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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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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