Soft Magnetic Properties of Nanostructured Vitroperm ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010

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Soft Magnetic Properties of Nanostructured Vitroperm Alloy Powder Cores Ján Füzer1 , Peter Kollár1 , Jana Füzerová2 , and Stefan Roth3 ˇ Department of Condensed Matter Physics, Faculty of Science, Institute of Physics, Safárik University, Koˇsice 04154, Slovakia Faculty of Mechanical Engineering, Technical University, Koˇsice 04200, Slovakia IFW Dresden, Institut für Metallische Werkstoffe, Dresden D-01069, Germany Magnetic properties of amorphous and nanocrystalline samples have been experimentally investigated. Rapidly quenched ribbons of Fe73 Cu1 Nb3 Si16 B7 (Vitroperm 800) were ball-milled and cryomilled to powder and warm-consolidated (at a pressure of 700 MPa) to get bulk compacts. It was found by investigating the influence of mechanical milling on the magnetic properties of powder samples prepared by milling that the alloy remains amorphous during the whole milling process. The frequency dependence of the coercivity and total core losses were studied. Investigation of the dc coercivity, magnetostriction, and electrical resistivity were done. Annealing at higher temperature causes a deterioration of dc soft magnetic properties. The higher coercivity of the as-prepared samples is mainly due to interfaces of the powder elements and internal stresses created by milling and consolidation that were decreased greatly after annealing. The frequency dependence of magnetic properties is also illustrated, and it is attributed mainly to the domain wall damping. The absolute values of losses and coercivity of Fe-based compacts are similar to that for Co-based compacts. We have prepared bulk samples in the form of the small cylinders with coercivity down to 13 A/m. These materials have more degrees of freedom for tailoring their magnetic properties due to their flexibility in shape and dimensions. Index Terms—Eddy current losses, magnetic cores, soft magnetic materials, Vitroperm alloys.

I. INTRODUCTION ANOCRYSTALLINE soft magnetic materials are recent magnetic materials, which nowadays may supersede power ferrite and amorphous materials for high-frequency applications in electronics. In the last decade, a new class of bulk metallic glasses (BMG) with promising soft magnetic properties prepared by different casting techniques has been intensively investigated [1]–[3]. Among the recently developed metallic glass systems, Fe-based alloys have attracted considerable attention related to their good soft magnetic properties with near-to-zero magnetostriction, high saturation magnetization, low core loss, and high permeability, which render the material a potential candidate for a variety of applications such as magnetic recording heads and electronic sensors [4]–[6]. Amorphous Vitroperm (Fe Cu Nb Si B ) alloy will transform into nanocrystalline material with optimum magnetic performances when it is annealed appropriately [7]. However, relatively less research has been conducted concerning the bulk Vitroperm samples [8]. One of the ways to prepare bulk material is compaction of powder produced by milling of amorphous or nanocrystalline ribbons [9], [10]. Ball milling technique has been successfully used to prepare many alloys in powder form, which are therefore suitable for compaction into a variety of shapes [11], [12]. In this paper, we report magnetic properties on the production and compaction of amorphous powder from Fe Cu Nb Si B amorphous ribbons.

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II. EXPERIMENT The ribbons Fe Cu Nb Si B (Vitroperm 800, provided by Vacuumschmelze Hanau) were milled or cryomilled using Manuscript received June 20, 2009; revised September 14, 2009; accepted September 21, 2009. Current version published January 20, 2010. Corresponding author: J. Füzer (e-mail: [email protected]). Digital Object Identifier 10.1109/TMAG.2009.2033337

a RETSCH PM4000 planetary ball mill. The milling was performed under Ar atmosphere with speed of 180 rpm at a ball-topowder mass ratio of 6:1 in hardened steel vials. Handling of the powder was done in a glove box with controlled atmosppm, H O ppm). The samples were conphere (O solidated at 700 MPa for 5 min at 500 C into cylinders with diameter of 10 mm and thickness of 3 mm. The dc coercivity of compacts was measured by a Förster Koerzimat. The saturation magnetostriction of the as-compacted bulk samples was measured by means of the strain gauge method. The thermal stability of the samples was analyzed by differential scanning calorimetry (DSC) using a NETZSCH DSC 404 C calorimeter at 20 K/min heating rate under flowing argon. The density of the consolidated bulk samples was measured using the Archimedes principle with bromoform as the immersion fluid. The electrical resistivity was measured by the Van der Pauw method. An axial hole with diameter of 5 mm was drilled into the disc, which produced a ring sample, and we have prepared coils with number of primary turns 20 and number of secondary turns 20 for ac (AMH 401 POD WALKER) measurement. A magnetic induction, B is sinusoidal when measuring the frequency-dependent hysteresis cycle. Annealing was carried out in a tubular furnace under Ar atmosphere. III. RESULTS The data presented here are relative to different experimental procedures, one corresponding to a milling at room temperature (sample R1) and the other corresponding to a cryomilling at temperature of liquid nitrogen (sample L1): • L1 sample—amorphous ribbon cryomilled for 6 h, consolidated at 500 C for 5 min; • R1 sample—amorphous ribbon milled for 6 h, consolidated at 500 C for 5 min. The compacts were annealed at temperatures 500 C, 520 C, 540 C, and 560 C; time of annealing was 60 min; and heating rate was 10 K/min in argon.

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Fig. 1. DSC scan of amorphous Fe Cu Nb Si B ribbon, powder-milled at room temperature (RT powder) and cryomilled powder (LN powder).

Fig. 3. Saturation magnetostriction as a function of annealing temperature determined by strain-gauge measurements.

Fig. 2. Electrical resistivity as a function of annealing temperature determined by van der Pauw method.

Fig. 4. DC coercivity as a function of annealing temperature determined for bulk samples.

The XRD analysis revealed that short-time (6 h) ball-milling of Fe Cu Nb Si B ribbons has no influence on its structure, and the powder remained amorphous [13]. Particle size of more than 95% of particles after milling at room temperature is from 50 to 300 m, but cryomilled powders have smaller particle sizes, from 20 to 150 m [14]. The resulting particle size distribution may affect the density of the compacted material. The compacted disk L1 has higher value of the density (6730 kg/m ) than disk R1 (6709 kg/m ). The DSC traces of the as-spun Fe Cu Nb Si B ribbon, RT powder, and LN powder (Fig. 1) show a sharp single exothermal crystallization C. The peak corresponds to the crystallization peak at of -Fe(Si) phase. To reduce the total losses, we have studied the electrical resistivity and the magnetostriction of all bulk samples obtained with different procedures. Fig. 2 shows the influence of the milling procedures and heat treatment on the resistivity. Thin air gaps are created between the particles during the compaction. These air gaps have a high electrical resistivity, leading to the increase of the electrical resistivity of the compact material more than twice, compared to the same alloy in the form of cm [14]). It is obvious that the samples the ribbon (about 115 that were annealed have lower values. Fig. 3 shows the influence of the milling procedures and heat treatment on saturation magnetostriction. Decreasing of the saturation magnetostriction is due to the transformation of amorphous into nanocrystalline

phase, and the resulting is affected by the magnetostriction of the forming nanocrystalline grains with different chemical composition. Near-to-zero magnetostriction in nanocrystalline Fe-base alloys requires a crystalline volume fraction with negative magnetostriction in order to compensate the positive value of the amorphous Fe-based matrix [7]. The dc and ac magnetic properties of the prepared bulk samples were studied. The changes in the dc coercivity with heat treatment were analyzed in both as-compacted and post-annealed disks. As can be seen from Fig. 4, the cryomilling of amorphous Fe Cu Nb Si B powder has negative influence on the value of the coercivity of the compacted samples. The , of the compacted samples decreases after ancoercivity, nealing reaching a minimum value 13.0 A/m for 520 C (sample R1) and 45 A/m for 500 C (sample L1), respectively. The value depends mostly on the surface and volume pinning of of magnetic domain walls. This contribution is proportional to the product of saturation magnetostriction and the amplitude of stress fluctuations [15]. Further annealing (540 C, 560 C) has no positive influence on the coercivity, and values are almost slight greater with increasing of the annealing temperature. The ac magnetic properties of the bulk samples were investigated to understand the effect of process parameters on the properties. The influence of annealing treatment on the hysteresis loops is given in Figs. 5 and 6. The shapes of the hysteresis loop,

FÜZER et al.: SOFT MAGNETIC PROPERTIES OF NANOSTRUCTURED VITROPERM ALLOY POWDER CORES

Fig. 5. Hysteresis loops of bulk cores (L1) in the as-prepared state and after : T. kHz, B annealing at various temperatures, f

= 10

=02

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Fig. 7. Frequency dependence of the core losses and the peak permeability for : T. The samples are after annealing sample L1 and sample R1 for B at 540 C.

=02

The eddy current losses as a contribution of the total losses are inversely proportional to the electrical resistivity of the samples. can be according to [4] expressed as The eddy current losses (1) where is effective dimension, (for the material prepared by casting, is thickness of the sample, for laminated material is maximum flux density, frethickness of the sheet), specific resistivity, density of the material, and quency, geometrical coefficient. For rectangular cross section of the ring, perpendicular to the direction of magnetic flux, is Fig. 6. Hysteresis loops of bulk cores (R1) in the as-prepared state and after annealing at various temperatures, f : T. kHz, B

= 10

=02

the rectangularity, as well the anisotropy are different with the initial powder material. The profile of the mechanical stress induced during milling and pressing also influences the hysteresis loops. Annealing at 500 C decreases the total power losses in both alloys by approximately a factor 2. During compaction, internal stresses are generated in the material. The hysteresis loss is partly due to stresses introduced in the material at compaction, which can impede domain wall movement. In practice, it is energetically favorable for domain walls to pass through certain imperfections such as stressed regions. Therefore, in order to reduce hysteresis in the Fe-based bulk samples, a stress-relieving heat treatment most often follows the compaction. Fig. 7 shows as a function of frethe core losses and peak permeability quency for L1 and R1 samples for flux density T. The frequency dependence of the peak permeability for R1 and L1 bulk samples decreases about 10 times on the continuous increase of the frequency from 300 Hz to 50 kHz. The variation of the magnetic peak permeability as a function of frequency is less pronounced for the sample L1, indicating this sample as more suitable candidates for frequency applications. The behavior of the core losses as a function of frequency for both samples is similar, regarding the nature of the electrical resistivity of the samples. The electrical specific resistivity ranges cm for sample L1 to 256 cm for sample R1. from 273

(2) where is width and height of the rectangle [4]. mm and For our prepared ring L1 with mm, the calculated value for is 10.8366. Separation of losses is complicated by the fact that classical eddy current losses cannot be calculated in a reliable way. Compacted materials are not homogeneous, and eddy currents are not restricted to the single sheets but may cover the full thickness or even the package of particles. If we take the largest , we obtain the value of of 2.48 W/g value of , i.e., kHz, T), which is higher value (sample L1, in comparison with experimentally measured total losses, of 0.41 W/g. It is important to remark that the values obtained for the coercivity and core losses are comparable and even lower than those reported for other bulk soft magnetic materials such as Ni-Fe or Ni-Fe-Mo permalloys [16] and Fe-Nb-B-Cu alloys [17]. IV. CONCLUSION We successfully synthesized bulk metallic Fe-based samples with different experimental procedures by consolidating prepared powders. Post-annealing of bulk samples enhances the soft magnetic properties by reducing coercivity, magnetostriction, and total losses. We have prepared bulk samples in the form of the small cylinders with dc coercivity down to 13 A/m.

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The magnetic properties of the prepared samples show dependence on their initial powder and annealing conditions. The relatively higher coercivity may be mainly due to the defects and internal stresses created by milling and consolidation. On the other hand, the porosity increases the electrical resistivity, which should be accompanied by a reducing of the core losses. The cryomilling of amorphous Fe Cu Nb Si B powder has positive influence on the ac magnetic properties of the compacted samples. These materials, prepared by consolidation, have more degrees of freedom for tailoring their applications due to their flexibility in shape and dimensions. Further improvement can be obtained by tailoring the magnetic properties’ changing composition and/or preparation procedure exploiting the wide possibility that consolidated bulk soft magnetic materials display. ACKNOWLEDGMENT The authors want to thank S. Kuszinski, H. Schulze, and M. Frey for technical assistance. This work was realized within the frame of the project “Centre of Excellence of Advanced Materials with Nano- and Submicron- Structure,” which is supported by the Operational Program “Research and Development” financed through the European Regional Development Fund. This work was also supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences, Project No. VEGA 1/4020/07. Special thanks to Mr. M. Vitovský of Vacuumschmelze GmbH & Co. KG Hanau, Germany, for providing of Vitroperm 800 samples. REFERENCES [1] J. Petzold, “Advantages of soft magnetic nanocrystalline materials for modern electronic applications,” J. Magn. Magn. Mater., vol. 242, pp. 242–245, 2002. [2] H. Chiriac and N. Lupu, “New FeNbB-based bulk amorphous and nanocomposite soft magnetic alloys,” IEEE Trans. Magn., vol. 41, no. 10, pp. 3289–3291, Oct. 2005.

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