Superparamagnetism and transport properties of ultrafine La2/3Ca1/3MnO3powders

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Abstract
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 (TB) obtained from the characteristic peak in the zero-field-cooled magnetization decreases with the measuring applied field, and can be well fitted by TB = aln (µ0H)-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 = ΔR/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 TB, 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.