Developing eco-friendly high-performance piezoceramics without lead has become one of the most advanced frontiers in interdisciplinary research. Although potassium sodium-niobate {(K,Na)NbO
3
, KNN} ...based ceramics are believed to be one of the most promising lead-free candidates, the relatively inferior piezoelectric properties and strong temperature dependency have hindered their development for more than 50 years since being discovered in the 1950s. It was not until 2014 that our group initially proposed a new phase boundary (NPB) that simultaneously improved the piezoelectric properties and temperature stability of non-textured KNN-based ceramics to the level of partly lead-based ceramics. The NPB has been then proved by some researchers and believed to pave the way for "
lead-free at last
" proposed by E. Cross (
Nature
, 2004,
432
, 24). However, the understanding of the NPB is still in its infancy, leaving many controversies, including the phase structure and physical mechanisms at the NPB as well as the essential difference when compared with other phase boundaries. In this context, we systematically summarized the origin and development of the NPB, focusing on the construction, structure and intrinsic trait of the NPB, the effects of the NPB on the performance, and the validity and related incipient devices of the NPB. Particularly, we concluded the phase structure and domain structure locating at the NPB, analyzed the physical mechanisms in depth, proposed the possible methods to further improve the performance at the NPB, and demonstrated the validity and scope of the NPB as well as the device application. Finally, we gave out our perspective on the challenges and future research of KNN-based ceramics with NPB. Therefore, we believe that this review could promote the understanding of the NPB and guide the future work of KNN-based ceramics.
A review of the newly emerging "new phase boundary" in potassium sodium niobate-based ceramics with high performance.
Reducing or even prohibiting the use of toxic lead in electronic devices has become one of the most cutting-edge topics in various disciplines. The recently proposed phase boundary engineering endows ...lead-free piezoceramics with comparable performances to that of some lead-based piezoceramics. However, the enhancement in performance hinges on the coexistence of multi-phases and complex domain structures, particularly the occurrence of nano-domains and polar nanoregions (PNRs). Although nano-domains have been significantly studied in lead-based piezoceramics, understanding the nano-domains and PNRs in lead-free piezoceramics is in its infancy and needs a systematic summary and in-depth analysis. Herein, we summarize the nano-domains and PNRs in three representative lead-free piezoceramics (
i.e.
, potassium sodium niobate, barium titanate, and sodium bismuth titanate), focusing on their effects on macro performance. First, we introduce the foundation and tools for the observation of the domains. Then, we summarize the variations in the nano-domains with phase structure, electric field, and temperature, and their effects on performance including piezoelectricity, strain, temperature stability, aging, and fatigue. Finally, we present our perspectives on the future of nano-domains, concentrating on nano-domain engineering. Therefore, this review can help better understand the nano-domains and PNRs in lead-free piezoceramics, and be used for the further development of high-performance lead-free piezoceramics.
A systematic review summarizing and analyzing the role of recently observed nano-domains and polar nanoregions (PNRs) in lead-free piezoceramics.
Recently, a type-II Weyl fermion was theoretically predicted to appear at the contact of electron and hole Fermi surface pockets. A distinguishing feature of the surfaces of type-II Weyl semimetals ...is the existence of topological surface states, so-called Fermi arcs. Although WTe2 was the first material suggested as a type-II Weyl semimetal, the direct observation of its tilting Weyl cone and Fermi arc has not yet been successful. Here, we show strong evidence that WTe2 is a type-II Weyl semimetal by observing two unique transport properties simultaneously in one WTe2 nanoribbon. The negative magnetoresistance induced by a chiral anomaly is quite anisotropic in WTe2 nanoribbons, which is present in b-axis ribbon, but is absent in a-axis ribbon. An extra-quantum oscillation, arising from a Weyl orbit formed by the Fermi arc and bulk Landau levels, displays a two dimensional feature and decays as the thickness increases in WTe2 nanoribbon.
Abstract
Spin–orbit torque has recently been intensively investigated for the purposes of manipulating the magnetization in magnetic nano-devices and understanding fundamental physics. Therefore, the ...search for novel materials or material combinations that exhibit a strong enough spin-torque effect has become one of the top priorities in this field of spintronics. Weyl semimetal, a new topological material that features open Fermi arc with strong spin–orbit coupling and spin–momentum locking effect, is naturally expected to exhibit an enhanced spin-torque effect in magnetic nano-devices. Here we observe a significantly enhanced spin conductivity, which is associated with the field-like torque at low temperatures. The enhancement is obtained in the
b
-axis WTe
2
/Py bilayers of nano-devices but not observed in the
a
-axis of WTe
2
/Py nano-devices, which can be ascribed to the enhanced spin accumulation by the spin–momentum locking effect of the Fermi arcs of the Weyl semimetal WTe
2
.
Room‐temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano‐spintronic devices. However, such skyrmion‐hosting materials are rare in ...nature. In this study, a self‐intercalated transition metal dichalcogenide Cr1+xTe2 with a layered crystal structure that hosts room‐temperature skyrmions and exhibits large THE is reported. By tuning the self‐intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out‐of‐plane to the in‐plane configuration are achieved. Based on the intercalation engineering, room‐temperature skyrmions are successfully created in Cr1.53Te2 with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion‐induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.
Magnetic skyrmions and large topological Hall effect are demonstrated in chromium telluride. The Curie temperature and magnetic anisotropy in Cr1+xTe2 can be controlled by the self‐intercalate concentration x. In Cr1.53Te2, which has a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy, room‐temperature skyrmions and topological Hall resistivity as large as ≈106 nΩ cm are observed.
•A new concept of tuning the trade-off between LRO and PNRs is proposed.•The concept relieves the sensitivity of d33 and enhances temperature stability.•d33 shows higher values at x = 0.9–0.95 and a ...high retention of > 83% at x = 0.8–1.1.•Suni of x = 1.1 is enhanced and has a high retention of ≥ 79% at T = 20–180 °C.
To relieve the sensitivity of piezoelectric coefficient (d33) to composition and strengthen temperature stability of strain in potassium sodium niobate {(K, Na)NbO3, KNN} ceramics, we proposed a new concept, tuning the trade-off between long-range ordering (LRO) and polar nanoregions (PNRs), and realized it by tailoring the content of bismuth (Bi) in an already-constructed multiphase coexistence, namely, 0.96(K0.48Na0.52)(Nb0.955Sb0.045)O3-0.04(BixNa4-3x)0.5ZrO3-0.3 mol%Fe2O3 ceramics. We obtained not only the high retention of > 83% at x = 0.80–1.10 in d33 but also higher d33 at x = 0.90–0.95, relieving the sensitivity of d33 to composition. We also obtained not only the enhanced strain but also the high retention of ≥ 79% over a wide temperature range of 20–180 °C at x = 1.10, irrespective of the electric field, strengthening the temperature stability. We demonstrated that high d33 values hinge on the trade-off between LRO and PNRs, and the enhanced temperature stability of strain originates from the diffused multiphase coexistence and the reduced contribution of domain switching. Therefore, the new concept helps further design high-performance KNN-based ceramics for practical application.
To improve the temperature stability and electrical properties of KNN‐based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), ...0.96K0.5Na0.5Nb0.96Sb0.04O3‐0.04(Bi1‐xLax)0.5Na0.5ZrO3 (0 ≤ x ≤ 1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La3+ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La3+ content leads to a change in phase structure, from a rhombohedral‐tetragonal (R‐T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La3+ (x = 0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80°C). In addition, we find that the ceramics with x = 0.2 exhibit a large piezoelectric constant (d33) of ~430 pC/N, a high Curie temperature (TC) of ~240°C and a fatigue‐free behavior (after 106 electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R‐T phase boundary and the appropriate addition of La3+.
Electrical manipulation of skyrmions attracts considerable attention for its rich physics and promising applications. To date, such a manipulation is realized mainly via spin-polarized current based ...on spin-transfer torque or spin-orbital torque effect. However, this scheme is energy consuming and may produce massive Joule heating. To reduce energy dissipation and risk of heightened temperatures of skyrmion-based devices, an effective solution is to use electric field instead of current as stimulus. Here, we realize an electric-field manipulation of skyrmions in a nanostructured ferromagnetic/ferroelectrical heterostructure at room temperature via an inverse magneto-mechanical effect. Intriguingly, such a manipulation is non-volatile and exhibits a multistate feature. Numerical simulations indicate that the electric-field manipulation of skyrmions originates from strain-mediated modification of effective magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Our results open a direction for constructing low-energy-dissipation, non-volatile, and multistate skyrmion-based spintronic devices.
The understanding of high piezoelectricity in potassium sodium niobate (KNN)-based ceramics with a new phase boundary has been limited to unpoled samples. Here, the phase structure, domain structure, ...and phenomenological theory were studied on both unpoled and poled samples by taking (0.99 −
x
)(K
0.48
Na
0.52
)(Nb
0.955
Sb
0.045
)O
3
-0.01SrZrO
3
-
x
(Bi
0.5
Ag
0.5
)ZrO
3
ceramics as an example. Shifting the phase transition temperatures to room temperature can result in the coexistence of a ferroelectric matrix containing an orthorhombic-tetragonal (O-T) coexisting phase and rhombohedral (R)-related polar nanoregions (PNRs), and then the miniature and nanoscale domain structure can be demonstrated. During the poling process, the R phase-related PNRs can facilitate domain switching and polarization rotation, resulting in a single domain structure and enhanced evidence of the R phase. Therefore, high piezoelectricity originates from a single domain feature as well as the diffused multi-phase coexistence in association with R phase related PNRs. This study provides a systematic approach to understand the physical mechanisms of enhanced piezoelectricity in KNN-based ceramics.
Unveiling the physical mechanisms of high performance in potassium sodium niobate-based ceramics from diffused multi-phase coexistence and a single domain feature.
Chiral magnets endowed with topological spin textures are expected to have promising applications in next‐generation magnetic memories. In contrast to the well‐studied 2D or 3D magnetic skyrmions, ...the authors report the discovery of 1D nontrivial magnetic solitons in a transition metal dichalcogenide 2H‐TaS2 via precise intercalation of Cr elements. In the synthetic Cr1/3TaS2 (CTS) single crystal, the coupling of the strong spin–orbit interaction from TaS2 and the chiral arrangement of the magnetic Cr ions evoke a robust Dzyaloshinskii–Moriya interaction. A magnetic helix having a short spatial period of ≈25 nm is observed in CTS via Lorentz transmission electron microscopy. In a magnetic field perpendicular to the helical axis, the helical spin structure transforms into a chiral soliton lattice (CSL) with the spin structure evolution being consistent with the chiral sine‐Gordon theory, which opens promising perspectives for the application of CSL to fast‐speed nonvolatile magnetic memories. This work introduces a new paradigm to soliton physics and provides an effective strategy for seeking novel 2D magnets.
Chiral helimagnetism and 1D magnetic solitons are discovered in Cr‐intercalated 2H‐TaS2. The chiral helimagnetic structure has an initial period of ≈25 nm, and it transforms into a chiral soliton lattice in an external magnetic field. The spatial period evolution of this topological spin texture obeys the chiral sine‐Gordon theory, revealing great potential for fast‐speed racetrack memories.
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