Showing papers on "Magnetoresistance published in 1990"

Oscillations in exchange coupling and magnetoresistance in metallic superlattice structures: Co/Ru, Co/Cr, and Fe/Cr.

Abstract: We report the discovery of antiferromagnetic interlayer exchange coupling and enhanced saturation magnetoresistance in two new metallic superlattice systems, Co/Cr and Co/Ru. In these systems and in Fe/Cr superlattices both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance are found to oscillate with the Cr or Ru spacer layer thickness with a period ranging from 12 \AA{} in Co/Ru to \ensuremath{\simeq}18--21 \AA{} in the Fe/Cr and Co/Cr systems.

Novel magnetoresistance effect in layered magnetic structures: Theory and experiment.

Abstract: Etude dans les multicouches ferromagnetiques/non magnetiques. L'approche theorique utilise l'equation de Boltzmann. La conductivite augmente lorsque l'aimantation des couches ferromagnetiques change d'un alignement parallele a un alignement antiparallele. Les contributions du volume et de l'interface sont etudiees numeriquement en fonction du libre parcours moyen des electrons et de l'epaisseur des couches. Etude des systemes (Fe/Cr) n /Fe(n=1, 2 et 4) et Co/Au/Co. Comparaison entre la theorie et l'existence

Large magnetoresistance of field-induced giant ferrimagnetic multilayers

Abstract: Multilayers consisting of two magnetic components with different anisotropies were prepared by successively depositing Co, Cu, Ni 80 Fe 20 and Cu layers. By applying a moderate field, the two magnetizations are oriented antiparallel with each other, and then the electric resistance significantly increases due to spin-dependent electron scattering. A large magnetoresistance change, 9.9% at 300 K, was observed in [Co(30 A)/Cu(50 A)/NiFe(30 A)/Cu(50 A)]×15.

Magnetic and transport properties of Fe/Cr superlattices (invited)

Abstract: We describe the magnetic and transport properties of Fe(001)/Cr(001) superlattices grown on GaAs (001) by molecular‐beam epitaxy and characterized by reflection high‐energy electron diffraction (RHEED), Auger spectroscopy, x‐ray diffraction, and electron microscopy. For Cr layers thinner than about 30 A the magnetic behavior reveals strong antiferromagnetic couplings between the Fe layers across the Cr layers. Polarized neutron diffraction experiments confirm the existence of an antiferromagnetic superstructure. We discuss the origin of the antiferromagnetic (AF) coupling. The Fe/Cr superlattices with AF interlayer coupling exhibit a giant magnetoresistance: when an applied field aligns the magnetizations of the Fe layers, the resistivity drops by a factor of 2 for some samples. This giant magnetoresistance can be ascribed to the spin dependence of the electron scattering by interfaces. We compare our results with the predictions of two recent theoretical models.

Magnetotransport through an antidot lattice in GaAs-AlxGa1-xAs heterostructures.

Abstract: Regular two-dimensional arrays of antidots with periodicities in the range of 200--500 nm are prepared in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures by means of Ga-focused ion-beam implantation. Transport measurements at low magnetic fields reveal a strong negative magnetoresistance originating from the localization of the electrons in the potential valleys between antidots. The low-temperature mobilities of the carriers deduced from the B=0 resistance are in the range of 1000--30 000 ${\mathrm{cm}}^{2}$/V s. Under illumination mobility changes by more than a factor of 20 can be achieved. For high magnetic fields well-defined Shubnikov--de Haas oscillations and quantum Hall plateaus are observed. Close to B=0 a very small structure in the magnetoresistance occurs reflecting the commensurability of the cyclotron diameter and the periodicity of the antidot array.

Semiclassical interpretation of the angular-dependent oscillatory magnetoresistance in quasi-two-dimensional systems

Abstract: In an attempt to understand angular-dependent oscillatory magnetoresistance phenomena recently discovered in organic conductors, calculations of magnetoresistance in quasi-two-dimensional systems were carried out in the framework of the Boltzmann transport theory. Calculated magnetoresistance curves show the angular-dependent oscillations reminiscent of those found experimentally. It is argued that the essential physics underlying the resistance oscillations lies in the angular dependence of the high field asymptotic behavior (saturation vs divergence) of semiclassical magnetoresistance which arises from Fermi surface topology.

Theory of magnetic superlattices: Interlayer exchange coupling and magnetoresistance of transition metal structures (invited)

Abstract: Theoretical calculations and models to explain two unusual features of Fe/Cr magnetically layered structures are presented: (1) Strong antiferromagnetic (AF) couplings between Fe layers separated by Cr layers have been found in Fe/Cr/Fe sandwiches and Fe/Cr superlattices. These AF couplings are too strong to be accounted for by dipolar interactions and have to be ascribed to exchange interactions through the Cr layers. The interlayer exchange coupling from numerical calculations of the electronic structure of Fe/Cr superlattices based on the local density approximation is derived. (2) Recently, giant magnetoresistance effects have been found in Fe/Cr magnetically layered structures for currents in the plane of the layers. The spin‐dependent scattering at the Fe/Cr interfaces that comes from interface roughness, as well as that in the bulk of the layers are considered. The resistivity of these magnetic superlattices are calculated by adapting the quantum treatment of the electrical conductivity of ultrathi...

Very large magnetoresistance effects induced by antiparallel magnetization in two ultrathin cobalt films

Abstract: Magnetoresistance (MR) and magnetization measurements by SQUID have been performed on Au/Co.Au/Co/Au multilayers in which the two ultrathin cobalt films have perpendicular magnetizations and different coercive fields Hc1 and Hc2 due to their different thicknesses. The two Co layers are magnetically uncoupled, as shown by their well separated square hysteresis loops. The MR exhibits a plateau for magnetic fields H such as Hc1

Rotational angle detecting sensor having a plurality of magnetoresistive elements located in a uniform magnetic field

Abstract: The invention is directed to an arrangement for detecting a rotational angle of a rotating shaft by a sensor. The sensor includes a substrate and a detecting element which has a plurality of blocks of magnetoresistance element deposited on a planar surface of the substrate. The sensor also includes a magneto member which has a pair of magnetic poles facing each side end of the substrate respectively and providing a magnetic field covering at least the planar surface of the substrate. The sensor is so arranged that one of the detecting element and the magneto member is mounted on the rotating shaft, and the other is disposed at a certain position relative to the rotating shaft. Accordingly, the blocks of magnetoresistance element are disposed always in a uniform magnetic field, so that the rotational angle is detected accurately.

Magnetoresistance of a two-dimensional electron gas in a strong periodic potential.

Abstract: We have investigated the magnetoresistance of a two-dimensional electron gas subjected to a periodic potential of variable amplitude. The periodic potential, of period 300 nm, is generated with use of a gate deposited over a layer of patterned resist, and its amplitude is controlled by the gate voltage. At low gate voltages, two series of oscillations periodic in inverse magnetic field are observed. One series, at low magnetic field, is due to the periodic potential and the other is the usual Shubnikov--de Haas oscillations. The application of a small gate voltage generates an increase in the amplitude of the low-field oscillations, followed by a quenching of these oscillations as the gate voltage is increased further. In addition, a low-field positive magnetoresistance is generated, becoming larger with increasing gate voltage. These effects are explained within a semiclassical model of electron transport. Also the Shubnikov--de Haas oscillations quench as the amplitude of the potential increases. This is explained in terms of a broadening of the Landau levels.

Field-Induced Destruction of Heavy Fermion State in URu2Si2

Abstract: High-field magnetization and magnetoresistance of URu 2 Si 2 single crystal are investigated up to 600 kOe. Three-step metamagnetic transitions are obtained with the transition fields of 358, 365 and 396 kOe at 1.3 K. The experimental results are explained quantitatively by introducing the model that the heavy Fermion state produced by the mixing of the f- and conduction electrons is destroyed by applying the field and that the exchange interactions between the recovered local f-electron magnetic moments on the uranium atom play an important role on the magnetic properties at high field. The frustration due to the exchange interactions induces the three-step metamagnetic transitions.

Two-dimensional weak localization in partially graphitic carbons

Abstract: The weak-localization phenomenon for two-dimensional (2D) electronic systems is invoked to explain the negative magnetoresistance as well as the low-temperature dependence of the resistivity of pyrocarbon samples heat treated between 2000 and 2600 \ifmmode^\circ\else\textdegree\fi{}C. The 2D character is found to originate from the random stacking of the graphene layers (turbostratic structure) characteristic of pregraphitic carbon materials. For a heat-treatment temperature (HTT) lower than 2200 \ifmmode^\circ\else\textdegree\fi{}C, x-ray analysis reveals that the structure is almost turbostratic, while the material exhibits a pronounced negative magnetoresistance. For higher HTT, 3D order typical of crystalline graphite increases, leading to a 2D-to-3D crossover and to a vanishing negative magnetoresistance.

Magnetoresistance magnetic head for perpendicular recording on a magnetic support

Abstract: A horizontal magnetic reading head for a perpendicular recording on a magnetic tape. A magnetic layer (50,51,52) is interrupted by two spacers arranged symmetrically with respect to the plane of a monopole (56). A U-shaped magnetoresistant element is constituted by two ribbons (34,35) parallel to the monopole and respectively located in the two spacers of the magnetic layer. The two ribbons are joined by a magnetoresistant bridge (36). Optionally, the head can have a conductor coil so that it could write in addition to read.

Observation of magnetic resonance modes of Fe layers coupled via intervening Cr (invited)

Abstract: Multiple‐frequency (2–14 GHz) ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes of a large set of single‐crystal Fe/Cr/Fe(001) sandwiches grown by molecular‐beam epitaxy The FMR data reveal two resonance modes which have complicated frequency dependencies for those samples that show antialigned Fe layers in zero applied field Such alignment is only found for samples with 12 A

High Field Magnetoresistance and de Haas-van Alphen Effect in CeSn3

Abstract: The magnetoresistance and the de Haas-van Alphen (dHvA) effect in CeSn 3 have been measured in the field up to 150 kOe and at temperature down to 0.45 K. The magnetoresistance does not show a tendency to saturate in all field directions. This result claims that CeSn 3 is a compensated metal with an equal carrier concentration of electrons and holes, and electron and hole Fermi surfaces possess no open orbits. In contrast to an uncompensated metal LaSn 3 , the 4 f electrons in CeSn 3 become itinerant electrons. This is consistent with the dHvA data which are almost explained by the modified Fermi surface model based on the result of band calculation where the 4 f electrons are treated the same as the usual s , p , d conduction electrons.

Magnetotransport study of Fe‐Cr‐Fe sandwiches grown on ZnSe(100)

Abstract: Magnetoresistance measurements have been performed on a set of molecular‐beam epitaxy‐grown Fe‐Cr‐Fe sandwiches with different chromium thicknesses, 12 A

Hopping transport in δ-doping layers in GaAs

Abstract: We consider hopping transport in a dilutely doped sheet of Si donors in GaAs. Samples are constructed as multiple \ensuremath{\delta} layers, spaced significantly farther apart than the average, in-plane donor separation. In this system we have determined activation energies and studied the positive magnetoresistance effect quantitatively. We provide a detailed examination of the negative-magnetoresistance effect that stems from quantum interference of neighboring hopping paths.

Magnetic breakdown and magnetoresistance oscillations in a periodically modulated two-dimensional electron gas.

Abstract: We develop a semiclassical theory for the magnetoresistance oscillations recently observed in two-dimensional systems with periodically modulated potentials. We show that these oscillations may be thought of as oscillations in the probability of magnetic breakdown. Our theory demonstrates a connection between the magnitude of the oscillations and the size of the positive magnetoresistance at weak magnetic fields.

Fluctuation conductivity and normal resistivity in YBa2Cu3Oy.

Abstract: Magnetic-field-dependent fluctuation conductivity in the {ital a}-{ital b} plane of single-crystal YBa{sub 2}Cu{sub 3}O{sub {ital y}} is evaluated using recent theories for layered superconductors considering both Aslamozov-Larkin and Maki-Thompson terms. The normal resistivity, without fluctuation conductivity effects, is also estimated in the temperature region above {ital T}{sub {ital c}} using the parameters obtained from the analysis of the field-dependent fluctuation conductivity. The result is substantially different from previously estimated values that are linearly extrapolated from high-temperature data.

Effects of high magnetic fields on charge-density waves in NbSe3.

Abstract: The magnetotransport properties of ${\mathrm{NbSe}}_{3}$ in the temperature range below the second charge-density-wave (CDW) onset of 59 K have been studied in magnetic fields up to 230 kG. At liquid-helium temperatures giant magnetoquantum oscillations caused by magnetic breakdown (MB) between the normal Fermi surface (FS) and open orbits on the nested sheets of the FS dominate the magnetotransport and are extremely sensitive to the pinned CDW configuration. Spatial variations in the phase of the pinned CDW change the local Fermi level and CDW gap, giving rise to a distribution of FS cross-sectional areas. These variations in FS cross section and CDW gap produce frequency shifts, amplitude modulations, and beat structures in the quantum oscillations observed in both the magnetoresistance and Hall effect. A model conductivity tensor has been developed describing the open-orbit network and MB interference as well as the closed-orbit contribution. The adjustable parameters of the model are the frequency of oscillation, the frequency-distribution spread \ensuremath{\Delta}F, and critical MB parameter ${\ensuremath{\omega}}_{0}$\ensuremath{\tau}.The model has been used to study the detailed configurations of the pinned CDW in nominally pure ${\mathrm{NbSe}}_{3}$ crystals and in ${\mathrm{NbSe}}_{3}$ crystals doped with Fe, Ni, and Co. Unique fits to the data can be generated, and the resulting FS distributions provide direct information on the local CDW domain structure produced by pinning and repinning the CDW or by deliberately introducing impurities. At temperatures in the range 10--59 K, the oscillation amplitude decreases rapidly as the scattering time decreases, and a new magnetoresistance enhancement is observed. The dc resistance anomaly associated with the CDW is enhanced by up to a factor of 4 in a magnetic field of 226 kG and, if assigned exclusively to FS obliteration, it would require an increase from 60% obliteration at H=0 to 92% at H=227 kG. Accurate measurements also indicate a small increase in the transition temperature of \ensuremath{\sim}0.5 K in a magnetic field of 226 kG. These results have been analyzed in terms of a theory proposed by Balseiro and Falicov in which a transverse magnetic field induces a more perfect nesting of the FS.Magnetic-field modifications of the electronic spectrum at the Fermi level can become large when the cyclotron energy \ensuremath{\Elzxh}${\ensuremath{\omega}}_{c}$ is on the order of the CDW gap \ensuremath{\Delta}. Both the enhancement of the magnetoresistance and the sign change in the Hall effect can be related to this mechanism. In addition to the magnetic-field-induced changes in the static CDW state, studies of the dynamics of CDW motion have also been carried out in magnetic fields up to 230 kG. The magnetoresistance response to the CDW motion can be modeled with equations similar to those of the Bardeen tunneling model. At high electric fields the enhanced magnetoresistance in the range 10--59 K is quenched; in the quantum-oscillation regime the enhanced dc magnetoresistance is quenched while the oscillation amplitude saturates. The large magnetoresistance in the static CDW state allows measurement of the threshold electric field down to low temperatures. For the nominally pure crystals the temperature dependence of ${E}_{T}$ in high magnetic fields follows a thermal-fluctuation model similar to that observed at H=0, and the magnitude of ${E}_{T}$ is not significantly changed from the values observed at H=0. At very low temperatures a departure from the dynamics of the tunneling model can give rise to a large region of zero dynamic resistance. This behavior is observed in the highest-purity crystals and is rapidly modified by impurities. The magnetic field facilitates the study of a large range of both static and dynamic CDW effects at low temperature: A systematic classification has been accomplished. Adequate theoretical models have been developed for several of the effects, although further refinement of the models and additional experimental confirmation are needed.

Evidence for Antiferromagnetic Coupling between Fe Layers through Cr from Neutron Diffraction

Abstract: Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.

Temperature dependence of magnetoresistance oscillations in a two-dimensional electron gas subjected to a periodic potential

Abstract: We have measured the magnetoresistance at various temperatures of a two-dimensional electron gas subjected to a one-dimensional periodic potential. A series of oscillations periodic in inverse magnetic field is observed at low fields due to a resonant drift of electrons in the periodic potential, while at higher fields the Shubnikov--de Haas (SdH) oscillations are observed. The low-field oscillations persist to a much higher temperature than the SdH oscillations. We argue that the low-field oscillations are quenched when the thermal smearing of the cyclotron orbit diameter is equal to the period of the potential. Using this simple model, we show that the temperature at which the low-field oscillations are quenched is larger than that for the SdH oscillations by a factor ${\mathit{k}}_{\mathit{F}}$a/2, where a is the period and ${\mathit{k}}_{\mathit{F}}$ the Fermi wave vector. In addition, an explicit expression for the temperature dependence of the low-field oscillations is calculated and compared with our experimental data. Excellent agreement is found between the predicted and observed temperature dependences.

Subband mobility of quasi‐two‐dimensional electrons in Si atomic layer doped GaAs

Abstract: Subband mobility and conductivity of quasi‐two‐dimensional electrons in Si atomic layer doped GaAs are estimated for the first time. The oscillations for different subbands in low‐temperature magnetoresistance are separated from each other by using the reverse Fourier transform technique. The mobility for each subband is then determined by fitting the field dependence of the amplitudes with conventional theory. A large subband mobility difference up to 20:1 is found. This is mainly due to strong screening. Furthermore, a partial conductivity for each subband is calculated and the importance of the shallower subbands in total current transport is clarified.

Galvanomagnetic phenomena and surface roughness in thin metallic films

Abstract: We study the galvanomagnetic properties of an electron gas in quasibidimensional structures such as thin films, submitted to a magnetic induction perpendicular to their surfaces. The confinement generates electronic quantum states that are gathered in a subband scheme. We determine the electron distribution functions in the different subbands by solving the appropriate set of Boltzmann equations. Then we derive the general expressions of transport coefficients and discuss the variations of the Hall effect and magnetoresistance with film thickness in the special case in which electrons are scattered by impurities and surface roughness. In thin metallic films where the correlation length describing surface roughness is less than the electron Fermi wavelength, we find a Hall constant proportional to thickness d and a magnetoresistance that follows a ${\mathit{d}}^{6}$ law.

Magnetoresistance and Hall effect in epitaxial Co-Au superlattices

Abstract: Resistivity, magnetoresistance, and the Hall effect have been measured in two epitaxially ordered Co-Au superlattices at temperatures between 2 and 295 K. One sample is composed of 60 bilayers of Co 5 \AA{} Au 16 \AA{} and has a magnetic easy axis perpendicular to the film plane, while the other contains 30 bilayers of Co (30 \AA{})/Au(16 \AA{}) and has an easy axis parallel to the film plane. The magnetoresistance effect of the first sample (\ensuremath{\sim}15%) is an order of magnitude larger than in the second sample. In addition, the Co(5 \AA{})/Au(16 \AA{}) sample exhibits a large Hall voltage in zero applied magnetic field.

New phenomena of magnetoresistance in ferrimagnetic Fe/Gd multilayers

Abstract: A new type of magnetoresistance characterized by its anomalous field dependence has been found in artificially multilayered ferrimagnets of Fe/Gd and of modified double‐structured {Fe/Gd}/{Fe/Gd}. The anomaly is attributed to the spin‐flop which remarkably takes place at fields less than 1 kOe even at room temperature and is particularly pronounced in the double‐structured multilayers. In coincidence with the spin‐flop, the magnetoresistance shows an up and down change against field.

Magnetoresistance of high- Tc superconductors in the fluctuation regime

Abstract: We study theoretically the fluctuation-induced electric conductivity in a magnetic field in the clean limit. Then we analyze the magnetoresistance observed in single-crystal Y-Ba-Cu-O samples. The present theory describes the observed magnetoresistance as well as the earlier dirty-limit theory by Aronov {ital et} {ital al}. (Phys. Rev. Lett. 62, 965 (1989)) but the value of the phase-relaxation time {tau}{sub {phi}} is two times larger than the one deduced within the dirty-limit theory.