Condensed Matter Physics

In order to examine contamination in the Chungnam Coal Mine area, rock, stream sediment and water samples were collected and analyzed for trace elements. In order to compile the drainage-based geochemical maps, catchment basins were calculated using aspects of DEM (data elevation model) and stream-line segments. The constructed DEMs with 3 types of resolution were examined, these need to be properly used for well-shaped and accurate catchment basins. The stream-line segments and sampling points were also considered as a target image for watershed segments and it was found that the stream-line segments as target images were better role at reflecting the catchment area rather than the sampling points. It is important that the geochemical databases were manipulated to present the various phenomena. Factor analysis in R-mode was used to integrate geochemical data, and the input data for factor analysis were the concentrations of 12 trace elements in stream sediment and 3 anion concentrations and pH values in stream water. Using a maximum likelihood scheme, eigenvalues that explained about 90% of total variance were established and 4 factors were selected. Factors 1 and 2 indicated relative enrichment of trace elements, factor 3 was explained by human activities of residents and factor 4 was related to the pH of stream water. The drainage-based geochemical maps with 4 kinds of factor scores were compiled.

The critical exponents and magnetocaloric properties of La 0.7 Sr 0.3 Mn 1Àx Ti x O 3 with x = 0 and 0.05 have been explored via magnetic measurements. The magnetic data analyzed in the critical region by using modified Arrott plots, the Kouvel-Fisher method, and the scaling hypothesis reveal values for the critical parameters of b = 0.3165 and 0.3027, c = 1.1121 and 1.203, and d = 4.7556 and 4.833 for x = 0 and x = 0.05 respectively. These values are in agreement with the three-dimensional (3D) Ising model. The n values obtained from the law DS max ¼ aðl 0 HÞ n are 0.648 and 0.685. The maximum values of the magnetic entropy change are 1.65 J/kg K and 1.46 J/kg K under a magnetic field change of 10 kOe for x = 0.00 and 0.05, respectively. These results can be explained in the scenario of the short-range interaction and the phase separation as well as the diluted magnetic effect in manganites.

We report the existence of a high temperature magnetic anomaly in the 3D Kitaev candidate material, β-Li2IrO3. Signatures of the anomaly appear in magnetization, heat capacity and muon spin relaxation measurements. The onset coincides with a re-ordering of the principal axes of magnetization which is thought to be connected to the onset of Kitaev-like correlations in the system. The anomaly also shows magnetic hysteresis with a spatially anisotropic magnitude that follows the spin-anisotropic exchange anisotropy of the underlying Kitaev Hamiltonian. We discuss possible scenarios for a bulk and impurity origin.

Charge order is now accepted as an integral constituent of cuprate high-temperature superconductors, one that is intimately related to other instabilities in the phase diagram including antiferromagnetism and superconductivity . Unlike nesting-induced Peierls-like density waves, the charge correlations in the CuO 2 planes have been predicted to display a rich momentum space topology depending on the detailed fermiology of the system . However, to date charge order has only been observed along the high-symmetry Cu-O bond directions. Here, using resonant soft X-ray scattering, we investigate the evolution of the full momentum space topology of charge correlations in T -Ln 2 CuO 4 (Ln=Nd, Pr) as a function of intrinsic electron doping. We report that, upon electron doping the parent Mott insulator, charge correlations first emerge in a hitherto-unobserved form, with full (C inf ) rotational symmetry in momentum-space. At higher doping levels, the orientation of charge correlations is sharply locked to the Cu-O bond high-symmetry directions, restoring a more conventional bidirectional charge order with enhanced correlation lengths. Through charge susceptibility calculations, we closely reproduce the drastic evolution in the topology of charge correlations across an antiferromagnetic quantum phase transition, highlighting the interplay between spin and charge degrees of freedom in electron-doped cuprates. Finally, using the established link between charge correlations and the underlying fermiology, we propose a revised phase diagram of T -Ln 2 CuO 4 with a superconducting region extending toward the Mott limit.

Magnetic honeycomb iridates are thought to show strongly spin-anisotropic exchange interactions which, when highly frustrated, lead to an exotic state of matter known as the Kitaev quantum spin liquid. However, in all known examples these materials magnetically order at finite temperatures, the scale of which may imply weak frustration. Here we show that the application of a relatively small magnetic field drives the three-dimensional magnet β-Li2IrO3 from its incommensurate ground state into a quantum correlated paramagnet. Interestingly, this paramagnetic state admixes a zig-zag spin mode analogous to the zig-zag order seen in other Mott-Kitaev compounds. The rapid onset of the field-induced correlated state implies the exchange interactions are delicately balanced, leading to strong frustration and a near degeneracy of different ground states.Materials with a Kitaev spin liquid ground state are sought after as models of quantum phases but candidates so far form either zig-zag or ...

A systematic study of the magneto-resistance (MR), magneto-reactance (MX), and magneto-impedance (MI) effects in single and multiple glass-coated amorphous Co 68 B 15 Si 10 Mn 7 microwires is reported. Our studies reveal that the MR, MX, and MI ratios and their corresponding magnetic field sensitivities strongly depend on the number of microwires in an array and on the distance between them. We find that increasing the number of microwires increases the MR and MI ratios and their field sensitivities (g R and g Z , respectively) but decreases the MX ratio and its field sensitivity (g X ). A similar trend is observed for the frequency dependence of these parameters. Increasing the distance between the wires is also found to decrease the MR and MI ratios but increase the MX ratio of the array considerably. From a sensor application perspective, it is interesting to note that for the case of a single microwire, the g X reaches a value as high as 960%/Oe at a frequency of 1 MHz, which is about 192 times of the g R or g Z ($5%/Oe), revealing the possibility of developing ultrahigh sensitivity magnetic field sensors based on the principle of the MX effect.

We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate γ-Li 2 IrO 3 (LIO), as well as the layered honeycomb iridate Na 2 IrO 3 (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, ∆R, as a function of time, t, temperature, T , and probe field polarization in both LIO and NIO. In LIO, ∆R(t, T ) is anisotropic and comprised of three T dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, ∆R(t, T ) has a single T dependent component that is strikingly similar to the electronic excitation component of ∆R in LIO. Through analysis and comparison of ∆R(t, T ) for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

Al and C-MgB2 doping adds an electron to the system for each atom. This extra electron fills up the sigma-bands thus diminishing the number of sigma-carriers; this has been the usual explanation for the Tc reduction. Nevertheless in this work we show that there is also a large reduction of anisotropy in the electrical conductivity due to the sigma-carriers which should also have an effect on the Tc reduction. Al and C doping produce a different Tc pattern; this difference can be largely explained by the relative shift between the sigma-bands and pi-bands. After adjusting to this shift there is a small but visible difference, at low doping Tc in the Al compounds drops faster than in the C compounds, this can be directly related to the faster loss of conductivity anisotropy in the Al compounds.

CePr mixed oxides were previously studied as supports for Water Gas Shift catalysts, considering their promising textural and redox properties arising from the presence of a non-stoichiometric oxide. A very small Ce 3+ fraction was observed on those oxides, suggesting that Pr 3+ ion could be responsible for the enhanced activity. Therefore, in-depth XPS analysis is conducted in the present work in order to determine the Pr 3+ fraction by fitting the XPS Pr 3d region. Much higher values of reduced Pr ions than Ce ions were observed, confirming the promoter role of the former.

Room-temperature photoreflectance in van Hoof structures together with atomic force microscopy has been used to determine the influence of InAs quantum dots on the Fermi-level pinning of ͕100͖ GaAs surfaces. We show that quantum dots with base lengths of 10 and 25 nm lead to the Fermi level being pinned approximately 250 meV deeper in the band gap, an effect which is reversible by either overgrowing the dots with GaAs or by selectively etching away the dots. These results are discussed within the framework of recent theoretical investigations of surface states due to polar facets associated with quantum dots. ͓S0163-1829͑99͒51944-9͔

Structure and growth mechanism of ultra-thin A1203 films on NiAI(001 ) have been investigated with 180°-neutral impact collision ion scattering spectroscopy, high resolution spot profile analysis of low energy electron diffraction, and scanning tunnelling microscopy (STM). Ordered A1203 films have been prepared by oxygen exposure at room temperature and subsequent annealing at T= 1200 K. The formation of epitaxial oxide films is unaffected by the initial composition or reconstruction of the substrate. The oxidized surface being covered by A1203 appears to be microscopically rough, reversible stepped with step heights of about 3 A and consists of a network of elongated equally distributed oxide stripes along ( ) and ( ) directions of the NiAl(001 ) surface. The lateral anisotropy of the oxide is probably caused by the build up of internal stress in the growth process. After oxygen saturation exposure at room temperature and subsequent annealing, regions with rather regular arranged parallel oxide stripes show up in the STM images (mean width ~ 27 A, period ~ 54 A). The corresponding low energy electron diffraction pattern shows the existence of orthogonal (2 x 1) antiphase domains with a (9 x 1) superstructure (period 26.02 A) close to the mean width of oxide stripes. Next to the oxide stripes, regrowing NiA1 terraces and thin layers of amorphous A1203 have been found.

It is well known that the (1x5) superstructure of Bi cuprate superconductors will be suppressed due to doping with Pb. Nevertheless, a Fermi surface map of Bi2-yP bySr2-xLaxCuO 6+δ (y = 0.4 and x = 0.4) determined by angular resolved photoemission (ARPES) revealed additional Fermi surface features. Low energy electron diffraction and X-ray diffraction of this sample showed no sign of any superstructure. Scanning tunneling microscopy (STM), on the other hand, revealed two distinct modulations of the charge density, one of (1x32) and a second of (6x6) periodicity. The wave vectors of both modulations have been extracted and used to simulate the corresponding Fermi surface, which is compared with the experimental one. The origin of these modulations is discussed in terms of dopant ordering.

We report a correlation between peaks observed in the resistance versus applied magnetic field and lowfrequency resistance noise for Ni 81 Fe 19 /Ag giant magnetoresistance systems with dc bias current density above 10 6 A/cm 2 . The magnetic noise has a 1/f -like spectrum and arises from time-dependent magnetic orientational fluctuations within the multilayer. The noise amplitude is strongly correlated with the observed magnetoresistance peaks. The data and a simple model for the fluctuations demonstrate the important role of fluctuations in determining magnetoresistance in such multilayer systems.

We report one of the first observations of lowfrequency noise of NiFe/Cu spin-valves. A ~t h o u g ~ the observed noise is very low in comparison to m ~~t ~~a y e r systems, it is typically concentrated in the linear region where such devices are expected to Increased noise is also associated with ~a r k h ~~s in the ~agnetores~stance trace. In devices one m i c ~o ~e t e r high, structures start to appear in the noise s perturbation to the simple llf-like noise observ devices. Although the large magnetoresistance of spin-valves make them attractive as magnetic read heads, this same sensitivity could also make them susceptible. to thermally activated noise. Due to processing effects, edge effects, and nonuniformities in the layer structure, there may be regions in a spinvalve that are weakly coupled to the main part of the device which could give rise to llf noise. These regions may be thought of as consisting of domains which are distinct from and in addition to the "single domain" of an ideal spin-valve. Additionally, due to strong magnetostatics, microdomains may also show llfnoise which is correlated with Barkhausen noise. In particular, as linear response is sought in sub-micrometer devices, either of two scenarios could result in increased noise. Domain sizes may be expected to decrease with a possible corresponding increase in their sensitivity to thermally driven noise. Alternatively, if domain sizes remain constant, unstable domains could come to dominate small device behavior. We have pursued observations of llj-type noise in NiFe/Cu spin valves. The noise we observe is typically concentrated near the low-field linear response region. The noise does not scale in an obvious manner with device height. We also find peaks in the noise near Barkhausen jumps in the magnetoresistance (MR) curve. These peaks may arise from thermally driven fluctuations of the large domain responsible for the Barkhausen resistance jump.

We report noise data for discontinuous Ni82Fe18/Ag multilayer test structures. Examination of the noise data for this material indicates that random telegraph fluchator (RTF) noise of the resistance is the predominant noise source. Analysis of the RTF noise in these structures presents an opportunity to estimate magnetic domain or magnetic cluster strengths and the domain-domain interactions.

We show that thin-film Ni82Fe18/Ag multilayer structures display multiple peaks in their magnetoresistance curves when biased at current densities above 106 A/cm2. These peaks appear for annealed and unannealed structures, and their number is correlated with the number of NiFe layers. At high bias currents, the peak positions shift linearly with the internal magnetic field created by the bias current. The peak positions extrapolate to nonzero fields at zero bias currents, providing an upper bound on the magnetic layer-layer coupling strength of J0≊10−20 J (kB×700 K). The peak positions do not shift with temperature over the range 200–375 K; their widths narrow with increasing temperature. The single-domain magnetic moment μ is estimated as 10−17 J/T (106 μB) from the peak widths of ∼0.8 kA/m.

The surface composition and structure of NiAI(001) has been investigated by 180°-neutral impact collision ion scattering spectroscopy (NICISS) and high resolution spot profile analysis of low energy electron diffraction (SPA-LEED). Long time annealing below 500 K results in a p(1 x 1) LEED pattern. The surface can be described by a defect enriched AI terminated surface. Another p(l x 1) LEED pattern appears after flashed annealing at 1400 K and rapid cooling at room temperature. The high temperature p(1 x 1) phase is however nearly perfectly Ni terminated. It has been found that the formation of the high temperature NiAI(00 l)p(1 x 1) phase is closely related to roughening of the topmost A1 layer by vacancy creation at high annealing temperatures. In the intermediate annealing temperature range about 800 K surface roughening results in an AI terminated missing row surface structure exposing a c(,f2 x 3x/2)R45 ° LEED superstructure.

We study the kinetics of diffusion-limited coalescence, A+A → A, and annihilation, A+A → 0, in the Bethe lattice of coordination number z. Correlations build up over time so that the probability to find a particle next to another varies from ρ 2 (ρ is the particle density), initially, when the particles are uncorrelated, to [(z -2)/z]ρ 2 , in the long-time asymptotic limit. As a result, the particle density decays inversely proportional to time, ρ ∼ 1/kt, but at a rate k that slowly decreases to an asymptotic constant value.

Quantum" resonances for longitudinal phonon propagation are reported here for propagation off the magnetic field direction in the quantum strong field limit (~w c > EF). The distinction between these "quantum" resonances and the resonances recently reported by Chock and Lee is examined. Chock and Lee have recently discussed longitudinal phonon propagation and have pointed out the existence of multiple coupled phonon-plasmon resonances in a quantizing magnetic field, which are associated with the interaction of ion plasma oscillations with "sound-like" electron plasmon resonances discussed by Ginzburg et al. for propagation parallel to the magnetic field. We wish to report additional coupled phonon-plasmon "quantum" resonances for longitudinal phonon propagation off the parallel direction, which are associated with "quantum" plasmon resonances near multiples of the cyclotron frequency discussed first by Mermin and Canel [3] and later elaborated by Horing . These additional coupled "quantum" resonances also lie near multiples of the cyclotron frequency, and they are clearly distinguished from those of Chock and Lee in that the "quantum" resonances do not exist for parallel propagation where the Chock and Lee resonances are active, and furthermore the "quantum" resonances are active in the quantum strong field limit (~w c > EF) where the Chock-Lee resonances do not exist, save for two which may be understood as the Bohm-Staver sound mode and the combined electron-ion principal plasma mode. Our analysis [5] of the phonon propagator following Baym [6] devolves upon the Bohm-Staver dis-to the magnetic field, we have found [5] "Bernstein" type resonances for longitudinal phonon propagation in all regimes of magnetic field strength, and these are devoid of natural damping.

The apparent inconsistencies among the theories of de Haas -van Alphen oscillations in the Knight shift due to Das and Sondheimer, to Stephen, and to Dolgopolov and Bystrik are resolved. The calculation of the Knight shift is treated by a method which goes beyond the semiclassical approximation used by these authors. It is found that for polyvalent metals the dominant magnetic Geld dependence may be due to the behavior of the electronic wave function, rather than to the density of states.

The interaction energy between two spins which are coupled with the conduction electrons via the hyperfine interaction is calculated at zero temperature for arbitrary magnetic field strength. The noninteracting electron system is bounded by a surface which is perpendicular to the applied mag- netic field. Analytic results are presented for a surface which is simulated by the infinite-barrier model with specular reflection. The interaction energy is also calculated for the nearly-free-electron model with specular-reflect&on boundary conditions.

The current situation in high-T c superconductivity of the hydrides H 3 S, LaH 10 , and related compounds has been considered from a methodological viewpoint. Both the physical theoretical and historical backgrounds have been presented and analyzed. The necessity of procedural purity in theory and the experimental protocol compliance has been discussed. It has been shown that it is too early now to fully recognize the sensational results on hydride superconductivity.

In most of seamless tube making industries, the useful life of mandrels for rotary forging is less than 350 perforation events; though in some cases, the tool may last longer than 1000 perforations. Being the first of a series of hot working steps, improvement during the piercing multiplies the benefits throughout the whole manufacturing process. Mandrels are cast in three metal bases: iron, nickel and cobalt; though lower costs support the use of iron base alloys, mostly when larger mandrel diameters are required. Mandrels lifespan is usually improved by the controlled growth, at high temperature, of a hard oxide film. The research reported in this work is related to the protective oxide films grown on mandrels for seamless tube rotary forging. 2 A laboratory-scale equipment has been entirely designed and built at INTEMA in order to study mandrels wear during the rotary piercing of steel billets. Hard coating oxides grown under a controlled atmosphere on mandrels surface were tested by this equipment, reproducing the wear conditions observed at industrial scale. Wear and oxide film evolution were studied by optical microscopy and energy-dispersive X-ray spectroscopy. Acquired data from lab-scale piercing experiments were analyzed using neural networks (selforganizing maps) to discover relationships among the 22 process parameters and the oxide film characteristics. This method of analysis may well be applied to any industrial component under multivariable hard coating wear conditions.

The electrochemical reduction of Ta 2 O 5 in a LiCl-Li 2 O molten salt system has been carried out in an electrolytic cell with an integrated cathode assembly. During the electrochemical run, Ta 2 O 5 reacts chemically with Li 2 O to form LiTaO 3 . The chronopotentiometry method was used to prepare the metallic tantalum from Ta 2 O 5 . The electrolysis of Li 2 O and the reduction of Ta 2 O 5 and/or LiTaO 3 take place simultaneously at the cathode part of an electrolysis cell. The recycle of Li 2 O by an O 2-diffusion occurs in the cell. The results suggest that the metallic tantalum is prepared successfully by an electrochemical reduction in the LiCl-Li 2 O molten salt.

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We determine the phase diagram of LaFe1-xCoxAsO single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of x = 0.03, it follows the phase diagram for F-doped polycrystals. Above x = 0.03, the F-doped samples become superconducting, whereas for Co-doping the structural and magnetic transitions can be observed up to x = 0.042, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to x = 0.056, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the magnetic fluctuations that reveal the interplay of nematicity and magnetism. Above the nematic ordering, the spin fluctuations in LaFe1-xCoxAsO are identical to those in Ba(Fe1-xCox)2As2, suggesting that nematicity in LaFeAsO is a result of the fluctuating spin density wave as well.

We determine the phase diagram of LaFe1-xCoxAsO single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of x = 0.03, it follows the phase diagram for F-doped polycrystals. Above x = 0.03, the F-doped samples become superconducting, whereas for Co-doping the structural and magnetic transitions can be observed up to x = 0.042, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to x = 0.056, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the magnetic fluctuations that reveal the interplay of nematicity and magnetism. Above the nematic ordering, the spin fluctuations in LaFe1-xCoxAsO are identical to those in Ba(Fe1-xCox)2As2, suggesting that nematicity in LaFeAsO is a result of the fluctuating spin density wave as well.

We investigate the spin-wave spectrum of a superlattice composed of two alternating ferromag- netic films with antiferromagnetic coupling at the interfaces. Depending on the value of tempera- ture and of an externally applied magnetic field, this system can exist in any of four distinct phases. The temperatureand field-dependent behaviors of the spin-wave modes in three of these phases are investigated. Propagation both perpendicular and parallel to the layers is considered. The frequen- cy of one mode is found to increase with increasing temperature, in contradiction to the usual ex- pectation; this behavior is explained in terms of a ferrimagnet analogy.

We calculate the absorption of infra-red radiation by a magnetic superlattice composed of alternating ferromagnetic films. Although the films are ferromagnetic, the interfacial interaction is antiferromagnetic (i.e. Fe/Gd or Co/Gd). As has been shown previously, such a system can exist in several distinct phases, and we calculate the power absorption for these different phases. The spin wave modes of the system were found and used to construct a Green's function via an eigenvector expansion. The incoming radiation was modeled as an oscillating magnetic field which was assumed to decay into the material. Given this driving field, the Green's function yields the response of the system. Our results indicate that a simple absorption measurement can be used to probe for surface and bulk magnetic phase transitions. Additionally, we can find the response of the system to a spatially uniform driving field and thus calculate the local susceptibility. We discuss the possibility of usi...

We analyze the bulk and surface spin-wave modes of a semi-infinite Fe/Gd superlattice. If an exter- nally applied magnetic field is increased from zero, one surface mode is driven soft, indicating a surface phase transition which occurs at a field value about 5 times lower than that necessary to cause a bulk phase transition. The phase transition is shown to nucleate at the surface and has a large penetration depth (on the order of a few hundred angstroms) even for fields just slightly above the value needed to induce the transition.

Fabrication and structural characterization of ordered magnetic nanodot arrays over large area 1 CHANG-PENG LI, IGOR V. ROSHCHIN, XAVIER BATLLE 2 , IVAN K. SCHULLER, Physics Dept., UCSD, La Jolla, CA, USA -Self-assembly of nanopores in anodized alumina is of much interest as a controlled fabrication method of magnetic nanostructures for fundamental studies and potential magnetic recording applications. Up to 10 micron thick Al films are e-beam evaporated on N-type Si substrate for porous alumina mask fabrication. By controlling anodization conditions, hexagonally ordered pores with 8-125 nm diameter and 20-160 nm periodicity are formed over ∼1 cm 2 area. SEM and AFM characterization shows that the pores are distributed within ∼10% standard deviation from the mean value. Fe magnetic nanodot arrays are fabricated by subsequent e-beam evaporation of Fe and mask lift-off. The smallest dot array fabricated this way is 44 nm, which corresponds to 0.4 Tbit/in 2 density. The nanodot periodicity is confirmed by small angle neutron scattering measurements. For nanoscale exchange bias studies, Fe/FeF 2 bilayer nanodot array are prepared using low angle Ar ion etching instead of the lift-off.

The coupling of electronic, magnetic, and structural properties between two dissimilar materials in contact can induce novel functionalities. Here we report on a drastic modification of the magnetization dynamics of thin Nickel films in Ni/V 2 O 3 bilayers. We performed temperature-dependent ferromagnetic resonance measurements across the first-order structural phase transition (SPT) of V 2 O 3 . The results show a strong coupling of the V 2 O 3 lattice dynamics to the magnon spectra of the Ni film in proximity. We have performed similar measurements across the second-order SPT in Ni/SrTiO 3 hybrids. In this later case, only a slight change of the static magnetization was found with no modification of the magnetization dynamics. Our results suggest that the phase coexistence across the first-order SPT of V 2 O 3 is responsible for the effects observed in the Ni/V 2 O 3 hybrids. This suggests the existence of similar effects in other hybrid materials with first-order structural phase transitions.

Antiferromagnet-ferromagnet (AF-F) bilayers exhibit exchange bias effect, i.e. a shift of the F hysteresis loop after a cooling down below the Neél temperature of the AF. We use SQUID magnetometry and magneto-optical Kerr effect (MOKE) to study the magnetization depth dependence and reversal processes in the F layer of FeF 2 (70nm)/F(70nm) (F=Fe, Ni, Py) samples. Since the MOKE penetration depth is about 35nm for F layer, it is possible to achieve depth dependent information on the F probing both sides of the layer. Analyzing the SQUID response arising from the whole sample and MOKE hystesis loops from both AF-F and air interfaces we show that a magnetic structure perpendicular to the interface is created in the F layer near the AF-F interface. The compression of this magnetic structure with the external field is a reversible process. Work supported by US-DOE.

Nanostructured materials have attracted much research over the recent years, as they provide the critical building blocks for the booming of nanoscience and nanotechnology. Nanostructures have novel properties due to the interplay between the size confinement of electrons in small structures and proximity effects. This is relevant for example in device miniaturization towards the Tbit/in recording density and in the thermal stability of the recorded bits. Nanofabrication by self-organized templates, in particular nanoporous alumina membranes, is being explored intensively due to the fact that it might enable mass production over large areas.

We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems.

The universality of the ac electrical transport in granular metals has been scarcely studied and the actual mechanisms involved in the scaling laws are not well understood. Previous works have reported on the scaling of capacitance and dielectric loss at different temperatures in Co-ZrO2 granular metals. However, the characteristic frequency used to scale the conductivity spectra has not been discussed, yet. This report provides unambiguous evidence of the universal relaxation behavior of Pd-ZrO2 granular thin films over wide frequency (11 Hz–2 MHz) and temperature ranges (40–180 K) by means of Impedance Spectroscopy. The frequency dependence of the imaginary parts of both the impedance Z″ and electrical modulus M″ exhibit respective peaks at frequencies ωmax that follow a thermal activation law, ωmax ∝ exp(T1/2). Moreover, the real part of electrical conductivity σ′ follows the Jonscher’s universal power law, while the onset of the conductivity dispersion also corresponds to ωmax. ...

Antiferromagnet-ferromagnet (AF-F) bilayers exhibit exchange bias effect, i.e. a shift of the F hysteresis loop after a cooling down below the Neél temperature of the AF. We use SQUID magnetometry and magneto-optical Kerr effect (MOKE) to study the magnetization depth dependence and reversal processes in the F layer of FeF 2 (70nm)/F(70nm) (F=Fe, Ni, Py) samples. Since the MOKE penetration depth is about 35nm for F layer, it is possible to achieve depth dependent information on the F probing both sides of the layer. Analyzing the SQUID response arising from the whole sample and MOKE hystesis loops from both AF-F and air interfaces we show that a magnetic structure perpendicular to the interface is created in the F layer near the AF-F interface. The compression of this magnetic structure with the external field is a reversible process. Work supported by US-DOE.

The exchange coupling between antiferromagnetic/ferromagnetic (AF/FM) materials shifts the hysteresis loop along the field axis by an amount known as exchange bias field. It is believed that the ferromagnetic thickness dependence of the exchange bias field follows an inverse proportionality law. This has experimentally and theoretically been confirmed for FM thicknesses below the FM domain wall width. In this work we demonstrate that this exchange bias dependence is broken for certain FM spin structures, even though in FM layers thinner than the FM domain wall width. We present experimental data of FeF 2 /FeNi bilayers that deviate from the inverse proportionality law, as well as a theoretical calculation that accounts for the results.

Presented is a quantitative study of the magnetic vortex state and the vortex core in sub-100 nm magnetic dots. Arrays of magnetic nanodots covering over 1 cm 2 are fabricated using self-assembled nanopores in anodized alumina. Transition from a vortex to a single domain state for the Fe dots is studied by magnetization measurements (SQUID, VSM, and MOKE) as a function of the dots size and magnetic field. Micromagnetic and Monte Carlo simulations confirm the experimental observations. Thermal activation and exchange bias have a large effect on the vortex nucleation field. Quantitative analysis of grazing incidence small angle neutron scattering measurements with polarization analysis, performed on 65 nm Fe dots yields the magnetization of the vortex core of 140±50 emu/cm 3 and its diameter of 19±4 nm, in agreement with the simulations results.