Co2FeAl Films Research Articles

Low magnetic damping constant in half-metallic Co2FeAl Heusler alloy thin films grown by molecular beam epitaxy

Half-metallic Co2FeAl Heusler alloy thin films with various thickness were prepared by MBE method and its magnetic and crystalline properties were investigated. At the thickness of 20 nm and above, good magnetic properties and high B2-order parameter was obtained. In addition, magnetic damping constant was evaluated from peak-to-peak linewidth measured by FMR and ultra-low magnetic damping constant below 0.001 was obtained at the film thickness of 30 nm. These results are thanks to the excellent half-metallicity in the Co2FeAl films prepared by MBE.

The enhanced ultrafast demagnetization at low temperature for MBE-grown Co2FeAl film on GaAs

The ultrafast demagnetization of Co2FeAl thin film grown on n-GaAs by molecular beam epitaxy was investigated with the time-resolved magneto-optical Kerr technique, by varying temperature, pump wavelength and fluence. A faster demagnetization with stronger magnetization quenching was observed at the on-resonant excitation of GaAs band-edge at 10 K. In addition, the pump fluence dependent ultrafast demagnetization at 10 K exhibits different behavior from that of the room temperature case. The enhanced spin angular momentum dissipation induced by the spin pumping effect was suggested to contribute to the ultrafast demagnetization for Co2FeAl thin film epitaxied on GaAs especially at low temperature, in addition to the local spin-flip scattering. Our results provide experimental proof for the ultrafast demagnetization facilitated by the spin pumping effect, which is of great importance for understanding the ultrafast spin angular momentum dissipation in ferromagnetic/semiconductor heterojunctions.

Significant Reorientation Transition of Magnetic Damping Anisotropy in Co2FeAl Heusler Alloy Films at Low Temperatures.

The temperature (T) dependences of magnetization dynamics, especially for magnetic damping anisotropy, have been systematically investigated in well-ordered Co2FeAl films with a biaxial anisotropy. It is found that the damping anisotropy factor Q, defined as the fractional difference of damping between the hard and easy axes, changes from 0.35 to -0.09 as T decreases from 300 to 80 K, performing a distinctive reorientation transition at T ∼ 200 K. Through the thickness-dependent damping measurement results, the damping anisotropy reorientation is verified to originate from the competitions between the intrinsic anisotropic distribution of bulk spin orbit coupling and the interfacial two-magnon scattering. The former governs the effective damping at high temperatures, while the latter with an opposite fourfold symmetry gradually plays a dominant role at reduced temperatures, leading to the transition of the Q value from positive to negative. The clear clarification of damping anisotropy variation as well as the underlying mechanism in this study would be of great importance for designing key spintronic devices with optimized dynamic magnetic properties.

The Gilbert damping of thickness-dependent epitaxial single-crystal Heusler Co2FeAl films at various temperatures

Gilbert damping in epitaxial Heusler Co2FeAl films with thickness varying from 3 nm to 9 nm are investigated by broadband ferromagnetic resonance (FMR) with a temperature range of 5 K–300 K. Gilbert damping shows a continuous decrease with the increasing thickness of Co2FeAl films. Moreover, an enhanced peak of the Gilbert damping is observed with increasing temperature up to approximately 50 K for 3 nm, 6 nm and 9 nm thick Co2FeAl films, which may be attributed to the spin reorientation transition at Co2FeAl/MgO interface. Further, we analyzed the linewidth with a superposition of a uniaxial and a fourfold anisotropy for all samples, which suggests that FMR linewidth in epitaxial Co2FeAl films stems from two-magnon scattering. The change of Gilbert damping is confirmed to originate from various order degrees of the B2 phase with the growth of CFA film, impacting the control of magnetic damping in spin-based nanodevices.

Resonant enhancement of magnetic damping driven by coherent acoustic phonons in thin Co2FeAl film epitaxied on GaAs

The magnetic dynamics of a thin Co2FeAl film epitaxially grown on GaAs substrate was investigated using the time- resolved magneto-optical Kerr measurement under an out-of-plane external field. The intrinsic magnetic damping constant, which should do not vary with the external magnetic field, exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co2FeAl/GaAs heterostructure. The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition, which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co2FeAl. Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect, suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.

Dynamic magnetic response of exchange-biased [formula omitted]FeAl/IrMn nanostructures

We report an experimental investigation of the quasi-static and dynamic magnetic properties of exchange-biased Co2FeAl/IrMn nanostructures. Specifically, we explore the exchange bias effect and the broadband ferromagnetic resonance spectrum of Co2FeAl/IrMn multilayers and verify their dependency with the thickness of the antiferromagnet. We observe an evolution of the dynamic magnetic response when samples are compared, and show it is experimentally tunable by varying the IrMn thickness and the angle between the external and bias fields. Our results thus provide further insights on the effect of antiferromagnetic thickness on the dynamical properties of Co2FeAl films. Additionally, they place exchange-biased Co2FeAl/IrMn nanostructures as interesting systems for applications based on the magnetization dynamics at high frequencies.

Comparison of high temperature growth versus post-deposition in situ annealing in attaining very low Gilbert damping in sputtered Co2FeAl Heusler alloy films

In this work, a detail study on the effectiveness of high substrate temperature (TS) during deposition versus in situ annealing of the films at high temperature (TA) after sputtering at room-temperature is compared with an objective to obtain Heusler alloy thin films of Co2FeAl (CFA) having optimally low αint (with an upper bound of 1.75 × 10-3) and 12–17% smaller MS by employing ion beam sputtering technique and Si(100) as substrates for current induced magnetization switching application. In each of the two series of CFA films, prepared with different choice of TS and TA (lying in 300–773 K range) were explored for optimally tailoring their structural, static and dynamic magnetization properties. Structural study revealed that although all the CFA films were polycrystalline but the post-deposition annealed films were structurally optimal with higher density (6.36 ± 0.09 g/cm3) and lower interface roughness (0.48 ± 0.03 nm) compared to the CFA films grown at high TS (6.23 ± 0.06 g/cm3, 0.61 ± 0.01 nm). In plane field-angle dependent longitudinal magneto optical Kerr effect (L-MOKE) study revealed the existence of a coupling between the weak uniaxial and biaxial magnetic anisotropies present in these films, attributed to the employed deposition geometry. Ferromagnetic resonance (FMR) spectroscopy measurements, performed in both the geometries - in-plane as well as out of plane, revealed a lowest value of 1.19 (±0.08) × 10-3 of αint in the film post-annealed at 773 K. In addition, the observed non-linear relation between the αint and the dynamical ‘g’-factor suggests that the contribution of spin orbit interaction to the damping is far less compared to the damping contribution from the DOS present near the Fermi level. It is remarkable to note that the as-grown CFA films sputtered at room temperature exhibited a record lowest value of 1.73 (±0.09) × 10-3 of αint. Attainment of such small value of Gilbert damping and having moderate magnetization in high spin polarized Co2FeAl Heusler films sputtered on the Si (100) substrate opens up their great application potential in future spintronics devices.

Low damping and small inhomogeneous linewidth broadening in [formula omitted] films

In this work we investigate structural, static and dynamics magnetic properties of Co2FeAl (CFA) films, deposited on Si (100) with Ti buffer and capping layers. We study the evolution of the coercive field, anisotropies, damping constant and linewidth broadening as a function of CFA film thickness in the range 5 nm to 200 nm. We obtain coercive fields weakly dependent on thickness, with values in the range 4.2 Oe to 7.8 Oe. All samples show roughly constant in-plane magnetic anisotropy, and out-plane anisotropy originated from the CFA/Ti interfaces. With increasing thickness we observe the emergence of rotatable anisotropy. We found magnetic damping parameters α between 2.56×10-3 and 3.45×10-3, and inhomogeneous linewidth ΔHo under 10.3 Oe for the whole range of CFA film thickness. These ΔH0 results are small when compared to the values reported for CFA in literature, specially for films with thickness above 100 nm. The present study contributes to a better understanding of the dynamic properties of magnetic CFA films, and open new potential applications in magneto-impedance sensors, RF shielding and others.

Manipulation of Gilbert damping in ultrathin half-metallic Co2FeAl1+x by composition-deficiency-compensation

Gilbert damping is of crucial importance for spintronic devices due to its practical effect on the response time and energy consumption. Lower damping would enable more energy-efficient excitations and, thus, less current is needed. The Gilbert damping constant of the Co2FeAl film, a half-metal material important for spintronics with its 100% spin polarization at the Fermi level, has shown an abnormal increase at the thin film limit due to the inter-diffusion of Al atoms. Here, we report that the Gilbert damping of ultra-thin Co2FeAl1+x films of nanometer thicknesses can be effectively tuned by delicately controlling the stoichiometric ratio during the growth. Gilbert damping has been found to be the lowest of 0.065 in Co2FeAl1+0.1, which is deduced by ∼50% compared to that in Co2FeAl. We have further found that the damping constant of the ultra-thin Co2FeAl1+0.1 film is restored to the value of 0.062 of the nominal stoichiometric Co2FeAl by compensating the Al composition deficiency, as supported by 10.37% of composition change from scanning transmission electron microscopy with energy-dispersive-spectroscopy. This work offers a unique path to manipulate the Gilbert damping constant in ultra-thin Co2FeAl films by Al concentration control.

Magnetic anisotropy and morphology of Co2FeAl films grown on sapphire surface and microstructures made from them

Thin films of Co2FeAl Heusler alloy were fabricated on R- and A-plane Al2O3 substrates with and without Mo seed layer. As the film growth temperature increases, the coercive field and the sharpness of the peaks on the magnetoresistance curve first increase rapidly and then sharply decrease, indicating structural changes in the films. Films grown on A-plane exhibit uniaxial magnetic anisotropy and those grown on R-plane – biaxial one. Patterns of domains being consistent with the magnetic anisotropy found in magnetoresistive experiments were observed using magnetic force microscopy on strips made of the films.

Magnetic anisotropy of half-metallic Co2FeAl ultra-thin films epitaxially grown on GaAs(001)

Single crystalline Co2FeAl films with different thicknesses varying from 3.6 to 10.6 nm have been grown on GaAs (001) using Molecule Beam Epitaxy (MBE). The magnetic characteristics were investigated by in-situ magneto-optical Kerr effect (MOKE). For all the samples, the angle dependent magnetization energy has a relatively high and steep peak around [110] direction which is the hard axis, and a wide basin from [11¯0] to [100] which is the range of the easy axis. More interestingly, the magnetic anisotropy includes a strong uniaxial component due to the Co2FeAl/GaAs interface, a cubic one from Co2FeAl crystalline structure, and an unexpected localized anisotropy term around the [110] direction. All the three anisotropy components overlap their own hard axis around [110] direction resulting in a steep energy barrier, which leads to unusual inverted hysteresis loops around [110]. Our findings add a building block for using half-metallic Co2FeAl thin films in the application of magnetic storage devices.

Field strength- and pump fluence-dependent magnetization dynamics of the Co2FeAl films epitaxially grown on the MgO substrates

The pump fluence- and amplitude of magnetic field-dependent demagnetization and dynamic processes are well studied in the Co2FeAl(CFA) films prepared on the MgO substrates at different temperatures by the time resolved magneto-optical Kerr effect (TR-MOKE). It is found that the demagnetization and the damping processes are strongly dependent on both the amplitude of the external fields and the pump fluences in low fields, resulting in a complex spin relaxation and damping behavior, which is mainly attributed to the non-uniform precession mode induced by the inhomogeneous magnetic anisotropy distribution and the anisotropy field competition. However, with increasing magnetic field gradually, the demagnetization and dynamic processes start to be dominated by the external field, and meanwhile the uniform spin-wave arises. More importantly, the dynamic properties are nearly independent of the amplitude of the magnetic fields and the pump fluences again. The constant damping parameter, which decreases slightly from 4.02 × 10−3 to 1.55 × 10−3 with grown temperature changing from 300 °C to 700 °C, is suggested to be an intrinsic characteristic of the CFA films, and can be attributed to the improvement of the atomic ordering and the crystal quality.

Excitation fluence dependence of spin-wave dynamics and intrinsic Gilbert damping in epitaxial Co2FeAl film

Spin wave dynamics and transient reflectivity measurement in Co2FeAl film are studied under different excitation fluences by the time resolved pump-probe experiment with the external field applied normal to the film. We found that the excitation fluence has only a slight influence on the frequency of the spin wave, but greatly modulates the lifetime and intrinsic Gilbert damping. The lifetime of spin wave decreases but that of coherent acoustic phonon increases with increasing excitation fluence. The enhancement of electron–phonon scattering with increasing excitation fluence is proven to the dominant origin of the enhancement of the intrinsic Gilbert damping.

Structure and magnetic research of the epitaxial Co2FeAl films on the MgO substrates

In this paper, a series of Co2FeAl(CFA) films are prepared on the MgO (001) substrates at different temperatures, and the structural and magnetic properties are well studied. Our results demonstrate that the room temperature-prepared CFA film is an amorphous structure with a very weak magnetic anisotropy and low saturated magnetization. However, when the grown temperature increases to about 300 °C, the CFA films start to be grown epitaxially on the MgO substrates with highly or some L21 ordered structures. Besides, magnetic measurements suggest that both the saturated magnetization and uniaxial anisotropy enhances slightly with increasing grown temperatures, which can be attributed to the improved crystal quality of the film and lattice tensile strain of the substrate, respectively.

Direct observation of high spin polarization in Co2FeAl thin films

We have studied the Co2FeAl thin films with different thicknesses epitaxially grown on GaAs (001) by molecular beam epitaxy. The magnetic properties and spin polarization of the films were investigated by in-situ magneto-optic Kerr effect (MOKE) measurement and spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES) at 300 K, respectively. High spin polarization of 58% (±7%) was observed for the film with thickness of 21 unit cells (uc), for the first time. However, when the thickness decreases to 2.5 uc, the spin polarization falls to 29% (±2%) only. This change is also accompanied by a magnetic transition at 4 uc characterized by the MOKE intensity. Above it, the film’s magnetization reaches the bulk value of 1000 emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.

Thickness-dependent enhancement of damping in Co2FeAl/β -Ta thin films

In the present work Co2FeAl (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300°C. A series of CFA films with different thic ...

The anisotropic effective damping of thickness-dependent epitaxial Co2FeAl films studied by spin rectification

Epitaxial Co2FeAl films with the thickness varying from 26.4 nm to 4.6 nm were grown on MgO(001) substrates by molecular beam epitaxy. Spin rectification was adopted to study the dynamic magnetic properties of the Co2FeAl films, considering the reported advantages of this technique with high thickness-independent sensitivity on samples. At a fixed microwave frequency, the in-plane angular dependent resonance fields and their linewidths exhibit a superposition of a uniaxial and a fourfold anisotropy for all samples. The results reveal an anisotropic damping behavior of the films. Along in-plane different azimuths of the films, frequency-dependent resonance-field linewidths were investigated. The anisotropic effective damping of the films with the thickness varying from 26.4 nm to 4.6 nm was then analyzed, which is contributed from the two-magnon scattering.

Annealing and thickness effects on magnetic properties of Co2FeAl alloy films

Co2FeAl (CFA) films in a wide thickness range between 2 and 100 nm are sputtered at room temperature. Perpendicular magnetic anisotropy (PMA) is achieved in the annealed structure of Pd/CFA/MgO with CFA thickness ranging between 2.3 and 4.9 nm. PMA as high as 2 × 106 erg/cm3 is demonstrated in the structures annealed in the temperature range between 300 and 350 °C. Positive contributions to the PMA made by the interfaces of Pd/CFA and CFA/MgO are identified. For the as-deposited structure of MgO/CFA/Ta with thick CFA alloy up to 5 nm or above a high effective saturation magnetization of 983.9 ± 30.1 emu/cc is derived from the fitting and an in-plane uniaxial magnetic anisotropy of 104 erg/cm3 in magnitude is revealed by angular dependent magnetic measurements. In addition to the increase in saturation magnetization, a fourfold cubic magnetic anisotropy is found to develop with annealing, in line with the improvement of the crystalline structure confirmed by X-ray diffraction measurements. Out results provide some useful information for the design of the CFA-based magnetoelectronic devices.

Increased magnetic damping in ultrathin films of Co2FeAl with perpendicular anisotropy

We estimated the magnetic damping constant α of Co2FeAl (CFA) Heusler alloy films of different thicknesses with an MgO capping layer by means of time-resolved magneto-optical Kerr effect and ferromagnetic resonance measurements. CFA films with thicknesses of 1.2 nm and below exhibited perpendicular magnetic anisotropy arising from the presence of the interface with MgO. While α increased gradually with decreasing CFA film thickness down to 1.2 nm, it was increased substantially when the thickness was reduced further to 1.0 nm. Based on the microstructure analyses and first-principles calculations, we attributed the origin of the large α in the ultrathin CFA film primarily to the Al deficiency in the CFA layer, which caused an increase in the density of states and thereby in the scatterings of their spins.

Perpendicular magnetic anisotropy of TiN buffered Co2FeAl/MgO bilayers

We investigated crystallographic and magnetic properties of TiN buffered Co2FeAl thin films. Room temperature deposition of the Heusler compound led to high crystalline ordering. The layer thickness and post annealing temperature dependence of the coercivity, squareness ratio, and the anisotropy constant of the thin Co2FeAl films were measured. High perpendicular magnetic anisotropy was observed for the 0.9 nm thin Co2FeAl film. A squareness ratio of one was reached at 325 °C post annealing temperature, which is lower compared to previous studies. It remained constant for temperatures up to 500 °C. Surface sensitive x-ray absorption and x-ray magneto circular dichroism measurements were carried out, in order to investigate the element specific magnetic moments. A magnetic moment of 5.4 ± 0.4 μB/f.u. for the 1 nm thin Co2FeAl was determined, which is in good agreement with the previously reported values for Cr or MgO buffered Co2FeAl.