Scrutinising magnetic disorder through metastable 3d- and 4f-nanostructured alloys

Mechanical milling of magnetic alloys 3d-(Fe) and 4f-based (Tb) results in the promotion of magnetic disorder. In Fe22Cu14Ag64 the nanoscopic arrangement has been revealed by a very comprehensive multipattern Rietveld analysis using a combination of four sets of data including X-ray and neutron diffraction. The nanostructure is formed by Fe61Cu39 particles of D = 4.6 nm with a collinear ferromagnetic structure in a metallic matrix constituted by D = 11.9 nm Ag nanoparticles. The creation of an ensemble of nanoparticles is favoured by the immiscible character of the starting metals. The nanostructured alloy presents a magnetic transition at around 160 K which cannot be considered as a pure single-domain blocking but affected by interparticle dipolar interactions. The latter is confirmed by the spin dynamics, displaying a critical slowing down of the AC-susceptibility and a definite peak of the non-linear susceptibility. When the matrix is alternatively formed by Au nanoparticles in the Fe14Au86 (D = 77 nm) and Fe10Cu10Au80 (D = 35 nm) alloys, the magnetic response resembles that of a reentrant state as the milling time is not enough to reduce the particle size, triggering ferromagnetic interparticle coupling enhanced by a multidomain magnetic structure. In milled 4f-alloys formed by miscible Tb and Al, as TbAl2, the production process promotes the magnetic disorder modifying the pure ferromagnetic order of the bulk alloy and simultaneously the existence of particles (D = 20 nm) which are affected by the structural disorder.

► Mechanical milling has been employed to produce 3d- and 4f-alloys in large quantities.
► In FeCuAg, we show a multipattern Rietveld. It is formed by Fe(Cu) nanoparticles in a Ag matrix as a super spin glass.
► The exchange of Ag by Au results in larger particles and the behaviour resembles that of a reentrant spin glass state.
► 150 h milled TbAl2 shows a disordered Curie transition and a freezing associated to the nanostructure.