arXiv subjects

Pham Nam Hai

Publications and source records attributed to Pham Nam Hai.

34 records · Page 2Linked to original sources

arXiv2017

A new class of ferromagnetic semiconductors with high Curie temperatures

Ferromagnetic semiconductors (FMSs), which have the properties and functionalities of both semiconductors and ferromagnets, provide fascinating opportunities for basic research in condensed matter physics and device applications. Over the past two decades, however, intensive studies on various FMS materials, inspired by the influential mean-field Zener (MFZ) model have failed to realise reliable FMSs that have a high Curie temperature (Tc > 300 K), good compatibility with semiconductor electronics, and characteristics superior to those of their non-magnetic host semiconductors. Here, we demonstrate a new n type Fe-doped narrow-gap III-V FMS, (In,Fe)Sb, in which ferromagnetic order is induced by electron carriers, and its Tc is unexpectedly high, reaching ~335 K at a modest Fe concentration of 16%. Furthermore, we show that by utilizing the large anomalous Hall effect of (In,Fe)Sb at room temperature, it is possible to obtain a Hall sensor with a very high sensitivity that surpasses that of the best commercially available InSb Hall sensor devices. Our results reveal a new design rule of FMSs that is not expected from the conventional MFZ model. (This work was presented at the JSAP Spring meeting, presentation No. E15a-501-2: https://confit.atlas.jp/guide/event/jsap2017s/subject/15a-501-2/advanced)

cond-mat.mtrl-sci↗

arXiv2016

Observation of spontaneous spin-splitting in the band structure of an n-type zinc-blende ferromagnetic semiconductor

Large spin splitting in the conduction band (CB) and valence band (VB) of ferromagnetic semiconductors (FMSs), predicted by the influential mean-field Zener model[1,2] and assumed in many spintronic device proposals[3-8], has never been observed in the mainstream p-type Mn-doped FMSs[9-15]. Here using tunnelling spectroscopy in Esaki-diode structures, we report the observation of such a large spontaneous spin-splitting energy (ΔE = 31.7 - 50 meV) in the CB bottom of n-type FMS (In,Fe)As, which is surprising considering the very weak s-d exchange interaction reported in several zinc-blende (ZB) type semiconductors[16,17]. The mean-field Zener model also fails to explain consistently the ferromagnetism and the spin splitting energy ΔE of (In,Fe)As, because we found that the Curie temperature (TC) values calculated using the observed ΔE are much lower than the experimental TC by a factor of 400. These results urge the need for a more sophisticated theory of FMSs. Furthermore, bias-dependent tunnelling anisotropic magnetoresistance (TAMR) reveals the magnetic anisotropy of each component of the (In,Fe)As band structure [CB, VB, and impurity band (IB)]. The results suggest that the energy range of IB overlaps with the CB bottom or VB top, which may be important to understand the strong s-d exchange interaction in (In,Fe)As[18,19].

cond-mat.mtrl-sci↗

arXiv2015

Spinodal nanodecomposition in magnetically doped semiconductors

This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, co-doping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear either in the form of nanodots (the {\em dairiseki} phase) or nanocolumns (the {\em konbu} phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.

cond-mat.mtrl-sci↗

arXiv2015

Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb

We investigate the crystal structure, transport and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1-x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1-x,Fex)Sb thin films with x lower or equal to 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1-x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1-x,Fex)Sb thin film with x = 14%, a sudden drop of mobility and TC was observed, which may be due to microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2015

Modulation of ferromagnetism in (In,Fe)As quantum wells via electrically controlled deformation of the electron wavefunctions

We demonstrate electrical control of ferromagnetism in field-effect transistors with a trilayer quantum well (QW) channel containing an ultrathin n-type ferromagnetic semiconductor (In,Fe)As layer. A gate voltage is applied to control the electron wavefunctions ϕi in the QW, such that the overlap of ϕi and the (In,Fe)As layer is modified. The Curie temperature is largely changed by 42%, whereas the change in sheet carrier concentration is 2 - 3 orders of magnitude smaller than that of previous gating experiments. This result provides a new approach for versatile, low power, and ultrafast manipulation of magnetization.

cond-mat.mtrl-sci↗

arXiv2015

Magnetization Process of the n-type Ferromagnetic Semiconductor (In,Fe)As:Be Studied by X-ray Magnetic Circular Dichroism

In order to investigate the mechanism of ferromagnetic ordering in the new n-type magnetic semiconductor (In,Fe)As co-doped with Be, we have performed X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic originating from Fe atoms incorporated into the Zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nano-scale ferromagnetic domains and a T-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nano-scale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nano-scale.

cond-mat.mtrl-sci↗

arXiv2013

Recent Progress in III-V based ferromagnetic semiconductors: Band structure, Fermi level, and tunneling transport

Spin-based electronics or spintronics is an emerging field, in which we try to utilize spin degrees of freedom as well as charge transport in materials and devices. While metal-based spin-devices, such as magnetic-field sensors and magnetoresistive random access memory using giant magnetoresistance and tunneling magnetoresistance, are already put to practical use, semiconductor-based spintronics has greater potential for expansion because of good compatibility with existing semiconductor technology. Many semiconductor-based spintronics devices with useful functionalities have been proposed and explored so far. To realize those devices and functionalities, we definitely need appropriate materials which have both the properties of semiconductors and ferromagnets. Ferromagnetic semiconductors (FMS), which are alloy semiconductors containing magnetic atoms such as Mn and Fe, are one of the most promising classes of materials for this purpose, and thus have been intensively studied for the past two decades. Here, we review the recent progress in the studies of the most prototypical III-V based FMS, p-type (GaMn)As, and its heterostructures with focus on tunneling transport, Fermi level, and bandstructure. Furthermore, we cover the properties of a new n-type FMS, (InFe)As, which shows electron-induced ferromagnetism. These FMS materials having zinc-blende crystal structure show excellent compatibility with well-developed III-V heterostructures and devices.

cond-mat.mes-hall↗

arXiv2013

Control of ferromagnetism by manipulating the carrier wavefunction in ferromagnetic semiconductor (In,Fe)As quantum wells

We demonstrated the control of ferromagnetism in a surface quantum well containing a 5-nm-thick n-type ferromagnetic semiconductor (In,Fe)As layer sandwiched between two InAs layers, by manipulating the carrier wavefunction. The Curie temperature (Tc) of the (In,Fe)As layer was effectively changed by up to 12 K (ΔTc/Tc = 55%). Our calculation using the mean-field Zener theory reveals an unexpectedly large s-d exchange interaction in (In,Fe)As. Our results establish an effective way to control the ferromagnetism in quantum heterostructures of n-type FMSs, as well as require reconsideration on the current understanding of the s-d exchange interaction in narrow gap FMSs.

cond-mat.mtrl-sci↗

arXiv2012

Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.

cond-mat.mtrl-sci↗

arXiv2012

Crystalline anisotropic magnetoresistance with two-fold and eight-fold symmetry in (In,Fe)As ferromagnetic semiconductor

We have investigated the anisotropic magnetoresistance (AMR) of (In,Fe)As ferromagnetic semiconductor (FMS) layers grown on semi-insulating GaAs substrates. In a 10 nm-thick (In,Fe)As layer which is insulating at low temperature, we observed crystalline AMR with two-fold and eight-fold symmetries. In a metallic 100 nm-thick (In,Fe)As layer with higher electron concentration, only two-fold symmetric crystalline AMR was observed. Our results demonstrate the macroscopic ferromagnetism in (In,Fe)As with magnetic anisotropy that depends on the electron concentration. Non-crystalline AMR is also observed in the 100 nm-thick layer, but its magnitude is as small as 10^-5, suggesting that there is no s-d scattering near the Fermi level of (In,Fe)As. We propose the origin of the eight-fold symmetric crystalline anisotropy in (In,Fe)As.

cond-mat.mtrl-sci↗

arXiv2011

Comment on "Reconciling results of tunnelling experiments on (Ga,Mn)As" arXiv:1102.3267v2 by Dietl and Sztenkiel

We comment on the recent paper "Reconciling results of tunnelling experiments on (Ga,Mn)As" arXiv:1102.3267v2 by Dietl and Sztenkiel. They claimed that the oscillations observed in the d2I/dV2-V characteristics in our studies on the resonant tunneling spectroscopy on GaMnAs, are not attributed to the resonant levels in the GaMnAs layer but to the two-dimensional interfacial subbands in the GaAs:Be layer. Here, we show that this interpretation is not able to explain our experimental results and our conclusions remain unchanged.

cond-mat.mtrl-sci↗

arXiv2011

Iron-based n-type electron-induced ferromagnetic semiconductor

Carrier-induced ferromagnetic semiconductors (FMSs) have been intensively studied for decades as they have novel functionalities that cannot be achieved with conventional metallic materials. These include the ability to control magnetism by electrical gating or light irradiation, while fully inheriting the advantages of semiconductor materials such as band engineering. Prototype FMSs such as (In,Mn)As or (Ga,Mn)As, however, are always p-type, making it difficult to be used in real spin devices. This is because manganese (Mn) atoms in those materials work as local magnetic moments and acceptors that provide holes for carrier-mediated ferromagnetism. Here we show that by introducing iron (Fe) into InAs, it is possible to fabricate a new FMS with the ability to control ferromagnetism by both Fe and independent carrier doping. Despite the general belief that the tetrahedral Fe-As bonding is antiferromagnetic, we demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS, a missing piece of semiconductor spintronics for decades. This achievement opens the way to realise novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.

cond-mat.mtrl-sci↗

arXiv2009

GaMnAs-based magnetic tunnel junctions with an AlMnAs barrier

We investigate the spin-dependent transport of GaMnAs-based magnetic tunnel junctions (MTJs) containing a paramagnetic AlMnAs barrier with various thicknesses. The barrier height of AlMnAs with respect to the Fermi level of GaMnAs is estimated to be 110 meV. We observe tunneling magnetoresistance (TMR) ratios up to 175% (at 2.6 K), which is higher than those of the GaMnAs-based MTJs with other barrier materials in the same temperature region. These high TMR ratios can be mainly attributed to the relatively high crystal quality of AlMnAs and the suppression of the tunneling probability near at the in-plane wave-vector k||=0.

cond-mat.mtrl-sci↗

arXiv2007

Spin valve effect by ballistic transport in ferromagnetic metal (MnAs) / semiconductor (GaAs) hybrid heterostructures

We demonstrate the spin valve effect by ballistic transport in fully epitaxial MnAs ferromagnetic metal / GaAs semiconductor / GaAs:MnAs granular hybrid heterostructures. The GaAs:MnAs material contains ferromagnetic MnAs nanoparticles in a GaAs matrix, and acts as a spin injector and a spin detector. Although the barrier height of the GaAs/MnAs interface was found to be very small, relatively large magnetoresistance was observed. This result shows that by using ballistic transport, we can realize a large spin valve effect without inserting a high tunnel barrier at the ferromagnetic metal / semiconductor interface.

cond-mat.mtrl-sci↗

arXiv2007

Reconfigurable Logic Gates Using Single-Electron Spin Transistors

We propose and numerically analyze novel reconfigurable logic gates using "single-electron spin transistors" (SESTs), which are single-electron transistors (SETs) with ferromagnetic electrodes and islands. The output characteristics of a SEST depend on the relative magnetization configuration of the ferromagnetic island with respect to the magnetization of the source and the drain, i.e., high current drive capability in parallel magnetization and low current drive capability in antiparallel magnetization. The summation of multiple input signals can be achieved by directly coupling multiple input gate electrodes to the SEST island, without using a floating gate. A Tucker-type inverter with a variable threshold voltage, a reconfigurable AND/OR logic gate, and a reconfigurable logic gate for all symmetric Boolean functions are proposed and simulated using the Monte Carlo method.

cond-mat.mtrl-sci↗

arXiv2007

Spin-dependent transport properties in GaMnAs-based spin hot-carrier transistors

We have investigated the spin-dependent transport properties of GaMnAs-based three-terminal semiconductor spin hot-carrier transistor (SSHCT) structures. The emitter-base bias voltage VEB dependence of the collector current IC, emitter current IE, and base current IB shows that the current transfer ratio alpha (= IC / IE) and the current gain beta (= IC / IB) are 0.8-0.95 and 1-10, respectively, which means that GaMnAs-based SSHCTs have current amplifiability. In addition, we observed an oscillatory behavior of the tunneling magnetoresistance (TMR) ratio with the increasing bias, which can be explained by the resonant tunneling effect in the GaMnAs quantum well.

cond-mat.mtrl-sci↗