Session G19: Focus Session: III-V Magnetic Semiconductors I

Prospects for high temperature ferromagnetism in (Ga,Mn)As semiconductors

We report on a comprehensive combined experimental and theoretical study of Courie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that $T_{c}$ in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn$_{Ga}$ acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10{\%} concentration of Mn$_{Ga}$. Based on the analysis of magnetotransport and magnetization data we find no fundamental obstacle to substitutional Mn$_{Ga}$ doping in high-quality materials beyond our current maximum level of 6.8{\%}, although this achievement will require further advances in growth control. Modest charge compensation does not limit the maximum $T_{c}$ possible in ferromagnetic semiconductors based on (Ga,Mn)As. Ref: Jungwirth et al. Phys. Rev. B 72, 165204 (2005).   

Spin and Charge Relaxations in Magnetic and Non-magnetic Narrow-gap Semiconductors

In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. Narrow gap semiconductors offer several scientifically unique features such as a small effective mass, a large g-factor, a high intrinsic mobility, and large spin-orbit coupling effects. We will discuss our recent time resolved studies on InSb quantum wells with symmetric and asymmetric confinement potentials and InMnSb ferromagnetic structures with different Mn contents. We have measured the carrier and spin dynamics by degenerate and non-degenerate pump-probe and Kerr effect spectroscopy at different temperatures and laser intensities in these unique structures with strong spin-orbit interaction. Supported by NSF-DMR-0507866, Jeffress-J748, NSF-DMR02-45227, NSF Grant No. DMR-0510056 and DMR-0520550   

Spin Waves in GaMnAs/GaAs Superlattices

We report on the spin-wave behavior of GaMnAs/GaAs superlattices as a function of the thickness of the nonmagnetic layer. Spin-wave frequencies were determined from time-domain magnetic Kerr measurements using a standard pump-probe setup. We studied two superlattices grown by molecular beam epitaxy on GaAs (001) substrates and, for comparison purposes, a single 120-nm-thick Ga$_{0.955}$Mn$_{0.045}$As layer. The superlattices consist of 6 repetitions of 20-nm-thick Ga$_{0.955}$Mn$_{0.045}$As layers separated by either $d~$=~3 or 6~nm of GaAs. For magnetic fields $H \quad <$~0.22 T, applied along the magnetic easy axis [100], the thin film and the superlattices exhibit a nearly $d$-independent mode of frequency \textit{$\nu $ }$\cong $ 5 + 33\textit{${\rm H}$} (GHz), whereas the sample with $d~$=~6~nm shows a second oscillation at \textit{$\nu $ }$\cong $ 7 + 33\textit{${\rm H}$} (GHz), which we assign to a higher-order confined spin-wave. The existence of this mode is tentatively ascribed to a reduction in the coupling between neighboring magnetic layers.   

Measurement of the $s$-$d$ exchange coupling in GaMnAs quantum wells

The discovery of ferromagnetism in zinc-blende III-V Mn-based compounds and the realization that this collective magnetic behavior is mediated by delocalized or weakly localized holes has given a technological impetus for developing a clear picture of the carrier-shell exchange couplings. Surprising results regarding the magnitude and sign of the exchange between $s$-like conduction band electrons and electrons localized in the $d$-shell of the Mn$^{2+}$ impurities point to deficiencies in the current theory which has successfully described experimentsin II-VI DMS within a higher regime of doping \footnote{R. C. Myers, M. Poggio, N. P. Stern, A. C. Gossard, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf {95}, 017204 (2005); M. Poggio, R. C. Myers, N. P. Stern. A. C. Gossard, and D. D. Awschalom, \textit{cond-mat/0503521}.}. Measurements of coherent electron spin dynamics in Ga$_{1-x} $Mn$_{x}$As/Al$_{0.4}$Ga$_{0.6}$As quantum wells with $0.003\% < x < 0.2\%$ show an antiferromagnetic (negative) $s$-$d$ exchange coupling, $N_0 \alpha$. The magnitude of $N_{0} \alpha$ varies as a function of width of the quantum well, which is consistent with a large and negative contribution due to kinetic exchange. MBE growth conditions are optimized in order to produce GaMnAs with low defect densities in which coherent spin dynamics can be observed optically, while at the same time incorporating enough Mn to make the $s$-$d$ coupling observable. Polarization-resolved photoluminescence and reflectance measurements reveal the effects of $p$-$d$ exchange between carriers in the valance band and spins localized on Mn sites. Light Mn doping is also seen to maximize the carrier electron spin lifetime, indicating the importance of the Dyakonov-Perel decoherence mechanism in these structures.   

Confinement engineering of s-d exchange interactions in GaMnAs quantum wells

Recent measurements of coherent electron spin dynamics have observed antiferromagnetic $s$-$d$ exchange coupling between conduction band electrons and electrons localized on Mn$^{2+}$ impurities in GaMnAs quantum wells \footnote{R. C. Myers, M. Poggio, N. P. Stern, A. C. Gossard, and D. D. Awschalom, \textit {Phys. Rev. Lett.} \textbf{95}, 017204 (2005).}. Here we discuss systematic measurements of the $s$-$d$ exchange interaction in Ga$_{1-x}$Mn$_{x}$As/Al$_{y}$Ga$_{1-y}$As quantum wells with different confinement potentials using time- resolved Kerr rotation spectroscopy. Extending previous investigations of the well width dependence of the $s$-$d$ exchange, $N_{0} \alpha$, we find that the magnitude of the exchange parameter, $N_{0} \alpha$, varies as a function of both well width and well depth ($y$). Both phenomena reduce to a general dependence on confinement energy, which is well-fit to a model taking into account the effect of kinetic exchange and band mixing on the exchange parameters.   

Controlled placement of Mn acceptors in GaAs (110) surfaces: Crystal orientation and spacing dependences for interactions between Mn acceptors

We report a controlled incorporation process substituting single Mn atoms into acceptor sites in GaAs (110) surfaces using a low temperature scanning tunneling microscope (STM). The incorporation process brings a Ga atom to the surface as an adsorbate loosely bound to the Mn that has replaced it in the lattice. Displacement of a Ga atom away from the substituted Mn via STM manipulation isolates the Mn atom in the surface layer acceptor configuration. In this acceptor configuration Mn atoms give rise to strong in-gap energy levels, with unusual and detailed spatial structure of highly anisotropic character. Modifications to this in-gap resonance level and its structure occur under controlled placement of two closely spaced Mn acceptors, revealing a striking dependence upon crystal orientation and spacing. For certain pair orientations, strong bonding/antibonding-like interactions split the in-gap level of the single Mn state, while the in-gap levels of other pairs of comparable spacing but different orientation are degenerate with the isolated Mn state.   

Magnetic and Chemical Depth Profiles of an Exchange-Biased MnO/GaMnAs Bilayer

There is a great deal of interest in development of high Curie temperature (Tc) ferromagnetic (FM) semiconductors for use in spintronic applications. GaMnAs may be a candidate for such applications, as it shows true long range FM order among Mn at Ga sites, and has a relatively high Tc (up to 173 K). For many device applications, the ability to pin the magnetization of a FM semiconductor to a preferred direction is highly desirable. This can be achieved by exchange coupling a GaMnAs layer to an adjacent antiferromagnetic (AFM) layer in order to produce an exchange-bias in the FM layer. We report on polarized neutron reflectivity and x-ray reflectivity studies of a GaMnAs film exchange biased by an AFM MnO overlayer. Using the depth sensitivity of these reflectivity techniques, we have chemically and magnetically characterized the MnO/GaMnAs interface, and have investigated the magnetization reversal process by comparing the magnetic depth profile at the coercive field to the profile at magnetic saturation.   

Asymmetric Planar Hall Effect in (Ga,Mn)As/MnO Exchange-Biased Structures

We report the observation of asymmetric Planar Hall Effect (PHE) in a series of exchange-biased (EB) (Ga,Mn)As/MnO ferromagnetic(FM)/antiferromagnetic(AFM) bilayer structures after field cooling. The process of magnetization reversal was systematically studied by PHE measurements by varying the direction of the in-plane applied magnetic field. The analysis of experiment results based on the Stoner-Wohlfarth model (including domain nucleation and expansion in the FM layer) revealed that two magnetization reversal mechanisms are present in these EB systems due to the exchange coupling between MnO and (Ga,Mn)As: the magnetization reversal undergoes either a full circle or a half circle, depending on the direction of the applied field relative to the cooling field. Our model is confirmed by the close agreement between experimental data and theoretical predictions for the angular dependence of the exchange bias and of the coercive field. Supported by NSF Grant DMR02-45227.   

Non-collinear giant magnetoresistance in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayers

Giant magnetoresistance (GMR) due to spin-dependent scattering has been extensively studied in ferromagnetic metal multilayers. However, the effect has eluded observation in ferromagnetic semiconductor multilayers where other sources of magnetoresistance (MR) are usually dominant. Here, we report the possible observation of GMR in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayers. Samples are designed such that the (Ga,Mn)As layers have distinct coercive fields and the p-GaAs spacer layer has a conductivity comparable to the (Ga,Mn)As layers. We measure longitudinal and transverse MR using Hall bars oriented along different crystalline directions and observe signatures consistent with GMR when the relative magnetization orientation in the two (Ga,Mn)As layers switches between collinear and non-collinear arrangements. Our measurements show that these signatures are distinct from other known sources of MR and further suggest that the observed GMR is associated with spin-dependent scattering at interfaces. Supported by DARPA, ONR and NSF.   

Magnetic anisotropy of (Ga,Mn)As formed by Mn ion implantation and pulsed-laser melting

We report on the magnetic anisotropy of (Ga,Mn)As formed by Mn ion implantation followed by pulsed-laser melting. Three specimens with different Mn doses have been systematically studied by SQUID magnetometry, magnetotransport and ferromagnetic resonance. The sample corresponding to the lowest Mn dose does not show any measurable magnetic anisotropy. With increasing Mn doses, however, complex magnetic anisotropy behavior clearly emerges. In particular, the high-Mn-dose samples show unambiguous in-plane uniaxial and cubic anisotropy, similar to that observed in (Ga,Mn)As layers grown by LT-MBE. As these samples were produced by a different processing method, this result lends credence to the hypothesis that the in-plane cubic symmetry breaking is intrinsic to the dilute (Ga,Mn)As phase. Furthermore, in contrast to LT-MBE grown (Ga,Mn)As, these specimens show small perpendicular uniaxial anisotropy, which can be interpreted by a small built-in compressive strain due to the absence of Mn interstitials characteristic of these specimens. Finally, we also observe that a super-paramagnetic phase with distinct perpendicular magnetic anisotropy exists in these specimens. An investigation into the origin of this second magnetic phase is in progress.   

Gating in ferromagnetic semiconductor

Ferromagnetic semiconductors have the potential of revolutionizing the way current electronic devices work: more so, because they are compatible with current fabrication lines and can easily be integrated with today technology. Particular interest lies in III-V Diluted Magnetic Semiconductor (DMS), where the ferromagnetism is hole-mediated and the Curie temperature can therefore be tuned by changing the concentration of free carriers\footnote{T. Dietl \textit{et al.}, Phys. Rev. B \textbf{63}, 195205 (2001)}. In these systems, most of the effort is currently applied toward the fabrication of devices working at room-temperature: this implies high carrier density accompanied by low mobility and short mean free path. We will report our results in exploring devices with low hole concentration and Curie temperature ($\sim$ 4 K) and we will discuss the effect of local gating\footnote{H. Ohno \textit{et al.}, Nature \textbf{408}, 944 (2000)} in light of possible applications to the fabrication of ferromagnetic quantum dots.   

Trapping and manipulating magnetic domain walls in patterned (Ga,Mn)As devices

We report magnetoresistance measurements that probe the trapping and manipulation of magnetic domain walls (DWs) in multi-element (Ga,Mn)As devices that include both vertically patterned (step-etched) microstructures [Yamanouchi et al, Nature 428, 539 (2004)] and laterally-patterned nanostructures. We exploit post-fabrication annealing to yield devices in which different elements have distinct switching fields, thus allowing us to establish stable antiparallel magnetization states separated by single DWs. We then use current pulses to manipulate DWs in these devices, and observe current-induced DW motion in some sample geometries. We describe ongoing efforts to map out the variation of electrically-driven DW motion with factors such as current amplitude, sample temperature and sample geometry. Supported by DARPA/ONR.   

2D patterned ferromagnetic semiconductors for planar spintronics

Fabrication of III-Mn-V ferromagnetic semiconductors by way of Mn ion implantation followed by pulsed laser melting (II-PLM) allows for the unique possibility of lateral patterning using optical and e-beam lithography. We have performed II-PLM on resist patterned substrates to create isolated ferromagnetically active Ga$_{1-x}$Mn$_{x}$As regions embedded in a GaAs substrate. We have prepared a uniform Ga$_{1-x}$Mn$_{x}$As film as well as a sample patterned with an array of Mn$^{+}$-implanted 100$\mu $m x 100$\mu $m squares that covers the equivalent of one-quarter the sample. The saturation magnetization of the patterned sample reflects this one-quarter implant area, and both samples display a T$_{C}$ of $\sim $100 K suggesting a similar maximum Mn concentration. The electrical and magnetic properties of this and other lateral structures consisting of sub-micron spaced active regions will be presented. This work was supported by the U.S. Department of Energy under contract No. DE-AC02-05CH11231.