Electronic phase separation in the rare-earth manganates(La1−xLnx)0.7Ca0.3MnO3(Ln = Nd,Gd and Y)

Abstract

Electron transport and magnetic properties of three series of manganates of theformula (La1−xLnx)0.7Ca0.3MnO3 withLn = Nd,Gd and Y, wherein only the average A-site cation radius ⟨rA ⟩ andassociated disorder vary, without affecting the Mn4+/Mn3+ratio, have been investigated in an effort to understand the natureof phase separation. All three series of manganates show saturationmagnetization characteristic of ferromagnetism, with the ferromagneticTc decreasing withincreasing x up toa critical value of x,xc (xc = 0.6,0.3, 0.2 respectively for Nd, Gd, Y). For x > xc,the magnetic moments are considerably smaller, showing a small increase aroundTM, the value ofTM decreasing slightlywith increase in x or decreasein ⟨rA ⟩.The ferromagnetic compositions (x ≤ xc)show insulator–metal transitions, while the compositions with x > xcare insulating. The magnetic and electrical resistivity behaviour of these manganatesis consistent with the occurrence of phase separation in the compositions aroundxc,corresponding to a critical average radius of the A-site cation, ⟨rAc ⟩, of 1.18 Å.Both Tcand TIMincrease linearly when ⟨rA ⟩ > ⟨rAc ⟩ or x ≤ xc,as expected of a homogeneous ferromagnetic phase. BothTc andTMdecrease linearly with the A-site cation size disorder as measured by the variance σ2. Thus, an increasein σ2favours the insulating AFM state. Percolative conduction is observed in thecompositions with ⟨rA ⟩ > ⟨rAc ⟩.Electron transport properties in the insulating regime for x > xcconform to the variable-range hopping mechanism. More interestingly, when x > xc,the real part of dielectric constant (ε′) reaches a high value(104 –106)at ordinary temperatures dropping to a very small (∼ 500)value below a certain temperature, the value of which decreases with decreasingfrequency.