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Magnetism in Ce2Fe17 at pressures up to 70 kbar
ARTICLE IN PRESS
Physica B 350 (2004) 63–65
Magnetism in Ce2Fe17 at pressures up to 70 kbar O. Prokhnenkoa,*, I. Goncharenkob, Z. Arnolda, J. Kamara! da b
a Institute of Physics AS CR, Na Slovance 2, 18221 Prague 8, Czech Republic Laboratoire Leon Brillouin, CEA-CNRS, CEA-Saclay, 91191 Gif Sur Yvette Cedex, France
Abstract The magnetic phase diagram of Ce2Fe17 at ambient pressure shows a ferromagnetic (FM) phase transforming into an incommensurate helical antiferromagnetic (AFM) one above YT=97 K, and becoming paramagnetic above TN=214 K. The FM ground state is highly unstable, dYT/dP= 24 K/kbar, whereas TN only moderately decreases with pressure P, dTN/dP= 1.7 K/kbar. Hence, pressures of the order of 100 kbar are necessary to suppress the magnetic ordered state in Ce2Fe17. The suppression of the FM phase under pressure is accompanied by the appearance of a new incommensurate antiferromagnetic (AFM1) phase at low temperatures. To investigate the stability of the magnetic state and the behaviour of the new pressure-induced AFM1 phase, powder neutron diffraction measurements were performed with P p 70 kbar. This completed the magnetic phase diagram of Ce2Fe17 in that region. The results clearly show that the pressure dependence of the N!eel temperature is linear up to 25 kbar. The strong decrease in the intensity of the magnetic (0 0 0)+ satellite with P gives clear evidence for the volume dependence of the Fe magnetic moments. Simultaneously, the incommensurate propagation vector s=(0, 0, tz) increases with pressure, reaching 0.72 r.l.u. at 4 K and 70 kbar. The results show the anisotropic behaviour of the exchange interactions between Fe moments in Ce2Fe17 and their complex dependence on volume changes. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12. q; 74.62.Fj; 75.25.+z; 71.20.Lp Keywords: Neutron diffraction; High pressure; Magnetic structure; Intermetallic compound
Recently, a strong dependence of the magnetic state of Ce2Fe17 intermetallics on interatomic distances has been revealed by magnetization [1] and neutron diffraction [2] measurements under external pressure. The FM ground state that is stable up to YT=97 K at ambient pressure can be completely suppressed by application of relatively low pressure PCE4 kbar. Simultaneously, the external pressure extends the range of stability of *Corresponding author. Tel.: +420-220-318-426; fax: +420233-343-184. E-mail address: [email protected] (O. Prokhnenko).
the AFM phase from the original one YToTo TN=214 K at ambient pressure down to the lowest temperatures at P>PC. Despite to a huge decrease of YT with pressure, dYT/dP= 24 K/kbar [2], the relevant decrease of the Ne! el temperature TN is rather small, dTN/dP= 1.7 K/kbar [1,3]. The last is comparable with decrease of ordering temperatures in other R2Fe17 compounds [4]. In the simplest case, assuming linear dependence of TN on pressure, one can expect that pressures higher than 100 kbar would be indispensable to suppress the magnetic ordered state in Ce2Fe17. Behaviour of the AFM incommensurate helix under pressure
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.254
ARTICLE IN PRESS O. Prokhnenko et al. / Physica B 350 (2004) 63–65 1200
Ce2Fe17 120 K
1000 800
(012)
600
(110) (003)
(000)+ (101)
is also complex. The previous neutron diffraction experiments performed at pressures up to 5 kbar [2] showed that the value of incommensurate propagation vector at 120 K is changed by external ( 1) at pressure from t1z=0.358 r.l.u. (0.029 A 1 ( 0 kbar up to t1z=0.510 r.l.u. (0.041 A ) at 5 kbar. Such significant change of t1z indicates that pressures higher than 30 kbar could shorten the spiral step to become identical with the crystal lattice unit cell. Moreover, the additional pressure induced changes in the original AFM incommensurate helix were detected under pressure above 3 kbar at temperature below 80 K. One additional ( 1 magnetic peak with the wave vector t2=0.079 A was observed in the neutron diffraction experiments in this pressure-temperature range [2]. The new high pressure phase was marked as AFM1. Complex magnetic structures of Ce2Fe17 and their strong dependence on interatomic distances motivated us to investigate the stability and character of magnetism in Ce2Fe17 in a region of very high pressures up to 70 kbar. Polycrystalline samples of Ce2Fe17 were prepared by the induction melting method with subsequent annealing at 1000 C for 2 weeks. The samples were tested by X-ray diffraction that confirmed the single phase Th2Zn17 type of crystal ( and structure with lattice parameters a=8.495 A ( c=12.416 A. Neutron diffraction experiments were carried out on the powdered Ce2Fe17 samples in the temperature range 4–300 K at pressures up to 70 kbar. The data were collected using the high pressure powder diffractometer G6.1 at the ( Laboratoire Leon Brillouin (Saclay) using 4.74 A wavelength. A sapphire anvil pressure cell was used to generate pressure. Use of very high pressures in the neutron diffraction experiment was achieved by an installation of a special focusing system with Ni–Ti supermirrors along the direction of the incident beam. For detailed description of the high-pressure neutron setup, see Ref. [5] and references therein. Despite to the experimental limitations caused by the small sample volume and incoherent scattering by the high pressure cell, we were able to observe a magnetic (0 0 0)+ satellite together with nuclear (1 0 1), (0 1 2), (1 1 0) and (0 0 3)
Intensity
64
0 kbar
400
25 kbar 200 40 kbar 70 kbar
0
10
20
30
40
50
60
70
80
90 100
2θ (degrees) Fig. 1. Neutron diffraction patterns of Ce2Fe17 at 120 K.
reflections. Other magnetic satellites are believed to exist [2,6], nevertheless, their intensities are too small to be detected even at ambient pressure (Fig. 1, 0 kbar). The neutron diffraction pattern at 120 K at ambient pressure perfectly resembles previous results [2,6]. They point to the incommensurate helical arrangement of magnetic moments. The moments are parallel within neighbouring atomic layers stacked along c-axis and change direction going from layer to layer. The propagation vector along c-axis amounts to ( 1). 0.367 r.l.u. (0.030 A Application of pressure significantly changes the magnetic structure. First of all, decrease of the intensity of (0 0 0)+ satellite (Fig. 1) clearly indicates strong pressure dependence of the Fe magnetic moments. The obtained value of q ln mFe/qp= 5.2 10 3 kbar 1 agrees well with the ones determined from magnetization measurements on R2Fe17 with R=Y, Gd, Ho, Er (varying between 8.8 10 3 and 2.6 10 3 kbar 1) [7] and with the theoretical one calculated from the first principles on Y2Fe17 ( 2.5 10 3 kbar 1) [8]. Shift of the (0 0 0)+ satellite towards the high diffraction angles (increase of the propagation vector) with pressure (Fig. 1) reflects an increase of the angle between magnetic moments in the neighbouring layers. Simultaneously, the Ne! el temperature decreases with pressure. Both the propagation vector and the Ne! el temperature show a remarkable deviation from the linear dependence on pressure in the pressure range above 25 kbar. The magnitude of the propagation
ARTICLE IN PRESS O. Prokhnenko et al. / Physica B 350 (2004) 63–65
TN
200
4 K, 70 kbar 175
150
(101)
150
Intensity
Temperature (K)
AFM
100 + τ2
AFM1
T
50
0
125
τ2-reflection
100 75
FM 0
Ce2Fe17
(000)+
200
PM
65
10
50
20
30
40
50
60
70
10
vector ‘‘saturates’’ at higher pressures reaching value of 0.72 r.l.u. at 4 K under the pressure of 70 kbar. As can be clearly seen from the phase diagram of Ce2Fe17 expanded up to 70 kbar (Fig. 2), the effect of pressure on TN becomes less pronounced with increasing pressure. The value of dTN/dP is reduced by factor of ten, dTN/dP= 0.17 K/kbar, at higher pressures. The detected change of the TN(P) slope coincides with the expected border of stability of the AFM1 phase [2] shown by a dashed line in Fig. 2. The additional satellite is clearly seen at pressure of 70 kbar in Fig. 3. Unfortunately, it is difficult to determine the temperature at which the satellite appears because of its small intensity. It seems it exists below TN already at P X 40 kbar. However, the absolute value of the propagation vector ( strongly corresponding to this peak, t2=0.075 A, supports the AFM1 structure reported in [2]. The value of t2 is slightly affected by temperature and pressure. The observed changes are rather typical for nuclear reflections. The obtained phase diagram indicates that magnetism in Ce2Fe17 can withstand quite large volume changes (B7%). Unlike the ferromagnetic phase, the pressure induced AFM1 phase is very stable. It was already noticed [2] and emphasised by these measurements that propagation vector, which usually varies strongly with temperature, stays practically unchanged in the AFM1 phase. While the increase of the propagation vector with pressure is a direct consequence of the enhancement of negative interlayer exchange interaction
30
40
50
2θ (degrees)
Pressure (kbar) Fig. 2. Magnetic phase P–T diagram of Ce2Fe17.
20
Fig. 3. Neutron diffraction pattern of Ce2Fe17 at 4 K under pressure of 70 kbar. The additional satellite corresponding to the AFM1 is marked by the arrow.
strength, the higher stability of the AFM1 phase above 40 kbar shows rather complex dependence of the exchange strength on the interatomic distances. Observed effects can be also related to the possible suppression of the anomalous behaviour of the c-lattice parameter (negative thermal expansion [9]) which plays a dominant role in magnetic properties of this compound. We acknowledge the support from Projects No. 202/02/0739 GA CR. References [1] I. Medvedeva, Z. Arnold, A. Kuchin, J. Kamar!ad, J. Appl. Phys. 86 (11) (1999) 6295. [2] O. Prokhnenko, C. Ritter, Z. Arnold, O. Isnard, J. Kamar!ad, A. Pirogov, A. Teplykh, A. Kuchin, J. Appl. Phys. 92 (1) (2002) 385. [3] K. Koyama, T. Goto, H. Fujii, N. Takeshita, N. Mori, H. Fukuda, Y. Janssen, J. Phys. Soc. Japan 67 (6) (1998) 1879. [4] Z. Arnold, J. Kamar!ad, P.A. Algarabel, B. Garcia-Landa, M.R. Ibarra, IEEE Trans. Magn. 30 (1994) 619. [5] I.N. Goncharenko, I. Mirebeau, A. Ochiai, Hyperfine Interactions 128 (2000) 225. [6] D. Givord, R. Lemaire, IEEE Trans. Magn. Mag-10 (1974) 109. [7] J. Kamar!ad, O. Mikulina, Z. Arnold, B. Garcia-Landa, M.R. Ibarra, J. Appl. Phys. 85 (1999) 4874. [8] T. Beuerle, M. F.ahle, J. Magn. Magn. Mater. 110 (1992) L29. [9] A.V. Andreev, A. Lindbaum, J. Alloys Compds. 297 (1–2) (2000) 43.
Physica B 350 (2004) 63–65
Magnetism in Ce2Fe17 at pressures up to 70 kbar O. Prokhnenkoa,*, I. Goncharenkob, Z. Arnolda, J. Kamara! da b
a Institute of Physics AS CR, Na Slovance 2, 18221 Prague 8, Czech Republic Laboratoire Leon Brillouin, CEA-CNRS, CEA-Saclay, 91191 Gif Sur Yvette Cedex, France
Abstract The magnetic phase diagram of Ce2Fe17 at ambient pressure shows a ferromagnetic (FM) phase transforming into an incommensurate helical antiferromagnetic (AFM) one above YT=97 K, and becoming paramagnetic above TN=214 K. The FM ground state is highly unstable, dYT/dP= 24 K/kbar, whereas TN only moderately decreases with pressure P, dTN/dP= 1.7 K/kbar. Hence, pressures of the order of 100 kbar are necessary to suppress the magnetic ordered state in Ce2Fe17. The suppression of the FM phase under pressure is accompanied by the appearance of a new incommensurate antiferromagnetic (AFM1) phase at low temperatures. To investigate the stability of the magnetic state and the behaviour of the new pressure-induced AFM1 phase, powder neutron diffraction measurements were performed with P p 70 kbar. This completed the magnetic phase diagram of Ce2Fe17 in that region. The results clearly show that the pressure dependence of the N!eel temperature is linear up to 25 kbar. The strong decrease in the intensity of the magnetic (0 0 0)+ satellite with P gives clear evidence for the volume dependence of the Fe magnetic moments. Simultaneously, the incommensurate propagation vector s=(0, 0, tz) increases with pressure, reaching 0.72 r.l.u. at 4 K and 70 kbar. The results show the anisotropic behaviour of the exchange interactions between Fe moments in Ce2Fe17 and their complex dependence on volume changes. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12. q; 74.62.Fj; 75.25.+z; 71.20.Lp Keywords: Neutron diffraction; High pressure; Magnetic structure; Intermetallic compound
Recently, a strong dependence of the magnetic state of Ce2Fe17 intermetallics on interatomic distances has been revealed by magnetization [1] and neutron diffraction [2] measurements under external pressure. The FM ground state that is stable up to YT=97 K at ambient pressure can be completely suppressed by application of relatively low pressure PCE4 kbar. Simultaneously, the external pressure extends the range of stability of *Corresponding author. Tel.: +420-220-318-426; fax: +420233-343-184. E-mail address: [email protected] (O. Prokhnenko).
the AFM phase from the original one YToTo TN=214 K at ambient pressure down to the lowest temperatures at P>PC. Despite to a huge decrease of YT with pressure, dYT/dP= 24 K/kbar [2], the relevant decrease of the Ne! el temperature TN is rather small, dTN/dP= 1.7 K/kbar [1,3]. The last is comparable with decrease of ordering temperatures in other R2Fe17 compounds [4]. In the simplest case, assuming linear dependence of TN on pressure, one can expect that pressures higher than 100 kbar would be indispensable to suppress the magnetic ordered state in Ce2Fe17. Behaviour of the AFM incommensurate helix under pressure
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.254
ARTICLE IN PRESS O. Prokhnenko et al. / Physica B 350 (2004) 63–65 1200
Ce2Fe17 120 K
1000 800
(012)
600
(110) (003)
(000)+ (101)
is also complex. The previous neutron diffraction experiments performed at pressures up to 5 kbar [2] showed that the value of incommensurate propagation vector at 120 K is changed by external ( 1) at pressure from t1z=0.358 r.l.u. (0.029 A 1 ( 0 kbar up to t1z=0.510 r.l.u. (0.041 A ) at 5 kbar. Such significant change of t1z indicates that pressures higher than 30 kbar could shorten the spiral step to become identical with the crystal lattice unit cell. Moreover, the additional pressure induced changes in the original AFM incommensurate helix were detected under pressure above 3 kbar at temperature below 80 K. One additional ( 1 magnetic peak with the wave vector t2=0.079 A was observed in the neutron diffraction experiments in this pressure-temperature range [2]. The new high pressure phase was marked as AFM1. Complex magnetic structures of Ce2Fe17 and their strong dependence on interatomic distances motivated us to investigate the stability and character of magnetism in Ce2Fe17 in a region of very high pressures up to 70 kbar. Polycrystalline samples of Ce2Fe17 were prepared by the induction melting method with subsequent annealing at 1000 C for 2 weeks. The samples were tested by X-ray diffraction that confirmed the single phase Th2Zn17 type of crystal ( and structure with lattice parameters a=8.495 A ( c=12.416 A. Neutron diffraction experiments were carried out on the powdered Ce2Fe17 samples in the temperature range 4–300 K at pressures up to 70 kbar. The data were collected using the high pressure powder diffractometer G6.1 at the ( Laboratoire Leon Brillouin (Saclay) using 4.74 A wavelength. A sapphire anvil pressure cell was used to generate pressure. Use of very high pressures in the neutron diffraction experiment was achieved by an installation of a special focusing system with Ni–Ti supermirrors along the direction of the incident beam. For detailed description of the high-pressure neutron setup, see Ref. [5] and references therein. Despite to the experimental limitations caused by the small sample volume and incoherent scattering by the high pressure cell, we were able to observe a magnetic (0 0 0)+ satellite together with nuclear (1 0 1), (0 1 2), (1 1 0) and (0 0 3)
Intensity
64
0 kbar
400
25 kbar 200 40 kbar 70 kbar
0
10
20
30
40
50
60
70
80
90 100
2θ (degrees) Fig. 1. Neutron diffraction patterns of Ce2Fe17 at 120 K.
reflections. Other magnetic satellites are believed to exist [2,6], nevertheless, their intensities are too small to be detected even at ambient pressure (Fig. 1, 0 kbar). The neutron diffraction pattern at 120 K at ambient pressure perfectly resembles previous results [2,6]. They point to the incommensurate helical arrangement of magnetic moments. The moments are parallel within neighbouring atomic layers stacked along c-axis and change direction going from layer to layer. The propagation vector along c-axis amounts to ( 1). 0.367 r.l.u. (0.030 A Application of pressure significantly changes the magnetic structure. First of all, decrease of the intensity of (0 0 0)+ satellite (Fig. 1) clearly indicates strong pressure dependence of the Fe magnetic moments. The obtained value of q ln mFe/qp= 5.2 10 3 kbar 1 agrees well with the ones determined from magnetization measurements on R2Fe17 with R=Y, Gd, Ho, Er (varying between 8.8 10 3 and 2.6 10 3 kbar 1) [7] and with the theoretical one calculated from the first principles on Y2Fe17 ( 2.5 10 3 kbar 1) [8]. Shift of the (0 0 0)+ satellite towards the high diffraction angles (increase of the propagation vector) with pressure (Fig. 1) reflects an increase of the angle between magnetic moments in the neighbouring layers. Simultaneously, the Ne! el temperature decreases with pressure. Both the propagation vector and the Ne! el temperature show a remarkable deviation from the linear dependence on pressure in the pressure range above 25 kbar. The magnitude of the propagation
ARTICLE IN PRESS O. Prokhnenko et al. / Physica B 350 (2004) 63–65
TN
200
4 K, 70 kbar 175
150
(101)
150
Intensity
Temperature (K)
AFM
100 + τ2
AFM1
T
50
0
125
τ2-reflection
100 75
FM 0
Ce2Fe17
(000)+
200
PM
65
10
50
20
30
40
50
60
70
10
vector ‘‘saturates’’ at higher pressures reaching value of 0.72 r.l.u. at 4 K under the pressure of 70 kbar. As can be clearly seen from the phase diagram of Ce2Fe17 expanded up to 70 kbar (Fig. 2), the effect of pressure on TN becomes less pronounced with increasing pressure. The value of dTN/dP is reduced by factor of ten, dTN/dP= 0.17 K/kbar, at higher pressures. The detected change of the TN(P) slope coincides with the expected border of stability of the AFM1 phase [2] shown by a dashed line in Fig. 2. The additional satellite is clearly seen at pressure of 70 kbar in Fig. 3. Unfortunately, it is difficult to determine the temperature at which the satellite appears because of its small intensity. It seems it exists below TN already at P X 40 kbar. However, the absolute value of the propagation vector ( strongly corresponding to this peak, t2=0.075 A, supports the AFM1 structure reported in [2]. The value of t2 is slightly affected by temperature and pressure. The observed changes are rather typical for nuclear reflections. The obtained phase diagram indicates that magnetism in Ce2Fe17 can withstand quite large volume changes (B7%). Unlike the ferromagnetic phase, the pressure induced AFM1 phase is very stable. It was already noticed [2] and emphasised by these measurements that propagation vector, which usually varies strongly with temperature, stays practically unchanged in the AFM1 phase. While the increase of the propagation vector with pressure is a direct consequence of the enhancement of negative interlayer exchange interaction
30
40
50
2θ (degrees)
Pressure (kbar) Fig. 2. Magnetic phase P–T diagram of Ce2Fe17.
20
Fig. 3. Neutron diffraction pattern of Ce2Fe17 at 4 K under pressure of 70 kbar. The additional satellite corresponding to the AFM1 is marked by the arrow.
strength, the higher stability of the AFM1 phase above 40 kbar shows rather complex dependence of the exchange strength on the interatomic distances. Observed effects can be also related to the possible suppression of the anomalous behaviour of the c-lattice parameter (negative thermal expansion [9]) which plays a dominant role in magnetic properties of this compound. We acknowledge the support from Projects No. 202/02/0739 GA CR. References [1] I. Medvedeva, Z. Arnold, A. Kuchin, J. Kamar!ad, J. Appl. Phys. 86 (11) (1999) 6295. [2] O. Prokhnenko, C. Ritter, Z. Arnold, O. Isnard, J. Kamar!ad, A. Pirogov, A. Teplykh, A. Kuchin, J. Appl. Phys. 92 (1) (2002) 385. [3] K. Koyama, T. Goto, H. Fujii, N. Takeshita, N. Mori, H. Fukuda, Y. Janssen, J. Phys. Soc. Japan 67 (6) (1998) 1879. [4] Z. Arnold, J. Kamar!ad, P.A. Algarabel, B. Garcia-Landa, M.R. Ibarra, IEEE Trans. Magn. 30 (1994) 619. [5] I.N. Goncharenko, I. Mirebeau, A. Ochiai, Hyperfine Interactions 128 (2000) 225. [6] D. Givord, R. Lemaire, IEEE Trans. Magn. Mag-10 (1974) 109. [7] J. Kamar!ad, O. Mikulina, Z. Arnold, B. Garcia-Landa, M.R. Ibarra, J. Appl. Phys. 85 (1999) 4874. [8] T. Beuerle, M. F.ahle, J. Magn. Magn. Mater. 110 (1992) L29. [9] A.V. Andreev, A. Lindbaum, J. Alloys Compds. 297 (1–2) (2000) 43.