Wenshan Zhan

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

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.

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

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.

A NOVEL HARD MAGNETIC MATERIAL FOR SINTERING PERMANENT-MAGNETS

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...

〈111〉 oriented and twin‐free single crystals of Terfenol‐D grown by Czochralski method with cold crucible

The growth of twin‐free single crystals of Tb0.3Dy0.7Fe2 with 〈111〉 orientation by the Czochralski method with cold crucible is reported. Various characterizations demonstrated the twin‐free and single‐crystalline quality of the crystals. The optimized growth parameters have been experimentally found to avoid constitutional supercooling. Decanting method was used to provide direct evidence that the unstable growth is the origin of the dendrite growth in this material and made the previous growth of 〈111〉 single crystal a failure.

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

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.

Magnetic properties and shape memory of Fe-doped Ni52Mn24Ga24 single crystals

Single crystals of a pseudoquaternary Heusler alloy, Ni52Mn16Fe8Ga24, have now been synthesized. The substitution of Fe for Mn strengthens the magnetic exchange interactions, increasing the Curie temperature to 381 K. The Fe also increases the Ni magnetic moment to 0.41 μB relative to that in the stoichiometric Heusler composition. The strain accompanying the martensitic transformation is 2.4% in zero field and can be enhanced to 4.2% by a field of 1.2 T. A field-induced strain of 1.15% has been induced along [001] in unstressed crystals with magnetic fields of 1.2 T applied at 291 K.

Intermartensitic transformation and magnetic-field-induced strain in Ni52Mn24.5Ga23.5 single crystals

We have found a complete thermoelastic intermartensitic transformation between modulated and unmodulated martensite in single-crystal Ni52Mn24.5Ga23.5. This intermartensitic transformation provides a much larger strain than that of the martensitic transformation. A giant switching-like strain of ±5.0% can be achieved by a small magnetic field of 0.2 T upon the intermartensitic transformation. In the modulated martensite, a large recoverable magnetic-field-induced strain of up to 1.2% has been obtained.

Structure and magnetocrystalline anisotropy of R2Fe17−xGax compounds with higher Ga concentration

The effect of Ga substitution for Fe in R2Fe17 (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Tm) compounds on the structure and magnetocrystalline anisotropy has been studied by means of x‐ray diffraction and magnetization measurements. Both iron sublattice anisotropy and rare earth sublattice anisotropy are found to be modified by the introduction of the gallium atoms. A uniaxial anisotropy is shown in R2Fe17−xGax (for R=Y, Gd, Tb, Dy, Ho, Er, and Tm) compounds with high Ga concentration, whereas a reversal change in the easy magnetization direction is observed in the samples for R=Sm. The contributions to the uniaxial orientation of the magnetization in these compounds result from not only the rare earth sublattice, but also the iron sublattice.

Magnetostrictive and magnetic properties of the pseudobinary compounds PrxTb1-xFe2 and Pr0.15TbxDy0.85-xFe2

The synthesis and magnetic and magnetoelastic properties of PrxTb1−xFe2 and Pr0.15TbxDy0.85−xFe2 are reported. The study on the crystalline structure, magnetization, and Curie temperature shows that PrFe2 has a lattice parameter of 7.463 A and a magnetic moment of 4.93 μB/f.u. With the addition of Pr, the anisotropy of TbFe2 decreases and the optimal magnetostriction with low anisotropy occurs on the Tb rich side for TbxDy1−xFe2. In addition, measurement of oriented samples has been carried out to analyze the magnetostrictive behavior.

Effect of low dc magnetic field on the premartensitic phase transition temperature of ferromagnetic Ni2MnGa single crystals

The temperature and field dependence of alternating-current (ac) susceptibility in a Ni50Mn25Ga25 single crystal was measured using an ac susceptometer with a dc magnetic field oriented in the [100] and [110] directions, respectively. It is found that even at very low fields the premartensitic transition temperature decreases monotonically with the increase of the applied field, and it decreases more rapidly with field applied along the [110] direction than with field along the [100] direction. In accord with a previous model, present results confirm that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnitude of the magnetoelastic interaction in the [110] direction is stronger than that in the [100] direction.