Weak Perpendicular Magnetic Anisotropy Research Articles

Tailoring Magnetic Anisotropy in Ultrathin Cobalt by Surface Carbon Chemistry

AbstractThe ability to manipulate magnetic anisotropy is essential for magnetic sensing and storage tools. Surface carbon species offer cost‐effective alternatives to metal‐oxide and noble metal capping layers, inducing perpendicular magnetic anisotropy in ultrathin ferromagnetic films. Here, the different mechanisms by which the magnetism in a few‐layer‐thick Co thin film is modified upon adsorption of carbon monoxide (CO), dispersed carbon, and graphene are elucidated. Using X‐ray microscopy with chemical and magnetic sensitivity, the in‐plane to out‐of‐plane spin reorientation transition in cobalt is monitored during the accumulation of surface carbon up to the formation of graphene. Complementary magneto‐optical measurements show weak perpendicular magnetic anisotropy (PMA) at room temperature for dispersed carbon on Co, while graphene‐covered cobalt exhibits a significant out‐of‐plane coercive field. Density‐functional theory (DFT) calculations show that going from CO/Co to C/Co and to graphene/Co, the magnetocrystalline and magnetostatic anisotropies combined promote out‐of‐plane magnetization. Anisotropy energies weakly depend on carbidic species coverage. Instead, the evolution of the carbon chemical state from carbidic to graphitic is accompanied by an exponential increase in the characteristic domain size, controlled by the magnetic anisotropy energy. Beyond providing a basic understanding of the carbon‐ferromagnet interfaces, this study presents a sustainable approach to tailor magnetic anisotropy in ultrathin ferromagnetic films.

Highly Tunable Perpendicular Magnetic Anisotropy and Anisotropic Magnetoresistance in Ru-doped La0.67Sr0.33MnO3 Epitaxial Films

Abstract As a prototypical half-metallic ferromagnet, La 0.67 Sr 0.33 MnO 3 (LSMO) has been extensively studied due to its versatile physical properties and great potential in spintronic applications. However, the weak perpendicular magnetic anisotropy (PMA) limits the controllability and detection of magnetism in LSMO, thus hindering the realization of oxide-based spintronic devices with low energy consumption and high integration level. Motivated by this challenge, we developed an experimental approach to enhance the PMA of LSMO epitaxial films. By cooperatively introducing 4 d Ru doping and a moderate compressive strain, the maximum uniaxial magnetic anisotropy in Ru-doped LSMO can reach 3.0×10 5 J/m 3 at 10 K. Furthermore, we also found a significant anisotropic magnetoresistance effect in these Ru-doped LSMO films, which is dominated by the strong PMA. Our findings offer an effective pathway to harness and detect the orientations of magnetic moments in LSMO films, thus promoting the feasibility of oxide-based spintronic devices, such as in spin valves and magnetic tunnel junctions.

Highly Tunable Perpendicular Magnetic Anisotropy and Anisotropic Magnetoresistance in Ru-Doped La0.67Sr0.33MnO3 Epitaxial Films

As a prototypical half-metallic ferromagnet, La0.67Sr0.33MnO3 (LSMO) has been extensively studied due to its versatile physical properties and great potential in spintronic applications. However, the weak perpendicular magnetic anisotropy (PMA) limits the controllability and detection of magnetism in LSMO, thus hindering the realization of oxide-based spintronic devices with low energy consumption and high integration level. Motivated by this challenge, we develop an experimental approach to enhance the PMA of LSMO epitaxial films. By cooperatively introducing 4d Ru doping and a moderate compressive strain, the maximum uniaxial magnetic anisotropy in Ru-doped LSMO can reach 3.0 × 105 J/m3 at 10 K. Furthermore, we find a significant anisotropic magnetoresistance effect in these Ru-doped LSMO films, which is dominated by the strong PMA. Our findings offer an effective pathway to harness and detect the orientations of magnetic moments in LSMO films, thus promoting the feasibility of oxide-based spintronic devices, such as spin valves and magnetic tunnel junctions.

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation.

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr1+ x Te2 with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr1.53 Te2 with a Curie temperature of 295K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106nΩ cm is observed at 290K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.

Magnetic anisotropy of the van der Waals ferromagnet Cr2Ge2Te6 studied by angular-dependent x-ray magnetic circular dichroism

The van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr ${L}_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be $3+$ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin ${t}_{2g}^{3}$ configuration of the ${\mathrm{Cr}}^{3+}$ ion. A large ($\ensuremath{\sim}0.2$ eV) trigonal crystal-field splitting of the ${t}_{2g}$ level caused by the distortion of the $\mathrm{Cr}{\mathrm{Te}}_{6}$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign opposite to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of ${\mathrm{CrI}}_{3}$.

Spin–orbit torques of an in-plane magnetized system modulated by the spin transport in the ferromagnetic Co layer

Spin–orbit torque (SOT) magnetoresistive random-access memory (MRAM) devices have been proposed for energy efficient memory and computing applications. New classes of materials such as antiferromagnets, topological insulators, and semimetals can generate spins with unconventional polarization and improve the efficiency of field-free SOT switching. In this work, we report significant changes in SOTs due to a Co thin film inserted in the Pt/Co/Mg/CoFeB heterostructures. Remarkably, the damping-like effective field has been enhanced by 7.4 times after inserting a thin Co layer with weak perpendicular magnetic anisotropy (PMA), while the field-like effective field is reduced to near zero value. Independent characterizations were performed to verify the presence of the changes in SOTs following spin modulation by the Co insertion layer. In addition, we found that the dynamic spin pumping coupling between Pt/Co with weak PMA and the in-plane CoFeB could significantly modulate the effective SOTs in the heterostructure, and this effect is dependent on the thickness of the spacer Mg through long-range spin-wave mediated coupling. Our work has experimentally demonstrated a new avenue to modulate SOTs with physically sputtered metal layers, and this finding is promising to enable flexible and efficient spin polarizations for MRAM devices.

Magnetic textures and singularities in ferri/ferromagnetic multilayers

The stacking of ferrimagnetic and ferromagnetic films leads to a competition between magnetic interactions that can stabilize Bloch points and other singularities at the interfaces. In this work, GdxCo1-x/NdCo5/GdyCo1-y trilayers, with different thicknesses, have been prepared. By tuning the stoichiometry of the Gd-Co outer layers, the Co magnetic sublattice dominates in one layer, whereas the opposite side is Gd dominated. In this way, in-depth magnetization domain walls can be created due to the balance between exchange and magnetostatic interactions. In addition, the weak perpendicular magnetic anisotropy of the Nd-Co central layer induces a stripe domain pattern, supporting the formation of Bloch points, meron-like textures, and vortex-antivortex pairs. The interaction between both types of structures, in-depth domain walls and magnetic singularities, has been studied by a combination of macroscopic Magneto-Optical Kerr Effect and Vibrating Sample Magnetometry measurements with microscopic Magnetic Force Microscopy and element selective magnetic Transmission soft X-ray Microscopy imaging. The results confirm marked changes in the magnetic properties of the trilayers, as compared with those single NdCo5 films, and the formation of complex magnetic textures, associated with partial magnetization reversal, where magnetic singularities are formed.

Magnetic switching behavior of each magnetic layer in perpendicular magnetic tunnel junctions

Magnetization of the reference layer (RL) in a normal magnetic tunnel junction (MTJ) has to be fixed below the switching field of the free layer. However, in the fabrication of magnetic random access memory (MRAM) chip consisting of huge number of MTJs, there often exist a few poor MTJ devices which show an abnormal magnetoresistance (RH) behavior. To eliminate chip fails and reduce bit error rate, in this paper magnetic switching behavior of each layer in perpendicular MTJs was studied through detailed RH measurements as a function of maximum applied field (Hmax). Switching behavior of each layer is evaluated through detailed analysis on the magnetostatic interactions of all magnetic layers in MTJs. As a result, three types of abnormal magnetic behaviors of RL and their related physical mechanisms were explored: 1), magnetization of RL tilts in elevated fields due to weak perpendicular magnetic anisotropy (PMA) below the switching field of free layer; 2), RL switches at a field below switching field of the free layer due to weak exchange coupling field (Hex) in the synthetic antiferromagnetic (SAF) structure; 3), RL switch takes place due to so-called flip flop behavior in the SAF structure. Relevant methods to overcome these abnormal magnetic switching behaviors are suggested and used in our successful fabrication of MRAM chips.

Ferrimagnetic insulators for spintronics: Beyond garnets

Ferrimagnetic insulators have gained much attention as material platforms with efficient magnetization dynamics. To date, epitaxial iron garnet thin films are the most widely used materials in the emerging field of “insulator spintronics.” However, further advances in this field require overcoming the disadvantages of garnets—e.g., their complex structure, high growth temperature, incompatibility with other crystalline materials, and relatively weak perpendicular magnetic anisotropy. In this Perspective, we make the case that epitaxial thin films of spinel ferrites and hexagonal ferrites are viable materials for insulator spintronics with complementary advantages over the oft-used garnets. Specifically, spinel ferrites have a simpler structure, can crystallize at lower temperatures, and are more amenable to coherent integration with various materials; hexagonal ferrites possess enormous perpendicular anisotropy of bulk origin, in contrast to garnets where the strength of anisotropy is restricted by interfacial strain. The expanded repertoire of materials for insulator spintronics will enable new physical insights and potential applications, beyond what is currently possible with garnets.

Temperature assistance of electric field-controlled spin-orbit torque-based magnetization switching in PMN-PT/FePt heterostructures.

We report the temperature assistance of electric field (E-field)-controlled spin–orbit torque (SOT)-based magnetization switching of L10-FePt films grown on a PbMg1/3Nb2/3O3–PbTiO3 (PMN–PT) (011) substrate, which generates considerable strain via piezoelectric effects of the PMN–PT substrate under E-field. Owing to large strain-induced effective field and weak perpendicular magnetic anisotropy (PMA) at a high temperature, E-field controls the PMA- and SOT-based magnetization switching more effectively. Driven by E-field, magnetization switching is detected by a magnetic optical Kerr (MOKE) microscope under a fixed perpendicular magnetic field. Furthermore, E-field modulates change of anomalous Hall resistance regularly, which enables us to achieve the bidirectional transmission of data by designing an E-field controlled SOT-based logical circuit. This study indicates an efficient way to fabricate potential E-field-controlled spintronic applications at high temperatures.

Effect of Mo monoatomic interlayer on magnetic properties of in-plane anisotropic Nd-Fe-B/Mo/FeCo nanocomposite multilayered films

The Nd-Fe-B/Mo/Fe-Co multilayered films on MgO(001) were fabricated by UHV sputtering method. The as-deposited films showed soft magnetic properties with weak perpendicular magnetic anisotropy, and after annealing above 650°C, these films showed in-plane magnetic anisotropy, good squareness, and hard magnetic properties with high coercivity of about 8 kOe. The (220) peak of Nd2Fe14B was identified in the XRD pattern after annealing of the film, which means that the c-axis, which is the easy axis of magnetization, points in the plane of the film, consistent with the measurement of the magnetization curve. The positive peak of ΔM plot of in-plane direction of the film annealed at 800°C was shown. This result suggests existence of exchange coupling between the Nd-Fe-B and Fe-Co layers in the in-plane anisotropic multilayered film with monoatomic Mo interlayer.

Tunable spin–orbit torque switching in antiferromagnetically coupled CoFeB/Ta/CoFeB

We demonstrate a tunable spin–orbit torque (SOT) switching in an antiferromagnetically coupled CoFeB/Ta/CoFeB trilayer through careful design of magnetic anisotropies, where the thicker bottom CoFeB layer has a relatively weak perpendicular magnetic anisotropy (PMA) and PMA of the upper CoFeB layer is robust. The unique anisotropy feature causes a strong sensitivity to the assistant field during SOT switching, resulting in a slanted magnetic moment and a highly tunable switching window defined by the difference in Hall resistance between two SOT switching states at zero current. By further reducing the PMA of bottom CoFeB, only the upper layer can be switched by SOT. Until now, there are three types of SOT switching in antiferromagnetically coupled ferromagnetic metal/nonmagnetic metal/ferromagnetic metal trilayers: simultaneous switching between two antiparallel states with strong PMA for both ferromagnetic layers, field-free switching where one layer has in-plane magnetization and the other has canted out-of-plane magnetization, and tunable switching reported in this work. Our findings enrich the physical phenomena in trilayer SOT and may have potential application in multilevel storage and neural computing.

Modulation of thermal stability and spin–orbit torque in IrMn/CoFeB/MgO structures through atom thick W insertion

Antiferromagnet (AFM)/ferromagnet (FM) systems such as IrMn/CoFeB/MgO enable spin–orbit-torque- (SOT-) induced switching of perpendicular magnetization in the absence of an external magnetic field. However, the low thermal stability, weak perpendicular magnetic anisotropy (PMA), and indistinctive SOT of these AFM/FM heterostructures pose challenges to the practical application. Here, through the insertion of a thin W layer between the IrMn and CoFeB layers, we show that much larger effective PMA fields are obtained with annealing stability to 300 °C, which is guaranteed by the prevention of Mn diffusion via W insertion as shown in spherical aberration corrected transmission electron microscopy and atomic-resolution electron energy-loss spectroscopy measurement results. Furthermore, the spin–orbit torque is effectively tuned by changing the W layer thickness via modulation of the interfacial spin–orbit coupling at IrMn/W/CoFeB interfaces, which was reported to degrade the interface spin transparency for the spin currents. Finally, field-free magnetization switching was achieved with comparable exchange bias fields to samples without W insertion. This work demonstrates an effective strategy for improving the performance of the thermally robust AFM-based SOT device.

Robust spin–orbit torques in ferromagnetic multilayers with weak bulk spin Hall effect

The spin currents generated by spin–orbit coupling in nonmagnetic metals or at interfaces with broken inversion symmetry are particularly important for devices with extraordinary performance. We have investigated the spin–orbit torques (SOTs) in Ru/Fe/HfO2 heterostructures with weak perpendicular magnetic anisotropy. Although the bulk spin Hall effect (SHE) of the Ru layer is smaller than that of Pt, Ta, or W, robust SOT-induced magnetization switching can also be realized. Through nonadiabatic harmonic measurements, the effective spin Hall angle in Ru/Fe/HfO2 heterostructures is calculated to be −0.045. We have explored the mechanisms of spin current generation, which could stem from the Ru/Fe interfaces other than the weak bulk SHE of Ru. Our results suggest the spin current generation mechanisms in ferromagnet/nonmagnetic metal heterostructures remain to be further explored.

Electric filed dependent valley polarization in 2D WSe2/CrGeTe3 heterostructure

Valleytronics in 2D transition metal dichalcogenides (TMDs) has received extensive interest as a promising field for information processing, storage, and logic operation applications. Here, we have investigated the electric field dependent valley polarization of 2D WSe2/CrGeTe3 heterostructure. The WSe2/CrGeTe3 system has an indirect band gap of 0.253 eV without spin–orbit coupling (SOC), and this is reduced to 21 meV with SOC. The WSe2/CrGeTe3 system has a weak perpendicular magnetic anisotropy of 0.05 meV cell−1, and the critical temperature is 38 K. The magnitude of the valley polarization is 3 meV without an electric field. For instance, we obtain the valley polarization of 9 meV if the electric field of 0.4 V Å−1 is applied from the CrGeTe3 to the WSe2, but it becomes 0.5 meV if the electric field direction is reversed even at the same intensity. We have found that the charge redistribution happens depending on the electric field direction. So, we attribute this feature to the electric field dependent valley polarization of the 2D WSe2/CrGeTe3 heterostructure.

Characterization of spin-orbit torque and thermoelectric effects via coherent magnetization rotation

Owing to the interplay between charge, spin, and orbits of electrons, spin-orbit torque (SOT) has attracted considerable interest during the past decade. Despite substantial progress, the existing quantification methods of SOT still have their respective restrictions on magnetic anisotropy, entanglement between SOT effective fields, and lack of corrections for the thermal gradient and planar Hall effect. Thus, it is still essential to accurately characterize SOT across diverse samples. In this study, with the aim of removing the above-mentioned restrictions and enabling universal SOT quantification, we report the characterization of sign and amplitude of SOT by angular measurement, during which magnetization coherently rotates as enforced by an external magnetic field. First, we validate the applicability of our angular measurement method in a perpendicularly magnetized [Co-Ni]/Pt heterostructure by showing excellent agreement with the results of conventional quantification methods. Remarkably, the thermoelectric effect, i.e., the anomalous Nernst effect (ANE) arising from the temperature gradient, can be self-consistently disentangled and quantified by field dependence. The superiority of this angular measurement method has been further demonstrated in a Cu/CoTb/Cu sample with large ANE but negligible SOT, and in a [Co-Ni]/Pt sample with weak perpendicular magnetic anisotropy (PMA), for which the conventional quantification methods face difficulty and even yield a fatal error. By providing a comprehensive and versatile way to characterize SOT and thermoelectric effects in diverse heterostructures, our results provide an important foundation for the spin-orbit study and interdisciplinary research of thermal spintronics.

The role of low Gd concentrations on magnetisation behaviour in rare earth:transition metal alloy films

The magnetisation reversal behaviour as a function of composition was studied in low rare earth concentration alloys. 30 nm thick rare earth:transition-metal films of composition GdxCo100−x, GdxFe100−x and Gdx(Co50Fe50)100−x were prepared by magnetron sputtering, where x ranged from 4 to 13 atomic%. Magnetisation behaviour was studied using MOKE and Hall hysteresis measurements. The magnetic reversal behaviour as a function of Gd content is strongly dependent on the transition metal. With increasing Gd content the film structure transitions from crystalline to amorphous and the saturation magnetisation decreases linearly. For GdCo, the reversal field, Hc, increases by less than a factor of two with Gd doping of 11%, while for Fe, the coercivity falls by a factor of ten with 8% Gd. This may be attributed to changes in the crystalline morphology. GdCoFe shows a similar trend with Gd doping for the in-plane reversal field to that of GdFe. With 13% Gd in Fe there is evidence indicating the presence of a weak perpendicular magnetic anisotropy, PMA. With Gd doping the anomalous Hall resistivity of Co, Fe and CoFe increases significantly with the largest increase observed for GdCoFe.

Two-pulse sub-ns switching scheme for advanced spin-orbit torque MRAM

The steady increase in performance and speed of modern integrated circuits is continuously supported by constant miniaturization of complementary metal-oxide semiconductor (CMOS) devices. However, a rapid growth of the dynamic and stand-by power due to transistor leakages becomes a pressing issue. A promising way to slow down this trend is to introduce non-volatility in circuits. The development of an electrically addressable non-volatile memory combining high speed and high endurance is essential to achieve these goals. To further reduce the energy consumption, it is essential to replace SRAM in modern hierarchical multi-level processor memory structures with a non-volatile memory technology. The spin-orbit torque magnetic random access memory (SOT-MRAM) combines non-volatility, high speed, and high endurance and is thus suitable for applications in caches. However, its development is still hindered by relatively high switching currents and the need of an external magnetic field for deterministic switching of perpendicularly magnetized layers. The switching by means of two orthogonal current pulses allows achieving deterministic sub-500 ps and magnetic field-free switching in perpendicularly magnetized rectangular structures. Complementing the two-pulse switching scheme with weak perpendicular interface-induced magnetic anisotropy reduces the switching current significantly for achieving sub-500 ps switching in in-plane structures.

Origin of Interfacial Magnetic Anisotropy in Ta/CoFeB/MgO and Pt/CoFeB/MgO Multilayer Thin Film Stacks

Interfacial perpendicular magnetic anisotropy (PMA) in Pt/Co40Fe40B20/MgO and Ta/Co40Fe40B20/MgO multilayer structures has been researched at various CoFeB layer thicknesses. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements show that while strong PMA is achieved in Ta/Co40Fe40B20/MgO structure for the thickness of Co40Fe40B20 range from 0.5 to 1.0 nm, a very weak PMA is observed in Pt/Co40Fe40B20/MgO film stack for the same Co40Fe40B20 thicknesses. Based on the experimental results, the interfacial anisotropy energy in Ta/Co40Fe40B20/MgO (1.17 erg/cm2) was found to be almost 2.5× larger than that in Pt/Co40Fe40B20/MgO (0.43 erg/cm2). It is accepted that the PMA comes from the seed/ferromagnetic and ferromagnetic/oxide layer interfaces. Thus, the origin of interfacial magnetic anisotropy can be hybridization and/or crystallinity properties at the interfaces in Ta- and Pt-seeded stacks. While the PMA is strong in Pt/Co/MgO structure, it becomes in-plane in Fe-rich Pt/CoFeB/MgO structure. It is deduced that hybridization between 3d-5d (Co-Pt) orbitals is more dominant in Pt/CoFeB interface than 2p-3d (O-Fe) hybridization at CoFeB/MgO interface in Pt-seeded stacks. In addition, the crystallinity was studied by performing high-resolution x-ray diffraction (HR-XRD) technique. The crystal orientations of Ta and Pt are found as (002) and (111), respectively. The ferromagnetic layer might be induced out-of-plane orientation in Ta-seeded stack due to observing highly crystallized MgO (001). Thus, another reason for in-plane magnetic anisotropy in Pt/Co40Fe40B20/MgO might be the absence of MgO crystallization because Pt is crystallized fcc (111) orientation.