Critical phenomenon of the near room temperature skyrmion material FeGe

Critical behaviors of van der Waals itinerant ferromagnet Fe3.8GaTe2

The critical behavior of the van der Waals ferromagnet Fe3.8GaTe2was systematically studied through measurements of isothermal magnetization, with the magnetic field applied along thec-axis. Fe3.8GaTe2undergoes a non-continuous paramagnetic to ferromagnetic phase transition at the Curie temperatureTc∼ 355 K. A comprehensive analysis of isotherms aroundTcutilizing the modified Arrott diagram, the Kouvel-Fisher method, the Widom scaling law, and the critical isotherm analysis yielded the critical exponent ofβ= 0.411,γ= 1.246, andδ= 3.99. These critical exponents are found to be self-consistent and align well with the scaling equation at high magnetic fields, underscoring the reliability and intrinsic nature of these parameters. However, the low-field data deviates from the scaling relation, exhibiting a vertical trend whenT Collapse

Key Words

• Fe3.8GaTe2
• critical behaviors
• critical exponent
• isothermal magnetization
• three-dimensional Heisenberg

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Affiliation(s)

Tianyang Yao School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
W L Qubie School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Pushpendra Kumar School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
Xu Bai School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Shixin Hu School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Desheng Xue School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China
Junli Zhang School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People's Republic of China

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Thermal Stability of Skyrmion Tubes in Nanostructured Cuboids

Magnetic skyrmions in bulk materials are typically regarded as two-dimensional structures. However, they also exhibit three-dimensional configurations, known as skyrmion tubes, that elongate and extend in-depth. Understanding the configurations and stabilization mechanism of skyrmion tubes is crucial for the development of advanced spintronic devices. However, the generation and annihilation of skyrmion tubes in confined geometries are still rarely reported. Here, we present direct imaging of skyrmion tubes in nanostructured cuboids of a chiral magnet FeGe using Lorentz transmission electron microscopy (TEM), while applying an in-plane magnetic field. It is observed that skyrmion tubes stabilize in a narrow field-temperature region near the Curie temperature (Tc). Through a field cooling process, metastable skyrmion tubes can exist in a larger region of the field-temperature diagram. Combining these experimental findings with micromagnetic simulations, we attribute these phenomena to energy differences and thermal fluctuations. Our results could promote topological spintronic devices based on skyrmion tubes.

Investigation of magnetic properties and colossal magnetoresistance in nanocrystalline doped manganite

Here in, we report structural, magnetic, and magneto-transport properties of nanocrystalline La0.6Ag0.2Bi0.2MnO3prepared using citrate sol-gel method. By using scanning and transmission electron microscopy measurements, the morphology and particle size of the sample have been confirmed. The Mn2p x-ray photoelectron spectroscopy spectra revealed the nanoparticles contained the coexistence of Mn3+and Mn4+ions with Mn3+/Mn4+ratio of 2:1. Field-cooled and zero field-cooled magnetization protocols with temperature span of 5 K-300 K, confirm the paramagnetic (PM) to ferromagnetic (FM) phase transition at critical temperature,TC∼ 146 K. The complete investigation of isothermal magnetization (130⩽T (K)⩽160,ΔT=2 K), Arrott plots, and magnetocaloric effect as well as quantitative analysis of second-order phase transition has been studied. The criticality at the PM-FM transition was examined for the sample, and the obtained critical exponents were verified for their reliability through the utilization of the scaling hypothesis and Kouvel-Fisher plot. We observed a large magnetic entropy change (∼7 J-Kg-1K-1) atTCupon 5 T magnetic field strength. The renormalized magnetic entropy change plots are found to collapse onto a single curve, thus verifying the universality of the sample. Above the metal-insulator transitions the electrical resistivity shows a small polaron hopping conduction mechanism, however, at low temperatures scattering mechanism dominates and the whole range was explained by the universal percolation model. The colossal value of negative MR is found to be 88% at 168 K under an applied field strength of 2 T. As a result of our experimental data, we can grasp the intuitive understanding of magnetic as well as transport properties in Bi-doped manganite systems potential for magnetic sensors and spintronics applications.

Origin of metamagnetism in skyrmion host Cu[Formula: see text]OSeO[Formula: see text]

Skyrmion host chiral Cu[Formula: see text]OSeO[Formula: see text] has attracted researchers due to several intriguing properties. Observation of metamagnetism in low-temperature and low-field makes the magnetic properties of Cu[Formula: see text]OSeO[Formula: see text] more complex. Here, we present an investigation on metamagnetism in Cu[Formula: see text]OSeO[Formula: see text] by analyzing its structural and magnetic properties. Study of magnetic properties reveal spin-flip of one of the Cu[Formula: see text] ions, embedded in square pyramidal CuO[Formula: see text] polyhedra, due to the development of strain in low-temperature and low-field regime. The spin-flip is found to be the main reason for field-induced first-order metamagnetic transition. Magnetic phase diagram of Cu[Formula: see text]OSeO[Formula: see text] has been constructed with the help of magnetization analyses. It is argued that the metamagnetic hysteretic field region may be low-temperature skyrmion phase with additional spiral and tilted-conical phases. A tricritical point has been observed in the phase diagram at which first-order metamagnetic hysteretic field range ceases to exist.

Different Critical Exponents on Two Sides of a Transition: Observation of Crossover from Ising to Heisenberg Exchange in Skyrmion Host Cu_{2}OSeO_{3}

We present experimental investigation on critical phenomena in Cu_{2}OSeO_{3} by analyzing the critical behavior of magnetization using a new method. This is necessary as a crossover from 3D Ising to 3D Heisenberg has been observed in Cu_{2}OSeO_{3}. The proposed method is applicable to explore the physics for a wide range of materials showing trivial or nontrivial critical behavior on two sides of the transition. A magnetic phase diagram has been constructed from the critical analysis. Multiple critical points due to multiple phases and transition between them have been observed in the phase diagram of Cu_{2}OSeO_{3}.

A theoretical strategy for pressure-driven ferroelectric transition associated with critical behavior and magnetoelectric coupling in organic multiferroics

In organic multiferroics, the charge or spin coupled to the lattice induces lattice symmetry breaking, which is responsible for the ferroelectric (FE) transition. We propose a quantum spin model to describe the ferroelectricity of organic multiferroics, in which the pressure-driven spin-lattice coupling is controlled both by a jump function and a pressure power function. The T-p phase diagram shows different scaling relationships at low and high pressure regions, respectively, which is in accordance with the experimental observation. It is found that the pressure can not only enhance the FE polarization, but also enhance the transition temperature T_c as well as the electrocaloric effect (ECE). The electrocaloric critical scaling laws are proposed to verify the order and universality of the FE transition based on the Banerjee and Franco's criteria. In addition, we propose a temperature mediated mechanism within an isentropic process based on the ECE combined with the pyromagnetic effect, together with multiple physically (magnetic field and pressure jointly) controlled means, to enhance the magnetoelectric coupling around room-temperature, which will provide thermodynamic and quantum controlled means for realizing multi-state logic memory.

Critical exponents and universal magnetic behavior of noncentrosymmetric Fe0.6Co0.4Si

The critical magnetic properties of a non-centrosymmetric B20 cubic helimagnet Fe_0.6Co_0.4Si are investigated using magnetization isotherms. It belongs to the 3D-Heisenberg universality class with short range magnetic coupling as inferred from the self-consistent critical exponents [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] in combination with exchange interaction [Formula: see text]. Itinerant magnetic nature of the compound is realized by the Rhodes-Wholfarth analysis. Field-induced weak first (para[Formula: see text]helical) to second (para[Formula: see text]field-polarized) order transition is reported to occur at low critical field due to the weak spin-orbit coupling arising from the weak Dzyaloshinksii-Moriya interactions. Our study suggests the distinct phenomenological magnetic structures for Fe-based cubic magnets (Fe_1-x Co _x Si and FeGe) and MnSi which cause contrasting physical properties.

Quantification of Magnetic Surface and Edge States in an FeGe Nanostripe by Off-Axis Electron Holography

Whereas theoretical investigations have revealed the significant influence of magnetic surface and edge states on Skyrmonic spin texture in chiral magnets, experimental studies of such chiral states remain elusive. Here, we study chiral edge states in an FeGe nanostripe experimentally using off-axis electron holography. Our results reveal the magnetic-field-driven formation of chiral edge states and their penetration lengths at 95 and 240 K. We determine values of saturation magnetization M_{S} by analyzing the projected in-plane magnetization distributions of helices and Skyrmions. Values of M_{S} inferred for Skyrmions are lower by a few percent than those for helices. We attribute this difference to the presence of chiral surface states, which are predicted theoretically in a three-dimensional Skyrmion model. Our experiments provide direct quantitative measurements of magnetic chiral boundary states and highlight the applicability of state-of-the-art electron holography for the study of complex spin textures in nanostructures.

Endotaxial growth of FexGe single-crystals on Ge(001) substrates

Through a 3D diffraction method combined with HRTEM images, we have successfully determined the specific phase of each Fe_xGe island grown on the Ge substrate.

Critical Behavior and Macroscopic Phase Diagram of the Monoaxial Chiral Helimagnet Cr1/3NbS2

Cr_1/3NbS₂ is a unique example of a hexagonal chiral helimagnet with high crystalline anisotropy, and has generated growing interest for a possible magnetic field control of the incommensurate spin spiral. Here, we construct a comprehensive phase diagram based on detailed magnetization measurements of a high quality single crystal of Cr_1/3NbS₂ over three magnetic field regions. An analysis of the critical properties in the forced ferromagnetic region yields 3D Heisenberg exponents β = 0.3460 ± 0.040, γ = 1.344 ± 0.002, and T_C = 130.78 K ± 0.044, which are consistent with the localized nature the of Cr³⁺ moments and suggest short-range ferromagnetic interactions. We exploit the temperature and magnetic field dependence of magnetic entropy change (ΔS_M) to accurately map the nonlinear crossover to the chiral soliton lattice regime from the chiral helimagnetic phase. Our observations in the low field region are consistent with the existence of chiral ordering in a temperature range above the Curie temperature, T_C < T < T*, where a first-order transition has been previously predicted. An analysis of the universal behavior of ΔS_M(T,H) experimentally demonstrates for the first time the first-order nature of the onset of chiral ordering.

Temperature and Magnetic Field Dependence of the Internal and Lattice Structures of Skyrmions by Off-Axis Electron Holography

The internal and lattice structures of magnetic Skyrmions in B20-type FeGe are investigated using off-axis electron holography. The temperature, magnetic field, and angular dependence of the magnetic moments of individual Skyrmions are analyzed. The internal Skyrmion shape is found to vary with applied magnetic field. In contrast, the inter-Skyrmion distance remains almost unchanged in the lattice phase over the studied range of applied field. The amplitude of the local magnetic moment is found to vary with temperature, while the Skyrmion shape does not change significantly. Deviations from a circular to a hexagonal Skyrmion structure are observed in the lattice phase, in agreement with the results of micromagnetic simulations.