Xiaoxi Zhu

Half-metallic properties for the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys: A first-principles study

The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 A. This is ...

Effect of site preference of 3d atoms on the electronic structure and half-metallicity of Heusler alloy Mn2YAl

The site preference of 3d atoms Y in Mn2YAl (Y = V, Fe, Co) alloys and its influence on their electronic structures and magnetism have been studied by first-principles calculations. The results prove that elements with more valence electrons than Mn tend to enter the A (0, 0, 0) and C (½, ½, ½) sites and elements with fewer electrons prefer the B (¼, ¼, ¼) site (Wyckoff positions). Meanwhile, it is found that for Mn2VAl and Mn2FeAl, a high spin polarization can be obtained whether the Y atom enters the (A, C) or the B site. In particular, Mn2VAl is half-metallic whether it forms the Cu2MnAl type or the Hg2CuTi type of structure. And a 100% spin polarization can be retained even when a 25% Mn–V antisite disorder occurs. This is quite preferable in practical applications. It is also found that the higher-valent element such as Co at the B (¼, ¼, ¼) site has opposite effects and tends to close the energy gap. Finally, a systemic summarization on the electronic and magnetic properties of Mn2YAl (Y = Ti, V, Cr, Mn, Fe and Co) alloys was made. All of them except for Mn2TiAl are predicted as half-metals. The calculated total spin moment is an integral value and increases from −3µB/f.u. for Mn2TiAl to +2µB/f.u. for Mn2CoAl with increasing number of valence electrons. This agrees with the Slater–Pauling curve quite well. All the Mn2YAl alloys studied here are ferrimagnets.

Ferromagnetism in the Mn-based Heusler alloy Mn2NiSb

A Mn-based Heusler alloy Mn2NiSb has been synthesized. First-principles calculations and experimental measurements were employed to investigate its electronic structure and magnetism. Theoretical calculation indicates a ferromagnetic coupling in Mn2NiSb and a total spin moment of 4.21μB/f.u., which agrees quite well with the saturation moment at 5 K. This ferromagnetic coupling arises from the larger lattice constant and more valence electrons compared with other Mn-based Heusler alloys. The partial spin moments for Mn (A), Mn (B), and Ni are 0.6, 3.30, and 0.3μB, respectively. The Curie temperature of Mn2NiSb is 647 K, indicating a strong exchange interaction between Mn atoms.

DISORDER ENHANCED MAGNETIC MOMENT IN Fe2CrAl RIBBONS

The electronic structure and magnetic properties of Fe2CrAl have been investigated theoretically and experimentally. The theoretical calculations show that there is an energy gap in the minority-spin band in the electronic structure of ordered Fe2CrAl with the L21 structure and that the spin-polarization ratio is as high as 93%. The calculated total spin moment is 0.97 μB/f.u. However, in melt-spun Fe2CrAl ribbons, the B2 type structure, an atomic-site disordered L21 structure, is found and this atomic site disorder gives rise to a magnetic moment of 2.02 μB in the ribbons which is much higher than the theoretical value. The calculations prove that the enhanced magnetic moment is mainly due to the Fe–Cr and Fe–Al types of disorder. In contrast, the Cr–Al type disorder has little influence on the total spin moment and the spin-polarization ratio.

The structural and magnetic properties of Mn2−xFexNiGa Heusler alloys

The effect of Fe substitution on the phase transformation and magnetic properties of Mn2NiGa has been studied. A single bcc phase was obtained in Mn2−xFexNiGa (x=0–0.6). With the substitution of Fe for Mn, the lattice constant decreases gradually. The martensitic transformation can be observed when x=0–0.3. Both the martensitic transformation and austenitic transformation temperatures decrease monotonically with increasing Fe content, which is different from common electron concentration dependence in Ni–Mn–Ga system. The saturation magnetizations Ms of Mn2−xFexNiGa in both austenitic and martensitic phases increase obviously with the doping of Fe, while the variation of TC shows an opposite tendency. Theoretical calculations indicate that both austenitic and martensitic phases are ferrimagnets. The Fe moment varies from positive to negative after the tetragonal distortion, which leads to the decrease of the saturation magnetization.