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

Abstract

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 intraparticle 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.

Glassy magnetic phase driven by short-range charge and magnetic ordering in nanocrystalline La1/3Sr2/3FeOSabyasachi1, Patra2, Majumdar3 et al. 2012Phys. Rev. BSelf Cite

“…The proper choice of maximum field (H max ) applied for recording a MH loop is crucial for obtaining H E and M E , because small H max may lead to minor loop effects. [35][36][37] The plots of H E and M E with H max are shown in the lower inset of Fig. 3.…”

Exchange bias in nano-ferrihydrite

“…This expression is valid at the fields noticeably higher than H* and the exponent  is determined by the structure of energy barriers [26]; in the case of ferrihydrite, the authors obtained the value of   1.5. The set of results obtained in this study agrees well with the experimental data on HC(Hmax) at the fields over ~2·H* at   1.5.…”

“…In [26], based on the concepts of field dependence of the energy barriers caused by the magnetic anisotropy overcome by the magnetic moment of a particle, the following expression for the HC(Hmax) dependence was proposed:…”

“…The question about the existence of exchange bias in ferrihydrite remains unanswered [26], since the observed magnetic hysteresis shift can be an artifact. Silva et al [26] attributed the discussed effect in ferritin and ferrihydrite, whose TB value is no more than ~30 K [26][27][28], to the existence of high-energy barriers for the uncompensated magnetic moment, which are induced by the effective magnetic anisotropy of particles upon cooling in magnetic field; i.e., the loop shift is, in fact, analogous to the minor loop effect.…”

Магнитные свойства наночастиц NiO: вклады антиферромагнитной и ферромагнитной подсистем в различных диапазонах магнитных полей до 250 kOe

“…Однако между AF-и FM-подсистемами может существовать обменная связь, что, в частности, должно приводить к наблюдаемому эффекту обменного смещения петли гистерезиса после охлаждения во внешнем поле. Это наблюдалось для NiO [4,29,30] и других AF-наночастиц [31][32][33][34][35]. Как пра-вило, при высоких температурах величина обменного смещения уменьшается [4,12,30], что и позволяет рас-сматривать AF-наночастицу как две независимые подси-стемы.…”