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Evolution of magnetic phase transition in intermetallic borocarbide Pr1−xDyxCo2B2C
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1058}1059
Evolution of magnetic phase transition in intermetallic borocarbide Pr Dy Co B C \V V M. El Massalami*, E.F. Chagas, R.E. Rapp IF-UFRJ, P. Box 68528, 21945-970 Rio de Janeiro, Brazil
Abstract The recently reported magnetic phase diagram (¹ versus de Gennes factor deG) of the body-centered tetragonal , RCo B C series indicates that compounds with R"Pr orders at a higher temperature (¹ +10.2 K) than that of the , isomorphs R"Nd (¹ "3.0 K), Er (¹ "4.0 K), Sm (¹ "6.0 K), Ho (¹ "5.4 K), Dy (¹ "7.8 K) (Rapp, El , , , , , Massalami, Phys. Rev. B 60 (1999) 3355; El Massalami et al., Phys. Rev. B, submitted). Since the deG of the latter compounds are higher than that of Pr compound, then such non-monotonic evolution of ¹ with deG deserves further , investigations. In that direction, we synthesized and investigated the magnetic phase diagram of the series Pr Dy Co B C (x"0.0, 0.09, 0.13, 0.345, 0.797, 1.0) wherein deG spans the interval from 0.8 (x"0) to 7.0819 (x"1). \V V The obtained unit-cell parameters are in excellent agreement with the Vegard's law, demonstrating that the Pr and Dy atoms substitute each other randomly. Magnetic measurements reveal that the saturated magnetic moment (at 2 K and 90 kOe) increases linearly with x (/x+4.7(5) ). Furthermore, for x(0.17, ¹ (x) decreases sharply with , x (¹ /x+!36(1) K), while for x* 0.17, ¹ (x) increases as ¹ /x+5.8(1) K. The overall evolution of ¹ with deG , , , , reproduces satisfactorily the behavior of the above cited RCo B C compounds. These results will be discussed in terms of the hybridization between the 4f and the conduction electrons. 2001 Elsevier Science B.V. All rights reserved. Keywords: Borocarbides; Hybridization; Substitutional e!ects, Rare earth intermetallic compounds
The systematic analysis of the magnetic features of the non-superconducting RCo B C [1,2] revealed that their magnetic transition points ¹ except of PrCo B C are , systematically lower than those of the isomorphous RNi B C [3,4,6] series and do not scale with the de Gennes factor, deG, "(g!1)J(J#1). PrCo B C, in particular, presents many anomalous features that are not in line with the tendency of other RCo B C: e.g. the peaks at ¹ of C (¹) and (¹) are exceptionally ,
broadened [1,2] and most strikingly, its ¹ (+10.2 K) is , higher than that of the isomorphs R"Nd (3.0 K), Er (4.0 K), Sm (6.0 K), Ho (5.4 K), Dy (7.8 K). Since deG of the latter compounds are higher than that of Pr compound, then such non-monotonic evolution of ¹ with the de , Gennes factor deserves further investigations. This work synthesized and investigated the magnetic phase diagram of the series Pr Dy Co B C (x"0.0, 0.09, 0.13, 0.345, \V V * Corresponding author. Fax: #55-21-562-7368. E-mail address: [email protected] (M. El Massalami).
0.797, 1.0) wherein deG spans the interval from 0.8 (x"0) to 7.0819 (x"1). Samples of Pr Dy Co B C were prepared by con\V V ventional argon arc-melt method and characterized structurally with room temperature Cu K powder X-ray di!raction. The AC susceptibility was measured with ! a mutual induction susceptometer (2 K(¹(300 K, "250}500 Hz, and H +1}15 Oe). Magnetization M was carried out in an extraction-type magnetometer (H(90 kOe, 1.8 K(¹(300 K). ¹ were determined , from the position of the maxima in !(M/H)/¹, ( )/¹, and (¹) (see Ref. [1,2]). ! ! Fig. 1 shows the variation of the unit-cell parameters with the e!ective lanthanide metallic radius. Such variation is in excellent agreement with the Vegard's law, demonstrating that the Pr and Dy atoms substitute each other randomly. Moreover, the substitution of the larger-sized Pr-atoms induces, just as observed in other borocarbides [3,4,6], an increase in both a-parameter and unit-cell volume with a concomitant decrease in c-parameter.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 6 8 - X
M. El Massalami et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1058}1059
Fig. 1. The metric parameters of the tetragonal unit cell of Pr Dy Co B C (symbol) evolve linearly with the e!ective \V V metallic r radius. Least-squares "ttings are denoted by solid lines. The corresponding cell parameters of Pr Dy Ni B C \V V are denoted by dashed lines [6,7].
Fig. 2. (Lower) ¹ of (Pr Dy )Co B C versus deG factor , \V V ("0.8#6.28x). Remarkably, the overall evolution of ¹ () , reproduces satisfactorily the behavior of RCo B C [1]. Com paring ¹ of PrCo B C (PrNi B C) to that of RCo B C , (RNi B C [5,6]) where R"Er, Ho, Dy, one infers that the hybridization in the former is much stronger than that in the Ni-based compound. (Upper) the saturated (H"90 kOe) and spontaneous (H"0 kOe) magnetic moment at 2 K versus the de Gennes factor. As a comparison, the saturation moment for PrNi B C and DyNi B C is, respectively, 0.81(3) and 8.47(9) [6].
1059
decreases sharply with x (¹ /x+!36(1) K) and , furthermore shows a saturated trend. In the second region (x'0.17), ¹ (x) increases almost linearly as , ¹ /x+5.8(1) K and (¹"2 K, H"90 kOe, x) , 1 augments linearly as /x+4.7(5) . It is to be emphasized that the evolution of the magnetic moments with concentration does not support an interpretation in terms of a compensation model wherein moments of Dy and Pr are oppositely oriented. The evolution of ¹ (x), (¹"2 K, H"0, x) and , 1 (¹"2 K, H"90 kOe, x) within the second region 1 can be understood from mean "eld arguments: magnetic properties are governed by an average of localized Pr and Dy moments. Thus, both ¹ (x) and the weighted average , of are expected, as con"rmed by experiments, to increase as the concentration of Dy-moment is increased. In contrast, the evolution of the magnetic features within x(0.17 is di!erent and anomalous: e.g. ¹ (x"0) is , higher than that of other RCo B C compounds though its (x"0) is lower than the theoretically expected Hundvalue of a Pr>-ion. In fact, these anomalous features are not uncommon among Pr-based intermetallics. As an example, the ordered magnetic moment of the isomorphous PrNi B C is unexpectedly small [6]. Furthermore, ¹ of , PrT Si (T"Fe, Ni, Cu) is much higher than its corre sponding Gd-isomorph. Both features were attributed to the 4f hybridization with the surrounding electrons [8]. Similarly for PrCo B C, we assume that hybridization is responsible for its anomalous magnetic features within the x(0.17 region: it is responsible for the high value of ¹ and the additional weak itinerant moment (x). The , ' addition of Dy leads to the destruction of (x) and to the ' abrupt reduction of ¹ (x). It is implied that the localized , component of the Pr-moment is responsible for the * mean "eld features observed for x'0.17, the value of is obtained from the linear extrapolation of the high * Dy-region, giving "1.2(1) . Then (x"0)" * ' (x"0)! "0.5(1) . * In conclusion, the evolution of features of Pr ,Dy Co B C suggests that on chemical variation \V V of x one is able to tune to any of deG of RCo B C within the range [0.8,7.08] and consequently reproduce its corresponding ¹ ; a universal ¹ versus deG curve for , , these RCo B C. References
The analysis of the measured magnetic curves (not shown) yields the determination (shown in Fig. 2) of ¹ , the saturated magnetic moment (¹"2 K, , 1 H"90 kOe), and the spontaneous magnetic moment (¹"2 K, H"0 kOe) using Arrot plots. The general 1 features of the evolution of ¹ (x), (¹"2 K, , 1 H"0 kOe, x) and (¹"2 K, H"90 kOe, x) reveals 1 two separate regions. The "rst region (x(0.17) re#ects a most interesting and dramatic manifestation of the anomalous behavior of Pr-based solid solution: ¹ (x) ,
[1] [2] [3] [4] [5] [6] [7] [8]
R.E. Rapp, M. El Massalami, Phys. Rev. B 60 (1999) 3355. M. El Massalami et al., Phys. Rev. B, submitted. T. Siegrist et al., Nature 367 (1994) 254. T. Siegrist et al., J. Alloys Compounds 216 (1994) 135. H. Eisaki et al., Phys. Rev. B 50 (1994) 647. J.W. Lynn et al., Phys. Rev. B 55 (1997) 6584. H. Takeya, M. Kunietz, J. Magn. Magn. Mater. 195 (1999) 366. A. Szytula, J. Leceiejwicz, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on Physics and Chemistry of Rare Earths, Vol. 12, North-Holland, Amsterdam, 1989, p. 133.
Evolution of magnetic phase transition in intermetallic borocarbide Pr Dy Co B C \V V M. El Massalami*, E.F. Chagas, R.E. Rapp IF-UFRJ, P. Box 68528, 21945-970 Rio de Janeiro, Brazil
Abstract The recently reported magnetic phase diagram (¹ versus de Gennes factor deG) of the body-centered tetragonal , RCo B C series indicates that compounds with R"Pr orders at a higher temperature (¹ +10.2 K) than that of the , isomorphs R"Nd (¹ "3.0 K), Er (¹ "4.0 K), Sm (¹ "6.0 K), Ho (¹ "5.4 K), Dy (¹ "7.8 K) (Rapp, El , , , , , Massalami, Phys. Rev. B 60 (1999) 3355; El Massalami et al., Phys. Rev. B, submitted). Since the deG of the latter compounds are higher than that of Pr compound, then such non-monotonic evolution of ¹ with deG deserves further , investigations. In that direction, we synthesized and investigated the magnetic phase diagram of the series Pr Dy Co B C (x"0.0, 0.09, 0.13, 0.345, 0.797, 1.0) wherein deG spans the interval from 0.8 (x"0) to 7.0819 (x"1). \V V The obtained unit-cell parameters are in excellent agreement with the Vegard's law, demonstrating that the Pr and Dy atoms substitute each other randomly. Magnetic measurements reveal that the saturated magnetic moment (at 2 K and 90 kOe) increases linearly with x (/x+4.7(5) ). Furthermore, for x(0.17, ¹ (x) decreases sharply with , x (¹ /x+!36(1) K), while for x* 0.17, ¹ (x) increases as ¹ /x+5.8(1) K. The overall evolution of ¹ with deG , , , , reproduces satisfactorily the behavior of the above cited RCo B C compounds. These results will be discussed in terms of the hybridization between the 4f and the conduction electrons. 2001 Elsevier Science B.V. All rights reserved. Keywords: Borocarbides; Hybridization; Substitutional e!ects, Rare earth intermetallic compounds
The systematic analysis of the magnetic features of the non-superconducting RCo B C [1,2] revealed that their magnetic transition points ¹ except of PrCo B C are , systematically lower than those of the isomorphous RNi B C [3,4,6] series and do not scale with the de Gennes factor, deG, "(g!1)J(J#1). PrCo B C, in particular, presents many anomalous features that are not in line with the tendency of other RCo B C: e.g. the peaks at ¹ of C (¹) and (¹) are exceptionally ,
broadened [1,2] and most strikingly, its ¹ (+10.2 K) is , higher than that of the isomorphs R"Nd (3.0 K), Er (4.0 K), Sm (6.0 K), Ho (5.4 K), Dy (7.8 K). Since deG of the latter compounds are higher than that of Pr compound, then such non-monotonic evolution of ¹ with the de , Gennes factor deserves further investigations. This work synthesized and investigated the magnetic phase diagram of the series Pr Dy Co B C (x"0.0, 0.09, 0.13, 0.345, \V V * Corresponding author. Fax: #55-21-562-7368. E-mail address: [email protected] (M. El Massalami).
0.797, 1.0) wherein deG spans the interval from 0.8 (x"0) to 7.0819 (x"1). Samples of Pr Dy Co B C were prepared by con\V V ventional argon arc-melt method and characterized structurally with room temperature Cu K powder X-ray di!raction. The AC susceptibility was measured with ! a mutual induction susceptometer (2 K(¹(300 K, "250}500 Hz, and H +1}15 Oe). Magnetization M was carried out in an extraction-type magnetometer (H(90 kOe, 1.8 K(¹(300 K). ¹ were determined , from the position of the maxima in !(M/H)/¹, ( )/¹, and (¹) (see Ref. [1,2]). ! ! Fig. 1 shows the variation of the unit-cell parameters with the e!ective lanthanide metallic radius. Such variation is in excellent agreement with the Vegard's law, demonstrating that the Pr and Dy atoms substitute each other randomly. Moreover, the substitution of the larger-sized Pr-atoms induces, just as observed in other borocarbides [3,4,6], an increase in both a-parameter and unit-cell volume with a concomitant decrease in c-parameter.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 7 6 8 - X
M. El Massalami et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1058}1059
Fig. 1. The metric parameters of the tetragonal unit cell of Pr Dy Co B C (symbol) evolve linearly with the e!ective \V V metallic r radius. Least-squares "ttings are denoted by solid lines. The corresponding cell parameters of Pr Dy Ni B C \V V are denoted by dashed lines [6,7].
Fig. 2. (Lower) ¹ of (Pr Dy )Co B C versus deG factor , \V V ("0.8#6.28x). Remarkably, the overall evolution of ¹ () , reproduces satisfactorily the behavior of RCo B C [1]. Com paring ¹ of PrCo B C (PrNi B C) to that of RCo B C , (RNi B C [5,6]) where R"Er, Ho, Dy, one infers that the hybridization in the former is much stronger than that in the Ni-based compound. (Upper) the saturated (H"90 kOe) and spontaneous (H"0 kOe) magnetic moment at 2 K versus the de Gennes factor. As a comparison, the saturation moment for PrNi B C and DyNi B C is, respectively, 0.81(3) and 8.47(9) [6].
1059
decreases sharply with x (¹ /x+!36(1) K) and , furthermore shows a saturated trend. In the second region (x'0.17), ¹ (x) increases almost linearly as , ¹ /x+5.8(1) K and (¹"2 K, H"90 kOe, x) , 1 augments linearly as /x+4.7(5) . It is to be emphasized that the evolution of the magnetic moments with concentration does not support an interpretation in terms of a compensation model wherein moments of Dy and Pr are oppositely oriented. The evolution of ¹ (x), (¹"2 K, H"0, x) and , 1 (¹"2 K, H"90 kOe, x) within the second region 1 can be understood from mean "eld arguments: magnetic properties are governed by an average of localized Pr and Dy moments. Thus, both ¹ (x) and the weighted average , of are expected, as con"rmed by experiments, to increase as the concentration of Dy-moment is increased. In contrast, the evolution of the magnetic features within x(0.17 is di!erent and anomalous: e.g. ¹ (x"0) is , higher than that of other RCo B C compounds though its (x"0) is lower than the theoretically expected Hundvalue of a Pr>-ion. In fact, these anomalous features are not uncommon among Pr-based intermetallics. As an example, the ordered magnetic moment of the isomorphous PrNi B C is unexpectedly small [6]. Furthermore, ¹ of , PrT Si (T"Fe, Ni, Cu) is much higher than its corre sponding Gd-isomorph. Both features were attributed to the 4f hybridization with the surrounding electrons [8]. Similarly for PrCo B C, we assume that hybridization is responsible for its anomalous magnetic features within the x(0.17 region: it is responsible for the high value of ¹ and the additional weak itinerant moment (x). The , ' addition of Dy leads to the destruction of (x) and to the ' abrupt reduction of ¹ (x). It is implied that the localized , component of the Pr-moment is responsible for the * mean "eld features observed for x'0.17, the value of is obtained from the linear extrapolation of the high * Dy-region, giving "1.2(1) . Then (x"0)" * ' (x"0)! "0.5(1) . * In conclusion, the evolution of features of Pr ,Dy Co B C suggests that on chemical variation \V V of x one is able to tune to any of deG of RCo B C within the range [0.8,7.08] and consequently reproduce its corresponding ¹ ; a universal ¹ versus deG curve for , , these RCo B C. References
The analysis of the measured magnetic curves (not shown) yields the determination (shown in Fig. 2) of ¹ , the saturated magnetic moment (¹"2 K, , 1 H"90 kOe), and the spontaneous magnetic moment (¹"2 K, H"0 kOe) using Arrot plots. The general 1 features of the evolution of ¹ (x), (¹"2 K, , 1 H"0 kOe, x) and (¹"2 K, H"90 kOe, x) reveals 1 two separate regions. The "rst region (x(0.17) re#ects a most interesting and dramatic manifestation of the anomalous behavior of Pr-based solid solution: ¹ (x) ,
[1] [2] [3] [4] [5] [6] [7] [8]
R.E. Rapp, M. El Massalami, Phys. Rev. B 60 (1999) 3355. M. El Massalami et al., Phys. Rev. B, submitted. T. Siegrist et al., Nature 367 (1994) 254. T. Siegrist et al., J. Alloys Compounds 216 (1994) 135. H. Eisaki et al., Phys. Rev. B 50 (1994) 647. J.W. Lynn et al., Phys. Rev. B 55 (1997) 6584. H. Takeya, M. Kunietz, J. Magn. Magn. Mater. 195 (1999) 366. A. Szytula, J. Leceiejwicz, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on Physics and Chemistry of Rare Earths, Vol. 12, North-Holland, Amsterdam, 1989, p. 133.