Significant Negative Magnetoresistance Research Articles

Sr2Mn0.5Hf1.5O6-δ double perovskite oxides: Structural, dielectric, magnetic, optical and transport properties

Double perovskite (DP) oxides with a chemical formula of A2B′B”O6 exhibit intriguing physical properties and structural flexibilities, which are widely used as a host structure to explore the novel candidates of half-metallic ferro-/ferrimagnetic materials. In this work, we have synthesized ferromagnetic Sr2Mn0.5Hf1.5O6-δ (SMHO) DP powders via solid-state reaction method. The powders had a magnetic Curie temperature up to 600 K, saturation magnetization of 0.59 emu/g at 2 K, and magnetic coercive field of 450 Oe. The SMHO ceramics displayed a butterfly-like magnetoresistance (MR)-H curve at 2 K due to the intergranular tunneling effect, and the MR (2 K, 7 T) value was −2.8 %. Structural investigations demonstrated the SMHO powders crystallized in an orthogonal crystal structure with Pnma symmetry. SEM images displayed a spherical morphology of the SMHO powders with particle size distribution of 100–200 nm. Quantitative EDS (energy dispersive X-ray spectra) data give out the percentage of Sr, Mn, Hf and O elements presented in approximately stoichiometric proportion. XPS spectra results confirmed the existence of Sr2+, Mn4+, and Hf4+/Hfx+(x＜4) ions in the powders, and oxygen elements were presented as the lattice oxygen and adsorbed oxygen species, respectively. Resistivity of the SMHO ceramics decreased continuously as the temperature increased from 2 K to 800 K, exhibiting a semiconductor behavior. The electrical transport data were well fitted by thermal activation model, small polaron hopping model, and Mott’s variable range hopping model, respectively in different temperature regions. Optical measurements gave out an indirect bandgap of 2.12 eV for the SMHO powders. Our present work demonstrates that the SMHO oxides are attractive in high temperature spintronic devices and solar cells owing to the integration of high TC, significant negative MR and optical bandgap located in the visible light window.

Magnetic frustration and paramagnetic state transport anomalies in Ho4RhAl and Er4RhAl: Possible test cases for newly identified roles of itinerant electrons

We report the results of magnetic, heat-capacity, electrical and magnetoresistance measurements on Ho4RhAl and Er4RhAl, characterized by 3 sites for rare-earths (R). The main conclusions are: (i) Antiferromagnetic ordering sets in at (TN = ) ~ 8.8 and ~ 4.0 K respectively. While Ho compound appears to enter into a complex spin-glass phase at T < TN (at nearly 5 K), spin-glass component appears to set in essentially almost at TN for the Er case. (ii) The loss of the spin-disorder contribution in the magnetically ordered state is not pronounced, mimicking that in Gd2PdSi3, a compound which now attracts interest in the area of topological Hall effect and magnetic skyrmions, indicating complex Fermi surface. (iii) There is a minimum in the temperature dependence of electrical resistivity in the case of only Ho above TN, but significant negative magnetoresistance is observed over a wide temperature range in the paramagnetic state increasing with decreasing temperature for both the cases. This finding establishes that these compounds belong to a select group of intermetallics in which spin-disorder contribution apparently increases gradually as one approaches respective TN with decreasing temperature. This could be an experimental signature for the effect due to classical spin-liquid above TN. In view of these properties analogous to those of Gd2PdSi3, it is of interest to investigate these 4:1:1 compounds further to understand possible unconventional roles of itinerant electrons, not only in the magnetically ordered state but also in the paramagnetic state, predicted by some theories in recent years for which this Gd compound is considered to be a classic example. Besides, magnetoresistance and isothermal entropy change (magnetocaloric effect) track each other. Isothermal entropy change at the peak in its plot versus temperature is large in comparison with that of the Gd analogue, possible implication of which is pointed out.

Successive destruction of charge density wave states by pressure in LaAgSb2

We comprehensively studied the magnetotransport properties of LaAgSb$_2$ under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at $T_{CDW1}\sim 210$ K and $T_{CDW2}\sim 190$ K at ambient pressure. With the application of pressure, both $T_{CDW1}$ and $T_{CDW2}$ were suppressed and disappeared at the critical pressures of $P_{CDW1}=3.0$--3.4 GPa and $P_{CDW2}=1.5$--1.9 GPa, respectively. At $P_{CDW1}$, the Hall conductivity showed a step-like increase, which is consistently understood by the emergence of two-dimensional hollow Fermi surface at $P_{CDW1}$. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the $c$ axis. Shubnikov--de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In $P<P_{CDW2}$, three major oscillation components, $\alpha$, $\beta$, and $\gamma$, were identified, whose frequencies were increased by application of pressure. The increment rate of these frequencies was considerably larger than that expected from the shrinkage of lattice constant, indicating the unignorable band modification under pressure. In the normal metallic phase above $P>P_{CDW1}$, we observed a single frequency of $\sim 48$ T with a cyclotron effective mass of 0.066 $m_0$, whose cross section in the reciprocal space corresponded to only 0.22\% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of $\sim 9.2$ T, which is significantly enhanced in the limited pressure range of $P_{CDW2}<P<P_{CDW1}$. The amplitude of this oscillation was anomalously suppressed in the high-field and low-temperature region, which cannot be explained by the conventional Lifshitz--Kosevich formula.

Low temperature spin polarized tunnelling magneto-resistance in La1-xCaxMnO3 (x=0.375 and 0.625) nanoparticles

The structural, electrical and magneto-transport properties of the La1-xCaxMnO3 (x = 0.375 and 0.625) nanoparticles (~ 50 nm) synthesized by the sol-gel method have been reported. X-Ray diffraction pattern recorded at room temperature confirms the growth of the samples in a single phase. In the absence of magnetic field, a metal to insulator (MI) transition is observed in x = 0.375 sample at 172 K (TMI), while the x = 0.625 sample shows a charge ordered (CO) behaviour at temperatures below 150 K (TCO). Both samples show a substantial negative magneto-resistance (MR). The fitting of the field-dependent MR isotherms by scaling law displays linear variation below TMI/TCO and quadratic above TMI/TCO. The fitting result suggests that a significant negative MR at low temperature is due to inter-grain spin polarized tunnelling across grain boundary and domain contribution.

Growth and Nanofabrication of All-Perovskite Superconducting/Ferromagnetic/Superconducting Junctions

We fabricate and study experimentally all-perovskite-oxide superconductor/ferromagnetic insulator/superconductor (S/FI/S) tunnel junctions made out of the high-temperature cuprate superconductor YBa2Cu3O7−y (YBCO) and the colossal magnetoresistive manganite LaMnO3 (LMO) in the ferromagnetic insulator state. YBCO/LMO/YBCO heterostructures with different LMO thicknesses (5, 10, and 20 nm) are grown epitaxially via pulsed laser deposition. Nanoscale S/FI/S junctions with sizes down to 300 nm are made by three-dimensional nano-sculpturing with focused ion beam. Junctions with a thick (20 nm) LMO barrier exhibit a large negative magnetoresistance below {T}_{Curie}sim 160 K, typical for colossal magnetoresistive manganites, as well as a kink in the current-voltage characteristics at large bias ( Vsim 1 –2 Volts), attributed to Zener-type tunneling. However, they do not show a measurable Josephson current. On the contrary, junctions with the thinnest 5-nm LMO barrier exhibit a large supercurrent and no signs of magnetism. The latter may indicate the presence of pinholes due to thickness inhomogeneity and/or a sim 2 nm dead magnetic layer at the YBCO / LMO interface caused, e.g., by interdiffusion or strain. The junction with an intermediate 10-nm LMO barrier exhibited a desired S/FI/S junction behavior with significant negative magnetoresistance and signatures of a small Josephson current.

Critical magnetic behavior and large magnetocaloric effect in Pr0.67Ba0.33MnO3 perovskite manganite

We report results of critical magnetic behavior and magnetocaloric investigations of the perovskite manganite Pr0.67Ba0.33MnO3. The compound exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature TC, and significant negative magnetoresistance in a wide temperature range. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition region. Magnetic entropy change ΔSM was estimated from isothermal magnetization data. We have found a remarkable large value of |ΔSM| around TC’s, of about 5.50J/kgK for μ0ΔH=4T and a large relative cooling power (~225J/kg). The analysis was done by using the modified Arrot plot (MAP) method. The values of the obtained critical exponents associated with this transition, β=0.366, γ=1.375 and δ=4.743 are close to those expected for the short range 3D Heisenberg model. The model was also confirmed by using |ΔSM|∝(μ0H)n, the field dependence of magnetic entropy change method. The large measured magnetocaloric effect is presumed to arise as a consequence of the sample preparation route.

Significant magneto-resistive effects in boron carbide thin films

We have found large room temperature magneto-resistance in boron carbides fabricated via electron-induced cross-linking of icosahedral closo-1,2-dicarbadodecaborane (ortho-carborane; 1,2-B10C2H12) in the presence of the aromatic compound 1,4-diaminobenzene (DAB). X-ray photoemission spectroscopy confirms that the electron beam irradiation leads to site-specific cross-linking of the ortho-carborane, with cross-linking between B sites non-adjacent to icosahedral carbon sites on the carborane moiety, and carbon sites on the diaminobenzene moiety. The I–V curves, as a function of external magnetic field, demonstrate that significant room temperature negative magneto-resistance (>50%) is possible in the resulting dielectric thin films. Inclusion of 1,4-diaminobenzene (DAB) is not essential for significant negative magneto-resistance, as crosslinking of ortho-carborane (1,2-B10C2H12 ) results in large negative magneto-resistance, over 100%, depending on bias voltage.

Transport, thermoelectric, and magnetic properties of a dense Cr2S3 ceramic

By using the spark plasma sintering technique, a dense Cr2S3−x ceramic, crystallizing in the trigonal P3¯1c space group of Cr2S3, has been prepared by starting from a powder of R3¯ Cr2S3 precursor. The Cr and S contents of the densified sample deviate from the 2/3 ratio, indicating the presence of extra Cr at the level of the deficient “Cr1/3S” layer of the Cr2S3 structure, thus leading to the Cr2S2.8 formula. In contrast to the large magnetoresistance associated to the ferromagnetic-like component in the stoichiometric rhombohedral precursor, no significant negative magnetoresistance could be measured in the trigonal, sulfur deficient, Cr2S3. However, the non stoichiometry and the densification induce a higher charge carrier concentration and a reduced contribution of the grain boundaries, respectively. This explains the low resistivity value, ρ300 K = 2.65 mΩ · cm. With the thermal conductivity remaining below 2 W · m−1K−1 up to 700 K, a thermoelectric figure of merit reaching zT = 0.12 at 675 K is achieved.

Correlation between the crystal structure and the Curie temperature in RCu3(Mn3Fe)O12 (R = rare-earth) complex perovskites

New members of the family of complex-perovskite oxides with the formula RCu(3)(Mn(3)Fe)O(12) (R = Ce, Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples have been prepared from citrate precursors treated under moderate pressure conditions (2-3.5 GPa) and 1000 °C in the presence of KClO(4) as an oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at 300 K and 2 K. These oxides crystallize in the cubic space group Im3 (no. 204). Mn(4+)/Mn(3+) and Fe(3+) occupy at random the octahedral B positions of the perovskite structure. These materials have also been characterized by magnetic and magnetotransport measurements. The observed enhancement of T(C) along the RCu(3)(Mn(3)Fe)O(12) series is understood as an effect of the chemical pressure on the (Mn,Fe)-O bonds as R(3+) size decreases. The semiconducting behaviour observed in all of the samples is related with the introduction of Fe at B position. Despite the drastic change of the transport properties, significant negative magnetoresistance values are observed in the Fe-containing compounds both at 10 K and 300 K.

From positive to negative magnetoresistance in graphene with increasing disorder

Artificial disorder was introduced gradually into monolayer graphene by controlling Ga+ ion irradiation and the corresponding electronic transport properties regulated by gate voltage, source-drain voltage, temperature, and magnetic field were studied experimentally. An unsaturated positive magnetoresistance (MR) up to 100% at ∼5 T was observed in as-fabricated graphene, while there is significant negative MR in disordered graphene. This phenomenon was attributed to the monocrystalline breaking and crystallite-boundary scattering in disordered graphene.

Magnetic and transport properties of amorphousGdxGe1−xalloys near the metal-insulator transition

The temperature and field dependence of magnetization and conductivity of amorphous Ge doped with Gd $(a\text{\ensuremath{-}}{\mathrm{Gd}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})$ has been measured for a wide range of $x$ $(0.08&lt;x&lt;0.25)$ near the metal-insulator transition. Magnetization and magnetic susceptibility measurements show strong magnetic interactions and a low temperature spin-glass freezing. High field magnetization and susceptibility per Gd atom in the paramagnetic state are significantly suppressed below that of noninteracting Gd, as observed previously for $a\text{\ensuremath{-}}\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Si}$ alloys. However, unlike $a\text{\ensuremath{-}}\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Si}$, the low field susceptibility does not fit a Curie-Weiss law and shows no significant dependence on composition. Conductivity measurements show that Gd causes localization of charge carriers below a characteristic temperature ${T}^{*}$, which also marks the onset of significant negative magnetoresistance. Both ${T}^{*}$ and the magnitude of the MR are significantly lower in $a\text{\ensuremath{-}}\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Ge}$ than in comparable $a\text{\ensuremath{-}}\mathrm{Gd}\text{\ensuremath{-}}\mathrm{Si}$ alloys. It is proposed that the large effects of the host matrix (Ge vs Si) are due to differences in both the band gap and dielectric constant, which cause changes in screening, thereby altering the effect of Gd magnetic moments on the localization of carriers and on the indirect mediated Gd-Gd exchange interactions.

New orthorhombic modification of equiatomic CePdAl

The crystal structure of a new low-temperature modification of CePdAl was determinedfrom single-crystal x-ray data: CePdAl-type; space group Pmmn (No. 59),Z = 14,oP42, a = 0.426 07 nm, b = 2.887 58 nm, c = 0.721 90 nm; RF = 0.048. Physical properties of orthorhombic CePdAl are governed by a mutualbalance of the RKKY interaction, the Kondo effect and crystallineelectric field splitting, resulting in antiferromagnetic ordering below≈2.5 K. Electronic transport is reminiscent of a textbook-like Kondo lattice, whichcomes along with a significant negative magnetoresistance of more than50% at low temperatures. Although geometrical frustration is absent whencompared to the hexagonal modification of CePdAl, the ordering temperatureTN is even smaller. A possible cause is enhanced Kondo interactions.

Magnetically controlled impurities in quantum wires with strong Rashba coupling

We investigate the effect of strong spin-orbit interaction on the electronic transport through non-magnetic impurities in one-dimensional systems. When a perpendicular magnetic field is applied, the electron spin polarization becomes momentum-dependent and spin-flip scattering appears, to first order in the applied field, in addition to the usual potential scattering. We analyze a situation in which, by tuning the Fermi level and the Rashba coupling, the magnetic field can suppress the potential scattering. This mechanism should give rise to a significant negative magnetoresistance in the limit of large barriers.

Weak localization and magnetoresistance of island-like thin copper films

We report magnetotransport and weak localization effects in island-like thin copper films. The samples show significant negative magnetoresistance (NMR) when a perpendicular magnetic field (H⊥&amp;gt;0.25 T) is applied. When the field is parallel to the film, the samples also show NMR for H∥&amp;gt;2.5 T, below which positive magnetoresistance (PMR) is found. The temperature dependence of the resistivity and NMR in both perpendicular and parallel fields are due to the weak localization effect in the two-dimensional (2D) system. The observation of NMR in the parallel field is in agreement with Altshuler and Aronov’s prediction [B. L. Altschuler and A. G. Aronov, Zh. Eksp. Teor. Fiz. Pis’ma Red. 33, 515 (1981)]. The PMR observed at low field is analyzed mainly in terms of the spin–orbit scattering, although other mechanisms may also be possible. It is suggested that the island-like geometry can significantly affect the localization and magnetotransport in 2D metallic films.

Thermally excited spin-disorder contribution to the resistivity ofLaCoO3

We have investigated the effect of the thermally induced spin-state transition on the electronic conduction mechanism in the perovskite ${\mathrm{LaCoO}}_{3}.$ The magnetotransport properties of this material are greatly influenced by this well studied spin-state transition. In fact, the excitation to a finite Co ion spin results in a thermal activation of spin disorder. This spin disorder is induced by thermal population of the nonzero spin states of the Co ions in zero magnetic field. The disorder is suppressed by a magnetic field leading to significant negative magnetoresistance effects, which are in effect thermally excited. We suggest that this material is an ideal testing ground for colossal magnetoresistive phenomena in a system where the complicating effects of lattice and magnetic phase transitions are not present.

Magnetic behavior of a new compound, Gd2PdGe3

We report here the synthesis of a new compound, Gd 2 PdGe 3 , crystallizing in a AlB 2 -derived hexagonal crystal structure and the results of electrical resistivity, magnetoresistance, heat-capacity and magnetic susceptibility measurements down to 2 K. The results suggest that this alloy orders antiferromagnetically below 10 K, exhibiting significant negative magnetoresistance over a wide temperature range above 10 K.

Magnetic field measurement within superconducting windings by CLTS temperature sensors

Magnetoresistance of the CLTS temperature sensor was determined in dependence on field and temperature. Based on its significant negative magnetoresistance and due to its small dimensions this sensor can be used to measure magnetic field within superconducting magnet windings. This method has been applied in the 3 coil arrangement of our superconducting TESPE torus magnet system and good agreement has been found with calculated field data.