Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles.
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.
Department of Physics, University of South Florida, Tampa, FL 33620.
Hafsa
H
Khurshid
Department of Physics, University of South Florida, Tampa, FL 33620.
Paula
P
Lampen-Kelley
Departament de Física Fonamental and Institut de Nanociència i Nanotecnologia (N2UB) Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain.
Òscar
Ò
Iglesias
Department of Physics, University of South Florida, Tampa, FL 33620.|BCMaterials Edificio No. 500, Parque Tecnológico de Vizcaya, Derio, Spain 48160.
Javier
J
Alonso
Department of Physics, University of South Florida, Tampa, FL 33620.
Manh-Huong
MH
Phan
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439.
Cheng-Jun
CJ
Sun
BCMaterials Edificio No. 500, Parque Tecnológico de Vizcaya, Derio, Spain 48160.|IMPMC-Université Pierre et Marie Curie F-75252 Paris- Cedex 5 &Université d'Orléans.
Marie-Louise
ML
Saboungi
Department of Physics, University of South Florida, Tampa, FL 33620.
Hariharan
H
Srikanth