Magnetic properties of nanocrystalline La_1-xMnO _3+δ manganites: Size effects

The magnetic properties of nanocrystalline manganites La _1-xMnO_3+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 5-320K, magnetic field up to 90kOe and under quasi-hydrostatic pressures up to 14.5kbar. 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 T _C>200K. 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 T_C, 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 T_C of La_1-xMnO _3+δ nanoparticles with a pressure coefficient of dT _C/dP≈1.9Kkbar^-1 for LMO20 and dT_C/dP≈1. 4Kkbar^-1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.