arXiv subjects

H. Munekata

Publications and source records attributed to H. Munekata.

At least 19 recordsLinked to original sources

arXiv2016

Pure circular polarization electroluminescence at room temperature with spin-polarized light-emitting diodes

We report the room-temperature electroluminescence (EL) with nearly pure circular polarization (CP) from GaAs-based spin-polarized light-emitting diodes (spin-LEDs). External magnetic fields are not used during device operation. There are two small schemes in the tested spin-LEDs: firstly, the stripe-laser-like structure that helps intensifying the EL light at the cleaved side walls below the spin injector Fe slab, and secondly, the crystalline AlOx spin tunnel barrier that ensures electrically stable device operation. The purity of CP is depressively low in the low current density (J) region, whereas it increases steeply and reaches close to the pure CP when J = 100 A/cm2. There, either right- or left-handed CP component is significantly suppressed depending on the direction of magnetization of the spin injector. Spin-dependent re-absorption, spin-induced birefringence and optical spin-axis conversion are suggested to account for the observed experimental results.

physics.optics↗

arXiv2016

Mechanism of photo-excited precession of magnetization in (Ga,Mn)As on the basis of time-resolved spectroscopy

Aiming at studying the mechanism of photo-excited precession of magnetization in ferromagnetic (Ga,Mn)As, magneto-optical (MO) and differential reflectivity (DR) temporal profiles are studied at relatively long and ultra-short time scales for samples with different Mn contents (x=0.01-0.11). As to the oscillatory MO profiles observed in the long time scale, simulation based on the LLG equation combined with two different MO effects confirms photo-inducement of the perpendicular anisotropy component p-Heff. As for the profiles observed in the ultra-short time scale, they are consistently explained in terms of the dynamics of photo-generated carriers, but not by the sudden reduction in magnetization. With those experimental results and analyses, new mechanism which accounts for the photo-induced p-Heff is addressed; photo-ionization like excitation of Mn(II). Namely, Mn(II) is excited into Mn(III) and e . It is discussed that such excitation tips magnetic anisotropy toward the out-of-plane direction through the inducement of orbital angular momentum and the gradient dMn(II*)/dz. Validity of the proposed mechanism is examined by estimating the efficiency of excitation on the basis of the Lambert-Beer law and the experimental p-Heff values, through which the efficiency of 1-10 ppm with the nominal optical cross section of around 5 x 10^12 m^2 are obtained.

cond-mat.mtrl-sci↗

arXiv2014

Dynamics of magnetization and carriers at the onset of the photo-excited precession of magnetization in (Ga,Mn)As

Photo-excited precession of magnetization in (Ga,Mn)As is investigated by measuring time-resolved magneto-optical response and transient differential reflectivity with pump-and-probe technique. In the time region less than 1 ps, rapidly oscillating and spike-like signals are observed, respectively, with excitation of below and above the GaAs band gap. Analysis with gyromagnetic model and autocorrelation function concludes that those signals are not attributed to ultrafast demagnetization but due to interference between pump and probe pulses incorporating sub-ps carrier dynamics characteristic of low-temperature grown semiconductors. Photo-ionization of Mn ions (Mn2+ -> Mn3+) is proposed as a mechanism which dynamically induces orbital angular momentum and affects hole-mediated magnetic anisotropy in (Ga,Mn)As.

cond-mat.mtrl-sci↗

arXiv2014

Circularly polarized light detector based on ferromagnet/semiconductor junctions

Helicity-dependent photocurrent delta-I has been detected successfully under experimental configuration that a circularly polarized light beam is impinged with a right angle on a cleaved sidewall of the Fe/x-AlOx/GaAs-based n-i-p double-heterostructure. The photocurrent delta-I has showed a well-defined hysteresis loop which resembles that of the magnetization of the in-plane magnetized Fe layer in the devices. The value of delta-I has been |delta-I|~0.2 nA at 5 K under the remnant magnetization state. Study on temperature dependence of the relative delta-I value at H = 0 has revealed that it is maximized at temperatures 125 - 150 K, and is still measurable at room temperature.

physics.optics↗

arXiv2014

Incidence Angle and Polarization Dependence of Photo-Induced FMR in Co/Pd Multilayers

Dependence of photo-induced FMR (phi-FMR) on incident angle of excitation and probing laser beams has been studied in a [Co (dCo = 0.78 nm) / Pd (dPd = 0.81 nm) ]5 multi-layer film with the aim to find experimentally the limitation of inducement and detection of magnetization dynamics with oblique light incidence. We have found, in the experiments changing the incident angle of a pump beam, that phi-FMR is observed up to the grazing incident angle of 88 degrees with p-polarized excitation pulses, whereas it disappears at the incidence angle of around 65 degrees with s-polarized excitation. As for the experiments changing the incident angle of a probe beam, phi-FMR disappears at the incidence angle of 65 degrees for both s- and p-polarizations, whereas it reappears with further increasing the angle for the p-polarization and vanishes at 75 degrees.

physics.optics↗

arXiv2014

A spin light emitting diode incorporating ability of electrical helicity switching

Fabrication and optical characteristics of a spin light-emitting-diode (spin-LED) having dual spin-injection electrodes with anti-parallel magnetization configuration are reported. Alternating a current between the two electrodes using a PC-driven current source has led us to the observation of helicity switching of circular polarization at the frequency of 1 kHz. Neither external magnetic fields nor optical delay modulators were used. Sending dc-currents to both electrodes with appropriate ratio has resulted in continuous variation of circular polarization between the two opposite helicity, including the null polarization. These results suggest that the tested spin-LED has the feasibility of a monolithic light source whose circular polarization can be switched or continuously tuned all electrically.

cond-mat.mtrl-sci↗

arXiv2013

Efficient spin injection through a crystalline AlOx tunnel barrier prepared by the oxidation of an ultra-thin Al epitaxial layer on GaAs

We report that an ultra-thin, post-oxidized aluminum epilayer grown on the AlGaAs surface works as a high-quality tunnel barrier for spin injection from a ferromagnetic metal to a semiconductor. One of the key points of the present oxidation method is the formation of the crystalline AlOx template layer without oxidizing the AlGaAs region near the Al/AlGaAs interface. The oxidized Al layer is not amorphous but show well-defined single crystalline feature reminiscent of the spinel gamma-AlOx phase. A spin-LED consisting of an Fe layer, a crystalline AlOx barrier layer, and an AlGaAs-InGaAs double hetero-structure has exhibited circularly polarized electroluminescence with circular polarization of P_{EL} = 0.145 at the remnant magnetization state of the Fe layer, indicating the relatively high spin injection efficiency (epsilon = 2P_{EL} / P_{Fe}) of 0.63.

cond-mat.mtrl-sci↗

arXiv2010

Above-Bandgap Magneto-optical Kerr Effect in Ferromagnetic GaMnAs

We have performed a systematic magneto-optical Kerr spectroscopy study of GaMnAs with varying Mn densities as a function of temperature, magnetic field, and photon energy. Unlike previous studies, the magnetization easy axis was perpendicular to the sample surface, allowing us to take remanent polar Kerr spectra in the absence of an external magnetic field. The remanent Kerr angle strongly depended on the photon energy, exhibiting a large positive peak at $\sim1.7$ eV. This peak increased in intensity and blue-shifted with Mn doping and further blue-shifted with annealing. Using a 30-band ${\bf k\cdot p}$ model with antiferromagnetic $s,p$-$d$ exchange interaction, we calculated the dielectric tensor of GaMnAs in the interband transition region, assuming that our samples are in the metallic regime and the impurity band has merged with the valence band. We successfully reproduced the observed spectra without \emph{ad hoc} introduction of the optical transitions originated from impurity states in the band gap. These results lead us to conclude that above-bandgap magneto-optical Kerr rotation in ferromagnetic GaMnAs is predominantly determined by interband transitions between the conduction and valence bands.

cond-mat.mtrl-sci↗

arXiv2008

Coherent manipulation of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As with successive optical pumping

We report dynamic control of magnetization precession by light alone. A ferromagnetic (Ga,Mn)As epilayer was used for experiments. Amplitude of precession was modulated to a large extent by tuning the time interval between two successive optical pump pulses which induced torques on magnetization through a non-thermal process. Nonlinear effect in precession motion was also discussed.

cond-mat.mtrl-sci↗

arXiv2008

Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic GaMnAs

We have studied ultrafast photoinduced demagnetization in GaMnAs via two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap midinfrared pump pulses strongly excite the valence band, while near-infrared probe pulses reveal sub-picosecond demagnetization that is followed by an ultrafast ($\sim$1 ps) partial recovery of the Kerr signal. Through comparison with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation of holes. We propose that the dynamical polarization of holes through $p$-$d$ scattering is the source of the observed probe signal. These results support the physical picture of femtosecond demagnetization proposed earlier for InMnAs, identifying the critical roles of both energy and spin relaxation of hot holes.

cond-mat.mtrl-sci↗

arXiv2007

Photo-induced precession of magnetization in ferromagnetic (Ga,Mn)As

Precession of magnetization induced by pulsed optical excitation is observed in a ferromagnetic semiconductor (Ga,Mn)As by time-resolved magneto-optical measurements. It appears as complicated oscillations of polarization plane of linearly-polarized probe pulses, but is reproduced by gyromagnetic theory incorporating an impulsive change in an effective magnetic field due to changes in magnetic anisotropy. It is inferred from the shape of the impulse that the changes in anisotropy result from non-equilibrium carrier population: cooling of hot photo-carriers and subsequent annihilation of photo-carriers.

cond-mat.mtrl-sci↗

arXiv2006

Mesoscopic conductance effects in InMnAs structures

Quantum corrections to the electrical conduction of magnetic semiconductors are comparatively unexplored. We report measurements of time-dependent universal conductance fluctuations (TDUCF) and magnetic field dependent universal conductance fluctuations (MFUCF) in micron-scale structures fabricated from two different In$_{1-x}$Mn$_{x}$As thin films. TDUCF and MFUCF increasing in magnitude with decreasing temperature are observed. At 4 K and below, TDUCF are suppressed at finite magnetic fields independent of field orientation.

cond-mat.mes-hall↗

arXiv2006

Detection of spin voltaic effect in a p-n heterojunction

Model calculation and experimental data of circularly-polarized-light-dependent photocurrent in a n-AlGaAs/p-InGaAs/p-GaAs heterostructure are reported. It is found that, under the appropriate forward bias condition, spin voltaic effect (SVE) can survive across the heterojunction and give rise to detectable polarization-dependent photocurrent signals which are greater than the signals due to the magnetic circular dichroism. Our analysis suggests that SVE can be enhanced by optimization of layer thickness, doping profile, and applied bias, making SVE favorable for the realization of a semiconductor-based polarization detector, a spin-photodiode (spin-PD).

cond-mat.mtrl-sci↗

arXiv2005

Electronic states and cyclotron resonance in p-type InMnAs and InMnAs/(Al,Ga)Sb at ultrahigh magnetic fields

We present a theoretical and experimental study on electronic and magneto-optical properties of p-type paramagnetic InMnAs dilute magnetic semiconductor alloys and ferromagnetic p-type InMnAs/(Al,Ga)Sb thin films in ultrahigh (> 100 T) external magnetic fields \textbf{B}. We use an 8 band Pidgeon-Brown model generalized to include the wavevector dependence of the electronic states along B as well as s-d and p-d exchange interactions with localized Mn d-electrons. In paramagnetic p-InMnAs alloys, we compute the spin-dependent electronic structure as a function of Mn doping and examine how the valence band structure depends on parameters such as the sp-d exchange interaction strength and effective masses. The cyclotron resonance (CR) and magneto-optical properties of InMnAs are computed using Fermi's golden rule. In addition to finding strong CR for hole-active polarization in p-type InMnAs, we also find strong CR for electron-active polarization. The electron-active CR in the valence bands results from transitions between light and heavy hole Landau levels and is seen in experiments. In ferromagnetic p-InMnAs/(Al,Ga)Sb, two strong CR peaks are observed which shift with position and increase in strength as the Curie temperature is approached from above. This transition takes place well above the Curie temperature and can be attributed to the increase in magnetic ordering at low temperatures.

cond-mat.stat-mech↗

arXiv2005

Theory of carrier dynamics and time resolved reflectivity in InMnAs/GaSb heterostructures

We present detailed theoretical calculations of two color, time-resolved pump-probe differential reflectivity measurements. The experiments modeled were performed on InMnAs/GaSb heterostructures and showed pronounced oscillations in the differential reflectivity as well as a time-dependent background signal. Previously, we showed that the oscillations resulted from generation of coherent acoustic phonon wavepackets in the epilayer and were not associated with the ferromagnetism. Now we take into account not only the oscillations, but also the background signal which arises from photoexcited carrier effects. The two color pump-probe reflectivity experiments are modeled using a Boltzmann equation formalism. We include photogeneration of hot carriers in the InMnAs quantum well by a pump laser and their subsequent cooling and relaxation by emission of confined LO phonons. Recombination of electron-hole pairs via the Schockley-Read carrier trapping mechanism is included in a simple relaxation time approximation. The time resolved differential reflectivity in the heterostructure is obtained by solving Maxwell's equations and compared with experiment. Phase space filling, carrier capture and trapping, band-gap renormalization and induced absorption are all shown to influence the spectra.

cond-mat.stat-mech↗

arXiv2005

Ultrafast Quenching of Ferromagnetism in InMnAs Induced by Intense Laser Irradiation

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct {\em demagnetization} processes -- fast ($<1$ ps) and slow ($\sim$100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences ($>10$ mJ/cm$^2$); the saturation value indicates a {\em complete quenching} of ferromagnetism on a sub-picosecond time scale. We attribute this fast dynamics to spin heating through $p$-$d$ exchange interaction between photo-carriers and Mn ions while the $\sim$100 ps component is interpreted as spin-lattice relaxation.

cond-mat.mes-hall↗

arXiv2005

Ion Irradiation Control of Ferromagnetism in (Ga,Mn)As

We report on a promising approach to the artificial modification of ferromagnetic properties in (Ga,Mn)As using a Ga$^+$ focused ion beam (FIB) technique. The ferromagnetic properties of (Ga,Mn)As such as magnetic anisotropy and Curie temperature can be controlled using Ga$^+$ ion irradiation, originating from a change in hole concentration and the corresponding systematic variation in exchange interaction between Mn spins. This change in hole concentration is also verified using micro-Raman spectroscopy. We envisage that this approach offers a means of modifying the ferromagnetic properties of magnetic semiconductors on the micro- or nano-meter scale.

cond-mat.mtrl-sci↗

arXiv2005

Propagating Coherent Acoustic Phonon Wavepackets in InMnAs/GaSb

We observe pronounced oscillations in the differential reflectivity of a ferromagnetic InMnAs/GaSb heterostructure using two-color pump-probe spectroscopy. Although originally thought to be associated with the ferromagnetism, our studies show that the oscillations instead result from changes in the position and frequency-dependent dielectric function due to the generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and their subsequent propagation into the GaSb. Our theory accurately predicts the experimentally measured oscillation period and decay time as a function of probe wavelength.

cond-mat.mes-hall↗