Microstructure and magnetic properties of field-annealed [Fe.sub.40.5][Co.sub.40.5][Nb.sub.7][B.sub.12] nanocrystalline alloy.

Krenicky, Tibor ; Fabian, Stanislav

Abstract: In this paper, we report on a beneficial effect of the external

magnetic field applied during the heat treatment on the soft magnetic character of the nanocrystalline Cu-free HITPERM-type alloy with nominal composition of [Fe.sub.40.5][Co.sub.40.5][Nb.sub.7][B.sub.12]. This material exhibits after the amorphous/crystalline transformation under applied field of 20 k[Am.sup.-1] the value of coercive field less than 10 [Am.sup.-1]. Moreover, good soft magnetic properties are reported to remain fairly stable also at elevated temperatures.

The influence of the presence of two different (Fe, Co) magnetic atom species in HITPERM as a benefit for a strong response of this material to the magnetic field annealing in the terms of atomic-pairs ordering theory is discussed.

Key words: soft magnetic materials, magnetic annealing, nanocrystalline materials

1. INTRODUCTION

Magnetic parameters of some of the nanocrystalline materials lie on or broaden the present limits and that is why such materials are called extremal. For example, the nanocrystalline alloy [Fe.sub.84][Nb.sub.7][B.sub.9] (NANOPERM) is magnetically soft material characterized by the value of coercitive force in the range of units [Am.sup.-1]. Change of one minor chemical component in the alloy causes dramatic jump in the alloy character--towards the opposite edge of the spectrum of feromagnetic materials as coercitive field of [Fe.sub.84][Nd.sub.7][B.sub.9] is about 6 times larger.

In the nanocrystalline Fe-Co-M-B(-Cu) alloys (M = Zr, Nb, Hf, ...), called also HITPERM, the Curie temperature of amorphous phase exhibits a substantial increase due to presence of Co and hence, the [alpha]'-FeCo (B2) nanograins remain magnetically well coupled up to high temperatures. The practical driving force behind the recent interest in HITPERM is demand of technological processes allowing us to tailor properties of the magnetically soft materials, which offer large magnetic induction and at the same time are capable of operation at high temperatures (Kulik et al., 2002). In order to further optimize the magnetic behaviour of HITPERM, it is important to better understand the influence of various processing techniques that can be used to change the magnetic properties of these alloys for specific applications, e.g. as transformers, sensors etc. One possible way, which could be employed for this purpose, is the annealing of the material under the presence of external magnetic field which determines the easy axis of magnetization in annealed samples (Chikazumi, 1964).

In many soft magnetic materials including various amorphous and crystalline alloys, the phenomenon of field annealing induced anisotropy is often explained in terms of magnetic atoms pair ordering mechanism. Generally, the various types of magnetization curves (with large or small squareness ratio) can be obtained after annealing in longitudinal or transverse magnetic fields (O'Handley, 1999).

According to the model for binary alloys (Neel, 1952), the directional diffusion takes place with a preferred direction of magnetic atom pairs imposed by the direction of magnetization during the annealing and/or subsequent cooling. Some conditions should be fulfilled when magnetic properties of an annealed alloy are to be changed by magnetic annealing. For the first, materials consistig of one type of magnetic atoms should not respond to such annealing as no preferred orientation of atomic pairs has to be achieved. But, HITPERM contains also non-magnetic atoms, often acting as impurities which can change the resulting magnitude of the effect. Furthermore, the value of the external magnetic field must be sufficient to saturate the material and has to be applied below of the Curie temperature of the material. On the other side, the value of the temperature should be high enough to allow diffusion of the atoms and also question of annealing time is very important as the mechanism of the magnetic annealing works in the way of the atomic re-arrangement, so the time must be long enough. Then magnetic annealing changes intrinsic energy in the sample and leaves directional anisotropy as a preferred orientation of magnetization, favouring one direction, the easy axis of magnetization.

In spite of a number of recent investigations on the HITPERM alloys, there is lack of experimental studies dealing with the evolution of the soft magnetic properties towards the high temperatures. In this context, we focuse a special attention on the elevated temperature magnetic behaviour of the nanocrystalline material and in particular on the stability of its properties.

2. EXPERIMENTAL

Master alloy of the nominal composition of [Fe.sub.40.5][Co.sub.40.5][Nb.sub.7][B.sub.12] have been prepared by arc-melting from elements of 99.95% purity and amorphous ribbon 6 mm wide and about 20 [micro]m thick was produced by planar flow casting technique in vacuum at the Institute of Physics, Slovak Academy of Sciences in Bratislava.

Annealing and magnetic characterization of the samples was performed at the Institute of Experimental Physics, Slovak Academy of Sciences in Kosice. Pieces of the ribbon (8 cm long) were isothermally annealed under a high vacuum for 1 hour at 773 K. The value of annealing temperature was high enough to produce crystalline phase (Skorvanek et al., 2006) but under the optimum--in order to study influence of further heat-treatment during measurements. In the LF-annealed samples, the furnace with the sample was inserted into the water-cooled solenoidal coil that provided a magnetic field of 20 k[Am.sup.-1] oriented along the ribbon length. After such annealing, the specimens were slowly cooled to room temperature in the presence of magnetic field. The typical cooling rate was 3 K[min.sup.-1]. For sake of comparison, the reference samples were annealed and cooled under the same conditions in zero magnetic field (ZF).

The soft magnetic behaviour was investigated by using a high temperature hysteresis loop tracer based on the flux-gate magnetometer in the temperatures ranging from room up to 700 K. Higher temperatures were not chosen in order to avoid further crystallization of the samples. After measuring the loop at highest temperature, another measurement at room temperature have been performed in order to compare the change in the magnetic properties of the sample with respect to long-lasting (some hours) application of the elevated temperature. The coercitive field of the samples was determined from measured hysteresis loops.

3. RESULTS AND DISCUSSION

[FIGURE 1 OMITTED]

The obtained hysteresis loops taken at room temperature are shown in Fig. 1. It is evident that the annealing of the sample in the presence of longitudinal magnetic field increases the loop squareness and reduces the coercive field from value 68 [Am.sup.-1] to 8 [A.sup.m-1]. Such behaviour could be expected if the magnetic atoms pair ordering is operative mechanism of induced anisotropy. As a driving force for directed atomic diffusion is internal, not external magnetic field, directional ordering effect can occur even if the alloy is heat-treated below its Curie temperature in the absence of an external magnetic field. In this case, the internal magnetic field of each domain in the sample governs the direction of atomic diffusion and the direction of the induced anisotropy differs from domain to domain. The consequence of this self-magnetic annealing is that the domains and domain walls tend to be stabilized in the positions they occupied during the annealing, which results often in undesirable increase of coercive field (Skorvanek et al., 2007). The fact that the field-annealed samples reveal a smaller coercivity than the samples annealed without field can thus be understood from more simple domain configuration due to anisotropy induced by external field with intensity large enough to saturate the sample and thus to unify internal magnetic fields (O'Handley, 1999).

Inset in Fig. 1 is used to emphasize the fact that the value of saturation magnetization [M.sub.s] is depending on the composition of the alloy only and thus cannot be affected by different process of magnetization.

A special attention was focused on the study of the stability of soft magnetic characteristics of LF-annealed samples at elevated temperatures. Fig. 2 shows the temperature evolution of the coercive field in temperature range from 293 K to 700 K for partially crystallized nanocrystalline samples annealed for 1 h at temperature 773 K. Values of coercive field [H.sub.c] in the case of the sample annealed in the presence of external field are markedly lower in comparison with the sample anneled in zero field and stay fairly stable in the whole measured range. Difference in the values measured at room influence of long-lasting measurements at elevated temperature taking some hours to complete on the sample with uncomplished crystallization. Such a behaviour is in good agreement with long-lasting measurements of Kulik's group (Kulik et al., 2002).

[FIGURE 2 OMITTED]

4. CONCLUSION

We have shown that the field annealing is very powerful tool to improve the soft magnetic properties of HITPERM. The coercive field value obtained after longitudinal field annealing below 10 [Am.sup.-1] is already close enough to coercivities of magnetically softest materials. This in combination with unique magnetic stability at elevated temperatures and possibility to tailor its characteristics by controlled inducing of anisotropy favourizes nanocrystalline HITPERM as a material with good applicability. Obtained results are in good agreement with the theory of directional order mechanism.

Observed shift in room-temperature coercivity before and after the heat-treatment at elevated temperature is in consensus with recent studies. The annealing temperature should be increased to accomplish the first crystallization step and thus avoid change in microstructure and stabilize magnetic properties of the material.

5. REFERENCES

Chikazumi, S. (1964). Physics of Magnetism, Chapter 12: Directional Order, John Wiley & Sons, ISBN 0471155357, New York, pp. 577-619

Kulik, T.; Wlazlowska, A.; Ferenc, J. & Latuch, J. (2002). Magnetically Soft Nanomaterials for High-Temperature Applications. IEEE Transactions on Magnetics Vol. 38, No. 5 (2002) pp. 3075-3077, ISSN 0018-9464

Neel, L. (1952). Theorie du trainage magnetique de diffusion. Journal de Physique et le Radium Vol. 13 (1952) pp. 249-264, ISSN 0368-3842

O'Handley, R.C. (1999). Modern Magnetic Materials: Principles and Applications, Chapter 14: Magnetic annealing and directional order, John Wiley & Sons, ISBN 0471155667, New York, pp. 517-556

Skorvanek, I.; Marcin, J.; Krenicky, T.; Kovac, J.; Svec, P. & Janickovic, D. (2006). Improved soft magnetic behaviour in field-annealed nanocrystalline Hitperm alloys. Journal of Magnetism and Magnetic Materials, Vol. 304 (2006) pp. 203-207, ISSN 0304-8853

Skorvanek, I.; Marcin, J.; Turcanova, J.; Wojcik, M.; Nesteruk, K.; Janickovic, D. & Svec, P. (2007). Field induced anisotropy and stability of soft magnetic properties towards high temperature in Co-rich nanocrystalline FeCoNbB alloys. Journal of Magnetism and Magnetic Materials, Vol. 310 (2007) pp. 2494-2496, ISSN 0304-8853