Abstract
Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.
Highlights
The intrinsic exchange bias observed in hollow γ -Fe2O3 hollow particles with d ~ 20 nm becomes a minor loop effect when the diameter is reduced to 10 nm as the large fraction of highly frustrated surface spins prevents saturation of the magnetization in fields up to several T8,10
In a recent study[11], we found that for d < 1 0 nm magnetic relaxation in a hollow particle ensemble was best described by a non-interacting particle model, as the dominant role of disordered surface spins and severely reduced particle magnetization rendered the influence of dipolar interactions negligible in determining low-temperature magnetic behavior
The results of our experimental observations and Monte Carlo simulations demonstrate that the freezing of disordered spins at the inner and outer surfaces leads to the development of spin-glass-like behaviors including memory, remanence, and aging effects as well as an exchange bias phenomenon in which surface spins play the role of an irreversible magnetic phase
Summary
The resulting spin-glass-like state, observed experimentally at low temperature, is reflected in the dependence of the coercive fields and remanent magnetizations on the field sweep rate—controlled in the simulation by the number of MCS. These quantities present a marked short-time dynamic evolution that stabilizes once surface spins attain their frozen state. A stronger contribution to the experimentally observed spin freezing and related properties can be expected from the outer surface layer of 15 nm hollow particles
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