H. C. Xuan

Low-field inverse magnetocaloric effect in Ni50−xMn39+xSn11 Heusler alloys

The low-field magnetic entropy changes in Ni50−xMn39+xSn11 alloys (x=5, 6, and 7) were investigated. The martensitic transition shifts to lower temperature with the increase of Mn concentration. Under an applied magnetic field of 10kOe, the magnetic entropy changes are 6.8, 10.1, and 10.4J∕kgK, for x=5, 6, and 7, respectively. The large entropy change in Ni50−xMn39+xSn11 can be attributed to the sharp magnetization change associated with the martensitic transition from a ferromagnetic parent phase to a weak-magnetic martensitic phase. The large low-field magnetic entropy change and low cost suggest Ni50−xMn39+xSn11 alloy as the promising magnetic refrigerant.

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.

Electric control of magnetism at room temperature.

In the single-phase multiferroics, the coupling between electric polarization (P) and magnetization (M) would enable the magnetoelectric (ME) effect, namely M induced and modulated by E, and conversely P by H. Especially, the manipulation of magnetization by an electric field at room-temperature is of great importance in technological applications, such as new information storage technology, four-state logic device, magnetoelectric sensors, low-power magnetoelectric device and so on. Furthermore, it can reduce power consumption and realize device miniaturization, which is very useful for the practical applications. In an M-type hexaferrite SrCo2Ti2Fe8O19, large magnetization and electric polarization were observed simultaneously at room-temperature. Moreover, large effect of electric field-controlled magnetization was observed even without magnetic bias field. These results illuminate a promising potential to apply in magnetoelectric devices at room temperature and imply plentiful physics behind them.

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.

The magnetostructural transformation and magnetocaloric effect in Co-doped MnNiGe1.05 alloys

A series of MnNi1−xCoxGe1.05 (x = 0, 0.03, 0.05, 0.07, 0.09, and 0.11) alloys were prepared by the arc-melting method. With increasing content of Co, a first-order magnetostructural transformation between the antiferromagnetic TiNiSi-type phase and the ferromagnetic Ni2In-type phase was observed. A magnetic and crystallographic phase diagram for MnNi1−xCoxGe1.05 alloys was proposed in this paper. Owing to the abrupt and large jump of magnetization around the magnetostructural transformation, MnNi1−xCoxGe1.05 (x = 0.07, 0.09, 0.11) alloys show large and positive magnetic entropy changes at relatively low 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.