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A mononuclear complex [Fe(tBu2qsal)2] has been obtained by a reaction between an Fe(II) precursor salt and a tridentate ligand 2,4-di(tert-butyl)-6-((quinoline-8-ylimino)methyl)phenol (tBu2qsalH) in the presence of triethylamine. The complex exhibits a hysteretic spin transition at 117 K upon cooling and 129 K upon warming, as well as light-induced excited spin-state trapping at lower temperatures. Although the strongly cooperative spin transition suggests substantial intermolecular interactions, the complex is readily sublimable, as evidenced by the growth of its single crystals by sublimation at 573 → 373 K and ∼10-3 mbar. This seemingly antagonistic behavior is explained by the asymmetric coordination environment, in which the tBu substituents and quinoline moieties appear on opposite sides of the complex. As a result, the structure is partitioned in well-defined layers separated by van der Waals interactions between the tBu groups, while the efficient cooperative interactions within the layer are provided by the quinoline-based moieties. The abrupt spin transition is preserved in a 20 nm thin film prepared by sublimation, as evidenced by abrupt and hysteretic changes in the dielectric properties in the temperature range comparable to the one around which the spin transition is observed for the bulk material. The changes in the dielectric response are in excellent agreement with differences in the dielectric tensor of the low-spin and high-spin crystal structures evaluated by density functional theory calculations. The substantially higher volatility of [Fe(tBu2qsal)2], as compared to a similar complex without tBu substituents, suggests that asymmetric molecular shapes offer an efficient design strategy to achieve sublimable complexes with strongly cooperative spin transitions.

With appropriate choice of ligands, Mn3-based single-molecule magnets (SMMs) can be covalently linked to form SMM dimers that exhibit either ferromagnetic (FM) or antiferromagnetic (AFM) ground sta...

We performed room-temperature far-infrared ($40--650\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra show enhanced transmittance after bromine doping and all spectra were fit to a Drude-Lorentz (DL) model. A decreased metallic conductivity along with a redshift of the lowest semiconducting gap was found with doped samples. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of nondispersive reflectance and subsequently compared with DL-model-derived spectra. The free-carrier density of the $n$-type powder and the electronic mean free path were estimated and compared with reported values for single-walled nanotubes and pyrolytic graphite.

We propose a chelation-assisted assembly of multidentate CNs into metal-organic nanoparticles (MONs). Multidentate CNs functionalized with coordination sites participate equally as organic linkers in MON construction, which is driven by chelation between metal ions and coordination sites. MONs assembled from Au nanoparticles display particle number- and size-dependent optical properties. In addition, the resulting CN-assembled MONs give evidence that assembly was dictated by the multidentate surface ligand rather than the size, shape or material of CNs. With this chelation-assisted strategy, it is possible to control the number of assembled CNs and build the connections between them.

Reviews of modern physics 91(3), 030001 (2019). doi:10.1103/RevModPhys.91.030001, Ninety years! As the first spinoff of the Physical Review launched in 1929, Reviews of ModernPhysics (RMP) has tracked the progress of physics during a time of explosive growth of the field.Originally one journal, the Physical Review has not only split into the more specialized journals weknow today, but the scope, breadth, and depth of topics covered could hardly have been imagined byJohn Tate, Sr., the editor in chief who started RMP. Yet RMP is still published quarterly. So whyhasn’t RMP grown at the same prodigious rate? The research that informs these reviews isprogressing faster, but writing a review that is broadly accessible and that will remain relevant fordecades has therefore become more difficult. RMP’s role is to find authors who are up to thischallenge.At the April 2019 editorial board meeting we resolved to reaffirm our mission of providing thecommunity with articles that capture the essence of the field, with introductions and conclusions thatare accessible to students (our most junior colleagues who will carry science into the future) and thatcommunicate how and why the topic has fascinated scientists and provided an arena for newparadigms, theories, and connections between subfields. To this end, we will implement even higherstandards for our submissions: not only will submitted manuscripts be reviewed by experts, they willalso be reviewed by qualified non-experts from nearby subfields to ensure broad accessibility.Though this is, of course, a greater burden on our referees and authors, we believe that it is necessaryto publish “classic” articles and keep RMP vital for many years to come.We look forward to continuing to serve the community., Published by APS, College Park, Md.

Combining single-molecule magnets (SMMs) and emergent two-dimensional substrates such as graphene may lead to device configurations that are promising for spintronics and quantum computing. However, to fully exploit the unique features of SMMs anchored to two-dimensional substrates, the choice of ligand attachments, which could affect the magnetic and electronic properties, is critical. In this work, we focus on hybrid junctions comprising CVD-grown graphene and [Mn 12O 12(O 2CR) 16(H 2O) 4]( R = CH 3 , CHCl 2) SMMs with different ligands. We find that [Mn 12O 12(O 2CCH 3) 16(H 2O) 4] SMMs barely change the graphene’s conductivity, while [Mn 12O 12(O 2CCHCl 2) 16(H 2O) 4] SMMs with more electronegative ligands, by means of charge transfer, remarkably modify the electronic transport in graphene as revealed by gate-voltage dependent magnetotransport measurements.

Thin films of barium titanate (BTO) are controllably grown to produce two different memory applications: ferroelectric Schottky diodes and resistive switching cells. The entire structure and fabrication of the two cells are identical, except for a slight adjustment to the calcination temperature for the BTO films. Films calcined at 800°C were ferroelectric and were used to create the ferroelectric Schottky diode. Conversely, films calcined at 700°C displayed resistive switching behavior and were used to create memristor cells. The ferroelectric Schottky diodes showed an ON/OFF ratio of over three orders of magnitude and the resistive switching cells showed an ON/OFF ratio of nearly two orders of magnitude, though both can likely be improved through electrode optimization. The identification of the critical process parameter shows that both memory cells can be fabricated on the same die with nearly identical processes, which would reduce process complexity for creating multiple advanced memories on a single die.

One of the subtlest phenomena in physics has been subjected to more than a century of experimental and theoretical investigations.

Layered transition metal dichalcogenide (TMD) materials, i.e. 1T-TiSe$_2$ and 2H-NbSe$_2$, harbor a second order charge density wave (CDW) transition where phonons play a key role for the periodic modulations of conduction electron densities and associated lattice distortions. We systematically study the transport and capacitance characteristics over a wide temperature range of Schottky barriers formed by intimately contacting freshly exfoliated flakes of 1T-TiSe$_2$ and 2H-NbSe$_2$ to \textit{n}-type GaAs semiconductor substrates. The extracted temperature-dependent parameters (zero-bias barrier height, ideality and built-in potential) reflect changes at the TMD/GaAs interface induced by CDW formation for both TMD materials. The measured built-in potential reveals chemical-potential shifts relating to CDW formation. With decreasing temperature a peak in the chemical-potential shifts during CDW evolution indicates a competition between electron energy re-distributions and a combination of lattice strain energies and Coulomb interactions. These modulations of chemical potential in CDW systems, such as 1T-TiSe$_2$ and 2H-NbSe$_2$ harboring second-order phase transitions, reflect a corresponding conversion from short to long-range order.

The transition metal dichalcogenide 1T-TaS$_2$ is well known to harbor a rich variety of charge density wave (CDW) distortions which are correlated with underlying lattice atom modulations. The long range CDW phases extend throughout the whole crystal and terminate with charge displacements at the crystal boundaries. Here we report on the transport properties and capacitance characteristics of the interface between freshly exfoliated flakes of 1T-TaS$_2$ in intimate van der Waals contact with \textit{n}-type GaAs substrates. The extracted barrier parameters (ideality, barrier height and built-in potential) experience pronounced changes across the Mott-CDW transition in the 1T-TaS$_2$. The CDW-induced changes in barrier properties are well described by a bond polarization model which upon decreasing temperature gives rise to an increased potential drop across the interfacial region due to the localization of carriers and a decreased dielectric constant., Comment: 17 Pages, 6 figures, Accepted by Applied Physics Letters

We report that the ferromagnetism of highly ordered pyrolytic graphite (HOPG) samples as measured by hysteretic magnetization loops can be diminished and eventually extinguished with sufficiently long high vacuum anneals at temperatures on the order of 2300 °C. Concomitant with the extinction of ferromagnetism, we observe an anneal-induced increase in grain size (accompanied by possible edge reconstruction) confirmed by X-ray diffraction measurements and improved transport properties, including lower in-plane and out-of-plane resistivity, higher electron and hole mobility and improved charge compensation. The implied reduction of defects and vacancies by annealing suggests that the ferromagnetism of pristine HOPG is correlated with localized states located at zigzag edges, vacancies and related defects.

We report on the use of bromine intercalation of graphite to perform in situ tuning of the Schottky barrier height (SBH) formed at many-layer-graphene (MLG) semiconductor interfaces. The intercalation of Br into MLG simultaneously increases interlayer separation between the graphene planes, while at the same time giving rise to an increase (decrease) in the free hole carrier density (Fermi energy) because of the transfer of electrons from carbon to bromine. The associated increase in the graphite work function results in an increase of the SBH, as manifested by lower forward/reverse current densities and higher depletion capacitances. These results are quantitatively understood within the context of the Schottky–Mott model and thermionic emission theory. The presented results have important implications for sensing and high power applications as well as the integration of carbon into semiconductors and carbon/graphene electronics.

Layered electrodes based on graphene or transition metal dichalcogenides have enriched the development of nanoelectronics due to their uniqueness in flexibility, transparency, thermal stability, and electronic structure. Here, we report on resistive switching behavior observed in graphite/Nb:SrTiO 3 (Gr/NbSTO) junctions. Straightforward in situ bromine intercalation of graphite modulates the transport properties of Gr/NbSTO devices, an effect which cannot be achieved using traditional metal electrodes. At low temperatures, the strong electric field dependence of the dielectric constant of NbSTO also plays an important role in further enhancing the resistive switching performance. Our findings here suggest that to optimize the performance and to perform more complex functions, tunability of the Fermi level of the layered graphite electrode in combination with the nonlinear dielectric constant of the NbSTO substrate is critically important for interface-type resistive switching devices.

Well-defined diblock copolymers of bicyclo[2.2.1]hept-5-ene-2-carboxylic acid oxiranylmethyl ester, having both anchoring and steric stabilizing blocks in a 1:1 ratio, have been prepared by ring-opening metathesis polymerization (ROMP). The epoxy-containing block copolymer stabilized in situ generated iron oxide (γ-Fe2O3) nanoparticles. The epoxy ester group provided strong chelation between the iron-oxide nanoparticle and the polymeric siderophores, producing a stable magnetic nanocomposite. The polymers were characterized by 1H NMR, GPC, TGA, and DSC. The morphology and crystalline structure of the maghemite−block copolymer nanocomposites were evaluated with TEM and XRD, revealing highly crystalline, monodisperse γ-Fe2O3 nanoparticles with an average size of 3−5 nm. Interactions between the maghemite nanoparticles and the polymer were observed by FTIR. SQUID magnetometric analysis of the nanocomposites demonstrated superparamagnetism at room temperature with high saturation magnetization.

We have used the “two-step” growth technique, quench condensing followed by an anneal, to grow ultra-thin films of silver on glass substrates. As has been seen with semiconductor substrates this process produces a metastable homogeneous covering of silver. By measuring the in situ resistance of the film during growth we are able to see that the low temperature growth onto substrates held at 100 K produces a precursor phase that is insulating until the film has been annealed. The transformation of the precursor phase into the final, metallic silver film occurs at a characteristic temperature near 150 K where the sample reconstructs. This reconstruction is accompanied by a decrease in resistance of up to 10 orders of magnitude.

Hall effect measurements were performed on epitaxial Co x Ti 1− x O 2– δ thin films grown on (0 0 1) LaAlO 3 by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M – H loop at room temperature for Co x Ti 1− x O 2– δ thin films for which x ⩾0.02. An anomalous Hall effect was observed for Co 0.10 Ti 0.90 O 2− δ films grown with the partial pressure of water P (H 2 O)=4×10 −4 Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti n O 2 n −1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO 2 thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti n O 2 n −1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.

The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy.

Thin films of strongly-correlated electron materials (SCEM) are often grown epitaxially on planar substrates and typically have anisotropic properties that are usually not captured by edge-mounted four-terminal electrical measurements, which are primarily sensitive to in-plane conduction paths. Accordingly, the correlated interactions in the out-of-plane (perpendicular) direction cannot be measured but only inferred. We address this shortcoming and show here an experimental technique in which the SCEM under study, in our case a 600 Angstrom-thick (La1-yPry)0.67Ca0.33MnO3 (LPCMO) film, serves as the base electrode in a metal-insulator-metal (MIM) trilayer capacitor structure. This unconventional arrangement allows for simultaneous determination of colossal magnetoresistance (CMR) associated with dc transport parallel to the film substrate and colossal magnetocapacitance (CMC) associated with ac transport in the perpendicular direction. We distinguish two distinct strain-related direction-dependent insulator-metal (IM) transitions and use Cole-Cole plots to establish a heretofore unobserved collapse of the dielectric response onto a universal scale-invariant power-law dependence over a large range of frequency, temperature and magnetic field., Comment: 32 pages, 4 figures, Supplementary section included, Submitted to Nature Physics

Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches

We report a giant linear magnetoelectric coupling in strained BiMnO3 thin films in which the disorder associated with an islanded morphology gives rise to extrinsic relaxor ferroelectricity that is not present in bulk centrosymmetric ferromagnetic crystalline BiMnO3. Strain associated with the disorder is treated as a local variable which couples to the two ferroic order parameters, magnetization M and polarization P. A straightforward "gas under a piston" thermodynamic treatment explains the observed correlated temperature dependencies of the product of susceptibilities and the magnetoelectric coefficient together with the enhancement of the coupling by the proximity of the ferroic transition temperatures close to the relaxor freezing temperature. Our interpretation is based on a trilinear coupling term in the free energy of the form L(PXM) where L is a hidden antiferromagnetic order parameter, previously postulated by theory for BiMnO3. This phenomenological invariant not only preserves inversion and time reversal symmetry of the strain-induced interactions but also explains the pronounced linear magnetoelectric coupling without using the more conventional higher order biquadratic interaction proportional to (PM)^2., Comment: 10 pages, 6 figures

Recent efforts on doping ZnO films for charge and spin functionality are reviewed, focusing on chemical doping for charge and spin device formation. Discussion includes the behavior of phosphorus as an acceptor and magnetism in transition metal-doped ZnO. Evidence for p-type behavior in phosphorus-doped (Zn,Mg)O grown by pulsed laser deposition is presented. The magnetic properties of ZnO co-doped with Mn and Sn are also discussed.

In this study, we demonstrate the use of bacterial templates of differing morphology for the facile synthesis of magnetic micron-sized spheres, tetrads of spheres, filaments, and coils. These microstructures were synthesized by a platinum catalyzed electroless deposition of nickel under mild aqueous conditions using D. radiodurans, genetically elongated E. coli, and R. rubrum as templates, respectively. Analysis by SEM/EDS, TEM, and SQUID magnetometry indicates the formation of superparamagnetic metallic microfilaments, which possess a blocking temperature of ¨70 K with no observable exchange anisotropy. XPS analysis of the metallic microfilaments supports the presence of NiB, which is diamagnetic, thus supporting the hypothesis that the morphologically unique microbial templates are encapsulated within a biosurface-bound NiB shell that contains embedded nickel nanoparticles (12–20 nm). The use of microorganisms as templates, which may be recombinantly modified, may therefore serve as alternative means for magnetic microstructure and core–shell synthesis.

The microstructural properties of secondary phase particles formed in epitaxial CoxTi1−xO2 anatase thin films grown on (0 0 1)LaAlO3 by a reactive RF magnetron co-sputter deposition are examined. These films exhibit ferromagnetic behavior in magnetization measurements, showing a M–H loop at room temperature with a saturation magnetization on the order of 0.7 μB /Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co2+ valence state. Cross-section electron microscopy reveals that a significant fraction of the cobalt segregates into Co–Ti–O secondary phase particles. Selected area electron diffraction shows that the secondary phase particles are cobalt-rich anatase. While the cobalt is concentrated in the segregated particles, local energy dispersive spectrometry indicates some Co throughout the film.

Epitaxial CoxTi1xO2 anatase thin films were grown on (001)LaAlO3 by a reactive RF magnetron co-sputter deposition with water vapor serving as the oxidant. The use of water as the oxygen source proves useful in growing oxygen-deficient, semiconducting CoxTi1xO2 by reactive sputter deposition, with undoped and Co-doped TiO2 thin films showing n-type semiconductor behavior, with carrier concentrations of 10 17 –10 18 cm 3 . Magnetization measurements of CoxTi1xO2 (x=0.07) thin films reveal ferromagnetic behavior in M-H loop at room temperature with a saturation magnetization on the order of 0.7 Bohr magnetons/Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co 2+ valence state. However, chemical analysis of surface structure indicates that a significant fraction of the cobalt segregates into a Co–Ti–O phase. This suggests that the ferromagnetic moments may reside in oxygen-bound Co that is segregated from the majority anatase phase. D 2005 Elsevier B.V. All rights reserved.

Recent results on achieving ferromagnetism in transition-metal-doped GaN, A1N and related materials are discussed. The field of semiconductor spintronics seeks to exploit the spin of charge carrier ...

Ion implantation is a valuable tool for introducing transition metal ions such as Cr, Mn, Fe, Co and Ni into a variety of semiconductors including AlN, GaN, GaP and SiC. High-transition-temperature ferromagnetic behaviour is found to be the rule rather than the exception. Implantation combined with magnetic screening techniques to determine hysteretic transition temperatures provides an effective procedure for rapidly determining whether particular combinations of magnetic dopants and host semiconductors are likely to display high-temperature ferromagnetic properties. Recent results on Cr, Mn and Co implanted into wide-bandgap AlN are presented and discussed with respect to their promise as carrier-mediated ferromagnets that might be useful for spintronics applications.

Implantation of Co or Mn into single-crystal BaTiO3(K), SrTiO3 or KTaO3(Ca), followed by annealing at 700 � C, produced ferromagnetic behavior over a broad range of transition metal concentrations. For BaTiO3, both Co and Mn implantation produced magnetic ordering temperatures near 300 K with coercivities 670 Oe. The M–T plots showed either a near-linear decrease of magnetization with increasing temperature for Co and a non-Brillouin shaped curve for Mn. No secondary phases were detected by high-resolution X-ray diffraction. The same basic trends were observed for both SrTiO3 and KTaO3, with the exception that at high Mn concentrations (� 5 at.%) the SrTiO3 was no longer ferromagnetic. Our results are consistent with recent reports of room temperature ferromagnetism in other perovskite systems (e.g. LaBaMnO3) and theoretical predictions for transition metal doping of BaTiO3 [Nakayama et al., Jap. J. Appl. Phys. 40 (2001) L1355].

Bulk single crystals of Sn-doped ZnO were implanted with Co or Mn at doses designed to produce transition metal concentrations of 3–5 at.% in the near-surface (� 2000 A region. The implantation was performed at � 350 Ct o promote dynamic annealing of ion-induced damage. Following annealing at 700 C, temperature-dependent magnetization measurements showed ordering temperatures of � 300 K for Co- and � 250 K for Mn-implanted ZnO. Clear hysteresis loops were obtained at these temperatures. The coercive fields were 6 100 Oe for all measurement temperatures. X-ray diffraction showed no detectable second phases in the Mn-implanted material. One plausible origin for the ferromagnetism in this case is a carrier-induced mechanism. By sharp contrast, the Co-implanted material showed evidence for the presence of Co precipitates with hexagonal symmetry, which is the cause of the room temperature ferromagnetism. Our results are consistent with the stabilization of ferromagnetic states by electron doping in transition metal-doped ZnO predicted by Sato and Katayama–Yoshida [Jpn. J. Appl. Phys. 40 (2001) L334]. This work shows the excellent promise of Mn-doped ZnO for potential room temperature spintronic applications. 2003 Elsevier Ltd. All rights reserved.

Semiconducting oxides offer the potential for exploring and understanding spin-based functionality in a semiconducting host material. Theoretical predictions suggest that carrier-mediated ferromagnetism should be favored for p-type material. Cu2O is a p-type, direct wide bandgap oxide semiconductor that may hold interest in exploring spin behavior. In this paper, the properties of Mn-doped Cu2O are described. Activities focused on understanding Mn incorporation during thin-film synthesis, as well as magnetic characterization. The epitaxial films were grown by pulsed-laser deposition. X-ray diffraction was used to determine film crystallinity and to address the formation of secondary phases. SQUID magnetometry was employed to characterize the magnetic properties. Ferromagnetism is observed in selected Mn-doped Cu2O films, but appears to be associated with Mn3O4 secondary phases. In phase-pure Mn-doped Cu2O films, no evidence for ferromagnetism is observed.

Both ion implantation and epitaxial crystal growth provide convenient methods of introducing transition metals such as Mn,Cr,Fe,Ni and Co into GaN, GaP, SiC and ZnO for creating dilute magnetic semiconductors exhibiting room temperature ferromagnetism. In this paper we review progress in wide band gap ferromagnetic semiconductors and the role of defects and doping on the resulting magnetic properties.

Implantation of Mn or Cr at doses of 3–5 × 1016 cm−2 into Si-doped Al0.24Ga0.76P epilayers on GaP substrates produced ferromagnetic ordering at temperatures up to 300 K. The results were similar to those obtained previously in p-type AlGaP(C), indicating that both electron and hole-doped AlGaP can exhibit ferromagnetism. In addition, the AlGaP results are similar to those for GaP so the magnitude of the bandgap was not the main parameter influencing the Curie temperature, in contradiction to predictions from some mean-field theories. Second phases were not observed by X-ray diffraction and were not responsible for the ferromagnetism.

Growth by molecular-beam epitaxy (MBE) of the dilute-magnetic alloy GaMnN is reported. The Mn concentration, as determined by Auger electron spectroscopy (AES), is found to be linear with increasing Mn-cell temperature up to ∼43at.%Mn. No second phases are observed for Mn levels below 9 at.%. The cubic-phase Mn4N is found to be the thermodynamically stable phase at the growth conditions used to produce GaMnN. Hysteresis in M versus H is observed in both GaMnN and GaMnN:C grown on both sapphire and metal-oxide chemical-vapor deposition (MOCVD) GaN at several growth temperatures. Magnetotransport results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis, indicating that Mn is incorporating into the GaN and forming the ferromagnetic-semiconductor GaMNN. Room-temperature hysteresis is obtained in magnetization measurements with an optimum Mn concentration of ∼3 at.%.

Existing semiconductor electronic and photonic devices utilize the charge on electrons and holes in order to perform their specific functionality such as signal processing or light emission. The relatively new field of semiconductor spintronics seeks, in addition, to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors. The ability to control of spin injection, transport and detection leads to the potential for new classes of ultra-low power, high speed memory, logic and photonic devices. The utility of such devices depends on the availability of materials with practical (>300 K) magnetic ordering temperatures. In this paper, we summarize recent progress in dilute magnetic semiconductors (DMS) such as (Ga, Mn)N, (Ga, Mn)P, (Zn, Mn)O and (Zn, Mn)SiGeN2 exhibiting room temperature ferromagnetism, the origins of the magnetism and its potential applications in novel devices such as spin-polarized light emitters and spin field effect transistors.

Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. T...

Results on the synthesis of ferromagnetic GaMnN and GaMnP by both Molecular Beam Epitaxy or implantation of Mn directly into p-GaN or GaP(C) at elevated temperatures to avoid amorphization will be described. There is a relatively broad range of growth conditions under which single-crystal, single-phase material may be obtained with significant Mn concentrations. The effects of background doping level are discussed, along with a comparison of results on direct implantation of Fe and Ni instead of Mn. Essential requirements for utilizing the spin of the electron in device structures include the ability to achieve efficient electrical spin injection and transport of spin-polarized carriers, along with effective detection of these carriers.

Epitaxial layers of ZnSiN2, ZnGe0.65Si0.35N2, and ZnGe0.31Si0.69N2 grown on Al2O3 substrates were implanted at 350 °C with high doses (5×1016 cm−2) of Mn+ ions and annealed at 700 °C. The implanted region did not appear to become amorphous and showed strong selected area diffraction patterns. Hysteresis was observed in magnetization versus field curves from all of the implanted samples. Differences in field-cooled and zero field-cooled magnetization persisted to temperatures of ∼200 K for ZnSiN2, and ∼280 K for both ZnGe0.31Si0.69N2 and ZnGe0.69Si0.31N2. The results are consistent with recent magnetic data from (ZnxMn1−x)GeP2, ZnSnAs2 and (CdxMn1−x)GeP2 and suggest that this class of materials may be promising for dilute magnetic semiconductor applications.

High concentrations ( � /10 21 cm � 3 ) of Ni were introduced into GaN and SiC by ion implantation at 350 8C. On subsequent annealing at 700 8C, there was more residual lattice damage in GaN compared to SiC. Both materials showed ferromagnetism with transition temperatures below 50 K. No secondary phases could be detected by transmission electron microscopy (TEM) or selected area diffraction in either GaN or SiC. The direct implantation process appears useful for studying ion/substrate combinations for potential spintronic applications. # 2002 Elsevier Science B.V. All rights reserved.

Structural and magnetic characteristics of Ni-implanted GaP were measured for doses in the range 3/5/10 16 cm 2 . After subsequent annealing at 700 8C, transmission electron microscopy (TEM) showed residual lattice damage which was more significant in highly carbon-doped epi layers as compared to bulk GaP substrates. The magnetization measurements showed two different contributions, one present at 5/75 K and the other below � /225 K. No secondary phase formation was detected in either type of GaP. # 2002 Elsevier Science B.V. All rights reserved.

Direct implantation of Fe, Ni or Mn at doses of 3–5×1016 cm−2 into p-type 6H-SiC substrates was carried out at a sample temperature of ∼350 °C. Subsequent annealing was performed at 700–1000 °C for 5 mins. Residual damage in the form of end-of-range defects and dislocation loops in the region from the surface to a depth of ∼0.20 μm were examined by transmission electron microscopy. To the sensitivity of both x-ray diffraction and selected area diffraction pattern analysis, no secondary phases could be detected. Signatures of ferromagnetism were observed in all the highest dose samples, with apparent Curie temperatures of 50 K (Ni), 250 K (Mn), and 270 K (Fe).

High concentrations (0.1-5 at.%) of Mn or Fe were introduced into the near-surface region (≤2000 A) of 6H-SiC substrates by direct implantation at ˜300°C. After annealing at temperatures up to 1000°C, the structural properties were examined by transmission electron microscopy (TEM) and selected-area diffraction pattern (SADP) analysis. The magnetic properties were examined by SQUID magnetometry. While the Mn-implanted samples were paramagnetic over the entire dose range investigated, the Fe-implanted material displayed a ferromagnetic contribution present at >175 K for the highest dose conditions. No secondary phases were detected, at least not to the sensitivity of TEM or SADP.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3–5×1016 cm−2) of Mn, Fe, or Ni and subsequent annealing at 700–1000 °C. The samples showed ferromagnetic contributions below temperatures ranging from 190–250 K for Mn to 45–185 K for Ni and 80–250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis for the doses we employed.

In p-GaN implanted with Mn (3 × 1016cm-2 at 250 keV), the material after annealing shows ferromagnetic properties below 250 K. Cross-sectional transmission electron microscopy (TEM) revealed the presence of platelet structures with hexagonal symmetry. These regions are most likely GaxMn1-xN, which produce the ferromagnetic contribution to the magnetization. In p-GaN implanted with Fe, the material after annealing showed ferromagnetic properties at temperatures that were dependent on the Fe dose, but were below 200 K in all cases. In these samples, TEM and diffraction analysis did not reveal any secondary phase formation. The results for the Fe implantation are similar to those reported for Fe doping during epitaxial growth of GaN.

We obtained metallic SrTiO3 with an atomically smooth surface, where the step height is close to the unit cell height of the crystal. After the surface treatments for generating the TiO2 terminated layer, the optimal conditions for vacuum annealing were found. The atomically smooth surface was verified by atomic force microscopy and lateral force microscopy. The temperature dependent resistance R(T) measured down to 52 mK indicates the metallic behavior, and its physical origin of the conduction was analyzed by fitting an equation involving electron-phonon and electron-electron scattering mechanisms. Our results are similar to R(T) reported for LaAlO3/SrTiO3 interfaces. In addition, the Hall effect measurement shows a clear resemblance between our reduced SrTiO3 and LaAlO3/SrTiO3 interfaces with sheet carrier density and Hall mobility. We expect that our treatments not only promote the usage of conducting SrTiO3 substrates for subsequent thin film growth but also contribute to the current research interes...

Oxide multilayer hetero-structure of BaFeO3-KTa0.47Nb0.53O3 have been grown using the pulsed laser deposition and the ex situ annealed in a flowing oxygen atmosphere. The multilayer has been characterized using X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Magnetic properties have been studied via SQUID measurements. Magnetization in the multilayer structure shows significant improvement compared to previous measurements of bare BaFeO3. Measuring magnetization as a function of temperature and fitting the resulting M–T curves to Bloch's 3/2 law, yields an estimated Curie temperature (TC) of around 425 K and a P factor of around 1.5, consistent with magnetization from a three dimensional crystal structure. In spite of using a room temperature ferroelectric KTa0.47Nb0.53O3 composition, no evidence of ferroelectricity was observed in multi-layer structure.

Optical and Hall-effect measurements have been performed on single crystals of Pb₀.₇₇Sn₀.₂₃Se, a IV-VI mixed chalcogenide. The temperature dependent (10–300 K) reflectance was measured over 40–7000 cm⁻¹ (5–870 meV) with an extension to 15,500 cm⁻¹ (1.92 eV) at room temperature. The reflectance was fit to the Drude-Lorentz model using a single Drude component and several Lorentz oscillators. The optical properties at the measured temperatures were estimated via Kramers-Kronig analysis as well as by the Drude-Lorentz fit. The carriers were p-type with the carrier density determined by Hall measurements. A signature of valence intraband transition is found in the low-energy optical spectra. It is found that the valence-conduction band transition energy as well as the free carrier effective mass reach minimum values at 100 K, suggesting temperature-driven band inversion in the material. Thus, density function theory calculation for the electronic band structure also make similar predictions.

Temperature dependent transport measurements on ultrathin antiferromagnetic Mn films reveal a heretofore unknown non-universal weak localization correction to the conductivity which extends to disorder strengths greater than 100 k$\Omega$ per square. The inelastic scattering of electrons off of gapped antiferromagnetic spin waves gives rise to an inelastic scattering length which is short enough to place the system in the 3d regime. The extracted fitting parameters provide estimates of the energy gap ($\Delta = 16$ K) and exchange energy ($\bar{J} = 320$ K).

We report measurements of Shubnikov-de Haas (SdH) oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, $F_{1}=65 \pm 4$ Tesla and $F_{2}=156 \pm 5$ Tesla. We also measured the infrared optical reflectance $\left(\cal R(\omega)\right)$ and Hall effect; we propose that the two frequencies correspond respectively to the inner and outer Fermi sheets of the Rashba spin-split bulk conduction band. The bulk carrier concentration was $n_{e}\approx1\times10^{19}$ cm$^{-3}$ and the effective masses $m_{1}^{*}=0.20 m_{0}$ for the inner and $m_{2}^{*}=0.27 m_{0}$ for the outer sheet. Surprisingly, despite its low effective mass, we found that the amplitude of $F_{2}$ is very rapidly suppressed with increasing temperature, being almost undetectable above $T\approx4$ K.