Modulated quasi-two-dimensional antiferromagnetic structures in La1 − y Nd y MnO3 + δ manganites

Abstract

The structural, electronic, and magnetic phase transitions induced by the isovalent substitution of the rare-earth Nd3+ ion for the La3+ ion with a larger radius have been investigated in the system of self-doped manganites La1 − yNdyMnO3 + δ (0 ≤ y ≤ 1; δ ∼ 0.1). For the average radius of the ions in A-sites of the lattice 〈rA〉 〈 1.19 A (y 〉 0.5), the phenomena revealed in the manganites are as follows: the ordering of Mn eg orbitals, the transition from the pseudocubic O* phase to the orthorhombic O’ phase, the opening of the dielectric Jahn-Teller gap, the frustration of the collinear ferromagnetic (FM) state, and the transition from the lowtemperature canted FM to canted antiferromagnetic (AFM) state of Mn spins. It is assumed that, in samples with neodymium concentrations y = 0.9 and 1.0 (〈rA〉 ≈ 1.16 A) at temperatures T < 12 K, there coexist A- and E-type modulated AFM states similar to the sinusoidal and helical structures of Mn spins, which were previously studied in RMnO3 multiferroics. The magnetic T-H phase diagrams of these samples are characteristic of quasi-two-dimensional antiferromagnets with a very low (zero) magnetic anisotropy in the ab planes. Under these conditions, the phase transition from the A-type AFM phase to the spin-flop state occurs in a relatively weak magnetic field. The AFM ordering of the Nd magnetic moments with a critical phase transition temperature TNd ≌ 6 K is induced in magnetic fields with a strength H ≥ 3.5 kOe. For the NdMnO3 + δ manganite in a magnetic field H = 10.7 kOe, the curves M(T) are characterized by additional very narrow peaks near temperatures T1 ≌ 4.5 K and T2 ≌ 5 K. The additional features revealed for the first time in the magnetization near T = 0 are assumed to be caused by the quantization of the spectrum of free holes in the ab planes by a strong magnetic field.