Magnetic nanoscopic correlations in the crossover between a superspin glass and a superferromagnet

Abstract

Collective behaviors in which the magnetic response depends not only on the individual constituents but also on their interactions are an area of active research. We have produced a paradigmatic system where DC magnetron sputtered FexAg100−x (x = 15, 35) nanogranular films exhibit a crossover between a superspin glass (SSG) state and superferromagnetism (SFM), where direct exchange interactions overcome the frustration. The systems have been studied by non-linear susceptibility (NLS) and small angle neutron scattering (SANS). The NLS measurements were carried out between 2 and 300 K, in the absence of a biasing magnetic field, with frequencies spanning two decades. These measurements shed light on the complex nature of the interactions and the intricate relationship between direct exchange and long range magnetic interactions. The use of SANS allows us to estimate qualitatively the lengthscale of the magnetic correlations, and therefore identify a clear difference between the collective "supermagnetic" states (i.e. SSG and SFM) while establishing links between the structure and the magnetic interactions.

Dipolar ferromagnetism in three-dimensional superlattices of nanoparticles

“…superlattices show an order/disorder transition from a superparamagnetic configuration to a true ferromagnetic state (i.e. not superferromagnetism 27,28 where exchange interactions amongst nanocrystallites dominate instead of the much weaker dipolar interactions) at a temperature that is consistent with theoretical expectations 14 .…”

Using small-angle scattering to guide functional magnetic nanoparticle design

“…This makes magnetic SANS an invaluable tool to study nanoscopic magnetic correlations in a large variety of MNP samples and other magnetically nanostructured systems. 192 3 Time-resolved in situ measurements Time-resolved studies with time resolution less than 100 ms are routinely possible on X-ray and neutron beamlines. This ranges from the observation of spontaneous nucleation and growth of particles, changes due to oxidation over time scales of several days, to the reorientation and switching behavior of the particle moment and the dynamic assembly of superstructures with a magnetic eld.…”

Unraveling Nanostructured Spin Textures in Bulk Magnets

“…[27,28] In this study, we introduce a new method to extract the underlying 2D correlation functions from 2D magnetic SANS patterns. This approach can be readily applied for the model-free analysis of diffuse magnetic SANS data in various research fields, including multiferroic alloys, [29] permanent magnets, [30] multilayer systems, [31] magnetic steels, [32] nanogranular magnetic films, [33] nanowire arrays, [34,35] ferrofluids, [36] and magnetic nanoparticles. [37,38] Furthermore, the numerical algorithm for the extraction of the 2D magnetic correlation function can be easily transferred to other experimental techniques where correlation functions are measured, such as spin-echo neutron scattering to study dynamics in the nanoelectron-volt energy range [39,40] and pair distribution function analysis in diffraction for information on atomic disorder, [41] and other complementary X-ray techniques, such as resonant soft X-ray magnetic scattering [42] and X-ray photon correlation spectroscopy.…”