Nguyen Van Khiem

Field dependence of zero-field-cooled magnetization of La1-xSrxCoO3 (x=0.05-0.5)

Zero-field-cooled magenetizations with field up to 50000e and temperature up to 250 K were measured for La 1-x Sr x CoO 3 compounds with Sr concentrations of x = 0.05, 0.15, 0.2, 0.3, 0.4 and 0.5. It is shown that all M ZC curves exhibit a maximum at a temperature T a (<T c ), which scales well with the power law T a /T c 1 - (C AT /T c )H.The variation of parameter n with the concentration x shows very clearly the existence of two regions bording at a value x c of around 0.18. For the low-Sr region the parameter n does not depend on x and its values are similar to those suggested by the Almaida. Thouless model for canonical spin glass (n = 2/3), whereas in the high x-region n increases monotonically from 0.23 to 0.58, correspondingly, for x varying from 0.2 to 0.5.

Electronic and magnetic phase diagram of La0.5Sr0.5Co1−xFexO3(0⩽x⩽0.6) perovskites

Spin dynamics and magnetic frustration effects in La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds were systematically studied by means of resistivity, dc magnetization, ac susceptibility, and electron-spin-resonance spectra. The Fe substitution caused a metal-semiconductor transition for x⩽0.3 compositions whereas the only semiconducting state behaved for x>0.3 compositions. With increasing Fe-doping levels, the ferromagnetic–paramagnetic transition temperature, TC, decreased from 250 (x=0) to 166K (x=0.4). A spin-glass behavior was observed for x>0.4 compositions and is attributed to the frustration of random competing exchange interactions, namely, the ferromagnetic Co3+–Co4+ double-exchange interaction and the antiferromagnetic interactions such as Co–O–Fe and Fe–O–Fe. The internal dynamic properties of the samples were elucidated by the electron-paramagnetic-resonance spectra. An electronic and magnetic phase diagram of La0.5Sr0.5Co1−xFexO3 is drawn up.

Spin dynamics and spin-glass state in Fe-doped cobaltites

A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponen...

Influence of A-site substitution on the EPR parameters of La/sub 0.7/A/sub 0.3/MnO/sub 3/ (A = Sr, Ba) compounds

In this paper, we investigate in detail influences of A-site substitution on the EPR parameters for La 0.7 Sr 0.3 MnO 3 (LSM) and La 0.7 Ba 0.3 MnO 3 (LBM) compounds by means of electron paramagnetic resonance (EPR) study. Magnetization and resistivity measurements were also performed. The Curie temperature T C is 365 and 322 K for LSM and LBM, respectively. In the whole temperature range studied, metallic behavior was observed. The minimum linewidth ΔH min is 402 and 378 Oe for LSM and LBM, respectively. Temperature dependences of the EPR linewidth ΔH pp (T) for the samples were analyzed by using the model of adiabatic hopping motion of small polarons, ΔH pp (T) = ΔH o + (A/T) exp(-E a /k B T). The activation energies E (1) a and E (2) a , obtained from ΔH pp (T) and EPR intensity I(T) data, are different. At temperatures T > T min , the Lande factor of both samples is the same (g 2), indicating that spin-spin interactions among electrons of Mn 3+ and Mn 4+ ions play a main role.

Electron-spin-resonance study of Y-doped Nd0.7Sr0.3MnO3 ceramics

We have used electron spin resonance (ESR) spectroscopy to investigate spin dynamics of Mn3+ and Mn4+ ions in (Nd1−xYx)0.7Sr0.3MnO3. Experimental data reveal that ferromagnetic (FM) resonance lines at temperatures T < Tmin become symmetrical-single lines in the Lorentzian shape as T ≥ Tmin, where Tmin is the temperature corresponding to the narrowest ESR linewidth. The temperature dependence of the resonant position above Tmin exhibits the presence of a spin-orbit interaction in addition to the spin-spin interaction. Based on one-phonon relaxation, the ESR linewidth versus temperature, ΔH(T), can be described by a linear function of ΔH(T) = A + BT. Also, the ESR intensity versus temperature, I(T), obeys an exponential function of I(T)=I0exp(Ea/kBT). It has been revealed that the coexistence of FM and anti-FM interactions with different strengths directly influences ESR spectra of (Nd1−xYx)0.7Sr0.3MnO3 as varying the Y content. This makes resonant spectra of Nd-based manganites different from La-based ones.

Spin dynamics and magnetic frustration effects in La1−xSrxCoO3 (0<x⩽0.5) compounds

La1−xSrxCoO3 (0<x⩽0.5) have been thoroughly studied by means of dc magnetization, ac susceptibilities, and electron-spin-resonance (ESR) spectra. Spin-glass behavior and its transition process for x<0.2 compositions appeared to occur at temperature Tg ranging from 14.6to68K, whereas for further strontium substitution the system was characterized by growing ferromagnetic clusters. The frequency dependence of the spin-glass freezing temperature Tf has been analyzed using the conventional critical slowing-down scaling law. The temperature dependence of Ta with respect to the external magnetic field Hex obeyed an exponential function of θa=Ta∕TC∝−Hexn. A complete magnetic phase diagram of La1−xSrxCoO3 is drawn up. Besides, the internal dynamics in the samples are elucidated by the ESR spectra. The results obtained provide more insights into the nature of spin dynamics and so-called magnetic frustration phenomena in such a cobaltate system.

Influence of annealing time on the magnetic properties of La0.8Sr0.2CoO3 compound

Abstract For the La 1− x Sr x CoO 3 compounds with x ⩽0.3, the formation of homogenous samples appear to depend strictly on the preparation conditions that cause the electronic and magnetic data reported scatter strongly. In the present contribution, the results of systematic investigation of the influence of annealing time ( t a ) on various magnetic parameters are reported for the x =0.2 substance, i.e. near the percolation threshold between spin-glass and cluster-glass phases. High Sr-concentration phases are initially formed at short t a but gradually transformed to lower Sr-concentration ones with longer t a . This transformation process apparently finishes at t a ≈200 h and homogenous samples with the purely expected stoichiometric phase can be obtained with longer t a . The results from transport measurements are also mentioned in comparison to the magnetic data.

The Role of Spin-Polarized Tunneling on Transport Properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 Nanocomposites (x = 0 ÷ 5wt%)

We have investigated the effect of artificial grain boundaries on the electromagnetic properties of (1–x)La0.7Ca0.3MnO3 + xAl2O3 nanocomposites (x x= 0 x÷ 5wt%). Based upon a spin-polarized tunneling mechanism we have proposed a phenomenological model to explain the observed electrical transport behavior over the whole temperature range (70 ÷ 300K), especially the gradual drop of metal-insulator transition temperature (Tp) as a function of increasing Al2O3 content, while the feromagnetic-paramagnetic transition temperature (TC) remains almost constant (TC x= 250K). A large low-field magneto-resistance was observed for all the composite samples and the largest magnetoresistance ratio was recorded for a composition with x x=0.01.

Magnetoresistance and magnetocaloric effects in La/sub 0.7/Sr/sub 0.3/Mn/sub 0.8/Ti/sub 0.2/O/sub 3/

The magneto-caloric effect in La/sub 0.7/Sf/sub 0.3/Mn/sub 0.8/Ti/sub 0.2/O/sub 3/ has been investigated. We have found the magnetic entropy value is /spl sim/0.74 J/kg K in an applied field of 4 T. The large magnetic entropy change produced by the abrupt reduction of magnetization is attributed to the strong coupling between spin and lattice in the magnetic ordering process.