Chapter 3
Collective Magnetic Behaviour
Roland Mathieu and Per Nordblad
Abstract The mechanisms responsible for magnetic interaction between nanoparticles are described and modelled in the previous chapter of this book. Here, the
collective superspin glass state resulting from such interaction is discussed, using
a collection of experimental results. Superspin glasses display qualitatively similar
dynamical magnetic properties as canonical spin glasses, including ageing, memory
and rejuvenation phenomena. In the Introduction, the dynamical properties of spin
and superspin glasses are illustrated and contrasted. These properties are discussed
in more detail in Case studies, taking into account the nanoparticle concentration,
size and size distribution, using results from studies of ferrofluids and compacts
of γ-Fe 2 O 3 particles. The Outlook section illustrates recent findings suggesting
that the temperature dependence of the low-field isothermal remanent magnetization (IRM) and magnetization as a function of magnetic field (hysteresis or M-H)
curves of superspin glasses include information on the superspin dimensionality
and magnetic anisotropy. The possibility to engineer nanocomposites with tailored
magnetic interaction and anisotropy is also discussed.
3.1 Introduction
Interaction between particles in an ensemble of magnetic nanoparticles causes collective behaviour [1–5]. Figure 3.1 shows the temperature dependence of the zerofield-cooled (ZFC) and field-cooled (FC) magnetization of a dilute and a concentrated assembly of 8 nm maghemite nanoparticles prepared from the same batch.
The magnetic response is altered by the dipolar interparticle interaction, yielding
slowing down of the magnetization dynamics at low temperatures and a nonzero
Weiss temperature (θ w ). The interparticle dipolar interaction transforms the nanoparticle system from being superparamagnetic to becoming a superspin glass (SSG). The
R. Mathieu (B) · P. Nordblad
Department of Materials Science and Engineering, Uppsala University, Box 35, 751 03 Uppsala,
Sweden
e-mail: roland.mathieu@angstrom.uu.se
© Springer Nature Switzerland AG 2021
D. Peddis et al. (eds.), New Trends in Nanoparticle Magnetism,
Springer Series in Materials Science 308,
https://doi.org/10.1007/978-3-030-60473-8_3
65
Collective Magnetic Behaviour
Roland Mathieu and Per Nordblad
Abstract The mechanisms responsible for magnetic interaction between nanoparticles are described and modelled in the previous chapter of this book. Here, the
collective superspin glass state resulting from such interaction is discussed, using
a collection of experimental results. Superspin glasses display qualitatively similar
dynamical magnetic properties as canonical spin glasses, including ageing, memory
and rejuvenation phenomena. In the Introduction, the dynamical properties of spin
and superspin glasses are illustrated and contrasted. These properties are discussed
in more detail in Case studies, taking into account the nanoparticle concentration,
size and size distribution, using results from studies of ferrofluids and compacts
of γ-Fe 2 O 3 particles. The Outlook section illustrates recent findings suggesting
that the temperature dependence of the low-field isothermal remanent magnetization (IRM) and magnetization as a function of magnetic field (hysteresis or M-H)
curves of superspin glasses include information on the superspin dimensionality
and magnetic anisotropy. The possibility to engineer nanocomposites with tailored
magnetic interaction and anisotropy is also discussed.
3.1 Introduction
Interaction between particles in an ensemble of magnetic nanoparticles causes collective behaviour [1–5]. Figure 3.1 shows the temperature dependence of the zerofield-cooled (ZFC) and field-cooled (FC) magnetization of a dilute and a concentrated assembly of 8 nm maghemite nanoparticles prepared from the same batch.
The magnetic response is altered by the dipolar interparticle interaction, yielding
slowing down of the magnetization dynamics at low temperatures and a nonzero
Weiss temperature (θ w ). The interparticle dipolar interaction transforms the nanoparticle system from being superparamagnetic to becoming a superspin glass (SSG). The
R. Mathieu (B) · P. Nordblad
Department of Materials Science and Engineering, Uppsala University, Box 35, 751 03 Uppsala,
Sweden
e-mail: roland.mathieu@angstrom.uu.se
© Springer Nature Switzerland AG 2021
D. Peddis et al. (eds.), New Trends in Nanoparticle Magnetism,
Springer Series in Materials Science 308,
https://doi.org/10.1007/978-3-030-60473-8_3
65