In-plane Easy Axis Research Articles

Microscopic analysis of the composition driven spin-reorientation transition in NixPd1−x/Cu(001)

The spin-reorientation transition (SRT) in epitaxial NixPd1−x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified.

Temperature dependence of the perpendicular magnetic anisotropy in Ta/Co2FeAl/MgO structures probed by Anomalous Hall Effect

We report a detailed study of the temperature dependence of the magnetic anisotropy in Ta/Co2FeAl/MgO structures by means of Anomalous Hall Effect measurements. The volume magnetic anisotropy, although negligible at room temperature, shows a non-negligible value at low temperatures and favors an in-plane easy magnetization axis. The surface magnetic anisotropy, which promotes the perpendicular magnetic easy axis, shows an increase from 0.76±0.05erg/cm2 at 300K, up to 1.08±0.04erg/cm2 at 5K, attributed to the evolution of the Co2FeAl layer saturation magnetization with temperature.

Graphene-Induced Magnetic Anisotropy of a Two-Dimensional Iron Phthalocyanine Network.

A single layer of flat-lying iron phthalocyanine (FePc) molecules assembled on graphene grown on Ir(111) preserves the magnetic moment, as deduced by X-ray magnetic circular dichroism from the Fe L2,3 edges. Furthermore, the FePc molecules in contact with the graphene buffer layer exhibit an enhancement of the magnetic anisotropy, with emergence of an in-plane easy magnetic axis, reflected by an increased orbital moment of the FePc molecules in contact with the C atoms in the graphene sheet. The origin of the increased magnetic anisotropy is discussed, considering the absence of electronic state hybridization, and the breaking of symmetry upon FePc adsorption on graphene.

Hysteresis Loops, Critical Fields and Energy Products for Exchange-spring Hard/soft/hard Trilayers

Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by a three-dimensional (3D) model for exchange-coupled Sm-Co/α-Fe/Sm-Co trilayers with in-plane collinear easy axes. These results are carefully compared with the popular one-dimensional (1D) micromagnetic models and recent experimental data. It is found that the results obtained from the two methods match very well, especially for the remanence and coercivity, justifying the calculations. Both nucleation and coercive fields decrease monotonically as the soft layer thickness L s increases while the largest maximum energy product (roughly 50 MGOe) occurs when the thicknesses of hard and soft layers are 5 ㎚ and 15 ㎚, respectively. Moreover, the calculated angular distributions in the thickness direction for the magnetic moments are similar. Nevertheless, the calculated nucleation and pinning fields as well as the energy products by 3D OOMMF are systematically smaller than those given by the 1D model, due mainly to the stray fields at the corners of the films. These demagnetization fields help the magnetic moments at the corners to deviate from the previous saturation state and facilitate the nucleation. Such an effect enhances as Ls increases. When the thicknesses of hard and soft layers are 10 ㎚ and 20 ㎚, respectively, the pinning field difference is as large as 30%, while the nucleation fields have opposite signs.

Magnetic properties of self-organized Co dimer nanolines on Si/Ag(110).

We demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.

Modification of structure and magnetic anisotropy of epitaxial CoFe2O4 films by hydrogen reduction

Heteroepitaxial CoFe2O4 (CFO) thin films with different thicknesses were deposited on MgO (001) substrates. The as-deposited CFO films show a clear switching of magnetic anisotropy with increasing film thickness. The thinner films (<100 nm) show a perpendicular magnetic anisotropy due to the out-of-plane compressive strain. The thicker films exhibit an in-plane easy axis owing to the dominating shape anisotropy effect. The magnetostriction coefficient of CFO films is estimated to be λ[001] = −188 × 10−6. Metallic CoFe2 films were obtained by annealing the as-deposited CFO films in forming gas (Ar 93% + H2 7%) at 450 °C. XRD shows that CoFe2 films are textured out-of-plane and aligned in-plane, owing to lattice matching between CoFe2 and MgO substrate. TEM results indicate that as-deposited films are continuous while the annealed films exhibit a nanopore mushroom structure. The magnetic anisotropy of CoFe2 films is dominated by the shape effect. The results demonstrate that hydrogen reduction can be effectively used to modify microstructures and physical properties of complex metal oxide materials.

Magnetic Anisotropy in (Ge,Mn)Te Layers

Magnetic anisotropy of (Ge,Mn)Te semiconductor layers exhibiting carrier-induced ferromagnetism was experimentally studied by ferromagnetic resonance (FMR) method. The one micron-thick monocrystalline (Ge,Mn)Te layers were grown on diamagnetic BaF2 (111) substrate by molecular beam epitaxy technique. The analysis of the angular dependence of the FMR resonant field carried out for the external magnetic field direction varying in the (1-10) and (11-2) crystal planes revealed the usual in-plane location of the magnetization easy axis for the Ge0.8Mn0.2Te layer with the cubic structure whereas the normal to the layer plane easy axis was found for the rhombohedral Ge0.9Mn0.1Te layer. These experimental findings are theoretically discused employing group-theoretical methods for various crystal structures as well as density functional theory (DFT) calculations of magnetization dependent contribution to the total electronic energy of (Ge,Mn)Te supercell composed of 64 atoms with Mn substituting Ge at fcc cation sublattice sites. The DFT calculations of (Ge,Mn)Te show an order of magnitude increase of uniaxial magnetic anisotropy in the rhombohedrally distorted layer as compared to the cubic one.

Magnetic properties of in-plane oriented barium hexaferrite thin films prepared by direct current magnetron sputtering

In-plane c-axis oriented Ba-hexaferrite (BaM) thin films were prepared on a-plane (112¯0) sapphire (Al2O3) substrates by DC magnetron sputtering followed by ex-situ annealing. The DC magnetron sputtering was demonstrated to have obvious advantages over the traditionally used RF magnetron sputtering in sputtering rate and operation simplicity. The sputtering power had a remarkable influence on the Ba/Fe ratio, the hematite secondary phase, and the grain morphology of the as-prepared BaM films. Under 80 W of sputtering power, in-plane c-axis highly oriented BaM films were obtained. These films had strong magnetic anisotropy with high hysteresis loop squareness (Mr/Ms of 0.96) along the in-plane easy axis and low Mr/Ms of 0.03 along the in-plane hard axis. X-ray diffraction patterns and pole figures revealed that the oriented BaM films grew via an epitaxy-like growth process with the crystallographic relationship BaM (101¯0)//α-Fe2O3(112¯0)//Al2O3(112¯0).

Co100−xFex magnetic thick films prepared by electrodeposition

Co–Fe films are grown onto plane pre-treated Cu foils; the effects of the alloy composition on the morphology and the crystal texture of the electrodeposited films and their anisotropic magnetic hysteresis properties are explored. Nucleation and crystallization mechanisms in these Co-rich layers are also investigated with pulse-reverse plating techniques, using the first cathodic pulse current–time transients. In the diffusion controlled regime the deposition mechanism is found to involve progressive nucleation with three-dimensional (3D) growth, except for the equiatomic Fe50Co50 solution where nucleation tends to become instantaneous. The different morphologies and size scales observed are described and correlated with coercivity. The films are electrodeposited onto electrochemically pre-treated Cu substrates from feeds of nominal Fe/Co mol ratios between 0/100 and 50/50. The composition of the deposited layers, as determined by energy dispersive X-ray spectroscopy, are quite close to the nominal values. Cyclic voltammetry determinations exhibit only a single reduction process on the cathode, indicating that a unique (Co100−xFex) phase grows. Depending on composition and on the substrate pre-treatment, these layers exhibit textures with features of different sizes. X ray diffraction patterns indicate that the nanostructures with Fe contents above 20at.% crystallize in a body-centered cubic cell, while samples with Fe contents below this value are fcc. Regarding the effect of composition on the morphology, Co and Co-rich layers are compact, with large (100–300nm) agglomerates of quite equiaxed, densely packed particles (average 50nm); as the iron content increases above 15at.%, faceted particles 100nm in size are observed. All the electrodeposited layers are soft ferromagnetic at room temperature, with an in plane easy axis; coercive fields/forces between 10mT and 71mT are measured at 300K.

Magnetic anisotropy of van der Waals absorbed iron(II) phthalocyanine layer onBi2Te3

A self-assembled iron(II) phthalocyanine single layer adsorbed on the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ was investigated by spin-polarized scanning tunneling microscopy and density functional theory calculations. Although the molecule-substrate interaction is dominated by a relatively weak van der Waals force, the local density of states of Fe was found to strongly depend on the adsorption sites, resulting in a supermolecular lattice. Spin-polarized measurements show that the magnetic moment of iron(II) phthalocyanine persists with an in-plane magnetic easy axis, which was further confirmed by density functional calculations.

Ferromagnetism of Fe3Sn and alloys.

Hexagonal Fe3Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K1 of 1.8 MJ/m3. However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. However, transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe3Sn is reported, as are some are magnetic and structural properties for the Fe3Sn2, and Fe5Sn3 compounds, which could be prepared as mm-sized single crystals.

Decimal Tunneling Magnetoresistance States in Fe/GaAlAs/GaMnAs Magnetic Tunnel Junction

We report the realization of ten stable tunneling magnetoresistance (TMR) states in a single device. To achieve ten resistance states, we have used a magnetic tunnel junction (MTJ) structure that consists of two magnetic layers, which are Fe and GaMnAs ferromagnetic layers. Owing to the two in-plane magnetic easy axes that result from strong cubic anisotropies in both Fe and GaMnAs layers, noncollinear magnetic configurations between two magnetic layers were realized, in addition to the parallel and antiparallel configurations. Such noncollinear magnetic configurations provide stable intermediate TMR values between two extreme values corresponding parallel and antiparallel configurations. The number of stable TMR values was further increased by forming multidomain structures in the MTJ structure. We demonstrate that we can obtain up to ten stable TMR values, and they can be controlled by applying the appropriate magnetic field sequences.

Magnetic Anisotropy in GeMnTe - ab initio Calculations

Recent magnetization and ferromagnetic resonance (FMR) measurements [1-4] show that in monocrystalline Ge1−xMnxTe layers grown on the BaF2 (111) substrate with x ≤ 0.1 the easy axis of magnetization is perpendicular to the layer. On the other hand, the usual in-plane easy axis due to dipolar interactions (shape anisotropy) is observed for polycrystalline and layered Ge1−xMnxTe with x ≥ 0.2. X-ray analysis suggests that the change of direction of the easy axis is connected with the change of crystal structure: the increase of the manganese content leads to change of the crystal symmetry from rhombohedral to cubic. Previous theoretical studies of anisotropy in semimagnetic semiconductors were based on the Zener model of ferromagnetism within effective–mass approach [5]. In the present work, first principles calculations of energy of magnetic anisotropy (EMA) in GeMnTe mixed crystals were performed using OpenMX package with fully relativistic pseudopotentials. The obtained results clearly indicate that EMA strongly depends on crystal structure, concentration of free holes and manganese content. The discussion of microscopic origin of magnetic anisotropy was conducted. The main conclusion is that the magnetic anisotropy is caused by interplay between spin – orbit and Coulomb interactions. The results show that the change of spin direction of manganese ions results in spatial redistribution of the electron charge. The discussion includes the differences between conducting and insulating cases, in particular the range of spin polarization caused by manganese ions. Finally, we point out the important role of chemical disorder on the macroscopic magnetic anisotropy.

Pulsed laser deposition of epitaxial yttrium iron garnet films with low Gilbert damping and bulk-like magnetization

Yttrium iron garnet (YIG, Y 3Fe5O12) films have been epitaxially grown on Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) substrates with (100) orientation using pulsed laser deposition. The films were single-phase, epitaxial with the GGG substrate, and the root-mean-square surface roughness varied between 0.14 nm and 0.2 nm. Films with thicknesses ranging from 17 to 200 nm exhibited low coercivity (<2 Oe), near-bulk room temperature saturation moments (∼135 emu cm−3), in-plane easy axis, and damping parameters as low as 2.2 × 10−4. These high quality YIG thin films are useful in the investigation of the origins of novel magnetic phenomena and magnetization dynamics.

In-plane anisotropic effect of magnetoelectric coupled PMN-PT/FePt multiferroic heterostructure: Static and microwave properties

The effects of the electric and magnetic field variation on multiferroic heterostructure were studied in this work. Thin films of polycrystalline Fe50Pt50 (FePt) were grown by dc-sputtering on top of the commercial slabs of lead magnesium niobate-lead titanate (PMN-PT). The sample was a (011)-cut single crystal and had one side polished. In this condition, the PMN-PT/FePt operates in the L-T (longitudinal magnetized-transverse polarized) mode. A FePt thin film of 20 nm was used in this study to avoid the characteristic broad microwave absorption line associated with these films above thicknesses of 40 nm. For the in-plane easy magnetization axis (01-1), a microwave magnetoelectric (ME) coupling of 28 Oe cm kV −1 was estimated, whereas a value of 42 Oe cm kV −1 was obtained through the hard magnetization axis (100). Insight into the effects of the in-plane strain anisotropy on the ME coupling is obtained from the dc-magnetization loops. It was observed that the trend was opposite along the easy and hard magnetic directions. In particular, along the easy-magnetic axis (01-1), a square and narrow loop with a factor of Mr/MS of 0.96 was measured at 10 kV/cm. Along the hard-magnetic axis, a factor of 0.16 at 10 kV/cm was obtained. Using electric tuning via microwave absorption at X-band (9.78 GHz), we observe completely different trends along the easy and hard magnetic directions; Multiple absorption lines along the latter axis compared to a single and narrower absorption line along the former. In spite of its intrinsic complexity, we propose a model which gives good agreement both for static and microwave properties. These observations are of fundamental interest for future ME microwave components, such as filters, phase-shifters, and resonators.

Electronic Structures and Magnetic Anisotropy Energies of Graphene with Adsorbed Transition-Metal Adatoms

We investigate the electronic structures of graphene with an adsorbed Fe, Co, or Ni adatom in the presence of spin–orbit coupling by density functional theory calculations. As a result of spin–orbit coupling, energy gaps are opened at the crossing points of spin-up and spin-down bands near the Dirac point in the cases of Fe and Co adatoms. In the case of the Ni adatom, the Rashba effect is observed with a k-dependent energy shift of the π bands at the bottom of the Fermi surface. The calculated magnetic anisotropy energies reveal that the Fe adatom exhibits an in-plane easy magnetization axis, while the Co adatom exhibits an out-of-plane easy axis.

Spin reorientation transition in Co/Au multilayers

We report a study about the spin reorientation transition (SRT) from perpendicular easy axis to in-plane easy axis of magnetization in Co/Au multilayers. A series of multilayers of Si/Au(100Å)/{[Co(tCo)/Au(20Å)]20}/Au(50Å) family were studied, with Co layer thickness varying between 6Å to 30Å. The thickness of the Au layer was chosen large enough in order to minimize the interlayer exchange coupling between Co layers. In such thick Au-layer samples the magnetic properties are mainly the result of competition between interlayer magnetostatic coupling due to stray field, perpendicular magnetic anisotropy and shape anisotropy. The effective anisotropy constant Keff and the second order anisotropy K2 were deduced from the fit of the resonant magnetic field obtained from out of plane dependence Ferromagnetic Resonance (FMR) experiments. To study the SRT, we have plotted the phase diagram between Keff and K2. The results show that SRT occurs through the metastable region with K2≤−½Keff, (Keff>0). It is interesting to note that FMR shows the coexistence of two modes with different anisotropy for small Co thickness, while for thick Co layers the modes have the same anisotropy. Moreover, in thick Co layer samples, volume and surface spin wave resonance (SWR) modes were also excited by the microwave field, around the perpendicular FMR geometry, giving a clear evidence of a magnetic coupling between the Co layers.

Voltage-driven hysteretic changes in magnetization in multiferroic Co/BTO composite thin films

Abstract Multiferroic Co/BaTiO3 layered composite thin films were grown on Nb-doped SrTiO3 single crystal by pulse laser deposition, in which the polycrystalline Co film was annealed under magnetic field to induce in-plane uniaxial magnetic easy axis. Voltage-induced magnetization changes along and perpendicular to the easy axis were measured by Magneto-Optical Kerr effect (MOKE) magnetometer without applied magnetic field. These changes in magnetization could be due to magnetoelectric coupling between Co and BaTiO3, and to possibly electro-optical effect of BTO if the MOKE laser could penetrate the top Co film. After excluding the electro-optical interference by analyzing the experimental results within an optical model, a hysteric loop of magnetization versus voltage is identified with a relative change in magnetization of about 8%.

Giant magnetic domains in amorphous SmCo thin films

The potential for tuning of magnetic properties and the exceptional uniformity are among the features that make amorphous magnetic materials attractive for technology. Here it is shown that the magnetization reversal in amorphous SmCo thin films takes place through the formation of giant magnetic domains, over a centimeter across. The domain structure is found to be dictated by the direction of the imprinted in-plane easy axis and the film boundaries. This is a consequence of the size of the anisotropy and the structural uniformity of the films, which also allows the movement of millimeter-long domain walls over distances of several millimeters. The results demonstrate the possibility of tailoring the magnetic domain structure in amorphous magnets over a wide range of length scales, up to centimeters. Moreover, they highlight an important consequence of the structural perfection of amorphous films.

Out-of-plane magnetic anisotropy in columnar grown Fe–Ni films

Polycrystalline thin films usually present magnetic anisotropy resulting from a conjunction of textures, residual stresses, surface effects, and magnetic dipole distribution. The shape anisotropy, which is caused by the magnetic dipole distribution, is dominant in most of the cases, and it forces the occurrence of in-plane easy axes for the magnetization. Contrary to this common expectation, we have found predominant out-of-plane easy axes in a series of Fe–Ni thin films produced by DC sputtering. Films with different thicknesses, from 40 to 1000nm, and different deposition temperatures have been tested and show similar results. These unusual characteristics are results of a particular columnar structure formed during the films growth. The magnetic characterization of the samples has been done by Mössbauer spectroscopy, magnetometry, and ferromagnetic resonance. The unusual anisotropy observed is not believed to be uniform along the film thickness. This interpretation comes from the comparison of the experimental results with hysteresis obtained by micromagnetic simulations. Five distinct configurations for the anisotropies have been simulated for this comparison.