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

Abstract

A series of GdCu 2 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 I m m a 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 ⟨ D ⟩ ≈ 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 ⟨ D ⟩ ≈ 25 nm sized NPs.

Exploring the Different Degrees of Magnetic Disorder in TbxR1−xCu2 Nanoparticle Alloys

“…The latter is revealed by the positive value of the paramagnetic Curie temperature θ P ≈ 20 K (see Table 2). The presence of a disordered magnetic phase already at the bulk state has also been shown in other Gd intermetallics, such as in pollycrystalline Gd 4 PtAl [36] or GdCu 2 [15], where the obtained θ P ≈ 20 K agrees well with the one obtained in our Tb 0.5 Gd 0.5 Cu 2 bulk. For the NPs, a clear freezing transition T f takes place (at around 20 K), leading to the formation of a Super Spin Glass state (SSG) [37].…”

“…For the NPs, a clear freezing transition T f takes place (at around 20 K), leading to the formation of a Super Spin Glass state (SSG) [37]. This evolution from a bulk AF state to a SSG for the NPs has already been shown in other systems, as in GdCu 2 [15] or in the 3d NiO compounds [38]. The SSG state gets more robust for smaller NP sizes [37,39], denoted by an increase for both magnetisation (1.5 times) and…”

“…Such a finding is concomitant with a progressive weakening of the FM interactions due to the increasing number of shell magnetic moments (disordered). Finally, the obtained µ e f f values do not display appreciable size-dependence, and lie slightly below the ones reported for parent TbCu 2 and GdCu 2 [15,16].…”

“…As it can be seen from Figure 3a, the ZFC and FC branches for the NPs display no trace of AF Néel transition, which is however present in the bulk state (see inset). The bulk T N = 47.2(1) K lies between those corresponding to bulk GdCu 2 (T N = 40.2(1) K [15]) and TbCu 2 (T N = 49.1(1) K [35]) alloys. The magnetisation value at this transition also lies between the parents' ones (almost 2.25 times larger than the one of GdCu 2 and 1/3 of the value of TbCu 2 ).…”

“…On the other hand, it has been showed that reducing the alloy size to the nanoscale affects the RKKY interactions among the magnetic moments, owing to both finite size effects [9][10][11] and the microstrain associated with grain boundaries [12][13][14]. These usually result in a combination of both frustration and random-site disorder that usually lead to the onset of a SG phase [15,16]. The question here that remains open is what happens to the disorder if one combines both worlds: dilution using 4 f dopant ions plus size reduction to the nanoscale.…”