetic do- mains have an out-of-plane orientation and spin reorientation or domain switching. The Presence of PMA and its depen- dence on physical variables such as temperature and strain governs spin reorientation transition SRT, where the mag- netic moment orientation changes from out of plane to in plane or vice versa. Realization of surface SRT on antiferro- magnetic materials is significant also from technological per- spective as a potential candidate for magnetic domain control in magnetoelectronic devices, 2 especially in spin-valve de- vices as a way to control the adjacent ferromagnetic domains using the interfacial exchange interactions or magnetic inter- layer coupling. 5,6 So far, mostly ferromagnetic systems are known to exhibit these properties which are rather well characterized. 7 There have been few studies available in lit- erature on the NiO100 thin-film system demonstrating the presence of PMA Ref. 8 and in-plane SRT Ref. 9 using spectroscopic technique. However in these studies, we get only average information about the system without any local information. Nickel Oxide NiO is considered to be a prototype anti- ferromagnetic system with bulk domain structure studied ex- tensively, both theoretically 10,11 and experimentally. 12–14 The easy axes of the Ni moments are determined to be along 112 ¯  directions and lying in the easy 111 plane leading to a ferromagnetic spin arrangement within the 111 plane with antiferromagnetic arrangement between adjacent 111 planes. The exchange striction resulting from this antiferro- magnetic spin arrangement leads to a rhombohedral contrac- tion along 111, yielding four different twin T domains characterized by different rhombohedral axis. Each T domain can further split into three spin S domains due to the dif- ferent possible 112 ¯  directions. NiO crystals show large magnetoelastic effect and this magnetoelastic term primarily governs the nature of antiferromagnetic domains and the do- main walls. 15 Gomonay et al. 4 theoretically investigated the general issues of antiferromagnetic domains and proposed that, the incompatibility between the surface and bulk mag- netostriction results in “elastic charges” at the surface pro- ducing an elastic stray field, determining the equilibrium magnetic domain structure. Apart from this intrinsic surface effect, there are other nonequilibrium effects on surfaces such as strain, dislocations etc. which are expected to modify the surface antiferromagnetic domain structure via the mag- netoelastic effect and can exhibit interesting phenomenon such as the presence of PMA and SRT. The motivation of the present work is to address these issues experimentally by studying the magnetoelastic nature of the antiferromagnetic domains on the freshly cleaved surface of NiO100 expect- ing the presence of induced surface strain field. In the present work, we use a combination of low energy electron micros- copy LEEM and photoemission electron microscopy PEEM, with LEEM providing the structural and morpho- logical information along with magnetic information ob- tained from x-ray magnetic linear dichroism XMLD as a PHYSICAL REVIEW B 80, 184408 2009 1098-0121/2009/8018/1844086 ©2009 The American Physical Society 184408-1