Nonferromagnetic State Research Articles

Signatures of fractional quantum anomalous Hall states in twisted MoTe2.

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field owing to intrinsic ferromagnetism1-3. In the presence of strong electron-electron interactions, fractional QAH (FQAH) states at zero magnetic field can emerge4-8. These states could host fractional excitations, including non-Abelian anyons-crucial building blocks for topological quantum computation9. Here we report experimental signatures of FQAH states in a twisted molybdenum ditelluride (MoTe2) bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole-filled moiré minibands. Using trion photoluminescence as a sensor10, we obtain a Landau fan diagram showing linear shifts in carrier densities corresponding to filling factor v = -2/3 and v = -3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of [Formula: see text] and [Formula: see text], respectively. Moreover, the v = -1 state exhibits a dispersion corresponding to Chern number -1, consistent with the predicted QAH state11-14. In comparison, several non-ferromagnetic states on the electron-doping side do not disperse, that is, they are trivial correlated insulators. The observed topological states can be electrically driven into topologically trivial states. Our findings provide evidence of the long-sought FQAH states, demonstrating MoTe2 moiré superlattices as a platform for exploring fractional excitations.

Certain exact many-body results for Hubbard model ground states testable in small quantum dot arrays

We present several interesting phenomena related to flatband ferromagnetism in the Hubbard model. The first is a mathematical theorem stating certain conditions under which a flatband ferromagnetic must necessarily be degenerate with a nonferromagnetic state. This theorem is generally applicable and geometry-independent, but holds only for a small number of holes in an otherwise filled band. The second phenomenon is a peculiar example where the intuition fails that particles prefer to doubly occupy low-energy states before filling higher-energy states. Lastly, we show a pattern of ferromagnetism which appears in small pentagonal and hexagonal plaquettes at filling factors of roughly 3/10 and 1/4. These examples require only a small number of lattice sites, and may be observable in quantum dot arrays currently available as laboratory spin qubit arrays.

Spin-flip-induced superfluidity in a ring of spinful hard-core bosons

The t - J Hamiltonian of the spinful hard-core bosonic ring in the Nagaoka limit is solved. The energy spectrum becomes quantized due to presence of spin, where each energy level corresponds to a cyclic permutation state of the spin chains. The ground state is true ferromagnetic when the ring contains N = 2, 3 spinful hard-core bosons; for all other N it is a mixture of the ferromagnetic and non-ferromagnetic states. This behaviour is different from the fermionic ring, where ground state is true ferromagnetic only for N = 3. It is shown that the intrinsic spin generated gauge fields are analogous to the synthetic gauge fields generated by rotation of either the condensate or the confining potential. It is argued that the low lying excited levels of the spin flipped states intrinsically support the superfluidity. Possible ways to experimentally verify these results are also discussed.

Structure, Thermal Stability, and Magnetism of Ni4N Thin Films

Herein, the synthesis, structure, thermal stability, and magnetic properties of Ni4N thin films are studied. Ni4N is difficult to synthesize in the correct chemical order and stoichiometry due to unfavorable thermodynamics. During the synthesis of Ni–N thin films, it is found that the substrate temperature (Ts) is a critical parameter affecting the growth of a Ni4N phase. The Ni4N phase transforms into an amalgamation of [Ni+Ni3N] phases even if the Ts rises just above 300 K. These results elucidate a correlation between atomic diffusion and Ts. N self‐diffusion measurements carried out using secondary‐ion mass spectroscopy indicate that substantial N self‐diffusion is occurring at low Ts. A comparison of N self‐diffusion coefficients in Fe–N, Co–N, and Ni–N indicates that N self‐diffusion in Ni–N lies between that of Fe–N and Co–N and exhibits a greater correlation with the enthalpy of formation. A transformation from the cubic–to‐tetragonal deformation in the Ni4N crystal lattice as a function of increasing N concentration is evident from the X‐ray diffraction and X‐ray absorption near‐edge spectroscopy measurements. Magnetization measurements confirm a nonferromagnetic state of Ni4N at 300 K, which transforms into a ferromagnetic state at 15 K.

Selective flattening of magnon bands in kagome-lattice ferromagnets with Dzyaloshinskii-Moriya interaction

There is growing interest in revealing exotic properties of collective spin excitations in kagome-lattice ferromagnets such as magnon Hall effects, topological magnon insulators, and flat magnon bands. Using the well-established nearest-neighbor Heisenberg ferromagnet model with Dzyaloshinskii-Moriya interaction (DMI), in this study we uncover intriguing new aspects in the selectivity and topology of flat magnon bands. Among the three magnon bands (except for the top one, which is flat in the absence of DMI), we observe that each of the three bands can be selectively flattened at the critical DMI of $$D=\pm\sqrt{3}J/3$$ and $$D=\pm\sqrt{3}J$$ . With a general DMI, the magnon bands become non-flat; however, there are nested lines that create a David star pattern for all three magnon bands whose flatness is robust during changing exchange coupling or DMIs. Contrary to prevailing belief, we show that each of the three flat bands is actually topologically trivial at critical DMIs. Furthermore, we show that while the middle band remains topologically trivial, for the other two bands, D = 0 corresponds to the topological phase transition where their Chern numbers get interchanged; when $$D=\pm\sqrt{3}J$$ , the system undergoes a phase transition to the nonferromagnetic state. These central findings increase our understanding of spin excitations for future magnonics applications.

High-throughput prediction of the ground-state collinear magnetic order of inorganic materials using Density Functional Theory

We present a robust, automatic high-throughput workflow for the calculation of magnetic ground state of solid-state inorganic crystals, whether ferromagnetic, antiferromagnetic or ferrimagnetic, and their associated magnetic moments within the framework of collinear spin-polarized Density Functional Theory. This is done through a computationally efficient scheme whereby plausible magnetic orderings are first enumerated and prioritized based on symmetry, and then relaxed and their energies determined through conventional DFT + U calculations. This automated workflow is formalized using the atomate code for reliable, systematic use at a scale appropriate for thousands of materials and is fully customizable. The performance of the workflow is evaluated against a benchmark of 64 experimentally known mostly ionic magnetic materials of non-trivial magnetic order and by the calculation of over 500 distinct magnetic orderings. A non-ferromagnetic ground state is correctly predicted in 95% of the benchmark materials, with the experimentally determined ground state ordering found exactly in over 60% of cases. Knowledge of the ground state magnetic order at scale opens up the possibility of high-throughput screening studies based on magnetic properties, thereby accelerating discovery and understanding of new functional materials.

Periodicity-dependent long range coulomb on-site repulsion in hydrogen adsorbed graphene: A DFT+U study

The long rang Coulomb on-site repulsion between π electrons in hydrogen (H) adsorbed graphene has been studied based on DFT + U calculations. This study demonstrates that the rigidity of π electronic structure of graphene is partially destroyed in the cases of H adsorption due to the introduction of un-paired electron in this system. The softness of π conjugation is largely depending on the concentrations as well as the periodicity of H adsorption. Due to the destruction of the rigidity of π conjugation, the Coulomb on-site repulsion increases. The results show that the Coulomb on-site repulsion between π electrons is very sensitive to the periodicity of H adsorption. A considerable enhancement of Coulomb on-site repulsion occurs for the systems with 3n×3n peroidcity. More importantly, the strength and the patterns of Coulomb on-site repulsion in the graphene plane are significantly size-dependent, which further induces the band gap opening at Fermi level. Especially for 6×6 system, the on-site repulsion induces a stabilized non-ferromagnetic state. Our results reveal for the first time that the considerable long rang Coulomb on-site repulsion between π electrons exists which plays an important role in the electronic characteristics of H adsorbed graphene. The finding in this investigation provides new insight into the many-body interaction between localized impurities with delocalized π electrons in graphene related materials.

Features of the Decomposition of Delta Ferrite in Nitrogen-Containing Austenitic Steels

Features of the decomposition of the ferromagnetic δ ferrite with the formation of nonferromagnetic constituents (γ and σ phases) at temperatures of 900–1200°C in a nitrogen-containing austenitic steel have been determined. The transformation of δ ferrite at high temperatures (1100–1200°C) occurs with the formation of Widmanstaatten austenitic crystals in the ferritic matrix. At lower temperatures (900–1000°C), the transformation of δ ferrite develops by discontinuous decomposition with the formation of colonies of alternating plates of paramagnetic γ and σ phases. In the course of formation of the nonferromagnetic state as a result of the decomposition of δ ferrite at 900–1000°C (for 1 h), a subgrain structure with an increased density of dislocations, which is typical of thermomechanical treatment, is retained in the steel previously strengthened by high-temperature deformation.

Large spontaneous Hall effects in chiral topological magnets

As novel topological phases in correlated electron systems, we have found two examples of non-ferromagnetic states that exhibit a large anomalous Hall effect. One is the chiral spin liquid compound , which exhibits a spontaneous Hall effect in a spin liquid state due to spin ice correlation. The other is the chiral antiferromagnets and that exhibit a large anomalous Hall effect at room temperature. The latter shows a sign change of the anomalous Hall effect by a small change in the magnetic field by a few 100 G, which should be useful for various applications. We will discuss that the magnetic Weyl metal states are the origin for such a large anomalous Hall effect observed in both the spin liquid and antiferromagnet that possess almost no magnetisation.

Magnetic inhomogeneity in a multiferroic EuTiO3thin film

We report on variable temperature magnetic force microscopy studies of a strain-enabled multiferroic EuTiO${}_{3}$ film epitaxially grown on a (110)-oriented DyScO${}_{3}$ substrate. Our temperature- and magnetic-field-dependent studies clearly reveal an inhomogeneous magnetic state with the coexistence of ferromagnetic and nonferromagnetic states at low magnetic fields, which provides a microscopic origin of the anomalous missing moment in previous studies [Lee et al., Nature (London) 466, 954 (2010)]. The spins of the nonferromagnetic phase can be aligned by modest magnetic fields (>1.5 T). The observed magnetic inhomogeneity probably originates from the coexistence of nearly degenerate magnetic ground states.

Small-angle neutron scattering study of magnetic ordering and inhomogeneity across the martensitic phase transformation in Ni50−xCoxMn40Sn10alloys

The Heusler-derived multiferroic alloy Ni${}_{50\ensuremath{-}x}$Co${}_{x}$Mn${}_{40}$Sn${}_{10}$ has recently been shown to exhibit, at just above room temperature, a highly reversible martensitic phase transformation with an unusually large magnetization change. In this work the nature of the magnetic ordering above and below this transformation has been studied in detail in the critical composition range $x$ $=$ 6--8 via temperature-dependent (5--600 K) magnetometry and small-angle neutron scattering (SANS). We observe fairly typical paramagnetic to long-range-ordered ferromagnetic phase transitions on cooling to 420--430 K, with the expected critical spin fluctuations, followed by first-order martensitic phase transformations to a nonferromagnetic state below 360--390 K. The static magnetization reveals complex magnetism in this low-temperature nonferromagnetic phase, including a Langevin-like field dependence, distinct spin freezing near 60 K, and significant exchange bias effects, consistent with superparamagnetic blocking of ferromagnetic clusters of nanoscopic dimensions. We demonstrate that these spin clusters, whose existence has been hypothesized in a variety of martensitic alloys exhibiting competition between ferromagnetic and antiferromagnetic exchange interactions, can be directly observed by SANS. The scattering data are consistent with a liquidlike spatial distribution of interacting magnetic clusters with a mean center-to-center spacing of 12 nm. Considering the behavior of the superparmagnetism, cooling-field and temperature-dependent exchange bias, and magnetic SANS, we discuss in detail the physical form and origin of these spin clusters, their intercluster interactions, the nature of the ground-state magnetic ordering in the martensitic phase, and the implications for our understanding of such alloy systems.

Soft Mode Driven Monoclinic Phase in Multiferroic BiFeO3

A monoclinic phase (Pm) BiFeO3 corresponding to the softening behavior of the E mode of the tetragonal phase (P4mm) was characterized by using first-principles method and factor group analysis. The calculated atomic structures, elastic constants and vibration frequencies at the Γ point of rhombohedra, tetragonal and monoclinic phases have an agreement with the available experimental and theoretical results. The results show the monoclinic phase is more dynamically stable than the tetragonal phase, and its ferromagnetic state more energetically stable than the non-ferromagnetic state, implying it might be a candidate of the multiferroic phase linking the R3c and P4mm symmetry of BiFeO3.

Superexchange induced canted ferromagnetism in dilute magnets

We argue, in contrast to recent studies, that the antiferromagnetic superexchange coupling between nearest neighbor spins does not fully destroy the ferromagnetism in dilute magnets with long-ranged ferromagnetic couplings. Above a critical coupling, we find a canted ferromagnetic phase with unsaturated moment. We have calculated the transition temperature using a simplified local random phase approximation procedure which accounts for the canting. For dilute magnetic semiconductors, such as GaMnAs, using ab initio couplings allows us to predict the existence of a canted phase and provides an explanation for the apparent contradictions observed in experimental measurements. Finally, we compared with previous studies that used Ruderman-Kittel-Kasuya-Yosida couplings and reported nonferromagnetic states when the superexchange is too strong. Even in this case the ferromagnetism should remain essentially stable in the form of a canted phase.

Magnetostructure of MnAs on GaAs revisited

The ferromagnetic to nonferromagnetic (α-β) phase transition in epitaxial MnAs layers on GaAs(100) is studied by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism photoemission electron microscopy in order to elucidate the nature of the controversial nonferromagnetic state of β-MnAs. In the coexistence region of the two phases the β phase shows a clear XMLD signal characteristic of antiferromagnetism. The nature and the possible causes of the elusiveness of this magnetic state are discussed.

Electronic structure, superconductivity and magnetism study of Cr 3GaN and Cr 3RhN

The intermetallic perovskite compounds Cr 3GaN and Cr 3RhN were theoretically investigated, using the Korringa–Kohn–Rostoker method, as possible candidates for MgCNi 3-like superconductors and being near the magnetic critical point. The large values of McMillan–Hopfield parameters indicate the possibility of superconductivity onset as estimated from electron–phonon coupling constant λ > 1 . Moreover, spin-polarised electronic structure calculations resulted in the non-ferromagnetic ground state in both compounds, in spite of the n ( E F ) values on Cr atoms close to the Stoner limit.

Surface effect on the magnetism of aMnPt3-type ordered surface alloy on Pt(001)

We grow a $\mathrm{Mn}{\mathrm{Pt}}_{3}$-type trilayer ordered surface alloy on Pt(001), $c(2\ifmmode\times\else\texttimes\fi{}2)$ $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Pt}∕1\mathrm{ML}$ $\mathrm{Pt}∕c(2\ifmmode\times\else\texttimes\fi{}2)$ $\mathrm{Mn}\ensuremath{-}\mathrm{Pt}∕\mathrm{Pt}(001)$, of which atomic structure is investigated by dynamic low energy electron diffraction analysis. Magneto-optic Kerr effect measurements of the surface alloy at $45\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ show no hysteresis, although bulk $\mathrm{Mn}{\mathrm{Pt}}_{3}$ ordered alloy is ferromagnetic. Ab initio total energy calculations based on density functional theory, rather, predict that ferromagnetic order is not the ground state for the surface alloy. We propose that the nonferromagnetic order of the surface alloy originates mainly from surface effects in regards to nonferromagnetic ground state also found for the near-surface layers of the bulk-terminated $\mathrm{Mn}{\mathrm{Pt}}_{3}(001)$.

Design and structural characterization of perpendicular magnetic superlattices

Various perpendicular ferromagnetic superlattices of the Co/Pd system were madeto tune magnetic properties and recording performance, dependent on factors likesputtering pressure, sputtering gas, bilayer number, sublayer thickness, depositiontemperature, annealing, type of seed layer, seed layer thickness and grainsize, and distribution. Structural analysis by x-ray diffraction and TEMshows columnar growth and orthorhombic (tetragonal) microcrystallinepacking that accounts for the ferromagnetic properties by polarization of theatoms during deposition, in contrast to the non-ferromagnetic state of thefcc crystal structure superlattice after high-temperature annealing. TheM–H loopis described by a recently developed analytical hysteresis model and the result issubstantiated by the structural analysis of grain sizes from multiple magneticphases.

Structural and magnetic properties of Fe/Rh(001) sputter deposited multilayers

We present details of the crystal structure and magnetic behavior of Fe/Rh (001) sputter deposited multilayer thin films. We find that the films undergo a structural phase transformation from a body-centered-tetragonal (bct) to a face-centered-tetragonal (fct) lattice which is concomitant with a magnetic transformation from a ferromagnetic to a nonferromagnetic state. We identify the Rh spacer thickness where both bct and fct phases first coexist as the critical thickness (tcrit). In the region prior to the critical thickness, both Fe and Rh are ferromagnetic with Rh moments of at least 1μB and Fe moments enhanced as much as 30% relative to their bulk value. In the postcritical region, we find the films to be nonferromagnetic. The experimental results are compared with theoretical calculations on similar structures.

Magnetic behavior of metastable fcc Fe-Cu after thermal treatments.

A ferromagnetic and supersaturated fcc ${\mathrm{Fe}}_{51}$${\mathrm{Cu}}_{49}$ solid solution has been obtained by mechanical alloying. After subsequent thermal treatments the fcc phase undergoes a spinodal decomposition which finally, at 780 K, yields a mixture of fcc and bcc phases. In this work, a systematic magnetic study is carried out on samples at different decomposition states in order to determine the process of transformation into the stable phases. We observe a 20% maximum diminution on the magnetic moment with increasing temperatures of the thermal treatment. The Mo\ifmmode\ddot\else\textasciidieresis\fi{}ssbauer spectrum taken at 8 K shows that 20% of the Fe atoms are in a nonferromagnetic state. On the other hand, upon heating up to 723 K the room-temperature coercive field increases dramatically to 640 Oe, and after cooling down to 10 K it decreases to 270 Oe. Deviations from the ${\mathit{T}}^{3/2}$ law in the temperature dependence of the magnetization have been observed. This behavior is explained by fluctuations in composition due to the spinodal decomposition, which lead to fluctuations of the magnetic order parameters, i.e., magnetic moment and Curie temperature.

Magnetic properties of fcc-Fe artificial superlattices

Fcc-Fe/Cu(M)(001) (M = Au,Ni) artificial superlattices were prepared. The interatomic distance of fcc-Fe was changed by the epitaxial growth of Cu(Au) or Cu(Ni) alloy layers. The magnetic moment of Fe is increased up to 2.7μ B for only 1% increment of interatomic distance. Furthermore, non-ferromagnetic state of fcc-Fe was observed at 1.8% decrement.