LaMn3+O3 is an AFM insulator with TN = 140 K but transforms into a FM insulator with Co2+ or Ni2+ substituted at the Mn-site in La2CoMnO6 and La2NiMnO6 (1), accompanied by a change of Mn valence from 3+ to 4+. However, LaMn0.96Mo0.04O3 is a metallic ferromagnet at low temperatures (2). The FM Curie temperature TC decreased only by 7 K upon 6% Mo6+: d0 substitution in La0.67Sr0.33Mn1-xMoxO3 (3). It was stated that Mo6+ doping induced Mn2+ (d5) which interacted with Mn3+(d4) through Zener’s double exchange giving rise to FM and metallic behavior. But, studies on Mo-doped manganites are yet rare. Here, we report the physical characterization of LaMn0.94Mo0.06O3 prepared by solid-state route. Fig.1(a) shows the temperature dependence of magnetization (M) and inverse susceptibility (Χ-1) measured under H = 1kOe. Fig. 1(b) shows the dc resistivity (ρ) and thermopower (S). Tc obtained from the inflection point of dM/dT is 237 K whereas the Χ-1 deviates from the Curie-Weiss behavior below 260 K. This is possibly due to Griffiths phase, i.e. nano-sizes FM clusters preformed above TC in the PM matrix. The ρ shows an IM transition with a peak at 235 K. However, S shows a peak around 258 K, which is about 20 K above the TC, suggesting that S is sensitive to nucleation of FM nano domains or short-range FM correlation. Fig. 1(c) shows ESR signal as a derivative microwave absorption as a function of dc magnetic field at selected temperatures near FM transition. The spectrum at 300 K is typical of ESR. While the spectrum at 300 K can be fitted to a single Lorentzian curve, the spectrum at 250 K reveals another low-intensity peak at a lower field (~1900 Oe) in addition to the ESR signal around 3200 Oe. The intensity of the ESR-signal decreases dramatically at 245 K but the intensity of the low-field peak increases. At 230 K, we do not see the ESR signal. These results indicate that the low field peak is due to FMR, but this feature start appearing above TC confirms that FM metallic nano domains are preformed above TC in the PM state and increase in size with lowering temperature and percolate leading to long-range FM and metallicity.
(1) G. Blasse, Ferromagnetic interactions in non-metallic perovskites, J. Phys. Chem. 26, 1969 (1965);S. Vasala and M. Karppinen, A2B’B’’O6 perovskites: A review, Prog. Soild State Chem. 42, 1 (2015).
(2) W. J. Lu et al., Induced ferromagnetism in Mo-substituted LaMnO3, Phys. Rev. B, 73, 174425 (2006).
(3) L. Chen et al., Critical behavior of Mo-doping La0.67Sr0.33M1-xMoxO3 perovskite, Physica B, 404,1879 (2009).
R. M. thanks the Ministry of Education, Singapore for supporting this work (Grant no. R144-000-442-114)
(a) T-dependence of M and X-1; (b) T-dependence of ρ and S; 1(c) ESR at selected T