Magneto-optical Transmission Research Articles

Irradiation Effects on Perpendicular Anisotropy Spin–Orbit Torque Magnetic Tunnel Junctions

We study the impact of irradiation on magnetic tunnel junction (MTJ) films with perpendicular magnetic anisotropy (PMA) and spin-orbit torque (SOT) switching using magneto-optical Kerr effect and transmission electron microscopy. Our results show that the thin-film stack is robust to gamma ionizing dose up to 1 Mrad(Si) and Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation fluences up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , showing SOT PMA MTJs are radiation-hard. But, at very high Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ion irradiation between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , reduced coercivity and eventually greatly reduced PMA are observed, corresponding with an increase in intermixing of the CoFeB-MgO layers, particularly at the lower CoFeB-MgO interface. These results agree with displacement damage modeling that predicts higher damage in the layers closer to the bottom heavy metal and substrate. Compared to spin transfer torque and in-plane anisotropy MTJs, needing a top-pinned stack, a thicker heavy metal layer, and perpendicular anisotropy that is pinned out of plane by interfaces, all could make SOT PMA MTJs more susceptible to damage at high doses.

Tunable Dirac interface states in topological superlattices

Relativistic Dirac fermions are ubiquitous in condensed matter physics. Their mass is proportional to the material energy gap and the ability to control and tune the mass has become an essential tool to engineer quantum phenomena that mimic high energy particles and provide novel device functionalities. In topological insulator thin films, new states of matter can be generated by hybridizing the massless Dirac states that occur at material surfaces. In this work, we experimentally and theoretically introduce a platform where this hybridization can be continuously tuned: the Pb1-xSnxSe topological superlattice. In this system, topological Dirac states occur at the interfaces between a topological crystalline insulator Pb1-xSnxSe and a trivial insulator, realized in the form of topological quantum wells (TQW) epitaxially stacked on top of each other. Using magnetooptical transmission spectroscopy on high quality MBE grown Pb1-xSnxSe superlattices, we show that the penetration depth of the TQW interface states and therefore their Dirac mass is continuously tunable with temperature. This presents a new pathway to engineer the Dirac mass of topological systems and paves the way towards the realization of emergent quantum states of matter using Pb1-xSnxSe topological superlattices.

Generalized treatment of magneto-optical transmission filters

This work has constructed a model for the theoretical analysis of generalized magneto-optical transmission filters for any relative angle between the light beam and the applied magnetic field. This model was experimentally validated and shows good agreement between the theoretical predictions and experimental spectra. In addition, the value of examining magneto-optical filters for arbitrary angle cases was demonstrated by revealing an optimized filter with an equivalent noise bandwidth of 0.56 GHz, which is approximately a factor of 2 better than those previously reported.

ANALYSIS OF FARADAY ROTATION AND MAGNETO-OPTICAL TRANSMISSION IN MONOLAYER GRAPHENE

The Faraday rotation angle and the magneto-optical transmission of light through the monolayer graphene with a magnetic field perpendicular to the plane of graphene are calculated within the framework of the low-energy effective quantum field theory of Dirac quasiparticles. The general results are applicable not only in the semi-classical, but also in the degenerate quantum regime. The theoretical results are in a good quantitative agreement with the available experimental data. The only fit parameter in the model is the value of the quasiparticle width.

X-ray magneto-optical polarization spectroscopy: an analysis from the visible region to the x-ray regime

An ultra-high vacuum compatible multipurpose chamber for magneto-optical reflection and transmission experiments with polarization analysis on magnetic systems is introduced. It is applicable in a broad photon energy range from the visible to the soft x-ray regime and for a wide angular range from grazing to normal incidence. It exploits a novel magnetization device based on rotating permanent magnets, which generates tuneable magnetic fields up to 570 mT in longitudinal, transverse and polar geometry. The unique combination of these features enables the feasibility of all typical magneto-optical spectroscopy techniques as T-MOKE, L-MOKE, P-MOKE, x-ray magneto optical linear dichroism, x-ray magnetic circular dichroism in reflection and Kerr polarization-spectroscopy, which is demonstrated for Co with focus on the Co 3p edges.

Nonlinear and magneto-optical transmission studies on magnetic nanofluids ofnon-interacting metallic nickel nanoparticles

Oxide free stable metallic nanofluids have the potential for various applicationssuch as in thermal management and inkjet printing apart from being a candidatesystem for fundamental studies. A stable suspension of nickel nanoparticles of ∼ 5 nm size has been realized by a modified two-step synthesis route. Structural characterizationby x-ray diffraction and transmission electron microscopy shows that the nanoparticles aremetallic and are phase pure. The nanoparticles exhibited superparamagnetic properties.The magneto-optical transmission properties of the nickel nanofluid (Ni-F) wereinvestigated by linear optical dichroism measurements. The magnetic field dependent lighttransmission studies exhibited a polarization dependent optical absorption, known asoptical dichroism, indicating that the nanoparticles suspended in the fluid arenon-interacting and superparamagnetic in nature. The nonlinear optical limiting propertiesof Ni-F under high input optical fluence were then analyzed by an open aperturez-scan technique. The Ni-F exhibits a saturable absorption at moderate laser intensitieswhile effective two-photon absorption is evident at higher intensities. The Ni-Fappears to be a unique material for various optical devices such as field modulatedgratings and optical switches which can be controlled by an external magnetic field.

Magneto-optical apparatus to measure ac magnetic susceptibility

A new setup to measure magnetic susceptibility versus the temperature, based on magneto-optical transmission techniques, has been developed. The advantage with respect to conventional systems is that it allows measurements in a wider range of frequencies (0.1–105 Hz). It is thought it can be used with transparent magnetic samples. Due to the dependence of the output signal on the magneto-optical properties of the materials under test, these properties must be known to achieve calibration.

Magnetic Domain Imaging with Scanning Near-field Optical Microscopy

An apparatus combining various scanning probe microscopy techniques is presented. It is dedicated to near-field magneto-optical (MO) and optical transmission imaging, and simultaneously gives topographical information. This MO scanning near-field optical microscope (SNOM), working in transmission-collection mode, measures the magneto-optical circular dichroism (MOCD) effect of the samples, using a tapered and metal-coated optical fiber probe. The performance of the apparatus is mainly determined by this fiber probe. Resolution of about 200 nm was obtained on 200 A thick Co/Pt and CoNi/Pt multilayer samples. MOCD-SNOM imaging artifacts are presented and discussed.

Resonant magneto-optical spin transitions in zinc-blende and wurtzite semiconductors.

A theoretical study of the resonant magneto-optical spin-flip transitions due to electrons bound to shallow donors is made. We present a general formalism of the integrated absorption coefficient for semiconductors having an arbitrary crystal structure. The model is based on the parity-violating spin-orbit interaction responsible for the active electric-dipole transition. Simple analytic expressions of the integrated absorption coefficient are obtained for the Faraday and Voigt field configurations and for any crystals having zinc-blende and wurtzite structure, respectively. A comparison between theory and far-infrared magneto-optical transmission measurements in InSb and ${\mathrm{Cd}}_{0.9}$${\mathrm{Mn}}_{0.1}$Se yields to the strength of the spin-orbit interaction and effective Bohr radius of the donors involved.

90° domains and coupling in Co/Cu giant magnetoresistance superlattices on Si(100) substrates

Giant magnetoresistance and interlayer coupling in Co/Cu superlattices grown by e-beam evaporation have been investigated with the combined three-axis magneto-optical Kerr effect, magnetotransport measurements, X-ray diffraction, and transmission electron microscopy. Correlations between 90/spl deg/ alignment of the magnetization in adjacent layers and magnetoresistance oscillation curves suggest that the GMR in superlattices with imperfect interfaces results from 90/spl deg/ relative orientation of domains in adjacent Co layers.

Violation of time-reversal symmetry in YBa2Cu3O7-x films.

We present a concept for the search for optical effects which indicates a violation of time-reversal symmetry in high-${\mathit{T}}_{\mathit{c}}$ superconductors. The idea is to investigate the reaction of the material on a magnetic field in a magneto-optical transmission experiment. Below 150 K an annealing field of 400 Oe induces an optical quantity which remains after the field is switched off and which depends on the sign of the field. This signature of broken time-reversal symmetry is confirmed in fields up to 50 kOe. Our results are consistent with a phenomenological theory which takes into account the presence of strains in films. The strains enable a switching of antiferromagnetic domains by magnetoelastic interactions. This interpretation is supported by the dependence of the optical response on \ensuremath{\Vert}H\ensuremath{\Vert}.

Photo-assisted tunneling at ferromagnet/semiconductor interfaces

We have measured the polarization-dependent photoresponse of ferromagnet-insulator-(III–V) semiconductor thin film tunnel structures as a function of wavelength. When the structures have no interfacial barrier, we find that the response agrees well with calculations of magneto-optical transmission through the magnetic overlayer. However, when a tunnel barrier is present and excitation nearly resonant with the semiconductor bandgap, we observe significant deviations from the magneto-optical effects. These deviations are attributed to spin-dependent electron transmission from the III–V semiconductor acting as a spin-polarized source of tunneling electrons.

Spin-dependent transmission at ferromagnet/semiconductor interfaces

We have measured the polarization-dependent photoresponse of ferromagnet-insulator-(III–V) semiconductor thin film tunnel structures as a function of wavelength. When the structures have no interfacial barrier, we find that the response agrees well with calculations of magneto-optical transmission through the magnetic overlayer. However, when a tunnel barrier is present and the excitation is nearly resonant with the semiconductor bandgap, we observe significant deviations from the magneto-optical effects. These deviations are attributed to spin-dependent electron transmission from the III–V semiconductor acting as a spin-polarized source of tunneling electrons.

High Pressure Far-Infrared Magnetooptical Studies of Sn and S Donors in GaAs

The diamond anvil cell technique is applied for magneto-optical far-infrared transmission experiments with LPE grown GaAs:Sn. The s-2p(+) intra-shallow-f-donor transition is investigated as a function of high hydrostatic pressure for Sn and residual S donors. Both donors are shallow up to 30 kbar. Above this pressure both donors become deep and shallow donor absorption is persistently bleached due to deep non-metastable (non DX-like) states of the donors entering the gap of GaAs.

ChemInform Abstract: Magnetic and Optical Properties of Rare Earth Garnets

Abstract Since their discovery and characterization, a huge magnetics research effort has been centered on the magnetic garnets. They have become the most studied and best understood class of magnetic materials, and continue to find their way into new technology. One new property that garnets have brought to magnetics is transparency in the visible and near infrared. Because light interacts with magnetization, this immediately made it possible to see the distribution of magnetization, to see the magnetic domain structure. This was crucial in the invention and development of magnetic bubble technology. There is much current interest in magneto-optical transmission devices such as infrared isolators using magnetic garnet bulk crystals and films.

Magnetic and optical properties of rare earth garnets

Since their discovery and characterization, a huge magnetics research effort has been centered on the magnetic garnets. They have become the most studied and best understood class of magnetic materials, and continue to find their way into new technology. One new property that garnets have brought to magnetics is transparency in the visible and near infrared. Because light interacts with magnetization, this immediately made it possible to see the distribution of magnetization, to see the magnetic domain structure. This was crucial in the invention and development of magnetic bubble technology. There is much current interest in magneto-optical transmission devices such as infrared isolators using magnetic garnet bulk crystals and films.

Infrared magnetooptic ? A tool for the determination of the effective mass

The analysis of the magnetooptical effects of heavily doped materials at the plasma edge yields the concentration dependence of the effective cyclotron mass. Therefore these experiments support the general diskussion about the nonparabolicity of a band, the position of the Fermi level at high degenerated semiconductors and the determination of the\(E(\vec k)\) dispersion function. Experimental results of the magnetooptical determinedm c * (N) function are compared with coresponding band structure calculations. A matrix calculation model, which describes the symmetrical magnetooptical transmission effects as well as the asymmetrical magnetooptical reflection effects of arbitrary successions of coherent films and incoherent substrates consitently, is used to determine free carrier density profilesN (z) of inhomogeneously doped semiconductors non-destructively. This application of the matrix formalism requires the knowledge of them* (N)-function. The influence of the effective cyclotron mass on thedifferential magnetooptical interference structures caused by buried density profiles is discussed.

Electron-electron interaction effects in bismuth

Measurements of the far infrared magneto-optical transmission spectra of bismuth at 311 and 337 μ and in fields up to 100 kG at 4°K are described. The sensitivity of the spectra of transmitted magneto plasma waves to the electron scattering in bismuth is discussed. Particular attention is drawn to excitonic effects associated with weakly allowed hole cyclotron resonance absorption lines observed in the spectra. A simple Cooper pair like argument is given to illustrate the nature of these inter subband excitons. The features entering into a more complete theory of the excitons are also briefly discussed.