The study of magnetic properties and relaxation processes in Co/Au bimetallic nanoparticles

• Co/Au fine nanoparticles prepared by reverse micelle as nanoreactor, controlling the size.
• Existence of superspin glass state confirmed from ac magnetic susceptibility study.
• Individual particles exhibit the collective behavior below glass temperature TSSG.
• Influence of diamagnetic shell on the magnetic properties of core–shell nanoparticles.

Co/Au bimetallic fine nanoparticles were prepared employing the method of microemulsion using reverse micelle as nanoreactor, controlling the particles size. Magnetic and structural properties of two different samples Co/Au1 and Co/Au2 with almost comparable size of Co core and different size of Au layer were studied. The investigation of magnetic relaxation processes present in the particles was carried out by means of ac and dc magnetization data obtained at different temperatures and magnitudes of magnetic field. We observed the existence of superspin glass state characterized by the strong inter-particle interactions in the nanoparticle systems. In this paper, we discuss the attributes of novel superspin glass magnetic state reflected on various features (saturated FC magnetization at low temperatures, shift of the Cole–Cole arc downwards) and calculated parameters (relaxation time, critical exponent zv ∼ 10 and frequency dependent criterion p < 0.05). Comparison of the magnetic properties of two studied samples show that the thickness of diamagnetic Au shell significantly influences the magnetic interactions and change the relaxation dynamics.

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