DSpace at VNU: EXAFS and EPR study of La0.6Sr0.2Ca0.2MnO3 and La0.6Sr0.2Ba0.2MnO3

Physica B 327 (2003) 183–186 EXAFS and EPR study of La0.6Sr0.2Ca0.2MnO3 and La0.6Sr0.2Ba0.2MnO3 Dong-Seok Yanga,*, A.N Ulyanovb, Manh-Huong Phanb, Ikgyun Kima, Byong-Keun Ahna, Jang Roh Rheec, Jung Sun Kimd, Chau Nguyene, Seong-Cho Yub a Physics Division, School of Science Education Chungbuk Naional University, Cheongju 361-763, South Korea b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea c Department of Physics, Sookmyung Women’s University, Seoul 140-742, South Korea d School of Electrical Engineering and Computer Science, Hanyang University, An San 425-791, South Korea e Center for Material Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract Extended X-ray absorption fine structure (EXAFS) analysis and electron-paramagnetic resonance (EPR) have been used to examine the local structure and the internal dynamics of La0.6Sr0.2Ca0.2MnO3 and La0.6Sr0.2Ba0.2MnO3 ( for both samples) and the Debye–Waller factors lanthanum manganites The Mn–O bond distance (B1.94 A ( for La0.6Sr0.2Ca0.2MnO3 and for La0.6Sr0.2Ba0.2MnO3, respectively) were obtained from (0.36  10À2 and 0.41  10À2A the EXAFS analysis The dependence of the EPR line width on dopant kind (Ca or Ba) showed a decrease of the spin– lattice interaction with an increase of the Curie temperature For both compositions, the EPR line intensity followed the exponential law ITị ẳ I0 expEa =kB Tị; deduced on the basis of the adiabatic polaron hopping model r 2002 Elsevier Science B.V All rights reserved Keywords: Magnetically ordered materials; Spin–lattice interaction; Electron paramagnetic resonance; EXAFS The local structure, internal dynamics and interactions between spin and lattice in lanthanum manganites are under discussion [1] These phenomena can be successfully studied by extended X-ray absorption fine structure (EXAFS) and electron-paramagnetic resonance (EPR) A local probe as given by the EXAFS is a useful tool to study the polarons and their dependence on temperature [2] and ionic size [3] At the same time, the resonance methods can give useful information about the internal dynamics of these *Corresponding author Tel.: +82-43-261-2724; fax: +8243-271-0526 E-mail address: dsyang@trut.chungbuk.ac.kr (D.-K Yang) colossal magneto-resistance (CMR) materials, especially near the Curie point EPR has been studied in a variety of lanthanum manganites Nevertheless, some aspects of the temperature dependence of IðTÞ; the EPR line intensity, are still in question Namely, IðTÞ decreased exponentially with increasing T [1,4], but was also found to be inversely proportional to T [5] In the present work, EXAFS and EPR measurements of La0.6Sr0.2Ba0.2MnO3 and La0.6Sr0.2Ca0.2MnO3 were carried out to study the local structure and the internal dynamics near Tc : The La0.6Sr0.2Ba0.2MnO3 (LSB) and La0.6Sr0.2Ca0.2MnO3 (LSC) samples were prepared by a ceramic technique XRD analysis showed the 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) 2 - 184 D.-K Yang et al / Physica B 327 (2003) 183–186 single-phase rhombohedral structure for both compositions EXAFS experiments were carried out at the 7C EC EXAFS beam line of the Pohang light source (PLS) in Korea PLS was operated with an electron energy of 2.5 GeV and the maximum current of 170 mA X-rays were monochromatized by a Si(3 1) double-crystal The higher harmonics were removed by a 15% detuning of the crystal The EXAFS spectrum was measured near the Mn K edge at the fluorescence mode The EPR measurements were performed at 9.2 GHz with a Jeol JES-TE300 ESR Spectrometer The measurements were carried out on samples of approximately equal mass (E2 mg), because the resonance line width depends on sample mass [5] The Curie temperature was obtained by the extrapolation of temperature dependence of magnetization (down to zero magnetization) and was found to be 35471 and 33871 K for LSB and LSC, respectively Fig shows the Fourier transforms of the EXAFS spectra, jrðRÞj; which gives the radial pair distribution of neighboring ions The first high peak is formed by the oxygen ions in the first neighbor shell and the second peak is formed by the La, Sr, Ba, Ca ions in the second neighbor shell The second peak for LSB is higher than that for LSC because the scattering amplitude of a Ba ion is larger than that of a Ca ion The local structural parameters were obtained using the conventional method [6] The average value of ( for the Mn–O bond distance was about 1.94 A both samples, and the dispersion, sMn2O ; was ( for LSB and LSC, 0.41  10À2 and 0.36  10À2 A respectively We assume that the larger value of s2Mn2O for LSB in comparison to LSC reflects the larger static dispersion, s2A ; of ions in the A (=La, Sr, Ba, Ca) position for LSB The EPR spectra showed a single Lorentzian line in the temperature range presented, with gE2 independent of temperature The peak-to-peak EPR line width, DH; showed a minimum at Tmin E1:1Tc and increased approximately linearly with temperature for both compositions (see Fig 2) Values of DHðTmin ) and b [b is the slope of DHðTÞ] are in agreement with those obtained, for example, in Ref [5]) The decrease of DH and b values with the increase of Tc indicates a decrease of the strength of the spin–lattice interaction It is in agreement with nuclear magnetic resonance [7] and magnetization measurements [7,8] The EPR line intensity, IðTÞ; was determined by double integration of the experimental derivative absorption curve The temperature dependencies of IðTÞ and 1=IðTÞ; for La0.6Sr0.2Ba0.2MnO3, are 600 ∆H (Oe) 500 400 LSC 300 LSB 200 100 Fig Fourier transforms of the EXAFS spectra for La0.6Sr0.2Ca0.2MnO3 (—) and La0.6Sr0.2Ba0.2MnO3 (- - -) 360 380 400 420 440 T(K) 460 480 Fig Temperature dependence of the EPR line width, DH; for La0.6Sr0.2Ca0.2MnO3 (LSC) and for La0.6Sr0.2Ba0.2MnO3 (LSB) D.-K Yang et al / Physica B 327 (2003) 183–186 Intensity, I(T) (arb.units) LSB 0.8 0.6 0.4 Θ 0.2 Inverse intensity, 1/I(T) (arb.units) 1.0 340 360 380 400 420 440 460 480 500 T(K) Fig Temperature dependencies of the EPR line intensity for La0.6Sr0.2Ba0.2MnO3: (’) IðTÞ; (J) inverse line intensity, 1=ITị; () tting curve ITị ẳ I0 expðEa =kB TÞ; and (- - -) Curie–Weiss law shown in Fig To fit the IðTÞ dependence some expressions describing the CMR material parameters were considered The EPR line width obeys DHp1=ðTwdc Þ [5], according to the spin–spin interaction model (wdc is the static susceptibility) At the same time, we have DHðTÞpsðTÞ [9] and, according to the adiabatic polaron hopping model [10], sðTÞpð1=TÞ expðÀEa =kB TÞ (s; Ea and kB are the conductivity, activation energy and Boltzmann constant, respectively) Taking into account also that EPR line intensity IðTÞpwdc [5], we nd ITị ẳ I0 expEa =kB Tị: 1ị This expression gives a good fit for the IðTÞ data obtained for both compositions By extrapolating the linear (high-temperature part) of the 1=IðTÞ dependence to zero value it is possible to obtain the Curie–Weiss temperature, Y (E357 K for both compositions) As one can see from Fig the Curie–Weiss law, IðTÞp1=ðT À YÞ; does not correctly describe the temperature dependence of the EPR line intensity in a wide temperature range around Tc : and the same is true for La0.6Sr0.2Ba0.2MnO3 sample The ln IðTÞ versus 1=T plots were almost linear It permitted to deduce the value of the activation energy Ea =0.22 eV and 185 0.26 eV for LSB and LSC, respectively These values are consistent with the ones reported in Ref [5] Polaron formation across the ferromagnetic– paramagnetic phase transition was observed [11] in a study of the La1ÀxAxMnO3 (A=Ca, Pb) perovskite system by the EXAFS spectroscopy The observed deviation of the experimental IðTÞ data from the Curie–Weiss law can be caused by the spin–lattice interaction in the vicinity of Tc : The fitting IðTÞ curves obtained using either the exponential or the Curie–Weiss law coincide at high temperatures The fact that the Curie–Weiss law can be applied there can be regarded as an indication for the decrease of the spin–phonon interaction at high temperatures In conclusion, the average distance and the Debye–Waller factor for the Mn–O bonds in the MnO6 octahedral were obtained for La0.6Sr0.2Ca0.2MnO3 and La0.6Sr0.2Ba0.2MnO3 by EXAFS analysis The EPR line intensity for both compositions followed the exponential law ITị ẳ I0 expEa =kB Tị deduced on the basis of the adiabatic polaron hopping model The deduced law describes the measured temperature dependence of the EPR line intensity in a more wide temperature range than the Curie–Weiss law and indicates the spin–lattice interaction in manganites near the Curie point The dependence of the EPR line width on dopant kind showed the decrease of spin–lattice interaction with the increase of Tc : Acknowledgements The Korean Research Foundation Grant (KRF-2001-005-D20010) supported the research at the Chungbuk National University References [1] J.M Coey, M Viret, S von Molnar, Adv Phys 48 (1999) 167 [2] A Lanzara, N.L Saini, M Brunelli, F Natali, A Bianconi, P Radaelli, S.W Cheong, J Phys Chem Solids 59 (1998) 2220 [3] D Louca, T Egami, W Dmowski, J.F Mitchell, Phys Rev B 64 (2001) 180403 186 D.-K Yang et al / Physica B 327 (2003) 183–186 [4] S.B Oseroff, M Torikachvili, J Singley, S Ali, S-W Cheong, S Schultz, Phys Rev B 53 (1996) 6521 * [5] M.T Causa, M Tovar, A Caneiro, F Prado, G Ibanez, * C.A Ramos, A Butera, B Alascio, X Obradors, S Pinol, ! 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The Curie temperature was obtained by the extrapolation of temperature dependence of magnetization (down to zero magnetization) and was found to be 35471 and 33871 K for LSB and LSC, respectively... decrease of DH and b values with the increase of Tc indicates a decrease of the strength of the spin–lattice interaction It is in agreement with nuclear magnetic resonance [7] and magnetization... transforms of the EXAFS spectra for La0.6Sr0.2Ca0.2MnO3 (—) and La0.6Sr0.2Ba0.2MnO3 (- - -) 360 380 400 420 440 T(K) 460 480 Fig Temperature dependence of the EPR line width, DH; for La0.6Sr0.2Ca0.2MnO3