Positive Magnetoresistance Effect Research Articles

High temperature annealing treatment on Co doped ZnO bulks

Zn0.95Co0.05O Bulk samples were synthesized by solid state reaction method and annealed by different processes. The Zn treated samples show ferromagnetism，whereas the vacuum annealed samples do not show ferromagnetism. The XRD patterns show that there is Co impurity phase in the sample after Zn treatment at 1100℃，which is the reason of the ferromagnetism observed. At low temperature，both the positive and negative magnetoresistance （MR） effect can be observed for all the samples. We conclude that the s-d exchange_interaction_induced spin-splitting gives rise to positive MR， while the formation of bound magnetic polarons and the magnetic field suppressed spin-disorder scattering are responsible for the negative MR behavior under high field.

Voltage-modulated negative and positive magnetoresistance in La0.7Sr0.3MnO3/Si heterostructure

Magnetotransport properties have been investigated in a La0.7Sr0.3MnO3/Si heterostructure, which exhibits good rectifying behaviour and voltage-modulated negative and positive magnetoresistance effects. The application of magnetic fields induces an increase in the current at a low bias voltage and a decrease at a high bias. This is caused by the effect of the voltage on the ferromagnetic state in the La0.7Sr0.3MnO3 film at the interface by the field-direction-dependent lattice distortion at a high bias voltage.

Room temperature ferromagnetism in Nd- and Mn-codoped ZnO films

Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pulsed laser deposition with the thickness ranging between 46 and 971 nm. The room temperature resistivity of the codoped films is independent of film thickness. Large positive magnetoresistance and clear anomalous Hall effect were observed at 5 K. Ferromagnetism with clear hysteresis up to 290 K was observed. Codoping is suggested to be an efficient method to introduce energy levels in the ZnO band gap to mediate the electron spins of the magnetic doping ions.

Positive colossal magnetoresistance effect in ZnO∕La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La0.7Sr0.3MnO3 and ZnO layers on a LaAlO3 (100) substrate using pulsed laser deposition. The ZnO∕La0.7Sr0.3MnO3 heterostructure exhibits good rectifying behavior and a positive colossal magnetoresistance (MR) effect over a temperature range of 77–280K. The maximum MR values are determined to be about 53.9% at H=0.5T and 36.4% at H=0.3T. A possible explanation is given in terms of the effect of magnetic fields on the depletion layer and the capture carriers effect at the interface.

Negative and linear positive magnetoresistance effect in silver-rich silver selenide

In the non-magnetic semiconductor silver selenide (Ag 2+ δ Se) with a minor heterogeneous silver excess (0.79 × 10 −4 < δ ≤ 1 × 10 −2) we measured either a saturating negative magnetoresistance (MR) effect, a linear positive MR effect or a superposition of both. This complex MR behavior depends on the amount of the silver metal excess, but is also strongly influenced by the thermal treatment of the samples. Excess silver that cannot be accommodated homogeneously in the silver selenide lattice creates structural heterogeneities and forms a microstructure which is controlled by the thermal annealing procedure. We suggest that small silver segregations at the grain boundaries of the silver selenide matrix (“nanofilms”) are responsible for the negative MR effect, whereas nanoscale silver particles within the silver selenide matrix (nanoparticles) cause a linear positive effect.

Anti-phase boundaries pinned abnormal positive magnetoresistance in Mg doped nanocrystalline zinc spinel ferrite

Ni and Mg substituted Zn 1− x M x Fe 2O 4 (M = Ni, Mg) nanocrystals with variable concentration were synthesized by thermal decomposition of EDTA precursor complexes at 600 °C. The crossover from negative to positive magnetoresistance (MR) is realized by replacing the doping ions from Ni to Mg in nanocrystalline zinc spinel ferrites. In Zn 1− x Ni x Fe 2O 4 nanocrystalline ferrite, the negative MR is originated from spin-polarized electron tunneling across the grain boundaries. In Zn 1− x Mg x Fe 2O 4 nanocrystalline ferrite, the electron hopping between the sublattices of A- and B-site is deeply decreased due to Mg doping, which hence suppresses the spin-polarized electron tunneling across the neighbouring grains. Moreover, the enhanced magnetic scattering across the antiferromagnetic-coupled anti-phase grain boundaries favors the abnormal positive MR effect. The combination of both the effects causes the abnormal positive MR effect.

Abnormal magnetoresistance inε−(Mn1−xFex)3.25Geantiferromagnets

The magnetic, transport and magnetotransport properties of antiferromagnetic epsilon-(Mn(1-x)Fe(x))(3.25)Ge (x=0.17 and 0.2) compounds are investigated systematically. For both compounds, a large positive magnetoresistance (MR) is observed in the temperature regime below the temperature (T(t)) for a transition from a triangular to a collinear antiferromagnetic spin configuration, while a negative magnetoresistance appears between T(t) and the Neel temperature T(N), and then the MR ratio turns to be positive again above T(N). Below T(t), the positive MR reaches a maximum ratio of 6.1% at 5 K at an applied magnetic field of 5 T for the epsilon-(Mn(0.83)Fe(0.17))(3.25)Ge compound. For the epsilon-(Mn(0.83)Fe(0.2))(3.25)Ge compound, the positive MR ratio reaches its maximum at 120 K and nearly vanishes at 5 K. The origin of the anomalous positive and negative MR effects is discussed in terms of the shrinkage of the orbits, the spin fluctuations, the spin configurations, and the magnetic transitions.

Epitaxial behavior and transport properties of PrBaCo2O5 thin films on (001) SrTiO3

Highly conductive PrBaCo2O5.5+δ thin films were grown on (001) SrTiO3 by pulsed laser deposition. Microstructural studies from synchrotron x-ray diffraction and transmission electron microscopy reveal that the as-grown PrBaCo2O5.5+δ films are a-axis oriented with excellent single crystallinity. Two domain structures with c axes in the plane directions parallel to the substrate surface were found with an atomic sharp interface. Transport property and isothermal magnetoresistance measurements for both as-grown and post-annealing were done to understand the physical properties of the films. An abnormal positive magnetoresistance effect was observed in the postannealed films, which suggests that the spins in the system are polarized easier.

Magnetoresistance in granular CrO2: Effects of variation in crystallographic and magnetic microstructure

We report magnetotransport measurements up to 5 Tesla on high purity sintered samples of granular CrO2 with a significantly enhanced grain size. The negative magnetoresistance (MR) as derived from RH isotherms is observed to be unhysteretic up to temperatures as high as 200 K. RH isotherms exhibit some unusual features between 240 and 290 K, including a positive MR and strong pinning effects. These features disappear above 290 K and are apparently related with the magnetic state of the insulating grain boundary. Qualitatively similar features with significantly enhanced MR are also observed when CrO2 is diluted with two antiferromagnetic oxides, Cr2O3 or Cr2O5. These results bring out the role played by the magnetic and crystallographic microstructure on the magnitude, sign, and hysteresis of the magnetoresistance in this technologically important material.

Large positive magnetoresistance effect below Curie temperature in In1.90−xMn0.1SnxO3

We report on the magnetoresistance (MR) and magnetic properties of In1.90−xMn0.1SnxO3 (0⩽x⩽0.06) oxide. All samples were found to be ferromagnetic below TC=46K. Sn doping changed In1.90Mn0.1O3 from an insulator to a highly conducting phase at 300K. A positive MR effect was observed over a wide temperature range just below TC. Calculated MR values reached a maximum of 20% at 5K. A change in the MR effect, from positive to negative, occurred under magnetic field H&amp;gt;4T at 5K. The results of x-ray absorption near-edge spectroscopy indicated that Mn ions are present both as Mn2+ and Mn4+.

One-pot sonochemical synthesis of Fe1−xCuxCr2S4 chalcospinels

We present a simple and convenient one step route to prepare a series of chalcospinels Fe1−xCuxCr2S4 employing high frequency sound waves. The magnetic and electronic transport properties of the as-prepared spinels were studied, and this simple, easy, and efficient synthetic route may further encourage wider application of Cu-doped Fe chalcogenides in solid state magnetoelectronic devices. Furthermore, the adopted synthetic route highlights a one-pot synthesis employing organometallic precursors such as metal carbonyls, ethylenedimine, and elemental sulfur. The ease of synthesizing such complex spinel structures of Fe1−xCuxCr2S4 chalcogenides with x=0.3, 0.4, and 0.6 has been highlighted in this work. The rapid cavitational collapse results in sulfur deficiency in the system, which drastically alters the conduction behavior. All the compositions studied show a negative magnetoresistance response except for x=0.3 which exhibit an unusual positive magnetoresistance effect. Such an effect has been attributed to the sulfur deficiency occurring during the rapid cavitational collapse. Our experimental results provide one effective method for synthesis of doped ternary chalcospinels exhibiting magnetoresistive properties.

Effect of spacer layer thickness on the magnetic and magnetotransport properties ofFe3O4∕Cu∕Ni80Fe20spin valve structures

We present a systematic and detailed study of the magnetization reversal process and magnetotransport properties of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}∕\mathrm{Cu}$ $({t}_{\mathrm{Cu}})∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ spin valve structures. A drastic change was observed in both the magnetic and transport properties as the thickness of the Cu spacer layer ${t}_{\mathrm{Cu}}$ was varied in the range $2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ensuremath{\leqslant}{t}_{\mathrm{Cu}}\ensuremath{\leqslant}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For ${t}_{\mathrm{Cu}}=2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the transport properties are mainly due to anisotropic magnetoresistance effects because of strong exchange coupling between the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ layers. For ${t}_{\mathrm{Cu}}\ensuremath{\geqslant}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, however, the transport properties are dominated by positive giant magnetoresistance (GMR) effects due to separate magnetization switching of the two magnetic layers. The GMR ratio decreases with increasing spacer layer thickness due to enhanced current shunting and scattering effects. We also observed that the GMR ratio has strong temperature dependence and decreases with increasing temperature due to spin flip scattering and electron-magnon interactions.

Spin polarization in Zn0.95Co0.05O:(Al,Cu) thin films

ZnCoO : Al, ZnCoO : (Al,Cu), and reference samples without Co have been grown by pulsed laser deposition to investigate the influence of Co-dopants and Cu-codopants on the magneto transport properties of ZnO. Positive magnetoresistance and anomalous Hall effect have been observed for ZnCoO : (Al,Cu). Versatile theoretical approaches for modelling positive magnetoresistance in ZnO-based diluted magnetic semiconductors are still to be developed. Negative magnetoresistance due to the scattering of spin-polarized charge carriers at isolated magnetic impurities has been observed in ZnO : (Al,Cu) without Co-dopants and successfully modelled. Cu-codopants enhance the positive and negative magnetoresistance in ZnCoO : Al and ZnO : Al, respectively, by one order of magnitude.

Magnetoresistance Effects Evidencing the π−d Interaction in Metallic Organic Conductors, (EDT-DSDTFVO)2•MX4 (M = Fe, Ga; X = Cl, Br)

The 2:1 salts of a new donor molecule, EDT-DSDTFVO with MX4- (M = Fe, Ga; X = Cl, Br) ions, were prepared. The crystal structures of the donor molecules had a beta-type packing motif. All the salts essentially exhibited metallic behaviors despite the small upturns in the resistances below 30-70 K. A large negative magnetoresistance (MR) effect [-14.7% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the FeCl4- salt, while a positive MR effect [+4.0% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the GaCl4- salt, suggesting that there is a pi-d interaction in the FeCl4- salt. The pressure application suppressed the resistivity upturns, increased the negative MR effect (-17.7% at 9.5 kbar) in the FeCl4- salt, and decreased the positive MR effect (+3.3% at 15 kbar) in the GaCl4- salt.

Magnetoresistance effects inSrFeO3−δ: Dependence on phase composition and relation to magnetic and charge order

Single crystals of iron(IV) rich oxides SrFeO(3-x) with controlled oxygen content have been studied by Moessbauer spectroscopy, magnetometry, magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in order to relate the large magnetoresistance (MR) effects in this system to phase composition, magnetic and charge order. It is shown that three different types of MR effects occur. In cubic SrFeO3 (x = 0) a large negative MR of 25% at 9 T is associated with a hitherto unknown 60 K magnetic transition and a subsequent drop in resistivity. The 60 K transition appears in addition to the onset of helical ordering at ~130 K. In crystals with vacancy-ordered tetragonal SrFeO(3-x) as majority phase (x ~0.15) a coincident charge/antiferromagnetic ordering transition near 70 K gives rise to a negative giant MR effect of 90% at 9 T. A positive MR effect is observed in tetragonal and orthorhombic materials with increased oxygen deficiency (x = 0.19, 0.23) which are insulating at low temperatures. Phase mixtures can result in a complex superposition of these different MR phenomena. The MR effects in SrFeO(3-x) differ from those in manganites as no ferromagnetic states are involved.

Positive magnetoresistance feature in ultrathin La 0.67Ca 0.33MnO 3 films

The ultrathin epitaxial La 0.67Ca 0.33MnO 3 (LCMO) films with thickness from 8 to 50 nm on (001) SrTiO 3 (STO) substrates were prepared and investigated, and the unique MR effects were distinctly observed. The positive magnetoresistance (MR) effects were observed in the high temperature region for all the present ultrathin films. The films with thickness in the range from 17 to 30 nm not only show the positive MR effect at high temperature (> T MI), but also in the low temperature region (≤96 K for 17 nm, ≤108 K for 30 nm). It is suggested that the positive MR effect occurred at low temperature could be attributed to the scattering of the domain wall on charge carriers, while the positive MR effect occurred at temperature higher than T MI is attributed to the transport dominated by the magnetic polarons which formed due to the trap of charge carriers with spin in local distorted lattice in such highly strained ultrathin LCMO films.

Magnetic-Field-Sensing Materials Composed of Metal–Semiconductor Hybrid Nanostructures

A huge positive magnetoresistance effect, about 4 orders of magnitude at room temperature, was discovered in metal-semiconductor hybrid nanostructures. The hybrid material consisting of metallic nanostructures that are fabricated on a GaAs substrate by ultra-high vacuum deposition method exhibits magnetic field-sensitive current-voltage characteristics. When a constant voltage above the threshold value, is applied to the film, a very steep change in the current, which we term magnetoresistive switch, is driven by the huge magnetoresistance effect under a relatively low magnetic field at room temperature. The magnetoresistance effect is very sensitive to the nanoscale morphology of the hybrid film; in other words, one can control the magnetoresistance function at will by modifying the nanostructure of this material. The origin of the magnetoresistive switch effect and its possible application as a magnetic field sensor is discussed.

Strain-induced properties of epitaxial VOx thin films

We have grown VOx thin films on different substrates in order to investigate the influence of epitaxial strain on the transport properties. We found that the electric conductivity is much larger for films grown under compressive strain on SrTiO3 substrates, as compared to bulk material and VOx films grown under tensile strain on MgO substrates. A clear crossover from metallic to semiconducting behavior is observed when increasing the oxygen content x. Apparently, the application of strain induces a Mott-Hubbard insulator-to-metal transition in VOx<1. The VOx/SrTiO3 films show an unexpected large positive magnetoresistance effect at low temperatures, which is not found in the VOx films grown under tensile strain on MgO or on a substrate with a similar lattice parameter.

Large magnetoresistance in (AA′) 2FeReO 6 double perovskites

We review the main structural, magnetic and magnetotransport properties of the intriguing (AA′) 2FeReO 6 magnetic double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic [(AA′) 2=Ba 2, Ba 1.5Sr 0.5, BaSr, Ba 0.5Sr 1.5] to tetragonal [(AA′) 2=Sr 2] and monoclinic [(AA′) 2=Ca 0.5Sr 1.5, CaSr, Ca 1.5Sr 0.5, Ca 2]. The Curie temperature increases anomalously from ≈303 K for Ba 2 to ≈522 K for Ca 2 in sharp contrast with the observed behaviour in the isostructural compounds (AA′) 2FeMoO 6. Other anomalous features in the (AA′) 2FeReO 6 series are: the large magnetic anisotropy, the large magnetoelastic coupling and the semiconducting behaviour of the monoclinic compounds. The monoclinic compounds undergo a structural/magnetic transition at T S below 125 K. Three different magnetoresistance mechanisms have been identified: the intergrain negative magnetoresistance effect, which is present across the whole series of compounds, and in the case of the monoclinic compounds below T S a negative magnetoresistance effect associated to the melting of the low-temperature phase and a positive magnetoresistance effect only present in (AA′) 2=Ca 2 below T ∼ 50 K .

The magnetoresistance of homogeneous and heterogeneous silver-rich silver selenide

The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (α-Ag2+δSe) is measured as a function of composition. Very small composition variations in the order of Δδ=10−6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2+δSe shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10−4&amp;lt;δ&amp;lt;1×10−2, a saturating negative MR effect, a linear positive MR effect, or a superposition of both can be measured. The microstructure of the silver-rich Ag2+δSe determines its complicated MR behavior. A heterogeneous silver selenide with a larger silver excess (δ&amp;gt;10−2) shows again an OMR effect.