M.P. Breijo

Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+)

Abstract We refer here to the electrical and magnetic properties of the Ln 1− x M x CoO 3 systems (Ln 3+ : La 3+ , M 2+ : Ca 2+ , Sr 2+ , Ba 2+ ; Ln 3+ : Nd 3+ , M 2+ : Sr 2+ ), paying special attention to those ferromagnetic compounds that display M–I transitions as temperature rises: La 1− x M x CoO 3 (M 2+ : Ca 2+ , Sr 2+ , Ba 2+ ) in the compositional interval x =0.2–0.3, and Nd 1− x Sr x CoO 3 , with x =0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical properties.

Synthesis, characterization, magnetism and transport properties of Nd1−xSrxCoO3 perovskites

Abstract We have prepared polycrystalline, single-phase Nd1−xSrxCoO3 materials (0≤x≤0.40) using the nitrate decomposition method and we have studied how their structural, magnetic and transport properties change upon Sr2+ doping. In this context we find that these series of compounds crystallize in GdFeO3-type perovskite structures whose cell volume increases as x gets higher. From the magnetic point of view, this doping makes the materials evolve towards a ferromagnetic behavior as x increases, so that for x>0.20 the samples are already ferromagnetic below a Tc∼225 K. Also, their electrical conductivity progressively increases with x. Nevertheless the materials remain semiconducting for x≤0.30 and only the sample with x=0.40 displays a M–I transition as a function of temperature (TMI∼160 K).

Spin dynamics of La0.85Sr0.15CoO3 perovskite

Abstract Dynamic magnetic properties of La 0.85 Sr 0.15 CoO 3 have been experimentally studied by AC-susceptibility and time-dependent zero-field-cooled magnetisation experiments. The onset of ferromagnetic order appears at T c ≈ 240 K and a frequency-dependent maximum of both components of the AC-susceptibility is observed at T ≈ 60 K indicating a low-temperature re-entrant spin glass phase. However, a dynamic scaling analysis of the behaviour does not yield critical slowing down but thermally activated dynamics and thus no finite temperature transition to a re-entrant spin-glass phase. Magnetic ageing, a signature of frustrated and disordered magnetic systems, is found to exist both above and below the frequency dependent maximum in the susceptibility.

Role of the rare-earth on the electrical and magnetic properties of cobalt perovskites

The magnetic and electrical properties of R1−xSrxCoO3 materials (R=La, Pr, Nd, Gd) (0⩽x⩽0.50) are studied. Rare-earths with angular momentum L≠0 enhance coercive field values. They are maximum for R=Nd (the highest L) and much smaller for Gd and La (L=0). The result is interpreted as the contribution of the L–S coupling in the rare-earth to the magnetic anisotropy of the system. Electrical conductivity is improved with Sr doping, but increases of rare-earth atomic number lead to decreases of conductivity. This is attributed to the increase of lattice distortions, due to the smaller R3+ ionic size, and to the effect of the rare-earth on the spin state of cobalt ions.

Effects of aging on the electrical resistivity of La1−xBxCoO3 perovskites (B=Sr, Ca, Ba; 0.20≤x≤0.30)

Abstract A study of the reliability of the title compounds as a basis for magnetoresistive sensors is presented. For this purpose we have carried out long-term systematic studies of the evolution of the resistive and magnetic properties of samples synthesized by both ceramic and wet chemical routes, allowing them to age during several months. It is observed that, although the magnetic properties remain unaltered, their resistivity does change appreciably. With time, samples that are initially metallic over all the measured temperature range ( T

Comparison between magnetic and electrical properties of La1-xSrxCoO3 (0 < x ⩽ 0.50) particles obtained by different synthesis methods

Abstract We have prepared polycrystalline La 1- x Sr x CoO 3 (0 x ⩽ 0.50) particles using the ceramic method and other alternative soft-chemistry techniques that yield materials with smaller particle size. The study of their magnetic and electrical properties shows differences between these series of samples, among them: changes in the compositional range for ferromagnetic behaviour, magnetic hardening as the particles get smaller; different composition for the metal-insulator transition, and changes in the conduction regime in the semiconducting interval. We try to interpret these differences taking into account the inhomogeneous electronic structure of these doped cobaltates.

Magnetic behaviour near the magnetic-phase transition in La1 − xSrxCoO3 (0.20 ⩽ x ⩽ 0.30) perovskites

Abstract Measurements of the temperature dependence of the magnetization (and parameters like coercivity and susceptibility) of La 1 − x Sr x CoO 3 (0.20 ⩽ x ⩽ 0.30) perovskites are presented. Particular attention is given to the interval of temperatures containing the ferro-paramagnetic phase transition. Curie temperature values lower than those reported previously and critical exponents are given. Additionally, the coercivity as well as the parameters of the law of approach to magnetic saturation are experimentally determined. Anomalies in Curie temperature, coercivity and law of approach to saturation are introduced confirming a non-conventional transition from ferromagnetic to paramagnetic states.

Anomalous electrical behavior in ferromagnetic La1−xBaxCoO3−δ materials (0<x≤0.50)

We prepared in this work La1−xBaxCoO3−δ (0<x≤0.50) compounds using the nitrate decomposition method, and focus in the study of the transport properties of the ferromagnetic compounds (x>0.15, Tc≈200 K) in the temperature range 77 K≤T≤300 K.We find that while for x<0.20 these materials show semiconducting behavior, their electrical conduction being dominated by small-polaron holes, for x>0.20 they show metallic behavior for T>100 K, with a transition to semiconducting behavior being observed for T ≤ 100 K. For x=0.20, small changes in the oxygen stoichiometry of the samples - due to slight variations in the thermal treatments - greatly affect the transport properties of the materials that can show either two metal-insulator transitions as the temperature increases or a semiconducting behavior. Very interestingly all these ferromagnetic samples are very sensitive to the polarity of the applied electrical current, and display peculiar “diodic” behaviors.All these observations are explained on the basis of an inhomogeneous electronic structure in the Ba-doped cobalt perovskites.