Journal of Magnetism and Magnetic Materials 226}230 (2001) 988}989
High-temperature elastic measurements in La Sr MnO at low frequency
S. Seiro , A. Ghilarducci , H. Salva *, M. Vasquez Mansilla , M. Saint-Paul, P. Lejay, P. Monceau Centro Atomico Bariloche (CNEA) and Instituto Balseiro, Av. Bustillo 9500, (8400) Bariloche, Argentina CRTBT (CNRS), Av. Martyrs 25, (38000) Grenoble, France
Abstract In the present work, we report elastic measurements performed on the manganese perovskite La Sr MnO , for which the ferromagnetic ordering temperature was determined to be 377 K, coincident with a metal}insulator transition. Measurements in the temperature range 250}480 K at various frequencies were performed, showing a weak dip in the modulus (1.9%) and no Q\ peak at the magnetic transition, unlike La Ca MnO . Two Q\ peaks were observed, one above ¹ (414.5$0.2 K at 1 Hz) and another below ¹ (303$6 K at 1 Hz). With increasing measurement frequency between 0.3 and 20 Hz, a shift to higher temperatures of the centers of both peaks was observed. Such shifts yielded an activation energy of 3.77$0.05 eV for the high-temperature peak and 0.56$0.07 eV for the low-temperature peak. Evidence was found that the high-temperature peak is not a simple relaxation process, but shows a relaxation time distribution. Relaxation peaks are attributed to jumps of oxygen ions within the crystal under applied stress. 2001 Elsevier Science B.V. All rights reserved. Keywords: Elasticity*internal friction; Magnetoresistance*ternary compounds; Magnetic transition
Known since the early 1950 s, manganite perovskites have revived interest during the last decade, because of their magnetoresistive properties, useful in magnetic recording technology, and for their rich magnetostructural phase diagram and the variety of phenomena involved (double- and super-exchange, charge and orbital ordering, Jahn}Teller distortions, etc.) [1]. As a function of temperature, these materials exhibit a metal}insulator transition near the ferromagnetic ordering temperature, ¹ . In this work, we study La Sr MnO , focusing on its elastic properties, as in our previous work on La Ca MnO [2]. Measurements were performed on a ceramic sample, in a sub-resonant inverted pendulum. The sample was forced to oscillate at constant frequency, varying the temperature at a rate of 0.5 K/min. from 250 to 480 K. Simultaneous two-wire resistivity
* Corresponding author. Fax: #54-2944-445-299. E-mail address:
[email protected] (H. Salva).
measurements were performed to determine the metal} insulator transition. A magnetization measurement was performed on another piece of the same preparation, at a low magnetic "eld (50 G) to determine ¹ , which was found to be 377 K (Fig. 1). DC resistivity at a sample oscillation frequency of 1 Hz exhibits an abrupt increase for ¹(¹ , as expected for the metal}insulator transition of this compound. Shear modulus at 1 Hz exhibits a small dip at ¹ , of about 1.9%, while no internal friction (Q\) peak is observed at that temperature. This behavior is qualitatively di!erent from that of La Ca MnO , which has instead a large positive step (4%) on decreasing the temperature below ¹ . As can also be seen in Fig. 1, a large internal friction e!ect (P ) is found at temperatures higher than ¹ (414.5$0.2 K), as well as its corresponding modulus jump (6.6%), which has no appreciable contribution to electrical resistivity. Another internal friction peak (P ), wider and smaller, is found below ¹ , at 303$6 K. After
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 1 6 9 - 0
S. Seiro et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 988}989
Fig. 1. From top to bottom: magnetization measurement at low magnetic "eld (50 G) used to determine the ferromagnetic transition; DC resistivity, showing the metal}insulator transition at ¹ ; internal friction Q\ showing peaks P and P and no e!ect at ¹ , and modulus (G) measurements showing a jump corresponding to P , a dip at ¹ , and no appreciable e!ect at P peak temperature.
subtracting the background, "tting was done to the peaks with the Debye (single relaxation time process) model [3]. The apparent activation energies were obtained, being 1.67$0.07 eV for the P peak, and 0.08$0.06 eV for the P peak. Varying the measurement frequency, a shift of peak temperatures was observed. Peak temperatures increased when measurement frequency was increased and vice versa. This behavior is consistent with the presence of stress-induced thermally activated relaxation processes [4]. However, the activation energy (Q) obtained by the Arrhenius plot of log 2f versus 1/¹ , where f is the measurement frequency and ¹ the peak temperature, was 3.77$0.05 eV for the P peak and 0.56$0.07 eV for the P peak (Fig. 2). These values are considerably higher than those obtained by individual "tting of the internal friction peaks, indicating that the processes cannot be described by a single relaxation time, but a relaxation time distribution must be considered. Also, for the
989
Fig. 2. Arrhenius plots for P peak (above) and P peak (below) with its obtained activation energy values.
P peak, the relationship between the relative modulus variation (G/G) and the height of the Q\ peak gives 4.9 which is di!erent from the factor 2 predicted by the Debye mode. This is consistent with the assumption of a relaxation time distribution. As a conclusion, the elastic constant behavior at the ferromagnetic transition is found to be qualitatively di!erent from that previously reported of La Ca MnO , and two relaxation peaks are found below and above ¹ , respectively. The e!ects are presumed to have a relaxation time distribution, to be determined. A possible origin of the P peak is the mechanism of interstitial oxygen ions ordering under applied stress.
References [1] Y. Tokura, Y. Tomioka, J. Magn. Magn. Mat. 200 (1999) 1. [2] A.A. Ghilarducci, H.R. Salva, R.D. SaH nchez, C. VaH zquez, Magnetism, magnetic materials and their applications, Mater. Sci. Forum 302}303 (1999) 139. [3] E. Carren o-Morelli, A.A. Ghilarducci, S.E. Urreta, Philos. Mag. A 79 (2) (1999) 293. [4] A.S. Nowick, B.S. Berry, `Anelastic Relaxation in Crystalline Solidsa, Materials Science Series, Academic Press, New York, 1972.
