Session K19: III-V Magnetic Semiconductors

An X-ray standing wave study of the diluted magnetic semiconductor Ga(Mn)As

We have combined the recently developed techniques of soft x-ray standing-wave angle-resolved photoemission (SW-ARPES) [Gray et al., EPL 104, 17004 (2013)] and hard x-ray ARPES (HARPES) [Gray et al., Nature Mat. 11, 957 (2012)] so as to be able to use single-crystal Bragg reflection to create the SW [Thiess et al., Sol. St. Comm. 150, 553 (2010)], thus permitting the first measurements of momentum- and element- resolved bulk electronic structure. The strengths of the SW-HARPES method are demonstrated using the dilute magnetic semiconductor Ga$_{(1-x)}$Mn$_x$As. A strong SW is generated by Bragg reflection of ca. 3 keV x-rays from the (111) planes of both undoped GaAs and Mn-doped thin films with x=0.05. Due to the uneven occupancy of (111) planes by either Ga(Mn) or As atoms, the element-specific band structure can be obtained with a help of the SW modulation in core levels. Apart from the site specific decomposition of the electronic structure, the SW measurements also confirmed a substitutional presence of Mn atoms at the Ga sites. This technique should be applicable to a broad range of complex materials.   

\textbf{Magnetism and Mn Clustering in (In, Mn)Sb Magnetic Semiconductors.}

Magnetic semiconductors doped with transition metal elements such as (In,Mn)Sb and (Ga,Mn)Sb are considered ideal systems for spintronic devices such as magnetic junction transistors. The magnetic behavior of these semiconductors is largely influenced by magnetic atom distribution, electronic structure, and chemical state. The Mn distribution and phase composition in (In, Mn)Sb films grown by metal-organic vapor phase epitaxy (MOVPE) were determined using X-ray photoelectron spectroscopy (XPS). From XPS the spin-orbit splitting energy of the Mn 2p core-level was found to increase with increasing Mn concentration. The measured magnetic moment per Mn atom decreases with increasing Mn concentration, which is attributed to atomic-scale clusters that are ferromagnetic or ferrimagnetic. The magnetic properties in conjunction with XPS analysis indicate that atomic-scale Mn clusters could be responsible for the high-temperature magnetism of greater than 400 K in (In,Mn)Sb. These results demonstrate the potential of modifying the magnetic properties of (In,Mn)Sb films by controlling Mn concentration or phase composition.   

Electronic structure near the Fermi level in the ferromagnetic semiconductor GaMnAs studied by ultrafast time-resolved light-induced reflectivity measurement\textbf{s}

The determination of the Fermi level ($E_{F})$ position is important to understand the origin of the ferromagnetism in ferromagnetic semiconductor GaMnAs. The recent transient reflectivity (TR) spectroscopy measurement, which is potentially sensitive to the absorption edges, indicated that the $E_{F}$ exists in the valence band [1]. However, the pump fluence in this study is rather high, and the accumulation of photo-carriers can shift the absorption edges. Thus, the definition of both the band gap and $E_{F}$ is obscure. Here, we have performed TR spectroscopy measurements on GaMnAs films with the pump fluence carefully controlled to suppress the accumulation of photo-carriers. The energy resolution of the TR spectrum was improved to 0.5 meV. The data shows light-induced change in the reflectivity spectra which is attributed to the band-gap renormalization and band filling. We have reproduced the observed TR spectra using the Kramers-Kronig relation and found the Mn-induced electronic states near the $E_{F}$ in the band gap. [1] T. de Boer et al., Phys. Rev. B 85, 033202 (2012).   

High Field Magnetic Circular Dichroism in Ferromagnetic InMnSb and InMnAs

An understanding of the fundamental interactions in narrow gap ferromagnetic semiconductors such as InMnAs and InMnSb has been developed primarily from static magnetization and electrical transport measurements. In this study, to provide a better understanding of the coupling of the Mn impurities to the conduction and valence bands through the sp-d exchange interactions, we have performed magnetic circular dichroism measurements (MCD) on MOVPE grown InMnAs and InMnSb. In our samples, the Mn content varies from 2\% to 10.7\% and all the samples have Curie temperatures above 300 K. The samples were photo-excited using a Quartz Tungsten Halogen lamp with energies ranging between 0.92-1.45 eV, and in magnetic fields up to 31 T. The temperatures ranged from 15-190 K. Comparison of the observed MCD with theoretical calculations provides a direct method to probe the band structure including the temperature dependence of the spin-orbit split-off bandgap and g-factors, as well as a means to estimate the sp-d coupling constants.   

Spin-dependent transport properties of a GaMnAs-based vertical spin metal-oxide- semiconductor field-effect transistor structure

Spin metal-oxide semiconductor field-effect transistors (spin MOSFETs) [1] are one of the most promising devices for the post-scaling era. In previous studies on spin MOSFETs[2,3], the drain-source current was controlled by the gate-source voltage and magnetization configuration of the source and drain; however, the magnetoresistance (MR) ratios (0.1{\%} [2] and 0.005{\%} [3] ) were too small to be put into practical applications, and thus spin MOSFET with a high MR ratio is strongly required. Here, we study a GaMnAs-based vertical spin-MOSFET structure. We successfully modulate the drain-source current $I_{\mathrm{DS}}$ by \textasciitilde 0.5 (--0.5) {\%} with a gate-source voltage of --10.8 ($+$10.8) V and also modulate $I_{\mathrm{DS}}$ by up to 60 {\%} with changing the magnetization configuration of the GaMnAs source/drain at 3.5 K. The MR ratio is more than two orders of magnitude higher than that obtained in the previous studies on spin MOSFETs[2,3][4] [1] S. Sugahara and M. Tanaka, APL 84, 2307 (2004). [2] R. Nakane \textit{et al}., JJAP 49, 113001 (2010). [3] T. Sasaki \textit{et al}., Phys. Rev. Appl. 2, 034005 (2014). [4] T. Kanaki \textit{et al}., submitted; arXiv:1510.07497.   

Ferromagnetism in Silicon Single Crystals with Positively Charged Vacancy Clusters

Defect-induced ferromagnetism provides an alternative for organic and semiconductor spintronics. Here, we investigated the magnetism in Silicon after neutron irradiation and try to correlate the observed magnetism to particular defects in Si. Commercially available p-type Si single crystal wafer is cut into pieces for performing neutron irradiations. The magnetic impurities are ruled out as they can not be detected by secondary ion mass spectroscopy. With positron annihilation lifetime spectroscopy, the positron trapping center corresponding to lifetime 375 ps is assigned to a kind of stable vacancy clusters of hexagonal rings (V6) and its concentration is enhanced by increasing neutron doses. After irradiation, the samples still show strong diamagnetism. The weak ferromagnetic signal in Si after irradiation enhances and then weakens with increasing irradiation doses. The saturation magnetization at room temperature is almost the same as that at 5 K. The X-ray magnetic circular dichroism further provides the direct evidence that Silicon is the origin of this ferromagnetism. Using first-principles calculations, it is found that positively charged V6 brings the spin polarization and the defects have coupling with each other.   

Microscopic understanding of spin current probed by shot noise

The spin currents is one of key issue in the spintronics field and the generation and detection of those have been intensively studied by using various materials. The analysis of experiments, however, relies on phenomenological parameters such as spin relaxation length and spin flip time. The microscopic nature of the spin current such as energy distribution and energy relaxation mechanism, has not yet well understood. To establish a better microscopic understanding of spin currents, I focused on the shot noise measurement which is well established technique in the field of mesoscopic physics [Y. M. Blanter and M. B \"{u}ttiker, Phys. Rep. 336, 1 (2000).]. Although there are many theoretically works about shot noise in the presence of spin currents, for example detection of spin accumulation [J. Meair, P. Stano, and P. Jacquod, Phys. Rev. B 84 (2011).], estimation of spin flip currents, and so on, these predictions have never been experimentally confirmed. In this context, we reported the first experimental detention of shot noise in the presence of the spin accumulation in a (Ga,Mn)As/tunnel barrier/n-GaAs based lateral spin valve device [T. Arakawa et al., Phys. Rev. Lett. 114, 016601 (2015).]. \\\\ Together with this result, we found however that the effective temperature of the spin current drastically increases due to the spin injection process. This heating of electron system could be a big problem to realize future spin current devices by using quantum coherence, because the effective temperature rise directly related to the destruction of the coherence of the spin current. Therefore, then we focused on the mechanism of this heating and the energy relaxation in a diffusive channel. By measuring current noise and the DC offset voltage in the usual non-local spin valve signal as a function of the spin diffusion channel length, we clarified that the electron-electron interaction length, which is the characteristic length for the relaxation of the electron system, is much longer than the spin relaxation length. In other words, the spin currents in such a semiconducting material can be strongly out of equilibrium. \\\\ In this invited talk, I will present a series of experimental work on the spin current in a (Ga,Mn)As/tunnel barrier/n-GaAs based lateral spin valve device, mainly probed by the current noise measurement. Finally I hope I will mention about our future plan to cool down the effective temperature of the spin current by using superconductivity.   

Physical Properties of Fe-doped Ba(Mn$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{\mathrm{2}}$Sb$_{\mathrm{2}}$ Single Crystals

BaMn$_{\mathrm{2}}$Sb$_{\mathrm{2}}$ forms the ThCr$_{\mathrm{2}}$Si$_{\mathrm{2}}$-type crystal structure and has the magnetic semiconducting ground state. In attempt to alter its ground-state properties, Mn is partially substituted by Fe resulting in Ba(Mn$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{\mathrm{2}}$Sb$_{\mathrm{2}}$. While the doped system remains the same structure for x $\le $ 0.5, its electrical and thermal conductivity decreases with increasing x, suggesting that doping-induced disorder plays an important role. Magnetically, we find that, with increasing x, the magnetic transition temperature T$_{\mathrm{M}}$ decreases (from 700 K for x $=$ 0 to 500 K for x $=$ 0.5) but magnetic susceptibility increases above and below T$_{\mathrm{M}}$. These and low-temperature magnetization anisotropy suggest the canted-antiferromagnetic configuration with net magnetic moment in BaMn$_{\mathrm{2}}$Sb$_{\mathrm{2}}$. The antiferromagnetic interaction is gradually suppressed upon Fe doping, leading to the enhanced ferromagnetic component in Ba(Mn$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})_{\mathrm{2}}$Sb$_{\mathrm{2}}$.   

Magnetic Coupling in FeBi$_{\mathrm{2}}$Se$_{\mathrm{4}}$ and FeSb$_{\mathrm{2}}$Se$_{\mathrm{4}}$ from first principles

Spintronic devices offer benefits in power efficiency and size reduction over current electronics, but require the development of semiconductor materials with favorable magnetic properties. Specifically, a high ferromagnetic-to-paramagnetic Curie transition temperature is required for spintronics operation at room temperature. FeBi$_{\mathrm{2}}$Se$_{\mathrm{4}}$ and FeSb$_{\mathrm{2}}$Se$_{\mathrm{4}}$ are two n and p-type magnetic semiconductors, respectively, with Curie transition temperatures of 450K. We employ first-principles calculations based on density functional theory to examine the magnetic coupling mechanisms in these materials. Our results indicate that antisite defects of Fe upon the Bi/Sb sites are crucial to the ferromagnetic coupling of the Fe magnetic moments in the crystals. This research was supported by the National Science Foundation CAREER award through Grant No. DMR-1254314. Computational Resources were provided by the DOE NERSC facility.   

Strain fields and electronic structure of CrN

Chromium nitride (CrN) has a promising future for its resistance to corrosion and hardness, and very interesting magnetic and electronic properties. CrN presents a phase transition in which the crystal structure, magnetic ordering and electronic properties change at a (Néel) temperature $\sim 280 K$. Thin films from different labs exhibit different conductance behavior at low temperature. We study the unusual electronic and magnetic properties of thin layers. For that purpose we develop a tight binding Hamiltonian based on the Slater-Koster approach, and estimate the interaction between the Cr-3d and N-2p orbitals, by analyzing the band structure and comparing it with ab initio calculations performed using the LSDA+U method [1]. These calculations show the system to behave as a semiconductor below the Néel temperature. Based on our model we calculate the effective masses and analyze the effect of strain fields in the electronic structure in order to understand the electronic behavior near the phase transition. [1] A. Herwadkar and W. Lambrecht, Phys. Rev. B 79(3), 035125 (2009).   

Observation of the room-temperature local ferromagnetism and its nanoscale growth in the ferromagnetic semiconductor GeFe

Group-IV-based ferromagnetic semiconductor GeFe is expected to be efficient spin injectors and detectors in group-IV-based semiconductor devices, because it can be epitaxially grown on Si and Ge substrates [1,2] and the $T_{\mathrm{C}}$ can be increased up to 210 K by annealing [3]; however, detailed microscopic understanding of the ferromagnetism is lacking. In this study, we have investigated the local magnetic properties of the GeFe$_{\thinspace }$films, using soft X-ray magnetic circular dichroism. We found that nanoscale local ferromagnetic regions formed in the high-Fe-content regions exist even at room temperature, well above the Curie temperature of 20 - 100 K. We also observed the intriguing nanoscale growth process of the local ferromagnetic regions in which they expand as temperature decreases, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature [4]. References [1] Y. Ban, Y. Wakabayashi et al., AIP Adv. \textbf{4}, 097108 (2014). [2] Y. K. Wakabayashi et al., Phys. Rev. B, \textbf{90}, 205209~(2014). [3] Y. K. Wakabayashi et al., J. Appl. Phys.~\textbf{116}, 173906~(2014). [4] Y. K. Wakabayashi et al., arXiv:1502.00118~(2015).   

Magnon-drag and thermomagnetic transport properties of Ca doped YIG

Yttrium-iron garnet (YIG) is an insulating ferromagnet commonly used to study various spin transport phenomena: in conjunction with a Pt film, it generates the well-known spin-Seebeck effect [1]. Because of the close relationship between the spin-Seebeck effect and the magnon-drag charge Seebeck effect [2], we investigate the thermoelectric transport properties of an electrically conducting bulk YIG crystal doped p-type with Ca. A large and sharp change in the thermopower of Ca:YIG near the Curie temperature has been observed, which is potentially explained by the magnon-drag model. We present the temperature dependence of electrical conductivity, magneto-thermopower, and Hall coefficient of Ca:YIG. Photo-excitation of the carriers from the valence band into the Ca level results in photoconductivity and photo-Seebeck effects as well. [1] Jin et al., \textit{Phys. Rev. B} 92, 054436 (2015) [2]~Lucassen et al., Appl. Phys. Lett. 99 262506 (2011)   

Infrared Kerr measurements on ferromagnetic silicon and silicon carbide

We measure the infrared (100-1000 meV) Kerr angle in ferromagnetic silicon and silicon carbide. The samples were either neutron irradiated or aluminum doped to induce ferromagnetic behavior. The samples are studied in the 10-300K temperature range at magnetic fields up to 7T. We also explore the dependence of the magneto-optical signal on samples with different irradiation exposure levels. This study provides new information on the optical, magnetic, and electronic properties of these materials. Work supported by NSF-DMR1410599 and the Helmholtz Postdoctoral Program PD-146.