Magnetically induced anisotropy in Co rich Finemet type nanocrystalline alloys

Journal of Alloys and Compounds 483 (2009) 560–562 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Magnetically induced anisotropy in Co rich Finemet type nanocrystalline alloys Aleksandra Kolano-Burian a,∗ , Roman Kolano a , Lajos K. Varga b a b Institute of Non-Ferrous Metals, Sowińskiego 5, 44-101 Gliwice, Poland Research Institute for Solid State Physics and Optics, 1525 Budapest, P.O.B. 49 Hungary a r t i c l e i n f o Article history: Received 30 August 2007 Received in revised form 11 July 2008 Accepted 18 July 2008 Available online 17 December 2008 Keywords: Amorphous materials Nanostructured materials Quenching Magnetic measurements a b s t r a c t The relationship between magnetic field induced anisotropy and magnetic properties in Co-doped Finemet type nanocrystalline alloys was studied. Low remanence ratio (Flat) B–H curves can be obtained by the transverse field annealing. The kinetics of the induced anisotropy was studied conducting the isotherm heat treatment at different annealing temperatures and treatment time. The activation energy of about 3–3.2 eV has been obtained, which is similar to the activation energy of an amorphous crystalline transformation in the Co-doped Finemet samples. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In many applications flat and linear magnetization curve is required, which can be obtained by gaped ferrite or better, by the transverse induced anisotropy of integer toroidal samples. However, the flatness measured as an inverse of the effective initial permeability (eff ) cannot be diminished by magnetic field annealing to below eff ∼20 000 for the ordinary Finemet composition. This is why, following an example of the Co based and zero lambda amorphous alloys (e.g. VITROVAC 6050 and 6030), the attempts are sometimes made to induce higher transverse anisotropy in Co-doped Finemet, where Fe is replaced with Co. The advantage of the Co-doped Finemet compared to the amorphous Co-based alloys consists in better thermal stability of the magnetic properties and in relatively lower Co concentration. Since the first heat treatment of the amorphous alloys in a magnetic field [1] flattens the hysteresis loop by the magnetic field annealing it is commonly used for producing toroidal samples from zero lambda amorphous alloys with an effective permeability (eff ) around 1500 and from ordinary Finemet with eff between 20 000 and 90 000. Yoshizawa has demonstrated recently [2] that the Co-doped Finemet containing 9 at.% Si is highly susceptible to flattening the magnetization curve by the transverse magnetic field annealing so that an induced anisotropy can be as high as 1800 J/m3 for the Fe8.8 Co70 Cu0.6 Nb2.6 Si9 B9 composition, which has almost as high Co/(Fe + Co) ratio (∼0.9) as that for the zero lambda amorphous composition VITROVAC 6050. Moreover, the flattening down to eff ∼ 200 was obtained at the expense of coercivity increase to about 30 A/m. This is why we have performed a systematic study of the Co-doped “ordinary” Finemet Fe73.5−x Cox Si13.5 B9 Nb3 Cu1 , which proved to be applicable at higher temperatures than the Co free Finemet [3,4]. In this work, we present results from a detailed study of the kinetics of the transverse anisotropy induced under isothermal conditions at different temperatures and annealing times. 2. Experimental The Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 and Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 amorphous ribbons were fabricated by melt-spinning technique. Before casting, chemical composition of the master alloys was examined using X-ray microanalyser. Amorphousness of the ribbons was checked by means of the X-ray diffractometry. Samples in a form of toroidal cores were prepared from the amorphous precursor ribbon and annealed at the temperatures between 380 and 500 ◦ C for different periods of time ranging from 1 to 320 min in a protective atmosphere of argon, in an external magnetic field HT of 500 kA/m oriented along the toroid axis. After the thermo-magnetic treatment, the AC (50 Hz) magnetic properties of the cores were measured using a computerized hysteresis loop tracer (REMACOMP C-100). The induced anisotropies Ku , were estimated from the measured anisotropy field HK , and magnetization Js data. The anisotropy field was evaluated from the remanence branch of the hysteresis loop using the singular point method in a version proposed by Barandiaran et al. [5]. 3. Results and discussion ∗ Corresponding author. Tel.: +4832 2380 251; fax: +4832 2316 933. E-mail address: olak@imn.gliwice.pl (A. Kolano-Burian). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.154 In order to obtain flat hysteresis loops characterized by the effective permeability eff even below 10,000 it was necessary A. Kolano-Burian et al. / Journal of Alloys and Compounds 483 (2009) 560–562 561 Table 1 An effect of the annealing temperature and time on the induced magnetic anisotropy Ku (A – Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 , B – Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 ). t (min) 1 10 20 40 80 160 240 320 Ta = 500 ◦ C Ta = 480 ◦ C Ta = 460 ◦ C Ta = 440 ◦ C Ta = 420 ◦ C Ta = 400 ◦ C Ta = 390 ◦ C Ta = 380 ◦ C A B A B A B A B A B A B A B A B 75 167 202 243 312 364 361 745 1044 1152 1147 1204 1271 1242 75 76 76 138 186 210 642 902 967 998 1060 1084 70 73 75 90 103 118 188 185 638 852 891 922 953 967 976 958 13 73 75 72 74 78 594 540 599 651 639 721 13 73 71 72 71 71 65 71 594 553 599 620 664 710 768 820 9 11 11 69 72 73 594 556 594 594 594 572 10 10 11 11 11 12 588 545 594 594 594 588 11 11 11 11 11 12 568 545 581 583 584 570 to conduct the annealing process in a presence of the transverse magnetic field. Hysteresis loops for both composition (Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 and Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 ) annealed in a transverse magnetic field of 500 kA/m were increasingly flat with the increase of the annealing temperature and time. From the hysteresis loops obtained for the samples heat-treated in a presence of the magnetic field, distributions of a transverse anisotropy field were determined as a second derivative of the returning branch from the saturation down to the remanent magnetization multiplied by (−H) [5]: p(HK ) = −H ∂2 M ∂2 H The induced magnetic anisotropy constant Ku was determined as Ku = Js HK /2. An effect of the annealing temperature and time on the induced magnetic anisotropy Ku is shown in Table 1. The value of Js was 0.86 and 1.08 for the Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 and Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 alloys, respectively. Figs. 1 and 2 show Ku as a function of t for different annealing temperatures. A strong increase in Ku was observed from the temperatures of 420 ◦ C and 460 ◦ C for the Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 and Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 compositions, respectively. The highest value of Ku was obtained for the temperature of 500 ◦ C after 160 min annealing. With further increase of the annealing time an increase in the induced annealing anisotropy was not observed. For the 9 at.% Si containing Co-doped Finemet, the value of Ku was about five times higher than in the case of 13 at.% Si containing Co-doped Finemet. The transverse induced magnetic anisotropy allows to reduce permeability to 780 and 340 for 13 at.% and 9 at.% Si containing Co doped Finemet, respectively. Figs. 3 and 4 show the Fig. 1. The dependence of Ku on t for Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 alloys annealed at the temperatures between 380 and 500 ◦ C in a transverse magnetic field HT of 500 kA/m. Fig. 2. The dependence of Ku on t for Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 alloys annealed at the temperatures between 380 and 500 ◦ C in a transverse magnetic field HT of 500 kA/m. dependence of ln(t − to ) on 1/Ta . The Kronmüller’s relaxation model of the induced anisotropy was used to calculate activation energy Q. Although our activation energy determination is affected by significant error (∼±0.5 eV), it can be assumed that for both compositions we have obtained the same activation energies, Q ∼ 3–3.2 eV, which is similar to the value obtained for the amorphous-crystalline transformation of the studied Co-doped Finemet alloys. It is worth mentioning that for the Co-free Finemet composition the activation Fig. 3. The dependence of ln(t − t0 ) on 1/Ta for the Fe14.7 Co58.8 Cu1 Nb3 Si13.5 B9 alloys taking ln(t − t0 ) necessary to induce the same Ku ∼ 175 J/m3 at different Ta . 562 A. Kolano-Burian et al. / Journal of Alloys and Compounds 483 (2009) 560–562 4. Conclusions The experimental values collected in Table 1 make it possible to select proper values of the annealing temperature and time for the required induced anisotropy (i.e. planned flatness of the magnetization curve). Starting from the amorphous state, the activation energy determined through the variation with time of the induced anisotropy (Ta being the parameter) proved to be in good agreement with the value obtained by the Kissinger plot of DSC measurements. Acknowledgements This study was partially financed by the Ministry of Education and Science as a Targeted Research Project, over the period 2005–2008. L.K. Varga was supported by the Hungarian OTKA grant K 62466. References Fig. 4. The dependence of ln(t − t0 ) on the 1/Ta for the Fe13.8 Co65 Cu0.6 Nb2.6 Si9 B9 alloys taking ln t necessary to induce the same Ku ∼ 900 J/m3 at different Ta . energy of devitrification is about ∼4.1 eV [6], i.e. a little bit higher than that for the Co-doped alloys. It turns out that we have actually measured the activation energy of crystallization through the induced anisotropy. The activation energy for the development of an induced anisotropy should be measured on previously nanocrystallized samples as it was done by Emura et al. [7]. They have found that an activation energy for the induced anisotropy was 2.9 eV in the case of Co-free nanocrystalline Finemet and 1.9 eV in the case of amorphous Finemet. [1] H.S. Chen, S.D. Ferris, E.M. Gyorgy, H.J. Leamy, R.C. Sherwood, Appl. Phys. Lett. 26 (1975) 402. [2] Y. Yoshizawa, S. Fujii, D.H. Ping, M. Ohnuma, K. Hono, Scr. Mater. 48 (2003) 863–868. [3] R. Kolano, A. Kolano-Burian, J. Szynowski, L. Varga, F. Mazaleyrat, T. Kulik, N. Wojcik, L. Winczura, L. Kubica, Mater. Sci. Eng. A 375 (377) (2004) 1072. [4] A. Kolano-Burian, T. Kulik, G. Vlasak, J. Ferenc, L.K. Varga, J. Magn. Magn. Mater. 272 (276) (2004) 1447. [5] J. Barandiaran, M. Vazquez, A. Hernando, J. Gonzalez, G. Rivero, IEEE Trans. Magn. 25 (1989) 3330. [6] L.K. Varga, E. Bakos, L.F. Kiss, I. Bakonyi, Mater. Sci Eng. A 179–180 (1994) 567. [7] M. Emura, A.M. Severino, A.D. Santos, F.P. Missell, IEEE Trans. Magn. 30 (1994) 4785–4787.