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Néel transition in the multiferroic BiFeO <sub/>3</sub> -0.25PbTiO <sub/>3</sub> nanoparticles with anomalous size effect
摘要： The role of size reduction on the structural parameters, antiferromagnetic transition temperature (TN), and spin reorientation transition temperature of BiFeO3-0.25PbTiO3 (BF-0.25PT) has been studied. Rietveld analysis using high resolution synchrotron x-ray powder diffraction data confirms that the space group of BF-0.25PT solid solutions is monoclinic Cc and not rhombohedral R3c for both bulk and nanocrystalline powders. This settles a longstanding controversy about the structure of these solid solutions toward the BiFeO3 rich end of the morphotropic phase boundary in the BiFeO3-xPbTiO3 system. Using magnetization and neutron powder diffraction data, we show that the Néel transition temperature (TN) of BF-0.25PT increases from 445 K for bulk to 480 K for 150 nm particle size. This is in marked contrast to the scaling theories of phase transitions in finite size systems. We also show that the spin reorientation transition occurring below TN in bulk monoclinic compositions like BF-0.25PT is suppressed in the nanocrystalline samples of ～150 nm particle size. Based on Rietveld refined structural parameters, we show that the asymmetry and non-linearity of the Fe-O-Fe superexchange pathways grow with decreasing particle size and that they exhibit a strong correlation with TN. We believe that the substantially enhanced Dzyaloshinskii-Moriya interaction with decreasing particle size as a result of asymmetric and non-collinear Fe-O-Fe superexchange pathways may be the key factor in raising the TN on decreasing the particle size. These observations present a new facet of type-I multiferroic materials, where superexchange pathways are intimately dependent on the ferroelectric distortion.
关键词： multiferroic,antiferromagnetic transition,BiFeO3-PbTiO3,size effect,Dzyaloshinskii-Moriya interaction,Néel transition,nanoparticles
更新于2025-09-23 15:23:52
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The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La1?xBax)(Zn1?xMnx)AsO in the Low Doping Regime
摘要： We investigated the magnetic properties of (La1?xBax)(Zn1?xMnx)AsO with x varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of x ≤ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to x = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for x ≤ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for x ≥ 0.02. The Mn-Mn spin interaction parameter |2J/kB| is estimated to be in the order of 10 K for both x ≤ 0.01 (antiferromagnetic ordered state) and x ≥ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems.
关键词： ferromagnetic ordering,ferromagnetic coupling,antiferromagnetic exchange interaction,carrier-mediated
更新于2025-09-23 15:22:29
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Pulsed-laser epitaxy of metallic delafossite PdCrO <sub/>2</sub> films
摘要： Alternate stacking of a highly conducting metallic layer with a magnetic triangular layer found in delafossite PdCrO2 provides an excellent platform for discovering intriguing correlated quantum phenomena. Thin film growth of delafossites may enable not only the tuning of the basic physical properties beyond what bulk materials can exhibit, but also the development of novel hybrid materials by interfacing with dissimilar materials, yet this has proven to be extremely challenging. Here, we report the epitaxial growth of metallic delafossite PdCrO2 films by pulsed laser epitaxy (PLE). The fundamental role of the PLE growth conditions, epitaxial strain, and chemical and structural characteristics of the substrate is investigated by growing under various growth conditions and on various types of substrates. While strain plays a large role in improving the crystallinity, the direct growth of epitaxial PdCrO2 films without impurity phases was not successful. We attribute this difficulty to both the chemical and structural dissimilarities with the substrate and volatile nature of the PdO sublayer, which make nucleation of the right phase difficult. This difficulty was overcome by growing CuCrO2 buffer layers before PdCrO2 films were grown. Unlike PdCrO2, CuCrO2 films were readily grown with a relatively wide growth window. Only a monolayer thick buffer layer was sufficient to grow the correct PdCrO2 phase. This result indicates that the epitaxy of Pd-based delafossites is extremely sensitive to the chemistry and structure of the interface, necessitating near perfect substrate materials. The resulting films are commensurately strained and show an antiferromagnetic transition at 40 K that persists down to as thin as 3.6 nm in thickness. This work provides key insights into advancing the epitaxial growth of the broader class of metallic delafossites for both studying the basic physical properties and developing new spintronic and computing devices.
关键词： delafossite,pulsed laser epitaxy,thin film growth,antiferromagnetic transition,PdCrO2
更新于2025-09-23 15:21:01
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Comparing the anomalous Hall effect and the magneto-optical Kerr effect through antiferromagnetic phase transitions in Mn <sub/>3</sub> Sn
摘要： In the non-collinear antiferromagnet Mn3Sn, we compare simultaneous measurements of the anomalous Hall effect (AHE) and the magneto-optical Kerr effect (MOKE) through two magnetic phase transitions: the high-temperature paramagnetic/antiferromagnetic (AF) phase transition at the N(cid:2)eel temperature (TN (cid:2) 420 K) and a lower-temperature incommensurate magnetic ordering at T1 (cid:2) 270 K. While both the AHE and MOKE are sensitive to the same underlying symmetries of the AF non-collinear spin order, we ?nd that the transition temperatures measured by these two techniques unexpectedly differ by approximately 10 K. Moreover, the applied magnetic ?eld at which the AF order reverses is signi?cantly larger when measured by MOKE than when measured by AHE. These results point to a difference between the bulk and surface magnetic properties of Mn3Sn.
关键词： antiferromagnetic phase transitions,anomalous Hall effect,magneto-optical Kerr effect,Mn3Sn
更新于2025-09-19 17:15:36
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Interplay of structure and charge order revealed by quantum oscillations in thin films of
摘要： The discovery of quantum oscillations in hole- and electron-doped cuprate families has underscored the importance of the Fermi surface in cuprate superconductivity. While the observed quantum oscillations in both families have revealed the presence of reconstructed Fermi surfaces, there remains an important distinction between the two. In hole-doped cuprates the oscillations are thought to arise from the effects of a charge density wave, while in the electron-doped cuprates it is thought that these oscillations occur from an antiferromagnetically reconstructed Fermi surface, despite the fact that the oscillations are observed in overdoped compounds, far from the putative antiferromagnetic critical point. In this work we study thin films of Pr2CuO4±δ, whose apparent doping can be finely tuned by annealing, allowing studies of quantum oscillations in samples straddling the critical point. We show that even though there is a mass enhancement of the quasiparticles, there are only small changes to the Fermi surface itself, suggesting that charge order is a more likely origin, with electronic correlations that are strongly dependent on the structural parameters.
关键词： antiferromagnetic critical point,Fermi surface,cuprate superconductivity,quantum oscillations,charge density wave
更新于2025-09-12 10:27:22
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Charge Dynamics and Metal–Insulator Transition in Perovskite SrIr <sub/>1?</sub><i> <sub/>x</sub></i> Sn <i> <sub/>x</sub></i> O <sub/>3</sub>
摘要： We investigate the variation of charge dynamics upon the metal–insulator transition for perovskite SrIr1?xSnxO3 by employing the optical spectroscopy. The Dirac semimetal of SrIrO3 turns into the antiferromagnetic insulator with a gap of 0.1 eV by the Sn-substitution, accompanying the reconstruction of electronic structure on an energy scale of 1 eV. The spectral intensity of optical excitation between Je? = 1=2-orbitals is signi?cantly reduced with increasing x, while that between the Je? = 3=2- and 1=2-orbital shows merely moderate x-dependence. We anticipate that the substituted Sn signi?cantly renormalizes the e?ective bandwidth of Je? = 1=2-orbital while modestly changing the Je? = 3=2-orbital.
关键词： charge dynamics,optical spectroscopy,electronic structure,Je? = 3=2-orbital,perovskite,metal–insulator transition,Dirac semimetal,Je? = 1=2-orbitals,SrIr1?xSnxO3,antiferromagnetic insulator,Sn-substitution
更新于2025-09-10 09:29:36
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Effect of cobalt substitution on the multiferroic characteristics of ferroelectric potassium sodium niobate (K0.5Na0.5NbO3) ceramics
摘要： The room-temperature multiferroic behavior of cobalt-substituted KNN [K0.5Na0.5Nb1?xCoxO3, 0.02 ≤ x ≤ 0.05] ceramics has been investigated. Both the X-ray diffraction and selected area electron diffraction analysis confirm that all the compositions found to be crystallized in the single phase (Orthorhombic, Amm2) without any formation of cobalt clusters and/or other secondary phases such as Co3O4. Co2+ oxidation state and the presence of oxygen vacancies in the samples have been studied through X-ray photoelectron spectroscopic measurements. The changes in the bandgap with the increase of Co concentration in KNN are noticed from UV–Visible absorption spectroscopy. Magnetic measurement on the samples reveals a dominant antiferromagnetic interaction attributed to the Co2+–Co2+ exchange interactions via F0-center and coupling between Co2+–Co2+ through O2?. The existence of exchange bias (EB) effect is also observed in KNaNb1?xCoxO3 (x = 0.03, 0.04, 0.05) samples from the magnetic measurements. In addition, it is found that defect complexes formed between Co′′′ Nb and oxygen vacancy lead to the enhanced dielectric, ferro, and piezoelectric properties of K0.5Na0.5Nb1?xCoxO3 ceramics.
关键词： Multiferroic,Cobalt-substituted KNN,Ferroelectric,Exchange bias,Antiferromagnetic
更新于2025-09-10 09:29:36
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Imaging Domains in a Zero-Moment Half Metal
摘要： We have a choice of methods for examining domains at the surface of a ferromagnet that depend on probing the stray ?eld distribution, but these methods do not work in antiferromagnets or compensated ferrimagnets, which produce no stray ?eld. The discovery of compensated ferrimagnetic half metals allows for the local magnetization state to be observed directly with polarized light. The example considered here, Mn2RuxGa, has two inequivalent but oppositely aligned Mn sublattices with equal and opposite moments, but only one of them contributes spin-polarized conduction electrons at the Fermi energy. The material looks like an antiferromagnet from the outside, but from the point of view of the electronic structure, it resembles a spin-polarized ferromagnetic metal. The anisotropy axis is perpendicular to the ?lm plane, which allows domains to be imaged directly by polar magneto-optic Kerr effect. The domain structure in a ?lm with a composition of Mn2Ru0.4Ga has been imaged in a Kerr microscope and hysteresis loops have been traced. Domains have dimensions of order 20 μm with meandering domain walls and a fractal dimension D f = 1.85. Our results open new direct imaging possibilities of magnetically ordered materials with no net moment.
关键词： compensated ferrimagnets,Mn2RuxGa (MRG),half metal,Kerr microscopy,Antiferromagnetic domain walls
更新于2025-09-10 09:29:36
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with pentagonal structure
摘要： Structure-property relationships have always been guiding principles for materials discovery. Here we explore the relationships to discover two-dimensional (2D) materials with the goal of identifying 2D magnetic semiconductors for spintronics applications. In particular, we report a density functional theory + U study of single-layer antiferromagnetic (AFM) semiconductor CoS2 with the pentagonal structure forming the so-called Cairo tessellation. We ?nd that this single-layer magnet exhibits an indirect band gap of 1.06 eV with electron and hole effective masses of 0.52 and 1.93 m0, respectively, which may lead to relatively high electron mobility. The hybrid density functional theory calculations correct the band gap to 2.24 eV. We also compute the magnetocrystalline anisotropy energy (MAE), showing that the easy axis of the AFM ordering is along the b axis with a sizable MAE of 153 μeV per Co ion. We further calculate the magnon frequencies at different spin-spiral vectors, based on which we estimate the N′eel temperatures to be 20.4 and 13.3 K using the mean ?eld and random phase approximations, respectively. We then apply biaxial strains to tune the band gap of single-layer pentagonal CoS2. We ?nd that the energy difference between the ferromagnetic and AFM structures strongly depends on the biaxial strain, but the ground state remains the AFM ordering. Although the low critical temperature prohibits the magnetic applications of single-layer pentagonal CoS2 at room temperature, the excellent electrical properties may ?nd single-layer semiconductor applications in optoelectronic nanodevices.
关键词： two-dimensional materials,magnon frequencies,N′eel temperature,biaxial strain,Cairo tessellation,antiferromagnetic semiconductor,density functional theory,band gap,magnetocrystalline anisotropy energy,pentagonal structure,spintronics
更新于2025-09-09 09:28:46
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Green Synthesis of Co <sub/>3</sub> O <sub/>4</sub> Nanorods for Highly Efficient Catalytic, Photocatalytic, and Antibacterial Activities
摘要： Cobalt oxide nanorods were successfully synthesized by a hot plate combustion method using the plant extract of Vitis vinifera. The plant extract as an alternative to toxic chemicals can be used generally as reducing and capping agents. The obtained nanorods were characterized by XRD, FT-IR, Raman, TEM, SAED, EDX, DRS, PL and VSM techniques for the structural, morphological, optical and magnetic properties. The XRD, FT-IR, Raman, EDX analysis confirmed the high purity of the sample. The TEM and SAED results showed the rod shape morphology of the sample. DRS and PL showed the band gap energy and emission at visible region. VSM showed the antiferromagnetic nature of the sample. The photocatalytic activities of the as-prepared cobalt oxide nanorods were investigated for the degradation of textile dying waste water. As per the standards of Indian pollution control board for industrial waste water let out into river bodies, the degradation reactions of waste water was found to be 250 mg/L at 150 min. Also, the same catalyst is used for the reduction of 4-nitrophenol and 4-nitroaniline using sodium borohydride as a reducing agent and it exhibits excellent reduction reaction, because of the high active surface sites. The time taken for the reduction reaction was 300 sec and 210 sec for 4-nitrophenol and 4-nitroaniline respectively. Also, the antibacterial activities towards the bacterial strains were studied and reported.
关键词： Cobalt Oxide,Antiferromagnetic,Antibacterial Studies,Catalytic Reduction,Vitis vinifera Plant Extract,Photocatalytic Degradation
更新于2025-09-04 15:30:14