Structural imperfections and magnetoresistive properties of the ceramic La0.6Sr0.2Mn1,2−xFexO3±δ

Abstract

The x-ray structural, resistive, magnetic, and Mn55 NMR methods are used to investigate the ceramic magnetoresistive lanthanum manganite perovskites La0.6Sr0.2Mn1,2−xFexO3 (x=0, 0.02, 0.05, and 0.1) annealed at 1170 and 1500°C. It is found that increasing the Fe content decreases the metal-semiconductor phase transition temperature Tms and the Curie temperature TC, and increases the peak of the magnetoresistive effect near these phase transitions and results in growth of the peak at low temperatures, where the magnetoresistive effect is due to tunneling transitions of carriers between crystallites. The wide asymmetric Mn55 NMR spectrum, whose resonance frequency shifts with increasing x to lower frequencies, confirms the high-frequency electron-hole exchange between the Mn3+ and Mn4+ ions and the high degree of lattice imperfections, including not only vacancies but also clusters. The hysteresis in the field dependences of the magnetization at 4.2K is due to a change in the fraction of low-spin Mn2+ in clusters, whose magnetism appears below 42K. The increase of the activation energy with increasing Fe content is explained by the influence of these ions on the structural defect density, the charge carrier concentration, and the electron-hole exchange between heterovalent manganese ions in B positions.

Phase separation, ferromagnetism and magnetic irreversibility in La1−xSrxMn1−yFeyO3

“…The first one is connected to a direct replacement of Mn 3 þ by Fe 3 þ ions [21], which do not support the DE interactions in the Fe 3 þ À Mn 4 þ pairs [16,22]. The second reason is increasing microscopic disorder due to Fe doping [20]. Although both effects are possible (as found e. g. in La 1À x Ca x Mn 1À y Fe y O 3 [23]), their influence on the dependence of T C on y in LSMFO remains unclear.…”

“…Investigations of LSMFO have revealed a high sensitivity of magnetic and transport properties to Fe doping [16][17][18][19][20]. In particular, it has been established a strong decrease of the FM Curie temperature, T C , with y, attributable to two different reasons [20].…”

“…In particular, it has been established a strong decrease of the FM Curie temperature, T C , with y, attributable to two different reasons [20]. The first one is connected to a direct replacement of Mn 3 þ by Fe 3 þ ions [21], which do not support the DE interactions in the Fe 3 þ À Mn 4 þ pairs [16,22].…”

“…Although both effects are possible (as found e. g. in La 1À x Ca x Mn 1À y Fe y O 3 [23]), their influence on the dependence of T C on y in LSMFO remains unclear. Another interesting feature of LSMFO is the magnetic irreversibility, pertinent to frustrated systems such as spin-glasses or cluster-glasses [24], and observed together with the FM transition [20]. Coexistence of the FM and glassy properties has been found in many other manganite perovskites like La 1À x Ca x MnO 3 [25], La 1À x Ca x Mn 1À y Fe y O 3 [23], La 1À x Ba x MnO 3 [26], Nd 0.7 Sr 0.3 MnO 3 [27] and Y 0.7 Ca 0.3 MnO 3 [28], although its connection to the phase separation is not understood well enough.…”

Thermochemistry of perovskites in the lanthanum–strontium–manganese–iron oxide system

“…For both A and B site doping, strong correlation between electronic properties and the valence and the ionic radius of the doping atoms has been observed [14]. The iron content strongly influences the magnetic and transport properties of LSMF [15][16][17] due to the progressive suppression of ferromagnetic ordering [12,15,18]. The number of Mn 4+ ions in LSMF is fixed not only by doping, but also by the oxygen nonstoichiometry () in the formula, La 1-x Sr x Mn 1-y Fe y O 3- .…”