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Single-Ion Anisotropy-Stabilized Short-Period Helimagnetism in Frustrated Chiral Co5TeO8
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Priya R. Baral1, 2, 3, *, Ravi Yadav3, Victor Ukleev2, 4, Thomas LaGrange3, Ivica Živković3, Wen H. Bi3, Marek Bartkowiak2, Robert Cubitt5, Nina-Juliane Steinke5, Vladimir Pomjakushin2, Yurii Skourski6, Henrik M. Rønnow3, Oleg V. Yazyev3, Arnaud Magrez3, Jonathan S. White2, *
Research. Vol 9 Article ID 1334
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Research. Vol 9 Article ID 1334
• RESEARCH ARTICLE •
Single-Ion Anisotropy-Stabilized Short-Period Helimagnetism in Frustrated Chiral Co5TeO8
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Priya R. Baral1, 2, 3, *, Ravi Yadav3, Victor Ukleev2, 4, Thomas LaGrange3, Ivica Živković3, Wen H. Bi3, Marek Bartkowiak2, Robert Cubitt5, Nina-Juliane Steinke5, Vladimir Pomjakushin2, Yurii Skourski6, Henrik M. Rønnow3, Oleg V. Yazyev3, Arnaud Magrez3, Jonathan S. White2, *
Affiliations
  • 1Department of Applied Physics and Quantum-Phase Electronics Center, The University of Tokyo, Tokyo 113-8656, Japan.
  • 2PSI Center for Neutron and Muon Sciences, 5232 Villigen PSI, Switzerland.
  • 3Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
  • 4Helmholtz-Zentrum Berlin für Materialien und Energie, D-12489 Berlin, Germany.
  • 5Institut Laue–Langevin, Grenoble 38000, France.
  • 6Dresden High Magnetic Field Laboratory (HLD-EMFL), 01328 Dresden, Germany.
Published: 2026-06-20 doi: 10.34133/research.1334
Outline
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Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii–Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here, we report that a cubic chiral insulator Co5TeO8 provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7 to 10 nm embedded in a complex phase diagram spanning 8 distinct phases. Capacitance anomalies throughout the phase diagram indicate ME coupling, pointing to the possibility of future E-field control of these textures. The temperature and field dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, ab initio calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co5TeO8 thus offers an experimental realization of sub-10-nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.

Priya R. Baral, Ravi Yadav, Victor Ukleev, Thomas LaGrange, Ivica Živković, Wen H. Bi, Marek Bartkowiak, Robert Cubitt, Nina-Juliane Steinke, Vladimir Pomjakushin, Yurii Skourski, Henrik M. Rønnow, Oleg V. Yazyev, Arnaud Magrez, Jonathan S. White. Single-Ion Anisotropy-Stabilized Short-Period Helimagnetism in Frustrated Chiral Co5TeO8[J]. Research, 2026 , 9 (6) : 1334 - . DOI: 10.34133/research.1334
Non-coplanar magnetic states—ranging from topological skyrmions to chiral vortex lattices—are central to modern spintronics owing to their emergent electrodynamics and robust stability against weak perturbations [1–4]. Yet, these intricate multi-Q architectures typically nucleate from a more fundamental parent phase: the single-Q spiral state [5,6]. As the elementary building block of non-coplanar magnetic states, the single-Q spiral emerges when competing interactions preclude collinear spin alignment [7,8], making a microscopic understanding of its stability essential for engineering complex topological phases [9,10]. Conventionally, this stability is attributed to specific exchange pathways governed by crystal symmetry and electronic structure of the material: the anti-symmetric Dzyaloshinskii–Moriya interaction (DMI) in chiral noncentrosymmetric hosts [11–13], geometric frustration [14,15] or Fermi-surface-nested Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions in centrosymmetric systems [16,17], and, in itinerant magnets, higher-order terms such as the biquadratic exchange interaction [18,19]. Crucially, while these diverse mechanisms successfully predict a wide array of noncollinear magnetic phases, they share a common paradigm in which spatial modulation is driven primarily by intersite exchange, with local energy scales typically treated as subleading.
Conversely, magnetic anisotropy—traditionally treated as a weak perturbative correction—can fundamentally restructure the magnetic energy landscape when its magnitude rivals the exchange energy scales [20–22]. In isotropic Heisenberg models, single-Q helices are generically favored as a simple harmonic modulation preserves the fixed-moment constraint, whereas multi-Q superpositions tend to generate amplitude modulations that require higher harmonics to restore normalization. However, this exchange cost can be offset by strong easy-axis single-ion anisotropy (SIA), which, aided by thermal softening of the longitudinal spin stiffness [20], promotes multi-Q formation. On frustrated lattices, this mechanism stabilizes a hierarchy of exotic phases, including the skyrmion lattice [23]. Moreover, in multi-sublattice magnets, inequivalent SIA tensors can compete with the leading exchange to promote noncollinearity [24], yet experimental realizations of anisotropy-dominated helimagnets remain elusive.
Here, we combine comprehensive experimental characterization with first-principles calculations to investigate helimagnetism in chiral Co5TeO8. Our density functional theory (DFT) calculation predicted a 127-meV charge-transfer gap, motivating the synthesis and investigation of its magnetic properties. Using magnetic and thermodynamic probes, we constructed a high-resolution multi-phase diagram consisting of 8 different phases. In addition, we employ neutron scattering, high-field magnetometry, and capacitance measurements to map this magnetic landscape and characterize compact field-tunable helical textures. These experiments indicate that site-dependent SIA, rather than DMI, is the leading interaction governing the stabilization of the helical state—a scenario further supported by ab initio many-body wavefunction calculations. Together with the experimental observations, these results suggest a framework for engineering compact magnetic textures in correlated oxides, with potential implications for magneto-electric (ME) spintronics.
To search for magnets with enhanced SIA, we turned our attention to Co-based systems, targeting spinel-type Co5TeO8. Although its magnetic ground state depends on structural polymorphism [25], we focused on the chiral cubic phase (P4132/P4332), where geometric frustration and broken inversion symmetry enable complex magnetic textures. The combination of these structural features with Co5TeO8’s partially filled d-orbitals, likely involving strong electronic correlations, motivated us to investigate its electronic structure.
To assess the metallic versus insulating character of its electronic structure, we performed DFT calculations within the local density approximation (LDA) + U framework, using the experimental crystal structure (see Fig. 1A). The LDA + U approach, incorporating on-site Coulomb repulsion via a Hubbard U parameter, has successfully reproduced band gaps in Mott insulators such as Na2IrO3 [26]. For Co5TeO8, while standard LDA predicts metallic behavior, introducing U = 3 eV opens a charge-transfer gap of 127 meV at the Γ-point (Fig. S1), a result robust to the choice of U. The predominant d-character of the bands near the Fermi energy indicates strong electronic correlations, implying sensitivity to external fields and potential ME coupling.
To explore magnetism of Co5TeO8, we synthesized a high-quality microcrystalline sample and performed comprehensive structural characterization. Structural analysis confirms that Co5TeO8 crystallizes in the chiral space group P4332 (or its enantiomorph P4132), with both inequivalent magnetic Co sites adopting a 2+ oxidation state as confirmed from electron energy loss spectroscopy (EELS) measurements (Section S2). Further structural details are provided in Section S1. The Co1 sublattice (12d sites) forms corner-sharing triangular motifs that interconnect with Co2 ions (8c sites) to complete a 3-dimensional framework of corner-sharing tetrahedra. This geometry generates strong magnetic frustration where competing exchange interactions between the 2 sublattices may suppress conventional magnetic order [27]. The combination of strong electronic correlations on the chiral frustrated lattice establishes Co5TeO8 as a platform for hosting nontrivial magnetic textures. As we demonstrate in the following sections, localized 3d electrons in Co5TeO8 enable tunable magnetic phenomena with a rich phase diagram.
High-resolution magnetometry reveals a complex magnetic energy landscape in Co5TeO8 with multiple phase transitions, as shown in Fig. 1C. The real component of ac susceptibility (χac′) exhibits a sharp peak at 44.9 K (THM), indicating onset of long-range magnetic order in phase I (see Fig. 2A). This transition displays clear dissipative character, as evidenced by a corresponding peak in imaginary susceptibility (χac′′). Below THM, 2 additional hysteretic regions emerge at Thys1 and Thys2, consistently observed in both ac susceptibility and dc magnetization. Applied magnetic fields cause these transitions to converge and shift toward THM (Fig. 2C and Section S3), suggesting renormalization effects similar to skyrmion lattice hosts [28–30].
Heat capacity measurements reveal contrasting thermodynamic signatures. While magnetometry shows pronounced features at all 3 transitions, heat capacity exhibits only weak anomalies at THM and Thys2, with no discernible signature at Thys1 (see Fig. 2B). This suppressed response despite clear magnetic signatures suggests minimal entropy changes governed by persistent short-range correlations, previously identified in Ref. [25]. Field-dependent magnetometry data reveal pronounced metamagnetic transitions below THM, becoming increasingly steeper at lower temperatures with broadened hysteresis, as shown in Fig. 2D. These field-driven responses coincide with sharp anomalies in electrical capacitance (Fig. 2E, inset, and Section S3), confirming ME coupling in Co5TeO8, which is crucial for functional control. Remarkably, magnetic saturation is absent even at 60-T pulsed fields (Fig. 2E), presumably reflecting frustration in the strongly-coupled tetrahedral framework where competing exchanges prevent forced ferromagnetic alignment [31]. This intricate behavior necessitates comprehensive neutron scattering to resolve the underlying magnetic structure.
To uncover the magnetic structure in Co5TeO8, we performed powder neutron diffraction (PND) measurements. Just below THM, magnetic satellites emerge around (h,h,h) and (h,h,0) nuclear reflections (see Fig. 3A), characterized by incommensurate propagation vectors QIC1a = G111 ± q(1) = G110 ± q(1), consistent with a previous report [25]. Notably, no magnetic satellites appear around Gh00 reflections within instrumental resolution. The systematic absences of (h,0,0) reflections (h ≠ 4n) confirm that the 41 (or 43) screw symmetry of the parent space group is retained, although overall cubic magnetic symmetry is broken by incommensurate modulation.
Small-angle neutron scattering (SANS) measurements provide complementary information about the magnetic order. Due to powder averaging, the discrete incommensurate Q-vectors manifest as ring-like scattering patterns on the 2-dimensional multidetector. Figure 3B and C show data at 44 and 2 K, respectively. The ring-like scattering centered at the Γ-point corresponds to QIC1 with |Q| = 0.903(2) nm−1 at THM and QIC2 with |Q| = 1.1(1) nm−1 below Thys2 (Fig. 3D). The corresponding pitch length at 2 K is λh = 5.71 ± 0.51 nm, increasing with temperature from λh (THM) = 6.96 ± 0.02 nm to λh (Thys1) = 9.66 ± 0.15 nm. These compact magnetic periodicities are among the shortest reported in chiral insulator magnets. See Table 1 for a thorough comparative list of most-relevant noncentrosymmetric magnets. However, the unpolarized neutron scattering measurements alone cannot distinguish between different spiral helicities. To determine whether these ultracompact spirals adopt proper-screw or cycloidal character, we performed polarization-analyzed SANS measurements as described in the following section.
Having established the compact spiral order, we performed uniaxial polarization-analyzed SANS to determine the helical spin texture. A schematic of the experimental setup is shown in Fig. 3E. The incoming neutron polarization vector is aligned (or anti-aligned) with the neutron beam. Non-spin-flip (NSF) and spin-flip (SF) scattering cross-sections are probed using a spin state analyzer placed between sample and detector. This configuration enables the decomposition of the magnetic interaction vector m⊥(Q) into components defined by the polarization axis. The NSF cross-section isolates the projection of m⊥(Q) parallel to the initial polarization Pi (as indicated in Fig. 3E), whereas the SF channel is sensitive to the component simultaneously orthogonal to both Pi and Q. Consequently, for a helimagnetic spiral characterized by moments distributed isotropically in the plane normal to Q, the scattering intensity is expected to be equally distributed between the NSF and SF channels. In contrast, for an easy-plane cycloidal structure with moments confined to the Q–x (or Q–z) plane, the magnetic scattering is expected to appear exclusively in the NSF (or SF) channel, respectively. See Ref. [32, 33] for more details. Since the ring-like scattering pattern (Fig. 3F) represents a superposition of multiple magnetic domains, each angular segment provides equivalent information about NSF and SF cross-sections, enabling systematic analysis across different Q-vector orientations.
Analysis of polarized SANS data at 40 K shows nearly equivalent intensities in both NSF and SF channels for phase I (Fig. 3G). This balanced contribution indicates that magnetic Fourier components lie predominantly in the plane perpendicular to Q with comparable amplitudes in both horizontal and vertical directions, consistent with helimagnetic spiral arrangement (schematic shown in inset of Fig. 3G). The helicity is retained down to Thys2 as deduced from equal NSF-SF intensities (Fig. 3H). More detailed analysis using narrower sector boxes reproduces the same result (Section S4). While helical modulation is expected due to the chiral crystal structure, the unusually short magnetic period sharply differentiates Co5TeO8 from archetypal DM-stabilized helimagnets [34–36]. Having confirmed the Bloch-type helical character, we next investigate how applied magnetic fields modify the helical pitch and induce phase transitions, as revealed by field-dependent SANS measurements.
To investigate the magnetic field response of the incommensurate phases, we systematically tracked the evolution of QIC1under applied transverse magnetic fields (μ0H ⊥ ki) using SANS. Starting from zero-field ring-like scattering at 36 K in phase I (Fig. 4A), applied magnetic field gradually transforms the ring into an intermediate phase characterized by arcs of scattering (Fig. 4B) before entering a well-defined conical phase at higher fields (Fig. 4C). The conical phase is clearly identified by a pair of spots with propagation vectors aligned parallel and antiparallel to the applied field, consistent with other cubic helimagnets [34,35]. Further field increase suppresses incommensurate satellites above 2.5 T, leading to field-polarized magnetic order. Notably, the absence of scattering intensity in sectors perpendicular to the applied field (around Ψ = 0° or 180° in Fig. 4D and teal sector box in Fig. 4E) definitively excludes formation of the conventional multi-Q skyrmion lattice observed in other chiral cubic magnets.
The intermediate phase (phase II) exhibits distinctive characteristics revealing complexity of underlying magnetic interactions. Unlike the conical phase with monotonic peak-width decay, the intermediate phase maintains nearly constant azimuthal intensity distribution across its stability range (Fig. 4E, inset). This behavior suggests stabilization of a multi-domain incommensurate structure with Q-vectors distributed between 〈110〉 and 〈111〉 directions, with the field only partially selecting preferred orientations. Remarkably, the incommensurate wavevector magnitude |Q(1)| increases systematically with applied field (see Fig. 4F), indicating that the spiral shortens under field rather than collapsing into ferromagnetic-type alignment. This field-driven reduction of helical pitch is rare among DMI-stabilized helimagnets, necessitating ab initio wavefunction-based calculations to understand microscopic magnetic interactions.
The Co–O coordination geometries in Co5TeO8 deviate significantly from ideal octahedral and tetrahedral environments with ∠Co1–O–Co1 and ∠Co2–O–Co2 bond angles of 94.51(1)° and 116.77(1)°, respectively. These distortions make prediction of exchange interactions via Goodenough–Kanamori–Anderson rules challenging, thus requiring explicit quantum chemistry calculations. Hence, we employed electrostatically embedded finite-size clusters extracted from the experimental crystal structure (see Fig. 1A), an approach well-suited for strongly correlated d-electron systems with highly localized electronic states [37–42]. We used the following spin Hamiltonian encompassing both on-site and intersite magnetic interactions:
H=∑ij=1,2JijSi·Sj+KijSi·Sj2+Dij·Si×Sj+∑iAiSi·êi2
The summation 〈i, j〉 runs over all nearest-neighbor bonds. The exchange parameters: bilinear (Jij), biquadratic (Kij), and antisymmetric DM interaction vectors (Dij (11, 12)) are bond-type-dependent, taking 3 distinct sets of values corresponding to Co1–Co1 (J11, K11, D11), Co2–Co2 (J22, K22, D22), and Co1–Co2 (J12, K12, D12) bonds, as shown in Fig. 1A. Ai is the SIA coefficient for site i, where êi represents the site-dependent anisotropy axis. Further details about our quantum chemistry calculations can be found in the Methods section and Section S5.
We find that all 3 bilinear exchange pathways are antiferromagnetic with J11 = 0.116 meV, J22 = 0.088 meV, and J12 = 0.208 meV (see Table S2), at least 20 times weaker than the thermal energy scale (kBTHM ∼ 3.9 meV) associated with THM, suggesting that the bilinear exchanges alone underestimate the long-range ordering temperature. The biquadratic interactions, in turn, are ferromagnetic-type with K11 = −8.4 μeV, K22 = −1.11 μeV, and K12 = −3.2 μeV, representing 5% to 8% of the corresponding bilinear strengths. Remarkably, the Co1–Co1 bonds exhibit exceptionally large DMI, with |D| = 0.099 meV yielding |D/J| ≃ 0.84, atypical for insulating chiral oxides, while Co2–Co2 bonds show negligible DMI (|D/J| ≃ 0.03) and Co1–Co2 bonds display intermediate values (|D/J| ≃ 0.102). As schematically shown in Fig. 1B, the calculated SIA constants are site-dependent, with A1 = −2.1 meV (easy-axis along [111] for Co1 at 12d sites) and A2 = 3.1 meV (xy-easy-plane for Co2 at 8c sites), representing the dominant energy scale, approximately 10 to 15 times larger than exchange interactions. In the absence of strong anisotropy in a single sublattice system without interaction, the canonical DMI helimagnet relation λh = 4πJa/D predicts λh ≃ 5.3 nm for Co1–Co1, coincidentally close to the observed range of 5.7 to 9.6 nm across the temperature-dependent phases. The same relation predicts λCo1–Co2 ≃ 45 nm and λCo2–Co2 ≃ 109 nm, both incompatible with experimental observations. The calculated energy hierarchy A ≫ J > D ≫ K, combined with competing antiferromagnetic exchanges on the 2-sublattice spinel-like structure, identifies Co5TeO8 as a frustrated magnetic system in which the SIA exceeds the exchange and DMI. In this regime, frustration and DMI drive the underlying finite-Q instability and chirality, while the dominant site-dependent SIA is proposed to lift the directional degeneracy and to favor a propagation vector selected from the otherwise nearly degenerate manifold.
This counterintuitive strong SIA can be traced to the electronic structure of high-spin Co2+ (d7, S = 3/2) in distinct local environments. For the octahedral Co1 sites (12d, t2g5eg2), the incompletely filled t2g manifold is expected to retain unquenched orbital angular momentum, in contrast to either fully filled or half-filled configurations. The Co1 octahedron exhibits a local ⟨111⟩-oriented distortion where the 6 Co–O coordinations comprise 2 short (2.01 Å), 2 intermediate (2.14 Å), and 2 long (2.17 Å) bond lengths, with the 2 short bonds subtending an expanded angle of 97.3° and the 2 long bonds subtending a contracted angle of 86.3°, consistent with compression along the pseudo-3-fold axis. This distortion splits the parent 4T1g term into 4A2g and 4Eg components in D3d symmetry. Mixed with spin-orbit coupling, this split manifold produces zero-field splitting of the S = 3/2 states with easy-axis character along the compression direction, yielding in our calculation A1 = −2.1 meV [43,44]. In contrast, the tetrahedrally coordinated Co2 (8c sites, eg4t2g3) presents a distinct scenario. The 4A2 ground term of tetrahedral Co2+ is an orbital singlet with no first-order orbital angular momentum. Nevertheless, Co2 occupies a trigonally compressed tetrahedron (C3v symmetry) with one short Co–O bond (1.91 Å) along the 3-fold axis and 3 longer bonds (2.02 Å), yielding O–Co–O angles of 106.6° and 112.2°. This distortion modifies both the energies and symmetries of the excited-state manifold (primarily 4T2) that couples to the ground state via spin-orbit interaction. Our ab initio calculations indicate that this second-order mechanism produces easy-plane anisotropy (A2 = 3.1 meV), whose magnitude appears to be enhanced by covalent mixing with heavy tellurium ligands. This overlap is expected to increase the effective spin-orbit coupling at Co2 sites and amplifies these second-order contributions despite the absence of first-order orbital effects. The insulating state with persistent correlations in Co5TeO8 is likely essential for preserving this strong SIA. In itinerant systems, electron delocalization into broad conduction bands quenches orbital angular momentum and averages over local distortions, substantially reducing magnetic anisotropy. This suppression is evidenced by the 4-fold reduction in near-metallic CoV2O4 compared to the wider-gap isostructural Mott insulator MnV2O4, where electron localization preserves the orbital physics [45–47]. The combination of Te-induced distortions at both Co sites, unquenched orbital moments at Co1, heavy-ligand spin-orbit enhancement at Co2, and Mott localization preserving the local electronic structure is proposed to produce the dominant site-dependent SIA.
The microscopic origin of sub-10-nm helimagnetism in Co5TeO8 presents a compelling materials physics puzzle: In the absence of strong anisotropy, canonical DMI helimagnets achieve λh ∼ 18 to 130 nm, yet Co5TeO8 realizes λh ∼ 5.7 to 9.6 nm despite only moderately enhanced |D/J| compared to archetypal chiral magnets (e.g., MnSi ∼ 0.12 [48]; Cu2OSeO3 ∼ 0.04 to 0.58 [31]). Our ab initio calculations point to a different mechanism in which site-dependent SIA, an order of magnitude larger than all exchange interactions, provides the leading energy scale. Three competing antiferromagnetic exchanges on the 2-sublattice structure create geometric frustration, producing a nearly flat E(Q) landscape over 0.8 to 1.3 nm−1. Within this degenerate manifold, dominant SIA is proposed to select the helical wavevector through Q-dependent orientational averaging, where shorter wavelengths reduce anisotropy penalties by allowing larger intersite spin canting that better accommodates the conflicting easy-axis (Co1) and easy-plane (Co2) local environments. While the canonical λh = 4πJa/D relationship using Co1–Co1 parameters coincidentally predicts λh ∼ 5.2 nm, this numerical agreement may mask distinct physics: Large DMI creates the helical instability and establishes chirality, but we argue that SIA provides the leading contribution to wavelength selection within the frustrated landscape. This SIA-selection mechanism is supported by T- and H-dependent neutron scattering data, detailed below. Additionally, weak ferromagnetic-type biquadratic interactions (5% to 8% of bilinear strength) lift residual degeneracies within the frustrated Q-space manifold and provide energy barriers between competing helical variants, enabling sharp phase transitions and hysteretic behavior. Although individual exchange bonds are modest, the experimentally observed THM is driven by collective stabilization from the full frustrated lattice and strong anisotropy, as captured by mean-field analysis (Section S5). This mechanism is intrinsically tied to the predicted insulating ground state with strong electronic correlations, which favors the localized orbital physics underlying the enhanced SIA while maintaining sufficient correlation strength to generate competing exchange pathways. In summary, Co5TeO8 thus appears to realize a DMI-initiated, SIA-enhanced, frustration-enabled compact helimagnet and may serve as a prototype for a class of anisotropy-dominated helimagnets in strongly correlated insulators.
Another indication of SIA dominance emerges from the field dependence of the helical wavevector. Co5TeO8 exhibits field-driven compression (dQ/dH > 0 as shown in Fig. 4F), not what is expected from canonical DMI helimagnets for which the modulation period is typically field independent to first approximation. In Co5TeO8, the field reorients the spin configuration, and on the frustrated 2-sublattice landscape, this favors shorter helical pitches that better accommodate the conflicting easy-axis (Co1) and easy-plane (Co2) environments. This distinctive field response is difficult to reconcile with a purely DMI-controlled wavelength selection and is most naturally explained if SIA provides the leading contribution. The synergy between frustration and SIA manifests dramatically in high-field magnetization, which fails to saturate even at 60 T (EZeeman ∼ 10.4 meV), as shown in Fig. 2E. This nonsaturation arises from geometric frustration creating competing exchange fields that reduce the effective molecular field, while calculated SIA barriers (∼24 T for Co1, ∼36 T for Co2) must be overcome to fully polarize spins. The contrasting anisotropy character of the 2 sublattices, combined with competing interactions, underlies the rich 8-phase magnetic diagram, where phase boundaries reflect competition between helical wavevector selection, rotation plane orientation, and Co1–Co2 sublattice coupling. Further experiments on Co5TeO8 single crystals combined with theoretical modeling are required to validate these points.
The SIA-dominated mechanism is further supported by 2 distinct temperature-induced features. First, at THM, long-range helical order emerges with a temperature-dependent wavevector |Q(T)| that evolves as thermal fluctuations partially quench the effective anisotropy. Second, upon cooling through a first-order lock-in transition at Tlock (= Thys2 ∼ 25 K) exhibiting substantial thermal hysteresis (Fig. 2A), the helical period becomes temperature independent and rigidly fixed at λh = 5.7 nm. The energy scale kBTlock ∼ 2.15 meV is comparable to the calculated Co1 single-ion barrier A1 (also naturally lower than A2), suggesting that reduction of thermal energy below this scale may trigger the transition and pins the system in the SIA-optimized configuration. This 2-transition phenomenology bears striking similarity to the archetypal Ising chain material CoNb2O6, which also exhibits an incommensurate–commensurate lock-in [49], but with a fundamental mechanistic difference. In CoNb2O6, ultra-strong crystal-field splitting (∼30 meV) quenches the spin to Seff = 1/2 and rigidly locks spin directions along a single axis, permitting only amplitude modulation of collinear ferromagnetic chains in the intermediate phase; lock-in is then driven by weak interchain coupling, occurring at an energy scale much smaller than the crystal-field splitting. In contrast, Co5TeO8’s moderate SIA preserves S = 3/2 and permits continuous spin rotation, resulting in helimagnetic spiral, with lock-in occurring at Tlock ∼ A1/kB via anisotropy saturation rather than interchain coupling. The hysteretic first-order character in Co5TeO8 points to an energy landscape with multiple metastable Q-states separated by nucleation barriers, consistent with the frustrated E(Q) manifold. Above Tlock, the system occupies a thermally accessible “soft” Q-state with broader distribution, while below Tlock, it nucleates into a “locked” Q-state with narrow, rigid distribution. This comparison supports the picture of SIA-dominated helical behavior with sufficient DMI and frustration in Co5TeO8, fundamentally distinct from DMI-controlled systems with monotonic changes in helical period [50].
The calculated anisotropy in Co5TeO8 places it in an intermediate regime within the broader landscape of chiral magnetism. At one extreme, archetypal cubic chiral magnets with weak magnetocrystalline anisotropy preserve near-degeneracy among multi-Q propagation vectors, enabling continuous Q(T) evolution and field-stabilized skyrmion lattices; metallic MnSi, Fe1-xCoxSi, and the insulating Cu2OSeO3 exemplify this regime. At the opposite extreme, CoNb2O6’s ultra-strong anisotropy restricts magnetic modulation to amplitude fluctuations of collinear order, suppressing all noncollinear textures and enforcing rigid uniaxial Ising behavior. Co5TeO8 lies between these limits where site-dependent SIA dominates exchange by at least an order of magnitude, yet remains small enough in absolute terms to preserve unquenched S = 3/2 and permit continuous spin rotation. Unlike weak-anisotropy systems where higher-harmonic coupling enables multi-Q states [23], the dominant SIA in Co5TeO8 strongly penalizes spin textures that deviate from locally optimized easy axes defined by the chiral crystal structure. A multi-Q superposition, which inherently requires spins to point in nearly isotropic directions to satisfy multiple wavevectors, incurs higher anisotropy cost compared to a single-Q helix that can coherently adapt its phase to the local anisotropy axes [21]. Consequently, the system is driven toward a single-Q harmonic state, effectively “locking” the magnetic texture to the structural chirality and suppressing complex multi-Q topologies. The lock-in transition and field-driven helix compression described above are absent in weaker-anisotropy systems, consistent with SIA playing the leading role. The β-Mn-type Co8Zn8Mn4, with comparable DMI but weaker anisotropy, provides direct comparison, as it exhibits a longer λh (Table 1) while retaining skyrmion phases, supporting the role of SIA in suppressing multi-Q skyrmion phases [51]. These findings suggest a transferable design principle: Systematically weakening SIA through selective dilution or enhancing DMI via heavier 4d/5d element substitution at Co2+ sites could generate skyrmion derivatives in isostructural compounds. More broadly, Co5TeO8 exemplifies how strong single-ion terms in frustrated geometries expand the accessible magnetic texture space beyond DMI paradigms, motivating exploration of anisotropy-engineered noncollinear states in other frustrated pyrochlore and spinel oxides with heavy transition metals. We note, however, that the present polycrystalline measurements do not permit a direct experimental determination of the full anisotropy tensor; accordingly, the microscopic interaction values should be viewed as semiquantitative estimates whose main importance lies in the inferred energy-scale hierarchy.
Our work establishes Co5TeO8 as a magnetic insulator hosting a short-period helical ground state, with calculations indicating the presence of strong electronic correlations. The stabilization mechanism is proposed to arise from a distinctive competition between site-dependent SIA, DMI and frustration, unlike that seen in archetypal cubic helimagnets. Our results suggest a design principle that links the local coordination geometry to the nanoscale spin texture. The interplay between SIA and DMI proposed here broadens the search for short-period helimagnets beyond compounds stabilized by DMI alone, extending it to material classes with weak to moderate anisotropy. Mixed coordination structures, such as Co-based chalcogenide spinels and β-Mn-type compounds, are therefore natural candidates for systematic exploration of this mechanism. The short-period helical texture itself may find practical use in 3 areas: first, in high-frequency nano-oscillators, where the insulating ground state avoids eddy-current losses and the antiferromagnetic background cancels the Magnus force, both favoring dynamical stability [52]; second, in reservoir computing, where the 2 crystallographically distinct Co sites could provide additional nonlinear degrees of freedom compared with single-sublattice magnetic domains [53]; third, in ME switching, where the insulating ground state allows electric field rather than current-driven control—an advantage for device miniaturization, contingent on sufficiently large ME tensor elements in Co5TeO8 [54]. Quantitative measurement of the Mott gap and the ME coefficients will therefore be needed to assess the practical viability of voltage-controlled manipulation of the helices. Further experiments to resolve the magnon band structure of these compact helices [55], to probe nonreciprocal thermal transport arising from broken inversion symmetry [56], and to measure electric field-driven phase transitions [57] would clarify the functional potential of Co5TeO8 and related materials for low-dissipation spintronics. More broadly, this work suggests that competing anisotropies between inequivalent magnetic sites, usually regarded as detrimental to magnetic order, can instead serve as a design principle for engineering compact spin textures in correlated insulators.
Density functional theory calculations were performed using the Vienna Ab-initio Simulation Package (VASP) [58]. Electron-core interactions are described using the projector-augmented wave method [59], while the Kohn–Sham wavefunctions for the valence electrons were expanded in a plane wave basis with a cutoff on the kinetic energy of 400 eV. LDA + U calculations were performed within the local density approximation framework with a Coulomb on-site repulsion U = 3 eV acting on the d shell of Co atoms following the rotationally invariant scheme proposed by Dudarev et al. [60]. The calculations included spin-orbit coupling and noncollinear magnetism to capture the essential magnetism of Co5TeO8. The cubic cell (a = 8.535 Å) had 56 atoms, and the Brillouin zone sampling used 100 k-points with the Methfessel–Paxton smearing (width 0.01 eV). The intrinsic C3 symmetry of the system was utilized, and static single-point energy calculations were performed without ionic relaxation. It should be noted that residual self-interaction errors inherent to LDA often result in slight underestimation of band gaps in systems with localized 3d electronic states. Thus, we may expect a slightly larger band gap in reality.
The pure phase microcrystalline Co5TeO8 sample was synthesized using conventional solid-state reactions. Stoichiometric amounts of CoCO3 (Aldrich, 99.999%) and TeO2 (Aldrich, 99.999%) were thoroughly mixed using a mortar. The resultant mixture was transferred to a platinum crucible and placed inside a high-temperature muffle furnace. The temperature of the furnace was slowly raised to 1000 °C at a rate of 100 °C per hour. The furnace was then dwelled for an hour at that temperature, followed by increasing the temperature to 1100 °C at a slower rate of 50 °C per hour. The powder was then sintered at this temperature for 72 h, followed by quenching to room temperature. The quenching process was necessary to avoid co-crystallization of Co3O4 spinel. Results presented throughout this article were obtained on the same batch of microcrystalline sample.
Microcrystalline Co5TeO8 grains with suitable size were mounted onto the goniometer head with cryo-loop obtained from MiTeGen, followed by installing on an Agilent SuperNOVA single-crystal diffractometer, equipped with Cu/Mo sealed tube x-ray source and an Atlas charge-coupled device detector. The temperature of the setup was controlled by an Oxford Cryostream 800 Plus system.
We performed 2 electron backscatter diffraction (EBSD) and energy-dispersive x-ray spectroscopy (EDX) experiments. For the first experiment, Co5TeO8 particles were distributed over a silver-pasted stub, and EBSD spectra were collected using a field emission gun (FEG) scanning electron microscope (SEM) Zeiss Gemini 450 equipped with Oxford Instruments UltimMax EDX detector and CMOS EBSD Symmetry camera. Data acquisition was performed with the AZtec system, and subsequent processing utilized the AZtecCrystal software suite. In the second experiment, Co5TeO8 particles were embedded in resin and cured at 180 °C for 5 min. Prior to EBSD and EDX analysis on the same specimen, a thin carbon layer was deposited in order to reduce the electron irradiation-induced charging effects.
DC magnetization as well as AC susceptibility measurements were performed using the VSM and ACMS-II option of a commercial Quantum Design (QD) 14 T Physical Property Measurement System (PPMS), as well as a Magnetic Property Measurement System (MPMS), MPMS3 from QD. The polycrystalline sample was enclosed in a standard polypropylene holder provided by QD. High-field magnetization experiments were performed in pulsed magnetic fields up to μ0H = 60 T at the Dresden High Magnetic Field Laboratory, which were normalized to laboratory measurements up to μ0H = 14 T.
Specific heat measurements were performed on a pelletized sample. In order to produce a pellet with higher structural stability, microcrystalline sample was first crushed in a mortar for about 5 min. The diameter and height of the disc-shaped pellet were made to be 3 and 0.5 mm, respectively. Measurements were performed using the Heat Capacity option of the 14 T PPMS, in the typical relaxation method. The pellet was mounted on the 3 × 3 mm2 sapphire plate provided by QD. Apiezon-N grease was used to ensure good thermal contact between the sample and the platform. Later, the long-pulse technique was also used to measure the sample heat capacity.
Capacitance measurements were performed on a pressed Co5TeO8 pellet of size 5 × 5 × 0.5 mm3, using an AH2700A ultraprecision capacitance bridge in zero dc field, and with a 1-kHz excitation and a voltage of 15 V. The sample mount was installed onto 9-T PPMS. Capacitance measurements were performed as a function of magnetic field at fixed temperatures in a sweep mode.
The experimental determination of the cation oxidation state in Co5TeO8 was carried out on a thin lamella prepared from a microcrystalline specimen using focused ion beam (FIB). We used the double aberration corrected, monochromated Titan Themis at the Interdisciplinary Centre for Electron Microscopy (CIME) of École Polytechnique Fédérale de Lausanne (EPFL). We acquired spectrum images (see the Supplementary Materials) in the dual EELS mode, which allowed for calibrating the absolute edge energy position and determining chemical shifts in the edge energies associated with changes in valence. We chose to acquire spectra at dispersions of 0.1 eV/channel and dwell time of 50 ms, which provided high energy resolution and produced well-defined L-edges above background in a single pixel spectrum but limited the electron beam interactions and radiation damage.
Wide-angle neutron powder diffraction measurements were carried out on a polycrystalline Co5TeO8 sample using the high-resolution powder diffractometer (HRPT) of the SINQ instrumental suite at Paul Scherrer Institute (PSI). Data were collected in both medium resolution and high-resolution modes, employing neutrons with a fixed wavelength of 2.45 Å. Each measurement was preceded by a 10-min thermal stabilization period to ensure equilibrium, followed by a 2-h acquisition of high-statistics data.
SANS experiments, without polarization analysis, were carried out using the SANS-I instrument of Swiss Spallation Neutron Source (SINQ) located at the Paul Scherrer Institute (PSI). About 200 mg of microcrystalline sample was mixed thoroughly with a 1:1 mixture of hydrogen-free FC-77 and FC-75 Fluorinert liquid and enclosed in a vanadium can. The Fluorinert freezes upon cooling to cryogenic temperatures, providing a quasi-strain-free matrix that maintains the random zero-field orientations of the sample grains in finite magnetic field. The can was then installed inside a 6.8-T horizontal field magnet. For all of our experiments, neutrons with wavelength 5 Å with a full width at half maximum (FWHM) spread Δλ/λ = 10% were used. The incoming neutron beam was collimated by a distance of 8 m before the sample, whereas the scattered neutrons were collected by a 2-dimensional 3He multidetector placed 3.5 m behind the sample.
Uniaxial polarization analysis was performed using the D33 SANS instrument at the Institut Laue-Langevin (ILL), Grenoble. The polycrystalline Co5TeO8 sample was embedded in hydrogen-free glue and encapsulated within a vanadium can. Measurements utilized longitudinal polarization geometry with the incident neutron beam collinear to the applied magnetic field. A weak longitudinal magnetic field of 50 mT was applied to the sample, while a 10-mT guide field maintained the neutron spin quantization axis parallel to the incident beam. Scattered neutrons were analyzed using a spin-polarized 3He cell with maximum polarization of 98.6%. Four neutron spin states (↓↓, ↑↓, ↑↑, and ↓↑) were systematically measured as a function of temperature during cooling. Neutron beam depolarization was monitored at 2-h intervals in the paramagnetic regime (T = 65 K). Data analysis was conducted using GRASANSP software [61].
We performed many-body wavefunction calculations on electrostatically embedded finite-size cluster models extracted from the experimental crystal structure. The crystallographic unit cell contains 2 distinct Co2+ sites: Co1 atoms in distorted octahedral coordination and Co2 atoms in tetrahedral coordination with oxygen ligands, as shown in Fig. 1A. Our computational models feature a central cluster containing 1 or 2 coordination polyhedra, with the active Co2+ centers treated using multireference wavefunctions. Adjacent Co polyhedra representing nearest-neighbor environments were included at the Hartree–Fock level to account for local charge distribution effects. Long-range electrostatic interactions were incorporated through an array of point charges fitted to reproduce the crystalline Madelung potential [62]. Electron correlation effects in the central clusters were described using complete-active-space self-consistent-field (CASSCF) and subsequent multi-reference configuration interaction (MRCI) calculations. The MRCI treatment incorporated single and double excitations from the Co2+ 3d valence orbitals and the bridging oxygen 2p orbitals. Co2+ oxidation states were confirmed through EELS measurements (see Section S2), providing the electronic configuration basis for all quantum chemical calculations. The multiplet structure obtained then entered into spin-orbit calculations to obtain spin-orbit coupled eigenstates and wavefunctions [63].
The generalized spin Hamiltonian considered to determine the local magnetic interactions in Co5TeO8 can be expressed as
H=HI + HII,
where HI denotes the coupling between the ith and jth nearest-neighbor sites bearing spin Si and Sj, respectively, while HII contains the contribution of the interactions within sites of spin Si. The intersite contribution, HI, reads as
HI=∑ij=1,2JijSi·Sj+KijSi·Sj2+Dij·Si×Sj
where Jij and Kij are the bilinear and biquadratic isotropic exchange couplings, respectively, and Dij is the antisymmetric DMI vector (11, 12). The intra-site contribution HII takes the following form:
HII=∑iAiSi·êi2
Here, Ai is the SIA. Further details about our quantum chemistry calculations can be found in Section S5.
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Year 2026 volume 9 Issue 6
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Article Info
doi: 10.34133/research.1334
  • Receive Date:2025-11-14
  • Online Date:2026-10-16
  • Published:2026-06-20
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History
  • Received:2025-11-14
  • Revised:2026-05-29
  • Accepted:2026-05-30
Affiliations
    1Department of Applied Physics and Quantum-Phase Electronics Center, The University of Tokyo, Tokyo 113-8656, Japan.
    2PSI Center for Neutron and Muon Sciences, 5232 Villigen PSI, Switzerland.
    3Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    4Helmholtz-Zentrum Berlin für Materialien und Energie, D-12489 Berlin, Germany.
    5Institut Laue–Langevin, Grenoble 38000, France.
    6Dresden High Magnetic Field Laboratory (HLD-EMFL), 01328 Dresden, Germany.

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* Address correspondence to: (P.R.B.);
* Address correspondence to: (J.S.W.)
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表12种不同金属材料的力学参数
科
Family
属数
Number of
genus
种数
Number of
species
占总种数比例
Percentage of
total species (%)
属
Genus
种数
Number of
species
占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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