Physica B 327 (2003) 241–243 Influence of P substitution for B on the structure and properties of nanocrystalline Fe73.5Si15.5Nb3Cu1B7ÀxPx alloys Nguyen Chaua,*, Nguyen Hoang Luonga, Nguyen Xuan Chiena, Phung Quoc Thanhb, Le Van Vub b a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract Amorphous ribbons of Fe73.5Si15.5Nb3Cu1B7ÀxPx (x ¼ 0; 1; 2; 3; and 4) have been prepared by rapid cooling on a single copper wheel The crystallization of a-Fe(Si) phase is independent of the P content in the alloys Based on Kissinger plots, the activation crystallization energies are determined The size of the nanoparticles crystallized on an amorphous matrix in heat-treated ribbons is found to be 10–12 nm The crystallization fraction is determined by using thermal-analysis equipment and we show that after 30 annealing, this fraction is over 80% The thermomagnetic curves measured between room temperature and 1000 K revealed clearly two magnetic phases: an amorphous phase at low temperatures and a crystalline one at high temperatures r 2002 Elsevier Science B.V All rights reserved Keywords: Crystallization kinetics; Grain size; Soft ferromagnetism; Nanocrystalline materials Great interest has been paid to nanocrystalline ferromagnets (FINEMET) since their invention by Yoshizawa and co-workers in 1988 [1] This is an attractive subject for scientists in both applied and fundamental research With two coexisting ordered magnetic phases, almost vanishing effective magnetostriction and very low magnetocrystalline anisotropy occur in the nanocrystalline state After an appropriate heat treatment on the as-cast amorphous ribbon, nanosize grains are embedded in the remaining amorphous matrix Owing to the grain size smaller than the ferromagnetic exchange lengths, the local magnetocrystalline anisotropy is averaged out over several grains, which reduces the effective anisotropy significantly [2] It was *Corresponding author Tel./fax: +84-4-858-9496 E-mail address: chau@cms.edu.vn (N Chau) shown that Cu and Nb play an important role to produce the nanocrystalline structure Moreover, Nb is ascribed to hinder the grain growth, which is one of the decisive factors to achieve the excellent soft magnetic properties In our previous work [3], we have studied the influence of Co on the structure and the properties of the alloys based on FINEMET Fe73.5ÀxCoxSi13.5B9Nb3Cu1 This work is performed to investigate the influence of P substituted for B on structure and properties of Vitroperm [4] The soft-magnetic ribbons Fe73.5Si15.5Nb3Cu1B7ÀxPx (x ¼ 0; 1; 2; and 4) have been prepared by rapid cooling on a single copper wheel The ribbons are 20 mm thick and 10 mm wide The structure of the as-cast ribbons and the annealed ones was examined by X-ray diffraction (D 5005, Brucker) The thermal analysis was 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N Chau et al / Physica B 327 (2003) 241–243 performed on a differential scanning calorimeter (SDT 2960-TA Instruments) The ribbons were annealed in vacuum Thermomagnetic curves of the ribbons were measured by a vibrating-sample magnetometer (VSM-DMS 880, Digital Measurement Systems) Hysteresis loop parameters were determined by automatic magnetic hysteresis graph (AMH-401A, Walker) Fig shows the X-ray diffraction patterns of as-cast Fe73.5Si15.5Nb3Cu1B7ÀxPx ribbons One can see that these patterns exhibit only one broad peak around 2y ¼ 451; showing that the ribbons of all these compositions are amorphous DSC measurements on as-cast amorphous ribbons were performed with increasing temperature at a rate of 201C/min in Ar atmosphere Fig presents the DSC curves of our ribbons One can see clearly that the first exothermic peak TP1 (corresponding to crystallization of a-Fe(Si) phase) does hardly depend on the P content substituted for B and occurs in the temperature interval 550–5571C With increasing P content in the ribbons, the second exothermic peak at 7261C for the sample with x ¼ (considered to correspond to the crystallization of Fe–B phase) is split into two peaks and shifted to lower temperatures with increasing x: This may suggest that if the ribbons contain P, the crystallization of the Fe2,3B and Fe2,3P occurs more easily Based on Kissinger plots we have evaluated the crystallization activation energy E1 of the a-Fe(Si) phase (corresponding to TP1 ) to be about 3.1–3.5 eV The value for E1 weakly depends on P content Calculations performed for the second and third peak showed that the crystallization activation energy decreases with increasing x: Fig X-ray diffraction patterns of as-cast Fe73.5Si15.5Nb3Cu1B7ÀxPx ribbons 557 o C 622 o C 550 o C 643 o C 550 o C 651 o C Heat Flow (a.u) 242 551 o C 55 1o C x =3 690 o C 726 o C 200 300 400 600 500 T (°C) x =4 700 x =2 x =1 x=0 800 900 Fig DSC curves of Fe73.5Si15.5Nb3Cu1B7ÀxPx ribbons For the first time Leu and Chin [5] have pointed out the use of the DTA apparatus for estimating the crystallization fraction wf : DHa À DHt wf ẳ ; 1ị DHa where DHa and DHt are the crystallization enthalpy of the as-cast alloy and of the alloy annealed for a time t; respectively Fig shows the DSC curves for Vitroperm ribbons (x ¼ 0) both as-cast and annealed in vacuum at 5401C for 30 Using expression (1) we derived a crystallization fraction of the aFe(Si) phase at peak TP1 to be wf =82% Apparently the crystallization fraction determines the magnetostriction of the ribbon while the grain size after annealing determines the magnitude of the effective magnetic anisotropy Both these factors play a decisive role in the softmagnetic properties of nanocomposite magnetic materials All compositions have been annealed at 5401C for 30 and subsequently examined by X-ray diffraction measurements From X-ray analysis, we can determine the average grain size of a-Fe(Si) formed after annealing according to the Scherrer expression [6] The size of nanograins crystallized on an amorphous matrix shows to be 10–12 nm and is not dependent on x: The crystallization kinetics of the ribbons can be observed by measurements of thermomagnetic curve Fig shows the MðTÞ curves of ribbon N Chau et al / Physica B 327 (2003) 241–243 Table Magnetic parameters of as-cast Fe73.5Si15.5Nb3Cu1B7ÀxPx ribbons 55 1.8 o C as-cast ribbon Heat Flow (a.u) 243 72 5.6 o C Composition x¼0 x¼1 x¼2 x¼3 x¼4 mi mm HC (Oe) Bm (G) 1000 16,000 0.12 6800 9700 13,000 0.15 6100 2500 15,000 0.196 4800 6400 10,000 0.32 7100 1700 12,000 0.235 3300 59 8.1 o C after annealing 72 6.7 o C Table Magnetic parameters of annealed Fe73.5Si15.5Nb3Cu1B7ÀxPx ribbons 200 300 400 500 600 700 800 900 T ( o C) Fig DSC curves for Fe73.5Si15.5Nb3Cu1B7 ribbons (as-cast and annealed at 5401C for 30 min) Composition x¼0 x¼1 x¼2 x¼3 x¼4 mi mm HC (Oe) Bm (G) 56,000 70,000 0.02 10,100 41,000 53,000 0.04 9300 39,000 42,000 0.038 7800 33,000 37,000 0.035 6700 40,000 43.000 0.08 6900 40 35 M (emu/g) 30 25 20 (1) (2) 15 10 100 200 300 400 500 600 700 T (oC) Fig Thermomagnetic curves of Fe73.5Si15.5Nb3Cu1B7 ribbon 1: heating cycle, 2: cooling cycle with x ¼ measured in magnetic field of 100 Oe The other samples display similar MðTÞ behavior It can be seen from Fig again that the ribbon is amorphous at room temperature When the temperature increases, the magnetization abruptly is reduced marking the Tc of the amorphous phase With further increasing temperature, the magnetization is small and constant over a large temperature interval up to a region where crystallization starts the crystallites of a-Fe(Si) lead to an increase of the magnetization On returning from high temperature a large amount of a-Fe(Si) grains are crystallized in the sample and this leads to a fast increase of the magnetization below Tc of aFe(Si) (curve of Fig 4) From measurements performed for the other compositions we observed that the Curie temperature of the amorphous phase varies around 3007101C Tables and show the characteristics of hysteresis loops of as-cast and annealed ribbons From these tables we can see that the magnetic characteristics of the hysteresis loops were evidently improved after annealing and the materials became ultrasoft magnetic nanocomposites Acknowledgements The authors are grateful to the Vietnam National Program for Natural Sciences for financial support References [1] Y Yoshizawa, S Oguma, K Yamauchi, J Appl Phys 64 (1988) 6044 [2] G Herzer, J Magn Magn Mater 112 (1992) 258 [3] N Chau, P.Q Thanh, N.H Luong, N.H Nghi, Invited paper presented at the Sixth Asean Science and Technology Week, Brunei 9/2001, to be published [4] J Petzold, J Magn, Magn Mater 242–245 (2002) 84 [5] M.S Leu, T.S Chin, MRS Symp Proc 557 (1999) 557 [6] B.D Cullity, Element of X-ray Diffraction, 2nd Edition, 1978, Addison-Wesley, Reading, MA, p 102 ... diffraction patterns of as-cast Fe73.5Si15.5Nb3Cu 1B7 ÀxPx ribbons One can see that these patterns exhibit only one broad peak around 2y ¼ 451; showing that the ribbons of all these compositions are... crystallization activation energy E1 of the a-Fe(Si) phase (corresponding to TP1 ) to be about 3.1–3.5 eV The value for E1 weakly depends on P content Calculations performed for the second and third peak... increasing P content in the ribbons, the second exothermic peak at 7261C for the sample with x ¼ (considered to correspond to the crystallization of Fe B phase) is split into two peaks and shifted