•A charge transfer from Ce ions to Mn ions induced by Ca substitution.•Abnormal properties on magnetism and electricity in La0.3Ce0.2CaxSr0.5-xMnO3.•Ca substitution changes both the chemical pressure and Ce ionic valence.•Existing a threshold value of Ca-substitution on the physical properties.The perovskite manganese oxides La0.3Ce0.2CaxSr0.5-xMnO3 (0 ≤ x ≤ 0.25) have been synthesized by a solid-state reaction, and their transport and magnetic properties have been systemically studied. It is found that the temperature dependent resistivity shows a maximum at Tmax below Curie temperature Tc with temperature decreasing, which is correlated to the Ce3+ spin-dependent scattering. Moreover, Tmax is almost unchanged for x ≤ 0.15, but decreases quickly and is closing on the temperature of Tc while x > 0.15, which indicates the increase of Ce ionic valence. Meanwhile, the saturation magnetic moment Ms decreases with x increasing to 0.15, and increases anomaly when x > 0.15. XANES results reveal that when x ≤ 0.15, the valence of Mn ion is almost unchanged, but when x > 0.15, it obviously shifts from tetravalence to trivalence with Ca substitution. Meanwhile, XPS spectra results indicate the similar trend of Ce valence changing from trivalence to tetravalence. It is suggested that Ca substitution changes not only the chemical pressure, but also the valence of Ce ion. There is a threshold value for the chemical pressure corresponding to Ca- substitution content at around x = 0.15 (or tv = 1.2395 Å), which is the starting point of charge transfer appearance in Ca-substituting La0.3Ce0.2Sr0.5MnO3 compound. The transport and magnetic properties of the half-doped manganties La0.3Ce0.2CaxSr0.5-xMnO3 are dominated by the competition/cooperation of Ce3+ spin-dependent scattering, interaction between Mn ions controlled by A-site ion-size, and the charge transfer from Ce ions to Mn ions.

•Re2NiMnO6 epitaxial films on (001) LAO were grown by a simple PAD method.•A FM transition exists in all the films.•FM transition temperature Tc decreases with the A-site ionic radius decreasing.•The Tc of the film is lower than that of the corresponding bulk.•The magnetic properties of the film are sensitive to the strain, A-site ionic radius.Re2NiMnO6 (Re = La, Pr, Nd, Sm, Y) epitaxial thin films grown on (001) LaAlO3 substrates were obtained via a simple polymer assisted deposition method. The XRD θ/2θ scans, ω scans, and φ scans indicate that the films are single-phase epitaxial films. It is found that there is a ferromagnetic (FM) transition with the temperature decreasing from room temperature for all the films. The Raman mode positions shift toward the low frequency with the substitution of La by a smaller rare earth element. The FM transition temperature Tc decreases as the A-site ionic radius decreasing. Moreover, all the FM transition temperatures (Tc) of the Re2NiMnO6 films are lower than those corresponding values of the bulks, indicating the ferromagnetic interaction weakening. It is suggested that the ferromagnetic interaction is affected by the strain existing in the film through affecting the B(B′)O6 octahedra and the cation disorder related to the ionic radius of Re, which can be used as an effective way to improve the magnetic properties of the film and obtain the higher Tc functional materials.

•LaCoO3 epitaxial films with different orientation were grown by a simple method.•Distortion of CoO6 octahedron is different in (1 0 0), (1 1 0) and (1 1 1) LCO films.•JT tetragonal distortion of CoO6 plays a crucial role in FM of LCO epitaxial films.•Higher JT tetragonal distortion of CoO6 corresponds to the higher FC magnetization.LaCoO3 (LCO) epitaxial nano-thin films with different thickness (∼30 nm, 60 nm, and 80 nm) were grown on (1 0 0), (1 1 0), and (1 1 1) oriented LaAlO3 (LAO) substrates, respectively, by a simple polymer assisted deposition method. X-ray diffraction analyses confirm that all the LCO films are epitaxially grown in accordance with the orientation of LAO substrates, with biaxial compressive strain in the ab plane. Due to the different strain directions, the Jahn–Teller (JT)-like tetragonal distortion of CoO6 octahedron is dominant in LCO film on (1 0 0) LAO, but the rotation of CoO6 octahedron is dominant in LCO film on (1 1 1) LAO, while it is intervenient between them in LCO film on (1 1 0) LAO. For the same oriented films, as a result of the lattice relaxation effect, the strain is slightly relaxed as the film thickness increasing. An obvious ferromagnetic (FM) transition at TC ∼ 85 K is observed in all the (1 0 0) oriented LCO films, but a very weak FM signal in (1 1 0) oriented LCO films and a nonmagnet-like characteristic in (1 1 1) oriented LCO films below ∼85 K. Combined with the structural analyses, it is found that the change of the FM signal is closely related to the stain-induced JT tetragonal distortion of CoO6 octahedron which can stabilize Co3+ ions in the intermediate spin (IS) state. The higher degree of the JT tetragonal distortion of CoO6 octahedron corresponds to the higher field-cooled (FC) magnetization. However, the rotation mode of CoO6 octahedron will weaken the degree of JT tetragonal distortion and reduce the population of IS Co3+.

Hall coefficient RH has been studied systematically in normal state MgC1−xBxNi3 (x = 0.05, 0.15, 0.20) and Mg1−yZnyCNi3 (y = 0, 0.75, 1.00). It is found that RH changes from negative to positive while increasing B content x from 0.15 to 0.20. For x = 0.20, the sample still shows a superconducting transition at TC ∼ 4.5 K. RH is negative for MgCNi3 and positive for ZnCNi3. The absolute value |RH| for MgCNi3 decreases with the temperature increasing above ∼150 K, whereas |RH| for ZnCNi3 increases with the temperature increasing from 4 K to 300 K. For Mg0.25Zn0.75CNi3, RH changes from positive to negative when decreasing temperature to ∼205 K. These results confirm the two-band model, which indicates that electronlike and holelike carriers are coexisting in MgCNi3 system.Highlights► Hall coefficient RH has been studied in normal state B-doped and Zn-doped MgCNi3. ► It is found that RH changes from negative to positive with B content increasing. ► RH is negative for MgCNi3 and positive for ZnCNi3. ► |RH| for MgCNi3 and ZnCNi3 shows an opposite trend with temperature above ∼150 K. ► The results confirm the coexistence of electronlike and holelike carriers.

Completely isotactic polyacrylonitrile (meso/meso triad >99%) has been synthesized successfully by radiation-induced inclusion polymerization of acrylonitrile in urea canals. The long-lived nature of the growing radical species and the linear dependence of molecular weight on conversion were observed. The molecular weight distribution of product polymers was relatively narrow (<1.5). The results indicate that the dual control of the molecular weight and the tacticity for inclusion polymerization of acrylonitrile in urea canals is achieved. In addition, the effect of experimental factors of inclusion polymerization on the isotacticity of product polymers was investigated. It is found that to ensure completely isotactic polyacrylonitrile, the following conditions should be met simultaneously: (1) acrylonitrile monomers are included in urea canals totally before γ-ray irradiation, (2) the temperature at the chain propagation step is lower than −90 °C, and (3) the heat of polymerization is removed effectively.

•LaCoO3 epitaxial thin films were grown by a simple polymer assisted deposition method.•All the LaCoO3 epitaxial films are found to be ferromagnetic below TC ≈ 85 K.•Ferromagnetism is enhanced with decreasing thickness of LaCoO3 film.•There is no relevant report on preparing LCO epitaxial film by the simple PAD method.LaCoO3 epitaxial films with different thicknesses (~ 20, 50 and 80 nm) were grown on (001) SrTiO3 substrates by a simple polymer assisted deposition method. X-ray diffraction analyses including θ/2θ symmetric scan, ω-scan and in-plane φ-scan indicate that single-phase (001) oriented LaCoO3 films with a pseudotetragonal structure were grown on (001) SrTiO3 substrates successfully, with a biaxial tensile strain from + 2.42% to + 2.60% and tetragonal distortion from 1.47% to 1.63%. Due to the lattice relaxation effect in epitaxial thin film, the biaxial tensile strain is slightly relaxed when the thickness of the LaCoO3 film increases, resulting in an increase of the c-axis constant in contrast to a decrease of the in-plane constants of the film. It is different from LaCoO3 bulk with a nonmagnetic ground state that all the epitaxial films exhibit a ferromagnetic transition at TC ~ 85 K. Combining with the structural and magnetic analyses, it is shown that the strain-induced ferromagnetism in LaCoO3 epitaxial films, corresponding to the higher spin states, origins from the decrease of the energy difference between eg and t2g levels, which is caused by an increase of the unit-cell volume and suppression of the CoO6 octahedral rotations. In addition, the change of FC curve with the thickness of the film reveals that the ferromagnetism is enhanced by the thickness decrease of LaCoO3 film due to the increase of the biaxial tensile strain.

Optical spectra are very sensitive to investigate strong charge correlations in transition-metal oxides. Recent studies on nanosized half-doped manganites frequently reported that the robust charge ordering present in the bulk is strongly weakened or suppressed by reducing the sample size to nanoscale; however, the origin of the novel phenomenon is not clearly understood until now. Here, we study this nanosize effect through infrared spectra on Nd0.5Ca0.5MnO3 nanoparticles with different particle sizes. Optical phonon modes demonstrate that the cooperative Jahn–Teller distortion associated with charge ordering is not only clearly observed in the large nanoparticles with a long-range charge-ordered transition but also visible in the small nanoparticles where such the transition is completely suppressed. Lattice distortion is interestingly found to be weakened at low temperatures but enhanced at the high-temperature paramagnetic regime by the size reduction, which is responsible for the change in magnetization with particle size. Detailed analysis on the optical density further reveals that size-induced suppression of the charge-ordered transition does not mean a close of the charge gap, which still persists with just a slight decrease in the energy and exhibits no obvious change in the dependence on the temperature. Localization of charge carriers increases with decreasing particle size, evidenced by the decrease in the effective number of carriers. Taken together, our findings give good evidence that a short-range charge-ordered state exists in the half-doped manganite nanoparticles when the long-range one is destroyed by the size reduction. More importantly, these features can be well ascribed to originate from the surface disorder effects. Our optical study provides deep insight into the nanosize effects on charge ordering in the transition-metal oxides.

The magnetic properties of ∼40 nm Nd0.5Ca0.5MnO3 and Sm0.5Sr0.5MnO3 nanoparticles are investigated by magnetometry and electronic spin resonance (ESR) spectroscopy. It is found that although their bulk counterparts have quite different magnetic properties at low temperatures, both the nanoparticles exhibit very similar magnetic behaviors, where the charge ordered transitions disappear and weak ferromagnetism emerges below about 100 K. A detailed analysis on the magnetic susceptibilities and the ESR linewidths reveals that for the two compounds the size reduction weakens both the ferromagnetic and antiferromagnetic interactions, and converts the long-range charge orderings to short-range ones. Moreover, the strength of the charge ordered correlations is observed to be not much affected by the size reduction. Based on the present results and the previous studies on various nanosized half-doped manganites, the magnetic phase diagram of the half-doped manganites with the particle sizes of ∼25–40 nm is established. We find that this diagram is very similar to those for the bulk near half-doped manganites with large quenched disorder, which allows us to propose that the reported exotic phenomena in the nanosized half-doped manganites should be mainly ascribed to surface disorder effect. These results may provide a deeper insight into the role of size reduction on the physics of half-doped manganites.

Recent studies on nanosized near half-doped manganites reported that the charge-ordered antiferromagnetic states present in their bulk counterparts are strongly suppressed and that ferromagnetic behaviors appear at low temperatures as the particle size is reduced to nanometer scale. In this paper, the effects of size reduction on the magnetic properties of Sm0.5Sr0.5MnO3, which shows a sharp first-order ferromagnetic transition and an inhomogeneous phase dominated by a charge-ordered antiferromagnetic state above TC in the bulk form, have been investigated. The Sm0.5Sr0.5MnO3 nanoparticles of ∼40 nm were prepared by a sol−gel method, and their magnetic properties were studied by magnetometry. It is found that similar suppression of the charge-ordered antiferromagnetic state by size reduction is also present in this system. Most interestingly, two novel phenomena associated with this suppression are observed in the nanoparticles. One is that the inhomogeneous phase above TC is dominated by the ferromagnetic interactions in place of the antiferromagnetic ones and can be well described as a Griffiths phase. The other is that the nature of the ferromagnetic transition changes from first-order to second-order. These results may provide a deeper insight into the physics of the effects of size reduction on the half-doped manganites.

The particle size effects on the charge and spin correlations in half-doped manganite Nd0.5Ca0.5MnO3, which exhibits a charge-ordered (CO) transition at 250 K in the bulk counterpart, have been investigated by magnetometry and electron spin resonance (ESR). Magnetic measurements show that reducing the particle size weakens the long-range CO transition, which completely disappears when the particle size is reduced down to 40 nm. Meanwhile, a weak ferromagnetic (FM) behavior appears at low temperatures and is gradually enhanced with the decrease of the particle size. The ESR intensities of the nanoparticles reproduce well these features. However, the temperature dependences of the ESR g-factor and line width exhibit the typical characteristics of the CO states in all the nanoparticles, even in the 40 nm sample, which suggests that, even though the long-range CO transition is completely suppressed by the size reduction, the CO state is still present in the short-range ordering form. Moreover, it is found that the onset temperatures of the CO states in all the nanoparticles are almost the same as that of the bulk, which strongly indicates that the strength of the CO correlations in this compound is not influenced much by the particle size. A detailed analysis on the magnetic susceptibilities and the ESR line width further reveals that both the antiferromagnetic (AFM) and the FM spin interactions are weakened by the size reduction, which suggests that the enhanced FM behavior in the nanoparticles is not due to the enhancement of the double-exchange FM interactions. We propose that, although the FM interactions are weakened, they gradually dominate over the AFM ones at low temperatures with the decrease of the particle size due to the more significant weakening of the latter by the size reduction, which, hence, gives rise to the development of the FM behaviors in the nanoparticles.

Perovskite manganite La1−xCaxMnO3 (x=1/3x=1/3, 1/2 and 2/3) nanoparticles with the average particle size of about 20 nm have been synthesized by a facile nonaqueous sol–gel method using methanol as a solvent and characterized by X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometer. Magnetic measurements reveal that although their bulk counterparts have quite different magnetic ground states, the three-nanosized samples exhibit similar ferromagnetic behaviors below about 270 K. This result implies that with the particle size reduced to nanoscale, the ferromagnetism for x=1/3x=1/3 is weaken, while it is enhanced, accompanied by the suppression of the charge ordering, for x=1/2x=1/2 and 2/3. Moreover, the exchange bias phenomena are observed in the two latter nanoparticles, which is of special interest for potential applications.

The structure, magnetic and electrical transport properties of La0.5Sr0.5MnO3 annealed in different atmosphere have been investigated. No evident change of structural symmetry and the Curie temperature is observed for the samples. The resistivity at zero magnetic field of the samples annealed in air and nitrogen exhibits a metal–insulator transition, while no metal–insulator transition is observed for the sample annealed in oxygen, and for which the resistivity decreases monotonously with increasing temperature. Surprisingly, when an external magnetic field is applied, a metal–insulator transition appears for the sample annealed in oxygen. It is suggested that the annealing atmosphere affects the competition between FM and AFM phases due to the change of Mn4+/Mn3+ ratio and the oxygen/cation vacancies, and has a great influence on the electrical transport properties of La0.5Sr0.5MnO3.

The noncovalent functionalization of boron nitride nanotubes (BNNTs) with perylene-derived molecules has been reported experimentally [Wang et al. J. Am. Chem. Soc. 2008, 130, 8144]. Here we study the structural and electronic properties for the perylene-derivative functionalized BNNTs using first-principles calculations. Our calculations highlight the electronic structure modifications of BNNT through the noncovalent functionalization and demonstrate that van der Waals interactions between the adsorbed perylene derivatives and host BN layers facilitate the functionalization. We also provide an explanation for the red-shift of optical adsorption bands observed in experiment and discuss improvements in theoretical calculations of noncovalently functionalized BNNTs.Keywords: ab initio calculations; absorption spectra; boron nitride nanotubes; electronic structures; noncovalent functionalization

The structural and magnetic properties of LaCoO3 nanoparticles with the particle size (D) ranging from ∼60 to 450 nm prepared by a sol−gel method are investigated in this paper. It is found that all the nanoparticles have rhombohedral structure as the bulk, while the volume of unit cell monotonically increases with the decrease of the particle size. Magnetic measurements reveal that in all the nanoparticles a weakly ferromagnetic behavior appears below ∼85 K, in agreement with recent studies on single crystals, powders, epitaxially strained thin films, and particles of this compound, and that the magnetic moment increases with reduction in particle size. In particular, both the unit cell volume and ferromagnetic moment show a nearly linear relation with 1/D, which allows us to assign the enhancement of the ferromagnetic moment in the nanoparticles to the lattice expansion. Moreover, from the linear relation, a significant but size-independent ferromagnetic moment can be obtained by extrapolating 1/D to zero, which is very close to the saturated magnetic moment previously reported for the single-crystal samples in the literatures. We propose that the ferromagnetic behavior usually observed in the single crystal and bulk polycrystalline LaCoO3 at low temperatures may be an intrinsically magnetic property of this material. Additionally, a paramagnetic phase is found to coexist with the ferromagnetic phase at low temperatures for all the nanoparticles and to show a similar dependence on the particle size as the ferromagnetic phase, which suggests that the paramagnetism arises from the higher spin-state Co3+ ions and may also be an intrinsic property of this material.

We describe a simple manipulation of an amphiphilic polymer, polyvinylacetone (PVKA, with ketalization degree DH = 0.549), mixed with tetrabutylammonium bromide (TBAB) that generates a series of self-assembled superstructures in the selective solvent composed of dimethylsulfoxide (DMSO) and acetone. The morphologies of those superstructures could be tuned by varying the ratio of DMSO/acetone in the solvent, as well as by varying the TBAB concentration in the original system. This procedure thus adds a facile yet effective tool for superstructure formation of polymer/surfactant complexes. These offer the probability of the research of PVKA on structural construction and utilization, and fabricating such assemblies' essential is not only to the production of novel devices but also to the understanding of fundamental phenomena at the nano-/microscale.

The structural, magnetic and transport properties of stripe ordered La5/3Sr1/3NiO4 nanoparticles synthesized by the sol–gel method were investigated. Compared with the bulk sample, it is found that the charge order is weakened in the nanosized particles. Accompanied by the disappearance of the antiferromagnetic spin-ordered state, a ferromagnetic-like phase emerges at ∼23 K. Furthermore, the spin glass/spin reorientation transition temperature TCATCA for the ∼100 nm particles is ∼65 K which is ∼12 K higher than that of the bulk. This suggests that instead of the stabilization of charge order, the structural disorder may have a dramatic effect on the complex magnetic transition in the stripe phase systems.

A series of Co and Mn doped MgB2 polycrystalline samples have been synthesized and studied by x-ray diffraction and Raman spectrum. It is found that for Co-doping the aa-axis of the lattice remains unchanged as the Co content increases, while the cc-axis shows a small increase. In contrast, for Mn-doping, the cc-axis shows an evident decrease as the Mn content increases, besides a small decrease of the aa-axis. The TcTc of Mn doped samples decreases quickly with increasing Mn content while that of Co-doped samples decreases rather slowly. The Raman spectra show a smaller change on Co content. For Mn-doped samples, however, the linewidth is obviously broadened, although the frequency is almost unchanged on an increase of xx. The variations of the lattice parameters and Raman spectra for Co doping reveal that the Co ion is divalent when substituted at the Mg site. It is suggested that additional structural disorder may be induced by a localized magnetic moment, which will further give an effect on TcTc and Raman linewidth, besides the effect of the magnetic moment.

Intrinsic defects, including single vacancy and Stone−Wales defects in a finite-length (8, 0) BN tube are investigated using density functional theory calculations. It is found that the stabilities of all defects are enhanced as they get close to the tube ends. We propose the possible physical origins for the defect energetic preference toward the tube ends and prove that the spontaneous electric polarization field of the BN tube can affect the stability of the defect by making the different electrostatic potential to the defect at different site. We also discuss the potential experimental consequences and provide the theoretical support for one experimental method for the synthesis of BN tubes.

Interaction of transition metal Fe atoms with pristine and defective (8, 0) boron nitride nanotubes have been investigated using density functional theory calculation. Our results indicate that a single Fe atom can adsorb on the outer wall of a perfect BN tube exothermically, and it can migrate along the tube surface easily, forming a stable Fe−Fe dimer for the double Fe atom adsorption. The presence of intrinsic defects, including single vacancy, divacancies, and Stone−Wales defects, can enhance the reactivity of BN tubes toward Fe. Special focus has been paid to the modulation on magnetic moment and spin configuration of atomic Fe adsorbed on the defect. Stable Fe−Fe dimer adsorption states exist on all intrinsic defects, characterized by distortion of local defect structures. Our findings propose a possible way for experiment to functionalize the BN tubes with transition metal iron.

The structural, transport and electron spin resonance properties of bulk and nanosized La0.875Sr0.125MnO3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below TCTC. In addition to a sharp peak in the vicinity of TCTC, another sharp peak close to TOO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.

The core-shell structural (BaTiO(C2O4)2/NH2CONH2) samples of giant electrorheological (ER) activity have been synthesized by a sol–gel method and studied by TEM, X-ray diffraction and IR spectroscopy. From the analysis of the X-ray diffraction, it is found that the component of samples is (BaTiO(C2O4)2/NH2CONH2) with a different content of urea. The observations of the TEM morphology show that (BaTiO(C2O4)2/NH2CONH2) particles are spheral with a size of around 80 nm and a surface coating with 5–10 nm thickness consisting of urea. The samples show an increasing tend of ER effect as the addition of urea increasing. Moreover, by IR spectroscopy study, it is revealed that some peaks of CO band and C–O band vibrations of BaTiO(C2O4)2 are split into two peaks after urea added. The extent of the splits increase with the increase of urea content, which is resulted by the interaction between the chemical bonds of urea and oxalate of BaTiO(C2O4)2. It is evident that the giant ER activity of the sample has a close relationship with the interfacial polarization between the core and the shell of (BaTiO(C2O4)2/NH2CONH2).TEM images of (BaTiO(C2O4)2/NH2CONH2) particles show the thickness of shell increasing as the urea addition increasing. The enhancement of giant ER activity relate with the increasing interfacial polarization of the core-shell structural (BaTiO(C2O4)2/NH2CONH2).

•XRD patterns and Raman spectroscopy confirm the Structural transformation of BiFeO3.•High-frequency dielectric properties of BiFeO3 are improved by Sr and Pb co-doping.•Saturation magnetization increases at RT with increasing Sr/Pb content.•Ferromagnetic property of BiFeO3 is enhanced by Sr and Pb co-doping.Bi1−x(Sr1/2Pb1/2)xFeO3 (0.10 ≤ x ≤ 0.30) multiferroic compounds have been synthesized by a conventional solid-state reaction. The influences of A-site Sr/Pb co-doping on the structure, dielectric and magnetic properties of BiFeO3 are investigated systematically. X-ray diffraction reveals that the crystal structure of Bi1−x(Sr1/2Pb1/2)xFeO3 transforms from the rhombohedral symmetry (space group R3c) to the cubic symmetry (space group Pm  3¯ m) with Sr/Pb concentration increasing. Meanwhile, the intensities of BiO bond vibrations continuously decrease and finally disappear. The RT polarization versus electric field (P-E) curves confim the ferroelectric nature for all the samples. Furthermore, it is found that the dielectric constant and dielectric loss tangent of the samples measured in the frequency range of 100–107 Hz decrease drastically at room temperature. A considerable reduction in the leakage current is observed for BFO with Sr/Pb doping content increasing. The ferromagnetic property enhances with the Sr/Pb substitution increasing, which is ascribed to the cooperation of the Fe2+OFe3+ double exchange interaction and the effective suppression of antiferromagnetic cycloidal spin structure caused by the crystalline structure transformation. All results indicate that the Sr and Pb co-doping can effectively improve the magnetic and high-frequency dielectric properties of the multiferroic BiFeO3 compounds.

The magnetic properties of ∼40 nm Nd0.5Ca0.5MnO3 and Sm0.5Sr0.5MnO3 nanoparticles are investigated by magnetometry and electronic spin resonance (ESR) spectroscopy. It is found that although their bulk counterparts have quite different magnetic properties at low temperatures, both the nanoparticles exhibit very similar magnetic behaviors, where the charge ordered transitions disappear and weak ferromagnetism emerges below about 100 K. A detailed analysis on the magnetic susceptibilities and the ESR linewidths reveals that for the two compounds the size reduction weakens both the ferromagnetic and antiferromagnetic interactions, and converts the long-range charge orderings to short-range ones. Moreover, the strength of the charge ordered correlations is observed to be not much affected by the size reduction. Based on the present results and the previous studies on various nanosized half-doped manganites, the magnetic phase diagram of the half-doped manganites with the particle sizes of ∼25–40 nm is established. We find that this diagram is very similar to those for the bulk near half-doped manganites with large quenched disorder, which allows us to propose that the reported exotic phenomena in the nanosized half-doped manganites should be mainly ascribed to surface disorder effect. These results may provide a deeper insight into the role of size reduction on the physics of half-doped manganites.