- Email: [email protected]
Spin fluctuations in YNi5 and CeNi5
cm ..__
Journal of Magnetism and Magnetic Materials 157/158 (1996) 627-628
Eo E3
!s
A
journal of magnetism
m A
Zgnetic materials
ELSEVIER
Spin fluctuations in YNi, and CeNi, M. Coldea
*,
D. Andreica, M. Bitu, V. Crisan
Faculty of Physics, Babes-Bolyai University, Kogalniceanu St. I, 3400 Cluj-Napoca, Romania
Abstract The temperature dependences of the magnetic susceptibilities of YNi, and CeNi, exchange-enhanced Pauli paramagnets and at high temperatures fit a Curie-Weiss law. The show broad maxima at T,, = 250 and 100 K, respectively, experimental data may be described in terms of the self-consistent renormalization theory of spin fluctuations. Keywords: DC susceptibility; Rare etih-transition
metal compounds; Spin fluctuations
The magnetic properties of exchange-enhanced Pauli paramagnets YNi, and CeNi, have been extensively studied for many years. The magnetic susceptibility of YNi, measured by Gignoux et al. [l] in the temperature range 1.4-300 K is constant at low temperature and decreases slowly in the vicinity of room temperature. Similar behaviour was also reported by Tazuke et al. [2], the only difference being that for T< 100 K the susceptibility increases slightly as the temperature decreases. This difference is due to the presence of magnetic impurities in the investigated samples. The CeNi, susceptibility shows a broad maximum around 100 K [l], similar to those observed in YCo, and LuCo, [3]. From measurements of the lattice parameters and X-ray absorption, it has been found that the valence state of Ce in CeNi, is close to 4 + . The susceptibility of YNi, and CeNi, arises mainly from the Ni 3d electrons which are close to the onset of ferromagnetism [I]. The Ni 3d band is still partially empty but does not show magnetic splitting. The thermal variation of the magnetic susceptibilities for the two compounds was explained by considering the position of the Fermi level in a zone of the density of states with a positive curvature. This curvature is more pronounced in CeNi, than in YNi,, which could explain the maximum observed in the magnetic susceptibility of CeNi, and the lack of it in YNi,. However, the magnetic properties of the isostructural compounds YNi, and CeNi, at finite temperatures are not yet well understood. In order to obtain further information on the magnetic behaviour of these compounds, we have extended the magnetic measurements in
* Corresponding author. Email: [email protected]; fax: +40&l-191906. 0304.8853/96/$15.00
the high-temperature range. The aim of this paper is to give an alternative explanation of the thermal variation of the magnetic susceptibility of YNi, an CeNi, in a large temperature range. The weakly and nearly ferromagnetic metals are correctly treated in the self-consistent renormalization @CR) theory of spin fluctuations [4]. This theory has revealed that only a small-q part of the wave-number-dependent susceptibility x, contributes to the temperature dependence of x in the nearly ferromagnetic metals. The average amplitude of the local spin fluctuations (SE) = 3k,TCp x, increases with temperature until it reaches an upper limit determined by the charge neutrality condition. The temperature dependence of x at low temperature is the result of the increase in local moments with increasing temperature. The amplitude (S:) of thermally excited longitudinal spin fluctuations saturates at a certain temperature T * , above which the susceptibility is governed by local moment-type fluctuations and therefore a CurieWeiss behaviour is observed. The exchange-enhanced Pauli paramagnets YCo,, LuCo, [5] and CoSe, [6] are some of the best systems where the characteristic features manifested by the temperature-induced local moments have been tested experimentally. The samples of YNi, and CeNi, were prepared by argon arc melting. The purity of the starting materials was 99.99% for Y and 99.9% for Ni and Ce. X-ray powder diffraction measurements showed that both compounds formed the hexagonal CaCu, structure. No extra lines were observed. The magnetic susceptibility was measured between 80 and 800 K by a Weiss-Forrer magnetic balance with the sensitivity of lo-’ emu/g. The temperature dependence of the magnetic susceptibilities and of the reciprocal susceptibilities (in the hightemperature range) for YNi, and CeNi, are shown in Fig.
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
SSDI 0304-8853(95)01045-9
M. Coldea et al. /Journal of Magnetism and Magnetic Materials 157/158 (1996) 627-628
628
!15
30
o
20
y+ " ~'"~-"
] ]
Oo o = -~
,,...r.o-~r~ . . . . ~ r ' = " - ~ t ~ . =
- t 10
"~-,=~.
_ _ ~i
os -6
E ¢U I
10 f
1I ,.__._____l
1
L____L ....
200
tOO T(K)
L____L-
600
I
"-
__j 800
Fig. 1. Thermal variation of the susceptibility (O), and of the reciprocal susceptibility for ([]) YNi 5. Open circles and crosses are data taken from Refs. [1] and [2], respectively.
1 and Fig. 2. For comparison, the values of the susceptibility reported in [1,2] are also shown. The x ( T ) curves for the two compounds show a broad maximum at Tmax ~ 250 and 100 K, respectively, and at high temperatures obey a Curie-Weiss law modified by a temperature-independent part X0, according to X = C / ( T O) + Xo. The paramagnetic Curie temperatures are negative and very large: 0--804 K for YNi 5 and 275 K for CeNi 5. The effective moments per Ni atoms determined from the Curie constants are 1.75 /x B in YNi 5 and 1.34 /xB in CeNi 5, considering that Ce is in a valence state close to 4 + . The effective Ni moment in YNi 5 is very close to that of N i *
,0F
]20
30
"t15
E 20-
"--
S "~
,o
o w
10 ~
0
,,~o,~ ~'
200
~,00 T{K)
,e-
600
BOO
Fig. 2. Thermal variation of the susceptibility (C)) and of the reciprocal susceptibility ( × ) for CeNi 5. The black dots are data taken from Ref. [1].
ion, considering only the spin contribution (1.73 /~B for 3d 9 configuration). The 1 / x ( T ) curves for YNi 5 and CeNi 5 present deviations from linearity at T * = 330 K, in accord with the predictions of the SCR theory of spin fluctuations. In both cases we have exchange-enhanced Panli paramagnefism at low temperature or below T * and the appearance of the temperature-induced local moments is very rapid. At T > T * the compounds behave as if they have local moments. In the case of YNi 5, when ( S 2) is saturated, the charge neutrality condition gives the effective moment txeff=g[S(S+ 1)] 1/2/z B, characteristic of the Ni + ion (S = 1/2), as experimentally observed. We believe that the maximum in the x ( T ) curve of YNi 5 was ltidden in the earlier investigations by the presence of magnetic impurities such as neighbouring phases. The maximum of the bulk susceptibility in CeNi 5 corresponds to a maximum of the susceptibility of Ni atoms in this compound. The decrease in the effective Ni moment and of Tma×, when the paramagnetic Curie temperature increases, may be attributed to the partial quenching of spin fluctuations in CeNi 5 in comparison with YNi 5. This is due to the 4 f - 5 d and 3 d - 5 d hybridizations. The presence of spin fluctuations in CeNi 5 was also inferred from the T 2 behaviour of the resistivity and of the magnetic susceptibility at low temperature, and a quenching of these spin fluctuations by a strong magnetic field was evidenced in point-contact spectroscopy [7]. In conclusion, the temperature dependence of the magnetic susceptibility of the isostructural compounds YNi s and CeNi 5 can be understood within the framework of the SCR theory by considering only small-q components of the spin fluctuations. Therefore. YNi 5 and CeNi 5 can be considered as nearly ferromagnetic metals with Xq enhanced only in a small- q region. References
[1] D Gignoux. F. Givord, R. Lemaire. H. Launois and F. Sayetat, J. Physique 43 (1982) 173. [2] Y. Tazuke. R. Nakabayashi. T. Hashimoto. T. Miyadai and S. Murayama, J. Magn. Magn. Mater. 104-107 (1992) 725. [3] R. Lemaire. Cobalt 33 (1966J 201. [4] T. Moriya, J. Magn. Magn. Mater. 14 (1979) 1 [5] E. Burzo. E. Gratz and V. Pop, J. Magn. Magn. Mater. 123 (1993) 159. [6] N Inoue and H. Yasuoka. Solid State Commun. 30 (1979) 341. [7] Y. Naidyuk. M. Reffers. A.G.M. Jansen. I.K. Yanson. P. Wyder. D. Gignoux and D. Schmitt. Int. J. Mod. Phys. B "7 (1993) 222.
Journal of Magnetism and Magnetic Materials 157/158 (1996) 627-628
Eo E3
!s
A
journal of magnetism
m A
Zgnetic materials
ELSEVIER
Spin fluctuations in YNi, and CeNi, M. Coldea
*,
D. Andreica, M. Bitu, V. Crisan
Faculty of Physics, Babes-Bolyai University, Kogalniceanu St. I, 3400 Cluj-Napoca, Romania
Abstract The temperature dependences of the magnetic susceptibilities of YNi, and CeNi, exchange-enhanced Pauli paramagnets and at high temperatures fit a Curie-Weiss law. The show broad maxima at T,, = 250 and 100 K, respectively, experimental data may be described in terms of the self-consistent renormalization theory of spin fluctuations. Keywords: DC susceptibility; Rare etih-transition
metal compounds; Spin fluctuations
The magnetic properties of exchange-enhanced Pauli paramagnets YNi, and CeNi, have been extensively studied for many years. The magnetic susceptibility of YNi, measured by Gignoux et al. [l] in the temperature range 1.4-300 K is constant at low temperature and decreases slowly in the vicinity of room temperature. Similar behaviour was also reported by Tazuke et al. [2], the only difference being that for T< 100 K the susceptibility increases slightly as the temperature decreases. This difference is due to the presence of magnetic impurities in the investigated samples. The CeNi, susceptibility shows a broad maximum around 100 K [l], similar to those observed in YCo, and LuCo, [3]. From measurements of the lattice parameters and X-ray absorption, it has been found that the valence state of Ce in CeNi, is close to 4 + . The susceptibility of YNi, and CeNi, arises mainly from the Ni 3d electrons which are close to the onset of ferromagnetism [I]. The Ni 3d band is still partially empty but does not show magnetic splitting. The thermal variation of the magnetic susceptibilities for the two compounds was explained by considering the position of the Fermi level in a zone of the density of states with a positive curvature. This curvature is more pronounced in CeNi, than in YNi,, which could explain the maximum observed in the magnetic susceptibility of CeNi, and the lack of it in YNi,. However, the magnetic properties of the isostructural compounds YNi, and CeNi, at finite temperatures are not yet well understood. In order to obtain further information on the magnetic behaviour of these compounds, we have extended the magnetic measurements in
* Corresponding author. Email: [email protected]; fax: +40&l-191906. 0304.8853/96/$15.00
the high-temperature range. The aim of this paper is to give an alternative explanation of the thermal variation of the magnetic susceptibility of YNi, an CeNi, in a large temperature range. The weakly and nearly ferromagnetic metals are correctly treated in the self-consistent renormalization @CR) theory of spin fluctuations [4]. This theory has revealed that only a small-q part of the wave-number-dependent susceptibility x, contributes to the temperature dependence of x in the nearly ferromagnetic metals. The average amplitude of the local spin fluctuations (SE) = 3k,TCp x, increases with temperature until it reaches an upper limit determined by the charge neutrality condition. The temperature dependence of x at low temperature is the result of the increase in local moments with increasing temperature. The amplitude (S:) of thermally excited longitudinal spin fluctuations saturates at a certain temperature T * , above which the susceptibility is governed by local moment-type fluctuations and therefore a CurieWeiss behaviour is observed. The exchange-enhanced Pauli paramagnets YCo,, LuCo, [5] and CoSe, [6] are some of the best systems where the characteristic features manifested by the temperature-induced local moments have been tested experimentally. The samples of YNi, and CeNi, were prepared by argon arc melting. The purity of the starting materials was 99.99% for Y and 99.9% for Ni and Ce. X-ray powder diffraction measurements showed that both compounds formed the hexagonal CaCu, structure. No extra lines were observed. The magnetic susceptibility was measured between 80 and 800 K by a Weiss-Forrer magnetic balance with the sensitivity of lo-’ emu/g. The temperature dependence of the magnetic susceptibilities and of the reciprocal susceptibilities (in the hightemperature range) for YNi, and CeNi, are shown in Fig.
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
SSDI 0304-8853(95)01045-9
M. Coldea et al. /Journal of Magnetism and Magnetic Materials 157/158 (1996) 627-628
628
!15
30
o
20
y+ " ~'"~-"
] ]
Oo o = -~
,,...r.o-~r~ . . . . ~ r ' = " - ~ t ~ . =
- t 10
"~-,=~.
_ _ ~i
os -6
E ¢U I
10 f
1I ,.__._____l
1
L____L ....
200
tOO T(K)
L____L-
600
I
"-
__j 800
Fig. 1. Thermal variation of the susceptibility (O), and of the reciprocal susceptibility for ([]) YNi 5. Open circles and crosses are data taken from Refs. [1] and [2], respectively.
1 and Fig. 2. For comparison, the values of the susceptibility reported in [1,2] are also shown. The x ( T ) curves for the two compounds show a broad maximum at Tmax ~ 250 and 100 K, respectively, and at high temperatures obey a Curie-Weiss law modified by a temperature-independent part X0, according to X = C / ( T O) + Xo. The paramagnetic Curie temperatures are negative and very large: 0--804 K for YNi 5 and 275 K for CeNi 5. The effective moments per Ni atoms determined from the Curie constants are 1.75 /x B in YNi 5 and 1.34 /xB in CeNi 5, considering that Ce is in a valence state close to 4 + . The effective Ni moment in YNi 5 is very close to that of N i *
,0F
]20
30
"t15
E 20-
"--
S "~
,o
o w
10 ~
0
,,~o,~ ~'
200
~,00 T{K)
,e-
600
BOO
Fig. 2. Thermal variation of the susceptibility (C)) and of the reciprocal susceptibility ( × ) for CeNi 5. The black dots are data taken from Ref. [1].
ion, considering only the spin contribution (1.73 /~B for 3d 9 configuration). The 1 / x ( T ) curves for YNi 5 and CeNi 5 present deviations from linearity at T * = 330 K, in accord with the predictions of the SCR theory of spin fluctuations. In both cases we have exchange-enhanced Panli paramagnefism at low temperature or below T * and the appearance of the temperature-induced local moments is very rapid. At T > T * the compounds behave as if they have local moments. In the case of YNi 5, when ( S 2) is saturated, the charge neutrality condition gives the effective moment txeff=g[S(S+ 1)] 1/2/z B, characteristic of the Ni + ion (S = 1/2), as experimentally observed. We believe that the maximum in the x ( T ) curve of YNi 5 was ltidden in the earlier investigations by the presence of magnetic impurities such as neighbouring phases. The maximum of the bulk susceptibility in CeNi 5 corresponds to a maximum of the susceptibility of Ni atoms in this compound. The decrease in the effective Ni moment and of Tma×, when the paramagnetic Curie temperature increases, may be attributed to the partial quenching of spin fluctuations in CeNi 5 in comparison with YNi 5. This is due to the 4 f - 5 d and 3 d - 5 d hybridizations. The presence of spin fluctuations in CeNi 5 was also inferred from the T 2 behaviour of the resistivity and of the magnetic susceptibility at low temperature, and a quenching of these spin fluctuations by a strong magnetic field was evidenced in point-contact spectroscopy [7]. In conclusion, the temperature dependence of the magnetic susceptibility of the isostructural compounds YNi s and CeNi 5 can be understood within the framework of the SCR theory by considering only small-q components of the spin fluctuations. Therefore. YNi 5 and CeNi 5 can be considered as nearly ferromagnetic metals with Xq enhanced only in a small- q region. References
[1] D Gignoux. F. Givord, R. Lemaire. H. Launois and F. Sayetat, J. Physique 43 (1982) 173. [2] Y. Tazuke. R. Nakabayashi. T. Hashimoto. T. Miyadai and S. Murayama, J. Magn. Magn. Mater. 104-107 (1992) 725. [3] R. Lemaire. Cobalt 33 (1966J 201. [4] T. Moriya, J. Magn. Magn. Mater. 14 (1979) 1 [5] E. Burzo. E. Gratz and V. Pop, J. Magn. Magn. Mater. 123 (1993) 159. [6] N Inoue and H. Yasuoka. Solid State Commun. 30 (1979) 341. [7] Y. Naidyuk. M. Reffers. A.G.M. Jansen. I.K. Yanson. P. Wyder. D. Gignoux and D. Schmitt. Int. J. Mod. Phys. B "7 (1993) 222.