Co2MnSi Films Research Articles

Near-interface electronic and magnetic states of insulator/Co2MnSi heterostructures probed by hard x-ray photoemission combined with x-ray total reflection

Depth-dependent electronic and magnetic states of AlOx and MgO capped Co2MnSi thin films were measured by using hard x-ray photoemission spectroscopy (HAXPES) combined with x-ray total reflection (TR). TR-HAXPES revealed that the near-interface electronic and magnetic states of Co2MnSi films differed from those of bulk measured in non-TR condition. The decrease of the Co and Mn magnetizations near the interface along the easy magnetization axis in the bulk region relative to those in the bulk region and the changes in the valence band profiles were experimentally detected by nondestructive HAXPES utilizing TR. The possible origin of the reduction of the Co and Mn magnetizations and the changes in the valence band profile near the AlOx/Co2MnSi interface was due to an enhanced spin-wave excitation originating from the weakened exchange interaction between the local magnetic moments compared to the bulk region of Co2MnSi, which can slightly modify the valence band electronic states, near the interface at a finite temperature. These results suggest that the combination of HAXPES with TR is useful to experimentally detect the electronic and magnetic states of near-interface and buried bulk regions in nondestructive way for insulator/ferromagnet heterojunctions. Published by the American Physical Society 2024

Superconducting fluctuation effect on epitaxially layered films of superconductor NbN and half-metallic Heusler alloy Co2MnSi

Superconducting properties were investigated in epitaxially layered films consisting of superconductor NbN and half-metallic Heusler alloy Co2MnSi (CMS). Temperature dependence of the electrical resistivity ρ(T) was measured by applying perpendicular magnetic fields to the surface of NbN/CMS films. With the increase of the CMS thickness dCMS, the upper critical field μ0Hc2 decreased monotonically, but the superconducting transition temperature Tc had the minimum of 10.1 K at dCMS ≈ 5 nm. The Tc behavior was in qualitative agreement with the theory of the π-coupling. The pair-breaking parameter δ determined by the superconducting fluctuation theory took the maximum at dCMS = 3 ∼ 5 nm, which would be related to the minimum of Tc. The experimental results reveal that the superconductivity of the NbN layer in NbN/CMS films is affected by the interplay between the superconducting NbN layer and the half-metallic CMS layer.

Epitaxy of half-metallic Heusler alloy Co2MnSi on Ge(111) substrate via a graphene interlayer with L21-ordered Co2MnSi

Cubic half-metallic Heusler alloy Co2MnSi (CMS) thin films were epitaxially grown on Ge(111) substrates via a hexagonal graphene interlayer through the combined use of magnetron sputtering for CMS and mechanical exfoliation for graphene. Despite the graphene interlayer being inserted between the CMS film and the Ge(111) substrate, CMS films were still grown epitaxially on Ge(111) substrates with extremely smooth and abrupt interfaces. Furthermore, 111 peaks of CMS were observed by x-ray diffraction measurement, indicating the L21 order for CMS thin films. The saturation magnetization (Ms) of CMS films annealed at 450 °C was 891 emu/cm3 at 10 K, which is correspondence to 87% of the theoretically predicted saturation magnetization value for half-metallic CMS.

Microstructure and phase formation of Heusler thin film compounds

Abstract Heusler compounds are promising materials to be applied effectively as highly spin-polarized medium (or components) in spintronic devices. To prove their chances the microstructure, the phase formation, and the chemical homogeneity of sputtered thin films of Co2MnSi and Co2Cr0.6Fe0.4Al were studied by in-situ TEM observations as well as by analytical characterizations. The observed grain growth and phase formation in dependence on temperature agrees well with the results of measurements of the film resistivity. The chemical homogeneity varies and depends on the deposition process during thin film preparation, especially the incorporation of oxygen is an unwanted effect.

Controlling the magnetic alignment at the MnGa/Co2MnSi interface: A DFT study

The MnGa/Co2MnSi is a common electrode in Magnetic Tunnel Junctions (MTJ). We study the MnGa/Co2MnSi interfaces using spin-polarized Density Functional Theory. We have focused on the interface stability and the effect of the interface arrangements -at the atomic scale- on its electronic and magnetic properties. Depending on how the atomic species interact at the interface, the MnGa/Co2MnSi electrode can show either antiferromagnetic or ferromagnetic behavior, modulating the spin transfer through the system. Antiferromagnetic ordering appears due to Mn-Mn interlayer interactions across the MnSi-Mn interface and is thermodynamically stable for Co-poor and Mn-rich growth conditions. Mn atoms have magnetic moments of −2.97 μB and 2.31 μB. On the other hand, a ferromagnetic behavior could be engineered under Co-rich and Mn-poor conditions, as demonstrated by the interface formation energy analysis. Such magnetic ordering emerges at the Co-Ga interface since direct Mn-Mn interlayer interactions disappear; Mn atoms have magnetic moments of ∼ 3.00 μB and ∼ 2.8 μB for the Co2MnSi and MnGa films, respectively. Our findings lay the foundations to guide experimental efforts in designing electrodes that could potentially generate efficient tunneling behavior in perpendicular Magnetic Tunnel Junctions, which are the heart of novel MRAMs.

Comparative study of Co2MnSi structural and surface morphological thin films on Si/SiO2

Thin films of Co2MnSi are grown on n-doped Si (100) and SiO2 (100) substrates by RF sputtering. The deposition time to grow the films is varied, once for ten minutes and another for an hour at a particular substrate temperature 600oC and keeping all the other parameters same. The Co2MnSi thin films deposited on Si and SiO2 are crystalline irrespective of the deposition time. The grains were round in the thin films deposited for 10 minutes and these grains are more consistently interconnected in the films deposited for 1 hour. This is supported by the surface roughness data from AFM. The rms roughness is found to be 4.82nm for Si for 10 minutes and 2.50nm for Si for 1 hour deposition that was observed over an area of 3µm2. Copyright © 2017 VBRI Press.

Unveiling transport properties of Co2MnSi Heusler epitaxial thin films with ultra-low magnetic damping

The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 - 10-3 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5 – 300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

Half-metallic nature of the low-temperature grown Co2MnSi films on SrTiO3

We experimentally show that half-metallic Co2MnSi films can be grown on SrTiO3(001) at around 350 °C by molecular beam epitaxy. Despite relatively low-temperature grown Co2MnSi films, L21-ordered structures, high saturation magnetic moments of ∼5.0 μB/f.u., and the negative sign of the anisotropic magnetoresistance (AMR) indicating half-metallic nature are obtained. For the 350°C-grown Co2MnSi films, a low Gilbert damping constant of ∼0.0035, implying the high spin polarization, is evaluated. This study will open a way for developing novel spintronic applications consisting of Co2MnSi and perovskite oxides such as high-performance interfacial multiferroic systems and efficient spin-to-charge conversion systems.

Magnetic and electrical properties of postannealed Co2MnSi Heusler alloy films

The Co2MnSi (CMS) Heusler alloy compound with ordered crystalline structure is attractive for a number of applications due to soft ferromagnetic properties, high Curie temperature (TC ≅ 985 K), high saturation magnetization (Ms ≅ 5.1 µB/f.u), and predicted full spin polarization of carriers. In this work, we report preparation and properties of nanocrystalline CMS films with thickness d = 120 nm magnetron sputtered onto Si(100) substrates followed by annealing in vacuum at Ta = 300 ÷ 500 °C. Nanocrystalline structure of partially ordered cubic B2 phase with grain diameter d = 15 ÷ 30 nm increasing gradually with Ta has been indicated after annealing at Ta ≥ 400 °C. The angle-dependent differential susceptibility measurements revealed an interesting evolution of magnetic properties of the films in a course of postdeposition annealing at Ta=500 °C. Formation of isolated FM particles demonstrating the angle-dependent coercivity Hc(α) following the well-known Stoner–Wohlfarth model has been indicated after 0.5 h annealing. However, the Kondorsky low Hc ~ 1/cosα demonstrating a strong pinning of domain walls has been certified for similar films after 1.5 h annealing. At the same time, long-term (~ 5 h) annealing at 500 °C revealed reduced magnetization of the films due to a decomposition of the material. Nevertheless, electrical transport measurements revealed low field magnetoresistance of these films of about 0.1% and 0.05% at 80 and 295 K, respectively, demonstrating tunneling of spin-polarized carriers between nanocrystalline grains.

Anomalous Nernst effect in Co2MnSi thin films

Separation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect (ANE) becomes non-negligible. Co2MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L21 order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co2MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the ANE is then investigated to inform future measurements of the spin Seebeck. A maximum ANE coefficient of 0.662 µV K−1 is found for an A2 disordered polycrystalline Co2MnSi film. This value is comparable to ordered Heusler thin films deposited onto single crystal substrates but obtained at a far lower fabrication temperature and material cost.

Great Differences between Low-Temperature Grown Co2FeSi and Co2MnSi Films on Single-Crystalline Oxides

Using extremely low synthesis temperatures in molecular beam epitaxy (MBE) conditions on MgO(001), SrTiO3(001), and MgAl2O4(001), we find great differences in the crystallinity and magnetic properties between Co-based full-Heusler-alloy Co2FeSi and Co2MnSi films. For example, while L21-ordered Co2FeSi films can be demonstrated on all the oxides even at less than 80 °C, only the B2-ordered and amorphous Co2MnSi films can be obtained. Considering the experimental results and the findings obtained from theoretical calculations, we can conclude that the stability of the Heusler/oxide interfaces and the alloying energy of the disordered Heusler alloys are two of the most important factors for the low-temperature growth of epitaxial Co2FeSi and Co2MnSi films on these oxides. We suggest that Co2FeSi is more suitable for low-temperature formed spintronic devices than Co2MnSi.

Interface alloying of ultra-thin sputter-deposited Co2MnSi films as a source of perpendicular magnetic anisotropy

Abstract Novel spin-electronic devices require electrodes that inject electrons with both high spin-polarization and perpendicular magnetic anisotropy (PMA). Several full-Heusler compounds are expected to be half-metallic ferromagnets, e.g. chemically-ordered L21 or B2 Co2MnSi. However, most cubic full-Heusler alloys have small magneto-crystalline anisotropy meaning that PMA is difficult to achieve in thin film geometries of devices. Addressing this limitation, Butler et al. (2014) calculate PMA and full spin-polarization in ultra-thin (2–3 nm) chemically-ordered Co2MnSi with an epitaxial coherent interface to rock-salt MgO (0 0 1). Experimentally, PMA in sputter-deposited ultra-thin films with full-Heusler compositions was reported though with adjacent layers of Pd or Pt. We investigate structural origins of such PMA using a test case of ultra-thin Co2MnSi films prepared by magnetron sputter-deposition, adjacent to a MgO layer to represent a tunneling barrier, and to a Pd buffer layer. We measure PMA, with an energy density of 7.8 ± 1.8 Merg/cc at 5 K, when a Pd layer is adjacent to Co2MnSi, following annealing in a narrow temperature range, around 350 °C. This ferromagnetism originates from nanometer scale regions, below 5 nm in size, having a relatively low saturation magnetization, 550 ± 50 emu/cc at low temperatures. Following thermal annealing, significant compositional intermixing between Co2MnSi films and adjacent layers, Co with Pd and Mn with MgO, was measured by electron energy-loss and angle resolved X-ray photoelectron spectroscopies. Aberration-corrected transmission electron microscopy shows that Co2MnSi does not crystallize while at the interface with Pd, nanometer-scale crystallites of FCC solid solution CoPd with {1 1 1} texture are identified. We conclude that these Pd rich CoPd crystallites, characterized by large magnetostriction, are a source for PMA.

Large negative anisotropic magnetoresistance in Co2MnGa Heusler alloy epitaxial thin films

Anisotropic magnetoresistance (AMR) effects in Cox(MnyGa1-y)100-x Heusler alloy thin films epitaxially grown on a MgO single-crystal substrate have been investigated by changing their composition in the ranges of x = 44.4–59.2 at. % and y = 0.44–0.55. Negative AMR ratios were observed at measurement temperatures from 5 to 300 K for all the films, indicating that Co2MnGa films have possible half-metallicity. The AMR ratio changed sensitively depending on the composition of the films. In the case of the film with y = 0.44, the AMR ratio peaked at around x = 50 at. %. In addition, the amplitude of the AMR ratio increased with decreasing y when x was approximately 50 at. %. As a notable result, large negative AMR ratios of −2.32% at 5 K and −0.82% at 300 K, whose amplitude is more than four times that of Co2MnSi films, were obtained for the Co−Mn−Ga films. These results suggest that the Co2MnGa films have a high spin polarization due to half-metallicity.

Fourfold symmetric anisotropic magnetoresistance in half-metallic Co2MnSi Heusler alloy thin films

In this study, we systematically investigated the anisotropic magnetoresistance (AMR) effect in half-metallic Co2MnSi Heusler alloy films epitaxially grown by molecular beam epitaxy. The fourfold symmetric AMR was observed in the temperature range of 25–275 K. In addition, the films exhibited a marked change in twofold symmetric AMR below 100 K. This specific temperature dependence of the AMR effect in Co2MnSi films can be caused by the tetragonal crystal field because of the distortion of the lattice at low temperatures. The influence of tetragonal distortion on both the AMR effect and half-metallicity is also discussed by first-principles calculations.

Spin wave propagation detected over [formula omitted] in half-metallic Heusler alloy Co2MnSi

The field of magnon spintronics offers a charge current free way of information transportation by using spin waves (SWs). Compared to forward volume spin waves for example, Damon-Eshbach (DE) SWs need a relatively weak external magnetic field which is suitable for small spintronic devices. In this work we study DE SWs in Co2MnSi, a half-metallic Heusler alloy with significant potential for magnonics. Thin films have been produced by pulsed laser deposition. Integrated coplanar waveguide (CPW) antennas with different distances between emitter and detection antenna have been prepared on a Co2MnSi film. We used a vector network analyzer to measure spin wave reflection and transmission. We observe spin wave propagation up to 100μm, a new record for half-metallic Heusler thin films.

Transport properties of epitaxial films for superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields

Transport properties were investigated for epitaxial films of superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields up to 17T. The superconducting transition temperature Tc of NbN/Co2MnSi/Au trilayer films was determined to be 16.1K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2MnSi/Au trilayer films. The activation energy U0(H) for vortex motion of the trilayer films in both magnetic fields was well fitted above 5T by the similar model with the exponents in the field dependence of the pinning force density. From the resistivity ρ(T) measurements under high magnetic fields, the upper critical field Hc2(0) at 0K was also deduced to be μ0Hc2∥(0)=23.2T for the parallel magnetic filed and μ0Hc2⊥(0)=15.8T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the half-metallic Co2MnSi layer.

Interface formation of epitaxial MgO/Co2MnSi(001) structures: Elemental segregation and oxygen migration

The interface formation in epitaxial MgO/Co2MnSi(001) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co2MnSi(001) layers using e-beam evaporation: a technique which is expected to oxidize the Co2MnSi layer somewhat due to the rise in oxygen partial pressure during MgO deposition while leaving the deposited MgO oxygen deficient. Not unexpectedly, we find that e-beam evaporation of MgO raises the oxygen background in the deposition chamber to a level that readily oxidizes the Co2MnSi surface, with oxygen bonding preferentially to Mn and Si over Co. Interestingly, this oxidation causes an elemental segregation, with Mn-Si effectively moving toward the surface, resulting in an MgO/Co2MnSi interface with a composition significantly differing from the original surface of the unoxidized Co2MnSi film. As MgO is deposited on the oxidized Co2MnSi, the Mn-oxides are reduced, while the Si oxide remains, and is only somewhat reduced after additional annealing in ultrahigh vacuum. Annealing after the MgO is grown on Co2MnSi causes oxygen to move away from the oxidized Co2MnSi interface toward the surface and into the MgO. This observation is consistent with an increase in the tunneling magnetoresistance ratio with post-growth annealing measured in fabricated magnetic tunnel junctions (MTJs). The findings are discussed in light of fabrication of MgO/Heusler based MTJs, where the exponential decay of tunneling probability with contact separation exemplifies the importance of the ferromagnet/tunnel barrier interface.

MnGa-based fully perpendicular magnetic tunnel junctions with ultrathin Co2MnSi interlayers

Because tetragonal structured MnGa alloy has intrinsic (not interface induced) giant perpendicular magnetic anisotropy (PMA), ultra-low damping constant and high spin polarization, it is predicted to be a kind of suitable magnetic electrode candidate in the perpendicular magnetic tunnel junction (p-MTJ) for high density spin transfer torque magnetic random access memory (STT-MRAM) applications. However, p-MTJs with both bottom and top MnGa electrodes have not been achieved yet, since high quality perpendicular magnetic MnGa films can hardly be obtained on the MgO barrier due to large lattice mismatch and surface energy difference between them. Here, a MnGa-based fully p-MTJ with the structure of MnGa/Co2MnSi/MgO/Co2MnSi/MnGa is investigated. As a result, the multilayer is with high crystalline quality, and both the top and bottom MnGa electrodes show well PMA. Meanwhile, a distinct tunneling magnetoresistance (TMR) ratio of 65% at 10 K is achieved. Ultrathin Co2MnSi films are used to optimize the interface quality between MnGa and MgO barrier. A strong antiferromagnetic coupling in MnGa/Co2MnSi bilayer is confirmed with the interfacial exchange coupling constant of −5erg/cm2. This work proposes a novel p-MTJ structure for the future STT-MRAM progress.

Broad-Band FMR Linewidth of Co2MnSi Thin Films with Low Damping Factor: The Role of Two-Magnon Scattering**Supported by the National Basic Research Program of China under Grant No 2015CB921502, the National Natural Science Foundation of China under Grant Nos 11474184 and 11174183, the Program for New Century Excellent Talents of China under Grant No NCET-10-0541, the Scientific Research Foundation for Returned

The low Gilbert damping factor, which is usually measured by ferromagnetic resonance, is crucial in spintronic applications. Two-magnon scattering occurs when the orthogonality of the ferromagnetic resonance mode and other degenerate spin wave modes was broken by magnetic anisotropy, voids, second phase, surface defects, etc., which is important in analysis of ferromagnetic resonance linewidth. Direct fitting to linewidth with Gilbert damping is advisable only when the measured linewidth is a linear function of measuring frequency in a broad band measurement. We observe the nonlinear ferromagnetic resonance linewidth of Co2MnSi thin films with respect to measuring frequency in broad band measurement. Experimental data could be well fitted with the model including two-magnon scattering with no fixed parameters. The fitting results show that two-magnon scattering results in the nonlinear linewidth behavior, and the Gilbert damping factor is much smaller than reported ones, indicating that our Co2MnSi films are more suitable for the applications of spin transfer torque.

Magnetic and Electrical Properties of Heusler Alloy Co2MnSi Thin Films Grown on Ge(001) Substrates via an Al2O3Tunnel Barrier

Heusler alloy Co2MnSi/Al2O3heterostructures on single-crystal Ge(001) substrates were prepared through magnetron sputtering for both Co2MnSi and Al2O3thin films as a promising candidate for future-generation semiconductor-based spintronic devices. Sufficiently high saturation magnetization 781 emu/cm3was obtained for the Co2MnSi thin film. Furthermore, the current versus voltage (I-V) characteristics showed that the tunneling conduction was dominant in Co2MnSi/Al2O3(2 nm)/Ge(001) heterostructure and theI-Vcharacteristics were slightly dependent on temperature. The conductance versus voltage (dI/dV-V) characteristics indicated that the potential barrier height at the Co2MnSi/Al2O3interface was almost equal to that at then-Ge/Al2O3interface for the prepared Co2MnSi/Al2O3/Ge(001) heterostructure.