- Email: [email protected]
27Al NMR studies in grain-aligned PrNi2Al5
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 371–373 www.elsevier.com/locate/jmmm
27
Al NMR studies in grain-aligned PrNi2 Al5
A. Ghoshray, R. Sarkar, B. Pahari, K. Ghoshray, B. Bandyopadhyay ECMP Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Available online 30 October 2006
Abstract 27 Al Knight shift (K) in grain aligned PrNi2 Al5 has been measured, in the temperature range 3.5–320 K, for two crystallographic nonequivalent Al sites. K becomes independent of temperature below 25 K in each case. wf ðTÞ for a Pr3þ ion in the orthorhombic crystal-field has been calculated to estimate the crystalline-electric-field (CEF) coefficients using K. The over-all crystal-field splitting ðDÞ is 660 K and CEF energy from the ground state to the first excited state is 45 K. r 2006 Elsevier B.V. All rights reserved.
PACS: 76.60.k Keywords: Knight shift; Crystalline electric field; s–f Exchange; PrNi2 Al5
1. Introduction The series of ternary intermetallics RNi2 Al5 (R ¼ La, Ce and Pr) deserve particular attention because of their interesting electronic properties. In this series, LaNi2 Al5 is generally taken as the reference material to estimate the 4f electron contribution. CeNi2 Al5 is a dense Kondo compound ðT K ¼ 4 KÞ with antiferromagnetic ordering T N ¼ 2:6 K [1]. Kondo effect along with the consideration of the crystalline electric-field effect (CEF) seems to be essential in the interpretation of the results. PrNi2 Al5 is, on the other hand, non-magnetic down to 2 K [2]. CEF in this case shows that it has a singlet ground state with the energy level of 4f electrons splits into nine singlets. The calculated result however, shows discrepancy (due to the presence of small impurities) in the reciprocal susceptibility below 10 K. Whereas, to explain specific heat data, consideration of only three lowest energy levels are sufficient to calculate C mag . In this paper, we report 27Al nuclear magnetic resonance (NMR) results in a grain aligned sample in the temperature range 3.5–320 K. NMR has an important advantage over the bulk susceptibility, because the Curie–Weiss term due to a dilute concentration of impurity spin hinders accurate estimation of w at low temperature. Corresponding author. Tel.: +91 33 23374321; fax:+91 33 23374637.
E-mail address: [email protected] (A. Ghoshray). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.079
But a random distribution of the impurities does not contribute to the NMR shift parameter. The results provide reasonably accurate CEF coefficients. 2. Experimental The single-phase aligned compound PrNi2 Al5 was prepared as in Ref. [3]. XRD patterns for an aligned sample and that of the random powder suggest that the majority of the grains in the aligned powder have their b-axis parallel to each other. 27Al NMR experiments were performed in a Bruker MSL 100 spectrometer with 7:04 T superconducting magnet. The direction of the applied field was always kept parallel to the b-axis in case of oriented sample. The spectrum was recorded by applying a p=2 t p=2 solid echo sequence. The spectra at a fixed magnetic field were obtained by exciting at small frequency steps, a narrow part of the spectrum, and recording in each step, the amplitude of the Fourier transformed spin echo signal. 3.
27
Al NMR in PrNi2 Al5
Fig. 1 shows the frequency swept 27Al spectra of PrNi2 Al5 at different temperatures. On lowering the temperature, the individual line broadens, however the basic nature of the spectrum remains same down to 3:5 K.
ARTICLE IN PRESS A. Ghoshray et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 371–373
Al(1), 1/2↔ -1/2
NMR spin-echo Intensity (arb.units)
4K
Al(1), 1/2↔ -1/2 Al(2), 1/2↔ -1/2
74
76
78
Al(2), -3/2↔ -5/2
Al(1), -3/2↔ -5/2
80
Al(2), -1/2↔ -3/2
Al(1), 1/2↔ 3/2
Al(1), 3/2↔ 5/2
Al(2), 3/2↔ 5/2
Al(2), 1/2↔ 3/2
300 K
Al(1), -1/2↔ -3/2
90 K
82
84
Frequency (MHz)
Fig. 1. Typical 27Al NMR spectra of grain aligned (b-axis) PrNi2 Al5 at 7:04 T at different temperatures. The continuous line indicates the theoretically fitted spectrum. The central transition for Al(1) and Al(2) are marked by the inclined arrows; the vertical arrows (#) indicate the satellite transitions.
Two pairs of clearly resolved satellite transitions consistent with the two inequivalent Al sites were observed except at very low temperature 4 K where the first satellite of the high frequency side of Al(2) and the second satellite of the Al(1) are merged on each other. Moreover, the well resolved central transition indicates that the Knight shift (K) for the two Al sites are completely different. The quadrupolar interaction parameters and the Knight shift are determined by fitting the experimental spectra as in Ref. [3]. The results yield values of nQ as 0.97 and 2:50 MHz and Z as 0.4 and 0.03, respectively, for Al(1) and Al(2) sites. The total Knight shift of a non-magnetic rare-earth based intermetallic compound can be written as, KðTÞ ¼ K 0 þ K s2f ðTÞ þ K dip ðTÞ,
(1)
where K 0 is due to non-4f contributions, K s2f ðTÞ is due to s–f exchange interaction and K dip is due to dipolar shift. Using the structural information and the bulk magnetic susceptibility, K dip for the Al(1) site has been estimated and has been subtracted from the total shift. However, for the Al(2) site, dipolar contribution could not be subtracted as the no principal axis coincides with the b-axis. Thus, pure hyperfine contribution KðTÞ in the uniform conduction electron polarization model is [4] KðTÞ ¼ K 0 ½1 þ ðgj 1ÞGwf ðTÞ=2Ngj m2B ,
(2)
where G is the s–f exchange energy constant. One should consider more accurate RKKY theory, then G would be replaced by 12pZG(0)SF ð2kF RÞ. Since ½KðTÞ K 0 / wf ðTÞ, a study ½KðTÞ K 0 provides a measurement of the temperature dependence of wf ðTÞ. Thus, the Knight shift data can be used to measure the paramagnetic susceptibility wf ðTÞ of the rare-earth ion. Fig. 2 shows the variation of K with temperature for the two Al sites for PrNi2 Al5 in the temperature range 320–3.5 K. Most important observation is that the shift continuously increases up to 25 K. Below this temperature the shift becomes temperature independent. Nevertheless, K follows Curie–Weiss (CW) law (‘‘A’’ and ‘‘B’’ curves in Fig. 2) above 50 K leading to yp ¼ 24 K, same as that reported in Ref. [2]. Inset of the Fig. 2 shows a linear variation of K with molar susceptibility w (measured with a b-axis aligned sample at 2 T in a SQUID magnetometer) in the temperature range 25–295 K. Similar discrepancy at low temperature has also been observed in case of CeNi2 Al5 compound. The linear region results the hyperfine field H hf for Al(1) and Al(2) sites as 1.95 and 0:89 KOe=mB , respectively. These values are close to that found in case of CeNi2 Al5 . In order to use Eq. (2) to fit the Knight shift data, we calculate wf ðTÞ for a Pr3þ ion (D2h site symmetry) in orthorhombic crystal-field using operator equivalent method [5]. The detail calculation would be presented elsewhere. The theoretical curve drawn through the data for Al (1) shown in Fig. 2 is a least-square fit giving parameters for PrNi2 Al5 of G ¼ 0:20 eV and the over-all crystal field splitting (DÞ ¼ 660 K, which is at least 100 K more than the
3.5 2 3
A
Shift (%)
Al(2), 1/2↔ -1/2
2.5
1.5 1 0.5 25 K
Shift (%)
372
0
2
1.5
0
0.02 0.04 χ (emu/mol)
0.06
B
1
0.5
Al(1)-expt. Al(2)-expt Al(1)-theo.
0 10
100 Temperature (K)
Fig. 2. Temperature dependence of 27Al Knight shifts for Al (1) and Al (2) sites of PrNi2 Al5 . The dotted line represents a least square fit to Eq. (2). ‘‘A’’ and ‘‘B’’ denote CW fitting. Inset shows the shift vs susceptibility plot along with the fitted curves.
ARTICLE IN PRESS A. Ghoshray et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 371–373
reported value [2]. However, the most important CEF energy from the ground state to the first excited state is 45 K, a similar value obtained from C mag . Finally, G value in PrNi2 Al5 is 2–3 times smaller than found in PrP/PrAs, where Pr3þ also has singlet ground state [4]. Acknowledgment R. Sarkar is thankful to CSIR, India for granting SRF.
373
References [1] Y. Isikawa, T. Mizushima, K. Oyabe, K. Mori, K. Sato, K. Kamigaki, J. Phys. Soc. Japan 60 (1991) 1869. [2] S. Akamaru, Y. Isikawa, J. Sakurai, K. Maezawa, H. Harima, J. Phys. Soc. Japan 70 (2001) 2049. [3] R. Sarkar, K. Ghoshray, B. Bandyopadhyay, A. Ghoshray, Phys. Rev. B 71 (2005) 104421. [4] E.D. Jones, Phys. Rev. 180 (1969) 455. [5] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Interscience Publishers, New York, 1965, p. 164.
Journal of Magnetism and Magnetic Materials 310 (2007) 371–373 www.elsevier.com/locate/jmmm
27
Al NMR studies in grain-aligned PrNi2 Al5
A. Ghoshray, R. Sarkar, B. Pahari, K. Ghoshray, B. Bandyopadhyay ECMP Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Available online 30 October 2006
Abstract 27 Al Knight shift (K) in grain aligned PrNi2 Al5 has been measured, in the temperature range 3.5–320 K, for two crystallographic nonequivalent Al sites. K becomes independent of temperature below 25 K in each case. wf ðTÞ for a Pr3þ ion in the orthorhombic crystal-field has been calculated to estimate the crystalline-electric-field (CEF) coefficients using K. The over-all crystal-field splitting ðDÞ is 660 K and CEF energy from the ground state to the first excited state is 45 K. r 2006 Elsevier B.V. All rights reserved.
PACS: 76.60.k Keywords: Knight shift; Crystalline electric field; s–f Exchange; PrNi2 Al5
1. Introduction The series of ternary intermetallics RNi2 Al5 (R ¼ La, Ce and Pr) deserve particular attention because of their interesting electronic properties. In this series, LaNi2 Al5 is generally taken as the reference material to estimate the 4f electron contribution. CeNi2 Al5 is a dense Kondo compound ðT K ¼ 4 KÞ with antiferromagnetic ordering T N ¼ 2:6 K [1]. Kondo effect along with the consideration of the crystalline electric-field effect (CEF) seems to be essential in the interpretation of the results. PrNi2 Al5 is, on the other hand, non-magnetic down to 2 K [2]. CEF in this case shows that it has a singlet ground state with the energy level of 4f electrons splits into nine singlets. The calculated result however, shows discrepancy (due to the presence of small impurities) in the reciprocal susceptibility below 10 K. Whereas, to explain specific heat data, consideration of only three lowest energy levels are sufficient to calculate C mag . In this paper, we report 27Al nuclear magnetic resonance (NMR) results in a grain aligned sample in the temperature range 3.5–320 K. NMR has an important advantage over the bulk susceptibility, because the Curie–Weiss term due to a dilute concentration of impurity spin hinders accurate estimation of w at low temperature. Corresponding author. Tel.: +91 33 23374321; fax:+91 33 23374637.
E-mail address: [email protected] (A. Ghoshray). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.079
But a random distribution of the impurities does not contribute to the NMR shift parameter. The results provide reasonably accurate CEF coefficients. 2. Experimental The single-phase aligned compound PrNi2 Al5 was prepared as in Ref. [3]. XRD patterns for an aligned sample and that of the random powder suggest that the majority of the grains in the aligned powder have their b-axis parallel to each other. 27Al NMR experiments were performed in a Bruker MSL 100 spectrometer with 7:04 T superconducting magnet. The direction of the applied field was always kept parallel to the b-axis in case of oriented sample. The spectrum was recorded by applying a p=2 t p=2 solid echo sequence. The spectra at a fixed magnetic field were obtained by exciting at small frequency steps, a narrow part of the spectrum, and recording in each step, the amplitude of the Fourier transformed spin echo signal. 3.
27
Al NMR in PrNi2 Al5
Fig. 1 shows the frequency swept 27Al spectra of PrNi2 Al5 at different temperatures. On lowering the temperature, the individual line broadens, however the basic nature of the spectrum remains same down to 3:5 K.
ARTICLE IN PRESS A. Ghoshray et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 371–373
Al(1), 1/2↔ -1/2
NMR spin-echo Intensity (arb.units)
4K
Al(1), 1/2↔ -1/2 Al(2), 1/2↔ -1/2
74
76
78
Al(2), -3/2↔ -5/2
Al(1), -3/2↔ -5/2
80
Al(2), -1/2↔ -3/2
Al(1), 1/2↔ 3/2
Al(1), 3/2↔ 5/2
Al(2), 3/2↔ 5/2
Al(2), 1/2↔ 3/2
300 K
Al(1), -1/2↔ -3/2
90 K
82
84
Frequency (MHz)
Fig. 1. Typical 27Al NMR spectra of grain aligned (b-axis) PrNi2 Al5 at 7:04 T at different temperatures. The continuous line indicates the theoretically fitted spectrum. The central transition for Al(1) and Al(2) are marked by the inclined arrows; the vertical arrows (#) indicate the satellite transitions.
Two pairs of clearly resolved satellite transitions consistent with the two inequivalent Al sites were observed except at very low temperature 4 K where the first satellite of the high frequency side of Al(2) and the second satellite of the Al(1) are merged on each other. Moreover, the well resolved central transition indicates that the Knight shift (K) for the two Al sites are completely different. The quadrupolar interaction parameters and the Knight shift are determined by fitting the experimental spectra as in Ref. [3]. The results yield values of nQ as 0.97 and 2:50 MHz and Z as 0.4 and 0.03, respectively, for Al(1) and Al(2) sites. The total Knight shift of a non-magnetic rare-earth based intermetallic compound can be written as, KðTÞ ¼ K 0 þ K s2f ðTÞ þ K dip ðTÞ,
(1)
where K 0 is due to non-4f contributions, K s2f ðTÞ is due to s–f exchange interaction and K dip is due to dipolar shift. Using the structural information and the bulk magnetic susceptibility, K dip for the Al(1) site has been estimated and has been subtracted from the total shift. However, for the Al(2) site, dipolar contribution could not be subtracted as the no principal axis coincides with the b-axis. Thus, pure hyperfine contribution KðTÞ in the uniform conduction electron polarization model is [4] KðTÞ ¼ K 0 ½1 þ ðgj 1ÞGwf ðTÞ=2Ngj m2B ,
(2)
where G is the s–f exchange energy constant. One should consider more accurate RKKY theory, then G would be replaced by 12pZG(0)SF ð2kF RÞ. Since ½KðTÞ K 0 / wf ðTÞ, a study ½KðTÞ K 0 provides a measurement of the temperature dependence of wf ðTÞ. Thus, the Knight shift data can be used to measure the paramagnetic susceptibility wf ðTÞ of the rare-earth ion. Fig. 2 shows the variation of K with temperature for the two Al sites for PrNi2 Al5 in the temperature range 320–3.5 K. Most important observation is that the shift continuously increases up to 25 K. Below this temperature the shift becomes temperature independent. Nevertheless, K follows Curie–Weiss (CW) law (‘‘A’’ and ‘‘B’’ curves in Fig. 2) above 50 K leading to yp ¼ 24 K, same as that reported in Ref. [2]. Inset of the Fig. 2 shows a linear variation of K with molar susceptibility w (measured with a b-axis aligned sample at 2 T in a SQUID magnetometer) in the temperature range 25–295 K. Similar discrepancy at low temperature has also been observed in case of CeNi2 Al5 compound. The linear region results the hyperfine field H hf for Al(1) and Al(2) sites as 1.95 and 0:89 KOe=mB , respectively. These values are close to that found in case of CeNi2 Al5 . In order to use Eq. (2) to fit the Knight shift data, we calculate wf ðTÞ for a Pr3þ ion (D2h site symmetry) in orthorhombic crystal-field using operator equivalent method [5]. The detail calculation would be presented elsewhere. The theoretical curve drawn through the data for Al (1) shown in Fig. 2 is a least-square fit giving parameters for PrNi2 Al5 of G ¼ 0:20 eV and the over-all crystal field splitting (DÞ ¼ 660 K, which is at least 100 K more than the
3.5 2 3
A
Shift (%)
Al(2), 1/2↔ -1/2
2.5
1.5 1 0.5 25 K
Shift (%)
372
0
2
1.5
0
0.02 0.04 χ (emu/mol)
0.06
B
1
0.5
Al(1)-expt. Al(2)-expt Al(1)-theo.
0 10
100 Temperature (K)
Fig. 2. Temperature dependence of 27Al Knight shifts for Al (1) and Al (2) sites of PrNi2 Al5 . The dotted line represents a least square fit to Eq. (2). ‘‘A’’ and ‘‘B’’ denote CW fitting. Inset shows the shift vs susceptibility plot along with the fitted curves.
ARTICLE IN PRESS A. Ghoshray et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 371–373
reported value [2]. However, the most important CEF energy from the ground state to the first excited state is 45 K, a similar value obtained from C mag . Finally, G value in PrNi2 Al5 is 2–3 times smaller than found in PrP/PrAs, where Pr3þ also has singlet ground state [4]. Acknowledgment R. Sarkar is thankful to CSIR, India for granting SRF.
373
References [1] Y. Isikawa, T. Mizushima, K. Oyabe, K. Mori, K. Sato, K. Kamigaki, J. Phys. Soc. Japan 60 (1991) 1869. [2] S. Akamaru, Y. Isikawa, J. Sakurai, K. Maezawa, H. Harima, J. Phys. Soc. Japan 70 (2001) 2049. [3] R. Sarkar, K. Ghoshray, B. Bandyopadhyay, A. Ghoshray, Phys. Rev. B 71 (2005) 104421. [4] E.D. Jones, Phys. Rev. 180 (1969) 455. [5] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Interscience Publishers, New York, 1965, p. 164.