- Email: [email protected]
Magnetization and specific heat of the Ho3Co compound
Journal of Magnetism and Magnetic Materials 258–259 (2003) 561–563
Magnetization and specific heat of the Ho3Co compound T. Palewskia,*, N.V. Tristana, K. Nenkova,b, K. Skokova,c, S.A. Nikitina,d a
International Laboratory of High Magnetic Fields and Low Temperatures, 95 Gajowicka Str., Wroclaw 53–421, Poland b Institut fur Festkorper–und Werkstofforschung, Postfach 270016, 01171 Dresden, Germany c Faculty of Physics, Tver State University, 33 Geljabova str., Tver 170002, Russia d Faculty of Physics, Moscow State University, Vorobievy Gory, Moscow 119899, Russia
Abstract The magnetization processes of the Ho3Co single crystal along three basic symmetry axes were investigated and possible magnetic structure is suggested. Specific heat measurements of the compound have been performed at temperature range of 2–300 K. The phonon conduction electron and magnetic contributions to the total specific heat are determined and discussed. Two peaks observed on the magnetic part of the specific heat correspond to magnetic ordering transition (TN ¼ 20 K) and Schottky anomaly (TSch B30 K). r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ho3Co single crystal; Magnetization; Specific heat; Entropy
1. Introduction The Ho3Co compound belongs to group of the isostructural R3M compounds (R=rare earth and M=Co or Ni) crystallizing to Fe3C-type orthorhombic structure. The preliminary study of magnetic properties of these compounds on polycrystalline samples disclosed their complex behavior in magnetic field [1]. The interest to these compounds increased considerably since the methods for single crystal growing has been developed. However, the Ho3Co compounds remain the least known and no data for a single crystal was obtained so far. The purpose of the work is to fill the gap and provide information on some magnetic and thermal properties of the Ho3Co compound.
2. Experimental The methods for sample preparation and quality controlling were described in Ref. [2]. The lattice *Corresponding author. Tel.: +48-61-34-69-71; fax: +48-6127-21-71. E-mail address: [email protected] (T. Palewski).
parameters of the Ho3Co compound were found to be a ¼ 0:695 nm, b ¼ 0:923 nm and c ¼ 0:621 nm. The magnetization measurements were carried out using a capacitance sensor magnetometer in static magnetic fields up to 13 T at temperatures between 1.5 and 75 K. The specific heat measurements were performed using Quantum Design PPMS Heat Capacity System in temperature range of 2–300 K.
3. Results and discussion The magnetization of a Ho3Co single crystal along main crystallographic directions at T ¼ 1:5 K is shown in Fig. 1. If an external magnetic field is applied along caxis, the typical for ferromagnetic materials field dependence of magnetization and hysteresis loop are observed. It is looks evident, that in Ho3Co the c-axis is the preferential direction with strong ferromagnetic moment along it. The existence of similar preferential direction was earlier discovered by neutron diffraction measurements for isostructural Er3Co (b-axis) [3] and Tb3Co (c-axis) [4] compounds. If the magnetic field is applied along a-axis, a number of peculiarities of magnetization are observed at Hcr ¼ 0:5; 2.0 and 2.9 T. It is possible that in zero field the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 4 6 - 0
T. Palewski et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 561–563
µ, µB/f.u.
15 10
T=1.5K H//a H//b H//c
5 0 0
1
2
3
4
5 µ0 H, T
6
7
8
9
Fig. 1. The magnetization of the Ho3Co compound along three principal crystallographic directions.
magnetic moment projections on a-axis have antiferromagnetic ordering with total magnetic moment equal to zero. An application of magnetic field leads to a number of discontinuous transformations of the original magnetic structure, i.e. the magnetic field increasing causes the number of transitions from one to another antiferromagnetic states. A complex behavior is also found if an external magnetic field is applied along b-axis. A sharp reversible increasing of the magnetization in initial field is accompanied by an irreversible metamagnetic transition at Hcr > 0:7 T. As a result of the combination, the sausage-type hysteresis loop is observed. The shape of the hysteresis loop as well as rapid reduction of the magnetic hysteresis along all axes with temperature increasing (at T ¼ 12 K no hysteresis is observed) are the evidences that the magnetization processes of the Ho3Co are accompanied by appearance of the domain structure with narrow domain walls and their movement in magnetic fields higher than an intrinsic coercivity of the domain walls. Similar situation was earlier proposed for Tb3Co compound [5]. For all crystallographic directions the saturation has not been achieved in applied magnetic fields. The magnetic moment per Ho atom showed it does not reach the theoretical value mHo=10 mB per holmium atom in pure metallic Ho at T-0 K even in magnetic field of 13 T (the calculation were performed assuming that Co atoms do not carry any magnetic moment). We suppose that even in strong applied magnetic field, the magnetic structure of the Ho3Co compound is canted and the field acts not only against anisotropy but also against exchange interactions because their values are comparable. The specific heat measurement of the Ho3Co compound is shown in Fig. 2. The sharp l-like maximum on the CP(T) curve at TN = 20 K corresponds to the magnetic ordering temperature. Since the low magnetic ordering temperature does not allow an accurate determination of the electronic specific heat coefficient g and the Debye temperature YD from the low
temperature range for this compound, a measurement of the specific heat of the nonmagnetic isostructural Y3Co compound was performed. The values gB15 mJ/ (mol K2) and YDB215 K estimated for Y3Co from the low temperatures area are in good agreement with previous data [6]. However, we have found that the best fittings for the wide temperature range T = 2300 K could be achieved with g = 15 mJ/(mol K2) and YD = 234 K. The Debye temperature for Ho3Co was then calculated by methods offered in [7], taking into account the different atom masses of the constituent elements, and was obtained as YD = 177 K. The magnetic part of the specific heat Cm was estimated by subtracting the conduction electron and phonon contributions Celþph ðTÞ from the total specific heat CP ðTÞ: The wide smooth maximum observed on the magnetic part around 30 K can be associated with Schottky like anomaly caused by splitting of the Ho3+ ions ground state multiplet due to crystal field effect. The Ho atom has large orbital moment ðL ¼ 6Þ and, therefore, not only low symmetry orthorhombic Fe3Ctype structure but also the anisotropy of the rare earth
Ho3Co
100 80
C, J/(mol-K)
Ho3Co
20
TN
60 40 20 0
0
50
100
150
200
250
300
T, K Fig. 2. The electron–phonon Celþph ðTÞ (dotted line) and magnetic Cm ðTÞ (dashed line) contributions to the total specific heat CP ðTÞ (solid line) of the Ho3Co compound.
80 Ho Co 3
Sm, J/(mol-K)
562
3Rln(17)
60
40
20
0
0
50
100
150
200
250
300
T, K Fig. 3. Temperature dependence of the entropy (solid line) and its maximum theoretical value Smax=3Rð2J þ 1Þ (dashed line) for the Ho3Co compound.
T. Palewski et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 561–563
element give rise to the crystal field effect which plays an important role for the compound. Similar large contribution to the magnetic part of the specific heat due to Schottky like effect was earlier observed for Er3Ni compound [8], whereas for the isostructural Gd3Co and Gd3Ni compounds, where Gd ion is isotropic and does not carry an orbital moment, the Schottky like effect is significantly lower [9]. The magnetic part of the entropy Sm was calculated by integrating Cm ðTÞ=T for the Ho3Co compound (see Fig. 3). The entropy tends to saturation at T > 200 K but does not reach the theoretical maximum value Smax ¼ 3Rlnð2J þ 1Þ; where J ¼ 8 is total angular momentum of holmium atom. This is a result of change in energy level system of the Ho3+ ions ground state multiplet and spin fluctuation contribution in magnetic part of the total specific heat.
Acknowledgements Authors are grateful to Dr. J. Stepien-Damm for ! for X-ray single crystal orientation, Prof. R. Horyn, diffraction measurements (they both are from Institute
563
of Low Temperatures and Structure Research, Wroclaw, Poland).
References [1] K.N. Taylor, G.J. Primavesi, J. Phys. F 1 (1971) L7. [2] N.V. Tristan, S.A. Nikitin, T. Palewski, K. Skokov, J. Warchulska, J. Alloys Comp. 334 (2002) 40. [3] D. Gignoux, R. Lemaire, D. Paccard, Solid State Commun. 8 (1970) 391. [4] N.V. Baranov, A.P. Vokhmyanin, A.V. Deryagin, V.V. Kelarev, A.N. Pirogov, V.A. Reimer, V.N. Syromiatnikov, Fiz. Met. Metallogr. 56 (1983) 261. [5] A.V. Deryagin, N.V. Baranov, V.A. Reimer, Sov. Phys. JETP 73 (1977) 1389. [6] N.V. Baranov, A.A. Yermakov, P.E. Markin, U.M. Possokhov, H. Michor, B. Weingartner, G. Hilscher, B. Kotur, J. Alloys Compounds 329 (2001) 22. [7] M. Bouvier, P. Lethuillier, D. Schmitt, Phys. Rev. B 43 (1991) 13137. [8] A. Takahashi, Y. Tokai, M. Sahashi, T. Hashimoto, Jpn. J. Appl. Phys. 33 (1994) 1023. [9] N.V. Tristan, S.A. Nikitin, T. Palewski, K. Nenkov, K. Skokov, J. Magn. Magn. Mater., this issue.
Magnetization and specific heat of the Ho3Co compound T. Palewskia,*, N.V. Tristana, K. Nenkova,b, K. Skokova,c, S.A. Nikitina,d a
International Laboratory of High Magnetic Fields and Low Temperatures, 95 Gajowicka Str., Wroclaw 53–421, Poland b Institut fur Festkorper–und Werkstofforschung, Postfach 270016, 01171 Dresden, Germany c Faculty of Physics, Tver State University, 33 Geljabova str., Tver 170002, Russia d Faculty of Physics, Moscow State University, Vorobievy Gory, Moscow 119899, Russia
Abstract The magnetization processes of the Ho3Co single crystal along three basic symmetry axes were investigated and possible magnetic structure is suggested. Specific heat measurements of the compound have been performed at temperature range of 2–300 K. The phonon conduction electron and magnetic contributions to the total specific heat are determined and discussed. Two peaks observed on the magnetic part of the specific heat correspond to magnetic ordering transition (TN ¼ 20 K) and Schottky anomaly (TSch B30 K). r 2002 Elsevier Science B.V. All rights reserved. Keywords: Ho3Co single crystal; Magnetization; Specific heat; Entropy
1. Introduction The Ho3Co compound belongs to group of the isostructural R3M compounds (R=rare earth and M=Co or Ni) crystallizing to Fe3C-type orthorhombic structure. The preliminary study of magnetic properties of these compounds on polycrystalline samples disclosed their complex behavior in magnetic field [1]. The interest to these compounds increased considerably since the methods for single crystal growing has been developed. However, the Ho3Co compounds remain the least known and no data for a single crystal was obtained so far. The purpose of the work is to fill the gap and provide information on some magnetic and thermal properties of the Ho3Co compound.
2. Experimental The methods for sample preparation and quality controlling were described in Ref. [2]. The lattice *Corresponding author. Tel.: +48-61-34-69-71; fax: +48-6127-21-71. E-mail address: [email protected] (T. Palewski).
parameters of the Ho3Co compound were found to be a ¼ 0:695 nm, b ¼ 0:923 nm and c ¼ 0:621 nm. The magnetization measurements were carried out using a capacitance sensor magnetometer in static magnetic fields up to 13 T at temperatures between 1.5 and 75 K. The specific heat measurements were performed using Quantum Design PPMS Heat Capacity System in temperature range of 2–300 K.
3. Results and discussion The magnetization of a Ho3Co single crystal along main crystallographic directions at T ¼ 1:5 K is shown in Fig. 1. If an external magnetic field is applied along caxis, the typical for ferromagnetic materials field dependence of magnetization and hysteresis loop are observed. It is looks evident, that in Ho3Co the c-axis is the preferential direction with strong ferromagnetic moment along it. The existence of similar preferential direction was earlier discovered by neutron diffraction measurements for isostructural Er3Co (b-axis) [3] and Tb3Co (c-axis) [4] compounds. If the magnetic field is applied along a-axis, a number of peculiarities of magnetization are observed at Hcr ¼ 0:5; 2.0 and 2.9 T. It is possible that in zero field the
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 1 4 6 - 0
T. Palewski et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 561–563
µ, µB/f.u.
15 10
T=1.5K H//a H//b H//c
5 0 0
1
2
3
4
5 µ0 H, T
6
7
8
9
Fig. 1. The magnetization of the Ho3Co compound along three principal crystallographic directions.
magnetic moment projections on a-axis have antiferromagnetic ordering with total magnetic moment equal to zero. An application of magnetic field leads to a number of discontinuous transformations of the original magnetic structure, i.e. the magnetic field increasing causes the number of transitions from one to another antiferromagnetic states. A complex behavior is also found if an external magnetic field is applied along b-axis. A sharp reversible increasing of the magnetization in initial field is accompanied by an irreversible metamagnetic transition at Hcr > 0:7 T. As a result of the combination, the sausage-type hysteresis loop is observed. The shape of the hysteresis loop as well as rapid reduction of the magnetic hysteresis along all axes with temperature increasing (at T ¼ 12 K no hysteresis is observed) are the evidences that the magnetization processes of the Ho3Co are accompanied by appearance of the domain structure with narrow domain walls and their movement in magnetic fields higher than an intrinsic coercivity of the domain walls. Similar situation was earlier proposed for Tb3Co compound [5]. For all crystallographic directions the saturation has not been achieved in applied magnetic fields. The magnetic moment per Ho atom showed it does not reach the theoretical value mHo=10 mB per holmium atom in pure metallic Ho at T-0 K even in magnetic field of 13 T (the calculation were performed assuming that Co atoms do not carry any magnetic moment). We suppose that even in strong applied magnetic field, the magnetic structure of the Ho3Co compound is canted and the field acts not only against anisotropy but also against exchange interactions because their values are comparable. The specific heat measurement of the Ho3Co compound is shown in Fig. 2. The sharp l-like maximum on the CP(T) curve at TN = 20 K corresponds to the magnetic ordering temperature. Since the low magnetic ordering temperature does not allow an accurate determination of the electronic specific heat coefficient g and the Debye temperature YD from the low
temperature range for this compound, a measurement of the specific heat of the nonmagnetic isostructural Y3Co compound was performed. The values gB15 mJ/ (mol K2) and YDB215 K estimated for Y3Co from the low temperatures area are in good agreement with previous data [6]. However, we have found that the best fittings for the wide temperature range T = 2300 K could be achieved with g = 15 mJ/(mol K2) and YD = 234 K. The Debye temperature for Ho3Co was then calculated by methods offered in [7], taking into account the different atom masses of the constituent elements, and was obtained as YD = 177 K. The magnetic part of the specific heat Cm was estimated by subtracting the conduction electron and phonon contributions Celþph ðTÞ from the total specific heat CP ðTÞ: The wide smooth maximum observed on the magnetic part around 30 K can be associated with Schottky like anomaly caused by splitting of the Ho3+ ions ground state multiplet due to crystal field effect. The Ho atom has large orbital moment ðL ¼ 6Þ and, therefore, not only low symmetry orthorhombic Fe3Ctype structure but also the anisotropy of the rare earth
Ho3Co
100 80
C, J/(mol-K)
Ho3Co
20
TN
60 40 20 0
0
50
100
150
200
250
300
T, K Fig. 2. The electron–phonon Celþph ðTÞ (dotted line) and magnetic Cm ðTÞ (dashed line) contributions to the total specific heat CP ðTÞ (solid line) of the Ho3Co compound.
80 Ho Co 3
Sm, J/(mol-K)
562
3Rln(17)
60
40
20
0
0
50
100
150
200
250
300
T, K Fig. 3. Temperature dependence of the entropy (solid line) and its maximum theoretical value Smax=3Rð2J þ 1Þ (dashed line) for the Ho3Co compound.
T. Palewski et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 561–563
element give rise to the crystal field effect which plays an important role for the compound. Similar large contribution to the magnetic part of the specific heat due to Schottky like effect was earlier observed for Er3Ni compound [8], whereas for the isostructural Gd3Co and Gd3Ni compounds, where Gd ion is isotropic and does not carry an orbital moment, the Schottky like effect is significantly lower [9]. The magnetic part of the entropy Sm was calculated by integrating Cm ðTÞ=T for the Ho3Co compound (see Fig. 3). The entropy tends to saturation at T > 200 K but does not reach the theoretical maximum value Smax ¼ 3Rlnð2J þ 1Þ; where J ¼ 8 is total angular momentum of holmium atom. This is a result of change in energy level system of the Ho3+ ions ground state multiplet and spin fluctuation contribution in magnetic part of the total specific heat.
Acknowledgements Authors are grateful to Dr. J. Stepien-Damm for ! for X-ray single crystal orientation, Prof. R. Horyn, diffraction measurements (they both are from Institute
563
of Low Temperatures and Structure Research, Wroclaw, Poland).
References [1] K.N. Taylor, G.J. Primavesi, J. Phys. F 1 (1971) L7. [2] N.V. Tristan, S.A. Nikitin, T. Palewski, K. Skokov, J. Warchulska, J. Alloys Comp. 334 (2002) 40. [3] D. Gignoux, R. Lemaire, D. Paccard, Solid State Commun. 8 (1970) 391. [4] N.V. Baranov, A.P. Vokhmyanin, A.V. Deryagin, V.V. Kelarev, A.N. Pirogov, V.A. Reimer, V.N. Syromiatnikov, Fiz. Met. Metallogr. 56 (1983) 261. [5] A.V. Deryagin, N.V. Baranov, V.A. Reimer, Sov. Phys. JETP 73 (1977) 1389. [6] N.V. Baranov, A.A. Yermakov, P.E. Markin, U.M. Possokhov, H. Michor, B. Weingartner, G. Hilscher, B. Kotur, J. Alloys Compounds 329 (2001) 22. [7] M. Bouvier, P. Lethuillier, D. Schmitt, Phys. Rev. B 43 (1991) 13137. [8] A. Takahashi, Y. Tokai, M. Sahashi, T. Hashimoto, Jpn. J. Appl. Phys. 33 (1994) 1023. [9] N.V. Tristan, S.A. Nikitin, T. Palewski, K. Nenkov, K. Skokov, J. Magn. Magn. Mater., this issue.