Flop Transition Research Articles

Single crystal growth and magnetic properties in triangular chain compound Ni2SiO4

Sub-centimeter-scale Ni2SiO4 single crystals were grown by a flux method with K2MoO4 as the flux. The obtained crystals exhibit high quality and were characterized using X-ray diffraction, chemical analysis, and magnetic properties, including high magnetic field (H) magnetization (M) measurements. The results of magnetic susceptibility and specific heat of a single crystal reveal significant magnetic anisotropy and antiferromagnetic ordering below TN = 30 K. High-field magnetization up to 43 T of polycrystalline shows six magnetic phase transitions at H1 = 14 T, H2 = 18.4 T, H3 = 23 T, H4 = 29 T, H5 = 32.5 T, and H6 = 37 T, forming a complex magnetic phase diagram with seven phases. Notably, the M exhibits a linear relationship with H during phases V (H4 < H < H5) and VII (H > H6), showing intercepts at 0 and half of magnetization saturation respectively, implying the presence of a spin flop transition and a 1/2 magnetization plateau. These novel phenomena in such a triangular chain system highlight complex physical nature, making Ni2SiO4 a promising candidate for studying intricate magnetic interactions.

Probing the weak limit of magnetocrystalline anisotropy through a spin‒flop transition in the van der Waals antiferromagnet CrPS4

The influence of magnetocrystalline anisotropy (MCA) on antiferromagnetism is elucidated through the characterization of the spin‒flop transition. However, due to a lack of suitable candidates for investigation, a detailed understanding of the preservation of the spin‒flop transition in the presence of low MCA energy remains elusive. In this study, we introduce CrPS4, which is a two-dimensional van der Waals antiferromagnet, as an ideal system to explore the exceedingly weak limit of the thermally-evolved MCA energy. By employing a uniaxially anisotropic spin model and fitting it to the experimental magnetic properties, we quantify the MCA energy and identify the discernible spin configurations in different magnetic phases. Notably, even at the limit of extremely weak MCA, with a mere 0.12% of the interlayer antiferromagnetic exchange interaction at T = 33 K, which is slightly below the Néel temperature (TN) of 38 K, the spin‒flop transition remains intact. We further establish a direct correlation between the visualized spin arrangements and the progressive reversal of magnetic torque induced by rotating magnetic fields. This analysis reveals the essential role of MCA in antiferromagnetism, thus extending our understanding to previously undetected limits and providing valuable insights for the development of spin-processing functionalities based on van der Waals magnets.

Spin flop transitions and complex magnetic behaviour in CoMoO4 powders and [formula omitted]-CoMoO4 thin films

Magnetic measurements were made on α-CoMoO4 powder and β-CoMoO4 thin film samples. α-CoMoO4 powder showed antiferromagnetic ordering below 13 K and two sharp spin flop transitions with complete magnetic saturation after the second transition (>5 T). A magnetic phase diagram for α-CoMoO4 was constructed, revealing three low-temperature magnetic phases: an antiferromagnetic phase at low field, an intermediate spin-flop phase, and a paramagnetic phase at high field. Two well-defined magnetisation plateaus were observed either side of the spin-flop phase: the first occurred with a moment per Co ion of 1/3 of spin saturation (where Co moments are completely aligned to the applied magnetic field), whereas the second occurred at a moment per Co ion matching calculations for spin saturation (3 μB per Co). No geometric origin for this factor of 1/3 could be found. Hysteresis was observed around the first spin-flop transition, indicating the presence of spin canting or magnetic domains under an applied field. The first successful fabrication of CoMoO4 thin films was achieved via RF magnetron sputtering, yielding β phase films which remain in the β phase upon cooling. The β-CoMoO4 thin films demonstrated antiferromagnetic ordering below 10 K. While two spin-flop transitions are seen in the β phase films (with a hysteresis around the first), spin saturation is not reached in fields up to 6 T, suggesting the existence of a third spin reorientation at higher field. A magnetic phase diagram was also constructed for the β-CoMoO4 film, which demonstrated the same three low-temperature magnetic phases, though with lower critical fields and transition temperatures. Comparisons with isostructural Ni and Mn molybdates showed that the key differences in the α and β phases arise from the different coordination of the Mo tetramers, indicating differences in magnetic properties in both phases arise from variations in the inter-tetramer exchange coupling.

Magneto-optical anisotropies of two-dimensional antiferromagnetic MPX3 from first principles

Here we systematically investigate the impact of the spin direction on the electronic and optical properties of transition metal phosphorus trichalcogenides (MPX3, M=Mn, Ni, Fe; X=S, Se) exhibiting various antiferromagnetic arrangements within the 2D limit. Our analysis based on the density functional theory and versatile formalism of Bethe-Salpeter equation reveals larger exciton binding energies for MPS3 (up to 1.1 eV in air) than MPSe3 (up to 0.8 eV in air), exceeding the values of transition metal dichalcogenides (TMDs). For the (Mn,Fe)PX3, we determine the optically active band-edge transitions, revealing that they are sensitive to in-plane magnetic order, irrespective of the type of chalcogen atom. We predict the anistropic effective masses and the type of linear polarization as important fingerprints for sensing the type of magnetic AFM arrangements. Furthermore, we identify the spin-orientation-dependent features such as the valley splitting, the effective mass of holes, and the exciton binding energy. In particular, we demonstrate that for MnPX3 (X=S, Se), a pair of nonequivalent K+ and K− points exists yielding the valley splittings that strongly depend on the direction of AFM aligned spins. Notably, for the out-of-plane direction of spins, two distinct peaks are expected to be visible below the absorption onset, whereas one peak should emerge for the in-plane configuration of spins. These spin-dependent features provide an insight into spin flop transitions of 2D materials. Finally, we propose a strategy for how the spin valley polarization can be realized in 2D AFM within a honeycomb lattice. Published by the American Physical Society 2024

Enhancement of spin–flop-induced magnetic hysteresis in van der Waals magnet (Fe1−xCox)5GeTe2

We have systematically studied magnetotransport properties in van der Waals (vdW) magnetic materials, (Fe1−xCox)5GeTe2, where the magnetic phase changes from the ferromagnetic with the perpendicular magnetic anisotropy (PMA; x = 0 , 0.05) or with the in-plane magnetic anisotropy (IMA; x = 0.19) to the antiferromagnetic (x = 0.46) with the PMA. We have demonstrated that such magnetic properties seen in bulk still remain even in thin film devices. An anomalous Hall resistance with magnetic hysteresis was clearly observed in the low Co substitution ( x = 0 , 0.05). The anomalous Hall effect was still observable for x = 0.19, but the magnetic hysteresis vanishes because of the IMA. In the antiferromagnetic region, there was no anomalous Hall effect in the low magnetic field range, but a clear hysteresis was observed at 2.5 T where the spin–flop transition takes place. This hysteresis can be seen only below 30 K and monotonically decreases with increasing temperature. We argue that the defects at a specific site in this system and also the resistance upturn below 30 K could be related to the hysteric behavior at the spin–flop transition. Our findings provide a recipe for the use of (Fe1−xCox)5GeTe2 with different Co substitutions to construct vdW magnetic devices.

Inducing skyrmion flop transitions in Co8Zn8Mn4 at room temperature

Magnetic skyrmions are topologically-protected spin textures that manifest in certain noncentrosymmetric ferromagnets under the right conditions of temperature and field. In thin-film skyrmion hosts, demagnetization effects combined with geometric confinement can result in two distinct types of spin textures: those with their axis of symmetry in the plane of the film (IP) and those with their axis pointing out-of-plane (OOP). Here we present Lorentz transmission electron microscopy evidence in conjunction with numerical modeling showing a flop transition between IP and OOP skyrmions in ${\mathrm{Co}}_{8}{\mathrm{Zn}}_{8}{\mathrm{Mn}}_{4}$ at room temperature. We show that this skyrmion flop transition is controllable via the angle of the external field relative to the film normal and we illustrate how this transition depends on thickness. Finally, we propose a skyrmion-writing device that utilizes the details of this transition.

Efficient characteristics of exchange coupling and spin–flop transition in Py/Gd bilayer using anisotropic magnetoresistance

Interlayer antiferromagnetic coupling rare-earth/transition-metal bilayer ferrimagnet systems have attracted much attention because they present different unusual temperature- and field-dependent nontrivial magnetic states and dynamics. These properties and the implementation of their applications in spintronics highly depend on the significant temperature dependence of the magnetic exchange stiffness constant A. Here, we quantitatively determine the temperature dependence of magnetic exchange stiffness APy−Gd and AGd in the artificially layered ferrimagnet consisting of a Py/Gd bilayer, using a measurement of anisotropic magnetoresistance of the bilayer thin film at different temperatures and magnetic fields. The obtained temperature dependencies of APy−Gd and AGd exhibit a scaling power law with the magnetization of Gd. The critical field of spin–flop transition and its temperature dependence can also be directly obtained by this method. Additionally, the experimental results are well reproduced by micromagnetic simulations with the obtained parameters APy−Gd and AGd, which further confirms the reliability of this easily accessible technique.

Spin-flip-driven anomalous Hall effect and anisotropic magnetoresistance in a layered Ising antiferromagnet

The influence of magnetocrystalline anisotropy in antiferromagnets is evident in a spin flip or flop transition. Contrary to spin flops, a spin-flip transition has been scarcely presented due to its specific condition of relatively strong magnetocrystalline anisotropy and the role of spin-flips on anisotropic phenomena has not been investigated in detail. In this study, we present antiferromagnet-based functional properties on an itinerant Ising antiferromagnet Ca0.9Sr0.1Co2As2. In the presence of a rotating magnetic field, anomalous Hall conductivity and anisotropic magnetoresistance are demonstrated, the effects of which are maximized above the spin-flip transition. Moreover, a joint experimental and theoretical study is conducted to provide an efficient tool to identify various spin states, which can be useful in spin-processing functionalities.

The evolution of structure and magnetic properties of GdCr1-Fe O3 (0 ≤ x ≤ 0.5) polycrystalline samples

In this paper, the evolution of crystal structures and magnetic properties of GdCr 1- x Fe x O 3 (0 ≤ x ≤ 0.5) samples with different doping levels was systematically investigated by powder X-ray diffraction, Fourier transform infrared spectra, positron annihilation spectra, and temperature-dependent and field-dependent magnetization measurements. The lattice constants and Cr(Fe)O 6 octahedral deformation increase with Fe doping showing an anomaly at x = 0.4 which is further confirmed by positron annihilation and Fourier transform spectra. Fe doping in GdCrO 3 can suppress the magnetization reversal in the parent phase as well as its antiferromagnetic ordering temperature T N at low doping levels but enhance T N at high doping levels. A possible spin flip or flop transition under field-cooled-cooling process occurs for the samples of x = 0.3 and 0.4 at low temperature. The coercive fields of weak ferromagnetic state with different doping levels at 10 and 100 K also exhibit a local maximum at around x = 0.4 which is accordingly correlated to the concentrations of the vacancy defects in the sample. • The information of cationic vacancy defects is analyzed by positron annihilation technique. • The correlation between lattice parameters and vacancy defects is unveiled. • Defects in GdCr 1- x Fe x O 3 samples have a great impact on their magnetic properties. • Chemical doping and defect engineering are effective tools to manipulate R CrO 3 materials.

Superpotentials from singular divisors

We study Euclidean D3-branes wrapping divisors D in Calabi-Yau orientifold compactifications of type IIB string theory. Witten’s counting of fermion zero modes in terms of the cohomology of the structure sheaf {mathcal{O}}_D applies when D is smooth, but we argue that effective divisors of Calabi-Yau threefolds typically have singularities along rational curves. We generalize the counting of fermion zero modes to such singular divisors, in terms of the cohomology of the structure sheaf {mathcal{O}}_{overline{D}} of the normalization overline{D} of D. We establish this by detailing compactifications in which the singularities can be unwound by passing through flop transitions, giving a physical incarnation of the normalization process. Analytically continuing the superpotential through the flops, we find that singular divisors whose normalizations are rigid can contribute to the superpotential: specifically, {h}_{+}^{bullet}left({mathcal{O}}_{overline{D}}right)=left(1,0,0right) and {h}_{-}^{bullet}left({mathcal{O}}_{overline{D}}right)=left(0,0,0right) give a sufficient condition for a contribution. The examples that we present feature infinitely many isomorphic geometric phases, with corresponding infinite-order monodromy groups Γ. We use the action of Γ on effective divisors to determine the exact effective cones, which have infinitely many generators. The resulting nonperturbative superpotentials are Jacobi theta functions, whose modular symmetries suggest the existence of strong-weak coupling dualities involving inversion of divisor volumes.

Correlated temperature and field dependent magnetization: Enhanced magnetic hysteresis upon Dy doping in La-based francisite Cu3La(SeO3)2O2Cl

Study of La-based cuprate francisite (LaCufr), an analogue compound of Cu3Bi(SeO3)2O2Cl, is interesting due to the interplay of competing magnetic interactions in this geometrically frustrated layered compound. The motivation is to investigate the role of rare earth metals with 4f electrons on Cu2+ ions with 3d electrons, i.e., to study a complex 3d-4f interaction. In this regard, pristine, 5, and 10 at% (atomic percentage) Dy doped La-based cuprate francisite [Cu3La1-xDyx(SeO3)2O2Cl; x = 0, 0.05, and 0.1] compounds were synthesized. Structural (X-ray diffraction i.e. XRD), electronic (X-ray absorption near edge structure i.e. XANES) and magnetic properties were investigated. XRD and XANES ensure phase purity and indicate dopant Dy atoms are more or less substitutionally incorporated at the La site. Magnetic properties were highlighted by unconventional magnetic hysteresis due to spin reorientation transition (SRT) and field-induced meta-magnetic transition (MMT) below 15 K. The signature of MMT was also observed from a temperature dependent magnetization (M−T) study, which is unique. The process of field dependent magnetization has been explored with a spin flop and spin flip transition. Dy doping enhances hysteresis and overall magnetization. The reason for this enhancement has been interpreted as identifying micro-interactions using an energy splitting scheme.

Two Magnetic Orderings and a Spin-Flop Transition in Mixed Valence Compound Mn3O(SeO3)3.

A mixed-valence manganese selenite, Mn3O(SeO3)3, was successfully synthesized using a conventional hydrothermal method. The three-dimensional framework of this compound is composed of an MnO6 octahedra and an SeO3 trigonal pyramid. The magnetic topological arrangement of manganese ions shows a three-dimensional framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Susceptibility, magnetization and heat capacity measurements confirm that Mn3O(SeO3)3 exhibits two successive long-range antiferromagnetic orderings with TN1~4.5 K and TN2~45 K and a field-induced spin–flop transition at a critical field of 4.5 T at low temperature.

Field-assisted sub-terahertz spin pumping and auto-oscillation in NiO

Spin pumping converting sub-terahertz electromagnetic waves to DC spin currents has recently been demonstrated in antiferromagnets (AFMs) with easy-axis magnetic anisotropy. However, easy-plane AFMs such as NiO, which are easier to prepare experimentally, are considered to be bad candidates for spin pumping because the Néel vector oscillation is linearly polarized, placing a major restriction on the material choice for practical applications. Through a case study of NiO, we show that an applied magnetic field below the spin–flop transition can substantially modify the polarization of the resonance eigenmodes, which enables coherent sub-terahertz spin pumping as strong as that in easy-axis AFMs. In addition, we find that an applied magnetic field can significantly reduce the threshold of Néel vector auto-oscillation triggered by spin-transfer torques. These prominent field-assisted effects can greatly facilitate spintronic device engineering in the sub-terahertz frequency regime.

Magnon polarons in spin Seebeck effect of easy axis antiferromagnets

The formation of magnon polarons, quanta of magnetoelastic waves, was found to be able to stimulate an enhancement or suppression in the magnetic field dependence of the spin Seebeck effect when the dispersion curve of the magnon becomes tangential to those of acoustic phonons. In the present work, we systematically analyzed the properties of the magnon spectrum in body-centered cubic easy-axis antiferromagnets with varying strength of exchange interaction and magnetic anisotropy, which allowed us to classify the antiferromagnets according to the number of solutions for the tangential condition between the dispersion curves of magnons and acoustic phonons. The anomaly features were found to occur only in the relatively weak magnetic field regime before spin flop transition. The manifestation of magnon–polaron-induced anomaly on the longitudinal spin Seebeck coefficient was also calculated directly from which a triple-peak feature, never observed or proposed before, was predicted. Our analysis also works for antiferromagnets with other magnetic lattices.

Geodesics in the extended Kähler cone of Calabi-Yau threefolds

We present a detailed study of the effective cones of Calabi-Yau threefolds with h1,1 = 2, including the possible types of walls bounding the Kähler cone and a classification of the intersection forms arising in the geometrical phases. For all three normal forms in the classification we explicitly solve the geodesic equation and use this to study the evolution near Kähler cone walls and across flop transitions in the context of M-theory compactifications. In the case where the geometric regime ends at a wall beyond which the effective cone continues, the geodesics “crash” into the wall, signaling a breakdown of the M-theory supergravity approximation. For illustration, we characterise the structure of the extended Kähler and effective cones of all h1,1 = 2 threefolds from the CICY and Kreuzer-Skarke lists, providing a rich set of examples for studying topology change in string theory. These examples show that all three cases of intersection form are realised and suggest that isomorphic flops and infinite flop sequences are common phenomena.

Magnetization reversal, field-induced transitions and H–T phase diagram of Y1−x Ce x CrO3

We report a systematic study of the magnetic phase diagram in the H–T plane, negative magnetization (NM), exchange interactions and field-induced spin–flop transitions in the distorted perovskite Y1−x Ce x CrO3. Locked AFM and weak-FM configurations in Γ4(G z , F y , A x ) phase of YCrO3 (S = 3/2 ground state) unlocks into the Γ2 (F z , G y , C x ; ) phase of the canted AFM and FM structures with the dilute substitution of Ce (x ⩾ 0.05). The asymmetric and symmetric exchange interaction (J AS ∼ 0.11 meV and J S ∼ 0.85 meV) between the trivalent Ce and Cr enable the positive quartic-anisotropy field ( ∼ 2.85 × 102 Oe) along with the second order anisotropy field ( ∼ 5.93 × 102 Oe). Unlike the pristine YCrO3 compound, the Ce incorporated system exhibits a giant fourth-order anisotropy constant (K 4 = 1.35 × 105 erg/c.c.) due to the asymmetric exchange interaction between the trivalent Ce–Cr which further lifts the free energy of the system and causes lag in the onset of AFM ordering showing the significant thermal hysteresis (ΔT ∼ 10 K) in the field-cooled (FC)-warming measurement protocol as compared to the FC-cooling mode. The H–T phase diagram, mapped from the isothermal magnetization data and differential magnetic susceptibility data with different measurement protocols clearly distinguishes three prominent regions below the T N (∼150 K), viz (i) long-range canted AFM + weak FM phase (Γ4 (G z , F y , A x )), (ii) Γ24 mixed phase and (iii) robust Γ2 (F z , G y , C x ; , ) AFM + FM phases. Tunable spin–flopped transition (∼ 30 kOe), significant negative exchange-bias field (H EB ∼ 2.5 kOe), huge coercive field (H C ∼ 22 kOe) and large NM (ΔM ∼ 280 emu/mole) are the unique characteristic features of the current investigated system.

Flops, Gromov-Witten invariants and symmetries of line bundle cohomology on Calabi-Yau three-folds

The zeroth line bundle cohomology on Calabi-Yau three-folds encodes information about the existence of flop transitions and the genus zero Gromov-Witten invariants. We illustrate this claim by studying several Picard number 2 Calabi-Yau three-folds realised as complete intersections in products of projective spaces. Many of these manifolds exhibit certain symmetries on the Picard lattice which preserve the zeroth cohomology.

CoMOF5(pyrazine)(H2O)2 (M = Nb, Ta): Two-Layered Cobalt Oxyfluoride Antiferromagnets with Spin Flop Transitions.

Two cobalt oxyfluoride antiferromagnets CoMOF5(pyz)(H2O)2 (M = Nb 1, Ta 2; pyz = pyrazine) have been synthesized via conventional hydrothermal methods and characterized by thermogravimetric (TGA) analysis, FTIR spectroscopy, electron spin resonance (ESR), magnetic susceptibility, and magnetization measurements at both static low field and pulsed high field. The single-crystal X-ray diffraction indicates both compounds 1 and 2 are isostructural and crystallize in the monoclinic space group C2/m with a two-dimensional Co2+ triangular lattice in the ab plane, separated by the nonmagnetic MOF5 (M = Nb 1, Ta 2) octahedra along the c-axis with large intertriangular-lattice Co···Co distance. Because of low dimensionality together with frustrated triangular lattice, compounds 1 and 2 exhibit no long-range antiferromagnetic order until ∼3.7 K. Moreover, a spin flop transition is observed in the magnetization curves at 2 K for both compounds, which is further confirmed by ESR spectra. In addition, the ESR spectra suggest the presence of a zero-field spin gap in both compounds. The high field magnetization measured at 2 K saturates at ∼7 T with Ms = 1.55 μB for 1 and 1.71 μB for 2, respectively, after subtracting the Van Vleck paramagnetic contribution, which is usually observed for Co2+ ions with pseudospin spin of 1/2 at low temperature. Powder-averaged magnetic anisotropy of g = 3.10 for 1 (3.42 for 2) and magnetic superexchange interaction J/kB = -3.2 K for 1 (-3.6 K for 2) are obtained.

Large magnetic anisotropy and field-induced spin-flop transition in Fe2(TeO3)2(SO4)·3H2O single crystals

Single crystals of Fe2(TeO3)2(SO4)·3H2O with an alternating spin-chain structure are successfully grown by a hydrothermal method. Magnetic and heat capacity measurements show that this compound possesses a short-range antiferromagetic order at ~55 K and a long-range antiferromagetic order at 32.5 K. A large magnetic anisotropy with magnetization easy c-axis is observed, in which a weak ferromagnetic component is observed along the a- and b-axes, while a spin flop transition is observed at applied field along the c-axis. Also, the magnetic anisotropy energy at 2 K is estimated to be 4.78(3) × 108 ergs/cm3. Such magnetic anisotropy may be due to a strong nonsymmetric distortion in the FeO6 octahedra.

New G2-conifolds in M-theory and their field theory interpretation

A recent theorem of Foscolo-Haskins-Nordström [1] which constructs complete G2-holonomy orbifolds from circle bundles over Calabi-Yau cones can be utilised to construct and investigate a large class of generalisations of the M-theory flop transition. We see that in many cases a UV perturbative gauge theory appears to have an infrared dual described by a smooth G2-holonomy background in M-theory. Various physical checks of this proposal are carried out affirmatively.