Huge Inverse Magnetization Generated by Faraday Induction in Nano-Sized Au@Ni Core@Shell Nanoparticles

Abstract

We report on the design and observation of huge inverse magnetizations pointing in the direction opposite to the applied magnetic field, induced in nano-sized amorphous Ni shells deposited on crystalline Au nanoparticles by turning the applied magnetic field off. The magnitude of the induced inverse magnetization is very sensitive to the field reduction rate as well as to the thermal and field processes before turning the magnetic field off, and can be as high as 54% of the magnetization prior to cutting off the applied magnetic field. Memory effect of the induced inverse magnetization is clearly revealed in the relaxation measurements. The relaxation of the inverse magnetization can be described by an exponential decay profile, with a critical exponent that can be effectively tuned by the wait time right after reaching the designated temperature and before the applied magnetic field is turned off. The key to these effects is to have the induced eddy current running beneath the amorphous Ni shells through Faraday induction.

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“…For example, physical methods have been reported for the synthesis of AgNi disordered mNPs, which exhibited substantial enhancement in their optical limiting efficiency [433]. Core-shell AuNi NPs with inverted segregation (Au cores covered by Ni) have shown huge magnetisation, which was maintained even after the formation of NiO on the surface [434]. The hybrid AuCo systems are consequently also expected to offer unique properties in different chemical orderings because of the possible magnetoresistance effect and optical-magnetic bifunctionalities [435].…”

Engineering and Scaling the Spontaneous Magnetization Reversal of Faraday Induced Magnetic Relaxation in Nano-Sized Amorphous Ni Coated on Crystalline Au

“…The dynamical behaviors of the two components can be very different, but the latter is frequently screened, as it is much smaller in amplitude and mixed in direction with the former. In a previous report [ 23 ], we have documented that huge inverse Faraday magnetizations can be induced in nano-sized amorphous Ni deposited on crystalline Au. However, there was no conclusive evidence found for the coexistence of magnetizations from slow spin dynamics and from inverse Faraday induction.…”

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“…48 Core-shell AuNi NPs with inverted segregation (Au cores covered by Ni) showed huge magnetisation, which was maintained even after the formation of NiO on the surface. 49 Furthermore, intermixed nanoalloys of magnetic 3d TM with Pt are potential materials to overcome the superparamagnetic limit due to their exceptional values of magnetic anisotropy, which can thus fulfil demanding requirements for high density information storage. Compared to the intermixed state of CoPt NPs, surface-segregated cuboidal counterparts have shown their magnetic moment reduced by 0.52 m B with a 19% reduction in magnetic anisotropy and, more pronouncedly, when adopting cuboctahedral morphology, reductions were of the order of 4.96 m B and 45%, respectively.…”