M. Kopcewicz and T. Kulik a ) Institute of Electronic Materials Technology, Warszawa, Wólczyńska Street 133, Poland, a ) Faculty of Materials Science and Engineering, Warsaw University of Technology, Warszawa, Wołoska Street 141, Poland, Rf-Mössbauer study of the magnetic properties of nanocrystalline FeNiZrB and FeNiCoZrB alloys

Introduction Nanocrystalline alloys are excellent soft magnetic materials with wide practical applications [1]. The main disadvantage: high brittleness. FeNiZrB alloy: show improved mechanical properties but at the expense of saturation magnetization of nanograins [2]. In order to increase the magnetization - Co is added. Aim of the current study: Investigation of magnetic properties of the nanocrystalline alloys: Fe 31 Ni 50 Zr 7 B 12 (Co-free alloy) and Fe 41 Ni 40-x Co x Zr 7 B 12 (x=10, 20) (Co 10 and Co 20 alloys) by means of the rf-Mössbauer technique. This technique permits us to study changes of magnetic anisotropy and to follow the variations of magnetostriction associated with the formation of nanocrystalline grains.

Experimental procedure Samples: amorphous alloys Fe 31 Ni 50 Zr 7 B 12 (Co-free alloy), Fe 41 Ni 30 Co 10 Zr 7 B 12 (Co 10 alloy), and Fe 41 Ni 20 Co 20 Zr 7 B 12 (Co 20 alloy), prepared by the melt quenching technique. The ribbons: 3 to 5 mm wide and about 25  m thick. The nanocrystalline alloys: formed by 1 hour annealing in vacuum of amorphous precursors (annealing temperatures: T a = 440 –770 o C). The rf-Mössbauer measurements: performed during sample exposure to a radio frequency (rf) magnetic field with a frequency of 61 MHz, and the intensity of which was varied between 0 and 20 Oe. The rf field was applied in the plane of the sample. The rf-Mössbauer investigations were accompanied by measurements of quasi- static hysteresis loops.

Rf-Mössbauer technique combines the Mössbauer effect with phenomena induced by an external radio-frequency magnetic field in ferromagnetic materials (see e.g., [3, 4, 5]). Effects induced by the rf field: 1. RF-SIDEBANDS EFFECT 2. RF-COLLAPSE EFFECT

Rf-Mössbauer technique 1. RF-SIDEBANDS EFFECT - frequency modulation of Mössbauer  -radiation due to rf- induced vibrations of Mössbauer atoms via magneto-acoustic coupling - magnetostriction Sidebands positions - given by  n  r intensities - frequency modulation (FM) model Rf-sidebands can be observed in ferromagnetic magnetostrictive materials (below the Curie point)

2. RF-COLLAPSE EFFECT - magnetic radio-frequency field H rf larger than the anisotropy field H A (H rf > H A ) - rf frequency  rf larger than Larmor frequency  L (  rf >  L )  fast reversal of magnetization  fast relaxation of the hyperfine field  hyperfine field is averaged to zero Magnetic hyperfine structure in Mössbauer spectrum disappears  the rf-collapsed spectrum: only QS doublet or a single line (the sample is in the ferromagnetic state). RF-collapse - very sensitive to the relation between H rf and H A - thus H A can be scanned by changing H rf RF-collapse can be observed in soft ferromagnetic materials Rf-collapse allows the separation of magnetic dipole and electric quadrupole hyperfine interactions in the ferromagnetic state

Rf-Mössbauer Results 1. Co-free alloy Conventional Mössbauer spectra : - as-quenched alloy- fully amorphous - nanocrystallization: at 470 – 670 o C The dominating phase - bcc-FeNi accompanied by the (FeNi) 23 B 6 phase. Rf-Mössbauer spectra - the as-quenched alloy and annealed at T a = 470 o C - rf-spectra - a collapsed doublet accompanied by rf sidebands. The alloy is magnetically very soft and magnetostrictive. - at T a  520 o C the collapsed pattern - single line - very soft bcc-FeNi phase; the rf sidebands vanished. - at T a  570 o C - rf-collapse incomplete. The Mössbauer spectra recorded before and after rf exposure are identical.

2. Co-containing alloys Co 10 alloy: As-quenched alloy: Addition of Co to the amorphous structure increases the anisotropy field. -The full collapse of mhfs occurs at higher intensity of the rf field (16 Oe). -The rf-sidebands of high intensity are observed. -A larger Co-content causes the instability of the amorphous phase during exposure to the rf field as a result of which the amorphous alloy crystallizes partly due to the rf field exposure. Co-containing nanocrystalline alloys: The anisotropy field strongly increased. -The rf-spectra of Co 10 alloy (T a  o C, rf field 20 Oe at 61 MHz) - a central doublet accompanied by partially narrowed mhfs. - No rf-sidebands. - At T a =620 o C: rf-spectra do not depend on the rf field intensity and show mhfs of FeCo phase (no rf-collapse, no rf-sidebands).

Co 20 alloy: As-quenched alloy: - rf collapsed pattern +intense rf sidebands Annealed alloy: - at T a  520 o C: noncollapsed mhfs (FeCo phase) (large anisotropy limits the rf collapse) + collapsed doublet (amorphous matrix) - at T a = 620 o C: rf-spectra do not depend on the rf field - mhfs of FeCo phase large anisotropy field The magnetostriction of the nanocrystalline alloy vanished; the rf sidebands are not observed.

The Mössbauer spectra recorded (without rf field) for the alloys (Co-free and Co-containing) before and after the rf field exposure: - are identical in the case of Co-free alloys, i.e., no rf induced crystallization; - are significantly different in the case of Co-containing alloys - rf- induced crystallization. The rf induced crystallization is attributed to mechanical deformations induced in the sample via magnetostriction (rf-sidebands effect).

Measurements of the quasi-static hysteresis loops [6] - the magnetic induction B (magnetization) and coercive fields H C were estimated. The hysteresis loops recorded for Fe 41 Ni 20 Co 20 Zr 7 B 12 alloy The coercivity in the Co- containing alloys strongly increased after annealing: Co 20 -alloy: The smallest coercivity, 15 A/m was observed at T a =520 o C, then increased to 783 A/m (T a =570 o C) and 4726 A/m (T a =620 o C). The largest value (13113 A/m) was reached at T a =670 o C. The changes in coercivity reflect the FeCo phase abundance in the alloys. The measurements of the hysteresis loops fully support the rf-Mössbauer results.

Conclusions The rf-Mössbauer results show that: The amorphous precursors are magnetically very soft. The anisotropy field increases with the introduction of Co - in the amorphous alloy - this effect is significant - in nanocrystalline alloys - this increase is dramatic. The rf-Mössbauer measurements are very useful for the estimation of magnetic anisotropy fields. The presence and the magnitude of the rf sidebands provide direct information regarding the magnetostriction of the alloys. The rf sidebands vanish when the magnetostriction becomes negligible due to formation of the nanocrystalline structure. A novel effect - the rf-induced crystallization, of nonthermal origin, of Co-containing alloys attributed to magnetostriction.

The rf field applied was able to induce the fast magnetization reversal (rf-collapse) only if the rf field intensity was markedly larger than the anisotropy field. The rf-collapse effect was suppressed when the anisotropy field became comparable to or larger than the rf field intensity. In the Co-containing alloys both coercive fields and anisotropy fields strongly increase at higher annealing temperatures.

References: 1. A. Makino, T. Hanai, A. Inoue, T. Masumoto, Mater. Sci. Eng. A 179/180, 552 (1994). 2. M. Kopcewicz, B. Idzikowski, J. Kalinowska, J. Appl. Phys. 94, 638 (2003). 3. M. Kopcewicz, Strutural Chem. 2, 313 (1991). 4. M. Kopcewicz, in G.J. Long, F. Grandjean (Eds) "Mössbauer Spectroscopy Applied to Inorganic Chemistry" vol. 3, Plenum, N. York, London, 1989, p M. Kopcewicz, J. Alloys and Comp. 382, 165 (2004). 6. T. Kulik, H.T. Savage, A. Hernando, J. Appl. Phys. 73, 6855 (1993). These results were presented at the Magnetism and Magnetic Materials Conference (MMM-2005), San Jose, CA, (USA) in November 2005 and were published in the Journal of Applied Physics 99, 08F112 (2006).