THERMAL STABILITY OF FIELD-AND STRESS-INDUCED ANISOTROPY IN NANOCRYSTALLINE Fe-BASED AND AMORPHOUS Co-BASED ALLOYS

Thermal stability of induced magnetic anisotropy (IMA) was studied in a course of sub- sequent annealings without any external effects for already field- or stress-annealed specimens of the nanocrystalline Fe73.sCulNb3Si13.sB9 and amorphous Fe3Co67Cr3SilsB9_ alloys. For these alloys the dependence of IMA thermal stability on the magnitude of the IMA constant (Ku) and temperature of stress-annealing was investigated. For the nano- crystalline alloy thermal stability of field- and stress-induced anisotropy with identical Ku was compared. It was shown that nanocrystalline specimens with identical Ku values after field- or stress-annealing have identical thermal stability of IMA. This can point to a similarity ofthe mechanisms of IMA formation after field- or stress-annealings. Thermal stability of stress-induced anisotropy in the nanocrystalline alloy with Ku value less than 1000 J/m and the amorphous alloy with Ku less than 100 J/m depends on the value of K. For both stress-annealed nanocrystalline and amorphous alloys magnetic anisotropy induced at higher temperatures is more stable because more long-range and energy-taking processes take place at these temperatures.


INTRODUCTION
It is known that amorphous and nanocrystalline materials show the highest soft magnetic properties. The required level of soft magnetic properties is reached by different kinds of anisotropies induced in these materials by various treatments. These are thermomagnetic treatments in d.c., a.c. and high frequency magnetic fields, quenching from the Curie temperature into water (Glazer et al., 1992;1994), thermomechanical treatment which is the only effective one in ferromagnetics with low Curie temperature (Kurlyandskaya et al., 1996). The practical use of soft magnetic materials with induced magnetic anisotropy is determined by their thermal stability. In present work the thermal stability of fieldand stress-induced anisotropy in the amorphous Fe3Co67Cr3SilsB12 and the nanocrystalline Fe73.sCulNb3Si13.sB9 alloys is studied, the thermal stability of these anisotropies is compared. This is important both for the estimation of the thermal stability of the magnetic properties and for understanding of the nature of the induced magnetic anisotropy (IMA).

EXPERIMENTAL
The alloy ribbons were produced by melt quenching on rotational disc in air. In order to achieve soft magnetic properties Fe-based amorphous ribbons were to be nanocrystallized by annealing at 530C for h (NCA). Thermomagnetic treatment (TMT) in transverse d.c. magnetic field H= 2.5.105 A/m always was carried out during NCA. Thermomechanical treatment (TMechT) was carried out either simultaneously with NCA (regime 1) or after NCA (regime 2) at different temperatures: 300C, 400C and 530C for 4, 2 and 2 h, respectively . For all Co-based amorphous specimens preliminary h annealing at 350C for stress releasing was performed. Field-annealing for this material is not effective because the Curie temperature is too low (160C) (Kurlyandskaya et al., 1996). Stress-annealing was carried out at 250C and 350C for 4 and h, respectively. These exposures were necessary for obtaining ultimate magnitudes of IMA constants. The magnitudes of IMA constants were determined from the hysteresis loops according to the formula Ku =.-1/2(MsHs), where Ms is the saturation magnetization, Hs is the magnetic field in which saturation magnetization is achieved.
Thermal stability of induced magnetic anisotropy was studied in a course of subsequent annealings without any external effects for already fieldor stress-annealed specimens of the alloys mentioned above.

RESULTS AND DISCUSSION
For the nanocrystalline alloy thermal stability ofthe magnetic anisotropy was studied using the samples after TMT with Ku-100 and 350 J/m 3 and after TMechT (regime 1) with Ku from 400 to 4000 J/m 3. The specimens with different Ku were annealed without any external effects for h at progressively increasing temperatures in steps of 50 K, starting from 200C, Ku was measured at room temperature after each annealing.
It was found that the specimens with Ku about 1000 J/m 3 and more had the most thermally stable magnetic anisotropy. It was stable during h annealing at 450C. For Ku less than 1000 J/m 3 the thermal stability depends on the magnitude of Ku; the less the magnitude, the less the temperature up to which Ku was stable. Maximal Ku magnitudes obtained after TMT usually were equal to 100-150J/m 3. These values were essentially less than the ones after TMechT. So, thermal stability of Ku which we can obtain after TMT is less than that after TMechT (see Fig. 1).
For some specimens Fig. displays  T, C FIGURE Relative Ku variation after stress and magnetic field-free annealing for h at different temperatures for the nanocrystalline alloy. Curves and 2: for Ku obtained after TMechT and equal to 4000 and 400J/m, respectively; curves 3 and 4: for K obtained after TMT and equal to 350 and 100 J/m3, respectively. the magnitude of Ku the less the h annealing temperature up to which Ku does not yet decrease (compare curves 1, 2, 3 and 4). It should be noticed that if the magnitudes of Ku obtained after TMT and TMechT are close to each other their thermal stabilities are equal, as well.(compare curves 2 and 3). It can point to a similarity of mechanisms of magnetic anisotropy induced after TMT and TMechT, as well.
Analogous investigations of the thermal stability of IMA for Co-based amorphous alloy show that magnetic anisotropy induced by TMechT is stable after h annealing without any stress at 240C for specimens with Ku values being equal or more than 100 J/m 3. For this amorphous alloy we considered the dependence of thermal stability on the magnitudes of Ku obtained after stress-annealing. The samples with Ku from 25 to 500 J/m 3 were prepared by TMechT at 350C for h with a from 80 to 1400 MPa, respectively. Figure 2 shows the change of Ku against annealing time at 260C without any load. It is seen that specimens with K from 500 (curve 1) to 100 J/m 3 (curve 2) have the same thermal stability; the experimental points are situated on the same curve.
The specimen with Ku 25 J/m 3 has less thermal stability; the respective curve 3 runs below curve 1. Thermal stability of stress-induced anisotropy in amorphous alloys with Ku values less than 100 J/m 3 depends on the Ku value: the less Ku, the less the thermal stability.
For the above-mentioned alloys the influence of TMechT temperature on the thermal stability of IMA was investigated. For the  Fig. 3). For the Co-based amorphous alloy after 10 h ofannealing without any load, Ku decreases by 50% ifa sample was stress-annealed at 350C and by 90% if the sample was stressannealed at 250C (Fig. 4). The higher thermal stability of IMA induced by TMechT at 350C for Co-based amorphous alloy and at 530C for the nanocrystalline alloy shows that an increase ofthe TMechT temperature not only accelerates the diffusion processes but gives rise to more longrange and energy-taking processes, as well. Indeed for Co-based amorphous alloys the activation energies, estimated for a change of 50% of Ku from the kinetic curves ofthe IMA relaxation, have the following values" 2.2 eV for IMA induced at 350C and 1.9 eV for IMA induced at 250C.

CONCLUSIONS
From the above the following conclusions can be made: 1. The induced magnetic anisotropy (IMA) is stable after h annealing at: (a) 450C for the nanocrystalline alloy after TMechT by regime with K value being equal to or more than 1000 J/m3; (b) 300C for the nanocrystalline alloy after TMT with K value about 100J/m3; (c) 240C for amorphous alloy with Ku value being equal to or more than 100 J/m 3.
2. Thermal stability of stress-induced anisotropy in the nanocrystalline alloy with Ku value less than 1000 J/m 3 and in the amorphous alloy with Ku value less than 100 J/m 3 depends on the value of Ku: the less Ku, the less the thermal stability. 3. It is shown that for both stress-annealed nanocrystalline by regime 2 and amorphous alloys magnetic anisotropy induced at higher temperatures is more stable because more long-range and energy-taking processes take place at higher temperature of TMechT. 4. Identical thermal stability for nanocrystalline specimens with identical Ku values after TMT and TMechT points to a similar nature of field-and stress-induced anisotropy.