Digital Ferromagnetic Heterostructures Research Articles

Spin-gapless and half-metallic ferromagnetism in potassium and calcium δ-doped GaN digital magnetic heterostructures for possible spintronic applications: insights from first principles

The reports previously issued predominantly paid attention to the d-block magnetic elements δ-doped digital magnetic materials. In this work, GaN δ-doped with non-magnetic main group s-block elements K and Ca as digital magnetic heterostructures were purposed and explored theoretically. We found that K- and Ca-embedded GaN digital alloys exhibit spin-gapless and half-metallic ferromagnetic characteristics, respectively. All compounds obey the Slater–Pauling rule with diverse electronic and magnetic properties. For these digital ferromagnetic heterostructures, spin polarization occurs in nitrogen within a confined space around the δ-doped layer, demonstrating a hole-mediated two-dimensional magnetic phenomenon.

Ab initioelectronic structure, optical, and magneto-optical properties of MnGaAs digital ferromagnetic heterostructures

We report on a theoretical study of the electronic, optical, and magneto-optical properties of digital ferromagnetic heterostructures based on Mn $\ensuremath{\delta}$-doped GaAs. We consider different structures corresponding to Mn contents within the range 12%--50% and we study how the system changes as a function of the doping concentration. Our first-principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local-field effect in the description of the optical absorption. We show that Mn $\ensuremath{\delta}$-doped GaAs shares many properties with the uniformly doped $\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{Mn}{}_{x}\mathrm{As}$ system, i.e., half-metallicity, similar absorption spectra, and moderate Kerr rotation angles in the visible spectral region.

Absence of half-metallicity in defect-free digital magnetic heterostructuresδ-doped with Cr and Mn

We present results of a combined density functional and many-body calculations for the electronic and magnetic properties of the defect-free digital ferromagnetic heterostructures obtained by doping GaAs with Cr and Mn. While local density approximation/(+U) predicts half-metallicity in these defect-free delta-doped heterostructures, we demonstrate that local many-body correlations captured by Dynamical Mean Field Theory induce within the minority spin channel non-quasiparticle states just above $E_F$. As a consequence of the existence of these many-body states the half-metallic gap is closed and the carriers spin polarization is significantly reduced. Below the Fermi level the minority spin highest valence states are found to localize more on the GaAs layers being independent of the type of electronic correlations considered. Thus, our results confirm the confinement of carriers in these delta-doped heterostructures, having a spin-polarization that follow a different temperature dependence than magnetization. We suggest that polarized hot-electron photoluminescence experiments might bring evidence for the existence of many-body states within the minority spin channel and their finite temperature behavior.

Theoretical investigations of defects in a Si‐based digital ferromagnetic heterostructure – a spintronic material

AbstractWe investigate the effects of two different forms of defects on the half‐metallic properties of the Mn/Si digital ferromagnetic heterostructure (DFH) (PRL 96, 027211 (2006)) using first principles algorithm based on density functional theory. The half metallicity is retained when the δ‐layer of the Mn atoms has 25% imperfection and even a vacancy. The cruci al properties of the DFH are robust against 25% defects (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A first-principles study of dual acceptors behaviors in Mn [formula omitted]-doped digital ferromagnetic heterostructures

The effect of the dual acceptors (Mn and Be) on ferromagnetism in δ -doped GaAs/AlAs digital ferromagnetic heterostructures (DFH) has been studied by ab initio calculations. The dependence of the electronic and magnetic properties on both doping acceptor positions in the well and in the barrier are reported in the two-dimensional hole gas system, respectively. Our results also show that the δ -doping Mn layer in the well edge of the DFH is potential for high Curie temperature of a diluted magnetic semiconductor (DMS). The hole distributions and the magnetic moments are analyzed in terms of their orbital projected density of states. The ferromagnetic coupling is attributed to the overlap of the hole gases and the magnetic layer. However, the Be δ -doping layer in AlAs barrier strongly suppresses the ferromagnetic coupling in the DFH, the origin is not from the electrical doping impurities, but rather be the result of some structural modifications due to the presence of the Be.

Design of Spintronic Materials with Simple Structures

A brief comparison of conventional electronics and spintronics is given. The key features of half metallic binary compounds with the zincblende structure are presented, using MnAs as an example. We discuss the interactions responsible for the half metallic properties. Special properties of superlattices and a digital ferromagnetic heterostructure incorporating zincblende half metals are also discussed.

X-ray magnetic circular dichroism characterization of GaN∕Ga1−xMnxN digital ferromagnetic heterostructure

The authors have investigated the magnetic properties of a GaN∕Ga1−xMnxN (x=0.1) digital ferromagnetic heterostructure (DFH) showing ferromagnetic behavior using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The Mn L2,3-edge XAS spectra were similar to those of Ga1−xMnxN random alloy thin films, indicating a substitutional doping of high concentration Mn into GaN. From the XMCD measurements, it was revealed that paramagnetic and ferromagnetic Mn atoms coexisted in the Ga1−xMnxN digital layers. Subtle differences were also found from the XMCD spectra between the electronic states of the ferromagnetic and paramagnetic Mn2+ ions. The ferromagnetic moment per Mn atom estimated from XMCD agreed well with that estimated from superconducting quantum interference device measurements, indicating that the ferromagnetic behavior of the GaN∕Ga1−xMnxN DFH sample arises only from substitutional Mn2+ ions in the Ga1−xMnxN digital layers and not from ferromagnetic precipitates.

The influence of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures

The effect of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures has been studied by ab initio calculations in combination with pseudopotential plane-wave method. The electronic and magnetic properties are shown as a function of the thickness of AlAs layers in the GaAs∕AlAs digital ferromagnetic heterostructures. It is found that all the structures show ferromagnetic alignment for the most favored configuration and their electronic structures are half metallicity. The exchange coupling constants N0β are estimated by using the spin splitting of the valence band. It is also shown that the charge density and the strong spin polarization are concentrated mostly on the magnetic layers for all structures. Furthermore, the hole distributions are analyzed in terms of their orbital projected density of states. The concentration of confined hole within the magnetic layer increases with increasing the additional potentials, which is responsible for the enhancement of ferromagnetic interaction in III–V digital ferromagnetic heterostructures.

Enhancement of ferromagnetic coupling in Mn∕GaAs digital ferromagnetic heterostructure by free-hole injection

We have studied the effect of the free-hole injection on the ferromagnetic coupling in the Mn∕GaAs digital ferromagnetic heterostructure (DFH) using ab initio electronic-structure methods. The DFH is modeled by a supercell periodically consisting of a δ-doped layer of MnAs and 15 layers of GaAs. The injection of free holes is simulated by assigning a range of missing electrons in unit cell. The δ-doped layer of Mn atoms in GaAs introduces three spin-polarized hole bands which are the consequence of hybridization between the d states of the Mn atoms and the p states of the nearest neighboring As atoms. These spin-polarized holes are confined to the vicinity of the MnAs layer. After the injection of free holes, the Fermi energy is lowered, consequently the number of spin-polarized holes in the layer of MnAs increases monotonously. Our results show the enhancement of the ferromagnetic coupling by the free-hole injection, which is in agreement with the experimental observation.

Electronic and magnetic properties of Mn∕Ge digital ferromagnetic heterostructures: An ab initio investigation

A digital ferromagnetic heterostructure composed of a δ-doped layer of Mn in a Ge substrate (Mn∕Ge-DFH) has been studied by using the density-functional theory within the generalized gradient approximation. We found that the ferromagnetic order in the Mn layer of the DFH is more favored than the antiferromagnetic order. However, the DFH exhibits metallic behaviors due to the small gap of the host semiconductor. In the majority spin channel, the hole carriers are originated from the hybridized states between the dt2g states of the Mn atom and the p state of its nearest neighbor Ge atoms. These hole states form three bands and are responsible for the ferromagnetic order. In the minority spin channel, the Fermi level is located below the valence band maximum of the host semiconductor, which gives rise to a number of hole carriers with the opposite spin direction in the host. We present the total and partial density of states and the band structures of the two spin channels. By fitting the exchange parameters, we predict the Curie temperature of this DFH.

Half-Metallic Digital Ferromagnetic Heterostructure Composed of aδ-Doped Layer of Mn in Si

We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

Microstructure of (Ga,Mn)As∕GaAs digital ferromagnetic heterostructures

We report on the microstructure of (Ga,Mn)As digital ferromagnetic heterostructures grown on GaAs (001) substrates by low-temperature molecular-beam epitaxy. The Mn concentration and the As4∕Ga beam equivalent pressure (BEP) ratio are varied in the samples containing periods of Mn sheets separated by thin GaAs spacer layers. Transmission electron microscopy studies reveal that decreasing the Mn doping concentration and reducing the BEP ratio lead to smaller composition fluctuations of Mn and more homogeneous (Ga,Mn)As layers with abrupt interfaces. Planar defects are found as the dominant defect in these heterostructures and their density is related to the magnitude of the composition fluctuation. These defects show a noticeable anisotropy in the morphologic distribution parallel to the orthogonal [110] and [11¯0] direction. Along the [11¯0] direction, they are stacking faults, which are preferentially formed in V-shaped pairs and nucleate at the interfaces between (Ga,Mn)As and GaAs layers. Along the [110] direction, the planar defects are isolated thin twin lamellae. The character of the planar defects and their configuration are analyzed in detail.

Infrared survey of the carrier dynamics in III-V digital ferromagnetic heterostructures

We report on the electromagnetic response of digital ferromagnetic heterostructures sDFHd: systems with d-doped MnAs layers separated by GaAs spacers of variable thickness syd. The gross features of the infrared conductivity of DFH samples are consistent with the notion that these digital structures are GaAs/ Ga 1˛xMnxAs superlattices. This conclusion is supported by a combination of spectral weight analysis and effective medium theory. The optical properties of DFH also provide insights into the evolution of their critical temperature with GaAs spacing. In DFH a low-lying gap materializes in the energy dependent conductivity, which is interpreted as a mobility gap resulting from Anderson localization.

Anisotropic distribution of stacking faults in (Ga,Mn)As digital ferromagnetic heterostructures grown by low-temperature molecular-beam epitaxy

We report on the microstructure of (Ga,Mn)As-based digital ferromagnetic heterostructures, which nominally consist of 40 periods of 0.75-monolayer (ML) Mn sheets between 17-ML GaAs spacer layers grown on GaAs(001) substrates by low-temperature molecular-beam epitaxy. Transmission electron microscopy studies reveal mainly stacking faults, which are preferentially coupled in V-shaped pairs with short intersecting lines along the [11¯0] direction. With increasing the V/III beam equivalent pressure ratio, a stronger laterally inhomogeneous distribution of the Mn atoms is detected along the sheets resulting in a larger local strain and thus in a higher density of stacking fault pairs. Their anisotropic distribution is explained by the energetically favorable Mn–As bonding configuration that is induced by the specific surface morphology appearing at the low growth temperature.

Effects of growth temperature on magnetism in digital ferromagnetic heterostructures

We report on the dependence of the magnetic and transport properties of ferromagnetic semiconductor superlattices on the temperature, ${T}_{G}$, of growth by molecular-beam-epitaxy, at low ${T}_{G}$ from $260\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}300\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The Curie temperature, ${T}_{C}$, of these $\mathrm{Mn}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ digital alloy structures shows nonmonotonic variation with ${T}_{G}$ that accompanies a steady increase in conductivity. We observe qualitative changes in the temperature-dependence of the anomalous Hall effect, with a striking sign reversal at temperatures comparable to ${T}_{C}$ in superlattices grown at higher ${T}_{G}$.

Optical and magnetic properties of (Ga 1− xMn x)N/GaN digital ferromagnetic heterostructures

(Ga 1− x Mn x )N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga 1− x Mn x )N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga 1− x Mn x )N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga 1− x Mn x )N/GaN DFHs were much larger than those in (Ga 1− x Mn x )N thin films. These results indicate that the (Ga 1− x Mn x )N/GaN DFHs hold promise for potential applications in spintronic devices.

Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures

Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures have been investigated by polarized neutron reflectometry and magnetometry. Tc of three samples with 20, 50, and 100 ML GaAs spacers ranges from 30 to 40 K. The saturation magnetization of three samples exhibits a pronounced tail extending over 50 K above Tc in addition to a temperature-independent background. For the 50 ML sample, PNR measurements show a similar tail but no background. These behaviors can be explained by a two-step ordering process. In the tail region, two-dimensional islands first individually become ferromagnetic. Long-range order develops as the temperature is decreased below Tc.

Structural engineering of ferromagnetism in III–V digital ferromagnetic heterostructures

We investigate the possibility of modulating the magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures (DFHs) via strain engineering. We p-dope DFHs below the compensation threshold of residual As antisites to achieve variations in strain without introducing free carriers and with relatively modest concentrations of impurity atoms. X-ray diffraction and superconducting quantum interference device measurements reveal a trend toward higher TC as the out-of-plane strain is increased. Additionally, we demonstrate a second method for strain engineering wherein DFHs are grown on anisotropically relaxed (Ga,In)As stressor layers. We show that the ferromagnetic properties are independent of strain in this regime and conclude that the structure-dependent modulation of magnetic properties in DFHs cannot be explained by simple strain effects alone.

Mechanism of enhanced ferromagnetism in delta-doped (Ga,Mn)As studied by ab initio electronic structure calculation

Abstract A mechanism of the enhanced ferromagnetic Curie temperature ( T C ) is proposed for (Ga,Mn)As digital ferromagnetic heterostructures (DFH) obtained by incorporating Mn delta-doping layer into GaAs based on the first-principles calculations. Our calculation shows a strong enhancement of T C for the DFH compared with that of random alloy with the same Mn concentration. In order to verify this enhancement mechanism of T C , we confirm that higher 3d density of states at the Fermi level upon delta-doping is responsible for the enhancement of T C due to the ferromagnetic double exchange interaction.

(Ga,Mn)As/AlAs digital ferromagnetic heterostructures

Abstract We present a density functional theory study of (Ga,Mn)As/AlAs digital ferromagnetic heterostructures, which can be obtained by δ-doping with Mn the GaAs layers of a short period GaAs/AlAs superlattice. In these structures band engineering allows us to improve the hole confinement in the Mn layer, and therefore to enhance the Mn–Mn ferromagnetic coupling over that of (Ga,Mn)As/GaAs heterostructures. In particular we investigate how the ferromagnetism depends on the thickness of the AlAs layer. Our analysis suggests that (Ga,Mn)As/AlAs heterostructures are good candidates for spintronics applications.