Exchange Bias Phenomenology and Models of Core/Shell Nanoparticles

Abstract

Some of the main experimental observations related to the occurrence of exchange bias in magnetic systems are reviewed, focusing the attention on the peculiar phenomenology associated to nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions posed by the experimental observations are presented and contrasted to existing theories and models for exchange bias formulated up to date. We also present results of simulations based on a simple model of a core/shell nanoparticle in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions, offering new insight on the microscopic origin of the experimental phenomenology. A detailed study of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally observed effects can be qualitatively accounted within the context of this model and allows also to quantify the magnitude of the loop shifts in striking agreement with the macroscopic observed values.

Constraining the Origins of the Magnetism of Lepidocrocite (γ-FeOOH): A Mössbauer and Magnetization Study

“…This behavior is typical of exchange-bias, a well-documented phenomenon observed in nanoparticles with a ferrimagnetic core and an antiferromagnetic outer shell (e.g., Iglesias et al, 2008), where the exchange field at the interface acts to shift the ferrimagnetic hysteresis loop. A similar observation was reported in Hirt et al (2002), where it was attributed either to an under-saturation of the sample (and thus a minor loop was measured) or to exchange bias between the bulk of the lepidocrocite particles (antiferromagnetic) and defective regions with uncompensated spins behaving like ferrimagnets.…”

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

“…An interesting class of nanoparticles (NP) is found when ferromagnetic (FM) and antiferromagnetic (AF) materials are combined together in a core/shell structure. The coupling at the interface between these two magnetic phases gives rise to the EB phenomenon [6][7][8][9][10], which is of fundamental interest and has found multiple technological applications depending on the specific composition and the characteristic size of the respective phases [11][12][13][14][15][16]. The pinning mechanism, that results from EB, has been commercially explored for magnetic field sensors and in modern magnetic read heads [17,18].…”

Energy dissipation in single-domain ferromagnetic nanoparticles: Dynamical approach

“…Single-domain ferromagnetic nanoparticles are of great interest due to their unique physical properties such as superparamagnetism [1,2], macroscopic quantum tunneling of magnetization [3,4], size-dependent characteristics [5], and exchange bias [6,7]. These and other nanoscale properties of ferromagnetic nanoparticles make them very attractive for applications, e.g., in high density data storage [8][9][10], spintronic devices [11][12][13], and biomedical engineering [14][15][16][17].…”