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Two magnetic states of Nd in Nd2(CoFe)14B —145Nd NMR study
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1405-1406 North-Holland
AI41
Two magnetic states of Nd in Nd2(CoFe)14B - 145Nd N M R study E. J~dryka
a,b,
M. W6jcik .,c and P. Panissod b
a Institute o f Physics, Polish Academy o f Sciences, 02668 Warsaw, Poland b I P C M S - G E M M E , 4, rue B. Pascal, 67070 Strasbourg, France ': University o f Connecticut, Storrs, C T 06268, USA
145Nd NMR has been studied in Nd2(COl_xFex)laB compounds in the full substitution range. From the first stages of iron substitution, the onset of a new NMR spectrum, shifted down in frequency by about 70 MHz and rapidly increasing in intensity, is observed. This discontinuous change of hyperfine field is explained by changes in its intra-ionic part resulting from a passage through a non-fully polarized state of Nd 3+ ion. NdeCo14B and Nd2Fe14B compounds and their alloys belong to the most studied magnetic materials over the past few years, due to their outstanding properties as permanent magnet materials. At low temperatures, they have a canted magnetic structure with the magnetization vector tilted away from the tetragonal c-axis by 12 ° in NdzCo14B and 32 ° in NdzFe14B. A spin reorientation towards uniaxial state takes place at 32 K in NdzCoI4B and 134 K in Nd2Fe14B , respectively. Our previous 59Co N M R studies in the mixed Ndz(CoFe)I4B compounds have revealed on one hand a rapid increase of the average magnetization canting angle upon dilution of Co sublattice with Fe and on the other - an unusual satellite structure in 59Co N M R spectra [1]. A strikingly similar satellite structure is observed in the (NdY)2Co14B system and readily interpreted as local environment phenomena due to the substitution of non-magnetic yttrium for Nd in the 4f sublattice which modifies local hyperfine field [2]. This observation suggests that the origin of satellites in Ndz(CoFe)14B is also connected with the 4f sublattice in some way. In order to have more direct information on hyperfine fields in Nd sites, we have performed a 145Nd N MR study in the mixed Ndz(CoFe)14B compounds in the full substitution range. Spin echo N M R spectra recorded at 4.2 K are presented in fig. 1. In the two extreme (non mixed) compositions, the spectra are identical to those reported earlier [3,4] and consist of two quadrupole split lines corresponding to the two Nd sites in the crystal lattice (4f and 4g). In NdzFel4B , the frequencies of the two signals are close to each other (490 and 510 MHz) and form a broad spectrum, while in Nd2Co14B two separate signals are observed around 545 MHz and 566 MHz, respectively. The quadrupole structure of the latter is not resolved. A new and unexpected result from this study is that the transition from Nd2Col4B to Nd2Fe14B does not occur in a way of a smooth and gradual change of hyperfine field on Nd site. Instead, with the addition of iron, a new signal is observed to built up steadily slightly below 500 MHz (the same frequency range as in NdzFe14B compound). The intensity of that signal
i
i
145Nd NMR Nd2(Col_ x Fex)t4 B
09
N
0
x=0.0
x=0.05 x=0.15
~
x=o.27
"~ 0 ~ ~:~ .~
x=0.40 x=0.51 x=0.63 x=0.75
03
x=0.85
450
i
i
500
550
x=0.95 x=l. O0 600
Frequency (MHz) Fig. 1. ~45Nd spin echo NMR spectra at 4.2 K in a series of Nd2(Co 1 xFex)14B compounds. increases very rapidly with iron content. For x = 0.2, already 50% of the overall signal intensity is shifted to this new line and for x = 0.5 a very small fraction of the signal is left in the original state. Fig. 2 represents the fraction of the total signal intensity which is contained in the two respective frequency ranges (below and above 530 MHz) as a function of composition. To explain this result one has to consider the origin of hyperfine field on Nd site. For 4f ion in a crystal lattice, it consists of two terms - intra-ionic, due to the on site magnetic moment (spin and orbital) and extraionic (mainly transferred) - connected with presence of other magnetic ions in the crystal lattice. Let us first consider the two extreme compositions: Nd2Fe14B and NdzCOl4B. The analysis of the properties of Nd 3+ ion
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
E. Jedryka et at / Two magnetic states of Nd in Nd2(CoFe)14B
1406
~4SNd NMR s i g n a l i n t e n s i t y in Ndz(Col-xFex)14B
~ 4
. . . . . . 0 , ~i . . . . 0. ,~ . . . . o ,~. . . . . . .
0
20
40
60
,
80
...... ,
100
Fe c o n c e n t r a t i o n x (%) Fig. 2. Integral intensity of the two contributions to 145Nd NMR spectrum in Nd2(Co 1 xFex)14B. Circles: signal contained between 530 and 570 MHz - Nd 3÷ in fully polarized state in purely Co environment. Squares: signal below 530 MHz - new state for Nd 3÷ ions with Fe neighbours (as described in text), superimposed on a contribution from Nd 3÷ ions in a predominantly Fe environment, which have returned to the fully polarized state.
in Nd2Fe14B compound performed in ref. [5] shows that its magnetic m o m e n t is equal to 3.23/x B, which is only slightly smaller than that of the free trivalent Nd ion - 3.27/x B (the highest possible moment). This means that the direction of a magnetic m o m e n t in the crystal lattice is collinear with the exchange field and the ion exists in fully polarized state. The intra-ionic part of hyperfine field will thus be equal to the value of hyperfine field for a free ion, which is known from the measurements of hyperfine splitting in E S R spectra and equals 4280 kOe [6]. Consequently, from the experimental hyperfine field values in Nd2Fe14B (3450 kOe (490 MHz) and 3600 kOe (510 MHz)) one obtains a transferred hyperfine field of 830 kOe in 4f site and 680 kOe in 4g site, the direction of which is opposite to that of the intraionic part in both sites. To calculate transferred hyperfine fields in Nd2Co14B compound, we use the scaling procedure as described in ref. [7] and, with the magnetic moments of ~zve = 2.2/x B and /Xco = 1.39/xB, we obtain the following values: BTR(4f) = 365 kOe and BTR(4g)= 266 kOe. These numbers added to the total hyperfine field as measured (3838 kOe (545 MHz) and 3986 kOe (566 MHz), respectively) give the value for the intra-ionic term very close to the value for a free ion (4280 kOe), meaning that Nd 3+ ion in N d z C o l 4 B is in a fully polarized state. A similar suggestion was implied in ref. [8]. The above considerations show clearly that in both unmixed compounds the magnetic m o m e n t of Nd 3+ ion is collinear with the exchange field. If this state was preserved throughout the whole substitution range, the difference in hyperfine field would result from a gradual change of the
transferred part only. At the same time, the experiment shows an immediate appearance of a new N M R spectrum, shifted down in frequency by about 70 MHz, from the very first stage of iron substitution. This corresponds to a hyperfine field lower by about 1000 kOe which is at least one order of magnitude more than any possible contribution from transferred field. The only possible explanation is that this change comes from the intra-ionic part of hyperfine field, and that in the intermediate compositions the Nd energy state is no longer a fully polarized state. This can be understood in the following way. It has been shown in ref. [8] that Fe changes locally crystal field parameters on Nd sites. At the same time, the exchange field remains to be driven mostly by Co and Co surrounded Nd ions. This implies for Nd ions, which have several Fe neighbours, that the favourable orientation from the local crystal field point of view is no longer collinear with the favourable orientation as determined by the exchange field. This leads to a different energy level scheme for those Nd ions which eventually results in a ground state different from the fully polarized one and gives rise to a new, much lower value of hyperfine field. With increasing Fe concentration, the exchange field rotates with the canting angle resulting in a switch back to a collinear (fully polarized) state in Fe rich compositions. In conclusion, from the abrupt change of the intraionic hyperfine field we demonstrate the coexistence of two different magnetic ground states for Nd in Fe substituted NdzCo]4B. Beside the fully polarized state as in undiluted NdzCo14B, the other ground state is explained by the competition between easy orientation due to crystal field interaction and that due to exchange interaction which are non-collinear for Nd ions having some Fe atoms as their neighbours. This study has been performed in cooperation between the Polish Academy of Sciences and Institut de Physique et Chimie des Materiaux de Strasbourg. References
[1] E. Jedryka, M. Wojcik, P. Panissod and K.H.J. Buschow, J. Appl. Phys. 67 (1990) 4586. [2] E. Jedryka, M. Wojcik, P. Panissod, A.T. Pedziwiatr and M. Slepowronski, J. Appl. Phys. 69 (1991) 6043. [3] E. Potenziani II, J. Appl. Phys. 58 (1985) 2764. [4] S. Nadolski, E. Machowska and E. Burzo, Solid State Commun. 71 (1989) 97. [5] R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mater. 80 (1989) 14. [6] M.A.H. McCausland and I.S. Mackenzie, Adv. Phys. 28 (1979) 305. [7] K. Erdmann and M. Rosenberg, J. Magn. Magn. Mater. 82 (1989) 273. [8] H.S. Li, J.P. Gavigan, J.M. Cadogan, D. Givord and J.M.D. Coey, J. Magn. Magn. Mater. 72 (1988) L241.
AI41
Two magnetic states of Nd in Nd2(CoFe)14B - 145Nd N M R study E. J~dryka
a,b,
M. W6jcik .,c and P. Panissod b
a Institute o f Physics, Polish Academy o f Sciences, 02668 Warsaw, Poland b I P C M S - G E M M E , 4, rue B. Pascal, 67070 Strasbourg, France ': University o f Connecticut, Storrs, C T 06268, USA
145Nd NMR has been studied in Nd2(COl_xFex)laB compounds in the full substitution range. From the first stages of iron substitution, the onset of a new NMR spectrum, shifted down in frequency by about 70 MHz and rapidly increasing in intensity, is observed. This discontinuous change of hyperfine field is explained by changes in its intra-ionic part resulting from a passage through a non-fully polarized state of Nd 3+ ion. NdeCo14B and Nd2Fe14B compounds and their alloys belong to the most studied magnetic materials over the past few years, due to their outstanding properties as permanent magnet materials. At low temperatures, they have a canted magnetic structure with the magnetization vector tilted away from the tetragonal c-axis by 12 ° in NdzCo14B and 32 ° in NdzFe14B. A spin reorientation towards uniaxial state takes place at 32 K in NdzCoI4B and 134 K in Nd2Fe14B , respectively. Our previous 59Co N M R studies in the mixed Ndz(CoFe)I4B compounds have revealed on one hand a rapid increase of the average magnetization canting angle upon dilution of Co sublattice with Fe and on the other - an unusual satellite structure in 59Co N M R spectra [1]. A strikingly similar satellite structure is observed in the (NdY)2Co14B system and readily interpreted as local environment phenomena due to the substitution of non-magnetic yttrium for Nd in the 4f sublattice which modifies local hyperfine field [2]. This observation suggests that the origin of satellites in Ndz(CoFe)14B is also connected with the 4f sublattice in some way. In order to have more direct information on hyperfine fields in Nd sites, we have performed a 145Nd N MR study in the mixed Ndz(CoFe)14B compounds in the full substitution range. Spin echo N M R spectra recorded at 4.2 K are presented in fig. 1. In the two extreme (non mixed) compositions, the spectra are identical to those reported earlier [3,4] and consist of two quadrupole split lines corresponding to the two Nd sites in the crystal lattice (4f and 4g). In NdzFel4B , the frequencies of the two signals are close to each other (490 and 510 MHz) and form a broad spectrum, while in Nd2Co14B two separate signals are observed around 545 MHz and 566 MHz, respectively. The quadrupole structure of the latter is not resolved. A new and unexpected result from this study is that the transition from Nd2Col4B to Nd2Fe14B does not occur in a way of a smooth and gradual change of hyperfine field on Nd site. Instead, with the addition of iron, a new signal is observed to built up steadily slightly below 500 MHz (the same frequency range as in NdzFe14B compound). The intensity of that signal
i
i
145Nd NMR Nd2(Col_ x Fex)t4 B
09
N
0
x=0.0
x=0.05 x=0.15
~
x=o.27
"~ 0 ~ ~:~ .~
x=0.40 x=0.51 x=0.63 x=0.75
03
x=0.85
450
i
i
500
550
x=0.95 x=l. O0 600
Frequency (MHz) Fig. 1. ~45Nd spin echo NMR spectra at 4.2 K in a series of Nd2(Co 1 xFex)14B compounds. increases very rapidly with iron content. For x = 0.2, already 50% of the overall signal intensity is shifted to this new line and for x = 0.5 a very small fraction of the signal is left in the original state. Fig. 2 represents the fraction of the total signal intensity which is contained in the two respective frequency ranges (below and above 530 MHz) as a function of composition. To explain this result one has to consider the origin of hyperfine field on Nd site. For 4f ion in a crystal lattice, it consists of two terms - intra-ionic, due to the on site magnetic moment (spin and orbital) and extraionic (mainly transferred) - connected with presence of other magnetic ions in the crystal lattice. Let us first consider the two extreme compositions: Nd2Fe14B and NdzCOl4B. The analysis of the properties of Nd 3+ ion
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
E. Jedryka et at / Two magnetic states of Nd in Nd2(CoFe)14B
1406
~4SNd NMR s i g n a l i n t e n s i t y in Ndz(Col-xFex)14B
~ 4
. . . . . . 0 , ~i . . . . 0. ,~ . . . . o ,~. . . . . . .
0
20
40
60
,
80
...... ,
100
Fe c o n c e n t r a t i o n x (%) Fig. 2. Integral intensity of the two contributions to 145Nd NMR spectrum in Nd2(Co 1 xFex)14B. Circles: signal contained between 530 and 570 MHz - Nd 3÷ in fully polarized state in purely Co environment. Squares: signal below 530 MHz - new state for Nd 3÷ ions with Fe neighbours (as described in text), superimposed on a contribution from Nd 3÷ ions in a predominantly Fe environment, which have returned to the fully polarized state.
in Nd2Fe14B compound performed in ref. [5] shows that its magnetic m o m e n t is equal to 3.23/x B, which is only slightly smaller than that of the free trivalent Nd ion - 3.27/x B (the highest possible moment). This means that the direction of a magnetic m o m e n t in the crystal lattice is collinear with the exchange field and the ion exists in fully polarized state. The intra-ionic part of hyperfine field will thus be equal to the value of hyperfine field for a free ion, which is known from the measurements of hyperfine splitting in E S R spectra and equals 4280 kOe [6]. Consequently, from the experimental hyperfine field values in Nd2Fe14B (3450 kOe (490 MHz) and 3600 kOe (510 MHz)) one obtains a transferred hyperfine field of 830 kOe in 4f site and 680 kOe in 4g site, the direction of which is opposite to that of the intraionic part in both sites. To calculate transferred hyperfine fields in Nd2Co14B compound, we use the scaling procedure as described in ref. [7] and, with the magnetic moments of ~zve = 2.2/x B and /Xco = 1.39/xB, we obtain the following values: BTR(4f) = 365 kOe and BTR(4g)= 266 kOe. These numbers added to the total hyperfine field as measured (3838 kOe (545 MHz) and 3986 kOe (566 MHz), respectively) give the value for the intra-ionic term very close to the value for a free ion (4280 kOe), meaning that Nd 3+ ion in N d z C o l 4 B is in a fully polarized state. A similar suggestion was implied in ref. [8]. The above considerations show clearly that in both unmixed compounds the magnetic m o m e n t of Nd 3+ ion is collinear with the exchange field. If this state was preserved throughout the whole substitution range, the difference in hyperfine field would result from a gradual change of the
transferred part only. At the same time, the experiment shows an immediate appearance of a new N M R spectrum, shifted down in frequency by about 70 MHz, from the very first stage of iron substitution. This corresponds to a hyperfine field lower by about 1000 kOe which is at least one order of magnitude more than any possible contribution from transferred field. The only possible explanation is that this change comes from the intra-ionic part of hyperfine field, and that in the intermediate compositions the Nd energy state is no longer a fully polarized state. This can be understood in the following way. It has been shown in ref. [8] that Fe changes locally crystal field parameters on Nd sites. At the same time, the exchange field remains to be driven mostly by Co and Co surrounded Nd ions. This implies for Nd ions, which have several Fe neighbours, that the favourable orientation from the local crystal field point of view is no longer collinear with the favourable orientation as determined by the exchange field. This leads to a different energy level scheme for those Nd ions which eventually results in a ground state different from the fully polarized one and gives rise to a new, much lower value of hyperfine field. With increasing Fe concentration, the exchange field rotates with the canting angle resulting in a switch back to a collinear (fully polarized) state in Fe rich compositions. In conclusion, from the abrupt change of the intraionic hyperfine field we demonstrate the coexistence of two different magnetic ground states for Nd in Fe substituted NdzCo]4B. Beside the fully polarized state as in undiluted NdzCo14B, the other ground state is explained by the competition between easy orientation due to crystal field interaction and that due to exchange interaction which are non-collinear for Nd ions having some Fe atoms as their neighbours. This study has been performed in cooperation between the Polish Academy of Sciences and Institut de Physique et Chimie des Materiaux de Strasbourg. References
[1] E. Jedryka, M. Wojcik, P. Panissod and K.H.J. Buschow, J. Appl. Phys. 67 (1990) 4586. [2] E. Jedryka, M. Wojcik, P. Panissod, A.T. Pedziwiatr and M. Slepowronski, J. Appl. Phys. 69 (1991) 6043. [3] E. Potenziani II, J. Appl. Phys. 58 (1985) 2764. [4] S. Nadolski, E. Machowska and E. Burzo, Solid State Commun. 71 (1989) 97. [5] R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mater. 80 (1989) 14. [6] M.A.H. McCausland and I.S. Mackenzie, Adv. Phys. 28 (1979) 305. [7] K. Erdmann and M. Rosenberg, J. Magn. Magn. Mater. 82 (1989) 273. [8] H.S. Li, J.P. Gavigan, J.M. Cadogan, D. Givord and J.M.D. Coey, J. Magn. Magn. Mater. 72 (1988) L241.