Strong ferromagnetic interlayer exchange couplings $J$ in perpendicularly magnetized systems are becoming increasingly desirable for applications. We study whether ferromagnetic interlayer exchange couplings can be measured by a combination of broadband ferromagnetic resonance methods and magnetometry hysteresis loops. For this we model the switching and the eigenexcitations in bilayer systems comprising a soft layer coupled to a thicker harder layer that possesses higher perpendicular magnetic anisotropy. For large $J >0$ the switching fields are essentially independent of $J$ but the frequency of the optical eigenmode of the bilayer and the linewidth of the acoustical and optical eigenmode are directly sensitive to the coupling. We derive a corpus of compact analytical expressions to analyze these frequencies, their linewidth and discuss the meaning thereof. We illustrate this corpus on a system mimicking the fixed layers of a magnetic tunnel junction meant for spin torque applications.
A perpendicularly magnetized ferromagnetic layer is an important building block for recent/future highdensity spintronic memory applications. This paper reports on the fabrication of perpendicularly magnetized Ni / Pt superlattices and the characterization of their structures and magnetic properties. The optimization of film growth conditions allowed us to grow epitaxial Ni / Pt (001) superlattices on SrTiO$_{3}$ (001) single crystal substrates. We investigated their structural parameters and magnetic properties as a function of the Ni layer thickness, and obtained a high uniaxial magnetic ani…
Spin waves in perpendicularly-magnetized Pt/Co/AlO$_x$/Pt ultrathin films with varying Co thicknesses (0.6-1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thicknesses. This nonreciprocity is attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), which is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant $D_s = -1.7 \pm 0.11$ pJ/m, which favors…
The use of epitaxial layers for domain wall-based spintronic applications is often hampered by the presence of pinning sites. Here, we show that when depositing Mn4N(10 nm) epitaxial films, the replacement of MgO(001) by SrTiO3(001) substrates allows minimizing the misfit, and to obtain an improved crystalline quality, a sharper switching, a full remanence, a high anisotropy and remarkable millimeter-sized magnetic domains, with straight and smooth domain walls. In a context of rising interest for current-induced domain wall motion in rare
We show that Py, a commonly-used soft ferromagnetic material with weak anisotropy, can become perpendicularly-magnetized while depositing on Ta buffer layer with Hf or Zr insertion layers (ILs) and MgO capping layer. By using two different approaches, namely harmonic voltage measurement and hysteresis loop shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic anisotropy are characterized. We find that though Ta has a significant spin Hall effect, the DL-SOT efficiencies are small in systems with the…
Perpendicularly magnetized Mn-Co-Ga-based thin films with high coercive field
采用磁控共溅射法在MgO衬底上生长了Mn $_{3-x}$ Co $_{x}$ Ga外延薄膜。根据Co含量的不同，其结构为四方晶系或立方晶系。研究了薄膜的饱和磁化强度和单轴磁各向异性$K_u$的组分依赖性。Mn $_{2.6}$ Co $_{0.3}$ Ga $_{1.1}$薄膜具有较高的$K_u$（1.2 MJ m $^{-3}$），磁矩为0.84 $μ_B$ .通过硬X射线光电子能谱获得价带谱。尖锐的峰值在立方的情况下，这是缺席的情况下，tetraquinion，证明了一个范霍夫奇点导致一个带Jahn-Teller效应与tetraquinion失真。观测结果与第一性原理计算吻合得很好。
The flow of in-plane current through ultrathin magnetic heterostructures can cause magnetization switching or domain wall nucleation owing to bulk and interfacial effects. Within the magnetic layer, the current can create magnetic instabilities via spin transfer torques (STT). At interface(s), spin current generated from the spin Hall effect in a neighboring layer can exert torques, referred to as the spin Hall torques, on the magnetic moments. Here, we study current induced magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. Dep…
Ferromagnetic films of L10-ordered MnGa and MnAl that exhibit giant perpendicular magnetic anisotropy and great controllability in the magnetism and structural disorders show promise not only in the applications in magnetic recording, permanent magnets and spintronics, but also in controllable studies of disorder-relevant electrical transport phenomena. In this article, we review the intriguing experimental observations of the orbital two-channel Kondo effect and anomalous Hall effect in L10-ordered MnGa and MnAl thin films with perpendicular magnetic anisotropy. We also give a perspective wit…
We demonstrate a ten-fold resonant enhancement of magneto-optical effects in perpendicularly magnetized $[$Co/Pt$]_{N}$ nanodots mediated by the excitation of optimized plasmon modes. Two magnetoplasmonic systems are considered; square arrays of $[$Co/Pt$]_{N}$ nanodots on glass and identical arrays on a Au/SiO2 bilayer. On glass, the optical and magneto-optical spectra of the nanodot arrays are dominated by the excitation of a surface lattice resonance (SLR), whereas on Au/SiO${}_{2}$, a narrow surface plasmon polariton (SPP) resonance tailors the spectra further. Both the SLR and SPP modes a…
Next-generation concepts for solid-state memory devices are based on current-driven domain wall propagation, where the wall velocity governs the device performance. It has been shown that the domain wall velocity and the direction of travel is controlled by the nature of the wall and its chirality. This chirality is attributed to effects emerging from the lack of inversion symmetry at the interface between a ferromagnet and a heavy metal, leading to an interfacial Dzyaloshinskii-Moriya interaction that can control the shape and chirality of the magnetic domain wall. Here we present direct imag…
We present an analytical theory to describe three-dimensional magnetic textures in perpendicularly magnetized magnetic multilayers that arise in the presence of magnetostatic interactions and the Dzyaloshinskii-Moriya interaction (DMI). We demonstrate that domain walls in multilayers develop a complex twisted structure, which persists even for films with strong DMI. The origin of this twist is surface-volume stray field interactions that manifest as a depth-dependent effective field whose form mimics the DMI effective field. We find that the wall twist has a minor impact on the equilibrium sky…
We report on reversible electric-field-driven magnetic domain wall motion in a Cu/Ni multilayer on a ferroelectric BaTiO$_3$ substrate. In our heterostructure, strain-coupling to ferroelastic domains with in-plane and perpendicular polarization in the BaTiO$_3$ substrate causes the formation of domains with perpendicular and in-plane magnetic anisotropy, respectively, in the Cu/Ni multilayer. Walls that separate magnetic domains are elastically pinned onto ferroelectric domain walls. Using magneto-optical Kerr effect microscopy, we demonstrate that out-of-plane electric field pulses across the…
Buckminsterfullerene (C60) can exhibit ferromagnetism at the interface (called as a spinterface) when it is placed next to a ferromagnet (FM). Formation of such spinterface happens due to orbital hybridization and spin polarized charge transfer at the interface. The spinterface can influence the domain size and dynamics of the organic/ferromagnetic heterostructure. Here, we have performed magnetic domain imaging and studied the relaxation dynamics in Pt/Co/C60/Pt system with perpendicular anisotropy. We have compared the results with its parent Pt/Co/Pt system. It is observed that presence of …
A study of the multilayer system MgO/CoFeB(1.1nm)/Ta($t$)/CoFeB(0.8nm)/MgO is presented, where the two CoFeB layers are separated by a Ta interlayer of varying thickness $t$. The magnetization properties deduced from complementary techniques such as superconducting quantum interference magnetometry, ferromagnetic resonance frequency measurements and Brillouin light scattering spectroscopy can be tuned by changing the Ta thickness between $t$=0.25 nm, 0.5 nm and 0.75 nm. For $t$=0.5 nm, a ferromagnetic coupling is observed, whereas for t=0.75 nm, the antiferromagnetic coupling needed to constru…
The demand of fast and power efficient spintronics devices with flexibility requires additional energy for magnetization manipulation. Stress/and strain have shown their potentials for tuning magnetic properties to the desired level. Here, we report a systematic study for the effect of both tensile and compressive stresses on the magnetic anisotropy (MA). Further the effect of stress on the domain structure and magnetization relaxation mechanism in a perpendicularly magnetized Co/Pt film has been studied. It is observed that a minimal in-plane tensile strain has increased the coercivity of the…
Spin-orbit torque can drive electrical switching of magnetic layers. Here, we report that at least for micrometer-sized samples there is no simple correlation between the efficiency of dampinglike spin-orbit torque (ξ_DL^j) and the critical switching current density of perpendicularly magnetized spin-current generator/ferromagnet heterostructures. We find that the values of ξ_DL^j based on switching current densities can either under- or over-estimated ξ_DL^j by up to tens of times in a domain-wall depinning analysis, while in the macrospin analysis based on the switching current density ξ_DL^…
We study the offset fields affecting the free layer of perpendicularly magnetized tunnel junctions. In extended films, the free layer offset field results from interlayer exchange coupling with the reference layer through the MgO tunnel oxide. The free layer offset field is thus accompanied with a shift of the free layer and reference layer ferromagnetic resonance frequencies. The shifts depend on the mutual orientation of the two magnetizations. The offset field decreases with the resistance area product of the tunnel oxide. Patterning the tunnel junction into an STT-MRAM disk-shaped cell cha…
We study bubble domain wall dynamics using micromagnetic simulations in perpendicularly magnetized ultra-thin films with disorder and Dzyaloshinskii-Moriya interaction. Disorder is incorporated into the material as grains with randomly distributed sizes and varying exchange constant at the edges. As expected, magnetic bubbles expand asymmetrically along the axis of the in-plane field under the simultaneous application of out-of-plane and in-plane fields. Remarkably, the shape of the bubble has a ripple-like part which causes a kink-like (steep decrease) feature in the velocity versus in-plane …
Ferrimagnets, which contain the advantages of both ferromagnets (detectable moments) and antiferromagnets (ultrafast spin dynamics), have recently attracted great attention. Here we report the optimization of epitaxial growth of a tetragonal perpendicularly magnetized ferrimagnet Mn2Ga on MgO. Electrical transport, magnetic properties and the anomalous Hall effect (AHE) were systematically studied. Furthermore, we successfully integrated high-quality epitaxial ferrimagnetic Mn2Ga thin films onto ferroelectric PMN-PT single crystals with a MgO buffer layer. It was found that the AHE of such a f…
A theoretical formula is derived for the threshold current to switch a perpendicular magnetization in a ferromagnet by the spin Hall effect. The numerical simulation of the Landau-Lifshitz-Gilbert equation indicates that magnetization switching is achieved when the steady-state solution of the magnetization in the presence of the current is outside an energetically unstable region. Based on the numerical result, an analytical theory deriving the threshold current is developed by focusing on the first-order perturbation to the unstable state. The analytical formula clarifies that the magnitude …
We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not ex…
Dynamics of domain growth driven by dipolar interactions in a perpendicularly magnetized ultrathin film
对垂直磁化的Fe/2 ML Ni/W（110）双金属薄膜的交流磁化率的测量表明，该系统支持的条纹状畴结构中畴生长和畴密度变化的动力学特征是清晰的。在0.20 K/s < R < 0.70 K/s范围内，随着升温速率的增加，磁化率峰向高温移动。使用松弛模型的分析表明，定量的动力学驱动的非平衡域密度在（近）零场（即偶极相互作用），和温度的变化是由于响应时间由局部域壁段的钉扎结构缺陷。由偶极相互作用驱动的畴密度弛豫的基本时间尺度比由外加场驱动的单个巴克豪森步骤的基本时间尺度慢10^5倍。基本时间尺度的增加反映了偶极子和塞曼能量的相对大小，以及需要许多局部畴壁段的相关运动来影响畴生长。
The Y3Fe5O12 (YIG) films with perpendicular magnetic anisotropy (PMA) have recently attracted a great deal of attention for spintronics applications. Here, we report the induced PMA in the ultrathin YIG films grown on (Gd2.6Ca0.4)(Ga4.1Mg0.25Zr0.65)O12 (SGGG) substrates by epitaxial strain without preprocessing. Reciprocal space mapping shows that the films are lattice-matched to the substrates without strain relaxation. Through ferromagnetic resonance and polarized neutron reflectometry measurements, we find that these YIG films have ultra-low Gilbert damping constant with a magnetic dead lay…
We report the first demonstration of the current-induced magnetization switching in a perpendicularly magnetized A1 CoPt single layer. We show that good perpendicular magnetic anisotropy can be obtained in a wide composition range of the A1 Co1-xPtx single layers, which allows to fabricate perpendicularly magnetized CoPt single layer with composition gradient to break the inversion symmetry of the structure. By fabricating the gradient CoPt single layer, we have evaluated the SOT efficiency and successfully realized the SOT-induced magnetization switching. Our study provides an approach to rea…
We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in the inverse-spinel ferrimagnet Ni$_{1-x}$Co$_{2+y}$O$_{4-z}$ (NCO) epitaxial films with the perpendicular magnetic anisotropy. We show that the oxygen pressure P$_{O2}$ during the film growth by pulsed laser deposition influences not only the cation stoichiometry (site-occupation) but also the cation valence state. Our XAS results show that the Ni in the O$_{h}$-site is in the intermediate valence state betw…
We investigate the Gilbert damping and the magnetization switching of perpendicularly magnetized FeCoB-based free layers embedded in tunnel junctions adequate for spin-torque operated memories. We study the influence of the boron content in MgO / FeCoB /Ta systems alloys on their Gilbert damping after crystallization annealing. Increasing the boron content from 20 to 30\% increases the crystallization temperature, thereby postponing the onset of elemental diffusion within the free layer. This reduction of the interdiffusion of the Ta atoms helps maintaining the Gilbert damping at a low level o…
Spin-orbit interaction derived spin torques provide a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. A basic and critical question for applications is the speed and efficiency of switching with nanosecond current pulses. Here we investigate and contrast the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micron scale Hall crosses consisting of very thin ($<1$ nm) perpendicularly magnetized CoFeB layers on $β$-Ta. While complete magnetization reversal is found at a …
We time-resolve the spin-transfer-torque-induced switching in perpendicularly magnetized tunnel junctions of diameters from 50 to 250 nm in the thermally activated regime. When the field and the spin-torque concur to favor the P to AP transition, the reversal yields monotonic resistance ramps that can be interpreted as a domain wall propagation through the device at velocities of 17 to 30 nm/ns; smaller cells switch hence faster. When the field hinders the P to AP transition, the switching is preceded by repetitive switching attempts, during which the resistance transiently increases until suc…
Tailoring Ultrathin Magnetic Multilayers at Terraced Topologically Insulating Interfaces for Perpendicularly Magnetized Domains
University of Leeds（利兹大学）；ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普尔顿实验室中子与μ子源）；Diamond Light Source（钻石光源）；Imperial College London（伦敦帝国理工学院）
Centro Atómico Bariloche（巴里洛切原子中心）；Instituto Balseiro, Universidad Nacional de Cuyo–CNEA（巴尔塞罗研究所，门多萨国立大学-国家原子能委员会）；Instituto de Nanociencia y Materiales de Aragón (INMA-CSIC)（阿拉贡纳米与材料研究所）；Universidad de Zaragoza（萨拉戈萨大学）
We use time-resolved measurement and modeling to study the spin-torque induced motion of a domain wall in perpendicular anisotropy magnets. In disc of diameters between 70 and 100 nm, the wall drifts across the disc with pronounced back-and-forth oscillations that arise because the wall moves in the Walker regime. Several switching paths occur stochastically and lead to distinct switching durations. The wall can cross the disc center either in a ballistic manner or with variably marked oscillations before and after the crossing. The crossing of the center can even occur multiple times if a ver…
We study how the shape of the spinwave resonance lines in rf-voltage induced FMR can be used to extract the spinwave density of states and the damping within the precessing layer in nanoscale tunnel junctions that possess perpendicular anisotropy. We work with a field applied along the easy axis to preserve the uniaxial symmetry of the system. We describe the set-up to study the susceptibility contributions of the spin waves in the field-frequency space. We then identify the maximum device size above which the spinwaves can no longer be studied in isolation as the linewidths of their responses…
Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly-magnetized typically requires an externally-applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study…
Spin-orbit torques (SOTs) have opened a new path to switch the magnetization in perpendicularly magnetized films and are of great interest due to their potential applications in novel data storage technology, such as the magnetic random access memory (MRAM). The effective manipulation of SOT has thus become an important step towards these applications. Here, current induced spin-orbit effective fields and magnetization switching are investigated in Pt/Ta/CoFeB/MgO structures with bilayer heavy metals. With a fixed thickness (1 nm) of the Ta layer, the magnitude and sign of current induced spin…
We study the evolution of perpendicularly magnetized tunnel junctions under 300 to 400 $^{\circ}$C annealing. The hysteresis loops do not evolve much during annealing and they are not informative of the underlying structural evolutions. These evolutions are better revealed by the frequencies of the ferromagnetic resonance eigenmodes of the tunnel junction. Their modeling provides the exchange couplings and the layers' anisotropies within the stack which can serve as a diagnosis of the tunnel junction state after each annealing step. The anisotropies of the two CoFeB-based parts and the two Co/…
We show by vector network analyzer ferromagnetic resonance measurements that low Gilbert damping α down to 0.006 can be achieved in perpendicularly magnetized MgO/CoFeB/MgO thin films with ultra-thin insertions of Ta in the CoFeB layer. While increasing the number of Ta insertions allows thicker CoFeB layers to remain perpendicular, the effective areal magnetic anisotropy does not improve with more insertions, and also comes with an increase in α.
We demonstrate current-induced bipolar switching in in-plane magnetized spin-valve devices that incorporate a perpendicularly magnetized spin polarizing layer. Further, hysteretic transitions into a state with intermediate resistance occur at high currents, again for both current polarities. These transitions are shown to be consistent with a macrospin model that considers a spin-polarized current that is tilted with respect to the free layer plane, due to the presence of spin-transfer torque from the polarizing layer. These unique switching characteristics, which have their origin in the nonc…
We investigated the temperature dependence of the switching current for a perpendicularly magnetized CoFeB film deposited on a nanocrystalline tungsten film with large oxygen content: nc-W(O). The spin Hall angle $|Θ_\mathrm{SH}| \approx 0.22$ is independent of temperature, whereas the switching current increases strongly at low temperature. We show that the nc-W(O) is insensitive to annealing. It thus can be a good choice for the integration of spin Hall driven writing of information in magnetic memory or logic devices that require a high-temperature annealing process during fabrication.
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 both the in-plane, but also the perpendicular magnetic anisotropy. This opens up a route towards electric field con…
We report on the occurrence of strong interlayer Dzyaloshinskii-Moriya interaction (DMI) between an in-plane magnetized Co layer and a perpendicularly magnetized TbFe layer through a Pt spacer. The DMI causes a chiral coupling that favors one-handed orthogonal magnetic configurations of Co and TbFe, which we reveal through Hall effect and magnetoresistance measurements. The DMI coupling mediated by Pt causes effective magnetic fields on either layer of up to 10-15 mT, which decrease monotonously with increasing Pt thickness. Ru, Ta, and Ti spacers mediate a significantly smaller coupling compa…
We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT efficiencies are larger for the trilayers as compared to the bilayers, confirming the enhanced values reported for the trilayers with perpendicularly magnetized Co. The maximum effective spin Hall angl…
