Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Abstract

Using photoemission electron microscopy combined with X-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF 2 , show that the spin structure is determined by the balance between the competing FM and AF magnetic energies.Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF 2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF 2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF 2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultra-high density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF 2 may be generic to other magnetic systems with complex non-collinear FM/AF spin structures.

Role of the antiferromagnetic bulk spins in exchange bias

“…Besides, PEEM in this case is sensitive to the additional PUM present on the side-walls of the antidots. These measurements proved that in antidot containing samples additional PEB domains appear, when the Zeeman energy of bulk PUM locally overcome the exchange energy at the FM/AFM interface [34]. Independent PEEM experiments also suggested the presence of additional bulk PUM on the artificially-created, exposed lateral AFM walls as a result of the antidot carving [34].…”

Dipole-induced exchange bias

“…We assume that the domains in the AFM, which are due to the energy balance and that originate EB, are created during field cooling and remain frozen even when the FM is fully saturated, as observed experimentally in exchange coupled bilayers. 24,[30][31][32][33] As shown in Fig. 2(a), during the field cooling process FM layers are fully saturated and magnetic domains are nucleated at AFM, simultaneously.…”

“…Our assumption about AFM domains is based on photoemission, electron microscopy and X-ray magnetic circular dichroism measurements, 30,34 by the formation of a two domain state, composed of uncompensated spins, as observed experimentally. 14,35,36 Since for large cooling fields the anisotropy energy is significantly larger than the dipolar and Zeeman energies, these domains remain frozen during the hysteresis cycle.…”

“…When the field is swept during the hysteresis cycle the magnetic domain formation on the FM is determined by the competition between the formation of large domains induced by the Zeeman interaction and the formation of small domains due to the local dipole fields. 30 The FM-AFM interaction energy density of two noninteracting FM domains is given by…”

Cooling-field dependence of dipole-induced loop bias

“…a single-domain approximation) [37]. (ii) The antiferromagnetic uncompensated magnetization in the vicinity of ferromagnet/antiferromagnet interface exists due to D IF and H FC [26,38], also based on the photoemission, electron microscopy, and x-ray magnetic circular dichroism measurements experimentally [39][40][41][42][43]. (iii) There are longrange dipolar interactions between ferromagnetic and antiferromagnetic domains.…”