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Normal state magnetic properties of superconducting single crystal YNi2B2C
PHYSlCA ELSEVIER
Physica C 341-348 (2000) 2025-2026
www.elsevier.nl/Iocate/physc
Normal State Magnetic Properties of Superconducting Single Crystal YNi2B2C B. K. Cho Department of materials science and technology, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712 South Korea The normal state magnetic properties of single crystal YNi2B2C have been investigated by field dependent isothermal magnetization (M) and temperature (T) dependent magnetic susceptibility (M/H --- Z) measurements. The ;~(T) data show a T-dependent anisotropic behavior. The )~(T) data is separated by the contributions of impurities, ~imp, diamagnetic core electrons, Z~°~, Van Vleck type interaction, Zvv, and conduction electron spins, ~ l . The weakly T-dependence ~(T) is explained by ~ l , from which the second derivative of density of state at Fermi energy is estimated. The comparison of the Pauli susceptibility, )~aauli(T), from this measurements with the one from electronic band structure calculation is performed.
The superconducting compounds, ReNizBzC (Re = rare earth elements), have been intensively studied for a last few years since the discovery of them at 1994 [1]. Although it has alternating layers of ReC and Ni2B2, it turned out to have three-dimensional electronic configuration based upon the electronic band structure calculation [2]. These compounds are of great interest because they exhibit not only relatively high superconducting transition temperature (T~) but also coexistence of superconductivity and magnetism. The significant interaction between superconductivity and magnetism is studied in transport and magnetic measurements [3]. In addition, the magnetic anisotropy in the normal state is studied in terms of crystalline electric field effects [4]. While the superconducting and magnetic properties and their interaction have been well characterized by transport and magnetic measurements, the magnetic properties in the normal state in YNi2B2C is the area to be studied more. Because the magnitude of the magnetic signal of YNi2B2C is quite small, it is important to measure single crystal sample to minimize extrinsic signal from impurity. The measurement of the single crystal is also essential to give the angular dependence of the magnetization. In this paper, we report the field (H) and temperature (T)
dependent magnetizations of single crystal of YNi2B2C and analyze the data in terms of various type of magnetic contribution. Single crystals of YNi2B2C have been grown via high temperature metal flux growth, using NieB as a solvent as described elsewhere [5]. Powder X-ray diffraction patterns of pulverized single crystals show the crystals to be single phase (only the most intense (2,1,1) line of Ni2B was seen and is due to small amounts of flux remaining on the surface of the crystal). The temperature dependent magnetic susceptibility ;~ = M/H, measured for Hlle (;0it) and H_l_c (;CLc) with H = 10 kG, is plotted in Fig. 1, which includes also the powder average )hvg(T) ( = 2 Z J 3 + ;0J3) and the difference, Z±¢ - ;0~c. The )~ values in Fig. I for both magnetic field orientations have been corrected for the contributions of ferromagnetic impurities, which are determined by M(H) isotherm, from the measured M(T), and then dividing by H. For the analysis of the Z values, several contributions, including the diamagnetism of the core electrons, ~¢o~, the Van Vleck type paramagnetism, ffv, and the conduction electron spin susceptibility, ;~el, i.e. Z = ;~co~ + f f v + Xccl. Since ZCel= ~LB2 I D(E) df/o~ dE where fiE) is Fermi distribution function and the X~°re and Zvv are assumed independent of temperature, the temperature dependence of Xc~
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01229-6
B.K. Cho /Physica C 341-348 (2000) 2025-2026
2026
2.5
2.0
1.5
~o
YNi,B,C
~'~ 1,0
0.5
o.o
.
.
.
i
.
.
.
100
i
.
.
.
200
,
•
300
.
•
400
Tempel"atum (K)
Figure 1. Magnetic susceptibility X of YNi2B2C single crystal versus temperature for HIIc (Xlz) and H.l_c (X_Le).The solid line is a fit of X = ct + 13T2 to the average x(T) data at T > 100 K. can occur in the approximation + OD(E) D(E)=
D(Er)
~ I E = E
2[
tgE 2
( E - E r )
IE=£~
The first term, the constant density of states at EF, gives the well-known Pauli susceptibility [Xra~u = ktB2 D(Ee)]. The second term, which is an odd function of energy near EE, gives zero contribution to z~,l. The third term will contribute to the temperature dependence in X~e~,leading to 6
OE
e=r, (kaT)2"
Fitting the expression X = ct + 13T2 to the x(T) at T > 100 K in Fig. 1, we estimate X¢°r* + X¢el +Xeu=li 2.1 x 10-4 cm3/mole and a negative curvature of D(E) near E = EF of OZD(E)[
~ -5.2×1021eV 3
To compare XPa~i with the electronic band structure calculation, Xb't~, we estimate Xc°~, based on a standard table, to be -0.36 x 10.4 cm3/mole.
If X~ej and X.... are assumed isotropic, the powder average of X±¢ - XI~ [= 2(X1¢ - X,J3] will give a lower limit of Xvv ~. 0.4 x 10.4 cm3/mole so that the contribution of f f v to ~ ] is almost compensated by Xcor*. Thus an upper limit for the powder average XPa~i is then = 2.1 x 10.4 cm3/mole. Comparing this value with the electronic band structure calculation result D(EF) 4.8/eV cell [Xbs~ ~ 1.53 x 10.4 cm3/mole] [2], the Stoner enhancement factor (rl) is found to be 0.4, where ffa.]i = zbstr(l + rl). The ffa,]i value can also be compared to the Sommerfeld parameter '/ [6], leading to the Wilson ration (zPauli//a B2)/(37/n2kB2) ~ 0.82 from which we can infer that electron-phonon interaction is stronger than the exchange interaction between electrons. In conclusion, the normal state ~(T) of YNi2B2C single crystal shows T-dependent anisotropic behavior. The x(T) values are analyzed in terms of several contributions, i.e. X c°r~, Xvv, and Zce~, leading to the negative curvature of D(E) o f - 5.2 x 102/eV 3 and relatively strong electron-phonon interaction. This work is supported by the Korea Science & Engineering Foundation through the Grant No. 1999-2-114-005 -5. REFERENCES
1. R. Nagarajan, et. al., Phys. Rev. Lett. 72 (1994) 274; R. J. Cava, et. al., Nature 367 (1994) 252. 2. W. E. Pickett, et. al., Phys. Rev. Lett., 23 (1994) 3702. 3. H. Eisaki, et. al., Phys. Rev, B 50 (1994) 647. 4. B. K. Cho, et. al., Phys. Rev. B 53 (1996) 2217. 5. B. K. Cho, et. al., Phys. Rev. B 52 (1995) 3684. 6. M. Movshovich, et. al., Physica C 227 (1994) 381.
Physica C 341-348 (2000) 2025-2026
www.elsevier.nl/Iocate/physc
Normal State Magnetic Properties of Superconducting Single Crystal YNi2B2C B. K. Cho Department of materials science and technology, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712 South Korea The normal state magnetic properties of single crystal YNi2B2C have been investigated by field dependent isothermal magnetization (M) and temperature (T) dependent magnetic susceptibility (M/H --- Z) measurements. The ;~(T) data show a T-dependent anisotropic behavior. The )~(T) data is separated by the contributions of impurities, ~imp, diamagnetic core electrons, Z~°~, Van Vleck type interaction, Zvv, and conduction electron spins, ~ l . The weakly T-dependence ~(T) is explained by ~ l , from which the second derivative of density of state at Fermi energy is estimated. The comparison of the Pauli susceptibility, )~aauli(T), from this measurements with the one from electronic band structure calculation is performed.
The superconducting compounds, ReNizBzC (Re = rare earth elements), have been intensively studied for a last few years since the discovery of them at 1994 [1]. Although it has alternating layers of ReC and Ni2B2, it turned out to have three-dimensional electronic configuration based upon the electronic band structure calculation [2]. These compounds are of great interest because they exhibit not only relatively high superconducting transition temperature (T~) but also coexistence of superconductivity and magnetism. The significant interaction between superconductivity and magnetism is studied in transport and magnetic measurements [3]. In addition, the magnetic anisotropy in the normal state is studied in terms of crystalline electric field effects [4]. While the superconducting and magnetic properties and their interaction have been well characterized by transport and magnetic measurements, the magnetic properties in the normal state in YNi2B2C is the area to be studied more. Because the magnitude of the magnetic signal of YNi2B2C is quite small, it is important to measure single crystal sample to minimize extrinsic signal from impurity. The measurement of the single crystal is also essential to give the angular dependence of the magnetization. In this paper, we report the field (H) and temperature (T)
dependent magnetizations of single crystal of YNi2B2C and analyze the data in terms of various type of magnetic contribution. Single crystals of YNi2B2C have been grown via high temperature metal flux growth, using NieB as a solvent as described elsewhere [5]. Powder X-ray diffraction patterns of pulverized single crystals show the crystals to be single phase (only the most intense (2,1,1) line of Ni2B was seen and is due to small amounts of flux remaining on the surface of the crystal). The temperature dependent magnetic susceptibility ;~ = M/H, measured for Hlle (;0it) and H_l_c (;CLc) with H = 10 kG, is plotted in Fig. 1, which includes also the powder average )hvg(T) ( = 2 Z J 3 + ;0J3) and the difference, Z±¢ - ;0~c. The )~ values in Fig. I for both magnetic field orientations have been corrected for the contributions of ferromagnetic impurities, which are determined by M(H) isotherm, from the measured M(T), and then dividing by H. For the analysis of the Z values, several contributions, including the diamagnetism of the core electrons, ~¢o~, the Van Vleck type paramagnetism, ffv, and the conduction electron spin susceptibility, ;~el, i.e. Z = ;~co~ + f f v + Xccl. Since ZCel= ~LB2 I D(E) df/o~ dE where fiE) is Fermi distribution function and the X~°re and Zvv are assumed independent of temperature, the temperature dependence of Xc~
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01229-6
B.K. Cho /Physica C 341-348 (2000) 2025-2026
2026
2.5
2.0
1.5
~o
YNi,B,C
~'~ 1,0
0.5
o.o
.
.
.
i
.
.
.
100
i
.
.
.
200
,
•
300
.
•
400
Tempel"atum (K)
Figure 1. Magnetic susceptibility X of YNi2B2C single crystal versus temperature for HIIc (Xlz) and H.l_c (X_Le).The solid line is a fit of X = ct + 13T2 to the average x(T) data at T > 100 K. can occur in the approximation + OD(E) D(E)=
D(Er)
~ I E = E
2[
tgE 2
( E - E r )
IE=£~
The first term, the constant density of states at EF, gives the well-known Pauli susceptibility [Xra~u = ktB2 D(Ee)]. The second term, which is an odd function of energy near EE, gives zero contribution to z~,l. The third term will contribute to the temperature dependence in X~e~,leading to 6
OE
e=r, (kaT)2"
Fitting the expression X = ct + 13T2 to the x(T) at T > 100 K in Fig. 1, we estimate X¢°r* + X¢el +Xeu=li 2.1 x 10-4 cm3/mole and a negative curvature of D(E) near E = EF of OZD(E)[
~ -5.2×1021eV 3
To compare XPa~i with the electronic band structure calculation, Xb't~, we estimate Xc°~, based on a standard table, to be -0.36 x 10.4 cm3/mole.
If X~ej and X.... are assumed isotropic, the powder average of X±¢ - XI~ [= 2(X1¢ - X,J3] will give a lower limit of Xvv ~. 0.4 x 10.4 cm3/mole so that the contribution of f f v to ~ ] is almost compensated by Xcor*. Thus an upper limit for the powder average XPa~i is then = 2.1 x 10.4 cm3/mole. Comparing this value with the electronic band structure calculation result D(EF) 4.8/eV cell [Xbs~ ~ 1.53 x 10.4 cm3/mole] [2], the Stoner enhancement factor (rl) is found to be 0.4, where ffa.]i = zbstr(l + rl). The ffa,]i value can also be compared to the Sommerfeld parameter '/ [6], leading to the Wilson ration (zPauli//a B2)/(37/n2kB2) ~ 0.82 from which we can infer that electron-phonon interaction is stronger than the exchange interaction between electrons. In conclusion, the normal state ~(T) of YNi2B2C single crystal shows T-dependent anisotropic behavior. The x(T) values are analyzed in terms of several contributions, i.e. X c°r~, Xvv, and Zce~, leading to the negative curvature of D(E) o f - 5.2 x 102/eV 3 and relatively strong electron-phonon interaction. This work is supported by the Korea Science & Engineering Foundation through the Grant No. 1999-2-114-005 -5. REFERENCES
1. R. Nagarajan, et. al., Phys. Rev. Lett. 72 (1994) 274; R. J. Cava, et. al., Nature 367 (1994) 252. 2. W. E. Pickett, et. al., Phys. Rev. Lett., 23 (1994) 3702. 3. H. Eisaki, et. al., Phys. Rev, B 50 (1994) 647. 4. B. K. Cho, et. al., Phys. Rev. B 53 (1996) 2217. 5. B. K. Cho, et. al., Phys. Rev. B 52 (1995) 3684. 6. M. Movshovich, et. al., Physica C 227 (1994) 381.