Reentrant spin glass transition in La0.96−yNdyK0.04MnO3: origin and effects on the colossal magnetoresistivity

– Magnetic, electric and structural properties of La0.96−yNdyK0.04MnO3+δ with 0≤ y ≤0.4 have been studied experimentally. A disordered magnetic state is formed as La is substituted by Nd, reflecting the competition between ferromagnetic (FM) double exchange and antiferromagnetic superexchange interactions. Key structural parameters are identified and correlated with changes in magnetic and electric properties. By application of a large magnetic field, spin disorder scattering is removed, creating a new magnetoresistance peak at a temperature lower than the first near-Tc peak. Time dependent zero-field-cooled magnetisation measurements have been performed for y=0.4 around this temperature. A reentrant spin glass (RSG) transition is evidenced, with low field ageing properties in both the RSG and FM phases, similar to those observed in archetypal spin glass materials. Introduction. – The doped manganite compounds R1−yAyMnO3 (R=rare earth; A=alkaline earth) have been intensively investigated concerning their colossal magnetoresistance (CMR) properties [1]. Simultaneous ferromagnetic (FM) and metallic properties in these manganese oxides are attributed to double exchange [2] (DE) interaction between pairs of Mn (t32ge 1 g) and Mn (t32g) ions. Substitution of cations with different sizes at the rare earth sites results in lattice distortions that may influence the ferromagnetic DE and the antiferromagnetic (AFM) superexchange interactions differently, for instance increasing the importance of the AFM interactions with respect to the FM interactions. This can create various forms of magnetic disorder, thereby also affecting the transfer integral for itinerant eg electrons which depends on the relative spin orientation of the localized t2g moments; tij ∝ cos(θij/2), where θij is the angle between moments i and j [3]. In systems with ferromagnetic DE interaction dominating, the AFM interactions may still cause magnetic disorder and frustration and the system is in this case expected to show a low temperature reentrant spin glass (RSG) phase [4, 5]. When such a system is cooled from a high temperature, it first exhibits a transition from a paramagnetic (PM) to a ferromagnetic (FM) phase. If the cooling proceeds, a transition to a RSG phase occurs. Indications of such a transition have already been reported for CMR compounds, but without presenting convincing experimental evidences for its existence. Usually, the authors consider that a low temperature cusp in the zero-field-cooled