Unusual Magnetoresistance Research Articles

Positive magnetoresistance in heterostructure composed of two oxides

A positive magnetoresistance (MR) has been discovered in the epitaxial p–n heterostructure we fabricated with Sr-doped LaMnO3 and Nbdoped SrTiO3 by laser molecular-beam epitaxy. The MR dependence on the bias voltage has been displayed at the temperatureof 130 and 190 K. The mechanism causing the unusual positive MR is proposed as the creation of the egion near the interface with electron filling in the t2g spin-down band in La0.9Sr0.1MnO3. Other puzzling MR features with bias voltage, temperature are well explained by the present scenario.

Current inhomogeneity effect in single-layer ferromagnetic antirectangular structures

The magnetotransport properties of antirectangular arrays (2×10μm2) embedded into 200-Å-thick continuous Ni80Fe20 films with the interhole spacing equal to 2μm have been investigated. We observed that the inhomogeneous current-density distribution, due to the placement of electrical contacts, caused an unusual positive magnetoresistance (MR) behavior for the in-plane longitudinal measurements. Finite element simulations on current-density distribution are in agreement with experimental data on small area antirectangular structures. For the continuous film, however, the current-density distribution does not play a significant role in determining the shape of the MR curve.

Large magnetoresistance in low temperature metallic region of manganite compounds (La 0.7−2 xEu x)(Ca 0.3Sr x)MnO 3 (0.05≤ x≤0.15)

Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La 0.7−2 x Eu x )(Ca 0.3Sr x )MnO 3 (0.05≤ x≤0.15), in the temperature range of 5–320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4–5 T. This large high-field MR occurs in the metallic region, far below the insulator–metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin–glass behavior, etc.

Observation of a near 100% magnetoresistance in (LaBi)CaMnO

Abstract The electrical transport properties of (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 was studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T, respectively. An obvious difference in temperature and magnetic field dependence of resistivity between the (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 and pure La 0.67 Ca 0.33 MnO 3 was observed. Especially, an enhanced magnetoresistance effect was obtained in the (La 0.8 Bi 0.2 ) 0.67 Ca 0.33 MnO 3 , such as, a near 100% magnetoresistance plateau within a temperature ranging from 10 to 180 K was observed in a 3 T field. The unusual large magnetoresistance value is encouraging as regard to the potential application of CMR materials. On the other hand, a significant hysteresis both in temperature and magnetic field dependence resistivity is simultaneously observed in the Bi-doped sample.

Crossover from positive to negative magnetoresistance in a spinel.

We present a new class of colossal magnetoresistance materials based on a series of frustrated spinels. The spin glass-like compound Zn0.95Cu0.05Cr2Se4, shows a field-induced transition to a ferromagnetic, which is associated with a highly unusual negative magnetoresistance effect (MR > 80%) in low magnetic field. At higher temperatures there is an unprecedented crossover to positive magnetoresistance (MR > 50%).

Magnetic response ofFe1−xCoxSialloys: A detailed study of magnetization and magnetoresistance

Results of dc magnetization and magnetoresistance measurements are presented for the heliferromagnetic ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ alloys with $x=0.35$ and 0.45. The field and temperature dependence of the magnetization suggests the presence of Stoner-like excitations in these alloys which in turn indicates significant itinerant character of the ferromagnetic state. Similarities with other weak itinerant electron ferromagnets are pointed out. While the relatively large and unusual positive magnetoresistance in the ferromagnetic state of these alloys has earlier been rationalized in terms of quantum interference effect, magnetism probably plays a role in the magnetoresistance of these alloys near the Curie temperature and even at temperatures well inside the paramagnetic regime.

Unusual hysteresis and giant low-field magnetoresistance in polycrystalline sample with nominal composition ofLa2/3Ca1/3Mn0.955Cu0.045O3

Unusual thermal and magnetic hysteresis is observed near the insulator-metal transition of ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{3}(x=4.5%).$ For the temperature range at which the hysteresis appears, the sample shows unusual giant magnetoresistance (MR) behavior even for a low field of $\ensuremath{\sim}0.3\mathrm{T}.$ The maximum MR has a value of $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}(H=0)$ as high as $\ensuremath{\sim}90%$ for the 0.3 T field. A possible discussion is presented by considering the sample as a granular system consisting of manganese grains surrounded by some surface layer created due to the Cu segregation towards the grain surface.

Band effect: A possible mechanism for large magnetoresistance in nonmagnetic materials

Motivated by a recently observed unusual magnetoresistance in sliver chalcogenides, we propose a mechanism for large magnetoresistance in nonmagnetic materials. We show that the Zeeman splitting can play a very important role in MR under certain conditions. In general, the electron density of states can be divided into two parts, one for spin-up electrons and the other for spindown. The two parts will shift against each other in an external magnetic field due to the Zeeman effect, leading to a substantial change in the effective carrier density when the original Fermi level is near the edge of the band and the Lande factor g of electrons is large such that the electron density of states at the Fermi level is sensitive to the external magnetic field.

Normal-state magnetoresistance ofSr2RuO4

We report measurements of the in-plane and out-of-plane magnetoresistance (MR) of single crystals of the layered perovskite superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ ${(T}_{c}\ensuremath{\approx}1\mathrm{K}).$ The transverse $c$-axis MR $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}(B)$ $(I\ensuremath{\parallel}c,B\ensuremath{\parallel}ab)$ varies linearly with field at low temperatures consistent with the behavior expected for a quasi-two-dimensional Fermi surface in the intermediate-field regime. $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}(T)$ shows a striking temperature dependence which arises from the competing effects of two separate MR contributions. At low temperatures, $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}$ is large and positive due to the orbital magnetoresistance of the carriers. At high temperatures, however, $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}$ is dominated by a negative MR term that is associated with the nonmetallic out-of-plane resistivity ${\ensuremath{\rho}}_{c}(T).$ We argue that the two contributions to $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}(B,T)$ originate from separate conduction channels along the $c$ axis and that the competition between the metallic and nonmetallic channels is responsible for the unusual temperature dependence of ${\ensuremath{\rho}}_{c}(T).$ Finally, we discuss the relevance of these findings to our current understanding of the unusual magnetoresistance of the high-${T}_{c}$ cuprates.

Magnetic properties of arrays of “holes” in Ni80Fe20films

We have studied the magnetization reversal and magnetoresistance behavior in lithographically defined structures based on a 400 Å Ni80Fe20 film. The structures consist of square arrays of holes with size d in the range from 0.5 to 15 μm fabricated using electron beam lithography and an optimized pattern transfer process. For the field applied along the intrinsic easy axis, a marked increase in the coercive field is observed as the hole size is decreased. This has been attributed to the pinning of the domain walls in the vicinity of the holes. However, for the field applied along the intrinsic hard axis direction, there is a marked increase in the remanence as the hole size is reduced due to the competition between the intrinsic uniaxial anisotropy field and the shape induced magnetic anisotropy field. Unusual magnetoresistance effects are observed as a function of orientation of applied field in submicron structures.

Magnetoresistance in thin films and bulks of layered-perovskite La2−2xCa1+2xMn2O7

Epitaxial thin-film and polycrystalline bulk samples of ferromagnetic La2−2xCa1+2xMn2O7 (x=0.3) with a layered perovskite structure have been examined with respect to their magnetotransport properties. In addition to a large magnetoresistance (MR) effect at temperatures around the metal–insulator transition, a-axis thin films exhibit unusual low-temperature MR behavior with apparent hysteresis in applied magnetic fields. The features in the bulk samples are a low-temperature MR effect without such hysteresis, and also an MR effect at high temperatures well above the metal–insulator transition temperature. The unique MR behavior depending on the sample form would reflect an anisotropic nature in the layered-perovskite ferromagnet with two-dimensional Mn-O networks.

Tetragonal domain structure and magnetoresistance of La1-xSrxCoO3.

${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CoO}}_{3}$ (LSCO) has recently been found to exhibit large magnetoresistance (MR) effect. In this paper, microstructures of LSCO thin films, which may provide important clues for understanding the unusual MR characteristics in these intrinsic magnetic oxides, were determined by transmission electron microscopy. An ordered, anisotropic perovskite-type structure n-LSCO was observed and its atomic model was determined. The ordered structure has a tetragonal cell with a La-Co-Sr-Co-[001] layered atom distribution along the c axis. This structure is intrinsic for the LSCO system and is resulted directly from the lattice substitution between La and Sr. The entire film is composed of [001], [010], and [100] anisotropic domains of sizes in the order of 30--200 nm. Microstructures of (011)- and (001)-type domain boundaries were determined. Although n-${\mathrm{La}}_{0.5}$${\mathrm{Sr}}_{0.5}$${\mathrm{CoO}}_{3}$ is the dominant structure, a tetragonal phase n-${\mathrm{La}}_{0.33}$${\mathrm{Sr}}_{0.67}$${\mathrm{CoO}}_{3}$ has also been identified, which is produced by both the lattice substitution between La and Sr and the variation of local chemical composition. These structures could play a key role in determining the MR properties of the materials which are believed due to the intrinsic coupled spin scattering of Co atoms in different atomic layers. \textcopyright{} 1996 The American Physical Society.

Annealing Effects on Transport and Magnetic Properties of Perovskite-Type Vanadium Oxide CaVO3

We report transport and magnetic properties for single-crystalline CaVO 3 , which are seriously affected by oxygen off-stoichiometry. The electrical resistivity of a stoichiometric sample is well fit by the relation ρ=ρ 0 + A T 2 in an extremely wide temperature range. Oxygen off-stoichiometry causes the systematic increment of both ρ 0 and A . Magnetoresistance data also change drastically due to the oxygen off-stoichiometry; the stoichiometric sample shows positive magnetoresistance with B 2 dependence, while a sample with a small number of oxygen vacancies shows an unusual negative magnetoresistance. Magnetic susceptibility of the latter sample shows Curie-Weiss behavior, while that of the former one shows Pauli paramagnetic behavior.

Unusual magnetoresistance of mesophase-pitch-derived graphite fibers

Detailed measurements of the magnetoresistance of two types of high tensile modulus mesophase-pitch-derived graphite fibers have been performed between 1.4 and 300 K in magnetic fields to 8 T. With increasing magnetic field at low temperatures, the transverse magnetoresistance of these fibers exhibits an unusual oscillatory behavior.