Heusler Films Research Articles

Large tunneling magnetoresistance effect at high voltage drop for Co-based Heusler alloy∕MgO∕CoFe junctions

Growth and magnetic characterization of thin films of Co2Cr0.6Fe0.4Al and Co2MnSi full-Heusler compounds are investigated. Thin films were deposited by magnetron sputtering at room temperature directly onto oxidized Si wafers. These Heusler films are magnetically very soft and ferromagnetic with Curie temperatures well above room temperature. Polycrystalline Co2Cr0.6Fe0.4Al Heusler films combined with MgO barriers and CoFe counter electrodes are structured to magnetic tunnel junctions and yield almost 50% magnetoresistance at room temperature. The magnetoresistance shows a strong bias dependence with the maximum occurring at a voltage drop well above 1V. This special feature is accompanied by only a moderate temperature dependence of the tunnel magnetoresistance.

Point-contact Andreev spectroscopy on thin [formula omitted] Heusler films

Heusler films with L 2 1 and B2 structure are deposited simultaneously on amorphous carbon films, Si(1 0 0) surfaces, and in situ cleaved InAs(1 1 0) surfaces by coevaporation of Ni and the alloy MnIn. Morphology, structure, and stoichiometry are investigated with transmission-electron microscopy, electron diffraction, and X-ray spectroscopy. The almost perfect lattice match supports highly oriented growth of Ni 2 MnIn on InAs, which is proven by electron diffraction under grazing incidence. The electrical resistivity of thin films on Si show metallic behavior. At temperatures of liquid helium point-contact Andreev reflection spectroscopy is performed on films grown on Si(1 0 0) and in situ cleaved InAs(1 1 0) surfaces yielding spin polarizations comparable to the ones of Fe, Ni, Co, and permalloy ( Ni 80 Fe 20 ) .

Interface magnetization of ultrathin epitaxial Co2FeSi(110)/Al2O3 films

Element-specific magnetic properties of ultrathin epitaxial Co2FeSi(110) films were measured using x-ray magnetic circular dichroism (XMCD). The epitaxial Heusler films were grown by RF magnetron sputtering on substrates. The magnetization of thicker films as determined by XMCD is smaller than expected for a half-metallic material. In addition, the magnetization decreases considerably for films thinner than 10 nm. The thickness dependence of the magnetic moment can be described by introducing a certain number of dead layers representing a deficiency of magnetization at the interfaces. Quantitative evaluation results in a dead layer thickness of 0.8 nm at room temperature, consisting of a temperature induced size effect of 0.1 nm and a surface effect of 0.15 nm at the top and 0.55 nm at the bottom interface.

Structural and magneto-transport characterization of Co2CrxFe1-xAl Heusler alloy films

We investigate the structure and magneto-transport properties of thin films of the Co_2Cr_xFe_(1-x)Al full-Heusler compound, which is predicted to be a half-metal by first-principles theoretical calculations. Thin films are deposited by magnetron sputtering at room temperature on various substrates in order to tune the growth from polycrystalline on thermally oxidized Si substrates to highly textured and even epitaxial on MgO(001) substrates, respectively. Our Heusler films are magnetically very soft and ferromagnetic with Curie temperatures up to 630 K. The total magnetic moment is reduced compared to the theoretical bulk value, but still comparable to values reported for films grown at elevated temperature. Polycrystalline Heusler films combined with MgO barriers are incorporated into magnetic tunnel junctions and yield 37% magnetoresistance at room temperature.

FMR investigations of half‐metallic ferromagnets

AbstractThin films of various half‐metallic ferromagnets, such as chromium dioxide (CrO2) and Heusler alloys (Co2Cr0.6Fe0.4Al, Co2MnSi) have been investigated by ferromagnetic resonance (FMR) technique. It is demonstrated that FMR is a very efficient method to study the nanoscale magnetic properties, in particular to probe the magnetic anisotropy and magnetic inhomogeneities of ferromagnetic thin films. Epitaxial CrO2 thin films of various thicknesses (25–535 nm) have been deposited on TiO2(100) substrates by chemical vapor deposition process. It is shown that the magnetic behavior of the CrO2 films results from a competition between the magnetocrystalline and strain anisotropies. For the ultrathin CrO2 film (25 nm) the magnetic easy axis switches from the c‐direction to the b‐direction of the rutile structure. Thin‐film Co2Cr0.6Fe0.4Al samples (25 nm or 100 nm) have been grown by DC magnetron sputtering either on unbuffered SiO2(100) substrates or on the substrates capped by a 50 nm thick V buffer layer. The effects of the vanadium buffer layer and of the film thickness are revealed by FMR studies of the Co2Cr0.6Fe0.4Al samples. Well‐resolved multiple spin‐wave modes are observed in the unbuffered Co2Cr0.6Fe0.4Al sample with a thickness of 100 nm and the exchange stiffness constant has been estimated. Thin films of Co2MnSi (4–100 nm) have been grown by DC sputtering on silicon substrates on top of a 42 nm thick V seed layer and capped either by Al2O3 or by Co and V layers. A set of the 80 nm thick films has been annealed at different temperatures in the range of 425–550 °C. FMR studies of the Co2MnSi samples shows that at the fixed annealing temperature (450 °C) the highest magnetization is observed in the sample with a thickness of 61 nm, while the thicker samples (100 nm) reveal not only a lower magnetization but greater magnetic inhomogeneity as well. An annealing treatment at T ≥ 450 °C is essential to obtain higher magnetization as well as uniform magnetic properties in the Co2MnSi films. Weak SWR modes have also been observed in the thick Heusler films. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heusler alloy/semiconductor hybrid structures

Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with III–V semiconductors, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi level. These alloys are categorized into half and full Heusler alloys dependent upon the crystalline structures, each of which is discussed both experimentally and theoretically in Sections 2 and 3, respectively. As an example, our recent studies on epitaxial L2 1 Co 2 Cr 1− x Fe x Al(0 0 1)/GaAs(0 0 1) hybrid structures are presented here. Both structural and magnetic characterizations on an atomic scale are typically carried out in order to prove the half-metallicity at RT as described in Section 4. Atomic ordering in the Heusler films is directly observed by X-ray diffraction and is also indirectly measured by the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarization of the Heusler alloy films are directly estimated by using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these alloy films in a spintronic device, diffusive spin-polarized electron transport may offer highly efficient spin injection across a direct interface between the Heusler alloy film and the semiconductor, while ballistic transport in a magnetic tunnel junction may further improve areal density of a magnetic random access memory with a large magnetoresistance ratio at RT as discussed in Section 5. A brief summary is provided at the end of this review.

Ｌ２１構造を有するＣｏ２Ｃｒ１−ｘＦｅｘＡｌ薄膜の結晶構造と磁気特性

We successfully grew Co2Cr1-xFexAl full Heusler films (0 ≤ x ≤ 1) with the L21 structure onto GaAs(001) substrates by achieving stoichiometry in an ultrahigh-vacuum molecular beam epitaxy chamber. The films develop epitaxial crystallinity with the relationship of Co2Cr1-xFexAl(001)‹110›||GaAs(001)‹110›, which induces very strong uniaxial magnetocrystalline anisotropy, except for x = 0. For x = 1, in particular, the film is almost a single phase and its magnetic moment per formula unit exhibits Slater-Pauling behavior. Films of this type were used to fabricate magnetic tunnel junctions with 8.8% and 4.9% tunnel magnetoresistance (TMR) ratios at room temperature for x = 1 with an Al-O tunnel barrier and x = 0.6 with a MgO barrier, respectively. The TMR ratio can be further enhanced by both realizing an L21 single phase and eliminating lattice distortion.

Soft X-ray resonant magnetic scattering studies on Co 2MnGe Heusler films

We report on soft-X-ray resonant magnetic scattering studies performed at the Mn and Co L 3,2 edges of Co 2MnGe Heusler films. The samples were prepared by RF-sputtering and have shown a ferromagnetic behavior with a coercive field of 30–70 Oe , depending on the layer thickness. The intensity of the scattered beam has been measured as a function of photon energy and magnetic field.

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature result in very smooth Co2MnSi Heusler films, enabling the Co2MnSi Heusler alloys to serve as electrodes in tunnel magnetoresistance structures.