Well-defined Co clusters embedded in an Ag matrix: A model system for the giant magnetoresistance in granular films

Well-defined granular Co/Ag films have been prepared by the co-deposition of in-beam prepared Co clusters and Ag atoms. In this way we were able to study the giant magnetoresistance (GMR) as a function of mean Co cluster size for a fixed Co cluster volume fraction v_cl as well as a function of v_cl for a fixed . Mean Co cluster size has been varied between and 6.9 nm, Co cluster volume fraction between 5 and 43%. The GMR was measured in-situ at T=4.2 K in magnetic fields 1.2 T. The analysis of the GMR data obtained from these studies clearly shows that spin-dependent scattering at the Co-cluster/Ag-matrix interface is the only relevant scattering mechanism causing the GMR in our well-defined samples. Received: 21 April 1998 / Received in final form: 17 May 1998 / Accepted: 18 May 1998     

Charge-density-wave instabilities and quantum transport in the monophosphate tungsten bronzes with m = 5 alternate structure

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m for m =5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at 160 K and a possible second one at 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below 30 K. Received 23 November 1999 and Received in final form 23 March 2000     

Galvanomagnetic properties of disordered Mn semi-Heusler phases with Anderson localisation

The Anderson localisation has been observed in Heusler-type solid solutions where Mn replaces Ti or V in NiTiSb or CoVSb compounds respectively, due to the atomic and magnetic disorders. The magnetoresistance, Hall effect, thermopower and electron spin resonance of these solutions are investigated. Strong anomalies appear for the concentration range where a carrier localisation occurs; the mobility of carriers is strongly reduced, the magnetoresistance scales with the resistivity and the susceptibility of solid solutions. Received 31 May 2000     

The temperature dependence of the perpendicular giant magnetoresistance in Co/Cu multilayered nanowires

A theory, based on earlier work by Valet and Fert, is first presented to describe the influence of temperature on the perpendicular giant magnetoresistance (GMR) in multilayers. Then we present GMR measurements performed at T=77 K and at room temperature on Co/Cu multilayered nanowires with layer thicknesses ranging from a few nm to 1 μm. We use our model to obtain a good quantitative fit to the experimental results in both the short spin diffusion length limit and out of this limit. We discuss the temperature dependence of the bulk parameters, the scattering spin asymmetry coefficient and spin diffusion length in the Co layers. Received: 25 January 1998 / Accepted: 6 May 1998     

Localization effects in the charge density wave state [4]of the quasi-two-dimensional monophosphate tungsten bronzes {(P{O_2})_4}{(W{O_3})_{2m}}(m = 7,8,9)

The monophosphate tungsten bronzes (PO ₂ ) ₄ (WO ₃ ) _2m are quasi-two-dimensional conductors which show charge density wave type electronic instabilities. We report electrical resistivity and magnetoresistance measurements down to 0.30 K and in magnetic fields up to 16 T for the m = 7, 8 and 9 members of this family. We show that these compounds exhibit at low temperature an upturn of resistivity and field dependences of the magnetoresistance characteristic of localization effects. We discuss the dimensionality of the regime of localization as m is varied. We show that for m =7, the regime is quasi-two-dimensional and three-dimensional for m = 8, 9. Received 16 September 1999     

Angular studies of the magnetoresistance in the density wave state of the quasi-two-dimensional purple bronze KMo<Subscript>6</Subscript>O<Subscript>17</Subscript>

The purple molybdenum bronze KMo₆O₁₇ is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.     

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)₂ClO₄  performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a, b) planes which is governed by the magnetic field component along the b^′ direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.     

Magnetic Bloch function and its application to FQHE

Summary In the present work we have constructed the orthonormal complete set of spatially extended magnetic Block functions in the subspace of the lowest landau level which can be uniquely represented by the Hilbert space of entire functions,i.e. the Fock-Bargman space, and discussed its application to the problem of fractional quantum Hall effect.     

Magnetic phase diagram of perovskite Mn oxides at T = 0

Minimizing total free energy by numerical calculations, we obtain the magnetic phase diagram of perovskite Mn oxides, such as with , Ca, Sr, etc. in the whole doping region from x =0 to x =1 at temperature T =0. It is discovered that a spiral state is stable in a low concentration of X ions while a canted state is stable in a high concentration of X ions, and a ferromagnetic phase can exist in the intermediate concentrations when the antiferromagnetic interaction is weak. The energy difference between spiral and canted states is found to be small when the Hund coupling is large. Magnetic field induced spiral/canted phase transition is considered as a possible mechanism of the colossal magnetoresistance (CMR) in the Mn oxides. Received: 11 July 1996 / Revised: 7 December 1996 / Accepted: 24 July 1997     

Mesoscopic charge density wave in a magnetic flux

The stability of a Charge Density Wave (CDW) in a one-dimensional ring pierced by a Aharonov-Bohm flux is studied in a mean-field picture. It is found that the stability depends on the parity of the number N of electrons. When the size of the ring becomes as small as the coherence length , the CDW gap increases for even N and decreases for odd N. Then when N is even, the CDW gap decreases with flux but it increases when N is odd. The variation of the BCS ratio with size and flux is also calculated. We derive the harmonics expansion of the persistent current in a presence of a finite gap. Received: 16 September 1997 / Received in final form: 12 November 1997 / Accepted: 13 November 1997     

Anomalous temperature dependence of resistivity in quasi-one-dimensional conductors in a strong magnetic field

We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (), when its effective dimensionality is one, and like a metal (), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor'kov and find that the theoretical results do not agree with the experimental observations. Received 20 October 1998     

Interfacial roughness and angle dependence of giant magnetoresistance in magnetic superlattices

Using the two-point conductivity formula, we numerically evaluate the giant magnetoresistance (GMR) in magnetic superlattices with currents in the plane of the layers (CIP), from which the effect of the interfacial roughness and magnetization configuration on the GMR is studied. With increasing interfacial roughness, the maximal GMR ratio is found to first increase and then decrease, exhibiting a peak at an optimum strength of interfacial roughness. For systems composed of relatively thick layers, the GMR is approximately proportional to ,where is the angle between the magnetizations in two successive ferromagnetic layers, but noticeable departures from this dependence are found when the layers become sufficiently thin. Received 21 September 1998 and Received in final form 22 December 1998     

On the cross-over from half-metal to normal ferromagnet in NiMnSb

Magnetism and transport properties of the semi-Heusler compound NiMnSb are re-examined in great details. A wide set of experiments (elastic and inelastic neutron scattering, static magnetic measurements, magnetoresistance, Hall effect, thermopower, FMR) have been performed on polycrystals, single crystals or single-crystalline thin films, and the results are analysed. Special emphasis is given to the magnetic excitations and to the relaxation mechanisms in this metallic ferromagnet. At low temperatures, all experimental results hint at the existence of a fully spin-polarized conduction band (half metallic state). At higher temperature (T > 80 K), but well below the Curie temperature (730 K), a cross-over to a usual metallic ferromagnetic state is evidenced and discussed. Received 10 January 2000     

Balance-equation approach to terahertz-field-driven magnetotransport in semiconductors

We investigate the magnetotransport in semiconductors under the influence of a dc or slowly-varying electric field, an intense polarized radiation field of terahertz frequency, and a uniform magnetic field, being in arbitrary directions and having arbitrary strengths. Effective force- and energy-balance equations are derived by using a gauge that the magnetic field and the high-frequency radiation field are described by a vector potential and the dc or slowly-varying field by a scalar potential, and by distinguishing the slowly-varying velocity from the rapidly-oscillating velocity related to the high-frequency field. These equations, which include the elastic photon process and all orders of multiphoton absorption and emission processes, are applied to the examination of the effect of a terahertz radiation on the magnetophonon resonance of the longitudinal resistivity in the transverse configuration in nonpolar and polar semiconductors. We find that the previous zero-photon resonance peaks are suppressed by the irradiation of the terahertz field, while many new peaks, which may be related to multiphoton absorption and emission processes, emerge and can become quite distinct, at moderately strong radiation field. Received 17 May 1999     

Hall effect plateaus and giant Shubnikov-de Haas oscillations in (BiSb)Telayered single crystals near the quantum limit

On bulk layered single crystals (Bi_0.25Sb_0.75)₂Te₃ with a hole concentration cm^-3 and a mobility cm²/Vs magnetoresistance and Hall effect investigations were performed in the temperature range T = 1.4 K ... 20 K in magnetic fields up to 18 T. For the magnetic field perpendicular to the layered structure giant Shubnikov-de Haas oscillations are measured; the positions of the maxima are triplets in the reciprocally scaled magnetic field. From the damping of the amplitudes with increasing temperature the cyclotron mass m _c = 0.12m ₀ is evaluated. Correlated with the SdH oscillations doublets of Hall effect plateaus (or kinks in low fields) are found. The weak well known Shubnikov-de Haas oscillations from the generally accepted multivallied highest valence band can be detected as a modulation on the giant oscillation. The high anisotropy of the SdH oscillations and their triplet structure in connection with the layered crystal structure lead us to suggest that the effects are caused by hole carrier pairing (mediated by the bipolaron mechanism) in quasi 2D sheets parallel to the crystal layer stacks. The measured Hall plateau resistances coincide with the quantum Hall effect values considering the number of layer stacks and the valley degeneracy of the 3D hole carrier reservoir. The ratio of spin splitting to Landau (cyclotron) splitting is observed to be . Received: 12 September 1997 / Revised: 8 January 1998 / Accepted: 22 January 1998     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

A phenomenological model of percolating magnetic nanostructures

Transport and magnetotransport properties were analysed systematically in percolating magnetic nanostructures such as Ni-rich and films. These granular magnetic films exhibit giant Hall effect. We identified features which are common and unique to these systems. Among the features are the correlation between a -like temperature dependent resistivity and a particle size distribution having a large fraction of small nanometer sized particles, and the power law dependence between the magnetoresistivity and the room temperature resistivity. Assuming the presence of nanometer sized particles in the percolating conduction channels whose contributions are sensitive to temperature and the external magnetic field, we developed a phenomenological model to explain all the common features. Received 4 November 1998     

Structural features of the La-Sr-Fe-Co-O system

A structural study has been performed on the La_0.8Sr_0.2Fe_xCo_1-xO₃ (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. ⁵⁷Fe M?ssbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La_0.8Sr_0.2Fe_xCo_1-xO₃ compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe³⁺. Received 22 January 2001     

Influence of preparation method on SrMoO4 impurity content and magnetotransport properties of double perovskite Sr2FeMoO6 polycrystals

The magnetic and electric properties of the Sr₂FeMoO₆ compound produced under different preparation conditions were studied. Depending on the preparation condition, a strong variation in the nonmagnetic SrMoO₄ impurity content was found, which in turn determined the metallic or semiconducting behavior of the resistivity of the Sr₂FeMoO₆ compound. There was also evidence that SrMoO₄ played a crucial role in modifying the low magnetic field intergrain tunneling magnetoresistance in Sr₂FeMoO₆. In addition, we have established a simple method to prepare the single phase Sr₂FeMoO₆ polycrystals.