57Fe Mössbauer study of RTiFe11Nx compounds

Solid State Communications, Vol. 83, No. 7, pp. 487-490, 1992. Printed in Great Britain.

0038-1098/92 $5.00 -t- .00 Pergamon Press Ltd

57Fe MOSSBAUER S T U D Y OF RTiFeliNx C O M P O U N D S Senlin Ge, Qi Pan, Guoqiang Yang, Jingsong Zhang, Yue Yang, Wei Tao and Yingchang Yang Department of Physics, Peking University, Beijing 100871, P.R. China and Linshu Kong Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China

(Received 6 January 1992 by P. Burlet) The 57Fe M6ssbauer spectra of some nitrides of the type RTiFeNNx (R = Gd, Tb, Dy, Ho and Y) have been measured at room temperature. The magnetic behaviour of Fe atoms at different sites in RTiFe~INx have been investigated. The interstitial nitrogen atoms lead to an increase both in effective hyperfine fields and in isomer shifts. It is also shown that there are slight differences in effective hyperfine fields for the various RTiFeI~N x compounds. 1. I N T R O D U C T I O N

2. E X P E R I M E N T A L

T H E R A R E earth-iron intermetallic compounds of the type RTiFe~t, can form nitrides by a proper thermal treatment. It is found that the interstitial nitrogen atoms have an important effect on the magnetic properties of the RTiFeI~N x series. By nitrogenation the Curie temperature is increased by nearly 200K, and the saturation magnetization is also raised [1]. Moreover, the magnetocrystalline anisotropy has a radical change. For the rare earth element with the second Stevens coefficient ~j < 0, such as Nd, Tb, Dy and Ho, the R sublattice anisotropy changes from an easy plane to an easy c-axis, while the sublattice anisotropy of the rare earth element with ~j > 0, such as Sm and Er, changes from an easy c-axis to an easy plane [2]. The large anisotropy in combination with the high Curie temperature and saturation magnetization makes the R T i F e ~ N x series the promising candidates for permanent magnets [3]. Neutron diffraction measurements made on the RTiFe~ Nx compounds have revealed that the nitrogen atoms occupy the 2b interstitial sites next to rare earth atoms [4]. In previous investigation, it had shown that the Fe atoms are located on three different sites 8i, 8j and 8f, and that the Ti atoms are restricted to the 8i positions [5]. In order to know how the Fe atoms are affected by the nitrogen insertion and to determine the magnetic properties on an atomic scale in RTiFet~ Nx, we have studied these nitrides by means of M6ssbauer effect. In the present paper we report the result of 57Fe M6ssbauer spectroscopy performed on RTiFe~IN x compounds.

The samples were prepared by arc melting of 99.5% pure primary materials in a purified argon atmosphere. X-ray diffraction showed the samples to be single phase. The melt samples were finely ground and nitrided by passing purified nitrogen gas at atmosphere pressure at 500°C for 2 h. After this treatment, the samples were investigated again by X-ray diffraction, showing that they had reacted with the nitrogen gas to form the nitrides. The magnetization curves were measured at room temperature with fields up to 70 kOe. The 57Fe M6ssbauer spectra of the powder samples were obtained at room temperature using a constantacceleration spectrometer, which utilized a 57Co (Rh) source and was calibrated with s-iron. The Mfssbauer spectra were fitted by least-squares techniques with Lorentzian lines. 3. RESULTS A N D DISCUSSION The RTiFeH compounds crystallize in the bodycentered tetragonal ThMn~2-type structure, and retain the same structure after a nitrogenation. The lattice parameters and unit cell volumes are listed in Table 1. The relative changes in unit cell volume upon a nitrogenation A V/V are slightly different for various rare earth compounds. Because the the 8i site occupation of the Ti atoms leads to different nearest neighbour configurations for various Fe sites and the 2b interstitial sites are not completely occupied in RTiFelt N x, it is a little difficult to fit the M6ssbauer spectra. For simplicity, we fit them with three independent Lorentzian sextets which

487

488

57Fe MOSSBAUER STUDY O F RTiFetiNx C O M P O U N D S

(a)

Vol. 83, No. 7

(b)

t

VelociW(ram / s)

Velocity(ram / s)

(c)

(d) ~o*,°o°

""

o °o

"

?h

¢. •

7

•

....

•

V

I

I

Velocity (mm / s)

I

I

I

I

Velocity (mm / s)

Fig. i. M6ssbauer spectra obtained at room temperature: (a) YTiFe~Nx; (b) YTiFe~]; (c) GdTiFej~Nx; (d) GdTiFe]~; (e) TbTiFe~lNx; (f) DyTiFel] Nx; (g) H o T i F e , Nx.

Table 1. Lattice parameter a and c, unit cell volume V and relative change in unit cell volume upon a nitrogenation AV/V

Compound

a (A)

c (A)

V (A 3)

YTiFell YTiFet i Nx GdTiFelt GdTiFell Nx TbTiFetl TbTiFel i Nx DyTiFel] D y T i F e , Nx HoTiFet~ HoTiFet~Nx ErTiFejt ErTiFe] ] Nx

8.503 8.611 8.548 8.595 8.537 8.581 8.521 8.570 8.506 8.561 8.495 8.548

4.800 4.821 4.800 4.782 4.808 4.798 4.799 4.790 4.799 4.787 4.795 4.792

347.0 357.5 350.7 353.3 350.4 353.3 348.4 351.2 347.2 350.8 346.0 350. I

A V / V (%)

3 0.7 0.8 0.8 1.0 1.2

correspond to the three nonequivalent iron sites in the crystal structure. Relative intensities of the lines in each subspectrum were constrained to the ratio 3 : 2 : ! : 1 : 2 : 3, as is required for a randomly oriented powder sample. The outer, middle and inner lines of each take different widths. The order of the internal hyperfine fields is taken to be i > j > f. This assignment is consistent with previous M6ssbauer studies of the ThMnt2-type compounds. The 5VFe M/Sssbauer spectra and the results of the fitting procedure are shown in Fig. 1. The fitting parameters are summarized in Table 2. It is shown that there is little variation in hyperfine fields when the rare earth is varied. As compared with the parent compounds, the average hyperfine fields increase by about 29 kOe for GdTiFe~Nx and about 52kOe for VTiFe~Nx. The difference may be due to more interstitial nitrogen atoms entering in the yttrium compound than in the gadolinium compound.

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STFe MOSSBAUER STUDY OF RTiFetiNx COMPOUNDS

489

(f)

(e)

.s-

,

.

~..,

• ..

o.

•

I

,'s

I

Velocity (mm / s)

Velocity (mm / s)

(g)

st

I

I

I

Velocity(ram/ s) Fig. 1. Continued. For the Y-Fe series compounds, the average hyperfine fields can be generally converted to iron moments by means of a conversion factor. The average hyperfine field of 287.44 kOe and the iron moment per atom of 1.82 #s in YTiFe~ Nx would lead to a conversion factor of 158 kOe/#B, which is close to the value of 147 kOe/#s deduced by Gubbens et al. [6].

The iron moment in the other RTiFe, Nx compounds can be derived from the A values, shown in Table 2. The average isomer shifts increase by 0.0776 mm/s for YTiFe~ N x and 0.1085 mm/s for GdTiFel, Nx. It is known that the volume effect on the iron isomer shift AI.S./AIn V is typically 1.0mm/s for close-packed structure. So the increase value caused by the volume

Table 2. 57Fe MOssbauer effect spectra data of RTiFell Nx and RTiFelt H (kOe)

GdTiFe, Nx GdTiFetl TbTiFell Nx DyTiFe, Nx HoTiFell Nx YTiFe, Nx YTiFe,

H (kOe)

8i

8j

8f

319.25 268.23 323.32 323.37 307.32 327.09 260.67

282.48 253.56 288.43 288.05 291.44 297.99 238.61

247.72 234.28 240.34 239.85 243.92 247.17 212.8

279.87 250.55 280.46 280.15 278.49 287.44 235.22

I.S. (mm/s)

-

0.1603 0.0518 0.0352 0.0372 0.1167 0.0440 0.1216

#a (kOe)

1.77 1.59 1.77 1.77 1.76 1.82 1.49

57Fe MOSSBAUER STUDY OF RTiFetiNx COMPOUNDS

490

expansion is 0.0210mm/s for YTiFe~INx and 0.0075mm/s for GdTiFe.Nx. This means that the average isomer shifts in RTiFe. Nx still increase without the volume expansion. The isomer shift in a M6ssbauer experiment can be expressed as: I.S,

const Ap(O)AR/R,

(1)

where Ap(O) =

p(O)absor~r

- - p(O)absorber,

(2)

and AR/R = - 1.4

REFERENCES 1.

2. 3. 4.

× 10 -3

for 57Fe.

(3)

It is proportional to the total s-electron density at the nucleus, and increases with increasing 3d occupation. From the 57Fe M6ssbauer data it follows that the 3d electron density in RTiFe. N x increases. This is consistent with the fact from magnetic measurements that the interstitial nitrogen atoms raise the iron moment in RTiFettN~. The previous work had showed that the band structure of RTiFe~ compounds has the same feature as that of iron metal [7]. On adding electrons to the system which spin-up and spin-down 3d sub-band are unfilled, the moment is raised.

Vol. 83, No. 7

5.

6. 7.

Ying-chang Yang, Sen-lin Ge, Xiao-dong Zhang, Lin-shu Kong, Qi Pan, Yong-tian Hou, Shuang Huang & Liu Yang, Proc. llth Int. Workshop on Rare-Earth Magnets and their Application. Vol. 2, p. 190 (1990). Ying-chang Yang, Xiao-dong Zhang, Lin-shu Kong, Qi Pan & Sen-lin Ge, Appl. Phys. Lett. 58, 2042 (1991). Ying-chang Yang, Xiao-dong Zhang, Lin-shu Kong, Qi Pan & Sen-lin Ge, Solid State Commun. 78, 317 (1991). Ying-chang Yang, Xiao-dong Zhang, Lin-shu Kong, Qi Pan & Sen-lin Ge, Solid State Commun. 78, 313 (1991). Ying chang Yang, Hong Sun, Linshu Kong, Jilian Yang, Yongfan Ding, Baisheng Zhang, Chuntang Ye, Lan Jin & Huiming Zhou, J. Appl. Phys. 64, 5968 (1988). P. Gubbens, J. van Aperdorn, A. van der Kraan & K. Buschow, J. Phys. F4, 921 (1974). Ying-chang Yang, Sen-lin Ge, Xiao-dong Zhang, Lin-shu Kong, Qui Pan, Ji-lian Yang, Bai-sheng Zhang, Yong-fan Ding & Chun-Tang Ye, Science in China B2, 113 (1991).