Electronic structures and magnetocrystalline anisotropy energies of ordered Co1-xNix alloys: a first principles study

The electronic structures and magnetocrystalline anisotropy(MA) of ordered hexagonal close-packed(hcp) Co1-xNix alloys are studied using the full-potential linear-augmented-plane-wave(FLAPW) method with generalized gradient approximation(GGA).Great changes of magnetocrystalline anisotropy energy(MAE) are gained with different Ni compositions.Also,in-plane magnetocrystalline anisotropy is obtained for Co 15 Ni in which the Snoek’s limit is exceeded.It is found that the changes of the symmetry of the crystal field on Ni induce small variations in band structures around the Fermi level under different compositions,which plays an important role in modulating the magnetization direction,where the hybridization between Co-3d and Ni-3d orbits is of special importance in deciding the magnetocrystalline anisotropy of itinerant states.The rigid-band model is inapplicable to explain the evolution of magnetocrystalline anisotropy energy with Ni composition,and it is also inadequate to predict the magnetocrystalline anisotropy energy through the anisotropy of the orbital magnetic moment.     

Mechanical and Magnetostrictive Properties of Fe—Doped Ni52Mn24Ga24 Single Crystals

To improve the magnetic and mechanical properties of a Heusler alloy of Ni52Mn24Ga24,iron was doped with some cotents,Single crystals of the pseudoquaternary Heusler alloy of Ni52Mn8Fe15Ga24 have been synthesized for mechanical and magnetostrictive measurements.The magnetostriction loops and stress-strain curves were measured under different coupled magnetic-mechanical loads.The experimental results show that the brittleness of the sample is clearly improved and Yong‘s modulus of 13.7GPa is obtained in the [001] direction due to the part substitution of Fe for Mn.Furthermore,the toughness and Vickers hardness of the sample are also given by use of the indentation technique.     

Annealing effect on magnetic anisotropy in ultrathin（Ga,Mn）As

We investigated the effect of low temperature annealing on magnetic anisotropy in 7-nm ultrathin Ga0.94Mn0.06As devices by measuring the angle-dependent planar Hall resistance(PHR).Obvious hysteresis loops were observed during the magnetization reversal through the clockwise and counterclockwise rotations under low magnetic fields(below 1000 Gs,1 Gs = 10-4 T),which can be explained by competition between Zeeman energy and magnetic anisotropic energy.It is found that the uniaxial anisotropy is dominant in the whole measured ferromagnetic range for both the as-grown ultrathin Ga0.94Mn0.06As and the annealed one.The cubic anisotropy changes more than the uniaxial anisotropy in the measured temperature ranges after annealing.This gives a useful way to tune the magnetic anisotropy of ultrathin(Ga,Mn)As devices.     

Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by firstprinciples calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in NigMn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

Mn-based permanent magnets

Mn-based intermetallic compounds have attracted much attention due to their fascinating structural and physical properties, especially their interesting hard magnetic properties. In this paper, we have summarized the magnetic and structural properties of Mn-based intermetallic compounds(Mn X, where X = Al, Bi, and Ga). Various methods for synthesizing single phases of MnAl, MnBi, and Mnx Ga were developed in our lab. A very high saturation magnetization of 125 emu/g,coercivity of 5 kOe, and maximum energy product(BH)max of 3.1 MG·Oe were achieved at room temperature for the pure τ-Mn–Al magnetic phase without carbon doping and the extrusion process. Low temperature phase(LTP) MnBi with a purity above 95 wt.% can be synthesized. An abnormal temperature coefficient of the coercivity was observed for the LTP MnBi magnet. Its coercivity increased with temperature from 100 K to 540 K, reached a maximum of 2.5 T at about540 K, and then decreased slowly to 1.8 T at 610 K. The positive temperature coefficient of the coercivity is related to the evolution of the structure and magnetocrystalline anisotropy field of the LTP MnBi phase with temperature. The LTP MnBi bonded magnets show maximum energy products(BH)max of 8.9 MG·Oe(70 kJ/m~3) and 5.0 MG·Oe(40 k J/m~3) at room temperature and 400 K, respectively. Ferrimagnetic Mnx Ga phases with L10 structures(x 2.0) and D022 structures(x 2.0) were obtained. All of the above structures can be described by a D0_(22) supercell model in which 2 a-Ga and 2 b-Mn are simultaneously substituted. The tetragonal D0_(22) phases of the Mnx Ga show high coercivities ranging from 7.2 kOe for low Mn content x = 1.8 to 18.2 kOe for high Mn content x = 3 at room temperature. The Mn_(1.2) Ga sample exhibits a room temperature magnetization value of 80 emu/g. The hard magnetic properties of coercivity_iH_c = 3.5 kOe, remanence Mr = 43.6 emu/g, and(BH)max = 2.5 MG·Oe were obtained at room temperature. Based on the above studies, we believe that Mn-based magnetic materials could be promising candidates for rare earth free permanent magnets exhibiting a high Curie temperature, high magnetocrystalline anisotropy, and very high coercivity.     

Quantum fluctuations of the antiferro-antiferromagnetic double-layer

This paper stuides the magnetization and quantum fluctuations of an antiferro-antiferromagnetic (AF-AF) doublelayer at zero temperature.It is found that the exchanges and anisotropy constants affect the quantum fluctuations of spins.If the anisotropy exists,there will be no acoustic energy branch in the system.The anisotropy constant,antiferromagnetic intralayer and interlayer coupling have important roles in a balance of the quantum competition.     

Influence of Tb on easy magnetization direction and magnetostriction of ferromagnetic Laves phase GdFe_2 compounds

The crystal structure,magnetization,and spontaneous magnetostriction of ferromagnetic Laves phase Gd Fe2 compound have been investigated.High resolution synchrotron x-ray diffraction(XRD) analysis shows that Gd Fe2 has a lower cubic symmetry with easy magnetization direction(EMD) along [100] below Curie temperature TC.The replacement of Gd with a small amount of Tb changes the EMD to [111].The Curie temperature decreases while the field dependence of the saturation magnetization(Ms) measured in temperature range 5–300 K varies with increasing Tb concentration.Coercivity Hc increases with increasing Tb concentration and decays exponentially as temperature increases.The anisotropy in Gd Fe2 is so weak that some of the rare-earth substitution plays an important role in determining the easy direction of magnetization in GdFe_2.The calculated magnetostrictive constant λ100 shows a small value of 37×10~(-6).This value agrees well with experimental data 30×10~(-6).Under a relatively small magnetic field,GdFe_2 exhibits a V-shaped positive magnetostriction curve.When the field is further increased,the crystal exhibits a negative magnetostriction curve.This phenomenon has been discussed in term of magnetic domain switching.Furthermore,magnetostriction increases with increasing Tb concentration.Our work leads to a simple and unified mesoscopic explanation for magnetostriction in ferromagnets.It may also provide insight for developing novel functional materials.     

Resonant tunnelling of the magnetization in molecular crystal of [（Mn1-xCrx）12O12 （CH3COO）16（H2O）4]·2CH3COOH·4H2O（x=0,0.03,0.04,0.05）

The quantum tunnelling of magnetization （QTM） in single crystals of the single molecule magnet （Mn1-xCrx）12- Ac （x=0, 0.03, 0.04, 0.05） has been investigated. In comparison with its parent Mnl2-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of Sz due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.     

Resonant tunnelling of the magnetization in molecular crystal of [（Mn1-xCrx）12O12 （CH3COO）16（H2O）4]·2CH3COOH·4H2O（x=0,0.03,0.04,0.05）

The quantum tunnelling of magnetization （QTM） in single crystals of the single molecule magnet （Mn1-xCrx）12- Ac （x=0, 0.03, 0.04, 0.05） has been investigated. In comparison with its parent Mnl2-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of Sz due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.