D. H. Wang

Large exchange bias field in the Ni–Mn–Sn Heusler alloys with high content of Mn

The exchange bias properties have been investigated in bulk Mn50Ni40−xSn10+x (x=0, 0.5, and 1) Heusler alloys with high content of Mn, in which the largest exchange bias field is up to 910 Oe for Mn50Ni40Sn10 alloy. In these alloys, the excess Mn atoms would occupy not only the Sn sites but also the Ni sites, and the moments of Mn on Sn or Ni sites are coupled antiferromagnetically to those on the regular Mn sites, respectively. The origin of this considerably large exchange bias field has been discussed.

Magnetostructural phase transition and magnetocaloric effect in off-stoichiometric Mn1.9−xNixGe alloys

Phase transitions and magnetic entropy changes are studied in Mn1.9−xNixGe (x=0.85, 0.855) alloys. In these off-stoichiometric alloys, the crystallographic transition temperature decreases remarkably due to the deficiency of transition-metal atoms, and, consequently, a magnetostructural transition from the antiferromagnetic TiNiSi-type structure to the ferromagnetic Ni2In-type structure is observed. Owing to the abrupt change in magnetization, large magnetic entropy changes are obtained. The effect of transition-metal vacancies on the phase transition temperature is discussed.

Effect of annealing on the martensitic transformation and magnetocaloric effect in Ni44.1Mn44.2Sn11.7 ribbons

The Ni44.1Mn44.2Sn11.7 ribbons were prepared by melt spinning. A single-phase austenite with L21 structure was confirmed in the melt-spun ribbons at room temperature. After the heat treatment, the martensitic transformation temperature increases obviously in the annealed ribbons. This method may be an effective way to tune the characteristic temperatures in the ferromagnetic shape memory alloys. Giant magnetic entropy changes are observed in the annealed ribbons. The peak values at 10kOe are 32.1 and 20.1J∕kgK, for the ribbons annealed at 1123 and 1173K, respectively.

The study of low-field positive and negative magnetic entropy changes in Ni43Mn46−xCuxSn11 alloys

A series of Ni43Mn46−xCuxSn11 (x=1, 2, and 3) alloys was prepared by the arc melting method. The martensitic transition shifts to a higher temperature with increasing Cu concentration. The isothermal magnetization curves around the martensitic transition temperature show a typical metamagnetic behavior. Under a low applied magnetic field of 10kOe, positive values of magnetic entropy change around the martensitic transition temperature are 14.1, 18.0, and 15.8J∕kgK for x=1, 2, and 3, respectively. While in the vicinity of the Curie temperature of the austenitic phase, these negative values are 1.1, 1.0, and 0.9J∕kgK for x=1, 2, and 3, respectively. The origin of the large entropy changes and the potential application for Ni43Mn46−xCuxSn11 alloys as a working substance for magnetic refrigeration are discussed.

Boron’s effect on martensitic transformation and magnetocaloric effect in Ni43Mn46Sn11Bx alloys

The most used method for changing the martensitic transformation temperatures in the ferromagnetic shape memory alloys is tuning the valence election concentration e∕a. In this paper, we report an alternative way, i.e., introducing few interstitial boron atoms in Ni43Mn46Sn11 alloy. The experimental results show that the martensitic transformation temperatures increase with the increasing boron content remarkably and large magnetic entropy changes can be obtained in these alloys. A possible origin of the enhanced martensitic transformation temperatures and large magnetic entropy changes is discussed in this paper.

Converse magnetoelectric effect in ferromagnetic shape memory alloy/piezoelectric laminate

In laminates, the converse magnetoelectric (CME) effect is often achieved by an elastic coupling between magnetostrictive and piezoelectric layers. Here the authors report on an alternative mechanism for obtaining CME. In a transition-metals-based ferromagnetic shape memory alloy/piezoelectric ceramic laminated composite, the stress-induced martensitic transformation is utilized to gain the magnetic changes, which gives rise to a giant CME effect consequently. The strong CME is observed at room temperature over a broad bandwidth, under weak magnetic bias and electric field.

Study of uniaxial magnetism and enhanced magnetostriction in magnetic-annealed polycrystalline CoFe2O4

Magnetic and magnetostrictive properties of magnetic-annealed polycrystalline CoFe2O4 were investigated. The magnetic hysteresis loops showed obvious uniaxiality with an induced easy direction parallel to the annealing field. Magnetic force microscopy study revealed that the domains were fixed by magnetic annealing. The uniaxial behavior was also observed in the magnetostrictive measurement, which showed a significantly enhanced magnetostriction of − 273 PPM when the external field was applied perpendicular to the annealing field direction. A physical mechanism for the effect of magnetic annealing on polycrystalline CoFe2O4 is developed, in which the induced uniaxiality is ascribed to the realignment of easy axes in polycrystals. The uniaxial behavior of magnetism and enhanced magnetostriction could be well explained by this model.

Effect of annealing on the martensitic transformation and magnetoresistance in Ni–Mn–Sn ribbons

Ni44.1Mn44.2Sn11.7 ribbons were prepared by melt spinning. After heat treatment, the martensitic transformation (MT) temperature increases obviously in the annealed ribbons. Large magnetoresistance (MR) was observed in these ribbons. Under the field of 50?kOe, the maximum values of negative MR were 22% for melt-spun ribbons at 240?K and 38% for annealed ribbons at 268?K, respectively. The annealing effect on MT and MR, together with the origin of the large MR, has been discussed in this paper.

The martensitic transformation, magnetocaloric effect, and magnetoresistance in high-Mn content Mn47+xNi43−xSn10 ferromagnetic shape memory alloys

A series of high-Mn content Mn47+xNi43−xSn10 (x=0, 1, 2, 3, 4, and 5) ferromagnetic shape memory alloys were prepared by arc melting method. The martensitic transformation were observed in these alloys, even the content of Mn is higher than 50 at. %. The phase transition temperature of these alloys can be adjusted by tuning the compositions of Ni and Mn. Large positive magnetic entropy change and negative magnetoresistance which originate from the magnetic-field-induced martensitic transformation are obtained in these alloys.

Investigation of the intermediate phase and magnetocaloric properties in high-pressure annealing Ni–Mn–Co–Sn alloy

The Ni–Mn–Co–Sn alloy is prepared by high-pressure annealing method. Besides the enhanced martensitic transformation temperature and the Curie temperature of austenite, an intermediate phase above the martensitic transformation is observed in this alloy. As a result, two successive magnetic entropy changes with the same sign are obtained around room-temperature, corresponding to the martensitic transformation and intermediate phase transition, respectively. The origin of the intermediate phase for high-pressure annealing Ni–Mn–Co–Sn alloy is discussed.