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Magnetic and electrical properties of antiferromagnetic (Gd1 − xYx)3Co single crystals
N
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 726-727
Journal of magnetism and magnetic materials
Magnetic and electrical properties of antiferromagnetic (Gdl - x Y x ) 3 C o single crystals N.V. Baranov*, A.A. Yermakov, P.E. Markin h~stitute oj' Ph3sics & Appl. Mathematics, Ural State U)~icersiO', Lenin Avenue 51, 620083 Ekaterinhurg, Russ'ia
Abstract Using magnetization and magnetoresistance measurements along the main axes of (Gd~ .~Y03Co single crystals wc have obtained the magnetic phase diagram of this system. The field-induced magnetic phase transitions from the antiferromagnetic (AF) to the ferromagnetic (F) state are accompanied by a giant magnetoresistance effect. Thc dilution of the Gd-sublattice leads to an unmonotonous change of the critical A F - F transition fields and to a change of the shape of the temperature dependencies of the electrical resistivity at high temperatures from saturation (at x = 0) to normal metallic behavior at high Y content. ! ' 1999 Elsevier Science B.V. All rights reserved. l ~ v w o r d s : Antiferromagnetism; Magnetic phase transitions: Electrical resistivity
Among the R3T (T = Co, Ni) series the compound Gd3Co with the orthorhombic Fe3C-type structure shows the highest N6el temperature TN of about 130 K and exhibits field-induced phase transitions from the antiferromagnetic (AF) to ferromagnetic (F) state in the relatively small magnetic fields (B < 1.5 T) [1]. When the field is applied along the b- or c-axis, the magnetization process in GdaCo is of the first order (shown in Fig. 1 (x = 0)). The magnetic order in GdsCo results from the indirect RKKY exchange interaction, while the Coatoms have no ordered magnetic moment at temperatures above 3.7 K. A significant change of the different physical properties: electrical resistivity, magnetostriction and sound velocity were observed at its field-induced A F - F transitions [1]. The aim of the present paper is to study the effect of diluting Gd in Gd3Co by nonmagnetic yttrium on magnetic phase transitions, electrical resistivity and magnetoresistance. The sample preparation and the measurements were performed as described in Ref. [1].
*Corresponding author. Tel.: + 7-3432-61-53-43;fax: + 73432-61-59-78; e-mail: [email protected].
As it follows from Fig. 1 the field-induced A F - F transitions along the c-axis of the (Gdl .,Y03Co single crystals with x ~<0.6 are accompanied by an abrupt decrease of the electrical resistivity which can be connected with the deformation of the Fermi surface owing to the disappearance of superzones. This suggestion is confirmed by the change of the electronic part of the specific heat at the A F - F transition [1]. An analogous behavior of magnetization and magnetoresistance was observed along the b-axis. The dilution of the Gd-sublattice by nonmagnetic yttrium gradually decreases the ordering t e m p e r a t u r e T N (see Fig. 2). However, the concentration dependencies of the critical transition fields measured along the c- and h-axis show an unmonotonous character with a maximum occurring at x ~ 0.3 (Fig. 2). This behavior can be associated with a nonequal probability of the substitution of Gd-ions by Y-ions on the different crystallographic positions 4c and 8d of the Fe3C-type structure: with increasing x, initially those Gd-ions can be substituted by Y-ions which are placed on the 4c sites (x = 1), followed by those located on the 8d sites. While the metamagnetic transition occurs when the applied magnetic field exceeds the intersublattice molecular field we can suggest that the exchange interaction of the Gdions on the 4c sites is weaker than those on the 8d sites.
0304-8853/99/$- see front matter ~C' 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 2 5 - 1
727
N.K Baranov et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 726-727
200
a)
03 x = O ~
x =0 -0.2
/--
0,6 0
100
200
T, K
Fig. 3. Temperature dependence of the electrical resistivity along the c-axis of (Gdl xYx)3Cosingle crystals.
!
. . . .
0
x~,o
I
,
2
3
,
i
B, T
Fig. 1. Field dependence of the magnetization (a) and magnetoresistance (b) measured at T = 4.2 K along the c-axis of (Gdl-xYx)3Co single crystals. 150
1.2
IO C
0.8
0 . . . . . 0.0 0.2 Gd3C°
0.4
0.6 x
0.8
0.0 1.0 Y3C°
Fig. 2. Concentration dependence of the Nrel temperature TN and the critical transition field Bc along the c-axis of (Gdl-xYx)3Co single crystals. Note that the unmonotonous change of some magnetic characteristics with Y-content was also observed in the isostructural system (Tbl-~Yx)3Co [2]. We have found that the concentration dependence of the difference Ap = PAF--PF between the electrical resistivity of (Gdx-xYx)3Co compounds (x ~<0.4) in the AF and F states and consequently the width of the energy gap in electronic spectrum correlates with the change of the intersublattice exchange interaction. In the paramagnetic region ( T > 150 K) the temperature dependence of the electrical resistivity of the corn-
pounds R3T (T = Co, Ni, Rh, Ir) shows a tendency towards saturation [1-3] which can be connected with the contribution from spin fluctuations in d-electron subsystem taking into account hybridization effects [4] and the large value of the coefficient 7 of the linear term of the low-temperature specific heat (about 110 mJ/mol K 2 for Gd3Co [1]). In the paramagnetic region an increase of the Y content in (Gdl-xYx)3Co leads to a change in the shape of the p(T) behavior from the saturation obtained at x = 0 to a normal metallic behavior at high Y concentrations (Fig. 3). Because the analogous results were obtained on (Tbt-xYx)3Co single crystals [2] we can suggest that in the R3T compounds the contribution from spin fluctuations may be influenced by the R-sublattice. This work was partly supported by the Russian Foundation for Fundamental Research (Project 97-02-16504) and the Ministry of Education (Project 97-0-73-170).
References [1] N.V. Baranov, A.V. Andreev, A.I. Kozlov, G.M. Kvashnin, H. Nakotte, H. Aruga Katori, T. Goto, J. AlloysComp. 202 (1993) 215. [-2] N.V. Baranov, A.V. Deryagin, A.I. Kozlov, E.V. Sinitsyn, Phys. Met. Metallogr. 61 (1986) 97. [3] E. Talik, J. Szade, J. Hemann, A. Winyarska, A. Winyarski, A. Chelkowski, J. Less-Common Met. 138 (1988) 129. [4] E. Talik, M, Neumann, J. Magn. Magn. Mater. 140-144 (1995) 795.
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 726-727
Journal of magnetism and magnetic materials
Magnetic and electrical properties of antiferromagnetic (Gdl - x Y x ) 3 C o single crystals N.V. Baranov*, A.A. Yermakov, P.E. Markin h~stitute oj' Ph3sics & Appl. Mathematics, Ural State U)~icersiO', Lenin Avenue 51, 620083 Ekaterinhurg, Russ'ia
Abstract Using magnetization and magnetoresistance measurements along the main axes of (Gd~ .~Y03Co single crystals wc have obtained the magnetic phase diagram of this system. The field-induced magnetic phase transitions from the antiferromagnetic (AF) to the ferromagnetic (F) state are accompanied by a giant magnetoresistance effect. Thc dilution of the Gd-sublattice leads to an unmonotonous change of the critical A F - F transition fields and to a change of the shape of the temperature dependencies of the electrical resistivity at high temperatures from saturation (at x = 0) to normal metallic behavior at high Y content. ! ' 1999 Elsevier Science B.V. All rights reserved. l ~ v w o r d s : Antiferromagnetism; Magnetic phase transitions: Electrical resistivity
Among the R3T (T = Co, Ni) series the compound Gd3Co with the orthorhombic Fe3C-type structure shows the highest N6el temperature TN of about 130 K and exhibits field-induced phase transitions from the antiferromagnetic (AF) to ferromagnetic (F) state in the relatively small magnetic fields (B < 1.5 T) [1]. When the field is applied along the b- or c-axis, the magnetization process in GdaCo is of the first order (shown in Fig. 1 (x = 0)). The magnetic order in GdsCo results from the indirect RKKY exchange interaction, while the Coatoms have no ordered magnetic moment at temperatures above 3.7 K. A significant change of the different physical properties: electrical resistivity, magnetostriction and sound velocity were observed at its field-induced A F - F transitions [1]. The aim of the present paper is to study the effect of diluting Gd in Gd3Co by nonmagnetic yttrium on magnetic phase transitions, electrical resistivity and magnetoresistance. The sample preparation and the measurements were performed as described in Ref. [1].
*Corresponding author. Tel.: + 7-3432-61-53-43;fax: + 73432-61-59-78; e-mail: [email protected].
As it follows from Fig. 1 the field-induced A F - F transitions along the c-axis of the (Gdl .,Y03Co single crystals with x ~<0.6 are accompanied by an abrupt decrease of the electrical resistivity which can be connected with the deformation of the Fermi surface owing to the disappearance of superzones. This suggestion is confirmed by the change of the electronic part of the specific heat at the A F - F transition [1]. An analogous behavior of magnetization and magnetoresistance was observed along the b-axis. The dilution of the Gd-sublattice by nonmagnetic yttrium gradually decreases the ordering t e m p e r a t u r e T N (see Fig. 2). However, the concentration dependencies of the critical transition fields measured along the c- and h-axis show an unmonotonous character with a maximum occurring at x ~ 0.3 (Fig. 2). This behavior can be associated with a nonequal probability of the substitution of Gd-ions by Y-ions on the different crystallographic positions 4c and 8d of the Fe3C-type structure: with increasing x, initially those Gd-ions can be substituted by Y-ions which are placed on the 4c sites (x = 1), followed by those located on the 8d sites. While the metamagnetic transition occurs when the applied magnetic field exceeds the intersublattice molecular field we can suggest that the exchange interaction of the Gdions on the 4c sites is weaker than those on the 8d sites.
0304-8853/99/$- see front matter ~C' 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 2 5 - 1
727
N.K Baranov et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 726-727
200
a)
03 x = O ~
x =0 -0.2
/--
0,6 0
100
200
T, K
Fig. 3. Temperature dependence of the electrical resistivity along the c-axis of (Gdl xYx)3Cosingle crystals.
!
. . . .
0
x~,o
I
,
2
3
,
i
B, T
Fig. 1. Field dependence of the magnetization (a) and magnetoresistance (b) measured at T = 4.2 K along the c-axis of (Gdl-xYx)3Co single crystals. 150
1.2
IO C
0.8
0 . . . . . 0.0 0.2 Gd3C°
0.4
0.6 x
0.8
0.0 1.0 Y3C°
Fig. 2. Concentration dependence of the Nrel temperature TN and the critical transition field Bc along the c-axis of (Gdl-xYx)3Co single crystals. Note that the unmonotonous change of some magnetic characteristics with Y-content was also observed in the isostructural system (Tbl-~Yx)3Co [2]. We have found that the concentration dependence of the difference Ap = PAF--PF between the electrical resistivity of (Gdx-xYx)3Co compounds (x ~<0.4) in the AF and F states and consequently the width of the energy gap in electronic spectrum correlates with the change of the intersublattice exchange interaction. In the paramagnetic region ( T > 150 K) the temperature dependence of the electrical resistivity of the corn-
pounds R3T (T = Co, Ni, Rh, Ir) shows a tendency towards saturation [1-3] which can be connected with the contribution from spin fluctuations in d-electron subsystem taking into account hybridization effects [4] and the large value of the coefficient 7 of the linear term of the low-temperature specific heat (about 110 mJ/mol K 2 for Gd3Co [1]). In the paramagnetic region an increase of the Y content in (Gdl-xYx)3Co leads to a change in the shape of the p(T) behavior from the saturation obtained at x = 0 to a normal metallic behavior at high Y concentrations (Fig. 3). Because the analogous results were obtained on (Tbt-xYx)3Co single crystals [2] we can suggest that in the R3T compounds the contribution from spin fluctuations may be influenced by the R-sublattice. This work was partly supported by the Russian Foundation for Fundamental Research (Project 97-02-16504) and the Ministry of Education (Project 97-0-73-170).
References [1] N.V. Baranov, A.V. Andreev, A.I. Kozlov, G.M. Kvashnin, H. Nakotte, H. Aruga Katori, T. Goto, J. AlloysComp. 202 (1993) 215. [-2] N.V. Baranov, A.V. Deryagin, A.I. Kozlov, E.V. Sinitsyn, Phys. Met. Metallogr. 61 (1986) 97. [3] E. Talik, J. Szade, J. Hemann, A. Winyarska, A. Winyarski, A. Chelkowski, J. Less-Common Met. 138 (1988) 129. [4] E. Talik, M, Neumann, J. Magn. Magn. Mater. 140-144 (1995) 795.