FIELD- AND STRESS-INDUCED MAGNETIC ANISOTROPY IN NANOCRYSTALLINE Fe-BASED AND AMORPHOUS Co-BASED ALLOYS

For nanocrystalline alloy Fe73.CuNb3Si3.B9 thermomechanical treatment was carried out simultaneously with nanocrystallizing annealing (1) or after it (2). It was shown that a change in magnetic properties for the case is essentially greater than for the case 2. Complex effect of thermomagnetic and thermomechanical treatments on magnetic properties was studied in the above-mentioned nanocrystalline alloy as well as in the amorphous alloy FeCo70.6SiB9.,. During the annealings both field and stress were aligned with the long side ofthe specimens. It was shown that the magnetic field, AC or DC, decreases an effect of loading. Moreover, the magnetic field, AC or DC, applied after stress-annealing can destroy the magnetic anisotropy already induced under load.


INTRODUCTION
Nanocrystalline Fe-based alloys are known as a unique magnetically soft material with a low coercive force, low magnetic losses, and high magnetic permeability, which is caused by very fine grain size (10-12 nm), random distribution ofcrystal axes ofgrains, and vanishing total magnetostriction. It was known that (Glazer et al., 1991) using thermornechanical treatment, magnetic anisotropy of the easy-plane type characteristic of Co-based alloys (Nielsen et al., 1985) may be * Corresponding author. 290 V. A. LUKSHINA et al. obtained in the Fe73.sCulNb3Si13.sB9 alloy. This work is a continuation of Glazer et al. (1991). Its aim is to study for Fe-based nanocrystalline alloy the effect of the conditions of thermomechanical and thermomagnetic treatments on the magnitude of induced magnetic anisotropy and to compare the data with those for Co-based amorphous alloy FesCo70.6Si15B9.4.

EXPERIMENTAL
The method of rapidly quenching the melt onto a rotating copper drum was used to prepare amorphous ribbons ofboth Feand Co-based alloys of 20 lm in thickness and mm in width. In order to obtain a nanocrystalline state, the Fe-based ribbons were annealed in air at 530C for h. Below, we will call this treatment nanocrystallizingannealing (NCA). For both alloys the thermomechanical (TMechT), or thermomagnetic (TMT), or thermomechanomagnetic (TMechMT) treatment, that involve annealing and cooling of the sample under a tensile load, or in a magnetic field, or upon both factors, was carried out in a vertical tubular furnace; a load was fastened to the ribbon using a special clamp and removed after the termination of the treatment. The longitudinal magnetic field H--400 Oe, DC or AC, was created by coil winded around the furnace. For Fe-based material the treatment was performed using two regimes: 1. The NCA was carried out simultaneously with TMechT, or TMT, or TMechMT. An amorphous sample was subjected to NCA with a tensile load, or a magnetic field, or both a load and a magnetic field simultaneously.
2. NCA and TMechT, or TMT, or TMechMT were performed sequentially; the sample that was preliminarily annealed to obtain the nanocrystalline condition was then subjected to one of three treatments.
From a ribbon subjected to such a treatment, samples of 80-100 mm in length were cut from a portion that was located in the zone of the controlled uniform heating. Magnetic properties (hysteresis loops) were measured by the ballistic method in a field directed along the ribbon. Similar to thermomagnetic treatment in transverse DC magnetic field the thermomechanical one increases the incline of the hysteresis loops; FIGURE Appearance of the hysteresis loop for Fe73.sCulNb3Si13.sB-specimens after stress-or field-treatment in transverse DC magnetic field. with increasing load, the slope of the loop increases (Glazer et al., 1991).
The constant of induced magnetic anisotropy Ku was determined from a relation Ku=-O.5MsHs (see Fig. 1), where Ms is the saturation magnetization and Hs is the saturating field (in which saturation magnetization is reached). The error of measuring Ku was 5-7%.

RESULTS AND DISCUSSION
The results of the investigations are shown in the figures and in the tables.
For nanocrystalline alloy Fig. 2 shows the variation of the constant of induced magnetic anisotropy depending on the load during TMechT at 530C for h according to regimes and 2 (curves and 2, respectively).
It can be seen that, first, the magnitude of Ku induced by TMechT increases with increasing load, the magnitude of Ku after TMechT by regime is substantially larger than that obtained after treatment by regime 2. The times for reaching maximum Ku upon treatment by regimes 1 and 2 are also substantially different. When TMechT is performed at 530C Ku reaches its maximum in a few minutes ifTMechT is combined with NCA, but only in more than h if TMechT follows NCA. It was noted also that ribbons treated by regime increase in length. The larger the load, the greater the elongation; at maximum loads it reaches 13-15 mm, which is equal to about 15% of the gage length of the ribbon. The elongation occurs in approximately the same  The Table I shows the Ku behavior for Fe-based specimens subjected to NCA simultaneously with TMechT in a magnetic field, AC or DC, at 530C for h. In addition, the percent of the Ku value decrease after TMechMT is shown in comparison with that after TMechT. One can see that the magnetic field decreases an effect of loading to 50 MPa; the less the load, the more the decrease. At loads more than 50 MPa the change of Ku is less than 5 % which is within the error of measuring Ku.  under load r=600MPa and cooling: under load cr=600MPa (a), without any load (b); without any load in longitudinal DC (c) or AC (d) magnetic field H 400 Oe.
The Table II shows the values of Ku after different coolings for both alloys. Also, it shows a percent of Ku decrease after cooling without any load and in magnetic fields in comparison with Ku after cooling under load. It is seen that after cooling without any load Ku decreases by 16% for Fe-based and 14% for Co-based alloys, and after cooling in a magnetic field much larger, particularly in AC one. It means that cooling in magnetic field can destroy magnetic anisotropy induced under load. CONCLUSION 1. The magnitude of the constant of induced magnetic anisotropy that can be obtained by TMechT increases with increasing tensile stresses.
2. The combined effect ofNCA and TMechT, all other conditions being the same, produces a greater induced anisotropy than that obtained when TMechT follows NCA.
3. Magnetic field can interfere with the induction of magnetic anisotropy during stress-annealing and destroy the already stress-induced one.