Magnetic properties of GdxDy1−xFe11.35Nb0.65 compounds and their nitrides

Journal of

,~.LOYS

AND COMPOUNDS ELSEVIER

Journal of Alloys and Compounds 226 (1995) 55-58

Magnetic properties of GdxDyl-xFe11.35Nbo.65compounds and their nitrides Kai-Ying Wang a,:~, Yi-Zhong Wang b, Bo-Ping Hu b, Jifan Hu b, Ruwen Zhao a, Wu-Yan Lai a, Zhen-Xi Wang b "Magnetism Laboratory, Institute of Physics, ChineseAcademy of Sciences, P.O. Box 603, Beijing 100080, People's Republic of China b San Huan Research Laboratory, ChineseAcademy of Sciences, P.O. Box 603, Beijing 100080, People's Republic of China Received 21 December 1994

Abstract A new series of compounds, GdxDyl_xFell.35Nbo.65,with a ThMn~2 structure was synthesized. The corresponding nitrides prepared by gas solid reaction retain the structure of their parent alloys, but with an expansion of 3% of the unit cell volume. Their Curie temperature increases by about 200 K. For the alloys, the spin-reorientation transition temperature Tsrl decrease, while the Curie temperature and the magnetocrystalline anisotropy field increase with the substitution of Gd for Dy. Upon nitrogenation, the spin reorientation of the nitrides disappear, and the magnetocrystalline anisotropy field of the nitrides decrease with increasing Gd content. Keywords: Synthesis; Magnetic properties; Nitrides

1. Introduction Recently the search for new magnetic materials has focused on the ternary compounds RFe~z_~Vlx ( R = r a r e earth, M:---Ti, V, Cr, Mo, W, Si and Re) and their nitrides [ 1-4], which crystallize in the tetragonal ThMnt2 structure. In this structure, the R atoms occupy the crystallographic 2a sites and the Fe atoms occupy the 8i, 8f and 8j sites while elements M partially occupy 8i sites. In this compound, a set of crystal-field coefficients was derived from the analysis of magnetization measurements on a DyFellTi single crystal [5]. The spin reorientation transitions occur at low temperature as a result of anisotropy competition between the R and Fe sublattices having different temperature dependence. After nitrogenation, the interstitial nitrogen atoms enter into the 2b site interstitial holes [6] and modify the magnetocrystalline anisotropy of the nitrides strongly [2-4]. In a previous investigation [ 7 ] we found that the rare earth iron-rich intermetallic compounds RFeI2_~Mx can be stabilized by small amounts (0.5 < x <0.75) of a third element (M = Nb), according to the diagonal position relationship of the elements in the periodic table [ 8 ]. Spin reorientation was observed in the Dy and Er alloys [9]. However, our recent studies for the Y and Gd alloys and their nitrides show that * Corresponding author. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All fights reserved

SSDI0925.8388(95)01593-0

the Fe sublattices are weakened by the nitrogen atoms [7]. Thus a comparative study of the series GdxDyl _ ~Fe11.35Nbo.65 and their nitrides may be helpful for a better understanding of the magnetocrystalline anisotropy in these systems. In the present paper, we report on the formation and the results of magnetic measurements of the compounds GdxDyl_xFe11.35Nbo.65and their nitrides. We focus our study on the effect of the substitution of Gd for Dy on the magnetic properties of the alloys and their nitrides.

2. Experiment details The samples GdxDyl _xFel 1.35Nbo.6~(x = 0.2, 0.4, 0.6, 0.8) were prepared by arc melting of stoichiometric amounts of the constituent elements (99.9%) in a water boat under an atmosphere of argon. The alloys were melted several times to ensure homogeneity. The nitrogenation was performed by heating the Gd~Dyl _xFe11.35Nbo.65powder in a pure nitrogen atmosphere at 773 K for about 3 h. The aligned samples were prepared by mixing the sample with epoxy resin, and aligning in a field of IT. X-ray diffraction measurements on powder samples were performed using Co Ka to determine the crystal structure. The thermomagnetic analysis was made on a vibrating sample magnetometer in a field of Bo = 0.04 T. The anisotropy field

K.-Y. Wang et al. /Journal of Alloys and Compounds 226 (1995) 55-58

56

Table 1 Structural data and magnetic properties of compounds Gd=Dy~_~Fe~~.35Nbo.6sand Gd,Dy~ _=Fel L35Nbo.65Ny Compounds

a (nm)

c (nm)

V (nm 3)

DyFeH.35Nbo.65 DyFe~l.35Nbo.65Nr Gdo.2Dyo.sFel L35Nbo.65 Gdo.2Dyo.sFelL35Nbo.65Ny Gdo.4Dyo.6FeH.3sNbo.65 Gdo.,,Dyo.6FeH.35Nbo.65Ny Gdo.6Dyo.4FeH.35Nbo.65 Gdo.6Dyo.4FeH.35Nbo.65Ny Gdo.sDyo.2FeH.35Nbo.65 Gdo.sDyo.2FeH.35Nbo.65Ny

0.8510 0.8624 0.8527 0.8661 0.8533 0.8658 0.8538 0.8659 0.8555 0.8656

0.4787 0.4817 0.4796 0.4823 0.4799 0.4837 0.4803 0.4844 0.4803 0.4855

0.3467 0.3582 0.3487 0.3618 0.3494 0.3626 0.3501 0.3632 0.3515 0.3638

T~ (K)

T~,I (K)

525 726 544 733 554 737 567 744 576 751

3.2 3.6 3.6 3.6 3.5

Ba (T)

235

[9] [9] 1.90 13.57 2.25 11.88 2.69 9.98 3.15 8.88

190 163 113 < 77

nitrides which contain small amounts of a-Fe. The alloys crystallize in the ThMn12-type structure. The R atoms occupy the 2a sites and the Fe alone occupy the 8f, 8i, 8j sites while the Nb atoms occupy partially the 8i sites [ 10]. The lattice parameters a, c and unit cell V, together with A V/V% are listed in Table 1. Typical X-ray diffraction patterns of Gdo.6Dyo.aFe]l.35Nbo.65 and its nitride are shown Fig. 1. It is clearly seen that the nitride retains its original tetragonal structure, but the diffraction peak positions shift to lower angles indicating lattice expansion. From Table 1, it can be observed that the lattice parameters also increase with the substitution of Gd for Dy for both the alloys and their nitrides. The increase of the lattice parameters is owing to the radius of the Gd atom being larger than that of the Dy atom. Fig. 2 shows the thermomagnetic curves of the compounds GdxDyl -xFel 1.35Nbo.65from 77 K to room temperature in the field of Bo = 0.04 T. The values of the spin-reorientation transition temperature are presented in Table 1. The spin reorien-

Co-K~

""06DY02Fo,,.3.,NbO. =~,., ~t ~

,ij g

e,i

A V/V%

v

eem

Gd0.6DY0.2Fel 1.35Nb0.65 Ny

GdxDYI'xFel1"35Nb0"65

~ Tsrl

X=0.2J X--0.4 "2 30

40

50

~ Tsrl

X=0.6

60

20 (degree)

I,,I

c~

Fig. 1. X-ray diffraction patterns of the Gdo.6Dyo.4FeH.35Nbo.65 and Gdo.6Dyo.4Fell.35Nbo.65Ny compounds.

X=O'8~ . ~

was derived from the magnetization curves parallel (Mii) and perpendicular (M±) to the alignment direction and by using the singular point detection technique (SPD). The nitrogen content was determined by chemical analysis. The absorbed nitrogen is about 1 N per formula for all the nitrides studied.

B0=O.04T 0

3. Results and discussion Based on X-ray diffraction and thermomagnetic analysis, all the samples are found to be single phase except for their

I

I

50

100

I , 150

I

I

200

250

30q

T (K) Fig. 2. The thermomagnetic curves of compounds GdxDy~-xFell.3sNbo.65 (x= 0.2, 0.4, 0.6, 0.8) from 77-300 K.

57

K.-Y. Wang et al. / Journal of Alloys and Compounds 226 (1995) 55-58

GdxDY! -xFel 1.35Nb0.65

X=0.8

[4].

X=0.6

=

X=0.4

c~

K=0.2

B0=0.04 T

L

K when the Dy atoms are replaced by Gd atoms at a rate of 0.2 atom f.u.- J. After nitrogenation, the Curie temperatures of the nitrides are about 200 K higher than those of the parent alloys, as observed also in RFeloV2N~. [2] and RFel]TiNy

I

I

I

I

300

400

500

600

T (K) Fig. 3. Thermomagneticcurves of the compoundsGdxDyl_xFelL35Nbo.65 (x = 0.2, 0.4, 0.6, 0.8) in the high temperaturerange. tations, which are deduced from the anomalous temperature variation of the thermomagnetic curves, are observed for all the samples in the GdxDy~ _xFel 1.35Nbo.65 series. It is clearly seen that the spin-reorientation transition temperature Tsr~ is shifted from Tsr 1 = 190 K for the x = 0.2 compound towards the low temperature range. For the x = 0.8 compounds, Tsr~ will be below 77 K. The decrease of Tsr can be understood from the contribution of the Dy sublattice to the magnetocrystalline anisotropy, which is gradually decreased with the substitution of Gd for Dy atoms. In addition, it can be observed in Fig. 2 that there also exists another cusp at 100 K for x--: 0.2 and at 140 K for x = 0.4. For the cusp of the x = 0.2 compound at 100 K, we assume that it corresponds to the easy :magnetization direction from the canted type to the planar type, as seen in DyFell.35Nbo.65 [9]. In the x = 0 . 2 compound it is also lower than the transition temperature (T~r2) of the latter compound. But we cannot explain the cusp at 140 K for the x = 0.4 compound. The spin reorientations have disappeared upon nitrogenation for all samples. The Curie temperature of the alloys GdxDyl - xFel L35Nbo.65 and their nitAdes are listed in Table 1. Fig. 3 shows typical thermomagnetic curves of these alloys. The sharp drop to the cr-Fe contribution corresponds to the Curie temperature. It can be seen that the Curie temperatures increase by about 10

The magnetocrystalline anisotropy fields of the alloys and their nitrides at room temperature were determined by extrapolating AM=M:M± versus Bo to zero and by the SPD technique. Fig. 4 shows the concentration dependence of the magnetocrystalline anisotropy of the alloys GdxDy~_xFell.35Nb0.65 and their nitrides. It can be seen that the magnetocrystalline anisotropy fields of the nitrides are obviously higher than those of the parent alloys. For the parent alloys, the magnetocrystalline anisotropy fields increase with substitution of Gd for Dy atoms. On the contrary, the magnetocrystalline field of the nitrides monotonically decreases from x = 0.2 to x = 0.8. The reason for the variation of the magnetocrystalline anisotropy fields of the alloys series is similar to that of the decrease of the spin reorientation transition temperature. It is due to the anisotropy of the Fe sublattice which becomes gradually dominant. After nitrogenation, the second-order coefficient A2o of the Dy sublattice changes from negative to positive, and its absolute value increases [4]. This enhances strongly the anisotropy field of the Dy sublattice. Thus the decreasing tendency of the anisotropy field from x = 0.2 to x = 0.4 for the nitrides only originates from the dilution of the Dy 3+ ions and the decrease of the anisotropy field of the Fe sublattice. In conclusion, we synthesized a new series of ThMn~2-type compounds having the composition GdxDyl _xFel t.35Nbo.65. The corresponding nitrides retain the structure of their parent alloy, but with an expansion of the unit cell volume by 3.0%.

14

12 10 A

Ira m

G

d

x

D

Y

l

~

8

6

GdxDyl-xFel1.35Nb0.65 2 0

0.0

I 0.2

I 0.4

I

I

0.6

0.8

1.0

X Fig.4. The Gd concentrationdependenceof the roomtemperatureanisotropy fieldsof GdxDy]-~Fell.35Nbo.65and Gd~Dyl-xFell.35Nbo.65Ny(x = 0.2, 0.4, 0.6, 0.8).

58

K.-Y. Wang et al./ Journal of AUoys and Compounds 226 (1995) 55-58

For the parent alloys, the spin-reorientation temperatures Tsrl decrease, whereas the magnetocrystalline anisotropy fields and Curie temperatures increase. After nitrogenation, the spin reorientations have disappeared, and the Curie temperatures have increased by about 200 K. The magnetocrystalline anisotropy fields gradually decrease from x = 0.2 to x = 0.8.

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[3] M. Anagnostu, C. Christides and D. Niarchos, Solid State Commun., 78 (1991) 681. [4] Y.C. Yang, X.D. Zhang, S.L. Ge, Q. Pan, L.S. Kong, J.L. Yang and C.T. Ye, J. Appl. Phys., 70 (1991) 6001. [5] B.P. Hu, H.-S. Li, J.M.D. Coey and J.P. Gavigan, Phys. Rev., B41 (1990) 2221. [6] Y.-Z. Wang, G.C. Hadjipanaysis, Z.X. Tang, W.B. Yeion, V. Papaefthymious, A. Moukarika and D.J. Sellmyer, J. Magn. Magn. Mater., 119 (1993) 41. [7] K.Y. Wang, Y.-Z. Wang, B.-P. Hu, J. Hu, W.-Y. Lai and Z.-X. Wang, J. Alloys Comp., in press. [8] G.N. Gilmore, A Modern Approach to Comprehensive Chemistry, 2nd edn., Stanly Thornes, 1979, p. 179. [9] K.-Y. Wang, Y.-Z. Wang, B.-P. Hu and W.-Y. Lai, IEEE Trans. Magn., 30 (1994) 4963. [lO] B.-P. Hu, K.-Y. Wang, Y.-Z. Wang, W.-Y. Lai and Z.-X. Wang, Phys. Rev. B, 51 (1995) 2905.