Magnetism of α′-FeN alloys and α″-(Fe16N2) Fe nitrides

ELSEVIER

M

journal of magnetism

IH A

Fnadgnetic materials

Journal of Magnetism and Magnetic Materials 135 (1994) 226-330

Magnetism of cx’-FeN alloys and CX”-( Fe,,N,) Fe nitrides M.Q. Huang, W.E. Wallace *, S. Simizu, S.G. Sankar Carnegie Mellon Research Institute, Carnegie Mellon University, Pittsburgh, PA 15213, USA

(Received 30 November 1993)

Abstract The d-FeN phase was prepared by treating Fe powder with NH,/H, gas mixtures at a temperature of - 665°C followed by a quench to cryogenic temperatures. Conversion of (Y to d to an extent exceeding 85% has been achieved. cx”-FeN is produced by treatment of (Y’ for - 1 to 2 h at 120-150°C. B,,, values obtained for CL- and d’-FeN are 23.5 and 26.6 kG, respectively. The latter corresponds to 2.94p,/Fe atom, a 34% enhancement over that of a-Fe.

1. Introduction When mixtures of NH, and H, are passed over Fe powder or thin films at elevated temperatures, a number of nitrides form [1,2]. One of these is y-FeN, often referred to as nitrogen austenite. In this, Fe is in a fee arrangement and N randomly occupies some of the octahedral interstices. As the nitrogen content increases, the nitrogens interact and at high concentrations they order to form y’-FeN or Fe,N. In y’-FeN, the iron is in a fee arrangement and the nitrogens occupy the octahedral interstices in an ordered manner. When the y-FeN which is formed at elevated temperatures is slowly cooled, it transforms into a mixture of a-Fe (or a-Fe with a small amount of N in solid solution) and Fe,N. However, if it is quenched to room temperature and below, the phase segregation into a-Fe and Fe,N is sup-

* Corresponding author. 0304-8853/94/$07,00 0 1994 Elsevier Science SSDI 0304-8853(94)00057-X

pressed and a phase designated as cx’ forms [l]. oi-FeN is isostructural with martensite and is often called nitrogen martensite [3]. If d-FeN is then heat treated at temperatures ranging from 120 to 15O”C, ordering of the nitrogens occurs [l] and a new phase forms [41. The new phase, designated d’, has the composition Fe,,N,. It is tetragonal and the unit cell dimensions are roughly double those of the (Y’phase. Japanese workers have formed the ct” phase in thin films of Fe. Using ion implantation or molecular beam epitaxy to introduce nitrogen into films, they observed a sharp rise in magnetization as nitrogen entered the Fe lattice [2]. Sugita et al. [5] estimated from this rise that oi’-FeN has a magnetic induction of 29 kG. This corresponds to an average Fe moment of 3.1pB, a 40% enhancement over that of a-Fe. Sugita et al. refer to Fe,,N, as a system with a giant magnetic moment. However, not all investigators have reported moment enhancement by nitrogenation. For example, Shoji et al. report no enhancement [5] and Coey et al. report [6] only small (N 10%)

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M.Q. Huang et al. /Journal

enhancement. This discrepancy, and the claim of very high induction for Fet6N2, provided motivation for the present study. As is indicated below, the present study shows a significant enhancement of the Fe moment by nitrogenation.

1200 1000

y Fe-N

600

N - 10.7 al % Tq - RT

0” c!

606

2

of cur-and a”-FeN alloys

m -

-0”

Wriedt, Gokcen and Natzinger [7] show that at temperatures above 6Oo”C, nitrogen austenite readily forms from Fe and nitrogen, provided that nitrogen is present at high pressures. The needed high pressure nitrogen can be generated from NH,/H, gas mixtures. Consider the reaction 2NH,( 1 atm) -

N2( 1 atm) + 3H,( 1 atm).

AC” for this reaction (in joules) = 32700 - 198(T - 298). AG” > 0 at 25°C and hence NH, is stable at room temperature with respect to its elements. However, at elevated temperatures, e.g., at 725% AG” = - 106 100 J and NH, is unstable with respect to decomposition into the elements. The equilibrium constant - 106 100 (8.31)998

K = exp [

1

= 3.3 X 105.

From this, one sees that if PH2= 0.9 atm and = 0.1 atm, the equilibrium pressure is -N!jOO atm. Fe is a well-known synthetic NH, catalyst 181and hence a high N, pressure (several thousand atmospheres) is expected at elevated

P

Table 1 Phases formed N/Fe [at%]

and magnetism Phases

t

c”

400

2. Preparation

227

of Magnetism and Magnetic Materials 135 (1994) 226-230

0 30

40

50

60

70

90

90

100

28

Fig. 1. X-ray diffraction pattern for y-FeN for which the quenching temperature (T,) is room temperature. The diffraction pattern indicates that the material is single phase.

temperatures when a mixture of NH, and H, is passed over Fe powder. y-FeN was formed by flowing a mixture of H, and NH, over Fe powder 6-9 Frn in size at temperatures of 660 to 670°C. The composition of the y-FeN (nitrogen austenite) was established from its lattice parameter or, in some cases, by noting the weight change. In the present study, nitrogen austenite was prepared containing IZ atoms of nitrogen per 100 atoms of Fe, where n ranged from 6.4 to 10.7. The formation of y-FeN is important since this phase is the precursor to the oi and CL”phases. Upon quenching the y phase with IZ= 10.7, it is observed that phase y-FeN was retained if the quenching was only to room temperature. (See

of o’-FeN present

w,l [wt%,l

a

M

MCCl

[emu/s1

[emu/s1

?y’- d) Kl

21

y

d

a [Al

a

c [Al

c/a

- 6.4

RT

3.615

2.857

3.022

1.057

- 87

210

- 240

-RT

- 8.1

RT LN

3.632 3.632

2.852 2.852

3.064 3.064

1.074

- 72 - 82

177 202

- 246

- RT-110

RT LN LHe

3.652 3.652 3.652

2.846 2.846

3.126 3.126

1.098

-0 - 51 - 56

3 133 146

- 10.7

PI

[Al

30-150 - 260

K

K

228

M.Q. Huang et al. /Journal

of Magnetism and Magnetic Materials 135 (1994) 226-230

Fig. 1.) However, upon further cooling, the martensitic transformation reported by Jack [3] was observed and substantial amounts of the LX’ phase were formed. The phases formed by quenching the n = 10.7 sample to various temperatures are summarized in Table 1. It is to be noted that for smaller values of n the transformation begins to occur at higher temperatures. This counterintuitive behavior will be returned to below in the discussion section. Following the procedure established by Jack, samples of d-FeN were heat treated at - 120150°C for 1 to 2 h. Under these conditions o” readily formed along with some o-Fe and y-FeN. The latter forms since the y-FeN is Fe-rich relative to the oi’ phase by - 15%. Hence, during the d to d’ transformation, some cx-Fe or a(FeN) is precipitated out. When the sample of nitrogen content 10.0 N/100 Fe is treated in this way, it transforms into a 3-phase mixture of mass composition 56% d’, 13% (Y and 31% y. The relative amounts of the three phases present were established by quantitative XRD measurements [91.

3. Magnetic

VSM measurements on the d, y mixtures are summarized in Table 1. The measured moments ranged from 133 to 210 emu per gram of sample and from 240 to 260 emu per gram of o’ phase. Thus magnetization of d-FeN is taken to be 250 emu/g f 5%, corresponding to an induction of 23.5 kG or 2.57pB/Fe atom. 3.2.The cl’ phase

Heat treating the d phase results in a partial conversion of this phase into a”-FeN. The occurrence of this conversion is indicated by (1) a rise in magnetization, (2) an alteration in lattice dimensions and (3) the appearance of a new diffraction line, the 213 line. As noted earlier, the heat-treated FeN alloy is converted into a 3-phase system. The quantities of the phases present [9], obtained from the intensities of the XRD lines, are given in Table 2. The fraction of a”-FeN

1.01

properties

,

I

I

I

I

of QL’-and a”-FeN alloys -

N - 8.1 at 5%

3.1. The cd phase

As noted above, there is a y to (Y’transformation when the y phase is quenched. This is evident from the altered XRD pattern and also from the magnetic behavior. This is brought out in the magnetization-temperature results plotted in Fig. 2. There is a rise in magnetization as temperature is lowered because the nonmagnetic y phase is being transformed into the strongly magnetic (Y’ phase. The transformation takes place over a range of temperatures and requires lower temperature for higher nitrogen conent. This latter behavior is unexpected on first sight. However, if one recalls that y’-FeN (i.e., Fe,N) is quite stable, the behavior of the high nitrogen y phase is less surprising. Most likely, the y phase is becoming increasingly stable as the nitrogen content is raised, so that a lower temperature is entailed to bring on the martensitic transition.

1.: ~__~____l____~__________

1

0

I

\

100

200

I 300

T(K)

Fig. 2. Magnetization versus temperature at low temperatures for Fe samples containing various amounts of nitrogen. The rise in magnetization at low temperatures is due to the conversion of y-FeN into d-FeN. The solid and dashed lines are for cooling and heating, respectively. The measurements plotted were obtained after the sample had been quenched to room temperature.

M.Q. Huang et al. /Journal of Magnetismand MagneticMaterials135 (1994) 226-230 Table 2 Phases present N/Fe [at%]

and magnetism

Phases [wt%]

present

Y

(Y

d’

-16 - 25 - 25 - 30 - 31 - 35 - 45 - 53

- 35 - 23 - 27 - 13 - 13 -13 -7 -8

-

-8.1 - 9.4 - 10.0

- 10.7

d’-Fe,,N,

49 52 48 59 56 52 48 39

Table 3 Magnetic

of a” samples

a[&

c [AI

5.721 5.714 5.715 5.719 5.718 5.714 5.713 5.715

6.286 6.291 6.286 6.291 6.290 6.285 6.290 6.285

M

M,,,

[emu

[emu

/sl

/sl

210 196 195 191 189 177 151 129

-

272 280 283 283 286 286 283 286

ranges from 39 to 59%. The measured magnetization varied from 129 to 210 emu per gram of samples and from 272 to 286 per gram of oi’-FeN, corresponding to an induction of 26.6 kG k 5% or 2.94~~ per Fe atom. This is approximately 34% higher than that of elemental Fe. 3.3. Magnetic

behavior

over a wide range of tem-

pera ture

The behavior of y-FeN at sub-room temperature is shown in Fig. 2. The properties of this system at elevated temperatures, up to 800°C have also been studied. Hot stage XRD and VSM have been used. The magnetization versus tem-

and/or

Temperature -

phase

[“Cl

275 495 620 680 770

transformations

229

when heating

y-FeN

Origin y-FeN + y’-FeN + u-Fe(N) a T, b of y’-FeN(Fe,N) y’ + a-Fe(N) a + y Evaporation of N from sample T, b of u-Fe

a This designation is used to indicate solid solution. b T, indicates Curie temperature.

a small amount

of N in

perature is shown in Fig. 3. There are five temperatures at which some kind of transformation is occurring. The nature of the various transitions is indicated in Table 3. The origin of the special temperature effects was initially inferred from VSM measurements and were later confirmed by hot stage XRD.

4. Conclusions The Fe moment in d- and o”-FeN is significantly larger than that of elemental Fe - about 34% larger in the CY”phase. The magnetic induction is slightly smaller than that obtained by Sugita et al. [5] but the two results are probably in agreement, considering the combined errors of the two studies.

Acknowledgements

This work was supported by a grant from the Army Research Office. Helpful advice by Prof. K.H. Jack, FRS in the early stages of this study is gratefully acknowledged.

06-

References

,,L,,i, 0

200”

,, 400

, ,i_J,i!j 600

800

T(C) Fig. 3. Magnetization-temperature plot for y-FeN 10.7 atoms of nitrogen per 100 atoms of Fe.

containing

[l] K.H. Jack, Proc. Roy. Sot. Al95 (1949) 41. [2] W.B. Pearson, Handbook of Lattice Spacings tures of Metals and Alloys, 1st ed. (MacMillan, 1958), pp. 984-987. [3] K.H. Jack, Proc. Roy. Sot. A208 (1951) 216. [4] K.H. Jack, Acta Cryst. 3 (1950) 392.

and StrucNew York,

230 [5] Y. Sugita, K. zono and M. [6] J.M.D. Coey, 171 H.A. Wriedt, letin of Alloy

M.Q. Huang et al. /Journal

of Magnetism and Magnetic Materials 135 (1994) 226-230

Mitsuoka, M. Komuro, H. Hoshiya, Y. KoHanazono, J. Appl. Phys. 70 (1991) 5977. private communication. N.A. Gokcen and R.H. Natzinger, The BulPhase Diagrams (1987) p. 1081.

[81 T. Takeshita, W.E. Wallace and R.S. Craig, J. Catalysis 44 (1976) 236. [9] B.D. Cullity, Elements of X-ray Diffraction (AddisonWesley, London, 1978) p. 411.