Barkhausen-like steps and magnetic frustration in doped La0.67-x Ax Ca0.33 Mn O3 (A=Ce,Y)

We present dc magnetization experiments performed on the doped, polycrystalline system La0.67-x Ax Ca0.33 Mn O3 (A=Ce,Y). The ceramic samples have been obtained by the nitrate decomposition route and structurally characterized by powder x-ray diffraction. Hysteresis loops reveal an exciting and nonrepetitive multistep structure of Barkhausen-like jumps in the low fields region, up to 50 Oe. We discuss our results in terms of a collective response of magnetic moments pinned by frustration due to the competition among different exchange interactions present in the compounds. A simple Ising Hamiltonian with in-plane ferromagnetic interactions and randomly mixed off-plane interactions is introduced to model this hypothesis. Computer simulations based on this model allow us to reproduce most of the features observed in the hysteresis curves, allowing a qualitative understanding of the complex magnetic behavior of this family of manganites.