Giant magnetoresistance in CaCu3Mn4O12-based oxides with perovskite-type structure

Ca0.5Na0.5Cu2.5Mn4.5O12, CaCu3-xMn4+xO12 (0.5 less than or equal to x less than or equal to 2) were prepared at 700 degrees C and 2600 PSI by combined sol-gel and high oxygen pressure methods. CaCu0.5Mn6.5O12 and CaMn7O12 (x = 2.5 and 3) were prepared by solid state reactions in evacuated quartz tub es at 900 degrees C, These compounds crystallize in a body-centered cubic v ariant of the perovskite structure except for CaMn7O12, which has rhombohed ral symmetry, The oxidation states of Cu and Mn were investigated by X-ray absorption spectroscopy and chemical analysis. The oxidation state of Mn is near 4+ in Ca0.5Na0.5Cu2.5Mn4.5O12 and decreases with increasing x, Ca0.5N a0.5Cu2.5Mn4.5O12, CaCu2.5Mn4.5O12 (x = 0.5), and CaCu1.5Mn5.5O12 (x = 1.5) undergo a semiconductor-to-metal transition (T-SM), CaCu2Mn5O12 (x = 1.0) is metallic, while the 2.0 less than or equal to x less than or equal to 3. 0 phases are semiconducting in the range 10-400 K, All of the materials (ex cept the x = 2.5 and 3.0) undergo a paramagnetic-to-ferromagnetic-like tran sition below their ordering temperatures (T-C). The CaCu0.5Mn6.5O12 and CaM n7O12 (x = 2.5 and 3.0) materials order antiferromagnetically at (T-N) 40 a nd 20 K, respectively. The magnetic and resistivity results are summarized in a T-x phase diagram. The ordering temperatures decrease with increasing Mn3+ content, hence the double exchange mechanism does not appear to govern the ferromagnetic ordering. The highest magnetoresistance reaches a maximu m of - 32% for Ca0.5Na0.5Cu2.5Mn4.5O12 and is - 28% for CaCu2.5Mn4.5O12 at 4.2 K and 5 T. In general, the magnetoresistance does not occur at the semi conductor-to-metal transition and it decreases smoothly with increasing tem perature or x, These materials show high sensitivity of the magnetoresistan ce at low applied magnetic fields and good temperature stability of the mag netoresistance, (C) 1999 Academic Press.