B. X. Gu

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 magnetic entropy changes in the Ni45.4Mn41.5In13.1 ferromagnetic shape memory alloy

The inverse magnetocaloric effect associated with the martensitic transition in the Ni45.4Mn41.5In13.1 Heusler alloy is reported. A large positive magnetic entropy change of 8J∕kgK under a low magnetic field of 10kOe is found near the martensitic transition temperature. This change originates from the first-order transition from a low-temperature weak-magnetic martensitic phase to a high-temperature ferromagnetic austenitic phase. The large low-field magnetic entropy change indicates a great potential of Ni–Mn–In alloys as working materials for magnetic refrigeration in a wide temperature range.

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 phase transitions, magnetocaloric effect, and magnetoresistance in Co doped Ni–Mn–Sb ferromagnetic shape memory alloys

The phase transitions, magnetocaloric effect, and magnetoresistance in Ni50−xCoxMn39Sb11 (x=0–11) ferromagnetic shape memory alloys were investigated. The temperatures of martensitic transformation and magnetic transition in austenitic phase depend strongly on the Co concentration, while the magnetic transition temperature in martensitic phase shows small dependence on alloy composition. For 7≤x≤9, the martensitic transformation is accompanied by a sudden change in magnetization. Large positive magnetic entropy changes and negative magnetoresistance near room temperature, which originate from the magnetic-field-induced transformation from the weak-magnetic high-resistance martensitic phase to the ferromagnetic low-resistance parent phase, are observed in these alloys. Our results indicate the potential application of Ni50−xCoxMn39Sb11 alloys in magnetic refrigeration and magnetic sensors.

Geometry dependence of the annealing effect on the magnetic properties of Fe48Co52 nanowire arrays

Highly ordered Fe48Co52 nanowire arrays with various interpore distances and diameters were fabricated by electrochemical deposition. The change of their magnetic properties after annealing has great dependences on the interpore distance and the wire diameter. The model of the prolate ellipsoid chain modified with the magnetostatic interaction is employed to explain this geometry dependence of the annealing effect. After annealing at 550 °C in H2 atmosphere, good hard magnetic properties (at room temperature, coercivity Hc = 3.89 kOe, squareness Mr/Ms = 0.954; at 78 K, Hc = 4.55 kOe, Mr/Ms = 0.954) were obtained in the arrays with a diameter of about 22 nm and an interpore distance of about 50 nm.

Large magnetic entropy changes and magnetoresistance in Ni45Mn42Cr2Sn11 alloy

The magnetic entropy changes and magnetoresistance were studied in Ni45Mn42Cr2Sn11 ferromagnetic shape memory alloy near the martensitic transition temperature. The substitution of Mn by Cr leads to the rapid decrease of the martensitic transition temperature. A large magnetic entropy change, ΔSM, of 15 J/kg K in a magnetic field of 10 kOe and a large negative MR of 45% in a magnetic field up to 50 kOe were observed in this alloy. The temperature and magnetic field induced martensitic transition should be the origin of large ΔSM and MR in Ni45Mn42Cr2Sn11 ferromagnetic shape memory alloy.

Anisotropy and Faraday effect in Co spinel ferrite films

Crystalline Co spinel ferrite films were prepared by rf sputtering on glass substrates without buffer layer and annealed at temperatures from 400 to 600 °C for 2 h in air. (100) texture was obtained in these films. The anisotropy constant, magnetic hysteresis loops, and magneto‐optical Faraday spectra of the films were measured. The texture became poor when the films were annealed below 500 °C, but the perpendicular anisotropy constant Ku, squareness ratio R=(Mr/Ms)⊥/(Mr/Ms)∥ increased and showed peak values of 2.2×105 ergs/cm3 and 0.94, respectively, at an annealing temperature Ta=500 °C. For the film annealed at 600 °C (higher than the Curie temperature, 520 °C, of Co spinel ferrite), the texture recovered, but Ku and R dropped. The Faraday rotation spectra of Co spinel ferrite films showed a broad peak centered around 720 nm. The peak value increased monotonically with increasing annealing temperature. These results may be related to the Co ion‐induced anisotropy and Co ion distribution in tetrahedral ...

Low-temperature magnetic properties of R0.7Pb0.3MnO3 (R=Nd and La) single crystals

Structural and magnetic properties of the Gd(Fe1-xNix)(11.3)Nb-0.7 compounds with x ranging from 0 to 0.175 have been investigated by means of X-ray diffraction and magnetic measurements. All compounds investigated crystallize in the ThMn12-type structure. Lattice parameters and unit-cell volume decrease slightly with increasing x. The substitution of Ni for Fe leads to a clear increase of Curie temperature from 596 K at x = 0.0 to 656 K at x = 0.175. The anisotropies at room temperature of these compounds are of easy axis. The anisotropy held B-a decreases clearly with increasing Ni content. The decrease speed of B-a with Ni content at 5 K is faster than at room temperature