- Email: [email protected]
Magnetic properties of (Ce1− xLa x)5Si2.69
Journal of Magnetism and Magnetic Materials 177-181 (1998) 423-424
~ 4 Journal of magnetism and magnetic ~i~ materials
ELSEVIER
Magnetic properties of (Cel-xLax)sSi2.69 Y. Ushida, T. Nishioka*, M. Kontani Department of Physics, Faculty of Science, Nagoya University, Nagoya 464-01, Japan
Abstract CesSi3, which possesses two inequivalent Ce sites, Cel and Ce2, shows two anomalies in the magnetic susceptibility and specific heat, and a meta-magnetic jump in the magnetization curve. In order to reveal the origin of the two low temperature anomalies and the meta-magnetism, AC magnetic susceptibility and magnetization measurements have been performed on (Ce~ _xLax)5Si2.69. By increasing x, the non-magnetic ground state in Ce2 changes into a magnetic one with ferromagnetic correlation at around x = 0.1. The antiferromagnetic (AF) ordering in Cel and the metamagnetism suddenly disappear for x > 0.1. These results suggest that the meta-magnetism is closely related with the AF ordering as well as the low-temperature anomalies. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Susceptibility - AC; Meta-magnetism
CesSi3 crystallizes in the tetragonal CrsB3-type structure with two inequivalent Ce sites, Cel on the 4c and Ce2 on the 16l site. In our previous papers [1 3], we have clarified that Cel orders antiferromagnetically (AF) below 12 K and Ce2 shows no magnetic ordering down to 0.5K. However, Ce2 brings about broad peaks at 2.5 K in the specific heat (C) and at ~ 3.5 K in the magnetic susceptibility (Z), and shows Kondo effect and valence fluctuation-like behavior 1,2]. Our recent neutron-scattering experiment also indicates Cel orders antiferromagnetically at 12 K and Ce2 does not order magnetically [4]. The magnetization curve below 4 K shows a meta-magneticjump at around 40 kOe. We have presented Ce2-dimer model in order to explain the anomalies at ~ 2.5 and ~ 3.5 K and the meta-magnetism 1-1]. Detailed measurements of C and Z down to 0.5 K and in magnetic fields have provided qualitative validity of the dimer model above 1 K, but large discrepancy below 1 K [2]. Furthermore, the meta-magnetism is suggested to be closely related with the AF ordering in Cel I-3]. In order to clarify the single-site and two-site effect for the lower-temperature anomalies and meta-magnetism, we have performed AC magnetic
*Corresponding author. Fax: + 81 52 789 2933; e-mail: [email protected].
susceptibility and magnetization measurements on (Cea - xLax)5Si2.69. The polycrystatline samples were prepared by melting constituent elements using an argon-arc furnace. These ingots were annealed at 800°C in evacuated quartz tubes for a week and, subsequently, they were quenched into water. X-ray powder diffraction has indicated that all the samples are single phase having the tetragonal CrsB3type structure. Fig. 1 shows the temperature dependence of ~( and zT per mole of Ce between 0.5 and 20 K. Substitution of 5% La for Ce changes the broad peak at around 3 K into a sharp cusp. The peak value increases with further substitution and attains the maximum value of 0.33 emu/Ce-mol for x = 0.20, which is seven times as large as that for x = 0. If the anomaly at 3.5 K is due to the single-site effect, then the peak value of Z per mole of Ce is independent of x. Therefore, the two-site effect is essential for the anomaly. We see from Fig. lb that the effective moment 'zT' shows a broad hump at 3.5 K for x = 0 and the hump turns into a relative sharp shoulder through a cusp with x. Especially, for 0.1 ~< x ~< 0.2, the value of z T increases with decreasing temperature, which arises only from ferromagnetic correlation. On the other hand, the N6el temperature (TN), which is clearly observed as a kink in z T versus T plot, remains unchanged for 0 ~< x ~<0.05, and suddenly disappears for x >~ 0.1. Since RKKY exchange interaction is a coupling between
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PI1 S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 9 8 0 - 3
E Ushida et al./ Journal of Magnetism and Magnetic Materials 177-181 (1998) 423-424
424
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Fig. 2. Magnetization curves at 1.3 K in the magnetic fields up to 150 kOe for (Cel -xLax)sSi2.69. 12
T [K]
;ii i i i i i i ~i ~
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Fig. 1. Temperature dependence of AC magneticsusceptibilities for (Cel_xLa:,)sSia.69 at 100Hz and at the AC-amplitude 3.6 Oe. (a) Xversus T plot and (b) z T versus T plot. The inset in (a) shows Xfor CesSiz.69 in enlarged scale.
10 8
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~
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0.4
0.6
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Ce ions, the strength is gradually weakened with decreasing Ce concentration (increasing x). Then TN should gradually decrease with x. Therefore, the experimental fact that TN shows sudden disappearance cannot be explained only by RKKY interaction. The fact that the AF state in Cel disappears for small x implies that La substitutes preferentially Cel site. Fig. 2 shows the magnetization curves per formula unit at 1.3 K in the magnetic fields up to 150 kOe. We can see that meta-magnetism has completely disappeared for x ~> 0.1. Since the AF transition also disappears for the same composition range, the meta-magnetism is closely connected with it. However, as we pointed out in our previous paper [3], the magnitude of the metamagnetic jump cannot be explained only by 15 mole of Cel. The saturation magnetization is about 0.8 #B/Ce for all x. The low-field differential magnetization, dM/dH, increases with x, even though Ce content decreases. This suggests that the character of Ce ions which induce meta-magnetism changes and the coupling between Cel and Ce2 is important. Fig. 3 presents a magnetic phase diagram of the (Cel xLax)sSi2.69 system. The open square (D) shows TN which was determined from the inflection point in z(T) and the peak in C(T). The open circles ((3) and open triangles (A) indicate TLT obtained from a peak temperature of Z (T) and C(T)/T, respectively. The value of TLC in z(T) is in good agreement with that in C(T) for x ~>0.10, but not for x < 0.10. Therefore, this phase diagram is divided into three regions: (i) region I, where Cel orders antiferromagnetically and Ce2 remains paramagnetic; and (ii) regions II and III, where a long-range ordering in Ce2 occurs. Since meta-magnetism is observed only in region I, it is closely connected with AF ordering in Cel and no magnetic ordering in Ce2. The region II is a crossover region. The AF ordering of Cel
Fig. 3. Magnetic phase diagram obtained from specificheat and magnetic susceptibility measurements. The symbol [] indicates the temperatures of the higher temperature anomaly in C and g. The symbols © and A indicate the temperatures of the lowertemperature anomalies in X and C, respectively. The character 'P' denotes paramagnetic state.
disappears and the Ce2 orders magnetically with strong ferromagnetic correlation. The region III should be an antiferromagnetic ground state. The value of TLT decreases monotonically with increasing x. These experimental results indicate the difficulty of Ce2-dimer model at least for the La-substituted system, because the Ce2-dimer model predicts that X and C show broad peaks at different temperatures and the metamagnetism is caused by Ce2. Since the disappearance of the meta-magnetism and the AF ordering in Cel occurs simultaneously, the coupling between Cel and Ce2 should be very important in this system. This seems to be closely related with the crystallographic feature. One of authors (TN) would like to express his sincere thanks to Prof. S. Chikazawa for AC magnetic susceptibility measurements.
References [1] M. Kontani, M. Senda, M. Nakano, J.M. Lawrence, K. Adachi, J. Magn. Magn. Mater. 70 (1987) 378. [2] T. Nishioka, Y. Ushida, S. Chikazawa, M. Kontani, J. Phys. Soc. Japan. 66 (1997), to be published. [3] T. Nishioka, Y. Ushida, S. Chikazawa, M. Kontani, Physica B 230-232 (1997) 189. [4] Y. Oohara et al., unpublished data.
~ 4 Journal of magnetism and magnetic ~i~ materials
ELSEVIER
Magnetic properties of (Cel-xLax)sSi2.69 Y. Ushida, T. Nishioka*, M. Kontani Department of Physics, Faculty of Science, Nagoya University, Nagoya 464-01, Japan
Abstract CesSi3, which possesses two inequivalent Ce sites, Cel and Ce2, shows two anomalies in the magnetic susceptibility and specific heat, and a meta-magnetic jump in the magnetization curve. In order to reveal the origin of the two low temperature anomalies and the meta-magnetism, AC magnetic susceptibility and magnetization measurements have been performed on (Ce~ _xLax)5Si2.69. By increasing x, the non-magnetic ground state in Ce2 changes into a magnetic one with ferromagnetic correlation at around x = 0.1. The antiferromagnetic (AF) ordering in Cel and the metamagnetism suddenly disappear for x > 0.1. These results suggest that the meta-magnetism is closely related with the AF ordering as well as the low-temperature anomalies. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Susceptibility - AC; Meta-magnetism
CesSi3 crystallizes in the tetragonal CrsB3-type structure with two inequivalent Ce sites, Cel on the 4c and Ce2 on the 16l site. In our previous papers [1 3], we have clarified that Cel orders antiferromagnetically (AF) below 12 K and Ce2 shows no magnetic ordering down to 0.5K. However, Ce2 brings about broad peaks at 2.5 K in the specific heat (C) and at ~ 3.5 K in the magnetic susceptibility (Z), and shows Kondo effect and valence fluctuation-like behavior 1,2]. Our recent neutron-scattering experiment also indicates Cel orders antiferromagnetically at 12 K and Ce2 does not order magnetically [4]. The magnetization curve below 4 K shows a meta-magneticjump at around 40 kOe. We have presented Ce2-dimer model in order to explain the anomalies at ~ 2.5 and ~ 3.5 K and the meta-magnetism 1-1]. Detailed measurements of C and Z down to 0.5 K and in magnetic fields have provided qualitative validity of the dimer model above 1 K, but large discrepancy below 1 K [2]. Furthermore, the meta-magnetism is suggested to be closely related with the AF ordering in Cel I-3]. In order to clarify the single-site and two-site effect for the lower-temperature anomalies and meta-magnetism, we have performed AC magnetic
*Corresponding author. Fax: + 81 52 789 2933; e-mail: [email protected].
susceptibility and magnetization measurements on (Cea - xLax)5Si2.69. The polycrystatline samples were prepared by melting constituent elements using an argon-arc furnace. These ingots were annealed at 800°C in evacuated quartz tubes for a week and, subsequently, they were quenched into water. X-ray powder diffraction has indicated that all the samples are single phase having the tetragonal CrsB3type structure. Fig. 1 shows the temperature dependence of ~( and zT per mole of Ce between 0.5 and 20 K. Substitution of 5% La for Ce changes the broad peak at around 3 K into a sharp cusp. The peak value increases with further substitution and attains the maximum value of 0.33 emu/Ce-mol for x = 0.20, which is seven times as large as that for x = 0. If the anomaly at 3.5 K is due to the single-site effect, then the peak value of Z per mole of Ce is independent of x. Therefore, the two-site effect is essential for the anomaly. We see from Fig. lb that the effective moment 'zT' shows a broad hump at 3.5 K for x = 0 and the hump turns into a relative sharp shoulder through a cusp with x. Especially, for 0.1 ~< x ~< 0.2, the value of z T increases with decreasing temperature, which arises only from ferromagnetic correlation. On the other hand, the N6el temperature (TN), which is clearly observed as a kink in z T versus T plot, remains unchanged for 0 ~< x ~<0.05, and suddenly disappears for x >~ 0.1. Since RKKY exchange interaction is a coupling between
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PI1 S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 9 8 0 - 3
E Ushida et al./ Journal of Magnetism and Magnetic Materials 177-181 (1998) 423-424
424
O.E !O.Z5
' (a)
oo6 ~
,
'
,
o.-oo, ' Ol.O
t ,1 o~o
"
0.3/~o.~s
I
o~
~0.2 -/z:5,~'~o~zs
0.10
(] 0.75
E
j~
O.
n 0
x=o
o.15 O.ZO
-~ 0.(
0.25
I
~ ;~] ,~ ~L
030
~® O.z .
~E 0.~
H^c=3.GOe
SO
x=O=
o.7._5
1 O0 H
150
[kOe]
o.15
0.4
.........
Fig. 2. Magnetization curves at 1.3 K in the magnetic fields up to 150 kOe for (Cel -xLax)sSi2.69. 12
T [K]
;ii i i i i i i ~i ~
o
E
Fig. 1. Temperature dependence of AC magneticsusceptibilities for (Cel_xLa:,)sSia.69 at 100Hz and at the AC-amplitude 3.6 Oe. (a) Xversus T plot and (b) z T versus T plot. The inset in (a) shows Xfor CesSiz.69 in enlarged scale.
10 8
6
~
4 2 0.2
0.4
0.6
0.8
0
x
Ce ions, the strength is gradually weakened with decreasing Ce concentration (increasing x). Then TN should gradually decrease with x. Therefore, the experimental fact that TN shows sudden disappearance cannot be explained only by RKKY interaction. The fact that the AF state in Cel disappears for small x implies that La substitutes preferentially Cel site. Fig. 2 shows the magnetization curves per formula unit at 1.3 K in the magnetic fields up to 150 kOe. We can see that meta-magnetism has completely disappeared for x ~> 0.1. Since the AF transition also disappears for the same composition range, the meta-magnetism is closely connected with it. However, as we pointed out in our previous paper [3], the magnitude of the metamagnetic jump cannot be explained only by 15 mole of Cel. The saturation magnetization is about 0.8 #B/Ce for all x. The low-field differential magnetization, dM/dH, increases with x, even though Ce content decreases. This suggests that the character of Ce ions which induce meta-magnetism changes and the coupling between Cel and Ce2 is important. Fig. 3 presents a magnetic phase diagram of the (Cel xLax)sSi2.69 system. The open square (D) shows TN which was determined from the inflection point in z(T) and the peak in C(T). The open circles ((3) and open triangles (A) indicate TLT obtained from a peak temperature of Z (T) and C(T)/T, respectively. The value of TLC in z(T) is in good agreement with that in C(T) for x ~>0.10, but not for x < 0.10. Therefore, this phase diagram is divided into three regions: (i) region I, where Cel orders antiferromagnetically and Ce2 remains paramagnetic; and (ii) regions II and III, where a long-range ordering in Ce2 occurs. Since meta-magnetism is observed only in region I, it is closely connected with AF ordering in Cel and no magnetic ordering in Ce2. The region II is a crossover region. The AF ordering of Cel
Fig. 3. Magnetic phase diagram obtained from specificheat and magnetic susceptibility measurements. The symbol [] indicates the temperatures of the higher temperature anomaly in C and g. The symbols © and A indicate the temperatures of the lowertemperature anomalies in X and C, respectively. The character 'P' denotes paramagnetic state.
disappears and the Ce2 orders magnetically with strong ferromagnetic correlation. The region III should be an antiferromagnetic ground state. The value of TLT decreases monotonically with increasing x. These experimental results indicate the difficulty of Ce2-dimer model at least for the La-substituted system, because the Ce2-dimer model predicts that X and C show broad peaks at different temperatures and the metamagnetism is caused by Ce2. Since the disappearance of the meta-magnetism and the AF ordering in Cel occurs simultaneously, the coupling between Cel and Ce2 should be very important in this system. This seems to be closely related with the crystallographic feature. One of authors (TN) would like to express his sincere thanks to Prof. S. Chikazawa for AC magnetic susceptibility measurements.
References [1] M. Kontani, M. Senda, M. Nakano, J.M. Lawrence, K. Adachi, J. Magn. Magn. Mater. 70 (1987) 378. [2] T. Nishioka, Y. Ushida, S. Chikazawa, M. Kontani, J. Phys. Soc. Japan. 66 (1997), to be published. [3] T. Nishioka, Y. Ushida, S. Chikazawa, M. Kontani, Physica B 230-232 (1997) 189. [4] Y. Oohara et al., unpublished data.