Perpendicular Magnetic Films Research Articles

Large enhanced thermal stability of perpendicular magnetic anisotropy films in magnetic tunnel junctions

For magnetic sensor and magnetic random access memory, the thermally tolerant need to be improved at a relatively high operation temperature. In this work, the correlations between the thermal stability and perpendicular magnetic anisotropy films with different heavy metal layers, structures and CoFeB compositions were investigated in details. It was found that the thin films adopting W or Mo heavy metal layer in the bottom structure, and the thin films adopting Mo heavy metal layer in the top structure exhibit a relatively good operation stability, no matter what the CoFeB composition is. Moreover, the thermally robust perpendicular magnetic anisotropy was obtained in the CoFeB with a relatively large Co atomic composition. According to the element distribution states analysis and oxidation conditions at CoFeB/MgO interface, the reasons are attributed to the structural- and composition-dependent Fe-O bonding. This research helps to design and optimize the ultra-low-power and thermally stable magnetic devices based on the suitable heavy metal layers, structures, and CoFeB compositions.

Composition dependence of anomalous Nernst effect in amorphous TbFeCo thin films with perpendicular magnetic anisotropy

In this study, we systematically investigated the anomalous Nernst effect in perpendicularly magnetized amorphous TbFeCo thin films with various compositions. It was found that the magnitude of the off diagonal thermopower (ODT), which corresponds to the anomalous Nernst effect, can be uniformly explained with respect to the Tb content regardless of the concentration above or below the compensation composition. The maximum ODT of 1.3 μV/K and the thermoelectric conductivity of 1.59 A/mK at room temperature were obtained, which is more significant than other perpendicular magnetic anisotropy thin films to achieve a large Nernst voltage for roll-type thermoelectric devices. By considering the thermoelectric tensor, Mott’s equation, and the scaling law, it was shown both experimentally and theoretically that the magnitudes of the first and second terms contributing to the anomalous Nernst effect are comparable. It was also found that the ODT of TbFeCo thin films is twice or more significant than the product of the Seebeck coefficient and the Hall angle. Furthermore, amorphous metals and Mn-alloys with a large Berry curvature are located above the relation that the ODT is twice the product of the Seebeck coefficient and the Hall angle, which means that amorphous metals are expected to enhance the ANE.

Investigation of deterministic and cumulative nature in helicity-dependent optical switching of ferrimagnetic Gd–Fe–Co films

The precise control of magnetization using circularly polarized laser pulses in perpendicularly magnetized films is a key approach to develop storage media. Although deterministic helicity-dependent magnetization switching (HDMS) in a single-pulse excitation has been observed in ferrimagnetic Gd–Fe–Co alloys, ferromagnetic [Co/Pt]N multilayers require cumulative effects of multiple pulses to achieve HDMS. In this study, we systematically investigated the single-shot and cumulative nature of HDMS using ferrimagnetic Gd–Fe–Co films with varying compositions and multilayer structures, in terms of angular momentum compensation, thermal diffusion, mobility of domain walls, and the size of the magneto-optical effect. These results provide insights for the realization of single-shot HDMS in various magnetic systems, such as ferromagnetic alloys.

Anomalous Nernst effect in perpendicularly magnetized τ-MnAl thin films

τ-MnAl is interesting for spintronic applications as a ferromagnet with perpendicular magnetic anisotropy due to its high uniaxial magnetocrystalline anisotropy. Here, we report on the anomalous Nernst effect of sputter deposited τ-MnAl thin films. We demonstrate a robust anomalous Nernst effect at temperatures of 200 and 300 K with a hysteresis similar to the anomalous Hall effect and the magnetization of the material. The anomalous Nernst coefficient of (0.6 ± 0.24) µV/K at 300 K is comparable to other perpendicular magnetic anisotropy thin films. Therefore, τ-MnAl is a promising candidate for spin-caloritronic research.

Field‐Free and Energy‐Efficient Switching of a Ferrimagnetic Insulator Through Orbital Currents of Copper

AbstractElectrically manipulating the magnetization of insulators presents exciting opportunities for fast and energy‐efficient spintronic devices. However, the existing approaches, which rely on spin‐orbit torque (SOT), invariably require an auxiliary field. Here field‐free current‐induced magnetization switching in perpendicularly magnetized Tm3Fe5O12 films is demonstrated. This is achieved through a magnetic hybrid structure, Tm3Fe5O12/Co40Fe40B20/Cu/SiO2, where the Cu layer acts as the source of orbital current, and the in‐plane magnetized Co40Fe40B20 layer functions as the converter of orbital‐to‐spin current. The interplay between the insulating and metallic magnetic layers not only yields a significant anomalous Hall signal for monitoring the Tm3Fe5O12 magnetization states, but also enables field‐free switching that is immune to the magnetic history of the structure. It also observes similar Tm3Fe5O12 switching in stacks with different spin/orbital current sources, with the SOT‐driven switching consuming substantially more power. This work establishes a pathway for achieving energetically efficient all‐electrical manipulation of insulator spins through orbital currents.

Reconfigurable nonreciprocal excitation of propagating exchange spin waves in perpendicularly magnetized yttrium iron garnet thin films

Reconfigurable nonreciprocal excitation of propagating exchange spin waves in perpendicularly magnetized yttrium iron garnet thin films

Magnetization reversal of [Co/Pd] perpendicular magnetic thin film dot on (Bi,La)(Fe,Co)O3 multiferroic thin film by applying electric field

A multilayer structure with a high-quality (Bi,La)(Fe,Co)O3 multiferroic thin film/[Co/Pd] perpendicular magnetic thin film dots was fabricated for demonstrating magnetization reversal of [Co/Pd] dots under an applied electric field. Although the magnetization direction of the multiferroic thin film was reversed under the electric field, the magnetic properties of the multiferroic thin films were generally low. If the multiferroic thin film in this structure can control the magnetization direction of the highly functional magnetic thin film under an electric field, high-performance magnetic devices with low power consumption are easily obtained. The magnetic domain structure of the [Co/Pd] dots fabricated on the (Bi,La)(Fe,Co)O3 thin film was analyzed by magnetic force microscopy (MFM). The structure was de-magnetized before the local electric-field application and magnetized after applying the field, showing reduced magnetic contrast of the dot. The line profile of the MFM image revealed a downward magnetic moment of 75%, which reversed to upward under the local electric field. Magnetic interaction between the (Bi,La)(Fe,Co)O3 and [Co/Pd] layers was also observed in magnetization hysteresis measurements. These results indicate that the magnetization direction of the [Co/Pd] dots was transferred through the magnetization reversal of the (Bi,La)(Fe,Co)O3 layer under a local electric field. That is, the magnetization of [Co/Pd] dots were reversed by applying a local electric field to the multilayer structure. This demonstration can potentially realize high-performance magnetic devices such as large capacity memory with low power consumption.

Perpendicular Magnetic Properties on hcp-Co<sub>80</sub>Pt<sub>20</sub> Stacked Films with Oxide Seed Layers

The influence of oxide seed layers on the magnetic properties and crystalline structures of hcp-Co80Pt20 perpendicular magnetic thin films were investigated for magneto-optical chemical and biological sensing system. Cr-O, Al-doped ZnO (AZO) and Al2O3 films were used as the oxide seed layers. The Cr-O layer was most effective to improve the perpendicular magnetic properties of Co-Pt. The Cr-O seed layer produced the largest values of the interfacial perpendicular magnetic anisotropy (KS) and perpendicular magnetocrystalline anisotropy (KV). The investigation in the crystalline and chemical bonding conditions suggested that the both perpendicular magnetic anisotropies were originated from the interaction of Co and oxide at the Co-Pt/Cr-O interface. The improvements of KS and KV on the Cr-O seed layer are attributed to the hybridization of Co-3d and O-2p orbits and the hcp(001)-CoPt crystal growth with self-orientation, respectively. We also demonstrated the magneto-optical cavity effect on the Co-Pt perpendicular magnetic stacked films with the Cr-O intermediate layer. The stacked films indicated a large magneto-optical response by optical interference and good perpendicular magnetic properties with an ideal square shaped hysteresis loop.

Spin wave diffraction model for perpendicularly magnetized films

We present a near-field diffraction model for spin waves in perpendicularly magnetized films applicable in any geometries of excitation fields. This model relies on Kalinikos–Slavin formalism to express the dynamic susceptibility tensor in k-space and calculate the diffraction patterns via inverse 2D-Fourier transform of the response functions. We show an excellent quantitative agreement between our model and MuMax3 micro-magnetic simulations on two different geometries of antennas. Our method benchmarks spin wave diffraction in perpendicularly magnetized films and is readily applicable for future designs of magnon beamforming and interferometric devices.

Real-time probing technique of domain wall dynamic in perpendicularly magnetized film

We present the study of a rarely mentioned method for measuring the magnetic domain wall velocity, which makes it possible to have a real-time probing of the domain wall movement in the perpendicularly magnetized thin film. We have compared this technique in detail with the most common Kerr imaging method. The comparison results show interesting differences if the spot size is too small. It can be explained by the dendritic shape of the domain wall. By changing the size spot, we propose a basic model that describes quite well the transit time in the laser spot as a function of its size and makes it possible to extract the velocity and depth of the dendrites. By generalizing our method, it helps people to understand magnetic domain wall dynamics from the temporal dimension and helps the academic community to obtain intrinsic domain wall motion characteristics in the film sample, ultimately promoting the development of spintronic devices.

Magnetic Field Magnitudes Needed for Skyrmion Generation in a General Perpendicularly Magnetized Film.

Due to its topological protection, the magnetic skyrmion has been intensively studied for both fundamental aspects and spintronics applications. However, despite recent advancements in skyrmion research, the deterministic creation of isolated skyrmions in a generic perpendicularly magnetized film is still one of the most essential and challenging techniques. Here, we present a method to create magnetic skyrmions in typical perpendicular magnetic anisotropy (PMA) films by applying a magnetic field pulse and a method to determine the magnitude of the required external magnetic fields. Furthermore, to demonstrate the usefulness of this result for future skyrmion research, we also experimentally study the PMA dependence on the minimum size of skyrmions. Although field-driven skyrmion generation is unsuitable for device application, this result can provide an easier approach for obtaining isolated skyrmions, making skyrmion-based research more accessible.

Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin–orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 °C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co– and Fe–O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.

Interfacial roughness driven manipulation of magnetic anisotropy and coercivity in ultrathin thulium iron garnet films

We have investigated the magnetic properties of thulium iron garnet (Tm3Fe5O12, TmIG) thin films grown on {111} gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates fabricated via the facing target sputtering method. The annealing temperature (Ta), thickness (tf), and roughness were systematically controlled in order to obtain the perfect magnetic properties of the film showing perpendicular magnetic anisotropy (PMA) and large coercive field, which are the requisites for non-volatile magnetic memory device applications. Here, we demonstrated that the Ta acts as the crucial parameter both for obtaining the high quality of the film surface and engineering the direction of the magnetic anisotropy from the in-plane to out-of-plane. Through the further optimization process with a function of the tf, we successfully achieved the best condition for the PMA films. Specifically, the sample fabricated at a tf of ≤ 20 nm and Ta of 1000 ℃ showed the perfect PMA. Furthermore, the surface roughness of the substrate mainly contributes to the enhancement of the coercive field of the perpendicular magnetic anisotropic films. We speculate that the drag force from the pinning energy induced by the interfacial roughness retarded the domain wall motion at the TmIG/GGG interface, resulting in enhanced coercivity.

Polarization transformation and destructive interference on subwavelength magnetic domains in magneto-plasmonic systems

We demonstrate magneto-optical (MO) polarization transformation due to surface plasmons in CoPt perpendicular magnetic films in the polar Kerr geometry. An extraordinary Kerr rotation angle (θK = ± 88.9°) that almost reaches the upper limit of polarization is produced in the attenuated total reflection (Kretschmann) configuration. P-polarized incident radiation is almost transformed upon reflection to s-polarized radiation, which may be out of phase depending on whether the magnetization of CoPt is up or down. Moreover, the reflected intensity may be drastically modulated by applying an external magnetic field. The reflectivity goes almost to zero in the demagnetized state and increases with increasing external magnetic field. This drastic optical response is attributed to the MO destructive interference produced by the subwavelength magnetic domain structure.

Mo-based perpendicularly magnetized thin films with low damping for fast and low-power consumption magnetic memory

Mo-based perpendicularly magnetized thin films with low damping for fast and low-power consumption magnetic memory

Polarized neutron reflectometry characterization of perpendicular magnetized Ho3Fe5O12 films with efficient spin-orbit torque induced switching

Rare-earth iron garnet films with perpendicular magnetic anisotropy (PMA) are important to develop magnon-based spintronic devices. In this study, high quality epitaxial Ho3Fe5O12 (HoIG) films with PMA are fabricated on (111) Y3(Sc2Ga3)O12 substrates by sputtering. The magnetization compensation temperature of HoIG films is determined to be 130 K, highly consistent with its bulk value. The magnetic dead layer at the interface is quantified as thin as about 0.6 nm through polarized neutron reflectivity. Furthermore, spin–orbit torque switching in HoIG/Pt films is realized with the threshold current density of 1.4 × 1011 A/m2 and assisting field as small as 10 Oe. These results provide one more alternative for spintronic materials using compensated ferrimagnetic insulators.

Switching between Magnetic Bloch and Néel Domain Walls with Anisotropy Modulations.

It has been shown previously that the presence of a Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films stabilizes Néel type domain walls. We demonstrate, using micromagnetic simulations and analytical modeling, that the presence of a uniaxial in plane magnetic anisotropy can also lead to the formation of Néel walls in the absence of a Dzyaloshinskii-Moriya interaction. It is possible to abruptly switch between Bloch and Néel walls via a small modulation of the in plane, but also the perpendicular, magnetic anisotropy. This opens up a route toward electric field control of the domain wall type with small applied voltages through electric field controlled anisotropies.

Defect-correlated skyrmions and controllable generation in perpendicularly magnetized CoFeB ultrathin films

Skyrmions have attracted significant interest due to their topological spin structures and fascinating physical features. The skyrmion phase arises in materials with a Dzyaloshinskii–Moriya interaction at interfaces or in volume of non-centrosymmetric materials. Although skyrmions have been demonstrated experimentally, the general critical intrinsic relationship among fabrication, microstructures, magnetization, and the existence of skyrmions remains to be established. Here, two series of CoFeB ultrathin films with controlled atomic scale structures are employed to reveal this relationship. The amount of defects was artificially tuned by inverting the growth order, and skyrmions were shown to be preferentially formed in samples with more defects. By utilizing first-order reversal curves, the stable region and the skyrmion densities can be efficiently controlled in the return magnetization loops. These findings establish a general internal link from sample preparation to skyrmion generation and provide a general method for controlling skyrmion density.

Controllable field-free switching of perpendicular magnetization through bulk spin-orbit torque in symmetry-broken ferromagnetic films

Programmable magnetic field-free manipulation of perpendicular magnetization switching is essential for the development of ultralow-power spintronic devices. However, the magnetization in a centrosymmetric single-layer ferromagnetic film cannot be switched directly by passing an electrical current in itself. Here, we demonstrate a repeatable bulk spin-orbit torque (SOT) switching of the perpendicularly magnetized CoPt alloy single-layer films by introducing a composition gradient in the thickness direction to break the inversion symmetry. Experimental results reveal that the bulk SOT-induced effective field on the domain walls leads to the domain walls motion and magnetization switching. Moreover, magnetic field-free perpendicular magnetization switching caused by SOT and its switching polarity (clockwise or counterclockwise) can be reversibly controlled in the IrMn/Co/Ru/CoPt heterojunctions based on the exchange bias and interlayer exchange coupling. This unique composition gradient approach accompanied with electrically controllable SOT magnetization switching provides a promising strategy to access energy-efficient control of memory and logic devices.

Magneto-optical surface plasmon resonances on perpendicular magnetic thin films consisting of CoPt/ZnO/Ag stacked nanolayers

Magneto-plasmonic phenomena on CoPt/ZnO/Ag stacked nanolayers were investigated in polar Kerr excitation geometry. The nanolayer displays an ideal square-shaped out-of-plane magnetic hysteresis loop with a large polar Kerr activity. The surface plasmon resonances (SPRs) induce drastic magneto-optical (MO) responses with a narrow linewidth and a sharp reversal of the Kerr polarity. The CoPt/ZnO/Ag nanolayer is a candidate for high-performance chemical sensor elements. For example, an MO‒SPR sensor with a Pd surface layer was applied for hydrogen detection. The Pd layer acts as a transducer for both hydrogen detection and plasmon response. Substantial MO responses to the exposure of hydrogen gas can be observed. The results show that the polarized sensing signal is not affected by the intensity of the incident light. The use of the new type of MO‒SPR element with polar Kerr activity leads to a stable chemical sensing system with a simple measurement configuration.