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Magnetoresistance (MR) is the magnetic field-induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field-induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m-3 and high mobility of 5.66 × 104 cm2 V-1 s-1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Low field magnetoresistance (LFMR) of double perovskite Sr 2 FeMoO 6 (SFMO) is low at room temperature. In this paper, a low-cost, facile, and effective way to enhance the LFMR is developed. Single-phase SFMO ceramics with 10% Fe/Mo anti-site defects have been prepared, and then soaked in glycerin/water (1/1, V/V) mixture for 0–14 days. The insulating glycerin tends to segregate into the SFMO grain boundary to form organic/inorganic composite. Such a post-treatment has no effects on magnetic properties, but can increase the resistivity by ∼500 times, as the result, remarkable enhanced LFMR is observed in composites. With 0.5 T magnetic field, the LFMR of the SFMO soaked for 14 days reaches −29.5% at 10 K and −11.0% at 300 K, more than 2 times larger than that of the SFMO without soaking, which are −12.0% at 10 K and −4.7% at 300 K. Based on the experimental results, a possible mechanism is proposed to explain the relation between the composite structure and the LFMR response. We believe our work not only provides a novel method to enhance the magnetoresistance in SFMO ceramics, but also sheds light on tuning transport/magnetoresistance in other perovskite ceramics.

Triple perovskite Sr3MFeMoO9 (M=Mn, Co, Ni) ceramics have been prepared under flowing N2 steam. All compositions are single phase and have B-site cation ordering. However, the composition with M=Co shows denser microstructures with smaller grains, the average grain sizes are 700, 500 and 650 nm for M=Mn, Co, and Ni, respectively. The chemical states of the surface and interior Mn, Co, Ni, Fe, and Mo cations are analyzed carefully. Both the magnetization-magnetic field curves and magnetization–temperature curves confirm that all compositions show antiferromagnetic behaviors. The Neel temperature of M=Mn and Ni compositions are determined to be 24.4 and 49.5 K, respectively. However, M=Co composition shows two magnetic transition temperatures at 70.9 and 180.7 K, which is possibly attributed to that the Co cations have intermediate spin-state (3d6, S=1) and high spin states (3d6, S=2). Furthermore, the compositions show semiconductor-like behaviors and neglectable magnetoresistance in whole measuring temperature range. Our results may be helpful for designing novel magnetic materials and for understanding magnetic couplings in triple perovskites.

Sr2CrWO6/Sr2Fe10/9Mo8/9O6 (SCWO/SFMO) superlattices with 4, 6, 7, 10 periods (abbreviated as S-1, S-2, S-3, and S-4) were prepared on (0 0 1) SrTiO3 (STO) substrates by pulsed laser deposition. All superlattices show macroscopic ferromagnetic behavior, and the magnetization increases with increasing period. The S-1 superlattice demonstrates semiconductor-like temperature-dependent resistivity in the whole measuring temperature range and negative magnetoresistance of -5.3% at 2 K with 2 T magnetic field, while the other superlattices illustrate metallic behaviors and increasing positive magnetoresistance of 223.1%, 275.4%, and 766.1% under the same conditions. This work not only provides a feasible way to tune the MR effect in magnetic perovskite oxides, but also may stimulate further work on artificially micro-structured thin films with designable magnetic properties.

Sr2FeMoO6 ceramics with different Fe/Mo antisite defect (ASD) concentrations and grain-boundary (GB) properties were prepared. The competitive and combining effects of GB and Fe/Mo ASD on the transport and magnetoresistance were discussed. The GB properties, that is, intergrain coupling strength, positively related with resistivity value, is extremely sensitive to the total flux of reducing gas, in general, lower total gas flux leads to larger resistivity, thus stronger intergrain coupling strength, and then the contributions of GB effect to low-field magnetoresistance (LFMR) increase appreciably regardless of the amount of ASD. In detail, when ASD concentration is less than 26%, LFMR is dominantly controlled by the GB effect. However, the suppressed ASD effect on LFMR behavior comes to play when the ASD content is larger than or equal to 26%, where the GB and ASD effects contribute together to the LFMR.

Double perovskite Sr 2 CrWO 6 films have been prepared on SrTiO 3 (111) substrates by pulsed laser deposition in high vacuum (10 −5 Pa). X-ray diffraction patterns indicate that the films are (111)-oriented. Both atomic force microscopy (AFM) and cross-section transmission electron microscopy (TEM) images prove that the films have very smooth surfaces. Detailed microstructures given by high resolution transmission electron microscopy (HRTEM) further confirm that the films are epitaxial with sharp and coherent substrate/film interface. Well saturated magnetization–magnetic field hysteresis loop is observed with the saturation magnetization of 1.2 μ B /formula unit at 10 K. The films show metallic transport behavior and large positive magnetoresistance (∼180% at 10 K). The structure–property relationship is discussed in detail.

Triple perovskite Sr3CrFeMoO9 was designed and prepared in reducing atmosphere. The ceramics are single phase with homogeneous, porous-like microstructure. The valences of the interior Cr, Fe, and Mo cations are +3, +2/+3, 0/+6, whereas that of the surface cations are +3, +3, and +6, respectively. The ceramics show well-saturated magnetization-magnetic field hysteresis loop, the saturation magnetizations are 1.0 and 0.6 μB per formula unit at 10 K and room temperature, respectively. The ferromagnetic Curie temperature is determined to be 385 K. Furthermore, semiconductor behavior in the measuring temperature range (10–300 K) and large negative magnetoresistance of −30.7 % at 10 K are observed. Our results may stimulate further works on room temperature ferromagnetism in triple perovskite.

A novel ferromagnetic triple perovskite Sr3CrFeMoO9 was designed and ultrathin films were fabricated. The films are highly strained, with a sharp film/substrate interface. The valences of the Cr, Fe and Mo cations are +3, +3 and +6, respectively. Well-saturated ferromagnetic hysteresis loops are observed at room temperature and 10 K, and the Curie temperature should be higher than 400 K, as indicated by the temperature–magnetization curve. The films show semiconductor-like behavior with almost negligible magnetoresistance. The possible spin configuration is analyzed based on experimental results.

Sr 2 Fe 1−0.5 x Mo 1−0.5 x Ir x O 6 ceramics ( x =0, 0.02, 0.04, 0.06, and 0.08) have been synthesized and the structures, magnetic and transport properties have been investigated. X-ray diffraction reveals a very low solution limit of x =0.06, which however, results in significantly decreased Fe/Mo ordering degree and thus decreased magnetization value. On the other hand, the Curie temperature ( T c ) rises from 390 K ( x =0) to 400 K ( x =0.02) firstly, and then decreases to 360 K and 340 K for the compositions with x =0.04 and 0.06, respectively. In addition, the resistivity and magnetoresistance drop drastically with increasing x . The composition with x =0 shows semiconductor transport behavior in the temperature range of 10–400 K, whereas the other compositions show a minimum resistivity around 120 K. These results are discussed in detail by considering the effects of Ir substitution.

Double perovskite Sr2Fe10/9Mo8/9O6 films with different thicknesses have been prepared on (111)-SrTiO3 substrates by pulsed laser deposition in flowing N2 atmosphere. The X-ray diffraction indicates the films have no impurity phases and are (111) oriented. The epitaxy and local Fe/Mo ordering are confirmed by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). Mixed valence states of interior Fe and Mo cations are observed by the x-ray photoelectron spectroscopy. Interestingly, the films show significantly thickness-dependent magnetization properties: at 10 K, the films show the saturated magnetization of 2.30 μB/f.u. (formula unit), 1.12 μB/f.u., and 0.50 μB/f.u., respectively. On the other hand, the thickest film shows a magnetoresistance of −13% at 120 K whereas other two samples show considerably higher resistivity and neglectable MR. The thickness effects on microstructure and properties have been discussed in detail.

Magnetoresistance (MR) of transition metal (TM)-doped and co-doped ZnO films prepared by a magnetron sputtering system has been discussed. For single-doped film, the values of H max (the field at which MR shows its maximum) increase as temperature increases. At a certain temperature, the values of H max increase from Mn-, Fe- to Co-doped films due to their different magnetic moments. For (Mn, Fe), (Mn, Co)-co-doped ZnO films, the MR behaviors show striking similarity as Mn-doped ZnO film. These results are interpreted in terms of the s–d spin-splitting mechanism. Large negative MR in high field is attributed to the formation of magnetic polarons (MPs). The temperature dependence of resistivity agrees well with the variable range hopping (VRH) mechanism of MPs.

In this Letter, we studied the electrical transport, magnetic property, magnetoresistance and anomalous Hall properties of La-, Sm-, Ho-, and Dy-doped quasi-two dimensional K0.58RhO2 single crystals. At low temperature (

Electrical property evolution of Bi2AE2Co2O8+δ single crystals (AE = Ca, Sr and Ba) is systematically explored. When AE changes from Ca to Ba, the electrical property of Bi2Ca2Co2O8+δ and Bi2Sr2Co2O8+δ demonstrates semiconductor-like properties. But Bi2Ba2Co2O8+δ shows the metallic behavior. Analysis of temperature-dependent resistance substantiates that from metallic Bi2Ba2Co2O8+δ to semiconductor-like Bi2Sr2Co2O8+δ can be attributed to Anderson localization. However the semiconductor behaviour of Bi2Sr2Co2O8+δ and Bi2Ca2Co2O8+δ is related to electronic correlations effect that is inferred by large negative magnetoresistance (∼70%). The theoretical electronic structures and valence X-ray photoemission spectroscopy substantiate that there is a relative large density of state around Fermi level in Bi2Ba2Co2O8+δ compared with other two compounds. It suggests that Bi2Ba2Co2O8+δ is more apt to be metal in this material system.

Using a facile method, Fe/Mo anti-site defect (ASD) concentration in double perovskite Sr2FeMoO6 is quantitatively controlled within a wide range of 2%–21%. Our systematic investigation demonstrates the significant effects of ASD on the physical properties, such as magnetization, magnetoresistance and the Curie temperature.

We report that the Fe-rich single-phase Sr2Fe1+x Mo1−x O6(x = 1/9, 1/6 and 1/3) ceramics can be synthesized in a N2 stream by the solid-state reaction method, while the usual Sr2FeMo2O6 (i.e. x = 0) can only be synthesized in a H2+Ar stream. X-ray diffraction shows that all ceramics are well crystallized with tetragonal symmetry. X-ray photoelectron spectroscopy results confirm that the chemical states of interior Fe and Mo cations are significantly more complex than those of the surface cations. With the increase in x, the materials change from ferrimagnetic (x = 0, 1/9, 1/6) to spin-glass-like behaviour (x = 1/3) and the magnetism decreases dramatically. The Curie temperature (T c) and the magnetoresistance (MR) at 10 K with a field of 2 T (T c, MR) are (400 K, −20.9%), (390 K, −10.3%) and (385 K, −4.5%) for x = 0, 1/9 and x = 1/6, respectively. The results are further analysed by Monte Carlo simulations. Our results show that high-T c ferrimagnetic double perovskites with good magnetic and transport properties can be obtained conveniently in a N2 stream instead of conventional H2+Ar, which not only provides a safe and low-cost way to synthesize these materials, but may also be helpful in decreasing the oxygen vacancy concentration.

Magnetic and transport properties were investigated in Zn0.94Mn0.03Co0.03O thin films prepared by dual radio frequency magnetron sputtering technique. A negative magnetoresistance (MR=[ρ(H)−ρ(0)]∕ρ(0)) of −95% at 2K and a positive MR of 48% at 10K were established in the films. The anomalous magnetic and magnetic-related transport properties in transition-metal-doped ZnO films are discussed in terms of magnetic polarons.

(Mn, Co)-codoped ZnO films have been synthesized on c-sapphire (0001) by a radio-frequency magnetron sputtering system in which two targets were sputtered together. X-ray-diffraction measurements indicate that the films are highly c-axis oriented. X-ray photon spectra show that the doped Mn and Co ions in (Mn, Co) ZnO films are both in the divalent states. The films show ferromagnetic behavior with a coercivity of about 90 Oe and a saturation moment of 0.11μB∕(0.3Mn2++0.7Co2+) at 300 K. In the lower temperatures between 5 and 20 K, a relatively large positive magnetoresistance over 10% was observed for (Mn0.03,Co0.07)Zn0.90O film. The number of carrier concentration is experimentally established to be 1.5613×1018cm−3 and the mobility to be 2.815cm2V−1s−1 for (Mn0.03,Co0.07)Zn0.90O film by Hall measurements at 300 K. The origins of the room-temperature magnetism and the large positive magnetoresistance are also discussed.