Magnetic properties of α′-FeNx and α″-Fe16N2 nitrides

Journal of Alloys and Compounds 274 (1998) 10–17

L

Magnetic properties of a9-FeN x and a0-Fe 1 6 N 2 nitrides ˇ a , *, Adriana Klarikova ´ ´ a , Ivo Paseka a , Karel Zaveta ´ ˇ b Petr Bezdicka a

b

Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, Pelleova 24, 160 00 Prague 6, Czech Republic ¨ ˇ ˇ ´ 2, Troja 190 00 Prague 9, Czech Joint Laboratory of Mossbauer Spectroscopy, Fac. of Math. and Phys., Charles University, V Holesovickach Republic Received 15 December 1997; received in revised form 15 February 1998

Abstract In the present work we studied the magnetic properties of a9-FeN x and a0-Fe 16 N 2 nitrides. Powder samples of nitrides were prepared by nitriding of iron powder in the mixture of H 2 and NH 3 . The desired a0-Fe 16 N 2 was prepared by the conventional method-tempering of samples containing a9-FeN x . The maximum content of a0-Fe 1 6 N 2 in the resulting mixture with a-Fe and g-FeN x was up to 60 wt %. The ¨ presence of individual phases was determined by XRD analysis and Mossbauer spectroscopy. The specific magnetic moment of samples was measured in SQUID magnetometer and that of individual phases was evaluated using data of the phase composition. Along with magnetic measurements the average magnetic moments of the present Fe atoms were evaluated from hyperfine fields derived from ¨ ¨ Mossbauer spectra. The average specific magnetization of a0-Fe 1 6 N 2 determined by XRD, Mossbauer phase analysis together with 21 ¨ SQUID measurements and from hyperfine fields of Mossbauer spectra were 246, 244 and 240 emu g , respectively.  1998 Elsevier Science S.A. ¨ spectroscopy Keywords: Iron nitrides; Synthesis; Magnetic properties; Fe-Mossbauer

1. Introduction The saturated magnetisation is one of the most important parameters of a magnetic material relevant to its application. The metallic a-Fe possesses a moment of 2.2 mB per atom due to the spontaneous splitting of the incompletely filled 3d band; this can be increased by alloying with Co up to |2.5 mB per atom. One of the ways how to enhance the local magnetic moments in a Fe based alloy, is to introduce an additional element and / or to change the distances of the interacting moments as realised in the Fe x N system. The magnetic properties of at least two phases from this system are of interest from the magnetic point of view [1]. These are the g9-Fe 4 N and in particular the highly metastable a0-Fe 16 N 2 first described by Jack [2]. The magnetic properties of the latter one have been attracting attention, since the first papers, where the preparation of a considerable amount of this phase by nitriding iron thin films was announced [3,4], reported unusually high value of magnetization ( m 53.0 mB ) as-

*Corresponding author.

0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00525-8

cribed to this phase from the experimentally determined magnetic moment of a mixture containing several ironbased phases. There appeared a great number of paper dealing with the magnetic properties of a0-Fe 16 N 2 in last few years. Some of them confirmed the high value of mB the other ones did not. It seems that only thin layers of iron with minority content of a0-Fe 16 N 2 posses higher values of mB . From the structural point of view the a0-Fe 16 N 2 phase is characterised by full and long-range ordering of the stoichiometric amount (1:8) of interstitial nitrogen. This ordering results in a distortion of the originally cubic lattice to a b.c.t. one with the lattice constants of a50.572 nm, c50.629 nm and c /a51.10. Three nonequivalent sites, (4d), (4e), and (8h), are formed for the iron atoms differing in local symmetries and distances from the nearest and next nearest N and Fe. Theoretical evaluation of the effect of introducing interstitial N into the Fe lattice on the magnetic moment pays attention to three possible effects of the presence of nitrogen: 1. change of the local density of states (DOS) of the 3d

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

electrons of Fe due to hybridisation with 2p electron wave function of N, 2. change of the distances of the first and second nearest neighbours connected to the change of the lattice constant and 3. change of the symmetry of the crystal lattice. These factors modify not only the DOS in the two spin subbands but also their spontaneous splitting giving rise to the change of the local moments. Several theoretical papers [5–7] agree on one typical feature – in one of the Fe sites in a0-Fe 16 N 2 structure, (4d), for which the N atoms are only found in the second co-ordination sphere, the local magnetic moment should be considerably increased with respect to the usual value of 2.2 mB found for iron with metallic bond in a phase. According to the method used, the local moment in this site of a0 was calculated to 2.74 to 2.99 mB with the average value equal to 2.83 mB . Both the experimental and theoretical results relevant to this problem were reviewed in detail in the panel discussion published in [8]. It followed from the review of Coey [1] that by the LMTO (Linearised muffin-tin orbital) calculation [9–15] the average magnetic moment of Fe in a0Fe 16 N 2 was found 2.41mB and in g0-Fe 4 N 2.28 mB . Powder nitrides with a content of a0-Fe 16 N 2 up to 50% were usually prepared by the above cited Jack’s method. It follows from the phase diagram that in the resulting mixture at least three components with various magnetic moments must be present. For the correct evaluation of the magnetic moment of a0-Fe 16 N 2 it is necessary to determine the content of individual phases with sufficient accuracy. Especially the exact determination of g phaseaustenite (with zero magnetic moment) content is very important. For the phase analysis of iron nitrides mixture it ¨ is possible to use XRD analysis and / or the Mossbauer spectroscopy. The use of the XRD method is complicated by the fact that most of the diffraction lines of the present phases overlap and their shape owing to the presence of strains originated during quenching is broadened and deformed. The fitting procedure usually used in the XRD phase analysis is then inaccurate. Also the possible inhomogeneity of the nitrogen distribution within the iron particles can cause the deformation of diffraction lines in ¨ the XRD analysis and of the Mossbauer spectra as well. These deformations can also influence the correct determination of the content of individual phases in the mixture of nitrides. The aim of the present paper is to compare the measurements of the magnetic moments with the analysis of ¨ Mossbauer spectra of samples prepared with the objective to attain as high concentration of the a0-Fe 16 N 2 phase as possible. This comparison should contribute to the determination of a reliable value of the magnetic moment of the a0-Fe 16 N 2 phase.

11

2. Experimental details The samples of iron nitrides with the content of nitrogen between 4.5 and 10 at % were prepared by nitriding the iron powder (30 to 70 m m in size) in a mixture of various proportions of NH 3 and H 2 at 923 K for 1 h. A longer time of nitriding did not cause a further increase of the nitrogen content in the sample. The flow of nitriding mixture was chosen so high that no great change in the composition of the gas mixture occurred. Rapid quenching to room or to liquid nitrogen temperature was subsequently followed by an anneal at the temperatures of 130 or 1508C for times ranging from 5 to 20 h. The concentration of nitrogen was determined from the increase in weight after nitriding, in some cases also by semi-micro Kjeldahl chemical analysis. The standard deviation in the nitrogen determination by the chemical analysis was 1.5%. The difference between the nitrogen determination from the increase in weight after nitriding and that by the chemical analysis was 5.1% on average. The nitrogen concentration was also verified by the XRD method using relations [16] between lattice parameters and the concentration of nitrogen a53.5751 0.008 c N for austenite and c /a5110.0091 c N for martensite (c N is the concentration of nitrogen expressed as the number of nitrogen atoms per 100 atoms of iron). Lattice parameters were determined with the XRD diffractometer SIEMENS D5005 using CoKa radiation. The presence of several phases in the samples, in particular a-Fe, g-FeN x , a9-FeN x and a0-Fe 16 N 2 was assumed; their relative amounts were determined by a standardless method based on the separation of individual diffraction lines by a fitting procedure, measuring their intensities and evaluating R factors [17,18]. ¨ The 57 Fe Mossbauer spectra were obtained from powdered samples in the transmission mode with the 5 7 Co in Cr matrix as the source moving in the constant-acceleration regime; all the spectra were only acquired at 300 K. The spectrometer was calibrated by means of a-Fe and the isomer shift was expressed with respect to this standard. The ratio of the magnitudes of the recoil-free fractions f for the ordered and disordered magnetic phases was experimentally determined with the help of a mixture of known quantities of a-Fe and g-FeN x ; this ratio was found to be rather close to unity. All the spectra were subsequently fitted with the help of the standard NORMOS program. The measurements of the specific magnetization s were performed in a Quantum Design MPMS 5 SQUID magnetometer on powdered samples placed in gelatine ampoules. The magnetic field was produced by a superconducting solenoid with maximum B up to 5 T; the temperature range of the magnetization measurements was from 10 to 300 K. The saturated moment calculated from the measurements of magnetisation was expressed either in terms of specific magnetization of the sample or of the relevant phase or alternatively as the average moment per

12

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

Fe atom making use of the quantitative chemical analysis of nitrogen.

3. Results and discussion

3.1. Preparation of martensite When nitriding a larger amount of iron (order of several grams) in a mixture of H 2 and NH 3 , there is always a great danger that sufficiently homogeneous samples are not formed. This is due to the fact that during nitriding the competition of two processes occurs, i.e. the decomposition of ammonia into nitrogen and hydrogen and the reaction of ammonia with iron resulting in the formation of nitrides. In order to obtain homogeneous samples it is necessary to keep the composition of nitriding mixture in the whole volume of the sample constant. Obviously, this is only possible to a certain extent. By a suitable choice of the particle size (large particles), thin layer of iron powder, and a high flow of the gas mixture we were able to prepare samples in which the difference in the nitrogen content on the surface and in the central part of the sample was in the average less then 7.5%. On the other hand the composition in the bulk of the particles is expected to be constant. If the diffusion coefficient of nitrogen in iron at 208C is DN 51.5310 220 and at 2008C DN 51.76310 215 m 2 s 2 1 [19] then at the nitriding temperature of 7008C the diffusion coefficient will be DN 53.16310 210 m 2 s 2 1 using for the evaluation of DN the linear extrapolation of the relation log DN vs. 1 /T, or DN 52.35310 29 (m 2 s 2 1 ) if we evaluated DN according to the relation DN 5D 0N exp(2E /RT ) with E5 89.45 kJ g atom [20]. For the time of nitriding equal to 1 h | 7–20 cm. This the mean diffusion distance l5(2Dt)1 / 2 5 value is by three orders of magnitude higher than the dimensions of the particles and consequently the concentration of nitrogen in particles should be constant. In spite of this Fall and Genin in [21] stated that, basing on ¨ the results of Mossbauer spectroscopy, the concentration of N in the film of 50 m m thickness is not homogeneous. It has been well known [22] that it is not possible to achieve the total transformation of austenite to martensite by quenching of austenite at the temperature of liquid nitrogen. The attainable degree of the transformation depends mainly on the nitrogen concentration and probably also on the particle size of iron powder. Fig. 1 shows the dependence of the martensite content on the nitrogen content in samples for our system (iron powder with the particle size ranging from 30 to 70 m m). Maximum content of martensite reaches about 90% but only at low concentrations of nitrogen (up to 6.5–7.0 at % N). The content of martensite steeply decreases at concentration around 8% N. This concentration is far lower than the maximum limit content of nitrogen in austenite (11.11 at % N).

Fig. 1. Dependence of the a0 martensite content on the N concentration in the sample. Curve A – j – the concentration of N determined from the lattice constant of austenite. Curve B – ? – the concentration of N determined from the weight increase after nitriding.

3.2. a0 -Fe16 N2 ; determination by XRD method a0-Fe 16 N 2 was prepared by tempering of martensite at temperatures 130 or 1508C. The concentration of a0Fe 16 N 2 was determined by the phase analysis of tempered samples by the procedure described above. The examples of XRD spectra of both tempered and non-tempered samples are shown in Fig. 2. The content of a0-Fe 16 N 2 can be also calculated using XRD data of martensite with the assumption that martensite is completely transformed stoichiometrically into a Fe and a0-Fe 16 N 2 and that the content of austenite does not change during tempering. The use of this method is possible owing to the fact that no martensite was found in the samples after tempering. The dependence of the concentration of the a0-Fe 16 N 2 phase on the total content of nitrogen in samples obtained by both methods is shown in Fig. 3. The variation of the a0-Fe 16 N 2 content in the range of the N concentration 6–7.5% is almost identical for both methods. At higher N concentrations the content of a0-Fe 16 N 2 determined by XRD analysis of tempered samples is, however, higher. To calculate the specific magnetization of a0-Fe 16 N 2 from the measured magnetic moment of the sample, it is of crucial importance to determine correctly its content of non-magnetic austenite. Fig. 4, that represents the variation of austenite content determined by various methods, shows that the accuracy of the determination of austenite is not high. Generally the highest content of austenite was found by the XRD analysis of tempered samples, while those determined by either XRD analysis of non-tempered ¨ samples or by the Mossbauer spectroscopy are significantly lower.

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

13

Fig. 4. Variation of the austenite content with N content. Curve A – ? – the austenite content determined from XRD data of tempered samples, curve B – j – the austenite content from both XRD data of non¨ tempered samples and Mossbauer spectroscopy data of tempered samples.

¨ 3.3. a0 -Fe16 N2 ; determination by Mossbauer spectroscopy Fig. 2. XRD spectra of non-tempered (A) and tempered nitride samples (B).

Fig. 3. Variation of the a0-Fe 16 N 2 content with the N content. Curve A – j – the a0-Fe 16 N 2 content determined from the XRD analysis of tempered samples, curve B – ? – the a0-Fe 16 N 2 calculated content from the XRD analysis of non-tempered samples.

For a plausible fit of the spectra it was necessary to employ up to eight sextets (three for various sites of a-Fe 16 N 2 , one for a-Fe, one for a defect phase, three for g9-Fe 4 N), in addition to a doublet and a singlet generally attributable to g-FeN x phase. The lines were assumed to have the Voigt profile consisting of a Gaussian distribution of Lorentzian shapes; the relative line intensities in the given sextet were fixed to the ratios following from random orientations of the local magnetic moments (3:2:1). During the fitting procedure the following quantities were left free: the line widths and their distributions, quadrupole splitting, isomer shift, and hyperfine field with the width of its distribution. When evaluating the spectra, two rather different approaches may be followed: one can either assume that the present phases possess the ideal structure and require that the relative occupations of different sites of each phase are fixed to the expected ratios, or alternatively interpret the sextet with characteristic hyperfine parameters as belonging to the site of the given type with only short range order analogous to that in the ideal structure. The latter view leaves the relative areas of the various sextets, and consequently the site occupations, free irrespective of their fixed ratios in the ideal structure. The distinction between the two procedures is particularly critical when treating the a0 phase with the aim to estimate its overall concentration. The quantitative evaluation of the amount of the various phases or site occupations was performed on the basis of the relative integral areas of the respective lines.

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

14

For the decomposition of some of the experimental spectra it was necessary to assume a contribution with rather broad distribution of hyperfine fields; we ascribed this sextet to the presence of a defect structure. The results of the fitting and further processing of our ¨ Mossbauer spectra are summarised in the Table 1. For the a0 phase we give the occupations derived separately for all the three different sites. Fig. 5 illustrates the change of the spectra after annealing the precursor. On one chosen spectrum we demonstrate in Fig. 6 the decomposition into the components belonging to the given phase and / or site.

3.4. Magnetic moment In the a0-Fe 1 6 N 2 phase the Fe atoms in different crystallographic sites possess various magnetic moments [1]. The problem of reliable comparison of the calculated moments or magnetization with the experiment is complicated by the fact that the single-phase samples only containing a0 phase were not yet prepared. The magnetization calculated from the magnetic measurement and ascribed to this phase depends on the experimentally derived concentrations and assumed magnetization of all the other magnetic and non-magnetic phases present in the mixture. ¨ We employed the Mossbauer spectra for the determination of the average magnetic moments per Fe atom and compared it with the measurement of the magnetic mo¨ ment in the SQUID magnetometer. The Mossbauer spectra were employed for this purpose in the following way: from the fitted integral intensities of the given sextet we took the relative occupations c i of the given site and the experimental value of the hyperfine field (Bhfi ) was assumed to be proportional to the average local moment in this site with the proportionality constant A i depending on the phase and

¨ Fig. 5. Mossbauer spectra of the precursor (A) and of the sample after annealing for 10 h (B).

site as critically listed in [1]. The average moment per iron atom is thus equal to , mFe . 5 Si c i A 21 i B hfi / Si c i .

Table 1 ¨ Phase composition determined from Mossbauer spectra and the moments , mF e . calculated from these spectra Sample

Heat treatment

a-Fe

4d type of

4e site

8h a0

Defect phase

g-FeN x

1 1 2 2 3 4 4 5 6 7 8 9 10 10 Average

T1 T2 T1 T2 T1 T1 T2 T1 T1 T1 T1 T1 T1 T2

33.5 26.6 18.3 16.0 18.8 18.3 18.4 18.8 23.9 47.5 32.7 32.0 32.1 35.7

11.0 11.3 6.7 8.2 10.6 8.9 8.8 10.0 11.7 9.1 11.1 11.4 13.5 11.1

16.9 12.3 13.3 11.2 15.8 14.5 14.2 15.5 16.9 12.7 15.4 15.4 14.7 17.5

17.7 18.5 18.7 16.7 18.1 18.4 17.4 21.1 17.2 15.0 18.9 17.0 18.9 15.0

10.7 17.7 7.0 11.5 15.4 8.2 9.4 12.1 14.9 8.1 12.1 13.8

10.2 13.6 26.3 24.3 21.3 27.3 26.6 22.4 15.4 7.5 9.8 10.4 20.8 20.7

g-Fe ´ 4N

9.7 12.1 4.4 5.1

, mF e . mB

, mFe . a01DF mB

sFe a01DF emu g 21

2.1–2.14 2.03–2.07 1.69–1.71 1.74–1.77 1.86–1.9 1.72–1.74 1.71–1.73 1.84–1.87 2.00–2.03 2.14–2.16 2.12–2.15 2.12–2.16 1.89 1.87

2.48 2.48 2.49 2.51 2.48 2.49 2.48 2.48 2.48 2.49 2.49 2.51 2.50 2.47 2.49

239.49 239.49 240.46 242.39 239.49 240.46 239.49 239.49 239.49 240.46 240.46 242.39 241.43 238.53 240.36

Abbreviation used: T1 heat treatment at 1508C for 5 h or at 1308C for 10 h in vacuum; T2 further tempering under the same conditions; , mF e . a01DF – magnetic moments related to the content of a01defect phase; s 99Fe a01DF emu g 21 the same values expressed as specific magnetizations in emu g 21 .

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

¨ Fig. 6. Decomposition of Mossbauer spectrum into the components belonging to: 1 – the fit, 2 – a0-Fe 16 N 2 (4d), 3 – a0-Fe 16 N 2 (4e), 4 – a0-Fe 16 N 2 (8 h), 5 – defect phase, 6 – a Fe, 7 – g Fe, 8 – g FeN x .

The moments calculated along these lines are listed in the Table 1 as , mF e . and may be compared with the moments derived from the magnetization measurements at 10 and / or 300 K. The relatively small difference of the moments measured at these two temperatures indicates that the Curie temperatures of all the magnetic phases present in considerable concentrations are rather high and the estimate of the saturated moment from the 300 K measurement is justified. As is seen from Table 1, the agreement of the magnetic moments derived by these two diverse methods is plausible. It is worth mentioning that the calculated moments , mFe . practically did not depend on the way, how the site occupations in the a0 phase were treated – whether they were fixed to the ratio corresponding to the ideally ordered structure of interstitial N atoms, or left free during the fitting procedure. In the last column of the Table 1 the average moment of Fe atom in the a0 phase is given as ¨ derived from the Mossbauer spectra with the site occupations indicated. Specific magnetization of individual samples were, as mentioned above, also measured by the SQUID magnetometer at temperatures 10 and 300 K at 0–5 T. All values listed in the Table 2 correspond to 5 T and 300 K as the

15

dependence of the magnetic moment on the intensity of the magnetic field is almost identical for all samples as well as the differences of moments at 10 and 300 K. Low values of standard deviation listed at some values of magnetic moment show that these measurements are rather precise. From the Table 2 it follows (i) that the maximum magnetic moment of the samples (containing austenite and martensite) reaches approximately only the value of the magnetic moment of pure iron, and (ii) the values of the magnetic moment of samples before and after tempering are in fact identical (the sum of differences of magnetic moments of non-tempered and tempered samples is only 3.7%). In other words, during tempering, at which a0-Fe 16 N 2 is formed, the total magnetic moment of the mixture of all the present phases did not appreciably change. The summary of the results concerning the formation of a0-Fe 16 N 2 identified by XRD method, magnetic moment of this phase derived from the measurements with the SQUID magnetometer and the comparison of these results ¨ with those of Mossbauer spectroscopy is listed in the Table 3. Listed data of the content of three present phases (in two samples even of four phases) in individual samples were obtained by the combination of XRD data of both tempered and non-tempered samples. The combination consisted in the determination of the austenite content in non-tempered samples and supposing that during tempering the content of this phase would not change, we used this value also in the evaluation of the phase composition of tempered samples. From the phase analysis of tempered samples we only used the determined ratio of the a-Fe and a0-Fe 16 N 2 contents. This procedure was adopted because the content of austenite determined by the XRD analysis of tempered samples is significantly higher than that derived both from the XRD analysis of non-tempered samples and ¨ Mossbauer spectroscopy of tempered samples (see Fig. 3). We consider the determination of austenite content from the data of non-tempered samples more exact as the phase analysis of two phase system objectively has to be more reliable than that of three phase system. The specific magnetization of a0-Fe 16 N 2 in the samples were evaluated taking into account that the specific magnetization other possible phases in the samples are as follow: a-Fe5218 emu g 21 , g9-Fe 4 N5178 (the mean value of data in [23]) g-FeN x 50. These values are summarised in Table 3. The average values of specific magnetization evaluated from the data in Table 3 were 238 emu g 21 and 236 emu g 21 using data of the phase com¨ position determined by XRD and Mossbauer spectroscopy methods, respectively. Comparing the data of sa0 listed in Table 3 it is seen that specific magnetization of samples containing also g9-Fe 4 N are significantly below-average. If the data of these samples were not included in the calculation of the average values then we obtained for the specific magnetization of a0-Fe 16 N 2 246 emu g 21 and 244 emu g 21 . On the other hand if we should use the data of the phase

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

16

Table 2 Specific magnetizations sF e of tempered and non-tempered nitride samples Sample

Content of N at %

Treatment

sF e emu g 21

1 1 1 2 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 10

5.51 5.51 5.51 7.76 7.76 7.76 7.57 7.57 7.87 7.87 7.47 7.47 6.90 6.90 5.43 5.43 6.90 6.90 7.04 7.04 8.70 8.70 8.70

U T1 T2 U T1 T2 U T1 U T1 U T1 U T1 U T1 U T1 U T1 U T1 T2

206.94 211.81 198.39 152.59 150.52 152.38 174.82 174.82 152.09 151.91 174.61 173.69 195.69 198.39 223.83 222.32 221.03 219.03 220.58 218.33 185.77 a 180.80 a 181.11 a Total DsF e

DsF e in %

Standard deviation 1.78 0.34

12.35 24.13 21.35 20.13 20.38

0.41

22.2

0.57 0.72

20.52 11.37 20.67 21.31 21.02 22.70 a

22.50 a 23.75

Abbreviation used: U – untreated samples, T1 heat treatment at 1508C 5 h or at 1308C 10 h in vacuum; T2 further tempering under the same conditions. a Values are not included in total DsF e as they were measured by another method.

composition determined from XRD analysis of tempered samples only for the evaluation of specific magnetization, we obtain 277.8629 and 284.8629 with excluding the data of samples containing g9-Fe 4 N, respectively. It is seen that these values are much higher than those obtained from ¨ Mossbauer spectroscopy and they are evidently incorrect.

4. Conclusions 1. The maximum content of the a0-Fe 16 N 2 phase prepared by nitriding and subsequent annealing of iron powder was up to 60 wt %. 2. The agreement of the phase determination in the

Table 3 ¨ The comparison of the concentrations of various phases derived from Mossbauer spectroscopy and XRD and that of magnetizations sa0 measured by SQUID and evaluated using the concentrations of a0-Fe 1 6 N 2 determined by both methods ¨ Mossbauer spectroscopy results

XRD results

Sample

Treatment

g-FeN x

a-Fe

a01defect phase

g9-Fe 4 N

sa0 emu g 21 a0

g-FeN x

a-Fe

a0

g9-Fe 4 N

sa0 emu g 21 a0

1 1 2 2 3 4 5 6 7 8 9 10 Average a Average b

T1 T2 T1 T2 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1

10.2 13.6 26.3 24.3 21.3 27.3 22.4 15.4 7.5 9.8 10.4 20.8

33.5 26.6 18.4 16.1 18.8 18.3 18.8 23.9 47.5 32.7 32.0 32.1

56.3 59.9 45.7 47.6 59.9 50.0 58.7 60.7 44.9 57.5 57.6 47.1

0.0 0.0 9.7 12.1 0.0 4.4 0.0 0.0 0.0 0.0 0.0 0.0

246.5 234.4 203.8 201.1 223.4 208.3 226.1 241.0 264.5 260.4 257.9 233.6 236.5620.2 244.2615.2

9.1 9.1 24.0 24.0 22.6 31.5 24.8 14.2 07.0 10.3 11.1 21.4

41.7 39.9 20.2 21.7 16.7 13.2 30.0 33.4 43.9 37.6 36.9 28.4

49.1 50.8 49.7 46.2 60.6 51.3 54.2 52.4 49.1 52.1 52.0 50.2

0.0 0.0 6.0 8.0 0.0 4.0 0.0 0.0 0.0 0.0 0.0 0.0

245.7 218.4 192.4 196.4 228.0 223.0 236.0 239.5 257.9 262.9 265.1 236.8 238.7620.5 246.2613.1

a

Average values without T2 results. Average values without T2 and samples without g9 content. The conditions of the heat treatments T1 and T2 are those referred to in the previous tables.

b

ˇ et al. / Journal of Alloys and Compounds 274 (1998) 10 – 17 P. Bezdicka

samples after nitriding and subsequent tempering de¨ rived from XRD and Mossbauer spectra was satisfactory. 3. The average magnetic moment of the mixture of nitrides did not appreciably change by tempering [24]. 4. The values of the average specific magnetization of a0-Fe 16 N 2 determined both from the magnetic measurements together with the phase analysis made by XRD ¨ and Mossbauer methods, and by evaluating the ¨ Mossbauer hyperfine fields are similar and they range among 240–246 emu g 21 , i.e. 2.49–2.55 mB . This value is in the satisfactory agreement with the theoretically derived average value of 2.41 mB [1].

Acknowledgements This work was supported by the Grant Agency of the ASCR under project No. A 403 2601.

References [1] J.M.D. Coey, J. Appl. Phys. 76 (1994) 6632. [2] K.H. Jack, Proc. Royal Soc. London, Ser. A 208 (1951) 216. [3] M. Komuro, Y. Konozo, M. Hanazono, Y. Sugita, J. Magn. Soc. Jpn. 13 (1989) 301.

17

[4] Y. Sugita, K. Mitsuoka, M. Komuro, H. Hochiya, Y. Kozono, M. Hanazono, J. Appl. Phys. 70 (1991) 5977. [5] S. Matar, Z. Phys. B87 (1992) 91. [6] S. Ishida, K. Kitawase, S. Fujii, S. Asano, J. Phys.: Condens. Matter 4 (1992) 765. [7] J. He, Y. Zhou, IEEE Trans. Mag. 31 (1995) 3668. [8] (Panel Discussion), J. Appl. Phys. 76 (1994) 6620. [9] A. Sakuma, J. Magn. Magn. Mater. 102 (1991) 127. [10] A. Sakuma, J. Phys. Soc. Jpn. 61 (1992) 223. [11] A. Sakuma, J. Phys. Soc. Jpn. 60 (1991) 2007. [12] S. Ishida, K. Kitawatase, J. Magn. Magn. Mater. 104–107 (1992) 1933. [13] B.I. Min, Phys. Rev. B 46 (1992) 8232. [14] C.A. Kuhnen, R.S. de Figueiredo, V. Drago, E.Z. da Silva, J. Magn. Magn. Mater. 111 (1992) 95. [15] J.M.D. Coey, K. O’Donnell, Q.Q.E. Touchais, J.H. Jack, J. Phys. Condens. Matter 6 (1994) L23. [16] M.Q. Huang, W.E. Wallace, S. Simizu, A.T. Pedziwiatr, R.T. Obermayer, S.G. Sankar, J. Appl. Phys. 75 (1994) 6574. [17] J. Fiala, Kovove Mater. 6 (1968) 579. ´ M. Cermak, ´ Kovove Mater. 19 (1981) 741. [18] J. Neuman, H. Hejdova, [19] J. Gonzon, J. Wegria, L. Habraken, Rev. Centre Recherches Metalurgiques B 33 (1973) 73. [20] R.V. Geld, R.A. Rjabov, Vodorod v metalach i splavach, Izd., Moskva, (1974), p. 117. [21] I. Fall, J.-M.R. Genin, Metall. Trans. 27A (1996) 2160. [22] K.H. Jack, J. Appl. Phys. 76 (1994) 6620. [23] M. Metzger, X. Bao, J. Appl. Phys. 76 (1994) 6626. [24] M. Takahashi, H. Shoji, H. Takahashi, H. Nashi, T. Wakiyama, M. Doi, M. Matsui, J. Appl. Phys. 76 (1994) 6642.