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Effect of substitution of vanadium in Nd2Fe14B studied by Mössbauer spectroscopy
Journal
of the Less-Common
Metals,
147 (1989)
79
- 87
EFFECT OF SUBSTITUTION OF VANADIUM BY MijSSBAUER SPECTROSCOPY R. KAMAL
and LOVLEEN
Department
of Physics,
(Received
June 16,1988;
Punjabi
University,
in revised form
Patiala 147002 August
79
IN Nd,Fe,,B
STUDIED
(India)
2,1988)
Summary The effect of an atomic replacement of iron by vanadium in Nd,Fe,,B is studied. The temperature dependence of the hyperfine field averaged over all the 57Fe sites Bhf, was determined from Mijssbauer spectroscopic measurements betwe& 150 and 650 K in Ndz(Fe,,,V,,),,B. It is observed that both & and the Curie temperature, T,, increase upon substitution by vanadium atoms. This behaviour of vanadium in Nd,Fe,,B is in contrast to that in o-Fe and other iron based alloys, where the presence of vanadium neighbours serves to decrease &. The variation of other hyperfine parameters at 57Fe nuclei is also studied as a function of temperature. A comparison is made between the substitution by vanadium in Nd,Fe,,B and substitution by other elements.
1. Introduction Bozorth et al. [l] have shown that in a-Fe the substitution of iron atomic neighbours by vanadium atoms decreases the magnetic hyperfine field, Bhf, at the iron atoms. Vincze and Griiner [2] have shown by Mbssbauer spectroscopy and continuous-wave nuclear magnetic resonance (NMR) methods that in iron-based alloys a vanadium impurity atom, whether at the nearest-neighbour site (nn site) or at the next nearestneighbour site, decreases the Bhf by 2.44(4) T. They have found a decrease in the average magnetic moment of the iron atoms, iire, which is proportional to the fraction of the substituted vanadium. Vanadium has been found [2] to be a well behaved impurity in the sense that it did not show anomalous behaviour of the Bhf [ 2,3], e.g. as observed in the case of manganese or nickel impurity. However, during a study of the effects of atomic replacements on the magnetic properties of a recently discovered novel alloy with applications as a permanent magnet, Nd,Fe,,B, Hirosawa et al. [4] have observed that the replacement of iron by vanadium leads to a slight increase in the Curie temperature, T, . This increase cannot be explained in terms of a dilution 0022-5088/89/$3.50
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in The Netherlands
80
model. However, the saturation magnetization, B,, was found [ 41 to drop by about 15% upon replacement of 10% of the iron atoms by vanadium. This contrasting behaviour of vanadium in o-Fe and iron-based alloys on the one hand, and in Nd,Fe,,B, on the other, needs further study in order to understand the nature of the exchange interactions and consequently the effect on the properties of Nd,Fe,,B and other isostructural alloys. In the present paper, the results obtained from Mijssbauer spectroscopic measurements in Nd,Fe,,V,.,),,B at temperatures between 150 and 650 K are given and discussed by comparing them with the results from other alloys.
2. Experimental details Appropriate amounts of the elements neodymium (purity 99.9%), iron (99.9%), boron and vanadium (99.99%) were melted twice in the presence of 1 ppm pure argon in an induction furnace. The ingots obtained were annealed in argon at 1200 (50) K for 10 h in a gas-fired furnace. Homogeneous samples of Nd,(Fe,gV,,)14B were obtained and checked by X-ray diffraction using a Debye-Scherrer camera and Cu Ko! radiation. The photographs showed lines at the positions [5] close to those in Nd,Fe,,B, and a few faint lines which are generally known to originate [6, 71 from Nd,. Fe4J34 * The alloy was crushed to a fine powder and mixed with BN to prepare a homogeneous disc of 1.5 cm* containing 20 mg cm-* of natural iron. Mijssbauer spectra were recorded between 250 and 650 K in the absorber mode using a conventional assembly [7 - 91 and then analysed by computer. The source used was “Co:Rh. The 1 - 2 million background counts per channel were accumulated in each two-fold spectrum at the different temperatures. Two Mijssbauer spectra consisting of 12 peaks were observed in an a-Fe foil of thickness 6 pm. These 12 peak positions were used to calibrate the channel numbers. The calibration was done by finding the appropriate coefficients for a third-order polynomial relationship assumed between the peak positions and the standard positions for a-Fe absorption peaks with the “Co:Rh source. Linearization of the velocity scale and summation of both halves were carried out to obtain the data for computer analysis. Analysis was made on the basis of the assumption given below. It was assumed that the spectrum consisted of six subspectra which were the Zeeman sextets below the T, . In addition, a central doublet of less than 5% area was presumed to account for a paramagnetic impurity present in the sample. Other assumptions in the analysis were: (i) B&c) > &(e), Bhf(j2)
> Bhf(jd,
Bhf(k2)
> Bhf(b)
and
(ii)
Bhf(j2)
> Bhf(k2),
where
&f(i)
represents the value of &f for the ith site (c, e, j,, j,, k, and k, are the six iron sites in the unit cell of Nd,Fei,B [ 6,10, ll]), and (iii) the peak on the extreme right side at about 6.5 mm s-l was assigned to the j, site rather than to the two sites, e and c, as was done earlier by Kamal and Andersson [6].
This assignment to j, is more conventional [12 - 141 than the others, although our group has reservations about this in view of recent theoretical work [ 151 on /,Lat the different sites.
3. Results Figure 1 shows the Miissbauer spectrum recorded at 148(5) K, the lowest temperature at which spectra were recorded. The measurements were carried out only above 148 K due to the occurrence [16] of a spin-reorientation at the lower temperatures in unsubstituted Nd,Fe,,B alloys which complicates the analysis. Table 1 gives the parameters corresponding to this spectrum. Figure 2 gives the temperature dependence of Bhf, the parameter obtained after averaging the internal hyperfine fields at all of the six sites in the Nd,( Fe, 9V,, 1)14Bunit cell. From the dependence of the parameters as a function of temperature the following observations could be made: (i) Tc is observed to be 630(5) K, and (ii) the temperature coefficient Q of Bhf in the vicinity of 300(50) K is -0.05% K-l (Table 2). This parameter is a measure [7 - 91 of the temperaNd+Fe
0.9 V 0.1 Jli
B
118(5lK
-6
_
VELOCITY
I
I
0 MM/S
-3
1 +6
-
Fig. 1. Miissbauer spectrum at 148(5) K. See text for recording and fitting details. The circles are the data points, and the solid line is the line after computer fitting with the parameters in Table 1, and with the assumption that impurity phase of the area was 5% of the total resonant absorption area, which causes only a central doublet in the whole range of temperatures studied between 148 and 650 K.
82 TABLE 1 The parameters Bnr, 6 (isomer shift relative to o-Fe) and &EQ (quadrupole splitting) in Ndz(Feo.eVo. r)r4B at 148(5) K, i.e. above the spin reorientation temperature where the spin axis still is along the c axis. Also given are f(i), the fractional area under the Zeeman sextet corresponding to the ith site ith site
(mm s-l)
AEdi) (mm s-l)
f(i)
(T) 34.4 33.8 33.6 27.1 31.1 31.4
0.13 0.12 0.00 -0.04 0.08 0.07
0.03 -0.06 -0.08 0.59 0.28 -0.24
0.18 0.31 0.14 0.12 0.06 0.18
S(i)
Bhf ti)
j2
k, kz e c jr
The error in 6 is f0.03 mm s-l, in Bhf 0.3 T, in A&J f0.06 mm s-l and in fk 0.02. See text for the assumptions used during the fitting of the spectrum. The line width was assumed to be equal at all the sites and was 0.30( 3) mm s-r in the spectrum at 148( 5) K.
Nd2 ( Feo.g”~.~ 1,,+B LO -
30-
f
0 ;; : -
20-
1;
I TO-
I
I
100
500
300 -
T CK)
700
_t
Fig. 2. Variation of average hyperfine field Bnf as a function of temperature T. The error expected is f0.3 T. The solid line is simply to guide the path of variation.
ture coefficient ,!i and other magnetization parameters. A comparison was made between the present results and those obtained in other isostructural alloys and the results are given in Table 2. This was done in order to facilitate a discussion on the effects of substitution by vanadium atoms with respect
83 TABLE
2
Comparison substituted
of Bhf, 6, (Y and T, between NdzFe14B
unsubstituted,
Nd2(Feo.aVe.
I)MB
vanadium-substituted
and cobalt-
NdzFer4B
Nd2(Feo,s&oo.
Tc (K) I& at 150(5) K(T) Bhf at 295( 5) K (T) 6 at 295(5) K (mm s-l) &( Bnf)%K- ’ (between 250 and 350 K)
630(5) 32.8(3) 31.3(3) 0.06(3) -0.05(2)
581(l) 32.7 30.4(2) -0.04( 2) -0.14(2)
675(5) 32.0(6) 28.8(ti) -0.08(2) -0.05( 1)
Reference
Present work
[5,6,131
[81
13)14B
to _other types of substitutions. Figure 3 gives the temperature dependence of 6, the average of isomer-shifts relative to o-Fe at the six sites.
0.1
i <
0.0
II I -
-
0.1
Iul
- 0.2
'I-
0.3
r 100 -
I
I
300
500 T(K)
I 70 0
-
Fig. 3. The variation of average isomershift temperature T. The error expected is +0.03 the path of variation.
6 (with respect to a-Fe) as a function of mm s-l and the solid line is simply to guide
4. Discussion 4.1. Evidence of substitution of vanadium in Nd,Fel$ According to the values of the heats of formation of rare-earth 3d transition metal alloys, the existence of inter-metallic alloys between neodymium and titanium, vanadium or chromium is unlikely [17]. However, the
present Mijssbauer study as well as the X-ray diffraction of our samples suggests that some substitution by vanadium atoms does indeed occur in the Nd,Fe,,B unit cell. This follows from (i) the changes noticed in Bhf, 8, T, and other parameters (Table 2) with respect to the values of these parameters in Nd,Fe,,B, and (ii) the ability to fit the Mossbauer spectrum (Fig. 1) with the restrictions mentioned, where we found parameters (Table 1) close to those in Nd,Fe,,B. Our spectra did not reveal any phase separation between 150 and 650 K. This became apparent as the relative intensity of the doublet in the central region of the spectrum (Fig. 1) remained practically unaffected as a function of temperature. The fact that Bhf is zero above 630 K shows the absence of any Fe, _xV, (3~< 0.1) alloy in the sample studied here as Tc > 630 K for x < 0.1 in Fe, -xV,. These results obtained after the substitution by vanadium atoms are different from those obtained for titanium atoms. In the latter case a phase separation of TiFe,was noticed [US]. The titanium was also found to be unable to change the parameters of the Zeeman sextets corresponding to Nd,Fe,,B significantly. A recent crystallographic and magnetization study [4] of substitution by vanadium atoms has also suggested that up to at least 15% vanadium atoms can replace the iron atoms in the Nd,Fe,,B unit cell. 4.2. Average internal hyperfine field B, The effect of substituting one vanadium atom into one of the eight nearest-neighbour positions in b.c.c. cw-Feis to reduce Bhf by 8% [ 11, causing a decrease of & by 2.44(4) T [2]. As the average number of near neighbours of iron in the - NdzFe,,B unit cell [16] is 9.64, by extrapolation, we expect a drop of Bhf by about 2 T when 10% of the vanadium atoms are replaced at the nearest sites in NdzFe14B. However, the results in Table 2 show no such drop in Bhf at 150 K. It is also observed that there is an increase in E, at 300 K equal to 1.4(5) T, which is expected due to the increased value (Fig. 2) of Tc upon the substitution by vanadium atoms compared to the value of 581(l) K [6] in the unsubstituted Nd,Fe,,B. Therefore, the Bhf value at low temperature, which can also be considered to be a measure of &+ at the iron sites if a linear relationship between Bhf and fir+ of 14.5 T pgl is assumed [ 191, gives evidence of a positive change in the case of Nd,Fe,$ as opposed to a negative change [l, 21 in o-Fe. Therefore, PF, appears to increase with 10% replacement of iron by vanadium in Nd,Fe,,B. Here &+ is the average magnetic moments of all iron atoms in the unit cell. An explanation of the above observation could be given as follows. At certain sites in the Nd,Fe,,B unit cell, like the j, site, short Fe-Fe distances (<0.24 nm) are observed [16] where the exchange interactions between the neighbouring atoms might be negative. When an atom with a higher atomic radius, like vanadium, is substituted at the corresponding nearest position, , there could be a net increase in the positive exchange interaction. This will result in a higher value of Tc and possibly in a higher value of iii+ in Nd,( Fe,gV, i)i4B compared to Nd,Fe14B. There is further evidence of a
85
similar effect from magnetization studies [ZO] in Y2Fei4_,Cu,B. There, iire remains unchanged up to a certain percentage of copper instead of decreasing as expected from the dilution model, and Tc is found to increase with copper concentrations up to 10%. Therefore, a possible explanation for the absence of a decrease in Bhf upon substitution by small amounts of vanadium is the reduction of the negative exchange interaction. 4.3. Curie Temperature T, The T, after 10% substitution by vanadium is found to increase by 50(5) K compared with that in Nd,Fei4B (Table 2). This increase can also be explained as above, i.e. it can be attributed as originating from a reduction in the negative exchange interactions as a consequence of the increased nearest distances. The increase in Tc influences LY@,~) in the vicinity of the working temperature range 250 - 350 K of the permanent magnets. The value of a(Bhf) in Table 2 is -0.05% K-l, which is at least twice as low as the temperature coefficient of the remanent magnetization a(&), in Nd,,Fe,,B, permanent magnets [5]. It is interesting to note that a similar decrease in (x was observed upon substitution of cobalt [5, 81. In other words, the substitution by vanadium in Nd-Fe-B magnets appears to be as beneficial as the substitution by cobalt, at least up to a concentration of 10%. 4.4. Isomer shift The variation of 6 with temperature (Fig. 3) is similar to that in Nd,Fe,,B [6, 141. An increase in 6 is, however, noticed (Table 2) compared to Nd,Fe,,B. The values of 5 increase with the average decrease in 4s electron density. The 4s electron density decreases due to two factors: (i) the increase in the near-neighbour distance from the 57Fe nuclei, and (ii) the increase in the 3d core polarization which causes more effective shielding of 4s electrons and therefore reduces the s-electron charge density at the nucleus. Therefore, the increase in near-neighbour distance, which can be postulated as due to the larger- atomic radii of the vanadium atoms, as well as the observed increase in B hf, predicts an increase in 6. Vanadium has a 5% larger atomic radius in its b.c.c. metallic state than iron has in cx-Fe metal A similar situation may exist in the Nd*Fe,,B structure and justify the assumption of the increase in near-neighbour distances. 4.5. Preferential substitution by vanadium The vanadium atoms are known [l] to substitute at random into the atomic positions in a-Fe. The relative intensities of the six subspectra at 148(5) K (Table 1) suggest accuracy ratios of 0.31(2), 0.14(2), 0.12(2), 0.18(2), 0.12(2) and 0.06(2) for the k,, k,, ji, jZ, e and c sites respectively. These may be compared with the iron occupancy ratio 0.28:0.28:0.14:0.14: 0.07:0.07 in the unit cell of unsubstituted Nd,Fe,,B [16]. If it is assumed that the recoilless fractions for the 57Fe nuclei at the six sites are equal at this temperature, it can be noted that iron atoms appear to occupy prefer-
86
entially the j, and k, sites at the expense of the kz site. The preferential occupation of the k, site by cobalt is known [21] to occur in the Nd,Fe,,B cell. The near-neighbour distances in unsubstituted Nd,Fe,,B are in the order j, > e > k, > j, > k, > c [6]. The preferential occupation of the c site by vanadium is unlikely in view of heat of formation considerations since vanadium would then have four neodymium neighbours. The preferential occupation of the k, site will reduce the net negative exchange interactions in accordance with the observed increase in Tc and Bhf (Table 2).
5. Conclusions The following evidence has been gathered from the present study. (i) The substitution of iron by vanadium in Nd,Fe,,B is possible, (ii) the replacement of 10% of the iron atoms by vanadium atoms results in an increase in the average hyperfine magnetic field at the “Fe nuclei, (iii) the temperature coefficient of the average field is smaller in tile substituted alloy, and (iv) the preferential occupation by vanadium of those iron sites which have neighbouring distances small enough to cause negative exchange interactions in Nd,Fei4B alloy is also noticed. The substitution of 10% vanadium for iron could perhaps be equally beneficial as the substitution by 10% cobalt applied in the manufacturing of Nd,Fe,,B permanent magnets. Experimental studies other than Mijssbauer spectroscopy are needed to confirm these findings.
Acknowledgments One of us (Lovleen) acknowledges a fellowship from the Council of Scientific and Industrial Research, India.
References 1 R. M. Bozorth, P. A. Wolf, D. D. Davis, V. B. Compton and J. H. Wernick, Phys. Rev., 122 (1961) 1157. 2 I. Vincze and G. Griiner, Phys. Rev. Lett., 28 (1972) 178. 3 T. E. Cranshaw, L. R. Walker and G. K. Wertheim, Phys. Rev. Lett., 13 (1964) 752. 4 S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura and M. Sagawa, IEEE Translotion J. Magn. Jpn., 1 (1986) 982. 5 M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, Proc. Zntermug. Conf. Hamburg, FRG, p. 25,1984. 6 R. Kamai and Y. Andersson, Phys. Rev. B, 32 (1985) 1756. 7 M. Rani and R. Kamal, J. Magn. Magn. Muter., 66 (1987) 379. 8 M. Rani and R. Kamal, J. Less-Common Met., 128 (1987) 343. 9 M. Rani, Ph.D. Thesis, Physics Department, Punjabi University, Patiala (1986). 10 D. Givord, H. S. Li and J. M. Moreau, Solid State Commun., 50 (1984) 497. 11 J. F. Herbst, J. J. Croat, F. E. Pinkerton and W. B. Yelon, Phys. Rev., B29 (1984) 4176.
87 12 H. Onodera, A. Fujita, H. Yamamoto, M. Sagawa and S. Hirosawa, J. Mugn. Magn. Mater., 68 (1987) 6. 13 H. Onodera, H. Yamauchi, M. Yamada, H. Yamamoto, M. Sagawa and S. Hirosawa, J. Magn. Magn. Mater., 68 (1987) 15. 14 K. H. J. Buschow, Mater. Sci. Rep., 1 (1986) 1. 15 B. Szpunar,Phys. Rev., B, 36 (1987) 3782. 16 D. Givord, H. S. Li and Perrier de la Blthie, Solid State Commun., 51 (1984) 857. 17 K. H. J. Buschow, Rep. hog. Phys., 40 (1977) 1179. 18 Lovleen and R. Kamal, J. Less-Common Met., 141 (1988) 83. 19 P. C. M. Gubbens, J. H. F. van Apeldoorn, A. M. Vander Kraan and K. H. J. Buschow, J. Phys. F, 4 (1974) 921. 20 A. T. Pedziwiatr, W. E. Wallace, E. Burzo and V. Pop, Solid State Commun., 61 (1987) 61. 21 J. F. Herbst and Y. B. Yelon,J. Appl. Phys., 60 (1986) 4224.
of the Less-Common
Metals,
147 (1989)
79
- 87
EFFECT OF SUBSTITUTION OF VANADIUM BY MijSSBAUER SPECTROSCOPY R. KAMAL
and LOVLEEN
Department
of Physics,
(Received
June 16,1988;
Punjabi
University,
in revised form
Patiala 147002 August
79
IN Nd,Fe,,B
STUDIED
(India)
2,1988)
Summary The effect of an atomic replacement of iron by vanadium in Nd,Fe,,B is studied. The temperature dependence of the hyperfine field averaged over all the 57Fe sites Bhf, was determined from Mijssbauer spectroscopic measurements betwe& 150 and 650 K in Ndz(Fe,,,V,,),,B. It is observed that both & and the Curie temperature, T,, increase upon substitution by vanadium atoms. This behaviour of vanadium in Nd,Fe,,B is in contrast to that in o-Fe and other iron based alloys, where the presence of vanadium neighbours serves to decrease &. The variation of other hyperfine parameters at 57Fe nuclei is also studied as a function of temperature. A comparison is made between the substitution by vanadium in Nd,Fe,,B and substitution by other elements.
1. Introduction Bozorth et al. [l] have shown that in a-Fe the substitution of iron atomic neighbours by vanadium atoms decreases the magnetic hyperfine field, Bhf, at the iron atoms. Vincze and Griiner [2] have shown by Mbssbauer spectroscopy and continuous-wave nuclear magnetic resonance (NMR) methods that in iron-based alloys a vanadium impurity atom, whether at the nearest-neighbour site (nn site) or at the next nearestneighbour site, decreases the Bhf by 2.44(4) T. They have found a decrease in the average magnetic moment of the iron atoms, iire, which is proportional to the fraction of the substituted vanadium. Vanadium has been found [2] to be a well behaved impurity in the sense that it did not show anomalous behaviour of the Bhf [ 2,3], e.g. as observed in the case of manganese or nickel impurity. However, during a study of the effects of atomic replacements on the magnetic properties of a recently discovered novel alloy with applications as a permanent magnet, Nd,Fe,,B, Hirosawa et al. [4] have observed that the replacement of iron by vanadium leads to a slight increase in the Curie temperature, T, . This increase cannot be explained in terms of a dilution 0022-5088/89/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
80
model. However, the saturation magnetization, B,, was found [ 41 to drop by about 15% upon replacement of 10% of the iron atoms by vanadium. This contrasting behaviour of vanadium in o-Fe and iron-based alloys on the one hand, and in Nd,Fe,,B, on the other, needs further study in order to understand the nature of the exchange interactions and consequently the effect on the properties of Nd,Fe,,B and other isostructural alloys. In the present paper, the results obtained from Mijssbauer spectroscopic measurements in Nd,Fe,,V,.,),,B at temperatures between 150 and 650 K are given and discussed by comparing them with the results from other alloys.
2. Experimental details Appropriate amounts of the elements neodymium (purity 99.9%), iron (99.9%), boron and vanadium (99.99%) were melted twice in the presence of 1 ppm pure argon in an induction furnace. The ingots obtained were annealed in argon at 1200 (50) K for 10 h in a gas-fired furnace. Homogeneous samples of Nd,(Fe,gV,,)14B were obtained and checked by X-ray diffraction using a Debye-Scherrer camera and Cu Ko! radiation. The photographs showed lines at the positions [5] close to those in Nd,Fe,,B, and a few faint lines which are generally known to originate [6, 71 from Nd,. Fe4J34 * The alloy was crushed to a fine powder and mixed with BN to prepare a homogeneous disc of 1.5 cm* containing 20 mg cm-* of natural iron. Mijssbauer spectra were recorded between 250 and 650 K in the absorber mode using a conventional assembly [7 - 91 and then analysed by computer. The source used was “Co:Rh. The 1 - 2 million background counts per channel were accumulated in each two-fold spectrum at the different temperatures. Two Mijssbauer spectra consisting of 12 peaks were observed in an a-Fe foil of thickness 6 pm. These 12 peak positions were used to calibrate the channel numbers. The calibration was done by finding the appropriate coefficients for a third-order polynomial relationship assumed between the peak positions and the standard positions for a-Fe absorption peaks with the “Co:Rh source. Linearization of the velocity scale and summation of both halves were carried out to obtain the data for computer analysis. Analysis was made on the basis of the assumption given below. It was assumed that the spectrum consisted of six subspectra which were the Zeeman sextets below the T, . In addition, a central doublet of less than 5% area was presumed to account for a paramagnetic impurity present in the sample. Other assumptions in the analysis were: (i) B&c) > &(e), Bhf(j2)
> Bhf(jd,
Bhf(k2)
> Bhf(b)
and
(ii)
Bhf(j2)
> Bhf(k2),
where
&f(i)
represents the value of &f for the ith site (c, e, j,, j,, k, and k, are the six iron sites in the unit cell of Nd,Fei,B [ 6,10, ll]), and (iii) the peak on the extreme right side at about 6.5 mm s-l was assigned to the j, site rather than to the two sites, e and c, as was done earlier by Kamal and Andersson [6].
This assignment to j, is more conventional [12 - 141 than the others, although our group has reservations about this in view of recent theoretical work [ 151 on /,Lat the different sites.
3. Results Figure 1 shows the Miissbauer spectrum recorded at 148(5) K, the lowest temperature at which spectra were recorded. The measurements were carried out only above 148 K due to the occurrence [16] of a spin-reorientation at the lower temperatures in unsubstituted Nd,Fe,,B alloys which complicates the analysis. Table 1 gives the parameters corresponding to this spectrum. Figure 2 gives the temperature dependence of Bhf, the parameter obtained after averaging the internal hyperfine fields at all of the six sites in the Nd,( Fe, 9V,, 1)14Bunit cell. From the dependence of the parameters as a function of temperature the following observations could be made: (i) Tc is observed to be 630(5) K, and (ii) the temperature coefficient Q of Bhf in the vicinity of 300(50) K is -0.05% K-l (Table 2). This parameter is a measure [7 - 91 of the temperaNd+Fe
0.9 V 0.1 Jli
B
118(5lK
-6
_
VELOCITY
I
I
0 MM/S
-3
1 +6
-
Fig. 1. Miissbauer spectrum at 148(5) K. See text for recording and fitting details. The circles are the data points, and the solid line is the line after computer fitting with the parameters in Table 1, and with the assumption that impurity phase of the area was 5% of the total resonant absorption area, which causes only a central doublet in the whole range of temperatures studied between 148 and 650 K.
82 TABLE 1 The parameters Bnr, 6 (isomer shift relative to o-Fe) and &EQ (quadrupole splitting) in Ndz(Feo.eVo. r)r4B at 148(5) K, i.e. above the spin reorientation temperature where the spin axis still is along the c axis. Also given are f(i), the fractional area under the Zeeman sextet corresponding to the ith site ith site
(mm s-l)
AEdi) (mm s-l)
f(i)
(T) 34.4 33.8 33.6 27.1 31.1 31.4
0.13 0.12 0.00 -0.04 0.08 0.07
0.03 -0.06 -0.08 0.59 0.28 -0.24
0.18 0.31 0.14 0.12 0.06 0.18
S(i)
Bhf ti)
j2
k, kz e c jr
The error in 6 is f0.03 mm s-l, in Bhf 0.3 T, in A&J f0.06 mm s-l and in fk 0.02. See text for the assumptions used during the fitting of the spectrum. The line width was assumed to be equal at all the sites and was 0.30( 3) mm s-r in the spectrum at 148( 5) K.
Nd2 ( Feo.g”~.~ 1,,+B LO -
30-
f
0 ;; : -
20-
1;
I TO-
I
I
100
500
300 -
T CK)
700
_t
Fig. 2. Variation of average hyperfine field Bnf as a function of temperature T. The error expected is f0.3 T. The solid line is simply to guide the path of variation.
ture coefficient ,!i and other magnetization parameters. A comparison was made between the present results and those obtained in other isostructural alloys and the results are given in Table 2. This was done in order to facilitate a discussion on the effects of substitution by vanadium atoms with respect
83 TABLE
2
Comparison substituted
of Bhf, 6, (Y and T, between NdzFe14B
unsubstituted,
Nd2(Feo.aVe.
I)MB
vanadium-substituted
and cobalt-
NdzFer4B
Nd2(Feo,s&oo.
Tc (K) I& at 150(5) K(T) Bhf at 295( 5) K (T) 6 at 295(5) K (mm s-l) &( Bnf)%K- ’ (between 250 and 350 K)
630(5) 32.8(3) 31.3(3) 0.06(3) -0.05(2)
581(l) 32.7 30.4(2) -0.04( 2) -0.14(2)
675(5) 32.0(6) 28.8(ti) -0.08(2) -0.05( 1)
Reference
Present work
[5,6,131
[81
13)14B
to _other types of substitutions. Figure 3 gives the temperature dependence of 6, the average of isomer-shifts relative to o-Fe at the six sites.
0.1
i <
0.0
II I -
-
0.1
Iul
- 0.2
'I-
0.3
r 100 -
I
I
300
500 T(K)
I 70 0
-
Fig. 3. The variation of average isomershift temperature T. The error expected is +0.03 the path of variation.
6 (with respect to a-Fe) as a function of mm s-l and the solid line is simply to guide
4. Discussion 4.1. Evidence of substitution of vanadium in Nd,Fel$ According to the values of the heats of formation of rare-earth 3d transition metal alloys, the existence of inter-metallic alloys between neodymium and titanium, vanadium or chromium is unlikely [17]. However, the
present Mijssbauer study as well as the X-ray diffraction of our samples suggests that some substitution by vanadium atoms does indeed occur in the Nd,Fe,,B unit cell. This follows from (i) the changes noticed in Bhf, 8, T, and other parameters (Table 2) with respect to the values of these parameters in Nd,Fe,,B, and (ii) the ability to fit the Mossbauer spectrum (Fig. 1) with the restrictions mentioned, where we found parameters (Table 1) close to those in Nd,Fe,,B. Our spectra did not reveal any phase separation between 150 and 650 K. This became apparent as the relative intensity of the doublet in the central region of the spectrum (Fig. 1) remained practically unaffected as a function of temperature. The fact that Bhf is zero above 630 K shows the absence of any Fe, _xV, (3~< 0.1) alloy in the sample studied here as Tc > 630 K for x < 0.1 in Fe, -xV,. These results obtained after the substitution by vanadium atoms are different from those obtained for titanium atoms. In the latter case a phase separation of TiFe,was noticed [US]. The titanium was also found to be unable to change the parameters of the Zeeman sextets corresponding to Nd,Fe,,B significantly. A recent crystallographic and magnetization study [4] of substitution by vanadium atoms has also suggested that up to at least 15% vanadium atoms can replace the iron atoms in the Nd,Fe,,B unit cell. 4.2. Average internal hyperfine field B, The effect of substituting one vanadium atom into one of the eight nearest-neighbour positions in b.c.c. cw-Feis to reduce Bhf by 8% [ 11, causing a decrease of & by 2.44(4) T [2]. As the average number of near neighbours of iron in the - NdzFe,,B unit cell [16] is 9.64, by extrapolation, we expect a drop of Bhf by about 2 T when 10% of the vanadium atoms are replaced at the nearest sites in NdzFe14B. However, the results in Table 2 show no such drop in Bhf at 150 K. It is also observed that there is an increase in E, at 300 K equal to 1.4(5) T, which is expected due to the increased value (Fig. 2) of Tc upon the substitution by vanadium atoms compared to the value of 581(l) K [6] in the unsubstituted Nd,Fe,,B. Therefore, the Bhf value at low temperature, which can also be considered to be a measure of &+ at the iron sites if a linear relationship between Bhf and fir+ of 14.5 T pgl is assumed [ 191, gives evidence of a positive change in the case of Nd,Fe,$ as opposed to a negative change [l, 21 in o-Fe. Therefore, PF, appears to increase with 10% replacement of iron by vanadium in Nd,Fe,,B. Here &+ is the average magnetic moments of all iron atoms in the unit cell. An explanation of the above observation could be given as follows. At certain sites in the Nd,Fe,,B unit cell, like the j, site, short Fe-Fe distances (<0.24 nm) are observed [16] where the exchange interactions between the neighbouring atoms might be negative. When an atom with a higher atomic radius, like vanadium, is substituted at the corresponding nearest position, , there could be a net increase in the positive exchange interaction. This will result in a higher value of Tc and possibly in a higher value of iii+ in Nd,( Fe,gV, i)i4B compared to Nd,Fe14B. There is further evidence of a
85
similar effect from magnetization studies [ZO] in Y2Fei4_,Cu,B. There, iire remains unchanged up to a certain percentage of copper instead of decreasing as expected from the dilution model, and Tc is found to increase with copper concentrations up to 10%. Therefore, a possible explanation for the absence of a decrease in Bhf upon substitution by small amounts of vanadium is the reduction of the negative exchange interaction. 4.3. Curie Temperature T, The T, after 10% substitution by vanadium is found to increase by 50(5) K compared with that in Nd,Fei4B (Table 2). This increase can also be explained as above, i.e. it can be attributed as originating from a reduction in the negative exchange interactions as a consequence of the increased nearest distances. The increase in Tc influences LY@,~) in the vicinity of the working temperature range 250 - 350 K of the permanent magnets. The value of a(Bhf) in Table 2 is -0.05% K-l, which is at least twice as low as the temperature coefficient of the remanent magnetization a(&), in Nd,,Fe,,B, permanent magnets [5]. It is interesting to note that a similar decrease in (x was observed upon substitution of cobalt [5, 81. In other words, the substitution by vanadium in Nd-Fe-B magnets appears to be as beneficial as the substitution by cobalt, at least up to a concentration of 10%. 4.4. Isomer shift The variation of 6 with temperature (Fig. 3) is similar to that in Nd,Fe,,B [6, 141. An increase in 6 is, however, noticed (Table 2) compared to Nd,Fe,,B. The values of 5 increase with the average decrease in 4s electron density. The 4s electron density decreases due to two factors: (i) the increase in the near-neighbour distance from the 57Fe nuclei, and (ii) the increase in the 3d core polarization which causes more effective shielding of 4s electrons and therefore reduces the s-electron charge density at the nucleus. Therefore, the increase in near-neighbour distance, which can be postulated as due to the larger- atomic radii of the vanadium atoms, as well as the observed increase in B hf, predicts an increase in 6. Vanadium has a 5% larger atomic radius in its b.c.c. metallic state than iron has in cx-Fe metal A similar situation may exist in the Nd*Fe,,B structure and justify the assumption of the increase in near-neighbour distances. 4.5. Preferential substitution by vanadium The vanadium atoms are known [l] to substitute at random into the atomic positions in a-Fe. The relative intensities of the six subspectra at 148(5) K (Table 1) suggest accuracy ratios of 0.31(2), 0.14(2), 0.12(2), 0.18(2), 0.12(2) and 0.06(2) for the k,, k,, ji, jZ, e and c sites respectively. These may be compared with the iron occupancy ratio 0.28:0.28:0.14:0.14: 0.07:0.07 in the unit cell of unsubstituted Nd,Fe,,B [16]. If it is assumed that the recoilless fractions for the 57Fe nuclei at the six sites are equal at this temperature, it can be noted that iron atoms appear to occupy prefer-
86
entially the j, and k, sites at the expense of the kz site. The preferential occupation of the k, site by cobalt is known [21] to occur in the Nd,Fe,,B cell. The near-neighbour distances in unsubstituted Nd,Fe,,B are in the order j, > e > k, > j, > k, > c [6]. The preferential occupation of the c site by vanadium is unlikely in view of heat of formation considerations since vanadium would then have four neodymium neighbours. The preferential occupation of the k, site will reduce the net negative exchange interactions in accordance with the observed increase in Tc and Bhf (Table 2).
5. Conclusions The following evidence has been gathered from the present study. (i) The substitution of iron by vanadium in Nd,Fe,,B is possible, (ii) the replacement of 10% of the iron atoms by vanadium atoms results in an increase in the average hyperfine magnetic field at the “Fe nuclei, (iii) the temperature coefficient of the average field is smaller in tile substituted alloy, and (iv) the preferential occupation by vanadium of those iron sites which have neighbouring distances small enough to cause negative exchange interactions in Nd,Fei4B alloy is also noticed. The substitution of 10% vanadium for iron could perhaps be equally beneficial as the substitution by 10% cobalt applied in the manufacturing of Nd,Fe,,B permanent magnets. Experimental studies other than Mijssbauer spectroscopy are needed to confirm these findings.
Acknowledgments One of us (Lovleen) acknowledges a fellowship from the Council of Scientific and Industrial Research, India.
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