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Molecular beam epitaxy (MBE) is a powerful tool in modern technologies, including electronic, optoelectronic, spintronic, and sensoric applications. The primary factor determining epitaxial heterostructure properties is the growth mode and the resulting atomic structure and microstructure. In this paper, we present a novel method for growing epitaxial layers and nanostructures with specific and optimized structural and magnetic properties by assisting the MBE process using electromagnetic and mechanical external stimuli: an electric field (EF), a magnetic field (MF), and a strain field (SF). The transmission of the external fields to the sample is realized using a system of specialized sample holders, advanced transfers, and dedicated manipulators. Examples of applications include the influence of MFs on the growth and anisotropy of epitaxial magnetite and iron films, the use of EFs for in situ resistivity measurements, the realization of in situ magneto-optic measurements, and the application of SFs to the structural modification of metal films on mica.
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In this work, the mechanochemical synthesis method was used for the first time to produce powders of the nanocrystalline Nd1.1Fe10CoTi compound from Nd2O3, Fe2O3, Co and TiO2. High-energy-milled powders were heat treated at 1000 °C for 10 min to obtain the ThMn12-type structure. Volume fraction of the 1:12 phase was found to be as high as 95.7% with 4.3% of a bcc phase also present. The nitrogenation process of the sample was carried out at 350 °C during 3, 6, 9 and 12 h using a static pressure of 80 kPa of N2. The magnetic properties Mr, µ0Hc, and (BH)max were enhanced after nitrogenation, despite finding some residual nitrogen-free 1:12 phase. The magnetic values of a nitrogenated sample after 3 h were Mr = 75 Am2 kg-1, µ0Hc = 0.500 T and (BH)max = 58 kJ·m-3. Samples were aligned under an applied field of 2 T after washing and were measured in a direction parallel to the applied field. The best value of (BH)max ~ 114 kJ·m-3 was obtained for 3 h and the highest µ0Hc = 0.518 T for 6 h nitrogenation. SEM characterization revealed that the particles have a mean particle size around 360 nm and a rounded shape.
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Magnetic van der Waals (vdW) materials have emerged as promising candidates for spintronics applications, especially after the recent discovery of intrinsic ferromagnetism in monolayer vdW materials. There has been a critical need for tunable ferromagnetic vdW materials beyond room temperature. Here, we report a real-space imaging study of itinerant ferromagnet Fe3GeTe2 and the enhancement of its Curie temperature well above ambient temperature. We find that the magnetic long-range order in Fe3GeTe2 is characterized by an unconventional out-of-plane stripe-domain phase. In Fe3GeTe2 microstructures patterned by a focused ion beam, the out-of-plane stripe domain phase undergoes a surprising transition at 230 K to an in-plane vortex phase that persists beyond room temperature. The discovery of tunable ferromagnetism in Fe3GeTe2 materials opens up vast opportunities for utilizing vdW magnets in room-temperature spintronics devices.
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Nd-Fe-B magnets, possessing the highest energy product, are extensively used in cutting-edge applications, including electrical machines and electrical vehicles. An environmentally benign and cost effective synthesis method of Cr alloyed Nd2 (Fe,Co)14 B magnetic nanoparticles using a dry mechanochemical process is reported. The method is solvent free, facile, energy efficient and scalable. The reduction of mixed oxides of Nd, Fe, Co, B and Cr is performed by using Ca. The coercivity (HC ) of the nanoparticles is found to depend on the dispersant content, with the highest value obtained for Nd2 (Fe11.25 Co2 Cr0.75 )B with 40 % CaO dispersant. The HC of isolated Nd2 (Fe11.25 Co2 Cr0.75 )B nanoparticles and nanoparticles embedded in a CaO matrix is found to be 11.5â kOe and 14.4â kOe, respectively, largest values for heavy rare earth free Nd-Fe-B nanoparticles with reasonable saturation and remanent magnetization, regardless of synthesis route. Considering the density of Nd2 Fe14 B, an energy product of 14.2â MGOe is obtained for the nanoparticles. The thermal coefficient of remanence and thermal coefficient of coercivity for aligned samples are -0.06 % and -0.29 %, respectively, in the temperature range between 100â K and 400â K. The spin reorientation temperature is found to be â¼30â K less than that of bulk Nd2 Fe14 B magnets.
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The tailoring of topological surface states in topological insulators is essential for device applications and for exploring new topological phase. Here, we propose a practical way to induce the quantum anomalous Hall phase and unusual metal-insulator transitions in Cr-doped Bi2Se3 films based on the model Hamiltonian and first-principles calculations. Using the combination of in-plane and plane-normal components of the spin along with external electric fields, we demonstrate that the topological state and band structures of topological insulating films exhibit rich features such as the shift of Dirac cones and the opening of nontrivial band gaps. We also show that the in-plane magnetization leads to significant suppression of inter-TSS scattering in Cr-doped Bi2Se3. Our work provides new strategies to obtain the desired electronic structures for the device, complementary to the efforts of an extensive material search.
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The effect of partially substituting Tin (Sn) at the Manganese (Mn) site ofCa3Mn2O7, viz.Ca3Mn2-xSnxO7withx=0.03,0.05, on its structural and magnetic properties have been investigated using synchrotron diffraction, neutron diffraction, and bulk magnetization measurements. It is observed that with a substitution ofx=0.03, the minor (≈8%) tetragonal (I4/mmm) structural phase that is present in the predominantly orthorhombic (Cmc21) undopedCa3Mn2O7, completely disappears. The compounds order antiferromagnetically, the ordering temperature decreases with increasing Sn-content, indicating a weakening of the antiferromagnetic exchange interactions. Interestingly, in the ordered state, the spin magnetic moments which were aligned along thea-axis of the unit cell in the undoped compound, are observed to have reoriented with their major components lying in theb - cplane in the Sn-doped compounds. The above influence of Sn-doping is seen to be stemming from a significant modification of the octahedral rotation and tilt mode geometry in the unit cell, that is known to be responsible for driving ferroelectricity in these compounds.
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Based on the proposed microscopic model, we investigate the multiferroic characteristics of VOX2 (X = Cl, Br, I) monolayers using a Green's function method. The dependence of the microscopic parameters of the ferroelectric system (pseudo-spin arrangement and flipping rate) on the magnitude and sign of the exchange magnetic interaction along the b-axis and the value of the Dzyaloshinskii-Moria vector have been investigated and qualitatively explained. The possibility of observing a spin-reorientation transition with a change in the character of spin ordering from antiferromagnetic to ferromagnetic is investigated. It is found that the antisymmetric magnetoelectric interaction may be responsible for the spin-reorientation transition without a change in the ordering of magnetic moments. Changing the sign of the exchange magnetic interaction along the b-axis leads to ferromagnetic ordering without observing a spin-reorientation transition. The dependence of isotropic and antisymmetric magnetic interactions on the microscopic parameters of the ferroelectric system is qualitatively explained. A mechanism for the occurrence of the spin-reorientation transition is presented based on the proposed microscopic model. The obtained results qualitatively coincide with Density Functional Theory calculations.
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Spin reorientation transition is an ubiquitous phenomenon observed in magnetic rare earth orthferrites RFeO3, which has garnered significant attention in recent years due to its potential applications in spintronics or magnetoelectric devices. Although a plenty of experimental works suggest that the magnetic interaction between R3+and Fe3+spins is at the heart of the spin reorientation, but a direct and conclusive theoretical support has been lacking thus far, primarily due to the challenging nature of handling R 4felectrons. In this paper, we explored DyFeO3as an example by means of comprehensive first principles calculations, and compared two different approaches, where the Dy 4felectrons were treated separately as core or valence states, aiming to elucidate the role of Dy 4felectrons, particularly in the context of the spin reorientation transition. The comparison provides a solid piece of evidence for the experimental argument that the Dy3+-Fe3+magnetic interactions play a vital role in triggering spin reorientation of Fe3+moments at low temperatures. The findings revealed here not only extend our understanding on the underlying mechanism for spin reorientation transition in RFeO3, but also highlight the importance of explicit description of R 4felectrons in rationally reproducing their structural, electronic and magnetic properties.
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The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.
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Rare-earth orthochromite is an interesting system in the view of its complex magnetic ordering due to competing interaction between different magnetic ions. Here, Fe-substituted SmCrO3samples were prepared by solid-state route to investigate their intriguing magnetic properties towards exploring its application potential. The magnetic studies revealed antiferromagnetic (AFM) ordering at Nèel temperature (TN) â¼ 181 K, magnetic compensation temperatures (TCOMP) at â¼137 K and 50 K, and spin-reorientation temperature (TSR) at 64 K in SmCr0.8Fe0.2O3sample. Additionally, the system exhibited negative magnetization under field-cooled conditions which allowed the field as well as temperature controllable magnetization switching behavior. Notably, the Fe-substituted SmCrO3sample displayed a remarkable exchange bias (HEB) value of â¼1.39 T at 10 K due to the coexistence of ferromagnetic and AFM ordering at different cationic sites. TheM-Hloops recorded under positive and negative field-cooled conditions ruled out the minor-loop effect. Theoretical models applied on the training effect studies confirmed the observed exchange-bias effect.
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High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii-Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties.
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We report the structural and magnetic properties of Nd1-xCexCrO3(x=0.05-0.175) single-phase samples to classify the influence of Ce substitution on the Nd-site. The electron density profile indicates the possible covalent nature of Cr-O bonds. The x-ray photoelectron spectroscopy confirms a mixed Ce valency with a constant ratio of Ce3+/Ce4+ions in all substituted compounds and the charge neutralization through the oxygen vacancies. The magnetization measurements reveal an increase in antiferromagnetic ordering temperature (TN) and spin-reorientation transition temperature (TSR) and unfold soft spin-reorientation attributed to diluted superexchange interactions upon Ce incorporation. The presence of mixed Ce ions induces the merging of the hysteresis loop with a significant exchange bias (EB) field. We demonstrate for the first time that the magnitude of the magnetization is different for the same applied field in positive and negative directions, indicating the existence of two different magnetic states. The difference between these two magnetic states possibly arises from the pinning of Cr3+spins, which requires an additional Zeeman energy for it to rotate. This maximum Zeeman energy from the normalized magnetic susceptibility vs. temperature curves correlates with the maximum EB field, validating unusual EB in these compounds.
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2D ferromagnetic chromium tellurides exhibit intriguing spin configurations and high-temperature intrinsic ferromagnetism, providing unprecedented opportunities to explore the fundamental spin physics and build spintronic devices. Here, a generic van der Waals epitaxial approach is developed to synthesize the 2D ternary chromium tellurium compounds with thicknesses down to mono-, bi-, tri-, and few-unit cells (UC). The Mn0.14 Cr0.86 Te evolves from intrinsic ferromagnetic behavior in bi-UC, tri-UC, and few-UC to temperature-induced ferrimagnetic behavior as the thickness increases, resulting in a sign reversal of the anomalous Hall resistance. Temperature- and thickness-tunable labyrinthine-domain ferromagnetic behaviors are derived from the dipolar interactions in Fe0.26 Cr0.74 Te and Co0.40 Cr0.60 Te. Furthermore, the dipolar-interaction-induced stripe domain and field-induced domain wall (DW) motion velocity are studied, and multibit data storage is realized through an abundant DW state. The magnetic storage can function in neuromorphic computing tasks, and the pattern recognition accuracy can reach up to 97.93%, which is similar to the recognition accuracy of ideal software-based training (98.28%). Room-temperature ferromagnetic chromium tellurium compounds with intriguing spin configurations can significantly promote the exploration of the processing, sensing, and storage based on 2D magnetic systems.
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The structure and the physical phenomena that occur in a crystal can be described by using a suitable set of symmetry-adapted modes. The classification of magnetic modes in crystals presented in Fabrykiewicz et al. [Acta Cryst. (2021), A77, 327-338] is extended to a classification of electric and toroidal (anapole) modes in crystals. These three classifications are based on magnetic point groups, which are used in two contexts: (i) the magnetic point group of the magnetic crystal class and (ii) the magnetic site-symmetry point group of the Wyckoff position of interest. The classifications for magnetic, electric and toroidal modes are based on the properties of the three generalized inversions: space inversion 1, time inversion 1' and the space-and-time inversion 1'. It is emphasized that none of these three inversions is more important than the other two. A new notation for symmetry operation symbols and magnetic point group symbols is proposed; each operation is presented as a product of one proper rotation and one generalized inversion. For magnetic, electric and toroidal orderings there are 64 modes: three pure ferro(magnetic/electric/toroidal) modes, 13 mixed ferro(magnetic/electric/toroidal) and antiferro(magnetic/electric/toroidal) modes, and 48 pure antiferro(magnetic/electric/toroidal) modes. The proposed classification of modes leads to useful observations: the electric and toroidal modes have many symmetry limitations similar to those already known for the magnetic modes, e.g. a continuous reorientation of the magnetic or electric or toroidal moments is possible only in triclinic or monoclinic symmetry. An antiferro(magnetic/electric/toroidal) ordering with a weak perpendicular ferro(magnetic/electric/toroidal) component is possible only in monoclinic or orthorhombic symmetry. The general classifications of magnetic, electric and toroidal modes are presented for the case of NdFeO3.
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A detailed crystal structure analysis, temperature and field dependence of magnetic characteristics and phonon instabilities for different compositions (0.1 ⩽x⩽ 0.5) of Dy1-xCexCrO3solid-solutions have been reported. All the investigated compounds exhibit distorted orthorhombic crystal structure with a distortion factor ofdOct/dCellâ¼ 6 × 10-3/3.5 ppm (forxâ¼ 0.2) forPbnmspace group that follows Vegard's law. The bonds between apical oxygen atoms (OA1) and Cr atoms stand more rigidly in comparison with the basal oxygen atoms (OB1/OB2) resulting the octahedral distortion and thereby causing the changes in phonon modes. The CrO6octahedral tilt angleθrotates with respect to the Miller pseudocubic axis [101] which varies from 10.36° (x= 0.1) to 12.25° (x= 0.5) and significantly influences the Ag(5) phonon stability by 3% for a change in A-site mean radius from 1.095 Å to 1.141 Å forx= 0.1 and 0.5, respectively. From the magnetization measurements we find that these series of compositions exhibit canted antiferromagnetic (AFM) ordering with Néel temperature,TN1that increases from 151.8 K (x= 0.1) to 162 K (x= 0.5) which also manifests as a significant reduction in the magneto-crystalline anisotropy (HKâ¼ 2.58 kOe â 2.07 kOe,K1â¼ 36.47 J m-3â 18.97 J m-3) while maintaining the stable Γ4(Gx,Ay,Fz) AFM configuration. Both Dzyaloshinskii-Moriya interaction method and modified Curie-Weiss law are employed to analyse the inverse paramagnetic susceptibility,χ-1(T>TN1). Further, we have evaluated the symmetric (JS) and antisymmetric exchange (DAS) constants, which show progressively increasing trend (JSâ 10.08 K to 11.18 K andDASâ 1.24 K to 1.73 K) with the incorporation of Ce inside the perovskite lattice. Furthermore, the role of Ce substitution on the low-temperature spin reorientation transition (TSRâ¼ 3.5 K â 16.8 K pertaining to the Γ25phase configuration) and emergence ofΓ2(Fx,Cy,Gz;FxR,CyR)weak-FM phase between 31 K and 45.5 K are discussed in consonance with the phonon spectra.
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We have provided the mesoscopic and microscopic understandings of polarity reversal of the magnetization or negative magnetization (NM) below TCOMP = 93 K in an exotic magnetic material containing three magnetic sublattices, viz., DyFe5Al7 crystallizing in ThMn12 structure, using neutron depolarization and neutron diffraction techniques. A full recovery of the neutron beam polarization at the TCOMP in a neutron depolarization experiment reveals a total compensation of magnetization inside the magnetic domains in the sample. The temperature-dependent neutron diffraction study under zero magnetic field has provided temperature dependencies of antiparallelly coupled Dy (MDy(2a)) and Fe (MFe(8f) and MFe(8j)) sublattice magnetic moments along [100] direction.Thedominance of |MDy(2a)| over total Fe moment, MFetotal = 4*|MFe(8f)| + |MFe(8j)|, below TCOMP leads to the NM in the compound. The magnetization versusmagnetic field curves below the TCOMP indicate the presence of field-induced spin reorientation in the compound. The magnetic field required for spin reorientation (HSR) is maximum at the lowest temperature and it decreases to zero as the temperature is increased to TCOMP. Interestingly, the compound shows a finite exchange-bias (HEB) below the TCOMP only, as evident from the field-cooled hysteresis loops, while at T > TCOMP,HEB is almost zero. The cooling-field (HCOOL) dependent study of HEB shows a slope change at HCOOL ~ HSR indicating a correlation of exchange-bias with spin-reorientation in the compound. This study, apart from revealing microscopic understanding of magnetic behavior of an exotic three magnetic sublattice system, provides a correlation among exchange-bias, magnetic compensation, and spin-reorientation phenomena.
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High resolution and high intensity neutron powder diffraction is used to study the ground state magnetic order and the spin reorientation transition in the orthoferrite DyFeO3. The transition from the high temperaturek= 0 Γ4(GxAyFz) to the low temperature Γ1(AxGyCz) type order of the Fe-sublattice is found atTSR= 73 K and does not show any thermal hysteresis. BelowTN2= 4 K the Dy-sublattice orders in an incommensurate magnetic structure withk= [0, 0, 0.028] while the Fe-sublattice keeps its commensurate Γ1type order. DyFeO3is the first orthoferriteRFeO3to possess an incommensurate magnetic order of the rare earth sublattice under zero field conditions; an important piece of information neglected in the recent discussion of its multiferroic properties.
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We have studied the magnetic structure, spin reorientation behaviour and dielectric properties of polycrystalline HoMn1-xFexO3(0.0 ⩽x⩽ 0.25) compounds using magnetization, neutron diffraction and dielectric measurements. These compounds crystallize predominantly in the hexagonal phase (P63cm) with a small phase fraction of the orthorhombic phase (Pnma) which increases with increase in dopant concentration and a total suppression of the hexagonal phase is observed atx= 0.25. Doping Fe at the Mn site leads to an increase in the spin reorientation temperature (TSR) from 33 K (x= 0) to 55 K (x= 0.1) while theTNremains nearly constant at 72 K. The magnetic structure of the hexagonal phase was found to be Γ4(P63'c'm) belowTNand Γ3(P63'cm') belowTSR. The magnetic ordering temperature of Ho3+ions at 2(a) site appears to coincide with theTSRonly in the case ofx= 0 sample. The Ho ions at 4(b) site are found to magnetically order below 8 K. TheTNof the Ho ions at both 4(b) and 2(a) sites do not appear to be affected by doping at the Mn site. The temperature variation of the Mn and Ho moments follow the Brillioun function dependence albeit with differing values of the molecular field constantλ0andλ1. Short range magnetic order alone was found for the completely orthorhombic sample (x= 0.25). An anomalous suppression of the dielectric constant (ε) atTNis observed in the case of hexagonal samples. Further, a linear correlation between Δε(=ε(T) -ε(0)) and the square of the antiferromagnetic momentM, is observed in these compounds.
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We report the evolution of coupled phonons and exchange Bias (EB) in perovskite-type Nd1-xEuxCrO3(x = 0.0, 0.05, and 0.10) samples by means of temperature-dependent Raman spectroscopy and dc magnetization measurements. We observed a non-monotonic behavior of theEBfield around the temperatureT*, which lies between the antiferromagnetic ordering temperature (TN) and spin-reorientation transition temperature (TSR). The temperature dependence of phonon modes related to antistretching and bending of CrO6octahedra and Nd3+/ Eu3+ion vibration belowTNconfirms the strong spin-phonon coupling. TheT*found from the non-monotonicity of theEBis imprinted with the additional anomaly observed in the low-temperature spin-phonon behavior. The phonon modes and phonon anomaly are also verified using the density functional theory-based calculations.
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Particle-like magnetic textures with nanometric sizes, such as skyrmions, are potentially suitable for designing high-efficiency information bits in future spintronics devices. In general, the Dzyaloshinskii-Moriya interactions and dipolar interactions are the dominant factors for generating nonlinear spin configurations. However, to stabilize the topological skyrmions, an external magnetic field is usually required. In this study, the spontaneous emergence of skyrmions is directly observed, together with the unique successive topological domain evolution during the spin reorientation transition in a neodymium-cobalt (NdCo5 ) rare-earth magnet. On decreasing the temperature, nanometric skyrmion lattices evolve into enclosed in-plane domains (EIPDs) similar to mini bar-magnets with size below 120 nm. The internal magnetization rotates with magnetic anisotropy, demonstrating the ability to manipulate the mini bar-magnets. The nanoscale EIPD lattices remain robust over the wide temperature range of 241-167 K, indicating the possibility of high-density in-plane magnetic information storage. The generation of spontaneous magnetic skyrmions and the successive domain transformation in the traditional NdCo5 rare-earth magnet may prompt application exploration for topological magnetic spin textures with novel physical mechanisms in versatile magnets.