Ultrathin Ferromagnetic Layers Research Articles

Spacer Thickness and Temperature Dependences of the Interlayer Exchange Coupling in a Co/Pt/Co Three-Layer Structure

Domain wall mobility as a function of nonmagnetic interlayer thickness and temperature was studied in ultrathin exchange-coupled ferromagnetic layers using magneto-optic Kerr microscopy. The system under study is a Pt/Co/Pt/Co/Pt heterostructure having perpendicular magnetic anisotropy and a middle Pt layer with spatially variable thickness. The ferromagnetic interaction between the Co layers is observed when the Pt interlayer thickness varies from 5 to 6 nm in a temperature range of 200–300 K. There is a certain interval of Pt layer thickness where domain walls in both ferromagnetic layers move independently. Nonlinear dependence of the domain wall displacement on the applied field was measured. It is shown that an equilibrium position of the relaxed domain wall depends on field, temperature, and the nonmagnetic interlayer thickness. This position is determined by the energy balance: (i) domain wall displacement provided by the applied field, (ii) interlayer exchange interaction in the area swept by the domain wall, and (iii) domain wall coercivity. The mechanism of domain wall stabilization in terms of independent wall motion near critical thickness was considered. It is found that both the coercivity of the Co layer and the critical thickness decrease at higher temperature, while the interlayer exchange constant J is changed weakly.

Voltage-controlled magnetic anisotropy effect through a LiF/MgO hybrid tunneling barrier

Improving the perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy (VCMA) properties are fundamentally important for the development of voltage-controlled magnetoresistive random access memories (VC-MRAM). Recently, we reported on a large increase in PMA at an Fe/MgO interface brought about by inserting an ultrathin LiF layer at the interface. In this paper, we investigate the PMA, VCMA, and TMR properties in MTJs with an Ir-doped ultrathin ferromagnetic layer and a LiF/MgO hybrid tunneling barrier. We observed a clear increase in the interfacial PMA by a factor of 2.5 when an ultrathin 0.25 nm LiF layer was inserted. A large VCMA coefficient, exceeding −300 fJ/Vm, was also achieved while maintaining the high TMR ratio and high interfacial PMA. These results demonstrate the high potential of interface engineering using ultrathin LiF layers for spintronic devices.

Probing optically induced spin currents using terahertz spin waves in noncollinear magnetic bilayers

The mechanism responsible for the generation of optically induced spin currents upon femtosecond laser-pulse excitation of ultrathin ferromagnetic layers is still a heavily debated topic. In this paper, the authors study the phase of spin-current induced terahertz spin waves in noncollinear bilayer structures to probe spin currents in Fourier space. They measure a significant laser-pulse energy dependence of the spin-wave phase, which is quantitatively consistent with a spin current that is proportional to the time derivative of the magnetization.

A ferromagnetic skyrmion-based nano-oscillator with modified perpendicular magnetic anisotropy

Perpendicular magnetic anisotropy can be induced when the atoms orbital anisotropy in the ultrathin ferromagnetic layer is reflected. Here, we computationally study a ferromagnetic skyrmion-based nano-oscillator model with a composite structure, where the skyrmion dynamics is controlled by modifying perpendicular magnetic anisotropy. The proposed nano-oscillator structure has two concentric circular areas with different anisotropy coefficients. When the anisotropy of the inner area is larger than that of the outer area, the inner area can provide a repulsive force acting on the skyrmion, leading to the motion of skyrmion along with the edge of the inner circular area. Three cases of skyrmion motion are found for different anisotropy coefficients. We also study the effects of each anisotropy coefficients and current density on the frequency of the skyrmion-based nano-oscillator. Our results provide a promising method to modulate the frequency of future skyrmion-based nano-oscillators.

Underlayer material dependent symmetric and asymmetric behavior of voltage-controlled magnetic anisotropy in CoFeB films

Voltage-controlled magnetic anisotropy (VCMA), observed at the interfaces of ultrathin ferromagnetic metallic films and oxide layer, has proven to be a useful tool for the development of all-electric field controlled spintronics devices. Here, we have studied the symmetric and asymmetric behavior of VCMA in CoFeB/MgO heterostructures, grown on different underlayer materials, by measuring ferromagnetic resonance using spin pumping and inverse spin Hall effect technique. We observe symmetric behavior of VCMA in CoFeB films with Ta underlayer, whereas a systematic transformation from symmetric to asymmetric behavior of VCMA with decreasing CoFeB thickness is observed for Pt underlayer. We speculate that the increased interfacial roughness, defects and strain of ultrathin CoFeB films with Pt buffer layer probably leads to the complicated band structure at CoFeB/MgO interface resulting in asymmetric behavior of VCMA. The observed symmetric behavior of VCMA in control samples justifies the role of interfacial roughness, defects and discards the role of oxide overlayer on the observed asymmetric behavior of VCMA in ultrathin CoFeB films.

Magnetic Skyrmion Spin‐Torque Nano‐Oscillators

The skyrmion oscillation frequency and steady orbit radius in ultrathin ferromagnetic layer of circular nanopillar are calculated in response to a spin‐polarized current. The analytical model of the rigid skyrmion motion is used. It is shown that the steady skyrmion oscillations with a frequency of about 1 GHz exist within finite range of the current. The case of the current perpendicular to the free layer plane and two types of the reference layer polarizations, uniform and vortex‐like, are considered. The first critical current can be very low in the case of zero difference between the skyrmion and vortex helicities. The second critical current is considered as a current of the skyrmion expelling from the free layer (dot). The oscillation frequency increases essentially with the current increasing. It is a function of the skyrmion equilibrium radius, dot radius, and the dot magnetic parameters.

Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy

Chiral magnetic textures in ultrathin perpendicularly magnetised multilayer film stacks with an interfacial Dzyaloshinskii-Moriya interaction have been the focus of much research recently. The chirality associated with the broken inversion symmetry at the interface between an ultrathin ferromagnetic layer and a heavy metal with large spin-orbit coupling supports homochiral N\'eel domain walls and hedgehog (N\'eel) skyrmions. Under spin-orbit torques these N\'eel type magnetic structures are predicted, and have been measured, to move at high velocities. However recent studies have indicated that some multilayered systems may possess a more complex hybrid domain wall configuration, due to the competition between interfacial DMI and interlayer dipolar fields. These twisted textures are expected to have thickness dependent N\'eel and Bloch contributions to the domain or skyrmion walls. In this work, we use the methods of Lorentz microscopy to measure quantitatively for the first time experimentally both; i) the contributions of the N\'eel and Bloch contributions and ii) their spatial spin variation at high resolution. These are compared with modelled and simulated structures which are in excellent agreement with our experimental results. Our quantitative analysis provides powerful direct evidence of the Bloch wall component which exists in these hybrid walls and will be significant when exploiting such phenomena in spintronic applications.

Far-Infrared Reflection from Heterostructures Made of Ultrathin Ferromagnetic Layers

Recent progress has been made in creating terahertz magnons using ultrathin ferromagnetic layers. Due to their short lifetimes, these can be difficult to measure. Here we detail a calculation that shows that infrared magnons can be detected using standard reflection and attenuated total reflection measurements from a thin-film heterostructure made up of alternating ultrathin magnetic and nonmagnetic layers. We use an entire-cell effective-medium calculation to find the magnetic permeability of the heterostructure and then use electromagnetic boundary conditions to calculate reflectivity as a function of frequency. There are appreciable dips in the reflectivity at infrared magnon resonance frequencies, for realistic material parameters. Moreover, the strong coupling of magnon photons indicates the possible use of 50 GHz–1 THz magnons in integrated signal-processing devices.

Controlled Current-Driven Bi-Directional Motion of Trains of Domain Walls Along a Ferromagnetic Strip

Domain wall (DW)-based magnetic memories where each bit is represented by a single magnetic domain have repeatedly been proposed to implement faster and denser memory devices. Multilayer systems where an ultrathin ferromagnetic layer is sandwiched between a heavy metal and an oxide allow particularly fast and synchronous current-driven DW dynamics. However, an essential prerequisite for their feasibility is the efficient and precise control of DW positions. The position of the DWs can be controlled by tailoring the perpendicular magnetic anisotropy (PMA) by means of ion irradiation. Nevertheless, the current-driven dynamics of trains of DWs in these systems has not been addressed yet, either theoretically or numerically. Here, we proposed a triangular PMA periodic profile defined by two extremal values, K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> <; K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ), and the anisotropy period, d, showing the proposal feasibility and its bi-directional features. This paper has been carried out by means of full micromagnetic simulations and a simplified 1-D model, updated to take into account the anisotropy profile. The results highlight the suitability of using an anisotropy modulated profile to efficiently and precisely define the DW equilibrium positions.

Enhanced magneto-optical effects in hybrid Ni-Si metasurfaces

The multifold enhancement of the Faraday effect induced by magnetic dipole and Voigt effect amplification due to electric dipole Mie resonances of the magnetophotonic metasurface is demonstrated. The values of the magneto-optical responses up to 0.8° and 0.5% are experimentally observed for the metasurface with an ultrathin ferromagnetic layer. The results can be used for the development of novel active magnetophotonic metadevices.

Modulated spin orbit torque in ultrathin ferromagnetic layer with different capping layers

In heavy metal (HM)/ferromagnetic metal (FM)/oxide heterostructures with perpendicular magnetic anisotropy (PMA), the magnetization can be efficiently controlled through spin orbit torque (SOT), which has attracted great attention since its applications in spintronic devices. In this work, we have investigated the SOT with modifying capping layers X in Pt/Co/X trilayers. In these systems, both the bottom Pt/Co and top Co/X interfaces are utilized to generate SOT via bulk SHE effect and/or Rashba effect. By the selecting of different capping layer X, one can expect an enhancement or cancellation of SOT contributions via interface effect, which provides an interesting prospect for enhancing the efficiency of SOT through a proper combination of materials in the structural engineering. Our experimental study explored the choice of the capping materials which influenced the magnetic behavior and current-induced magnetization switching process.

Symmetry broken spin reorientation transition in epitaxial MgO/Fe/MgO layers with competing anisotropies

The observation of perpendicular magnetic anisotropy (PMA) at MgO/Fe interfaces boosted the development of spintronic devices based on ultrathin ferromagnetic layers. Yet, magnetization reversal in the standard magnetic tunnel junctions (MTJs) with competing PMA and in-plane anisotropies remains unclear. Here we report on the field induced nonvolatile broken symmetry magnetization reorientation transition from the in-plane to the perpendicular (out of plane) state at temperatures below 50 K. The samples were 10 nm thick Fe in MgO/Fe(100)/MgO as stacking components of V/MgO/Fe/MgO/Fe/Co double barrier MTJs with an area of 20 × 20 μm2. Micromagnetic simulations with PMA and different second order anisotropies at the opposite Fe/MgO interfaces qualitatively reproduce the observed broken symmetry spin reorientation transition. Our findings open the possibilities to develop multistate epitaxial spintronics based on competing magnetic anisotropies.

Large Uniaxial Anisotropy Induced in Soft Ferromagnetic Thin Films by Oblique Deposition of Underlayer

We produced a large uniaxial anisotropy $(H_{k})$ in ultrathin (1.2–2.4 nm) soft ferromagnetic layers through the oblique deposition of a nanometer-thick metallic underlayer. Fe, Ni, and Ni80Fe20 (Permalloy) films deposited on a 5 nm thick Nb layer grown at an oblique angle (up to 60 ${}^{\circ}$ ) exhibit an $H_{k}\approx 40-500$ Oe and have properties that are nearly ideal for use as a fixed layer in magnetic devices, such as for magnetic memory. A much higher induced $H_{k}$ is observed in Ni (500 Oe) and Fe (300 Oe) compared to Ni-Fe (40 Oe). We used this technique to increase the switching field ( $H_{{\rm{sw}}}$ ) by 3–5 times in an array of pattered Fe or Ni-Fe bits, and achieved an excellent relative switching distribution of about 0.05–0.06. We also controllably enhanced the $H_{k}$ in the fixed layer of patterned spin-valve devices for superconducting Josephson-junction magnetic random-access memory and produced a large difference in $H_{{\rm{sw}}}$ between the fixed and free layers. Different spacer layers between the Nb and the magnetic layers either propagate (for Ru) or reduce (for Cu, Ru/Al, ion-milled Ru) the effect of the oblique Nb. This technique was also effectively used in synthetic antiferromagnet structures and, therefore, has the potential to eliminate the need for an antiferromagnetic pinning layer when it degrades magnetic device performance.

Micromagnetic Simulations of Magnetization Spatial Distribution in Ultrathin Cobalt Layers With Gradient Magnetic Anisotropy

Spatial distribution of magnetization in an ultrathin ferromagnetic Co layer with a lateral gradient of magnetic anisotropy, while approaching spin reorientation transition (SRT) (from perpendicular to in-plane magnetization alignment), has been investigated by means of micromagnetic simulations in the 2-D mode, using OOMMF software. The geometry of the out-of-plane-magnetized domains (parallel stripes, labyrinth, and bubbles) has been found to be dependent on both the initial distribution of magnetization and the direction of the applied magnetic field. A fast decrease in the domain size has been observed while moving toward SRT. In the experiment, Pt/Co/Pt layers with initial in-plane magnetization orientation have been irreversibly modified by femtosecond laser pulses. In the irradiated spot, rings with induced perpendicular magnetic anisotropy have been formed, resulting in an appearance of several local SRTs. Magnetic domain structure in the SRT regions has been visualized using magnetic force microscopy. The experimental observations are qualitatively explained by the results of the micromagnetic simulations.

Micromagnetic modeling of the shielding properties of nanoscale ferromagnetic layers

Ferromagnetic shields are widely used to concentrate magnetic fields in a target region of space. Such shields are also used in spintronic nanodevices such as magnetic random access memory and magnetic logic devices. However, the shielding properties of nanostructured shields can differ considerably from those of macroscopic samples. In this work, we investigate the shielding properties of nanostructured NiFe layers around a current line using a finite element micromagnetic model. We find that thin ferromagnetic layers demonstrate saturation of magnetization under an external magnetic field, which reduces the shielding efficiency. Moreover, we show that the shielding properties of nanoscale ferromagnetic layers strongly depend on the uniformity of the layer thickness. Magnetic anisotropy in ultrathin ferromagnetic layers can also influence their shielding efficiency. In addition, we show that domain walls in nanoscale ferromagnetic shields can induce large increases and decreases in the generated magnetic field. Therefore, ferromagnetic shields for spintronic nanodevices require careful design and precise fabrication.

Self-current induced spin-orbit torque in FeMn/Pt multilayers.

Extensive efforts have been devoted to the study of spin-orbit torque in ferromagnetic metal/heavy metal bilayers and exploitation of it for magnetization switching using an in-plane current. As the spin-orbit torque is inversely proportional to the thickness of the ferromagnetic layer, sizable effect has only been realized in bilayers with an ultrathin ferromagnetic layer. Here we demonstrate that, by stacking ultrathin Pt and FeMn alternately, both ferromagnetic properties and current induced spin-orbit torque can be achieved in FeMn/Pt multilayers without any constraint on its total thickness. The critical behavior of these multilayers follows closely three-dimensional Heisenberg model with a finite Curie temperature distribution. The spin torque effective field is about 4 times larger than that of NiFe/Pt bilayer with a same equivalent NiFe thickness. The self-current generated spin torque is able to switch the magnetization reversibly without the need for an external field or a thick heavy metal layer. The removal of both thickness constraint and necessity of using an adjacent heavy metal layer opens new possibilities for exploiting spin-orbit torque for practical applications.

Critical Dzyaloshinskii–Moriya interaction energy density for the skyrmion states formation in ultrathin ferromagnetic layer

We investigate the condition for the skyrmion state formation in ultrathin ferromagnetic layers with the Dzyaloshinskii–Moriya interaction (DMI). By using micromagnetic simulations with DMI term, we found skyrmion state is a ground state instead of a collinear single domain when DMI is larger than a critical value. When the skyrmion arrays are formed, a specific length scale of skyrmion is governed the spin configurations. We found that the critical DMI energy density for the skyrmion state is related with not only crystalline anisotropy energy, but also the dipole–dipole interaction.

Spin-orbit torque in a bulk perpendicular magnetic anisotropy Pd/FePd/MgO system.

Spin-orbit torques, including the Rashba and spin Hall effects, have been widely observed and investigated in various systems. Since interesting spin-orbit torque (SOT) arises at the interface between heavy nonmagnetic metals and ferromagnetic metals, most studies have focused on the ultra-thin ferromagnetic layer with interface perpendicular magnetic anisotropy. Here, we measured the effective longitudinal and transverse fields of bulk perpendicular magnetic anisotropy Pd/FePd (1.54 to 2.43 nm)/MgO systems using harmonic methods with careful correction procedures. We found that in our range of thicknesses, the effective longitudinal and transverse fields are five to ten times larger than those reported in interface perpendicular magnetic anisotropy systems. The observed magnitude and thickness dependence of the effective fields suggest that the SOT do not have a purely interfacial origin in our samples.

Dynamic selective switching in antiferromagnetically-coupled bilayers close to the spin reorientation transition

We have designed a bilayer synthetic antiferromagnet where the order of layer reversal can be selected by varying the sweep rate of the applied magnetic field. The system is formed by two ultra-thin ferromagnetic layers with different proximities to the spin reorientation transition, coupled antiferromagnetically using Ruderman-Kittel-Kasuya-Yosida interactions. The different dynamic magnetic reversal behavior of both layers produces a crossover in their switching fields for field rates in the kOe/s range. This effect is due to the different effective anisotropy of both layers, added to an appropriate asymmetric antiferromagnetic coupling between them. Field-rate controlled selective switching of perpendicular magnetic anisotropy layers as shown here can be exploited in sensing and memory applications.

Large and robust electrical spin injection into GaAs at zero magnetic field using an ultrathin CoFeB/MgO injector

Binary information encoded within the spin of carriers can be transferred into corresponding right- or left-handed circularly polarized photons emitted from an active semiconductor medium via carrier-photon angular momentum conversion. In order to attain maximized spin injection at out-of-plane magnetic remanence, a number of material systems have been explored as possible solid-state spin injectors. However, the circular polarization $({P}_{\mathrm{C}})$ of emitted light was still limited at 3--4% at remanence. Here, we demonstrate a sizable electroluminescence circular polarization from a III-V-based spin light-emitting diode at zero magnetic field with a perpendicular spin injector consisting of an ultrathin CoFeB ferromagnetic layer (1.2 nm) grown on a MgO tunnel barrier (2.5 nm). The maximum value of ${P}_{\mathrm{C}}$ measured at zero field is as large as 20% at 25 K and still 8% at 300 K. These types of ultrathin perpendicular spin injectors are of great interest (i) to realize the electrical switching of the magnetization of the injector layer owing to the advanced spin-transfer torque properties of the CoFeB layer and (ii) to be directly embedded in optical cavities for spin lasers due to their very low optical absorption loss.