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Title: Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

Disorder among surface spins largely dominates the magnetic response of ultrafine magnetic particle 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 becomes 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 with bulk coordination. Monte Carlo simulations of a hollow particle reproducing the experimental morphology 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. Lastly, our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by themore » surface spins in magnetic hollow nanostructures.« less
Authors:
 [1] ;  [1] ;  [2] ;  [3] ;  [1] ;  [4] ;  [5] ;  [1]
  1. Univ. of South Florida, Tampa, FL (United States)
  2. Univ. of Barcelona (Spain)
  3. Univ. of South Florida, Tampa, FL (United States); BCMaterials Edificio No. 500, Derio (Spain)
  4. Argonne National Lab. (ANL), Argonne, IL (United States)
  5. BCMaterials Edificio No. 500, Derio (Spain); Univ. Pierre et Marie Curie, Paris (France)
Publication Date:
OSTI Identifier:
1237916
Grant/Contract Number:
AC02-06CH11357; FG02-07ER46438
Type:
Accepted Manuscript
Journal Name:
Scientific Reports
Additional Journal Information:
Journal Volume: 5; Journal ID: ISSN 2045-2322
Publisher:
Nature Publishing Group
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS Exchange bias; Hollow nanoparticles; Spin glass; Surface spins