Abstract
We report results of magnetization and ac susceptibility measurements down to very low fields on a single crystal of the perovskite manganite, La$_{0.82}$Ca$_{0.18}$MnO$_3$. This composition falls in the intriguing ferromagnetic insulator region of the manganite phase diagram. In contrast to earlier beliefs, our investigations reveal that the system is magnetically (and in every other sense) single-phase with a ferromagnetic ordering temperature of $\sim$ 170 K. However, this ferromagnetic state is magnetically frustrated, and the system exhibits pronounced glassy dynamics below 90 K. Based on measured dynamical properties, we propose that this quasi-long-ranged ferromagnetic phase, and associated superspin glass behavior, is the true magnetic state of the system, rather than being a macroscopic mixture of ferromagnetic and antiferromagnetic phases as often suggested. Our results provide an understanding of the quantum phase transition from an antiferromagnetic insulator to a ferromagnetic metal via this ferromagnetic insulating state as a function of $x$ in La$_{1-x}$Ca$_x$MnO$_3$, in terms of the possible formation of magnetic polarons.
Highlights
Hole-doped perovskite manganites of the general formula A1−xBxMnO3 (A 1⁄4 trivalent lanthanide; B 1⁄4 divalent alkali metal) with a low doping, generally 0.05 < x < 0.22 for A 1⁄4 La and B 1⁄4 Ca, are of fundamental interest because they constitute the few examples of ferromagnetic insulators, unlike the ones with higher x or a larger bandwidth, which are ferromagnetic and metallic, or with a lower x, which are insulating and antiferromagnetic [1]
Based on measured dynamical properties, we propose that this quasi-long-ranged ferromagnetic phase, and the associated superspin glass behavior, is the true magnetic state of the system, rather than being a macroscopic mixture of ferromagnetic and antiferromagnetic phases, as often suggested
Our results suggest that the magnetic and electrical properties with hole doping in low-bandwidth manganites evolve from an antiferromagnetic insulator (AFI) at the lowest doping levels via a frustrated ferromagnetic insulator for intermediate doping to the ferromagnetic metallic (FMM) at higher doping
Summary
Hole-doped perovskite manganites of the general formula A1−xBxMnO3 (A 1⁄4 trivalent lanthanide; B 1⁄4 divalent alkali metal) with a low doping, generally 0.05 < x < 0.22 for A 1⁄4 La and B 1⁄4 Ca, are of fundamental interest because they constitute the few examples of ferromagnetic insulators, unlike the ones with higher x or a larger bandwidth, which are ferromagnetic and metallic, or with a lower x, which are insulating and antiferromagnetic [1]. [9,10] observed a nondiverging magnetic correlation length and signatures of short-range magnetic polarons using neutron-scattering experiments, while a much more recent work [11] suggested an ideal three-dimensional Heisenberg ferromagnetic ground state, based on the values of the critical exponents that they obtain for the FMI composition of LCMO Both theory and experiments suggest the formation of local lattice distortions or lattice polarons in the ordered state leading to a nanoscale inhomogeneity that is starkly different from the chemical or macroscopic electronic phase separation [10,12,13,14,15,16]. We discuss the microscopic nature and origin of the ferromagnetic insulating phase, as well as the transition to ferromagnetic metal for larger hole doping
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