Superparamagnetism and transport properties of ultrafine La2/3Ca1/3MnO3 powders

Magnetic and transport properties of ultrafine La2/3Ca1/3MnO3 (LCMO) powders synthesized by mechanical alloying with a grain size of about 18 nm have been investigated. It is found that the powder sample is superparamagnetic above 95 K. The blocking temperature (T-B) Obtained from the characteristic peak in the zero-field-cooled magnetization decreases with the measuring applied field, and can be well fitted by T-B = a ln(mu H-0) - b. The spontaneous magnetization of the powder sample is only 38% of that in the corresponding bulk LCMO. Unlike conventional bulk LCMO, the powder compact is insulating from 5 K to 300 K. The low-temperature magnetoresistance (MR), defined as MR = DeltaR/R(0) (R(0) being the zero-field resistance), is approximately 100% under a field of 6 T, much higher than that of the conventional bulk LCMO (50-60%). When the temperature is above T-B, the low-field MR associated with the spin-polarized tunnelling between grains cannot be observed; however, the high-field MR increases linearly with the applied field. The small grain size and the surface layer in which many oxygen vacancies and defects induced by high-energy milling exist are suggested to be responsible for the present observations.