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Improved soft magnetic properties by laser de-vitrificationof Fe–Si–B amorphous magnetic alloys

Casey Smith a, Shravana Katakamb, Soumya Nag a, Xi Chen c, Raju V. Ramanujan c,Narendra B. Dahotre b, Rajarshi Banerjee a,n

a Center for Advanced Research and Technology, Department of Materials Science and Engineering, University of North Texas, Denton, TX 76201, USAb Laboratory of Laser Material and Processing and Synthesis, Department of Materials Science and Engineering, University of North Texas, Denton,TX 76201, USAc School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore, Singapore

a r t i c l e i n f o

Article history:Received 20 January 2014Accepted 1 February 2014Available online 11 February 2014

Keywords:CrystallizationLaser processingSoft magnet

a b s t r a c t

Devitrification of Fe–Si–B melt-spun ribbons by laser annealing, instead of typical furnace-annealing, ledto substantially refined α-Fe(Si) nanocrystals nucleating within the amorphous matrix with a largenumber density of nucleation sites. This microstructural refinement due to laser devitrification leads tosubstantially improved soft magnetic properties with a drastic, 25 times, reduction in the coercivity,while maintaining the saturation magnetization values comparable to the furnace-annealed condition,making laser-annealing a very attractive devitrification route for these soft magnetic alloys.

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1. Introduction

Fe–Si–B based amorphous alloys have been studied for manyyears due to their excellent soft magnetic properties. It is wellknown from these studies that a superior soft magnetic materialcan be attained by nucleating a nanocrystalline phase within theamorphous matrix [1–8]. Typically, partial devitrification of amor-phous alloys is achieved through furnace annealing at varioustemperatures and times after melt spinning. Such heat treatmentscan lead to soft magnetic property degradation due to coarsemicrostructures. One reason for this degradation is the depen-dence of coercivity on the grain size. Based on the experimentalmeasurements of coercivity for a large number of differentamorphous magnetic alloys, Herzer concluded that coercivity islowest for grain sizes which are either more than 100 μm or lessthan 40 nm [7,8]. The previously mentioned studies have foundthat a fine dense distribution of nanocrystals can be achieved in acomposition of Fe73.5Si13.5B9Nb3Cu1. This metallic glass, oftenreferred to as FINEMET, has a partially de-vitrified microstructureconsisting of α-Fe(Si) nanocrystals between 10 and 15 nm in sizeembedded in an amorphous matrix. It is well known that theaddition of Cu to the Fe–Si–B alloy results in clustering of thisalloying element, with the Cu-rich clusters acting as potentnucleation sites for the α-Fe(Si) nanocrystals, while Nb restrictscoarsening of these nanocrystals [9–15]. Thus these alloying

additions result in a much finer microstructure with much lowercoercivity but somewhat lower saturation magnetization com-pared to ternary Fe–Si–B based compositions [16]. This can beattributed in part to the lower concentration of Fe in FINEMET.

The bulk of the research done so far has relied on furnaceannealing for devitrification. However, recent studies on laser-induced nanocrystallization has revealed that after irradiationwith a continuous wave laser beam with a Gaussian profile, thereis a transition in microstructure across the laser track, from partialdevitrification in the center of the track to complete devitrificationat the edge of the track [17,18]. The behavior and nature of thislaser-induced nanocrystallization has been previously modeledand rationalized [17,18]. Furthermore, the laser devitrificationprocess is site-specific, opening up many possibilities of designingpatterned crystallized regions that could lead to a wide array ofcomposite functional applications, an aspect not been addressed inprevious studies. The focus of the present study is on laser-induced devitrification of amorphous Fe–Si–B alloys and its impacton the soft magnetic properties. A direct comparison of micro-structure and magnetic properties of laser devitrified versusfurnace annealed amorphous alloys has been made.

2. Experimental procedure

Most of this study was conducted on melt-spun ribbon sam-ples, with a nominal composition of Fe77.5Si13.5B9 and an averagethickness of 20 mm, prepared in a laboratory scale melt-spinning

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Materials Letters

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n Corresponding author. Tel.: þ1 940 891 6812.E-mail address: Rajarshi.Banerjee@unt.edu (R. Banerjee).

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setup. Heat treatments were conducted in a tube furnace at 823 Kfor 60 min under an argon atmosphere. Laser devitrification wascarried out by scanning parallel tracks with a continuous waveytterbium-doped Nd-YAG laser beam at a wavelength of 1.064 mm,a speed of 500 mm/s, and a laser power density of 0.33 J/mm2. Thelateral spacing between the centers of neighboring laser trackswas maintained at 0.6 mm. Since the estimated laser track widthwas also 0.6 mm, the entire ribbon surface was laser treated. Thinfilm transmission electron microscopy (TEM) samples and needleshaped atom probe tomography (APT) samples were preparedusing an FEI Nova 200 focused ion beam system (FIB/SEM). TEMcharacterization was done on an FEI Tecnai F20 TEM with a fieldemission source. APT was conducted on an Imago LEAP 3000XHR through laser ionization with a pulse fraction of 0.2, pulse rateof 160 kHz, temperature of 30 K, and a pressure of 1�10�10 Pa.Magnetization measurements were performed by means of avibrating sample magnetometer (VSM-Lakeshore 7404) using amaximum magnetic field of 1 T.

3. Results and discussion

The x-ray diffraction patterns of both furnace annealed (Fig. 1(a))and laser devitrified (Fig. 1(b)) ribbons are nearly identical exhibitingthe (0 1 1), (0 0 2), and (1 1 2) peaks corresponding to the α-Fe(Si)phase. Transmission electron micrographs (TEM) from furnaceannealed (823 K/60 min) samples (Fig. 2(a)) exhibit the formationof a dendritic microstructure within the amorphous matrix. Thesedendrites were identified as α-Fe(Si) from the electron diffractionpattern and are in the size range of 50–200 nm. Fig. 2(b) reveals

a drastically different microstructure after laser devitrification ofanother sample from the same amorphous Fe–Si–B ribbon.

The α-Fe(Si) crystals in the laser devitrified ribbon are found tobe 31710 nm. Fig. 2(c) and (d), are bright-field and dark-field pairof TEM images showing the highly refined nanocrystal sizes in thelaser devitrified sample.

Atom probe tomography reconstructions of the furnace annealedribbon (Fig. 3(a)) shows sections of α-Fe(Si) crystals (red regions) inthe same size range as revealed by the TEM micrographs, sur-rounded by the B-rich amorphous regions (blue regions). The twophases in this three-dimensional reconstruction have been deli-neated using an isoconcentration surface (or isosurface in short)corresponding to 6 at% B. Fig. 3(b) shows the averaged compositionalprofiles for Fe, Si, and B, measured across the same isosurfaces, usingthe proximity histogram analysis. The partitioning of Si into the α-Fe(Si) crystals and B into the amorphous matrix occurs over a region of�2 nm across the α-Fe(Si)/amorphous interface. When the ribbon islaser devitrified instead of furnace annealed, the reconstructions(Fig. 3(c)) show a much more refined microstructure, in agreementwith the TEM results. Comparing the compositional profiles for Fe, Si,and B after laser devitrification (Fig. 3(d)) with furnace annealing(Fig. 3(b)), shows Si and B partitioning in the former over a region of�3.0 nm, somewhat more diffuse in the laser devitrified case ascompared to the furnace annealed case.

Vibrating sample magnetometer results for the furnaceannealed and laser devitrified ribbons are shown in Fig. 4. Thesaturation magnetization for both samples is quite similar (differ-ence of 16.73 emu/g) while there is a drastic difference in thecoercivity values (difference of 70.55 G). These measurementsclearly reveal that the difference in the saturation magnetization(Ms) values for laser devitrified and furnace annealed Fe–Si–Balloys with the same nominal composition is within the range of5–10%, while the coercivity (Hc) of the laser devitrified alloy is �1to 3% that of the furnace annealed alloy. The reasons for thisdrastic reduction in coercivity by laser devitrification have beendiscussed below.

According to the Herzer model, the grain size has a stronginfluence on coercivity [7,8], and when grain sizes are less than40 nm, the coercivity drastically drops. This is due to ferromagneticexchange interactions allowing the randomly aligned moments tocancel out. Therefore, in systems with crystal sizes below the ferro-magnetic exchange length, a group of randomly aligned momentsaverage out and the overall contribution of unaligned moments to themagneto-crystalline anisotropy is consequently drastically reduced.Since magneto-crystalline anisotropy is the primary factor determin-ing coercivity, the reduction in magneto-crystalline anisotropy due toreduced grain size leads to low coercivity values. The typical exchangelength for Fe-20 at% Si crystals, calculated from the exchange stiffnessand the magneto-crystalline anisotropy factor was determined to be�35 nm [7]. Since the α-Fe(Si) crystals in both the furnace annealedand laser devitrified ribbons contain �19 at% Si, based on the atomprobe studies, the ferromagnetic exchange length in the presentexperimental case should be very similar to the predicted value for20 at% Si. In case of the large dendrites formed from furnace annealing,the α-Fe(Si) crystals are much larger than the exchange length,resulting in a substantial value of magneto-crystalline anisotropy.However, after laser devitrification, the substantially refined α-Fe(Si)nanocrystals are able to benefit from the cancellation of ferromagneticexchange interactions, and therefore the coercivity is quite drasticallyreduced, as seen in the VSM results (Fig. 4).

The magneto-elastic anisotropies should also be minimizedto enhance soft magnetic properties, which ideally require extre-mely low values of saturation magnetostriction. The two primaryfactors for reducing magnetostriction are high crystalline volumefraction and Si content greater than 16 at% in α-Fe(Si). Magnetos-triction of the composite can be evaluated using a standard rule of

Fig. 1. XRD results from laser devitrification and furnace annealed Fe77.5Si13.5B9ribbons. Identical Fe3Si peaks are found in both cases.

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Fig. 2. TEM micrographs of Fe77.5Si13.5B9 after being (a) furnace annealed and ((b)–(d)) laser devitrified. Reduction in nanocrystal size is very apparent.

Fig. 3. APT reconstructions and compositional charts of Fe77.5Si13.5B9 after ((a) and (b)) furnace annealing and ((c) and (d)) laser devitrification. The black, red and blue linesrefer to Fe, Si and B concentrations respectively ((b) and (d)). The nanocrystal (red) size difference between the two cases is once again apparent. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)

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mixtures equation,

λs � ðxcrÞðλFeSis Þþð1�xcrÞðλams Þ: ð1ÞThe volume fraction of the crystalline phase is xcr while the

magnetostrictions of the α-Fe(Si) grains and the amorphous matrixare λsFeSi and λsam, respectively. The amorphous matrix magnetostric-tion is well known to be highly positive while the α-Fe(Si) magne-tostriction is ideally negative for Si contents above 16 at% [19]. Basedon this equation, to reduce the positive amorphous term andincrease the negative crystalline term, the crystalline volume fractionshould be increased. Based on the TEM observations (Fig. 2), itappears that the volume fraction of the crystalline α-Fe(Si) phase ishigher in case of the laser devitrified sample compared to furnace-annealed sample. This can be possibly attributed to substantiallyhigher number density of nucleation sites in case of laser devitrifica-tion. As previously mentioned, α-Fe(Si) crystals containing �16 at%of Si lead to a negative magnetostriction term. The APT compositioncharts in Fig. 2(b) and (d) show that the α-Fe(Si) crystals after bothfurnace annealing and laser devitrification have Si concentrationsaround 19 at%. So both methods of devitrification lead to negativevalues of magnetostriction arising from the α-Fe(Si) crystals.

4. Conclusion

Laser devitrification of amorphous Fe–Si–B alloys resulted in sub-stantially higher number density of nucleation sites for the α-Fe(Si)nanocrystals compared to furnace annealing. Consequently the averagesize of nanocrystals in the laser devitrified condition was 31710 nm,substantially smaller than 50–200 nm dendritic crystals formed byfurnace annealing. This highly refined microstructure obtained by laserdevitrification, resulted in a dramatic reduction in the coercivity as largeas two orders of magnitude. The results presented in this paper clearly

indicate that the same microstructural refinement and consequentreduction in coercivity can be achieved in ternary Fe–Si–B alloyswithout Cu and Nb alloying additions via laser devitrification.

Acknowledgements

The authors acknowledge Center for Research and Technology atthe University of North Texas for microstructure characterization andauthors SK and NBD for financial support from NSF (CMMI 0969249).

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Fig. 4. VSM results show a large coercivity change between furnace annealing and laser devitrification. Saturation magnetization is relatively similar for both cases.

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