Abstract
In nanoscale magnetic systems, the possible coexistence of structural disorder and competing magnetic interactions may determine the appearance of a glassy magnetic behavior, implying the onset of a low-temperature disordered collective state of frozen magnetic moments. This phenomenology is the object of an intense research activity, stimulated by a fundamental scientific interest and by the need to clarify how disordered magnetism effects may affect the performance of magnetic devices (e.g., sensors and data storage media). We report the results of a magnetic study that aims to broaden the basic knowledge of glassy magnetic systems and concerns the comparison between two samples, prepared by a polyol method. The first can be described as a nanogranular spinel Fe-oxide phase composed of ultrafine nanocrystallites (size of the order of 1 nm); in the second, the Fe-oxide phase incorporated non-magnetic Au nanoparticles (10–20 nm in size). In both samples, the Fe-oxide phase exhibits a glassy magnetic behavior and the nanocrystallite moments undergo a very similar freezing process. However, in the frozen regime, the Au/Fe-oxide composite sample is magnetically softer. This effect is explained by considering that the Au nanoparticles constitute physical constraints that limit the length of magnetic correlation between the frozen Fe-oxide moments.
Highlights
Magnetic systems classifiable as ‘disordered’ have the common property that the constituent magnetic moments undergo, at a critical temperature, a collective freezing along essentially random directions, giving rise to a low-temperature quasi-degenerate frozen state
AuMNP (b), for two different waiting times, tw = 30 s and tw = 10,800 s (i.e., 3 h). Both the samples investigated show a magnetic behavior typical of the disordered magnetism Both the samples investigated a magnetic behavior of curves the disordered magnetism phenomenology
We have studied and compared the magnetic behavior of the MNP and AuMNP samples
Summary
Magnetic systems classifiable as ‘disordered’ have the common property that the constituent magnetic moments undergo, at a critical temperature, a collective freezing along essentially random directions, giving rise to a low-temperature quasi-degenerate frozen state. This phenomenology requires basic ingredients, which are topological disorder, mixed and competing magnetic interactions, and random local anisotropy. In a canonical spin glass, this magnetic correlation length increases more and more on reducing temperature across the freezing one, and in principle, it reaches an infinite extension in the final, low-temperature frozen regime. It is worth mentioning that, depending on the specific system, the frozen regime can develop out of a ferromagnetic state of the spins (re-entrant behavior) [5,6,7,8]
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