TOPICAL REVIEW — Magnetism, magnetic materials, and interdisciplinary research
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.
It has long been noticed that special lattices contain single-electron flat bands (FB) without any dispersion. Since the kinetic energy of electrons is quenched in the FB, this highly degenerate energy level becomes an ideal platform to achieve strongly correlated electronic states, such as magnetism, superconductivity, and Wigner crystal. Recently, the FB has attracted increasing interest because of the possibility to go beyond the conventional symmetry-breaking phases towards topologically ordered phases, such as lattice versions of fractional quantum Hall states. This article reviews different aspects of FBs in a nutshell. Starting from the standard band theory, we aim to bridge the frontier of FBs with the textbook solidstate physics. Then, based on concrete examples, we show the common origin of FBs in terms of destructive interference, and discuss various many-body phases associated with such a singular band structure. In the end, we demonstrate real FBs in quantum frustrated materials and organometallic frameworks.
Ferromagnetic transition has generally been considered to involve only an ordering of magnetic moment with no change in the host crystal structure or symmetry, as evidenced by a wealth of crystal structure data from conventional X-ray diffractometry (XRD). However, the existence of magnetostriction in all known ferromagnetic systems indicates that the magnetic moment is coupled to the crystal lattice; hence there is a possibility that magnetic ordering may cause a change in crystal structure. With the development of high-resolution synchrotron XRD, more and more magnetic transitions have been found to be accompanied by simultaneous structural changes. In this article, we review our recent progress in understanding the structural change at a ferromagnetic transition, including synchrotron XRD evidence of structural changes at the ferromagnetic transition, a phenomenological theory of crystal structure changes accompanying ferromagnetic transitions, new insight into magnetic morphotropic phase boundaries (MPB) and so on. Two intriguing implications of non-centric symmetry in the ferromagnetic phase and the first-order nature of ferromagnetic transition are also discussed here. In short, this review is intended to give a self-consistent and logical account of structural change occurring simultaneously with a ferromagnetic transition, which may provide new insight for developing highly magneto-responsive materials.
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.