Uniaxial Perpendicular Magnetic Anisotropy Research Articles

Effects of interfacial oxygen diffusion on the magnetic properties and thermal stability of Pd/CoFeB/Pd/Ta heterostructure

We investigated the effects of annealing temperatures (TA) on a Pd (5 nm)/CoFeB (10 nm)/Pd (3 nm)/Ta (10 nm) multilayer structure. The as-deposited sample showed an amorphous state with in-plane uniaxial magnetic anisotropy (UMA), resulting in low coercivity and moderate Gilbert damping constant (α) values. Increasing TA led to crystallization, forming bcc-CoFe (110) crystals, which increased in-plane coercivity and introduced isotropic magnetic anisotropy, slightly reducing the α. The two-fold UMA persists up to 600 °C, and the thermal stability of the in-plane magnetic anisotropy remains intact even TA = 700 °C. The TA significantly influenced the magnetic properties such as in-plane saturation magnetization (Ms//), in-plane and out-of-plane coercivities (Hc// and Hc⊥), and in-plane effective magnetic anisotropy energy density (Keff). Above 600 °C, Keff decreased, indicating a transition towards uniaxial perpendicular magnetic anisotropy. Interfacial oxidation and diffusion from the Ta capping layer to the Pd/CoFeB/Pd interfaces were observed, influencing chemical bonding states. Annealing at 700 °C, reduced oxygen within TaOx through a redox reaction involving Ta crystallization, forming TaB, PdO, and BOx states. Ferromagnetic resonance spectra analysis indicated variations in resonance field (Hr) due to local chemical environments. The α reduction, reaching a minimum at 300 °C annealing, was attributed to reduced structural disorder from inhomogeneities. Tailoring magnetic anisotropy and spin dynamic properties in Pd/CoFeB/Pd/Ta structures through TA-controlled oxygen diffusion/oxidation highlights their potential for SOT, DMI, and magnetic skyrmion-based spintronic devices.

Validity of DFT-based spin-orbit torque calculation for perpendicular magnetic anisotropy in iron thin films

In this study, the validity of our DFT-based self-developed JunPy+SOT calculation method has been rigorously confirmed by the perpendicular magnetic anisotropy (PMA) induced effective magnetic anisotropy field of 5.6 kOe in a 1.5 nm-thick iron thin film. The angular dependence of the magnetic anisotropy energy (MAE) and the total spin-orbit torque (SOT) indicate the uniaxial PMA corresponding to the out-of-plane rotations of the magnetization. Our results agree with the conventional MAE calculation but provide deeper insights into atomistic spin dynamics of local magnetic moments. The main advantage of this method is that the well-decomposed layer-resolved SOT and SOC-dominated spin current accumulation, which exhibit maximum magnitude but opposite sign at the surface, provide comprehensive physical understanding in angular momentum transfer between spin and orbital, competition between SOT and spin current accumulation, and precise spin torque acting on each local magnetic moment that is crucial for the atomistic spin dynamics especially in magnetic heterostructures.

Strong perpendicular magnetic anisotropy in epitaxial D022-Mn3+xGa ultrathin films

Ferrimagnets are highly sought-after materials thanks to their strong perpendicular magnetic anisotropy (PMA) and low magnetization. For example, they are promising materials for current-induced magnetization switching applications. Ferrimagnetic D022-Mn3Ga integrates these properties with a low damping constant and a high Curie temperature. Here, we report a complete analysis of the magnetization and the effective PMA evolution with the thickness of the epitaxial D022-Mn3+xGa ultrathin films. The crystals were grown by magnetron sputtering following two methods: reactive deposition epitaxy and solid phase epitaxy. We found that the uniaxial PMAs and the thickness of the dead magnetic layers differ strongly upon the employed growth method, while saturation magnetizations remain unchanged. Structural defects at the interface due to lattice mismatch and growth method, explain our results. We expose objective estimations of the thermal stability factor Δ, based on the effective PMA in the ultrathin film context. Taking into account our results, D022-Mn3+xGa, in this form, can be employed as the free layer on magnetic tunnel junctions.

Influence of twin-crystal structures on the temperature dependence of magneto-optic Kerr effect and magnetic anisotropy in epitaxial Ni thin films

We report on the enhanced Curie temperature and magnetic anisotropy observed in the temperature dependence of magneto-optic Kerr effect (MOKE) in epitaxial Ni films grown on single-crystal c-cut sapphire Al2O3 (0001) substrates. X-ray diffraction (XRD) revealed the epitaxial growth of Ni (111) on Al2O3 (0001) substrate while XRD azimuthal scans indicated the presence of 60°-rotated twin-crystal structures of fcc Ni in ABC…and ACB… stacking in the Ni/Al2O3 (0001) film. Twin-crystal structures were also revealed from the surface morphology of the Ni (111) layer in the Ni/Al2O3 (0001) film. MOKE hysteresis loops indicated higher Kerr remanence with lower coercivity for the Ni/Al2O3 (0001) film as compared to the polycrystalline Ni film grown on Si (100) substrate under the same conditions. Azimuthal MOKE hysteresis loops confirmed the presence of 2-fold in-plane magnetic anisotropy in the Ni/Al2O3 (0001) film. Temperature dependent MOKE measurements in the range of 300– 680 K revealed a power-law dependence of the Kerr remanence similar to magnetization. The Curie temperature (TC) of the Ni/Al2O3 film was found to be almost 10 K higher than bulk Ni. In- and out-of-plane magnetization revealed a uniaxial perpendicular magnetic anisotropy with the easy axis along the Ni/Al2O3 film plane. The calculated values of the anisotropy constants revealed a high degree of magnetocrystalline anisotropy in the epitaxial Ni/Al2O3 (0001) film. A close correlation of MOKE and magnetization hysteresis loops indicated similar surface and volume magnetic behaviors in the thin films. The enhanced temperature dependent MOKE, magnetocrystalline anisotropy and magnetization could be attributed to the twin-crystal structures in the epitaxial Ni/Al2O3 film.

Investigation of perpendicular magnetic anisotropy in CoFeMnSi based heterostructures

In this work, perpendicular magnetic anisotropy (PMA) is realized in MgAl2O4(MAO)/CoFeMnSi(CFMS)/MgAl2O4/Ti heterostructures annealed at 300 °C and 400 °C. We show that interfacial PMA is very responsive to the annealing temperature and the CFMS layer thickness (tCFMS). A large PMA is achieved for tCFMS = 2.0 nm. An improvement in the out-of-plane saturation magnetization (Ms⊥) with PMA is found to get remarkably stimulated by the bottom MgAl2O4/CFMS interface. Besides, a uniaxial PMA is observed for an as-deposited stack with tCFMS ≤ 1.5 nm. Annealing at 400 °C significantly promoted adequate interfacial oxidation with the improved thermal stability of the heterostructure. Through X-ray photoelectron spectroscopy (XPS), the formation of Co–O bonds occurring as a result of the hybridization of Co-3dZ2 and O-2pZ orbitals at the interface shared by CFMS with MAO is identified as the microscopic origin of the observed PMA in these stacks. These findings indicate that the CFMS and MAO-based structures have immense potential for the next generation spintronic devices.

Intrinsic DMI-free skyrmion formation and robust dynamic behaviors in magnetic hemispherical shells

We performed finite-element micromagnetic simulations to examine the formation of skyrmions without intrinsic Dzyaloshinskii–Moriya interaction (DMI) in magnetic hemispherical shells. We found that curvature-induced DM-like interaction allows for further stabilization of skyrmions without the DMI in curved-geometry hemispherical shells for a specific range of uniaxial perpendicular magnetic anisotropy (PMA) constant Ku. The larger the curvature of the shell, the higher the Ku value required for the formation of the skyrmions. With well-stabilized skyrmions, we also found in-plane gyration modes and azimuthal spin-wave modes as well as an out-of-plane breathing mode, similarly to previously found modes for planar geometries. Furthermore, additional higher-frequency hybrid modes were observed due to coupling between the gyration and azimuthal modes. This work provides further physical insight into the static and dynamic properties of intrinsic DMI-free skyrmions formed in curved-geometry systems.

Ultrafast Magnetization Precession in Perpendicularly Magnetized L10-MnAl Thin Films with Co2MnSi Buffer Layers**Supported by the National Key R&D Program of China (Grant No. 2018YFB0407601), the Key Research Project of Frontier Science of Chinese Academy of Sciences (Grant Nos. QYZDY-SSW-JSC015 and XDPB12), and the National Natural Science Foundation of China (Grant Nos. 11834013, 11874349, and 11774339).

Perpendicularly magnetized L10-MnAl thin films with Co2MnSi buffer layers were prepared on GaAs (001) substrates by molecular-beam epitaxy (MBE). The samples with high crystalline quality show a maximum uniaxial perpendicular magnetic anisotropy constant of 1.4 × 107 erg/cm3. Ultrafast spin dynamics with a magnetization precession frequency up to 200 GHz was investigated by using time-resolved magneto-optical Kerr effect (TRMOKE) measurements, from which the Gilbert damping constant α of epitaxial L10-MnAl thin films is evaluated to be less than 0.0175. This work provides an important reference for analyzing the current-induced magnetization switching process in MnAl-based spintronic devices.

Effect of Electric Field on the Exchange-Stiffness Constant in a Co12Fe72B16 Disk-Shaped Nanomagnet 65 nm in Diameter

Electric field control of magnetism in metallic ferromagnets continues to receive close scrutiny for ultralow-energy spintronic devices. This study uses thermally excited ferromagnetic resonance to investigate the electric field effect on sub-100-nm magnetic tunnel junctions. The characteristic appearance of saturation magnetization, exchange-stiffness constant, and uniaxial perpendicular magnetic anisotropy in the mode-frequency splitting provides an effective means to selectively analyze the electric field effect on the exchange-stiffness constant. This insight is useful for controlling domain size, ${T}_{C}$, and other magnetic properties with an electric field.

Magnetic and structural properties of L10 Mn-Ga epitaxially grown islands

Magnetic and structural properties of L10 Mn53Ga47 islands epitaxially grown onto MgO(1 0 0) substrates are discussed. The samples were fabricated by sputter-deposition at a substrate temperature Ts during deposition. The samples deposited at Ts above 500 °C were found to form the island structure of Volmer-Weber type. The average size of the islands fabricated at Ts = 600 °C is around 200 nm in width and 60 nm in height. The XRD and TEM analyses indicate that those islands are of the L10 phase with the 〈0 0 1〉 axis perpendicular to the substrate surface. The lattice constants a and c are 3.91 ± 0.01 Å and 3.65 ± 0.02 Å, respectively, which are larger by 0.8%, and smaller by 1.5% respectively, as compared to the bulk values of the same composition. The order parameter of these islands is found to increase from 0.7 to 0.8 for Ts = 450–500 °C, and then remains nearly constant (≈0.8) for higher Ts. The M-H hysteresis loops exhibit a high squareness for all the samples for Ts = 450–600 °C. The saturation magnetization Ms is about 220 and 350 emu/cm3 for Ts = 450 and 600 °C, respectively. The out-of-plane coercivity Hc is 5 and 13 kOe for Ts = 450 and 600 °C, respectively. The temperature dependence of Ms for the sample made at Ts = 600 °C was measured and fitted to an empirical relation in order to estimate Tc. The estimated Tc around 600 K is in reasonable agreement with the reported value of bulk L10 Mn53Ga47. The uniaxial perpendicular magnetic anisotropy constant Ku estimated by the out-of-plane torque curves is around 1.1 × 107 and 8.3 × 106 erg/cm3 at 5 and 300 K, respectively. The initial magnetization curves of these islands exhibit a typical pinning mode for the coercivity mechanism. A phenomenological discussion of the coercivity is presented on the basis of Kronmuller’s formula. It is found that the coefficient α and effective demagnetizing factor Neff in the formula by Kronmuller are approximately 0.38 and 12. For the magnetic anisotropy mechanism, the power law relation between Ku(T) and Ms(T) is examined. It is found that the exponent n for Ku(T)∝Ms(T)n varies from 2.3 to 3.5 for T from 350 to 5 K. However, the n in a whole temperature range from 5 to 350 K is 2.6. The n = 2.6 suggests a deviation from the single-ion mechanism, and is consistent with the theoretical predictions of Ga contribution.

Conversion of FeCo from soft to hard magnetic material by lattice engineering and nanopatterning

The development of magnetic materials with large uniaxial magnetic anisotropy (Ku) and high saturation magnetization has attracted much attention in various areas such as high-density magnetic storage, spintronic devices, and permanent magnets. Although FeCo alloys with the body-centred cubic structure exhibit the highest Ms among all transition metal alloys, their low Ku and coercivity (Hc) make them unsuitable for these applications. However, recent first-principles calculations have predicted large Ku for the FeCo films with the body-centred tetragonal structure. In this work, we experimentally investigated the hard magnetic properties and magnetic domain structures of nanopatterned FeCo alloy thin films. As a result, a relatively large value of the perpendicular uniaxial magnetic anisotropy Ku = 2.1 × 106 J·m−3 was obtained, while the Hc of the nanopatterned FeCo layers increased with decreasing dot pattern size. The maximum Hc measured in this study was 4.8 × 105 A·m−1, and the corresponding value of μ0Hc was 0.60 T, where μ0 represented the vacuum permeability.

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.

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.

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).

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.

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.

Perpendicular magnetic anisotropy in Ta/CoFeB/MgO systems synthesized on treated SiN/SiO2 substrates for magnetic memories

Ta(5nm)/CoFeB(1nm)/MgO(2nm) stacks have been deposited by magnetron sputtering on top of 8" Si/SiN/SiO2 substrates, where the effects of different passivation steps related to copper damascene processes have been simulated. It is shown that excellent thickness uniformity of the deposited stack is achieved on 8" areas, and strong uniaxial perpendicular magnetic anisotropy Ku up to 0.5–1×106J/m3 develops in the 1nm CoFeB following thermal annealing at 300°C for 2h, irrespective of the adopted passivation treatment. The CoFeB coercive field varies from 0.21 to 0.51kA/m for substrate surface roughness in the range of 0.45–0.55nm. These results are of relevance in the view of integrating into standard complementary metal-oxide semiconductor process, advanced magnetic memory concepts based on perpendicular magnetic anisotropy in ultrathin layers.

Characterization of coercivity at a recording speed of granular media for thermally assisted recording

As a fundamental study of thermally assisted recording, remanence coercivity at recording speed and the energy barrier for magnetization switching for granular media were evaluated as a function of temperature T. We characterized those for CoPtCr-oxide granular media as a trial examination. Remanence coercivity was measured at measurement time t′ of 1 s (using an electromagnet) and 10−5 s (pulse field); they were defined, respectively, as Hr and HrP for T of 300–473 K. Assuming that the value of t′ at a recording speed is 1 ns and the value of remanence coercivity at the recording speed, Hr(1 ns), was evaluated by fitting Hr and HrP to Sharrock’s equation. Moreover, the values of the energy barrier for magnetization switching, ΔE, were estimated. The value of Hr(1 ns) at T = 573 K, which is a typical temperature for thermally assisted recording, as estimated by extrapolation was about 6.2 kOe for the single medium and 5.5 kOe for the stacked medium. These values were about 1 order of magnitude larger than those of Hr at T = 573 K. Moreover, the reduction ratio of ΔE on increasing T showed good agreement with that of perpendicular uniaxial magnetic anisotropy, Ku. These results indicate that a large Ku reduction on increasing T was necessary to enhance the reduction of recording coercivity at elevated recording temperatures.