The magnetization reversal in exchange-biased ferromagnetic-antiferromagnetic (FM-AFM) bilayers is investigated. Different reversal pathways on each branch of the hysteresis loop, i.e., asymmetry, are obtained both experimentally and theoretically when the magnetic field is applied at certain angles from the anisotropy direction. The range of angles and the magnitude of this asymmetry are determined by the ratio between the FM anisotropy and the interfacial FM-AFM exchange anisotropy. The occurrence of asymmetry is linked with the appearance of irreversibility, i.e., finite coercivity, as well as with the maximum of exchange bias, increasing for larger anisotropy ratios. Our results indicate that asymmetric hysteresis loops are intrinsic to exchange-biased systems and the competition between anisotropies determines the asymmetric behavior of the magnetization reversal.

It is well known that localized surface plasmon resonances (LSPRs) greatly influence the optical properties of metallic nanostructures. The spectral location of the LSPR is sensitive to the shape, size, and composition of the nanostructure, as well as on the optical properties of the surrounding dielectric. [1] The latter effect has been used to develop different types of optical biosensors for which biological reactions near the surface of the nanostructure can be monitored through the changes in the frequency of the LSPR. [2][3][4][5][6] The induced electromagnetic field associated with the LSPR is greatly enhanced at the metal/dielectric interface, a phenomenon that is the basis for various types of surface-enhanced spectroscopy, such as surface-enhanced Raman scattering. [7] Furthermore, metallic nanoparticles have been shown to have light-guiding capabilities on the nanometer scale. This makes them suitable for the development of nano-optic devices. [8] The overwhelming majority of LSPR studies have focused on Au or Ag nanoparticles because these metals have suitable optical constants for application with visible wavelengths of light. However, once the morphology and composition of a nanostructure have been fixed, it is difficult to change or control the LSPR properties by external means, which would be desirable for the development of active nanoplasmonic devices. One way to overcome this problem could be to embed the metal nanostructure in an active medium, such as a liquid crystal, [9] which can be controlled by an external electrostatic field, or a ferromagnetic garnet, [10,11] which can be moderated by a magnetic field. An alternative approach could be to let the controlling field act directly on the metallic nanostructure, for instance, using nanoparticles made of ferromagnetic metals. Such metals have strong magneto-optical (MO) activity, that is, their optical properties change markedly even if the applied magnetic field is weak. Unfortunately, this high optical absorption results in a strong damping of any intrinsic LSPR that prevents the development of active plasmonic devices made solely of ferromagnetic metals. A promising route forward could be to combine ferromagnetic materials that would promote strong MO activity with noble metals that could induce plasmonic response. The large enhancement and spatial localization of the electromagnetic field associated with the LSPR suggest that a strong enhancement of the MO properties should be possible. [12] Several attempts to develop these kinds of structures have been carried out using different chemical synthesis methods to fabricate complex onion-like nanoparticles made of noble metals and ferromagnetic materials. [13][14][15][16] These systems do exhibit LSPRs, but so far no MO activity has been reported. On the other hand, continuous thin films made of Au/Co/Au trilayers were found to lead simultaneously to well-defined propagating surface plasmon polaritons and to strong MO activity at low magnetic fields. [17] Moreover, such composite structure...

Growth regimes of gold thin films deposited by magnetron sputtering at oblique angles and low temperatures are studied from both theoretical and experimental points of view. Thin films were deposited in a broad range of experimental conditions by varying the substrate tilt angle and background pressure, and were analyzed by field emission scanning electron microscopy and grazing-incidence small-angle x-ray scattering techniques. Results indicate that the morphological features of the films strongly depend on the experimental conditions, but can be categorized within four generic microstructures, each of them defined by a different bulk geometrical pattern, pore percolation depth and connectivity. With the help of a growth model, a microstructure phase diagram has been constructed where the main features of the films are depicted as a function of experimentally controllable quantities, finding a good agreement with the experimental results in all the studied cases.

Exchange bias, referred to the interaction between a ferromagnet ͑FM͒ and an antiferromagnet ͑AFM͒, is a fundamental interfacial magnetic phenomenon, which is key to current and future applications. The effect was discovered half a century ago, and it is well established that the spin structures at the FM/AFM interface play an essential role. However, currently, ad hoc phenomenological anisotropies are often postulated without microscopic justification or sufficient experimental evidence to address magnetization-reversal behavior in exchange-bias systems. We advance toward a detailed microscopic understanding of the magnetic anisotropies in exchange-bias FM/AFM systems by showing that symmetry-breaking anisotropies leave a distinct fingerprint in the asymmetry of the magnetization reversal and we demonstrate how these emerging anisotropies are correlated with the intrinsic anisotropy. Angular and vectorial resolved Kerr hysteresis loops from FM/AFM bilayers with varying degree of ferromagnetic anisotropy reveal a noncollinear anisotropy, which becomes important for ferromagnets with vanishing intrinsic anisotropy. Numerical simulations show that this anisotropy naturally arises from the inevitable spin frustration at an atomically rough FM/AFM interface. As a consequence, we show in detail how the differences observed for different materials during magnetization reversal can be understood in general terms as originating from the interplay between interfacial frustration and intrinsic anisotropies. This understanding will certainly open additional avenues to tailor future advanced magnetic materials.

We present in this work our current understanding on magnetoplasmonic structures, that is, systems whose constituents exhibit simultaneously magnetic and plasmonic properties. We analyze both the influence of the plasmon resonance on the magneto-optical properties of the system and the ability of the magnetic field to modulate the plasmon properties. In particular we show how, in magnetoplasmonic systems sustaining localized or propagating surface plasmons, the associated electromagnetic field enhancement gives rise to an enhancement of the magneto-optical activity. On the other hand, we have analyzed the modulation of the propagating surface plasmon polariton wavevector in noble metal/ferromagnet/noble metal trilayers by an external magnetic field. These phenomena can be addressed as new concepts for the development of active plasmonic devices.

The growth of Ti thin films by the magnetron sputtering technique at oblique angles and at room temperature is analysed from both experimental and theoretical points of view. Unlike other materials deposited in similar conditions, the nanostructure development of the Ti layers exhibits an anomalous behaviour when varying both the angle of incidence of the deposition flux and the deposition pressure. At low pressures, a sharp transition from compact to isolated, vertically aligned, nanocolumns is obtained when the angle of incidence surpasses a critical threshold. Remarkably, this transition also occurs when solely increasing the deposition pressure under certain conditions. By the characterization of the Ti layers, the realization of fundamental experiments and the use of a simple growth model, we demonstrate that surface mobilization processes associated to a highly directed momentum distribution and the relatively high kinetic energy of sputtered atoms are responsible for this behaviour.

Surface plasmon polariton ͑SPP͒ excitation effects on the magneto-optical ͑MO͒ activity of Au capped Ag/Co/Ag trilayers are studied as a function of Co thickness. An enhancement of the transverse MO Kerr signal under SPP excitation as compared with that obtained without SPP excitation is measured with a maximum value of 150 times obtained for the trilayer with 8 nm Co. Such enhancement on the magneto-optical activity due to SPP excitation is also five times higher than that obtained in Au/Co/Au trilayers in similar conditions. The lower optical absorption in the studied range and the sharper plasmon resonance of Ag vs Au are responsible for these values. On the other hand, magnetic field-induced SPP wavevector modulation ͑⌬k / k͒ SPP is studied for these trilayers and compared both with previous results in the Au/Co/Au system as well as with the theory. In the wavelength considered here, the obtained values are similar for both Ag-and Au-based structures and on the order of 10 −4 , pinpointing the role of the magnetic layer on the SPP wavevector modulation.

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