MnAl Films Research Articles

Structural Phase Transitions in Al-Rich C-Doped Mnal Thin Films

We study the crystal structure of carbon-doped Al-rich MnAl thin films deposited on Si substrates. The effects of carbon content and vacuum heat treatment parameters are studied. It is shown that the carbon content, in combination to heat treatment, allows to tailor structural phase transitions in the films. The main phases detected are Al2Mn3, pure Mn, and pure C. As carbon content increases, the amount of Al2Mn3 phase decreases and the content of pure crystallized Mn phase increases. In addition, it is shown that as the heat treatment temperature increases – up to 500 °C – the Al2Mn3 phase content increases, whereas a pure C phase appears at lower temperatures.

Optical and Electrical Properties of Mn-doped and Mn-Al co-doped ZnO Thin Films Annealed at Different Temperatures for Photosensor Applications

Optical and Electrical Properties of Mn-doped and Mn-Al co-doped ZnO Thin Films Annealed at Different Temperatures for Photosensor Applications

Highly (001) oriented MnAl thin film fabricated on CoGa buffer layer

5 nm- and 15 nm-thick (001) oriented MnAl films were fabricated on CoGa buffer layers with various thermal treatments. The insertion of the CoGa layer was effective to obtain the square out-of-plane hysteresis loop even in the MnAl thickness of 5 nm. Highly (001) oriented MnAl film was obtained by depositing Mn and Al on CoGa at a substrate temperature of 200°C followed by annealing at 500°C. The perpendicular magnetic anisotropy was estimated to be 7.4±0.2 and 8.5±0.4 Merg/cc for 5 nm- and 15 nm-thick MnAl, respectively. Lower anisotropy in 5 nm-thick MnAl may be due to the interdiffusion between the MnAl and CoGa layers.

Epitaxial L10-MnAl Thin Films With High Perpendicular Magnetic Anisotropy and Small Surface Roughness

L10-ordered MnAl thin films were epitaxially grown by sputtering technique. The substrate and annealing temperature dependences of the structural and magnetic properties of the films were systematically investigated to improve the magnetic properties and surface roughness. A low substrate temperature and subsequent post-annealing are the useful process to obtain MnAl films with both high magnetic anisotropy $K_{u}$ and small surface roughness $R_{a}$ . We have successfully fabricated MnAl thin films deposited on MgO substrates and Cr90Ru10 buffer layers with the very high $K_{u}$ of 13 Merg/cm3, relatively small magnetization of 500 emu/cm3, and small $R_{a}$ of 0.34 nm by optimization of the substrate and post-annealing temperatures. The obtained MnAl films will be greatly useful to realize the high-density spin-transfer-torque magnetic random access memory.

Epitaxial Heusler superlattice Co2MnAl/Fe2MnAl with perpendicular magnetic anisotropy and termination-dependent half-metallicity

Single-crystal Heusler atomic-scale superlattices that have been predicted to exhibit perpendicular magnetic anisotropy and half-metallicity have been successfully grown by molecular beam epitaxy. Superlattices consisting of full-Heusler Co$_2$MnAl and Fe$_2$MnAl with one to three unit cell periodicity were grown on GaAs (001), MgO (001), and Cr (001)/MgO (001). Electron energy loss spectroscopy maps confirmed clearly segregated epitaxial Heusler layers with high cobalt or high iron concentrations for samples grown near room temperature on GaAs (001). Superlattice structures grown with an excess of aluminum had significantly lower thin film shape anisotropy and resulted in an out-of-plane spin reorientation transition at temperatures below 200 K for samples grown on GaAs (001). Synchrotron-based spin resolved photoemission spectroscopy found that the superlattice structure improves the Fermi level spin polarization near the X point in the bulk Brillouin zone. Stoichiometric Co$_2$MnAl terminated superlattice grown on MgO (001) had a spin polarization of 95%, while a pure Co$_2$MnAl film had a spin polarization of only 65%.

Mn induced 1 × 2 reconstruction in the τ-MnAl(0 0 1) surface

We report on first principles total energy calculations to describe the structural, electronic and magnetic properties of MnAl(0 0 1) surfaces. We have concentrated in structural models having 1 × 1 and 1 × 2 periodicities, since recent experiments of the similar MnGa(0 0 1) surface have found 1 × 1 and 1 × 2 reconstructions. Our calculations show the existence of two stable structures for different ranges of chemical potential. A 1 × 1 surface is stable for Al-rich conditions, whereas a Mn-induced 1 × 2 reconstruction appears after increasing the Mn chemical potential up to Mn-rich conditions. It is important to notice that experimentally, Mn rich conditions are important for improved magnetic properties. The Mn layers in both structures have ferromagnetic arrangements, but they are aligned antiferromagnetically with the almost no magnetic Al atoms. Moreover, the on top Mn atoms, which produce the 1 × 2 reconstruction, align antiferromagnetically with the second layer Mn atoms. These findings are similar to those obtained experimentally in MnGa thin films grown by molecular beam epitaxy. Therefore, this method could also be used to grow the proposed MnAl films.

Structural characterization and magnetic properties of L10-MnAl films grown on different underlayers by molecular beam epitaxy

We grow MnAl films on different underlayers by molecular beam epitaxy (MBE), and investigate their structural and magnetic properties. L10-ordered MnAl films were successfully grown both on an MgO(0 0 1) single-crystalline substrate and on an Mn4N(0 0 1) buffer layer formed on MgO(0 0 1) and SrTiO3(0 0 1) substrates. For the MgO substrate, post rapid thermal annealing (RTA) drastically improved the crystalline quality and the degree of L10-ordering, whereas no improvement in the crystallinity was achieved by altering the substrate temperature (TS) during MBE growth. However, high-quality L10-MnAl films were formed on the Mn4N buffer layer by simply varying TS. Structural analysis using X-ray diffraction showed MnAl on an MgO substrate had a cubic structure whereas MnAl on the Mn4N buffer had a tetragonal structure. This difference in crystal structure affected the magnetic properties of the MnAl films. The uniaxial magnetic anisotropy constant (Ku) was drastically improved by inserting an Mn4N buffer layer. We achieved a perpendicular magnetic anisotropy of Ku = 5.0 ± 0.7 Merg/cm3 for MnAl/Mn4N film on MgO and 6.0 ± 0.2 Merg/cm3 on STO. These results suggest that Mn4N has potential as an underlayer for L10-MnAl.

Electrical Transport of Perpendicularly Magnetized L10-MnGa and MnAl Films

Ferromagnetic films of [Formula: see text]-ordered MnGa and MnAl that exhibit giant perpendicular magnetic anisotropy and great controllability in the magnetism and structural disorders show promising applications not only in magnetic recording, permanent magnets and spintronics, but also in controllable studies of disorder-relevant electrical transport phenomena. In this paper, we review the intriguing experimental observations of the orbital two-channel Kondo effect and anomalous Hall effect in [Formula: see text]-ordered MnGa and MnAl thin films with perpendicular magnetic anisotropy. We also give a perspective with regards to the future technological and fundamental applications of these perpendicularly magnetized Mn-based binary alloy films.

Cobalt substituted L10-MnAl thin films with large perpendicular magnetic anisotropy

The Co composition dependences of the structural and magnetic properties of L10-(MnAl)1−xCox alloy films were investigated. The lattice constants of (MnAl)1−xCox films gradually changed with increasing Co content while maintaining the L10-ordered structure below x = 0.08. The saturation magnetization gradually decreased with increasing Co content, and perpendicular magnetic anisotropy was observed below x = 0.08. In addition, Co substitution markedly improved the surface roughness of the films by decreasing the substrate temperature of (MnAl)1−xCox films. We found that both a high magnetic anisotropy and a small surface roughness can be obtained by the substitution of Co atoms into MnAl films.

The fabrication and characterization of MnAl/FeCo composite thin films with enhanced maximum energy products

This paper reported a method to increase the maximum energy products of MnAl thin films through magnetic exchange coupling. MnAl and MnAl/FeCo thin films were deposited through DC magnetron sputtering on Si wafer substrate. The morphology of the thin films was investigated through SEM, TEM and AFM, and the magnetic properties were investigated through VSM. Results show that there is an effective magnetic exchange coupling between MnAl and FeCo thin films when the FeCo layer is no larger than 8nm. The magnetic exchange coupling was demonstrated by smooth magnetic hysteresis loops, enhanced maximum energy products, and positive peaks in the Henkel Plots. The results reported in this paper could be used to further increase the magnetic performance of MnAl thin films.

Enhanced saturation magnetization in perpendicular L1–MnAl films upon low substitution of Mn by 3d transition metals

In order to enhance the saturation magnetization (Ms) of a L10−MnAl alloy, which is a promising candidate for use in rare-earth free permanent magnets, this work assesses perpendicular L10−MnAl films in which Mn is substituted by the 3d transition metals (TMs) Fe, Co, Ni, or Cu. In the films for which the TM is Fe or Ni, the lattice constant, a, of the L10−(Mn, TM)Al phase increases from 0.390 to 0.397 nm with a substitution amount, x, of 5 at. %. Simultaneously, the lattice constant, c, decreases from 0.355 to 0.350 nm, such that the lattice ratio, c/a, changes from 0.91 to 0.88. Upon increasing x to more than 5 at. %, an unknown phase (either Mn or another Mn−Al phase) forms together with the L10−(Mn, TM)Al phase. The Ms of the MnAl film increases from 360 to more than 400 emu/cm3 with an increasing x when substituting Fe or Ni for Mn. In addition, although a MnAl film without substitution shows a Ms of 360 emu/cm3 and a Hc of 4.1 kOe, a MnAl film substituted with Fe at x = 2.5 at. % exhibits a Ms of 400 emu/cm3 and a Hc of 3.8 kOe. These results suggest that the lattice changes induced in the L10−MnAl alloy upon substituting Fe or Ni for Mn, corresponding to an expansion of the interatomic distance between (Mn, TM) atoms, are an effective means of enhancing the Ms value of the alloy.

Engineering the polar magneto-optical Kerr effect in strongly strained L10–MnAl films

We report the engineering of the polar magnetooptical (MO) Kerr effect in perpendicularly magnetized L10–MnAl epitaxial films with remarkably tuned magnetization, strain, and structural disorder by varying substrate temperature (Ts) during molecular-beam epitaxy growth. The Kerr rotation was enhanced by a factor of up to 5 with Ts increasing from 150 to 350 °C as a direct consequence of the improvement of the magnetization. A similar remarkable tuning effect was also observed on the Kerr ellipticity and the magnitude of the complex Kerr angle, while the phase of the complex Kerr angle appears to be independent of the magnetization. The combination of the good semiconductor compatibility, the moderate coercivity of 0.3–8.2 kOe, the tunable polar MO Kerr effect of up to ~0.034°, and giant spin precession frequencies of up to ~180 GHz makes L10–MnAl films a very interesting MO material. Our results give insights into both the microscopic mechanisms of the MO Kerr effect in L10–MnAl alloys and their scientific and technological application potential in the emerging spintronics and ultrafast MO modulators.

Magneto-transport and domain wall scattering in epitaxy L1 MnAl thin film

This work demonstrated two different kinds of magneto-transport behaviors in epitaxial L10 MnAl film as a function of temperature. The magneto-resistance ratio (MR) was negative and exhibited evident enhancement in the resistivity at coercive fields above 175 K. The MR enhancement was attributed to the increase in the magnetic domain walls based on the quantitative correlation between the domain density and the resistivity. Below 175 K, the MR was positive and showed a quadratic dependence on the external magnetic field, which implied that the MR was dominated by Lorentz effects.

<inline-formula> <tex-math notation="LaTeX">${L}{\text {1}}_{\text {0}}$ </tex-math></inline-formula>-Ordered MnAl Thin Films With High Perpendicular Magnetic Anisotropy Using TiN Underlayers on Si Substrates

Ferromagnetic L1 0 -ordered τ-phase MnAl thin films have been shown to exhibit high perpendicular magnetic anisotropy (K u > 107 ergs/cm3), moderate saturation magnetization (M 0 -ordering for perpendicular anisotropy in MnAl thin films has proven challenging and highly sensitive to deposition conditions and underlayers used [2]. Using MgO underlayers, we previously demonstrated MnAl thin films with high perpendicular magnetic anisotropy (PMA) on Si substrates for the first time [3]. However, MgO is an insulator, which presents two major challenges. First, MgO must be RF-sputtered, which results in low deposition rates, re-sputtering, and therefore the potential for MgO contamination in undesirable locations. Second, since it is an insulator, MgO impedes the use of MnAl film stacks as the bottom electrode in a magnetic tunnel junction (MTJ) for spintronic applications.

Spin gapless semiconductor like Ti2MnAl film as a new candidate for spintronics application

A novel Heusler ferrimagnet Ti2MnAl film has been grown on Si(001) substrate using magnetron sputtering. Characteristics of its magnetic and transport properties reveal the spin‐gapless‐semiconductor (SGS) nature of the stoichiometric Ti2MnAl, in agreement with theoretical prediction. The as‐grown SGS‐like Ti2MnAl film demonstrated high Curie temperature, nearly compensated ferrimagnetic properties with small coercivity and low magnetization. It also showed semiconductor‐like behavior at room temperature allowing good compatibility with commercial Si‐based semiconductor. In this regards, Ti2MnAl film is a potential candidate material for spintronics application, especially for the minimization of energy consumption of device. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Fabrication of MnAl thin films with perpendicular anisotropy on Si substrates

For the first time, perpendicularly magnetized L10-ordered MnAl thin films were demonstrated using a MgO seed layer on Si substrates, which is critical to making spintronic devices. Fabrication conditions were selected by systematically varying sputtering parameters (film thickness, DC sputtering power, in situ substrate temperature, and post-annealing temperature) and investigating structural and magnetic properties. Strong perpendicular magnetic anisotropy with coercivity Hc of 8 kOe, Ku of over 6.5 × 106 erg/cm3, saturation magnetization Ms of 300 emu/cm3, and out-of-plane squareness Mr/Ms of 0.8 were achieved. These MnAl film properties were obtained via DC magnetron sputtering at 530 °C, followed by 350 °C annealing under a 4 kOe magnetic field oriented perpendicular to the film plane.

Modifications of Structure and Magnetic Properties of ${\boldsymbol {L}}{1}_{{0}}$ MnAl and MnGa Films by Kr+ Ion Irradiation

$L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase MnGa and MnAl films with (001) orientation were fabricated by magnetron sputtering. Both films were confirmed to exhibit high perpendicular magnetic anisotropies of 7-8× 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cc and magnetizations of ~450 emu/cc. For the MnGa film, a very flat surface with an average roughness of 0.2 nm was obtained, whereas for the MnAl film, the island growth was confirmed. The X-ray magnetic circular dichroism spectra of these films revealed a significant difference in spectral shape between the perpendicular and inplane magnetized cases. The multiplet peaks appeared in the spectra taken by applying the field normal to the film plane, while such peaks were not clearly seen in the inplane magnetized spectra, possibly owing to the large magnetic anisotropy observed in MnGa and MnAl films. On the other hand, we confirmed a small <;L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> >/2<;S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> >(0.04) and no systematic change of <;L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> >/2<;S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> > between the perpendicular and inplane magnetized cases. Finally, 30 keV Kr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion irradiation into the films was carried out to modify the magnetic properties of MnGa and MnAl films. In both films, the ion irradiation with a quite low dose of 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was confirmed to change the magnetism from ferromagnetic to paramagnetic because of the phase change from L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordered phase to disordered phase. With the significant variation of the magnetism by a low dose of ion irradiation, MnGa and MnAl are considered to be candidates for planar bit-patterned media fabricated by ion irradiation.

Fabrication of L10-Ordered MnAl Films for Observation of Tunnel Magnetoresistance Effect

We succeeded in fabricating L10-ordered MnAl films with a high perpendicular magnetic anisotropy energy of 107 erg/cm3 and a small average film roughness of 0.4 nm by using a molten Mn–Al sputtering alloyed target and optimizing the substrate temperature. In addition, we investigated the tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) with the prepared L10-ordered MnAl electrode. The TMR effect was observed at RT in an MTJ with a very thin Co50Fe50 layer inserted into the MnAl electrode and MgO tunneling barrier interface. This is the first observation of the TMR effect in MTJs with an L10-ordered MnAl electrode.

Perpendicularly magnetized τ-MnAl (001) thin films epitaxied on GaAs

Perpendicularly magnetized τ-MnAl films have been epitaxied on GaAs (001) by molecular-beam epitaxy. Crystalline quality and magnetic properties of the samples were strongly dependent on growth temperature. The highest coercivity of 10.7 kOe, saturation magnetization of 361.4 emu/cm3, perpendicular magnetic anisotropy constant of 13.65 Merg/cm3, and magnetic energy product of 4.44 MGOe were achieved. These tunable magnetic properties make MnAl films valuable as excellent and cost-effective alternative for not only high density perpendicular magnetic recording storage and spintronics devices but also permanent magnets.

Recovery of the chemical ordering in L1 MnAl epitaxial thin films irradiated by 2 MeV protons

Epitaxial MnAl films with a high chemical ordering were synthesized and characterized during a series of irradiations by 2 MeV protons (H+). The chemical ordering was first reduced to a minimum at a total fluence (TF) of 1 × 1015 H+/cm2, and consequently was recovered at the final total fluence of 2 × 1015 H+/cm2. We attributed the recovery of chemical ordering to thermal effects and the enhanced diffusion caused by the high energy protons. In addition, the damages by the protons have little effect on the magnetic scattering processing in MnAl characterized by the anomalous Hall effect.