Laser Processing of Soft Magnetic Metal Amorphous Nanocomposites

Abstract

Crystallization from an amorphous precursor has been historically exploited to synthesize metastable soft magnetic nanocomposite systems comprised of transition metal rich ferromagnetic nanocrystals embedded within an intergranular amorphous phase [1,2]. Subsequent thermal processing steps are designed to produce a highly refined microstructure with grain sizes in the nm-range to enable effective exchange averaging and highly reversible magnetization processes as described by the random anisotropy model [3,4]. Successful realization of superior soft magnetic properties results from ubiquitous nucleation and self-limiting growth of nanocrystals due to expulsion of glass formers to the intergranular amorphous matrix, thereby requiring significant amounts of metalloids and early transition metals which reduce the effective saturation magnetization.Recent work has discovered the balance between nucleation and growth can alternatively be optimized through highly controlled thermal processing steps involving extremely rapid heating and subsequent cooling rates on the order of 102-103K/s referred to as “flash annealing”, such that a high density of nucleation sites can be achieved with limited nanocrystal growth even for alloy chemistries which do not produce nm-scale microstructures for conventional annealing [5]. High heating and cooling rates generated during laser radiation have offered great advantages for rapid thermal processing and surface treatment of ferromagnetic amorphous ribbons [6–8].In this work, we report on pulsed-laser processing of a series of Fe-based and Co-based amorphous ribbons with emphasis being placed on phase transformations within the laser-irradiated regions as well as impacts on surrounding regions due to the detailed thermal effects associated with the laser pulse.Amorphous ribbons with nominal compositions Fe73.5Si13.5Nb3B9Cu1 and Co77.2Fe1.4Mn1.4Nb4B14Si2 were used for laser experiments. Laser annealing has been carried out using a Q-smart pulsed Nd-YAG (wavelength of 1064 nm) and laser beam diameter around 0.6 mm. The laser power was varied from 50-150 mJ and the time delay associated with each laser power has been calculated using the FWHM of the laser output pulse. Microstructure and crystalline phases after laser annealing were characterized using both XRD and SEM. Temperature profiles achieved during laser annealing have been estimated using FEA heat transient analysis.Fig. 1 shows XRD patterns of as-cast and laser annealed Fe- and Co-based ribbons at different laser power. As can be seen, as-cast and samples annealed at 60 mJ are fully amorphous, whereas samples treated at higher laser power (i.e., at 100mJ and above) present a mixture of boride and α-Fe phases with an average grain size of the latter being on the order of 8-10 nm estimated from the FWHM of the corresponding crystalline peak. In conventional annealing methods, the formation of boride phases are typically observed only after reaching secondary crystallization temperatures in these alloy systems. Because the borides are already observed in laser annealed samples with significant volume fraction for the lowest pulse power levels at which crystalline peaks can be detected, a need exists to revisit the crystallization mechanism in light of the detailed thermal histories experienced across and surrounding the irradiated region and the Gaussian distribution of the laser beam across the irradiated surface. Laser energy density decreases exponentially in the radial direction and has a maximum at the center of the laser spot leading to greater temperature values at the center and a temperature gradient from the center towards the edges. In addition, laser beam intensity also produces a stress distribution which can be associated with its Gaussian profile resulting in an inhomogeneous stress across the beam diameter, tensile at the center and compressive at the edges. Such temporally and spatially dependent temperature and stress profiles can alter free volume, nucleation rate, and diffusion kinetics leading to rapid crystallization events and alternative pathways towards the crystalline phase.Temperature as a function of time of a simulated laser-irradiated spot at the top surface and below the top surface of an amorphous ribbon is shown in Fig. 2. The temporal temperature profile suggests temperatures above 2000K at the top surface which is beyond the melting point and most likely results in a local melting at the top surface layer with crystallization possible upon cooling. Temperatures estimated approximately 2 microns below the surface (within the skin penetration depth of Fe at the optical wavelength of Nd-YAG) are on the order of 1000K and likely not to reach melting temperatures, consistent with the surface deformation observed as illustrated in the SEM images of Fig. 2c. These temperatures significantly decrease within the distance from the center towards the edge of the laser spots suggesting diffusion of heat from the centers outwards and therefore different crystallization kinetics are expected. **