Mapping of Magnetic Metastable States in Synthetic Antiferro- and Ferrimagnets with Perpendicular Magnetic Anisotropy

Magnetic multilayers (MLs) with perpendicular magnetic anisotropy (PMA), such as Co/Pt or
Co/Pd, are the host materials for a variety of metastable magnetic configurations, e.g. aligned or labyrinth
stripe domains, bubble domains and their mixtures. The magnetic morphology at remanence mostly depends
on the specific demagnetization routine using an external magnetic field [1], whereas the characteristic size of
the magnetic objects (domain period or bubble radius) is mostly determined by the design parameters of the
magnetic MLs (e.g. the thickness of Co layer, periodicity and the number of repeats X) [1,2]. Interleaving the
Co/Pt blocks by Ru or Ir layers, which promote antiferromagnetic (AF) interlayer coupling between PMA ML
blocks, results in a class of artificial magnetic structures, known as synthetic antiferromagnets (SAFs) with
PMA. The AF interlayer exchange energy alters the typical energy balance, thus modifying the morphology
and characteristic size of magnetic domain states in PMA SAFs [1]. Stabilisation of magnetic bubble domain
states in SAFs is of particular practical interest [3], whereas other new magnetic configurations are of interest
for fundamental research. Here we present the mapping of magnetic states in SAFs for different
demagnetization protocols and ML parameters by means of monitoring the remanent magnetization Mr during
the AC or DC demagnetization process itself and performing magnetic force microscopy (MFM) imaging at the
intermediate states of interest (Fig. 1). The aligned stripe domain state is characterised by almost zero Mr,
whereas bubble domains [4] and mixed states (Fig. 1) manifest themselves by an enhanced Mr. The magnetic
states for SAFs with tunable parameters such as Co thickness and repetition number X will be presented. We
will also discuss the magnetic behaviour in synthetic ferrimagnets, where the two ML block sub-lattices have
different magnetic moments.
References: [1] O. Hellwig, A. Berger, J. B. Kortright, JMMM, Vol. 319, p.13-55 (2007)
[2] I. Lemesh, F. Büttner, G. S. D. Beach, Phys. Rev. B, Vol. 95, p.174423 (2017)
[3] K. Chesnel, A. S. Westover, C. Richards, Phys. Rev. B, Vol. 98, p.224404 (2018)
[4] A. Hubert, R. Schäfer, Magnetic Domains, Springer Berlin (2009)