Magnetic textures in hemispherical thin film caps with in-plane exchange bias

Abstract

Hemispherical caps of in-plane exchange biased IrMn/CoFe layer systems have been fabricated on top of regularly arranged spherical silica particles by magnetron sputtering, creating magnetic Janus particles. In this thin film layer system cap, the magnetic shape anisotropy of the topographically non-flat hemispheres competes with the unidirectional anisotropy induced by the exchange bias. The magnetic properties of this non-trivial system have been investigated by longitudinal magneto-optical Kerr effect magnetometry, where a characterization method has been developed considering both the curved layer system and the signal contributions of flat parts of the sputtered thin film system. Both remagnetization curves, from Kerr magnetometry and the magnetic force microscopy images, reveal an onion state in the magnetic caps of the ensemble. Additional micromagnetic simulations show a stabilization of the onion state due to the introduced unidirectional anisotropy also in individual hollow hemispheres as compared to the vortex state exhibited by purely ferromagnetic caps.

Translatory and rotatory motion of exchange-bias capped Janus particles controlled by dynamic magnetic field landscapes

“…For superparamagnetic beads, however, a reorientation of the particles’ magnetic moment by an external magnetic field either occurs by Néel (pure spin reorientation) or Brownian relaxation (reorientation by physical rotation) 38 . For the present MJPs with a spatially fixed remnant magnetic moment in the exchange-biased cap (magnetic onion state 16 ) only Brownian relaxation is possible as long as the external field does not induce a magnetization reversal in the particle. As the cap is optically opaque its direction has been determined from the recorded videos by an automated method employing machine learning 37 .…”

“…Exchange-biased MJPs ( d = 3 µm) with a nominal cap-pole thickness of 55 nm 16 were dispersed in distilled water and put into a microfluidic chamber on top of a magnetically patterned, topographically flat EB thin film system, which serves as the substrate. The EB substrate possesses a magnetic head-to-head (hh)/tail-to-tail (tt) parallel-stripe domain pattern (see Fig.…”

“…the effective magnetic moment. This can be intuitively understood by considering the magnetization distribution of the attributed onion state 16 . Here, the effective magnetic moment is mainly governed by the in-plane component of the magnetization, since out-of-plane components from the two equatorial poles of the cap cancel each other.…”

“…Further works highlight the flexibility of externally controlling magnetic MJP rotation due to their asymmetric structure 9 . Besides other known fabrication methods for MJPs 10 – 12 , the magnetic functionalization can be achieved by directly depositing a magnetic thin film system on top of self-assembled, non-magnetic spheres 13 leading to the formation of a magnetic cap covering one half of the original particle 14 – 16 . The magnetic ground state of these caps (vortex, onion or out-of-plane) can be controlled by variation of the particle size, cap thickness and material composition 14 , 17 .…”

“…In the present work, we make use of the same technique for studying the motion behavior of magnetically engineered MJPs, which were fabricated by sputtering an EB thin film system on top of spherical silica beads. The additional unidirectional anisotropy, originating from the EB, was introduced to stabilize a magnetic onion state inside the cap in remanence, instead of a normally preferred vortex state 14 , 16 , 17 for the studied particle size. For the onion state, magnetic moments align tangentially to the cap’s surface following its curvature, while the macroscopic net moment points along the EB direction (see sketch of Fig.…”