Magnetic phase transitions and magnetoresistance in the La0.7Ba0.3(Mn1-xMex)O-3 (Me = Fe, Al) single crystals and La0.7Sr0.3(Mn1-xMex)O-3 (Me = Fe, Cr, Al) ceramics

The single crystals of La0.7Ba0.3(Mn1-xFex)O-3 (x less than or equal to 0.2 8) and La0.7Ba0.3(Mn1-xAlx)O-3 (x less than or equal to0.15) compositions w ere grown using flux method and characterized by X-ray, electrical and magn etization measurements. The Fe-doping above x = 0.2 destroys a long range f erromagnetic order thus leading to a spin glass state. It is found that ins ulating spin glasses exhibit a large magnetoresistance in the paramagnetic region which is comparable to that for ferromagnetic crystals showing metal -insulator transition close to T-c. The magnetic behavior of La0.7Ba0.7(Mn1 -xMx)O-3 (Me = Fe, Cr, Al) ceramics is in agreement with superexchange magn etic interactions via oxygen. (C) 2001 Published by Elsevier Science B.V.