Comment on  Low-temperature transport properties of non-stoichiometric La_{0.95  x}Sr_xMnO₃ 

Abstract

In a recent paper (Michalopolou A., Syskakis E. and Papastaikoudis C., 2001 J. Phys.: Cond. Matter, 13 11615) A. Michalopolou et al reported the measurements of electrical resistivity and specific heat at zero magnetic field carried out on polycrystalline non-stoichiometric La 0.95-x Sr x MnO 3 manganites. In particular, authors had attributed the low temperature behavior of resistivity (shallow minimum and slight upturn at lowest temperatures) to 3D electron-electron interaction enhanced by disorder, using results of numerical fittings of resistivity versus temperature dependencies at the interval 4.2 -40 K. It is shown in this comment that such analysis may be not valid for polycrystalline manganites where relatively strong grain boundary effects might mask weak contribution of quantum effects to low temperature resistivity. The crucial test of applicability of the theory of quantum corrections to conductivity in this case is the resistive measurements under non-zero magnetic field.

Comment on papers “Effect of Ag substitution on the transport property and magnetoresistance of LaMnO3” [J. Magn. Magn. Mater. 248 (2002) 26] and “Possible magnetic phase separation in Ru-doped La0.67Ca0.33O3” [J. Magn. Magn. Mater. 257 (2003) 195]

“…4 from Ref. [11]), which fully accords with previously observed behavior of GBinduced LT effect in polycrystalline ceramics and bicrystal epitaxial manganite films [8][9][10]. (ii) The magnetic field influences on two 'splitted' peaks of rðT; 0Þ ( Fig.…”

“…But, according to the claim of the authors, additional AF coupling is strong enough to modify drastically the rðT; 0Þ dependences [12]. The prominent feature of GB effects [8][9][10] is that LT minimum of resistivity is enhanced with x (see Fig. 3 from Ref.…”

“…In relatively good (with large size of grains) polycrystalline samples, the former peak at T max degenerates to a 'shoulder' at ToT C and only a sharp maximum of rðT; 0Þ is observed near T C [5][6][7]. Except for this, the prominent feature that distinguishes the polycrystalline from single crystalline La 1Àx A x MnO 3 is a shallow minimum of rðT; 0Þ; which occurs at a temperature T min well below T C [7][8][9]. The results presented in Ref.…”

“…The GB effects on DC conductivity of the above systems at temperatures below its Curie points are well known-see Refs. [5,6,[8][9][10]. The usual methods to eliminate (or minimize) such effects, i.e.…”

Low-Temperature Intergranular Spin Transport in $${\mathbf {La_{0.5}Ca_{0.4}Li_{0.1}MnO_3}}$$ La 0.5 Ca 0.4 Li 0.1 MnO 3 Manganite Under High Magnetic Field $$\mathbf {(1\,T\le H \le 14\,T)}$$ ( 1 T ≤ H ≤ 14 T )

“…The SPT mechanism in manganites under weak magnetic fields is studied extensively [1][2][3][4][5][6][7][8], while the SPT at high fields are found to be out of due attention [7,9]. On the other hand, the interpretation of physical mechanisms for a low-temperature minimum (LTM) of resistivity occurring in manganites still remains in question and no consensus exists on the nature of this phenomenon [10][11][12][13][14][15][16][17][18][19][20][21][22]. Thus, all studies of the LTM in manganites can be divided into three groups:…”