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

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

The magnetic refrigeration technique based on the magnetocaloric effect (MCE) has attracted increasing interest because of its high efficiency and environment friendliness. in this article, our recent progress in exploring effective MCE materials is reviewed with emphasis on the MCE in the LaFe13-xSix-based alloys discovered by us. These alloys show large entropy changes over a wide temperature range near room temperature. The effects of magnetic rare-earth doping, interstitial atoms and high pressure on the MCE have been systematically studied. Special issues, such as appropriate approaches to determining the MCE associated with the first-order magnetic transition, the depression of magnetic and thermal hysteresis, and the key factors determining the magnetic exchange in alloys of this kind, are discussed. The applicability of giant MCE materials to magnetic refrigeration near ambient temperature is evaluated. A brief review of other materials with significant MCE is also presented.

Recent Progress in Exploring Magnetocaloric Materials

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

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New materials for micro-scale sensors and actuators An engineering review

A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31 °C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.

/pdf/giant-magnetic-field-induced-strains-in-heusler-alloy-nimnga-4dy0zf6uns.pdf

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

The magnetic and magnetotransport properties of the perovskite La0.67Ca0.33Mn0.9Fe0.1O3 have been investigated, and the spin-glass behavior with a spin freezing temperature of 42 K has been well confirmed for this compound. A metal-to-insulator transition and colossal magnetoresistance have been observed near its spin freezing temperature; besides, the insulator behavior has been found to reappear at lower temperature. The formation of ferromagnetic and antiferromagnetic clusters and the competition between them with the introduction of Fe3+ ions, which do not participate in the double-exchange process, have been suggested to explain the experimental results.

Colossal magnetoresistance of spin-glass perovskite La0.67Ca0.33Mn0.9Fe0.1O3

We have demonstrated a low temperature surface spin-glass layer in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ high-pressed nanocompacts. The related core-shell magnetic structure arises during the compaction, which is evident by the field dependence of the freezing temperature following the de Almeida--Thouless relationship and obvious exchange bias effect. Moreover, we have also found a critical cooling field, above which both the surface spin-glass behavior and the exchange bias effect abruptly disappeared.

Surface spin glass and exchange bias in Fe 3 O 4 nanoparticles compacted under high pressure

We have discovered that the substitution of Ga or Si for Fe in Sm2Fe17Cx helps the formation of high‐carbon rare‐earth iron compounds with 2:17‐type structure. We have succeeded in preparing Sm2Fe15M2Cx (M=Ga, x=0, 1.0, 2.0, and 3.0; M=Si, x=0, 0.5, 1.0, and 1.5) compounds with Th2Zn17‐type structure by arc melting. The carbides are single phase except for Sm2Fe15Ga2C3.0, which contains a few percent of α‐Fe. The Curie temperature TC of Sm2Fe15Si2Cx compounds is found to increase from 550 to 590 K, as x increases from 0 to 1.5. For Sm2Fe15Ga2Cx, TC increases with x from 565 K for x=0 to 635 K for x=2.0, and then decreases with x. Room‐temperature saturation magnetization of these carbides is in excess of 100 emu/g and has a small dependence on carbon content. All compounds of Sm2Fe15M2Cx studied in this work except for Sm2Fe15Si2 exhibit an easy c‐axis anisotropy at room temperature and show an anisotropy field of higher than 90 kOe for x≥1.0. The present work suggests the possibility of producing high‐pe...

A novel hard magnetic material for sintering permanent magnets

Stress-free and two-way thermoelastic shape memory, with 1.2% strain and 6 K temperature hysteresis, has been found in single crystalline Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2 T applied along the measurement direction, or changed to 1.5% expansion by the 1.2 T applied perpendicular to the measurement direction. For achieving a large deformation, the magnetic field exhibits a more evident contribution than an external stress on this material. These characteristics can be attributed to the low level of internal stress and the preferential orientation of the martensitic variants.

/pdf/stress-free-two-way-thermoelastic-shape-memory-and-field-2ewiebfrbq.pdf

Wen-shan Zhan

Papers

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

Colossal magnetoresistance of spin-glass perovskite La0.67Ca0.33Mn0.9Fe0.1O3

Surface spin glass and exchange bias in Fe 3 O 4 nanoparticles compacted under high pressure

A novel hard magnetic material for sintering permanent magnets

Stress-free two-way thermoelastic shape memory and field-enhanced strain in Ni52Mn24Ga24 single crystals