[Paper Review] High temperature magnetic stabilization of cobalt nanoparticles by an antiferromagnetic proximity effect

J. A. De Toro, Daniel P. Marques+5 more|arXiv (Cornell University)|Jul 13, 2015

Magnetic properties of thin films64 references73 citations

TL;DR

This study demonstrates that cobalt nanoparticles (5–7 nm) embedded in a NiO matrix achieve magnetic blocking temperatures above 400 K—over 10× higher than in isolation—via a proximity effect between a low-Néel-temperature, high-anisotropy CoO shell and a high-Néel-temperature, low-anisotropy NiO matrix. The resulting effective antiferromagnetic coupling stabilizes the Co core’s magnetization against thermal fluctuations through exchange bias, with mean-field modeling confirming the mechanism.

ABSTRACT

Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications in ultra-high density recording among other applications. Here we demonstrate that low blocking temperature ferromagnetic (FM) Co nanoparticles (TB<70 K) become magnetically stable above 400 K when embedded in a high N\\'eel temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable TB enhancement is due to a magnetic proximity effect between a thin CoO shell (with low N\\'eel temperature, TN; and high anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix (with high TN but low KAFM). This proximity effect yields an effective AFM with an apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed TB increase. Mean-field calculations provide a semi-quantitative understanding of this magnetic- proximity stabilization mechanism.

Motivation & Objective

• To overcome the limitation of low blocking temperatures (TB) in ferromagnetic Co nanoparticles due to thermal fluctuations.
• To explore whether a proximity effect between two antiferromagnetic materials (CoO and NiO) could enhance magnetic stability beyond room temperature.
• To investigate the role of interfacial exchange coupling between a CoO shell and a NiO matrix in stabilizing the magnetic moment of the Co core.
• To develop a mean-field model to explain the observed enhancement in blocking temperature and exchange bias.

Proposed method

• Synthesis of Co/CoO core/shell nanoparticles (5–7 nm) via inert gas condensation and RF-sputtering, with controlled particle size via cluster source power and helium carrier gas.
• Embedding the nanoparticles in a high-Néel-temperature (520 K) NiO matrix to form a composite system (S-series), with Nb-doped samples as reference (R-series).
• Magnetic characterization using SQUID magnetometry to measure field-cooled/zero-field-cooled (FC/ZFC) magnetization curves and hysteresis loops at various temperatures.
• Quantitative analysis of exchange bias (HE) and blocking temperature (TB) from M(H) loops and ZFC/FC curves, with background subtraction from Si substrate.
• Development of a mean-field molecular model to calculate staggered magnetization in Co and Ni atoms across spherical shells, incorporating interfacial exchange coupling and anisotropy.
• Estimation of effective CoO shell thickness (0.25 nm) from fitting HE(T) data to the model, assuming isolated nanograins rather than a continuous shell.

Experimental results

Research questions

• RQ1 Can the magnetic stability of small Co nanoparticles be enhanced beyond room temperature using an antiferromagnetic proximity effect?
• RQ2 What is the role of the CoO shell’s anisotropy and the NiO matrix’s Néel temperature in enabling high-TB stabilization?
• RQ3 How does interfacial exchange coupling between CoO and NiO lead to an effective antiferromagnet with enhanced blocking temperature?
• RQ4 To what extent can mean-field modeling quantitatively explain the observed exchange bias and blocking temperature enhancement?

Key findings

• The blocking temperature (TB) of Co/CoO-NiO nanoparticles exceeds 400 K (TB(S50He) > 400 K), representing a more than 10-fold increase compared to reference Co/Nb samples (TB(R50He) ≈ 35 K).
• At 300 K, the S50He sample exhibits a finite coercivity (µ0HC ≈ 6 mT) and exchange bias (µ0HE ≈ 14 mT), confirming non-superparamagnetic behavior.
• The exchange bias (HE) remains finite up to at least 400 K, indicating that the exchange-coupled core-shell system maintains its magnetic memory beyond room temperature.
• The CoO shell thickness is estimated to be 0.25 nm per nanograin, based on fitting the HE(T) data to the mean-field model, consistent with TEM observations of small, isolated oxide grains.
• Mean-field calculations reproduce the experimental trend of Co magnetization suppression at the surface and enhanced stability in the CoO/NiO interface, without fitting parameters.
• The effective antiferromagnetic behavior arises from synergy between the high-TN NiO matrix and the high-anisotropy CoO shell, creating a composite with enhanced effective anisotropy and blocking temperature.