Journal of Magnetism and Magnetic Materials 254–255 (2003) 486–488
Influence of early stages of nanocrystallization on the low- temperature magnetic properties of nanocrystalline ribbons
L.F. Kiss
a,*, J. Kov a ! c $
b, A. Lovas
caResearch Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary
bInstitute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04353 Ko! sice, Slovak Republic&
cDepartment of Vehicles Manufacturing and Repair, Budapest University of Technology and Economics, H-1111 Bertalan L. u. 2., Budapest, Hungary
Abstract
A correlation was found between the change of the saturation magnetization (Ms) and spin wave stiffness constant (Dsp) as a function of annealing temperature for FINEMET-type glassy precursors (Fe73.5Nb3Si13.5B9Cu1), annealed in the temperature range 400–5401C for times up to 1 h. During irreversible structural relaxation both Ms and Dsp increase. During the evolution of the nanocrystalsMsdecreases whileDspincreases.
r2002 Elsevier Science B.V. All rights reserved.
Keywords: Metallic glasses; Nanocrystalline materials; Finemet; Saturation magnetization; Spin waves-constant
Nanocrystalline alloys including FINEMET-type (Fe73.5Nb3Si13.5B9Cu1) and NANOPERM-type (FeZrB- Cu) ribbons proved to be excellent soft magnetic materials [1]. They are two-phase alloys composed of BCC nanograins ofB10 nm embedded in an amorphous matrix, produced by partial crystallization from amor- phous precursors. A lot of investigations have been performed on these alloys in the final nanocrystallization state and the excellent softness was interpreted by averaging out the magnetocrystalline anisotropy of the nanocrystals via exchange coupling [2]. Relatively few works have been published on the evolution of the magnetic properties in the early stages of nanocrystalli- zation [3–10]. In order to understand better the physical processes taking place during nanocrystallization, in this paper the parallel evolution of the saturation magnetiza- tion and spin wave stiffness constant was studied for a FINEMET-type alloy after different heat treatments below and around the nanocrystallization temperature.
The FINEMET-type amorphous ribbon (Fe73.5Nb3- Si13.5B9Cu1) was prepared by melt-spinnig and subse-
quently annealed in the temperature range 400–5401C for times up to 1 h in protective Ar atmosphere. The saturation magnetization (Ms) and spin wave stiffness constant (Dsp) of the samples obtained were determined from the fit of the temperature dependence of magne- tization (M) by Bloch’s law [11]. The temperature dependence ofM was measured by a vibrating sample magnetometer in the temperature range 4.2–300 K.
Figs. 1 and 2 show the saturation magnetization and spin wave stiffness constant as a function of the annealing temperature (Ta) for samples heat treated for 1 h, respectively. Both quantities increase up to Ta¼4201C: According to previous investigations irre- versible structural relaxation of the amorphous pre- cursor takes place at this stage of annealing [6]. The increase in Ms due to structural relaxation has been detected in Fe-based glasses [12], which was attributed to changing degree of chemical disorder. This phenom- enon has also been observed recently in the same FINEMET-type alloy [6]. It was interpreted on the basis of a model [13] which describes the irreversible structural relaxation in amorphous materials in terms of annihilation of compressed (p-type defects) and stretched (n-type defects) regions (with respect to an average density). During relaxation the latter process
*Corresponding author. Tel.: +36-1-392-2222/1296; fax:
+36-1-392-2215.
E-mail address:kiss1@power.szfki.kfki.hu (L.F. Kiss).
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PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 8 6 2 - 4
dominates, resulting in a slightly more compact struc- ture. Theoretical calculations for FCC and amorphous iron [14,15] show that below a threshold Fe atomic distance the Fe atomic moments moderately decrease, whereas they increase rapidly above it. Therefore, during annihilation the increase of the Fe atomic moments in the p-type defects caused by the increasing Fe atomic volumes can overweigh the decrease of the moments in the n-type defects where the Fe atomic volumes decrease. The increase ofMsdue to relaxation preceding nanocrystallization was not observed in previous works [3–5] because this process is already finished at their lowest annealing temperature (Ta¼4501C for 1 h).
The increase of the spin wave stiffness constant during irreversible structural relaxation can be easily explained in accordance with the literature [4,5]. In as-quenched amorphous state the fluctuations of the exchange
integral lead to a reduced Dsp: In the course of the relaxation the high fluctuations of the atomic distances are reduced, causing the observed increase ofDsp(up to Ta¼4201C).
The decrease of Ms above Ta¼4401C can be attributed to the beginning of nucleation and growth of the nanocrystals [6]. During this at least three processes take place, eachcontributing to the decrease ofMs:First, the continuous Si diffusion into the BCC nanograins leads to the enrichment of Si in the Fe(Si) nanocrystals, loweringMs:Second, it is well known that the Fe3Si phase forms an ordered DO3 phase in the FINEMET-type alloys [16]. Since the Fe atoms in the ordered phase have lower magnetic moments than in the disordered one [17], the ordering of the Fe(Si) phase of the nanograins reduces Ms either. Third, as nano- crystallization proceeds, the influence of Nb enrichment in the amorphous matrix on reducing Ms cannot be excluded either.
At Ta¼4401C the spin wave stiffness constant suddenly drops. However, the significance of this feature is not yet clear since it is determined only by one experimental point. As nanocrystallization proceeds, the ordering of the initially disordered Fe(Si) phase can lead to the increase ofDsp;reducing the fluctuations of the exchange integral. This increase of Dsp is further enhanced by the reduction of the volume fraction of the amorphous phase and by the growth of the Fe(Si) grains, in line withthe literature [3–5].
The isothermal evolution of the saturation magnetiza- tion and spin wave stiffness constant atTa¼5301C vs.
annealing time (ta) is shown in Figs. 3 and 4, respec- tively. The monotonous decrease ofMsand increase of Dsp as a function of ta clearly reflects the process of nucleation and growthof the nanocrystals described above. The influence of the irreversible structural Fig. 1. Saturation magnetization as a function of annealing
temperature for FINEMET-type alloy annealed for 1 h.
Fig. 2. Spin wave stiffness constant as a function of annealing temperature for FINEMET-type alloy annealed for 1 h.
Fig. 3. Saturation magnetization as a function of annealing time for FINEMET-type alloy annealed at 5301C.
L.F. Kiss et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 486–488 487
relaxation prior to nanocrystallization cannot be ob- served at this high annealing temperature since relaxa- tion takes place in a very short time interval.
This work has been supported by the Hungarian Scientific ResearchFund (OTKA) throughGrant T 030753 and by the Slovak Scientific Research Fund (VEGA) through Grant 2/1149/21.
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Fig. 4. Spin wave stiffness constant as a function of annealing time for FINEMET-type alloy annealed at 5301C.
L.F. Kiss et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 486–488 488
