Exchange-bias and magnetic anisotropy fields in core-shell ferrite nanoparticles

Magnetic anisotropy engineering in onion-structured metal oxide nanoparticles combining dual exchange coupling and proximity effects

A series of exchange-coupled magnetic nanoparticles combining several magnetic phases in an onion-type structure were synthesized by performing a three-step seed-mediated growth process. Iron and cobalt precursors were alternatively decomposed in high-boiling-temperature solvents (288-310 °C) to successively grow CoO and Fe3-δO4 shells (the latter in three stages) on the surface of Fe3-δO4 seeds. The structure and chemical composition of these nanoparticles were investigated in depth by combining a wide panel of advanced techniques, such as scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy-spectrum imaging (EELS-SI), 57Fe Mössbauer spectrometry, and X-ray circular magnetic dichroism (XMCD) techniques. The size of the nanoparticles increased progressively after each thermal decomposition step, but the crystal structure of core-shell nanoparticles was significantly modified during the growth of the second shell. Indeed, the antiferromagnetic CoO phase was progressively replaced by the CoFe2O4 ferrimagnet due to the concomitant processes of partial solubilization/crystallization and the interfacial cationic diffusion of iron. A much more complex chemical structure than that suggested by a simple size variation of the nanoparticles is thus proposed, namely Fe3-δO4@CoO-CoFe2O4@Fe3-δO4, where an intermediate Co-based layer was shown to progressively become a single, hybrid magnetic phase (attributed to proximity effects) with a reduction in the CoO amount. In turn, the dual exchange-coupling of this hybrid Co-based intermediate layer (with high anisotropy and ordering temperature) with the surrounding ferrite (core and outer shells) stabilized the particle moment well above room temperature. These effects allow for the production of Fe oxide-based magnetic nanoparticles with high effective anisotropy, thus revealing the potential of this strategy to design rare-earth-free permanent nanomagnets at room temperature.

Probing the strongly correlated magnetic state of Co2C nanoparticles at low temperatures usingμSR

Co2C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. An earlier study (Royet al2021J. Phys.: Condens. Matter33375804) showed that a pellet made from Co2C NPs exhibits exchange bias (EB) effect below a temperature,TEB= 50 K and a spin glass (SG) feature emerges belowTSG= 5 K. In the current study we use magnetic, electrical transport, specific heat, and muon spin rotation (μSR) measurements to explore further the magnetic properties of a pellet made with 40 nm diameter pure Co2C NPs. We uncover the onset of Kondo localization at Kondo temperatureTK(= 40.1 K), which is close to the onset temperature (TEB) of the EB effect. A crossover from the Kondo-screened scenario to the Ruderman-Kittel-Kasuya-Yosida interaction-dominated regime is also observed forT Collapse

Key Words

• Kondo effect
• exchange bias
• magnetic nanoparticles
• muon spin relaxation
• spin glass

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Affiliation(s)

Nirmal Roy Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
P C Mahato Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
Suprotim Saha Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
M Telling ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom
J S Lord ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom
D T Adroja ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
S S Banerjee Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

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A numerical study on the interplay between the intra-particle and interparticle characteristics in bimagnetic soft/soft and hard/soft ultrasmall nanoparticle assemblies

A mesoscopic scale approach and the Monte Carlo (MC) method have been employed to study the exchange bias behaviour of MnFe₂O₄ (soft)/maghemite (soft) and CoFe₂O₄ (hard)/maghemite (soft) nanoparticles (NPs) of size ∼ 3 nm in dense and diluted assemblies at low temperatures. The analysis of our MC results clearly shows that in the powder samples the contribution to the exchange bias field (H _ex) and the coercivity (H _c) comes mainly from the intraparticle core/shell structure in the hard/soft sample and that the interplay between the internal characteristics and the interparticle interactions is more important in the soft/soft samples where the dipolar strength is enhanced. In the diluted frozen ferrofluid samples where interparticle exchange interactions are absent and the role of the dipolar interactions is not significant the exchange bias effects are reduced, and they come from the intra particle structure. The variation of H _ex and H _c with the applied cooling field well reproduces the experimental findings and sheds light on the key mechanisms of the observed magnetic behaviour. Our results demonstrate the possibility to control the magnetic behaviour of nanostructures by using properly chosen core/shell bimagnetic nanoparticles.