Ultrathin Ferromagnets Research Articles

Ubiquity of the spin-orbit induced magnon nonreciprocity in ultrathin ferromagnets

Ubiquity of the spin-orbit induced magnon nonreciprocity in ultrathin ferromagnets

Above-room-temperature intrinsic ferromagnetism in ultrathin van der Waals crystal Fe3+xGaTe2

Two-dimensional (2D) van der Waals (vdW) magnets are crucial for ultra-compact spintronics. However, so far, no vdW crystal has exhibited tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin regime. Here, we report the tunable above-room-temperature intrinsic ferromagnetism in ultrathin vdW crystal Fe3+xGaTe2 (x = 0 and 0.3). By increasing the Fe content, the Curie temperature (TC) and room-temperature saturation magnetization of bulk Fe3+xGaTe2 crystals are enhanced from 354 to 376 K and 43.9 to 50.4 emu·g−1, respectively. Remarkably, the robust anomalous Hall effect in 3-nm Fe3.3GaTe2 indicates a record-high TC of 340 K and a large room-temperature perpendicular magnetic anisotropy energy of 6.6 × 105 J m−3, superior to other ultrathin vdW ferromagnets. First-principles calculations reveal the asymmetric density of states and an additional large spin exchange interaction in ultrathin Fe3+xGaTe2 responsible for robust intrinsic ferromagnetism and higher TC. This work opens a window for above-room-temperature ultrathin 2D magnets in vdW-integrated spintronics.

(Invited) Bulk Traps in Layered 2D Gate Dielectrics

Ultra-thin ferromagnets, when coupled with magnetoelectric or multiferroic materials, could potentially enable highly energy-efficient electric field control of the magnet for use in nanoelectronic memories. Substitutional doping of magnetic impurities in monolayer transition metal dichalcogenides (TMDs) may be a promising way to create 2D ferromagnets but, according to theoretical calculations, require high doping levels (10-20 %) to achieve above room temperature (RT) Curie temperature. Room-temperature ferromagnetism has been reported for very low doping levels (0.1-1%), in conflict with the theoretical calculations, and always in the presence of high concentrations of defects, making it unclear if the dopants alone are responsible for this ferromagnetism. In this work, we use a combination of molecular beam epitaxy (MBE), plan-view TEM, XRD, XPS, Raman, and magnetometry to study iron and vanadium doping in monolayer WSe2. We show that optimally grown monolayers with up to 35% doping and low defect density show no ferromagnetism. Interestingly, ferromagnetism is observed when these monolayers contain a significant amount of selenium vacancies (Sevac), intentionally created via a post-growth heat treatment process, and magnetism is seen to scale with heating time/vacancy concentration. Moreover, even undoped WSe2 shows similar ferromagnetism for Sevac > 1014 cm-2. This ferromagnetism can later be quenched by filling these vacancies via Se annealing. For films with low Se vacancy concentration, TEM analysis reveals significant clustering of the V and Fe dopants which likely suppresses the ferromagnetism predicted by theory - a problem that has previously plagued III-V based dilute magnetic semiconductors as well. We go so far as to claim that all of the above room-temperature ferromagnetism reports in the literature thus far are due to Se vacancies and not magnetic doping.

Giant Spin-Orbit Induced Magnon Nonreciprocity in Ultrathin Ferromagnets.

The propagation characteristics of fermionic and bosonic quasiparticles determine the fundamental transport properties of solids and are of great technological relevance for designing logic devices. In particular, nonreciprocity, which describes that a quasiparticle flows preferably along a certain direction of a symmetry path, is an essential requirement to realize logic architectures, e.g., switches, diodes, transistors, etc. Here we introduce a mechanism, which leads to giant nonreciprocity of ultrafast terahertz magnons in ferromagnetic films with a large spin-orbit coupling. The mechanism is based on the competition between the exchange and spin-orbit scattering. We anticipate that the effect can be used to excite nonreciprocal or even unidirectional magnons in a large class of ultrathin films and nanostructures grown on substrates with a large spin-orbit coupling.

Ultra-thin ferromagnets with large magnetic anisotropy by assembling MnCl3 superatoms on SbAs monolayer

Two-dimensional (2D) magnetic materials with large magnetic anisotropy are crucial for developing thermally stable, miniaturized, and low-power spintronic devices. However, naturally existing 2D materials with intrinsic anisotropic magnetization are scarce. Employing a bottom-up primitive assembly approach and first-principles calculations, we find that the adsorption of MnCl3 superatoms not only induces magnetism in non-magnetic monolayer SbAs but also hardens it (large out-of-plane magnetic anisotropy). The MnCl3/SbAs system becomes a ferromagnetic half-metal with high Curie temperature. Interestingly, the delocalized p orbitals of Sb/As not only mediate the long-range magnetic exchange but also contribute to the significant magnetic anisotropic energy. The magnetic exchange and anisotropy of the system can be efficiently modulated by changing the assembly mode of MnCl3 on the SbAs surface. This study presents an alternative concept and method for the development of 2D ferromagnetic materials with uniaxial magnetic anisotropy, which holds promise for spintronic device applications.

Large voltage-controlled magnetic anisotropy effect in magnetic tunnel junctions prepared by deposition at cryogenic temperatures

We investigated the influence of the buffer material and a cryogenic temperature deposition process on the voltage-controlled magnetic anisotropy (VCMA) effect for an ultrathin CoFeB layer in bottom-free type MgO-based magnetic tunnel junctions prepared by a mass production sputtering process. We used Ta and TaB buffers and compared the differences between them. The TaB buffer enabled us to form a flat and less-contaminated CoFeB/MgO interface by suppressing the diffusion of Ta with maintaining a stable amorphous phase. Furthermore, the introduction of cryogenic temperature deposition for the ultrathin CoFeB layer on the TaB buffer improved the efficiency of the VCMA effect and its annealing tolerance. Combining this with interface engineering employing an Ir layer for doping and a CoFe termination layer, a large VCMA coefficient of −138 ± 3 fJ/Vm was achieved. The developed techniques for the growth of ultrathin ferromagnet and oxide thin films using cryogenic temperature deposition will contribute to the development of high-performance spintronic devices, such as voltage-controlled magnetoresistive random access memories.

Strong spin-lattice entanglement in cobaltites

Strongly correlated electronic system contains strong coupling among multi-order parameters and is easy to efficiently tune by external field. Cobaltite (LaCoO<sub>3</sub>) is a typical multiferroic (ferroelastic and ferromagnetic) material, which has been extensively investigated over decades. Conventional research on cobaltites has focused on the ferroelastic phase transition and structure modulation under stress. Recently, researchers have discovered that cobaltite thin films undergo a paramagnetic-to-ferromagnetic phase transition under tensile strain, however, its origin has been controversial over decades. Some experimental evidence shows that stress leads the valence state of cobalt ions to decrease, and thus producing spin state transition. Other researchers believe that the stress-induced nano-domain structure will present a long-range ordered arrangement of high spin states, which is the main reason for producing the ferromagnetism of cobalt oxide films. In this paper, we review a series of recent researches of the strong correlation between spin and lattice degrees of freedom in cobalt oxide thin films and heterojunctions. The reversible spin state transition in cobalt oxide film is induced by structural factors such as thin-film thickness, lattice mismatch stress, crystal symmetry, surface morphology, interfacial oxygen ion coordination, and oxygen octahedral tilting while the valence state of cobalt ions is kept unchanged, and thus forming highly adjustable macroscopic magnetism. Furthermore, the atomic-level precision controllable film growth technology is utilized to construct single cell layer cobaltite superlattices, thereby achieving ultra-thin two-dimensional magnetic oxide materials through efficient structure regulation. These advances not only clarified the strong coupling between lattice and spin order parameters in the strongly correlated electronic system, but also provided excellent candidate for the realization of ultra-thin room temperature ferromagnets that are required by oxide spintronic devices.

Significant suppression of two-magnon scattering in ultrathin Co by controlling the surface magnetic anisotropy at the Co/nonmagnet interfaces

To enable suppression of two-magnon scattering (TMS) in nanometer-thick Co (ultrathin Co) layers and realize low-magnon damping in such layers, the magnon damping in ultrathin Co layers grown on various nonmagnetic seed layers with different surface magnetic anisotropy (SMA) energies are investigated. We verify the significantly weak magnon damping realized by varying the seeding layer species used. Although TMS is enhanced in ultrathin Co on Cu and Al seeding layers, the insertion of a Ti seeding layer below the ultrathin Co greatly suppresses the TMS, which is attributed to suppression of the SMA at the interface between Co and Ti. The Gilbert damping constant of the ultrathin Co layer on Ti (3 nm), 0.020, is comparable to the value for bulk Co, although the Co layer thickness here is only 2 nm. Realization of such weak magnon damping can open the door to tunable magnon excitation, thus enabling coupling of magnons with other quanta such as photons, given that the magnetization of ultrathin ferromagnets can be tuned using an external electric field.

Dependency of Anisotropy Field of Cofeb Layer on the Thickness of W Insert Layer for Manipulating Size of Skyrmions

Magnetic skyrmions are expected to offer novel functionalities for high density and low power spintronics devices in memory, logic and neuromorphic computing. [1] Since the recent observation at room temperature in which the skyrmion is stabilized by interfacial Dzyaloshinskii-Moriya interaction (DMI), many researches have focused on the creation and manipulating the motion of skyrmion in sub-100 nm size. [2-3] Observation of skyrmion sizes of sub-100 nm were reported at room temperature [4-5], however, there is little work on controlling the size of the skyrmions. In this work, we designed a novel structure, as shown in Figure 1, to manipulate the size of the skyrmion in engineering perspective. First, we investigated the roughness of the W seed layer depending on the sputtering power using atomic force microscopy (AFM). Then, we investigated the dependency of thickness of the W insert layer on the H k of CoFeB layer using vibrating sample magnetometer (VSM). The size of the skyrmions were determined by magneto-optical-kerr-effect (MOKE). Acknowledgement This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT (2021M3F3A2A01037733) Reference [1] Nagaosa N. and Tokura Y., Nat. Nanotechnol. 8, 899-911 (2013)[2] Seonghoon W. et al. Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets, Nat. mat. 15, 501-506 (2016)[3] Wanjun J. et al. Science, 349(2645), 283-286 (2015)[4] Aranda A.R. et al. Journal of Magnetism and Magnetic Materials, 465, 471-479 (2018)[5] Mouad F. et al. Physics Letters A, 384(13), 126260 (2020)Figure 1. Dependency of skyrmion size on thickness of W insert. (a) skymion PMA structure with W insert, (b) Hk depending on W thickness, and (c) skymion formation under external field of 3.5 Oe Figure 1

Enhancing the ferromagnetic interlayer coupling between epitaxial SrRuO3 layers

Magnetic interlayer coupling is a key ingredient in designing magnetic multilayers with functionalities that reach out to the realm of applications. In epitaxial ferromagnetic (FM) oxide multilayers, the magnetic interlayer coupling is, however, less studied and its prediction is often a challenging task. Ultrathin FM ${\mathrm{SrRuO}}_{3}$ epitaxial films with perpendicular magnetic anisotropy, interfaced with suitable oxides, may be susceptible of forming skyrmions. Hence, a strong FM interlayer coupling would be beneficial to achieve uniform switching behavior of a ${\mathrm{SrRuO}}_{3}$-based multilayer. Previous studies reported that the coupling of two ${\mathrm{SrRuO}}_{3}$ layers separated by a non-FM oxide spacer is at best weakly FM and the two FM layers switch at markedly different fields. Here we study the magnetic interlayer coupling between two FM ${\mathrm{SrRuO}}_{3}$ layers separated by ultrathin ${\mathrm{LaNiO}}_{3}$ in epitaxial heterostructures grown on ${\mathrm{SrTiO}}_{3}$(100) single crystals. We found that FM ${\mathrm{SrRuO}}_{3}$ layers separated by 2 monolayers (MLs) thick ${\mathrm{LaNiO}}_{3}$ show weak FM interlayer coupling of about $106\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{J}/{m}^{2}$ at 10 K. The coupling becomes strongly FM for four MLs thick (about 1.6 nm) ${\mathrm{LaNiO}}_{3}$ spacers and the two ${\mathrm{SrRuO}}_{3}$ layers reverse their magnetization at a common value of the perpendicular magnetic field. This is likely due to a transition of the ${\mathrm{LaNiO}}_{3}$ spacer from insulating to metallic, as its thickness increases.

Magnetic properties and the interfacial Dzyaloshinskii-Moriya interaction in exchange biased Pt/Co/NixOy films

We show that a trilayer magnetic structure, consisting of a heavy metal (Platinum) as a buffer, an ultrathin ferromagnet (Cobalt) as a functional layer, and an antiferromagnetic insulator (Nickel oxide, NixOy) as a capping, is a host of intriguing physical phenomena desirable for spintronics. X-ray photoelectron spectroscopy revealed changes in the ratio of NixOy phases with different stoichiometry. Due to strong spin–orbit coupling, the Pt layer induces perpendicular magnetic anisotropy in Co, breaks its structural inversion symmetry, and causes an additive effect of Pt/Co and Co/NixOy interfaces to the effective interfacial Dzyaloshinskii-Moriya interaction (Deff). Manipulation the magnetic anisotropy can be realized by the ferromagnetic layer thickness variation and modification of the Co/NixOy interface. The NixOy capping layer induces the exchange bias in Co, which effective field |Beb| can be controlled in the range from 0 to 24 mT. The magnitude and direction of Beb does not affect Deff. The increase of Ar pressure leads to structural transformation of the Co/NixOy interface and, consequently, to decrease of the surface Dzyaloshinskii-Moriya interaction (Ds), the source of which is the NiO phase. We demonstrate that the studied Pt/Co/NixOy system has a number of functional properties useful for future logic and memory applications.

Asymmetric skyrmion-antiskyrmion production in ultrathin ferromagnetic films

Ultrathin ferromagnets with frustrated exchange and the Dzyaloshinskii-Moriya interaction can support topological solitons such as skyrmions and antiskyrmions, which are metastable and can be considered particle-antiparticle counterparts. When spin-orbit torques are applied, the motion of an isolated antiskyrmion driven beyond its Walker limit can generate skyrmion-antiskyrmion pairs. Here, we use atomistic spin dynamics simulations to shed light on the scattering processes involved in this pair generation. Under certain conditions a proliferation of these particles and antiparticles can appear with a growth rate and production asymmetry that depend on the strength of the chiral interactions and the dissipative component of the spin-orbit torques. These features are largely determined by scattering processes between antiskyrmions, which can be elastic or result in bound states or annihilation.

Nonvolatile Electric Control of the Anomalous Hall Effect in an Ultrathin Magnetic Metal

AbstractLow‐dimensional magnetism has been boosted by the recent discovery of the ferromagnetism in layered two‐dimensional (2D) semiconductors. Although the macroscopic magnetic moments of 2D ferromagnets are weak, the anomalous Hall effect (AHE) can serve as a versatile electric probe to their magnetic properties. Here, the nonvolatile electric‐field manipulation of the AHE in an ultrathin metallic ferromagnet with perpendicular magnetic anisotropy at room temperature is reported, which is achieved by the electrostatic modulation of the longitudinal resistivity via a ferroelectric substrate without varying magnetization. Therefore, this work demonstrates an electric‐field‐controlled room‐temperature memory device based on the zero‐magnetic‐field anomalous Hall resistance of an ultrathin ferromagnet. More importantly, the experimental results disentangle magnetization and anomalous Hall resistance. As a result, the study reveals a linear decrease of anomalous Hall conductivity with normal conductivity, which is distinct from previous scaling relations. Accordingly, this work manifests a universe avenue to harnessing the AHE in low‐dimensional magnetic materials.

Emergent Dynamics of Artificial Spin-Ice Lattice Based on an Ultrathin Ferromagnet.

We present high-frequency dynamics of magnetic nanostructure lattices, fabricated in the form of "artificial spin-ice", that possess magnetically frustrated states. Dynamics of such structures feature multiple resonance excitation that reveals rich and intriguing microwave characteristics, which are highly dependent on field-cycle history. Geometrical parameters such as dimensions and ferromagnetic layer thickness, which control the interplay of different demagnetizing factors, are found to play a pivotal role in governing the dynamics. Our findings are highlighted by the evolution of unique excitations pertaining to magnetic frustration, which are well supported by static magnetometry studies and micromagnetic simulations.

Collective coordinate descriptions of magnetic domain wall motion in perpendicularly magnetized nanostructures under the application of in-plane fields

Manipulation of magnetic domain walls in nanostructures can be used to improve the capabilities of the next generation of memory and sensing devices. Materials of interest for such devices include heterostructures of ultrathin ferromagnets sandwiched between a heavy metal and an oxide, where spin–orbit coupling and broken inversion symmetry give rise to the Dzyaloshinskii-Moriya interaction (DMI), stabilizing chiral domain walls. The efficiency of the motion of these chiral domain walls may be controlled using in-plane magnetic fields. This property has been used both for measurement of DMI strength, and for improved performance in applications. While micromagnetic simulations are able to accurately predict domain wall motion under in-plane fields in these materials, collective coordinate models such as the q-ϕ and q-ϕ-χ models fail to reproduce the micromagnetic results. In this theoretical work, we present a set of extended collective coordinate models including canting in the domains, which better reproduce micromagnetic results, and improve our understanding of the effect of in-plane fields on magnetic domain walls. These models are used in conjunction with micromagnetic simulations to develop simpler descriptions of DW motion under specific conditions. Our new models and results help in the development of future domain wall based devices based on perpendicularly magnetized materials.

Enhanced interfacial Dzyaloshinskii-Moriya interaction and isolated skyrmions in the inversion-symmetry-broken Ru/Co/W/Ru films

An enhancement of the spin-orbit effects arising on an interface between a ferromagnet (FM) and a heavy metal (HM) is possible through the strong breaking of the structural inversion symmetry in the layered films. Here, we show that an introduction of an ultrathin W interlayer between Co and Ru in Ru/Co/Ru films enables to preserve perpendicular magnetic anisotropy (PMA) and simultaneously induce a large interfacial Dzyaloshinskii-Moriya interaction (iDMI). The study of the spin-wave propagation in the Damon-Eshbach geometry by Brillouin light scattering spectroscopy reveals the drastic increase in the iDMI value with the increase in W thickness (tW). The maximum iDMI of −3.1 erg/cm2 is observed for tW = 0.24 nm, which is 10 times larger than for the quasi-symmetrical Ru/Co/Ru films. We demonstrate the evidence of the spontaneous field-driven nucleation of isolated skyrmions supported by micromagnetic simulations. Magnetic force microscopy measurements reveal the existence of sub-100-nm skyrmions in the zero magnetic field. The ability to simultaneously control the strength of PMA and iDMI in quasi-symmetrical HM/FM/HM trilayer systems through the interface engineered inversion asymmetry at the nanoscale excites new fundamental and practical interest in ultrathin ferromagnets, which are a potential host for stable magnetic skyrmions.

Current-driven dynamics and inhibition of the skyrmion Hall effect of ferrimagnetic skyrmions in GdFeCo films

Magnetic skyrmions are swirling magnetic textures with novel characteristics suitable for future spintronic and topological applications. Recent studies confirmed the room-temperature stabilization of skyrmions in ultrathin ferromagnets. However, such ferromagnetic skyrmions show an undesirable topological effect, the skyrmion Hall effect, which leads to their current-driven motion towards device edges, where skyrmions could easily be annihilated by topographic defects. Recent theoretical studies have predicted enhanced current-driven behavior for antiferromagnetically exchange-coupled skyrmions. Here we present the stabilization of these skyrmions and their current-driven dynamics in ferrimagnetic GdFeCo films. By utilizing element-specific X-ray imaging, we find that the skyrmions in the Gd and FeCo sublayers are antiferromagnetically exchange-coupled. We further confirm that ferrimagnetic skyrmions can move at a velocity of ~50 m s−1 with reduced skyrmion Hall angle, |θSkHE| ~ 20°. Our findings open the door to ferrimagnetic and antiferromagnetic skyrmionics while providing key experimental evidences of recent theoretical studies.

Temperature Dependence of Magnetic Excitations: Terahertz Magnons above the Curie Temperature.

When an ordered spin system of a given dimensionality undergoes a second order phase transition, the dependence of the order parameter, i.e., magnetization on temperature, can be well described by thermal excitations of elementary collective spin excitations (magnons). However, the behavior of magnons themselves, as a function of temperature and across the transition temperature T_{C}, is an unknown issue. Utilizing spin-polarized high resolution electron energy loss spectroscopy, we monitor the high-energy (terahertz) magnons, excited in an ultrathin ferromagnet, as a function of temperature. We show that the magnons' energy and lifetime decrease with temperature. The temperature-induced renormalization of the magnons' energy and lifetime depends on the wave vector. We provide quantitative results on the temperature-induced damping and discuss the possible mechanism, e.g., multimagnon scattering. A careful investigation of physical quantities determining the magnons' propagation indicates that terahertz magnons sustain their propagating character even at temperatures far above T_{C}.

Collective coordinate models of domain wall motion in perpendicularly magnetized systems under the spin hall effect and longitudinal fields

Recent studies on heterostructures of ultrathin ferromagnets sandwiched between a heavy metal layer and an oxide have highlighted the importance of spin-orbit coupling (SOC) and broken inversion symmetry in domain wall (DW) motion. Specifically, chiral DWs are stabilized in these systems due to the Dzyaloshinskii-Moriya interaction (DMI). SOC can also lead to enhanced current induced DW motion, with the Spin Hall effect (SHE) suggested as the dominant mechanism for this observation. The efficiency of SHE driven DW motion depends on the internal magnetic structure of the DW, which could be controlled using externally applied longitudinal in-plane fields. In this work, micromagnetic simulations and collective coordinate models are used to study current-driven DW motion under longitudinal in-plane fields in perpendicularly magnetized samples with strong DMI. Several extended collective coordinate models are developed to reproduce the micromagnetic results. While these extended models show improvements over traditional models of this kind, there are still discrepancies between them and micromagnetic simulations which require further work.

Spin Wave Power Flow and Caustics in Ultrathin Ferromagnets with the Dzyaloshinskii-Moriya Interaction.

The Dzyaloshinskii-Moriya interaction in ultrathin ferromagnets can result in nonreciprocal propagation of spin waves. We examine theoretically how spin wave power flow is influenced by this interaction. We show that the combination of the dipole-dipole and Dzyaloshinskii-Moriya interactions can result in unidirectional caustic beams in the Damon-Eshbach geometry. Morever, self-generated interface patterns can also be induced from a point-source excitation.