Magnetic properties of nanocrystalline La_1−<i>x</i>MnO_3+δmanganites: size effects

Abstract

The magnetic properties of nanocrystalline manganites La1−xMnO3+δ with particle size of 20 (LMO20), 25 (LMO25), and 30 nm (LMO30), prepared by the citrate method, have been investigated in the temperature range 5–320 K, magnetic field up to 90 kOe and under quasi-hydrostatic pressures up to 14.5 kbar. The studies involve sequential zero-field-cooled magnetization (M) measurements followed by magnetization measurements during cooling in the same magnetic field (H) and complementary measurements of ac susceptibility. Additional measurements of M versus H were carried out at ambient and applied pressures. All nanoparticles exhibit a paramagnetic to ferromagnetic transition (PFT) at a Curie temperature TC>200 K. It was found that the relative volume of the ferromagnetic phase increases for larger particle size and approaches a value of about 93% for LMO30. The real part of the ac susceptibility of sample LMO20 exhibits strong frequency dependence in a wide temperature range below TC, whereas for sample LMO30 only relatively weak frequency dependence was observed. The magnetization of sample LMO30 exhibits a PFT of second order; the type of transition could not be established for the smaller particles. It was found that an applied pressure enhances the TC of La1−xMnO3+δ nanoparticles with a pressure coefficient of dTC/dP≈1.9 K kbar−1 for LMO20 and dTC/dP≈1.4 K kbar−1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.