STUDY OF MAGNETIC PROPERTIES OF ISOTOPICALLY ENRICHED 56 FE AT DIFFERENT MAGNETIC FIELD STRENGTH VALUES

The magnetic properties of polycrystalline samples of isotopically enriched iron with the content of isotope 56 Fe 99.945±0.002 at. % were investigated at different magnetic field strength values. The samples of monoisotopic iron were melted in the environment of argon and annealed in hydrogen. The same measurements were carried out on the samples of iron with natural isotopic composition. The quantity and composition of impurities in the samples of natural and monoisotopic iron were equal. The magnetic properties were measured using an automatic induction vibro-magnetometer MagEq MNMS 216 and an automatic measuring complex MK-3E in accordance with GOST 8.377-80 and GOST 12119.1-98. It was found that the value of the saturation magnetization J s of 56 Fe is higher by 10.87% than that of nat Fe at room temperature, and by 11.35% at 100 K. The values of saturation induction В s and residual magnetic induction B r for 56 Fe are higher than for nat Fe. The difference in the values of the coercive force Н с for the nat Fe and 56 Fe samples is due to the difference in the grain size of the samples and in the purity of the materials. = 400 Hz: when magnetic field strength increases from 210 to 8884 A/m, the losses during re-magnetization increase by about 52.5 times at f=50 Hz and about 275 times at 400 Hz. The same is observed for the 56 Fe sample as well.


INTRODUCTION
Authors [1] found that the fundamental magnetic properties of natural ( nat Fe) and monoisotopic ( 56 Fe) iron are almost the same. The excess of saturation magnetization (Js) for 56 Fe in comparison with nat Fe is 4.6%, the differences in the values of the coercive force (Hc), residual magnetic induction (Br) and maximum magnetic permeability (µmax) are explained by the difference in the content of impurities. This work is a continuation of work [1]. When carrying out it, the same objects of research were used as in [1], but they additionally annealed in high-purity dried hydrogen. The quasi-static and dynamic magnetic properties of the studied samples of natural ( nat Fe) and monoisotopic ( 56 Fe) iron were measured in this work at different values of the magnetic field strength and at different frequencies of the alternating magnetic field. The results obtained were compared with each other.
The aim of this work is to obtain new data on the fundamental and structure-sensitive magnetic properties of nat Fe and 56 Fe with decreased concentration of impurities in the samples and at increased values of the magnetic field strength.

MATERIALS AND METHODS
For the study, we used previously prepared 56 Fe and nat Fe samples [1], the isotopic composition of which is shown in Table 1. Two types of samples were made, differing in shape and geometric dimensions. The first type of samples had the shape of a parallelepiped and was intended to measure the magnetic properties by the method of vibration magnetometry [2][3][4]. The second type of samples had the shape of a ring with an outer diameter D = 15 mm, an inner diameter d = 11.5 mm, and a height h = 4.0 mm and is intended for determining the magnetic properties by the induction-pulse method in quasi-static and dynamic modes in accordance with GOST 8.377-80 and GOST 12119.1-96 [5,6]. All samples were additionally annealed in a stream of dry hydrogen of grade "B" (purity 99.9999%) at a temperature of 1100°C for 6 hours, after which they were analyzed for impurities.
Before determining the impurity composition, in order to exclude the influence of possible surface contamination, the surface of the samples was etched for 2 hours at 30⁰С in a 15% solution of hydrochloric acid of the high purity grade, further purified using a Savillex DST-1000 sub-boiling distillation system.
The analysis of the impurity composition of iron samples was carried out by laser mass spectroscopy using an EMAL-2 device (USSR).
The magnetic properties on parallelepiped samples with dimensions of 1.5x1.5x3.5 mm were measured using an automatic induction vibromagnetometer MagEq MNMS 216 (Russia) with a control program based on the LabVIEW8 package. The measurements were carried out at various temperatures in the range of 100-350K with a step of 50K. The maximum magnetic field strength was 1.79 T. Using the constructed hysteresis curves we judged the the saturation magnetization (Js) of the measured sample.
Measurements of magnetic properties in quasi-static and dynamic modes were carried out on ring samples with an outer diameter D = 15 mm, an inner diameter d = 13.5 mm, and a height of h = 4.0 mm using an automatic measuring complex MK-3E (Russia) in accordance with GOST 8.377-80 and GOST 12119.1-98. The magnetic field strength varied from 108 to 10810 A/m. The frequency of alternating magnetic field during measurements in the dynamic mode was 50, 60, and 400 Hz. The values of the coercive force (Hc), residual magnetic induction (Br), saturation induction (Bs), rectangularity coefficient of the hysteresis loop (Kr) and losses during magnetization reversal (P) were determined. The relative error of measurments for a confidence coefficient of 0.95 was ± 2% for Hc, ± 2% for Br, and ± 2% for Bs. The relative measurement error the points of the magnetic hysteresis loop and the main magnetization curve by induction is ± 1.5%, by the magnetic field strength is ± 2%.

RESULTS AND DISCUSSION
The results of the analysis of the impurity composition of the nat Fe and 56 Fe samples before and after their annealing in hydrogen, determined by mass spectrometry on the EMAL-2 device, are shown in Table 2.
Comparing the results from Table 2, with the purity of the samples used for measurements in [1], it can be noted that additional annealing led to a significant (by 3 orders of magnitude) decrease in the oxygen content, the carbon content decreased by 8-10 times, and etching of the samples removed most of surface contamination. After annealing, the content of impurities in nat Fe and 56 Fe for most of the impurity elements is within the same limits and leveled off.  Table 3 shows the values of the saturation specific magnetization (σs) obtained on nat Fe and 56 Fe samples at various temperatures in the range 100-350K using a MagEq MNMS 216 vibromagnetometer. In Table 3 Δ is the difference, % in σs values for these samples (Δ,%). It can be seen from Table 3 that the values of the saturation specific magnetization, σs, for nat Fe at all measurement temperatures are lower than for 56 Fe. The difference in values depending on the measurement temperature is 10.87-11.35%. Accordingly, the saturation magnetization defined as Js = σs•q, where q is the density of the material of the measured sample (for both samples, the value q = 7.78 g/cm 3 ), for nat Fe at all measurement temperatures will be lower than for 56 Fe [7][8][9][10]. Moreover, the difference in Js values depending on the measurement temperature will also be 10.87-11.35%. It should be noted that in [1] this difference for room temperature was 4.6%.
The magnetic properties of ring-shaped nat Fe and 56 Fe samples after their annealing in hydrogen, measured in the quasi-static mode at various values of the magnetic field strength, are given in Table 4. It can be seen from Table 4  It can be seen from Figure 4 that the losses during re-magnetization of samples (P) of both nat Fe and 56 Fe at f = 50 Hz and 60 Hz less dependent on the magnetic field strength than at f = 400 Hz: when magnetic field strength increases from 210 to 8884 A/m, the losses during re-magnetization increase by about 52.5 times at f=50 Hz and about 275 times at 400 Hz. The same is observed for the 56 Fe sample as well.
As we see, the changes in the magnetic properties of the nat Fe and 56 Fe samples with an increase in the strength H and frequency f of the alternating magnetic field are identical. The difference is only in the values of these magnetic properties.

1)
The saturation magnetization Js of 56 Fe is higher than that of nat Fe in the temperature range from 100 to 350 K. At room temperature, this excess (Δ) is 10.87%. When the temperature changes to 100K, Δ increases to 11.35%.

2)
The values of saturation induction Вs and residual magnetic induction Br for 56 Fe are higher than for nat Fe. This excess Δ depends on the strength of the magnetic field H. With an increase in H from 500 to 8000A / m, the value of Δ for Вs decreases from 3.65 to 1.72%, and for Br, increases from 5.69 to 7.03%. 3) The magnitude of the specific losses during re-magnetization P, for nat Fe and 56 Fe samples at any value of the strength of the alternating magnetic field H, strongly depends on its frequency f. An increase in f with a simultaneous increase in H leads to a strong increase in P.

4)
The difference in the values of the coercive force Нс for the nat Fe and 56 Fe samples is due to the difference in the grain size of the samples and in the purity of the materials with respect to impurities and, first of all, with respect to carbon.

5)
The dependence of the value of P on H and f correlates with the dependence of Hc on H and f. A decrease in Hc should lead to a decrease in P and can be achieved by further increasing the purity of the material of the samples in terms of soluble (mainly interstitial) and insoluble impurities.