Structural and magnetic properties of (Sm1−xPrx)3Fe27.5Ti1.5 [x=0.2, 0.5, 0.8, 1.0] and their nitrides

Journal of Alloys and Compounds 352 (2003) 6–15

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Structural and magnetic properties of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 [x50.2, 0.5, 0.8, 1.0] and their nitrides a a, a b b b V.R. Shah , G. Markandeyulu *, K.V.S. Rama Rao , M.Q. Huang , K. Sirisha , M.E. McHenry a

Magnetism and Magnetic Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India b Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 -3890, USA Received 20 August 2002; received in revised form 23 September 2002; accepted 23 September 2002

Abstract (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 [x50.2, 0.5, 0.8, 1.0] compounds belonging to the novel 3:29 class have been prepared in order to investigate the effects of the combination of rare earths having opposite signs of second-order Stevens coefficient on their structural and magnetic properties. The amounts of different phases have been estimated from the Rietveld analysis of powder X-ray diffractograms. The changes in the easy magnetization direction in these compounds are explained as the resultant of two competing anisotropies from the rare earths occupying the two crystallographically inequivalent sites, 2a and 4i. Nitrogenation has increased the saturation magnetization and Curie temperature which could be due to magnetovolume effect and the changes in the densities of states in the 3d band. Changes in the easy 57 ¨ magnetization direction upon nitrogenation are due to the modification of rare earth anisotropy. Fe Mossbauer spectra have been analyzed in a Wigner–Seitz cell approach.  2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Permanent magnets; Gas–solid reaction; Magnetic measurements

1. Introduction The new class of R (rare earth)–TM (transition metal) intermetallics namely, R 3 (Fe,M) 29 (3:29 compounds) are of current interest in the field of permanent magnet materials [1,2]. These compounds crystallize in monoclinic symmetry with A2 /m space group [3,4] with two crystallographically inequivalent R sites and 11 inequivalent Fe sites. This structure has been shown to be derivable from the RTM 5 -unit (CaCu 5 -type, hexagonal) by replacing a fraction of rare earths by a pair of Fe atoms as in the case of well studied R 2 Fe 17 (2:17 compounds) and RFe 11 Ti (1:12 compounds). Due to this structural relationship, the 3:29 phase is formed by alternative stacking of 2:17 and 1:12 type segments [5]. The two inequivalent R sites, viz. 2a and 4i have 1:12 and 2:17 like environments, respectively [6]. For a systematic study of the structural and magnetic properties of 3:29 compounds, both from the fundamental and application point of view, we have initiated three *Corresponding author. Tel.: 191-44-257-8677; fax: 191-44-2570509 / 257-0545. E-mail address: [email protected] (G. Markandeyulu).

series of compounds. Co substitution for Fe for the first time in any 3:29 compound, has been effected in the series Pr 3 (Fe 12x Co x ) 27.5 Ti 1.5 , and single phase compounds were obtained [7]. The Co and Ti occupancies and the quantities of the different phases present were determined from neutron diffraction and EXAFS studies [8]. Co substitution resulted in a considerable increase in Curie temperature. The rare earth sublattice anisotropy is found to dominate over the Fe sublattice anisotropy at room temperature and the EMD is found to be along the b-axis for Pr 3 (Fe 12x Co x ) 27.5 Ti 1.5 and away from the b-axis for other concentrations [9,10]. The nature of the Fe sublattice anisotropy and the strength of the different exchange interactions have been investigated in the series (Y 12x Gd x ) 3 Fe 27.5 Ti 1.5 [11]. The Fe sublattice anisotropy has planar nature with the EMD lying in the a–b plane of the CaCu 5 -type unit. Li et al. [12] have predicted that, in 3:29 compounds, the sign of the dominant crystal field coefficient A 02 is different at the two inequivalent rare earth sites, being positive at the 2a site and negative at the 4i site. As the anisotropy constant K1 is proportional to A 02 and the second-order Stevens coefficient (aJ ), it was suggested that, if two rare earths with opposite signs of aJ preferentially occupy these

0925-8388 / 03 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01090-3

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

sites, the rare earth anisotropy could increase. Following this, we have chosen Pr (aJ ,0) and Sm (aJ .0) and carried out detailed structural and magnetic studies in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and the results are presented in this paper. In addition, the influence of interstitial modification by nitrogen on the structural and magnetic properties of these compounds has been investigated.

2. Experimental details The alloys were prepared by arc-melting the constituent elements several times in high pure argon atmosphere and homogenizing in the temperature range 1050–11008C for 3 days and subsequent quenching in ice–water mixture. The nitrides of the samples were prepared by heating fine powders in high pure nitrogen atmosphere of 2.5 bars at 5008C for 5 h, using a nitrogenation facility developed by Suresh and Rama Rao [13]. The nitrogen content was estimated from the increase in the weight after nitrogenation. Structural characterization was carried out by the Rietveld analysis of powder X-ray diffraction patterns taken using Fe Ka radiation in step scan mode using a computer program DBWS9411 [14]. The Curie temperatures were determined from the derivatives of M versus T plots, from the data obtained using a VSM in the temperature range 300–900 K and in a field of 400 Oe. Magnetically aligned samples were prepared by arresting fine powder (,20 mm) in a fast drying epoxy resin in a magnetic field of 2.5 T. Magnetization measurements were carried out up to 5 T using a Quantum Design (MPMS) ¨ SQUID magnetometer. 57 Fe Mossbauer spectra were re-

7

¨ corded at 300 K, using a FAST Comtec Mossbauer spectrometer.

3. Results and discussion

3.1. Structural analysis The results of the Rietveld refinement for (Sm,Pr) 3 Fe 27.5 Ti 1.5 compounds (parent compounds) are shown in Fig. 1a. The compounds with x50.5, 0.8, 1.0 have formed in monoclinic Nd 3 (Fe,Ti) 29 -type crystal structure having A2 /m space group. Traces of a-Fe were detected in the case of x50.5 and 1.0 compounds. In the case of the x50.2 compound, an impurity of |30 wt.% of the 1:12 phase was observed. The lattice parameters of the compounds are given in Table 1. The XRD patterns of the (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y compounds are shown in Fig. 1b. The compounds retain the monoclinic crystal structure after nitrogenation with unit cell volume expansions ranging from 5.4 to 6.0% as shown in Table 1.

3.2. Magnetization The magnetization measurements on powder samples of the parent compounds and their nitrides have been carried out at 10 and 300 K and are shown in Fig. 2. At 10 K, the magnetization for the x50.5 compound tends to saturate at 5 T, whereas for the x51.0 compound it does not saturate. This is a consequence of the larger magnetic moment on Pr compared to Sm along with the ferromagnetic coupling of

Fig. 1. X-ray diffractograms of (a) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and (b) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y . The observed (dots), calculated (line) and difference patterns obtained from the Rietveld analysis of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 are shown.

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V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

Table 1 Lattice parameters, unit cell volume and Curie temperature in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and their nitrides Compound

(Sm 0.8 Pr 0.2 ) 3 Fe 27.5 Ti 1.5 (Sm 0.8 Pr 0.2 ) 3 Fe 27.5 Ti 1.5 N 5 (Sm 0.5 Pr 0.5 ) 3 Fe 27.5 Ti 1.5 (Sm 0.5 Pr 0.5 ) 3 Fe 27.5 Ti 1.5 N 4.4 (Sm 0.2 Pr 0.8 ) 3 Fe 27.5 Ti 1.5 (Sm 0.2 Pr 0.8 ) 3 Fe 27.5 Ti 1.5 N 4.3 Pr 3 Fe 27.5 Ti 1.5 Pr 3 Fe 27.5 Ti 1.5 N 5

Unit cell parameters a ˚ (A)

b ˚ (A)

c ˚ (A)

b (8)

10.637 10.872 10.641 10.883 10.659 10.897 10.647 10.911

8.573 8.736 8.558 8.748 8.605 8.757 8.600 8.765

9.784 9.917 9.749 9.913 9.764 9.912 9.755 9.918

96.957 97.57 96.926 97.58 96.895 97.71 96.913 97.72

the Sm / Pr and Fe sublattices. However, at 300 K, the magnetization of both the compounds saturates maintaining their relative values as at 10 K. The magnetization in the nitrides increased with respect to their parent compounds. The basic mechanism responsible for the increase is the enhancement in the Fe magnetic moment due to the localization of 3d moments (as a consequence of unit cell

Volume ˚ 3 (A)

DV /V (after nitrogenation) (%)

TC (K)

882 934 884 935 889 937 887 940

5.9

462 741 443 727 409 716 393 711

5.8 5.4 6.0

volume expansion) and the volume dependence of the magnetization [15].

3.3. Curie temperature The Curie temperature of the parent compounds and their nitrides are tabulated in Table 1. The Curie temperature decreases with increase in Pr concentration in both parent and nitride compounds which can be understood, in mean field approach [16], as due to the reduction in the contribution from Sm / Pr–Fe, Sm / Pr–Sm / Pr exchanges with increase in Pr concentration. These contributions are proportional to the de-Gennes factor [( g 2 1)2 hJ(J 1 1)j] of the respective rare earth involved and this factor is small for Pr compared to Sm. There is a considerable increase in the Curie temperatures (by |300 K) of parent compounds after interstitial modification by nitrogen. This is explained as being due to the magnetovolume effect and associated changes in the 3d band structure. According to the spin fluctuation theory [17], the Curie temperature of a R–Fe compound is proportional to M0 /x0 , where M0 is the zero temperature magnetic moment and x0 the exchange susceptibility given by,

F

G

1 1 1 x 21 0 5 ]]] 1 ]] 2 I ]] 2N↑ (EF ) N↓ (EF ) 2m B2

Fig. 2. Magnetization curves of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y at (a) 10 K and (b) 300 K.

and

(1)

where, N↑ (EF ) and N↓ (EF ) are the densities of states (DOS) at the Fermi level. Jaswal et al. [18], from band structure calculations, have shown that there is reduction in the DOS at the Fermi level after nitrogenation. This together with the increase in Fe moment explains the large increase in Curie temperature. The relationship between the volume (V ) of the unit cell and Curie temperature (T C ) of R–TM intermetallics has been investigated [19,20]. A combined model of localized and itinerant d electrons was assumed and it was suggested that only a fraction of 5% or less of the 3d electrons are truly itinerant while the rest can be considered as localized. Since the interstitial C and N expand the unit cell of the parent compounds, the changes in T C upon interstitial

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

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modification are associated with the volume changes. Studies on the Curie temperature on the structurally related 2:17 nitrides and carbides [19,20] have shown a linear dependence of G on T C , where G was evaluated as T C( y) 2 T C( 0) ]]]] T C( 0) d ln T C G 5 ]] 5 ]]]] d ln V Vy 2V0 ]] V0

(2)

and y is the interstitial element concentration. Valleanu et al. [21] have studied the dependence of G on the 2:17 interstitial carbides and nitrides and found that for low carbon concentrations it has a T 22 C dependence and for high carbon / nitrogen concentrations ($2), it has a linear dependence on T C . In (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 compounds, the variation of G with Curie temperature is shown in Fig. 3. It is observed that G shows a linear dependence of T C of the form G 541.05–0.068 T C which is similar to that reported for 2:17 carbides and nitrides [21,22] and is in good agreement with the combined model of localized and itinerant d electrons.

Fig. 4. The two inequivalent rare earth atoms with 2:17 (R) and 1:12 (T) like environments. The three principal axes are shown.

3.4. Easy magnetization direction ( EMD) The magnetocrystalline anisotropy and hence the EMD of R 3 (Fe,M) 29 compounds is decided by the nature of R

Fig. 3. G versus T C plots of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y .

and the site it occupies. The local environments of the two crystallographically inequivalent rare earth sites 2a and 4i are shown in Fig. 4 [6]. The R at 2a site has a 1:12 like environment and that at 4i has a 2:17 like environment. The three principal axes, viz. c r (rhombohedral (2:17) c-axis), c t (tetragonal (1:12) c-axis) and b m (monoclinic (3:29) b-axis) are also shown in the figure. At the 2a site (1:12 like), the anisotropy is axial with EMD along c t if R has negative aJ [23]. At the 4i site (2:17 like), the anisotropy is axial (very weak) with EMD along c t if R has positive aJ and is planar with EMD in a plane perpendicular to c t if R has negative aJ [24]. The nature of magnetocrystalline anisotropy was determined from the XRD of magnetically aligned samples. XRD of magnetically aligned (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 compounds are shown in Fig. 5a. In the Pr-rich compounds (x50.8 and 1.0) the EMD has a strong preference for the b-axis, as seen from the enhancement of the relative intensity of the (0 4 0) reflection. For the x50.2 and 0.5 compounds, a strong preference of the EMD towards (4 0 ] 2) is seen. Yang et al. [25] reported such a preferential direction in Sm 3 (Fe,Ti) 29 compound as well. These results can be understood as being due to the net effect of the anisotropies from the two inequivalent R sites as discussed below. Since Sm has positive aJ , the strong axial anisotropy of the 2a site (along c t ) dominates over the weak axial anisotropy at the 4i site and therefore, the resultant EMD ] will be along c t . It has been reported that (4 0 2) (monoclinic) corresponds to c t [26]. A similar behaviour is expected from the Sm-rich [x,0.5] compounds too. The ] strong preference of (4 0 2) in the case of the x50.5

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

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Fig. 5. X-ray diffractograms of the magnetically aligned (a) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and (b) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y compounds.

compound (Fig. 5a) suggests that the anisotropy at the 2a site dominates over that at the 4i site. When R5Pr (negative aJ ), the rare earth anisotropies at 2a and 4i sites are perpendicular to c t and c r , respectively. Since the b-axis (b m ) is perpendicular to both c t and c r , the EMD becomes the b-axis, resulting in a large enhancement of the (0 k 0) reflections of the magnetically aligned samples. Similar is the case for the x50.8 compound as ] well. A weak reflection (2 3 1) is also seen in these compounds after magnetic alignment. This can be taken as an indication that the Fe sublattice anisotropy has planar nature [11]. It is of interest to compare the XRD of magnetically aligned 3:29 compounds of R5Sm, Pr, with those of their 1:12 and 2:17 counterparts. The relative intensity of (0 0 1) 1:12 reflection is enhanced after magnetic alignment in Sm(Fe,Ti) 12 compounds [24]. However, in Sm 2 Fe 17 , even though the R anisotropy favours c t , it is weak compared to the planar Fe h

34 3 5

k l

h

2:17

l

3 2] 5 2 ] 5

0 2 ] 5 5 2 ] 1:12 5

34 3 k

3 ] 5

1 ] 2 1 ] 2 0

3 2] 10 3 ] 10 4 ] 5

1

0 4 0 ] 5

0

1 2] 5

h

43 4

(3)

k l

3:29

h

43 4 k

l

(4) 3:29

sublattice anisotropy and this results in the enhancement of the (h k 0) 2:17 reflections [24]. Han et al. [27] have obtained the interrelation between the Miller indices in 1:12, 2:17 and 3:29 setting by the transformation matrices. ] ] Using the matrices (2 3 1) 3:29 , (0 4 0) 3:29 and (4 0 2) 3:29 ] transform to (3 0 0) 2:17 , (2 2 0) 2:17 and (3 3 0) 2:17 and (3 0 1) 1:12 , (4 0 0) 1:12 , and (0 0 2) 1:12 , respectively. Fig. 5b shows the XRD of magnetically aligned ] ] (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y . The (4 0 2) and (3 0 4) reflections are strong, whereas, for the x50.8 compound, ] the (4 0 2) reflection is the most prominent one. For the x50.5 and 0.2 compounds, the EMD is along (2 0 4). The interstitial nitrogen influences the rare earth anisotropy significantly. Fig. 4 shows the nitrogen near neighbours of the two inequivalent rare earths. The two nitrogen near neighbours of R at the 2a site are situated along c t , whereas the three nitrogen near neighbours of R at 4i site are situated in a plane perpendicular to c t . Drawing an analogy from the case of the nitrides of the 1:12 compounds, it can be said that, for the rare earth at 2a site (1:12 like), the anisotropy becomes planar (perpendicular to c t ) upon nitrogenation for R having positive aJ [24]. Similarly at the 4i site (2:17 like), the axial anisotropy is enhanced upon nitrogenation and favours c t for rare earths having positive aJ and remains planar (plane perpendicular to c t ) for R having negative aJ [24]. In the case of Pr-rich compounds (x51.0 and 0.8), the anisotropy at the 2a site becomes axial upon nitrogenation and the EMD is along c t . The anisotropy at the 4i site remains planar resulting in the enhancement of the intensi] ty of the (4 0 2) reflection after magnetic alignment. For

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

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Sm-rich compounds, the anisotropy at the 4i site becomes strongly uniaxial (favouring c r ), whereas that at the 2a site becomes planar after nitrogenation. It has been reported that c r corresponds to (2 0 4) [24]. For the x50.5 compound, the EMD shifts to almost along (2 0 4). Since out of the six R atoms, four occupy the 4i site and two occupy the 2a site, the change in EMD at x50.5 probably would have originated from the preferential occupation of Sm atoms at the 4i site. For x50.2, the intensity of the (2 0 4) reflection is further enhanced. The first-order anisotropy constant for the rare earth is given by [28] 3 K1 (R) 5 2 ]kr 24f lk3J 2Z 2 J(J 1 1)laJ A 02 2

(5)

where, r 4f is the radius of the 4f orbital and J and JZ are the total and z components of the angular momentum vector and the terms within k l represent the thermal averages of the respective quantities. A preferential substitution of Sm (aJ positive) at the 4i site (A 02 negative) and Pr (aJ negative) at the 2a site (A 02 positive), makes the product (aJ A 20 ) negative and makes the respective rare earth anisotropy uniaxial. However, since (2 0 4) and (4 0 ] 2) directions are mutually perpendicular, the resultant anisotropy is determined by the prominent one among the two. This nature of magnetocrystalline anisotropy can also be seen from the magnetization of the magnetically oriented samples, along and perpendicular to the alignment directions. Fig. 6a,b shows the magnetization on oriented samples of x50.5 and 1.0 and their nitrides at 10 K. A large anisotropy in the x51.0 compound is seen compared to the x50.5 compound. This arises due to the competing anisotropies as explained earlier. While a first order magnetization process (FOMP) is seen in the x51.0 compound, it is absent in the x50.5 compound. Fig. 7 shows the temperature variation of x 9 in the temperature range 13–300 K for all the samples. In the Pr 3 Fe 27.5 Ti 1.5 compounds, two broad transitions, one at 280 K and other at 152 K, were observed, whereas, in the (Sm 0.2 Pr 0.8 ) 3 Fe 27.5 Ti 1.5 compound, a sharp transition around 180 K was observed. However, no transition is observed in the case of x50.5 compound. In Nd 3 (Fe,Ti) 29 compounds, Pareti et al. [29] from magnetization measurements, Kalogirou et al. [30] from ac magnetic susceptibility measurements and Morelon et al. [31] from the temperature variation of linear thermal expansion coefficient and ac magnetic susceptibility, have reported two transitions, one around 230 K and the other around 150 K. Pareti et al. [29] have attributed the transition around 233 K to be due to a spin reorientation from the monoclinic a-axis (at room temperature) to a direction away from the a-axis below 233 K. In Tb 3 (Fe,Ti) 29 , Ibarra et al. [32] have observed similar transitions around 250 and 150 K from ac susceptibility measurements. In the Pr 3 (Fe,Ti) 29 and (Sm 0.2 Pr 0.8 ) 3 Fe 27.5 Ti 1.5 compounds, the EMD is

Fig. 6. M versus H plots of magnetically aligned (a) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 (x50.5 and 1.0) and (b) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y (x50.5 and 1.0) compounds at 10 K.

almost along the b-axis at room temperature. Therefore, the transitions observed, respectively, at 280 and 180 K, correspond to a spin reorientation indicating that the EMD may bend away from the b-axis below these temperatures. In the case of the x50.5 and 0.2 compounds, no transitions ] were observed suggesting that the EMD is along (4 0 2) in the entire temperature range investigated. The transition observed in Pr 3 Fe 27.5 Ti 1.5 at 153 K could be due to the formation of another magnetic phase. Kalogirou et al. [30] have attributed the transition at 150 K in Nd 3 Fe 27.5 Ti 1.5 , to domain wall motion. Asti [33], in his review on FOMP, has linked the domain wall motion to FOMP in uniaxial systems. In Nd 3 Fe 27.5 Ti 1.5 , in fact, a FOMP below 155 K and extending down to |10 K has been reported by Pareti et al. [29]. In Pr 3 Fe 27.5 Ti 1.5 too, a FOMP has been observed at 10 K [10], from magnetization measurements. Therefore, it may be inferred that in Pr 3 Fe 27.5 Ti 1.5 , a FOMP may be present up to a tempera-

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

12

ture of 152 K. This transition was not observed in the case of the x50.2, 0.5 and 0.8 compounds.

3.5.

Fig. 7. Temperature variation of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 compounds.

ac

susceptibility

( x 9)

of

57

¨ Fe Mossbauer studies

¨ The 57 Fe Mossbauer spectra of (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and their nitrides at 300 K and are shown in Fig. 8a,b, respectively. All the spectra were fitted with four sextets for the 3:29 phase (based on a Wigner–Seitz cell analysis), three sextets for 1:12 phase and one sextet for a-Fe wherever necessary. The intensity ratio of the four sextets has been constrained to 1:1.8:2.2:1.3, based on the neutron diffraction result of Pr 3 Fe 27.5 Ti 1.5 [8]. The same number of sextets was employed for both the parent compounds and the nitrides. The Wigner–Seitz (WS) cell volumes were computed using a computer code BLOKJE [34]. The number of sextets for 3:29 phase was derived from the number of near neighbours of the Fe atoms and their average bond lengths. The atomic positions are designated following Kalogirou et al. [3]. The 11 inequivalent Fe atoms can be grouped into four with Fe 3 , Fe 6 and Fe 7 in group 1, Fe 1 , Fe 2 and Fe 11 in group 2, Fe 4 , Fe 8 and Fe 9 in group 3 and Fe 5 , Fe 10 in group 4, respectively, having 13, 10, 10 and 9 near neighbours (NN). Groups 2 and 3 atoms have the same NN. In the case of the parent compounds, a direct correlation between the average hyperfine field (Hhf ) and the NN is found to exist. The hyperfine fields of the different groups can be arranged in the following sequence: Hhf (group 1).Hhf (group 2).Hhf (group 3).Hhf (group 4). The Hhf at room temperature, of each of the groups and their weighted average as well, decrease with increasing Pr

¨ Fig. 8. Mossbauer spectra of (a) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and (b) (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y compounds at 300 K.

V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

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Table 2 57 Fe hyperfine fields and the weighted averages in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y at 300 K Group

Hyperfine field (kOe) x50.2

1 2 3 4 Weighted average

x50.5

x50.8

x51.0

y50

y55

y50

y54.4

y50

y54.3

y51.0

y55

264 240 213 200 226

279 326 337 280 313

254 222 203 190 214

296 323 326 265 308

252 211 192 177 204

270 327 324 280 307

247 209 188 176 201

262 323 320 274 302

concentration, as shown in Table 2. This decrease could be due to (i) the decrease in Curie temperature upon Pr substitution and more importantly, (ii) a reduction in the Fermi contact field with increasing Pr content. When Pr is substituted for Sm, there will be a weakening of the 4f–3d coupling, owing to the smaller value of ( gR 21)J of Pr compared to Sm, which in turn reduces the polarization of s-electrons at the Fe nucleus. A similar behaviour has been reported in (Sm 12x Pr x ) 2 Fe 17 compounds [35]. A large increase in the hyperfine field is observed in the nitrogenated compounds compared to their parent counterparts (Table 2), illustrating the large enhancement in the Fe magnetic moment and Curie temperature after interstitial modification by nitrogen. However, the direct correlation between the Hhf and the near neighbours defined by WS cell volume is not observed in the nitrogenated compounds. The hyperfine fields for the groups 2 and 3 are found to be larger compared to those of groups 1 and 4 of the nitrides. Hu et al. [4] have discussed the structural properties of Nd 3 (Fe,Ti) 29 and its nitride from neutron diffraction studies and reported that the unit cell undergoes an anisotropic expansion upon nitrogenation. The WS vol-

umes and the average bond lengths, associated with the different atoms, before and after nitrogenation are examined from their data and are tabulated in Table 3. A very similar expansion of the unit cell is observed in the present compounds too and therefore, the structural details of the neutron diffraction results are extended to the present compounds. An overall increase in WS volume is observed upon nitrogenation (although in two cases, viz. Fe 6 and Fe 4 it has become reduced, the reason for which is not apparent). The change in WS volume is found to be high for the group 4 atoms followed by a substantial variation in the average bond length. This together with the smaller number of NN can explain the observed low value of Hhf of group 4. However, the observed low values of the hyperfine field in group 1 atoms are less obvious. The highest NN and only a marginal increase in the average bond length cannot explain the observed low values of hyperfine fields. One possible reason could be the following: group 1 contains dumb bell Fe–Fe bonds, some of which are very short and can result in large values of Hhf at these sites. After interstitial modification, these bonds become dilated as a result of the unit cell expansion and

Table 3 Average bond lengths (kBLl) and Wigner–Seitz (WS) cell volume before and after nitrogenation for Nd 3 (Fe,Ti) 29 (calculated from the neutron diffraction data [4]) Group

Atoms

Before nitrogenation

After nitrogenation

kBLl ˚ (A)

WS volume ˚ 3 (A)

kBLl ˚ (A)

WS volume ˚ 3 (A)

% Change in kBLl

% Change in WS volume

1

Fe 3 Fe 6 Fe 7

2.706 2.713 2.716

12.79 12.86 12.73

2.728 2.748 2.740

13.12 12.71 13.39

10.8 11.1 10.9

12.6 21.2 15.2

2

Fe 1 Fe 2 Fe 11

2.501 2.515 2.510

11.54 11.43 11.44

2.535 2.567 2.558

11.72 12.18 12.01

11.4 12.1 11.9

11.6 16.6 15.0

3

Fe 4 Fe 8 Fe 9

2.579 2.598 2.595

11.99 12.06 11.93

2.607 2.643 2.645

11.69 12.20 11.99

11.1 11.7 11.9

21.9 11.2 10.5

4

Fe 5 Fe 10

2.579 2.565

12.13 12.21

2.663 2.635

13.31 12.84

13.3 12.7

19.7 15.2

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V.R. Shah et al. / Journal of Alloys and Compounds 352 (2003) 6–15

therefore, can result in low values of Hhf . The group 2 atoms have a large change in the WS cell volume, which however is compensated by a smaller average bond length, and an opposite behaviour is observed in the group 3 atoms. Long et al. [36] have reported a similar behaviour in the observed hyperfine fields of Pr 2 Fe 17 compounds, where the direct correlation exists between hyperfine field and the NN, whereas in the case of nitrides this correlation is not found to exist. A decrease in the weighted average of the hyperfine field kHhf l (by |3.5%) with increasing Pr concentration may be due to the combined effect of the decrease in the Curie temperature and the weakening of the 4f–3d coupling upon Pr substitution. The isomer shift (IS) for Fe observed in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 and their nitrides at 300 K is shown in Fig. 9. The weighted average of the isomer shift kISl does not vary significantly (only | 20.08 mm / s) in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 compounds. However, after nitrogenation, an increase of IS, by |0.13 mm / s is observed. This increase could be due to an overall increase in the WS volume due to the lattice expansion caused by the interstitial modification with nitrogen as well as the reduction of s-electron density due to the nitrogen in the neighbourhood [35]. The variation of the weighted average of quadrupole splitting (kQSl) as a function of Pr concentration for the parent compounds and the nitrides at 300 K is also given in Fig. 9. The sign of kQSl remains the same for the parent compounds, whereas it changes its sign in the case of nitrides at around x50.5. As mentioned earlier, the EMD at room temperature shows a very strong preference for (0 4 0) for the Pr-rich ] compounds and a change to (4 0 2) for the Sm-rich parent ] compounds and from (4 0 2) to (2 0 4) in their respective ] nitrides. It has been reported that (0 4 0) and (4 0 2) are in the a–b plane and (2 0 4) is along the c-axis of the 1:5 setting (CaCu 5 -type). Therefore, the change in sign of kQSl in the case of the nitrides at x50.5 might be an indication of the change in the EMD from the a–b plane to

the c-axis (in the 1:5 setting). A similar behaviour of the kQSl was also observed in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 N y and was attributed to a change in EMD from basal plane to the c-axis of the rhombohedral structure [35].

4. Conclusions The novel class of 3:29 compounds and their nitrides with different signs of the second-order Stevens coefficient aJ were prepared and characterized by Rietveld analysis. The saturation magnetization and Curie temperature increase with interstitial modification by nitrogen, due to the magnetovolume effect and the associated changes in the 3d band structure. The variation of the EMD is understood from the resultant of the rare earth anisotropies from the two inequivalent rare earth sites. Nitrogen coordination at these two sites strongly influences the rare earth anisotropy. Two magnetic phase transition observed at |280 K for x50.1 and that at |180 K for x50.8 compounds in the temperature variation of ac magnetic susceptibility of the parent compounds are attributed to spin reorientation transitions, whereas the transition at |150 K for the x51.0 compound is attributed to the onset of a FOMP. The ¨ Mossbauer spectra were analyzed by a Wigner–Seitz cell approach. A direct correlation exists between the nearest neighbours and the hyperfine fields in the case of the parent compounds, whereas it is not observed in the case of nitrides. An indication of the spin-orientation is observed from change in sign of the weighted average of the quadrupole splitting in the case of nitrides.

Acknowledgements The authors thank the Indian Institute of Technology Madras for supporting this program. This work is supported in part by the Air Force Office of Scientific Research, Air Force Material Command, USAF, under grant F49620-96-1-0454. The US Government is authorized to reproduce and distribute reprints for Government purposes not withstanding any copyright notation thereon.

References

Fig. 9. Variation of the weighted average of kQSl and kISl with Pr concentration in (Sm 12x Pr x ) 3 Fe 27.5 Ti 1.5 compounds and their nitrides at 300 K.

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