Structural and magnetic properties of the half-doped manganite Ho0.5Ca0.5MnO3: a comparison in bulk and in nanometric powders.

Half-doped manganites derived by hole doping via chemical substitution at rare earth (RE) site of a REMnO3 parent phase are widely studied for their rich physics, emerging from a outstanding variety of interconnected structural, electronic and magnetic phase transitions which lead to a plethora of competing ordered ground states, either metallic ferromagnetic (FM), or insulating antiferromagnetic (AFM) with possible orbital-ordering and/or charge-ordering. The resulting physical effects include colossal magnetoresistance and magnetoelectricity. Since great modifications of the phase transitions are expected at the nanoscale, the study of nanosized samples has major relevance. In particular, attention has been paid to the destabilization in nanoparticles of the charge-ordered AFM phase in favor of the charge-disordered FM one. In this work, the structural and magnetic properties of bulklike (micrometric) and nanometric (? 12÷15 nm) powders of Ho0.5Ca0.5MnO3 are presented and compared. Samples were prepared by standard solid state reaction and by Pechini-modified sol-gel method, respectively. Neutron diffraction patterns collected in the temperature range 10 K < T< 300 K indicate two main common features: i) both micro and nanometric samples have Pbnm orthorhombic symmetry with relationships c/?2 < a < b among lattice parameters, characteristic of the so-called O? structure originated by a strong cooperative Jahn-Teller effect, inducing orbital ordering and distortion of the MnO6 octahedra; ii) regardless to particle size, magnetic Bragg reflections develop below T = 110 K, corresponding to a pseudo-CE AFM supercell, which implies a charge ordering transition with chessboard arrangement of two inequivalent Mn species in the (001) crystallographic planes. Therefore there appears to be a charge disproportionation between neighbouring Mn sites towards Mn3+ and Mn4+, also at the nanoscale. SQUID magnetometry too indicates several common properties: i) magnetization versus applied field curves M(H) reveal weak ferromagnetism below the Néel temperature in both samples, with a tiny hysteresis cycle most likely associated to a spin canting. The absence of exchange bias, in fact, makes a strong case against coexisting FM and AFM phases. A significant difference between micro and nanometric Ho0.5Ca0.5MnO3 is present in the low field magnetization curves versus temperature M(T): quite surprisingly, the magnetic signal corresponding to charge ordering could be detected only for the micrometric sample (at about 250 K). Taken together, our results indicate that predominant ground state in nanometric Ho0.5Ca0.5MnO3 is bulklike charge-ordered AFM. In this particular manganite, therefore, the bulk properties are not suppressed but at most hindered at the nanoscale. At the same time, they suggest that the use of different complementary experimental techniques is mandatory for the correct characterization of the ground state of nanosized half-doped manganites.