P. Fumagalli

Optical and Magnetic Properties of Hexagonal Arrays of Subwavelength Holes in Optically Thin Cobalt Films

In this study, we present our experimental results on the optical, magnetic, as well as magneto-optic properties of hexagonal arrays of subwavelength holes in optically thin cobalt films. Different meshes were used with hole diameters ranging between 220 and 330 nm while the interhole distance has been kept constant at 470 nm. The hole pattern modifies completely the magnetic behavior of the cobalt films; it gives rise to an increase of the coercive field of the in-plane magnetization with increasing hole diameter and to the appearance of out-of-plane magnetization components. Magneto-optic measurements show a spectacular magneto-optic response at wavelengths where surface plasmon-polaritons are supported by the structure as deduced in optical measurements. The experiments demonstrate the ability to artificially control the magnetic and thus the magneto-optic properties in hole array structures.

Preparation of (In,Mn)As(Ga,Al)Sb magnetic semiconductor heterostructures and their ferromagnetic characteristics

A series of III‐V‐based magnetic semiconductor heterostructures, p‐type (In,Mn)As/(Ga,Al)Sb, has been grown by molecular beam epitaxy. Studies on magnetotransport and magneto‐optical properties show that perpendicular ferromagnetic order occurs in the heterostructures with thin (In,Mn)As layers. The origin is discussed in terms of both carrier‐ and strain‐induced effects.

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Quantitative determination of the local Kerr rotation by scanning near-field magneto-optic microscopy

By using a polarization-modulation technique in combination with reflection-mode scanning near-field optical microscopy we are able to determine quantitatively the local polar Kerr rotation on thin magnetic films. As an example, the magneto-optic contrast of magnetic domains thermomagnetically written on a Co/Pt sample is discussed. The size of the magnetic domains is 3×0.5 μm2 and the angular resolution of the local Kerr rotation is better than 0.1°. This measurement technique will make it possible to investigate, e.g., the fine structure of domain walls by scanning near-field optical microscopy.

Magneto-optic enhancement and magnetic properties in Fe antidot films with hexagonal symmetry

The magneto-optic and magnetic properties of hexagonal arrays of holes in optically thin iron films are presented. We analyze their dependence on the hole radius and compare the results to a continuous iron film of the same thickness. We observe a large enhancement of the magneto-optic Kerr rotation with respect to that of the continuous film, at frequencies where surface-plasmon excitations are expected. The spectral position of the Kerr maxima can be tuned by the size and the distance between the holes. Additional simulations are in very good agreement with the experiment and thus confirm the effect of the surface plasmons on the Kerr rotation. The altering of the magnetic properties by the hole array is also visible in the hysteretic behavior of the sample where a significant hardening is observed.

Surface plasmons and magneto-optic activity in hexagonal Ni anti-dot arrays.

The influence of surface plasmons on the magneto-optic activity in a two-dimensional hexagonal array is addressed. The experiments were performed using hexagonal array of circular holes in a ferromagnetic Ni film. Well pronounced troughs are observed in the optical reflectivity, resulting from the presence of surface plasmons. The surface plasmons are found to strongly enhance the magneto-optic response (Kerr rotation), as compared to a continuous film of the same composition. The influence of the hexagonal symmetry of the pattern on the coupling between the plasmonic excitations is demonstrated, using optical diffraction measurements and theoretical calculations of the magneto-optic and of the angular dependence of the optical activity.

Magneto-optic properties of macroscopic ferrimagnets

A new class of two phased materials, called macroscopic ferrimagnets, displays many properties associated with ferrimagnetism including magnetic compensation points. A typical material system which shows this phenomenon is cobalt containing /spl cong/100 /spl Aring/ precipitates of EuS. The negative exchange in these systems is relatively weak so the remanent magneto-optic properties show the ferrimagnetic state, but at fields above 2 to 3 T, alignment of both the Co and EuS with the field can occur. In addition to the usual magnetic compensation point at which the sign of the magneto-optic loop reverses, we observe a second, magneto-optic, compensation point which occurs when the Kerr rotation of the two phases are equal and opposite. >

Structural, magnetic, and spectroscopic magneto-optical properties aspects of Pt–Co multilayers with intentionally alloyed layers

Polycrystalline Pt–Co multilayers with intentionally alloyed layers were grown by e-beam evaporation on polyimide, Si, and glass substrates. X-ray diffraction spectra show that the multilayered structure of the samples degrades as compared to conventional Pt–Co multilayers. This degradation depends on the compositional variation of the intentionally alloyed layers. Magnetometry measurements reveal enhanced magnetization values for all samples, exceeding even by 90% the one of bulk Co, at a temperature of 10 K. This is attributed to both Pt-induced and enhanced Co magnetic moments, according to x-ray magnetic circular dichroism experiments. Spectroscopic magneto-optic measurements reveal large negative polar Kerr rotation maxima at photon energies between 4 and 4.4 eV due to the intense magneto-optic response of spin-polarized Pt.

Magneto-optics of thin magnetic films composed of Co nanoparticles

The magneto-optic properties of 8, 10, and 12 nm diameter Co particles deposited on Si and Al substrates are investigated at room temperature in the photon-energy range from 0.8 to 4.8 eV using the magneto-optic Kerr effect. The 10 and 12 nm particles give rise to a magneto-optic signal, while the 8 nm particles show virtually no magneto-optic response at room temperature. The spectral features of the polar Kerr rotation depend on particle size and are at variance from those of a reference spectrum of a thick Co film. For the 10 nm particles an aging effect on a time scale of months is observed through the magneto-optic signature. The magneto-optic signal also depends on the arrangement of the particles within the film of nanoparticles. The magnetization of the 10 and 12 nm particle films is analyzed in terms of a Langevin function. From this analysis, the effective magnetic core of the 10 nm particles is estimated as 4.5 nm, while the 12 nm particles appear to show ferromagnetic coupling.

A new magneto-optic enhancement effect in macroscopic ferrimagnets

Optical and magneto-optic properties at room temperature of Co/sub 1-x/(EuS)/sub x/ evaporated thin films are presented over a wide photon-energy range. Co/sub 1-x/(EuS)/sub x/ belongs to a new class of phase-separated magnetic materials, called macroscopic ferrimagnets, in which two magnetic phases couple antiferromagnetically across the phase boundary. Polar Kerr rotations of up to 2/spl deg/ are observed in Co rich samples (x/spl les/0.3) at photon energies of 4.5 eV. The high Kerr rotation coincides with a distinct decrease in reflectivity suggesting an optical enhancement effect. A model calculation indicates a new type of enhancement based on the relative index of refraction at the phase boundary between the Co matrix and the EuS precipitate particles.