Temperature-dependent vacuum tunneling spectroscopy of rare-earth manganates showing colossal magnetoresistance and charge ordering

Abstract

We report here the results of vacuum tunneling spectroscopy of rare-earth manganates which show colossal magnetoresistance (CMR) and charge ordering (CO). Three samples with different hole concentrations and average A site cation radii have been studied. A charge-ordering gap (∆ CO ) opens up in the density of states (DOS) near E F , below the charge-ordering transition. A gap comparable to the transport gap opens up in the paramagnetic state also. There is an absence of gap only in the ferromagnetic metallic state. We have measured the temperature dependence of these quantities.Since the recent discovery of colossal magnetoresistance (CMR) in hole-doped rare-earth manganates, their novel transport properties have been investigated extensively [1,2]. These rare-earth manganates with general formula RE (1−x) A x MnO 3 (RE = La, Nd, Pr, Sm, etc. and A = Ca, Sr, Ba, Pb, etc.) show a fine interplay of magnetic, Coulomb and lattice interactions which manifest themselves through such phenomena as insulator-metal transitions, CMR and charge ordering (CO). Mn ions in these compounds have mixed valency with the Mn 4+ /Mn 3+ ratio ≈ x/(1 − x), depending on the exact value of the oxygen stoichiometry. These oxides belong to the ABO 3 perovskite class of structure where the A site is occupied by the Re or M ions and the B site by the Mn ions. For certain values of x ≈ 0.2-0.3 these materials show CMR close to the ferromagnetic transition temperature, T C , when fields of a few tesla are applied. The stability of the ferromagnetic metallic state depends on the bandwidth W which can be controlled through external pressure [3] or alternatively through the average radius of an A-site cation ( r A ) [4]. As r A becomes smaller the lattice is distorted leading to a smaller transfer matrix and a smaller W. This in turn reduces the T C . When r A is sufficiently small the ferromagnetic T C vanishes, as in solids with RE = Nd, Pr and A = Ca [4]. Lattice effects play an important role in both carrier transport and magnetism in these materials by con- * trolling the bandwidth and the Jahn-Teller (J-T) distortion around the Mn 3+ ions, which results in strong electron-lattice interaction.For materials with smaller A-site cations ( r A ≤ 1.25 Å) near half-filling (x ≈ 1/2), the Coulomb interaction can overcome the kinetic energy of the electrons leading to arrangement of the Mn 3+ and Mn 4+ in alternate lattice sites. This CO transition is associated with large lattice distortions and leads to localization of the carrier and the material becomes insulating below the transition temperature, T CO [5]. For materials like Nd 0.5 Sr 0.5 MnO 3 ( r A ≈ 1.22 Å) the CO transition is associated with the onset of antiferromagnetic ordering i.e. T CO ≈ T N , the Néel temperature [5]. In materials with even smaller r A (≤ 1.18 Å) the CO transition is more gradual and the AFM ordering may take place at a lower temperature [6,7]. Lattice distortion (through the J-T effect) plays an important role in ushering in the CO insulating state. The orthorhombic...