Techniques to enhance magnetic permeability in microwave absorbing materials

Recent developments in permanent magnetism are summarized, considering both intrinsic and extrinsic properties. After a general introduction to permanent magnetism, several classes of materials are discussed in the light of future improvements. Emphasis is on magnets rich in Fe, Co, and Mn. The search for new magnetic compounds with improved magnetization, Curie temperature, and anisotropy is accompanied by the need to realize a microstructure that ensures high coercivity. This need refers to both bulk magnets, where hcp Co and tetragonal FeNi are briefly discussed as negative and positive examples, respectively, and to aligned hard–soft nanocomposites. A very recent concept is imaginary magnetic hardness, which reflects easy-plane magnetism and may be exploited in some ferromagnetic compounds. In aligned two-phase nanostructures, soft-in-hard geometries are better than hard-in-soft geometries, and different shapes behave different in the first and second quadrants of the hysteresis loops. Both intrinsically and extrinsically, the most important task is to maximize the hard phase anisotropy while maintaining a high magnetization. Anisotropy field and magnetic hardness can be maximized by choosing a small magnetization, but this strategy is detrimental to the energy product. The last section deals with the behavior of permanent magnets above room temperature, with emphasis on nanoscale effects. Throughout the chapter, current research trends are critically evaluated, and several common misconceptions are dispelled.

Permanent Magnets: History, Current Research, and Outlook

Preparation and properties of nanosize MnZn ferrite from δ-FeOOH

Herein, the spinel Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4 and 0.6) powder samples have been prepared by the solid-state reaction method. We have carried out the measurements of crystal structure, element analysis, material characterization, magnetic property and Curie temperature using the X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer, and the first-principles calculations within the framework of the density functional theory (DFT). The EDS measurement indicates that the Co1-xZnxFe2O4 powder samples have been successfully synthesized and exhibited the cubic spinel structures. Both the lattice constant and crystallite size increase with the Zn concentration due to the larger ionic radius of Zn2+ ion than the Co2+ ion. The concentration ratio of the Co2+ and Co3+ ions can be predicted by the distribution of cations between the A and B sites by the XPS measurement. For the magnetic properties, the residual magnetization, coercivity and Curie temperature decrease monotonically as the Zn concentration increases, while the saturation magnetization initially increases and then decreases at the room temperature. For the Co0.8Zn0.2Fe2O4 sample, the magnetic saturation reaches the maximum value of 62.98 Am2kg-1, due to a large amount of the Co3+ ions. The adequate replacement of Zn ion for the Co site can improve the magnetic properties of spinel Co1-xZnxFe2O4 powders, and effectively regulates the Curie temperature.

Study of Structural and Magnetic Properties of Spinel Zn Doped Cobalt Ferrites

Anisotropy and Crystal Field

Magnetic materials can be divided into two kinds,metal magnetic materials which have ferromagnetic order and non\|metal magnetic materials such as ferrites which have mostly ferrimagnetic order and semiconducting properties.Before the fifties,metal magnetic materials were the most widely used.However,after the 1950's,non\|metal magnetic materials gradually replaced metal magnetic materials except for FeSi alloy which is used in the electric power industry because it has higher saturation magnetization than ferrites.In the nineties,new metal magnetic materials,such as alloys and compounds of 3d\|(4f,4d,5d,5f\:)elements,amorphous magnetic materials and nanostructured magnetic materials surpassed ferrites with their superior characteristics.

Du You

Papers

The development of magnetic materials