Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material ...realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi
Te
with alternating septuple MnBi
Te
and quintuple Bi
Te
layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi
Te
is a Z
antiferromagnetic TI with two types of surface states associated with the MnBi
Te
or Bi
Te
termination, respectively. Additionally, its superlattice nature may make various heterostructures of MnBi
Te
and Bi
Te
layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi
Te
make it an ideal system to investigate rich emergent phenomena.
Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations or hidden magnetic ...quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe, detwinned by mounting on a BaFe
As
substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors Q
= (±1, 0) at low energies E = 6-11 meV in the normal state. This two-fold (C
) anisotropy is reduced at lower energies, 3-5 meV, indicating a gapped four-fold (C
) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.6 meV) at Q
with incommensurate scattering around 5-6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.
Neutron scattering experiments have been performed to elucidate magnetic properties of the quasicrystal approximant Au70Si17Tb13, consisting of icosahedral spin clusters in a body-centered-cubic ...lattice. Bulk magnetic measurements performed on the single crystalline sample unambiguously confirm long-range ordering at TC = 11.6 ± 1 K. In contrast to the simple ferromagnetic response in the bulk measurements, single crystal neutron diffraction confirms a formation of intriguing non-collinear and non-coplanar magnetic order. The magnetic moment direction was found to be nearly tangential to the icosahedral cluster surface in the local mirror plane, which is quite similar to that recently found in the antiferromagnetic quasicrystal approximant Au72Al14Tb14. Inelastic neutron scattering on the powdered sample exhibits a very broad peak centered at ℏω ≃ 4 meV. The observed inelastic spectrum was explained by the crystalline-electric-field model taking account of the chemical disorder at the fractional Au/Si sites. The resulting averaged anisotropy axis for the crystalline-electric-field ground state is consistent with the ordered moment direction determined in the magnetic structure analysis, confirming that the non-coplanar magnetic order is stabilized by the local uniaxial anisotropy.
The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe_{2}Se_{3} and BaFe_{2}S_{3} have been studied using neutron diffraction. With identical crystal structure and ...similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe_{2}Se_{3} the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe_{2}S_{3} undergoes a quantum phase transition where an abrupt increase of Néel temperature by more than 50% occurs at about 1 GPa, accompanied by a jump in the ordered moment. With its spin structure unchanged, BaFe_{2}S_{3} enters an enhanced magnetic phase that bears the characteristics of an orbital selective Mott phase, which is the true neighbor of superconductivity emerging at higher pressures.
Abstract
Ferrotoroidal order, which represents a spontaneous arrangement of toroidal moments, has recently been found in a few linear magnetoelectric materials. However, tuning toroidal moments in ...these materials is challenging. Here, we report switching between ferritoroidal and ferrotoroidal phases by a small magnetic field, in a chiral triangular-lattice magnet BaCoSiO
4
with tri-spin vortices. Upon applying a magnetic field, we observe multi-stair metamagnetic transitions, characterized by equidistant steps in the net magnetic and toroidal moments. This highly unusual ferri-ferroic order appears to come as a result of an unusual hierarchy of frustrated isotropic exchange couplings revealed by first principle calculations, and the antisymmetric exchange interactions driven by the structural chirality. In contrast to the previously known toroidal materials identified via a linear magnetoelectric effect, BaCoSiO
4
is a qualitatively new multiferroic with an unusual coupling between several different orders, and opens up new avenues for realizing easily tunable toroidal orders.
Multiferroics materials, which exhibit coupled magnetic and ferroelectric properties, have attracted tremendous research interest because of their potential in constructing next-generation ...multifunctional devices. The application of single-phase multiferroics is currently limited by their usually small magnetoelectric effects. Here, we report the realization of giant magnetoelectric effects in a Y-type hexaferrite Ba
Sr
Mg
Fe
O
single crystal, which exhibits record-breaking direct and converse magnetoelectric coefficients and a large electric-field-reversed magnetization. We have uncovered the origin of the giant magnetoelectric effects by a systematic study in the Ba
Sr
Mg
Fe
O
family with magnetization, ferroelectricity and neutron diffraction measurements. With the transverse spin cone symmetry restricted to be two-fold, the one-step sharp magnetization reversal is realized and giant magnetoelectric coefficients are achieved. Our study reveals that tuning magnetic symmetry is an effective route to enhance the magnetoelectric effects also in multiferroic hexaferrites.Control of the electrical properties of materials by means of magnetic fields or vice versa may facilitate next-generation spintronic devices, but is still limited by their intrinsically weak magnetoelectric effect. Here, the authors report the existence of an enhanced magnetoelectric effect in a Y-type hexaferrite, and reveal its underlining mechanism.
A quantum spin liquid is a state of matter where unpaired electrons’ spins, although entangled, do not show magnetic order even at the zero temperature. The realization of a quantum spin liquid is a ...long-sought goal in condensed-matter physics. Although neutron scattering experiments on the two-dimensional spin-1/2 kagome lattice ZnCu3(OD)6Cl2 and triangular lattice YbMgGaO4 have found evidence for the hallmark of a quantum spin liquid at very low temperature (a continuum of magnetic excitations), the presence of magnetic and non-magnetic site chemical disorder complicates the interpretation of the data. Recently, the three-dimensional Ce3+ pyrochlore lattice Ce2Sn2O7 has been suggested as a clean, effective spin-1/2 quantum spin liquid candidate, but evidence of a spin excitation continuum is still missing. Here, we use thermodynamic, muon spin relaxation and neutron scattering experiments on single crystals of Ce2Zr2O7, a compound isostructural to Ce2Sn2O7, to demonstrate the absence of magnetic ordering and the presence of a spin excitation continuum at 35 mK. With no evidence of oxygen deficiency and magnetic/non-magnetic ion disorder seen by neutron diffraction and diffuse scattering measurements, Ce2Zr2O7 may be a three-dimensional pyrochlore lattice quantum spin liquid material with minimum magnetic and non-magnetic chemical disorder.
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.
A 2-Q antiferromagnetic order of the ferromagnetic dimers was found below T
= 2.9 K in the Shastry-Sutherland lattice BaNd
ZnS
by single crystal neutron diffraction. The magnetic order can be ...understood by the orthogonal arrangement of local Ising Nd spins, identified by polarized neutrons. A field was applied along 1 -1 0 to probe the observed metamagnetic transition in the magnetization measurement. The field decouples two magnetic sublattices corresponding to the propagation vectors q
= (½, ½, 0) and q
= (-½, ½, 0), respectively. Each sublattice shows a "stripe" order with a Néel-type arrangement in each single layer. The "stripe" order with q
remains nearly intact up to 6 T, while the other one with q
is suppressed at a critical field H
~1.7 T, indicating a partial disorder. The H
varies with temperature and is manifested in the H-T phase diagram constructed by measuring the magnetization in BaNd
ZnS
.
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