A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead-zirconate-titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

The depolarization temperature T d of piezoelectric materials is an important figure of merit for their application at elevated temperatures. Until now, there are several methods proposed in the literature to determine the depolarization temperature of piezoelectrics, which are based on different physical origins. Their validity and inter-correlation have not been clearly manifested. This paper applies the definition of depolarization temperature as the temperature of the steepest decrease of remanent polarization and evaluates currently used methods, both in terms of this definition and practical applicability. For the investigations, the lead-free piezoceramics (1-y)(Bi 1/2 Na 1/2 TiO 3 -xBi 1/2 K 1/2 TiO 3 )ÀyK 0.5 Na 0.5 NbO 3 in a wide compositional range were chosen. Results were then compared to those for BaTiO 3 and a commercial Pb(Zr,Ti)O 3 -based material as references. Thermally stimulated depolarization current and in situ temperature-dependent piezoelectric coefficient d 33 are recommended to determine T d according to the proposed definition. Methods based on inflection point of the real part of permittivity or the peak in dielectric loss give consistently higher temperature values.

A high‐temperature dielectric, (1–x)(0.6Bi1/2Na1/2TiO3–0.4Bi1/2K1/2TiO3)–xK0.5Na0.5NbO3, off the morphotropic phase boundary of the parent matrix 0.8Bi1/2Na1/2TiO3–0.2Bi1/2K1/2TiO3, has been developed for application as a high‐temperature capacitor. In addition to temperature‐dependent permittivity and dielectric loss, DC conductivity and field‐dependent permittivity are reported. These properties are correlated with temperature‐dependent structure data measured at different length scales using Raman spectroscopy and neutron diffraction. It is suggested that all materials investigated are ergodic relaxors with two types of polar nanoregions providing different relaxation mechanisms. The most attractive properties for application as high‐temperature dielectrics are obtained in a material with x = 0.15 at less than 10% variation of relative permittivity of about 2100 between 54°C and 400°C.

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The nearzero moment is a result of the nearly equal and opposing spin and orbital moments in the 6 H 5/2 ground state of the Sm 3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L 2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.

Conventional wisdom expects that making semiconductors ferromagnetic requires doping with magnetic ions, and that superconductivity cannot coexist with magnetism. However, recent concerted efforts exploring new classes of materials have established that intrinsic ferromagnetic semiconductors exist and that certain types of strongly correlated metals can be ferromagnetic and superconducting at the same time. Here we show that the trifecta of semiconducting behavior, ferromagnetism and superconductivity can be achieved in a single material. Samarium nitride (SmN) is a well-characterised intrinsic ferromagnetic semiconductor, hosting strongly spin-ordered 4f electrons below a Curie temperature of 27 K. We have now observed that it also hosts a superconducting phase below 4 K when doped to electron concentrations above 10 21 cm −3 . The large exchange splitting of the conduction band in SmN favors equal-spin triplet pairing with p-wave symmetry. An analysis of the robustness of such a superconducting phase against disorder leads to the conclusion that the 4f bands are crucial for superconductivity, making SmN a heavy-fermion-type superconductor.

The structural behavior of ceramic solid solutions (1-x)Na ½ Bi ½ TiO 3 -xK ½ Bi ½ TiO 3 (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudorhombohedral (R) and KBT-like pseudo-tetragonal (T) phases was observed at x≈0.2. For x<0.2, both local and average room-temperature structures are similar to those in NBT. Simultaneous long-range anti-phase and short-range in-phase octahedral rotations average, resulting in effective anti-phase a -a -c -tilting, which yields monoclinic symmetry when probed by X-ray diffraction (XRD). For these compositions, polar ordering is coupled to anti-phase octahedral rotations so that tilting and ferroelectric (FE) domains coincide. Compositions with x>0.2 exhibit a tetragonal-like distortion; however, complex splitting of reflections in XRD patterns suggests that the actual symmetry is lower than tetragonal. For 0.2≤x≤0.5, in-phase octahedral tilting a) is present but confined to the nanoscale, while for x>0.5, the structure becomes untilted. In-phase tilting evolves above the ferroelectric transition and occurs around a non-polar (a or b) axis of the average T structure. The onset of polar order has no significant effect on the coherence length of in-phase tilting, which suggests only weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry (Imm2) of assemblages of coherent in-phase tilted nanodomains. Near the MPB, the coexistence of extended R-and T-like regions is observed, but lattice distortions within each phase are small, yielding narrow peaks with a pseudo-cubic appearance in XRD. The temperature of the FE phase transition exhibits a minimum at the MPB. The structured diffuse scattering observed in electron diffraction patterns for all the compositions suggests that polar order in NBT-KBT solid solutions is modulated away from the average displacements refined using powder diffraction data.2

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems, where the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromagnetic Gd-Gd, Sm-Sm and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of X-ray magnetic circular dichroism (XMCD) is used in seperate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization. PACS numbers: 75.25-j, 75.47.-m, 75.50.Pp 1 arXiv:1504.04425v1 [cond-mat.mtrl-sci] 17 Apr 2015 I. INTRODUCTION An inhomogeneous, twisted magnetic ordering commonly occurs near interfaces between ferromagnetic materials, due to competing interactions which favor opposing alignments of the magnetization. These phases are types of engineered domain walls, and thus have important implications for spintronics applications, where current-driven domain wall motion is an active area of research. 1-4 So far, twisted phases are known to manifest in diverse magnetic systems, 5-12 however these all fall under the conventional spin-dominant paradigm of magnetism where the orbital moment plays no significant role. Competing interactions in the presence of a dominant orbital moment have so far remained unexplored, yet the opportunity now exists within the rare-earth nitride (REN) series, where orbital-dominant magnetism is possible due to strong spin-orbit coupling of the 4f electrons. Forming a series of mostly intrinsic ferromagnetic semiconductors, 13-19 the RENs are already integrated within spintronic devices, 20,21 and thus provide a novel system for studying competing interactions. The rare-earth elements, comprising the series across which the 4f shell is filled, have been of interest for nearly a century. They are most commonly found in the trivalent state in a wide range of compounds, including the RENs. The 4f shell, with l = 3, comprises seven distinct orbital states, −3 m l 3, and with the spin degeneracy a total of 14 single-electron states. Gd 3+ has a half filled shell, for which Hund's rules state that the seven electrons fill all of the orbital states with spin-up electrons; L = 0 and S = J = 7/2. It thus has a purely spin moment of 7 µ B . The indirect exchange interaction aligns the spins below a Curie temperature of about 50 K, rising to 70 K under heavy donor doping, 22 but the spherical symmetry of...

Europium nitride is semiconducting and contains nonmagnetic Eu3+, but substoichiometric EuN has Eu in a mix of 2+ and 3+ charge states. We show that at Eu2+ concentrations near 15%-20% EuN is ferromagnetic with a Curie temperature as high as 120 K. The Eu3+ polarization follows that of the Eu2+, confirming that the ferromagnetism is intrinsic to the EuN which is, thus, a novel diluted magnetic semiconductor. Transport measurements shed light on the likely exchange mechanisms.