Nanoscale ferromagnetism in phase-separated manganites

Journal of Solid State Chemistry, 2002

An experimental study of the low T magnetic field dependence of the magnetization, specific heat and resistivity for an orbital and charge-ordered perovskite manganite doped in the Mn-site is presented. Well-defined steps are found in all these three physical properties. The abruptness of these jumps for To10 K suggests some similarities to the Barkhausen effect and to the martensitic transition. This is a new illustration of the rich physics related to the phase separation. # 2002 Elsevier Science (USA) Due to the richness of their physics (1), the perovskite manganites of formula Ln 1Àx Ae x MnO 3 , where Ln and Ae are a trivalent lanthanide and a divalent alkaline-earth cations, respectively, have been the focus of intensive research these last years. Earlier it was proposed by Zener that the double-exchange (DE) model can explain the simultaneous paramagnetic (PM) to ferromagnetic (FM) and insulator (I) to metal (M) transitions occurring at T C in these Mn 3+ (t 3 2g e 1 g )/Mn 4+ (t 3 2g e 0 g ) based 3D structures (2). However, this DE model cannot explain alone the colossal magnetoresistance (CMR) observed for these oxides, which exhibit resistivity drops by several orders of magnitude as shown for Sr-or La-doped Pr 0.7 Ca 0.3 MnO 3 (3, 4). In fact for the Ln 0.7 Ae 0.3 MnO 3 manganites, below a critical size of the A site cation, charge and orbital ordering appears, and phase separation phenomena are observed when cooling the samples from room temperature. This is the case of Pr 0.7 Ca 0.3 MnO 3 whose single-phase paramagnetic insulating state (PMI) is progressively transformed into a mixture of two phases coexisting at the microscopic scale (10-100 nm), an insulating orbitally charge-ordered (OO-CO) phase, and a ferromagnetic metallic (FMM) phase (5, 6). The OO/CO phenomena in these manganites are thought to be the result of the tendency of their Jahn-Teller Mn 3+ cations to separate in the form of alternating Mn 3+ and Mn 4+ stripes, for which the ordered d 2 z orbitals are rotated by 901 in two successive Mn 3+ planes . The Jahn-Teller nature of Mn 3+ species is thus responsible for the strong coupling between charge, spin and the lattice in these materials (8). In the phase-separated state, the Pr 0.7 Ca 0.3 MnO 3 insulator is amenable to a metallic state by different external sources [electron irradiation, X ray, pressure, magnetic field, electrical field (6, 9-11)] and its high sensibility to several kinds of perturbations is understood as a partial melting of the OO/CO insulating regions into FMM ones which allows to cross the percolation threshold (from I to M) to yield CMR properties.

Physical Review B, 2010

The structural and magnetic properties of Ca 0.8 Sm 0.16 Nd 0.04 MnO 3 have been investigated by synchrotron x-ray powder diffraction in pulsed magnetic fields in connection with resistivity and magnetization measurements ͑in static and pulsed magnetic fields͒. Below 100 K, the spontaneous ͑B =0͒ low-temperature phase is found to be structurally and magnetically phase segregated, a major antiferromagnetic monoclinic P2 1 / m phase coexisting with a minor antiferromagnetic orthorhombic Pnma phase containing ferromagnetic clusters. Upon the application of a magnetic field, two magnetic transitions occur: a first transition without structural changes at low field, showing the superparamagnetic like behavior of ferromagnetic domains, followed by a metamagnetic transition. The latter is clearly accompanied by field-induced structural changes, the orthorhombic phase growing at the expense of the monoclinic one.

Low Temperature Physics, 1999

Low Temperature Physics, 2005

Magnetic and structural phase diagrams of the La0.88MnOx, La1−xSrx(Mn1−x/2Nbx/2)O3, Nd1−xCaxMnO3, and Bi1−xCaxMnO3 series, constructed on the basis of x-ray, neutron powder diffraction, Young’s modulus, magnetization and resistivity measurements, are presented. It is shown that the main factor controlling the antiferromagnet-ferromagnet phase transition in the manganites is a type of an orbital state. The results are discussed in the framework of structurally driven magnetic phase separation model.

Physical Review B, 2010

We examined thin epitaxial films Lasl8_yPryCa3l8Mn03 (LPCMO:y=O.275-0.3) in situ by Lorentz transmission electron microscopy (TEM) and other microscopy methods. Clear evidence was obtained for the competing two-phase coexistence of anti ferromagnetic charge-ordered (CO) and ferromagnetic (FM) phases that exhibit mesoscale phase separation below the metal-to-insulator transition (MIT) at~164 K. In addition, we observed some regions of mixed CO-and FM-domain contrast attributed earlier to formation of the new CO-FM phase. Using in situ heating/cooling TEM experiments, we interpret this effect as the interfacial wetting phenomenon inherent to the first-order CO-FM phase transition, rather than to the formation of new CO-FM phase. It is evidenced by the partial magnetic melting of CO phase at interfaces with the FM phase, thereby creating charge-disordered spin-glass metastates. For coexisting CO-and FM-domain configurations, we directly refined the relationship between charge-orbital and spin-ordering vectors, consistent with FM moments pinned by (lOl)-crystal twins in LPCMO films. We also discuss the striking linear dependence observed below MIT for the log-resistance behavior and the CO fraction in LPCMO directly measured by TEM. Such linear dependence does not follow typical percolation equations, suggesting that percolation model needs further revisions for transport description of manganites.

Physical Review B, 2003

In view of recent experiments, indicating the spatial coexistence of conducting and insulating regions in the ferromagnetic metallic phase of doped manganites, we propose a refined mixedphase description. The model is based on the competition of a double-exchange driven metallic component and a polaronic insulating component, whose volume fractions and carrier concentrations are determined self-consistently by requiring equal pressure and chemical potential. The resulting phase diagram as well as the order of the phase transition are in very good agreement with measured data. In addition, modelling the resistivity of the mixed, percolative phase by a random resistor network, we obtain a pronounced negative magnetoresistance in the vicinity of the Curie temperature TC. 75.47.Gk, 71.38.Ht, 71.30.+h

Physical Review Letters, 2006

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase [the so-called current-carrier density collapse (CCDC)] has accounted for the first-order ferromagnetic phase transition, colossal magnetoresistance (CMR), isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of CCDC. The present quantitative description of resistivity is obtained without any fitting parameters by using the experimental resistivities far away from the transition and the experimental magnetization, and essentially model independent.

Journal of Alloys and Compounds, 2013

We study the nature of short-range magnetic interactions observed in the paramagnetic phase of colossal magnetoresistance compounds. Our results reveal that ferromagnetic-like interaction between Mn ions cannot be explained by the conventional double exchange mechanism. The results show evidence that the e g electrons are localized in Mn 3+ ions regardless the introduction of holes leading to ferromagnetic/antiferromagnetic superexchange-like interactions.

Physical Review B, 2003

We report neutron scattering results on the spin dynamics in the paramagnetic ͑PM͒ and ferromagnetic ͑FM͒ states of the 50% hole-doped manganites Pr 1/2 Sr 1/2 MnO 3 and Nd 1/2 Sr 1/2 MnO 3. In the PM phase, these systems exhibit two kinds of diffuse scatterings: an isotropic quasielastic diffuse scattering and a dynamical ridge-type, i.e., two-dimensional FM diffuse scattering. With decreasing temperature, both systems enter the pure FM metallic state, but the spin dynamics in the FM state possess a strong A-type antiferromagnetic feature. In fact, Nd 1/2 Sr 1/2 MnO 3 enters a canted antiferromagnetic phase at lower temperature through a second order phase transition. These behaviors in spin dynamics demonstrate the existence of the static d x 2 Ϫy 2-type orbital ordering in the PM and FM states of 50% hole-doped manganites which have relatively larger one-electron bandwidth.

New Journal of Physics, 2010