- Email: [email protected]
Temperature dependence of 55Mn NMR spectra in Pr1 −xBaxMnO3
~
N
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 518-519
J~
Journalof mnad gnetlsm magnetic mateflals
Temperature dependence of 55Mn NMR spectra in Pr - Ba MnO3 M.M. Savosta ", P. Novfik b'* ~Donetsk Institute ~[Physics & Technics, Rosy Luremhourg 72. 340114, Donetsk, Ukraine bhlstitute ~?['Physics. Cukrovarnicka 10, 162 53 Praha 6. (~ech Republic
Abstract N M R on 5SMn nuclei was measured in 430 M H z were observed at 61 K. For x = localized and delocalized holes is observed x = 0.3. For x = 0.2 the signal is too weak, rights reserved.
Pr~ -,BaxMnO3 (x - 0.2, 0.25). Complex N M R spectra between 290 and 0.25 the transformation of N M R spectrum due to the transition between when the temperature is increased, similar to that reported previously for to measure the temperature dependence. C 1999 Elsevier Science B.V. All
Keywords. Manganite perovskites: N M R
In our recent paper [1] the temperature dependence of 5SMn N M R in two ferromagnetic manganites Pro.,Cao.15Sro.15MnO3 and Pro.vBao.3MnO3 was reported. In these compounds Mn has the same formal valency and also the Curie temperatures are similar. Yet, the N M R spectra have different character. In the (Pr, Ca, Sr) c o m p o u n d the spectrum consists of a single motionally narrowed line in the whole temperature interval studied, corresponding to either delocalized or rapidly hopping (J~,op>>J~) electron holes, while in the (Pr, Ba) c o m p o u n d at 77 K the main contribution to the N M R spectrum is produced by the Mn 3~ and Mn 4+ ions (localized holes or holes with jhop< 140 K. The present paper extends the study of the Pr~ xBa~MnO3 system with different concentration of the Mn 4+ ions (x = 0.2, 0.25) and we also present results obtained at T = 61 K. The compounds in question are ferromagnetic. Tc increases with the content of barium, from -~ 140 (x = 0.2)
*Corresponding author. [email protected].
Fax:
0304-8853/99/$ - see front matter PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 6 4 - 6
+420-2-3123184: e-mail:
to 175 K (x = 0.3) [1,2]. Fig. 1 shows ~SMn spin-echo spectra taken at 61 K. For x = 0.25 and 0.3 the spin-echo amplitude is extrapolated to r = 0, where r is the time between exciting and refocusing RF pulses. For x = 0.2 the signal/noise ratio is poor, so that extrapolation was not possible and the spectrum taken at r -~ 3 j_ts is only shown. The spin-echo decay time is short and T 2 ( M n 3 + ) < T2(Mn "~+) [1,3], therefore tile relative intensity of the high-frequency spectrum, which contains mainly Mn 3 + signal, is strongly reduced. Taking this into account the spectra are qualitatively similar for all three values of x. F r o m a decomposition of the spectra for x - 0.25 and 0.3 the contribution of localized Mn 3 + ions {.lr~ -~ 375 MHz), localized Mn 4+ ions ( f ~ ~ 315 MHz) and rapidly hopping holes {.lr~, ~-- 340 MHzt may be determined. For both values of x the position of the line caused by the rapidly hopping holes is closer to the average of Mn 4 + and Mn 3 + frequencies than to their center of gravity. This is expected if the electron hole hops between restricted number of sites only. The fraction of Mn sites contributing to the motionally narrowed line is larger for x = 0.25 comparing to x - 0.3 which is in agreement with a larger disorder in the later compound. Temperature dependence of the N M R spectra of x - 0.25 and 0.3 samples is displayed in Fig. 2. For
1999 Elsevier Science B.V. All rights reserved.
M.~L Savosta, P. Nov~tk / Journal of Magnetism and Magnetic Materials 196-197 (1999) 518-519 . . . .
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Fig. 1. 55Mn NMR spectra in Prl-xBaxMnO3 at T = 61 K, x = 0.2 (V), 0.25 (Vq),0.3 (O). For x = 0.25 and 0.3 the spectra are decomposed into three lines (full curves). Areas of the lines corresponding to Mn 3+ and Mn 4+ are in the ratio as expected from the composition and nominal valency of the constituents. The dashed curves are the fits.
a)
I/,~=#'~
L
°g"~,~
j 320
360
400
Frequency (MHz)
440
280
158K/
"',,,.2.°6K2 320
n I i 300
118 K
n n i I t i i i I n J i i 350 400 Frequency (MHz)
Frequency (MHz)
280
. . . .
61 K
~
4
519
360
400
440
Frequency ( M H z )
Fig. 2. The temperature dependence of 55Mn NMR spectra in Prl xBaxMnO3, x = 0.25 (a) and x = 0.3 (b). For x = 0.25 the spectra are decomposed into three lines (full curves) as described in the text. The dashed curves are the fits.
x = 0.25 a d e c o m p o s i t i o n of the spectra on three Gaussian lines is s h o w n (for analogous decomposition of x = 0.3 spectra see Ref. [-1]). It is seen that the motional
Fig. 3. Pr0.~sBao.25MnOa. Variation of the relaxation time T2 throughout the NMR spectrum for several temperatures.
narrowing starts near 100 K and the picture is similar to the case of x = 0.3 sample. Unfortunately, the M n N M R signals could not be observed above ~ 120 K because of too short Tz (T2 < 1 las, Fig. 3). Nevertheless, the results obtained indicate that for b o t h x = 0.25 and 0.3, the electron hole m o t i o n is thermally activated at the temperatures of our measurement. F o r x = 0.25 the frequency dependence of the s p i n - s p i n relaxation time T2 at several temperatures was also studied (for similar results on x = 0.3, see Ref. [-1]). Fig. 3 d o c u m e n t s that the difference between M n 3 + and M n 4+ ions gradually vanishes when T is increased. The authors are indebted to Dr. Z. Jirfik and to Dr. J. Hejtmfinek for providing the polycrystalline sampies. This work was supported by the grant 202/97/1018 of G r a n t Agency of the Czech Rupublic.
References [-1] M.M. Savosta, P. Novfik, Z. Jirfik, J. Hejtmfinek, M. Mary~ko, Phys. Rev. Lett. 79 (1997) 4278. [2] Z. Jirfik, E. Pollert, A.F. Andersen, J.C. Grenier, P. Hagenmuller, Eur. J. Solid State Inorg. Chem. 27 (1990) 421. [3] G. Matsumoto, J. Phys. Soc. Jpn. 29 (1970) 615.
N
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 518-519
J~
Journalof mnad gnetlsm magnetic mateflals
Temperature dependence of 55Mn NMR spectra in Pr - Ba MnO3 M.M. Savosta ", P. Novfik b'* ~Donetsk Institute ~[Physics & Technics, Rosy Luremhourg 72. 340114, Donetsk, Ukraine bhlstitute ~?['Physics. Cukrovarnicka 10, 162 53 Praha 6. (~ech Republic
Abstract N M R on 5SMn nuclei was measured in 430 M H z were observed at 61 K. For x = localized and delocalized holes is observed x = 0.3. For x = 0.2 the signal is too weak, rights reserved.
Pr~ -,BaxMnO3 (x - 0.2, 0.25). Complex N M R spectra between 290 and 0.25 the transformation of N M R spectrum due to the transition between when the temperature is increased, similar to that reported previously for to measure the temperature dependence. C 1999 Elsevier Science B.V. All
Keywords. Manganite perovskites: N M R
In our recent paper [1] the temperature dependence of 5SMn N M R in two ferromagnetic manganites Pro.,Cao.15Sro.15MnO3 and Pro.vBao.3MnO3 was reported. In these compounds Mn has the same formal valency and also the Curie temperatures are similar. Yet, the N M R spectra have different character. In the (Pr, Ca, Sr) c o m p o u n d the spectrum consists of a single motionally narrowed line in the whole temperature interval studied, corresponding to either delocalized or rapidly hopping (J~,op>>J~) electron holes, while in the (Pr, Ba) c o m p o u n d at 77 K the main contribution to the N M R spectrum is produced by the Mn 3~ and Mn 4+ ions (localized holes or holes with jhop< 140 K. The present paper extends the study of the Pr~ xBa~MnO3 system with different concentration of the Mn 4+ ions (x = 0.2, 0.25) and we also present results obtained at T = 61 K. The compounds in question are ferromagnetic. Tc increases with the content of barium, from -~ 140 (x = 0.2)
*Corresponding author. [email protected].
Fax:
0304-8853/99/$ - see front matter PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 6 4 - 6
+420-2-3123184: e-mail:
to 175 K (x = 0.3) [1,2]. Fig. 1 shows ~SMn spin-echo spectra taken at 61 K. For x = 0.25 and 0.3 the spin-echo amplitude is extrapolated to r = 0, where r is the time between exciting and refocusing RF pulses. For x = 0.2 the signal/noise ratio is poor, so that extrapolation was not possible and the spectrum taken at r -~ 3 j_ts is only shown. The spin-echo decay time is short and T 2 ( M n 3 + ) < T2(Mn "~+) [1,3], therefore tile relative intensity of the high-frequency spectrum, which contains mainly Mn 3 + signal, is strongly reduced. Taking this into account the spectra are qualitatively similar for all three values of x. F r o m a decomposition of the spectra for x - 0.25 and 0.3 the contribution of localized Mn 3 + ions {.lr~ -~ 375 MHz), localized Mn 4+ ions ( f ~ ~ 315 MHz) and rapidly hopping holes {.lr~, ~-- 340 MHzt may be determined. For both values of x the position of the line caused by the rapidly hopping holes is closer to the average of Mn 4 + and Mn 3 + frequencies than to their center of gravity. This is expected if the electron hole hops between restricted number of sites only. The fraction of Mn sites contributing to the motionally narrowed line is larger for x = 0.25 comparing to x - 0.3 which is in agreement with a larger disorder in the later compound. Temperature dependence of the N M R spectra of x - 0.25 and 0.3 samples is displayed in Fig. 2. For
1999 Elsevier Science B.V. All rights reserved.
M.~L Savosta, P. Nov~tk / Journal of Magnetism and Magnetic Materials 196-197 (1999) 518-519 . . . .
i
. . . .
i
. . . .
i
. . . .
10
,
t
.
.
.
.
i
. . . .
J
o
~
.~_
W
-771(
~
7
~
92 K
2
Q.
ininimililli 0 250
300
350
400
450
Fig. 1. 55Mn NMR spectra in Prl-xBaxMnO3 at T = 61 K, x = 0.2 (V), 0.25 (Vq),0.3 (O). For x = 0.25 and 0.3 the spectra are decomposed into three lines (full curves). Areas of the lines corresponding to Mn 3+ and Mn 4+ are in the ratio as expected from the composition and nominal valency of the constituents. The dashed curves are the fits.
a)
I/,~=#'~
L
°g"~,~
j 320
360
400
Frequency (MHz)
440
280
158K/
"',,,.2.°6K2 320
n I i 300
118 K
n n i I t i i i I n J i i 350 400 Frequency (MHz)
Frequency (MHz)
280
. . . .
61 K
~
4
519
360
400
440
Frequency ( M H z )
Fig. 2. The temperature dependence of 55Mn NMR spectra in Prl xBaxMnO3, x = 0.25 (a) and x = 0.3 (b). For x = 0.25 the spectra are decomposed into three lines (full curves) as described in the text. The dashed curves are the fits.
x = 0.25 a d e c o m p o s i t i o n of the spectra on three Gaussian lines is s h o w n (for analogous decomposition of x = 0.3 spectra see Ref. [-1]). It is seen that the motional
Fig. 3. Pr0.~sBao.25MnOa. Variation of the relaxation time T2 throughout the NMR spectrum for several temperatures.
narrowing starts near 100 K and the picture is similar to the case of x = 0.3 sample. Unfortunately, the M n N M R signals could not be observed above ~ 120 K because of too short Tz (T2 < 1 las, Fig. 3). Nevertheless, the results obtained indicate that for b o t h x = 0.25 and 0.3, the electron hole m o t i o n is thermally activated at the temperatures of our measurement. F o r x = 0.25 the frequency dependence of the s p i n - s p i n relaxation time T2 at several temperatures was also studied (for similar results on x = 0.3, see Ref. [-1]). Fig. 3 d o c u m e n t s that the difference between M n 3 + and M n 4+ ions gradually vanishes when T is increased. The authors are indebted to Dr. Z. Jirfik and to Dr. J. Hejtmfinek for providing the polycrystalline sampies. This work was supported by the grant 202/97/1018 of G r a n t Agency of the Czech Rupublic.
References [-1] M.M. Savosta, P. Novfik, Z. Jirfik, J. Hejtmfinek, M. Mary~ko, Phys. Rev. Lett. 79 (1997) 4278. [2] Z. Jirfik, E. Pollert, A.F. Andersen, J.C. Grenier, P. Hagenmuller, Eur. J. Solid State Inorg. Chem. 27 (1990) 421. [3] G. Matsumoto, J. Phys. Soc. Jpn. 29 (1970) 615.