In colossal magnetoresistive (CMR) materials, magnetic fields of several tesla are typically required to exhibit large changes in electrical resistance, and hence, materials should be engineered to provide a more sensitive MR response at lower fields for their viability in practical applications. Enhanced low-field magnetoresistance (LFMR) was observed in highly ferromagnetic ∼20 nm La 0.71 Sr 0.29 MnO 3 particles synthesized by the nonaqueous sol−gel route. The enhanced LFMR of the nanoparticles (NPs) reaches 29.8% at 30 K with 50 mT, and the high-field magnetoresistance was 56% with a 5000 mT applied field. The large LFMR effect can be attributed to the spin-polarized tunneling across the ∼1.3 nm thick natural grain boundaries. The weaker MR effect below 30 K was attributed to the re-entrant spin glass at the core of the NPs and surface spin glass phase that could be eliminated with lower and higher applied fields, respectively. The magnetic and MR properties of the NPs were compared to those of the corresponding ∼2 μm bulk material with the same elemental composition. These results provide insight into the role of particle size, grain boundaries, and spin glass phases on the MR properties, and the consequence of this finding is useful for the potential fabrication of LFMR devices.

A bimagnetic nanostructure was designed where the antiferromagnetic (AFM) NiO nanoparticles (NPs) are confined within the pores of a mesoporous ferrimagnetic (FiM) CoFe2O4 matrix. An amount of 3.4 wt % of 9 ± 1 nm NiO NPs was inserted into pores of 35 ± 5 nm clustered CoFe2O4 NPs when the −NH3 + groups of cysteamine on the NiO NP surface electrostatically bind to the −OSO3 – of sodium dodecyl sulfate (SDS) attached to CoFe2O4 NPs. The role of in situ embedded NiO NPs is 3-fold: (i) to nearly double the saturation magnetization (M S) and coercivity (H C) by suppressing the frozen disordered spins on the surface of CoFe2O4 NPs surrounding the NiO NPs inside the pores at the cost of enhanced FiM ordering, (ii) to introduce AFM/FiM exchange coupling by breaking the spin glass surface layer to provide exchange bias (EB) of 233.0 ± 0.2 Oe at 5 K with a cooling field of 2 T, and (iii) to provide symmetry to the asymmetric nature of the hysteresis loop of CoFe2O4. In the absence of cooling field, the pristine CoFe2O4 NP porous matrix shows hysteresis loop shifts of >1000 Oe and asymmetric magnetization reversal which are uncommon in spinel oxides.

planar architectures of inorganic materials have attracted great attention owing to their fascinating physical properties. However, nanosheets of the technologically relevant doped rare-earth manganites are still elusive. Stacked 10−14 nm thick nanosheets of phase pure Pr 1−x Ca x MnO 3 (PCMO; x = 0.3 and 0.49) were obtained by decomposition of carbon coated CaCO 3 /MnCO 3 microsheets and Pr 2 O 2 CO 3 aggregates, the latter synthesized under pressure at 500−800 °C. The PCMO nanosheets had flatter Mn−O−Mn tilt angles in the MnO 6 octahedra and ferromagnetic (FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between the antiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FM moments and SEB were possible due to the long-range magnetic interactions in the stacked 2D arrangement of Mn 3+ /Mn 4+ d-electron spins, where charge ordering was completely suppressed. The SEB behavior was dependent on the initial magnetization process and the direction of rotation of the random spins at the AF/FM interface.

Although the magnetic phase diagrams of bulk and thin film samples of Pr 1−x Ca x MnO 3 (x ≤ 0.5) are widely explored, few works have been published on the magnetic properties of nanoparticles, especially in the lightly doped regime. In this paper, microwave irradiation was used to synthesize the Pr 0.977 Ca 0.023 MnO 3 and Pr 0.964 Ca 0.036 MnO 3 Manganite phases with Pnma space group in the form of anisotropic nanoparticles. The phase identification, structural characteristics, and formation of the nanostructures were analyzed by Rietveld analysis of the X-ray diffraction patterns, electron microscopy, and combining thermal analysis with infrared spectroscopy, respectively. Transport measurements on the annealed samples revealed the insulating nature, and electrical conduction occurs through thermally activated hopping of small polarons. The Mn−O c −Mn tilt angles in the MnO 6 octahedra show considerable flattening (160.9°and 167.6°for x = 0.023 and x = 0.036, respectively), possibly enhancing the electronic double exchange and promoting ferromagnetism. Ferromagnetic ordering of Mn spins was indeed observed below 109 K, and interestingly, the magnetic moment for x = 0.036 was 3.92 μ B /f.u. at 5 K, which is higher than the saturated Mn magnetic moment (3.8 μ B ). The enhanced magnetization is attributed to ordering of the Pr spins.

No abstract