- Email: [email protected]
Anomalous metamagnetic behavior of TbCu2Ge2 single crystal
ARTICLE IN PRESS
Physica B 346–347 (2004) 112–116
Anomalous metamagnetic behavior of TbCu2Ge2 single crystal Toru Shigeokaa,*, Masataka Shiraishia, Hiroyuki Mitamura, Yoshiya Uwatokob, Tetsuya Fujiwarab, Tuneaki Gotob b
a Faculty of Science, Yamaguchi University, Yamaguchi 753-8512, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa-shi, Chiba 277-8581, Japan
Abstract The magnetic characteristics of the single crystal compound TbCu2Ge2 have been studied by magnetic susceptibility and magnetization measurements. The susceptibility shows two anomalies at TN=12.3 K and Tt=9.3 K. The magnetic easy direction is the [1 1 0] direction in the basal plane. The magnetization along the easy direction in the basal plane shows a multi-step (at least four steps) metamagnetic process. No significant change of the process is observed at Tt. A one-step metamagnetic transition appears around 17 T in the hard c-axis magnetization process. This transition persists above TN up to 25 K where the susceptibility shows a broad peak. The origin is unsolved yet. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Cr; 75.30.Kz; 75.30.Gw Keywords: TbCu2Ge2; Metamagnetic transition; High-field magnetization process; Antiferromagnetism
1. Introduction The ternary compounds RCu2Ge2 (R=rare earth) belong to the large family having the tetragonal ThCr2Si2-type crystal structure (space group: I4/mmm) [1]. Most of them show antiferromagnetic order and a great variety of magnetic behavior [2,3]. An interesting magnetic behavior of a TbCu2Ge2 single crystal has been reported by Song et al. [4]. Two magnetic transitions appear at TN=12.3 K and Tt=9.6 K. A reorientation of the Tb moments from [1 1 0] to [1 0 0] in the basal plane at Tt has been proposed *Corresponding author. Tel.: +81-83-933-5674; fax: +8183-933-5768. E-mail address: [email protected] (T. Shigeoka).
from the result of resonant and non-resonant X-ray magnetic scattering. The antiferromagnetic structure, reported from powder neutron diffraction studies [5,6], is characterized by the wave vector k= (1/2, 0, 1/2), having Tb moments parallel to the [1 1 0] direction. No change in the wave vector at Tt is detected; only one transition at TN was revealed. There is no evidence for the spin reorientation from neutron studies. A metamagnetic behavior has been reported from magnetization measurement on a poly-crystalline compound [7]. The details, however, are unknown yet. In order to elucidate the magnetic transitions and metamagnetic behavior of TbCu2Ge2, a detailed magnetic study has been carried out on a TbCu2Ge2 single crystal. The single crystal has been grown by the tri-arc Czochralski method. The single phase nature has
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.01.031
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
been confirmed by X-ray powder diffraction. High-field magnetization measurements up to 47 T have been performed using a pulse magnet. Low-field magnetization below 7 T and magnetic susceptibility measurements have been carried out using a superconducting magnet by a sample extracting and a SQUID magnetometer. All magnetic measurements have been carried out at the Institute for Solid State Physics, University of Tokyo.
2. Results and discussion The temperature dependences of the magnetic susceptibility along the main symmetry axes of the tetragonal cell are shown on a TbCu2Ge2 single crystal for low temperatures in Fig. 1. The main features obtained are similar to those reported by Song et al. [4] although they showed the susceptibility along only two directions, parallel and perpendicular to the basal plane. A strong anisotropy is observed between the easy direction of [1 1 0] in the basal plane and the hard direction of [0 0 1], the c-axis. Within the basal plane, an anisotropy is evidenced between the [1 1 0] direction and the [1 0 0] direction at low temperatures, but it is weak. The susceptibility along the [1 1 0] direction is larger than that of the [1 0 0] direction
at low temperatures. The susceptibilities along the [1 0 0] direction and the [1 1 0] direction intersect at about 8.7 K; that of [1 0 0] becomes larger than that of [1 1 0] above 8.7 K. These behaviors indicate that the magnetic moments direct the [1 1 0] direction at low temperatures, which is consistent with reports by neutron studies [4,5]. And if the proposed spin reorientation is realized; a magnetic moment leans gradually out [1 1 0] to [1 0 0] with increasing temperature. The susceptibility behavior means the moment angle with respect to [1 0 0] becomes smaller than p/4 above 8.7 K. Two anomalies, indicating magnetic transitions, are seen at Tt=9.3 K and TN=12.3 K along all directions although that of [0 0 1] is very small. Both temperatures are in good agreement with the previous result of 9.6 and 12.3 K [3]. The latter is the antiferromagnetic ordering temperature, the Ne! el temperature. It has been proposed that the moment, gradually tilting with increasing temperature, reaches the [1 0 0] direction at Tt [3]. The magnetic susceptibility for high temperature up to 300 K is shown in Fig. 2. Along the [0 0 1] direction, an additional anomaly, a broad peak, appears around 25 K, which may indicate the existence of some magnetic correlation. The reciprocal susceptibility along all directions is linear above about 100 K in the paramagnetic region; it follows the Curie–Weiss law (see the inset of Fig. 2). The effective magnetic moment
0.8
0.8 TbCu2Ge2
0.7
0.6
χ (µB/T-f.u.)
χ (µB/T-f.u.)
B=0.1 T 0.5 [110]
0.4 0.3
[001]
[110]
0.5
[100]
0.4
5
10
15
20
25
10
[100] [110]
5 0 0
0.3
50 100 150 200 250 300 T (K)
[001]
0.1 0
TbCu2Ge2 B=0.1 T [001]
15
0.2
[100]
0.2
B=0.1 T
0.6
20
1/χ (µB/T-f.u.)
TbCu2Ge2
0.7
0.1
113
30
35
40
T (K) Fig. 1. Temperature dependence of the magnetic susceptibility along the main symmetry axes of the tetragonal cell at low temperatures of a TbCu2Ge2 single crystal.
0
0
50
100
150
200
250
300
T (K) Fig. 2. The temperature dependence of the magnetic susceptibility in the wider temperature range up to 300 K. The inset shows the reciprocal susceptibility.
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
transition are 2.1mB, 3.9mB, 5.3mB and 7.0mB which correspond to (5/18) Ms, (7/18) Ms, (12/18) Ms and (16/18) Ms, respectively (the value of Ms will be given below). It is worth noting that the moment values have the common denominator of 18. This supposes that a spin-flip is responsible for the transitions. These transitions smear out with increasing temperature. Each transition shows a hysteresis. This multi-step behavior is difficult to explain on the basis of the simple antiferromagnetic structure reported, suggesting a long period antiferromagnetic one. 10 [110]
8 6 [100]
B
estimated is 9.2 mB which is smaller than the value for a Tb3+ free ion. Paramagnetic Curie temperatures Y[1 0 0], Y[1 1 0] and Y[0 0 1] are 3.9, 0.6 and – 27.6 K, respectively, indicating a strong antiferromagnetic coupling along the [0 0 1] direction and a weak ferromagnetic coupling along the [1 0 0] direction which are consistent with the reported antiferromagnetic structure. The magnetization as a function of applied magnetic field along the main symmetry axes at 1.7 K below 7 T is shown in Fig. 3. A strong anisotropy between directions in the basal plane and the c-axis is obvious. The magnetization along the hard direction of the c-axis increases almost linearly with a small gradient and reaches only 1.0 mB at 7 T due to a strong antiferromagnetic interaction along the c-axis. Within the basal plane, the magnetization behavior is more complex. In the [1 0 0] magnetization process, a metamagnetic transition around 2.5 T and a change in the slope at 4.5 and 6.0 T appear, followed by a gradual increase. The [1 1 0] process is most complex; four metamagnetic transitions can be seen though it is not so sharp due to thermal effects. The transition fields, which are defined by peak fields in the dM/dB vs. B curve as shown in the inset of Fig. 3, are Bc1=3.8 T, Bc2=4.6, Bc3=6.0 and Bc4=6.8 T in the ascending process. The moment values just after each
M (µ /f.n.)
114
4 [001]
TbCu2Ge2
2
T=1.7[K] 0 0
10
20
30
40
50
B (T) Fig. 4. High-field magnetization curves along the main symmetry axes at 1.7 K.
8
5 4
TbCu2Ge2
[110]
8
T=1.7[K] 7 [100] 0
1
2
3
4
5
6
7
B(T)
3
[110]
2 1 1
2
3
4
5
[100]
6
5 4 [001]
3 2
[001] 0 0
[110]
6
B
B
M (µ /f.u.)
6
3.5 3 2.5 2 1.5 1 0.5 0
M (µ /f.u.)
dM/dB(µB/T)
7
7
B (T) Fig. 3. Magnetization as a function of applied magnetic field in low fields along the main symmetry axes at 1.7 K of the TbCu2Ge2 single crystal. The inset shows the dM/dB vs. B plot along [1 1 0].
TbCu Ge 2 2
1
T=10.0[K]
0 0
5
10
15
20
25
30
B (T) Fig. 5. High-field magnetization curves along the main symmetry axes above Tt at 10 K.
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
8 7 [110]
B
M (µ /f.u.)
6
[100]
5 4
[001]
3 2
TbCu2Ge2
1
T=15 K
0 0
5
10
15
20
25
30
B (T) Fig. 6. High-field magnetization curves along the main symmetry axes above TN at 15 K.
8 7
TbCu2Ge2
B
M (µ /f.u.)
6
[110]
T=30 K
[100]
5 4 [001]
3 2 1 0
0
5
10
15
20
25
30
B (T) Fig. 7. High-field magnetization curves along the main symmetry axes at 30 K.
High-field magnetization up to 47 T under pulsed magnetic field at 1.7 K and up to 30 T at 10, 15, 30 K are shown in Figs. 4,5,6 and 7, respectively. It is obvious that the easy magnetization direction is [1 1 0] in high fields at all temperatures, which is consistent with neutron studies [3,4], but it does not support a spinreorienation transition proposed for the transition at Tt. At 1.7 K (see in Fig. 3), the magnetization along [1 1 0] is almost saturated around 10 T after the four metamagnetic transitions in low fields. At maximum field of 47 T, the moment value reaches
115
Ms=8.6 mB which is close to the Tb3+ free ion moment and is in good agreement with the neutron result [5,6]. The [1 0 0] magnetization process shows a change in the slope around 8 T. The moment value at the knee is about 5.8mB which is close to Ms cos(p/4)=6.1mB (here, the angle is that between the a-axis and the magnetic moment), indicating that a rectangular configuration of two moments coupled antiferromagnetically is favored in the basal plane. This suggests that some effects to stabilize the configuration such as quadrupolar effects, proposed to PrCu2Ge2 [8], play a role in this compound. At 10 K above Tt, both processes in the basal plane are still antiferromagnetic. The easy magnetization direction becomes [1 0 0] in low fields as evidenced in Fig. 5. This change of the easy direction is consistent with the intersection of both susceptibilities as mentioned above. Above TN, both processes are identical within the experimental accuracy and become paramagnetic (see in Fig. 6). On the other hand, along the c-axis perpendicular to the basal plane, a one-step metamagnetic transition appears around the critical field Bc of 16 T. Above Bc, the magnetization is almost saturated. This magnetization process should be considered as follows; magnetic moments firstly tilt gradually with increasing field from the [1 1 0] direction to the magnetic field direction of the c-axis below Bc. At Bc, all moments flop simultaneously to the c-axis by a cooperative phenomenon. It should be noted that this metamagnetic transition still survives above TN in the paramagnetic region as evidenced in Fig. 6 although thermal effects smear it out. The critical field Bc is independent on temperature. This transition disappears around 25 K where the broad peak appears in the w-T curve (see in Fig. 2). This anomalous metamagnetic transition may come from a short-range order due to the strong antiferromagnetic coupling along the c-axis indicated by the large negative paramagnetic Curie temperature of Y[0 0 1]. No evidence of the shortrange order is, however, obtained yet. The magnetization processes along all directions become paramagnetic already at 30 K as shown in Fig. 7. Here, a strong anisotropy between the c-axis and that in the basal plane is still observed.
ARTICLE IN PRESS 116
T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
In summary, high-field magnetization measurements have been performed on a TbCu2Ge2 single crystal as well as low-field magnetization and susceptibility measurements. The existence of two magnetic transitions at Tt=9.3 K and TN=12.3 K is confirmed. Although the former is proposed to be a spin reorientation, a clear evidence is not obtained from this magnetic study. The susceptibility along the c-axis shows another anomaly, a broad peak, around 25 K above TN. A multi-step (at least four steps) metamagnetic transition appears in the easy [1 1 0] magnetization process, caused by spin-flip, suggesting a long period antiferromagnetic structure. A very peculiar phenomenon appears along the hard c-axis; a one-step metamagnetic transition, that survives above TN, appears. It may originate from a short-range order due to antiferromagnetic correlations. The details are, however, unknown yet. To understand this magnetic behavior completely, further study is need.
References [1] A. Szytula, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North-Holland, Amsterdam, 1991, pp. 85–180. [2] D. Gignoux, D. Schmitt, in: K.A. Gschneidner, L. Eyrings (Eds.), Handbook of the Physics and Chemistry of Rare Earths, Vol. 20, Elsevier Science, Amsterdam, 1995, pp. 293–424 (Chapter 138). [3] D. Gignoux, D. Schmitt, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 10, North-Holland, Amsterdam, 1997, pp. 239–414. [4] C. Song, D. Johnson, D. Wermeille, A.I. Goldman, S.L. Budo’ko, I.R. Fisher, P.C. Canfield, Phys. Rev. B 64 (2001) 224414. [5] P.S. Schobinger, A. Niggli, J. Phys. Chem. Solids 45 (1984) 695. [6] H. Pinto, M. Melamud, M. Kuznietz, H. Shaked, Phys. Rev. B 31 (1985) 508. [7] P.A. Kotsanidis, J.K. Yakinthos, Solid State Commun. 40 (1981) 1014. [8] I. Das, E.V. Sampathkumaran, J. Phys.: Condens. Matter 6 (1994) L557.
Physica B 346–347 (2004) 112–116
Anomalous metamagnetic behavior of TbCu2Ge2 single crystal Toru Shigeokaa,*, Masataka Shiraishia, Hiroyuki Mitamura, Yoshiya Uwatokob, Tetsuya Fujiwarab, Tuneaki Gotob b
a Faculty of Science, Yamaguchi University, Yamaguchi 753-8512, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa-shi, Chiba 277-8581, Japan
Abstract The magnetic characteristics of the single crystal compound TbCu2Ge2 have been studied by magnetic susceptibility and magnetization measurements. The susceptibility shows two anomalies at TN=12.3 K and Tt=9.3 K. The magnetic easy direction is the [1 1 0] direction in the basal plane. The magnetization along the easy direction in the basal plane shows a multi-step (at least four steps) metamagnetic process. No significant change of the process is observed at Tt. A one-step metamagnetic transition appears around 17 T in the hard c-axis magnetization process. This transition persists above TN up to 25 K where the susceptibility shows a broad peak. The origin is unsolved yet. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Cr; 75.30.Kz; 75.30.Gw Keywords: TbCu2Ge2; Metamagnetic transition; High-field magnetization process; Antiferromagnetism
1. Introduction The ternary compounds RCu2Ge2 (R=rare earth) belong to the large family having the tetragonal ThCr2Si2-type crystal structure (space group: I4/mmm) [1]. Most of them show antiferromagnetic order and a great variety of magnetic behavior [2,3]. An interesting magnetic behavior of a TbCu2Ge2 single crystal has been reported by Song et al. [4]. Two magnetic transitions appear at TN=12.3 K and Tt=9.6 K. A reorientation of the Tb moments from [1 1 0] to [1 0 0] in the basal plane at Tt has been proposed *Corresponding author. Tel.: +81-83-933-5674; fax: +8183-933-5768. E-mail address: [email protected] (T. Shigeoka).
from the result of resonant and non-resonant X-ray magnetic scattering. The antiferromagnetic structure, reported from powder neutron diffraction studies [5,6], is characterized by the wave vector k= (1/2, 0, 1/2), having Tb moments parallel to the [1 1 0] direction. No change in the wave vector at Tt is detected; only one transition at TN was revealed. There is no evidence for the spin reorientation from neutron studies. A metamagnetic behavior has been reported from magnetization measurement on a poly-crystalline compound [7]. The details, however, are unknown yet. In order to elucidate the magnetic transitions and metamagnetic behavior of TbCu2Ge2, a detailed magnetic study has been carried out on a TbCu2Ge2 single crystal. The single crystal has been grown by the tri-arc Czochralski method. The single phase nature has
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.01.031
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
been confirmed by X-ray powder diffraction. High-field magnetization measurements up to 47 T have been performed using a pulse magnet. Low-field magnetization below 7 T and magnetic susceptibility measurements have been carried out using a superconducting magnet by a sample extracting and a SQUID magnetometer. All magnetic measurements have been carried out at the Institute for Solid State Physics, University of Tokyo.
2. Results and discussion The temperature dependences of the magnetic susceptibility along the main symmetry axes of the tetragonal cell are shown on a TbCu2Ge2 single crystal for low temperatures in Fig. 1. The main features obtained are similar to those reported by Song et al. [4] although they showed the susceptibility along only two directions, parallel and perpendicular to the basal plane. A strong anisotropy is observed between the easy direction of [1 1 0] in the basal plane and the hard direction of [0 0 1], the c-axis. Within the basal plane, an anisotropy is evidenced between the [1 1 0] direction and the [1 0 0] direction at low temperatures, but it is weak. The susceptibility along the [1 1 0] direction is larger than that of the [1 0 0] direction
at low temperatures. The susceptibilities along the [1 0 0] direction and the [1 1 0] direction intersect at about 8.7 K; that of [1 0 0] becomes larger than that of [1 1 0] above 8.7 K. These behaviors indicate that the magnetic moments direct the [1 1 0] direction at low temperatures, which is consistent with reports by neutron studies [4,5]. And if the proposed spin reorientation is realized; a magnetic moment leans gradually out [1 1 0] to [1 0 0] with increasing temperature. The susceptibility behavior means the moment angle with respect to [1 0 0] becomes smaller than p/4 above 8.7 K. Two anomalies, indicating magnetic transitions, are seen at Tt=9.3 K and TN=12.3 K along all directions although that of [0 0 1] is very small. Both temperatures are in good agreement with the previous result of 9.6 and 12.3 K [3]. The latter is the antiferromagnetic ordering temperature, the Ne! el temperature. It has been proposed that the moment, gradually tilting with increasing temperature, reaches the [1 0 0] direction at Tt [3]. The magnetic susceptibility for high temperature up to 300 K is shown in Fig. 2. Along the [0 0 1] direction, an additional anomaly, a broad peak, appears around 25 K, which may indicate the existence of some magnetic correlation. The reciprocal susceptibility along all directions is linear above about 100 K in the paramagnetic region; it follows the Curie–Weiss law (see the inset of Fig. 2). The effective magnetic moment
0.8
0.8 TbCu2Ge2
0.7
0.6
χ (µB/T-f.u.)
χ (µB/T-f.u.)
B=0.1 T 0.5 [110]
0.4 0.3
[001]
[110]
0.5
[100]
0.4
5
10
15
20
25
10
[100] [110]
5 0 0
0.3
50 100 150 200 250 300 T (K)
[001]
0.1 0
TbCu2Ge2 B=0.1 T [001]
15
0.2
[100]
0.2
B=0.1 T
0.6
20
1/χ (µB/T-f.u.)
TbCu2Ge2
0.7
0.1
113
30
35
40
T (K) Fig. 1. Temperature dependence of the magnetic susceptibility along the main symmetry axes of the tetragonal cell at low temperatures of a TbCu2Ge2 single crystal.
0
0
50
100
150
200
250
300
T (K) Fig. 2. The temperature dependence of the magnetic susceptibility in the wider temperature range up to 300 K. The inset shows the reciprocal susceptibility.
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
transition are 2.1mB, 3.9mB, 5.3mB and 7.0mB which correspond to (5/18) Ms, (7/18) Ms, (12/18) Ms and (16/18) Ms, respectively (the value of Ms will be given below). It is worth noting that the moment values have the common denominator of 18. This supposes that a spin-flip is responsible for the transitions. These transitions smear out with increasing temperature. Each transition shows a hysteresis. This multi-step behavior is difficult to explain on the basis of the simple antiferromagnetic structure reported, suggesting a long period antiferromagnetic one. 10 [110]
8 6 [100]
B
estimated is 9.2 mB which is smaller than the value for a Tb3+ free ion. Paramagnetic Curie temperatures Y[1 0 0], Y[1 1 0] and Y[0 0 1] are 3.9, 0.6 and – 27.6 K, respectively, indicating a strong antiferromagnetic coupling along the [0 0 1] direction and a weak ferromagnetic coupling along the [1 0 0] direction which are consistent with the reported antiferromagnetic structure. The magnetization as a function of applied magnetic field along the main symmetry axes at 1.7 K below 7 T is shown in Fig. 3. A strong anisotropy between directions in the basal plane and the c-axis is obvious. The magnetization along the hard direction of the c-axis increases almost linearly with a small gradient and reaches only 1.0 mB at 7 T due to a strong antiferromagnetic interaction along the c-axis. Within the basal plane, the magnetization behavior is more complex. In the [1 0 0] magnetization process, a metamagnetic transition around 2.5 T and a change in the slope at 4.5 and 6.0 T appear, followed by a gradual increase. The [1 1 0] process is most complex; four metamagnetic transitions can be seen though it is not so sharp due to thermal effects. The transition fields, which are defined by peak fields in the dM/dB vs. B curve as shown in the inset of Fig. 3, are Bc1=3.8 T, Bc2=4.6, Bc3=6.0 and Bc4=6.8 T in the ascending process. The moment values just after each
M (µ /f.n.)
114
4 [001]
TbCu2Ge2
2
T=1.7[K] 0 0
10
20
30
40
50
B (T) Fig. 4. High-field magnetization curves along the main symmetry axes at 1.7 K.
8
5 4
TbCu2Ge2
[110]
8
T=1.7[K] 7 [100] 0
1
2
3
4
5
6
7
B(T)
3
[110]
2 1 1
2
3
4
5
[100]
6
5 4 [001]
3 2
[001] 0 0
[110]
6
B
B
M (µ /f.u.)
6
3.5 3 2.5 2 1.5 1 0.5 0
M (µ /f.u.)
dM/dB(µB/T)
7
7
B (T) Fig. 3. Magnetization as a function of applied magnetic field in low fields along the main symmetry axes at 1.7 K of the TbCu2Ge2 single crystal. The inset shows the dM/dB vs. B plot along [1 1 0].
TbCu Ge 2 2
1
T=10.0[K]
0 0
5
10
15
20
25
30
B (T) Fig. 5. High-field magnetization curves along the main symmetry axes above Tt at 10 K.
ARTICLE IN PRESS T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
8 7 [110]
B
M (µ /f.u.)
6
[100]
5 4
[001]
3 2
TbCu2Ge2
1
T=15 K
0 0
5
10
15
20
25
30
B (T) Fig. 6. High-field magnetization curves along the main symmetry axes above TN at 15 K.
8 7
TbCu2Ge2
B
M (µ /f.u.)
6
[110]
T=30 K
[100]
5 4 [001]
3 2 1 0
0
5
10
15
20
25
30
B (T) Fig. 7. High-field magnetization curves along the main symmetry axes at 30 K.
High-field magnetization up to 47 T under pulsed magnetic field at 1.7 K and up to 30 T at 10, 15, 30 K are shown in Figs. 4,5,6 and 7, respectively. It is obvious that the easy magnetization direction is [1 1 0] in high fields at all temperatures, which is consistent with neutron studies [3,4], but it does not support a spinreorienation transition proposed for the transition at Tt. At 1.7 K (see in Fig. 3), the magnetization along [1 1 0] is almost saturated around 10 T after the four metamagnetic transitions in low fields. At maximum field of 47 T, the moment value reaches
115
Ms=8.6 mB which is close to the Tb3+ free ion moment and is in good agreement with the neutron result [5,6]. The [1 0 0] magnetization process shows a change in the slope around 8 T. The moment value at the knee is about 5.8mB which is close to Ms cos(p/4)=6.1mB (here, the angle is that between the a-axis and the magnetic moment), indicating that a rectangular configuration of two moments coupled antiferromagnetically is favored in the basal plane. This suggests that some effects to stabilize the configuration such as quadrupolar effects, proposed to PrCu2Ge2 [8], play a role in this compound. At 10 K above Tt, both processes in the basal plane are still antiferromagnetic. The easy magnetization direction becomes [1 0 0] in low fields as evidenced in Fig. 5. This change of the easy direction is consistent with the intersection of both susceptibilities as mentioned above. Above TN, both processes are identical within the experimental accuracy and become paramagnetic (see in Fig. 6). On the other hand, along the c-axis perpendicular to the basal plane, a one-step metamagnetic transition appears around the critical field Bc of 16 T. Above Bc, the magnetization is almost saturated. This magnetization process should be considered as follows; magnetic moments firstly tilt gradually with increasing field from the [1 1 0] direction to the magnetic field direction of the c-axis below Bc. At Bc, all moments flop simultaneously to the c-axis by a cooperative phenomenon. It should be noted that this metamagnetic transition still survives above TN in the paramagnetic region as evidenced in Fig. 6 although thermal effects smear it out. The critical field Bc is independent on temperature. This transition disappears around 25 K where the broad peak appears in the w-T curve (see in Fig. 2). This anomalous metamagnetic transition may come from a short-range order due to the strong antiferromagnetic coupling along the c-axis indicated by the large negative paramagnetic Curie temperature of Y[0 0 1]. No evidence of the shortrange order is, however, obtained yet. The magnetization processes along all directions become paramagnetic already at 30 K as shown in Fig. 7. Here, a strong anisotropy between the c-axis and that in the basal plane is still observed.
ARTICLE IN PRESS 116
T. Shigeoka et al. / Physica B 346–347 (2004) 112–116
In summary, high-field magnetization measurements have been performed on a TbCu2Ge2 single crystal as well as low-field magnetization and susceptibility measurements. The existence of two magnetic transitions at Tt=9.3 K and TN=12.3 K is confirmed. Although the former is proposed to be a spin reorientation, a clear evidence is not obtained from this magnetic study. The susceptibility along the c-axis shows another anomaly, a broad peak, around 25 K above TN. A multi-step (at least four steps) metamagnetic transition appears in the easy [1 1 0] magnetization process, caused by spin-flip, suggesting a long period antiferromagnetic structure. A very peculiar phenomenon appears along the hard c-axis; a one-step metamagnetic transition, that survives above TN, appears. It may originate from a short-range order due to antiferromagnetic correlations. The details are, however, unknown yet. To understand this magnetic behavior completely, further study is need.
References [1] A. Szytula, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North-Holland, Amsterdam, 1991, pp. 85–180. [2] D. Gignoux, D. Schmitt, in: K.A. Gschneidner, L. Eyrings (Eds.), Handbook of the Physics and Chemistry of Rare Earths, Vol. 20, Elsevier Science, Amsterdam, 1995, pp. 293–424 (Chapter 138). [3] D. Gignoux, D. Schmitt, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 10, North-Holland, Amsterdam, 1997, pp. 239–414. [4] C. Song, D. Johnson, D. Wermeille, A.I. Goldman, S.L. Budo’ko, I.R. Fisher, P.C. Canfield, Phys. Rev. B 64 (2001) 224414. [5] P.S. Schobinger, A. Niggli, J. Phys. Chem. Solids 45 (1984) 695. [6] H. Pinto, M. Melamud, M. Kuznietz, H. Shaked, Phys. Rev. B 31 (1985) 508. [7] P.A. Kotsanidis, J.K. Yakinthos, Solid State Commun. 40 (1981) 1014. [8] I. Das, E.V. Sampathkumaran, J. Phys.: Condens. Matter 6 (1994) L557.