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Itinerant magnetism of LaNi3
Journal of Magnetism and Magnetic Materials 140-144 (1995) 209-210
ELSEWIER
Itinerantmagnetismof LaNi3 Y. Tazuke ay*, S. Murayama b, K. Nishiyama ‘, K. Nagamine ’ ofEngineering, Iboraki UniLlersify, Hitachi, lbaraki 316, Japan b Muroran Inslitute of Technology, Muroran 050, Japan ’ hfeson Science Laboratory, Faculty of Science, Unicersiry of Tokyo, Tokyo 113, Japan a Faculty
Abstract IaNi, shows a non-ferromagmetic ordered phase in zero field below about 50 K, and metamagnetism in non-zero 8eld. Zero-field p,SR spectra show an antiferromagnetic phase beiow about 59 K. From the longitudinal-field l&R spectra, small Ni moments of about 0.03~~ are estimated.
Rare earth-nickel (R-Ni) intermetallic compounds show various magnetic behaviors. As the Ni composition decreases in Y-Ni compounds, ferromagnetism disappears for YNi,, weak ferromagnetism appears for YaNi, and YNi,, and fmally the weak ferromagnetism disappears for YNi, [l]. This behavior has been explained by a two-peak band structure [2]. Compared to the Y-Ni compounds, the magnetic properties of La-Ni compounds have not been well studied. La,Ni, was reported to be an antiferromagnet with metamagnetic behavior [3]. Recently we have shown that below about 50 K LaNi, is a ferromagnet above a critical field H,,,, which depends on temperature T [4]. The state for H < H,, since it is not a ferromagnetic state, has not been clarified by magnetization measurements alone. Muon spin relaxation (p+SR) measurements were done on a polyorystalline sample of LaNi, at the Muon Science Laboratory of the University of Tokyo installed at KEK, Tsukuba. Fig. 1 shows the relaxation of muon polarization at room temperature. The horizontal line shows an instrumental asymmetry. The relaxation is governed by nuclear relaxation due to La nuclei and by spin relaxation due to Ni moments. Since LaNi, is paramagnetic at room temperature, it is reasonable to assume that the nuclear relaxation is dominant at 300 K. The relaxation is described by a relaxation function G(t) multiplied by an initial asymmetry Ai. In the present case, G(t) is the well-known KuboToyabe function [5]: G&t)
= 4
t
i(l
-A%‘)
potentia1 is low. There are three possible sites: I, II and III, in the LaNi3 crystal IS]. Site I is at the center of three lSh-Ni atoms, site II is at the center of two 15La atoms, and site III is at the center of three fit-La atoms. The dashed curve in Fig. 1 shows the expected relaxation assuming site I. The calculated relaxation is slower than the experimzrr!al data, and the difference may be due to the Ni moments. Similar results are obtained for sites II and III. Since relaxations are faster than the experimental data for sites other than these, the muon sites are restricted to these three siles. Since there are only small differences among the three relaxation functions corresponding to the three sites, we can only conclude that the muons stop at the three sites. The differences between the values calculated by Eq. (1) and the experimental values are attributed to the Ni moments. The effect of the Ni moments on the relaxation can be described by an exponential function, and the combined relaxation function is, G(f)
= GKT( t) exp( -VT),
(2)
exp( - fA2t2),
(1) where A is a measure of nuclear dipolar field. The positive muons would stop at the sites where the electrostatic
l
Corresponding author. Fax:
-1.81~LY+37?809.
Fig. 1. Time variation of the asymmetry ratio for LaNi, at 300 K. The dots are data; the horizontal line is the instrumental asymmetry. and the curves show calculated values.
0304~8853/95/$09SO 0 1995 Elsevier Science B.V. AH rights reserved ssD10304-8853(94)01072-2
210
Y. Tazuke et al./.lournal @Magnetism and Magnetic Materials 140-W
6
fl9951209-210
0 -
CLl-
;
:o
: l
*iz
0 1OOK a 5K
ono5g l
0
L.ong.‘~eld Fig. 2. Temperature dependence of the initial asymmehy.
where T is the relaxation time due to the Ni moments. The solid curve in Fig. 1 shows the values calculated with Q* (2). As T decreases, the difference between the data and the calculated values with E& (1) increases, showing that the effect of Ni moments becomes strong. In addition to the effect of the Ni described by Eq. (21, the initial asymmetry Ai changes between 70 and SO K, as shown in Fig. 2. The abrupt change is interpreted as follows. Some site(s) are subjected to a strong enough field below about 50 K such that muons on these sit& relax faster than the minimum time window of the spectrometer (0.02 Jd. For other site(s) the Ni moments would not affect the strong field on the muons, and the relaxation is described bY Es. (2). The interpretation of more than one muon sites is consistent with the former conclusion that the muons can stop at the three sites. It is reasonable that the strong field arises from an ordered state of the Ni moments- Taking into account previous results 143, the zero-field state of LaNi, is antiferromagnetic, and the
(Oe)
-
200
Fig. 4. Dependence of the final asymmetries at lC!O and 5 K on longitudinal field strength. transition temperature TN lies between 50 and 70 K. Assuming that the muons stop at the three sites with equal probability, the average relaxation time T.~ is calculated to be 29 fis at 300 K, and about 5 ks below 100 K. p.SR experiments with an applied magnetic field parallel to the muon beam were done at 100 and 5 K. Fig. 3 shows an example of data at 10 K with H = 5 Oe. The muon polarization relaxes exponentially, with the final polarization increasing with the field strength. Fig. 4 shows the field dependence of the final polarization, which is nearly constant at 200 Oe. This behavior is explained as follows. The muon polarization is locked to the applied field and the effects of the random field due to La nuclei and Ni moments disappear. Since the effect of La nuclei almost disappears at around 10 Oe, the saturating field of 200 Oe is due to Ni moments, which are estimated to be about 0.06~~ by assuming a magnetic dipolar interaction between Ni moments and the muon moment. This value is of the same order of the ferromagnetic m+Jment of 0.1~~ per Ni atom below TN for H > II,,,. We plan to do detailed +SR measurements in the future. References [l] D. Gignoux. F. Givord, R. Lemaire and F. Tassel, J. LessCommon Metals 94 (1983) 1. [2] M. Shimizu, J. lnoue and S. Nagasawa, J. Phys. F 14 (1984) 2673. [3] K.H.J. Buschow, 1. Magn. Magn. Mater. 40 (1983) 224; F.T.
Parker and H. Oesterreicher, J. Less-Common Metals 90 (1983) 127. Fig. 3. Time variation of the asymmetry ratio for LaNi, at 100 K with a longitudinal field of 5 Oe.
141Y. Tazuke, R. Nakabayashi, S. Murayama, T. Sakakibara and T. Goto, Physica B 186-188 (1993) 956. [S] R. Lemaire and D. Paccard, Bull. Sot. Fr. Mineral. Cristal-
logr. 92 (1969) 9.
ELSEWIER
Itinerantmagnetismof LaNi3 Y. Tazuke ay*, S. Murayama b, K. Nishiyama ‘, K. Nagamine ’ ofEngineering, Iboraki UniLlersify, Hitachi, lbaraki 316, Japan b Muroran Inslitute of Technology, Muroran 050, Japan ’ hfeson Science Laboratory, Faculty of Science, Unicersiry of Tokyo, Tokyo 113, Japan a Faculty
Abstract IaNi, shows a non-ferromagmetic ordered phase in zero field below about 50 K, and metamagnetism in non-zero 8eld. Zero-field p,SR spectra show an antiferromagnetic phase beiow about 59 K. From the longitudinal-field l&R spectra, small Ni moments of about 0.03~~ are estimated.
Rare earth-nickel (R-Ni) intermetallic compounds show various magnetic behaviors. As the Ni composition decreases in Y-Ni compounds, ferromagnetism disappears for YNi,, weak ferromagnetism appears for YaNi, and YNi,, and fmally the weak ferromagnetism disappears for YNi, [l]. This behavior has been explained by a two-peak band structure [2]. Compared to the Y-Ni compounds, the magnetic properties of La-Ni compounds have not been well studied. La,Ni, was reported to be an antiferromagnet with metamagnetic behavior [3]. Recently we have shown that below about 50 K LaNi, is a ferromagnet above a critical field H,,,, which depends on temperature T [4]. The state for H < H,, since it is not a ferromagnetic state, has not been clarified by magnetization measurements alone. Muon spin relaxation (p+SR) measurements were done on a polyorystalline sample of LaNi, at the Muon Science Laboratory of the University of Tokyo installed at KEK, Tsukuba. Fig. 1 shows the relaxation of muon polarization at room temperature. The horizontal line shows an instrumental asymmetry. The relaxation is governed by nuclear relaxation due to La nuclei and by spin relaxation due to Ni moments. Since LaNi, is paramagnetic at room temperature, it is reasonable to assume that the nuclear relaxation is dominant at 300 K. The relaxation is described by a relaxation function G(t) multiplied by an initial asymmetry Ai. In the present case, G(t) is the well-known KuboToyabe function [5]: G&t)
= 4
t
i(l
-A%‘)
potentia1 is low. There are three possible sites: I, II and III, in the LaNi3 crystal IS]. Site I is at the center of three lSh-Ni atoms, site II is at the center of two 15La atoms, and site III is at the center of three fit-La atoms. The dashed curve in Fig. 1 shows the expected relaxation assuming site I. The calculated relaxation is slower than the experimzrr!al data, and the difference may be due to the Ni moments. Similar results are obtained for sites II and III. Since relaxations are faster than the experimental data for sites other than these, the muon sites are restricted to these three siles. Since there are only small differences among the three relaxation functions corresponding to the three sites, we can only conclude that the muons stop at the three sites. The differences between the values calculated by Eq. (1) and the experimental values are attributed to the Ni moments. The effect of the Ni moments on the relaxation can be described by an exponential function, and the combined relaxation function is, G(f)
= GKT( t) exp( -VT),
(2)
exp( - fA2t2),
(1) where A is a measure of nuclear dipolar field. The positive muons would stop at the sites where the electrostatic
l
Corresponding author. Fax:
-1.81~LY+37?809.
Fig. 1. Time variation of the asymmetry ratio for LaNi, at 300 K. The dots are data; the horizontal line is the instrumental asymmetry. and the curves show calculated values.
0304~8853/95/$09SO 0 1995 Elsevier Science B.V. AH rights reserved ssD10304-8853(94)01072-2
210
Y. Tazuke et al./.lournal @Magnetism and Magnetic Materials 140-W
6
fl9951209-210
0 -
CLl-
;
:o
: l
*iz
0 1OOK a 5K
ono5g l
0
L.ong.‘~eld Fig. 2. Temperature dependence of the initial asymmehy.
where T is the relaxation time due to the Ni moments. The solid curve in Fig. 1 shows the values calculated with Q* (2). As T decreases, the difference between the data and the calculated values with E& (1) increases, showing that the effect of Ni moments becomes strong. In addition to the effect of the Ni described by Eq. (21, the initial asymmetry Ai changes between 70 and SO K, as shown in Fig. 2. The abrupt change is interpreted as follows. Some site(s) are subjected to a strong enough field below about 50 K such that muons on these sit& relax faster than the minimum time window of the spectrometer (0.02 Jd. For other site(s) the Ni moments would not affect the strong field on the muons, and the relaxation is described bY Es. (2). The interpretation of more than one muon sites is consistent with the former conclusion that the muons can stop at the three sites. It is reasonable that the strong field arises from an ordered state of the Ni moments- Taking into account previous results 143, the zero-field state of LaNi, is antiferromagnetic, and the
(Oe)
-
200
Fig. 4. Dependence of the final asymmetries at lC!O and 5 K on longitudinal field strength. transition temperature TN lies between 50 and 70 K. Assuming that the muons stop at the three sites with equal probability, the average relaxation time T.~ is calculated to be 29 fis at 300 K, and about 5 ks below 100 K. p.SR experiments with an applied magnetic field parallel to the muon beam were done at 100 and 5 K. Fig. 3 shows an example of data at 10 K with H = 5 Oe. The muon polarization relaxes exponentially, with the final polarization increasing with the field strength. Fig. 4 shows the field dependence of the final polarization, which is nearly constant at 200 Oe. This behavior is explained as follows. The muon polarization is locked to the applied field and the effects of the random field due to La nuclei and Ni moments disappear. Since the effect of La nuclei almost disappears at around 10 Oe, the saturating field of 200 Oe is due to Ni moments, which are estimated to be about 0.06~~ by assuming a magnetic dipolar interaction between Ni moments and the muon moment. This value is of the same order of the ferromagnetic m+Jment of 0.1~~ per Ni atom below TN for H > II,,,. We plan to do detailed +SR measurements in the future. References [l] D. Gignoux. F. Givord, R. Lemaire and F. Tassel, J. LessCommon Metals 94 (1983) 1. [2] M. Shimizu, J. lnoue and S. Nagasawa, J. Phys. F 14 (1984) 2673. [3] K.H.J. Buschow, 1. Magn. Magn. Mater. 40 (1983) 224; F.T.
Parker and H. Oesterreicher, J. Less-Common Metals 90 (1983) 127. Fig. 3. Time variation of the asymmetry ratio for LaNi, at 100 K with a longitudinal field of 5 Oe.
141Y. Tazuke, R. Nakabayashi, S. Murayama, T. Sakakibara and T. Goto, Physica B 186-188 (1993) 956. [S] R. Lemaire and D. Paccard, Bull. Sot. Fr. Mineral. Cristal-
logr. 92 (1969) 9.