Jens J. Paggel

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.

Structure and magneto-optic Kerr measurements of epitaxial MnSi films on Si(111)

MnSi films are grown by evaporation of Mn onto Si(111) substrates under ultrahigh-vacuum conditions. Films are characterized with real- and reciprocal-space surface-science techniques such as scanning tunneling microscopy and low-energy electron diffraction. The bulk structure is determined ex situ by transmission electron microscopy (TEM). Thin (<60A Mn-deposited) MnSi films show a regularly modulated surface due to a 3% lattice mismatch of the interfaces. TEM confirms epitaxial growth and demonstrates smooth, atomically flat interfaces. Temperature-dependent ex situ measurements of the magneto-optic Kerr effect show ferromagnetism with an in-plane easy axis magnetic anisotropy for 60- and 100-A-thick Mn films.

Combined transmission electron microscopy and cathodoluminescence studies of degradation in electron-beam-pumped Zn1-xCdxSe/ZnSe blue-green lasers

We explored degradation in electron-beam-pumped Zn1−xCdxSe/ZnSe laser structures by combining cathodoluminescence (CL) measurements in a scanning electron microscope with transmission electron microscopy. The rate of degradation, measured as the decrease of the emitted CL intensity under electron bombardment, depends critically on the threading dislocation density and on the strain in the quantum well. Degradation occurs via the formation of dark spot defects, which are related to bombardment-induced networks of dislocation loops in the quantum well. These degradation defects often initiate where threading dislocations cross the quantum well. We propose a self-supporting dislocation climb mechanism activated by nonradiative recombination to explain the formation and propagation of the degradation defects.

Magnetite particles studied by Mössbauer and magneto-optical Kerr effect

We present results of transmission electron microscopy, Mossbauer spectroscopy, magneto-optical Kerr effect, and magnetization measurements on magnetite-based nanoparticles M0.5Fe2.5O4 (M=Fe, Co, Mn, and Ni). Room-temperature Mossbauer spectra show ferrimagnetic behavior of the particles and suggest a preferential occupation of the B (octahedral) magnetite site by Ni and Mn cations. Magnetization measurements reveal a homogeneous magnetic-size distribution irrespective of the substitution elements. The influence of the chemical nature of the substituent on the magneto-optical Kerr spectrum is investigated. Magneto-optical magnetization loops reveal that particles in the frozen solution have the same magnetically active size as those deposited on Al substrates.

Evidence of exchange-induced spin polarization in the magnetic semiconductor EuS

The semiconducting character and the large spin moment of 7/2 μB of europium sulfite are important preconditions for applications in the field of spintronics. However, the low Curie temperature (TC=16.5 K) excludes EuS from commercial application. We have studied a variety of Co/EuS multilayer samples deposited in a molecular beam epitaxy system with varying numbers of layers and different single-layer thickness. Magneto-optic Kerr effect measurements at low temperatures (10 K) indicate an antiferromagnetic coupling between EuS and Co moments.

Evidence of exchange-induced spin polarization in the semiconductor EuS at 300K

We report on exchange-induced spin polarization in the magnetic semiconductor EuS at room temperature studied by x-ray magnetic-circular-dichroism measurements. This turns EuS into a promising material for spin-injection applications. The system investigated is a Co∕EuS multilayer grown on top of a Co∕Pt multilayer on a Si(111) substrate. The EuS moments are aligned antiparallel to the Co moments in a narrow region at the Co∕EuS interface. We attribute this exchange-induced alignment to a magnetic proximity effect which is closely related to the magnetic exciton originally proposed by Kasuya [IBM J. Res. Dev.14, 214 (1970)] to explain ferromagnetism in EuS.

Magneto-optic properties of thin EuS/Co and EuS/Cu films on Si(111) substrates

Thin EuS films of 5.5 and 11 nm have been grown at 300 K on epitaxial Co(111) and Cu(111) surfaces on Si(111) substrates. The EuS layers show volumetric reflections in the RHEED pattern indicating that the EuS is deposited in form of small crystallites on the Co or Cu surface. The magneto-optic properties are investigated by polar Kerr spectroscopy at 300 K and 10 K. At 10 K, a clear signature of the intra-Eu 4f-5d transition is observed at 1.8 eV and at 4.2 eV. Below 1 eV, the magneto-optic contribution of the Co layer is dominant. The influence of the Co or Cu layer on the magnetic ordering of the EuS layer is studied by measuring the Kerr spectra as a function of temperature. The layered structures exhibit an in-plane magnetic anisotropy. Estimates of the paramagnetic EuS contribution to the spectrum at 300 K suggest only a weak coupling between the EuS film and the Co layer.

Growth of Mn-Bi films on Si(111): Targeting epitaxial MnBi

MnBi is of high interest for application as active layer in magneto-optic data storage media for several decades. Here the molecular-beam-epitaxy growth of Mn-Bi films on Si(111) surfaces is investigated under ultrahigh vacuum conditions by means of reflection high-energy electron diffraction, low-energy electron diffraction with spot-profile analysis, Auger-electron spectroscopy, magneto-optic Kerr spectroscopy, and transmission electron microscopy to reveal the microscopic film structure, film composition, and magnetic properties. The film parameters and growth protocols are varied but none of the chosen conditions lead to the formation of MnBi from the two elements. The affinity of Mn to the substrate material Si must be regarded as major reason for the fact that no MnBi forms.

TRANSMISSION ELECTRON MICROSCOPY AND CATHODOLUMINESCENCE STUDIES OF EXTENDED DEFECTS IN ELECTRON-BEAM-PUMPED ZN1-XCDXSE/ZNSE BLUE-GREEN LASERS

We report on studies of extended defects in electron-beam-pumped Zn1-xCdxSe/ZnSe blue and blue-green laser structures. To establish a direct correlation between the local luminescence properties and the presence of structural defects, the same thin foil samples were sequentially examined by transmission electron microscopy (TEM) and cathodoluminescence (CL) microscopy. The majority of the non-radiative defects were found to have one or more threading dislocations in their vicinity. Stacking faults, as a rule, did not give rise per se to non-radiative recombination centers. In several instances we observed nonradiative defects by CL with no extended defect counterparts in TEM

Polarization-dependent measurement of the near-field distribution of a waveguide with subwavelength aperture

The understanding of the near field is essential for scanning near-field optical microscopy. We present here a simple model experiment to examine the polarization of the near field in the proximity of a subwavelength aperture. We make use of microwaves, a fast diode, and different apertures. This allows us to model a polarization-sensitive scanning near-field optical microscope by mapping the field intensity around the aperture in the near-field region.