Magnetic Microstructure of Sintered Nd-Fe-B Magnets Made from Casting Strips

The magnetic microstructures of two Dy-AI substituted sintered Nd-Fe-13 magnets with the different nominal compositions of Nd12.2Vy0.6Fe80.4Al0.7B6 (at.%) (composition-A,C-A) and Nd13.7Dy0.6Fe78.8Al0.7B6.2(at.%)(composition-B,C-13) prepared by strip casting technique have been revealed by using a magnetic force microscope. The magnetic properties of sintered C-B magnets are worse than that of C-A sintered magnets. In particular, the value of density products (BH)max for sintered C-A magnets is about 32% higher than that of C-B magnets, which is reflected by their quite different magnetic microstructures. We believe that for the C-B samples, the inappropriate composition and thus the redundant Nd2Fe17(B) phase of the casting strips make its final magnetic microstructures worse than the C-A, and then deteriorates the performance of the C-B magnets.     

Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI_3

Van der Waals magnet VI_3 demonstrates intriguing magnetic properties that render it great for use in various applications.However,its microscopic magnetic structure has not been determined yet.Here,we report neutron diffraction and susceptibility measurements in VI_3 that revealed a ferromagnetic order with the moment direction tilted from the c-axis by ～36° at 4 K.A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed,before the magnetic order completely disappears at T_C=50 K.The refined magnetic moment of ～1.3μ_B at 4 K is much lower than the fully ordered spin moment of 2μ_B/V~(3+),suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin-orbit coupling in VI_3.This results in strong magnetoelastic interactions that make the magnetic properties of VI_3 easily tunable via strain and pressure.     

Magnetic and Crystalline Microstructures of the Sr-La-Co M-type Ferrites by Magnetic Force Microscopy

With proper erosion of sample surface and making a grid of crystal boundaries, the magnetic and crystal1ine microstruct ures of Sr-La-Co M-type ferrites of nominal composition of La0.2 Sr08Fe11.8Co0.2 O19 (at. %) have been investigated by using magnetic force microscopy. By calculating average diameter and thickness, and average alignment degree of the grains of the ferrites, recognizing their domain patterns and calculating the proportion of grains with single domain or multi-domains, the Sr-La-Co M-type ferrites with high magnetic performance have been characterized at microcrystalline and micromagnetic level. In addition, we has interpreted why the grains always present plate-like domains.     

Tailoring the structural and magnetic properties of Cu-doped ZnO by c-axis pressure

The structural and magnetic properties of the Cu-doped ZnO(ZnO:Cu) under c-axis pressure were studied using first-principle calculations. It was found that the ZnO:Cu undergoes a structural transition from Wurtzite to Graphite-like structure at a c-axis pressure of 7–8 GPa. This is accompanied by an apparent loss of ferromagnetic stability, indicating a magnetic transformation from a ferromagnetic state to a paramagnetic-like state. Further studies revealed that the magnetic instability is closely related to the variation in crystalline field originated from the structural transition, which is in association with the overlapping of spin–charge density between the Cu2+ and adjacent O2-.     

Evolution of domain structure in Fe_3GeTe_2

Two-dimensional(2D)magnets provide an ideal platform to explore new physical phenomena in fundamental magnetism and to realize the miniaturization of magnetic devices.The study on its domain structure evolution with thickness is of great significance for better understanding the 2D magnetism.Here,we investigate the magnetization reversal and domain structure evolution in 2D ferromagnet Fe₃GeTe₂(FGT)with a thickness range of 11.2-112 nm.Three types of domain structures and their corresponding hysteresis loops can be obtained.The magnetic domain varies from a circular domain via a dendritic domain to a labyrinthian domain with increasing FGT thickness,which is accompanied by a transition from squared to slanted hysteresis loops with reduced coercive fields.These features can be ascribed to the total energy changes from exchange interaction-dominated to dipolar interaction-dominated with increasing FGT thickness.Our finding not only enriches the fundamental magnetism,but also paves a way towards spintronics based on 2D magnet.     

Maximizing Thermal Entanglement of XY Model Using Site-Dependent Magnetic Fields in Specific Directions

We study the thermal entanglement by means of concurrence in a two-qubit isotropic XY model in the presence of site-dependent external magnetic fields in arbitrary directions. We find that at a given temperature and magnetic field strength, the mirror symmetry of the two fields about the x-y plane is a necessary condition for maximum entanglement. However, if there is no constraint on the field strengths, then the necessary condition for maximum entanglement reduces to the configuration that the two fields are vertical, anti-parallel and with the same strength. We also investigate the anisotropic XY model and find that the above conclusion more or less holds.     

Barrier Li Quantum Dots in Magnetic Fields

The methods for the few-body system are introduced to investigate the states of the barrier Li quantum dots (QDs) in an arbitrary strength of magnetic field. The configuration, which consists of a positive ion located on the z-axis at a distaneed from the two-dimensional QD plane (the x-y plane) and three electrons in the dot plane bound by the positive ion, is called a barrier Li center. The system, which consists of three electrons in the dot plane bound by the ion,is called a barrier Li QD. The dependence of energy of the state of the barrier Li QD on an external magnetic field B and the distance d is obtained. The angular momentum L of the ground states is found to jump not only with the variation orB but also with d.     

X-Ray Magnetic Circular Dichroism Measurement of Fe-Co Alloy Films Prepared by Electrodeposition

The macro- and micro-magnetic properties of Fe-Co alloy films eletrodeposited on CaAs（100） are studied by synchrotron radiation x-ray magnetic circular dichroism （XMCD） in combination with the magneto-optical Kerr effect （MOKE） measurements and magnetic force microscopy （MFM）. The orbital and spin magnetic moments of each element in the Fe-Co alloy are determined by the sum rules of XMCD. Element-specific hysteresis loops （ESHL） are obtained by recording the La MCD signals as a function of applied magnetic field. MOKE results reveal that the amorphous films are magnetically isotropic in the surface plane. The MFM image shows that the dimension of the magnetic domains is about 1-2 #m, which is much larger than that of the grains, indicating that there are intergranular correlations among these grains. Both ESHL and MOKE hysteresis loops indicate the strong ferromagnetic coupling of Fe and Co in the alloy films.     

Exchange interaction and demagnetization process of high-abundance rare-earth magnets sintered using dual alloy method

The 2:14:1-type rare-earth(RE)-Fe-B permanent magnets prepared by the dual alloy method have been found to possess much superior magnetic properties to those prepared by the single alloy method,providing an appealing route to promote the utilization of high-abundance RE elements Ce and La and balance the use of the RE source.However,the relationship between magnetic interactions among different 2:14:1 main phases and superior magnetic properties is still unclear.In this study,we investigated the magnetic interactions and reversal field distribution in these magnets using first-order reversal curve(FORC)images.The FORC images showed that(Nd,Pr)27.8(La,Ce)2.7FebalM1.4B1.0(S-9)and(Nd,Pr)19.5(La,Ce)11.0FebalM1.4B1.0(S-36)have the characteristics of multiple main phases.The reverse magnetic fields corresponding to the soft and hard main phases,as well as the associated exchange coupling,were highly dependent on the La Ce content.The higher the La Ce content,the weaker the exchange coupling and the more asynchronous the demagnetization process.In addition,the FORC images indicated that the magnetization reversal process also varies with La Ce content,where the nucleation and propagation of reversed domains dominant in the S-9 magnet,while the domain propagation in the S-36 magnet is considerably suppressed.Additional micromagnetic simulations also revealed that the coercivity and exchange coupling of multi-main-phase magnets decrease with increasing La Ce content,correlating well with the experimental results.These findings may not only contribute to a better understanding of the complex magnetic interactions between the soft and hard phases and how they affect macroscopic magnetic properties but also help in improving the magnetic performance of the RE-Fe-B magnets with high La Ce content.     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

MFM studies of magnetic domain patterns in bulk barium ferrite (BaFe12O19) single crystals

Magnetic domain patterns in bulk barium ferrite (BaFe₁₂O₁₉; BaM) single crystals on the basal plane and the prism plane were measured and studied by magnetic force microscopy (MFM). The surface domain pattern is in the form of flowers or star on the basal plane and long elongated spikes or stripe domains on the prism plane. The change in domain structure with applied field (H_app) and the thickness (T) dependence on domain width (δ) was observed. The domain width decreased from 32 to 9 μm for the crystals of 800-100 μm thicknesses, respectively.     

Observation of magnetic domain structure in Terfenol-D by scanning electron acoustic microscopy

The magnetic domain structure in oriented Tb_0.3Dy_0.7Fe_1.92 (Terfenol-D) is investigated by scanning electron acoustic microscopy (SEAM) in a wide frequency range from 75 to 530 kHz. Both secondary electron image and electron acoustic image can be obtained in situ simultaneously. By changing the modulation frequencies, the SEAM can be used as an effective nondestructive method to observe not only the surface topography and domain structure but also the subsurface domain structure and defects. The magnetic domain structure is verified by magnetic force microscopy (MFM). Furthermore, magnetic domains can be observed in both linear and nonlinear imaging modes by SEAM. The contributions to the image contrast are related to the signal generation through the piezomagnetic coupling mechanism, magnetostrictive coupling mechanism, and thermal-wave coupling mechanism.     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

A study on the order–disorder phase transformations of rapidly solidified Sm–Co-based permanent magnets

The structural transformation from the metastable, disordered TbCu₇-type SmCo₇ structure to the equilibrium, ordered Th₂Zn₁₇-type Sm₂Co₁₇ structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu₇-type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm₂Co₁₇ structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.     

Magnetic domain structure in small diameter magnetic nanowire arrays

Fe_0.3Co_0.7 alloy nanowire arrays were prepared by ac electrodepositing Fe²⁺ and Co²⁺ into a porous anodic aluminum oxide (PAO) template with diameter about 50 nm. The surface of the samples were polished by 100 nm diamond particle then chemical polishing to give a very smooth surface (below ±10 nm/μm²). The morphology properties were characterized by SEM and AFM. The bulk magnetic properties and domain structure of nanowire arrays were investigated by VSM and MFM respectively. We found that such alloy arrays showed strong perpendicular magnetic anisotropy with easy axis parallel to nanowire arrays. Each nanowire was in single domain structure with several opposite single domains surrounding it. Additionally, we investigated the domain structure with a variable external magnetic field applied parallel to the nanowire arrays. The MFM results showed a good agreement with our magnetic hysteresis loop.     

High-temperature oxidation resistance and magnetic properties of Si-doped Sm2Co17-type magnets at 500 °C

The high-temperature oxidation resistance and magnetic properties of Si-doped Sm₂Co₁₇-type magnets at 500 °C were systematically investigated. The Sm(Co_0.76, Fe_0.1, Cu_0.1, Zr_0.04)₇Si_x (x=0–0.6) magnets were prepared by the conventional powder metallurgical technique. It is found that the addition of silicon in the Sm₂Co₁₇-type magnet can remarkably improve its oxidation resistance. Moreover, a small amount of silicon addition can also increase its high-temperature intrinsic coercivity. A maximum intrinsic coercivity of 6.7 kOe at 500 °C was obtained for the Sm₂Co₁₇-type magnet with Si content x=0.4, whose high-temperature maximum energy product loss was about 2.5 times smaller than pure Sm₂Co₁₇-type magnet after oxidation at 500 °C for 100 h, indicating the enhanced oxidation resistance. Its corresponding Curie temperature and saturation magnetization are about 723.9 °C and 7.4 kG, respectively.     

Spin domain mapping of a CrO2 thin film using spin-polarized current microscopy

We investigate spin domain mapping of a CrO₂ thin film using spin-polarized current microscopy at room temperature, where conductive atomic force microscopy (CAFM) with a CrO₂-coated tip is used. The nanoscale spin domains of the CrO₂ thin film were crosschecked by magnetic force microscopy (MFM). Notably, the CAFM exhibits the spin domains of the CrO₂ thin film with higher resolution than the MFM, which may result from a local point contact between the nanoscale CrO₂-coated tip and surface of the CrO₂ thin film.     

Growth temperature dependent evolutions of microstructural, optical and magnetic properties of Zn0.75Co0.25O films

Zn0.75Co0.25O films are fabricated via reactive electron beam evaporation. The influence of growth temperature on the microstructural, optical and magnetic properties of Zn0.75Co0.25O films is investigated by using x-ray diffraction, selected area electron diffraction, field emission scanning electron microscope, high resolution transmitting electron microscope, photoluminescence （PL）, field dependent and temperature dependent DC magnetization, and x-ray photoelectron spectroscopy （XPS）. It is shown that Zn0.75Co0.25O films grown at low temperatures （250-350℃） are of single-phase wurtzite structure. Films synthesized at 300 or 350℃ reveal room temperature （RT） ferromagnetism （FM）, while su for 250℃ fabricated films is found above 56 K. PL and XPS investigations show favour towards the perspective that the O-vacancy induced spin-split impurity band mechanism is responsible for the formation of RT FM of Zn0.75Co0.25O film, while the superparamagnetism of 250℃ fabricated film is attributed to the small size effect of nanoparticles in Zn0.75Co0.25O film.     

Effect of nonuniform mechanical stresses on the domain structure of iron borate

A magnetooptic method is used to study the effect of nonuniform radial mechanical stresses on the domain structure, magnetic susceptibility, and magnetic hysteresis loops of a FeBO₃ single crystal. When a magnetic field is applied in the basal plane of FeBO₃ along the stress vector, a system of tapered domains appears in the crystal during magnetization. These domains exist in a certain temperature-dependent field range H₀ ≤ H ≤ H _c . The appearance of a system of tapered domains is found to substantially affect the technical magnetization of a stressed crystal. The results obtained are discussed within the thermodynamic theory of a domain structure. A theoretical model used is shown to adequately describe the experimental temperature and field dependences of the ratio \({D \mathord{\left/ {\vphantom {D {\sqrt L }}} \right. \kern-\nulldelimiterspace} {\sqrt L }}\) (where D and L are the average width and length of a tapered domain, respectively). The calculated value of D is approximately 1.3 times smaller than the experimentally observed domain width.