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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...

Single phase epitaxial pure ␥Ј-Fe 4 N films are grown on MgO ͑001͒ by molecular beam epitaxy of iron in the presence of nitrogen obtained from a radio frequency atomic source. The epitaxial, single phase nature of the films is revealed by x-ray diffraction and by the local magnetic environment investigated by Mössbauer spectroscopy. The macroscopic magnetic properties of the ␥Ј-Fe 4 N films are studied in detail by means of transverse Kerr effect measurements. The hysteresis loops are consistent with the cubic atomic structure, displaying easy ͓100͔ magnetization directions. The films are single domain at remanence, and the reversal is dominated by 180°or 90°domain wall nucleation and propagation, depending on the applied field direction. When 90°domain walls are responsible for the magnetization reversal, this proceeds in two stages, and the measured coercive fields vary accordingly. Magnetic domain observations reveal the two distinct reversal -driven by 180°or 90°domain walls-modes displaying large domains, of the order of mm. From magnetometer techniques, the saturation magnetization, 0 M s , is measured to be 1.8 T. A magneto-optical torque technique is used to obtain a value of the anisotropy constant of 2.9ϫ10 4 J/m 3 .

A complete study of the magneto-optical (MO) and magnetoplasmonic properties of both epitaxial and polycrystalline Au/Fe/Au multilayers as a function of Fe thickness is reported. An increase for both the MO activity without surface-plasmon-polariton (SPP) excitation and the magnetic modulation of the SPP wave vector for epitaxial structures compared to the polycrystalline ones is observed. This difference is related with the larger interface roughness of the polycrystalline structures that as a consequence, exhibit a reduction in the Fe MO constants. A comparative study of the MO activity with SPP excitation for both epitaxial and polycrystalline structures is also presented, being the key factor in the deposition of the adequate layer thickness to achieve optimum SPP excitation with independence of their crystalline nature.

Nanohole arrays in Ni films have been prepared by a replica/antireplica method based on anodic alumina membranes. The nanohole arrays exhibited long range ordering with hexagonal symmetry, the hole distance was kept constant ͑105 nm͒, and the hole diameter and the film thickness were varied between 50 and 70 nm and 55 and 600 nm, respectively. The magnetic domain structures of such samples have been studied by analyzing magnetic force microscopy images at remanent state. Different domain structures have been observed depending on the geometrical characteristics of the films. The experimental results have been interpreted with the help of micromagnetic simulations.