SCIENCE CHINA Physics, Mechanics & Astronomy © Science China Press and Springer-Verlag Berlin Heidelberg 2012 phys.scichina.com www.springerlink.com *Corresponding author (email: hfding@nju.edu.cn) • Review • January 2013 Vol.56 No.1: 70–84 Progress of Projects Supported by NSFC·Spintronics doi: 10.1007/s11433-012-4975-3 Thickness-driven spin reorientation transition in ultrathin films MIAO BingFeng 1 , MILLEV YonkoTimtchev 2 , SUN Liang 1 , YOU Biao 1 , ZHANG Wei 1 & DING HaiFeng 1* 1 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, 22 Hankou Rd., Nanjing 210093, China; 2 American Physical Society, 1 Research Road, Ridge, New York, USA Received November 27, 2012; accepted December 11, 2012; published online December 26, 2012 We review recent studies by different experimental means of ultrathin films, exhibiting thickness-driven spin reorientation transitions (SRTs). The stage is set by determining, via phenomenological thermodynamic description, of the relevant phase diagrams for the possible types of SRT with and without applied magnetic field. Suitable representation may be chosen such that best use is made of the linear character (under thickness variation) of the system’s path in anisotropy space. The latter in- volves higher-order bulk and surface anisotropies in a substantial way. We examine sensitive experimental techniques for the detection and quantification of SRTs, such as hysteresis measurements with magneto-optical Kerr effect (MOKE), micromag- netic studies utilizing scanning electron microscopy with polarization analysis (SEMPA), photoemission electron microscopy (PEEM) and spin-polarized low-energy electron microscopy (SPLEEM) as well as ac magnetic susceptibility measurements via MOKE. Key issues are conclusively discussed including the identification of reliable experimental fingerprints about whether a given SRT proceeds via a phase of coexistence or via a cone (canted) phase. We demonstrate how the application of the general theoretical ideas to carefully designed measurements leads to the determination of the most important material pa- rameters in any ultrathin-film SRT, namely, the surface (interface) magnetic anisotropy constants. The review concludes by our personal outline for future promising work on SRTs. ultrathin magnetic films, spin reorientation transitions, magnetic anisotropy PACS number(s): 75.70.-i, 75.30.Gw, 75.30.Kz, 75.40.-s Citation: Miao B F, Millev Y T, Sun L, et al. Thickness-driven spin reorientation transition in ultrathin films. Sci China-Phys Mech Astron, 2013, 56: 7084, doi: 10.1007/s11433-012-4975-3 A spin reorientation transition (SRT) is, generally, a cross- over between different states of macroscopic magnetic order (antiferromagnetic, canted, ferrimagnetic or ferromagnetic), which is effected through a rotation of the macroscopic magnetization into a new equilibrium direction under varia- tion of experimentally controllable, or at least identifiable, parameters such as temperature, stress, film thickness, etc [1]. Whenever such variations affect the balance between competing and often subtle contributions to magnetic ani- sotropy, the energetically preferred axis of magnetization (spin axis) may change orientation. The study of SRTs in the bulk has a long history, mostly focused on the tempera- ture-driven reorientations [2,3]. Recently, much of the pro- gress in the study of magnetism and magnetic materials has been driven by the fundamental and practical interest to explore the meso- and nanoscales. Suffice it to mention the discovery and fast technological implementation of the gi- ant magnetoresistance effect, which opened up the new field of spintronics [4–6]. Much of its proven and future capabili- ties, as well as those of ultra-dense magnetic data storage [3,7], depend crucially on the reliable control of the mag- netization orientation in one or more thin or ultrathin mag- netic layers of the sometimes rather intricate multilayer sandwich structures. Thus, having a clear and detailed un-