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Formation of a magnetic-polaron lattice in the low-carrier-density system CeP
Physica B 206 & 207 (1995) 783-785
ELSEVIER
Formation of a magnetic-polaron lattice in the low-carrier-density system CeP M. Kohgi a'*, T. Osakabe a'b, T. Suzuki a, Y. Haga a, T. Kasuya a, Y. Morii b aDepartment of Physics, Tohoku University, Sendai 980-77, Japan bJapan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
Abstract
Neutron diffraction experiments revealed that CeP shows several long period layered magnetic structures composed of F7 and Fs Ce (0 0 1) layers under magnetic field along [0 0 1] as a function of H and T. The phase diagram obtained is discussed in terms of the lattice formation of strong magnetic polarons with F8 symmetry.
1. Introduction
The NaCl-type compounds Ce-monopnictides are compensated semi-metals with charge carriers composed of hole states around the F-point, which originate mainly from pnictogen p-electrons, and conduction electron states around the X-point, which originate mainly from Ce 5d-electrons [1]. They have attracted much attention because they show anomalous thermal and magnetic properties including Kondoeffect-like anomalies, although their carrier density is very small (10-2-10-3/f.u.) [2]. Since the low carrier density comes from their band structure, it is expected that pure samples of these compounds will exhibit essentially intrinsic properties of low carrier density systems. Very recently, high quality single crystals of CeP were grown, and interesting features were found in this compound [3-6]. Kasuya et al. suggested that the magnetic polaron effect is important in these compounds due to the combined effect of Wigner crystallization and strong p - f mixing [7]. The results of neutron scattering experiments demonstrated that a strong magnetic polaron lattice is actually realized in CeP in a magnetic field at low temperatures [4]. In this
* Corresponding author.
paper, the temperature dependence of the magnetic polaron lattice is discussed based on the neutron scattering experiments.
2. Results and discussion
The experiments were performed at the triple axis spectrometer TAS-1 installed at the 2G beam hole of the JRR-3M reactor in J A E R I , Tokai. The sample was a single crystal of size about 3 x 4 x I mm 3 which was obtained from the same batch ( C e P # 3 ) as that used in the previous experiment. A horizontal magnetic field up to 6 T was applied along the [0 0 1] axis. First, we summarize briefly the low temperature properties of CeP. A t zero-magnetic-field, CeP shows a typical type-I antiferromagnetic ordering below the N6el temperature of T N = 10.5 K, where each ferromagneticaUy coupled (00 1) Ce layer stacks antiferromagnetically in the direction of the magnetic moment perpendicular to the plane. The magnetic moment per Ce atom was estimated to be 0.8 - 0.1p,a which is close to that of a F7 crystal field state Ce 3÷ ion (0.7/za). However, the dependence of static quantities such as the specific heat, resistivity or magnetization on the temperature and applied external
0921-4526/95/$09.50 (~) 1995 Elsevier Science B.V. All rights reserved S S D I 0921-4526(94)00583-4
784
M. Kohgi et al. / Physica B 206 & 207 (1995) 783-785
i
1500
I
10
CeP #3 / cl
[--
/
T= 1.5K H=2.7T
Tc2
a~;
II 5
AF
o
(2,2,2+~)
o
(3,3,3+~)
"
i
i
,1 ~ I 1 JI
~ "t~' " ', ' ", l ~ l
2o.~ - 0.7 p.B
,
f
I
~ ~ ~ ,'
~ ~
t t t t
500
HclI 0
1000
.~
Hc2
i
CeP
I
I
10
20
30
T (g) Fig. 1. Phase diagram of CeP under magnetic field along [0,0,1] (see text). field is anomalous [3]. The H - T phase diagram for the sample CeP#3 obtained from them is shown schematically in Fig. 1 for magnetic fields below 10T. The compound also shows anomalous properties above 10 T [5], but they are not discussed here. The phase boundaries determined from the peak positions of the specific heat are denoted as Tcl(H ) and Tc2(H ). Below Tc2 , a ferromagnetic component appears. The phase boundaries H c l ( T ) and Hc2(T ) were determined from the field values where step-wise increases in magnetization were observed. The phases for Hc~ < H < H c z and T < T c ~ , H > H c 2 and T < T c ~ , and Tc~ < T < Tc2, are denoted as I, II, and III, respectively, as shown in Fig. 1. Although there are some mixed phases between them they are not shown here for the sake of simplicity. Very interesting magnetic structures were observed in these phases by neutron diffraction experiments under a magnetic field. The results in phases I and II at 5 K have already been reported in Ref. [4]. The scattering pattern in each phase was composed of a ferromagnetic component and an array of satellite peaks along the direction of the applied magnetic field (11[0,0,1]). The pattern in Phase I obtained at 1.5 K in the measurement at TAS-1 is shown in Fig. 2, where the measurement was carried out by applying the magnetic field after the sample was cooled down to a low temperature in zero applied field, as in the previous experiment. The positions of the satellite peaks are given by multiples of wave vector (2~r/a)& where a is the lattice parameter and ~ = 0 . 1 8 2 ~ 2 / 1 1 . The proposed magnetic structure in
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Fig. 2. Scattering pattern in phase I. The inset shows the proposed magnetic structure of CeP in phase I. phase I is also shown as an inset in Fig. 2. Here, double ( 0 0 1 ) layers containing Ce atoms with an almost saturated moment value of - 2 / z a per Ce along the applied field are periodically located among the antiferromagnetically coupled F7 Ce layers with a moment value of about 0.7/xB per Ce in a sequence of eleven ( 0 0 1 ) layers. In phase II, only the fifth satellite peak intensity becomes small. Thus, it was suggested that the direction of the magnetic moment of the F7 Ce lattice as well as that of the antiferromagnetic stacking had flopped from the direction of the applied field, without changing the order of the 2/za Ce layers, in order to gain Zeeman energy. The actual magnetic structure of the F7 lattice in phase II has not yet been discovered. In the previous paper, the 2/zB layers were interpreted as the 4fFs state caused by the magnetic polaron effect. Thus, the layers are called simply as the ' Fs layers' hereafter. In phase III, as shown in Fig. 3(a), a similar scattering pattern as that in phase II was observed when the sample was heated to the temperature region Tc~ < T < Tc2 from phase I under a magnetic field, indicating that a long period magnetic polaron lattice still exists in phase III with the same sequence as that in phases I or II. The intensity of the fifth satellite peak, which is a good measure of the antiferromagnetic ordering in the F7 Ce layers, decreases gradually with increasing temperature from phase I to phase III. Thus, the antiferromagnetic ordering in the F7 Ce layers which are sandwiched by the Fs layers was thought to become very weak above Tc~ with a
M. Kohgi et al. / Physica B 206 & 207 (1995) 783-785 i
i
i
i
]
,--.600
CeP
.d
o t2,2,z~ (3,3.1+~)
(a)
c~ 400
t2
T=10K H=2.7T
"~ 200 0 I
,-.600 .4
t_
(b) 400
"~ 2oo-
A r
.
0.0
T--10K H=5.3T
t
t
t
I
0.2
0.4
0.6
0.8
"
I
1.0
1.2
Fig. 3. Scattering patterns in phase III (T= 10K) at (a) H = 2 . 7 T a n d (b) H = 5 . 3 T . second-order-transition-like behavior. However, the intensity of the fifth satellite peak shows rather a long tail on the high temperature side than Tc~ which is defined as the peak position of the specific heat. In contrast, the intensities of the other satellite peaks became a little smaller with increasing temperature and then disappeared suddenly at Tca. Thus, the phase boundary Tca(H ) corresponds to the disappearance of the F8 magnetic polaron lattice, and has a first order transition character, in good agreement with the static measurements [3]. Interestingly, a slightly different spin structure is also seen in phase III. Fig. 3(b) shows the scattering pattern obtained with increasing magnetic field from 2.7 to 5.3T at the same temperature ( T = 10K). Here, satellite peaks appear at multiples of (2~r/a)~' instead of (2~r/a)& where 8' = 2 / 1 0 . This indicates that the period of the polaron lattice becomes ten layers rather than eleven layers, which is seen in the zero-field-cooling measurement. A similar pattern
785
with a ten-layer period can be seen in phase II if the sample is cooled down into phase II from the paramagnetic region under an applied field. The situation is a little different in the case of phase I: if the sample is cooled down in an applied field into phase I, the pattern becomes just an intermediate between those of ten and eleven layer periodicity. These facts indicate that the interaction between the F8 magnetic polaron lattice and the G Ce lattice is relatively weak compared to those in each lattice and the former lattice prefers a ten Ce layer period than an eleven one, if the effect of ordering in the latter is negligible. It is now clear that the anomalous phase diagram of CeP under a magnetic field originates from the interplay of the formation of the strong magnetic polaron lattice with Fs symmetry and the antiferromagnetic ordering in the F7 lattice. For T > Tc2(H), the system becomes paramagnetic. However, the magnetic polaron effect was thought to play still an important role (weak polaron state) in the anomalous thermal properties in CeP [7,8]. A more detailed report on the experimental results will be published in a separate paper.
References
[1] T. Kasuya, O. Sakai, J. Tanaka, H. Kitazawa and T. Suzuki, J. Magn. Magn. Mater. 63-64 (1987) 9. [2] T. Suzuki, in: Physical Properties of Actinide and Rare Earth compounds, JJAP Series 8 (Publication office, Japanese Journal of Applied Physics, Tokyo, 1993) p. 267. [3] Y. Haga, Y.S. Kwon, T. Suzuki and T. Kasuya, unpublished. [4] M. Kohgi, T. Osakabe, K. Kakurai, T. Suzuki, Y. Haga and T. Kasuya, Phys. Rev. B 49 (1994) 7068. [5] T. Kuroda, K. Sugiyama, Y. Haga, T. Suzuki, A. Yamagishi and M. Date, Physica B 186-188 (1993) 396. [6] N, M6ri, Y. Okayama, H. Takahashi, Y. Haga and T. Suzuki, Physica B 186-188 (1993) 444. [7] T. Kasuya, Y. Haga, K.S. Kwon and T. Suzuki, Physica B 186-188 (1993) 9; T. Kasuya, T. Suzuki and Y. Haga, J. Phys. Soc. Japan 62 (1993) 2549. [8] T. Kasuya and Y. Haga, Solid State Commun., in press.
ELSEVIER
Formation of a magnetic-polaron lattice in the low-carrier-density system CeP M. Kohgi a'*, T. Osakabe a'b, T. Suzuki a, Y. Haga a, T. Kasuya a, Y. Morii b aDepartment of Physics, Tohoku University, Sendai 980-77, Japan bJapan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
Abstract
Neutron diffraction experiments revealed that CeP shows several long period layered magnetic structures composed of F7 and Fs Ce (0 0 1) layers under magnetic field along [0 0 1] as a function of H and T. The phase diagram obtained is discussed in terms of the lattice formation of strong magnetic polarons with F8 symmetry.
1. Introduction
The NaCl-type compounds Ce-monopnictides are compensated semi-metals with charge carriers composed of hole states around the F-point, which originate mainly from pnictogen p-electrons, and conduction electron states around the X-point, which originate mainly from Ce 5d-electrons [1]. They have attracted much attention because they show anomalous thermal and magnetic properties including Kondoeffect-like anomalies, although their carrier density is very small (10-2-10-3/f.u.) [2]. Since the low carrier density comes from their band structure, it is expected that pure samples of these compounds will exhibit essentially intrinsic properties of low carrier density systems. Very recently, high quality single crystals of CeP were grown, and interesting features were found in this compound [3-6]. Kasuya et al. suggested that the magnetic polaron effect is important in these compounds due to the combined effect of Wigner crystallization and strong p - f mixing [7]. The results of neutron scattering experiments demonstrated that a strong magnetic polaron lattice is actually realized in CeP in a magnetic field at low temperatures [4]. In this
* Corresponding author.
paper, the temperature dependence of the magnetic polaron lattice is discussed based on the neutron scattering experiments.
2. Results and discussion
The experiments were performed at the triple axis spectrometer TAS-1 installed at the 2G beam hole of the JRR-3M reactor in J A E R I , Tokai. The sample was a single crystal of size about 3 x 4 x I mm 3 which was obtained from the same batch ( C e P # 3 ) as that used in the previous experiment. A horizontal magnetic field up to 6 T was applied along the [0 0 1] axis. First, we summarize briefly the low temperature properties of CeP. A t zero-magnetic-field, CeP shows a typical type-I antiferromagnetic ordering below the N6el temperature of T N = 10.5 K, where each ferromagneticaUy coupled (00 1) Ce layer stacks antiferromagnetically in the direction of the magnetic moment perpendicular to the plane. The magnetic moment per Ce atom was estimated to be 0.8 - 0.1p,a which is close to that of a F7 crystal field state Ce 3÷ ion (0.7/za). However, the dependence of static quantities such as the specific heat, resistivity or magnetization on the temperature and applied external
0921-4526/95/$09.50 (~) 1995 Elsevier Science B.V. All rights reserved S S D I 0921-4526(94)00583-4
784
M. Kohgi et al. / Physica B 206 & 207 (1995) 783-785
i
1500
I
10
CeP #3 / cl
[--
/
T= 1.5K H=2.7T
Tc2
a~;
II 5
AF
o
(2,2,2+~)
o
(3,3,3+~)
"
i
i
,1 ~ I 1 JI
~ "t~' " ', ' ", l ~ l
2o.~ - 0.7 p.B
,
f
I
~ ~ ~ ,'
~ ~
t t t t
500
HclI 0
1000
.~
Hc2
i
CeP
I
I
10
20
30
T (g) Fig. 1. Phase diagram of CeP under magnetic field along [0,0,1] (see text). field is anomalous [3]. The H - T phase diagram for the sample CeP#3 obtained from them is shown schematically in Fig. 1 for magnetic fields below 10T. The compound also shows anomalous properties above 10 T [5], but they are not discussed here. The phase boundaries determined from the peak positions of the specific heat are denoted as Tcl(H ) and Tc2(H ). Below Tc2 , a ferromagnetic component appears. The phase boundaries H c l ( T ) and Hc2(T ) were determined from the field values where step-wise increases in magnetization were observed. The phases for Hc~ < H < H c z and T < T c ~ , H > H c 2 and T < T c ~ , and Tc~ < T < Tc2, are denoted as I, II, and III, respectively, as shown in Fig. 1. Although there are some mixed phases between them they are not shown here for the sake of simplicity. Very interesting magnetic structures were observed in these phases by neutron diffraction experiments under a magnetic field. The results in phases I and II at 5 K have already been reported in Ref. [4]. The scattering pattern in each phase was composed of a ferromagnetic component and an array of satellite peaks along the direction of the applied magnetic field (11[0,0,1]). The pattern in Phase I obtained at 1.5 K in the measurement at TAS-1 is shown in Fig. 2, where the measurement was carried out by applying the magnetic field after the sample was cooled down to a low temperature in zero applied field, as in the previous experiment. The positions of the satellite peaks are given by multiples of wave vector (2~r/a)& where a is the lattice parameter and ~ = 0 . 1 8 2 ~ 2 / 1 1 . The proposed magnetic structure in
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Fig. 2. Scattering pattern in phase I. The inset shows the proposed magnetic structure of CeP in phase I. phase I is also shown as an inset in Fig. 2. Here, double ( 0 0 1 ) layers containing Ce atoms with an almost saturated moment value of - 2 / z a per Ce along the applied field are periodically located among the antiferromagnetically coupled F7 Ce layers with a moment value of about 0.7/xB per Ce in a sequence of eleven ( 0 0 1 ) layers. In phase II, only the fifth satellite peak intensity becomes small. Thus, it was suggested that the direction of the magnetic moment of the F7 Ce lattice as well as that of the antiferromagnetic stacking had flopped from the direction of the applied field, without changing the order of the 2/za Ce layers, in order to gain Zeeman energy. The actual magnetic structure of the F7 lattice in phase II has not yet been discovered. In the previous paper, the 2/zB layers were interpreted as the 4fFs state caused by the magnetic polaron effect. Thus, the layers are called simply as the ' Fs layers' hereafter. In phase III, as shown in Fig. 3(a), a similar scattering pattern as that in phase II was observed when the sample was heated to the temperature region Tc~ < T < Tc2 from phase I under a magnetic field, indicating that a long period magnetic polaron lattice still exists in phase III with the same sequence as that in phases I or II. The intensity of the fifth satellite peak, which is a good measure of the antiferromagnetic ordering in the F7 Ce layers, decreases gradually with increasing temperature from phase I to phase III. Thus, the antiferromagnetic ordering in the F7 Ce layers which are sandwiched by the Fs layers was thought to become very weak above Tc~ with a
M. Kohgi et al. / Physica B 206 & 207 (1995) 783-785 i
i
i
i
]
,--.600
CeP
.d
o t2,2,z~ (3,3.1+~)
(a)
c~ 400
t2
T=10K H=2.7T
"~ 200 0 I
,-.600 .4
t_
(b) 400
"~ 2oo-
A r
.
0.0
T--10K H=5.3T
t
t
t
I
0.2
0.4
0.6
0.8
"
I
1.0
1.2
Fig. 3. Scattering patterns in phase III (T= 10K) at (a) H = 2 . 7 T a n d (b) H = 5 . 3 T . second-order-transition-like behavior. However, the intensity of the fifth satellite peak shows rather a long tail on the high temperature side than Tc~ which is defined as the peak position of the specific heat. In contrast, the intensities of the other satellite peaks became a little smaller with increasing temperature and then disappeared suddenly at Tca. Thus, the phase boundary Tca(H ) corresponds to the disappearance of the F8 magnetic polaron lattice, and has a first order transition character, in good agreement with the static measurements [3]. Interestingly, a slightly different spin structure is also seen in phase III. Fig. 3(b) shows the scattering pattern obtained with increasing magnetic field from 2.7 to 5.3T at the same temperature ( T = 10K). Here, satellite peaks appear at multiples of (2~r/a)~' instead of (2~r/a)& where 8' = 2 / 1 0 . This indicates that the period of the polaron lattice becomes ten layers rather than eleven layers, which is seen in the zero-field-cooling measurement. A similar pattern
785
with a ten-layer period can be seen in phase II if the sample is cooled down into phase II from the paramagnetic region under an applied field. The situation is a little different in the case of phase I: if the sample is cooled down in an applied field into phase I, the pattern becomes just an intermediate between those of ten and eleven layer periodicity. These facts indicate that the interaction between the F8 magnetic polaron lattice and the G Ce lattice is relatively weak compared to those in each lattice and the former lattice prefers a ten Ce layer period than an eleven one, if the effect of ordering in the latter is negligible. It is now clear that the anomalous phase diagram of CeP under a magnetic field originates from the interplay of the formation of the strong magnetic polaron lattice with Fs symmetry and the antiferromagnetic ordering in the F7 lattice. For T > Tc2(H), the system becomes paramagnetic. However, the magnetic polaron effect was thought to play still an important role (weak polaron state) in the anomalous thermal properties in CeP [7,8]. A more detailed report on the experimental results will be published in a separate paper.
References
[1] T. Kasuya, O. Sakai, J. Tanaka, H. Kitazawa and T. Suzuki, J. Magn. Magn. Mater. 63-64 (1987) 9. [2] T. Suzuki, in: Physical Properties of Actinide and Rare Earth compounds, JJAP Series 8 (Publication office, Japanese Journal of Applied Physics, Tokyo, 1993) p. 267. [3] Y. Haga, Y.S. Kwon, T. Suzuki and T. Kasuya, unpublished. [4] M. Kohgi, T. Osakabe, K. Kakurai, T. Suzuki, Y. Haga and T. Kasuya, Phys. Rev. B 49 (1994) 7068. [5] T. Kuroda, K. Sugiyama, Y. Haga, T. Suzuki, A. Yamagishi and M. Date, Physica B 186-188 (1993) 396. [6] N, M6ri, Y. Okayama, H. Takahashi, Y. Haga and T. Suzuki, Physica B 186-188 (1993) 444. [7] T. Kasuya, Y. Haga, K.S. Kwon and T. Suzuki, Physica B 186-188 (1993) 9; T. Kasuya, T. Suzuki and Y. Haga, J. Phys. Soc. Japan 62 (1993) 2549. [8] T. Kasuya and Y. Haga, Solid State Commun., in press.