Exchange bias in ferrite hollow nanoparticles originated by complex internal magnetic structure.

EMILIO DE BIASI; ENIO LIMA JR.; JOSE M VARGAS ; ROBERTO DANIEL ZYSLER; JORDI ARBIOL ; ALFONSO IBARRA; GERARDO F GOYA; M RICARDO IBARRA

Materials Research Express

IOP publishing Ltd

Año: 2015

>Iron-oxide hollow nanospheres (HNS) may present unusual magnetic behavior as a consequence oftheir unique morphology. Here, we report the unusual magnetic behavior of HNS that are 9 nm indiameter. The magnetic properties of HNS originate in their complex magnetic structure, as evidencedby Mössbauer spectroscopy and magnetization measurements. We observe a bias in the hysteresiswhen measured at very low temperature in the field cooling protocol (10 kOe). In addition, dc (static)and ac (dynamic) magnetization measurements against temperature and applied field reveal afrustrated order of the system below 10 K. High-resolution transmission electron microscopy(HRTEM) studies reveal that the HNS are composed of small crystalline clusters of about 2 nm indiameter, which behave as individual magnetic entities. Micromagnetic simulations (using conjugategradient in order to minimize the total energy of the system) reproduce the experimentally observedmagnetic behavior. The model considers the hollow particles as constituted by small ordered clustersembedded in an antiferromagnetic environment (spins localized outside the clusters). In addition, thesurface spins (in both inner and outer surfaces of the HNS) are affected by a local surface anisotropy.The strong effective magnetic anisotropy field of the clusters induces the bias observed when thesystem is cooled in the presence of a magnetic external field. This effect propagates through theexchange interaction into the entire particle.