Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet their execution has been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi 2 Te 3 and Bi 2 Se 3 bulk single crystals, as well as impurities on their surfaces. Here we present the preparation of Bi 2 Te 3 thin films that are insulating in the bulk and the fourpoint probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of the order of 10 12 cm −2 only, and mobilities up to 4,600 cm 2 /Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and X-ray photoemission analysis, and temperature-dependent four-point probe conductivity measurement all in situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states.topological insulator | molecular beam epitaxy | thin films | in situ four-point conductance | Dirac electrons T he observation of a quantum spin Hall effect due to edge states in 2D HgTe/CdTe quantum wells (1-3) has opened the field of topological states in matter without external magnetic fields applied. In 3D materials, the existence of metallic surface states with massless Dirac dispersion, locked helical spin polarization, and a simultaneous bulk insulating behavior is the hallmark of a topological insulator state (TI) (4-7). For Bi 2 Te 3 and Bi 2 Se 3 (8-10), their presence and topological protection were inferred from angle-resolved photoelectron spectroscopy (ARPES) experiments (8, 9), Shubnikov-deHaas (SdH) oscillations, and weak antilocalization in magnetotransport (11), as well as quasiparticle interference with scanning tunneling microscopy (STM) (12, 13).The Bi 2 Te 3 and Bi 2 Se 3 TI materials, however, have a severe problem that has thus far hindered four-probe conductivity measurements involving the Dirac surface states (SSs) (10,14,15): the presence of vacancies and anti-site defects in the bulk seems unavoidable, with the result that the bulk conductivity will overwhelm the contribution of the surface states (16). A remedy often used thus far has been the application of counter doping (e.g., with Ca, Sn, or Pb) (9, 10, 17-19) or gating (20-22) of exfoliated samples. Another promising way to solve the problem is to use thin films (23, 24) so that the surface to bulk ratio of the conduction is increased. By varying the growth parameters, one can change the film from p-to n-type (25, 26), suggesting that it should be possible to make the films consistently insulating. Another important aspect is that the conductivity studies reported thus far (27-29) have their samples exposed to air for mounting of the contacts before the four-probe conductivity...

We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg 1/3 Nb 2/3 O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of µ0HAF = 1.8 T is found to change by µ0 HAF / ε ∼ -520 mT % −1 under reversible biaxial strain ( ε) at 80 K in a [La0.7Sr0.3MnO3(22Å)/SrRuO3(55Å)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain, leading to a larger net coupling of SrRuO3 layers at the interface. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface for the strength of the apparent antiferromagnetic coupling. We discuss our results in the framework of available models.

Electrostriction is a property of all the dielectric materials where an applied electric field induces a mechanical deformation proportional to the square of the electric field. The magnitude of the effect is usually minuscule. However, recent discoveries of symmetry-breaking phenomena at interfaces opens up the possibility to extend the electrostrictive response to a broader family of dielectric materials. 1,2 Here, we engineer the electrostrictive effect by epitaxially depositing alternating layers of

A surge in interest of oxide-based materials is testimony for their potential utility in a wide array of device applications and offers a fascinating landscape for tuning the functional properties through a variety of physical and chemical parameters. In particular, selective electronic/defect doping has been demonstrated to be vital in tailoring novel functionalities, not existing in the bulk host oxides. Here, an extraordinary interstitial doping effect is demonstrated centered around a light element, boron (B). The host matrix is a novel composite system, made from discrete bulk LaAlO :LaBO compounds. The findings show a spontaneous ordering of the interstitial B cations within the host LaAlO lattices, and subsequent spin-polarized charge injection into the neighboring cations. This leads to a series of remarkable cation-dominated electrical switching and high-temperature ferromagnetism. Hence, the induced interstitial doping serves to transform a nonmagnetic insulating bulk oxide into a ferromagnetic ionic-electronic conductor. This unique interstitial B doping effect upon its control is proposed to be as a general route for extracting/modifying multifunctional properties in bulk oxides utilized in energy and spin-based applications.

Thin films of various half‐metallic ferromagnets, such as chromium dioxide (CrO2) and Heusler alloys (Co2Cr0.6Fe0.4Al, Co2MnSi) have been investigated by ferromagnetic resonance (FMR) technique. It is demonstrated that FMR is a very efficient method to study the nanoscale magnetic properties, in particular to probe the magnetic anisotropy and magnetic inhomogeneities of ferromagnetic thin films. Epitaxial CrO2 thin films of various thicknesses (25–535 nm) have been deposited on TiO2(100) substrates by chemical vapor deposition process. It is shown that the magnetic behavior of the CrO2 films results from a competition between the magnetocrystalline and strain anisotropies. For the ultrathin CrO2 film (25 nm) the magnetic easy axis switches from the c‐direction to the b‐direction of the rutile structure. Thin‐film Co2Cr0.6Fe0.4Al samples (25 nm or 100 nm) have been grown by DC magnetron sputtering either on unbuffered SiO2(100) substrates or on the substrates capped by a 50 nm thick V buffer layer. The effects of the vanadium buffer layer and of the film thickness are revealed by FMR studies of the Co2Cr0.6Fe0.4Al samples. Well‐resolved multiple spin‐wave modes are observed in the unbuffered Co2Cr0.6Fe0.4Al sample with a thickness of 100 nm and the exchange stiffness constant has been estimated. Thin films of Co2MnSi (4–100 nm) have been grown by DC sputtering on silicon substrates on top of a 42 nm thick V seed layer and capped either by Al2O3 or by Co and V layers. A set of the 80 nm thick films has been annealed at different temperatures in the range of 425–550 °C. FMR studies of the Co2MnSi samples shows that at the fixed annealing temperature (450 °C) the highest magnetization is observed in the sample with a thickness of 61 nm, while the thicker samples (100 nm) reveal not only a lower magnetization but greater magnetic inhomogeneity as well. An annealing treatment at T ≥ 450 °C is essential to obtain higher magnetization as well as uniform magnetic properties in the Co2MnSi films. Weak SWR modes have also been observed in the thick Heusler films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)