full-Heusler Alloy Thin Films Research Articles

Soft magnetism in single phase Fe3Si thin films deposited on SrTiO3(001) by pulsed laser deposition

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition.

Spin gapless semiconducting behavior in inverse Heusler Mn2-xCo1+xAl (0[formula omitted]x[formula omitted]1.75) thin films

We correlate the structural, electrical, and magnetotransport properties of co-sputtered Mn2-xCo1+xAl full Heusler alloy thin films (0≤x≤1.75) in terms of Co/Mn concentration variation concerning the spin gapless semiconducting (SGS) behavior. The alloy thin films are found to stabilize in B2 order for near stoichiometric films, i.e. (x = 0 and x = 1), with the gradual change in the ordering and lattice parameter through Mn concentration variation. Magnetization measurements in Mn2-xCo1+xAl thin films reveal the ferromagnetic and ferrimagnetic character for x = 1.75, 1.5, 1.25 & 1, and x = 0, 0.5 & 0.75, respectively. The longitudinal resistivity measurement revealed that the films exhibit semiconducting behavior with a change in sign of the temperature coefficient of resistance with temperature. The anomalous Hall conductivity values for the Mn2-xCo1+xAl thin films are extracted from the Anomalous Hall effect (AHE) measurements. The non-saturating positive MR (linear in H) is being reported for the first time in the Mn2CoAl thin films. The value of the AHE coefficient and positive MR together serve as a piece of experimental evidence for the SGS character in the thin film. The SGS behavior becomes predominant at higher Mn concentration. Highly resistive thin films with ferromagnetic (ferrimagnetic) character in Co2MnAl (Mn2CoAl) could be beneficial for semiconductor spintronics, where we need a good resistive element to match up with Silicon base substrate.

Synthesis and transport properties of FET based on Heusler alloy thin films formed by rapid thermal annealing

In this work we show a preparation technique of Co2FeSi full-Heusler alloy thin films on silicon-on-insulator (SOI) substrates, employing rapid thermal annealing (RTA). The films of the Co2FeSi alloy were formed by a silicidation reaction, caused by RTA, between the ultrathin SOI (001) layer and the Fe/Co layers deposited on it. It is assumed that this technology is compatible with the process of formation of a half-metal source-drain in an advanced CMOS and SOI technology and will be applicable for the manufacture of a source-drain of a field-effect transistor. Schottky barrier field-effect transistors (FET) with a back-gate, based on silicon nanowires with source and drain of a Co2FeSi film, synthesized on an SOI substrate, were manufactured. The transport properties of the device were investigated.

Growth dependent magnetization reversal in Co2MnAl full Heusler alloy thin films

Angular dependent magnetization reversal has been investigated in Co2MnAl (CMA) full Heusler alloy thin films grown on Si(100) at different growth temperatures (Ts) by DC-magnetron sputtering. An M-shaped curve is observed in the in-plane angular (0°–360°) dependent coercivity (ADC) by magneto-optical Kerr effect measurements. The dependence of the magnetization reversal on Ts is investigated in detail to bring out the structure-property correlation with regards to ADC in these polycrystalline CMA thin films. This magnetization reversal (M-shaped ADC behavior) is well described by the two-phase model, which is a combination of Kondorsky (domain wall motion) and Stoner Wohlfarth (coherent rotation) models. In this model, magnetization reversal starts with depinning of domain walls, with their gradual displacement explained by the Kondorsky model, and at a higher field (when the domain walls merge), the system follows coherent rotation before reaching its saturation following the Stoner Wohlfarth model. Further, the analysis of angular dependent squareness ratio (Mr/Ms) indicates that our films clearly exhibited twofold uniaxial anisotropy, which is related to self-steering effect arising due to the obliquely incident flux during the film-growth.

Anomalous Hall effect in ion-beam sputtered Co2FeAl full Heusler alloy thin films

Investigations of temperature dependent anomalous Hall effect and longitudinal resistivity in Co2FeAl (CFA) thin films grown on Si(100) at different substrate temperature Ts are reported. The scaling of the anomalous Hall conductivity (AHC) and the associated phenomenological mechanisms (intrinsic and extrinsic) are analyzed vis-à-vis influence of Ts. The intrinsic contribution to AHC is found to be dominating over the extrinsic one. The appearance of a resistivity minimum at low temperature necessitates the inclusion of quantum corrections on account of weak localization and electron-electron scattering effects whose strength reduces with increase in Ts. The study establishes that the optimization of Ts plays an important role in the improvement of atomic ordering which indicates the higher strength of spin-orbit coupling and leads to the dominant intrinsic contribution to AHC in these CFA full Heusler alloy thin films.

Effect of substrate on the structural and magnetic properties of DC sputtered Co2FeSi full Heusler alloy thin films

Co2FeSi thin films were deposited on GaAs (100), MgO (001) and Si (111) substrates using ultra high vacuum DC magnetron sputtering by varying the substrate temperature. Structural and magnetic properties were examined for all the films. The results showed that at higher value of substrate temperature exhibits L21 ordering of Co2FeSi. Re-entrant ferromagnetism and spin glass behavior was observed for Co2FeSi/Si and Co2FeSi/GaAs. From the overall temperature dependent magnetic measurements, the Co2FeSi/MgO exhibit good thermal stability, high saturation magnetization and low coercivity.

ChemInform Abstract: Perpendicular Magnetic Anisotropy in Co‐Based Full Heusler Alloy Thin Films

AbstractReview: 59 refs.

Perpendicular Magnetic Anisotropy in Co-Based Full Heusler Alloy Thin Films

Half-metallic Co-based full Heusler alloys have been qualified as promising functional materials in spintronic devices due to their high spin polarization. The lack of perpendicular magnetic anisotropy (PMA) is one of the biggest obstacles restricting their application in next generation ultrahigh density storage such as magnetic random access memory (MARM). How to induce the PMA in Co-based full Heusler alloy thin films has attracted much research interest of scientists. This paper presents an overview of recent progress in this research area. We hope that this paper would provide some guidance and ideas to develop highly spin-polarized Co-based Heusler alloy thin films with PMA.

Si 上に作製した Co<sub>2</sub>FeAl<sub>0.5</sub>Si<sub>0.5</sub> フルホイスラー合金薄膜の結晶構造とスピン伝導特性

We investigated the crystal structure and spin conduction property of Co2FeAl0.5Si0.5 (CFAS) full-Heusler alloy thin films deposited on Si substrate. The multilayer films with different barrier layer were deposited by RF magnetron sputtering system to compare their spin conduction properties. Spin injection signals were observed only with Si sub.//Mg/MgO/CFAS structure. It is considered that the crystal structure near the interface changed by insertion of Mg layer affected the spin conduction property.

MBEによりSi基板上に成膜されたCo2FeAl0.5Si0.5薄膜の結晶構造と磁気特性

We investigated the structural and magnetic properties of Co2FeAl0.5Si0.5 (CFAS) full-Heusler alloy thin films deposited on Si/SiO2/(001)-oriented MgO strucutre by molecular beam epitaxy. The MgO layer deposited at 300°C grew with an enough (001)-orientation to obtain high spin polarization on Si/SiO2 substrate. The CFAS thin films on Si/SiO2/(001)-oriented MgO structure showed only (220) peaks in XRD measurements. However, those films also showed high magnetization, same as the magnetization of L21-ordered CFAS on MgO substrates. Therefore, it is considered that the CFAS thin films on Si/SiO2/(001)-oriented MgO structure have the high ordered structure.

Structural and Magnetic Properties of Co<SUB>2</SUB>FeAl<SUB>0.5</SUB>Si<SUB>0.5</SUB> Full-Heusler Alloy Thin Films on GaAs Substrates

We investigated the structural and magnetic properties of Co2FeAl0:5Si0:5 (CFAS) thin films on GaAs substrates. The CFAS thin films grew epitaxially on GaAs with CFAS (100)[100] k GaAs (100)[100] relation and formed an ordered L21 structure with substrate heating deposition at TSUB 1⁄4 300 C, 400 C and post annealed at TPA 1⁄4 400 C. Moreover, we confirmed the no reaction between CFAS and GaAs. The CFAS thin films on GaAs showed strong uniaxial magnetic anisotropy with an easy axis of 1⁄2110 CFAS (1⁄2110 GaAs) direction and the magnetic anisotropy generally increased by decreasing the thickness of CFAS. The magnetic moment of the CFAS film deposited with substrate heating at 300 C was approximately 4.8 mB/f.u. at room temperature. [doi:10.2320/matertrans.MBW201004]

Quantitative analysis of atomic disorders in full-Heusler Co2FeSi alloy thin films using x-ray diffraction with Co Kα and Cu Kα sources

The authors developed a new analysis technique for atomic disorder structures in full-Heusler alloys using x-ray diffraction (XRD) with Co Kα and Cu Kα sources. The technique can quantitatively evaluate all the atomic disorders for the exchanges between X, Y, and Z atoms in full-Heusler X2YZ alloys. In particular, the technique can treat the DO3 disorder that cannot be analyzed by ordinary Cu Kα XRD. By applying this technique to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing (RTA), RTA-temperature (TA) dependence of the atomic disorders was revealed. The site occupancies of Co, Fe, and Si atoms on their original sites were 98%, 90%, and 93%, respectively, for the film formed at TA=800 °C, indicating that the RTA-formed Co2FeSi film had the L21 structure with the extremely high degree of ordering.

Analysis of L21-ordering in full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing

The authors developed a new analysis approach for evaluation of atomic ordering in full-Heusler alloys, which is extension of the commonly used Webster model. Our model can give accurate physical formalism for the degree of atomic ordering in the L21 structure, including correction with respect to the fully disordered A2 structure, i.e., the model can directly evaluate the degree of L21 ordering under a lower ordering structure than the complete B2-ordering structure. The proposed model was applied to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing. The film formed at TA=800 °C showed a high degree of L21 ordering of 83% under a high degree of B2 ordering of 97%.

Tunnel magnetoresistance for magnetic tunnel junctions with Co2FeAl0.5Si0.5 full Heusler electrodes fabricated by molecular beam epitaxy system

The authors fabricated Co2FeAl0.5Si0.5 full Heusler alloy thin films by using a molecular beam epitaxy system on Cr buffered MgO single crystal substrates and investigated their structural and magnetic properties. It was revealed that Co2FeAl0.5Si0.5 films formed ordered L21 and B2 structures after annealing above 500 °C and below 400 °C, respectively. Then magnetic tunnel junctions with these electrodes and Al-oxide or MgO barriers were also fabricated, and the bias voltage dependence of tunnel magnetoresistance ratio was investigated. It was found that the tunnel magnetoresistance ratio of the junction with Al-oxide barrier has a larger asymmetric bias voltage dependence compared with that with MgO barrier, and the negative tunnel magnetoresistance ratio at high negative bias voltage is observed for both junctions.

Formation of Si- and Ge-based Full-Heusler Alloy Thin Films using SOI and GOI Substrates for the Half-metallic Source and Drain of Spin Transistors

The paper presents a novel preparation technique for Si- and Ge-based half-metallic full-Heusler alloy thin films, utilizing silicon-on-insulator (SOI) and germanium-on-insulator (GOI) substrates, respectively. Full-Heusler Co2FeSi (Co2FeGe) alloy thin films were successfully formed by thermally activated silicidation (germanidation) reaction between an ultra-thin SOI (GOI) layer and Co/Fe layers deposited on it. This technique can easily produce fully ordered L21 structure that is necessary for the half-metallicity of full-Heusler alloys. The proposed technique is compatible with metal source/drain formation process in advanced CMOS technology and would be applicable to the fabrication of the half-metallic source/drain of MOSFET type of spin transistors.

Formation of Si- and Ge-based Full-Heusler Alloy Thin Films using SOI and GOI Substrates for the Half-metallic Source and Drain of Spin Transistors

not Available.

Characterization of half-metallic L21-phase Co2FeSi full-Heusler alloy thin films formed by rapid thermal annealing

The authors developed a preparation technique of Co2FeSi full-Heusler alloy thin films with the L21-ordered structure on silicon-on-insulator (SOI) substrates, employing rapid thermal annealing (RTA). The Co2FeSi full-Heusler alloy films were successfully formed by RTA-induced silicidation reaction between an ultrathin SOI (001) layer and Fe∕Co layers deposited on it. The highly (110)-oriented L21-phase polycrystalline full-Heusler alloy films were obtained at the RTA temperature of 700°C. Crystallographic and magnetic properties of the RTA-formed full-Heusler alloy films were qualitatively the same as those of bulk full-Heusler alloy. The proposed technique is compatible with metal source∕drain formation process in advanced complementary metal-oxide semiconductor technology and would be applicable to the fabrication of the half-metallic source∕drain of metal-oxide-semiconductor field-effect transistor type of spin transistors.

Structural and Magnetic Properties of Off-Stochiometric Ni-Mn-Al Heusler Alloy Thin Film

The initials results on growth and structural properties of Ni-Mn-Al full Heusler alloy thin films on silicon substrates deposited by RF magnetron sputtering is reported in this paper. Good crystallinity in the film is obtained by optimizing the sputtering parameters. The as-deposited film was post-annealed in vacuum in the temperature range between 150 °C, 250 °C and 450 °C for 60 min. It is observed that as deposited film shows nanocrystalline in nature. The film annealed at 450 °C shows L21 structure. The magnetic properties of the NiMnAl films at room temperature are measured by vibrating sample magnetometer [VSM]. It is found that the annealed samples shows clear saturating loop whereas the as prepared film is paramagnetic in nature.

Highly Spin-Polarized Tunneling in Fully Epitaxial Magnetic Tunnel Junctions Using Full-Heusler Alloy Co$_2$Cr$_0.6$Fe$_0.4$Al Thin Film and MgO Tunnel Barrier

Highly spin-polarized tunneling with tunnel magnetoresistance (TMR) ratios of 90% at room temperature and 240% at 4.2 K was demonstrated for fully epitaxial magnetic tunnel junctions fabricated using a cobalt-based full-Heusler alloy Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Al (CCFA) thin film having a composition close to the stoichiometric one and a MgO tunnel barrier. A high tunneling spin polarization of 0.79 at 4.2 K was obtained for the epitaxial CCFA films from the TMR ratios. This adds to the promise of the fully epitaxial MTJ as a key device structure for utilizing the intrinsically high spin polarizations of Co-based full-Heusler alloy thin films

Fabrication of fully epitaxial magnetic tunnel junctions using cobalt-based full-Heusler alloy thin film and their tunnel magnetoresistance characteristics

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co2Cr0.6Fe0.4Al (CCFA) and Co2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600 °C. X-ray pole figure measurements of the CCFA films indicated that these films were epitaxial and crystallized in the B2 structure. X-ray pole figure measurements of the annealed CMG films showed (111) peaks with four-fold symmetry, which provides direct evidence that these films were epitaxial and crystallized in the L21 structure. Furthermore, cross-sectional transmission electron microscope images of a fabricated CMG film indicated that it was single-crystalline. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.23 nm rms for 100 nm thick CCFA films and 0.26 nm rms for 45 nm thick CMG films. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier and a Co50Fe50 (CoFe) upper electrode. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. Reflection high-energy electron diffraction patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially in both the CCFA/MgO/CoFe and CMG/MgO/CoFe MTJ layer structures. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios, for MTJs using a full-Heusler alloy, of 42% at RT and 74% at 55 K. On the other hand, the microfabricated epitaxial CMG/MgO/CoFe MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 14% at RT and 70% at 7 K. These results confirm the promise of epitaxial MTJs as a key device structure for clarifying and utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films.