Uniaxial Perpendicular Anisotropy Research Articles

Oxygen deficiency and cooling field driven vertical hysteretic shift in epitaxial SrRuO3/SrTiO3 heterostructures

SrRuO3 thin films have been epitaxially grown on SrTiO3 substrates using a pulsed laser deposition technique. By adjusting the oxygen partial pressure during deposition, a sharp drop in the Curie temperature (TC) of 95 K and vertical magnetization shift (MShift) of 82.7% in the hysteresis loop was observed due to the oxygen deficiency induced lattice distortion that modifies the strong hybridization of p-d orbitals and perpendicular uniaxial magnetic anisotropy. In particular, the vertical hysteretic shift can also be effectively tuned by the applied cooling field, and thus, we obtained a giant and complete MShift of 106% with a large volume of pinned Ru4+ moments. These findings reveal the critical role played by intrinsic oxygen defects and extrinsic cooling field in controlling magnetic couplings in this perovskite-type complex oxide system.

Magnetization reversal and magnetoresistance behavior of exchange coupled SrRuO3 bilayer

Magnetic interlayer coupling of a bilayer structure composed of a tetragonal phase SrRuO3 (T-SRO) and a monoclinic phase SRO (M-SRO) was investigated by means of magnetization and magneto-transport measurements. The T-SRO showed large uniaxial perpendicular anisotropy and M-SRO exhibited longitudinal anisotropy. The thickness of the M-SRO top layer was varied and the bottom T-SRO layer remained unchanged. Magnetic hysteresis (M-H) loops showed that as thickness of the M-SRO layer is 4 nm, the M-SRO layer was fully perpendicularly coupled to the T-SRO layer. As the thickness of M-SRO further increased to 8 nm and above, the magnetization reversals of T- and M-SRO phase were clearly distinguished. The angular dependent magnetoresistance (MR) of the bilayers showed consistent results with the M-H loops, which suggests that angle dependent MR may offer a new way for the investigation of exchange coupling between two magnetic layers.

Ultra-low magnetic damping of perovskite La0.7Sr0.3MnO3 thin films

The perovskite La0.7Sr0.3MnO3 (LSMO) films grown on different substrates were investigated by an angle resolved broadband ferromagnetic resonance technique. All films exhibited a four-fold magnetocrystalline anisotropy, which is in accord with the crystal structure. Moreover, a perpendicular uniaxial anisotropy changed from the (001)pc easy plane to the [001]pc easy direction when the strain of LSMO films varies from tensile to compressive. The ultra-low magnetic damping constant of 5.2 × 10−4 was obtained for a 44.6 nm LSMO film on an NdGaO3 (110) substrate. The breathing Fermi surface model in which the damping constant is proportional to the density of states at Fermi energy is the dominant mechanism for the intrinsic magnetic relaxation.

Ferromagnetic Resonance Study of Fe/Cu Multilayer Thin Film

Fe/Cu/Fe multilayer thin film was grown on Si (100) substrate by using magnetron sputtering technique at room temperature. Dynamic and static magnetisations of the film have been investigated using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques in the temperature range of 10–300 K. From the room-temperature in-plane FMR measurements, a growth-induced uniaxial magnetic anisotropy was observed. Out-of-plane FMR measurements exhibited a large magnetic anisotropy due to a large saturation magnetisation of Fe. A computer code was written to simulate the experimental FMR data and to obtain the magnetic parameters of the Fe/Cu/Fe multilayer thin film. g value, effective magnetisation, uniaxial anisotropy field and perpendicular anisotropy constant from the fitting of the angular dependence of the resonance field at both the in-plane and out-of plane geometries were determined. The exchange bias effect was observed from the low-temperature VSM measurements. The saturation magnetisation and coercive field decrease with increasing temperature due to the increase of the thermal fluctuations.

Skyrmion core size dependence as a function of the perpendicular anisotropy and radius in magnetic nanodots

A detailed analytical and numerical analysis of the skyrmion core size dependence as a function of the uniaxial perpendicular anisotropy and radius in magnetic nanodots has been carried out. Results from micromagnetic calculations show a non-monotonic behavior between the skyrmion core size and the uniaxial perpendicular anisotropy. The increment of the radius reduces the skyrmion core size at constant uniaxial perpendicular anisotropy. Thus, these results can be used for the control of the core sizes in magnetic artificial skyrmion crystals or spintronic devices that need to use a skyrmion configuration at room temperature.

Interfacial Dzyaloshinskii-Moriya interaction studied by time-resolved scanning Kerr microscopy

We investigate the influence of the interfacial Dzyaloshinskii-Moriya interaction (DMI) on the propagation of Damon-Eshbach spin waves in micrometer-sized Pt(2 nm)/Co(0.4 nm)/Py(5 nm)/MgO(5 nm) stripes. We use time-resolved scanning Kerr microscopy to image the spin waves excited by a microwave antenna and to directly access their dispersion. The presence of an interfacial DMI manifests itself in an asymmetry in the dispersion for counterpropagating spin waves which reverses sign upon reversal of the direction of the externally applied magnetic field. From this asymmetry we deduce the strength of the interfacial DMI. Micromagnetic simulations confirm that the observed difference in the wave numbers and the signature of the asymmetry are characteristic for the occurrence of an interfacial DMI at the Pt/Co interface and cannot be explained by the uniaxial perpendicular magnetic anisotropy field originating from the same interface.

Magnetic properties of epitaxial bismuth ferrite-garnet mono- and bilayers

Magnetic properties of Bi1.5Gd1.5Fe4.5Al0.5O12 (84nm) and Bi2.8Y0.2Fe5O12 (180nm) films epitaxially grown on gallium-gadolinium garnet (GGG) single crystal (111) substrate as well as Bi1.5Gd1.5Fe4.5Al0.5O12/Bi2.8Y0.2Fe5O12 bilayer were investigated using ferromagnetic resonance technique. The mismatch of the lattice parameters of substrate and magnetic layers leads to formation of adaptive layers which affect on the high order anisotropy constant of the films but practically do not affect on uniaxial perpendicular magnetic anisotropy The magnetic properties of the bilayer film were explained in supposition of strong exchange coupling between magnetic layers taking into account film-film and film-substrate elastic interaction.

Increased uniaxial perpendicular anisotropy in tetragonally distorted FeCo-Ti-N films

Tetragonally distorted FeCo-Ti-N films were prepared on a Rh(001) underlayer. Uniaxial perpendicular anisotropy was observed for FeCo-Ti-N films with a thickness of 23.5 nm, and the c/a lattice distortion reached 1.08. The anisotropy constant (Ku) was estimated to be a minimum of 0.57 MJ·m−3 (5.7 × 106 ergs·cm−3). These results show that the combination of Ti and N additive elements could be effective for inducing lattice distortion in FeCo-based alloys on a Rh under layer, and that the lattice distortion increases Ku.

ELECTRONIC, STRUCTURAL AND MAGNETIC PROPERTIES OF Co2FeAl THIN FILMS FOR POTENTIAL SPINTRONIC APPLICATIONS

This paper presents an overview concerning the electronic, structural and magnetic properties of Co 2 FeAl (CFA) thin films. We first used ab initio calculations of the electronic structure in order to discuss the half-metallicity of this compound. Involving a correlated structural-magnetic analysis, we then illustrate, experimentally, the effect of the thickness as well as the annealing temperature on the magnetic and structural properties of CFA films epitaxially grown on MgO (001) single crystal substrates. The X-ray diffraction shows that in our samples having the CFA(001)[110]// MgO (001)[100] epitaxial relation, the chemical order is enhanced as the thickness and the annealing temperature (T a ) are increased. Ferromagnetic resonance measurements reveal further dynamic magnetic properties. The gyromagnetic factor, estimated at 29.2 GHz/T, is both thickness and annealing temperature independent. The in-plane anisotropy results from the superposition between a dominant fourfold symmetry term, as expected for cubic crystal symmetry of the alloy, and a small uniaxial term. The fourfold anisotropy decreases with increasing thickness and annealing temperature. The exchange stiffness constant is thickness independent but increases with T a . In addition, the effective magnetization varies linearly with T a and with the inverse CFA thickness. This is due to the presence of perpendicular uniaxial anisotropy, estimated around -1.8 erg/cm2 at T a = 600°C and 1.05 erg/cm2 at T a = 265°C, respectively. Frequency and angular dependences of the FMR linewidth show two magnon scattering and mosaicity contributions which depend on the CFA thickness and T a . A Gilbert damping coefficient as low as 0.0011 is found for samples annealed at 600°C. Finally, we illustrate that these films can be used as ferromagnetic electrodes in sputtered epitaxial magnetic tunnel junctions (MTJ) based on MgO (001) tunnel barriers. These MTJs show an improvable TMR ratio around 95% at room temperature.

Strong Perpendicular Uniaxial Magnetic Anisotropy in Tetragonal Fe<sub>0.5</sub>Co<sub>0.5</sub> Films of Artificially Ordered B2 State

We studied the relationship between the magnetic anisotropy properties and crystal structure of an artificial B2 state of Fe 0.5 Co 0.5 epitaxial films on a Rh(001) seed layer with a tetragonal distortion structure, which exhibited large perpendicular uniaxial magnetic anisotropy over a wide range of thicknesses and c/a ratios at room temperature. We obtained the bulk anisotropy energy of the Fe 0.5 Co 0.5 films, which reached 2.5× 10 7 erg/cm 3 at c/a=1.20. The magnetization easy axis of the Fe 0.5 Co 0.5 films transferred from out-of-plane to in-plane when the thickness was increased from 24 to 30 monolayers. This is due to the lattice relaxation caused by the increase of the total thickness. We observed that the cubic anisotropy component around the in-plane of the films was much smaller than the perpendicular effective magnetic anisotropy, which demonstrates that our films exhibited the uniaxial anisotropy property. Furthermore, we found the bulk anisotropy values of our artificial B2-state Fe 0.5 Co 0.5 to be larger than those of the A2 state under the same lattice distortion. Thus, we suggest that tuning the degree of crystal order of FeCo alloys is an effective way to experimentally enhance their perpendicular uniaxial magnetic anisotropy.

Successful fabrication of high Ku columnar CoPt-SiO2 granular film sputtered under high substrate temperature

High substrate temperature sputtered CoPt-SiO2 granular films with columnar grain structure, high perpendicular uniaxial magnetic anisotropy (Ku⊥), and low stacking faults (SFs) are reported. By introducing a CoCr-SiO2 buffer layer onto a Ru layer, SiO2 segregates to Ru grains boundaries and CoCr grains grow epitaxially onto Ru grains. Consequently, the bumpy surface morphology of the underlayer is maintained even though the temperature of the substrate heating is subsequently elevated to around 400 °C. Therefore, the CoPtCr magnetic grains of the CoPtCr-SiO2 granular recording layer can grow epitaxially with columnar structure upon the CoCr grains of the CoCr-SiO2 buffer layer. For deposition of CoPt-SiO2 (no Cr) granular films at the substrate temperature below 400 °C, the increase of SFs can be suppressed such that Ku⊥ can be kept high at around 6.5 × 106 ergs/cm3 (Kugrain of around 9.0 × 106 ergs/cm3, where Kugrain is referred to uniaxial magneto-crystalline anisotropy of the magnetic grains when oxide material is excluded from the granular layer).

Magnetic Properties of Fe/WRe (001) Multilayers

Following recent theoretical predictions on perpendicular uniaxial magnetocrystalline anisotropy in multilayers of 5 monolayers of Fe and 2 monolayers of W x Re100–x with tetragonally strained components, we have measured the magnetization and detailed atomic structure in a series of such samples with x = 20–80 at%. Although the achieved strain in the WRe layers is smaller than in the modelled case, we do observe a positive uniaxial magnetocrystalline anisotropy of around 0.6 MJ/m3 at 40 K, after accounting for the in-plane shape anisotropy. This causes the out-of-plane saturation field to be reduced considerably compared to a pure Fe thin film, while the saturation magnetization is maintained at a value which is enhanced by at least 30% from what would be expected from the multilayer composition alone. The coercivity is low, on the order of 6 kA/m in all samples.

The role of inhomogeneity of perpendicular anisotropy in magnetic properties of ultra thin CoFeB film

In this work, the hysteresis loops of ultra thin CoFeB films with perpendicular anisotropy are simulated using a micromagnetic model with microstructure included. An inhomogeneous region of the perpendicular uniaxial anisotropy constant K is introduced in the model, and this is the key factor to explain why the in-plane saturation field is several thousand Oersted, while the perpendicular coercivity can be on the order of 100 Oersted. Tendency of the simulated loops with increasing total simulation size reveals that this model is reliable, and the calculated loops basically agree with experiments. The effects of different K distribution, defect sizes, defect patterns, and higher order magnetic anisotropy on simulated M-H loops are also discussed in the paper.

Annealing temperature and thickness dependence of magnetic properties in epitaxial L1-Mn1.4Ga films

Mn1.4Ga films with high perpendicular magnetic anisotropy and high crystalline quality were grown on MgO substrates with Cr buffer layer using molecular beam epitaxy. The crystalline structure and the surface morphology of the films have been systematically investigated as functions of in-situ annealing temperature (Ta) and film thickness. It is found that the magnetic properties can be largely tuned by adjusting Ta. As Ta increases, both saturation magnetization (Ms) and uniaxial perpendicular magnetic anisotropy constant (Ku) increase to the maximum values of 612 emu/cc and 18 Merg/cc at 300 °C, respectively, and then decrease. The morphology also changes with Ta, showing a minimum roughness of 2.2 Å at Ta = 450 °C. On the other hand, as the thickness increases, Ms and Ku increase while coercivity decreases, which indicates there is a magnetic dead layer with a thickness of about 1.5 nm at the interfaces. The detailed examination on the surface morphology of the films with various thicknesses shows a complicated film growth process, which can be understood from the relaxation mechanism of the interfacial strain.

Magnetic anisotropy in nano-crystalline BaFe12O19 thin films deposited under various applied magnetic fields

BaFe12O19 thin films have been deposited on p-type Si (1 0 0) substrate employing pulsed laser deposition technique. The films have been deposited under the magnetic field of 0.15, 0.3, 0.45 and 0.6T applied transverse to the plume, keeping the substrate temperature constant at 300°C. The aim is to investigate the effect of external magnetic fields on the structural, magnetic, morphological and optical properties of the deposited thin films.A perpendicular uniaxial magnetic anisotropy is observed for the films deposited under B=0.15 to 0.6T. The estimated value of anisotropy field has increased with the increase in the applied field. The film deposited under 0.6T, also exhibited perpendicular anisotropy. The perpendicular loop saturation magnetization increases from 168 to 227.5×103A/m as the external field is increased from B=0.15 to 0.6T, respectively. The coercivity of in-plane measurements increases with the increase in the magnetic field. The films have grains of the order of few tens of nano meters on the surface. The thickness and band gap energy of the thin films increases monotonically as the applied field during deposition is increased. The maximum values of thickness and band gap energy achieved under 0.6T, are 128nm and 2.98eV, respectively.

Co2FeAl thin films grown on MgO substrates: Correlation between static, dynamic, and structural properties

Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have been deposited on MgO(001) substrates by magnetron sputtering and then capped by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA axis is normal to the substrate and that all the CFA films exhibit full epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$ phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer measurements show that, depending on the field orientation, one or two-step switchings occur. Moreover, the films present a quadratic MOKE signal increasing with the CFA thickness, due to the increasing chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse susceptibility and torque (TBIIST) measurements reveal that the in-plane anisotropy results from the superposition of a uniaxial and of a fourfold symmetry term. The fourfold anisotropy is in accord with the crystal structure of the samples and is correlated to the biaxial strain and to the chemical order present in the films. In addition, a large negative perpendicular uniaxial anisotropy is observed. Frequency and angular dependences of the FMR linewidth show two magnon scattering and mosaicity contributions, which depend on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found.

On the theory of a supercritical state of ferromagnetic films with uniaxial perpendicular anisotropy and (001) single-crystal films with positive constants of magnetocrystalline anisotropy

A model of the magnetization distribution in thin ferromagnetic films that are in the so-called supercritical state has been considered. The model makes it possible to take into account the real distribution of the magnetization more precisely in ferromagnetic films than open and closed models. The theoretical results have been compared to the experimental data.

Tailoring the magnetic anisotropy of thin film permalloy microstrips by combined shape and induced anisotropies

The sensitivity of magnetic sensors based on the Giant Magneto-Impedance effect can be enhanced using high permeability materials with a well-defined but small transverse anisotropy. We describe an experimental study performed on multilayered, permalloy-based thin films deposited by sputtering under a magnetic field that produces a homogeneous uniaxial anisotropy in the plane of the film. Patterning of the deposit by photolithographic methods into strip-shaped samples (with their long direction perpendicular to the induced anisotropy) establishes a longitudinal shape anisotropy that competes with the transversal one induced at deposition. The combination and competition of the two mutually perpendicular uniaxial anisotropies result in an effective one with a reduced magnitude (the difference between both of them) in the transversal direction. As the strength of the shape anisotropy is determined by the relation between width and length of the stripe, the magnitude of the effective anisotropy can be conveniently modulated by adequately selecting the aspect ratio of the patterned sample. The hysteresis loops measured by Kerr effect magnetometry confirm that the effective transversal anisotropy field can be reduced from 5 to 1 Oe which should concomitantly increase the sensitivity of thin film magneto-impedance sensors.

Magnetic anisotropies in ultrathin bismuth iron garnet films

Ultrathin bismuth iron garnet Bi3Fe5O12 films were grown epitaxially on (001)-oriented gadolinium gallium garnet substrates. Film thickness varied from two to three dozens of unit cells. Bi3Fe5O12 films grow pseudomorphically on substrates up to a thickness of 20nm, and then a lattice relaxation occurs. Magnetic properties of the films were studied as a function of bismuth iron garnet thickness. The magnetization and cubic anisotropy decrease with decreasing film thickness. The uniaxial magnetocrystalline anisotropy is constant for all film thicknesses. For two unit cell thick films, the easy magnetization axis changes from in-plane to perpendicular to the plane direction. Such a reorientation takes place as a result of the competition of constant uniaxial perpendicular anisotropy with weakening film magnetization.

The role of octahedral tilting in the structural phase transition and magnetic anisotropy in SrRuO3 thin film

We present a stoichiometry-dependent structural phase transition in SrRuO3 film on SrTiO3 substrate. The oxygen stoichiometry in the films was varied by changing the oxygen partial pressure P(O2) during the deposition process. For SrRuO3 films with P(O2) ≥ 60 mTorr, they exhibited a pseudo-orthorhombic structure with in-plane uniaxial magnetic anisotropy. On the other hand for films with P(O2) ≤ 45 mTorr, the tetragonal SrRuO3 phase with a perpendicular uniaxial magnetic anisotropy was stabilized at room temperature. The big difference in the magnetic anisotropy of these two SrRuO3 phases was shown to be closely linked to their respective RuO6 octahedral rotation patterns: the RuO6 octahedra rotate differently along the two orthogonal in-plane directions in the pseudo-orthorhombic phase, whereas in the tetragonal phase only octahedral rotations around z-axis are present and the octahedral tilts along the in-plane axes are diminished. First-principles calculations show that such a suppression of the RuO6 octahedra tilting in the tetragonal phase arises from the oxygen vacancies at the octahedral apex (along z-axis). This work demonstrates that the stoichiometry plays an important role in determining the octahedral rotations and tilts in the perovskite materials, which may induce new phases with distinctively different structural symmetry and physical property.