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Spin reorientation and magnetic anisotropy in Y2Co17−xCrx (x = 1.17–3.0) compounds
Journal of Alloys and Compounds 399 (2005) 208–213
Spin reorientation and magnetic anisotropy in Y2Co17−xCrx (x = 1.17–3.0) compounds B. Fuquan a,b,∗ , O. Tegus a,b , W. Dagula a,b , E. Br¨uck a , F.R. de Boer a , K.H.J. Buschow a a
b
Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Department of Physics, Inner Mongolia Normal University, Huhhot 010022, PR China Received 28 February 2005; accepted 16 March 2005 Available online 20 April 2005
Abstract Spin reorientation transitions and magnetic anisotropy in Y2 Co17−x Crx (x = 1.17–3.0) compounds have been investigated by means of X-ray diffraction and magnetization measurements. The powder X-ray diffraction patterns show that most samples crystallize as a single phase with the rhombohedral Th2 Zn17 -type structure. However, in the compound Y2 Co14 Cr3 the Th2 Zn17 phase coexist with the hexagonal Th2 Ni17 -type phase. The lattice parameters a and c hardly change and the unit cell volume V increases slightly with increasing Cr content. The X-ray diffraction patterns of the aligned powder of the samples have confirmed that at room temperature the compound with x = 1.17 has planar anisotropy, but the compounds with x = 1.76, 2.34 and 3.00 have uniaxial anisotropy. Spin reorientation phenomena occur in all of the compounds. With increasing Cr content, the Curie temperature, the spin reorientation temperature, the spontaneous magnetization, and the anisotropy constant K2 of the Y2 Co17−x Crx (x = 1.17–3.0) compounds decrease strongly while the anisotropy constant K1 increases in the range of x from 1.17 to 2.34 and then decreases in the range of x from 2.34 to 3.00. © 2005 Elsevier B.V. All rights reserved. Keywords: Spin reorientation; Uniaxial anisotropy; Planar anisotropy
1. Introduction In the past decades, rare-earth (R) transition-metal (T) compounds of the R2 T17 type with T = Fe or Co have been attracting much attention due to possible practical applications of these compounds as high energy-product magnets [1–11]. A strong uniaxial magnetic anisotropy is required for achieving high coercivity, while the binary R2 Co17 compounds exhibit a weak uniaxial anisotropy only for the compounds with R = Sm, Er and Tm. Recently, it was found that the substitution of Al, Ga and Si for Fe could not only significantly increase the Curie temperature of R2 Fe17 , but it was also found that such substitutions can cause a change in the easy magnetic direction (EMD) from the basal plane to the c-axis ∗
Corresponding author. Tel.: +31 20 5256310; fax: +31 20 5255788. E-mail address: [email protected] (B. Fuquan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.042
in R2 T17−x Mx (R = Y, Er, Pr, Ho, Dy; T = Fe, Co; M = Al, Ga, Si) [11–16]. Zhang et al. [17] studied the influence of substitution of small amounts of Ti, V, Cr, Cu, and Mo for Co on the magnetic properties of Y2 Co17 and found that the magnetic anisotropy can be correlated with the metallic radius of the element substitution for Co. These studies have shown that the substitution of various elements offers the possibility of different preferred site occupations in the crystal structure, which can have a pronounced influence on the magnetic anisotropy of the compounds. In order to understand the origin of the magnetic anisotropy and to optimize the magnetic properties of the compounds, further systematic investigation of the influence of the substitution elements and their contents on the crystal structure and the magnetic properties of R2 (T,M)17 -type compounds is still needed. In the previous study [18] we investigated the influence of Cr on the magnetic properties of the compounds Sm2 Co17−x Crx (x = 1.17–3.0).
B. Fuquan et al. / Journal of Alloys and Compounds 399 (2005) 208–213
In this paper, the spin reorientation and anisotropy properties of the Y2 Co17−x Crx (x = 1.17–3.0) compounds are presented.
The ingots of Y2 Co17−x Crx (x = 1.17–3.0) compounds were prepared by arc melting the starting elements with purity of at least 99.9%. The ingots were wrapped in Ta foil and annealed at 1373 K for 120 h in an Ar atmosphere of 100 mbar, and then quenched in water. X-ray diffraction (XRD) with Cu K␣ radiation was used for the determination of the crystal structure, anisotropy and the lattice parameters of the compounds. The thermomagnetic analyses (TMA) were performed in a low field about 5 mT in a SQUID magnetometer in the temperature range from 5 K to room temperature and in a home made Faraday balance from room temperature to above the corresponding Curie temperatures, respectively. The magnetization curves of aligned powder of the Y2 Co17−x Crx (x = 1.17–3.0) compounds were measured in a SQUID magnetometer in applied fields up to 5 T at 5, 100, 200 and 300 K. In order to investigate the magnetic anisotropy of the Y2 Co17−x Crx (x = 1.17) compound with planar anisotropy, fine powder of the compounds were mixed with epoxy resin and filled into a plastic tube of cylindrical shape. Then the plastic tube was mounted on a motor to rotate the tube around its axis perpendicular to an applied field of about 1.5 T until the epoxy resin was solidified. The hard magnetization direction (HMD) was aligned in this way to be along the axis of the tube. The magnetization curves were measured with the SQUID magnetometer in a field applied parallel or perpendicular to the alignment direction of the sample. For the samples with x = 1.76, 2.34 and 3.00, the fine powder of each sample was mixed with epoxy resin and filled into two plastic tubes with cylindrical shape. One of the plastic tubes was placed into an external field of about 1.5 T with its axis parallel to the field direction and the other plastic tube was placed in same external magnetic field with its axis perpendicular to the field direction until the epoxy resin was solidified. The two mentioned procedures were adopted to prepare samples for measurement of the magnetization in the EMD and in the HMD. The values of the spontaneous magnetization M0 at 5, 100, 200 and 300 K were derived from the M–H curves by extrapolating to H = 0. In the cases, the magnetization curves were obtained from aligned powder measured in the SQUID. The anisotropy constants K1 and K2 of the samples with uniaxial anisotropy were determined by using the Sucksmith–Thompson [19] equation: B 2K1 4K2 = + M 2 + N, µ0 M µ0 Ms2 µ0 Ms4
and plotting B/M versus M2 from values obtained in small fields applied perpendicular to the easy direction. In a similar way the planar anisotropy were determined by using the Klein–Menth–Perkins [20] equation: B 4K2 2K1 + 4K2 + = M 2 + N, µ0 M µ0 Ms2 µ0 Ms4
2. Experimental
(1)
209
(2)
where M is the magnetization with a field applied perpendicular to the EMD for uniaxial anisotropy and the easy magnetization plane for planar anisotropy, respectively, B is the applied field, Ms is the saturation magnetization of the sample, and N is the demagnetization factor.
3. Results and discussion All of the samples investigated are almost single-phase compounds crystallizing in the rhombohedral Th2 Zn17 -type structure. Fig. 1 shows the XRD patterns of the random powder and aligned powder of the Y2 Co17−x Crx (x = 1.17–3.00) compounds. It can be seen that the compounds with x = 1.17 has planar anisotropy (as shown in Fig. 1(b)) and that the compounds with x = 1.76, 2.34 and 3.00 have uniaxial anisotropy (as shown in Fig. 1(c) and (d)) at room temperature, respectively. If x = 3.00, the peak (2 0 3) of the hexagonal Th2 Ni17 type structure appears in the XRD spectra, as shown in Fig. 1(e). This means that the rhombohedral and hexago-
Fig. 1. X-ray diffraction patterns of the magnetically aligned and the random powder of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
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Table 1 The lattice parameters and the Curie temperature, the spin reorientation temperature, the spontaneous magnetization of Y2 Co17−x Crx (x = 1.17–3.0) compounds on temperature range from 5 to 300 K x
a (nm)
c (nm)
V (nm3 )
0 1.17 1.76 2.34 3.00
0.8351a 0.8333 0.8346 0.8348 0.8341
1.2200a 1.2171 1.2161 1.2161 1.2173
0.7368b 0.7316 0.7336 0.7338 0.7340
a b
TC (K)
1167a 990 855 708 525
Tsr (K)
1000a 485 237 177 111
M0 (×10−4 T m3 /kg) 5K
100 K
200 K
300 K
– 1.31 1.14 0.99 0.76
– 1.29 1.12 0.98 0.73
– 1.28 1.11 0.96 0.67
– 1.27 1.09 0.91 0.59
Data from Ref. [6]. ៝ × c៝ . Data was calculated with V = (៝a × b)
nal structures coexist in the sample. The values of the lattice parameters a, c and V obtained from the XRD data of the Y2 Co17−x Crx (x = 1.17–3.0) compounds increase slightly with increasing x, as listed in Table 1 and as shown in Fig. 2(a)–(c). Fig. 3 shows the temperature dependence of the magnetization of Y2 Co17−x Crx (x = 1.17–3.0) measured in a field of 5 mT in the temperature range from 5 K to above the corresponding Curie temperature of the compounds. It can be seen that a spin reorientation phenomenon is observed in all of the compounds. It can also be seen that the compounds are ordered ferromagnetically, the Curie temperature TC , the spin reorientation temperature Tsr decreasing rapidly with increasing Cr concentration. The values of all these quantities are also listed in Table 1 and are also shown in Fig. 2(d) and (e). The experimental as well as theoretical studies have shown that the substitution of T atoms in R2 Co17−x Tx and R2 Fe17−x Tx are not at random [21–28], depending on the nature of the T component. The T atoms may show a strong preference for one or more of the available crystallographic sites (6c, 18f, 18h, 9d). The expansion of the unit-cell volume V is due to the fact that the metallic radius of Cr is larger than that of Co. The spontaneous magnetization M0 and the saturation magnetization Ms derived from the magnetization curves
Fig. 2. The lattice parameters a, c, unit cell volume V, and the concentration dependence of the Curie temperature TC and the spin reorientation temperature Tsr of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
Fig. 3. Temperature dependence of the magnetization of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
B. Fuquan et al. / Journal of Alloys and Compounds 399 (2005) 208–213
211
Table 2 The saturation magnetization, the anisotropy constant of Y2 Co17−x Crx (x = 1.17–3.0) compounds at 5 K and the saturation magnetization, the anisotropy constant of Y2 Co15.83 Cr1.17 compound on temperature range from 5 to 300 K x
Ms (T m3 /kg) (5 K)
K1 (kJ/m3 ) (5 K)
K2 (kJ/m3 ) (5 K)
T (K)
Ms (T m3 /kg) (x = 1.17)
K1 (kJ/m3 ) (x = 1.17)
K2 (kJ/m3 ) (x = 1.17)
1.17 1.76 2.34 3.00
1.33 × 10−4 1.31 × 10−4 1.11 × 10−4 0.78 × 10−4
−8.5 18.7 30.5 8.6
17.6 15.9 13.5 3.7
5 100 200 300
1.33 × 10−4 1.32 × 10−4 1.30 × 10−4 1.16 × 10−4
−8.5 −7.1 −3.2 −2.5
17.6 15.8 12.9 8.6
of Y2 Co17−x Crx (x = 1.17–3.0) compounds measured along the EMD are listed in Tables 1 and 2, respectively. It can be seen that the values of M0 decrease monotonically with increasing Cr content and decrease monotonically with increasing temperature, too. As an example, the curves of Y2 Co15.83 Cr1.17 compound from 5 to 300 K and the curves of the Y2 Co17−x Crx (x = 1.17–3.0) compounds at 300 K
measured along the EMD and the HMD are shown in Fig. 4. The values of the anisotropy constant K1 and K2 obtained from B/M versus M2 plots by using Eqs. (1) and (2) are listed in Table 2. It is clear that the values of the K1 of Y2 Co17−x Crx compounds at 5 K increase with increasing Cr content x from 1.17 to 2.34 and changes its sign from
Fig. 4. The magnetization curves of Y2 Co15.83 Cr1.17 from 5 to 300 K and the magnetization curves of Y2 Co17−x Crx (x = 1.17–3.0) compounds measured along the HMD and the EMD in applied fields up to 5 T.
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Fig. 5. Concentration dependence of the anisotropy constant K1 and K2 of Y2 Co17−x Crx (x = 1.17–3.0) compound at 5 K.
tallize as a single phase of the rhombohedral Th2 Zn17 -type structure. The lattice parameters a, c change hardly while the unit-cell volume V increases slightly with increasing Cr content. The XRD patterns of the aligned powder of the samples have confirmed that the compounds Y2 Co17−x Crx with x = 1.17 exhibit planar anisotropy, but those with x = 1.76, 2.34 and 3.00 show uniaxial anisotropy. Spin reorientation phenomena occur in all of the compounds. The values of TC , Tsr , the spontaneous magnetization M0 , and the anisotropy constant K2 at 5 K of the Y2 Co17−x Crx (x = 1.17–3.0) compounds decrease strongly with increasing Cr content x, but the anisotropy constant K1 at 5 K of the compounds appears as a maximum value (30.1 kJ/m3 ) on x = 2.34. The anisotropy constant K1 of the Y2 Co15.83 Cr1.17 compound increases almost monotonically while the anisotropy constant K2 decreases monotonically with increasing temperature from 5 to 300 K.
Acknowledgments This work has been performed in the scientific exchange program between The Netherlands and the PR China.
References
Fig. 6. Temperature dependence of the anisotropy constant K1 and K2 of Y2 Co15.83 Cr1.17 compound.
negative to positive between x = 1.17 and 1.76, and decrease with increasing Cr content x from 2.34 to 3.00 (as shown in Fig. 5) while the K2 values of the compounds decrease monotonically with increasing Cr content. It also can be seen that the anisotropy constant K1 of the Y2 Co15.83 Cr1.17 compound increases almost monotonically while the anisotropy constant K2 decreases monotonically with increasing temperature from 5 to 300 K (as shown in Fig. 6). It was reported that Fe atoms have a tendency to preferential site occupation in the Nd2 Co17−x Tx (T = Fe, Cr) compounds [29,30], so the change of the anisotropy of the Y2 Co17−x Crx compounds due to Cr substitution, may be similar to the case of Fe substitution in Y2 Co17−x Fex given the fact the Cr and Fe have almost the same atomic volumes. It is interesting to note that also in Y2 Co17−x Fex . The room temperature anisotropy changes from easy plane to easy axis for small values of x.
4. Conclusions In summary, the occurrence of spin re-orientation transitions and anisotropy behaviour of Y2 Co17−x Crx (x = 1.17–3.0) compounds have been investigated. The powder X-ray diffraction patterns show that most samples crys-
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Spin reorientation and magnetic anisotropy in Y2Co17−xCrx (x = 1.17–3.0) compounds B. Fuquan a,b,∗ , O. Tegus a,b , W. Dagula a,b , E. Br¨uck a , F.R. de Boer a , K.H.J. Buschow a a
b
Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Department of Physics, Inner Mongolia Normal University, Huhhot 010022, PR China Received 28 February 2005; accepted 16 March 2005 Available online 20 April 2005
Abstract Spin reorientation transitions and magnetic anisotropy in Y2 Co17−x Crx (x = 1.17–3.0) compounds have been investigated by means of X-ray diffraction and magnetization measurements. The powder X-ray diffraction patterns show that most samples crystallize as a single phase with the rhombohedral Th2 Zn17 -type structure. However, in the compound Y2 Co14 Cr3 the Th2 Zn17 phase coexist with the hexagonal Th2 Ni17 -type phase. The lattice parameters a and c hardly change and the unit cell volume V increases slightly with increasing Cr content. The X-ray diffraction patterns of the aligned powder of the samples have confirmed that at room temperature the compound with x = 1.17 has planar anisotropy, but the compounds with x = 1.76, 2.34 and 3.00 have uniaxial anisotropy. Spin reorientation phenomena occur in all of the compounds. With increasing Cr content, the Curie temperature, the spin reorientation temperature, the spontaneous magnetization, and the anisotropy constant K2 of the Y2 Co17−x Crx (x = 1.17–3.0) compounds decrease strongly while the anisotropy constant K1 increases in the range of x from 1.17 to 2.34 and then decreases in the range of x from 2.34 to 3.00. © 2005 Elsevier B.V. All rights reserved. Keywords: Spin reorientation; Uniaxial anisotropy; Planar anisotropy
1. Introduction In the past decades, rare-earth (R) transition-metal (T) compounds of the R2 T17 type with T = Fe or Co have been attracting much attention due to possible practical applications of these compounds as high energy-product magnets [1–11]. A strong uniaxial magnetic anisotropy is required for achieving high coercivity, while the binary R2 Co17 compounds exhibit a weak uniaxial anisotropy only for the compounds with R = Sm, Er and Tm. Recently, it was found that the substitution of Al, Ga and Si for Fe could not only significantly increase the Curie temperature of R2 Fe17 , but it was also found that such substitutions can cause a change in the easy magnetic direction (EMD) from the basal plane to the c-axis ∗
Corresponding author. Tel.: +31 20 5256310; fax: +31 20 5255788. E-mail address: [email protected] (B. Fuquan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.03.042
in R2 T17−x Mx (R = Y, Er, Pr, Ho, Dy; T = Fe, Co; M = Al, Ga, Si) [11–16]. Zhang et al. [17] studied the influence of substitution of small amounts of Ti, V, Cr, Cu, and Mo for Co on the magnetic properties of Y2 Co17 and found that the magnetic anisotropy can be correlated with the metallic radius of the element substitution for Co. These studies have shown that the substitution of various elements offers the possibility of different preferred site occupations in the crystal structure, which can have a pronounced influence on the magnetic anisotropy of the compounds. In order to understand the origin of the magnetic anisotropy and to optimize the magnetic properties of the compounds, further systematic investigation of the influence of the substitution elements and their contents on the crystal structure and the magnetic properties of R2 (T,M)17 -type compounds is still needed. In the previous study [18] we investigated the influence of Cr on the magnetic properties of the compounds Sm2 Co17−x Crx (x = 1.17–3.0).
B. Fuquan et al. / Journal of Alloys and Compounds 399 (2005) 208–213
In this paper, the spin reorientation and anisotropy properties of the Y2 Co17−x Crx (x = 1.17–3.0) compounds are presented.
The ingots of Y2 Co17−x Crx (x = 1.17–3.0) compounds were prepared by arc melting the starting elements with purity of at least 99.9%. The ingots were wrapped in Ta foil and annealed at 1373 K for 120 h in an Ar atmosphere of 100 mbar, and then quenched in water. X-ray diffraction (XRD) with Cu K␣ radiation was used for the determination of the crystal structure, anisotropy and the lattice parameters of the compounds. The thermomagnetic analyses (TMA) were performed in a low field about 5 mT in a SQUID magnetometer in the temperature range from 5 K to room temperature and in a home made Faraday balance from room temperature to above the corresponding Curie temperatures, respectively. The magnetization curves of aligned powder of the Y2 Co17−x Crx (x = 1.17–3.0) compounds were measured in a SQUID magnetometer in applied fields up to 5 T at 5, 100, 200 and 300 K. In order to investigate the magnetic anisotropy of the Y2 Co17−x Crx (x = 1.17) compound with planar anisotropy, fine powder of the compounds were mixed with epoxy resin and filled into a plastic tube of cylindrical shape. Then the plastic tube was mounted on a motor to rotate the tube around its axis perpendicular to an applied field of about 1.5 T until the epoxy resin was solidified. The hard magnetization direction (HMD) was aligned in this way to be along the axis of the tube. The magnetization curves were measured with the SQUID magnetometer in a field applied parallel or perpendicular to the alignment direction of the sample. For the samples with x = 1.76, 2.34 and 3.00, the fine powder of each sample was mixed with epoxy resin and filled into two plastic tubes with cylindrical shape. One of the plastic tubes was placed into an external field of about 1.5 T with its axis parallel to the field direction and the other plastic tube was placed in same external magnetic field with its axis perpendicular to the field direction until the epoxy resin was solidified. The two mentioned procedures were adopted to prepare samples for measurement of the magnetization in the EMD and in the HMD. The values of the spontaneous magnetization M0 at 5, 100, 200 and 300 K were derived from the M–H curves by extrapolating to H = 0. In the cases, the magnetization curves were obtained from aligned powder measured in the SQUID. The anisotropy constants K1 and K2 of the samples with uniaxial anisotropy were determined by using the Sucksmith–Thompson [19] equation: B 2K1 4K2 = + M 2 + N, µ0 M µ0 Ms2 µ0 Ms4
and plotting B/M versus M2 from values obtained in small fields applied perpendicular to the easy direction. In a similar way the planar anisotropy were determined by using the Klein–Menth–Perkins [20] equation: B 4K2 2K1 + 4K2 + = M 2 + N, µ0 M µ0 Ms2 µ0 Ms4
2. Experimental
(1)
209
(2)
where M is the magnetization with a field applied perpendicular to the EMD for uniaxial anisotropy and the easy magnetization plane for planar anisotropy, respectively, B is the applied field, Ms is the saturation magnetization of the sample, and N is the demagnetization factor.
3. Results and discussion All of the samples investigated are almost single-phase compounds crystallizing in the rhombohedral Th2 Zn17 -type structure. Fig. 1 shows the XRD patterns of the random powder and aligned powder of the Y2 Co17−x Crx (x = 1.17–3.00) compounds. It can be seen that the compounds with x = 1.17 has planar anisotropy (as shown in Fig. 1(b)) and that the compounds with x = 1.76, 2.34 and 3.00 have uniaxial anisotropy (as shown in Fig. 1(c) and (d)) at room temperature, respectively. If x = 3.00, the peak (2 0 3) of the hexagonal Th2 Ni17 type structure appears in the XRD spectra, as shown in Fig. 1(e). This means that the rhombohedral and hexago-
Fig. 1. X-ray diffraction patterns of the magnetically aligned and the random powder of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
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Table 1 The lattice parameters and the Curie temperature, the spin reorientation temperature, the spontaneous magnetization of Y2 Co17−x Crx (x = 1.17–3.0) compounds on temperature range from 5 to 300 K x
a (nm)
c (nm)
V (nm3 )
0 1.17 1.76 2.34 3.00
0.8351a 0.8333 0.8346 0.8348 0.8341
1.2200a 1.2171 1.2161 1.2161 1.2173
0.7368b 0.7316 0.7336 0.7338 0.7340
a b
TC (K)
1167a 990 855 708 525
Tsr (K)
1000a 485 237 177 111
M0 (×10−4 T m3 /kg) 5K
100 K
200 K
300 K
– 1.31 1.14 0.99 0.76
– 1.29 1.12 0.98 0.73
– 1.28 1.11 0.96 0.67
– 1.27 1.09 0.91 0.59
Data from Ref. [6]. ៝ × c៝ . Data was calculated with V = (៝a × b)
nal structures coexist in the sample. The values of the lattice parameters a, c and V obtained from the XRD data of the Y2 Co17−x Crx (x = 1.17–3.0) compounds increase slightly with increasing x, as listed in Table 1 and as shown in Fig. 2(a)–(c). Fig. 3 shows the temperature dependence of the magnetization of Y2 Co17−x Crx (x = 1.17–3.0) measured in a field of 5 mT in the temperature range from 5 K to above the corresponding Curie temperature of the compounds. It can be seen that a spin reorientation phenomenon is observed in all of the compounds. It can also be seen that the compounds are ordered ferromagnetically, the Curie temperature TC , the spin reorientation temperature Tsr decreasing rapidly with increasing Cr concentration. The values of all these quantities are also listed in Table 1 and are also shown in Fig. 2(d) and (e). The experimental as well as theoretical studies have shown that the substitution of T atoms in R2 Co17−x Tx and R2 Fe17−x Tx are not at random [21–28], depending on the nature of the T component. The T atoms may show a strong preference for one or more of the available crystallographic sites (6c, 18f, 18h, 9d). The expansion of the unit-cell volume V is due to the fact that the metallic radius of Cr is larger than that of Co. The spontaneous magnetization M0 and the saturation magnetization Ms derived from the magnetization curves
Fig. 2. The lattice parameters a, c, unit cell volume V, and the concentration dependence of the Curie temperature TC and the spin reorientation temperature Tsr of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
Fig. 3. Temperature dependence of the magnetization of Y2 Co17−x Crx (x = 1.17–3.0) compounds.
B. Fuquan et al. / Journal of Alloys and Compounds 399 (2005) 208–213
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Table 2 The saturation magnetization, the anisotropy constant of Y2 Co17−x Crx (x = 1.17–3.0) compounds at 5 K and the saturation magnetization, the anisotropy constant of Y2 Co15.83 Cr1.17 compound on temperature range from 5 to 300 K x
Ms (T m3 /kg) (5 K)
K1 (kJ/m3 ) (5 K)
K2 (kJ/m3 ) (5 K)
T (K)
Ms (T m3 /kg) (x = 1.17)
K1 (kJ/m3 ) (x = 1.17)
K2 (kJ/m3 ) (x = 1.17)
1.17 1.76 2.34 3.00
1.33 × 10−4 1.31 × 10−4 1.11 × 10−4 0.78 × 10−4
−8.5 18.7 30.5 8.6
17.6 15.9 13.5 3.7
5 100 200 300
1.33 × 10−4 1.32 × 10−4 1.30 × 10−4 1.16 × 10−4
−8.5 −7.1 −3.2 −2.5
17.6 15.8 12.9 8.6
of Y2 Co17−x Crx (x = 1.17–3.0) compounds measured along the EMD are listed in Tables 1 and 2, respectively. It can be seen that the values of M0 decrease monotonically with increasing Cr content and decrease monotonically with increasing temperature, too. As an example, the curves of Y2 Co15.83 Cr1.17 compound from 5 to 300 K and the curves of the Y2 Co17−x Crx (x = 1.17–3.0) compounds at 300 K
measured along the EMD and the HMD are shown in Fig. 4. The values of the anisotropy constant K1 and K2 obtained from B/M versus M2 plots by using Eqs. (1) and (2) are listed in Table 2. It is clear that the values of the K1 of Y2 Co17−x Crx compounds at 5 K increase with increasing Cr content x from 1.17 to 2.34 and changes its sign from
Fig. 4. The magnetization curves of Y2 Co15.83 Cr1.17 from 5 to 300 K and the magnetization curves of Y2 Co17−x Crx (x = 1.17–3.0) compounds measured along the HMD and the EMD in applied fields up to 5 T.
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Fig. 5. Concentration dependence of the anisotropy constant K1 and K2 of Y2 Co17−x Crx (x = 1.17–3.0) compound at 5 K.
tallize as a single phase of the rhombohedral Th2 Zn17 -type structure. The lattice parameters a, c change hardly while the unit-cell volume V increases slightly with increasing Cr content. The XRD patterns of the aligned powder of the samples have confirmed that the compounds Y2 Co17−x Crx with x = 1.17 exhibit planar anisotropy, but those with x = 1.76, 2.34 and 3.00 show uniaxial anisotropy. Spin reorientation phenomena occur in all of the compounds. The values of TC , Tsr , the spontaneous magnetization M0 , and the anisotropy constant K2 at 5 K of the Y2 Co17−x Crx (x = 1.17–3.0) compounds decrease strongly with increasing Cr content x, but the anisotropy constant K1 at 5 K of the compounds appears as a maximum value (30.1 kJ/m3 ) on x = 2.34. The anisotropy constant K1 of the Y2 Co15.83 Cr1.17 compound increases almost monotonically while the anisotropy constant K2 decreases monotonically with increasing temperature from 5 to 300 K.
Acknowledgments This work has been performed in the scientific exchange program between The Netherlands and the PR China.
References
Fig. 6. Temperature dependence of the anisotropy constant K1 and K2 of Y2 Co15.83 Cr1.17 compound.
negative to positive between x = 1.17 and 1.76, and decrease with increasing Cr content x from 2.34 to 3.00 (as shown in Fig. 5) while the K2 values of the compounds decrease monotonically with increasing Cr content. It also can be seen that the anisotropy constant K1 of the Y2 Co15.83 Cr1.17 compound increases almost monotonically while the anisotropy constant K2 decreases monotonically with increasing temperature from 5 to 300 K (as shown in Fig. 6). It was reported that Fe atoms have a tendency to preferential site occupation in the Nd2 Co17−x Tx (T = Fe, Cr) compounds [29,30], so the change of the anisotropy of the Y2 Co17−x Crx compounds due to Cr substitution, may be similar to the case of Fe substitution in Y2 Co17−x Fex given the fact the Cr and Fe have almost the same atomic volumes. It is interesting to note that also in Y2 Co17−x Fex . The room temperature anisotropy changes from easy plane to easy axis for small values of x.
4. Conclusions In summary, the occurrence of spin re-orientation transitions and anisotropy behaviour of Y2 Co17−x Crx (x = 1.17–3.0) compounds have been investigated. The powder X-ray diffraction patterns show that most samples crys-
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