Article pubs.acs.org/cm
Enhanced Low-Field Magnetoresistance in La0.71Sr0.29MnO3 Nanoparticles Synthesized by the Nonaqueous Sol−Gel Route Anustup Sadhu and Sayan Bhattacharyya* Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur 741252, Nadia, WB, India S Supporting Information *
ABSTRACT: 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 La0.71Sr0.29MnO3 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. fields, a property known as low-field magnetoresistance (LFMR).5,7−9 LFMR is associated with a large number of grain boundaries having noncollinear spin structure,10,11 good connectivity between the grains,11 and the smallest possible grain size with a high saturation magnetic moment.10,12 Though LFMR is usually more pronounced at low temperatures, the utility of this effect is uncertain in practical applications unless it is observed at room temperature.8,13 Combining all these features in a single nanostructured material had been so far elusive. The major focus has been on the thickness and type of grain boundaries, because they decouple the neighboring ferromagnetic (FM) grains and provide an energy barrier for spin-polarized tunneling of electrons, the key point for strengthening the LFMR effect.14 Enhanced LFMR was observed by incorporating artificial grain boundaries in FM manganite/spacer/FM manganite trilayers. The highest LFMR of 45−50% was obtained below 20 mT at 4.2 K for La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3 thin film junctions. 6,15 The LFMR of composite films such as the La0.67Sr0.33MnO3/ZnO film was 23.9% at 500 mT and 10 K.7 The contribution of the grain boundary toward a 23−27% LFMR at 20−100 mT and 77 K was explained by designing a single SrTiO3 bicrystal substrate grain boundary into an epitaxial film of La0.7Ca0.3MnO3.9 However, LFMR with natural
1. INTRODUCTION The application of colossal magnetoresistive (CMR) materials in high-density information storage, spin valves, and sensor technologies relies on the efficiency of controlling the spin polarization at the atomic scale by applying electric and magnetic fields.1,2 Of particular importance have been the La1−xAxMnO3 (A = Ca2+, Sr2+, or Ba2+; x = 0.3−0.5) phases that demonstrate stable magnetic ordering up to high temperatures and CMR properties for practical data storage applications.3,4 Among these doped rare-earth manganites, La1−xSrxMnO3 (LSMO; x ∼ 0.3) is extremely attractive for the underlying physics involved at the nanoscale for their exploration in potential devices.5 When an x amount of Sr2+ displaces La3+ from the lattice sites, the charge balance in LSMO is maintained by the coexistence of 1 − x Mn3+ and x Mn4+ ions. Both Mn3+ (3d4) and Mn4+ (3d3) ions have their t2g orbitals filled with three electrons, and the extra electron in Mn3+ lies in the eg orbital. Because of strong intra-atomic exchange in Mn3+, the eg1 electron remains ferromagnetically coupled to the local spin (S = 3/2) of the t2g3 orbital, according to Hund’s rule.6 CMR is observed when electron transport takes place via the eg1 electron of Mn3+ ions by applying a large magnetic field that can suppress the thermal magnetic disorder. Thus, the practical utilization of the CMR property is limited because of the requirement of very high fields (several tesla). To overcome this problem, considerable research effort has been devoted to examining the large decrease in electrical resistance caused by the application of
