- Email: [email protected]
EPR and magnetization of Gd2BaNiO5
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1681-1682
~
,~ ELSEVIER
, ~
Journalof magnetism
and
magnetic materials
EPR and magnetization of Gd2BaNiO 5 A. Butera
a,*, M.T. Causa a, M. Tovar a S.B. Oseroff b, S.-W. Cheong c
a Centro At6mico Bariloche and Instituto Balseiro, Comisi6n Nacional de Energla At6mica and Universidad Nacional de Cuyo, 8400 San Carlos de Bariloche, Argentina b San Diego State University, San Diego, CA 92182, USA c AT & T Bell Laboratories, Murray Hill, NJ 07974, USA
Abstract We report EPR and magnetization for Gd2BaNiO 5 single crystals. The high-temperature susceptibility gives /xeff = 7.9/za/mol Gd and OCd = 11 K (JGdc,d/kB = 0.6 K). The coupling of 1-D Ni chains through the G d - N i interaction induces 3-D antiferromagnetic order with TN = 55(3) K, indicated by a divergence of the EPR linewidth. A mean field analysis gives J G d _ N i / k B = 3 K.
R2BaNiO 5, with R = Y and rare earths, usually form in an orthorhombic structure [1] that presents isolated chains of NiO 6 flattened octahedra, sharing corners along the a-axis, being good examples of 1-D antiferromagnetic (AF) systems [2]. The N i - O distance is very short along the chains and a strong superexchange coupling is expected between Ni moments: JNiNi//kB = 285 K has been determined [2] for R = Y. Since there is no direct coupling between neighboring chains, Y2BaNiO5 does not show magnetic order, at least down to 1.8 K [3]. The R ions occupy layers parallel to the ab-plane (see Fig. 1) and, when they carry a magnetic moment, provide a superexchange channel for interchain interaction and 3-D magnetic ordering is achieved. In the case of R = Er, neutron scattering experiments [3] have shown that long-range AF order appears below TN = 33 K. In this work we show, through dc magnetization and EPR measurements, that the Gd moments induce magnetic order in Gd2BaNiO 5. Using a mean field description for the interchain coupling we determined the strength of the G d - N i coupling. Single crystals of Gd2BaNiO 5 have been grown. The magnetization measurements have been made using a SQUID magnetometer and the EPR spectra have been measured with a Bruker ESP 300 Spectrometer. Above T = 60 K the magnetic susceptibility is isotropic, following a Curie-Weiss law, x ( T ) = C / ( T + 0 ) , with C = 16.24 emu K / m o l and (9 = 14.5 K. The susceptibility in the isostructural compound Y2BaNiO5 is small. After subtracting x(Y2BaNiO 5) from x(Gd2BaNiOs) , we ob-
tained /zeff = 7.9 /xa/mol Gd, which corresponds to the free-ion value for Gd 3+. The corrected Curie-Weiss temperature is O6a = 11 K. At room temperature the EPR spectrum consist of a single line with g = 2. This line is isotropic for T > 400 K with a peak-to-peak linewidth, AHpp= 1.9 kG. The linewidth decreases with decreasing temperatures down to T = 120 K, where it reaches a minimum value, AHpo = 1.3 kG, as shown in Fig. 2. At lower temperatures AHpp(T) shows a divergent behavior, which defines the N6el temperature, Trq --- 55 K. The intensity of the EPR line, I(T) = [AHpp(T)]2h(T), where h(T) is the normalized height for the EPR derivative line, follows a Curie-Weiss temperature dependence with ~9 = 11 K, as shown in the inset of Fig. 2. The EPR line is no longer observed below 55 K.
G
"w" \
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(-" i
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et
* Corresponding author. Fax: (54)-(994) 61006; email: [email protected]. 0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01 187-7
~}~ B a
Od
. 0
Fig. 1. Crystal structure for Od2 BaNiO5.
A. Butera et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1681-1682
1682
4000
~
We may write XGtd(T)= C ~ / ( T - O o a ) for T>> O ~ , which indicates that AF ordering is expected for the Gd sublattice. For the Ni ions, the one-dimensional character of the crystal structure prevents the AF long-range order of the system, as observed [2] for Y2BaNiOs. In this case, XtNi(T) is not expected to diverge at any finite temperature. On the other hand, the coupling of 1-D Ni chains through the Gd paramagnetic ions is able to induce long range order at TN, which results from the zeros of the determinant for the set of homogeneous equations (2). Then,
.~" '
3000 T(K]
2000 <3 1000 TN
Gd2BaNiO5
0 100
200
300
400
Temperalure
500
600
(K)
Fig. 2. EPR linewidth versus T as measured for 9 GHz (O) and 35 GHz (+), with H IIa. The inset shows the C-W behavior of the integrated intensity. The susceptibility becomes anisotropic below TN, as shown in Fig. 3 for different orientations of the external field. A discontinuity at Tr = 23 K indicates a reorientation of the magnetic moments. A mean field expression for the total free energy per f.u. may be written as
9-= [4Xoa(T)]-XlMGd I 2 + [4XtGa(T)] - I l M ~
12
+ [2XNi(T)]-IlMNI I 2 + [2XtNi(T)] -11MN*i I 2 --
2~/edNiMGfd• MNti -- 2yGdNiMGd • MNi
--
(Mad + MNi ) • H ,
(1)
where Mod, MNi are the uniform and M~d, MNti are the staggered Gd and Ni sublattice magnetizations, respectively. The corresponding susceptibilities are Xod(T), Xotd(T), XNi(T) and Xtsi(T). The intersublattice coupling is described by YOdii = ZJodNi/2 gad gNi Jt/'BNA" The equilibrium conditions &~qr/0MGd,Ni = 0 . ~ / 0MGtd,Ni= 0 give a set of linear equations where MGd,Ni and MGtd,Ui become decoupled. We have [2X*od(r)]-I MGa *
-
2~GdNiMNi
=
0,
(2)
,
MNi -- 2TGdNiMGCd= 0.
o.s o % ~ '-~
o.4
"
Gd2BaNi05 ~
0.3
' e e
•
[Ilia I
0.2 0.1
0.0
+
20
40
60
80
(3)
If we estimate an order of magnitude for XtNi(TN) using the expression of Ref. [4] for the uniform susceptibility of a ferromagnetic Heisenberg chain, we obtain JGdNi/kB = 3 K, assuming z = 4, which is the number of Gd neighbors in the bc plane. Below TN, nNti is expected to grow rapidly towards saturation since XfNi(T) is a fast-growing function for decreasing T. Once the Ni sublattice reaches its saturation value (/.tNi = 1.54/x B in the case of Er2BaNiO 5 [3]), the effective field on the Gd sublattice Herf = ' Y a d N i M N i ---- 20 kG. This field is not enough to saturate the Gd moments until much lower temperatures are reached, as is the case for Er 2 BaNiO 5. However, this staggered field polarizes the Gd lattices, creating a significant nonzero staggered magnetization in the absence of external fields. This polarization of the Gd ions increases with decreasing T and modifies the uniform magnetization of the ordered system. It is reduced from the high-temperature C - W law and becomes strongly anisotropic below TN, as observed in Fig. 3. A detailed analysis of the temperature dependence of the magnetic susceptibility, including the behavior at the reorientation transition at Tr = 23 K, will be published elsewhere. Acknowledgements: We acknowledge interesting discussions with Dr B. Alascio and partial support received from the NSF/CONICET international cooperation program.
R e f e r e n c e s
TN ~'e
( r N - OGd) = 8CGa X{D(TN)(YOaNi) 2.
100
T e n l p e r a ~ u r e ( 1,2 ) Fig. 3. Magnetic susceptibility for Od2BaNiO 5. The continuous line is an extrapolation of the high-temperature Curie-Weiss law.
[1] S. Schiffer and Hk. Miiller-Buschbaum, Z. Anorg. Allgem. Chem. 532 (i986) 10. [2] J. Darriet and L.P Regnault, Solid St. Commun. 86 (1993) 409. [3] J.A. Alonso, J. Amador, J.L Martinez, I. Rasines, J. Rodrignez-Carvajal and R. Sfiez Puehe, Solid St. Commun. 76 (1990) 467. [4] P. Schlottmann and Kong-Ju-Bock Lee, in: Magnetic Properties of Low-Dimensional Systems II, eds. L.M. Falicov, F. Mejia-Lira and J.L. Morfin-Lrpez, Springer Proc. Phys. 50 (1990) 205.
~
,~ ELSEVIER
, ~
Journalof magnetism
and
magnetic materials
EPR and magnetization of Gd2BaNiO 5 A. Butera
a,*, M.T. Causa a, M. Tovar a S.B. Oseroff b, S.-W. Cheong c
a Centro At6mico Bariloche and Instituto Balseiro, Comisi6n Nacional de Energla At6mica and Universidad Nacional de Cuyo, 8400 San Carlos de Bariloche, Argentina b San Diego State University, San Diego, CA 92182, USA c AT & T Bell Laboratories, Murray Hill, NJ 07974, USA
Abstract We report EPR and magnetization for Gd2BaNiO 5 single crystals. The high-temperature susceptibility gives /xeff = 7.9/za/mol Gd and OCd = 11 K (JGdc,d/kB = 0.6 K). The coupling of 1-D Ni chains through the G d - N i interaction induces 3-D antiferromagnetic order with TN = 55(3) K, indicated by a divergence of the EPR linewidth. A mean field analysis gives J G d _ N i / k B = 3 K.
R2BaNiO 5, with R = Y and rare earths, usually form in an orthorhombic structure [1] that presents isolated chains of NiO 6 flattened octahedra, sharing corners along the a-axis, being good examples of 1-D antiferromagnetic (AF) systems [2]. The N i - O distance is very short along the chains and a strong superexchange coupling is expected between Ni moments: JNiNi//kB = 285 K has been determined [2] for R = Y. Since there is no direct coupling between neighboring chains, Y2BaNiO5 does not show magnetic order, at least down to 1.8 K [3]. The R ions occupy layers parallel to the ab-plane (see Fig. 1) and, when they carry a magnetic moment, provide a superexchange channel for interchain interaction and 3-D magnetic ordering is achieved. In the case of R = Er, neutron scattering experiments [3] have shown that long-range AF order appears below TN = 33 K. In this work we show, through dc magnetization and EPR measurements, that the Gd moments induce magnetic order in Gd2BaNiO 5. Using a mean field description for the interchain coupling we determined the strength of the G d - N i coupling. Single crystals of Gd2BaNiO 5 have been grown. The magnetization measurements have been made using a SQUID magnetometer and the EPR spectra have been measured with a Bruker ESP 300 Spectrometer. Above T = 60 K the magnetic susceptibility is isotropic, following a Curie-Weiss law, x ( T ) = C / ( T + 0 ) , with C = 16.24 emu K / m o l and (9 = 14.5 K. The susceptibility in the isostructural compound Y2BaNiO5 is small. After subtracting x(Y2BaNiO 5) from x(Gd2BaNiOs) , we ob-
tained /zeff = 7.9 /xa/mol Gd, which corresponds to the free-ion value for Gd 3+. The corrected Curie-Weiss temperature is O6a = 11 K. At room temperature the EPR spectrum consist of a single line with g = 2. This line is isotropic for T > 400 K with a peak-to-peak linewidth, AHpp= 1.9 kG. The linewidth decreases with decreasing temperatures down to T = 120 K, where it reaches a minimum value, AHpo = 1.3 kG, as shown in Fig. 2. At lower temperatures AHpp(T) shows a divergent behavior, which defines the N6el temperature, Trq --- 55 K. The intensity of the EPR line, I(T) = [AHpp(T)]2h(T), where h(T) is the normalized height for the EPR derivative line, follows a Curie-Weiss temperature dependence with ~9 = 11 K, as shown in the inset of Fig. 2. The EPR line is no longer observed below 55 K.
G
"w" \
/ /
(-" i
z)
et
* Corresponding author. Fax: (54)-(994) 61006; email: [email protected]. 0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01 187-7
~}~ B a
Od
. 0
Fig. 1. Crystal structure for Od2 BaNiO5.
A. Butera et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1681-1682
1682
4000
~
We may write XGtd(T)= C ~ / ( T - O o a ) for T>> O ~ , which indicates that AF ordering is expected for the Gd sublattice. For the Ni ions, the one-dimensional character of the crystal structure prevents the AF long-range order of the system, as observed [2] for Y2BaNiOs. In this case, XtNi(T) is not expected to diverge at any finite temperature. On the other hand, the coupling of 1-D Ni chains through the Gd paramagnetic ions is able to induce long range order at TN, which results from the zeros of the determinant for the set of homogeneous equations (2). Then,
.~" '
3000 T(K]
2000 <3 1000 TN
Gd2BaNiO5
0 100
200
300
400
Temperalure
500
600
(K)
Fig. 2. EPR linewidth versus T as measured for 9 GHz (O) and 35 GHz (+), with H IIa. The inset shows the C-W behavior of the integrated intensity. The susceptibility becomes anisotropic below TN, as shown in Fig. 3 for different orientations of the external field. A discontinuity at Tr = 23 K indicates a reorientation of the magnetic moments. A mean field expression for the total free energy per f.u. may be written as
9-= [4Xoa(T)]-XlMGd I 2 + [4XtGa(T)] - I l M ~
12
+ [2XNi(T)]-IlMNI I 2 + [2XtNi(T)] -11MN*i I 2 --
2~/edNiMGfd• MNti -- 2yGdNiMGd • MNi
--
(Mad + MNi ) • H ,
(1)
where Mod, MNi are the uniform and M~d, MNti are the staggered Gd and Ni sublattice magnetizations, respectively. The corresponding susceptibilities are Xod(T), Xotd(T), XNi(T) and Xtsi(T). The intersublattice coupling is described by YOdii = ZJodNi/2 gad gNi Jt/'BNA" The equilibrium conditions &~qr/0MGd,Ni = 0 . ~ / 0MGtd,Ni= 0 give a set of linear equations where MGd,Ni and MGtd,Ui become decoupled. We have [2X*od(r)]-I MGa *
-
2~GdNiMNi
=
0,
(2)
,
MNi -- 2TGdNiMGCd= 0.
o.s o % ~ '-~
o.4
"
Gd2BaNi05 ~
0.3
' e e
•
[Ilia I
0.2 0.1
0.0
+
20
40
60
80
(3)
If we estimate an order of magnitude for XtNi(TN) using the expression of Ref. [4] for the uniform susceptibility of a ferromagnetic Heisenberg chain, we obtain JGdNi/kB = 3 K, assuming z = 4, which is the number of Gd neighbors in the bc plane. Below TN, nNti is expected to grow rapidly towards saturation since XfNi(T) is a fast-growing function for decreasing T. Once the Ni sublattice reaches its saturation value (/.tNi = 1.54/x B in the case of Er2BaNiO 5 [3]), the effective field on the Gd sublattice Herf = ' Y a d N i M N i ---- 20 kG. This field is not enough to saturate the Gd moments until much lower temperatures are reached, as is the case for Er 2 BaNiO 5. However, this staggered field polarizes the Gd lattices, creating a significant nonzero staggered magnetization in the absence of external fields. This polarization of the Gd ions increases with decreasing T and modifies the uniform magnetization of the ordered system. It is reduced from the high-temperature C - W law and becomes strongly anisotropic below TN, as observed in Fig. 3. A detailed analysis of the temperature dependence of the magnetic susceptibility, including the behavior at the reorientation transition at Tr = 23 K, will be published elsewhere. Acknowledgements: We acknowledge interesting discussions with Dr B. Alascio and partial support received from the NSF/CONICET international cooperation program.
R e f e r e n c e s
TN ~'e
( r N - OGd) = 8CGa X{D(TN)(YOaNi) 2.
100
T e n l p e r a ~ u r e ( 1,2 ) Fig. 3. Magnetic susceptibility for Od2BaNiO 5. The continuous line is an extrapolation of the high-temperature Curie-Weiss law.
[1] S. Schiffer and Hk. Miiller-Buschbaum, Z. Anorg. Allgem. Chem. 532 (i986) 10. [2] J. Darriet and L.P Regnault, Solid St. Commun. 86 (1993) 409. [3] J.A. Alonso, J. Amador, J.L Martinez, I. Rasines, J. Rodrignez-Carvajal and R. Sfiez Puehe, Solid St. Commun. 76 (1990) 467. [4] P. Schlottmann and Kong-Ju-Bock Lee, in: Magnetic Properties of Low-Dimensional Systems II, eds. L.M. Falicov, F. Mejia-Lira and J.L. Morfin-Lrpez, Springer Proc. Phys. 50 (1990) 205.