There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Review of the magnetocaloric effect in manganite materials

Magnetocaloric effect: From materials research to refrigeration devices

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models

A good field to develop new materials with half metallicity is the quaternary Heusler alloys. The preferred route is to combine the compounds that have been already grown in Heusler structure. As a typical example, the quaternary LiMgPdSb-type CoFeMnSi have been investigated in detail. For the quaternary LiMgPdSb-type compounds, three nonequivalent structures exist. From the calculated density of state (DOS) results, it can be seen that one superstructure shows half metallicity. The second superstructure has a pseudogap at the Fermi level. The third superstructure shows metallic behavior. Based on the analysis of the measured XRD pattern and magnetization curve, we can deduce that the CoFeMnSi compound is crystallized in the structure where half metallicity occurs.

New quarternary half metallic material CoFeMnSi

Prediction of half-metallic properties for the Heusler alloys Mn2CrZ (Z=Al, Ga, Si, Ge, Sb): A first-principles study

CO sensor based on Au-decorated SnO2 nanobelt

With a high Curie temperature and low entropy change, the magnetic-field-induced martensitic transformation has been obtained in ferromagnetic shape memory alloys MnNiGa by doping a small amount of Co. Due to the ferromagnetic activation effect of Co, a large amount of antiferromagnetically aligned Mn moments are turned into ferromagnetic ordering, which is verified by our electronic structural calculation and experimental observation. Consequently, the magnetization rises up to 70emu∕g and the magnetization difference between two phases increases about ten times, resulting in a considerable dT∕dH of 4K∕T and a well-defined reversed transformation induced by a magnetic field.

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Magnetic-field-induced martensitic transformation in MnNiGa:Co alloys

We present an investigation of the magnetic entropy /spl Delta/S and resistivity /spl rho/, and the relation between them, for La/sub 0.67/Ca/sub 0.33/MnO/sub 3/ film. We found that the maximum /spl Delta/S is approximately 6 J/kg/spl middot/K, and the maximum magnetoresistance (defined as /spl rho/(0)//spl rho/(H)-1) is about 400% under a field /spl mu//sub o/H of 5 T. A careful analysis gives an equation of the form /spl Delta/S=-/spl alpha//spl int//sub 0//sup H/[/spl part/ln(/spl rho/)//spl part/T]/sub H/dH (/spl alpha/=21.74 emu/g), which relates magnetic order to transport behavior of the compounds. This result reveals the predominant role of magnetic polarons on the magnetoresistive property of manganites. It also provides an alternative method to determine magnetic entropy change on the basis of resistive measurements.

Yangxian Li

Papers

New quarternary half metallic material CoFeMnSi

Prediction of half-metallic properties for the Heusler alloys Mn2CrZ (Z=Al, Ga, Si, Ge, Sb): A first-principles study

CO sensor based on Au-decorated SnO2 nanobelt

Magnetic-field-induced martensitic transformation in MnNiGa:Co alloys

Relation between magnetic entropy and resistivity in La/sub 0.67/Ca/sub 0.33/MnO/sub 3/