Particle size effect on magnetotransport properties of nanocrystalline Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples.     

Magnetic and electrical properties of zinc selenide crystals containing disorder

The electrical and magnetic properties of ZnSe single crystals containing disorder have been studied between temperatures 290K and 900K. The study of the magnetic properties has been extended to low temperatures (100K). Paramagnetism has been found to appear at high temperatures (460–900K). From the fact that this paramagnetism is proportional to e^?δE/kT, it is suggested that localized states of single occupancy are created by thermal excitation. The study of the magnetic properties has been of help in ascertaining the nature of the transport (band conduction or hopping conduction) and in finding the hopping energy and excitation energy separately. It has also been shown from this that both band conduction and hopping conduction exist simultaneously in the sample. A study of the thermo electric power (t.e.p.) shows that below 450K current is carried by electrons in the conduction band and above by hopping of holes.     

Thermomagnetic hysteresis and electrical transport in amorphous films

We report resistivity and magnetization measurements on an amorphous Ni₇₄Mn₂₄Pt₂ thin film in the temperature range of 3–300 K. Two significant features are apparent in both the magnetic susceptibility and electrical resistivity. A low-temperature (low-T) anomaly is observed at about 40 K, where a cusp appears in the resistivity, while a concomitant step-like increase in zero-field-cooled (ZFC) magnetization (M) appears with increasing temperature. The low-T anomaly is attributed to a crossover from a pure re-entrant spin-glass within individual domains to a mixed ferro-spin-glass regime at lower temperatures. By contrast, the high-temperature (high-T) anomaly, signaled by the appearance of hysteresis below 250 K, corresponds to the freezing of transverse spins in individual domains acting independently. Between the low-T and high-T anomalies a small but discernable magnetic hysteresis is observed for warming vs. cooling in the field-cooled (FC) case. This behavior clearly indicates the presence of domain structure in the sample, while the disappearance of this hysteresis at lower temperatures indicates the complete freezing of the spin orientation of these domains. According to these results, we have divided the magnetic state of this sample into three regions: at temperatures above 250 K, the sample behaves like a soft ferromagnet, exhibiting M vs. H loops with very small hysteresis (less than 5 Oe). As the temperature is lowered into the intermediate region (the range 40–250 K), spins become frozen randomly and progressively within the individual domains. These domains behave independently, rather than as a cooperative behavior of the sample. Weak irreversibility sets in, indicating the onset of transverse spin freezing within the domains. At temperatures below 40 K, the M vs. H loops exhibit larger hysteresis, for both the ZFC and FC cases, as in a pure spin-glass. We have also demonstrated giant noise in the resistivity at temperatures just below 250 K. Such noise can originate from fluctuations of the domains near the film surface because of competing effective bulk and surface anisotropy fields. The large observed amplitude may be explained by means of a large ferromagnetic anisotropy in the resistivity due to the large spin–orbit effect seen in NiMn systems. Finally, the low-T peak in the resistivity has been analyzed using Fisher and Langer's expression based on the Friedel Model proposed for critical transitions in transition metals (s–d systems). The fitted results are in satisfactory agreement with the predictions of this model.     

Electrical and magnetic properties of ε-Fe3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe₃N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe₃O₄ phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe₃N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ(T) for compacted ε-Fe₃N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

Two magnetic regimes in doped ZnO corresponding to a dilute magnetic semiconductor and a dilute magnetic insulator

Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.     

Contribution of electron-nuclear interaction to the residual resistivity of metals

In nonmagnetic metals the spin-spin interaction of the electrons and nuclei makes a strongly magnetic field and temperature T dependent contribution to the residual resistivity. The nuclei act as magnetic impurities. For magnetic metals (Tb, Ho, Dy) with a high internal magnetic field, the nuclear contribution to the resistivity vanishes at low temperatures T, where the nuclear spins are ordered, and saturates at high temperatures T, where the nuclear spins are disordered—the analog of the Schottky effect for the nuclear specific heat. The electron-nuclear interaction can destroy superconductivity in metals with low critical magnetic fields under conditions of ferromagnetic ordering of the nuclear spins. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 193–197 (10 August 1996)     

High-temperature properties of the manganites: Manifestation of a paramagnetic-phase inhomogeneity?

An analysis was made of the magnetic susceptibility, electrical resistivity, and magnetoresistance of (La_1?yPr_y)_0.7Ca_0.3MnO₃ samples differing in Pr content and enriched in the oxygen isotope ¹⁸O. At high temperatures, all samples were paramagnetic insulators, while below 60 K, part of them transferred to a ferromagnetic metallic state. All the samples exhibit practically identical behavior of the susceptibility, resistivity, and magnetoresistance in the high-temperature region, despite a noticeable difference between their properties at low temperatures; more specifically, the magnetoresistance grows quadratically with magnetic field within a broad range of temperatures and magnetic fields and scales with increasing temperature close to 1/T⁵. A combined analysis of the magnetic susceptibility and magnetoresistance indicates the possible existence of an inhomogeneous state with considerable ferromagnetic correlations in the paramagnetic region.     

In-plane thermal conductivity of Nd2CuO4: evidence for magnon heat transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (kappa(ab)) of high-quality monocrystalline Nd2CuO4. Isothermal measurements of the field dependence of kappa(ab) at low temperatures (2 K相似文献   

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

Electrical and magnetic properties of antimony sulphide (Sb2S3) crystals and the mechanism of carrier transport in it

The principal magnetic susceptibilities, electrical conductivities and thermoelectric powers of Sb₂S₃ single crystals have been studied in the temperature range 100–550 K. An analysis of the results on the basis of its anomalous crystal structure and of the proposition of the hopping process of conduction at higher temperatures, account for the discrepancy between findings from magnetic and electrical observations.     

Influence of Fe doping on electrical properties of LCMO

The polycrystalline samples La_0.67Ca_0.33Mn_(1?x)Fe _x O₃ (x?=?0.00,?0.01,?0.03, and 0.1) have been grown in single phase by solid state route. The analysis of the reaction has been done by thermogravimetry and differential thermal analysis measurements. DC electrical resistivity measurements have been carried out down to 15?K. The samples with x?=?0.00, 0.01, and 0.03 exhibit metal–insulator (MI) transition at temperatures 221.5?K, 217?K, and 215?K respectively, whereas the sample with x?=?0.1 is insulating in nature for entire temperature range. Interestingly, the electric transport properties of these samples are not consistent with their magnetic phase transitions and the samples show MI transition at a temperature, T _MI, which is significantly lower than the paramagnetic to ferromagnetic transition temperature (T _c). The resistivity data below T _MI has been analyzed using the empirical relation ρ?=?ρ₀?+?ρ₁ T ⁿ and the data above this temperature has been analyzed using two existing models, Mott's variable range hopping model and spin polaronic conduction model.     

Collapse of 5 f -Electron Ferromagnetism in UPtAl Under High Pressures

UPtAl exhibits a ferromagnetic ordering of U magnetic moments at temperatures below T C =42.5 K. The magnetic-ordering transition is accompanied by an anomaly in the temperature dependence of electrical resistivity. This allows us to determine the value of Curie temperature from 𝜌 vs. T data that can be measured at very high pressures, at which reliable magnetization measurements are difficult. We report on resistivity measurements performed on an UPtAl single crystal under hydrostatic pressures p h 8 GPa. It was observed that the initial increase of T C with p becomes gradually reduced for p >2 GPa until the maximum T C value of , 48 K is reached between 4 and 6 GPa that is followed by a progressively developing downturn of the T C ( p ) curve. The latter result is attributed to the approaching collapse of the U 5 f -moment ferromagnetism. Low-temperature resistivity data point to a rapidly reduced magnetic anisotropy at highest pressures.     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

A study of the magnetic and dielectric properties of Y Fe0.5Cr0.5O3

Y Fe_0.5Cr_0.5O₃ ceramics have been synthesized by a conventional solid-state reaction. Powder X-ray diffraction shows that this compound possesses an orthorhombic structure with Pnma space group. It exhibits a high magnetic transition temperature at around 250 K with weak ferromagnetic behavior below this temperature. A dielectric relaxation following the Arrhenius law found in the Y Fe_0.5Cr_0.5O₃ compound can be attributed to the charge carrier hopping conduction.     

Structural stability and electrical conductivity of amorphous Ge-metal alloy films

Stable homogeneous amorphous alloy¹ films of Ge with different concentrations of Al, Cu and Fe have been prepared by the simultaneous vapor deposition technique. Ge-Metal films are amorphous up to a concentration of ～ 40 at.% Al, ～ 20 at.% Cu and ～ 20 at.% Fe. The cyclic annealing and crystallization temperature of these films show that whereas Al increases the stability of the amorphous phase, the addition of Cu and Fe decreases it. The electrical resistivity decreases gradually with increasing Al content. In contrast, a rapid decrease in the electrical resistivity is observed for the Ge-Cu and Ge-Fe systems. The thermoelectric power (TEP) of Ge-Cu and Ge-Fe system assumes small values ～ few μV/deg for concentrations greater than few atomic percent. Ge-Al system exhibits large positive thermoelectric power at all compositions. The temperature dependence of the electrical resistivity of these alloy films show that the addition of Cu and Fe to Ge results in a drastic decrease in the activation energy of conduction whereas the addition of Al increases the activation energy. Ge-Al films exhibit intrinsic like conduction in the temperature range 100–300 K. The Ge-Cu and Ge-Fe films exhibit hopping conduction from 100–300 K and the related density of states is up to 100 times larger than in pure a-Ge films.     

Conductivity anisotropy and localization of charge carriers in CuFeTe2 single crystals with a layered structure

The temperature dependences of the electrical resistivity of CuFeTe₂ semiconductor single crystals with a layered structure are investigated parallel and perpendicular to the plane of the crystal layers in the temperature range 5–300 K. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range studied are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed energy band. In the low-temperature range, the electrical conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity charge carriers is determined. The density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures     

钽掺杂二氧化钛薄膜的光电性能研究

采用脉冲激光沉积法 (PLD), 以石英玻璃为衬底制备了钽掺杂TiO₂薄膜并研究了薄膜样品的光电性质. 沉积氧气分气压从0.3 Pa变化到0.7 Pa时薄膜样品的帯隙变化范围是3.26 eV到3.49 eV. 通过测量电阻率随温度的变化关系确定了薄膜内部的主要导电机理. 在150 K到210 K温度范围内, 热激发导电机理是主要的导电机理; 而在10 K到150 K范围内; 电导率随温度的变化复合Mott的多级变程跳跃模型 (VRH); 在210 K到300 K范围内, 电阻率和exp(b/T)^1/2呈正比关系. 关键词： 2')" href="#">Ta掺杂TiO₂ 脉冲激光沉积法 薄膜 导电机理     

Multiple magnetic transitions in single crystals of Ce_(12)Fe_(57.5)As_(41) and La_(12)Fe_(57.5)As_(41)

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41).The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds.Both compounds exhibited multiple magnetic orders within 2-300 K and metamagnetic transitions at various fields.Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41),respectively,followed by antiferromagnetic type spin reorientations near Curie temperatures.The magnetic properties underwent complex evolution in the magnetic field for both compounds.An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce_(12)Fe_(57.5)As_(41).The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure.A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce_(12)Fe_(57.5)As_(41).A temperature-field phase diagram was present for these two rare earth systems.In addition,a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150-300 K,which is rarely found in 3D-based compounds.It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe_{1? x} Ni _x O₃ (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Mott's variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe³⁺–O–Fe³⁺ and Fe³⁺–O–Ni³⁺ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.