New progress in the research on MnGa ferromagnetic filmswith giant perpendicular anisotropy |
From: PublishDate:2013-06-15 Hits: |
Magnetic materials with high coercivity (Hc), perpendicular magnetic anisotropy (PMA) and magnetic energy product have great application potential in ultrahigh-density perpendicular magnetic recording media (~10Tb/inch2), high-performance permanent magnets and ferromagnetic electrodes of spintronic devices with high magnetic-noise immunity and thermal stability. Spintronic devices with high perpendicular magnetic anisotropy include spin valves and magnetic tunneling junctions for nonvolatile, high-speed, and low-power-consuming magnetic random access memories, magnetic sensors and HDD reader with both high space-resolution and sensitivity, high-power oscillators, spin-LED and spin-FET. L10 ordered MnGa ferromagnetic alloys (Fig 1(a)) are theoretically expected to have amazing properties including giant perpendicular anisotropy, robust magnetization, large energy product, high spin polarization and ultralow magnetic damping constant. These outstanding properties make this kind of materials promising for ultrahigh density magnetic recording, permanent magnets and spintronics applications. Recently, Prof. Jianhua Zhao’s group, at the Institute of Semiconductor, Chinese Academy of Sciences has made an important progress in L10-Mn1.5Ga. Prof. Zhao and her PhD student Lijun Zhu successfully prepared homogenous L10-Mn1.5Ga single-crystalline films on semiconductor GaAs (001) by molecular-beam epitaxy. These L10-Mn1.5Ga films exhibited the pronounced properties including perpendicular Hc up to 4.3 T, perpendicular magnetocrystalline anisotropy with a maximum of 21.7 Merg/cm3, energy product comparable with ferrite magnets and nearly perfectly square hysteresis loops. These room-temperature magnetic characteristics make these L10-Mn1.5Ga films multifunctional as pronounced and cost-effective alternative for not only perpendicular magnetic recording bits with areal density over 27 Tb/inch2, but a variety of novel devices with high magnetic-noise immunity and thermal stability like spin-torque MRAMs and oscillators pillars below 5 nm in dimension. Moreover, this kind of material is also promising to be developed for economical and environment-friendly permanent magnets and replace the expensive rare-earth magnets widely used today. The degree of chemical ordering, strains and defects are of great importance in understanding the origins and mechanisms of physical properties of alloys. Prof. Zhao’s group performed X-ray diffraction experiments at 4B9A Diffraction station in Beijing Synchronal Radiation Faculty,and calculated the degree of chemical ordering, strains and defects levels of MnGa films,which provide powerful evidence for explaining the origins of ultrahigh coercivity in this kind of materials. In perfect uniaxial magnets, the coercivity is determined by the magnetic anisotropy Ku. However, the coercivities of MnGa films have a quite different dependence on growth temperature from that of Ku, indicating important contributions from structural imperfections. From Figs.1(c)-(f), it can be found that the degree of chemical ordering, and defects level (FWHM) have nearly the same dependence on growth temperature with that of coercivity, and the lattice constants c of these films are much smaller than bulk values, indicating strong strains and tetragonal distortion. Therefore, the conclusion could be made that the ultrahigh coercivity in these MnGa films are from the combination of giant perpendicular anisotropy and structural imperfections including chemical disorder, lattice defects and strains. Fig 1. (a) Unit cell of L10 -MnxGa (1 < x < 1.8) bulks ; (b)Synchrotron radiation X-ray diffraction θ-2θ patterns of L10 -Mn1.5Ga films grown at 100, 150, 200, 250 and 300 oC, respectively;growth temperature dependence of (c) perpendicular magnetic anisotropy Ku, (d) degree of chemical ordering S, (e) full width at the half maximum FWHM of MnGa (002) peaks and (f) lattice constant c. The blue dash line in (f) presents the c value of 3.69 Å for L10 –Mn1.5Ga bulk This work was published in Advanced Materials, website:http://onlinelibrary.wiley.com/doi/10.1002/adma.201200805/full.
Article: Lijun Zhu, Shuaihua Nie, Kangkang Meng, Dong Pan, Jianhua Zhao*, Houzhi Zheng. Multifunctional L10-Mn1.5Ga Films with Ultrahigh Coercivity, Giant Perpendicular Magnetocrystalline Anisotropy and Large Magnetic Energy Product, Advanced Materials, 24 (33), 4547-4551, 2012.
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