Quantum theory of the magneto-optical effect and magnetization of Nd-substituted yttrium iron garnet

The magneto-optical and magnetic properties of Nd ³⁺ ions in Y ₃Fe ₅O ₁₂ garnet are analyzed by using quantum theory. In the spontaneous state, the magneto-optical effects originate mainly from the intra-ionic electric dipole transitions between the 4 f ³ and 4 f ²5d states split by the spin-orbit, crystal field, and superexchange interactions. For the excited configuration, the coupling scheme of Yanase is extended to the Nd ³⁺ ion. The magneto-optical resonance frequencies are mainly determined by the splitting of the 5d states induced by the crystal field. The theoretical results of both Nd magnetization and Faraday rotation are in good agreement with experiments. The observed Faraday rotation is proved to be of the paramagnetic type. Although the value of the magneto-optical resonance frequency derived from a macroscopic analysis is approximately confirmed by our theoretical study, a new assignment about the transitions associated with this resonance is unambiguously determined. The spin-orbit coupling of the ground configuration has a great influence on both the Faraday rotation and magnetization, but, unlike the theoretical results obtained in some metals and alloys, the relation between the Faraday rotation and the spin-orbit coupling strength is more complex than a linear one. The magnitude of the magneto-optical coefficient increases as the spin-orbit interaction strength of the ground configuration decreases when the strength is not very weak. Finally, the temperature dependence of the magneto-optical coefficient and the effect of the mixing of different ground-term multiplets induced by the crystal field are analyzed. Received 8 November 2000     

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     

Optimized factor of merit of the magneto-optical Kerr effect of ferromagnetic thin films

This paper deals with the optimization of the factor of merit of the magneto-optical Kerr effect of a resonant multilayer cavity including a ferromagnetic film. This optimization is of interest in the context of optical storage technology. Using numerical simulations based on the Green's dyadic technique, we discuss a route to obtain magneto-optical multilayers with a vanishing ellipticity and factors of merit (with respect to the bulk magnetic material) larger than 3 on a broad range of wavelengths, significantly higher than the actual state of the art. Received 21 May 1999     

High spin state of Mn in an ideal monolayer on Ag(001)

We studied the magnetic properties of ultra-thin Mn films deposited on Ag (001) held at 80 K with soft X-ray absorption and magnetic circular dichroism. The observed shape and branching ratio of the Mn 2p absorption edge as a function of Mn coverage demonstrate that, up to , the Mn adopts a stable high spin state similar to the Mn atom Hund's rule ⁶ S _5/2 ground state. Above this coverage a rapid transition from localized high spin to itinerant low spin behavior of the Mn 3d electrons is evidenced. Magnetic circular dichroism shows no sign of long range ferromagnetic order in these films at 80 K. The data, first confirm the large atomic-like local magnetic moment, and second are in line with the in-plane antiferromagnetic order, reported recently (Phys. Rev. B 57, 1141 (1998)), for Mn in the nearly ideal on-top Mn monolayer formed by 0.9 ML deposited at 80 K. Received: 4 May 1998     

Enhanced magneto-optical Kerr effects in nanocrystalline Sc-doped CoFe2O4 thin films

Nanocrystalline CoFe_2−xSc_xO₄ (x=0-0.4) thin films were prepared on silicon substrates at reduced temperature by a sol-gel process, and the doping effects of scandium on the microstructure, magnetism and polar magneto-optical Kerr effects of the as-deposited films were examined. It was shown that the intensities of both of the Kerr rotation peaks increase with the doping content x of Sc³⁺. The increase for the peak at 540 nm is due to the decrease of the electrostatic polarization of O²⁻ resulting from the relatively large radius of Sc³⁺, and that for the peak at 620 nm was a result of the migration of Co²⁺ from octahedral to tetrahedral sites in the presence of the dopant of Sc³⁺.     

Polar Kerr rotation of the ferromagnet

Magneto-optical data on EuB ₆ , a ferromagnet with a Curie temperature T _C ∼ 15 K, are presented and discussed in detail. We have measured the polar Kerr rotation, covering a spectral range from the infrared up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields between 0 and 10 T. The Kerr rotation in high fields and at low temperatures is enormous. Our observations, which implicitly reflect the large magnetoresistive effects, are shown to discriminate between the spectroscopic response of localized and itinerant electronic states. Our data analysis is based on the phenomenological Lorentz-Drude model, following from the classical dispersion theory and appropriately extended to magneto-optical experiments. Received 30 January 2003 / Received in final form 21 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: degiorgi@solid.phys.ethz.ch     

Magnetic behavior and role of the antiphase boundaries in Fe3O4 epitaxial films sputtered on MgO (001)

Magnetite Fe₃O₄ films were grown on single crystal MgO (001) substrates using facing target sputtering technique. Conversion Electron M?ssbauer Spectroscopy and magneto optical polar Kerr spectra have confirmed the stoichiometric repartition of Fe cations corresponding to the inverse spinel structure and the electronic structure characteristic of bulk Fe₃O₄. Hysteresis loops carried out at room temperature show that, in a 1 T applied magnetic field, only 60% of the saturation magnetization is detected. This behavior is discussed in correlation to the antiphase boundaries (APBs) observed by electron microscopy. Magnetic force microscopy studies show that magnetic domains are larger than the mean distance between APBs. Received 2 July 2001     

Magneto-optical Kerr effect in bilayer structures

This paper describes the magneto-optical effects and the reflectivity behaviors of bilayers based on magnetic and isotropic ()/anisotropic () layers under the condition of total internal reflection. In the framework of Green's dyadic technique, we show accurately the optical properties of anisotropic layers deposited on a substrate. We present numerical simulations which account for the variation of angle of incidence at the HeNe laser wavelength. The Kerr rotation is found to increase significantly around the optical modes in total reflection. We also discuss the importance of anisotropic effects due to the crystallization of the dielectric material () which occur in the reflectivity and Kerr rotation spectra. Received 26 January 2000     

Magnetism of alloyed models of Cr(Mn) V and Mn Cr overlayers on V (001) substrates

The magnetism is calculated for substitutional alloyed of Cr_x(Mn_x)V_1-x monolayers (MLs) and Cr_xMn_1-x on V (001) surface with a variety of concentrations (x = 0.25, 0.50 and 0.75). The substitutional surface alloys were treated by an artificial super cell construction. Parallel magnetic ordering is obtained for all the considered structures. The surface net magnetization increases in terms of Cr (Mn) concentration in Cr_x(Mn_x)V_1-x/V (001) system, while no serious variations occur in Cr_xMn_1-x/V (001). Vanadium atoms at the interfacial layers acquire appreciable magnetic moments antiferromagneticlly (AF) coupled with the surface moments. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: jkalifa@sci.ju.edu.jo     

Many-body Green's function theory for thin ferromagnetic anisotropic Heisenberg films: treatment of the exchange anisotropy

The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier work, on the magnetic properties of thin ferromagnetic films are compared. Received 31 October 2002 Published online 7 May 2003     

Spin-polarized tunneling in ferromagnetic double barrier junctions

Spin-polarized tunneling in FMS/M/FMS double tunnel junctions where FMSs are ferromagnetic semiconductor layers and M is a metal spacer is studied theoretically within the single-site coherent potential approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated in the framework of the s-f model. The spin polarization in the FMS layers is observed to oscillates as a function of the number of atomic planes in the spacer layer. Amplitude of these oscillations decreases with increasing the exchange interaction in FMS layers. Received 9 June 2001 and Received in final form 20 August 2001     

Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H _ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to k _B T) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity H _C(H _C < H _ani), - a soft dipole regime (anisotropy energy of the order of k _B T) for maghemite nanoparticles with a relaxation time independent of the field intensity H _C(H _C > H _ani). Received 5 June 2000 and Received in final form 8 January 2001     

Phase diagrams of Ising films with competing interactions

The axial next-nearest-neighbour Ising (ANNNI) model of finite thickness is studied. Using mean-field theory, Monte Carlo simulations, and low-temperature analyses, phase diagrams are determined, with a distinct phase diagram for each film thickness. The robustness of the phase diagrams against varying the couplings in the surface layers is analysed. Received 19 March 2002 and Received in final form 2 April 2002 Published online 6 June 2002     

Exchange coupling in ultrathin epitaxial yttrium iron garnet films

Magnetic exchange coupling has been observed for ultrathin films of yttrium iron garnet (Y₃Fe₅O₁₂ or YIG). Single-crystalline YIG films were prepared on yttrium aluminium garnet (Y₃Al₅O₁₂ or YAG) substrates by pulsed laser deposition. (111) and (110) oriented substrates were used. Film thicknesses were varied from 180 ? to 4600 ?. Epitaxial growth of YIG on YAG was obtained in spite of the lattice mismatch of 3%. Magnetic hysteresis loops recorded for ultrathin YIG films have a “bee-waist” shape and show a coupling between two different magnetic phases. The first phase is magnetically soft YIG. A composition study by secondary ion mass spectroscopy shows the second phase to be Y₃Fe_5-xAl_xO₁₂ due to the interdiffusion of Fe and Al at the film/substrate interface. This compound is known to be magnetically harder and to have weaker magnetization than YIG. The coupling of the two phases leads to a hysteresis loop displacement at low temperatures. This displacement varies differently with film thickness for two substrate orientations. Assuming an interfacial coupling, the maximal interaction energy is estimated to be about 0.17 erg/cm² at 5 K for (111) oriented sample. Received 3 June 2002 / Received in final form 7 October 2002 Published online 27 January 2003 RID="a" ID="a"Presently at LPM, Université H. Poincaré, BP 239, 54506 Vandœuvre-lès-Nancy e-mail: popova@lpm.u-nancy.fr     

Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms. The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good approximation for realistic coupling strengths. We apply the new method to monolayers with spin and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some significant differences. Received 19 June 2000 and Received in final form 2 August 2000     

Strain effect on transport properties of <Emphasis Type="Bold">SrRuO</Emphasis><Subscript><Emphasis Type="Bold">3</Emphasis></Subscript> films grown by laser MBE

Transport properties of SrRuO₃ thin films were studied as a function of the epitaxial strain. SrRuO₃ films were grown on (100) SrTiO₃ substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples thickness has been varied from a few unit cells to above 1000 ? while monitoring RHEED intensity oscillations. In thicker films epitaxial strain was found to be progressively relaxed. SrRuO₃ relaxed films (thickness 1000 ?) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 ? it disappears. Films thinner than 80 ? show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial role of the strain effect for conducting and magnetic properties of SrRuO₃. Received 16 July 2001 and Received in final form 22 October 2001     

Magnon scattering by a symmetric atomic well in free standing very thin magnetic films

A theoretical model is presented for the study of the scattering of magnons at an extended symmetric atomic well in very thin magnetic films. The thin film consists of three cubic atomic planes with ordered spins coupled by Heisenberg exchange, and the system is supported on a non-magnetic substrate, and considered otherwise free from magnetic interactions. The coherent transmission and reflection scattering coefficients are derived as elements of a Landauer type scattering matrix. Transmission and reflection scattering cross sections are hence calculated specifically, as a function of the varying local magnetic exchange on the inhomogeneous boundary. Detailed numerical results for the individual incident film magnons, and for the calculated overall magnon conductance, show characteristic transmission properties, with associated Fano resonances, depending on the magnetic boundary conditions and on the magnon incidence.     

Magneto-optical study of the orientation confinement of particles in ferrolyotropic systems

Ferrosmectics and ferrohexagonals are magnetic liquid crystals which consist of a lamellar and a hexagonal phase in which magnetic nanoparticles are incorporated. The magnetic field variation and the relaxation of the linear optical birefringence of ferrosmectics and ferrohexagonals and its time relaxation are measured in different geometries and compared with that of a ferrofluid sample. We interpret our data by the existence of a mean orientation of the magnetic particles in the lyotropic structure, which appears to be non-random in zero field. We conclude that the magnetic moments of the particles are preferentially aligned in the plane of the ferrosmectic layers and along the axis of the ferrohexagonal cylinders, respectively. To account for this preferred alignment, we propose that some of the particles in the ferrosmectic are adsorbed on the surfactant layers with their moment aligned in the lamellar plane, while the orientation of the moments in the ferrohexagonal is restricted to a cone around the cylinder axis. Received 31 December 1998 and Received in final form 22 September 1999     

A new type of temperature driven reorientation transition in magnetic thin films

We present a new type of temperature driven spin reorientation transition (SRT) in thin films. It can occur when the lattice and the shape anisotropy favor different easy directions of the magnetization. Due to different temperature dependencies of the two contributions the effective anisotropy may change its sign and thus the direction of the magnetization as a function of temperature may change. Contrary to the well-known reorientation transition caused by competing surface and bulk anisotropy contributions the reorientation that we discuss is also found in film systems with a uniform lattice anisotropy. The results of our theoretical model study may have experimental relevance for film systems with positive lattice anisotropy, as e.g. thin iron films grown on copper.     

Tetragonal structure of thin nickel films on Cu(001)

The crystallographic structure of thin Ni films deposited on Cu(001) has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). Taking advantage of the linear polarization of the synchrotron radiation, we have shown that Ni adopts the Cu lattice parameter parallel to the interface. This lateral expansion induces a longitudinal compression of the unit cell, leading to a face centered tetragonal structure of the Ni films from 3 to 10 monolayers. The temperature dependence of the EXAFS oscillations has allowed to measure strain inside the Ni layers. Received 22 December 1998