S. von Molnar

Bio: S. von Molnar is an academic researcher from Florida State University. The author has contributed to research in topics: Magnetic semiconductor & Magnetization. The author has an hindex of 35, co-authored 126 publications receiving 16014 citations. Previous affiliations of S. von Molnar include IBM & Max Planck Society.

Spintronics: a spin-based electronics vision for the future.

Abstract: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

Mixed-valence manganites

Abstract: Mixed-valence manganese oxides (R1-χAχ)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, ...

Diluted magnetic III-V semiconductors

Abstract: A new diluted magnetic III-V semiconductor of ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$As (x\ensuremath{\le}0.18) has been produced by molecular-beam epitaxy. Films grown at 300 \ifmmode^\circ\else\textdegree\fi{}C are predominantly ferromagnetic and their properties suggest the presence of MnAs clusters. Films grown 200 \ifmmode^\circ\else\textdegree\fi{}C, however, are predominantly paramagnetic, and the lattice constant decreases with increasing Mn composition; both are indicative of the formation of a homogeneous alloy. These films have n-type conductivity and reduced band gaps.

Magnetotransport properties of p-type (In,Mn)As diluted magnetic III-V semiconductors.

Abstract: Magnetotransport properties of p-type (In,Mn)As, a new diluted magnetic semiconductor based on a III-V semiconductor, are studied. The interaction between the holes and the Mn 3d spins is manifested in the anomalous Hall effect, which dominates the Hall resistivity from low temperature (0.4 K) to nearly room temperature, and in the formation of partial ferromagnetic order below 7.5 K, which is a cooperative phenomenon related to carrier localization. The coexistence of remanent magnetization and unsaturated spins as well as the large negative magnetoresistance at low temperatures is explained by the formation of large bound magnetic polarons.

Evidence for two-band magnetotransport in half-metallic chromium dioxide

Abstract: Magnetotransport measurements were made on patterned, (110) oriented ${\mathrm{CrO}}_{2}$ thin films grown by the high-pressure, thermal decomposition of ${\mathrm{CrO}}_{3}$ onto rutile substrates. The low-temperature Hall effect exhibits a sign reversal from positive to negative as the magnetic field is increased above 1 T, which may be interpreted within a simple two-band model as indicating the presence of highly mobile $({\ensuremath{\mu}}_{h}=0.25{\mathrm{m}}^{2}/\mathrm{V}\mathrm{}\mathrm{s})$ holes as well as a much larger number of less mobile electrons $(n=0.4$ electrons/Cr). Between 50 and 100 K, the field at which the sign reversal occurs rapidly increases and a contribution from the anomalous Hall effect becomes significant, while the large, positive transverse magnetoresistance (MR) observed at low temperatures changes over to a predominantly negative MR. These changes correlate with a thermally activated dependence in the resistivity of the form ${T}^{2}{e}^{\ensuremath{-}\ensuremath{\Delta}/T}$ with $\ensuremath{\Delta}\ensuremath{\approx}80\mathrm{K},$ reflecting the lack of temperature dependence in the resistivity at low temperatures and a ${T}^{2}$ behavior above 100 K. The high mobilities at low temperature which result in the observed positive MR reflect the suppression of spin-flip scattering expected for a half-metallic system. However, the changes in magnetotransport above the temperature $\ensuremath{\Delta}$ must be due to the onset of spin-flip scattering, even though ${k}_{B}\ensuremath{\Delta}$ is much less than the expected energy gap in the minority spin density of states. The significance of $\ensuremath{\Delta}$ is discussed in terms of recent models for another half-metallic system, the perovskite manganites, and the possible formation of ``shadow bands.''

Spintronics: a spin-based electronics vision for the future.

Abstract: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors

Abstract: Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

Making Nonmagnetic Semiconductors Ferromagnetic

Abstract: REVIEW Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors currently in use for devices. Low solubility of magnetic elements was overcome by low-temperature nonequilibrium molecular beam epitaxial growth, and ferromagnetic (Ga,Mn)As was realized. Magnetotransport measurements revealed that the magnetic transition temperature can be as high as 110 kelvin. The origin of the ferromagnetic interaction is discussed. Multilayer heterostructures including resonant tunneling diodes (RTDs) have also successfully been fabricated. The magnetic coupling between two ferromagnetic (Ga,Mn)As films separated by a nonmagnetic layer indicated the critical role of the holes in the magnetic coupling. The magnetic coupling in all semiconductor ferromagnetic/nonmagnetic layered structures, together with the possibility of spin filtering in RTDs, shows the potential of the present material system for exploring new physics and for developing new functionality toward future electronics.

Electromagnetically induced transparency : Optics in coherent media

Abstract: Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.