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We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned, thus providing a means to establish exchange bias.

When a ferromagnet is in proximity with an antiferromagnet, lateral length scales such as the respective magnetic domain sizes drastically affect the exchange bias. Bilayers of FeF 2 and either Ni, Co or Fe are studied using SQUID and spatially resolved MOKE. When the antiferromagnetic domains are larger than or comparable to the ferromagnetic domains, a local, non-averaging exchange bias is observed. This gives rise to unusual and tunable magnetic hysteresis curves.PACS numbers: 75.70.-i, 75.60.-d, 75.25.+z, 75.30.-m Understanding the relevant length scales that characterize a particular phenomenon or type of interaction in a material is one of the most important issues in physics. This becomes less obvious and more intriguing when two dissimilar materials are in contact. In such cases, the proximity effect is often observed, where one material modifies the properties of the other. Because of the finite extent of electron wave functions, the proximity effect is typically described as the spatial variation of an order parameter across the interface. For example, the proximity effect that occurs in superconductor-normal bilayers, is characterized by the decay of the superconducting

Independently exchange biased subsystems can coexist in ferromagnet/antiferromagnet bilayers after various field-cooling protocols. We find well separated double hysteresis loops in FeF2∕Ni bilayers for intermediate cooling fields, while for small or large cooling fields a negatively or positively shifted single loop, respectively, are encountered. The antiferromagnet breaks into a bidomain state with opposite signs but equal magnitude of bias acting on the ferromagnet. This idea is supported by micromagnetic simulations. Experiments are presented, where thermally activated motion of these antiferromagnetic domain boundaries can be achieved.

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a b s t r a c tThis "Critical Focused Issue" presents a brief review of experiments and models which describe the origin of exchange bias in epitaxial or textured ferromagnetic/antiferromagnetic bilayers. Evidence is presented which clearly indicates that inner, uncompensated, pinned moments in the bulk of the antiferromagnet (AFM) play a very important role in setting the magnitude of the exchange bias. A critical evaluation of the extensive literature in the field indicates that it is useful to think of this bulk, pinned uncompensated moments as a new type of a ferromagnet which has a low total moment, an ordering temperature given by the AFM Néel temperature, with parallel aligned moments randomly distributed on the regular AFM lattice.

Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet/ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrinsic broken symmetry of the system, which results in local incomplete domain walls parallel to the interface in reversal to negative saturation of the FM. The magneto-optical Kerr effect unambiguously confirms such an asymmetric reversal and a depth-dependent FM domain wall in accord with the magnetometry and simulations. DOI: 10.1103/PhysRevLett.96.217205 PACS numbers: 75.25.+z, 75.60.Jk, 75.70.Cn, 78.20.Jq Exchange coupling between a ferromagnet (FM) and an antiferromagnet (AF) has been intensely studied due to the fundamental interest in inhomogeneous magnetic systems and its central role as a magnetic reference in various devices. In most magnetic systems, time reversal symmetry is present and manifested by a symmetric magnetization curve relative to the origin. This symmetry also requires that the magnetization reversal from positive to negative saturation be identical to the reverse process. However, in a FM/AF system, exchange bias (EB) develops below the AF Néel temperature T N producing a shift (H EB ) of the hysteresis loop along the magnetic field axis [1]. Therefore, with the shift breaking the time reversal symmetry, magnetization reversal symmetry is no longer required. In fact, asymmetric reversal was observed by polarized neutron reflectometry [2], photoemission electron microscopy [3], magneto-transport [4], magneto-optical indicator film [5], and magneto-optical Kerr effect [6]. In some systems the reversal along the decreasing branch is dominated by transverse magnetic moments, a phenomenon interpreted as due to coherent magnetic rotation. The absence of transverse moments in the increasing branch reversal was interpreted as domain wall propagation [2,3]. Different, even opposite, scenarios were also found [6][7][8]. Despite the well established experimental evidence and proposed theoretical models [9][10][11], the origin of this asymmetry remains a controversial and highly debated issue [12]. This situation is further complicated by the lack of knowledge of the interface, crystal imperfections, complex FM and AF anisotropy energies, and training effect. While these factors are important for each individual system, the fundamental connection of the reversal asymmetry to the broken symmetry intrinsic in the inhomogeneous system is overlooked.In this Letter, we investigate a simple model system using a variety of experimental techniques combined with numerical simulations. We establish a critical link between this unusual reversal asymmetry with the time reversal asymmetry in these systems. Namely, in reversal toward the two FM saturated states, the intrinsic asymmetry gives rise to different competing mechanisms, thus different reversal processes.Fe...

Magnetic nanostructures have been widely studied due to their potential applicability into several research fields such as data storage, sensing and biomedical applications. Focusing on the biomedical aspect, some new approaches deserve to be mentioned: cell manipulation and separation, contrast-enhancing agents for magnetic resonance imaging, and magnetomechanically induced cell death. This work focuses on understanding three different magnetic nanostructures, disks in the vortex state, synthetic antiferromagnetic particles and nanowires, first, by explaining their interesting properties and how they behave under an applied external field, before reviewing their potential applications for each of the aforementioned techniques.