Uniaxial Perpendicular Anisotropy Research Articles

Temperature dependent interfacial-coupling and magnetization reversal of La2/3Sr1/3MnO3 film with inverted hysteresis features

Temperature-dependent magnetic anisotropy and interfacial coupling of La2/3Sr1/3MnO3 (LSMO) epitaxial films with inverted hysteresis (IHLs) features are investigated using a broadband ferromagnetic resonance (FMR) spectrometer. Through the dispersion relation obtained along [100], [110] and [010] axes, a simplified method is proposed to investigation in-plane magnetic anisotropy by broadband FMR in a cryostat. According to this method, the perpendicular uniaxial anisotropy, magneto-crystalline anisotropy, and interfacial coupling between the LSMO film and hard-magnetic interface layer are investigated as a whole. The measured value of interfacial coupling is −17.6 Oe at 300 K, close to the bias field found in minor loop, i.e., −18.4 Oe. The interfacial coupling is enhanced with the dropping of temperatures, and the value of interfacial coupling changed to −112 Oe at 10 K. A Stoner-Wohlfarth-like model is employed to calculate the hysteresis loops of the films based on the measured values of in-plane magnetic anisotropy and interfacial-coupling. The calculation reveals the mechanism of magnetization reversal and the origin of IHLs found in LSMO films.

Skyrmionium dynamics on a racetrack in the presence of a magnetic defect

Exotic topological textures known as skyrmioniums can stabilize in different types of ferromagnetic nanostructures. They possess a topological charge of Q=0 and can move in straight lines unaffected by the skyrmion Hall effect. In this work, using micromagnetic simulations, we study the behavior of a skyrmionium when a polarized spin current transports it along a racetrack in the presence of a semicircular magnetic defect with different Dzyaloshinskii–Moriya interaction values. As a first step, we obtained the perpendicular uniaxial anisotropy (Kz) and the Dzyaloshinskii–Moriya constants (Dint) for which it is possible to form a skyrmionium in the racetrack. Our results show that the higher the Kz value, the higher the Dint values necessary to stabilize the skyrmionium. Furthermore, interaction energies between the skyrmionium and the semicircular defect are presented. The results show that the interaction energy can be attractive or repulsive, depending on the Dzyaloshinskii–Moriya defect interaction constant (Dintdef). Finally, we conclude that using these types of defects and a spin-polarized current, the motion of the skyrmionium can be either modified in a specific way or even blocked.

Magnetocaloric effect at the reorientation of the magnetization in ferromagnetic multilayers with perpendicular anisotropy

We investigate the magnetocaloric effect obtained by the rotation of a magnetic field applied to an exchange-coupled multilayer system composed of two different ferromagnetic (FM) materials. We specifically consider a system in which the two FMs have perpendicular uniaxial anisotropy axes and utilize conditions which yield a reorientation of the total magnetization when compensation between the anisotropies of the two layers occurs. We calculate the consequent entropy change associated with the “artificial” reorientation. By using known parameters from MnBi and Co we predict an entropy change of Δs=0.34 J kg−1 K−1 for perfect coupling. Lastly, we study the behavior of the multilayer under a rotating magnetic field via a micromagnetic model. When the layer thicknesses are of the order of the local domain wall width, the magnetic field-induced entropy change can be obtained with magnetic fields one order of magnitude lower than in the uncoupled case.

Spin reorientation induced large spin memory loss at Py/Pd interface

Achieving spin current switching functionality is crucial for the development next-generation low power information storage. In this study, the spin reorientation and temperature dependence of spin Hall angle θSH in the Permalloy (Py)/Pd bilayer were investigated by using ferromagnetic resonance, spin pumping, inverse spin Hall effect, and quantum interference transport. The uniaxial ferromagnetic perpendicular magnetic anisotropy (PMA) induced by spin reorientation persists at the Py/Pd interface below 30 K. This PMA further enhances the interfacial spin scattering, leading to a reduction of injected spin current, as indicated by the underestimated θSH values. These experimental results demonstrate that the interfacial spin reorientation at the ferromagnet/heavy metal interface, commonly employed in spintronic devices, causes a significant spin memory loss effect. Our findings provide valuable insights into the influence of interlayer spin configuration on spin transport, which can be utilized in the rational design of spintronic devices based on pure spin current.

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.

Twofold and fourfold anisotropies in zinc ferrite thin films investigated by ferromagnetic resonance

By means of the ferromagnetic resonance (FMR) technique (at the $X$ band) we investigated the magnetic properties of ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ (ZFO) films grown on different substrates. Angular dependence of the FMR field showed the existence of twofold and fourfold anisotropies for the same material, grown on Si(111), MgO(100), and STO(100), respectively. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization ${M}_{\mathrm{eff}}$, cubic magnetocrystalline anisotropy field, in-plane and out of plane uniaxial anisotropy fields, and the $g$ factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/${\mathrm{SrTiO}}_{3}$ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, on the order of tens of oersteds. The FMR linewidth ($\mathrm{\ensuremath{\Delta}}H$) fitted from FMR data shows $\mathrm{\ensuremath{\Delta}}H$ values above 190 Oe for all samples, indicating high damping of this material to be $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ for the ZFO/MgO. The present analysis compared the FMR data obtained for the in-plane and out of plane measurements. Likewise, the effective magnetization and perpendicular uniaxial anisotropy field by FMR were compared with the magnetic response obtained by a vibrating sample magnetometer.

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.

Limitations of the macrospin approximation of materials with inhomogeneous perpendicular anisotropy

Experimental investigations of the perpendicular anisotropy in thin films that are interpreted using a macrospin model often require the inclusion of a higher-order anisotropy contribution. However, recent ferromagnetic resonance experiments on [Co/Ni]N multilayers indicate that the macrospin model cannot explain the full angular dependence in this system. Using micromagnetic simulations of a system with lateral variations of the second-order perpendicular uniaxial anisotropy, we show that while the macrospin model is able to capture the average properties of the system along high symmetry orientations by including a higher-order anisotropy, the model fails to reproduce the full angle dependence. Our studies provide another indication that higher-order anisotropies may not be intrinsic to these systems but instead may be caused by inhomogeneities.

Influence of interfacial magnetic ordering and field-cooling effect on perpendicular exchange bias and magnetoresistance in nanoporous IrMn/[Co/Pd] films

We have studied systematically the effect of field cooling on the magnetic properties of continuous and porous IrMn/[Co/Pd] films. It is found that the coexistence of two ferromagnetic (FM) phases in the porous film, namely, hard-magnetic and soft-magnetic ones, with significantly different magnetic properties relates to the role of pore edges and modifies its magnetic and magnetoresistive properties. It is shown that annealing of the films with their subsequent cooling in an external magnetic field applied for aligning the magnetic moments in the antiferromagnetic (AFM) IrMn layer improves effectively the uniaxial perpendicular anisotropy of the [Co/Pd] layer and induces unidirectional anisotropy in its hard-magnetic regions, blocking simultaneously the soft-magnetic parts by pinning their magnetic moments along the film plane. Magnetoresistance of both continuous and porous films is found to be determined mainly by electron–magnon scattering, whereas the complex morphology of the porous film providing different orientations of exchange coupling at the AFM/FM interface in different film regions modifies significantly the spin-dependent electron transport. The revealed asymmetry of the field dependences of magnetoresistance is attributed both to unidirectional magnetic anisotropy of the FM layer and its splitting into magnetically nonequivalent regions in the porous films. The origin of the observed phenomenon is associated with a local influence on the orientation of AFM magnetic moments by an adjacent ferromagnet.

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.

Creating skyrmions and skyrmioniums using oscillating perpendicular magnetic fields

Magnetic skyrmions and skyrmioniums are exotic magnetic configurations that have several potential applications in Spintronics. In this work, using micromagnetic simulations, we show that it is possible to create metastable skyrmions and skyrmioniums in an isolated cobalt nanodisk, that has as its minimum energy configuration a single domain with perpendicular magnetization. First, we have determined the ground state of the nanodisk and the frequencies of the spin wave modes of skyrmions and skyrmioniums for different values of perpendicular uniaxial anisotropy constant Kz and Dzyaloshinskii-Moriya exchange constant Dint. Next, in order to create a skyrmion or skyrmionium, we have applied an oscillating perpendicular magnetic field with a frequency equal to that of the spin wave modes. Our results show that it is possible to switch from single domain to skyrmion or skyrmionium, tuning parameters such as the intensity of the applied perpendicular magnetic field, and its duration.

Temperature dependence of magnetic anisotropy in heavily Fe-doped ferromagnetic semiconductor (Ga,Fe)Sb

We study the temperature dependence of magnetic anisotropy (MA) of ferromagnetic semiconductor (Ga0.7,Fe0.3)Sb thin films with thicknesses of 20 nm and 40 nm using ferromagnetic resonance. With decreasing temperature from 300 to 10 K, the easy magnetization axis of the 40-nm-thick film changes from in-plane to perpendicular at 250 K, while that of the 20-nm-thick film lies in the film plane at all measurement temperatures (10–300 K). In the 40-nm-thick film, the uniaxial perpendicular anisotropy (K2⊥) significantly increases with decreasing temperature and surpasses the thin-film shape anisotropy, leading to perpendicular magnetic anisotropy (PMA). We present a possible scenario that the increase in K2⊥ originates from the formation of the Fe-rich nanocolumns with the perpendicular shape anisotropy, which induces the PMA in the whole film at low temperatures due to carrier-mediated exchange interactions between Fe spin in the Fe-rich nanocolumns and Fe spin in the (Ga,Fe)Sb matrix. Our findings on the temperature dependence of MA of heavily Fe-doped (Ga,Fe)Sb provide insights into the mechanism of high-Curie-temperature ferromagnetism.

Influence of the dipolar interaction in the creation of skyrmions in coupled nanodisks

Abstract In the present work, using micromagnetic simulation, we show that the dipolar magnetic interaction effect plays a very important role in the process of creation of skyrmions in a coupled system of nanodisks. First, we have determined the magnetic ground state in an isolated nanodisk for different values of perpendicular uniaxial anisotropy (PUA) and Dzyaloshinskii-Moriya interaction (DMI). Next, we have applied perpendicular pulse polarized current densities (J) and found that it is possible to create a metastable Neel skyrmion from a nanodisk whose ground state is a single magnetic domain. From these results, we obtained a phase diagram of polarized current intensity vs. time of application of the current pulses, for different values of parameters such as PUA, DMI, and distance between the nanodisks. Our results show that, depending on the separation distance between the nanodisks, the current density required to create a skyrmion changes, due to the dipolar magnetic interaction.

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.

Writing and storing information in an array of magnetic vortex nanodisks using their azimuthal modes

The switching of a vortex core of a single disk in an array of a multilayer system is investigated by micromagnetic simulation. We found that the perpendicular uniaxial anisotropy decreases the frequencies of the azimuthal mode in disks with magnetic vortex configuration. We obtained a phase diagram of magnetic field intensity vs. frequency of the azimuthal mode, as a function of the value of perpendicular uniaxial anisotropy. We demonstrated that rotating magnetic fields (CW and CCW) with frequency equal to azimuthal modes can be used to switch the vortex core of single disks in a disk array. This allows obtaining different memory states with a single array of nanodisks, and therefore writing information through the application of rotating fields.

Temperature Dependence of Magnetic Properties of a Ultrathin Yttrium-Iron Garnet Film Grown by Liquid Phase Epitaxy: Effect of a Pt Overlayer

Liquid phase epitaxy of an 18 nm thick yttrium-iron garnet (YIG) film is achieved. Its magnetic properties are investigated in the 100–400 K temperature range, as well as the influence of a 3 nm thick Pt overlayer on them. The saturation magnetization and the magnetocrystalline cubic anisotropy of the bare YIG film behave similarly to bulk YIG. A damping parameter of only a few $10^{-4}$ is measured, together with a low inhomogeneous contribution to the ferromagnetic resonance linewidth. The magnetic relaxation increases upon decreasing temperature, which can be partly ascribed to impurity relaxation mechanisms. While it does not change its cubic anisotropy, the Pt capping strongly affects the uniaxial perpendicular anisotropy of the YIG film, in particular at low temperatures. The interfacial coupling in the YIG/Pt heterostructure is also revealed by an increase of the linewidth, which substantially grows by lowering the temperature.

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.

Reduced Models for Ferromagnetic Thin Films with Periodic Surface Roughness

We investigate the influence of periodic surface roughness in thin ferromagnetic films on shape anisotropy and magnetization behavior inside the ferromagnet. Starting from the full micromagnetic energy and using methods of homogenization and Gamma -convergence, we derive a two-dimensional local reduced model. Investigation of this model provides an insight into the formation mechanism of perpendicular magnetic anisotropy and uniaxial anisotropy with an arbitrary preferred direction of magnetization.

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.