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NMR study of the onset of magnetic ordering in CeAl3
Journal of Magnetism and Magnetic Materials 76 & 77 (1988) 465-466 North-Holland, Amsterdam
465
N M R S T U D Y OF T H E O N S E T OF M A G N E T I C O R D E R I N G IN CeAI 3
H. N A K A M U R A , Y. K I T A O K A , K. A S A Y A M A Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan
and J. F L O U Q U E T Centre de Recherches sur les Tres Basses Temperatures, Centre National de la Recherche Scientifique, 38042 Grenoble Cedex, France The magnetic field effect on the magnetically ordered state of CeA13 below T N = 1.2 K has been investigated by 27A1 nuclear magnetic resonance. It is reported that the spin structure is modified by a low magnetic field of 3 kOe.
It has been believed that CeA13 is a prototype of the heavy fermion (HF) systems with a nonmagnetic ground state. Recently, however, a ~SR study revealed the development of antiferromagnetic correlation in CeA13 below 2 K [1]. Furthermore, an antiferromagnetic ordering is successively found in other H F compounds such as CeInCu 2 [2,3], CeCuzSi 2 [4,5] and UPt 3 [6] which were believed to have a non-magnetic or superconducting ground state. These results imply that either a magnetic ordering or superconductivity is realized in the ground state of the H F compounds or that both states coexist as in CeCu2Si 2 and UPt 3. In a previous paper [7], a 27A1 N M R investigation proved that CeA13 undergoes an antiferromagnetic-like transition at the Nrel temperature, T N = 1.2 K. As shown in fig. 1, the line width of 27A1 spectrum increases markedly with decreasing temperature below 1.2 K and the nuclear relaxation rate, 1 / T 1, exhibits a sharp peak at 1.2 K. In order to observe a magnetic ordering by N M R , it is essential to carry out an experiment such a low frequency that the N M R linewidth is not affected by an inhomogeneous broadening due to the anisotropic Knight shift. Fig. 2(a) shows the 27A1 N M R spectrum at 0.98 M H z and 1.2 K, which is well articulated by the first order quadrupole effect. At 0.4 K, the spectrum is remarkably broadened around the same Knight shift as that at 1.2 K as seen in fig. 2(b), possessing a characteristic shape consisting of three components, namely, a narrow one, a broader and the broadest with the line width of about 1 kOe. It was pointed out that the characteristic line shape may be associated with something like a modulated spin structure which gives rise to different hyperfine fields at A1
sites [7]. The magnitude of the ordered moment was estimated to be 0.3/tB/Ce at the maximum. In order to clarify a magnetic field effect on the magnetically ordered state found in CeA13, we here report the magnetic field dependence of 27A1 N M R spectrum. For N M R measurement, a polycrystal sample of CeA13 is crushed into fine powder with a smaller size than the skin depth. N M R measurement was made by a phase-coherent-type pulsed spectrometer. Fig. 2(c) shows the spectrum at 0.4 K and 9.2 M H z where the resonance field is one order larger
(a)
CeA[ 3
ol.c
0.98MHz
,E \~'I~'~\~\ I TN
-~0.5 o
o'.s I mN •"-. • •
~I0
I
I 0.1
, I Ip ,,,~l 1
. . . . . . . . . . . .
..
CeA[ 3 0.98 MHz
, J I ll,,l] 10
I I [r ll,I 100
T (K)
Fig. 1. (a) Temperature dependence of 1/10 linewidth for 27A1 spectrum at 0.98 MHz. (b) Temperature dependence of spinlattice relaxation rate, 1/T1, for 27A1 at 0.98 MHz.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
466
It. N a k a m u r a et al. / N M R study qf magnetic order m CeAI 3
CeA[3
m
n
/
•
~. (c) 9.2MHz
..-. \
:>, •~
l
~,
I
l
7s
i
l
,¢'
I
l#
~
J
: ~
~,,,,t'
I
k
z
8.5
]
J
(b) O.98MHz
gS (a) 0.98MHz i !.2K
/
0.5
1.0 H (k0e)
1.5
Fig. 2. :TAI spin-echo spectra at (a) 1.2 K and 0.98 MHz, (b) 0.4 K and 0.98 MHz, and (c) 0.4 K and 9.2 MHz.
than that at 0.98 MHz. As seen in the figure, the 1/10 linewidth of the spectrum characterizing the broadest component is invariant, whereas both the narrow and the broader components are not discriminated owing to the line broadening of the broader component. To present a detailed field dependence of the spectrum, both the 1 / 1 0 and 1 / 2 line widths at 0.4 K are shown in fig. 3 as a function of the external magnetic field. It should be noted that the 1 / 2 width begins to increase above 3 kOe and the narrow spectrum is smeared o u t j u s t around 3 kOe. Although the spin structure CeAI 3 2~[
0.4K
o vl.0
+
£23
--+-
~/10 WIDTH
0.5 f
9-+ * ~ [
0
t
~
1/2 WIDTH I
I
5 H (kOe)
I
t
1
[
10
Fig. 3. External magnetic field dependences of 1 / 2 and 1/10 linewidth of 27A1 spectra at 0.4 K.
is not yet identified, it seems to be modified around the critical magnetic field, H~ = 3 kOe. Next, we tentatively try to extract some possible spin structures from the characteristic shape of the observed ~-TAIspectrum by referring other antiferromagnetic H F compounds. First, we apply a collinear antiferromagnetic structure found in an isostructural compound (UTh)Pt) [8], which yields two magnetic sites of AI. In this case, the :TAI spectrum is expected to consist of two components with different hyperfine fields generated by the magnetic ordering, which is not successful in explaining the observed complicated line shape consisting of three components (fig. 2(b)) and its magnetic field dependence (fig. 3). Accordingly, it is plausible to take into account of some complex spin structures such as an incommensulate spindensity-wave (SDW) in CeA12 [9] or a double-q structure in CeB~ [10]. It is noteworthy that these types of structures are modified by the magnetic field. In conclusion, a low critical field of H~ = 3 kOe observed in the field dependence of the line shape characterizes the magnetic state appearing in CeA13 below 1.2 K. For a further detailed experiment, a single crystal is required. References [1] S. Barth, H.R. Ott. E.N. Gygax. B. Hitti, E. kippelt. A. Schenck, C. Baines, B. van den Brandit, T. Konter and S. Mango, Phys. Rev, LeU. 59 (1987) 2991. [21 S. Takagi, T. Kimura. N. Sato, T. Satoh and T. Kasuya, J. Phys. Soc. Japan. 57 (1988) 1562. [3] H. Nakanmra, Y. Kitaoka, K, Asayama, Y. Onuki and T. Komatsubara. J. Phys. Soc. Japan. 57 (1988) 2276. [4] Y.J. Uemura, W.J. Kossler, X.H. Yu, H.E. Schone, J.R. Kempton, C,E. Stronach, S. Barth, F.N. Gygax, B. Hitti, A. Schenck, C. Baines, W.E. Lankford, Y. Onuki and 3". Komatsubara, Physica C 153-155 (1988)455. [5] H. Nakamura, Y. Kitaoka. H. Yamada, K. Asayarna, J. Magn. Magn. Mat. 76 & 77 (1988) 517. [6] (3. Aeppli, E,. Bucher, C. Broholm, J.K. Kjems, J. Baumann and J. Hufnagl, Phys. Rev. Lett. 60(1988) 615. [71 H. Nakamura, Y. Kitaoka, K. Asayama and J. Elouquet, J, Phys. Soc. Japan. 57 (1988) 2644. [8] A.I. Goldman, G. Shirane, G. Aeppli. B. Batlogg and E. Bucher, Phys. Rev. B34 (19861 6564. [91 B. Barbara, J.X. Boucherle, J.I,. Buew~z, M.F. Rossignol and J. Scbweizer, Solid State C o m m u n . 24 (1977) 481, ibid 29 (1979) 810. [101 J.M. Fffantin, J. Rossat-Mignod, P. Burlet, tt. Bartholin, S. Kunii and T. Kasuya, J. Magn. Magn. Mat. 47 & 48 (1985) 145.
465
N M R S T U D Y OF T H E O N S E T OF M A G N E T I C O R D E R I N G IN CeAI 3
H. N A K A M U R A , Y. K I T A O K A , K. A S A Y A M A Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan
and J. F L O U Q U E T Centre de Recherches sur les Tres Basses Temperatures, Centre National de la Recherche Scientifique, 38042 Grenoble Cedex, France The magnetic field effect on the magnetically ordered state of CeA13 below T N = 1.2 K has been investigated by 27A1 nuclear magnetic resonance. It is reported that the spin structure is modified by a low magnetic field of 3 kOe.
It has been believed that CeA13 is a prototype of the heavy fermion (HF) systems with a nonmagnetic ground state. Recently, however, a ~SR study revealed the development of antiferromagnetic correlation in CeA13 below 2 K [1]. Furthermore, an antiferromagnetic ordering is successively found in other H F compounds such as CeInCu 2 [2,3], CeCuzSi 2 [4,5] and UPt 3 [6] which were believed to have a non-magnetic or superconducting ground state. These results imply that either a magnetic ordering or superconductivity is realized in the ground state of the H F compounds or that both states coexist as in CeCu2Si 2 and UPt 3. In a previous paper [7], a 27A1 N M R investigation proved that CeA13 undergoes an antiferromagnetic-like transition at the Nrel temperature, T N = 1.2 K. As shown in fig. 1, the line width of 27A1 spectrum increases markedly with decreasing temperature below 1.2 K and the nuclear relaxation rate, 1 / T 1, exhibits a sharp peak at 1.2 K. In order to observe a magnetic ordering by N M R , it is essential to carry out an experiment such a low frequency that the N M R linewidth is not affected by an inhomogeneous broadening due to the anisotropic Knight shift. Fig. 2(a) shows the 27A1 N M R spectrum at 0.98 M H z and 1.2 K, which is well articulated by the first order quadrupole effect. At 0.4 K, the spectrum is remarkably broadened around the same Knight shift as that at 1.2 K as seen in fig. 2(b), possessing a characteristic shape consisting of three components, namely, a narrow one, a broader and the broadest with the line width of about 1 kOe. It was pointed out that the characteristic line shape may be associated with something like a modulated spin structure which gives rise to different hyperfine fields at A1
sites [7]. The magnitude of the ordered moment was estimated to be 0.3/tB/Ce at the maximum. In order to clarify a magnetic field effect on the magnetically ordered state found in CeA13, we here report the magnetic field dependence of 27A1 N M R spectrum. For N M R measurement, a polycrystal sample of CeA13 is crushed into fine powder with a smaller size than the skin depth. N M R measurement was made by a phase-coherent-type pulsed spectrometer. Fig. 2(c) shows the spectrum at 0.4 K and 9.2 M H z where the resonance field is one order larger
(a)
CeA[ 3
ol.c
0.98MHz
,E \~'I~'~\~\ I TN
-~0.5 o
o'.s I mN •"-. • •
~I0
I
I 0.1
, I Ip ,,,~l 1
. . . . . . . . . . . .
..
CeA[ 3 0.98 MHz
, J I ll,,l] 10
I I [r ll,I 100
T (K)
Fig. 1. (a) Temperature dependence of 1/10 linewidth for 27A1 spectrum at 0.98 MHz. (b) Temperature dependence of spinlattice relaxation rate, 1/T1, for 27A1 at 0.98 MHz.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
466
It. N a k a m u r a et al. / N M R study qf magnetic order m CeAI 3
CeA[3
m
n
/
•
~. (c) 9.2MHz
..-. \
:>, •~
l
~,
I
l
7s
i
l
,¢'
I
l#
~
J
: ~
~,,,,t'
I
k
z
8.5
]
J
(b) O.98MHz
gS (a) 0.98MHz i !.2K
/
0.5
1.0 H (k0e)
1.5
Fig. 2. :TAI spin-echo spectra at (a) 1.2 K and 0.98 MHz, (b) 0.4 K and 0.98 MHz, and (c) 0.4 K and 9.2 MHz.
than that at 0.98 MHz. As seen in the figure, the 1/10 linewidth of the spectrum characterizing the broadest component is invariant, whereas both the narrow and the broader components are not discriminated owing to the line broadening of the broader component. To present a detailed field dependence of the spectrum, both the 1 / 1 0 and 1 / 2 line widths at 0.4 K are shown in fig. 3 as a function of the external magnetic field. It should be noted that the 1 / 2 width begins to increase above 3 kOe and the narrow spectrum is smeared o u t j u s t around 3 kOe. Although the spin structure CeAI 3 2~[
0.4K
o vl.0
+
£23
--+-
~/10 WIDTH
0.5 f
9-+ * ~ [
0
t
~
1/2 WIDTH I
I
5 H (kOe)
I
t
1
[
10
Fig. 3. External magnetic field dependences of 1 / 2 and 1/10 linewidth of 27A1 spectra at 0.4 K.
is not yet identified, it seems to be modified around the critical magnetic field, H~ = 3 kOe. Next, we tentatively try to extract some possible spin structures from the characteristic shape of the observed ~-TAIspectrum by referring other antiferromagnetic H F compounds. First, we apply a collinear antiferromagnetic structure found in an isostructural compound (UTh)Pt) [8], which yields two magnetic sites of AI. In this case, the :TAI spectrum is expected to consist of two components with different hyperfine fields generated by the magnetic ordering, which is not successful in explaining the observed complicated line shape consisting of three components (fig. 2(b)) and its magnetic field dependence (fig. 3). Accordingly, it is plausible to take into account of some complex spin structures such as an incommensulate spindensity-wave (SDW) in CeA12 [9] or a double-q structure in CeB~ [10]. It is noteworthy that these types of structures are modified by the magnetic field. In conclusion, a low critical field of H~ = 3 kOe observed in the field dependence of the line shape characterizes the magnetic state appearing in CeA13 below 1.2 K. For a further detailed experiment, a single crystal is required. References [1] S. Barth, H.R. Ott. E.N. Gygax. B. Hitti, E. kippelt. A. Schenck, C. Baines, B. van den Brandit, T. Konter and S. Mango, Phys. Rev, LeU. 59 (1987) 2991. [21 S. Takagi, T. Kimura. N. Sato, T. Satoh and T. Kasuya, J. Phys. Soc. Japan. 57 (1988) 1562. [3] H. Nakanmra, Y. Kitaoka, K, Asayama, Y. Onuki and T. Komatsubara. J. Phys. Soc. Japan. 57 (1988) 2276. [4] Y.J. Uemura, W.J. Kossler, X.H. Yu, H.E. Schone, J.R. Kempton, C,E. Stronach, S. Barth, F.N. Gygax, B. Hitti, A. Schenck, C. Baines, W.E. Lankford, Y. Onuki and 3". Komatsubara, Physica C 153-155 (1988)455. [5] H. Nakamura, Y. Kitaoka. H. Yamada, K. Asayarna, J. Magn. Magn. Mat. 76 & 77 (1988) 517. [6] (3. Aeppli, E,. Bucher, C. Broholm, J.K. Kjems, J. Baumann and J. Hufnagl, Phys. Rev. Lett. 60(1988) 615. [71 H. Nakamura, Y. Kitaoka, K. Asayama and J. Elouquet, J, Phys. Soc. Japan. 57 (1988) 2644. [8] A.I. Goldman, G. Shirane, G. Aeppli. B. Batlogg and E. Bucher, Phys. Rev. B34 (19861 6564. [91 B. Barbara, J.X. Boucherle, J.I,. Buew~z, M.F. Rossignol and J. Scbweizer, Solid State C o m m u n . 24 (1977) 481, ibid 29 (1979) 810. [101 J.M. Fffantin, J. Rossat-Mignod, P. Burlet, tt. Bartholin, S. Kunii and T. Kasuya, J. Magn. Magn. Mat. 47 & 48 (1985) 145.