Commenton "Giant magnetoresistance of the La_1–xAg_xMnO₃ polycrystalline inhomogeneous granular system"[Appl. Phys. Lett. 77, 723 (2000)]

V. L. Joseph Joly, P. A. Joy,^a)and S. K. Date

Physical and Materials Chemistry Division, National Chemical LaboratoryPune 411008, India

Received: 28 November 2000; accepted: 16 April 2001

In a recent letter,¹ Tang et al. have reported thesynthesis and giant magnetoresistance properties of the substituted manganate, La_1–xAg_xMnO₃.It has been shown that single perovskite phase is obtainedfor x0.25 and two phases containing a magnetic perovskitephase and a nonmagnetic phase which is Ag metal areobserved for x>0.25. This comment is to show that thereported Ag substituted compositions with x<0.25 are self-doped manganates La_1–xMnO_3–.The higher T_cs observed for these compositions are due tooxidation of Mn^{3 +} to Mn^{4 +} because of the presence ofAg during synthesis, as reported for thin film samples ofLa_1–xMnO_3–.²^,³ Even for x = 0.2, we have observed the presence ofAg metal in the powder x-ray diffraction (XRD) pattern anda self-doped composition La_0.8MnO_3– shows enhanced T_c after annealing witha smaller amount of Ag₂O at low temperatures.

Powder XRDpatterns of a self-doped composition La_0.8MnO_3–(LMO) and another composition withx = 0.2 of Ag (LAMO), synthesized under identical conditions as reportedby Tang et al.,¹ are compared in Fig. 1. Identical XRDpatterns corresponding to a rhombohedral structure are ob-tained for bothLMO and LAMO, with a slightly smaller lattice parameter forLAMO. The most intense reflection of Ag is expected at38.1° using Cu K alpha radiation and a weak reflection from the(113) set of planes from the rhombohedral lattice of La_1–xMnO_3–is also observed close to the same 2 theta value. Therefore,it is difficult to detect the presence of Ag inthe low-Ag containing compositions reported in Ref. 1, as thepeak at 2 theta = 38.1° may be indexed to the reflection fromthe rhombohedral lattice. The inset of Fig. 1 shows theexpanded XRD patterns of LMO and LAMO in this 2 theta range. The relative intensities of all other reflections are comparablefor both compounds and therefore the enhanced intensity of thepeak at 38.1° for LAMO is due to the overlappingreflection from Ag. Similarly, a very weak peak corresponding tothe second most intense reflection of Ag is also observedat 44.3° in the XRD pattern of LAMO.

Figure 1.

A pelletof La_0.8MnO_3– was annealed at 850 °C after adding Ag₂O correspondingto x = 0.1 (Ag–LMO). Zero-field cooled (ZFC) magnetization curves of LMO andLAMO are shown in Fig. 2 along with the magnetizationcurve of Ag–LMO. It may be seen that Ag–LMO alsoshows T_c almost identical to that of LAMO. Earlier studieson thin film samples of La_1–xMnO_3– have indicated that T_cis considerably increased after annealing with Ag (Ref. 2) orin oxygen³ and this is due to the decrease inthe Mn^{3 +}/Mn^{4 +} ratio due to incorporation of additional oxygen inthe structure. A similar situation happens in the present casealso. Ag₂O releases oxygen which oxygenates La_1–xMnO_3– thereby increasing T_cdue to increase in the Mn^{4 +} content. The slightly lowervalue of the lattice parameter obtained for LAMO is consistentwith the increase in the Mn^{4 +} content as reported.³ Therefore,the conclusion that Ag in LaMnO₃ forms a single perovskitephase is questionable.

Figure 2.

REFERENCES

FIGURES

Full figure (10 kB)

Fig. 1. PowderXRD patterns of LMO and LAMO. Inset: the overlapping reflectionsfrom Ag and LMO. First citation in article

Full figure (6 kB)

Fig. 2. ZFC magnetization (H = 50 Oe) curves of LMO, LAMO,and Ag–LMO. First citation in article

FOOTNOTES

^aAuthor to whom correspondence should be addressed; electronicmail: joy@dalton.ncl.res.in

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Comment on "Giant magnetoresistance of the La1–xAgxMnO3 polycrystalline inhomogeneous granular system" [Appl. Phys. Lett. 77, 723 (2000)]

V. L. Joseph Joly, P. A. Joy,a) and S. K. Date