Abstract
Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the ...evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4
d
ferromagnetic oxide SrRuO
3
. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO
3
films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm
2
V
−1
s
−1
—combined with first-principles electronic structure calculations establishes SrRuO
3
as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.
Improper ferroelectricity (trimerization) in the hexagonal manganites RMnO3 leads to a network of coupled structural and magnetic vortices that induce domain wall magnetoelectricity and magnetization ...(M), neither of which, however, occurs in the bulk. Here we combine first-principles calculations, group-theoretic techniques and microscopic spin models to show how the trimerization not only induces a polarization (P) but also a bulk M and bulk magnetoelectric (ME) effect. This results in the existence of a bulk linear ME vortex structure or a bulk ME coupling such that if P reverses so does M. To measure the predicted ME vortex, we suggest RMnO3 under large magnetic field. We suggest a family of materials, the hexagonal RFeO3 ferrites, also display the predicted phenomena in their ground state.
Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such ...materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3-the geometric ferroelectric with the greatest known planar rumpling-we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially-from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.
Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO
)
/(LuFe
O
)
superlattices. A challenge to understanding ...the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe
→ Fe
charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe
O
layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.
Abstract
Pb
M
O
3
(
M
= 3
d
transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6
s
and ...transition metal 3
d
orbitals. However, the detailed structure and physical properties of PbFeO
3
remain unclear. Herein, we reveal that PbFeO
3
crystallizes into an unusual 2
a
p
× 6
a
p
× 2
a
p
orthorhombic perovskite super unit cell with space group
Cmcm
. The distinctive crystal construction and valence distribution of Pb
2+
0.5
Pb
4+
0.5
FeO
3
lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb
2+
and Pb
4+
) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the
a
-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the
b
-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe
3+
magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.
The coupling between the magnetic and electric dipoles in multiferroic and magnetoelectric materials holds promise for conceptually novel electronic devices. This calls for the development of local ...probes of the magnetoelectric response, which is strongly affected by defects in magnetic and ferroelectric ground states. For example, multiferroic hexagonal rare earth manganites exhibit a dense network of boundaries between six degenerate states of their crystal lattice, which are locked to both ferroelectric and magnetic domain walls. Here we present the application of a magnetoelectric force microscopy technique that combines magnetic force microscopy with in situ modulating high electric fields. This method allows us to image the magnetoelectric response of the domain patterns in hexagonal manganites directly. We find that this response changes sign at each structural domain wall, a result that is corroborated by symmetry analysis and phenomenological modelling, and provides compelling evidence for a lattice-mediated magnetoelectric coupling. The direct visualization of magnetoelectric domains at mesoscopic scales opens up explorations of emergent phenomena in multifunctional materials with multiple coupled orders.
We explored lithium ion transport phenomena in a cation-deficient quadruple perovskite LiCuTa3O9 based on ab initio calculations and experiments. Here, we investigated the stability of ...cation-ordered/disordered phases by employing a Li–Cu–vacancy ternary lattice model and canonical Monte Carlo simulations. Our studies predict the formation of interconnected chains of lithium and vacancies, extending along all crystallographic axes, in the most stable cation-ordered phase. We have investigated and discussed the mechanism of lithium diffusion through this system by calculating the activation energy associated with the localized nearest neighbor lithium hoppings between active sites. In our investigations, we measured lithium ion conductivity in the epitaxial thin films of LiCuTa3O9 fabricated on YAlO3 (110). The film, as investigated by reciprocal space mapping of X-ray diffraction, showed in-plane lattice anisotropy induced by the substrate. Notably, a small anisotropy in lithium transport was also observed.
Negative thermal expansion (NTE) is an intriguing physical phenomenon. Layered Ca2RuO4 exhibits giant NTE over a wide temperature range from 200 to 400 K, which makes it attractive for fundamental ...research and industrial applications. However, a clear physical understanding is lacking for the appearance of NTE over such a wide temperature range and the oxygen-content-dependent switch from NTE to positive thermal expansion (PTE). Herein, we present insights into the average crystal structure, local structure, and electronic and orbital states of Ca2RuO4. Surprisingly, a previously overlooked monoclinic distortion is identified by electron diffraction and synchrotron X-ray diffraction (SXRD). X-ray absorption fine structure (XAFS) and synchrotron X-ray pair distribution function (PDF) analyses show large local distortions in monoclinic Ca2RuO4. Moreover, local stress on Ru cations is confirmed by the existence of over-bonding states, which relaxes along with NTE. Theoretical calculations indicate that dxy orbital ordering and disordering in the monoclinic structure are the origins of NTE. Moreover, interstitial oxygen plays a critical role in stabilizing elongated RuO6 and locally breaks the dxy orbital ordering, facilitating the occurrence of PTE. This work elucidates the electronic and orbital states in NTE materials with defective lattices and provides a different route to designing unconventional NTE materials.
The pyrochlore-type Ca2Bi2O7 and Sr2Bi2O7 have been synthesized from a low-temperature hydrothermal route using NaBiO3·nH2O as a starting material. The crystal structures of these compounds were ...refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca2Bi2O7 and Sr2Bi2O7, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.
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