Session A14: Focus Session: Spins in Semiconductors - Magnetic Semiconductors I

Oxygen Annealing Studies of SnO$_{2}$:Co Thin Films Deposited by RF Sputtering

We report on post-deposition oxygen annealing studies of SnO$_{2}$:Co thin films to examine the origin of the room temperature ferromagnetism (RTFM) observed in such materials. Materials are deposited on r-cut sapphire substrates by RF sputtering from a doped target with 5 at.{\%} Co nominal concentration. Magnetization measurements reveal that as-grown samples in Ar atmosphere are non-magnetic at RT. However, by annealing them in low O$_{2}$ pressure (10$^{-4 }$- 2x10$^{-4}$ Torr), the saturation moment increases to $\sim $0.78 $\mu _{B}$/Co at RT, somewhat lower than the expected value for Co$^{2+}$ ions. This verifies that the Co ions are incorporated in the matrix. X-ray diffraction data show a decrease in crystallinity for the most magnetic samples annealed in O$_{2 }$at 2x10$^{-4}$ Torr. To confirm this, further structural and temperature-dependent magnetic measurements for various annealing protocols are underway to determine the nature of magnetism in SnO$_{2}$:Co sputtered thin films.   

Embedded Ferromagnetic GdN Nano-Islands in GaN by Molecular Beam Epitaxy

Nano-islands of GdN are embedded into a GaN matrix by plasma-assisted molecular beam epitaxy. X-ray diffractometry shows that the cubic rocksalt islands are (111) oriented to the c-axis of the hexagonal wurtzite GaN matrix. Cross-sectional scanning transmission electron microscopy allows for the study of island formation, which occurs after 1.2 monolayers of GdN coverage, forming discrete particles, which the GaN matrix grows epitaxially around. Magnetometry reveals two ferromagnetic phases, one due to the GdN particles with Curie temperature of 70K and an anomalous phase with ferromagnetism persistent to room temperature. This room temperature ferromagnetic phase is strongly anisotropic, with out of plane magnetization nearly 300{\%} larger than in-plane at fields less than 1T. Optical characterization reveals that GdN, when strained to GaN, is a semiconductor with direct and indirect bandgaps at 1.2eV and 0.75eV, respectively.   

Synthesis and Characterization of Co-doped ZnO Dilute Magnetic Semiconducting Nanorods

Transition-metal doped ZnO dilute magnetic semiconducting nanomaterials are considered as ideal systems for carrying out research in the field of spintronics as they can successfully combine magnetism and electronics in a single substance. ZnO is a wurtzite-type wide-bandgap semiconductor of the II-VI semiconductor group with band gap energy of 3.37 eV. Hydrothermal synthesis of undoped ZnO and Co-doped ZnO nanorods is carried out using aqueous solutions of Zn(NO$_{3})_{2}$.6H$_{2}$O, Co(C$_{2}$H$_{3}$OO)$_{2}$.4 H$_{2}$O, and using NH$_{4}$OH as hydrolytic catalyst. Nanomaterials of different sizes and shapes were synthesized by varying the process parameters such as molarity (0.15M, 0.3M, 0.5M) and pH (8-11) of the precursors, growth temperature (130$^{\circ}$C), and annealing time during the hydrothermal Process. Structural, morphological, optical and magnetic properties are studied using various techniques such as XRD, SEM, UV-vis spectroscopy, and SQUID magnetometer. XRD and SEM studies reveal nanorods with hexagonal wurtzite structure with length in the range of 200 to 500 nm, and cross section in the range of 30 to 60 nm. Detailed structural, optical, and magnetic properties will be discussed in this presentation.   

Defect-Induced High-$T_{C}$ Ferromagnetism and Spinodal Nanodecomposition in MgO

Based on a first-principles study of the magnetic properties of MgO with Mg vacancies [1,2], we calculated the electronic structures and exchange coupling constants for Monte Carlo Simulation (MCS) of the Curie temperature (\textit{Tc}). We also performed MCS of the spinodal nanodecomposition based on the calculated chemical pair interactions between the vacancies. In this study, it was found that hole-doping by Mg vacancies leads to a ferromagnetic ground state, invoking long-range magnetic interaction, and that \textit{Tc} can reach room temperature at sufficient vacancy concentrations of 15 at.{\%} under a homogeneously distributed condition. However, it was also found that the chemical pair interactions between vacancies are significantly attractive and that the system can form super-paramagnetic clusters of vacancies with strong ferromagnetic coupling in the clusters. These results suggest that, by the spinodal nanodecomposition, the \textit{Tc} or blocking temperature ($T_{B})$ can be enhanced and reach room temperature at smaller vacancy concentrations compared with those estimated for room-temperature ferromagnetism under the homogeneous distribution condition.\\[4pt] [1] M. Seike, et al. Jpn. J. Appl. Phys. 50, 090204 (2011).\\[0pt] [2] K. Sato, et al. Rev. Mod. Phys. 82, 1633 (2010).   

Gd in GaN: the role of magnetic vacancy clusters

Five years after the experimental discovery of colossal magnetic moments and ferromagnetic ordering above room temperature in gadolinium doped gallium nitride the identification of its microscopic origin is still not accomplished. Here, we are proposing a new model explaining the origin: the clustering of magnetic gallium vacancies. First, we show that such clustered gallium vacancies indeed can preferentially occur by utilizing a simplified growth model, for which we provide the input by large-scale first-principles Green function calculations. The same calculations reveal that the dangling bond nitrogen states around gallium vacancies become significantly spin-polarized. Moreover, we are able to extract a rich set of information on the magnetic exchange interactions between those spin-polarized atoms. These exchange interactions are the basis for our study on the thermal behavior of magnetic vacancy clusters by means of Monte-Carlo simulations. We present the resulting magnetic properties of our simulations and highlight important similarities to the experiment that all point at gallium vacancy clusters as the origin of the experimentally observed magnetic properties in GaN:Gd.   

Photoluminescence studies of various magnetic phases in (Zn,Mn)Te/ZnSe QDs

We have carried out a magneto-PL study of a single layer of (Zn,Mn)Te/ZnSe quantum dots. The CW PL from a previously studied 5 layer sample exhibits high circular polarization but very small Zeeman splitting. Time-resolved PL exhibited a temperature independent temporal red shift, associated with the magnetic polaron formation. These data have been interpreted as due to the presence of a antiferromagnetic phase for the Mn ions. In the present study, the CW magneto-PL spectra exhibit high circular polarization, but significantly larger Zeeman splittings. Furthermore the Zeeman splitting depends strongly on temperature and vanishes at 50 K. These results indicate the presence of a different magnetic phase for the Mn ions in the single layer QD sample. Time-resolved PL experiments suggest the formation of magnetic polarons in this sample.   

Controlling Curie Temperature of (GaMn)As Through Location of the Fermi Level Within the Impurity Band

This talk will address the unresolved issue of ferromagnetism in the ferromagnetic semiconductor GaMnAs, a material whose understanding is of central importance to semiconductor-based spintronics. The above unresolved issue concerns the question of whether the ferromagnetic order in GaMnAs is mediated by valence band holes or by holes residing in the impurity band. The results to be presented are based on the investigation of a wide range of GaMnAs samples using a comprehensive set of experiments that include magnetization, electrical transport and magneto-optics, along with studies of microscopic composition by channeling Rutherford back-scattering and particle-induced x-ray emission. These experiments show unambiguously that the holes underlying ferromagnetic order in GaMnAs reside in the impurity band; and that it is not only the concentration of Mn and of holes, but also the specific location of the Fermi level in the impurity band that establishes the Curie temperature of this material. Specifically, we show that having the Fermi level near the middle of the impurity band, where the states are most extended, is more important for raising Tc than increases in the effective Mn or hole concentrations. Since the location of the Fermi level can be controlled by a variety of means both during and after growth, this new understanding automatically provides practical guidelines for increasing the critical temperature.   

Interstitial Cu Codoping Method for High Curie Temperature (Ga,Mn)As

Based on first principles calculations, we propose a solubility control method of magnetic impurities in dilute magnetic semiconductors (DMSs). We show that donor atoms, such as Cu, introduced at the interstitial sites in GaAs enhance the solubility of Mn. As a result, Mn can be doped to more than 20 percent in GaAs in the thermal equilibrium condition [1]. Due to the carrier-induced nature of the ferromagnetism in DMSs, the ferromagnetism is suppressed because of the compensation of hole from Mn acceptors by the codoped interstitial Li or Cu. In order to recover the ferromagnetism, we propose low temperature annealing after crystal growth to remove only the interstitials. Our NEB(Nudged Elastic Band method) calculation results show that the effective migration barrier of Cu in GaMnAs is about 0.2eV. This value is small compared with the migration barrier of Li in GaMnAs (about 0.5eV). Even if Li, it is possible to diffuse Li in (Ga$_{0.7}$,Mn$_{0.3}$)As at 0.12 micron in 24 hours [2]. In case of Cu, therefore, we can expect further annealing distance than Li case because of the low migration barrier.\\[4pt][1] H. Fujii, et al.:Appl. Phys. Express 4 (2011) 043003.\\[0pt] [2] L. Bergqvist, K. Sato: Phys. Rev. B 83, 165201 (2011)   

Tight binding study of single ion magnetic anisotropy of $\rm{Mn^{2+}}$ in Ga(Mn)As

Bulk uni-axial magnetic anisotropy of Ga(Mn)As observed in experiments has not been well understood as much as cubic magnetic anisotropy in the same material. We propose that the uni-axial anisotropy arises due to the coupling of local lattice distortions around $\rm{Mn^{2+}}$ impurity ion to its spin state through spin-orbit coupling of holes bound to the impurity ion. We model the coupling using nearest-neighbor tight-binding and many-body perturbation theory. The model includes intra-atomic Coulomb interaction inside $\rm{Mn^{2+}}$ ion, spin-orbit interaction of holes at the $\Gamma$ point, $p-d$ hopping interaction between $\rm{Mn^{2+}}$ ion $d$ orbitals and As ion $p$ orbitals, and strain due to local lattice distortions. We observe breaking of tetrahedral symmetry around the $\rm{Mn^{2+}}$ ion when the system is paramagnetic. We explore the effect of this broken symmetry in stabilizing certain magnetization directions through spin-orbit coupling in the ferromagnetic regime.   

Infrared to visible magneto-optical effects in (Ga,Mn)As

(Ga,Mn)As is perceived as a model material for future low-power spintronics devices due to its carrier mediated ferromagnetic properties. Despite the extensive theoretical and experimental studies, the energy band structure and the mechanism of ferromagnetic order (of Mn spins) still remains a matter of controversy [Ohya, Nature Physics 2011; Jungwirth, PRL 2010]. In our study, we employ magneto-optical Faraday and Kerr measurements to probe the character of the states near the Fermi energy, which is expected to be responsible for Mn-ordering. We also study the spectral dependence of magnetic linear dichroism that is mainly sensitive to the states mediating the Mn-Mn interaction [Kimel, PRL 2005]. The measurements are performed from the infrared to visible range (100~-- 2600~meV) on high quality samples with different Mn concentration (1.5 -- 14~{\%}) with Curie temperatures up to 190~K. The results are compared with the predictions of $k.p$ theory with antiferromagnetic $p-d$ exchange. We acknowledge the financial support provided by NSF-DMR1006078 and Faculty of Mathematics and Physics, Charles University in Prague.   

Spin-Transfer-Torque Driven Domain Wall Motion in (Ga,Mn)(As,P)

Precise control of domain wall (DW) motion in magnetic materials is a prerequisite for the realization of novel non-volatile and down-scalable logic/memory devices which promise to overcome the limitations of current technologies. While magnetic fields are the obvious choice for DW manipulation, in spin-orbit (SO) coupled materials, electric fields provide an additional means of control via current-induced spin torque. We extend the existing theoretical framework used to describe magnetization dynamics in uniform ferromagnets (FM) to dilute FM semiconductors. Analogous to the study of homogeneous systems, we compute the current-induced internal fields (CIF) corresponding to the spin torques and perform a quantitative analysis of the effect of CIFs on DW motion by solving the phenomenological Landau-Lifshitz-Gilbert equations. Microscopic calculations based on an accurate description of the SO coupling effects are used to estimate the observed anisotropies. Our theoretical efforts are complemented by experimental studies in the SO coupled FM (Ga,Mn)(As,P).   

Magnetic properties of Ga1-xMnxAs/Ge heterojunctions

It has been shown that the incorporation of Mn in substitutional and interstitial positions of GaAs is linked to the Fermi level of GaMnAs during the growth [1]. Additionally, experiments with samples that are doped with either Be or Si reveal the role of the Fermi level in determining Tc [2]. To further investigate this effect, we deposited Ge in varying thicknesses on top of GaMnAs layers. Germanium is very nearly lattice matched to GaMnAs, and the valence band offset of the two materials ($\sim$0.54 eV) places the top of the valence band as well as the Mn acceptor level of GaMnAs significantly below the top of the valence band of Ge. Thus, when Ge is grown on GaMnAs, the incorporation of Mn has already been fixed during its growth, but the holes are drained off into Ge. SQUID measurements on these samples show that the Tc of the GaMnAs drops very rapidly when layers of Ge are deposited over it, the decrease in Tc scaling roughly with the thickness of the Ge layers. This behavior is consistent with the expected ``draining away'' of holes from the GaMnAs layer into the Ge. Results of our efforts to fine-tune the amount of holes removed from GaMnAs by Ge will be presented.\\[4pt] [1] Wojtowicz et al., Physica E 25, 171 (2004).\\[0pt] [2] Cho et al., JAP 103, 07D132 (2008); APL 93, 262505 (2008).   

Spin-dependent Transport in GaAs/MnAs Core/shell Nanowires

Hybrid GaAs/MnAs core/shell nanowires synthesized by molecular beam epitaxy [APL {\bf 97}, 072505 (2010)] are of potential interest for proof-of-concept room temperature nanospintronics applications. Magnetic order in these nanostructures is directly influenced by a unique competition between the magnetocrystalline and shape anisotropies in MnAs. We report four probe measurements of the temperature dependence of the resistivity and the anisotropic magnetoresistance (AMR) in single nanowires over a temperature range 1 K - 300 K and in magnetic fields ranging up to 80 kOe, applied both parallel and perpendicular to the nanowire axis. We used the measured AMR in conjunction with micromagnetic simulations to reveal the detailed magnetization reversal process in the MnAs nanoshell. The micromagnetic simulations also provide insights into interesting structures for spin engineering at the nanoscale. Supported by NSF-MRSEC and ONR.