Coercivity mechanism in permanent magnets has been debated for many years. In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast. Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes. By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes. However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well. Nucleation and pinning are the two main coercivity mechanisms in permanent magnets. The former signifies the beginning of the magnetic reversal, whilst the latter completes it. Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field (self-pinning), which has the attributes of both traditional nucleation and pinning. Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.
Mn-based intermetallic compounds have attracted much attention due to their fascinating structural and physical properties, especially their interesting hard magnetic properties. In this paper, we have summarized the magnetic and structural properties of Mn-based intermetallic compounds (MnX, where X=Al, Bi, and Ga). Various methods for synthesizing single phases of MnAl, MnBi, and MnxGa were developed in our lab. A very high saturation magnetization of 125 emu/g, coercivity of 5 kOe, and maximum energy product (BH)max of 3.1 MG·Oe were achieved at room temperature for the pure τ-Mn-Al magnetic phase without carbon doping and the extrusion process. Low temperature phase (LTP) MnBi with a purity above 95 wt.% can be synthesized. An abnormal temperature coefficient of the coercivity was observed for the LTP MnBi magnet. Its coercivity increased with temperature from 100 K to 540 K, reached a maximum of 2.5 T at about 540 K, and then decreased slowly to 1.8 T at 610 K. The positive temperature coefficient of the coercivity is related to the evolution of the structure and magnetocrystalline anisotropy field of the LTP MnBi phase with temperature. The LTP MnBi bonded magnets show maximum energy products (BH)max of 8.9 MG·Oe (70 kJ/m3) and 5.0 MG·Oe (40 kJ/m3) at room temperature and 400 K, respectively. Ferrimagnetic MnxGa phases with L10 structures (x < 2.0) and D022 structures (x > 2.0) were obtained. All of the above structures can be described by a D022 supercell model in which 2a-Ga and 2b-Mn are simultaneously substituted. The tetragonal D022 phases of the MnxGa show high coercivities ranging from 7.2 kOe for low Mn content x=1.8 to 18.2 kOe for high Mn content x=3 at room temperature. The Mn1.2Ga sample exhibits a room temperature magnetization value of 80 emu/g. The hard magnetic properties of coercivity iHc=3.5 kOe, remanence Mr=43.6 emu/g, and (BH)max=2.5 MG·Oe were obtained at room temperature. Based on the above studies, we believe that Mn-based magnetic materials could be promising candidates for rare earth free permanent magnets exhibiting a high Curie temperature, high magnetocrystalline anisotropy, and very high coercivity.
Hot deformation is one of the primary methods for fabricating anisotropic rare earth permanent magnets. Firstly, rapidly quenched powder flakes with a nanocrystal structure are condensed into fully dense isotropic precursors using the hot-pressing process. The prepared isotropic precursors are then hot-deformed to produce high-anisotropy uniaxial bulk rare earth permanent magnets and a highly textured structure is produced via this process. The resulting magnets possess many advantages such as near-net-shape, outstanding corrosion resistance, and ultrafine-grain structure. The influence of the preparation parameters utilized in the hot-pressing and deformation processes on the magnetic properties and microstructure of the permanent magnets are systemically summarized in this report. As a near-net-shape technique, the hot deformation process has notable advantages with regard to the production of irregular shapes, especially for radially oriented ring-shaped magnets with high length-diameter ratios or thin walls. The difficulties associated with the fabrication of crack-free, homogeneous, and non-decentered ring-shaped magnets are substantially resolved through an emphasis on mold design, adjustment of deformation parameters, and application of theoretical simulation. Considering the characteristics of hot-deformed magnets which include grain shape and size, anisotropic distribution of intergranular phases, etc., investigation and improvement of the mechanical and electric properties, in addition to thermal stability, with the objective of improving the application of hot-deformed magnets or ring-shaped magnets, is of practical significance.
Experimental and theoretical researches on nanostructured exchange coupled magnets have been carried out since about 1988.Here,we review the structure and magnetic properties of the anisotropic nanocomposite soft/hard multilayer magnets including some new results and phenomena from an experimental point of view.According to the different component of the oriented hard phase in the nanocomposite soft/hard multilayer magnets,three types of magnets will be discussed:1) anisotropic Nd2Fe14B based nanocomposite multilayer magnets,2) anisotropic SmCo5 based nanocomposite multilayer magnets,and 3) anisotropic rare-earth free based nanocomposite multilayer magnets.For each of them,the formation of the oriented hard phase,exchange coupling,coercivity mechanism,and magnetic properties of the corresponding anisotropic nanocomposite multilayer magnets are briefly reviewed,and then the prospect of realization of bulk magnets on new results of anisotropic nanocomposite multilayer magnets will be carried out.
In this paper, we review the magnetic properties and magnetocaloric effects (MCE) of binary R-T (R=Pr, Gd, Tb, Dy, Ho, Er, Tm; T=Ga, Ni, Co, Cu) intermetallic compounds (including RGa series, RNi series, R12Co7 series, R3Co series and RCu2 series), which have been investigated in detail in the past several years. The R-T compounds are studied by means of magnetic measurements, heat capacity measurements, magnetoresistance measurements and neutron powder diffraction measurements. The R-T compounds show complex magnetic transitions and interesting magnetic properties. The types of magnetic transitions are investigated and confirmed in detail by multiple approaches. Especially, most of the R-T compounds undergo more than one magnetic transition, which has significant impact on the magnetocaloric effect of R-T compounds. The MCE of R-T compounds are calculated by different ways and the special shapes of MCE peaks for different compounds are investigated and discussed in detail. To improve the MCE performance of R-T compounds, atoms with large spin (S) and atoms with large total angular momentum (J) are introduced to substitute the related rare earth atoms. With the atom substitution, the maximum of magnetic entropy change (Δ SM), refrigerant temperature width (Twidth) or refrigerant capacity (RC) is enlarged for some R-T compounds. In the low temperature range, binary R-T (R=Pr, Gd, Tb, Dy, Ho, Er, Tm; T=Ga, Ni, Co, Cu) intermetallic compounds (including RGa series, RNi series, R12Co7 series, R3Co series and RCu2 series) show excellent performance of MCE, indicating the potential application for gas liquefaction in the future.
The magnetocaloric effect (MCE) in many rare earth (RE) based intermetallic compounds has been extensively investigated during the last two decades, not only due to their potential applications for magnetic refrigeration but also for better understanding of the fundamental problems of the materials. This paper reviews our recent progress on studying the magnetic properties and MCE in some binary or ternary intermetallic compounds of RE with low boiling point metal(s) (Zn, Mg, and Cd). Some of them exhibit promising MCE properties, which make them attractive for low temperature magnetic refrigeration. Characteristics of the magnetic transition, origin of large MCE, as well as the potential application of these compounds are thoroughly discussed. Additionally, a brief review of the magnetic and magnetocaloric properties in the quaternary rare earth nickel boroncarbides RENi2B2C superconductors is also presented.
The magnetocaloric effect (MCE) of RTSi and RTAl systems with R=Gd-Tm, T=Fe-Cu and Pd, which have been widely investigated in recent years, is reviewed. It is found that these RTX compounds exhibit various crystal structures and magnetic properties, which then result in different MCE. Large MCE has been observed not only in the typical ferromagnetic materials but also in the antiferromagnetic materials. The magnetic properties have been studied in detail to discuss the physical mechanism of large MCE in RTX compounds. Particularly, some RTX compounds such as ErFeSi, HoCuSi, HoCuAl exhibit large reversible MCE under low magnetic field change, which suggests that these compounds could be promising materials for magnetic refrigeration in a low temperature range.
Recent discoveries in the synthesis and applications of magnetic vortex nanorings/nanodiscs in theranostic applications are reviewed. First, the principles of nanomagnetism and magnetic vortex are introduced. Second, methods for producing magnetic vortex nanorings/nanodiscs are presented. Finally, theranostic applications of magnetic vortex nanorings/nanodiscs are addressed.
Recent progress of the preparation and applications of superparamagnetic iron oxide (SPIO) clusters as magnetic resonance imaging (MRI) probes is reviewed with regard to their applications in labeling and tracking cells in vivo, in diagnosis of cardiovascular diseases and tumors, and in drug delivery systems. Magnetic nanoparticles (NPs), especially SPIO nanoparticles, have long been used as MRI contrast agents and as an advantageous nanoplatform for drug delivery, taking advantage of their unique magnetic properties and ability to function at the molecular and cellular levels. Due to advances in nanotechnology, various means to control SPIO NPs' size, composition, magnetization and relaxivity have been developed, as well as ways to usefully modify their surface. Recently, self-assembly of SPIO NP clusters in particulate carriers–such as polymeric micelles, vesicles, liposomes, and layer-by-layer (LbL) capsules–have been widely studied for application as ultrasensitive MRI probes, owing to their remarkably high spin-spin (T2) relaxivity and convenience for further functionalization.
Controllable formation and manipulation of domain walls in one-dimensional (1D) nanostripes underpins a promising type of emergent spintronic device. Magnetic skyrmion is topologically stable whirlpool-like spin texture and is expected to replace familiar domain wall phenomena to build such devices, owing to its prominent features including small size, topological stability and the small critical current required to move it. It is thus essential to understand skyrmions' properties in such a nanostructured element. In this paper, we mainly give fundamental insight into this issue. Experimental achievements in the formation and stability of individual skyrmions in the nanostripe are outlined in detail.
We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a CoFeB/MgO/CoFeB magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.
Progress in surface modification of magnetic nanoparticles (MNPs) is summarized with regard to organic molecules, macromolecules and inorganic materials. Many researchers are now devoted to synthesizing new types of multi-functional MNPs, which show great application potential in both diagnosis and treatment of disease. By employing an ever-greater variety of surface modification techniques, MNPs can satisfy more and more of the demands of medical practice in areas like magnetic resonance imaging (MRI), fluorescent marking, cell targeting, and drug delivery.
The interdependences of preparation conditions, magnetic and crystal structures, and magnetocaloric effects (MCE) of the MnFePGe-based compounds are reviewed. Based upon those findings, a new method for the evaluation of the MCE in these compounds, based on differential scanning calorimetry (DSC), is proposed. The MnFePGe-based compounds are a group of magnetic refrigerants with giant magnetocaloric effect (GMCE), and as such, have drawn tremendous attention, especially due to their many advantages for practical applications. Structural evolution and phase transformation in the compounds as functions of temperature, pressure, and magnetic field are reported. Influences of preparation conditions upon the homogeneity of the compounds' chemical composition and microstructure, both of which play a key role in the MCE and thermal hysteresis of the compounds, are introduced. Lastly, the origin of the “virgin effect” in the MnFePGebased compounds is discussed.
We review the recent progress on the magnetic skyrmions in chiral magnetic materials. The magnetic skyrmion is a topological spin configuration with localized spatial extent, which could be thought of as an emergent rigid particle, owing to its particular topological and chiral properties. Static skyrmionic configurations have been found in various materials with different transport and thermodynamic properties. The magnetic skyrmions respond to externally applied fields in a very unique way, and their coupling to other quasiparticles in solid-state systems gives rise to the emergent electrodynamics. Being not only theoretically important, the magnetic skyrmion is also very promising to be the information carrier in next generation spintronic devices.
