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Superconductivity and magnetism in the Er(Rh1-xCox)4B4 system
519 Journal of Magnetism and Magnetic Materials 31-34 (1983) 519-520 SUPERCONDUCTIVITY AND MAGNETISM M. I S I N O , K. T S U N O K U N I ,
I N T H E Er(Rh I_ xCox)4B4 S Y S T E M
H. I W A S A K I a n d Y. M U T O
The Research Institute for lron, Steel and Other Metals, Tohoku University, Sendai 980, Japan
Rh-rich compounds (x < 0.3) of the LuRu4B4 type are antiferromagnetic superconductors. T~ decreases steeply between x = 0 and 0.1 with increasing x while TN changes rather continuously. No superconductivity appears in compounds with x > 0.3. An unidentified ferromagnetic phase appears in the intermediate region (0.3 < x < 0.7).
Recently, the interplay between superconductivity and magnetism has been studied in a n u m b e r of ternary borides, LT4B4, where L is a lanthanoid and T is a transition element. Polytypes often appear in the borides. The interaction between superconducting electrons consisting mainly of d-electrons of T atoms and magnetic moments of L ions is relatively so weak that a variety of phenomena have been reported: ErRh4B 4 of the L u R u 4 B 4 type is an antiferromagnetic superconductor, as reported separately [1]. In this paper the effect of the substitution of Rh atoms by Co ones in ErRh4B 4 of the LuRu4B 4 type is studied on both superconductivity and magnetism. Pseudoternary borides which contain Co have never been studied. Er(Rhl_xCox)4B 4 compounds were prepared as follows. First, both ErRh4B 4 and ErCo4B 4 were synthesized by arc-melting. Then, the correct proportion of both compounds was arc-melted to get the desired pseudoternary compound. Obtained ingots were annealed at 1050°C for ten days. Three types of crystal structures appeared: the crystal structure in the Rh-rich
:
o.2 !
I
0
I
Z
3 T
4
tK)
Fig. I. Temperature dependence of the upper critical field H~2 of Er(Rh l_xCox)4 compounds of the LuRu4B4 type. 0304-8853/83/0000-0000/$03.00
side (x = 0, 0.1, 0.2 and 0.3) is the LuRh4B 4 type [2], while that in the Co-rich side (x = 0.7, 0.8 and 1) is the CeCo4B 4 type [3]; unexpectedly, a new tetragonal crystal structure appears in the intermediate region (x = 0.4 and 0.5), and the X-ray diffraction data show that it exists as an impurity phase even in Rh-rich samples, especially in a sample with x = 0.3. This new phase is found to be ferromagnetic from the magnetization measurement. The superconducting transition is detected by an ac four-probe technique of the resistivity measurement. No superconductivity is observed in compounds with x > 0.3. Fig. 1 shows Hc2(T) curves of three Rh-rich compounds. He2 is determined by a linear extrapolation to zero resistance. They are superconducting down to the lowest temperature in our measurement, that is, to 9 m K for samples with x = 0.1 and 0.2, and to 0.45 K for a sample with x = 0.3. Hc2(T) curves attain a maximum and have a dip with decreasing temperature. In superconductors where the antiferromagnetism is confirmed by neutron diffraction [4], it is found that superconductivity persists down below the N6el temperature Try; magnetic effects supress the superconductivity, and the temperature dependence of H~2 changes discontinuously at a temperature just below T~. For a sample with x = 0 [1] the magnetization measurement shows that it is an antiferromagnetic superconductor and that it is not so bad to estimate T N from a dip in Hc2(T ). Therefore, Rh-rich samples are considered to be antiferromagnetic superconductors, and T N is estimated from a dip in H~2(T ). Fig. 2 shows the concentration dependence of both T~ and TN; values of a compound with x = 0 were taken from ref. [1]. It should be noted that T~ decreases largely between 0 and 0.1 with increasing x, while T~ changes rather continuously. T~ has already been observed to decrease precipitously at a critical concentration xc in L(Rh]_xRux)4B 4 systems: x¢ is 0.5 when L is Er [5] and 0.3 when L is Dy [6]. A similar phenomenon has also been found in borides of the CeCo4B 4 type: x¢ is 0.5 in L(Rh l_xlrx)4B4 when L is Er, Ho or Dy [7]. The magnetic transition temperature ever measued in the
© 1983 N o r t h - H o l l a n d
M. lsino et al. / The Er(Rh~_ xCox)4B4system
520
in Tc but have rather little effect on the magnetic interaction between L ions. Our data show that in pseudotenary Er(Rh l_xT;)4B4 systems the effect of the substitution of Rh atoms by Co ones on T~ is much larger when compared with that by Ru or Ir ones. It is s u p p o s e d that x c is b e t w e e n 0 and 0.1 in Er(Rhl_xCox)4B4 system, and a study on this point is in progress. A Co atom has two characteristics when compared with a Rh, Ru and Ir atom. Its atomic radius is a little smaller and its electronegativity is more positive than those of the others which are almost equal to each other. Therefore, it seems natural that the substitution of Rh atoms by Co ones has much larger effect than that by Ru or Ir ones, though it has not yet been made clear what causes a precipitous decrease in T¢.
t4
TN 0
0.!
0.2 0.3 x Fig. 2. Concentration dependence of the superconducting transition temperature Te and the N~el temperature TN in Er(Rhl_xCox)4B4 compounds of the LuRu4B 4 type. Vertical bars display the width of the superconducting transition.
above-mentioned pseudoternary systems changes continuously at xc with increasing x. A kind of "structural transformation" may happen at xc, though the superstructure and the order-disorder transition has already been denied [5,7]; Yvon and Griittner [8] suggested that the B 2 dimer plays an important role. It is thought that the "structural transformation" causes a change in the electronic density of states at the Fermi level and then
References [1] H. Iwasaki, M. Isino, K. Tsunokuni and Y. Muto, J. Magn. Magn. Mat. 31-34 (1983) 521. [2] D.C. Johnston, Solid State Commun. 24 (1977) 699. [3] Yu. Kuz'ma and N.S. Bilonizhko, Soviet Phys.-Cryst. 16 (1972) 897. [4] W. Thomlinson, G. Shirane. D.E. Moncton, M. Ishikawa and O. Fischer, Phys. Rev. B23 (1981) 4455. [5] H.E. Horng and R.N. Shekon, in: Ternary Superconductors, eds. G.K. Shenoy, B.D. Dunlap and F.Y. Fradin (North-Holland, New York, 1981) p. 213. [6] H.C. Hamaker and M.B. Maple, Physica 108B (1981) 757. [7] H.C. Ku, B.T. Matthias and H. Barz, Solid State Commun. 35 (1979) 937. [8] K. Yvon and A. Grtittner, in: Superconductivity in d- and f-Band Metals, eds. H. Suhl and B. Maple (Academic Press, New York, 1980) p. 515.
I N T H E Er(Rh I_ xCox)4B4 S Y S T E M
H. I W A S A K I a n d Y. M U T O
The Research Institute for lron, Steel and Other Metals, Tohoku University, Sendai 980, Japan
Rh-rich compounds (x < 0.3) of the LuRu4B4 type are antiferromagnetic superconductors. T~ decreases steeply between x = 0 and 0.1 with increasing x while TN changes rather continuously. No superconductivity appears in compounds with x > 0.3. An unidentified ferromagnetic phase appears in the intermediate region (0.3 < x < 0.7).
Recently, the interplay between superconductivity and magnetism has been studied in a n u m b e r of ternary borides, LT4B4, where L is a lanthanoid and T is a transition element. Polytypes often appear in the borides. The interaction between superconducting electrons consisting mainly of d-electrons of T atoms and magnetic moments of L ions is relatively so weak that a variety of phenomena have been reported: ErRh4B 4 of the L u R u 4 B 4 type is an antiferromagnetic superconductor, as reported separately [1]. In this paper the effect of the substitution of Rh atoms by Co ones in ErRh4B 4 of the LuRu4B 4 type is studied on both superconductivity and magnetism. Pseudoternary borides which contain Co have never been studied. Er(Rhl_xCox)4B 4 compounds were prepared as follows. First, both ErRh4B 4 and ErCo4B 4 were synthesized by arc-melting. Then, the correct proportion of both compounds was arc-melted to get the desired pseudoternary compound. Obtained ingots were annealed at 1050°C for ten days. Three types of crystal structures appeared: the crystal structure in the Rh-rich
:
o.2 !
I
0
I
Z
3 T
4
tK)
Fig. I. Temperature dependence of the upper critical field H~2 of Er(Rh l_xCox)4 compounds of the LuRu4B4 type. 0304-8853/83/0000-0000/$03.00
side (x = 0, 0.1, 0.2 and 0.3) is the LuRh4B 4 type [2], while that in the Co-rich side (x = 0.7, 0.8 and 1) is the CeCo4B 4 type [3]; unexpectedly, a new tetragonal crystal structure appears in the intermediate region (x = 0.4 and 0.5), and the X-ray diffraction data show that it exists as an impurity phase even in Rh-rich samples, especially in a sample with x = 0.3. This new phase is found to be ferromagnetic from the magnetization measurement. The superconducting transition is detected by an ac four-probe technique of the resistivity measurement. No superconductivity is observed in compounds with x > 0.3. Fig. 1 shows Hc2(T) curves of three Rh-rich compounds. He2 is determined by a linear extrapolation to zero resistance. They are superconducting down to the lowest temperature in our measurement, that is, to 9 m K for samples with x = 0.1 and 0.2, and to 0.45 K for a sample with x = 0.3. Hc2(T) curves attain a maximum and have a dip with decreasing temperature. In superconductors where the antiferromagnetism is confirmed by neutron diffraction [4], it is found that superconductivity persists down below the N6el temperature Try; magnetic effects supress the superconductivity, and the temperature dependence of H~2 changes discontinuously at a temperature just below T~. For a sample with x = 0 [1] the magnetization measurement shows that it is an antiferromagnetic superconductor and that it is not so bad to estimate T N from a dip in Hc2(T ). Therefore, Rh-rich samples are considered to be antiferromagnetic superconductors, and T N is estimated from a dip in H~2(T ). Fig. 2 shows the concentration dependence of both T~ and TN; values of a compound with x = 0 were taken from ref. [1]. It should be noted that T~ decreases largely between 0 and 0.1 with increasing x, while T~ changes rather continuously. T~ has already been observed to decrease precipitously at a critical concentration xc in L(Rh]_xRux)4B 4 systems: x¢ is 0.5 when L is Er [5] and 0.3 when L is Dy [6]. A similar phenomenon has also been found in borides of the CeCo4B 4 type: x¢ is 0.5 in L(Rh l_xlrx)4B4 when L is Er, Ho or Dy [7]. The magnetic transition temperature ever measued in the
© 1983 N o r t h - H o l l a n d
M. lsino et al. / The Er(Rh~_ xCox)4B4system
520
in Tc but have rather little effect on the magnetic interaction between L ions. Our data show that in pseudotenary Er(Rh l_xT;)4B4 systems the effect of the substitution of Rh atoms by Co ones on T~ is much larger when compared with that by Ru or Ir ones. It is s u p p o s e d that x c is b e t w e e n 0 and 0.1 in Er(Rhl_xCox)4B4 system, and a study on this point is in progress. A Co atom has two characteristics when compared with a Rh, Ru and Ir atom. Its atomic radius is a little smaller and its electronegativity is more positive than those of the others which are almost equal to each other. Therefore, it seems natural that the substitution of Rh atoms by Co ones has much larger effect than that by Ru or Ir ones, though it has not yet been made clear what causes a precipitous decrease in T¢.
t4
TN 0
0.!
0.2 0.3 x Fig. 2. Concentration dependence of the superconducting transition temperature Te and the N~el temperature TN in Er(Rhl_xCox)4B4 compounds of the LuRu4B 4 type. Vertical bars display the width of the superconducting transition.
above-mentioned pseudoternary systems changes continuously at xc with increasing x. A kind of "structural transformation" may happen at xc, though the superstructure and the order-disorder transition has already been denied [5,7]; Yvon and Griittner [8] suggested that the B 2 dimer plays an important role. It is thought that the "structural transformation" causes a change in the electronic density of states at the Fermi level and then
References [1] H. Iwasaki, M. Isino, K. Tsunokuni and Y. Muto, J. Magn. Magn. Mat. 31-34 (1983) 521. [2] D.C. Johnston, Solid State Commun. 24 (1977) 699. [3] Yu. Kuz'ma and N.S. Bilonizhko, Soviet Phys.-Cryst. 16 (1972) 897. [4] W. Thomlinson, G. Shirane. D.E. Moncton, M. Ishikawa and O. Fischer, Phys. Rev. B23 (1981) 4455. [5] H.E. Horng and R.N. Shekon, in: Ternary Superconductors, eds. G.K. Shenoy, B.D. Dunlap and F.Y. Fradin (North-Holland, New York, 1981) p. 213. [6] H.C. Hamaker and M.B. Maple, Physica 108B (1981) 757. [7] H.C. Ku, B.T. Matthias and H. Barz, Solid State Commun. 35 (1979) 937. [8] K. Yvon and A. Grtittner, in: Superconductivity in d- and f-Band Metals, eds. H. Suhl and B. Maple (Academic Press, New York, 1980) p. 515.