Effects of high vacancy concentrations on the magnetic properties of La1−xMn1−yO3 (0.02⩽x, y⩽0.13)

Abstract

It has been found that the synthesis conditions and subsequent annealing treatments can significantly change the magnetic and transport behavior of the colossal magnetoresistive materials of the general formula La1−zAzMnO3, where A is a divalent ion. In order to clarify the role of vacancies within this structure, resistivity and magnetization measurements have been carried out on a series of samples derived from the parent compound LaMnO3, with La and Mn vacancies introduced by systematically varying the oxygen annealing conditions. Previous studies have shown that for a given carrier concentration, the Curie temperature of the paramagnetic to ferromagnetic transition (Tc) increases as the tolerance factor of the perovskite structure increases and then begins to decrease slowly for even higher tolerance factors. Generally, Tc also increases with the Mn4+ content, consistent with a double exchange mechanism responsible for ferromagnetism. In this study Tc was found to decrease as the vacancy concentration (and therefore the Mn4+ concentration) and tolerance factor both increase. The magnetic and transport data are discussed in the light of structural information obtained from neutron diffraction studies. In the present study it is found that La and Mn vacancies play a significant role in determining the physical properties of these materials: for high vacancy concentrations (x,y⩾0.080) the magnetic properties are analogous to those of spin glasses associated with disorder and frustration. Semiconductorlike resistivity was observed for all samples at all temperatures.

Structure, magnetic and transport properties of La1−xBixMnO3

“…Both spin glass (SG) behavior and metamagnetic canted spin (MCS) behavior were found in lower and high d regions, respectively. Spin glass behavior was also observed in highly defective La 1Àx Mn 1Ày O 3 [21]. In LBMO, the magnetization behavior of the x ¼ 0:4 and 0.5 samples is different from the SG behavior because we didn't observe the cusp in their ZFC curves, which is a characteristic feature of SG behavior [22].…”

Specific heat and magnetic order inLaMnO3+δ

“…As mentioned in the introduction, the evolution from FM to spin glass features for increasing oxygen content in LaMnO 3+δ was previously observed in the literature. 6,8 Moreover, it is most interesting to note in Fig. 1 that the results for the two FM samples show a doublepeak structure and a frequency dependence of the imaginary component, χ ′′ .…”

“…By varying the oxygen stoichiometry the resulting compounds display a wide variety of structural and magnetic phases, previously studied by x-rays and neutron scattering, 6,7 as well as magnetic and transport measurements. 6,8 It is well known that the low-temperature magnetic phase of non-stoichiometric LaMnO 3+δ changes from AF to FM for small values of δ due to the DE interaction caused by the presence of Mn 4+ ions in the sample. However, unlike the cation-doped systems, the material remains insulating at all temperatures, and the FM transition temperature decreases for increasing content of Mn 4+ .…”

“…For large values of δ the FM order is suppressed, and the low-temperature phase is better described by a spin-glass-like state. 6,8 The competing effect between cation and manganese vacancies makes LaMnO 3+δ a model system for studying magnetic interactions and disorder effects in mixedvalence manganites. In order to achieve a better understanding of the low temperature properties of this system we have performed magnetic and specific-heat measurements in three different samples of non-stoichiometric LaMnO 3+δ .…”

On the La–Cr–O Phase Change During Methane Catalytic Combustion Investigated with in situ HT-XRD

“…/Mn 4? ratio in LaMnO 3?d [21][22][23][24]. High temperature synthesis led to samples with low over-stochiometry while low temperature synthesis yielded high cationic vacancy concentrations.…”