arXiv subjects

N. J. O. Silva

Publications and source records attributed to N. J. O. Silva.

5 recordsLinked to original sources

arXiv2013

Pressure effects in hollow and solid iron oxide nanoparticles

We report a study on the pressure response of the anisotropy energy of hollow and solid maghemite nanoparticles. The differences between the maghemite samples are understood in terms of size, magnetic anisotropy and shape of the particles. In particular, the differences between hollow and solid samples are due to the different shape of the nanoparticles and by comparing both pressure responses it is possible to conclude that the shell has a larger pressure response when compared to the core.

cond-mat.mtrl-sci↗

arXiv2011

Shifted loops and coercivity from field imprinted high energy barriers in ferritin and ferrihydrite nanoparticles

We show that the coercive field in ferritin and ferrihydrite depends on the maximum magnetic field in a hysteresis loop and that coercivity and loop shifts depend both on the maximum and cooling fields. In the case of ferritin we show that the time dependence of the magnetization also depends on the maximum and previous cooling fields. This behavior is associated to changes in the intra-particle energy barriers imprinted by these fields. Accordingly, the dependence of the coercive and loop shift fields with the maximum field in ferritin and ferrihydrite can be described within the frame of a uniform-rotation model considering a dependence of the energy barrier with the maximum and the cooling fields.

cond-mat.mtrl-sci↗

arXiv2009

Temperature dependence of antiferromagnetic susceptibility in ferritin

We show that antiferromagnetic susceptibility in ferritin increases with temperature between 4.2 K and 180 K (i. e. below the Néel temperature) when taken as the derivative of the magnetization at high fields ($30\times10^4$ Oe). This behavior contrasts with the decrease in temperature previously found, where the susceptibility was determined at lower fields ($5\times10^4$ Oe). At high fields (up to $50 \times10^4$ Oe) the temperature dependence of the antiferromagnetic susceptibility in ferritin nanoparticles approaches the normal behavior of bulk antiferromagnets and nanoparticles considering superantiferromagnetism, this latter leading to a better agreement at high field and low temperature. The contrast with the previous results is due to the insufficient field range used ($< 5 \times10^4$ Oe), not enough to saturate the ferritin uncompensated moment.

cond-mat.str-el↗

arXiv2007

Evidence of random magnetic anisotropy in ferrihydrite nanoparticles based on analysis of statistical distributions

We show that the magnetic anisotropy energy of antiferromagnetic ferrihydrite depends on the square root of the nanoparticles volume, using a method based on the analysis of statistical distributions. The size distribution was obtained by transmission electron microscopy, and the anisotropy energy distributions were obtained from ac magnetic susceptibility and magnetic relaxation. The square root dependence corresponds to random local anisotropy, whose average is given by its variance, and can be understood in terms of the recently proposed single phase homogeneous structure of ferrihydrite.

cond-mat.mtrl-sci↗

arXiv2004

Relevance of a magnetic moment distribution and scaling law methods to study the magnetic behavior of antiferromagnetic nanoparticles

In antiferromagnetic nanoparticles magnetization the linear component, $χ_{AF}$H superposed to the saturation one usually complicates the fit of experimental data. We present a method based on scaling laws to determine the variation of $χ_{AF}$ with temperature and to find the temperature dependence of the average magnetic moment $<μ>$, without any assumption on both the magnetization dependence on field or the moment distribution function, whose relevance can also be estimated. We have applied this method to ferritin and found that $<μ>$ decreases with increasing temperature and that a distribution function cannot be ignored. The fit with Langevin magnetization law and lognormal moment distribution functions yielded parameters close to those estimated with the scaling method. We also show that in general if the distribution is ignored, and a single particle moment $μ_p$ is used, $μ_p$ presents an artificial systematic increase with temperature. This calls the attention to the necessity of evaluating the effect of a size distribution before concluding about the physical nature of the parameters variation.

cond-mat.mtrl-sci↗