X Y Zeng

Origins of both insulator–metal transition and colossal magnetoresistance in doped manganese perovskites

Based on experiments of both transport and paramagnetic resonance for (La1−xYx)2/3Ca1/3MnO3, the ground state above Tc and possible origins of both insulator–metal transition and colossal magnetoresistance near Tc are discussed. Modeling the system as a network of junctions, each consisting of a paramagnetic region sandwiched between two ferromagnetic domains, a phenomenological expression is proposed for resistance as a function of temperature and magnetic field. We show that the observed transport and magnetotransport phenomena can be quantitatively explained by the present model for the whole temperature range studied.

Exchange-narrowing spin-spin interaction in the paramagnetic regime of perovskite manganites studied through the EPR measurements for (La, Y)2/3(Ca, Sr, Ba)1/3MnO3 with a wide span of Tc

The electron paramagnetic resonance (EPR) spectrum as a function of T is experimentally studied for (La,Y)2/3 (Ca,Sr,Ba)1/3 MnO3 with a wide span of TC . In the real paramagnetic regime (T Tmin ), the spectrum consists of a single line with the resonance field being independent of T . The resonance linewidth is given by Hpp = H pp ,min +b (T -Tmin ). The exchange-narrowing spin-spin interaction and the spin-lattice interaction are explained to be responsible for the terms of H pp ,min and b (T -Tmin ), respectively. Based on the exchange-narrowing mechanism, the values of H pp ,min are directly deduced from the TC -values, which are well consistent with the experimental H pp ,min -values. This confirms the possibility that the exchange-narrowing spin-spin interaction exists in the ABO3 -type perovskite manganites.

Electrical transport and low-field magnetoresistance in the series of mixed polycrystals (1-m)La2/3Ca1/3MnO3 + mLa2/3Sr1/3MnO3

The perovskites of La2/3Ca1/3MnO3 (LCMO) and La2/3Sr1/3MnO3 (LSMO) were synthesized by the sol-gel method. The two perovskites were mixed into polycrystalline materials, (1-m)LCMO + mLSMO, where m is the relative weight ratio of LSMO. The compound with m~0.3-0.5 has the lowest semiconductor-metal transition temperature Tp and the largest low-T resistivity. Moving away from this ratio, Tp increases while the low-T resistivity decreases with increasing or decreasing m. Measurements of the magnetoresistance (MR) show that the mixed polycrystals exhibit a sizable MR effect in low magnetic fields which increases upon cooling and reaches its largest value for T?0?K. It is also interesting to observe the almost T-independent MR effect over a wide temperature window below 300?K for the mixed materials with smaller m.

Monte Carlo simulation of colossal magnetoresistance in doped manganese perovskites

Resistance as a function of temperature and applied magnetic field for doped manganese perovskites is simulated on the basis of a random-resistor-network model. The network, consisting of ferromagnetic metallic particles with number density p and paramagnetic insulating particles with number density 1-p, is generated through the Monte Carlo method. Approximating p by the reduced magnetization (m) determined from the mean-field theory, we show that the simulation can yield the main features of the colossal magnetoresistance in doped manganese perovskites. Comparisons between simulated and experimental data are also presented for (La1-xYx)2/3Ca1/3MnO3 (x = 0.2). The excellent agreement between the simulations and experiments gives strong support to the present approach.

Quantitative explanation for the electrical conductivity in (La1-xYx)2/3Ca1/3MnO3

The electrical conductivity of single-phase (La1-xYx)2/3Ca1/3MnO3 ceramic samples shows activated insulating behaviour at high temperatures, and a transition to metallic behaviour at low temperatures. On the basis of paramagnetic resonance experiments, we model the conductive channel as a series of paramagnetic regions and ferromagnetic domains along the direction of current flow. It is shown that the electrical conductivity can be explained quantitatively for the whole temperature range studied.

Phase Separation and Transport Behavior in La0.5Ca0.5–xBaxMnO3

We show that for La 0.5 Ca 0.5-x Ba x MnO 3 , the sample is single phase with an orthorhombic structure for x x d . The observed phase separation is a chemically driven inhomogeneity caused by the large size-mismatch (σ 2 ) which occurs as σ 2 > 1.42 x 10 -4 nm 2 . In the single-phase region, increasing x to >0.14 causes a transition to metallic state, and the magnetoresistance near the transition shows behavior similar to that in double-exchange manganite oxides. In the two-phase region, although the compound also shows an insulator-metal transition, both transport and magnetoresistance show behavior completely different from that in the single-phase region. Some anomalous transport and magnetoresistance behaviors are also revealed at low temperatures. A possible phase diagram is presented as a function of T and x and the experimental findings are discussed according to different ground states.

Perovskite LaBaCo2O5+δ (LBCO) single-crystal thin films for pressure sensing applications

Perovskite LaBaCo2O5+δ (LBCO) single-crystal films were deposited on (001) MgO substrates by a magnetron sputtering method and processed into Pirani sensors for investigation of pressure measurements. In comparison to the poly-crystal film deposited under the same condition, the single-crystal LBCO films exhibited rather a large temperature coefficient of resistance and a high sensitivity in response to pressure. The LBCO sensors with dimensions of 30 to 200 μm, which are different from resistor-on-dielectric membrane or micro-beam structures, demonstrated to be capable of making response to the pressures ranging from 5 × 10–2 to 105 Pa with a real dynamic range of 3 to 2 × 103 Pa.

Magnetoresistance and paramagnetic anomalies in La(2-x)/3Ba(1+x)/3Mn1-xCuxO3 (x = 0.2)

Magnetoresistance (MR) measurements for La(2 - x )/3 Ba(1 + x )/3 Mn1 - x Cux O3 (x = 0.2) show that the MR effect becomes observable at Tonset (~ 200 K), sharply increases on further cooling and reaches a maximum at Tc (~ 105 K). Whereas paramagnetic (PM) resonance experiments reveal that PM anomalies occur just below Tonset . Above Tonset , the g -value keeps constant and the resonance linewidth Hpp shows a linear temperature-dependent behaviour, while below Tonset , both the g -value and Hpp anomalously increase on cooling. It is interesting to find that at the range of Tc < T < Tonset , both g and Hpp show temperature-dependent behaviours similar to that for the MR against T . These findings suggest the same underlying physical origin for the MR effect and the PM anomalies.