arXiv subjects

N. Samarth

Publications and source records attributed to N. Samarth.

At least 37 records · Page 2Linked to original sources

arXiv2010

Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet/Semiconductor Heterostructures

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically-injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations exceeding 4 pi radians during transport.

cond-mat.mes-hall↗

arXiv2010

Localization and the Anomalous Hall Effect in a "Dirty" Metallic Ferromagnet

We report magnetoresistance measurements over an extensive temperature range (0.1 K $\leq T \leq$ 100 K) in a disordered ferromagnetic semiconductor (\gma). The study focuses on a series of metallic \gma~ epilayers that lie in the vicinity of the metal-insulator transition ($k_F l_e\sim 1$). At low temperatures ($T < 4$ K), we first confirm the results of earlier studies that the longitudinal conductivity shows a $T^{1/3}$ dependence, consistent with quantum corrections from carrier localization in a ``dirty'' metal. In addition, we find that the anomalous Hall conductivity exhibits universal behavior in this temperature range, with no pronounced quantum corrections. We argue that observed scaling relationship between the low temperature longitudinal and transverse resistivity, taken in conjunction with the absence of quantum corrections to the anomalous Hall conductivity, is consistent with the side-jump mechanism for the anomalous Hall effect. In contrast, at high temperatures ($T \gtrsim 4$ K), neither the longitudinal nor the anomalous Hall conductivity exhibit universal behavior, indicating the dominance of inelastic scattering contributions down to liquid helium temperatures.

cond-mat.mtrl-sci↗

arXiv2009

Interlayer and interfacial exchange coupling in ferromagnetic metal/semiconductor heterostructures

We describe a systematic study of the exchange coupling between a magnetically hard metallic ferromagnet (MnAs) and a magnetically soft ferromagnetic semiconductor (GaMnAs) in bilayer and trilayer heterostructures. An exchange spring model of MnAs/GaMnAs bilayers accounts for the variation of the exchange bias field with layer thickness and composition. We also present evidence for hole-mediated interlayer exchange coupling in MnAs/p-GaAs/\GaMnAs trilayers and study the dependence of the exchange bias field on the thickness of the spacer layer.

cond-mat.mtrl-sci↗

arXiv2008

Quasi-reversible Magnetoresistance in Exchange Spring Tunnel Junctions

We report a large, quasi-reversible tunnel magnetoresistance in exchange-biased ferromagnetic semiconductor tunnel junctions wherein a soft ferromagnetic semiconductor (\gma) is exchange coupled to a hard ferromagnetic metal (MnAs). Our observations are consistent with the formation of a region of inhomogeneous magnetization (an "exchange spring") within the biased \gma layer. The distinctive tunneling anisotropic magnetoresistance of \gma produces a pronounced sensitivity of the magnetoresistance to the state of the exchange spring.

cond-mat.mtrl-sci↗

arXiv2008

Scaling analysis of the magnetoresistance in Ga_{1-x}Mn_xAs

We compare experimental resistivity data on Ga_{1-x}Mn_xAs films with theoretical calculations using a scaling theory for strongly disordered ferromagnets. All characteristic features of the temperature dependence of the resistivity can be quantitatively understood through this approach as originating from the close vicinity of the metal-insulator transition. In particular, we find that the magnetic field induced changes in resistance cannot be explained within a mean-field treatment of the magnetic state, and that accounting for thermal fluctuations is crucial for a quantitative analysis. Similarly, while the non-interacting scaling theory is in reasonable agreement with the data, we find clear evidence in favor of interaction effects at low temperatures.

cond-mat.mtrl-sci↗

arXiv2007

Spin Valve Effect in Self-exchange Biased Ferromagnetic Metal/Semiconductor Bilayers

We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs/(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, re- sulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs/p- GaAs/(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.

cond-mat.mtrl-sci↗

arXiv2007

Random telegraph noise from magnetic nanoclusters in the ferromagnetic semiconductor (Ga,Mn)As

Measurements of the low frequency electrical noise in the ferromagnetic semiconductor (Ga,Mn)As reveal an enhanced integrated noise at low temperature. For moderate localization, we find a 1/f normalized power spectrum density over the entire range of temperatures studied (4.2K < T < 70K). However, for stronger localization and a high density of Mn interstitials, we observe Lorentzian noise spectra accompanied by random telegraph noise. Magnetic field dependence and annealing studies suggest that interstitial Mn defects couple with substitutional Mn atoms to form nanoscale magnetic clusters characterized by a net moment of about 20 Bohr magnetons whose fluctuations modulate hole transport.

cond-mat.mtrl-sci↗

arXiv2007

Onset of ferromagnetism in low-doped GaMnAs

We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of GaMnAs and compare with experimental measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originates from disorder in the Mn distribution as well as the presence of As antisite defects. The experimentally-determined hopping energy at the Curie temperature is roughly constant over a series of samples whose conductivities vary more than 10^4 and whose hole concentrations vary more than 10^2. Thus in this regime the hopping energy is an excellent predictor of the Curie temperature for a sample, in agreement with the theory.

cond-mat.mtrl-sci↗

arXiv2007

The Influence of Magnetic Domain Walls on Longitudinal and Transverse Magnetoresistance in Tensile Strained (Ga,Mn)As Epilayers

We present a theoretical analysis of recent experimental measurements of magnetoresistance in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. The model reproduces the field-antisymmetric anomalies observed in the longitudinal magnetoresistance in the planar geometry (magnetic field in the epilayer plane and parallel to the current density), as well as the unusual shape of the accompanying transverse magnetoresistance. The magnetoresistance characteristics are attributed to circulating currents created by the presence of magnetic domain walls.

cond-mat.mtrl-sci↗

arXiv2007

Non-collinear Spin Valve Effect in Ferromagnetic Semiconductor Trilayers

We report the observation of the spin valve effect in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayer devices. Magnetoresistance measurements carried out in the current in plane geometry reveal positive magnetoresistance peaks when the two ferromagnetic layers are magnetized orthogonal to each other. Measurements carried out for different post-growth annealing conditions and spacer layer thickness suggest that the positive magnetoresistance peaks originate in a noncollinear spin valve effect due to spin-dependent scattering that is believed to occur primarily at interfaces.

cond-mat.mtrl-sci↗

arXiv2006

Tuning alloy disorder in diluted magnetic semiconductors in high fields to 89 T

Alloy disorder in II-VI diluted magnetic semiconductors (DMS) is typically reduced when the local magnetic spins align in an applied magnetic field. An important and untested expectation of current models of alloy disorder, however, is that alloy fluctuations in many DMS compounds should increase again in very large magnetic fields of order 100 tesla. Here we measure the disorder potential in a Zn$_{.70}$Cd$_{.22}$Mn$_{.08}$Se quantum well via the low temperature photoluminescence linewidth, using a new magnet system to 89 T. Above 70 T, the linewidth is observed to increase again, in accord with a simple model of alloy disorder.

cond-mat.mtrl-sci↗

arXiv2006

Internal magnetic fields in thin ZnSe epilayers

Strain induced spin-splitting is observed and characterized using pump-probe Kerr rotation spectroscopy in n-ZnSe epilayers grown on GaAs substrates. The spin-splitting energies are mapped out as a function of pump-probe separation, applied voltage, and temperature in a series of samples of varying epilayer thicknesses and compressive strain arising from epilayer-substrate lattice mismatch. The strain is independently quantified using photoluminescence and x-ray diffraction measurements. We observe that the magnitude of the spin splitting increases with applied voltage and temperature, and is highly crystal direction dependent, vanishing along [1 1-bar 0].

cond-mat.mtrl-sci↗

arXiv2006

Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor

The band structure of a prototypical dilute ferromagnetic semiconductor, Ga$_{1-x}$Mn$_{x}$As, is studied across the phase diagram via optical spectroscopy. We prove that the Fermi energy ($E_{F}$) resides in a Mn induced impurity band (IB). This conclusion is based upon careful analysis of the frequency and temperature dependence of the optical conductivity ($σ_{1}(ω,T)$). From our analysis of $σ_{1}(ω,T)$ we infer a large effective mass ($m^*$) of the carriers, supporting the view that conduction occurs in an IB. Our results also provide useful insights into the transport properties of Mn-doped GaAs.

cond-mat.str-el↗

arXiv2006

Current-Induced Polarization and the Spin Hall Effect at Room Temperature

Electrically-induced electron spin polarization is imaged in n-type ZnSe epilayers using Kerr rotation spectroscopy. Despite no evidence for an electrically-induced internal magnetic field, current-induced in-plane spin polarization is observed with characteristic spin lifetimes that decrease with doping density. The spin Hall effect is also observed, indicated by an electrically-induced out-of-plane spin polarization with opposite sign for spins accumulating on opposite edges of the sample. The spin Hall conductivity is estimated as 3 +/- 1.5 Ohms**-1 m**-1/|e| at 20 K, which is consistent with the extrinsic mechanism. Both the current-induced spin polarization and the spin Hall effect are observed at temperatures from 10 K to 295 K.

cond-mat.mes-hall↗

arXiv2006

Antisite effect on ferromagnetism in (Ga,Mn)As

We study the Curie temperature and hole density of (Ga,Mn)As while systematically varying the As-antisite density. Hole compensation by As-antisites limits the Curie temperature and can completely quench long-range ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by molecular beam epitaxy without substrate rotation in order to smoothly vary the As to Ga flux ratio across a single wafer. This technique allows for a systematic study of the effect of As stoichiometry on the structural, electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than 1.5% Mn, a strong deviation from Tc ~ p^0.33 is observed. Our results emphasize that proper control of As-antisite compensation is critical for controlling the Curie temperatures in (Ga,Mn)As at the low doping limit.

cond-mat.mtrl-sci↗

arXiv2006

Artificial "spin ice" in a geometrically frustrated lattice of nanoscale ferromagnetic islands

We report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the lowtemperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

cond-mat.dis-nn↗

arXiv2005

A Tunable Anomalous Hall Effect in a Non-Ferromagnetic System

We measure the low-field Hall resistivity of a magnetically-doped two-dimensional electron gas as a function of temperature and electrically-gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the paramagnetic polarization of the sample, in analogy to the ferromagnetic anomalous Hall effect. The data are consistent with skew-scattering of carriers by disorder near the crossover to localization.

cond-mat.mes-hall↗

arXiv2005

Enhancement of Spin Coherence using Q-factor Engineering in Semiconductor Microdisk Lasers

Semiconductor microcavities offer unique means of controlling light-matter interactions, which have led to the development of a wide range of applications in optical communications and inspired proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities - a new and promising research field - have revealed novel effects such as polarization beats, stimulated spin scattering, and giant Faraday rotation. Here, we study the electron spin dynamics in optically-pumped GaAs microdisk lasers with quantum wells (QWs) and interface-fluctuation quantum dots (QDs) in the active region. In particular, we address the question of how the electron spin dynamics are modified by the stimulated emission in the disks, and observe an enhancement of the spin lifetime when the optical excitation is in resonance with a high quality (Q ~ 5000) lasing mode. This resonant enhancement, contrary to what is expected from the Purcell effect observed in the cavities, is then manipulated by altering the cavity design and dimensions.

cond-mat.mtrl-sci↗