Cr-buffered MgO Research Articles

Structural and magnetic properties of epitaxial thin films of the equiatomic quaternary CoFeMnSi Heusler alloy

We report the structural and magnetic properties of CoFeMnSi equiatomic quaternary Heusler alloy thin films. The epitaxial growth of the single-crystalline films with full $B2$ and partial $L{2}_{1}$ ordering on the Cr-buffered MgO(001) substrate was achieved using the in situ postannealing at temperature (${T}_{\mathrm{a}}$) of $500\ensuremath{-}600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. A saturation magnetization value of about $3.5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (where f.u. represents formula unit) was obtained at room temperature for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is very close to the value of $3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ reported previously for a bulk sample. Ferromagnetic resonance unveiled that negligible extrinsic relaxation due to magnetic softness as well as small intrinsic spin relaxation, i.e., the Gilbert damping constant of 0.005, which is smaller than that of permalloy. The spin magnetic moments for Co, Fe, and Mn atoms for the sample with ${T}_{\mathrm{a}}=600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ deduced from the x-ray magnetic circular dichroism were in agreement with those previously reported in the bulk sample, the effects of Fe-Mn swapping and Mn antisite disorder were discussed.

Room temperature growth of ultrathin ordered MnGa films on a CoGa buffer layer

Tetragonal ultrathin (1–5 nm) ordered MnGa films on a CsCl-type CoGa buffer layer were fabricated by a sputtering method. The (001)-CoGa layer was first deposited on a Cr-buffered MgO substrate and then annealed in-situ at 500 °C. The ultrathin MnGa film deposited on the CoGa buffer layer formed the L10 structure with very small roughness even when grown at room temperature. In addition, the films showed well-squared perpendicular magnetization hysteresis curves even when the film thickness was as little as 1 nm. The obtained results are important for the development of the MnGa-based spin-transfer torque devices for Gbit class magnetic random access memory and high frequency applications.

Large perpendicular magnetic anisotropy at Fe/MgO interface

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

The microstructure, electronic, and magnetic characterization of highly ordered Co2MnSi thin films deposited on MgO substrate

Co2MnSi thin films are usually deposited on Cr-buffered MgO substrate. In this work, Co2MnSi thin films were deposited directly on MgO substrate and annealed at 350 °C to 650 °C to investigate the evolution of microstructure, electronic, and magnetic properties. Due to nearly perfect B2-ordering and very good L21-ordering, the Ms increases to 1029 emu/cc and Hc decreases to 27 Oe, respectively, after annealing at 650 °C. The large residual resistance ratio (RRR = 2.40) and small magnetic damping constant (α = 0.0039) further demonstrates that nearly perfect B2-ordering, good L21 ordering, and nearly perfect film quality have been achieved.

Fabrication of L1-MnAl perpendicularly magnetized thin films for perpendicular magnetic tunnel junctions

Structural and magnetic properties of MnAl thin films with different composition, growth temperature, and post-annealing temperature were investigated. The optimum condition for fabrication of L10-MnAl perpendicularly magnetized thin film deposited on Cr-buffered MgO single crystal substrate was revealed. The results of x ray diffraction indicated that the MnAl films annealed at proper temperature had a (001)-orientation and L10-ordered structure. The L10-ordered films were perpendicularly magnetized and had a large perpendicular anisotropy. In addition, low surface roughness was achieved. For the optimized fabrication condition, the saturation magnetization Ms of 600 emu/cm3 and perpendicular magnetic anisotropy Ku of 1.0 × 107 erg/cm3 was obtained using the Mn48Al52 target at deposition temperature of 200 °C and post-annealing temperature of 450 °C.

Half-metallic Fe2CrSi and non-magnetic Cu2CrAl Heusler alloys for current-perpendicular-to-plane giant magneto-resistance: First principle and experimental study

We present results of our first principle and experimental study on new Heusler alloys candidates for half-metallic electrode and nonmagnetic spacer in current-perpendicular-to-plane giant magneto-resistance devices. The electronic structures of Fe2CrSi and Cu2CrAl are characterized by high density of states at the Fermi level in the majority spin band with a good match in their majority band structures near the Fermi level. The calculated interfacial spin polarization in Fe2CrSi/Cu2CrAl (100) superlattice is ∼80%. We have also epitaxially grown ferromagnetic A2 Fe-Cr-Si and nonmagnetic L21 Cu-Cr-Al (100) films on Cr-buffered MgO(100) substrates.

Fabrication of Magnetic Tunnel Junctions With Co$_{2}$ FeSi Heusler Alloy and MgO Crystalline Barrier

Tunneling magnetoresistance (TMR) characteristics of magnetic tunnel junctions with a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode and an MgO crystalline barrier have been investigated. Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy electrode grown on Cr-buffered MgO(100) substrate starts to have an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> 2 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> structure when annealed above 420degC. In the cases of CoFeB/MgO/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi junctions, a high TMR ratio of 158% has been achieved after annealing at 350degC, which is obtained by coherent tunneling between the electrodes and barrier, not by the half-metallic nature of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy. However, the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy electrode degrades the TMR ratio when compared with an amorphous CoFeB electrode and the bottom Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode makes more degradation of the TMR ratio than the top Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode. The major reason for the low TMR ratio in Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi-based junctions is the broken epitaxial relationship between the bottom Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrodes and the MgO crystalline barrier, which is investigated by a cross-sectional transmission electron micrograph.

Fabrication of Co2MnAl Heusler Alloy Epitaxial Film Using Cr Buffer Layer

Thin films of the full-Heusler compound, Co2MnAl, were grown on Cr-buffered MgO(001) substrates at different growth temperature and post-annealing temperature by magnetron sputtering. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements show that Cr-buffer layer can largely improve the surface morphology and structural quality of Co2MnAl film. The (001)-oriented epitaxial growth and B2 structure were confirmed by XRD measurements for all prepared samples. The smallest surface roughness (∼2 Å) and an identical Ms value to the bulk value were obtained in the sample which was deposited at ambient temperature and post-annealed at 300°C.