Pressure Effects in Granular La0.7Ca0.3?xSrxMnO3

phys. stat. sol. (a) 189, No. 2, 281–285 (2002)

Pressure Effects in Granular La0.7Ca0.3–xSrxMnO3 I. S. Maksimov (a, b), E. B. Nyeanchi (a), Yu. V. Medvedev (b), A. N. Ulyanov (b), N. I. Mezin (b), and B. Sundqvist (a) (a) University of Umea, Department of Experimental Physics, S-90187, Umea, Sweden (b) Donetsk Physical and Technical Institute, National Academy of Sciences of Ukraine, 72 R. Luxemburg St., 83114 Donetsk, Ukraine (Received May 1, 2001; accepted September 30, 2001) Subject classification: 64.70.Kb; 75.30.Kz; 75.30.Vn; 75.50.Cc; S10.15 This paper discusses the effect of hydrostatic pressure up to 0.8 GPa on magnetic properties of manganites La0.7Ca0.3––xSrxMnO3 (0
x
0.3, Dx = 0.03) and magnetoresistance data in an applied magnetic field of 5.0 T. Application of pressure enlarges the temperature range of the ferromagnetic phase. Curie temperature, TC, as a function of pressure and the temperature of resistance maximum, Tp, showed an anomaly for x = 0.15. The slope of pressure dependence of TC for x < 0.15 and x > 0.15 is higher than for x = 0.15. Dependence of temperature Tp on x consists of two curves: for x
0.15 and for x  0.15. There is a sharp bend on the Tp curve at x = 0.15. The structural phase transition from orthorhombic phase (x < 0.15) to rhombohedral one (x  0.15) corresponds to that concentration.

Introduction High pressure is a useful tool to study the electronic and magnetic properties of manganese perovskite Ln1––xMxMnO3 (where Ln = La, Y, Pr, . . . are three-valent rare earth metals; M = Ca, Sr, Ba, . . . are the two-valent metals). At ambient pressure, the resistivity of manganites with the definable doping gradually increases with decreasing temperature and then sharply drops near the ferromagnetic transition temperature, TC, indicating a transition from paramagnetic non-metal to ferromagnetic metal [1]. Temperature TC usually increases with pressure with a slope of a few K/kbar [1, 2]. Lattice structure plays a significant role in the transport and magnetic properties of those manganites (see, e.g., review [1]). The compounds with doping level x = 0.3 have been studied carefully [1, 3]. In particular, if M = Ca, then the value of magnetoresistive effect, MR = ––[R(T, 0) –– R(T, H)]/ R(T, 0), is maximum, but the temperature Tm is minimum (R(T, 0) and R(T, H) are the resistance at temperature T at zero and H value of external field, respectively). If the M = Sr, then TC is maximum and the MR value is relatively small. Beside that, near the A, the lanthapoint where the average ionic radius hrAi in A position equals to 1.227  num manganites reveal the concentration structural phase transition [4] from orthorhombic (Pbnm) phase to rhombohedral (R3c) one. The A position is a position of rare earth three-valent and two-valent metals in the perovskite cell. In manganites A corresponds to x = 0.15 and La0.7Ca0.3––xSrxMnO3 the average ionic radius hrAi = 1.227  for that concentration a sharp change, like jump, of ferromagnetic ordering temperature was revealed [5]. In the present work, in order to investigate more carefully the properties of the manganites La0.7Ca0.3––xSrxMnO3 near the concentration phase transition the pressure dependencies of susceptibility up to 0.8 GPa and the magnetoresistance measurement were carried out. We observed an anomaly in the pressure dependencies of TC for sample with x = 0.15 and a change of the resistivity peak temperature, Tp, dependence on x at x = 0.15.

# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002

0031-8965/02/18902-0281 $ 17.50þ.50/0

282

I. S. Maksimov et al.: Pressure Effects in Granular La0.7Ca0.3––xSrxMnO3

Experimental Samples of composition La0.7Ca0.3––xSrxMnO3 (x = 0.0, 0.03, 0.06, ..., 0.3) were prepared using oxide La2O3, carbonates CaCO3, SrCO3 and metallic manganese by means of nitrate technology. The solutions of initial components were evaporated and the powders obtained were pressed into pellets. After that the pellets were subjected to annealing for 80 h at temperature 950  C with intermediate grinding and pressing. The final annealing of samples was made at 1150  C for 20 h. The synthesized samples were single phase as observed by X-ray diffraction analyses [5]. The samples measured had a cylindrical shape with typical dimensions 2  6 mm2 (radius and length). To investigate the pressure dependence of temperature TC we measured the magnetic susceptibility c under pressure. A sample piece was placed in a system of coils (3 mm in diameter and 12mm in length) and the susceptibility was determined as the difference in the inductance of two counter-wound secondary coils. A field of 37 Oe was applied by the outer primary coil at a frequency of 1 kHz. The sample temperature was measured by a thermocouple placed near the coils. The hydrostatic pressure was generated inside a teflon cell using a cylinder and piston apparatus that maintained a constant pressure over the entire temperature range. Silicone oil (DC 200) was used as the pressure medium and the pressure was monitored continuously. The pressure value was controlled by the calibrated manganin coil. Resistivity measurements were performed using the standard four-probe direct current method with a current of about 5 mA. Measurements were carried out as a function of temperature at zero and 5.0 T applied magnetic field. The temperature was controlled to an accuracy of 0.1 K by means of an Oxford Instruments’ Maglab 2000 cryostat system. Results and Discussion All La0.7Ca0.3––xSrxMnO3 samples showed an insulator–metal transition as the temperature is decreased. The resistance peak temperature, Tp, increases steadily with the increase of Sr content (with increasing x value). One can see from Fig. 1 that the Tp dependence on x consists of two curves: for x
0.15 and for x  0.15. There is a sharp bend on the Tp curve at x = 0.15. The structural phase transition (Pbnm) –– (R3c) corresponds to that x content.

Fig. 1. The resistance peak temperature, Tp, as a function of x value in zero field. The line is a guide to the eye

phys. stat. sol. (a) 189, No. 2 (2002)

283

Fig. 2. Magnetoresistance MR = ––[R(T, 0) –– R(T, H)]/R(T, 0) of La0.7Ca0.3––xSrxMnO3 samples as a function of temperature. External magnetic field H = 5.0 T

Figure 2 shows the magnetoresistive effect, MR, of the series as a function of temperature. The applied magnetic field was 5.0 T. The MR as a function of temperature shows a maximum near Tp. As the temperature decreases further, the MR steadily increases. As x changes, there is a change in the average ionic radius hrAi of the cation in the A position of perovskite. The magnitude of hrAi is determined as hrAi = Syiri, where ri is the ionic radius in the A position and yi is the relative population of the A position by the corresponding ion (Syi = 1). All ionic radii were taken according to Shennon [6] for ninefold surrounding of ion in A position.

Fig. 3. The ac susceptibility vs. temperature at various pressures for La0.7Ca0.3––xSrxMnO3 (x = 0). The inset shows the dc/dT dependence vs. temperature

284

I. S. Maksimov et al.: Pressure Effects in Granular La0.7Ca0.3––xSrxMnO3 Fig. 4. Change in TC of La0.7Ca0.3––xSrxMnO3 due to applied pressure. The lines are guides for the eye

The lattice volume contraction induced by an applied pressure has a strong influence on the shift of the transition temperature TC. It is illustrated in Fig. 3 by the temperature dependencies of susceptibility of La0.7Ca0.3MnO3 for different values of pressure. The value of TC was obtained as the point with the maximum slope of susceptibility c versus temperature. The application of pressure, p, increases the overlap of the Mn orbitals as well as the Mn–O–Mn bond angle [3]. It increases the temperature range of the ferromagnetic phase through the pressure-enhanced transfer interaction of the holes generated by doping. The pressure dependence of temperature change DTC is shown in Fig. 4, where DTC = TC –– TCp, TC and TCp is the ferromagnetic ordering transition temperature at zero and high pressure, respectively. TC is an almost linear function of pressure. The value of DTC(p) and the slope of DTC(p) are nonmonotonous functions on the x value. The sample with x = 0.15 showed the lowest slope and the lowest maximum value of DTC(p). We believe that this anomalous behaviour, as well as the anomaly in temperature Tp on x, is related to a structural phase transition. This anomaly is more visible in Fig. 5, where it can be seen that the point x = 0.15 is a peculiar point in the temperature TC on concentration x and on pressure p dependencies.

Fig. 5. TC vs. strontium concentration x in La0.7Ca0.3––xSrxMnO3 under different pressures. The lines are guides for the eye

phys. stat. sol. (a) 189, No. 2 (2002)

285

Summary and Conclusion A systematic study of the pressure and magnetic field effects on the magnetic and transport properties of ceramic La0.7Ca0.3––xSrxMnO3 (0
x
0.3) were carried out. The magnetic susceptibility strongly depends on pressure. This is related to the overlap between adjacent Mn orbitals and thus to the Mn–O–Mn bond angle. The application of pressure extends the temperature range of the ferromagnetic phase. In addition we associate the observed anomaly in the pressure dependence of TC and of the temperature of peak resistance Tp near the concentration A) to the structural orthorhombic–rhombohedral phase transix = 0.15 (hrAi  1.225  tion.

References [1] J.M.D. Coey, M. Viret, and S. von Molnar, Adv. Phys. 48, 167 (1999). [2] K.V. Kamenev, G.J. McIntyre, D. McK Paul, M.R. Lees, and G. Balakrishnan, Phys. Rev. B 57, R6775 (1998). [3] P.G. Radaelli, G. Iannoe, and M. Marezio, Phys. Rev. B 56, 8265 (1997). [4] P.G. Radaelli, M. Marezio, and H.Y. Hwang, J. Solid State Chem. 122, 444 (1996). [5] A.N. Ulyanov, G.V. Gusakov, V.A. Borodin, N.Yu. Starostyuk, and A.B. Mukhin, Solid State Commun. (to be published). [6] R.D. Shennon, Acta Cryst. A 32, 751 (1976).