Showing papers on "Magnetoresistance published in 2001"

Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide

Abstract: Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.

Conditions for efficient spin injection from a ferromagnetic metal into a semiconductor

Abstract: We adapt the spin accumulation model of the perpendicular transport in metallic magnetic multilayers to the issue of spin injection from a ferromagnetic metal (F) into a semiconductor (N). We show that the problem of the conductivity mismatch between F and N can be solved by introducing a spin dependent interface resistance (tunnel junction preferably) at the $F/N$ interfaces. In the case of a $F/N/F$ structure, a significant value of the magnetoresistance can be obtained if the junction resistance at the $F/N$ and $N/F$ interfaces is chosen between two threshold values depending on the resistivity, spin diffusion length and thickness of N. The problem is treated for various geometries (vertical or lateral $F/N/F$ structures).

High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties

Abstract: Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 297 eV and 371 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 325 eV Different magnetoresistance behavior was observed for the samples codoped with n-type carriers Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K

Spin-polarized current induced switching in Co/Cu/Co pillars

Abstract: We present experiments of magnetization reversal by spin injection performed on pillar-shaped Co/Cu/Co trilayers The pillars (200×600 nm2) are fabricated by electron beam lithography and reactive ion etching Our data for the magnetization reversal at a threshold current confirm previous results on similar pillars In addition, we present another type of experiment that also clearly evidences the control of the magnetic configuration by the current intensity Our interpretation is based on a version of the Slonczewski model in which the polarization of the current is calculated in the Valet–Fert model of the giant magnetoresistance with current applied perpendicular to plane

Method of writing to scalable magnetoresistance random access memory element

Abstract: A method to switch a scalable magnetoresistive memory cell including the steps of providing a magnetoresistive memory device sandwiched between a word line and a digit line so that current waveforms can be applied to the word and digit lines at various times to cause a magnetic field flux to rotate the effective magnetic moment vector of the device by approximately 180°. The magnetoresistive memory device includes N ferromagnetic layers that are anti-ferromagnetically coupled. N can be adjusted to change the magnetic switching volume of the device.

Non-Fermi-liquid nature of the normal state of itinerant-electron ferromagnets

Abstract: A century of research on magnetic phenomena had led to the view that the normal state of itinerant-electron ferromagnets such as Fe, Ni and Co could be described in terms of the standard model of the metallic state or its extension known as the nearly ferromagnetic Fermi liquid theory. In recent years, however, a large body of observations has accumulated from various complex intermetallic systems that raises the possibility that this assumption might be wrong. Here we examine this issue by means of high-precision measurements of the electrical transport and magnetic properties of pure ferromagnets-in particular, MnSi-in which the Curie temperature is tuned towards absolute zero by the application of hydrostatic pressure. With this method, it is possible for us to study the normal state over an extraordinarily large range of temperature of up to five orders of magnitude above the Curie temperature. Our results using MnSi reveal a particularly striking combination of properties-most notably a T3/2 power law for the resistivity-showing clearly that the normal state of this itinerant-electron ferromagnet cannot be described in terms of the standard model of metals.

Large Tunneling Magnetoresistance in GaMnAs / AlAs / GaMnAs Ferromagnetic Semiconductor Tunnel Junctions

Abstract: We have observed very large tunneling magnetoresistance (TMR) in epitaxially grown ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}/\mathrm{AlAs}/{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ ferromagnetic semiconductor tunnel junctions. Large TMR ratios more than $70%$ (maximum $75%$) were obtained in junctions with a very thin ( $\ensuremath{\le}1.6\mathrm{nm}$) AlAs tunnel barrier when the magnetic field was applied along the $[100]$ axis in the film plane. The TMR was found to rapidly decrease with increasing barrier thickness, which is explained by calculations assuming that the parallel wave vector of carriers is conserved in tunneling.

Electrically variable multi-state resistance computing

Abstract: An electrically operated, overwritable, multivalued, non-volatile resistive memory element is disclosed. The memory element includes a two terminal non-volatile memory device in which a memory film material is included, and a circuit topological configuration is defined. The memory device relates generally to a unique new electrically induced variable resistance effect, which has been discovered in thin films of colossal magnetoresistive (CMR) oxide materials. The memory material is characterized by: 1) the electrical control of resistance through the application of short duration low voltage electrical pulses at room temperature and with no applied magnetic field; 2) increase of the resistance or decrease of the resistance depending on the polarity of the applied pulses; 3) a large dynamic range of electrical resistance values; and 4) the ability to be set at one of a plurality of resistance values within said dynamic range in response to selected electrical input signals so as to provide said single cell with multibit/multivalued storage capabilities. The memory element includes a circuit topology to construct a ROM/RAM configuration. The features of the memory element circuit are: 1) the ability to set and then measure the resistance of the two terminal multi-valued memory devices with negligible effects of sampling voltage and current; and 2) the ability to step up or down the resistance value, i. e., to set one of multiple number of resistance states, with repeated applications of pulses of varying amplitude.

Electronic transport properties of nanographite ribbon junctions

Abstract: The electronic transport properties through junctions connecting nanographite ribbons of different or same width are investigated by means of the Landauer-B\"uttiker approach using a tight binding model. Graphite ribbon with zigzag boundary has a single conducting channel of edge states in the low-energy regime. The electrical conductance as a function of the chemical potential shows a rich structure with sharp dips of zero conductance. This perfect reflectivity originates from twofold degenerate resonant levels, i.e., flux states visible in the formation of strong current-current correlation with a Kekul\'e-like vortex pattern. At each energy of conductance-zeros, this degeneracy yields the formation of standing waves in the scattering region of the junctions. The origin of zero-conductance resonances is also discussed by the standard scattering matrix approach, and the similarities between the nanographite ribbon junctions and the asymmetric Aharanov-Bohm ring connected to current leads are pointed out. Since the zero-conductance resonances are connected with the time-reversal symmetry of the system, the application of a magnetic field removes these zero-conductance dips, yielding a pronounced negative magnetoresistance.

Indication of ferromagnetism in molecular-beam-epitaxy-derived N-type GaMnN

Abstract: Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnN is reported. The GaMnN contains 7.0% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction and reflection high-energy electron diffraction. Both magnetic and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaN and forming the ferromagnetic semiconductor GaMnN. At 25 K the anomalous Hall term vanishes, indicating a Curie temperature between 10 and 25 K.

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

Abstract: We present tunneling experiments on Fe(001)/MgO(20 A)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

Magnetoresistance of Half-Metallic Oxide Nanocontacts

Abstract: Magnetoresistive effects $(R(0)\ensuremath{-}R(H))/R(H)$ exceeding $500%$ are found at room temperature in a field of 7 mT in nanocontacts between ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ crystallites. The shape of the $I(V)$ curve depends on field and the magnitude of the magnetoresistance is correlated with the resistance, the largest effects occurring when $Rg100\mathrm{k}\ensuremath{\Omega}$. The explanation proposed involves hopping transport of spin-polarized electrons through a narrow domain wall pinned at the nanocontact; spin pressure on the domain wall pushes it out into the electrode, leading to the nonlinearity of the $I(V)$ characteristic. Application of current-induced wall motion in a simple fast-switching magnetic memory element is proposed.

Gd5(SixGe1–x)4: An Extremum Material

Abstract: The large shear displacements of atomic layers in Gd5(SixGe1–x)4 materials, coupled with the change of crystallographic symmetry and magnetic order, characterizes these transformations as magnetic–martensitic, which are extremely rare. The start and the end of the magnetic–martensitic transitions depends strongly on the direction of change (i.e., increasing or decreasing) of either or both the temperature and magnetic field. These profound bonding, structural, electronic, and magnetic changes, which occur in the Gd5(SixGe1–x)4 system, bring about some extreme changes of the materials' behavior resulting in a rich variety of unusually powerful magneto-responsive properties, such as the giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance.

Giant magnetoresistance-based eddy-current sensor

Abstract: The purpose of this paper is to introduce a new eddy-current testing technique for surface or near-surface defect detection in nonmagnetic metals using giant magnetoresistive (GMR) sensors. It is shown that GMR-based eddy-current probes are able to accurately detect short surface-breaking cracks in conductive materials. The self-rectifying property of the GMR sensor used in this study leads to a simplified signal conditioning circuit, which can be fully integrated on a silicon chip with the GMR sensor. The ability to manufacture probes having small dimensions and high sensitivity (220 mV/mT) to low magnetic fields over a broad frequency range (from dc up to 1 MHz) enhances the spatial resolution of such an eddy-current testing probe. Experimental results obtained by scanning two different probes over a slotted aluminum specimen are presented. General performance characteristics are demonstrated by measurements of surface and subsurface defects of different sizes and geometries. Dependence of the sensor output on orientation, liftoff distance, and excitation intensity is also investigated.

Thin film magnetoresistive sensors

Abstract: Preface. 1 AMR Sensors. Anisotropic magnetoresistive effect in thin ferromagnetic films. Essential information about magnetoresistive effects. The origin of anisoptropic magnetoresistance in ferromagnetic metals. Thin ferromagnetic film as the magnetic field sensor. Applying the Stoner-Wohlfarth model for analysis of magnetorestive effects. Biasing and stabilizing techniques. The biasing and stabilizing fields in ferromagnetic magnetoresistors. The real thin film - the multidomain structure and the dispersion of anisotropy. The sensor biased by the hard magnetic layer. The sensors biased by the current conducting layer. Biasing by exchange coupled antiferromagnetic layer. The soft adjacent layers (SAL) biasing technique. Dual element sensors. AC biasing techniques. The reverse mode of biasing. Design and performances of AMR sensors. The influence of the sample geometry on the MR effect. The influence of magnetization non-uniformity on the MR effect. Technological factors affecting the performances of AMR sensors. The design and construction of AMR sensors. The performances of AMR sensors. References. 2 GMR Sensors. Giant magnetoresistive effects. An historical review and the main terms. Oscillatory exchange coupling in the magnetic multilayers. Other coupling effects in multilayer structures. Other coupling effects in multilayer structures. Theoretical models of giant magnetoresistance. Ferromagnetic multilayers as magnetic field sensors. Various types of GMR structures. Structures with antiferromagnetic coupling. Granular GMR structures. Spin-valve structures with asymmetric magnetic layers (uncoupled structures). Spin Valve structures with exchange-biased layer. Current perpendicular to plane (CPP) structures. Magnetic tunnel junction (MTJ) structures. Giant magnetoimpedance (GMI) structures. Preparation, design and properties of GMR sensors. The deposition of thin film GMR structures. Technological factors affecting the performances of GMR sensors. The shape effects in thin film GMR devices. Design and construction of GMR sensors. References. 3 Applications of Magnetorisistive Sensors. Magnetic measurements. Magnetometers and compasses. Gradiometers, magnetic anomaly detection. Electrical measurements. Current transducers. Electrical transducers, switching and logic elements. Magnetoresistive elements in data storage applications. Magnetic Random Access Memory (MRAM) devices. Magnetic card readers. Magnetoresistive heads for tape and disk applications. Unshielded magnetoresistive reading heads. Shielded magnetoresistive heads. Yoke-type magnetoresistive heads. Transducers of mechanical values. Transducers of linear displacement. Transducers of angular position. Measurements of rotational speed. Materials testing and magnetic field imaging. Material testing by means of thin film magnetoresistors. Magnetic imaging systems. References. List of Symbols. List of Abbreviations. Index

Magnetoresistance effect element, magnetic head, magnetic head assembly, magnetic storage system

Abstract: Disclosed are a high-sensitivity and high-reliability magnetoresistance effect device (MR device) in which bias point designing is easy, and also a magnetic head, a magnetic head assembly and a magnetic recording/reproducing system incorporating the MR device. In the MR device incorporating a spin valve film, the magnetization direction of the free layer is at a certain angle to the magnetization direction of a second ferromagnetic layer therein when the applied magnetic field is zero. In this, the pinned magnetic layer comprises a pair of ferromagnetic films as antiferromagnetically coupled to each other via a coupling film existing therebetween. The device is provided with a means of keeping the magnetization direction of either one of the pair of ferromagnetic films constituting the pinned magnetic layer, and with a nonmagnetic high-conductivity layer as disposed adjacent to a first ferromagnetic layer on the side opposite to the side on which the first ferromagnetic layer is contacted with a nonmagnetic spacer layer. With that constitution, the device has extremely high sensitivity, and the bias point in the device is well controlled.

Magnetoresistance effect element, magnetic head and magnetic recording and/or reproducing system

Abstract: There is provided a practical magnetoresistance effect element which has an appropriate value of resistance, which can be sensitized and which has a small number of magnetic layers to be controlled, and a magnetic head and magnetic recording and/or reproducing system using the same. In a magnetoresistance effect element wherein a sense current is caused to flow in a direction perpendicular to the plane of the film, a resistance regulating layer is provided in at least one of a pinned layer, a free layer and an non-magnetic intermediate layer. The resistance regulating layer contains, as a principal component, an oxide, a nitride, a fluoride, a carbide or a boride. The resistance regulating layer may be a continuous film or may have pin holes. Thus, it is possible to provide a practical magnetoresistance effect element which has an appropriate value of resistance, which can be sensitized and which has a small number of magnetic layers, while effectively utilizing the scattering effect depending on spin.

Magnetoresistance of Mn:Ge ferromagnetic nanoclusters in a diluted magnetic semiconductor matrix

Abstract: We have fabricated a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute ferromagnetic semiconductor matrix. The clusters form for growth temperatures of ∼300 °C with an average diameter and spacing of 100 and 150 nm, respectively. While the clusters dominate the magnetic properties, the matrix plays a subtle but interesting role in determining the transport properties. Variable range hopping at low temperatures involves both nanoclusters and MnGe sites, and is accompanied by a negative magnetoresistance attributed in part to spin-dependent scattering analogous to metallic granular systems.

Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3

Abstract: The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the s...

Strong ferromagnetism and weak antiferromagnetism in double perovskites: Sr2FeMO6 (M = Mo, W, and Re)

Abstract: Double perovskites Sr2FeMO6 (M = Mo and Re) exhibit significant colossal magnetoresistance even at room temperature due to the high Curie temperatures (419 and 401 K). However, such a high Curie temperature is puzzling, given the large separation between

Large magnetoresistance using hybrid spin filter devices

Abstract: A magnetic "spin filter" tunnel barrier, sandwiched between a non-magnetic metal and a magnetic metal, is used to create a new magnetoresistive tunnel device, somewhat analogous to an optical polarizer-analyzer configuration. The resistance of these trilayer structures depends on the relative magnetization orientation of the spin filter and the ferromagnetic electrode. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%.

Fermi surface nesting and nanoscale fluctuating charge/orbital ordering in colossal magnetoresistive oxides.

Abstract: We used high-resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered colossal magnetoresistive oxide La1.2Sr1.8Mn2O7. With these parameters, the calculated in-plane conductivity is nearly one order of magnitude larger than the measured direct current conductivity. This discrepancy can be accounted for by including the pseudogap, which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and to many other properties are the parallel straight Fermi surface sections, which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations, which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange.

Interface ferromagnetism in oxide superlattices of CaMnO3/CaRuO3

Abstract: Oxide superlattices composed of antiferromagnetic insulator layers of CaMnO3 (10 unit cells) and paramagnetic metal layers of CaRuO3 (N unit cells) were fabricated on LaAlO3 substrates by pulsed-laser deposition. All the superlattices show ferromagnetic transitions at an almost identical temperature (TC∼95 K) and negative magnetoresistance below TC. Each magnetization and magnetoconductance of the whole superlattice at 5 K is constant and independent of CaRuO3 layer thickness when normalized by the number of the interfaces between CaMnO3 and CaRuO3. These results indicate that the ferromagnetism shows up only at the interface and is responsible for the magnetoresistance.

Electrical transport in doped multiwalled carbon nanotubes

Abstract: The effects of doping, electron coherence, and electron correlation on the transport properties of boron-doped multiwalled carbon nanotubes are studied. Substitutional boron lowers the Fermi level of the tubes and increases the number of participating conduction channels without introducing strong carrier scattering. From 300 to about 50 K, the tubes show metallic behavior with weak electron-phonon coupling. At lower temperatures the resistance increases, and a zero-bias anomaly is observed. The magnetoresistance is now negative indicating the importance of coherent back-scattering processes. The coherence lengths are measured and dephasing is found to involve weakly inelastic electron-electron collisions. The temperature dependence of the resistance as well as the other low temperature observations can be accounted for by one-dimensional weak-localization theory.

Martensitic accommodation strain and the metal-insulator transition in manganites

Abstract: In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance ${(10}^{10}%)$ in low magnetic fields.

Enhanced room-temperature magnetoresistance in La0.7Sr0.3MnO3-glass composites

Abstract: In this letter, we report the improved magnetotransport properties of La0.7Sr0.3MnO3-borosilicate glass composite with different weight percents of glass. All the composites showed ferromagnetic nature at room temperature. The microstructure of these composites was seen using a scanning electron microscope. The microstructure was reconfirmed using spot energy dispersive x-ray analysis. We observe an enhancement of the low-field magnetoresistance (<200 Oe) at room temperature for the optimal composition of 25 wt % of glass. It is argued that glass layer separating the grain boundaries may be acting as barrier for spin-polarized tunneling, thereby enhancing the low-field magnetoresistance.

Interplay of structure and transport properties of sodium-doped lanthanum manganite

Abstract: The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La1-xNaxMnO3 (0.07≤x≤0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07≤x≤0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La1-xNaxMnO3 compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO6 octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT2 behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

Surface spin-glass behavior in La2/3Sr1/3MnO3 nanoparticles

Abstract: The low-temperature magnetic and transport properties of La2/3Sr1/3MnO3 nanoparticles have been investigated. It is found that a surface spin-glass behavior exists in La2/3Sr1/3MnO3 nanoparticles, which undergo a magnetic transition to a frozen state below 45 K. The low-temperature surface spin-glass behavior exists even at the highest field used (H=50 kOe). Moreover, the spin-glass-like transition disappears for particles above 50 nm. In addition, the suppressed low-field magnetoconductivity (LFMC) observed at low temperature for nanosized La2/3Sr1/3MnO3 is obviously lower than the expected upper limit of LFMC, 1/3, for polycrystalline manganites, which is proposed to arise from the higher-order tunneling through the insulating spin-glass-like surface layers.

Ultrafast conductivity dynamics in colossal magnetoresistance manganites.

Abstract: Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4–1.0 THz) of La0.7M0.3MnO3 thin films (M Ca, Sr) from 10 K to 0.9Tc reveal a two-component relaxation. A fast, 2 ps, conductivity decrease arises from an optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. We show that, for T o Tc, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate near Tc. The observation of “colossal” negative magnetoresistance (CMR) in the hole-doped manganite perovskites (R12xDxMnO3, where, e.g., R La, Nd and D Ca, Sr) demonstrates the sensitivity of electronic conduction to the underlying magnetic structure in these materials [1,2]. Experimental and theoretical work has also revealed the importance of the lattice and orbital degrees of freedom in determining the electronic properties of CMR materials above and below Tc [3,4]. Nonetheless, it is still not clear, especially for T o Tc, what the relative importance of phonons is in comparison to double exchange in determining s. Ultrafast optical spectroscopy has provided significant insight into electron dynamics in metals [5–7], and more recently, transition metal oxides [8 –10]. Using similar ultrafast techniques, we address the relative contributions of spin fluctuations and phonons in determining the conductivity in the manganites from 10 Kt o0.9Tc. Terahertz time-domain spectroscopy is an ultrafast optical technique in which electric field transients are used to measure the complex conductivity of a material. Since this is a coherent technique, a sample can be optically excited and then probed with a terahertz (THz) pulse to measure induced conductivity changes with picosecond (ps) resolution. We use this method, known as timeresolved terahertz spectroscopy (TRTS), to measure ps conductivity transients in La0.7Ca0.3MnO3 (LCMO) and La0.7Sr0.3MnO3 (LSMO) thin films. The dynamics occur on two time scales. A fast, 2 ps, conductivity decrease arises from optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. Our results demonstrate that, at low temperatures, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate close to Tc. The TRTS experiments were performed on LCMO and LSMO epitaxial thin films grown on LaAlO3 substrates using pulsed laser deposition [11]. For very thin films (150 A), island growth can alter the film properties, but the thicker films used in these experiments (1000 A) display bulk behavior [12]. Magnetization measurements

Antisite defects and magnetoresistance in Sr2FeMoO6 double perovskite

Abstract: We report a detailed study of the magnetic and magnetotransport properties of Sr2FeMoO6 ceramics having a controlled concentration of antisite (AS) defects. It is found that a high-field differential susceptibility exists in all samples, which increases with AS. Similarly, a high-field magnetoresistivity develops and mimics the differential susceptibility. These observations suggest that antisite defects promote some magnetic frustration. High-resolution electron microscopy studies have allowed observation of the existence of antiphase boundary defects in the Sr2FeMoO6 structure.