We present theoretical and experimental studies that explain the observed strong enhancement of the magneto-optical (MO) Faraday rotation in all-metal core-shell Co-Ag nanoparticles (NPs) attributed to localized surface plasmon resonance (LSPR). We also explain why the optical absorption and MO spectra peaks appear blue-shifted with increased Co core size while keeping the NP size constant. Further, we demonstrate direct correlation between the strong LSPR induced electromagnetic fields and the enhanced MO activity of the NPs.

We demonstrate magnetic field control of surface plasmon excitations in noble-metal/ferromagnetic/noble metal trilayers, analogous to the effects previously observed in semiconductor structures. We show that the coupling of an external magnetic field to the surface plasmon-polariton wave vector is greatly enhanced in the metallic structure due to the ferromagnetic nature of one of its constituents. The observed coupling could be used to modulate the surface plasmon response in ultrasensitive spectroscopic applications. DOI: 10.1103/PhysRevB.76.153402 PACS number͑s͒: 78.20.Ls, 73.20.Mf, 78.66.Bz, 42.25.Bs Surface plasmon-polariton ͑SPP͒ modes are electromagnetic excitations localized at the interface between two media, one with positive and the other with negative dielectric constant. These modes may appear at the interface between a degenerate semiconductor and a dielectric or between a metal and a dielectric. In the former case, due to the low value of the plasma frequency of the semiconductor, the frequencies of the SPPs are restricted to the far infrared range, whereas in the second case the SPP modes can have frequencies varying from the far infrared to the visible range. The propagation characteristics of the SPPs and their EM field distribution depend strongly on the optical properties and interface morphology of the system. This dependence has been exploited in different optical contexts such as light guiding at the subwavelength scale, 1-3 optical switching, 4 biochemical sensing, 5 or nanometer resolved far-field optical microscopy. 6 To date SPPs are commonly considered as passive, i.e., insensitive to the magnetic field and just depending on the optical and geometrical properties of the system. In this work we demonstrate the control of SPP excitations in metallic trilayer structures by means of an external magnetic field. We show that the coupling of the magnetic field to the wave vector of the plasmon is greatly enhanced by the ferromagnetic nature of the trilayer structure. This effect was first studied theoretically in semiconductor-based SPPs 7-9 and in metals. 10 The effect of the magnetic field on the properties of the SPP modes depends on the relative orientation of the applied magnetic field with respect to the wave vector of the SPP. In particular, we will show that when the magnetic field is applied perpendicular to the direction of propagation of the SPP and parallel to the interface, it modifies the dispersion relation of the SPP mode in such a way that the dispersion relation depends on the k direction ͓i.e., w͑k͒ w͑−k͔͒. Experimentally this magnetic field induced nonreciprocity has been observed on semiconductor-based SPPs, 11 but not yet in metallic systems. This is due to the high magnetic field needed to observe magnetic field induced effects on metallic based SPPs.One way to reduce the required external magnetic field is to incorporate ferromagnetic metals. Due to the magnetooptical ͑MO͒ activity that many ferromagnetic materials exhibit at low magnetic fields, surface magnetopl...

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed ͑hcp͒ form which we believe is stabilized by heteroepitaxial growth on the ͑001͒ face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value ͑about 2.5 nm thick with a lateral size of ϳ5 nm͒. The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c / a ratio ͑ϳ6% larger than ideal hcp͒ is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.

We present correlated experimental and theoretical studies on the magnetic field modulation of Surface Plasmon Polaritons (SPPs) in Au/Co/Au trilayers. The trilayers were grown by sputter deposition on glass slides with the Co films placed at different distances from the surface and with different thickness. We show that it is possible to tailor Au/Co/Au trilayers with the critical thickness needed for optimum excitation of SPPs leading to large localized electromagnetic fields. The modification of the SPP wave vector by externally applied magnetic fields was investigated by measuring the magneto-optical activity in transverse configuration. In addition, using magneto-optics as a tool we determined the spatial distribution of the SPP generated electromagnetic fields within Au/Co/Au samples by analyzing the field-dependent optical response, demonstrating that it is possible to excite SPPs that exhibit large electromagnetic fields that are also magneto-optically active and therefore can be modulated by externally applied magnetic fields.

The propagation of surface plasmon polaritons (SPPs) on gratings can be experimentally modified when magneto-optically active materials are used. We have observed this effect in Au-Co thin-film bilayers where a polymer grating was patterned on the upper surface. In addition, Au-Co-Au trilayers were grown on polycarbonate gratings, and the Co layer thicknesses and placement from the upper interface were varied to further investigate this effect. We show that the Co layer must be tailored to balance optical absorption and magneto-optical activity, while the pitch of the grating can be adjusted to tune the angular dependence of the field-dependent SPP excitation.