Fingerprints of surface magnetism in Cr$_{2}$O$_{3}$ based exchange bias heterostructures

Magnetoelectric materials experienced a recent revival as promising components of novel spintronic devices [1, 2, 3]. Since the magnetoelectric (ME) effect is relativistically small in traditional antiferromagnetic (AF) compounds like Cr$_{2}$O$_{3}$ (max. \textit{$\alpha $}\textit{zz }\textit{$\approx $}4ps/m) and also cross-coupling between ferroic order parameters is typically small in the modern multiferroics, it is a challenge to electrically induce sufficient magnetization required for the envisioned device applications. In exchange bias systems the bias field depends critically on the AF interface magnetization. Hence, a strong relation between the latter and the surface magnetization of the free Cr$_{2}$O$_{3 }$pinning layer can be expected. Our recent research indicates that there are surface magnetic phase transitions in free Cr$_{2}$O$_{3}$ (111) films accompanying surface structural phase transitions. Well defined AF interface magnetization is initialized through ME annealing to T=20K. Subsequently, the interface magnetization is thermally driven through phase transitions at T=120 and 210K. Their effects on the exchange bias are studied in Cr$_{2}$O$_{3}$ (111)/CoPt films with the help of polar Kerr and SQUID magnetometry. [1] P. Borisov et al. Phys. Rev. Lett. 94, 117203 (2005). [2] Ch. Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). [3] R. Ramesh et al. 2007 \textit{Nature Materials }6 21. Financial support by NSF through Career DMR-0547887, MRSEC DMR-0820521 and the NRI.