Hongguo Zhang

Crystal structure and magnetism of the MnxGa (1.15 ≤x ≤ 2.0) rare-earth-free permanent magnet system

Room temperature neutron powder diffraction has been used to investigate the chemical structure and magnetic ordering of a series of tetragonal (I4/mmm #139) MnxGa (1.15 ≤ x ≤ 2.0) alloys. Initially (x 1.5 Mn also appears on the 2a site. The manganese atoms on the 4d site carry an almost constant moment of 2.16(6) μB/Mn. The loss of magnetisation seen with increasing Mn content is shown to be the result of large (∼3u2002μB/Mn), antiparallel Mn moments on the 2b, and later 2a sites, and not to a reduction of the Mn moment on the 4d sites.

Effect of phase composition on crystal texture formation in hot deformed nanocrystalline SmCo5 magnets

In the present study, bulk anisotropic nanocrystalline SmCo5 magnets were prepared by hot press and subsequent hot deformation method. Effect of phase composition on texture and magnetic properties are presented, based on which the mechanism of plastic deformation and texture formation during the hot deformation process is discussed. The SmCo5 magnets were prepared by hot deformation, excessive Sm of 2.5 wt% and 10 wt% was added to compensate the weight loss due to Sm evaporation. Our analyses reveal that the phase composition is one of the most important parameters that determine the texture of SmCo5 magnets. It is therefore suggested that the existence of 2:17 phase and its phase transformation undermined the crystal texture formation as well as the magnetic properties of nanocrystalline SmCo5 magnets.

Experimental and first-principles determination of the magnetocrystalline anisotropy in MnxGa

Singular point detection (SPD) for the determination of the anisotropy field (BA) using a conventional magnetometer is demonstrated. We then follow the composition dependence of BA in MnxGa using a combination of SPD measurements complemented by first-principles density functional theory (DFT) calculations. We find excellent quantitative agreement for 1.2≤x≤u20091.8, but observe a marked departure for x≤1.2. We suggest that the deviation from ideal behaviour might be associated with site disorder at low excess Mn.

The Magnetic and Crystal Structure of Mn x Ga (1.15 ≤ x ≤ 1.8) Alloys

Neutron powder diffraction patterns measured above TC have been used to determine the location of the excess Mn in MnxGa (1.15u2009≤u2009xu2009≤u20091.8). This information has then been used to constrain the fits to neutron powder diffraction patterns measured at ambient temperature and so determine unambiguously the Mn moments in this system. We find that Mn randomly occupies the two Ga sites (2a and 2b) in the I4/mmm structure and propose that it is more appropriate to use a simpler structure based on the P4/mmm space group with a reduced unit cell. In this structure the two Ga sites are formally equivalent (they occupy the 1a site while Mn occupies the 1d site). Our experimental observations are supported by DFT calculations. Below TC we find that the Mn(1d) moment is constant at 2.45(3) μB, while Mn on the 1a site carries a slightly larger moment (~3 μB) that is coupled antiparallel to the Mn(1d) moments, leading to the observed drop in magnetisation with increasing Mn content in MnxGa.

Disorder-Induced Enhancement of Magnetic Properties in Ball-Milled Fe 2 CrAl Alloy

First principles calculation for the energies and magnetic moments of Fe₂CrAl with various structures has been carried out. The contradiction between theoretical and experimental magnetic moment of Fe₂CrAl has been illustrated. The L2₁-based structure has the lowest energy and thus should be formed during synthesis, while the native magnetic Cr-clusters are responsible for the extra large magnetic moment. Based on these calculations, a metastable and partially disordered B2_CD structure has been achieved in a ball-milled Fe₂CrAl alloy. It shows a 300 K higher T_C and a 1.5 μ_B higher M_S, respectively, compared with the arc-melted samples with L2₁-based structures.

The irreversible structural change in Mn1.1Fe0.9P0.8Ge0.2: Evidence for a magnetic driver

Neutron powder diffraction measurements complemented by first-principles density functional theory (DFT) calculations have been used to study the irreversible change that accompanies the reversible magnetocaloric transition (MCT) in Mn1.1Fe0.9P0.8Ge0.2. We observe the growth and loss of long-period antiferromagnetism as we pass through the MCT for the first time and the development of significant strain in the cycled material. We attribute both the reversible and irreversible changes to the distance dependence of the Mn-Mn exchange in the Mn-P(Ge) ab–plane layers.

Microstructure and Improved Coercivity of Mn 1.33 Ga Nanoflakes by Surfactant-Assisted Ball Milling

In recent years, MnxGa alloys with tetragonal structure have attracted much attention because of the erratic fluctuation of rare earth supply and especially the tunable magnetic properties from fer-rimagnetic to ferromagnetic by varying Mn content. It is theoretically estimated that MnGa compound in L10 structure owes high anisotropy constant Ku of 26 Mergs/cm3 and magnetization Ms of 845 emu/cm3, and thus can be regarded as a potential candidate for rare-earth-free permanent magnets. At present, surfactant-assisted high-energy ball milling (HEBM) method has been regarded as one of the effective ways to realize high coercivity for magnetic materials with large anisotropy. It was reported that high-aspect-ratio single-crystal or textured polynanocrystalline flakes of Sm-Co-based permanent magnetic materials with a submicron or nanosize thickness and high coercivity can be achieved. In this study, textured rare-earth-free Mn1.33Ga polycrystalline nanoflakes were prepared by surfactant-assisted HEBM in heptane with 15wt. % oleic acid (OA), and the effects of milling time on structure and magnetic properties were also investigated.