Effect Of Interlayer Exchange Coupling Research Articles

High Efficiency and Flexible Modulation of Spintronic Terahertz Emitters in Synthetic Antiferromagnets.

Spintronic terahertz (THz) emitters based on synthetic antiferromagnets (SAFs) of FM1/Ru/FM2 (FM: ferromagnet) have shown great potential for achieving coherent superposition and significant THz power enhancement due to antiparallel magnetization alignment. However, key issues regarding the effects of interlayer exchange coupling and net magnetization on THz emissions remain unclear, which will inevitably hinder the performance improvement and practical application of THz devices. In this work, we have investigated the femtosecond laser-induced THz emission in Pt (3)/CoFe (3)/Ru (tRu = 0-3.5)/CoFe (tCoFe = 1.5-10)/Pt (3) (in units of nm) films with compensated and uncompensated magnetic moments. Antiferromagnetic (AF) coupling occurs in the Ru thickness ranges of 0.2-1.1 and 1.9-2.3 nm, with the first peak (tRu = 0.4 nm) of the AF coupling field (Hex) significantly higher than that of the second peak (2.0 nm). Rather high THz amplitude is found for the samples with strong AF coupling. Nevertheless, despite the same remanence ratio of zero, the THz amplitude for the symmetric SAF films declines significantly as the tRu decreases from 0.8 to 0.4 nm, which is mainly ascribed to the noncolinear magnetization vectors due to the increased biquadratic coupling term. Specifically, we demonstrate that an asymmetric SAF structure with a dominant FM layer is more favored than the completely compensated one, which could generate significantly enhanced THz electric field with well-controlled polarity and intensity. In addition, as the temperature decreases, the THz emission intensity increases for the SAF samples of tRu = 0.9 nm with negligible biquadratic coupling, which is contrary to the decreasing trend of the tRu = 0.4 nm sample and has been attributed to the greatly enhanced Hex.

Correlation between interlayer exchange coupling and domain wall velocity in multilayered magnetic wires

The effect of interlayer exchange coupling (IEC) in {Tb/Co}4/Rh/{Co/Tb}4/Pt multilayered wires on the domain wall (DW) structure and DW velocity was investigated. It was found that the stability of the Néel-type DWs was enhanced and the velocity of the current-induced DW movement increased with an increase in the antiferromagnetic IEC strength. The micromagnetic simulation based on the Landau–Lifshitz equation clarifies that the direction of the magnetic moment in the DWs depends on the antiferromagnetic IEC strength, and the stability of the Néel-type DW increases with an increase in the antiferromagnetic IEC strength. As a consequence of this stability, the spin-orbital torque affects effectively on the magnetic moment of the DWs and the DW velocity increases. These results imply that the DW velocity in magnetic domain-based memories can be controlled by the antiferromagnetic IEC strength.

Effect of interlayer exchange coupling in spin-torque nano oscillator

The dynamics of the magnetization of the free layer in a spin-torque nano oscillator (STNO) influenced by a noncollinear alignment between the magnetizations of the free and pinned layers due to an interlayer exchange coupling has been investigated theoretically. The orientations of the magnetization of the free layer with that of the pinned layer have been computed through the macrospin model and they are found to match well with experimental results. The bilinear and biquadratic coupling strengths make the current to switch the magnetization between two states or oscillate steadily. The expressions for the critical currents between which oscillations are possible and the critical bilinear coupling strength below which oscillations are not possible are derived. The frequency of the oscillations is shown to be tuned and increased to or above 300 GHz by the current, which is the largest to date among nanopillar-shaped STNOs.

Anisotropic coercivity and the effects of interlayer exchange coupling in CoFeB/FeRh bilayers

In an amorphous CoFeB layer, coercivity becomes anisotropic with fourfold symmetry when the CoFeB layer exchange couples to an FeRh layer. The angular dependence of coercivity of the CoFeB layer coincides with the in-plane easy-axis direction of the FeRh layer and experiences a ${45}^{\ensuremath{\circ}}$ shift with the occurrence of a metamagnetic phase transition of the FeRh layer from antiferromagnetism at room temperature to ferromagnetism at 400 K. The intriguing phenomena are well reproduced by our unbiased Monte Carlo simulation. The interfacial exchange and anisotropy energies, as well as the interfacial magnetization in the CoFeB/FeRh bilayer, are disentangled to demonstrate the strong dependence of the imprinting of anisotropy in the CoFeB layer on the interfacial exchange coupling. The evolution of the easy-axis direction of the induced anisotropy arises from the reconstruction of the interfacial exchange energy profile accompanied with the change of the magnetic state of FeRh, which governs the magnetization reversal of the CoFeB layer at both branches. Moreover, the imprinting is further applicable for the uniaxial magnetocrystalline anisotropy. This work not only presents the possibility of directly duplicating anisotropy between dissimilar materials, but it also provides a powerful tool to probe the hidden magnetic structures and/or the properties of materials that have weak magnetism, such as antiferromagnetic materials.

Stabilization of coupled Dzyaloshinskii domain walls in fully compensated synthetic anti-ferromagnets

We examine the combined effects of interlayer exchange coupling (IEC) and the interfacial Dzyaloshinskii-Moriya Interaction (DMI) on the structure of magnetic domain walls in fully compensated synthetic anti-ferromagnets (SAFs). Ir-based SAFs with ferromagnetic (FM) layers based on [Pt/(Co/Ni)M]N were characterized by Lorentz transmission electron microscopy (LTEM). The multi-layer design of the individual ferromagnetic layers enables control of the interfacial Dzyaloshinskii-Moriya interaction (via ‘M’) and, in turn, the structure and chirality of domain walls (DWs). We compare the Fresnel-mode LTEM images in SAF designs with only a change in the purported strength of the DMI. The existence of anti-ferromagnetically coupled Dzyaloshinskii domain walls (DWs) in a high DMI SAF is confirmed through application of in-situ perpendicular magnetic field and sample tilt. This conclusion is based on a unique set of conditions required to observe contrast in Fresnel-mode LTEM, which we outline in this document.

All-Silicon Switchable Magnetoelectric Effect through Interlayer Exchange Coupling.

The magnetoelectric (ME) effect originating from the effective coupling between electric field and magnetism is an exciting frontier in nanoscale science such as magnetic tunneling junction (MTJ), ferroelectric/piezoelectric heterojunctions etc. The realization of switchable ME effect under external electric field in d0 semiconducting materials of single composition is needed especially for all-silicon spintronics applications because of its natural compatibility with current industry. We employ density functional theory (DFT) to reveal that the pristine Si(111)-3×3 R30° (Si3 hereafter) reconstructed surfaces of thin films with a thickness smaller than eleven bilayers support a sizeable linear ME effect with switchable direction of magnetic moment under external electric field. This is achieved through the interlayer exchange coupling effect in the antiferromagnetic regime, where the spin-up and spin-down magnetized density is located on opposite surfaces of Si3 thin films. The obtained coefficient for the linear ME effect can be four times larger than that of ferromagnetic Fe films, which fail to have the reversal switching capabilities. The larger ME effect originates from the spin-dependent screening of the spin-polarized Dirac fermion. The prediction will promote the realization of well-controlled and switchable data storage in all-silicon electronics.

Magnetostatic spin wave modes in trilayer nanowire arrays probed using ferromagnetic resonance spectroscopy

We investigate the spin wave modes in asymmetric trilayer $[\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}\phantom{\rule{0.16em}{0ex}}(10\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Cu}({t}_{\mathrm{Cu}})/\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}\phantom{\rule{0.16em}{0ex}}(30\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]$ nanowire structures as a function of the Cu thickness $({t}_{\mathrm{Cu}})$ in the range from 0 to 20 nm using perpendicular ferromagnetic resonance (pFMR) spectroscopy. For ${t}_{\mathrm{Cu}}=0\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, corresponding to the 40 nm thick single layer $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$ nanowires, both the fundamental and first order modes are observed in the saturation region. However, for the trilayer structures, two additional modes, which are the fundamental and first order optical modes, are observed. We also found that the resonance fields of these modes are markedly sensitive to the Cu thickness due to the competing effects of interlayer exchange coupling and magnetostatic dipolar coupling. When the ${t}_{\mathrm{Cu}}\ensuremath{\ge}10\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, the fundamental optical mode is more pronounced. Our experimental results are in quantitative agreement with the dynamic micromagnetic simulations.

Effect of interlayer exchange coupling on magnetic chiral structures

We numerically investigated the effect of interlayer exchange coupling on magnetic chiral structures, such as a helical/cycloidal spin structure and magnetic skyrmion crystal (SkX), which are produced in a magnetic system involving the Dzyaloshinskii-Moriya interaction (DMI). We report the existence of a phase transition where the length scale of magnetic structure discontinuously changes, and that there can be a novel magnetic structure around the phase boundary that exhibits double-ordering lengths of magnetic structure. Therefore, the system has multiple ground phases determined by the ratio of interlayer exchange coupling strength and DMI strength. Furthermore, we investigated the critical condition of the external perpendicular field required for the SkX. The critical field is significantly reduced under the effect of interlayer exchange coupling, which can stabilize the SkX without the external field.

Synthetic Antiferromagnetic Nanoparticles as Potential Contrast Agents in MRI

We present the top-down synthesis of a novel type of MRI T2 contrast agent with great control over size and shape using a colloidal lithography technique. The resulting synthetic antiferromagnetic nanoparticles (SAF-NPs) yield improved relaxivities compared to superparamagnetic iron oxide alternatives (SPIONs). For T2 weighted imaging, the outer sphere relaxation theory has shown that the sensitivity of a T2 contrast agent is dependent on the particle size with an optimal size that exceeds the superparamagnetic limit of SPIONs. With the use of the interlayer exchange coupling effect, the SAF-NPs presented here do not suffer from this limit. Adjusting the outer sphere relaxation theory for spherical particles to SAF-NPs, we show both theoretically and experimentally that the SAF-NP size can be optimized to reach the r2 maximum. With measured r2 values up to 355 s(-1) mM(-1), our SAF-NPs show better performance than commercial alternatives and are competitive with the state-of-the-art. This performance is confirmed in an in vitro MRI study on SKOV3 cells.

Effect of interlayer exchange coupling on magnetization reversal process in ECC media with high coercivity

Effect of interlayer exchange coupling on time­evolutional magnetization reversal process in ECC media with high coercivity was investigated by utilizing micromagnetic simulation. In the case of intermediate value of the interlayer exchange coupling, during the application of printing field, magnetization in the soft layer initially reverses and magnetization in the hard layer follows that of the soft layer. Furthermore, after removal of printing field, magnetization in the soft layer is reversed again according with that in the hard layer.

Magnetotransport properties of ferromagnetic semiconductor GaMnAs-based superlattices

Two series of GaMnAs/GaAs superlattices (SSs) comprised of ferromagnetic semiconductor GaMnAs layers and non-magnetic GaAs spacers were investigated by the electronic transport measurements with an external magnetic field applied in the plane of the sample. The two SL series consisted of specimens that were structurally the same, but the GaAs spacers layers of one series were doped by Be, while in the second series the spacers were undoped. Although in field scans taken at 4 K all SLs showed a typical anisotropic magnetoresistance (MR) behavior dominated by magnetic anisotropy, similar to that normally observed in single GaMnAs ferromagnetic layers grown on GaAs (001) substrates, some of the SLs showed signatures of interaction between the GaMnAs layers in the form of broadening of the MR hysteresis. The effect of inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers became clear in the MR data taken at 30 K, where the strength of the magnetic anisotropy was reduced to the level of the inter-layer interaction. Specifically, MR measurements on two of the Be-doped SLs (BD2 and BD3) exhibited conspicuously large values of resistance at zero field, along with transitions of magnetization with negative coercive fields. The observation of these features in GaMnAs/GaAs SLs indicates the presence of spontaneous antiferromagnetic (AFM) inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers. The study further revealed that the IEC in the GaMnAs multilayers strongly depended on the properties of the non-magnetic GaAs spacers, such as their thickness and the density of carriers in the layers. Importantly, these IEC effects occurred on a longer range than that expected from current theoretical studies.

Resonance frequency in ferromagnetic superlattices

The resonance frequency in two-layer and three-layer ferromagnetic superlattices is studied, using the Callen's Green function method, the Tyablikov decoupling approximation and the Anderson–Callen decoupling approximation. The effects of interlayer exchange coupling, anisotropy, external magnetic field and temperature on the resonance frequency are investigated. It is found that the resonance frequencies increase with increasing external magnetic field. In a parameter region of the asymmetric system, each sublayer corresponds to its own resonance frequency. The anisotropy of a sublayer affects only the resonance frequency corresponding to this sublayer. The stronger the anisotropy, the higher is the resonance frequency. The interlayer exchange coupling affects only the resonance frequencies belonging to the sublayers connected by it. The stronger the interlayer exchange coupling, the higher are the resonance frequencies. All the resonance frequencies decrease as the reduced temperature increases. The results direct the method to enhance and adjust the resonance frequency of magnetic multilayered materials with a wide band.

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

Magnetization reversal and interlayer coupling in Co50Fe50 nanomagnets

We investigated magnetization reversal mechanism in elliptical shaped nanomagnets made from single layer and pseudospin valve Co50Fe50 films. The structures were fabricated using deep ultraviolet lithography and the lift-off process. We observed that the magnetization reversal process of the single layer elements is strongly dependent on the film thickness. For thickness tCoFe=10 nm, the magnetization reversal process is dominated by a systematic coherent rotation, whereas for tCoFe=60 nm, the reversal process is mediated by vortex nucleation, displacement, and annihilation. By exploiting the thickness dependence of the magnetization reversal process, pseudospin valve nanomagnets from two Co50Fe50 thicknesses (10 and 60 nm) were fabricated. We also investigated the effect of interlayer exchange coupling in pseudospin valve structures by varying the Cu spacer layer (tCu). For tCu≤5 nm, the two ferromagnetic layers are found to be strongly coupled by exchange interaction. The strength of the coupling is significantly dependent on temperature. For tCu≥20 nm, the two Co50Fe50 layers are antiferromagnetically coupled at 300 K. As the temperature is reduced below 50 K, we observed a clear transition from antiferromagnetic to ferromagnetic coupling.

Effects of interlayer exchange coupling on transport properties of Co∕Pd metallic multilayers with perpendicular magnetic anisotropy

Ru-mediated interlayer exchange coupling (IEC) was introduced in Co∕Pd(111) metallic multilayers with perpendicular magnetic anisotropy. The magnetic and electronic transport properties were investigated. The magnetically controlled Co∕Pd(111) multilayers exhibited unique resistivity changes. This is quite common in perpendicular antiferromagnetically (AF) coupled systems where the AF coupled multilayer stacks switch separately during field reversal. However, we found that even within the stacks, the individual ferromagnetic layers switch separately, such that the AF configurations are present during reversal even within the same multilayer stack. On considering the magnetic reversal and transport properties of the Co∕Pd multilayers, the IEC was found to cause antiferromagnetic alignment inside a multilayer stack that was sandwiched between two Ru layers. This phenomenon is mainly due to the thicker Pd films between the Co layers that allow more decoupling of individual Co layers than those employed in most of the previous studies.

Micromagnetic simulation study on exchange-coupled composite media

The magnetic dynamics of the total recording system with a single pole head, recording layer (RL), and semihard layer (SHL) or soft underlayer were examined by a three-dimensional micromagnetic model to understand the recording characteristics of the composite perpendicular media. The effects of the exchange strength and the SHL properties on read/write properties and the possibility of the composite media for high recording density were investigated. By comparing the three different cases: (i) single layer, (ii) conventional double-layered perpendicular media, and (iii) exchange-coupled composite media, the improvement of writability and signal-to-noise ratio (SNR) was studied. The effect of interlayer exchange coupling (Jex) between the RL and the SHL on SNR in the exchange-coupled composite media will be discussed. The effect of nonmagnetic grain-boundary region in the RL and the SHL on read/write properties will be also discussed.

Planar Hall effect and magnetoresistance in Co/Cu multilayer films

The magnetization reversal process and magnetotransport properties of [Co (10 nm)/Cu ( t Cu)/Co (10 nm)] 2 multilayer films have been investigated using a combination of planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) measurements. We have studied the effects of interlayer exchange coupling in the Co/Cu multilayer films by varying the thickness of the Cu spacer layer in the range 0≤ t Cu≤15 nm for fixed Co thickness. We observed marked changes in both the magnetic and magnetotransport properties for varied t Cu due to the effect of interlayer exchange coupling. For t Cu=5 nm, a complex magnetic spin state develops in the PHE output which may be attributed to anti-ferromagnetic coupling between the adjacent magnetic layers in contrast with ferromagnetic coupling observed for t Cu=2 nm. We have also made a direct comparison of AMR and PHE voltages as a function of the orientation of the constant applied field relative to the current direction. We observed that PHE output voltage is extremely sensitive to exact spin orientation when compared with AMR voltage measured simultaneously on the same device. When the applied field is less than the switching field of the device PHE voltage reveals a departure from the sin 2 θ behaviour predicted by theory due to domain wall propagation.

Effect of Interlayer Exchange Coupling on the Curie Temperature in Ga1-xMnxAs Trilayer Structures

We have investigated the effect of interlayer exchange coupling on the Curie temperature of ferromagnetic (FM) layers in Ga1-xMnxAs(Be)/GaAs/Ga1-xMnxAs trilayer structures. The FM layers in the trilayers contained the same concentration of Mn (x≈0.046), but the top ferromagnetic layer was additionally doped with Be. For this x the Curie temperature observed on a control single-layer sample of Be-doped Ga0.954Mn0.046As is lower than that of an undoped sample. However, in the case of trilayers, we observed that the Curie temperature of the Ga0.954Mn0.046As(Be) layer increased with decreasing thickness of the GaAs spacer separating the two FM layers. This increase in Curie temperature is due to the exchange coupling between the ferromagnetic layers mediated by the spin-polarized free carriers (holes) through the thin insulating spacer.

Dynamic magnetic susceptibility of a two-layer film in a strong magnetic field

A theoretical study is reported of the effect of interlayer exchange coupling on the resonance properties of a two-layer magnetic film with “easy-axis” and “easy-plane” anisotropic layers in a strong tilted magnetic field. The dependence of the resonance fields on the tilting angle of the external magnetic field to the film has been obtained, the tensor of integrated high-frequency film susceptibility has been found, and its dependence on the strength and orientation of the external field, as well as on layer thickness, has been analyzed. The results obtained agree with the available experimental data.

Influence of exchange coupling on the Ni magnetization in Co/Cu/Ni trilayers

The X-ray magnetic circular dichroism has been used to study trilayers consisting of two ferromagnets (Ni, Co) separated by a non-magnetic spacer (Cu). The coupling of Ni and Co layers was found to be both ferromagnetic and anti-ferromagnetic depending on spacer thickness. Two magnetization curves vanishing at two different temperatures were recorded for Ni and Co in a coupled trilayer. For Ni, this temperature is larger than the Curie temperature ( T C) of the Cu-capped Ni layer, due to the effect of interlayer exchange coupling.