IMPACT OF PHOSPHORUS FOR BORON SUBSTITUTION ON MAGNETIC PROPERTIES OF MAGNETOSTRICTIVE FINEMETS

Reduction of Si percentage in Fe-Nb-Cu-B-Si alloys known as Finemets results in 1.5 T saturation, surpassing so the standard Sirich compositions. However the other soft-magnetic properties are worse due mainly to magnetostriction and the consequences of socalled macroscopic heterogeneity. Therefore phosphorus has been added to the detriment of boron to test, whether this could be the way to suppress the undesired properties of Si-poor Finemets. Phosphorus appears to reduce the vulnerability of the ribbon surfaces to environmental influences at non-vacuum annealing and improves parameters like coercivity and magnetic anisotropy. Crystallization kinetics character remains preserved and critical temperatures change only negligibly.


INTRODUCTION
Soft-magnetic materials produced by rapid quenching from the melt known as Finemets (basic composition Fe-Nb-Cu-B-Si) are already of interest for a lot of researchers and electrotechnical developers because of their widely used magnetic properties [1].Magnetic properties of specific compositions greatly profit from thermal treatment resulting in partial crystallization when nanoscale crystalline grains are embedded in the amorphous rest of the alloy.Apart from the "classic" Finemet composition containing 13.5-15.5at% of Si, new alternatives are sought to obtain higher saturation induction (B s ) without much sacrifice of increasing coercivity and magnetostriction.Whereas the earlier development line exchanged metals and their percentage (Co for Fe), the other way is to try also exchanging metalloids and/or glass-formers.One of the latest achievements of the later way is B s as high as 1.8 -1.9 T shown by a composition with P addition and reduced Si percentage [2].So far less attention has been paid to the consequences of so-called macroscopic heterogeneity (MH), which builds up preferentially during the thermal treatment of magnetostrictive Si-poor compositions [3] devoted for nanocrystallization.The MH goes along with mutual stress between surfaces and ribbon interior and shows significant effect on the resulting magnetic anisotropy -characteristic hard-ribbon-axis (HRA) contribution appears [4].The above-mentioned quest for the improvement of saturation without significant collateral detriments to other properties, MH inclusive, led us to the choice of a Si-poor Finemet with partial substitution of B by P to study how P affects the properties of higher-B s Finemets.Our work on the "parent" Si-poor material (without P) points to the presence of surface oxides [5] capable of supporting MH and so the question how can phosphorus affect the MH is opened by the P for B substitution too.

MATERIALS AND METHODS
Planar-flow casting on air was used to prepare the ribbons of 10 mm width and 18 -26 µm thickness following in the text, labels to identify the composition are used as follows: Fe 78 Nb 3 Cu 1 B 13.5 Si 4.5 -FM(Si4.5),Fe 74 Nb 3 Cu 1 B 13 Si 9 -FM(Si9), Fe 78 Nb 3 Cu 1 P 3 B 10 Si 5 -FM-P(Si5), Fe 74 Nb 3 Cu 1 P 3 B 10 Si 9 -FM-P(Si9).Amorphous ribbons were submitted to differential scanning calorimetry (DSC) in Perkin-Elmer DSC7 under Ar atmosphere to determine the temperature of crystallization onset T x , temperature of first crystallization T p and Curie temperature T C .Thermo-gravimetric analysis (TGA) at applied magnetic field was performed to check more details of T C .The nanocrystalline state and ensuing magnetic properties were obtained by annealing at 500 and 540°C for 1 hour in vacuum and in Ar (without preceding furnace evacuation).We measured the hysteresis loops of 10 cm long strips at 21 Hz sine-H excitation in Helmholtz coils.Raman spectroscopy investigation of strip surfaces used the confocal system with 632.8 nm monochromatic laser radiation.Capacitive sensor measured the magnetostrictive strain to facilitate the determination of coefficients of saturation magnetostriction λ s .

Structure and transformation
Very similar transformation process has been observed for P-containing as well as for P-free ribbons by DSC measurement at 10 K/min and 40 K/min rate.As it is seen from Fig. 1, all the curves show the typical regions: structural relaxation, Curie temperature, structural rearrangement at 350-450°C [6] and the first crystallization peaks, which are merely few degrees lower for the P-containing alloys when compared to the P-free alloys.Nanocrystallization process is governed by graingrowth kinetics, which is often observed in Finemet-type alloys [7].Si-poor Finemets crystallize to the bcc-Fe(Si) phase moreover, the alloy with 9 at.% Si provides additionally the DO 3 superstructure too [3].The comparison of critical temperatures and Curie temperatures measured by DSC method at the two rates is shown in Table 1.Curie temperatures of FM-P alloys determined from DSC are lower by about 2°C when compared to TGA measurement.We can hypothesize, that the applied field affected the structural relaxation at TGA.The TGA results are seen in Fig. 2. Considerable increase of T C is noted for FM-P(Si9) alloy as compared to FM(Si9) (see Table 1).

Magnetic properties
Hysteresis loops at 21 Hz for FM-P(Si5) and FM(Si4.5)strips annealed 500°C and 540°C in vacuum during 1 hour.The loop shape is very similar but significant decrease of coercivity (from 16.8 to 6.7 A/m and from 11.3 to 4.8 A/m) was found for both the annealing temperatures if FM and FM-P are compared, respectively.The comparison of loops for all alloy compositions after annealing at 500°C is shown in Fig. 3.For alloy FM-P(Si9) is the effect of P for B substitution different as to the impact on magnetic properties.Hysteresis loops after vacuum annealing show modest increase of coercivity (~ 3 A/m) but higher saturation induction (B s = 1.36 T) than FM(Si9).Fig. 2 TGA measurements of the indicated Finemets with P at 10 K/min rate Annealing in Ar is the treatment that supports MH buildup more than vacuum annealing.Generally, the characteristic HRA anisotropy contribution is seen on a loop as its tilt.The lower temperature (500°C) Ar annealing results in tilted loops as seen in Fig. 5.The tilt increases after higher annealing temperature 540°C (not shown).Still, FM-P(Si5) exhibits less-slant loops than FM(Si4.5)for both the annealing temperatures.We assume that this is due to a reduced impact of MH in Pcontaining alloys.Actually, FM-P(Si9) does not show the characteristic HRA tilt after 500°C Ar anneal, somewhat rounder knees are only visible (see Fig. 4).However the 540°C annealing lets the phosphorus to reduce MH effects in FM-P(Si9) no more -the coercivity increases significantly and the tilt appears.The reason is to be sought possibly in a diverse crystallization -see Fig. 1.The details are unavailable so far.

Raman spectroscopy
Neither side (air-or wheel-) of vacuum-annealed strips show a well-resolved Raman spectrum.Only for sporadic dark spots graphitic double peak at 1300 and 1600 cm -1 was recorded [5].However, on the strips annealed at Ar atmosphere we observed well-expressed peaks on both sides (more intensive on air side) after 500°C and 540°C anneal of FM-P(Si5), and after 540°C anneal of FM-P(Si9).The characteristic Raman spectra (see Fig. 5) correspond with bands of oxides Fe 2 O 3 and Fe 3 O 4 as it was previously studied on FM(Si4.5)ribbon [5].Fig. 5 Raman spectra of Finemet containing 5 at.% Si with substituted P (air side) after different-temperature annealing as is indicated above.The applied beam was ~4 mW power

Magnetostriction
All the Si-poor Fe-based Finemets show considerable magnetostriction -e.g.FM(Si4.5)λ s ~ 25 ppm and FM(Si9) λ s ~ 19 ppm in the as-cast state.These "isotropic" numeric values for the coefficient of saturation magnetostriction are gained by the standard evaluation based on the formula: where the symbol meaning external magnetizing field oriented parallel (||) and transverse (⊥ = 90°) to the ribbon axis.Apart from some uncertainty at judging of saturation to obtain λ s , the orthogonal components plotted together provide the helpful information about magnetic anisotropy, which is not always available from hysteresis loops.In particular, as-cast FM-P(Si5) was observed to saturate poorly and anisotropically.Whereas FM-P(Si5) shows some reduction of its λ s = 21.7 ppm if compared with its FM counterpart, FM-P(Si9) with λ s = 19.5 ppm does the other way.Thus -no significant difference of ascast magnetostriction is due to the phosphorus presence in the alloys.The magnetostriction of the P-free Finemets decreases markedly (more than by 50%) due to annealing.Similar behaviour is expected for the FM-Ps although FM-P(Si9) shows surprisingly weakly tilted loop -see Fig. 4. Since magnetoelastic anisotropy is proportional to the product of stress and magnetostriction coefficient, it remains to be disclosed, whether this response to annealing is due to lower MH-coupled stress or to lower magnetostriction.

CONCLUSIONS
Substitution of phosphorus for boron has produced marked effects on the magnetic properties of Si-poor Finemets.No impact of the substitution on structural transformation to nanocrystalline state was identified.
Significant decrease of coercivity and better soft magnetic properties were observed for alloy with 5 at.% Si, especially after vacuum annealing.Higher saturation magnetic induction B s (1.36 T) and increased T C was found for the higher-Si alloy.The effects of macroscopic heterogeneity after Ar annealing appear significantly reduced by phosphorus, especially after 500°C annealing.Although showing considerable magnetostriction and not free of macroscopic heterogeneity, the substitution of phosphorus for boron brings better soft-magnetic properties useful in the production of electrotechnical devices.

Fig. 1
Fig. 1 DSC measurements at 40 K/min of Finemets with and without P

Fig. 3 Fig. 4
Fig. 3 Comparison of hysteresis loops of FM-P and FM alloys annealed at 500°C for 1 hour in vacuum

Table 1
Critical temperatures calculated form DSC measurements Beata Butvinová maiden name de Ronzyová, born 1955 in Nitra (Slovakia), graduated from the Faculty of Natural Sciences, J.A. Comenius University Bratislava, from the branch of Experimental Physics in 1980.Then, she joined the Institute of Physics, Slovak Academy of Sciences Bratislava.She received her RNDr.degree from Comenius University of Bratislava and CSc.= PhD degree in 1981 and 1987, respectively.Her current interests are in soft-magnetic properties research of materials prepared by rapid quenching of the melt and operates several experimental methods.From 1998 she works in IP SAS Bratislava as senior researcher.Pavol Butvin born 1948 in Bratislava studied physics, specialized in solid state physics and graduated from Faculty of Natural Sciences, J.A. Comenius University Bratislava in 1971.He joined the Institute of Physics, Slovak Academy of Sciences (SAS) Bratislava the same year and he is still active there as department deputy head and principal investigator of various projects.In 1980, he acquired his RNDr and CSc (=PhD) degrees from the University and SAS, respectively.He was a member of the group awarded by the State Prize in 1988.From 1973 on, his interests are in magnetism of metals as well as in applied research of novel magnetic materials.In the recent years, he specializes in less-common properties (heterogeneity) of modern soft-magnetic materials.Emília Illeková was born in 1949 in Prague.In 1972 she graduated with distinction the study of applied physics at the Faculty of Natural Sciences of the Comenius University in Bratislava.Also there she defended her RNDr. in 1975.She obtained the scientific degree CSc.(= PhD.) in the field of kinetics of phase transitions in the solid phase in 1981 at the Slovak Academy of Sciences.In 2006 she was graduated for Doctor of Physical and Mathematical Sciences (DrSc.) at the Comenius University, her doctoral thesis.Since 1972 she has been working in the Institute of Physics of SAS Bratislava in the Laboratory of Thermal Analysis, her actual position is the chief research scientist.Her scientific research is focusing at material research of glasses, especially metallic glassy ribbons, mechanically alloyed powders and bulk metallic glasses.She is specialized in kinetics of transformations in glasses and thermodynamics of noncrystalline and metastable solids.Peter Švec, Sr. was born in Bratislava, Czechoslovakia, in 1955, graduated from the Faculty of Electrical Engineering, Slovak University of Technology, Bratislava, specialization Solid State Physics, in 1979 and received his PhD degree in solid state physics in 1986 and DrSc. in 2004 from the Institute of Physics of the Slovak Academy of Sciences in Bratislava, where he is still active as department head and projects leader.His research interests are physics of metals, metastable disordered systems, especially those prepared by rapid quenching from the melt, phase transitions and quantitative structure analysis on atomic-scales.Gabriel Vlasák was born 1937 in Vinodol, Slovakia, graduated from J.A. Comenius University Bratislava (branch of Experimental Physics) and acquired his RNDr.degree from the same university in 1961.He gained his CSc.(=PhD) degree from Institute of Physics Slovak Academy of Sciences Bratislava in 1977.Since 1994 he works at IP SAS Bratislava as senior researcher.His interest is in mechanical and magnetoelastic properties of amorphous and nanocrystalline materials.Dušan Janičkovič graduated from Comenius University Bratislava and acquired his RNDr degree in 1981.Since 1984 he works as research-methods engineer in rapid solidification technology and amorphous metals production.He supervises the Laboratory of Rapid Solidification at Institute of Physics, SAS.Current filed of interest: development of new composite amorphous materials prepared by in-situ rapid solidification of the melt.Magdaléna Kadlečíková (PhD) born in Nitra in 1956, graduated in solid state physics in 1980 and received her PhD in electronics and vacuum technology, both from STU, in 1986.At present a senior scientist.Her research activities include nanotechnology and materials analysis.