Low-field magnetoresistance in the pyrochlore Tl2Mn2O7

Influence of heat sintering on the physical properties of bulk La0.67Ca0.33MnO3 perovskite manganite: role of oxygen in tuning the magnetocaloric response

The effect of heat treatments on bulk poly-crystalline La0.67Ca0.33MnO3 perovskite manganite is presented, to explore the possible enhancement in magnetocaloric performance. Samples were prepared via conventional solid-state reaction route with annealing and sintering at various temperatures. Detailed measurements of temperature-dependent and field-dependent magnetization were carried out to estimate the Curie point and order of magnetic transition. The increased sintering temperature results in a steep transition near the TC, and establishes the magnetic sensitivity as well as the active zone for substantial magnetocaloric performance, at about 168.2% for the LCM9 (sintered at 900 °C) sample. The cause for the significant improvement in the magnetic and magnetocaloric response is brought to light using detailed X-ray photoelectron spectroscopy (XPS) analysis, highlighting the role of oxygen in modifying the Mn3+/Mn4+ charge ratio. The maximum value of the isothermal magnetic entropy change for the optimized sample is found to be 6.4 J kg-1 K-1, achieved at 269 K, while temperature-averaged entropy change (TEC) values, TEC(ΔTH-C = 3 K) and TEC(ΔTH-C = 5 K), of 6 J kg-1 K-1 and 5.2 J kg-1 K-1, respectively, were obtained with a low magnetic field change of 20 kOe. The obtained isothermal entropy change at low field for the optimized La0.67Ca0.33MnO3 sample is higher than that of pure Gd and most oxide-based materials. The relative cooling power (RCP) value is around 93 J kg-1 (ΔH = 20 kOe). The order of the phase transition is examined with universal scaling; the scaled entropy change curves confirm the collapse onto a single curve for LCM9, asserting second-order character, whereas the breakdown of the curve with a dispersion relation (d) of 101.1% at Θ = -5 confirms the onset of intrinsic first-order nature in the case of the high-temperature-sintered samples. Calorimetry measurements show thermal hysteresis of 2.4 K and 7.1 K for LCM11 (sintered at 1100 °C) at ramp rates of 5 K min-1 and 10 K min-1, respectively, confirming the first-order nature of the magnetic transition.

SrV0.3Fe0.7O2.8: A Vacancy-Ordered Fe-Based Perovskite Exhibiting Room-Temperature Magnetoresistance

We report room-temperature (RT) magnetoresistance (MR) in a novel Fe-based perovskite, SrV0.3Fe0.7O2.8. This compound contains ordered oxygen vacancies in every fifth primitive perovskite (111)p plane, leading to a layered structure consisting of triple-octahedral and double-tetrahedral layers. Along with the oxygen vacancies, the transition-metal ions are also ordered: the octahedral sites are occupied by 100% of Fe ions, while the tetrahedral sites are occupied by 25% of Fe ions and 75% of V ions. As a result, SrV0.3Fe0.7O2.8 forms a magnetically striped lattice in which the octahedral layers with 100% of magnetic Fe ions are separated by the diluted magnetic layer. The compound exhibits weak ferromagnetism and shows a large negative MR (-5% at 3 T) at RT, despite the small saturation moment (0.4 μB/Fe atom). Thus, this type of layered compound is promising for further large MR by an increase of magnetization through chemical substitution.

Insights into cationic ordering in Re-based double perovskite oxides

Cationic ordering in Sr₂FeReO₆ (SFRO) and Sr₂CrReO₆ (SCRO) is investigated using magnetic property measurement, atomic-scale imaging, and first-principles calculations. We find that the nature of cationic ordering strongly depends on the host oxides, although they have the same crystal symmetry and chemical formula. Firstly, adding Re is effective to enhance the cationic ordering in SFRO, but makes it worse in SCRO. Secondly, the microscopic structure of antisite (AS) defects, associated with the level of cationic ordering, is also distinguishable; the AS defects in SFRO are clustered in the form of an antiphase-boundary-like feature, while they are randomly scattered in SCRO. Interestingly, we observe that the clustered AS defects deteriorate the ferromagnetism more than the scattered defects. Our findings elevate the importance of the AS defect configuration as well as the amount of defects in terms of magnetic property.

Ammonia and iron cointercalated iron sulfide (NH3)Fe0.25Fe2S2: hydrothermal synthesis, crystal structure, weak ferromagnetism and crossover from a negative to positive magnetoresistance

(NH₃)Fe_0.25Fe₂S₂ is successfully synthesized, which behaves as a ferromagnetic semiconductor and exhibits a novel crossover from a negative to positive magnetoresistance.

Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd₂Ti₂O₇ pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

Low field magnetotransport in manganites

The perovskite manganites with generic formula RE(1-x)AE(x)MnO(3) (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator-metal/paramagnetic-ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g. structure, phase diagram, double-exchange mechanism, Jahn-Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g. pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

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%).

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.