Investigating the Size and Microstrain Influence in the Magnetic Order/Disorder State of GdCu2 Nanoparticles.

E M Jefremovas,D P Rojas,A García-Prieto,J Alonso,J I Espeso,M De La Fuente Rodríguez,M L Fernández-Gubieda,L Fernández Barquín,J Rodríguez Fernández

doi:10.3390/nano10061117

Abstract

A series of GdCu nanoparticles with controlled sizes ranging from 7 nm to 40 nm has been produced via high-energy inert-gas ball milling. Rietveld refinements on the X-ray diffraction measurements ensure that the bulk crystalline structure is retained within the nanoparticles, thanks to the employed low milling times ranging from t = 0.5 to t = 5 h. The analysis of the magnetic measurements shows a crossover from Superantiferromagnetism (SAF) to a Super Spin Glass state as the size decreases at NP size of 18 nm. The microstrain contribution, which is always kept below 1%, together with the increasing surface-to-core ratio of the magnetic moments, trigger the magnetic disorder. Additionally, an extra contribution to the magnetic disorder is revealed within the SAF state, as the oscillating RKKY indirect exchange achieves to couple with the aforementioned contribution that emerges from the size reduction. The combination of both sources of disorder leads to a maximised frustration for 25 nm sized NPs.

Highlights

Rare Earth (R) intermetallic alloys constitute excellent systems for studying the 4 f -orbital magnetism, the eventual effect of crystal field (CEF) and the indirect exchange RKKY interactions that develop within these systems [1]
This is associated with both the size reduction and the appearance of a microstrain η introduced by the milling process [27,28]
The analysis has revealed how the AF bulk state gets progressively destroyed with milling time, whereas a disordered magnetic contribution emerges due to both size reduction and increasing microstrain

Summary

Introduction

Rare Earth (R) intermetallic alloys constitute excellent systems for studying the 4 f -orbital magnetism, the eventual effect of crystal field (CEF) and the indirect exchange RKKY interactions that develop within these systems [1]. In the very recent years, magnetic nanoparticles (MNPs) containing Rare Earths in their composition have been proposed as promising candidates in applications mainly connected to magnetic separation and biomedicine [5,6] Among these MNPs, Gd-based ones are especially interesting for biomedical applications, e.g., as contrast agents for MRI imaging [7,8]. It is surprising that the outburst of magnetic Nanoscience at the turn of the century has not paid much attention to the magnetism at its basis in 4 f -based compounds, in general, and to the intermetallics in particular It has only been very recently, around a decade ago, that some works started to unveil the magnetic properties of R-intermetallics at the nanoscale in the form of collections of magnetic nanoparticles (NPs) [9,10,11]. These results underlined the importance of carefully defining the nanometric structure to understand whatever nanomaterial is analysed

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Conclusion