Using Landau-Ginzburg-Devonshire theory and effective medium approximation, we analytically calculate typical dependences of the pyroelectric and electrocaloric coefficients on external electric field, temperature and radius for spherical single-domain ferroelectric nanoparticles. The considered physical model corresponds to the nanocomposite with small fraction of ferroelectric nanoparticles.Within the framework of the analytical model we establish how the size changes determine the ferroelectric thin films [31, 32, 33, 34], multilayers [35, 36, 37] and other low-dimensional materials [38] can be very different from those of single crystals [39].As it is known [40,41], the polar materials in adiabatic conditions are characterized by the PEE (charge or electric field generation under temperature change) and by the inverse ECE (temperature change under application or removal of an electric field). The vivid manifestation of PEE and ECE in ferroelectrics is a consequence of the strong temperature dependence of the spontaneous polarization [42,43,44], especially in the vicinity of phase transitions [45,46] or near the morphotropic phase boundary [47]. This property is the basis for the widespread applications of ferroelectric materials for pyroelectric detectors and energy converters, as well as for realizing their potentiality in modern electrocaloric converters [48,49,50].At present, ECE and PEE in ferroelectric crystals, ceramics and polymers, thin films and multilayer structures are the objects of intensive theoretical, experimental, and applied studies.Nevertheless, ECE and PEE in ferroelectric nanoparticles are relatively poor studied. The possible reason is the strong influence of size effects via depolarization field [24] and polarization-strain coupling [20, 25] on the polarization distribution, ferroelectric transition temperature, dielectric, PE, and EC properties. There are several studies directed on the elucidation of the features of the PEE and ECE in nanowires, nanotubes [51,52,53,54], and nanoparticles [55]. However, the analytical description of ECE and PEE in the most "technological" spherical nanoparticles and nanocomposites, allowing for depolarization and incomplete screening effect, is still missing.Using the LGD theory and effective medium approximation, this work analyzes typical dependences of the polarization, dielectric permittivity, PE and EC coefficients on external electric field, temperature, and radius for spherical ferroelectric nanoparticles covered by a semiconducting shell and placed in a dielectric medium. The considered physical model corresponds to a nanocomposite "nanoparticles-matrix" with a small fraction (less than 10%) of the ferroelectric nanoparticles.The manuscript has the following structure. Problem statement containing free energy and basic equations with boundary conditions is formulated in Section II. Section III introduces approximate analytical expressions for the transition temperature, EC temperature change, heat capacity, and related physical quantities. Siz...

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We studied magnetostatic response of the Bi 0.9 La 0.1 FeO 3 – KBr composites (BLFO-KBr) consisting of nanosized (≈100 nm) ferrite Bi 0.9 La 0.1 FeO 3 (BLFO) conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is rather small (less than 15 wt%) the magnetic response of the composite is very weak and similar to that observed for the BLFO (pure KBr matrix without Bi 1-x La x FeO 3 has no magnetic response as anticipated). However, when the fraction of KBr increases above 15%, the magnetic response of the composite changes substantially and the field dependence of magnetization reveals ferromagnetic-like hysteresis loop with a remanent magnetization about 0.14 emu/g and coercive field about 1.8 Tesla (at room temperature). Nothing similar to the ferromagnetic-like hysteresis loop can be observed in Bi 1-z La z FeO 3 ceramics with z ≤ 0.15, which magnetization quasi-linearly increases with magnetic field. Different physical mechanisms were considered to explain the unusual experimental results for BLFO-KBr nanocomposites, but only those among them, which are highly sensitive to the interaction of antiferromagnetic Bi 0.9 La 0.1 FeO 3 with ionic conductor KBr, can be relevant.