Mössbauer spectroscopy uses the resonant absorption of nuclear radiation by 57Fe to probe the electronic and internal magnetic structure of iron based magnetic materials. The technique has a characteristic measuring time of 10 ns enabling investigation of spin relaxation phenomena in nanoscale particles; and determination of their magnetic properties in the absence of externally applied magnetic fields. We report on Mössbauer studies of γ-Fe2O3 nanoparticles synthesized within hexagonally packed mesoporous MCM-41 aluminosilicate matrices with cylindrical pores of 2.5 nm diameter. Data analysis allowed differentiation of particle-matrix interfacial versus particle-core interior iron sites. Interfacial iron atoms experience large electric field gradients resulting in quadrupole splitting values of ΔEq (surface) = (1.25 ± 0.05) mm/s, while core atoms exhibit smaller values of δEq (core) = (0.73± 0.05) mm/s at room temperature. Similarly, differences were observed in the values of the internal hyperfine fields at low temperatures indicating reduced strength in the exchange interactions at the particle surface, with interfacial atoms experiencing internal fields Hhf (surface) = (458 ± 1) kOe reduced relative to the core Hhf (core) = (488 ±1) kOe at 4.2 K. Particle/matrix interactions at the surface appear to perturb the electronic interactions deeper into the core than the magnetic exchange interactions.

We give an exposition of the principles of quantum computing (logic gates, exponential parallelism from polynomial hardware, fast quantum algorithms, quantum error correction, hardware requirements, and experimental milestones). A compact description of the quantum Fourier transform to find the period of a function—the key step in Shor's factoring algorithm—illustrates how parallel state evolution along many classical computational paths produces fast algorithms by constructive interference similar to Bragg reflections in x-ray crystallography. On the hardware side, we present a new method to estimate critical time scales for the operation of a quantum computer. We derive a universal upper bound on the probability of a computation to fail due to decoherence (entanglement of the computer with the environment), as a function of time. The bound is parameter-free, requiring only the interaction between the computer and the environment, and the time-evolving state in the absence of any interaction. For a simple model we find that the bound performs well and decoherence is small when the energy of the computer state is large compared to the interaction energy. This supports a recent estimate of minimum energy requirements for quantum computation.

We use High Resolution Electron Microscopy together with Electron Energy Loss Spectroscopy to analyze the crystallography and the chemical configuration of a Co/SrTiO3 interface in a Co/SrTiO3/La2/3Sr1/3MnO3 magnetic tunnel junction.

PACS: 75.47.-m, 75.70.Cn, 68.37.Lp, 79.20.Uv

We have synthesized and characterized doped manganite trilayers composed of La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO). Because of the isostructural nature of the Sr and Ca doped manganites, trilayers exhibit good epitaxy as observed in X-ray diffraction. Magnetization measurements reveal magnetization values consistent with bulk values. In-plane transport of these trilayers reveals anisotropic magnetoresistance and high field negative magnetoresistance attributed to the suppression of spin fluctuations at high fields.

Non-local damping was investigated by Ferromagnetic Resonance (FMR) using ultrathin magnetic single and double layer structures prepared by Molecular Beam Epitaxy (MBE). The double layer structures show magnetic damping which is caused by spin transport across a normal metal spacer (N). In double layer structures a thin Fe layer, F1, was separated from a thick Fe layer, F2, by a Au(001) spacer. The interface magnetic anisotropies separated the FMR fields of F1 and F2 by a big margin allowing one to investigate FMR in F1 while F2 had a negligible angle of precession, and vice versa. The Fe films in magnetic double layers acquire non-local interface Gilbert damping. It will be shown that the precessing magnetic moments act as spin pumps and spin sinks. This concept was tested by investigating the FMR linewidth around an accidental crossover of the resonance fields for the layers F1 and F2. There is another possible mechanism for non-local damping which is based on a “breathing Fermi surface” of the spacer. The temperature dependence of the non-local damping indicates that this mechanism is weak in Au spacers. Surprisingly the Au spacer acts as an additional impedance for the spin pump mechanism. Finally, it will be shown that electron-electron correlations in a Pd spacer can lead to a significant enhancement of the non-local damping.

We have performed both Hall effect, and magnetoresistance measurements on thin films of Permalloy (Py 10 nm) and Py(tPy)/Cu(tCu)/Py(tPy) multilayers deposited on thermally oxidized Si substrates, where tPy=4 and 10 nm and tCu=4 and 8 nm. The measurements were made at room temperature in a setup that allows us to perform both Hall effect and magnetoresi stance measurements. The Hall effect measurements were performed varying the angle, Δθ, between the magnetic field direction and the normal to the film plane from 0 to 90 degrees. The measured voltages present hysteresis loops at low magnetic field even for Δθ=0°. From these measurements we can obtain some information regarding the magnetic properties of our samples.

The technique of small angle neutron scattering (SANS) is ideally suited for determining the length scale of magnetic correlations in nanocrystalline materials. The additional use of polarized neutrons also allows for a clear separation between magnetic and non-magnetic scattering. The temperature dependence of the SANS cross-section from a two-phase alloy, consisting of both amorphous and nano-crystalline parts, Fe89Zr7B3Cu1, has been measured in the temperature range from 293 to 500 K. The SANS measurements are accompanied by bulk magnetization and Mössbauer transmission data. In this range of temperatures, the magnetic contrast between the nanocrystalline and amorphous phases, which are both magnetic, changes dramatically. This phase contrast increases up to 380 K, which is the proposed decoupling temperature for the inter-granular exchange stiffness. Above this temperature, the contrast levels off slowly, being totally dominated by the decreasing magnetization of the nanocrystalline phase.

Magnetotransport properties of granular CrO2 /Cr2O3 films made of CrO2 crystals covered by 1– 2 nm native insulating Cr2O3 are presented. Electrical properties of a limited number of grains measured in series and parallel (10 to 15 grains) reveal intergrain tunneling characteristics. At lowest temperatures, a well pronounced zero bias anomaly indicates that impurities in the junctions block the electronic flow.

Hysteresis in the IV curves are observed at intermediate temperatures on zero-field cooled samples. Changing the polarity of a short excitation pulse (100ns) of amplitude smaller than 1 V triggers a change in the zero-bias resistance by 10–50%. These states are stable and well reproducible in the temperature interval ranging from 100K to 250K. Applying an external magnetic field cancels the IV hysteresis. The resistance of the devices in the kΩ range, the potential high-speed for writing and reading the resistance sate, make these systems interesting candidates for magnetic non-volatile memories.

The magnetic proximity effect (MPE) has attracted the attention of theorists and experimentalists for at least three decades. Lately, the relevance of the effect for the development of nanodevices has revived interest on the subject. Here we review how the field has evolved, centering our attention on metal-metal and metal-insulator systems. We describe, and critically compare, the different theoretical approaches that have been put forward, as well as their limitations. An evaluation of the relationship between existing theories and available experimental results is also attempted.

We report high frequency electron paramagnetic resonance (EPR) investigations of a series of high spin (total spin up to S = 10) manganese and nickel complexes which have been shown to exhibit single molecule magnetism, including low temperature (below ∼ 1K) hysteresis loops and resonant magnetic quantum tunneling. A cavity perturbation technique enables high sensitivity oriented single crystal EPR measurements spanning a very wide frequency range (16 to 200+ GHz). Fitting of the frequency and field orientation dependence of EPR spectra allows direct determination of the effective spin Hamiltonian parameters. Studies on a range of materials with varying (approximately axial) site symmetries facilitates an assessment of the role of transverse anisotropy (terms in the Hamiltonian that do not commute with ŝz) in the magnetic quantum tunneling phenomenon.

We introduce a random field model with anisotropic interfacial exchange for ferromagnet / antiferromagnet bilayers and calculate average coupling energies and coupling directions for different grain or domain sizes. The model is shown to reconcile Malozemoff's random field Ising model for exchange bias with the spinflop coupling mechanism. Furthermore, we find that for small and mid size domains, the average coupling direction strongly varies between different interfacial configurations. We show how this behavior can lead to rotating anisotropies in systems with very strong magnetocrystalline anisotropies, where the use of superparamagnetic grains or domains in the antiferrofmagnet is not justified.

The transport properties of Fe(001)/Cr/Fe(001) trilayers are discussed with respect to the influence of transition metal impurities in form of layers. We are able to show that the periodicity of the giant magnetoresistance is directly influenced by the interlayer exchange coupling (IEC). Furthermore, it is observed that the behavior of the IEC strongly depends on whether an impurity overlayer of Mn or V is used. It turns out that the size of the GMR is only little effected by 3d-transition metal impurities, which is in agreement with the experimental findings. The electronic and magnetic properties of the trilayers have been investigated within the fully relativistic, spin-polarized SKKR method and the LDA. The transport properties of the Fe/Cr/Fe systems have been derived from the fully relativistic spin-polarized Kubo-Greenwood equation.

A modified sol-gel technique was used to synthesize nanocomposites of Ni:SiO2 which resulted in Ni nanoparticles embedded in a SiO2 amorphous matrix. Transmission electron microscopy TEM analysis were performed to study the structure and morphology of the magnetic powders. The Ni particles were found to have a good dispersion and a controlled particle size distribution, with average particle radius of ∼ 3 nm. A detailed characterization of the magnetic properties was done through magnetization measurements M(T,H) in applied magnetic fields up to ± 7 T and for temperatures ranging from 2 to 300 K. The superparamagnetic (SPM) behavior of these metallic nanoparticles was inferred from the temperature dependence of the magnetization. The blocking temperature TB, as low as 20 K, was found to be dependent on Ni concentration, increasing with increasing Ni content. The SPM behavior above the blocking temperature TB was confirmed by the collapse of M/MS vs. H/T data in universal curves. These curves were fitted to a log-normal weighted Langevin function allowing us to determine the distribution of magnetic moments. Using the fitted magnetic moments and the Ni saturation magnetization, the radii of spherical particles were determined to be close to ∼ 3 nm, in excellent agreement with TEM analysis. Also, magnetic hysteresis loops were found to be symmetric along the field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer. In addition, for the most diluted samples, the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles. This was confirmed by the temperature dependence of the coercivity field that obeys the relation HC(T) = HC0 [1-(T/TB)1/2] below TB with HC0 ∼ 780 Oe.

The electronic properties of arrays and isolated magnetite nanocrystals were studied using tunneling spectroscopy. Macroscopic tunnel junctions were used to study stacked arrays of the nanocrystals. The temperature dependent resistance measurements showed an abrupt increase of the resistance around 100 K, attributed to the Verwey metal-insulator transition, while the current-voltage characteristics exhibit a sharp transition from an insulator gap to a peak in the density of states near the Fermi energy. This conductance peak was sensitive to in-plane magnetic field showing large magnetoresistance. The tunneling spectra obtained on isolated particles using a Scanning Tunneling Microscope exhibit a gap-like structure below the transition temperature that gradually disappeared with increasing temperature, ending with a small peak structure around zero bias.

The structure of the antiferromagnet (AF) in the Ta/ MnxPt1-x/Co /Ta system must have the face-centered tetragonal CuAu structure in order to obtain a measurable exchange bias (HE). The formation of this structure has an onset temperature of approximately 240 °C. Increasing the annealing temperature increases the amount of CuAu ordering, which increases HE, and also increases the MnPt/Co interdiffusion, which decreases HE. These two competing processes result in an optimum annealing temperature that maximizes HE.

Single-molecule magnets (SMM) are molecules that function as single-domain nanomagnets. SMMs have been characterized with a ground-state spin ranging from S = 4 to S = 13. A few SMMs have been identified that have half-integer spin ground states. [Cation][Mn12O12(O2CR)16(H2O)4] complexes, where R is some substituent, are SMMs that have either a S = 19/2 or 21/2 ground state. Quantum tunneling of magnetization (QTM) is observed for these half-integer-spin Kramers [Mn12]- degenerate SMMs in zero external magnetic field, as well as for a class of S = 9/2 Mn4 SMMs. The presence of QTM in zero external field is attributed to a transverse component of a nuclear spin field, dipolar interactions and intermolecular exchange interactions. The Landau-Zener method is used to measure the tunnel splitting as a function of transverse magnetic field for a single crystal of the S = 9/2 SMM [Mn4O3(OSiMe3)(OAc)3(dbm)3]. Spin parity dependent QTM is established. The effect of a magnetic exchange interaction between two S = 9/2 Mn4 SMMs upon QTM was studied for another compound. The hydrogen bonding and Cl…Cl contacts within a supramolecularly linked [Mn4]2 dimer lead to a weak antiferromagnetic exchange interaction between the two S = 9/2 SMMs. This interaction causes a shift (exchange bias) from zero field for the magnetic field at which QTM occurs.