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139 La NQR study of antiferromagnetic La2NiO4+δ and La2CoO4−δ
Physica B 165&166 (1990) 1313-1314 North-Holland
139La NQR
STUD~OF
ANTIFERROMAGNETIC La2Ni04+6 AND La2Co04-6
Shinji WADA. Tatsuo KOBAYASHI. Makoto KABURAGI. Tsuyoshi KAJITANIA. Syoichi HOSOYAA, Tsuguo FUKUDAA Shigehumi ONODERAB, Yoshihiro YAMADAc. Kazuyuki SHIBUTANID and Rikuo OGAWAD College of Liberal Arts. Kobe University, Nada, Kobe 657. Japan AInstitute for Material Research. Tohoku University. Katahira 2. Sendai 980. Japan BSendai National College of Technology. Kamiayashi. Sendai 989-31. Japan CHimeji Institute of Technology. Syosha, Himeji 671-22. Japan DTechnical Development Group, Kobe Steel. LTD .• Nishi-Ku. Kobe 673-02, Japan 139La NQR study has been performed on La2Ni04+6 and La2Co04-6. Experimental spectra at low temperatures provided an evidence for the existence of antiferromagnetic ordering in both of them. The internal field at La site in La2Ni04+6 (19.3 kOe. II c-axis) and in La2Co04-6 (30.5 kOe. II c-axis) was significantly large comparing with that in La2Cu04-, (1.0 kOe ~c-axis), suggesting an importance of exchange interactions via apex oxygen atoms.
1. INTRODUCTION Rare earth metal oxide family of formula La2M04 (M=Cu.Ni,Co) shows some interesting properties such as metal-semiconductor and structural phase transitions. and antiferromagnetic (AF) ordering (TN ~250K. ~650K. ~275K). In the past year, greater attention has been concentrated on these oxides because of the discovery of the high-Tc superconductivity in-doped La2Cu04 .1) We have investigated 139La NQR in La2Ni04 21 and La2Co04 in order to make a microscopic and comparative study on their magnetic properties with that in La2Cu04 and also to understand magnetic peculiarities in the parent oxide of the high-Tc superconductor. 2. EXPERIMENTAL 2.1. Sample preparation Ceramic sample of La2Ni04+1 was synthesized by solid state reaction in air. As the structural and magnetic properties have been known to depend on the stoichiometry3) 6. several samples with different 6 were prepared, e.g. sample D(6~0.03) annealed in high vacuum at 1000' C for 91 hrs., and sample O(6~0.07) at 550' C for 100 hrs. in a mixture of 02 (20%) and Ar(80%) gas under 1000 atm. Single crystal of La2Co04-1 was prepared by the coprecipitation method detailed elsewhere. Magnetic susceptibility X(T) for sample D showed a small and broad cusp at ~125K and a sharp peak at ~20K. while X(T) for sample 0 had only a small cusp at ~45K. X-ray diffraction at room temperature (RT) showed that sample D (deoxygenated) of La2Ni04+1 and La2Co04-1 are in orthorhombic phase. while sample 0 (oxygenated) of La2Ni04+d is in t~tragonal phase.
2.2. NQR spectrrrm The NQR spectrum for La2Ni04+1 taken at 1.4K for sample D and sample 0 is shown in Fig.l(a) and l(b). As can be seen from the figures. a small change in 6 leads to a drastic change in the spectrum. For sample D. each of the resonance lines was found to shifts towards lower frequencies with increasing T as shown typically in Fig.2. The NQR spectrum for La2Co0 4- 1 at 4.2 K is shown in Fig.1(c). which has a complex structure. Dominant peaks. however, provide a similar pattern to that for sample D of La2Ni04+1. 2.3 Analysis of NQR spectrum The main peaks in Fig.l can be analyzed as a NQR spectrum originated in a La site in case of Zeeman term HM » quadrupole term HQ. Set of parameters VQ, Hint. 8 obtained are summarized in Table I, where 8 is the polar angle of Hint to the principal axis.of the electric field gradient. Table 1 La2Cu04-1 4 ) La2Ni04+1 2 ) La2 C004-1 Crystal 6 Spin VQ (MHz) Hi n t (kOe) 6
(. )
Ortho. 1/2 6.38 1.02 78.0
Ortho.
Tetra.
~.03
~.07
1 1.8 19.3 ~5
1 2.7 17.8 77.5
Ortho. 3/2 2.0 30.5 ~5
2.3. Nuclear spin-lattice relaxation The 139La spin-lattice relaxation time TI in sample D of La2Ni04+1 was measured at the central peak [(1/2)~~(-1/2)1. The T dependence of T,-l is shown in Fig.3 on a log-
0921-4526/90/$03.50 @ 1990 - Elsevier Science Publishers B.V. (North-Holland)
S. Wada et aI.
1314
log scale. Tl - 1 is proportional to T2. 6 below ~20K, while shows a maximum at ~ 30K. 3. DISCUSSION The AF ordering in La2M04 was evidenced by the existence of the internal field Hi n\ at La sites. The significantly large values of Hi n \ in La2 Ni04 +d and La2 C004 - d ,however, cannot be simply explained by the dipole field originated in the spins at Ni or Co sites, in contrast to Hi n\ in La2Cu04-d. 4 . 5) The exchange interactions through apex oxygen atoms may play more important role in the Ni and Co system than in the Cu system. The di rection of Hi n \ in both of the orthorhombic La2Ni04+6 and La2Co04-d is nearly perpendicular to the basal M-O plane. This result seems to disagree with the Ni moment parallel to the plane which was concluded from the neutron diffraction (ND) experiments. 6 ) For sample D, our ND experiment at low T showed a magnetic peak in the direction of the M-O plane which disappeared above ~100K.
•
6
8
10
(a)
12
14
16
18
20
(c)
8
10
12
14
16
18
MHz
20
22
24
26
Figure 1 Spin-echo amplitude of '39La NQR was plotted by solid circles; (a) sample D of La2 NiOd +d at 1.4K, (b) sample 0 of La2NiOd+6 at 4.2K, (c) La2CuOd-d at 4.2K. Solid line gives a spectrum calculated utilizing the set of parameters listed in Table 1.
l-v(O)/v(T)
)-1 T-1( 1 5
10- 1
10 10
10- 2
1
T
10 2
(K)
Figure 2 Temperature dependence of 1-v(T)/v(0) of central line for sample D of La2Ni04+d.
10- 3 3 10 1
•
00
•
\
10- 1 10- 2
10- 3 10
..,:• ,
0
0
•
• T
10 1 10 2 10 3 (K)
Figure 3 Temperature dependendence of T,-\ of 139La in sample D of La2NiOd+6.
This result is considered to be related to the small cusp in X(T) at ~125K. On the other hand, as can be seen from Fig.2, the NQR experiment clearly showed that TN lie well above RT. Low T X-ray diffraction experiments revealed an occurrence of phase transitions from orthorhombic to low T tetragonal successively with decreasing T. These results suggest a coexistence of two phases with different crystallographic and magnetic structure originated in a possible distribution of the oxygen concentration. In sample D of La2Ni04+6, the reduction of the internal field [l-HintlTl/HintlO)] is proportional to T2.3 below ~200K and Tt-\ is to T2.6 below 20K. These dependence on Tare close to ~T2 and ~T3, respectively, which were predicted by the spin wave theory for 3D AF system 7 ) • These results indicate that, generally speaking, Ni 2+ is in the localized spin state, in contrast with the itinerant behaviors of Cu 2+ in La2Cu04-d. 51 The maximum in T\-\ at ~30K may be closely related to the sharp peak in x(T) at ~20K. REFERENCES (1) J.G.Bednorz and K.A.Muller, Z.Phys. B64(1986)189. (2) S.Wada and T.Kobayashi et al., J.Phys.Soc.Jpn., 58(1989)2658. (3) J.Rodriguez-Carajal, J.L.Matinez and P.Pannetier, Phys.Rev. B38(1988)7148. (4) Y.Kitaoka and S.Hiramatsu et al., Jpn.J.Appl.Phys. 26(1987)L397. (5) H.Nishihara and H.Yasuoka et al., Phys.Soc.Jpn. 56(1987)4556. (6) G.H.Lander, P.G.Brown, J.Spalek and J.M. Honig, Phys.Rev. B40(1989)4463. (7) T.Moriya,Progr.Theor.Phys. 16(1956)641.
139La NQR
STUD~OF
ANTIFERROMAGNETIC La2Ni04+6 AND La2Co04-6
Shinji WADA. Tatsuo KOBAYASHI. Makoto KABURAGI. Tsuyoshi KAJITANIA. Syoichi HOSOYAA, Tsuguo FUKUDAA Shigehumi ONODERAB, Yoshihiro YAMADAc. Kazuyuki SHIBUTANID and Rikuo OGAWAD College of Liberal Arts. Kobe University, Nada, Kobe 657. Japan AInstitute for Material Research. Tohoku University. Katahira 2. Sendai 980. Japan BSendai National College of Technology. Kamiayashi. Sendai 989-31. Japan CHimeji Institute of Technology. Syosha, Himeji 671-22. Japan DTechnical Development Group, Kobe Steel. LTD .• Nishi-Ku. Kobe 673-02, Japan 139La NQR study has been performed on La2Ni04+6 and La2Co04-6. Experimental spectra at low temperatures provided an evidence for the existence of antiferromagnetic ordering in both of them. The internal field at La site in La2Ni04+6 (19.3 kOe. II c-axis) and in La2Co04-6 (30.5 kOe. II c-axis) was significantly large comparing with that in La2Cu04-, (1.0 kOe ~c-axis), suggesting an importance of exchange interactions via apex oxygen atoms.
1. INTRODUCTION Rare earth metal oxide family of formula La2M04 (M=Cu.Ni,Co) shows some interesting properties such as metal-semiconductor and structural phase transitions. and antiferromagnetic (AF) ordering (TN ~250K. ~650K. ~275K). In the past year, greater attention has been concentrated on these oxides because of the discovery of the high-Tc superconductivity in-doped La2Cu04 .1) We have investigated 139La NQR in La2Ni04 21 and La2Co04 in order to make a microscopic and comparative study on their magnetic properties with that in La2Cu04 and also to understand magnetic peculiarities in the parent oxide of the high-Tc superconductor. 2. EXPERIMENTAL 2.1. Sample preparation Ceramic sample of La2Ni04+1 was synthesized by solid state reaction in air. As the structural and magnetic properties have been known to depend on the stoichiometry3) 6. several samples with different 6 were prepared, e.g. sample D(6~0.03) annealed in high vacuum at 1000' C for 91 hrs., and sample O(6~0.07) at 550' C for 100 hrs. in a mixture of 02 (20%) and Ar(80%) gas under 1000 atm. Single crystal of La2Co04-1 was prepared by the coprecipitation method detailed elsewhere. Magnetic susceptibility X(T) for sample D showed a small and broad cusp at ~125K and a sharp peak at ~20K. while X(T) for sample 0 had only a small cusp at ~45K. X-ray diffraction at room temperature (RT) showed that sample D (deoxygenated) of La2Ni04+1 and La2Co04-1 are in orthorhombic phase. while sample 0 (oxygenated) of La2Ni04+d is in t~tragonal phase.
2.2. NQR spectrrrm The NQR spectrum for La2Ni04+1 taken at 1.4K for sample D and sample 0 is shown in Fig.l(a) and l(b). As can be seen from the figures. a small change in 6 leads to a drastic change in the spectrum. For sample D. each of the resonance lines was found to shifts towards lower frequencies with increasing T as shown typically in Fig.2. The NQR spectrum for La2Co0 4- 1 at 4.2 K is shown in Fig.1(c). which has a complex structure. Dominant peaks. however, provide a similar pattern to that for sample D of La2Ni04+1. 2.3 Analysis of NQR spectrum The main peaks in Fig.l can be analyzed as a NQR spectrum originated in a La site in case of Zeeman term HM » quadrupole term HQ. Set of parameters VQ, Hint. 8 obtained are summarized in Table I, where 8 is the polar angle of Hint to the principal axis.of the electric field gradient. Table 1 La2Cu04-1 4 ) La2Ni04+1 2 ) La2 C004-1 Crystal 6 Spin VQ (MHz) Hi n t (kOe) 6
(. )
Ortho. 1/2 6.38 1.02 78.0
Ortho.
Tetra.
~.03
~.07
1 1.8 19.3 ~5
1 2.7 17.8 77.5
Ortho. 3/2 2.0 30.5 ~5
2.3. Nuclear spin-lattice relaxation The 139La spin-lattice relaxation time TI in sample D of La2Ni04+1 was measured at the central peak [(1/2)~~(-1/2)1. The T dependence of T,-l is shown in Fig.3 on a log-
0921-4526/90/$03.50 @ 1990 - Elsevier Science Publishers B.V. (North-Holland)
S. Wada et aI.
1314
log scale. Tl - 1 is proportional to T2. 6 below ~20K, while shows a maximum at ~ 30K. 3. DISCUSSION The AF ordering in La2M04 was evidenced by the existence of the internal field Hi n\ at La sites. The significantly large values of Hi n \ in La2 Ni04 +d and La2 C004 - d ,however, cannot be simply explained by the dipole field originated in the spins at Ni or Co sites, in contrast to Hi n\ in La2Cu04-d. 4 . 5) The exchange interactions through apex oxygen atoms may play more important role in the Ni and Co system than in the Cu system. The di rection of Hi n \ in both of the orthorhombic La2Ni04+6 and La2Co04-d is nearly perpendicular to the basal M-O plane. This result seems to disagree with the Ni moment parallel to the plane which was concluded from the neutron diffraction (ND) experiments. 6 ) For sample D, our ND experiment at low T showed a magnetic peak in the direction of the M-O plane which disappeared above ~100K.
•
6
8
10
(a)
12
14
16
18
20
(c)
8
10
12
14
16
18
MHz
20
22
24
26
Figure 1 Spin-echo amplitude of '39La NQR was plotted by solid circles; (a) sample D of La2 NiOd +d at 1.4K, (b) sample 0 of La2NiOd+6 at 4.2K, (c) La2CuOd-d at 4.2K. Solid line gives a spectrum calculated utilizing the set of parameters listed in Table 1.
l-v(O)/v(T)
)-1 T-1( 1 5
10- 1
10 10
10- 2
1
T
10 2
(K)
Figure 2 Temperature dependence of 1-v(T)/v(0) of central line for sample D of La2Ni04+d.
10- 3 3 10 1
•
00
•
\
10- 1 10- 2
10- 3 10
..,:• ,
0
0
•
• T
10 1 10 2 10 3 (K)
Figure 3 Temperature dependendence of T,-\ of 139La in sample D of La2NiOd+6.
This result is considered to be related to the small cusp in X(T) at ~125K. On the other hand, as can be seen from Fig.2, the NQR experiment clearly showed that TN lie well above RT. Low T X-ray diffraction experiments revealed an occurrence of phase transitions from orthorhombic to low T tetragonal successively with decreasing T. These results suggest a coexistence of two phases with different crystallographic and magnetic structure originated in a possible distribution of the oxygen concentration. In sample D of La2Ni04+6, the reduction of the internal field [l-HintlTl/HintlO)] is proportional to T2.3 below ~200K and Tt-\ is to T2.6 below 20K. These dependence on Tare close to ~T2 and ~T3, respectively, which were predicted by the spin wave theory for 3D AF system 7 ) • These results indicate that, generally speaking, Ni 2+ is in the localized spin state, in contrast with the itinerant behaviors of Cu 2+ in La2Cu04-d. 51 The maximum in T\-\ at ~30K may be closely related to the sharp peak in x(T) at ~20K. REFERENCES (1) J.G.Bednorz and K.A.Muller, Z.Phys. B64(1986)189. (2) S.Wada and T.Kobayashi et al., J.Phys.Soc.Jpn., 58(1989)2658. (3) J.Rodriguez-Carajal, J.L.Matinez and P.Pannetier, Phys.Rev. B38(1988)7148. (4) Y.Kitaoka and S.Hiramatsu et al., Jpn.J.Appl.Phys. 26(1987)L397. (5) H.Nishihara and H.Yasuoka et al., Phys.Soc.Jpn. 56(1987)4556. (6) G.H.Lander, P.G.Brown, J.Spalek and J.M. Honig, Phys.Rev. B40(1989)4463. (7) T.Moriya,Progr.Theor.Phys. 16(1956)641.