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Structural and magnetic properties of DyCo5−xGax compounds
Journal of Alloys and Compounds 491 (2010) 18–21
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and magnetic properties of DyCo5−x Gax compounds J.Q. Li a,∗ , W.Q. Ao a , F.S. Liu a , W.H. Zhang a , J.L. Yan b a
College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China College of Materials Science and Engineering, Guangxi University and Key Laboratory of Nonferrous Metal Materials and New Processing Technology, Ministry of Education, Guangxi University, Nanning, Guangxi 530004, PR China
b
a r t i c l e
i n f o
Article history: Received 16 September 2009 Received in revised form 25 October 2009 Accepted 26 October 2009 Available online 5 November 2009 Keywords: Rare earth alloys and compounds Crystal structure Magnetic properties
a b s t r a c t The structural and magnetic properties of DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15, 1.45 and 1.75, have been investigated by X-ray diffraction and magnetic measurements. Powder X-ray diffraction analysis revealed that the DyCo5−x Gax samples are stabilized in single phase with a hexagonal CaCu5 type structure (space group P6/mmm) for 0.55 ≤ x ≤ 1.45. The substitution of Co by Ga in this compound increases its lattice parameters a, c and cell volume V but decreases the 3d-sublattice moment and the 3d uniaxial anisotropy. As result, the compensation temperature Tcomp increases from 199 K (for the sample with x = 0.55) to 260 K (for the sample with x = 1.45). Two spin reorientation transitions, at TSR1 = 460 and TSR2 = 725 K, respectively, for the sample with x = 0.55, only one at TSR3 = 466 K for the sample with x = 0.85 while none for the samples with x = 1.15 or less were found. The Curie temperature decreases with increasing Ga content. The total magnetization for the compound increases at low temperature (e.g. 100 K) but decreases at high temperature with increasing Ga content (e.g. 300 K). © 2009 Elsevier B.V. All rights reserved.
1. Introduction Rare earth-transition metal (R-T) intermetallics represent an important group of compounds with interesting magnetic and hydrogen absorption properties [1]. The interesting magnetic performances of the intermetallic compounds formed by alloying rare earth and 3d transition metals are due to the combination of the complementary characteristics of 3d-itinerant and 4f-localized magnetism. Their technological importance as well as interesting properties resulted in a continuous experimental and theoretical research [1,2] over the last several decades. Among the R-T intermetallics, hexagonal Haucke compounds (CaCu5 -type structure, space group of P6/mmm) of the RCo5 composition are one of the most interesting subclasses, which were widely studied from a fundamental viewpoint but also for their possible applications as permanent magnets [3]. The compound DyCo5 is high temperature phase which decomposes into Dy2 Co7 and Dy2 Co17 by eutectoid reaction at about 1130 ◦ C [4]. In our investigation of Dy–Co–Ga ternary system, we found that the DyCo5−x Gax compounds are stable in CaCu5 -type structure for 0.55 ≤ x < 1.75 at 773 K. The investigations of the structural and magnetic properties for DyCo4 M (M = Al, Ga) [5,6], DyCo5−x Cux [7] and Dy1−x Yx Co5 compounds indicate that the substitutions of non-magnetic atoms, Al, Ga and Cu for Co, or Y for Dy have remarkable influences on both crystal structure and magnetic properties of the compound, such as the Curie
temperatures (Tc), compensation temperatures Tcomp , spin reorientation transitions TSR and magnetization. To further study the effects of the substitution of Ga for Co on both crystal structural and magnetic properties of DyCo5 in a wide composition range which has not been studied before, we studied the structural and magnetic properties of the DyCo5−x Gax compounds from x = 0.55–1.45. 2. Experimental details The polycrystalline DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15, 1.45 and 1.75 were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in an atmosphere of pure argon. The alloy samples were prepared from the starting materials Dy (99.99 wt.%), Co (99.95 wt.%) and Ga (99.999 wt.%). The alloy buttons were re-melted at least three times to ensure complete fusion and homogeneous composition. No composition analysis was carried out since the weight lost of the sample was less than 1% during the preparation. The alloy buttons were sealed in evacuated quartz tubes and annealed at 950 ◦ C for 2 weeks, and then cooled to 500 ◦ C, kept at 500 ◦ C for 5 days and quenched in liquid nitrogen. The X-ray powder diffraction (XRD) data were collected by a Bruker D8 Advance SS/18 kW diffractometer with Cu K␣ radiation. JADE 5.0 and Topas 3.0 software were used for phase analysis and structure refinement. The temperature dependence of the magnetization was measured using a vibrating sample magnetometer (Nanjing University VSM-HH20). The Curie temperature was identified as the minimum of the first derivative of the magnetization with respect to temperature.
3. Results and discussion 3.1. Crystal structure
∗ Corresponding author. E-mail address: [email protected] (J.Q. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.225
The powder X-ray diffraction patterns of the DyCo5−x Gax alloys with x = 0.55, 0.85, 1.15, 1.45 and 1.75, shown in Fig. 1, revealed that
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
19
Fig. 1. XRD patterns for the alloys DyCo5−x Gax with x = 0.55 (a), 0.85 (b), 1.15 (c), 1.45 (d) and 1.75 (e).
Fig. 2. Rietveld refinement for the XRD patterns of the alloys DyCo4.15 Ga0.85 .
the alloys are virtually single phase with a hexagonal CaCu5 -type structure (space group P6/mmm) for 0.55 ≤ x ≤ 1.45, but contain a small amount of a second phase as well as the CaCu5 -type phase for x = 1.75. The second phase seems to be a Dy–Co–Ga ternary
compound with the same structure of Sm2 Se3 . The XRD patterns of the CaCu5 -type phase in the samples DyCo5−x Gax move to the smaller 2 as comparing to that of the DyCo5 card obviously, indicating the expansion of the lattice as the substitution of Ga for
Table 1 Structure, refined parameters for the compound DyCo5−x Gax with x = 0.85 and 1.45. x
0.85
1.45
Space group Structure type Cell parameters (nm) Volume of unit cell (nm3 ) Calculated density (g cm−3 ) Reliability factors
P6/mmm CaCu5 a = 0.49499 (3), c = 0.40432 (2) V = 0.08579 (1) 8.960 (1) Rexp = 12.67, GOF = 1.24 Rexp = 7.40, GOF = 1.39
a = 0.49860 (9), c = 0.40638 (7) V = 0.08749 (4) 8.912 (4) Rp = 12.07, Rwp = 15.73 Rp = 7.46, Rwp = 10.29
Position
1a (0, 0, 0) 2c (1/3, 2/3, 0) 3g (1/2, 0, 1/2)
Atomic parameters Occupation
Occupation
Dy Co Co0.72 Ga0.28
Dy Co0.83 Ga0.17 Co0.63 Ga0.37
20
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
Fig. 3. Crystal structure for the compounds DyCo5−x Gax with CaCu5 -type structure.
Co in DyCo5 . Rietveld refinement of the XRD data was performed using the Topas 3.0 program. The representative refinement results for DyCo4.15 Ga0.85 are shown in Fig. 2 and listed in Table 1. The refinement results show the samples DyCo5−x Gax contain the single phase with a hexagonal CaCu5 -type structure mainly, together with about 0.1–0.5 wt.% Dy2 O3 . The very small amount of the oxide Dy2 O3 (less than 0.5 wt.%) may be occurred during sample preparation which does not much affect the investigation of structural and magnetic properties for the DyCo5−x Gax . The rare earth Dy atom occupies 1a (0, 0, 0) site, two Co atoms occupy 2c (1/3, 2/3, 0) and 3g (1/2, 0, 1/2) sites, and the Ga atoms prefer to occupy the 3g sites in the CaCu5 structure, given in Fig. 3. Almost all Ga atoms occupy the 3g sites for the lower Ga content (x < 1.0), whereas the most Ga atoms occupy the 3g sites and the rest being located at 2c sites for the higher Ga content (x > 1.0), which is in agreement with previous neutron diffraction investigations of DyCo4 Ga [5]. For example, the occupation of Ga atom in 2c site is 0 for the sample with x = 0.85, while 0.17 for the sample with x = 1.45 (Table 1). The compositional dependence of lattice parameters a, c, c/a and unit cell V for DyCo5−x Gax are show in Fig. 4(a)–(c). Both the lattice parameters a and c increase upon the substitution of Ga for Co because of the larger atomic radius of Ga (Fig. 4(a)). The c/a ratio increases slightly on lower Ga substitution from x = 0.55 to x = 1.15, but decreases significantly on higher Ga substitution from x = 1.15 to x = 1.75 (Fig. 4(b)), which may due to higher occupation of Ga in 2c site for higher Ga substitution. The expansion of the unit cell lattice is more pronounced along the a axis. This anisotropic expansion of the unit cell lattice along the a axis may be responsible for the nonstability of structures having a high Ga concentration. 3.2. Magnetic properties Fig. 5 shows the temperature dependence of the magnetization (M–T curves) for the samples DyCo5−x Gax with x = 0.55, 0.85, 1.15 and 1.45 measured in the applied field of 0.1 T and the temperature range from 80 to 850 K. The separated M–T curve of DyCo4.15 Ga0.85 , shown in Fig. 5(b), indicates the same magnetic behavior with that of the DyCo4 Ga reported in Ref. [5]. A compensation transition and a spin reorientation transition are found in DyCo4.15 Ga0.85 at Tcomp = 233 K and TSR = 466 K, respectively, about 67 and 66 K lower than those of DyCo4 Ga reported in Ref. [5]. The previous analysis of powder neutron diffraction indicated that the Dy-sublattice magnetization is anti-parallel to the Co-sublattice magnetization and much more temperature dependence than that of Co-sublattice in this compound [5]. Almost cancellation of Dy- and Co-sublattice magnetization occurs at its compensation temperature Tcomp . The spin reorientation transition occurs when the magnetic moments rotate continuously with temperature near temperature TSR , which is originated from the competition between axial Co (3d) and planar Dy (4f) magnetocrystalline anisotropic in this compound. The compensation temperatures Tcomp of compounds DyCo5−x Gax increases
Fig. 4. Compositional dependence of the lattice parameters a and c (a), c/a (b) and the unit volume V (c) for the DyCo5−x Gax .
from 199 K (for the sample with x = 0.55) to 260 K (for the sample with x = 1.45) due to the decrease of the 3d-sublattice moment by substitution of Ga for Co in this compound [8]. Two spin reorientation transitions, at TSR1 = 325 K and TSR2 = 367 K, respectively, were observed in compound DyCo5 [9]. The two spin reorientation transitions increase to TSR1 = 460 and TSR2 = 725 K, respectively, for the Ga substituted sample DyCo5−x Gax with x = 0.55. Only one at TSR3 = 466 K for the sample DyCo5−x Gax with x = 0.85 while none for x = 1.15 or less were found. It reflects that the weakening of the 3d uniaxial anisotropy upon Ga substitution. The Curie temperature (Tc) of DyCo5−x Gax decreases with Ga substitution from Tc = 595 K (for the sample with x = 0.85) to 367 K (for the sample with x = 1.45). The Curie temperature is mainly determined by the 3d–3d exchange interaction [5]. Here its decrease is due to the weakening of the 3d–3d exchange interaction by substitution of Ga for Co. Since decreases of the 3d-sublattice magnetic moment with increasing Ga content which is anti-parallel to that of the dysprosium
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
21
dominates that of the transition metal (3d) sublattice at the temperature lower than the compensation temperature Tcomp whereas the opposite situation occurs at the temperature higher than Tcomp . Therefore, the decrease of the 3d-sublattice moment by substitution of Ga for Co in this compound leads to increase of the total magnetization at low temperature, e.g. 100 K, but decrease the total magnetization at high temperature, e.g. 300 K. 4. Conclusion We studied the crystal structural and magnetic properties of the DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15 and 1.45. The substitution of Co by Ga increases its lattice parameters a, c, and cell volume but decreases its 3d-sublattice moment. It leads to increase its compensation, spin reorientation but decrease its Curie temperature. The total magnetization for the compound increases at lower its compensation temperature but decreases at higher its compensation temperature with increasing Ga content. Acknowledgments The work was supported by the National Natural Science Foundation of China (nos.: 50871070 and 50861004) and Shenzhen Science and Technology Research Grant (nos. 200726 and JC200903120081A). References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. Temperature dependence of the magnetization (M–T curves) for the DyCo4−x Fex Ga with x = 0.55, 0.85, 1.15 and 1.45 (a) and separated M–T curve of DyCo4.15 Ga0.85 (b) measured in the applied field of 0.1 T and the temperature range from 80 to 850 K.
sublattice [10], the magnetization of the dysprosium (4f) sublattice
K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179–1256. M. Richter, J. Phys. D: Appl. Phys. 31 (1998) 1017–1048. K.J. Strnat, Ferromagn. Mater. 4 (1988) 131. Y.C. Chuang, Z. Metalkd. 85 (1994) 104–108. V. Klosek, C. Zlotea, O. Isnard, Phys. J. Condens. Matter 15 (2003) 8327–8337. V. Klosek, C. Zlotea, O. Isnard, Physica B 350 (2004) E155–E158. D. Banerjee, D. Bahadur, K.G. Suresh, et al., Physica B 378–380 (2006) 1091–1092. [8] N.P. Thuy, N.M. Hong, T.D. Hien, J.J.M. Franse, in: S.G. SankAr (Ed.), Proceeding of the 11th International Workshop RE-Magnets and 6th International Symposium on Magn. Anis. Coerc. RE-TM Alloys, Part II, 1996, p. 60. [9] T. Tsushima, M. Ohokoshi, J. Magn. Magn. Mater. 31–34 (1983) 197– 198. [10] H. Mayot, O. Isnard, F. Grandjean, G.J. Long, J. Appl. Phys. 103 (2008) 093917.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Structural and magnetic properties of DyCo5−x Gax compounds J.Q. Li a,∗ , W.Q. Ao a , F.S. Liu a , W.H. Zhang a , J.L. Yan b a
College of Materials Science and Engineering, Shenzhen University and Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China College of Materials Science and Engineering, Guangxi University and Key Laboratory of Nonferrous Metal Materials and New Processing Technology, Ministry of Education, Guangxi University, Nanning, Guangxi 530004, PR China
b
a r t i c l e
i n f o
Article history: Received 16 September 2009 Received in revised form 25 October 2009 Accepted 26 October 2009 Available online 5 November 2009 Keywords: Rare earth alloys and compounds Crystal structure Magnetic properties
a b s t r a c t The structural and magnetic properties of DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15, 1.45 and 1.75, have been investigated by X-ray diffraction and magnetic measurements. Powder X-ray diffraction analysis revealed that the DyCo5−x Gax samples are stabilized in single phase with a hexagonal CaCu5 type structure (space group P6/mmm) for 0.55 ≤ x ≤ 1.45. The substitution of Co by Ga in this compound increases its lattice parameters a, c and cell volume V but decreases the 3d-sublattice moment and the 3d uniaxial anisotropy. As result, the compensation temperature Tcomp increases from 199 K (for the sample with x = 0.55) to 260 K (for the sample with x = 1.45). Two spin reorientation transitions, at TSR1 = 460 and TSR2 = 725 K, respectively, for the sample with x = 0.55, only one at TSR3 = 466 K for the sample with x = 0.85 while none for the samples with x = 1.15 or less were found. The Curie temperature decreases with increasing Ga content. The total magnetization for the compound increases at low temperature (e.g. 100 K) but decreases at high temperature with increasing Ga content (e.g. 300 K). © 2009 Elsevier B.V. All rights reserved.
1. Introduction Rare earth-transition metal (R-T) intermetallics represent an important group of compounds with interesting magnetic and hydrogen absorption properties [1]. The interesting magnetic performances of the intermetallic compounds formed by alloying rare earth and 3d transition metals are due to the combination of the complementary characteristics of 3d-itinerant and 4f-localized magnetism. Their technological importance as well as interesting properties resulted in a continuous experimental and theoretical research [1,2] over the last several decades. Among the R-T intermetallics, hexagonal Haucke compounds (CaCu5 -type structure, space group of P6/mmm) of the RCo5 composition are one of the most interesting subclasses, which were widely studied from a fundamental viewpoint but also for their possible applications as permanent magnets [3]. The compound DyCo5 is high temperature phase which decomposes into Dy2 Co7 and Dy2 Co17 by eutectoid reaction at about 1130 ◦ C [4]. In our investigation of Dy–Co–Ga ternary system, we found that the DyCo5−x Gax compounds are stable in CaCu5 -type structure for 0.55 ≤ x < 1.75 at 773 K. The investigations of the structural and magnetic properties for DyCo4 M (M = Al, Ga) [5,6], DyCo5−x Cux [7] and Dy1−x Yx Co5 compounds indicate that the substitutions of non-magnetic atoms, Al, Ga and Cu for Co, or Y for Dy have remarkable influences on both crystal structure and magnetic properties of the compound, such as the Curie
temperatures (Tc), compensation temperatures Tcomp , spin reorientation transitions TSR and magnetization. To further study the effects of the substitution of Ga for Co on both crystal structural and magnetic properties of DyCo5 in a wide composition range which has not been studied before, we studied the structural and magnetic properties of the DyCo5−x Gax compounds from x = 0.55–1.45. 2. Experimental details The polycrystalline DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15, 1.45 and 1.75 were prepared by arc melting using a non-consumable tungsten electrode and a water-cooled copper tray in an atmosphere of pure argon. The alloy samples were prepared from the starting materials Dy (99.99 wt.%), Co (99.95 wt.%) and Ga (99.999 wt.%). The alloy buttons were re-melted at least three times to ensure complete fusion and homogeneous composition. No composition analysis was carried out since the weight lost of the sample was less than 1% during the preparation. The alloy buttons were sealed in evacuated quartz tubes and annealed at 950 ◦ C for 2 weeks, and then cooled to 500 ◦ C, kept at 500 ◦ C for 5 days and quenched in liquid nitrogen. The X-ray powder diffraction (XRD) data were collected by a Bruker D8 Advance SS/18 kW diffractometer with Cu K␣ radiation. JADE 5.0 and Topas 3.0 software were used for phase analysis and structure refinement. The temperature dependence of the magnetization was measured using a vibrating sample magnetometer (Nanjing University VSM-HH20). The Curie temperature was identified as the minimum of the first derivative of the magnetization with respect to temperature.
3. Results and discussion 3.1. Crystal structure
∗ Corresponding author. E-mail address: [email protected] (J.Q. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.225
The powder X-ray diffraction patterns of the DyCo5−x Gax alloys with x = 0.55, 0.85, 1.15, 1.45 and 1.75, shown in Fig. 1, revealed that
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
19
Fig. 1. XRD patterns for the alloys DyCo5−x Gax with x = 0.55 (a), 0.85 (b), 1.15 (c), 1.45 (d) and 1.75 (e).
Fig. 2. Rietveld refinement for the XRD patterns of the alloys DyCo4.15 Ga0.85 .
the alloys are virtually single phase with a hexagonal CaCu5 -type structure (space group P6/mmm) for 0.55 ≤ x ≤ 1.45, but contain a small amount of a second phase as well as the CaCu5 -type phase for x = 1.75. The second phase seems to be a Dy–Co–Ga ternary
compound with the same structure of Sm2 Se3 . The XRD patterns of the CaCu5 -type phase in the samples DyCo5−x Gax move to the smaller 2 as comparing to that of the DyCo5 card obviously, indicating the expansion of the lattice as the substitution of Ga for
Table 1 Structure, refined parameters for the compound DyCo5−x Gax with x = 0.85 and 1.45. x
0.85
1.45
Space group Structure type Cell parameters (nm) Volume of unit cell (nm3 ) Calculated density (g cm−3 ) Reliability factors
P6/mmm CaCu5 a = 0.49499 (3), c = 0.40432 (2) V = 0.08579 (1) 8.960 (1) Rexp = 12.67, GOF = 1.24 Rexp = 7.40, GOF = 1.39
a = 0.49860 (9), c = 0.40638 (7) V = 0.08749 (4) 8.912 (4) Rp = 12.07, Rwp = 15.73 Rp = 7.46, Rwp = 10.29
Position
1a (0, 0, 0) 2c (1/3, 2/3, 0) 3g (1/2, 0, 1/2)
Atomic parameters Occupation
Occupation
Dy Co Co0.72 Ga0.28
Dy Co0.83 Ga0.17 Co0.63 Ga0.37
20
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
Fig. 3. Crystal structure for the compounds DyCo5−x Gax with CaCu5 -type structure.
Co in DyCo5 . Rietveld refinement of the XRD data was performed using the Topas 3.0 program. The representative refinement results for DyCo4.15 Ga0.85 are shown in Fig. 2 and listed in Table 1. The refinement results show the samples DyCo5−x Gax contain the single phase with a hexagonal CaCu5 -type structure mainly, together with about 0.1–0.5 wt.% Dy2 O3 . The very small amount of the oxide Dy2 O3 (less than 0.5 wt.%) may be occurred during sample preparation which does not much affect the investigation of structural and magnetic properties for the DyCo5−x Gax . The rare earth Dy atom occupies 1a (0, 0, 0) site, two Co atoms occupy 2c (1/3, 2/3, 0) and 3g (1/2, 0, 1/2) sites, and the Ga atoms prefer to occupy the 3g sites in the CaCu5 structure, given in Fig. 3. Almost all Ga atoms occupy the 3g sites for the lower Ga content (x < 1.0), whereas the most Ga atoms occupy the 3g sites and the rest being located at 2c sites for the higher Ga content (x > 1.0), which is in agreement with previous neutron diffraction investigations of DyCo4 Ga [5]. For example, the occupation of Ga atom in 2c site is 0 for the sample with x = 0.85, while 0.17 for the sample with x = 1.45 (Table 1). The compositional dependence of lattice parameters a, c, c/a and unit cell V for DyCo5−x Gax are show in Fig. 4(a)–(c). Both the lattice parameters a and c increase upon the substitution of Ga for Co because of the larger atomic radius of Ga (Fig. 4(a)). The c/a ratio increases slightly on lower Ga substitution from x = 0.55 to x = 1.15, but decreases significantly on higher Ga substitution from x = 1.15 to x = 1.75 (Fig. 4(b)), which may due to higher occupation of Ga in 2c site for higher Ga substitution. The expansion of the unit cell lattice is more pronounced along the a axis. This anisotropic expansion of the unit cell lattice along the a axis may be responsible for the nonstability of structures having a high Ga concentration. 3.2. Magnetic properties Fig. 5 shows the temperature dependence of the magnetization (M–T curves) for the samples DyCo5−x Gax with x = 0.55, 0.85, 1.15 and 1.45 measured in the applied field of 0.1 T and the temperature range from 80 to 850 K. The separated M–T curve of DyCo4.15 Ga0.85 , shown in Fig. 5(b), indicates the same magnetic behavior with that of the DyCo4 Ga reported in Ref. [5]. A compensation transition and a spin reorientation transition are found in DyCo4.15 Ga0.85 at Tcomp = 233 K and TSR = 466 K, respectively, about 67 and 66 K lower than those of DyCo4 Ga reported in Ref. [5]. The previous analysis of powder neutron diffraction indicated that the Dy-sublattice magnetization is anti-parallel to the Co-sublattice magnetization and much more temperature dependence than that of Co-sublattice in this compound [5]. Almost cancellation of Dy- and Co-sublattice magnetization occurs at its compensation temperature Tcomp . The spin reorientation transition occurs when the magnetic moments rotate continuously with temperature near temperature TSR , which is originated from the competition between axial Co (3d) and planar Dy (4f) magnetocrystalline anisotropic in this compound. The compensation temperatures Tcomp of compounds DyCo5−x Gax increases
Fig. 4. Compositional dependence of the lattice parameters a and c (a), c/a (b) and the unit volume V (c) for the DyCo5−x Gax .
from 199 K (for the sample with x = 0.55) to 260 K (for the sample with x = 1.45) due to the decrease of the 3d-sublattice moment by substitution of Ga for Co in this compound [8]. Two spin reorientation transitions, at TSR1 = 325 K and TSR2 = 367 K, respectively, were observed in compound DyCo5 [9]. The two spin reorientation transitions increase to TSR1 = 460 and TSR2 = 725 K, respectively, for the Ga substituted sample DyCo5−x Gax with x = 0.55. Only one at TSR3 = 466 K for the sample DyCo5−x Gax with x = 0.85 while none for x = 1.15 or less were found. It reflects that the weakening of the 3d uniaxial anisotropy upon Ga substitution. The Curie temperature (Tc) of DyCo5−x Gax decreases with Ga substitution from Tc = 595 K (for the sample with x = 0.85) to 367 K (for the sample with x = 1.45). The Curie temperature is mainly determined by the 3d–3d exchange interaction [5]. Here its decrease is due to the weakening of the 3d–3d exchange interaction by substitution of Ga for Co. Since decreases of the 3d-sublattice magnetic moment with increasing Ga content which is anti-parallel to that of the dysprosium
J.Q. Li et al. / Journal of Alloys and Compounds 491 (2010) 18–21
21
dominates that of the transition metal (3d) sublattice at the temperature lower than the compensation temperature Tcomp whereas the opposite situation occurs at the temperature higher than Tcomp . Therefore, the decrease of the 3d-sublattice moment by substitution of Ga for Co in this compound leads to increase of the total magnetization at low temperature, e.g. 100 K, but decrease the total magnetization at high temperature, e.g. 300 K. 4. Conclusion We studied the crystal structural and magnetic properties of the DyCo5−x Gax compounds with x = 0.55, 0.85, 1.15 and 1.45. The substitution of Co by Ga increases its lattice parameters a, c, and cell volume but decreases its 3d-sublattice moment. It leads to increase its compensation, spin reorientation but decrease its Curie temperature. The total magnetization for the compound increases at lower its compensation temperature but decreases at higher its compensation temperature with increasing Ga content. Acknowledgments The work was supported by the National Natural Science Foundation of China (nos.: 50871070 and 50861004) and Shenzhen Science and Technology Research Grant (nos. 200726 and JC200903120081A). References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. Temperature dependence of the magnetization (M–T curves) for the DyCo4−x Fex Ga with x = 0.55, 0.85, 1.15 and 1.45 (a) and separated M–T curve of DyCo4.15 Ga0.85 (b) measured in the applied field of 0.1 T and the temperature range from 80 to 850 K.
sublattice [10], the magnetization of the dysprosium (4f) sublattice
K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 1179–1256. M. Richter, J. Phys. D: Appl. Phys. 31 (1998) 1017–1048. K.J. Strnat, Ferromagn. Mater. 4 (1988) 131. Y.C. Chuang, Z. Metalkd. 85 (1994) 104–108. V. Klosek, C. Zlotea, O. Isnard, Phys. J. Condens. Matter 15 (2003) 8327–8337. V. Klosek, C. Zlotea, O. Isnard, Physica B 350 (2004) E155–E158. D. Banerjee, D. Bahadur, K.G. Suresh, et al., Physica B 378–380 (2006) 1091–1092. [8] N.P. Thuy, N.M. Hong, T.D. Hien, J.J.M. Franse, in: S.G. SankAr (Ed.), Proceeding of the 11th International Workshop RE-Magnets and 6th International Symposium on Magn. Anis. Coerc. RE-TM Alloys, Part II, 1996, p. 60. [9] T. Tsushima, M. Ohokoshi, J. Magn. Magn. Mater. 31–34 (1983) 197– 198. [10] H. Mayot, O. Isnard, F. Grandjean, G.J. Long, J. Appl. Phys. 103 (2008) 093917.