Strong In-plane Uniaxial Magnetic Anisotropy Research Articles

FePd2Te2: An Anisotropic Two-Dimensional Ferromagnet with One-Dimensional Fe Chains.

Two-dimensional (2D) magnets have attracted significant attention in recent years due to their importance in the research on both fundamental physics and spintronic applications. Here, we report the discovery of a new ternary compound FePd2Te2. It features a layered quasi-2D crystal structure with 1D Fe zigzag chains extending along the b-axis in the cleavage plane. Single crystals of FePd2Te2 with centimeter size could be grown. Density functional theory calculations, mechanical exfoliation, and atomic force microscopy on these crystals reveal that they are 2D materials that can be thinned down to ∼5 nm. Magnetic characterization shows that FePd2Te2 is an easy-plane ferromagnet with TC ∼ 183 K and strong in-plane uniaxial magnetic anisotropy. Magnetoresistance and the anomalous Hall effect demonstrate that ferromagnetism could be maintained in FePd2Te2 flakes with large coercivity. A crystal twinning effect is observed by scanning tunneling microscopy which makes the Fe chains right angle bent in the cleavage plane and creates an intriguing spin texture. Besides, a large electronic specific heat coefficient of up to γ ∼ 32.4 mJ mol-1 K-2 suggests FePd2Te2 is a strongly correlated metal. Our results show that FePd2Te2 is a correlated anisotropic 2D magnet that may attract multidisciplinary research interests.

Strong uniaxial magnetic anisotropy in Fe3Si thin films

Thin Fe3Si films grown on Si/SiO2 substrates by ion-beam and pulsed laser deposition possess soft ferromagnetic behavior and strong in-plane uniaxial magnetic anisotropy. The material of the capping layer and deposition of the buffer layer do not affect the presence and character of obtained anisotropy. The structural studies showed that Fe3Si film deposited on amorphous SiO2 layer is polycrystalline while the same layer grown on polycrystalline Ta buffer layer is amorphous. It has also been found that uniaxial anisotropy is similar for amorphous and crystalline DO3 Fe3Si films. Magnetic properties yield a magnetization saturation of both amorphous and crystalline films as 1050 emu/cm3 at room temperature and coercive field not exceeding 5 Oe. The origin of such an anomalous behavior were discussed in terms of deposition technique features and parameters, substrate and capping layer type and resulting film parameters.

Exploring the shear strain contribution to the uniaxial magnetic anisotropy of (Ga,Mn)As

We present an experimental and k⋅p theoretical study on the origin of the strong in-plane uniaxial magnetic anisotropy in (Ga,Mn)As layers, unexpected from the cubic crystalline structure. The symmetry lowering can be accounted for by structural or effective shear strains. We find theoretically out-of-plane and in-plane magnetic anisotropy constants being linear with the shear strain. Searching for a real shear strain arising from lattice relaxation, we perform two types of measurements: anomalous x-ray diffraction and strain-induced optical birefringence, at room temperature. Working on a strongly anisotropic (Ga,Mn)As layer, the estimated ϵxy=10−4 was not found although it lied an order of magnitude above the detection threshold. This ensemble of results indicates as unlikely a relaxation-driven uniaxial anisotropy. As previously suggested theoretically, the magnetic symmetry-lowering could instead originate from the anisotropic incorporation of Mn atoms during growth. This would yield a perfectly in-plane matched lattice, with an anisotropy that could nevertheless be modeled as an effective shear strain and modified by an external shear stress, in agreement with the existing experimental literature.

Investigation of magnetic properties in thick CoFeB alloy films for controllable anisotropy

CoFeB alloy material has attracted interest for its wide uses in magnetic memory devices and sensors. We investigate magnetic properties of thick Co40Fe40B20 films in the thickness range from 10 to 100 nm sandwiched by MgO and Ta layers. Strong in-plane uniaxial magnetic anisotropy is revealed in the as-deposited amorphous films by angular dependent magnetic measurements, and the growth-induced anisotropy is found to strongly depend on the film thickness. A fourfold cubic magnetic anisotropy develops with annealing, as a result of improved crystalline structure in films confirmed by X-ray diffraction measurements. The observed magnetic properties can be explained by the superposition of the uniaxial and additional cubic magnetic anisotropy, tuned by annealing temperature.

Building oriented nano-sheets in thin films of Co–MT (MT = V, Cr, Cu, Zn, Cd, Hf) and the generation and enhancement of their magnetic anisotropy

Thin films of Co-rich transition metal or metal Co–MT (MT = V, Cr, Cu, Zn, Cd, Hf) compounds were fabricated as a superposition of oblique nano-sheets (≈300 x 80 × 5 nm for each nano-sheet) from a mixture of Co and MT plasmas. Because the X-ray diffraction patterns of all of the films showed a similar nano-crystalline microstructure, the oblique nano-sheets were visualized by high-resolution transmission electron microscopy. This nano-morphology generated strong in-plane uniaxial magnetic anisotropy, UMA, as we explained in this work, for all the as-deposited films. The evolution of this magnetic anisotropy was studied at temperatures up to 450 °C. Three different cases were observed after these heat treatments depending on the MT component of the film: (1) UMA was lost for temperatures ≈ 400 °C, (2) UMA retained the same value until the highest temperature was reached, or (3) UMA was enhanced by a factor greater than 2 for the highest temperature. This behavior can be explained by considering of the various processes of structural relaxation and crystallization produced by heat treatment and affected by the nano-sheet morphology.

In-plane Uniaxial Anisotropy and Magnetization Reversal Mechanism of FeCo Films by Strip Pattern

Strip-like FeCo films were patterned by a traditional lithograph process from intrinsically isotropic continuous FeCo films. The strip-patterned FeCo film shows a strong in-plane uniaxial magnetic anisotropy with easy axis along the length direction of the strip. The angular dependences of remanence ratio, switching field, and coercivity indicate that the magnetization reversal mechanism of the strip-patterned FeCo film is coherent rotation and domain wall depinning when the applied field is near the hard axis and easy axis, respectively. The consistency of the experimental hysteresis loops of the strip-patterned FeCo film and calculated hysteresis loops with a simple in-plane uniaxial anisotropy model indicates that the strip-patterned FeCo film behaves as a single domain. The absence of the domain wall and the strong in-plane anisotropy field make the strip-patterned FeCo films have much potential for high-frequency application.

Optimization of high frequency characteristics in Co-Ta thin films

Magnetic properties and high frequency characteristics of binary Co100−xTax (x = 0–24) thin films, prepared by using the oblique sputtering method under an external magnetic field of 1200 Oe have been studied. All the Co100−xTax films are easy to form amorphous phase, and thus exhibit strong in-plane uniaxial magnetic anisotropy, high resistivity, and low coercivity. With increasing x, the saturation magnetization (4πMs) and coercivity along the easy and hard axes (Hce and Hch) are decreased, while the magnetic anisotropy field (Hk) reaches the maximum value of 578 Oe for the films with the composition to become fully amorphous phase. Due to large Hk, high ferromagnetic-resonance frequency (fFMR) of 4.7–6.6 GHz is obtained for Co100−xTax (x = 11–24) films. The optimal magnetic properties of 4πMs = 10.9 kG, Hce = 6.2 Oe, Hch = 6.0 Oe, Hk = 507 Oe, and high resonance characteristics of fFMR = 6.4 GHz, respectively, are reached for Co84Ta16 films. Finally, high fFMR of 5.4–7.2 GHz and Hk of 434–617 Oe can be tuned by the modification of the sputtering parameters. This result reflects the benefit of Co100−xTax (x = 11–22) films for above 5 GHz applications.

Structural control of magnetic anisotropy in a strain-driven multiferroic EuTiO3thin film

Octahedral distortion plays a key role in engineering the physical properties of heterostructures composed of perovskite oxides. We observe a strong in-plane uniaxial magnetic anisotropy in a strain-enabled multiferroic EuTiO3 thin film epitaxially grown on a (110)o DyScO3 substrate. First principles calculations show that the magnetic anisotropy is closely correlated with the uniaxial TiO6 octahedral tilting and the ferroelectric polarization of the film, indicating potential strong magnetoelectric coupling in the strain-engineered multiferroic system.

Epitaxial Fe films on ZnSe(001): effect of the substrate surface reconstruction on the magnetic anisotropy

It is well known that Fe films deposited on a c(2 × 2)-reconstructed ZnSe(001) surface show a strong in-plane uniaxial magnetic anisotropy. Here, the effect of the substrate reconstruction on the magnetic anisotropy of Fe has been studied by in situ Brillouin light scattering. We found that the in-plane uniaxial anisotropy is strongly reduced for Fe films grown on a (1 × 1)-unreconstructed ZnSe substrate while the in-plane biaxial one is nearly unaffected by the substrate reconstruction. Calculations of magnetic anisotropy energies within the framework of ab initio density functional theory reveal that the strong suppression of anisotropy at the (1 × 1) interface occurs due to complex atomic relaxations as well as the competing effects originating from magnetocrystalline anisotropy and dipole–dipole interactions. For both sharp and intermixed c(2 × 2) interfaces, the magnetic anisotropy is enhanced compared to the (1 × 1) case due to the further lowering of symmetry. The theoretical results are in agreement with the experimental findings.

Strong uniaxial in-plane magnetic anisotropy of (001)- and (011)-orientedLa0.67Sr0.33MnO3thin films onNdGaO3substrates

Epitaxial La0.67Sr0.33MnO3 (LSMO) ferromagnetic thin films were coherently grown on NdGaO3 (NGO) substrates with different crystal orientations of the surface plane. On the (110)o- and (001)o-oriented substrates, the film grows in the (001)pc orientation, and on the (100)o-, (010)o-, and (112)o-oriented substrates the film is (011)pc oriented (we will use subindices o and pc for the orthorhombic and pseudocubic crystal structures, respectively). The lattice parameters and pseudocube angles of the deformed LSMO pseudocube have been determined from x-ray diffraction measurements. The in-plane magnetic easy and hard directions of these films have been determined from the dependence of the remnant magnetization on the angle of the in-plane applied field. For all substrate orientations there is a strong in-plane uniaxial magnetic anisotropy, determined by the crystal directions of the substrate surface. The easy and hard magnetic-anisotropy directions are explained consistently by the (bulk) inverse magnetostriction model, except for the film on NGO (112)o.

Brillouin Light Scattering Study of the Magnetic Anisotropy in bcc-Fe(100) Ultrathin Films Grown on GaAs(100) Surfaces With Different Reconstructions

We investigated the magnetic anisotropy of ultrathin bcc-Fe (001) films with a fcc-Au(001) cover layer, grown onto GaAs (001) (4 times 1) and (2 times 6)/(3 times 6) reconstructed surfaces. The magneto-optical Kerr effect hysteresis loops revealed existence of a strong in-plane uniaxial magnetic anisotropy in the Fe films grown on both the surfaces. Details of the in-plane uniaxial anisotropy (KU), cubic anisotropy (KC), and effective demagnetization field (4piMeff) were studied by Brillouin light scattering method. We observed a stronger reduction of KU, KC, and 4piMeff in thinner Fe layers (below 1.3 nm) grown on both the surfaces. In the case of Fe films grown on GaAs (4 times 1) surface, anisotropies observed were smaller than that for the (2 times 6)/(3 times 6) surface. This reflects the role of surface structure in determining the magnetic anisotropies.

In-Plane Uniaxial Magnetic Anisotropy of [(InyGa1-y)1-xMnx]As Characterized by Planar Hall Effect

The in-plane magnetic anisotropy was studied for a quaternary alloy ferromagnetic semiconductor [(InyGa1-y)1-xMnx]As thin film with x=0.12 and y=0.54 grown on an InP(001) substrate using planar Hall effect measurements. The present [(In0.54Ga0.46)0.88Mn0.12]As thin film shows a large planar Hall effect resistance change of 210 Ω at 2.8 K. It was found that the film has strong in-plane uniaxial magnetic anisotropy (Ku:K1>100:1) at 2.8 K, and its easy magnetization axis is along the [110] direction in the whole temperature range below its Curie temperature (< 100 K). This strong in-plane uniaxial magnetic anisotropy as well as the large planer Hall effect will be a potential advantage for spin-electronic devices.

MICROSTRUCTURES AND MAGNETIC PROPERTIES OF COBALT THIN FILMS

Cobalt thin films deposited by radio frequency sputtering were investigated. Microstructures of the Co films were analyzed by XRD and TEM. The results show that films microstructure varies with the variation of the sputter gas pressure P Ar . Magnetic properties measured by VSM show that all the films possess relatively high saturation magnetization 4πMs, and strong in-plane uniaxial magnetic anisotropy field Hk. Co films deposited below 0.3 Pa show soft magnetic properties and its microwave permeability was measured by vector network analyzer in the 0.1–5 GHz range.

Magnetic properties of self-organized lateral arrays of (Fe,Ag)∕Mo(110) nanostripes

We report the fabrication of self-organized arrays of Fe nanostripes with a lateral period of 3.5nm by sequential deposition of Fe and Ag on Mo(110). The wires display a strong in-plane uniaxial magnetic anisotropy along their length and are superparamagnetic above TB=185±15K. The large value of nucleation volumes, which is inferred from the analysis of the thermal dependence of coercivity below TB, suggests the existence of interactions between the wires.

Magnetic and transport properties of epitaxial Co/sub 2/MnSi films

We report on magnetic and magnetotransport properties of single-crystalline Co/sub 2/MnSi films grown by using conventional pulsed laser deposition (PLD) method on moderately heated semiconductor GaAs(001) substrates. The films exhibit the expected high Curie temperature and a strong in-plane uniaxial magnetic anisotropy with the easy axis of magnetization along the [1-10] direction. A saturation magnetization of 950 emu/cm/sup 3/ was measured at 5 K. The temperature dependence of the resistivity shows metallic behavior down to low temperatures. At 5 K, a rather large negative magnetoresistance (MR) of 1.26% has also been observed.

Magnetic study of epitaxial Fe∕InGaAs∕InP(100) deposited by ion-beam sputtering

Fe layers, 3, 6, 10, and 25 nm thick, were epitaxially deposited by ion-beam sputtering on InGaAs∕InP(100) wafers. For the 3-nm-thick layer, the sample shows a strong in-plane uniaxial magnetic anisotropy along the [110] direction between 5 and 300 K. The 6-nm film exhibits competition between the uniaxial magnetic anisotropy and the magnetic anisotropy of the bulk bcc Fe. The fourfold magnetic anisotropy of the bulk Fe dominates for the 10-nm Fe film. A decrease of the magnetization is observed for the thinner sample as compared to the bulk. This decrease is discussed in terms of Fe thickness, interface effect, diffusion effect, and possible phases at the interface with the semiconductor.

Uniaxial magnetic anisotropy tuned by nanoscale ripple formation: Ion-sculpting of Co/Cu(001) thin films

We have investigated the growth of surface nanostructures on a Co/Cu(001) film and the growth of Co films on a nanostructured Cu(001) substrate as well as the effect of nanoscale pattern formation on the film magnetic properties. Here we demonstrate by scanning tunneling microscopy measurements and magneto-optic Kerr effect hysteresis curves that low-temperature grazing-incidence ion sputtering can be used to induce the formation of nanoscale ripples which reduce the four-fold symmetry of the Co film to two-fold, thus generating a strong in-plane uniaxial magnetic anisotropy. The nanostructures and the associated uniaxial magnetic anisotropy were found to be stable up to room temperature.

Onset of magnetic anisotropy in epitaxial Fe films on GaAs(001)

It was shown previously that ultrathin Fe films epitaxially grown on GaAs(001) exhibit a strong in-plane uniaxial magnetic anisotropy which turns out to be a pure interface contribution with an anisotropy constant KUS, expressed as an energy per unit area, which is constant in a wide thickness range. However, for films thinner than ∼10 monolayers (ML), KUS decreases with decreasing thickness when measured at 300 K. In order to eliminate effects of thermal excitations, Fe(001) films grown on GaAs(001) by molecular beam epitaxy were investigated by superconducting quantum interference device magnetometry at low temperature. The extrapolated room temperature values and the ground state data both indicate that KUS vanishes at t=2.5 ML. This is the thickness at which the onset of ferromagnetism takes place, i.e., where the Curie temperature TC becomes nonzero. This might be interpreted as a verification of the Mermin–Wagner theorem in the sense that long-range ferromagnetic order is stabilized by the magnetic anisotropy. It is discussed whether the onset of ferromagnetism is indeed triggered by the appearance of magnetic anisotropy or if there is a common origin of both phenomena. Finally, it is found that the uniaxial anisotropy does not vanish at TC, but persists up to temperatures of ∼1.5 TC. This means that KUS does not scale with a certain power of the spontaneous magnetization. The disparity between the persistence of the anisotropy above TC and its disappearance below the critical thickness of 2.5 ML is discussed.

Correlation between magnetism and structural relaxation in thin Fe(001) films patterned by the atomic saw method

We present detailed extended x-ray-absorption fine-structure (EXAFS) studies carried out on 50-\AA{} epitaxial thin films grown by molecular-beam epitaxy on MgO(001) substrate prior and after structuration into ribbons by the so-called ``atomic saw'' method. Because of interfacial strain due to lattice mismatch (+3.8%), the crystallographic structure of the as-deposited film is demonstrated to be body-centered tetragonal with lattice constants $a=2.915\ifmmode\pm\else\textpm\fi{}0.015\AA{}$ and $c=2.82\ifmmode\pm\else\textpm\fi{}0.01\AA{}.$ This structure is 2% expanded in plane and -1.6% compressed along the surface normal as compared to the bulk Fe one. After structuration, the patterned 50-\AA{} Fe films show a strong in-plane uniaxial magnetic anisotropy (close to 2.5 kOe). The EXAFS studies provide the clear evidence that the dislocation slipping process enhances a uniaxial relaxation $\ensuremath{\delta}=\ensuremath{-}2.5%$ of the elastic strain field in the Fe film along the direction perpendicular to the ribbons. Through magnetoelastic effects, this relaxation is clearly demonstrated to be at the source of the observed magnetic anisotropy. This paper emphasizes the power of the ``phase derivative'' method in the analysis of EXAFS spectra for the determination of crystalline parameters in the case of bcc-type structures.

Magnetic properties in epitaxial binary iron and ternary iron–cobalt silicide thin films grown on Si(1 1 1)

Binary Fe(Si 1− x Fe x ) iron and ternary Fe 3− y Co y Si iron cobalt silicide thin films (thickness : 200 Å) with local CsCl structure epitaxially grown at room temperature on Si(1 1 1) have been studied through X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE) vector magnetometry. The binary Fe(Si 1− x Fe x ) films have ferromagnetic properties at room temperature in the range 0.09⩽ x<1 and the cobalt-substituted Fe 3− y Co y Si films are ferromagnetic as well in the whole range 0< y<3 at room temperature. The off-normal evaporation geometry leads to strong in-plane uniaxial magnetic anisotropy (including the non-epitaxial pure cobalt silicide Co 3Si) favored by the absence of orientation dependence of the fourth-order cubic term of the magnetocrystalline anisotropy energy in (1 1 1) crystallographic plane. The easy axis corresponds invariably to the direction normal to the incidence plane of the atomic flux during evaporation. The substitution of iron by silicon in Fe(Si 1− x Fe x ) films increases the coercive field but does not strongly affect the uniaxial anisotropy constant. In Fe 3− y Co y Si alloys, a similar trend is observed and the increase of coercive force may be related to defects generated by alloying. The change of the magnetic moments of Fe and Co versus y, as obtained by XMCD, can be understood in terms of simple models.