Influence of interfacial roughness on exchange bias in core-shell nanoparticles

Exchange bias is a phenomenon that has attracted a great deal of interest in the 50 years following its discovery, but that is still lacking a deep theoretical understanding of its origin in core-shell nanoparticles. We present calculations of ferro-antiferromagnetic core-shell nanoparticles with roughened interfaces and demonstrate a wide dispersion in the calculated exchange-bias field caused by the roughening. Furthermore, we show that the magnitude of the exchange-bias field is strongly correlated with the net interfacial moment in the antiferromagnet, proportional to the degree of the interfacial roughness. This provides new insight into the origins of exchange bias in core-shell nanoparticles.

Figure 1

Hysteresis loops and visualizations (right) of four typical particles, each with different interfaces. Each interface structure gives rise to different hysteresis, with different degrees of exchange bias. The general trend shown in the visualizations is greater intermixing between the core and shell leads to a larger exchange-bias effect.

Figure 2

Plot of the calculated exchange-bias field against the net interfacial moment in the AFM shell. Line shows a linear fit to the data.

Figure 3

Collective hysteresis loop for 128 noninteracting exchange-biased nanoparticles with aligned easy axes. The loop displays an average exchange bias but shows little relation to the individual loops of the particles. The collective loop crucially omits information regarding the statistical distribution of exchange-bias fields of individual particles.