A-site dependent percolative thermopower and Griffiths phase in Pr(0.7−x)HoxSr0.3MnO3 (x=0.0, 0.04, 0.08, and 0.1)
TL;DR: In this article, the Griffiths temperature at which ferromagnetic metallic clusters nucleate above TC was evaluated and it was shown that Griffiths phase extends over a higher temperature range above TC with increase in the variance of ionic radii of the A site.
Abstract: We report our analysis of thermopower S(T) of Pr(0.7−x)HoxSr0.3MnO3 (x=0, 0.04, 0.08, and 0.1) compounds within a percolative framework and correlate the results with the existence of a Griffiths phase. The Griffiths temperature, at which ferromagnetic metallic clusters nucleate above TC, is evaluated. We further show that the Griffiths phase extends over a higher temperature range above TC with increase in the variance of ionic radii of the A site.
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TL;DR: In this article, the structural, magnetic and electrical properties of polycrystalline ceramics were investigated using conventional solid-state method and their structural and electrical transport properties were investigated.
Abstract: Polycrystalline Pr 0.75 Na 0.25- x K x MnO 3 ( x = 0, 0.05, 0.10, 0.15 and 0.20) ceramics were prepared using conventional solid-state method and their structural, magnetic and electrical transport properties were investigated. Magnetization versus temperature measurements showed un-substituted sample exhibited paramagnetic behavior with charge-ordered temperature, T CO around 218 K followed by antiferromagnetic behavior at transition temperature, T N ∼ 170 K. K + -substitution initially weakened CO state for x = 0.05–0.10 then successfully suppressed the CO state for x = 0.15–0.20 and inducing ferromagnetic-paramagnetic transition with Curie temperature, T C increased with x . In addition, deviation of the temperature dependence of inverse magnetic susceptibility curves from the Curie-Weiss law suggests the existence of Griffiths phase-like increased with x . Magnetization versus magnetic field curves show existence of hysteresis loops at T x = 0) and T x = 0.05–0.10), which related to metamagnetic transition occurring at critical field. Electrical resistivity measurements showed an insulating behavior for x = 0 sample while for x = 0.05–0.20 samples showed metal-insulator transition and transition temperature, T MI increased with x . The increased in T C and T MI are attributed to the increase in tolerance factor which indicates reduction in MnO 6 octahedral distortion consequently enhanced double exchange interaction.
18 citations
TL;DR: The anomalous magnetic properties of Nd0.75Sr1.25CoO4 have been investigated by means of dc magnetization, ac susceptibility and electron spin resonance measurements as discussed by the authors.
Abstract: The magnetic properties of Nd0.75Sr1.25CoO4 have been investigated by means of dc magnetization, ac susceptibility and electron spin resonance measurements. The specimen presented a deviation from the Curie–Weiss law at TG~230 K, ferromagnetic behaviour below TC~179 K, and a cusp structure at TSG~18 K in dc magnetization. The cusp was confirmed to be due to the existence of spin-glass states, and the deviation was argued to be a Griffiths singularity. Comparing the magnetic properties of the analogous layered cobalt oxides, the anomalous magnetic features of the specimen were attributed to the introduction of rare-earth ions, Nd3+.
14 citations
TL;DR: In this paper, the effect of electron beam (EB) irradiation on the structural, electrical transport and thermal properties of Pr0.8Sr0.2MnO3 manganites has been investigated.
Abstract: In this communication, the effect of electron beam (EB) irradiation on the structural, electrical transport and thermal properties of Pr0.8Sr0.2MnO3 manganites has been investigated. Rietveld refinement of XRD data reveals that all samples are single phased with orthorhombic distorted structure (Pbnm). It is observed that the orthorhombic deformation increases with EB dosage. The Mn–O–Mn bond angle is found to increase with increase in EB dosage, presumably due to strain induced by these irradiations. Analysis on the measured electrical resistivity data indicates that the small polaron hopping model is operative in the high temperature region for pristine as well as EB irradiated samples. The electrical resistivity in the entire temperature region has been successfully fitted with the phenomenological percolation model which is based on phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. The Seebeck coefficient (S) of the pristine as well as the irradiated samples exhibit positive values, indicating that holes is the dominant charge carriers. The analysis of Seebeck coefficient data confirms that the small polaron hopping mechanism governs the thermoelectric transport in the high temperature region. In addition, Seebeck coefficient data also is well fitted with the phenomenological percolation model. The behavior in thermal conductivity at the transition is ascribed to the local anharmonic distortions associated with small polarons. Specific heat measurement indicates that electron beam irradiation enhances the magnetic inhomogeneity of the system.
13 citations
TL;DR: In this paper, the magnetic properties of polycrystalline Sr 1.5 Nd 0.5 MnO 4 compound are discussed in detail, based on the magnetization, susceptibility, and isotherm.
Abstract: The Sr 1.5 Nd 0.5 MnO 4 compound was synthesized by the solid state reaction method. The X-ray diffraction (XRD) and magnetic measurement have been used to study the physical and chemical properties of the Sr 1.5 Nd 0.5 MnO 4 compound. The XRD data has been analyzed by Jana refinement technique. The refinement has revealed that the compound crystallizes in tetragonal symmetry with Pmmm space group. The magnetic properties of polycrystalline Sr 1.5 Nd 0.5 MnO 4 compound are discussed in detail, based on the magnetization, susceptibility, and isotherm. The specimen presents a ferromagnetic (FM) property with T C =245 K, and a Griffiths phase at T G =275 K which gives as the existence of ferromagnetic clusters in the paramagnetic domain. A big divergence between zero fields cooled (ZFC) and field cooled (FC) M (T)is observed at low temperature below of blocking temperature. This indicated a thermomagnetic irreversibility behavior. In addition, the zero field cold (ZFC) curve shows a clear cusp at low temperature (35 K) which is generally related to a spin glass or a cluster-glass state. The variation of magnetocaloric Δ S M as function of magnetic field ( μ 0 H ) has deduced that the compound Sr 1.5 Nd 0.5 MnO 4 obeys the mean field. According to the variation of resistivity as a function of temperature, the compound presents a semiconductor behavior. The electric transport behaviors obey to variable range hopping model. From this model, we can determinate the increase of activation energy E a when the temperature increases.
13 citations
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TL;DR: In this paper, it was shown that both electrical conduction and ferromagnetic coupling in these compounds arise from a double exchange process, and a quantitative relation was developed between electrical conductivity and the Ferromagnetic Curie temperature.
Abstract: Recently, Jonker and Van Santen have found an empirical correlation between electrical conduction and ferromagnetism in certain compounds of manganese with perovskite structure. This observed correlation is herein interpreted in terms of those principles governing the interaction of the $d$-shells of the transition metals which were enunciated in the first paper of this series. Both electrical conduction and ferromagnetic coupling in these compounds are found to arise from a double exchange process, and a quantitative relation is developed between electrical conductivity and the ferromagnetic Curie temperature.
5,097 citations
TL;DR: In this paper, a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions, including phase diagrams of manganite models, the stabilization of the charge/orbital/spin ordered half-doped correlated electronics (CE)-states, the importance of the naively small Heisenberg coupling among localized spins, the setup of accurate mean-field approximations, and the existence of a new temperature scale T∗ where clusters start forming above the Curie temperature, the presence of stripes in the system, and many others.
Abstract: The study of the manganese oxides, widely known as manganites, that exhibit the “colossal” magnetoresistance effect is among the main areas of research within the area of strongly correlated electrons. After considerable theoretical effort in recent years, mainly guided by computational and mean-field studies of realistic models, considerable progress has been achieved in understanding the curious properties of these compounds. These recent studies suggest that the ground states of manganite models tend to be intrinsically inhomogeneous due to the presence of strong tendencies toward phase separation, typically involving ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating domains. Calculations of the resistivity versus temperature using mixed states lead to a good agreement with experiments. The mixed-phase tendencies have two origins: (i) electronic phase separation between phases with different densities that lead to nanometer scale coexisting clusters, and (ii) disorder-induced phase separation with percolative characteristics between equal-density phases, driven by disorder near first-order metal–insulator transitions. The coexisting clusters in the latter can be as large as a micrometer in size. It is argued that a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions. The main phenomenology of mixed-phase states appears to be independent of the fine details of the model employed, since the microscopic origin of the competing phases does not influence the results at the phenomenological level. However, it is quite important to clarify the electronic properties of the various manganite phases based on microscopic Hamiltonians, including strong electron–phonon Jahn–Teller and/or Coulomb interactions. Thus, several issues are discussed here from the microscopic viewpoint as well, including the phase diagrams of manganite models, the stabilization of the charge/orbital/spin ordered half-doped correlated electronics (CE)-states, the importance of the naively small Heisenberg coupling among localized spins, the setup of accurate mean-field approximations, the existence of a new temperature scale T∗ where clusters start forming above the Curie temperature, the presence of stripes in the system, and many others. However, much work remains to be carried out, and a list of open questions is included here. It is also argued that the mixed-phase phenomenology of manganites may appear in a large variety of compounds as well, including ruthenates, diluted magnetic semiconductors, and others. It is concluded that manganites reveal such a wide variety of interesting physical phenomena that their detailed study is quite important for progress in the field of correlated electrons.
2,927 citations
TL;DR: It is proposed that in addition to double-exchange physics a strong electron-phonon interaction arising from the Jahn-Teller splitting of the outer Mn $d$ level plays a crucial role.
Abstract: The ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system with $02\ensuremath{\lesssim}x\ensuremath{\lesssim}04$ has traditionally been modeled with a ``double-exchange'' Hamiltonian in which it is assumed that the only relevant physics is the tendency of carrier hopping to line up neighboring spins We present a solution of the double-exchange model, show it is incompatible with many aspects of the data, and propose that in addition to double-exchange physics a strong electron-phonon interaction arising from the Jahn-Teller splitting of the outer Mn $d$ level plays a crucial role
2,302 citations
TL;DR: The results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes inThe Mn-O-Mm bond angle.
Abstract: A detailed study of doped LaMn${\mathrm{O}}_{3}$ with fixed carrier concentration reveals a direct relationship between the Curie temperature ${T}_{c}$ and the average ionic radius of the La site $〈{r}_{A}〉$, which is varied by substituting different rare earth ions for La. With decreasing $〈{r}_{A}〉$, magnetic order and significant magnetoresistance occur at lower temperatures with increasing thermal hysteresis, and the magnitude of the magnetoresistance increases dramatically. These results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes in the Mn-O-Mn bond angle.
1,654 citations
TL;DR: In this paper, it was shown that in a class of randomly diluted Ising ferromagnets the magnetization fails to be an analytic function of the field at a range of temperatures above that at which spontaneous magnetization first appears.
Abstract: It is shown that in a class of randomly diluted Ising ferromagnets the magnetization fails to be an analytic function of the field $H$ at $H=0$ for a range of temperatures above that at which spontaneous magnetization first appears.
1,206 citations