arXiv subjects

H. Munekata

Publications and source records attributed to H. Munekata.

33 records · Page 2Linked to original sources

arXiv2005

High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N

We have studied the electronic structure of the diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N ($x$ = 0.0, 0.02 and 0.042) grown on Sn-doped $n$-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn $L$-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new state were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2$p$ core level and the Mn 3$d$ partial density of states were analyzed using configuration-interaction calculation on a MnN$_{4}$ cluster model. The deduced electronic structure parameters reveal that the $p$-$d$ exchange coupling in Ga$_{1-x}$Mn$_{x}$N is stronger than that in Ga$_{1-x}$Mn$_{x}$As.

cond-mat.mtrl-sci↗

arXiv2004

Effect of Ga$^{+}$ irradiation on magnetic and magnetotransport properties in (Ga,Mn)As epilayers

We report on the magnetic and magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As modified by Ga$^{+}$ ion irradiation using focused ion beam. A marked reduction in the conductivity and the Curie temperature is induced after the irradiation. Furthermore, an enhanced negative magnetoresistance (MR) and a change in the magnetization reversal process are also demonstrated at 4 K. Raman scattering spectra indicate a decrease in the concentration of hole carriers after the irradiation, and a possible origin of the change in the magnetic properties is discussed.

cond-mat.mtrl-sci↗

arXiv2004

Current-induced magnetization reversal in a (Ga,Mn)As-based magnetic tunnel junction

We report current-induced magnetization reversal in a ferromagnetic semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared by molecular beam epitaxy on a p-GaAs(001) substrate. A change in magneto-resistance that is asymmetric with respect to the current direction is found with the excitation current of 10^6 A/cm^2. Contributions of both unpolarized and spin-polarized components are examined, and we conclude that the partial magnetization reversal occurs in the (Ga,Mn)As layer of smaller magnetization with the spin-polarized tunneling current of 10^5 A/cm^2.

cond-mat.mtrl-sci↗

arXiv2004

Ultrahigh-Field Hole Cyclotron Resonance Absorption in InMnAs Films

We have carried out an ultrahigh-field cyclotron resonance study of p-type In1-xMnxAs films, with Mn composition x ranging from 0% to 2.5%, grown on GaAs by low-temperature molecular-beam epitaxy. Pulsed magnetic fields up to 500 T were used to make cyclotron resonance observable in these low-mobility samples. The clear observation of hole cyclotron resonance is direct evidence of the existence of a large number of itinerant, effective-mass-type holes rather than localized d-like holes. It further suggests that the p-d exchange mechanism is more favorable than the double exchange mechanism in this narrow gap InAs-based dilute magnetic semiconductor. In addition to the fundamental heavy-hole and light-hole cyclotron resonance absorption appearing near the high-magnetic-field quantum limit, we observed many inter-Landau-level absorption bands whose transition probabilities are stronglydependent on the sense of circular polarization of the incident light.

cond-mat.mtrl-sci↗

arXiv2004

Ultrafast Photoinduced Softening in a III-V Ferromagnetic Semiconductor for Non-thermal Magneto-Optical Recording

Through time-resolved two-color magneto-optical Kerr spectroscopy we have demonstrated that photogenerated transient carriers decrease the coercivity of ferromagnetic InMnAs at low temperatures. This transient ``softening'' persists only during the carrier lifetime ($\sim$ 2 ps) and returns to its original value as soon as the carriers recombine to disappear. We discuss the origin of this unusual phenomenon in terms of carrier-enhanced ferromagnetic exchange interactions between Mn ions and propose an entirely nonthermal scheme for magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2003

Determining carrier densities in InMnAs by cyclotron resonance

Accurate determination of carrier densities in ferromagnetic semiconductors by Hall measurements is hindered by the anomalous Hall effect, and thus alternative methods are being sought. Here, we propose that cyclotron resonance (CR) is an excellent method for carrier density determination for InMnAs-based magnetic structures. We develop a theory for electronic and magneto-optical properties in narrow gap InMnAs films and superlattices in ultrahigh magnetic fields oriented along [001]. In n-type InMnAs films and superlattices, we find that the e-active CR peak field is pinned at low electron densities and then begins to shift rapidly to higher fields above a critical electron concentration allowing the electron density to be accurately calibrated. In p-type InMnAs, we observe two h-active CR peaks due to heavy and light holes. The lineshapes depend on temperature and line broadening. The light hole CR requires higher hole densities and fields. Analyzing CR lineshapes in p-films and superlattices can help determine hole densities.

cond-mat.mes-hall↗

arXiv2003

Electronic States and Cyclotron Resonance in n-type InMnAs

We present a theory for electronic and magneto-optical properties of n-type In(1-x)Mn(x)As magnetic alloy semiconductors in a high magnetic field, B. We use an 8-band Pidgeon-Brown model generalized to include the wavevector (Kz) dependence of the electronic states as well as s-d and p-d exchange interactions with localized Mn d-electrons. Calculated conduction-band Landau levels exhibit effective masses and g factors that are strongly dependent on temperature, magnetic field, Mn concentration (x), and Kz. Cyclotron resonance (CR) spectra are computed using Fermi's golden rule and compared with ultrahigh-magnetic-field (> 50 T) CR experiments, which show that the electron CR peak position is sensitive to x. Detailed comparison between theory and experiment allowed us to extract s-d and p-d exchange parameters, alpha and beta. we find that not only alpha but also beta affects the electron mass because of the strong interband coupling in this narrow gap semiconductor. In addition, we derive analytical expressions for the effective masses and g facors within the 8-band model. Results indicate that (alpha - beta) is the crucial parameter that determines the exchange interaction correction to the cyclotron masses. These findings sould be useful for designing novel devices based on ferromagnetic semiconductors.

cond-mat.mes-hall↗

arXiv2003

Photo-induced spin dynamics in ferromagnetic semiconductor $p$-(Ga,Mn)As

Spin dynamics in ferromagnetic $p$-(Ga,Mn)As ($x$ = 0.011, $T_{C}$ = 30 K) has been studied by carefully comparing the decay time of the photo-induced reflectivity change with the transient behavior of polar Kerr rotation induced by photo-generated carrier spins with a femtosecond light pulse of various polarizations. As to the rising process, the rate of Kerr rotation is found comparable to the generation rate of spin-polarized carriers. For the decay process, the Kerr rotation and reflectivity signal both show the same decay rate at above the $T_{C}$, whereas, below the $T_{C}$, the former becomes slower than the latter. The magnitude of Kerr rotation suggests that 10$^{2}$ Mn spins are revolved by injecting one hole spin. On the basis of these observations, collective rotation of ferromagnetically coupled Mn spins is discussed in terms of $p$-$d$ exchange interaction and successive transverse spin relaxation. Development of another long-lived behavior under external perpendicular magnetic fields is also disclosed.

cond-mat.mtrl-sci↗

arXiv2003

Cyclotron Resonance in Ferromagnetic InMnAs/(Al,Ga)Sb Heterostructures

We report the observation of hole cyclotron resonance (CR) in InMnAs/(Al,Ga)Sb heterostructures in a wide temperature range covering both the paramagnetic and ferromagnetic phases. We observed two pronounced resonances that exhibit drastic changes in position, linewidth, and intensity at a temperature higher than the Curie temperature, indicating possible local magnetic ordering or clustering. We attribute the two resonances to the fundamental CR transitions expected for delocalized valence-band holes in the quantum limt. Using an 8-band {\bf k$\cdot$p} model, which incorporates ferromagnetism within a mean-field approximation, we show that the temperature-dependent CR peak shift is a direct measure of the carrier-Mn exchange interaction. Significant line narrowing was observed at low temperatures, which we interpret as the suppression of localized spin fluctuations.

cond-mat.mes-hall↗

arXiv2003

Relation among concentrations of incorporated Mn atoms, ionized Mn acceptors, and holes in p-(Ga,Mn)As epilayers

The amount of ionized Mn acceptors in various p-type Mn-doped GaAs epilayers has been evaluated by electrochemical capacitance-voltage measurements, and has been compared systematically with concentrations of incorporated Mn atoms and holes for wide range of Mn concentration (10^17 ~ 10^21 cm^-3). Quantitative assessment of anomalous Hall effect at room temperature is also carried out for the first time.

cond-mat.mtrl-sci↗

arXiv2002

Ultrafast optical manipulation of ferromagnetic order in InMnAs/GaSb

We have performed a two-color time-resolved magneto-optical Kerr effect (MOKE) study of a ferromagnetic InMnAs/GaSb heterostructure. We observed ultrafast photo-induced changes in the MOKE signal induced by a large density of spin-polarized transient carriers created only within the InMnAs layer using intense 140 fs mid-infrared pulses. Our data clearly demonstrates that magnetic properties, e.g., remanence and coercivity, can be strongly modified. The dependence of these changes on the time delay, pump polarization, pump intensity, and sample temperature is discussed.

cond-mat.mtrl-sci↗

arXiv2002

Cyclotron Resonance of Itinerant Holes in Ferromagnetic InMnAs/GaSb Heterostructures

We report the first observation of hole cyclotron resonance (CR) in ferromagnetic InMnAs/GaSb heterostructures both in the high-temperature paramagnetic phase and the low-temperature ferromagnetic phase. We clearly resolve two resonances that exhibit strong temperature dependence in position, linewidth, and intensity. We attribute the two resonances to the so-called fundamental CR transitions expected for delocalized holes in the valence band in the magnetic quantum limit, demonstrating the existence of $p$-like itinerant holes that are describable within the Luttinger-Kohn effective mass theory.

cond-mat.mtrl-sci↗

arXiv2002

Ultrahigh field electron cyclotron resonance absorption in In$_{1-x}$Mn$_x$As films

We have carried out an ultrahigh field cyclotron resonance study of $n$-type In$_{1-x}$Mn$_x$As films, with Mn composition $x$ ranging from 0 to 12%, grown on GaAs by low temperature molecular beam epitaxy. We observe that the electron cyclotron resonance peak shifts to lower field with increasing $x$. A detailed comparison of experimental results with calculations based on a modified Pidgeon-Brown model allows us to estimate the {\em s-d} and {\em p-d} exchange coupling constants, $α$ and $β$, for this important III-V dilute magnetic semiconductor system.

cond-mat.mtrl-sci↗

arXiv2002

Electronic structure of In$_{1-x}$Mn$_x$As studied by photoemission spectroscopy: Comparison with Ga$_{1-x}$Mn$_x$As

We have investigated the electronic structure of the $p$-type diluted magnetic semiconductor In$_{1-x}$Mn$_x$As by photoemission spectroscopy. The Mn 3$d$ partial density of states is found to be basically similar to that of Ga$_{1-x}$Mn$_x$As. However, the impurity-band like states near the top of the valence band have not been observed by angle-resolved photoemission spectroscopy unlike Ga$_{1-x}$Mn$_x$As. This difference would explain the difference in transport, magnetic and optical properties of In$_{1-x}$Mn$_x$As and Ga$_{1-x}$Mn$_x$As. The different electronic structures are attributed to the weaker Mn 3$d$ - As 4$p$ hybridization in In$_{1-x}$Mn$_x$As than in Ga$_{1-x}$Mn$_x$As.

cond-mat.mtrl-sci↗

arXiv2002

Ferromagnetic semiconductor (In,Ga,Mn)As with Curie temperature above 100 K

We have grown (InyGa1-y)1-xMnxAs ferromagnetic semiconductor layers with Mn composition x up to 0.13 on InP substrates by molecular beam epitaxy. Near the lattice-matched composition, i.e., y ~ 0.53, the Curie temperature increases linearly with the ferromagnetically effective Mn composition x_eff, following the empirical equation T_C=1300x_eff. We obtained Curie temperatures above 100 K when x is relatively high (x>0.1; x_eff>0.08) and the hole concentration is in the order of 10^19 cm^-3.

cond-mat.mtrl-sci↗