Complex Magnetic Order Research Articles

Complex magnetic ordering in the noncentrosymmetric metal PrCuGe3

We report on the successful synthesis of PrCuGe3 which crystallizes in the tetragonal BaNiSn3-type structure (space group: I4mm) with lattice parameters, a = 4.1470(2) Å and c = 10.185(3) Å. The material is characterized using dc magnetic susceptibility χ(T), ac magnetic susceptibility χac(T), isothermal magnetization M(B), specific heat Cp(T), and electrical resistivity ρ(T) measurements. PrCuGe3 undergoes a long-range ferromagnetic ordering at TC = 23 K, which is followed by another transition below Ttr = 17 K. TC has a quite small anomaly in Cp(T) and ρ(T) despite displaying large associated anomalies in both χ(T) and χac(T) and a sizable ferromagnetic ordered moment. Instead the transition at Ttr has a much larger anomaly in Cp(T) and ρ(T) but essentially no signal in χ(T) and χac(T). Such observations in PrCuGe3 are nontrivial. A Sommerfeld coefficient γ = 375 mJ/(mol K2) is extracted from specific heat which indicates a significant enhancement of the quasi-particle mass at low temperature. Hence, PrCuGe3 exemplifies a system with the coexistence of complex magnetic ordering and a putative heavy-fermion state.

Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix in EuIn2As2

Understanding and manipulating emergent phases, which are themes at the forefront of quantum-materials research, rely on identifying their underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic order influences the electronic band structure and can lead to exotic topological effects. However, identifying symmetry of a magnetically ordered phase can pose a challenge, particularly in the presence of small domains. Here we introduce a multimodal approach for determining magnetic structures, which combines symmetry-sensitive optical probes, scattering, and group-theoretical analysis. We apply it to EuIn2As2, a material that has received attention as a candidate axion insulator. While first-principles calculations predict this state on the assumption of a simple collinear antiferromagnetic structure, subsequent neutron-scattering measurements reveal a much more intricate magnetic ground state characterized by two coexisting magnetic wave vectors reached by successive thermal phase transitions. The proposed high- and low-temperature phases are a spin helix and a state with interpenetrating helical and Néel antiferromagnetic order termed a “broken helix,” respectively. Employing a multimodal approach, we identify the magnetic structure associated with these two phases of EuIn2As2. We find that the higher-temperature phase is characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower-temperature structure is similar to the broken helix, with one important difference: Because of local strain, the relative orientation of the magnetic structure and the lattice is not fixed. Consequently, the symmetry required to protect the axion phase is not generically protected in EuIn2As2, but we show that it can be restored if the magnetic structure is tuned with uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions that account for the complex magnetic order in EuIn2As2. Our work highlights the importance of a multimodal approach in determining the symmetry of complex order parameters. Published by the American Physical Society 2024

Advancing Heteroanionicity in Zintl Phases: Crystal Structures, Thermoelectric and Magnetic Properties of Two Quaternary Semiconducting Arsenide Oxides, Eu8Zn2As6O and Eu14Zn5As12O.

Two novel quaternary oxyarsenides, Eu8Zn2As6O and Eu14Zn5As12O, were synthesized through metal flux reactions, and their crystal structures were established by single-crystal X-ray diffraction methods. Eu8Zn2As6O crystallizes in the orthorhombic space group Pbca, featuring polyanionic ribbons composed of corner-shared triangular [ZnAs3] units, running along the [100] direction. The structure of Eu14Zn5As12O crystallizes in the monoclinic space group P2/m and its anionic substructure can be described as an infinite "ribbonlike" chain comprised of [ZnAs3] trigonal-planar units, although the structural complexity here is greater and also amplified by disorder on multiple crystallographic positions. In both structures, the O2- anion occupies an octahedral void with six neighboring Eu2+ cations. Formal electron counting, electronic structure calculations, and transport properties reveal the charge-balanced semiconducting nature of these heteroanionic Zintl phases. High-temperature thermoelectric transport properties measurements on Eu14Zn5As12O reveal relatively high resistivity (ρ500K = 8 Ω·cm) and Seebeck coefficient values (S500K = 220 μV K-1), along with a low concentration and mobility of holes as the dominant charge-carriers (n500K = 8.0 × 1017 cm-3, μ500K = 6.4 cm2/V s). Magnetic studies indicate the presence of divalent Eu2+ species in Eu14Zn5As12O and complex magnetic ordering, with two transitions observed at T1 = 21.6 K and T2 = 9 K.

Complex magnetic ordering in EuAl4–A 151Eu Mössbauer study

151Eu Mössbauer spectroscopy has been used to investigate the behaviour of EuAl4 through the four magnetic transitions that occur below 16 K. We find clear evidence for the first transition (TN1, the onset of order) where an incommensurate modulated magnetic structure appears, and the third (TN3) where the modulation disappears at the tetragonal → orthorhombic structural transition. We see no changes at the lowest transition (TN4) but find that the modulation amplitude passes through a maximum at TN2. Data on the isostructural but magnetically simpler EuGa4 are also presented for comparison.

Bipolar magnetic switching and large exchange-bias in Fe-substituted SmCrO3

Rare-earth orthochromite is an interesting system in the view of its complex magnetic ordering due to competing interaction between different magnetic ions. Here, Fe-substituted SmCrO3 samples were prepared by solid-state route to investigate their intriguing magnetic properties towards exploring its application potential. The magnetic studies revealed antiferromagnetic (AFM) ordering at Nèel temperature (T N) ∼ 181 K, magnetic compensation temperatures (T COMP) at ∼137 K and 50 K, and spin-reorientation temperature (T SR) at 64 K in SmCr0.8Fe0.2O3 sample. Additionally, the system exhibited negative magnetization under field-cooled conditions which allowed the field as well as temperature controllable magnetization switching behavior. Notably, the Fe-substituted SmCrO3 sample displayed a remarkable exchange bias (H EB) value of ∼1.39 T at 10 K due to the coexistence of ferromagnetic and AFM ordering at different cationic sites. The M–H loops recorded under positive and negative field-cooled conditions ruled out the minor-loop effect. Theoretical models applied on the training effect studies confirmed the observed exchange-bias effect.

Observation of negative terahertz photoconductivity in strongly correlated electron-doped CaMnO3 thin film

Electron-doped Ca0.96Ce0.04MnO3 (CCMO) possesses a unique band structure and exhibits a giant topological Hall effect contrary to other correlation-driven manganites known for insulator-to-metal transition, magnetoresistance, complex magnetic order, etc. The interaction mechanisms among the fundamental entities and their dynamical evolutions responsible for this unusual topological phase are yet to be understood. Here, we employ time-averaged and sub-picosecond time-resolved terahertz (THz) spectroscopy to explore the low-energy steady-state and ultrafast carrier dynamics, respectively, to unravel the complexity of charge carriers during their transition from a non-equilibrium state to the ground state in CCMO thin film. The THz optical conductivity confirms the presence of dichotomic charge carriers, i.e. heavy and light carriers throughout the temperature range of 15–300 K. A rare observation of both positive and negative photoconductivities along with a sharp crossover between the two resolved to a few picoseconds of illumination confirms the formation of polaron with a lifetime of a few nanoseconds. These optical evidences of dichotomic charge carriers, along with manipulation of the sign of photoconductivity induced by dynamics of related quasiparticles could facilitate a new mechanism for ultrafast optoelectronic switching devices.

Electronic and magnetic properties of TATA-DNA sequence driven by chemical functionalization.

The TATA box is a promoter sequence able to interact directly with the components of the basal transcription initiation machinery. We investigate the changes in the electronic and magnetic properties of a TATA-DNA sequence when functionalized with different chemical groups; using the first-principles density functional theory specifically, the TATA-DNA sequences were functionalized with methyl groups (CH3 , methylation), amino groups (NH2 , amination), imine groups (NH, imination), chloroamine groups (NCl2 , chloramination), H-adatom (hydrogenation), and Cl-adatom (chlorination). The functional groups were anchored at nitrogen atoms from adenine and oxygen atoms from thymine at sites pointed as reactive regions. We demonstrated that chemical functionalization induces significant changes in charge transfer, hydrogen bond distance, and hydrogen bond energy. The hydrogenation and imination increased the hydrogen bond energy. Results also revealed that the chemical functionalization of DNA molecules exhibit a ferromagnetic ground state, reaching magnetization up to 4.665 μB and complex magnetic ordering. We further demonstrated that the functionalization could induce tautomerism (proton migration in the base pair systems). The present study provides a theoretical basis for understanding the functionalization further into DNA molecules and visualizing possible future applications.

Intertwined charge and spin orders through the exchange-interaction on a square lattice

We study exotic electronic charge orders and magnetic spin orders in a lattice model which encloses conducting electrons coupled with localized magnetic moments via the exchange-like interaction. By means of numerical simulations based on kernel polynomials method combined with Langevin dynamics, we systematically map out phase diagrams versus exchange coupling strength. At the half-filling case n=1/2 (one electron per lattice site on average), a robust antiferromagnetic insulator is formed at low temperature regime. Away from the half-filling, competition between itinerant electrons and local spin moments drives intertwined charge and spin orders. As a concrete example, at the three-quarter filling n=3/4, we identify various spin orders such as incommensurate stripe order and a long-sought double-Q state. Especially, we demonstrate that such double-Q magnetic order leads to a Dirac semi-metal phase on the square lattice, which is identified by the electronic density of states and optical conductivity. Additionally, we also uncover the spin dynamics of magnetic orders via Landau-Lifshitz method. These findings demonstrate that the interplay between itinerant electrons and preformed local spin moments via the exchange interaction is able to produce complex magnetic orders and electronic charge orders.

Magnetotransport as a probe for the interplay between Sm and Fe magnetism in SmFeAsO

The complex magnetic ordering of parent compounds of most unconventional superconductors is crucial for the understanding of high-temperature superconductivity (SC). Within this framework, we have performed temperature-dependent magnetotransport experiments on a single crystal of SmFeAsO, a parent compound of iron pnictide superconductors. We observe multiple features in the measured transport properties at temperatures below the antiferromagnetic (AFM) ordering of Sm, , which evolve with in-plane magnetic field, suggesting a rich variety of metamagnetic transitions never before observed in this compound. Considering that transport mainly involves Fe d orbitals at the Fermi level, these findings suggest that the features originate from magnetic transitions of the Fe moments sublattice, which in turn may be induced by magnetic transitions of the Sm moments sublattice via the interaction between Fe and Sm moments. We outline a possible scenario in which the Fe moments, strongly affected by the Sm ordering below T NSm, reorder to an in-plane canted AFM structure, which is washed out by the application of an in-plane magnetic field up to 9 T. Our work shows that transport properties are a valuable tool for investigating magnetic ordering in iron pnictide parent compounds, where the interplay of magnetism and SC is believed to be the origin of high-temperature SC.

Skyrmionic spin structures in layered Fe5GeTe2 up to room temperature

The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temperature is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be observed even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic density functional theory and spin-model calculations. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets.

Nano-scale collinear multi-Q states driven by higher-order interactions

Complex magnetic order arises due to the competition of different interactions between the magnetic moments. Recently, there has been an increased interest in such states not only to unravel the fundamental physics involved, but also with regards to applications exploiting their unique interplay with moving electrons. Whereas it is the Dzyaloshinskii-Moriya interaction (DMI) that has attracted much attention because of its nature to induce non-collinear magnetic order including magnetic-field stabilized skyrmions, it is the frustration of exchange interactions that can drive magnetic order down to the nano-scale. On top of that, interactions between multiple spins can stabilize two-dimensional magnetic textures as zero-field ground states, known as multi-Q states. Here, we introduce a two-dimensional itinerant magnet with various competing atomic-scale magnetic phases. Using spin-polarized scanning tunneling microscopy we observe several zero-field uniaxial or hexagonal nano-scale magnetic states. First-principles calculations together with an atomistic spin model reveal that these states are stabilized by the interplay of frustrated exchange and higher-order interactions while the DMI is weak. Unexpectedly, it is found that not only non-collinear magnetic states arise, but that higher-order interactions can also lead to collinear nano-scale multi-Q states.

Complex Magnetic Order in Topochemically Reduced Rh(I)/Rh(III) LaM0.5Rh0.5O2.25 (M = Co, Ni) Phases.

Topochemical reduction of the cation-disordered perovskite oxides LaCo0.5Rh0.5O3 and LaNi0.5Rh0.5O3 with Zr yields the partially anion-vacancy ordered phases LaCo0.5Rh0.5O2.25 and LaNi0.5Rh0.5O2.25, respectively. Neutron diffraction and Hard X-ray photoelectron spectroscopy (HAXPES) measurements reveal that the anion-deficient phases contain Co1+/Ni1+ and a 1:1 mixture of Rh1+ and Rh3+ cations within a disordered array of apex-linked MO4 square-planar and MO5 square-based pyramidal coordination sites. Neutron diffraction data indicate that LaCo0.5Rh0.5O2.25 adopts a complex antiferromagnetic ground state, which is the sum of a C-type ordering (mM5+) of the xy-components of the Co spins and a G-type ordering (mΓ1+) of the z-components of the Co spins. On warming above 75 K, the magnitude of the mΓ1+ component declines, attaining a zero value by 125 K, with the magnitude of the mM5+ component remaining unchanged up to 175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co1+ cations in MO4 square-planar and MO5 square-based pyramidal coordination sites. LaNi0.5Rh0.5O2.25 shows no sign of long-range magnetic order at 2 K – behavior that can also be explained on the basis of the d-orbital occupation of the Ni1+ centers.

C-type and Γ2 antiferromagnetic orders in the complex magnetic ordering of Yb0.6Sr0.4MnO3

C-type and Γ2 antiferromagnetic orders in the complex magnetic ordering of Yb0.6Sr0.4MnO3

Emergence of exchange bias effect in double perovskite La2Cu0.9Fe0.1IrO6 originated from Fe doping

The effect of partial electron doping (10% Fe) in the antiferromagnetic (AFM) double perovskite La2CuIrO6 (LCO) has been investigated at the macroscopic level using X-ray diffraction (XRD), and DC Magnetic measurements to study the evolution of complex magnetic order. The experimental observations indicate a significant modification in the nature of microscopic magnetic interaction induced by charge imbalance resulting in strong ferromagnetic (FM) ordering in the parent antiferromagnetic LCO. The nature of magnetic interaction results in a cluster-glass state (CG) with novel zero-field-cooled exchange bias (ZEB) and conventional exchange bias (CEB) effect at low temperature. Obtained ZEB field value is 0.735 kOe at 3 K, while an unusually large CEB field of 9.24 kOe for a bias cooling field (HB) of 50 kOe has been observed at the same temperature. The doped LCO also shows the training effect in field cooled M(H) measurements, following the frozen spin relaxation behavior, indicating the cluster-glass (CG) nestle in the interface/pinning boundary. The emerged complex exchange interaction among different magnetic ordering, i.e., FM, AFM, and CG, significantly influences the CEB as well as ZEB.

Unusual Magnetic Features in Two-Dimensional Fe5GeTe2 Induced by Structural Reconstructions.

Recent experiments on Fe5GeTe2 suggested the presence of a symmetry breaking of its conventional crystal structure. Here, using density functional theory calculations, we elucidate that the stabilization of the (√3 × √3)R30° supercell structure is caused by the swapping of Fe atoms occurring in the monolayer limit. The swapping to the vicinity of Te atoms is facilitated by the spontaneous occurrence of Fe vacancy and its low diffusion barrier. Our calculated magnetic exchange parameters show the simultaneous presence of ferromagnetic and antiferromagnetic exchange among a particular type of Fe atom. The Fe sublattice projected magnetization obtained from Monte Carlo simulations clearly demonstrates an exotic temperature-dependent behavior of this Fe type along with a large canting angle at T = 0 K, indicating the presence of a complex noncollinear magnetic order. We propose that the low-temperature crystal structure results from the swapping between two sublattices of Fe, giving rise to peculiar magnetization obtained in experiments.

Multipolar exchange interaction and complex order in insulating lanthanides

In insulating lanthanides, unquenched orbital momentum and weak crystal-field (CF) splitting of the atomic $J$ multiplet at lanthanide ions result in a highly ranked (multipolar) exchange interaction between them and a complex low-temperature magnetic order not fully uncovered by experiment. Explicitly correlated ab initio methods proved to be highly efficient for an accurate description of CF multiplets and magnetism of individual lanthanide ions in such materials. Here we extend this ab initio methodology and develop a first-principles microscopic theory of multipolar exchange interaction between $J$ multiplets in $f$-metal compounds. The key point of the approach is a complete account of Goodenough's exchange mechanism along with traditional Anderson's superexchange and other contributions, the former being dominant in many lanthanide materials. Application of this methodology to the description of the ground-state order in the neodymium nitride with rocksalt structure reveals the multipolar nature of its ferromagnetic order. We found that the primary and secondary order parameters (of ${T}_{1u}$ and ${E}_{g}$ symmetry, respectively) contain non-negligible $J$-tensorial contributions up to the ninth order. The calculated spin-wave dispersion and magnetic and thermodynamic properties show that they cannot be simulated quantitatively by confining to the ground CF multiplet on the Nd sites. Our results demonstrate that the ab initio approach to the low-energy Hamiltonian represents a powerful tool for the study of materials with complex magnetic order.

Superconducting YAu3Si and Antiferromagnetic GdAu3Si with an Interpenetrating Framework Structure Built from 16-Atom Polyhedra.

Investigations of reaction mixtures REx(Au0.79Si0.21)100–x (RE = Y and Gd) yielded the compounds REAu3Si which adopt a new structure type, referred to as GdAu3Si structure (tP80, P42/mnm, Z = 16, a = 12.8244(6)/12.7702(2) Å, and c = 9.0883(8)/9.0456(2) Å for GdAu3Si/YAu3Si, respectively). REAu3Si was afforded as millimeter-sized faceted crystal specimens from solution growth employing melts with composition RE18(Au0.79Si0.21)82. In the GdAu3Si structure, the Au and Si atoms are strictly ordered and form a framework built of corner-connected, Si-centered, trigonal prismatic units SiAu6. RE atoms distribute on 3 crystallographically different sites and each attain a 16-atom coordination by 12 Au and 4 Si atoms. These 16-atom polyhedra commonly fill the space of the unit cell. The physical properties of REAu3Si were investigated by heat capacity, electrical resistivity, and magnetometry techniques and are discussed in the light of theoretical predictions. YAu3Si exhibits superconductivity around 1 K, whereas GdAu3Si shows a complex magnetic ordering, likely related to frustrated antiferromagnets exhibiting chiral spin textures. GdAu3Si-type phases with interesting magnetic and transport properties may exist in an extended range of ternary RE–Au–Si systems, similar to the compositionally adjacent cubic 1/1 approximants RE(Au,Si)∼6.

Learning a compass spin model with neural network quantum states

Neural network quantum states provide a novel representation of the many-body states of interacting quantum systems and open up a promising route to solve frustrated quantum spin models that evade other numerical approaches. Yet its capacity to describe complex magnetic orders with large unit cells has not been demonstrated, and its performance in a rugged energy landscape has been questioned. Here we apply restricted Boltzmann machines (RBMs) and stochastic gradient descent to seek the ground states of a compass spin model on the honeycomb lattice, which unifies the Kitaev model, Ising model and the quantum 120° model with a single tuning parameter. We report calculation results on the variational energy, order parameters and correlation functions. The phase diagram obtained is in good agreement with the predictions of tensor network ansatz, demonstrating the capacity of RBMs in learning the ground states of frustrated quantum spin Hamiltonians. The limitations of the calculation are discussed. A few strategies are outlined to address some of the challenges in machine learning frustrated quantum magnets.

Complex magnetic ordering in RE5Pd2In4 (RE = Tb-Tm) compounds investigated by neutron diffraction and magnetometric measurements

The RE5Pd2In4 (RE = Tb-Tm) compounds crystallize with the orthorhombic Lu5Ni2In4-type crystal structure (Pbam space group). In this work we report results of structural and magnetic studies by means of X-ray and neutron diffraction as well as dc and ac magnetometric data. Magnetic susceptibility and neutron diffraction data revealed rare-earth moments order at low temperatures with complex magnetic structures showing a cascade of temperature-induced transitions. The magnetic ordering temperatures are found to be 97, 88, 28.5, 16.5 and 4.3 K for RE = Tb, Dy, Ho, Er and Tm, respectively. Magnetic structures related to the propagation vector k→1=[0,0,0] are found just below the magnetic ordering temperatures in majority of the investigated compounds (RE = Tb-Er). Below the Curie temperature TC they have purely ferromagnetic character in Tb5Pd2In4 and Dy5Pd2In4. A ferrimagnetic order finally sets at lower temperatures in Dy5Pd2In4, while in Ho5Pd2In4 two magnetic phases related to k→1 are observed: the antiferromagnetic one (phase I) and the ferrimagnetic one (phase II, coexisting with phase I at lower temperatures). Er5Pd2In4 is a canted antiferromagnet with additional ferromagnetic component developing at lower temperatures. A purely antiferromagnetic component of magnetic structure with enlarged magnetic unit cell appears with decreasing temperature in Tb5Pd2In4 (k→2=[0,12,0] and k→3=[0,12,12]) while in Ho5Pd2In4 such component (k→4=[14,0,0]) is present within whole temperature range below the magnetic ordering temperature. Magnetic structure of Tm5Pd2In4, exceptionally, has no k→1 component, but is an antiferromagnetic incommensurate one related to two propagation vectors: k→5=[0.073(3),0.451(1),12] and k→6=[0,0.335(2),12]. In majority of the compounds (RE = Tb-Er) the first rare-earth 4g site (noted as 4g1) orders at lower temperature than two remaining sites (2a and 4g2). The direction of the magnetic moments depends on rare-earth element involved and indicates an influence of single-ion anisotropy in the crystalline electric field (CEF).

Structure, Magnetic Properties, and 151 Eu Mössbauer Spectra of the Indium‐rich Phase Eu 3 Rh 2 In 15

Structure, Magnetic Properties, and ¹⁵¹ Eu Mössbauer Spectra of the Indium‐rich Phase Eu ₃ Rh ₂ In ₁₅