Quantum theory of the magneto-optical effect induced by Ni ions in barium ferrites

To reveal the physical origin of the giant magneto-optical enhancement of Ni²⁺ ions in barium ferrite, quantitative calculations of the contributions of both the intra-ionic electric dipole transition between the 3d⁸ and 3d^{7 4p} configurations of the Ni²⁺ ions and the intra-ionic electric dipole transition induced by odd-parity crystal field terms are presented. It is deduced that the transition is important in the origin of the considered magneto-optical enhancement. The most important factor is the Ni-Fe superexchange interaction; since it is strong enough, the Faraday rotation produced by the Ni²⁺ ions is large though the energy difference between the 3d⁸ and 3 d7 4 p configurations is large. It is demonstrated that though the intra-ionic electric dipole transition does produce Faraday rotation peaks in the visible range, their magnitude is too small to explain the observed Faraday rotation. The effect of the spin-orbit interaction on the Faraday rotation is analysed. The spin-orbit interaction of the ground configuration plays a very important role in the occurrence of Faraday effects, but the Faraday rotation does not increase linearly with the strength of the spin-orbit coupling. On the contrary, the spin-orbit interaction of the excited configuration has almost no effect on the Faraday rotation. It is shown that the mixing of the different multiplets of the ground term induced by the crystal field has a great influence on the magneto-optical properties. Received 7 January 1998     

Magnetogyrotropy in twisted CdS crystals

In this paper the optical activity (gyrotropy) induced by a simultaneous action of a magnetic field and a torsion in hexagonal CdS crystals has been investigated. The crystal having undergone to both external influences is placed between two crossed polarizers. The spectral course of transmittance of this system near the isotropic point of CdS (the wavelength where the birefringence changes its sign) is studied in the experiment. An explanation of the experimental results is given by using the model of elliptical birefringence in anisotropic gyrotropic crystals. Taking into account the superposition of both external influences, it has been proved that the torsion generates gyrotropy whose parameter changes its sign in the isotropic point.     

The electric field modulation by three-dimensional crack on fused silica subsurface

The main factor of laser-induced damage is the modulation to electromagnetic field of laser by the crack on the subsurface. In this paper, a three-dimensional crack model on the exit surface is presented. Three-dimensional finite-difference time-domain (FDTD) method is employed to simulate the electric field intensity distribution in the vicinity of crack on fused silica subsurface. The roles of the crack width, depth, length and the gradient angle in the modulation to the incident light field are analyzed in detail. Results show that the crack size plays an important role in the electric modulation. With the increasing depth and width, the peak value of maximal electric field intensity appears in fused silica. However, the maximal electric field intensity tends to be a constant when the crack length reaches 1 μm. Besides, the enhancement of light intensity becomes obvious when total internal reflection occurs in fused silica. Our calculated results provide an advisable theoretical criterion to the corresponding experiment.     

Magnetic field effects on the optical properties of an azo-dye doped liquid crystal

Homeotropically aligned nematic liquid crystals doped with azo-dye were subjected both to a linear polarized light of a He–Ne laser and to a magnetic field perpendicular to the incident light beam. We found that the emerging light was elliptically polarized when using magnetic field strengths higher than the threshold value for the magnetic Freedericksz transition. The light transmission, the rotatory power (induced by azo-dye) and the ellipticity varied quasiperiodically when increasing magnetic field strength. The number and positions of maxima and minima depend on the cell thicknesses. Changes in the phase difference between the emergent ordinary and extraordinary rays were computed from the experimental data and the magnetic field dependence of the birefringence was determined.     

The BMV experiment: a novel apparatus to study the propagation of light in a transverse magnetic field

In this paper, we describe in detail the BMV (Biréfringence Magnétique du Vide) experiment, a novel apparatus to study the propagation of light in a transverse magnetic field. It is based on a very high finesse Fabry-Perot cavity and on pulsed magnets specially designed for this purpose. We justify our technical choices and we present the current status and perspectives.     

Magneto-optical Kerr Effect in Co/Alq3 Granular Films

A series of Co/Alq3 granular film samples are prepared using co-evaporating technique. The microstructures and magnetic properties are investigated. The Kerr spectra and optical constants of Cox（Alq3）1-x （0.19 ≤ x ≤ 1） granular films are measured using a magneto-optical Kerr spectrometer and a spectroscopic ellipsometer in the 1.5-4.5 eV photon energy range. The Kerr rotation reaches the maximum value of 0.5° in the x = 0.65 sample. The relationship between enhanced Kerr rotation and optical constants is discussed, and the concentration dependence of MOKE spectra is also investigated.     

Some consequences of relativistic effects upon optical spectra

Large spin–orbit interaction produces large orbital magnetic moments in narrow energy bands. Since the orbital character of the wave functions is more important in orbital than in spin magnetism, the limitations of the local spin density approximation become evident. It is possible to keep the orbital dependence of the exchange interactions by using an orbital polarization scheme or by using Hartree–Fock theory with screened Slater integrals for exchange. This leads to an enhancement of the calculated orbital moment when the magnetism is strong. Recently calculated magnetic moments and calculated sum rules for X-ray magnetic circular dichroism in US are described. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Magneto-optic polar Kerr and Faraday effects in magnetic superlattices

Magneto-optic (MO) effects in magnetic multilayers with periodic regions are discussed assuming normal incidence of polarized waves and polar magnetization (Faraday and polar Kerr effects). From the (4 × 4)-matrix formalism simplified analytical expressions linear in the off-diagonal permittivity tensor element are obtained with no loss in accuracy. The MO effects are expressed as weighted sums of contributions from individual layers. Approximate expressions are given for the case when the thickness of magnetic layers is much smaller than the radiation wavelength. The procedure is illustrated on superlattices consisting of symmetric sandwiches of ultrathin magnetic and non-magnetic metallic layers. Partial support from the Grant Agency of Czech Republic # 202/00/0761 is acknowledged.     

Polar and longitudinal magnetooptical Kerr effects in magnetic film/spacer/magnetic substrate system

The paper deals with the electromagnetic theory of longitudinal and normal incidence polar magnetooptical Kerr effects (MOKE) in sandwich structures consisting of a magnetic film on a thick magnetic substrate separated by a nonmagnetic spacer. These structures are employed in fundamental studies of magnetic exchange coupling, tunnel magnetoresistance, spin polarized current,etc., as well as in the design of the magnetoelectronic devices,e.g., magnetic random access memories. The results are expressed in terms of the Jones reflection matrix. This makes the analysis of the observed MOKE in various experimental set-ups easier. To first order in off-diagonal elements of the permittivity tensor, the analytical expressions approximate the results obtained using rigorous matrix formalisms with a little loss in accuracy, provided the magnetooptical effects quadratic in the offdiagonal elements are negligible. The total MOKE is expressed as a a sum of components originating from magnetic film and magnetic substrate, respectively. The components enter the analytical expressions with different phases and may be identified separately by a proper control of the polarization state and photon energy of the incident radiation, angle of incidence, nonmagnetic spacer thickness, and radiation wavelength. The formulae are derived without any restriction on the magnetic film thickness. However, for a rapid evaluation of the trends the MOKE response is also treated under the assumption that the thickness of the magnetic film is much smaller than the radiation wavelength. This work has been partially supported by Grant Agency of the Czech Republic (#202/97/1180 and #202/00/0761), Barrande Project and Grant Agency of Charles University.     

Buffer effects of Ag layers on magneto-optical Co/Ge(1 0 0) ultrathin films

Magnetic properties of the Co/Ag/Ge(1 0 0) films grown at room temperature and 200 K were studied by the surface magneto-optical Kerr effect (SMOKE). More than 1.5 monolayer Ag buffer layers not only effectively block the interdiffusion between the capped Co layers and the Ge(1 0 0) substrate but also stabilize the magnetic phase. The temperature and thickness dependence on coercivity measurements show that interactions upon the interfaces are strongly correlated to the microstructures.     

Magnetic-field behavior of heterogeneous magnetic materials

Results on the investigation of the magnetic properties of nanocrystalline Co/Ni/Fe and Fe/Zr/Fe thin-film systems are presented. The study of the magnetic properties of the examined samples was carried out employing magneto-optical micromagnetometer with a surface sensitivity about of 20 nm of the thickness depth and a vibrating sample magnetometer. The examined samples were revealed to exhibit hysteresis loops of complicated forms. These data were explained by the magnetostatic and exchange interactions between the layers in heterogeneous magnetic materials.     

Magnetic Properties of Exchange-Coupled Dual-Layer SmTbCo Thin Films

Exchange-coupled SmTbCo dual-layer media are prepared by an r.f magnetron sputtering system and their magnetic properties are investigated. The prepared SmTbCo dual layer is composed of a 340 emu/cm^3 TM-rich readout layer and a 5.80 kOe RE-rich memory layer, meeting the requirements of high saturation magnetization and large coercivity for hybrid recording. Through exchange coupling, the coercivity of the high-saturation- magnetization SmTbCo layer is greatly enhanced from 1.85 to 5.96 kOe. The calculated interface wall energy for Sm6.65Tb12.35Co81 （20nm）/Sm1.22Tb42.16Co56.62 （20hm） is about 3.85erg/cm62. The reversal magnetization of the SmTbCo exchange-coupling dual-layer films is analysed based on a micro-magnetic model.     

Laser-frequency stabilization using forward scattering

Frequency stabilization of a single-mode dye laser is demonstrated using a simple magneto-optical forward scattering method. The dye laser was locked to the 2p₄–3s_s2, = 633 nm neon transition. Heterodyne beat-frequency measurements against a¹²⁷I₂-He-Ne meter standard laser showed a frequency stability of a few MHz.     

Magneto-Optic Fiber Gratings Useful for Dynamic Dispersion Management and Tunable Comb Filtering

Intelligent control of dispersion management and tunable comb filtering in optical network appfications can be performed by using magneto-optic fiber Bragg gratings （MFBGs）. When a nonuniform magnetic field is applied to the MFBG with a constant grating period, the resulting grating response is equivalent to that of a conventional chirped grating. Under a linearly nonuniform magnetic field along the grating, a linear dispersion is achieved in the grating bandgap and the maximal dispersion slope can come to 1260ps/nm2 for a l0-mm-long fiber grating at 1550nm window. Similarly, a Gaussian-apodizing sarnpled MFBG is also useful for magnetically tunable comb filtering, with potential application to clock recovery from return-to-zero optical signals and optical carrier tracking.     

Zeeman spectroscopy of the Zn acceptor in InP

Magneto-spectroscopy to 17.5 T of the Zn acceptor impurity in InP in the Faraday configuration using unpolarised radiation is reported. Four components of the G line have been followed to higher fields than previously, allowing the g factors of the ground and the first excited states to be determined more reliably. The Zeeman splitting of the D line is reported—three principal components are evident. A hitherto unreported third series of Zeeman transitions is observed at energies just above the reststrahlen band.     

Reduction of optical absorption at a wavelength of around 0.8 μm in LPE garnet (TmBiCa)3(FeGaPt)5O12

Liquid-phase epitaxy (LPE) garnet films (TmBiCa)₃(FeGaPt)₅O₁₂ have been grown using only Bi₂O₃ as the flux so that the film containing Bi gives high specific Faraday rotation. The film does not contain Pb, which may affect optical absorption. The optical absorption coefficient at 810 nm has been effectively reduced by doping Ca in the melt. Our data show that a minimum level of and of the anisotropy constantK _u and also the maximum of the electrical resistivity are achieved when Ca²⁺ replaces Fe²⁺. Fe²⁺ results from Pt⁴⁺ incorporation in the film due to a Bi₂O₃ flux attack on a Pt crucible. Using a compensated film, of 58 cm^–1 and a figure of merit of 9deg/dB were obtained.     

How itinerant are 3d electrons in transition metals? A study of magnetic circular dichroism in 2p photoemission

We show how the spectral structure of the magnetic circular dichroism (MCD) in photoemission (PE) can be analysed using a sum rule for the sublevel orbital moment. For itinerant 3d metals, such as Co, Fe and Ni, the MCD in the 2p PE does not agree with a single-particle model. On the other hand, a final-state impurity model calculation gives an excellent agreement with the experimental results for the Ni 2p PE of nickel metal. This indicates that inter-configurational mixing must be taken into account to understand the detailed MCD structure in metallic systems. Vice versa, MCD-PE can help to explain the complicated behavior of electrons in correlated materials. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Dependence of the heavy-hole exciton reduced mass on quantum-well size in InGaAs/GaAs heterostructures

We present low temperature photoluminescence investigations of the exciton ground state of In_0.14Ga_0.86As/GaAs quantum wells (QW) in the presence of pulsed magnetic fields up to 50 T. The exciton in-plane reduced mass and the heavy-hole in-plane mass are determined from the best fit of theoretical calculations to the magnetic field dependence of PL peaks. When the QW thickness decreases, their masses increases due to valence-band mixing effect.     

Optics of circularly polarized light waves in one-dimensional magneto-photonic crystals: Theory

A theory is presented for propagation of electromagnetic waves through one-dimensional magnetic Bragg structures (magneto-photonic crystals). Within the self-consistent Green-function technique the transfer matrices and magneto-optical characteristics are derived in terms of circularly polarized waves propagating in periodical arrays of alternating magnetic and dielectric layers. For finite-thickness magneto-photonic crystals, the Faraday rotation and other magneto-optical responses are demonstrated to change considerably in the spectral range of stop-bands, the magnetic modulation of the in-plane reflection intensity being essentially enhanced.     

A sensitive detection method for magnetization-induced second-harmonic generation under an externally applied field

We describe the instrumentation related to the first observation of magnetization-induced enhancement of surface second-harmonic generation (SHG) from the paramagnetic Si(111)-7×7 surfaces. A judicious choice of polarization and sample orientation enabled us to isolate the magnetic-field-dependent tensor element of the nonlinear susceptibility. A conductive liquid-nitrogen system, coupled to an ultrahigh-vacuum system that is immune to the high magnetic field of 10 T, cooled the sample to about 120 K. A high extinction that is necessary to detect the magnetization-induced SHG (MSHG) was accomplished by minimizing stray optical effects such as Faraday rotation, photoluminescence and thermal birefringence in all optical components, with extra care taken for the UHV window. Consistent and stable operation of this sensitive measurement system permitted experiments involving MSHG at multiple wavelengths and temperatures. Probing the magnetization-induced optical nonlinearity was validated by quenching the surface states by oxidation of the surface layers. From MSHG measurements at two wavelengths resonant with different surface-state transitions, we were able to suggest that the observed MSHG is proportional to the number of dangling bond states of Si(111)-7×7. Received: 15 January 2002 / Published online: 2 May 2002