Strong Spin Frustration Research Articles

Observation of Topological Hall Effect in a Chemically Complex Alloy.

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.

Near Room-Temperature Intrinsic Exchange Bias in an Fe Intercalated ZrSe2 Spin Glass.

Some magnetic systems display a shift in the center of their magnetic hysteresis loop away from zero field, a phenomenon termed exchange bias. Despite the extensive use of the exchange bias effect, particularly in magnetic multilayers, for the design of spin-based memory/electronics devices, a comprehensive mechanistic understanding of this effect remains a longstanding problem. Recent work has shown that disorder-induced spin frustration might play a key role in exchange bias, suggesting new materials design approaches for spin-based electronic devices that harness this effect. Here, we design a spin glass with strong spin frustration induced by magnetic disorder by exploiting the distinctive structure of Fe intercalated ZrSe2, where Fe(II) centers are shown to occupy both octahedral and tetrahedral interstitial sites and to distribute between ZrSe2 layers without long-range structural order. Notably, we observe behavior consistent with a magnetically frustrated and multidegenerate ground state in these Fe0.17ZrSe2 single crystals, which persists above room temperature. Moreover, this magnetic frustration leads to a robust and tunable exchange bias up to 250 K. These results not only offer important insights into the effects of magnetic disorder and frustration in magnetic materials generally, but also highlight as design strategy the idea that a large exchange bias can arise from an inhomogeneous microscopic environment without discernible long-range magnetic order. In addition, these results show that intercalated TMDs like Fe0.17ZrSe2 hold potential for spintronic technologies that can achieve room temperature applications.

Strong spin frustration and magnetism in kagomé antiferromagnets LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu)

To study the effects of lanthanide ions on the geometrically frustrated antiferromagnets and their magnetic properties, we grew high-quality single crystals of LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu) by hydrothermal method and studied their crystal structures and magnetic properties. The refinements of the crystal structure referred to the powder x-ray diffraction data show that LnCu3(OH)6Br3 adopt a Kapellasite-type layer structure, which is isostructural to their chlorine analogue. Magnetic susceptibilities demonstrate that LnCu3(OH)6Br3 have strong antiferromagnetic coupling and a pronounced magnetic frustration effect. Magnetization measurements indicate canted antiferromagnetic ordering of Cu2+ ions around 16 K within the kagomé plane and weak ferromagnetic coupling. Moreover, shoulder-like anomalies in specific heat around 16 K could be a signature of emergent of magnetic ordering. The low-temperature negative magnetization and specific heat of LnCu3(OH)6Br3 (Ln = Nd, Sm, and Eu) indicate that Ln 3+ ions induce more exotic magnetic ground state properties.

Signatures of spin-liquid state in a 3D frustrated lattice compound KSrFe2(PO4)3 with S = 5/2

A quantum spin-liquid is a spin disordered state of matter in which spins are strongly correlated and highly entangled with low-energy excitations. It has been often found in two-dimensional S = ½, highly frustrated spin networks but rarely observed in three-dimensional (3D) frustrated quantum magnets. Here, KSrFe2(PO4)3, forming a complicated 3D frustrated lattice with a spin moment S = 5/2, is investigated by thermodynamic, neutron diffraction measurements and electronic structure calculations. Despite the relatively sizable Curie–Weiss temperature θCW = −70 K, a conventional magnetic long-range order is confirmed to be absent down to 0.19 K. The magnetic heat capacity data follow the power-law behavior at the lowest temperature region, supporting gapless excitations in a 3D spin-liquid state. Strong geometrical spin frustration responsible for the spin-liquid feature is understood as originating from the almost comparable five competing nearest-neighbor antiferromagnetic exchange interactions, which form the complicated 3D frustrated spin network. All these results suggest that the compound KSrFe2(PO4)3, representing a unique 3D spin frustrated network, could be a rare example of forming a gapless spin-liquid state even with a large spin moment of S = 5/2.

Synthesis and magnetic properties of two fluorophosphates A3Fe4(PO4)2F9 (A = K+ and NH4+) with a tetrahedral spin-cluster chain structure

The fluorophosphate compounds A3Fe4(PO4)2F9 (A ​= ​K+ and NH4+) were found to exhibit a channel framework constructed from {Fe4O8F10} tetrahedral clusters and PO4 tetrahedra, featuring a tetramer chain structure along the a-axis. Here these two compounds were synthesized by a fluorine-rich hydrothermal method. The purity of the samples was characterized by single crystal and powder XRD, EDS, and FT-IR measurements. The results of magnetic and heat capacity measurements confirmed that both compounds possess an antiferromagnetic ordering at ∼10 ​K and ∼6 ​K with Weiss temperatures of −341.5(8) K and −334.3(5) K, respectively, indicating strong spin frustration in the systems due to large frustration factors of ∼34 and ∼56. The presence of spin frustration is suggested to arise from the competition of very close magnetic interactions within the tetrahedral Fe4 clusters.

Understanding of the bond covalency nature with ionic electronegativity in perovskite cuprates La2CuMO6-x (M = Ti, Mn, Ru)

A bond valence sums (BVS) modified ionic electronegativity (EN) scale was suggested for better accommodating the double perovskite La2CuMO6-x (M ​= ​Ti, Mn, Ru) with mixed bonding nature. Both the M ​− ​O bond covalency fraction (f) and the cooperative B-site sublattice distortion are highlighted to connect structure and physical properties. Particularly, the Ti-based compound hosts the largest M/O ionic EN difference and hence yields the smallest f value while the Ru-based analogue exhibits the opposite result. As the consequence, robust spin disordering accompany with poor dc conductivity is convinced in the Ti-based sample owing to its featuring B-site ionic arrangement and profound suppression of 3d(Ti)-2p(O) orbital hybridization. For the Ru-based sample, strong ferromagnetic spin frustration and low charge carrier hopping potential might be essentially ascribed to its large f value through enhancing orbital degeneracy between 4d(eg)-2p σ and 4d(t2g)-2p π bonding states. The Mn-based sample with the moderate f value exhibits the similar magnetic and electric behaviors as those of Ru-based sample. Both the d(M)-p(O) orbital hybridization and the M ​− ​O bond covalency could be tuned by rationally selecting the M cation with proper ionic EN value. This work provides a deep understanding of the bond covalency nature in perovskite cuprates and might help to develop novel electronic functional materials.

Approach toward Iron(II) Coordination Polymers Based on Chain Motifs with Thiolate or Mixed Thiolate/Carboxylate Bridges: Structures and Magnetic Properties.

The search for iron-sulfur-based coordination polymers (CPs) has become an attractive field in recent years. Here we demonstrate how it is possible to synthesize new iron-sulfur-based CPs by solvothermal reactions of [CpFe(CO)2]2 (Cp = cyclopentadienyl) with two positional isomeric ligands 6-mercaptonicotinic acid (6-H2mna) and 2-mercaptoisonicotinic acid (2-H2mina) in different mixed-solvent systems. The reactions afforded, in moderate yields, a variety of desired CPs, namely, [Fe(6-Hmna)2] (1), [Fe3(6-Hmna)2(6-mna)2] (2), [Fe2(6-mna)2]·H2O (3), and [Fe(2 mina)(H2O)] (4 and 5). The structures of these compounds have been characterized by single-crystal X-ray diffraction, which reveals that they all contain 1D chain motifs of iron held together in different ways by thiolate or mixed thiolate/carboxylate bridges. These chains are further connected through the ligand backbones to form 3D networks of 1-3 and 5 and a 2D sheet of 4. Moreover, magnetic investigations indicate that both 1 and 4 display canted antiferromagnetic behavior with weak ferromagnetism, while 2 and 5 possess short-range antiferromagnetic order at ∼20 K. CP 3 exhibits paramagnetic behavior down to 2 K with strong spin frustration.

Exchange Bias in a Layered Metal-Organic Topological Spin Glass.

The discovery of conductive and magnetic two-dimensional (2D) materials is critical for the development of next generation spintronics devices. Coordination chemistry in particular represents a highly versatile, though underutilized, route toward the synthesis of such materials with designer lattices. Here, we report the synthesis of a conductive, layered 2D metal–organic kagome lattice, Mn3(C6S6), using mild solution-phase chemistry. Strong geometric spin frustration in this system mediates spin freezing at low temperatures, which results in glassy magnetic dynamics consistent with a rare geometrically frustrated (topological) spin glass. Notably, we show that this geometric frustration engenders a large, tunable exchange bias of 1625 Oe in Mn3(C6S6), providing the first example of exchange bias in a coordination solid or a topological spin glass. Exchange bias is a critical component in a number of spintronics applications, but it is difficult to rationally tune, as it typically arises due to structural disorder. This work outlines a new strategy for engineering exchange bias systems using single-phase, crystalline lattices. More generally, these results demonstrate the potential utility of geometric frustration in the design of new nanoscale spintronic materials.

Structure and frustrated magnetism of the two-dimensional triangular lattice antiferromagnet Na2BaNi(PO4)2 * *Project supported by the National Natural Science Foundation of China (Grant No. 11804137) and the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2020YQ03 and ZR2018BA026).

A new frustrated triangular lattice antiferromagnet Na2BaNi(PO4)2 was synthesized by high temperature flux method. The two-dimensional triangular lattice is formed by the Ni2+ ions with S = 1. Its magnetism is highly anisotropic with the Weiss constants θ CW = –6.615 K (H ⊥ c) and –43.979 K (H ∥ c). However, no magnetic ordering is present down to 0.3 K, reflecting strong geometric spin frustration. Our heat capacity measurements show substantial residual magnetic entropy existing below 0.3 K at zero field, implying the presence of low energy spin excitations. These results indicate that Na2BaNi(PO4)2 is a potential spin liquid candidate with spin-1.

Extraordinary magnetic properties of double perovskite Eu2CoMnO6 wide band gap semiconductor

Some novel magnetic behaviours in double perovskite Eu2CoMnO6 (ECMO) have been reported. The x-ray photoemission spectroscopy study shows the presence of mixed valence states of transition metal ions. The UV–visible absorption spectroscopic study suggests that the ECMO has a direct wide band gap. A second-order magnetic phase transition as a sudden jump in the magnetization curve has been observed around 124.5 K. The large bifurcation between the zero field cooling and field cooling, suggests existence of strong spin frustration in the system. The inverse DC susceptibility confirms the presence of the Griffiths like phase. Sharp steps in magnetization have been observed in the M–H curve at 2 K, which vanishes on increasing temperature. The AC susceptibility study demonstrates the Hopkinson like effect as well as the presence of volume spin-glass-like behaviour. The temperature dependent Raman spectrum shows the presence of spin–phonon coupling.

Spin-glass behavior in Co-based antiperovskite compound SnNCo3

Spin-glass (SG) behavior is observed in the antiperovskite compound SnNCo3, which is confirmed by the relevant measurements and specific parameters [τ0 = 1.397 × 10−11 s, T0 = 2.705 K, zv = 6.857 for χ′(T)]. The typical relaxation behavior of isothermal remanent magnetization (MIRM) exhibits strong spin frustration in the system. The δM plots illustrate the strong exchange coupling and demagnetizing interaction around the freezing temperature (Tf). Besides, the density functional theory calculations demonstrate that the antiferromagnetic ground has lower energy than that of the ferrimagnetic and ferromagnetic states. Our calculations exhibit that the electrons of Co-3d play a dominant role around the Fermi energy (EF), while Sn-5p and N-2p are mainly distributed from −8 eV to −5 eV and −5 eV to 5 eV, respectively. The interactions of Sn-Co and N-Co disclose the magnetic ground state and bond nature of the compound, which should be an evidential response to the SG behavior in SnNCo3.

Na2Mn3Se4: Strongly Frustrated Antiferromagnetic Semiconductor with Complex Magnetic Structure

A new ternary selenide, Na2Mn3Se4, was prepared by a stoichiometric reaction between Na2Se4 and metallic Mn at 923 K. Crystal structure determination revealed a new structure type, built of alternating layers of Na+ ions and [Mn3Se4]2- anionic slabs. Band structure calculations indicate that Na2Mn3Se4 is an indirect band gap semiconductor with Eg = 1.59 eV, although a direct band gap is only marginally larger, at 1.64 eV. The material shows antiferromagnetic (AFM) ordering at 27 K, while the Weiss constant of ∼-400 K suggests much stronger nearest-neighbor AFM exchange between the Mn sites. This discrepancy is attributed to the strong spin frustration caused by a triangulated arrangement of the Mn sites in the [Mn3Se4]2- layer. The magnetic frustration leads to the stabilization of a complex AFM ordered structure with non-collinear arrangement of the Mn magnetic moments, as established from neutron diffraction data.

Fullerene and endometallofullerene Kagome lattices with symmetry-forced spin frustration.

Salts of fullerene C60˙- and endometallofullerene Sc3N@Ih-C80˙- radical anions with Bu3MeP+ cations ((Bu3MeP+)3(C60˙-)3·C6H4Cl2 (1) and (Bu3MeP+)3(Sc3N@Ih-C80˙-)3·C6H4Cl2 (2)) have been obtained. The C3 symmetry of the Bu3MeP+ cation provides 2D Kagome lattices with an equilateral triangle arrangement of fullerenes in accordance with trigonal crystal symmetry P31m. The C60˙- and Sc3N@Ih-C80˙- radical anions preserve their monomeric forms in 1 and 2 with the S = 1/2 spin state down to 1.9 K. The close packing of the fullerene radical anions results in strong antiferromagnetic coupling of the spins with Weiss temperatures of -108 K for 1 and -43 K for 2. Compound 1 is a rare example of a magnetic system in which in spite of the strong magnetic coupling of spins no long-range ordering is observed down to 1.9 K. The 13C NMR spectra of the 13C enriched sample of 1 support the absence of the antiferromagnetic ordering of spins down to 1.5 K. Thus, the crystals of 1 preserve large spin frustration forced by the trigonal symmetry. Therefore, compound 1 is a promising candidate for the first observation of a quantum spin liquid (QSL) state in a fullerene-based system. Isostructural salt 2 is the first compound that contains monomeric paramagnetic Sc3N@Ih-C80˙- radical anions stable down to 1.9 K, which show strong spin frustration. These data indicate the ability of endometallofullerenes to give exotic magnetic systems such as QSLs.

Physical properties of MgAl2O4, CoAl2O4, NiAl2O4, CuAl2O4, and ZnAl2O4 spinels synthesized by a solution combustion method

MgAl2O4, CoAl2O4, NiAl2O4, CuAl2O4, and ZnAl2O4 spinels have been synthesized by a solution combustion method following with secondary calcination. These aluminates were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, and scanning electron microscope. As-prepared samples show pure spinel phase with good crystalline, vivid colour, fine particle size, and porous structure. Phenomenon of anti-site disorder in these samples has been investigated, which shows dependence on the radius of the A-site ions. Magnetic properties and heat capacities of these samples were further characterized by magnetic property measurement system and physical property measurement system. Among the magnetic compounds, NiAl2O4 is typical antiferromagnetic, CoAl2O4 shows interesting spin-glass-like ground state, and CuAl2O4 exhibits characteristic of spin liquid due to strong spin frustration and site inversion effects in the system. Heat capacities of these compounds are further studied. The values of the Debye and Einstein temperatures are given through fitting the data. Furthermore, the magnetic contribution ΔCm and magnetic entropies ΔSm of the magnetic samples are obtained after subtraction of the phonon contribution.

Origin of Ferrimagnetism and Ferroelectricity in Room-Temperature Multiferroic ϵ−Fe2O3

In devices, being able to control magnetism with electric rather than magnetic fields is very appealing, and magnetoelectric multiferroic materials are key to this functionality. \ensuremath{\epsilon}-FeO is a room-temperature multiferroic featuring both strong magnetization and strong ferroelectricity simultaneously, but the origin of this multiferroicity is unclear. This work shows that the compound's ferrimagnetism arises from strong spin frustration, and that a low-energy transition state is responsible for switchable ferroelectricity. At a higher level, these results also suggest how to search for other high-performance multiferroics, for use in next-generation data storage.

Low-Temperature Lattice Effects in the Spin-Liquid Candidate κ-(BEDT-TTF)2Cu2(CN)3

The quasi-two-dimensional organic charge-transfer salt κ -(BEDT-TTF) 2 Cu 2 (CN) 3 is one of the prime candidates for a quantum spin-liquid due the strong spin frustration of its anisotropic triangular lattice in combination with its proximity to the Mott transition. Despite intensive investigations of the material’s low-temperature properties, several important questions remain to be answered. Particularly puzzling are the 6 K anomaly and the enigmatic effects observed in magnetic fields. Here we report on low-temperature measurements of lattice effects which were shown to be particularly strongly pronounced in this material (R. S. Manna et al., Phys. Rev. Lett. 2010, 104, 016403)). A special focus of our study lies on sample-to-sample variations of these effects and their implications on the interpretation of experimental data. By investigating overall nine single crystals from two different batches, we can state that there are considerable differences in the size of the second-order phase transition anomaly around 6 K, varying within a factor of 3. In addition, we find field-induced anomalies giving rise to pronounced features in the sample length for two out of these nine crystals for temperatures T < 9 K. We tentatively assign the latter effects to B-induced magnetic clusters suspected to nucleate around crystal imperfections. These B-induced effects are absent for the crystals where the 6 K anomaly is most strongly pronounced. The large lattice effects observed at 6 K are consistent with proposed pairing instabilities of fermionic excitations breaking the lattice symmetry. The strong sample-to-sample variation in the size of the phase transition anomaly suggests that the conversion of the fermions to bosons at the instability is only partial and to some extent influenced by not yet identified sample-specific parameters.

Highly Spin-Frustrated Magnetism in the Topochemically Prepared Triangular Lattice Cluster Magnets Na3 A2 (MoO4 )2 Mo3 O8 (A=In, Sc).

The physical properties of novel cluster-based triangular lattice antiferromagnets Na3 A2 (MoO4 )2 Mo3 O8 (A=In, Sc), synthesized through a topochemical Na-intercalation to nonmagnetic Na2 A2 (MoO4 )2 Mo3 O8 , are reported. The S=1/2 [Mo3 ]11+ clusters form a regular triangular lattice, which gives the magnetic system a strong geometrical spin frustration effect. Despite the strong antiferromagnetic couplings among [Mo3 ]11+ clusters, they show no long-range magnetic orderings down to 0.5 K with the finite residual magnetic entropy. The ground states of Na3 A2 (MoO4 )2 Mo3 O8 have been characterized as a quantum spin liquid, owing to the strong spin frustration of cluster spins on the triangular lattice.

Design and Preparation of a Quantum Spin Liquid Candidate κ-(ET)2Ag2(CN)3 Having a Nearby Superconductivity

Abstract Similar to the first quantum spin liquid (QSL) candidate, κ-(ET)2Cu2(CN)3 (1), newly prepared κ-(ET)2Ag2(CN)3 (2) is a dimer Mott insulator with a QSL ground state at ambient pressure and exhibits metallic and superconducting states next to the QSL state under pressure, where ET is bis(ethylenedithio)tetrathiafulvalene. The packing of the ET dimer, which corresponds to a single spin site (S = 1/2), in 2 is similar to that in 1. Salt 2 afforded a two-dimensional equilateral triangular spin lattice having strong geometrical spin frustration (t′/t = 0.967) similar to 1 (t′/t = 1.09), where t and t′ are the interdimer transfer interactions. The geometric relationship between the spin site and the opening in the anion layer in 2 is considerably different from that in 1, resulting in more dispersed packing of ET dimers, narrower bandwidth W, and larger on-site Coulomb repulsion energy U for 2 than 1. As a consequence, 2 has a higher electron correlation U/W, more robust QSL state for wider pressure range and is allocated far away from the itinerant region compared to those of 1. The narrower bandwidth is consistent with a higher superconducting critical temperature for 2 than 1 under pressure.

Layered Cu7(TeO3)2(SO4)2(OH)6 with Diluted Kagomé Net Containing Frustrated Corner-Sharing Triangles.

The half-spin Kagomé antiferromagnet is one of the most promising candidates for the realization of a quantum spin liquid state because of its inherent frustration and quantum fluctuations. The search for candidates for quantum spin liquids with novel spin topologies is still a challenge. Herein, we report a new diluted Kagomé lattice in Cu7(TeO3)2(SO4)2(OH)6, showing a 9/16-depleted triangle lattice, where the corner-sharing triangle units [Cu5(OH)6O8] are separated by CuO2(OH)2. Magnetic measurements show that the title compound does not exhibit long-range antiferromagnetic order down to 2 K, suggesting strong spin frustration with f > 19.

Strong spin frustration and negative magnetization in LnCu3(OH)6Cl3 (Ln = Nd and Sm) with triangular lattices: the effects of lanthanides.

Herbertsmithite- and kapellasite-type compounds with triangular lattices (i.e. Kagomé) as the most promising candidates for realizing the exotic quantum spin liquid (QSL) state have recently attracted significant attention in condensed matter physics and materials science but are often adversely affected by dimensional imperfections arising from significant cation mixing. Also, interaction mechanisms between the Kagomé lattices and ionic impurities remain unclear. Herein we report on the synthesis, crystal structures and magnetic properties of a new class of kapellasite-type compounds LnCu3(OH)6Cl3 (Ln = Nd and Sm) with two overlapped triangular lattices. These compounds are characterized by the triangular lattices of Cu2+ superimposed by another triangular lattice of paramagnetic Ln3+. The magnetic properties of LnCu3(OH)6Cl3 feature strong spin frustrations as well as antisymmetrical Dzyaloshinskii-Moriya interactions resulting in canted antiferromagnetic ordering with the Néel temperature (TN) of ∼20 K and ∼18 K for NdCu3(OH)6Cl3 and SmCu3(OH)6Cl3, respectively. Moreover, negative magnetization at low temperatures was firstly observed in Kagomé lattice compounds, arising from geometrical spin frustration and competing interactions within two overlapped triangular lattices.