Exchange bias in Co/CoO core-shell nanowires: Role of antiferromagnetic superparamagnetic fluctuations

High aspect ratio Co/CoO nanotubes (NTs) were obtained by potentiostatic electrodeposition of Co inside nanoporous alumina templates followed by the natural oxidation of their inner walls. Magnetic measurements performed at low temperatures after field cooling the samples from above its blocking temperature (T B $ 220 K), evidenced the existence of exchange bias (EB) coupling between the Co ferromagnetic outer wall and the CoO antiferromagnetic inner wall of the NTs. A decrease in the magnitude of the EB field was measured at T < T B when cycling the Co/CoO NT arrays through consecutive hysteresis loops. This decrease is known as the training effect (TE) and is here studied in the 6 K T < T B temperature range. The TE was fitted using the recursive Binek formula, giving small values for the characteristic decay rate of the training behavior, and evidencing a decrease of EB with increasing antiferromagnetic layer thickness. A phenomenological theory for the temperature dependence of the TE in exchange biased systems was applied for the first time to core-shell nanotubular structures. The good agreement obtained between the experimental results and the theoretical data, provided a strong confirmation of the qualitative correctness of the spin configuration relaxation model used in these systems.

Section: Introductionsupporting

confidence: 70%

Section: Introductionmentioning

confidence: 99%

Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

Proença

Ventura

Sousa

et al. 2013

“…Keeping the nanowires in polyol solution avoids any air exposure and therefore prevents metal from oxidation.In this condition we could verify the absence of the exchange bias phenomenon from the magnetization reversal inside the nano-objects (author?) [31] (i.e. no shift of the hysteresis cycles was observed even at low temperature (author?)…”

Section: Methodsmentioning

confidence: 99%

“…This shift is due to exchange bias effect coming from the coupling of the AFM CoO or NiO shell together with the FM Co 80 Ni 20 core (author?) [31]. Keeping the nanowires in polyol solution avoids any air exposure and therefore prevents metal from oxidation.In this condition we could verify the absence of the exchange bias phenomenon from the magnetization reversal inside the nano-objects (author?)…”

Section: Methodsmentioning

confidence: 99%

Morphology control of the magnetization reversal mechanism in Co80Ni20 nanomagnets

Mercone

Zighem

Léridon

et al. 2015

Self Cite

Nanowires with very different size, shape, morphology and crystal symmetry can give rise to a wide ensemble of magnetic behaviors whose optimization determines their applications in nanomagnets. We present here an experimental work on the shape and morphological dependence of the magnetization reversal mechanism in weakly interacting Co80Ni20 hexagonal-close-packed nanowires. Non-agglomerated nanowires (with length L and diameter d) with a controlled shape going from quasi perfect cylinders to diabolos, have been studied inside their polyol solution in order to avoid any oxidation process. The coercive field HC was found to follow a standard behavior and to be optimized for an aspect ratio L d > 15. Interestingly, an unexpected behavior was observed as function of the head morphology leading to the strange situation where a diabolo shaped nanowire is a better nanomagnet than a cylinder. This paradoxical behavior can be ascribed to the growth-competition between the aspect ratio L d and the head morphology ratio d D (D being the head width). Our experimental results clearly show the importance of the independent parameter (t = head thickness) that needs to be considered in addition to the shape aspect ratio ( L d ) in order to fully describe the nanomagnets magnetic behavior. Micromagnetic simulations well support the experimental results and bring important insights for future optimization of the nanomagnets morphology.

“…They have been extensively studied both experimentally and theoretically [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] with variable length and width. The theoretical work is mainly based on the numerical approach of micromagnetics or semi-analytical approaches for solving the extended magnetostatic model.…”

Section: B Magnetic State Of An Isolated Chainmentioning

confidence: 99%

Ferromagnetic resonance of magnetostatically coupled shifted chains of nanoparticles in an oblique magnetic field

Bastardis

Déjardin

Vernay

et al. 2016

We investigate the ferromagnetic resonance characteristics of a magnetic dimer composed of two shifted parallel chains of iron nanoparticles coupled by dipolar interactions. The latter are treated beyond the point-dipole approximation taking into account the finite size and arbitrary shape of the nano-elements and arbitrary separation. The resonance frequency is calculated as a function of the amplitude of the applied magnetic field and the resonance field is computed as a function of the direction of the applied field, varied both in the plane of the two chains and perpendicular to it. We highlight a critical value of the magnetic field which marks a state transition that should be important in magnetic recording media.