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Register a new userProbing the core and shell contributions to exchange bias in Co/Co$_3$O$_4$ nanoparticles of controlled size
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Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for the exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with the total averaged diameter around $sim$ 27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co$_3$O$_4$ crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias $H_E$ and coercive $H_C$ fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both the phases. Results of our atomistic Monte Carlo simulations of the particles with the same size and compositions as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal giving rise to the maximum in $H_E$ and $H_C$.
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We present an atomistic model of a single nanoparticle with core/shell structure that takes into account its lattice strucutre and spherical geometry, and in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and interfacial regions. By means of Monte Carlo simulations of the hysteresis loops based on this model, we have determined the range of microscopic parameters for which loop shifts after field cooling can be observed. The study of the magnetic order of the interfacial spins for different particles sizes and values of the interfacial exchange coupling have allowed us to correlate the appearance of loop asymmetries and vertical displacements to the existence of a fraction of uncompensated spins at the shell interface that remain pinned during field cycling, offering new insight on the microscopic origin of the experimental phenomenology.
We present a numerical simulation study of the exchange bias (EB) effect in nanoparticles with core/shell structure aimed to unveil the microscopic origin of some of the experimental phenomenology associated to this effect. In particular, we have focused our study on the particle size and field cooling dependence of the hysteresis loop shifts. To this end, hysteresis loops after a field cooling process have been computed by means of Monte Carlo simulations based on a model that takes into account the peculiar properties of the core, shell and interfacial regions of the particle and the EB and coercive fields have been extracted from them. The results show that, as a general trend, the EB field $h_{EB}$ decreases with increasing particle size, in agreement with some experimental observations. However, closer inspection reveals notable oscillations of $h_{EB}$ as a function of the particle radius which we show to be closely related to the net magnetization established after field cooling at the interfacial shell spins. For a particle with ferromagnetic interface coupling, we show that the magnitude and sign of $h_{EB}$ can be varied with the magnetic field applied during the cooling process.
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 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 with striking agreement with the macroscopic observed values.
Naturally occurring spin-valve-type magnetoresistance (SVMR), recently observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the maximal resistance from the coercivity of the sample. Here we present the evidence that SVMR can be engineered in specifically designed and fabricated core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4 as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show that this provides a magnetically switchable tunnel barrier that controls the magnetoresistance of the system, instead of the magnetic properties of the magnetic grain material, Fe3O4, and thus establishing the feasibility of engineered SVMR structures.
We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in amorphous zirconia matrix. The deposition method allows controlling the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2x10^{-5} to 10^{-1} mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoOx, with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. For oxygen pressure of about 10-3 mbar the ratio between the FM and AFM phases is optimum with an exchange bias field about 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also at the origin of the induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.
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