Ferromagnetic Bilayers Research Articles

From helical state to chiral state in ferromagnetic bilayer graphene

Abstract We explore topological phases in biased ferromagnetic bilayer graphene, formed by bilayer graphene subjected to an external ferromagnetic exchange field, under a magnetic field. The most likely way to obtain a variety of distinct broken symmetry topological phases is proposed by means of ferromagnetic exchange field. Both spin-filtered quantum Hall and quantum spin Hall (QSH) phases are found. Edge modes in this QSH phase carry charge, spin and valley currents. When both time reversal and inversion symmetries are broken, the QSH phase remains robust against weak disorder. Moreover, topological phase transition from helical phase to chiral phase can be driven by simply tuning bias voltage or Fermi energy. A few possible experimental realizations are also discussed.

Spin wave isolator based on frequency displacement nonreciprocity in ferromagnetic bilayer

We demonstrated the spin wave isolator using bilayer ferromagnetic media comprising single crystalline and poly-crystalline yttrium iron garnet films, which can control the propagation frequency of magnetostatic waves by the direction of applied magnetic field. This isolator's property does not depend on their thickness then this can be downsized and integrated for nano-scale magnonic circuits. Calculated dispersion relationship shows good agreement with measured one.

Spin Pumping Induced Electric Voltage

Spin pumping in a ferromagnetic (FM) and nonmagnetic (NM) metallic bilayer generates an electric voltage whose origins have been attributed to the inverse spin Hall effect (ISHE) in the NM layer. We show that the electric voltage or current from the spin pumping is not limited to the ISHE of the NM layer; instead, there are two other previously unidentified contributions: one from the interface and the other from the FM layer. By using our newly developed spin pumping formalism, we compute the relative contributions of these three sources of the induced electric voltages. When we apply our results to a NiFe/Pt bilayer, we find the contribution from the strong spin-orbit coupled interface, known as the inverse Edelstein effect (IEE), can be as large as or even larger than that from the NM layer. Our finding indicates that the experimental estimation of the spin Hall angle and spin diffusion length of Pt should include both IEE and ISHE.

A Minimal tight-binding model for ferromagnetic canted bilayer manganites.

Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La2−2xSr1+2xMn2O7 (x = 0.38), transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO2 planes and the mixing of the |x2 − y2 > and |3z2 − r2 > Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of kz dispersion are included.

Magnetic domain wall coercivity and aftereffect in antiferromagnetic/ferromagnetic bilayers

Exchange bias and coercivity are investigated in antiferromagnetic (AFM)/ferromagnetic (FM) bilayers by steepest descent and Monte Carlo simulations. For a given range of the AFM anisotropy, a magnetic domain wall parallel to the interface is created in the AFM layer. This domain wall propagates along the AFM thickness and eventually gets pinned at the negative saturation due to the intrinsic Peierls potential of the AFM material. We demonstrate that this pinning and its associated dissipation is one possible source of coercivity in such exchange coupled bilayers. Monte Carlo simulations were also carried out to investigate the influence of thermal fluctuations, which induce a random walk of the domain wall throughout the AFM layer, leading to magnetic aftereffect. Depending on the experimental characteristic measurement time and temperature, the domain wall can eventually be expelled from the AFM layer leading to the reversal of the entire AFM spin lattice. This contribution to the exchange bias phenomenon particularly explains quite well the enhancement of coercivity in exchange coupled bilayers at low temperature, even in polycrystalline samples.

Superharmonic Long-Range Triplet Current in a Diffusive Josephson Junction

We study the Josephson current through a long ferromagnetic bilayer in the diffusive regime. For noncollinear magnetizations, we find that the current-phase relation is dominated by its second harmonic, which corresponds to the long-range coherent propagation of two triplet pairs of electrons.

From quasi-two-dimensional metal with ferromagnetic bilayers to Mott insulator with G-type antiferromagnetic order in Ca3(Ru1−xTix)2O7

We report the electronic and magnetic phase diagram of Ca${}_{3}$(Ru${}_{1\ensuremath{-}x}$Ti${}_{x}$)${}_{2}$O${}_{7}$. With Ti doping, the system evolves from a quasi-two-dimensional metal with ferromagnetic (FM) bilayers coupled antiferromagnetically along the $c$ axis (AFM-b) for $x=0$, to a weakly localized state for $0<x<0.05$, and finally to a Mott insulator with G-type antiferromagnetic (G-AFM) order for $x\ensuremath{\ge}0.05$. The magnetic state switching from the AFM-b to the G-AFM occurs in the weakly localized state near $x=0.03$. We show that such a magnetic transition is controlled by the charge carrier itinerancy and can be understood in light of competing interactions between FM double exchange and AFM superexchange. An incommensurate component is also observed due to competing magnetic interactions.

Magnetism of Nano-Graphene with Defects: A Monte Carlo Study

The effect of the defects on magnetic properties of a bilayer Ising ferromagnetic antiferromagnetic model is studied by Monte Carlo simulations, for a nano-graphene lattice with spins that can take the values σ=3/2 and S=5/2. We consider two ferromagnetic and antiferromagnetic bilayers with N=42 spins, with a random number of defects. We only consider the nearest-neighbor interactions between the site i and j on each layer. The effects of the defects on magnetization are investigated for fixed temperature, crystal field, and magnetic field values. The thermal dependency of each layer magnetization is calculated for fixed defect rate values K3 and K5, the crystal field, and external magnetic field. The magnetization hysteresis loops for several rate defects are also investigated as a function of the external magnetic field.

Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies

The magnetization reversal processes are discussed for exchange-coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect, consisting of a shift of the in-plane minor hysteresis loops along the field axis, was observed at room temperature after in-plane saturation. This effect was associated with specific ferromagnetic domain configurations experimentally determined by polarized neutron reflectivity. On the other hand, perpendicular exchange bias effect was revealed from the out-of-plane hysteresis loops and it was attributed to residual domains in the magnetically hard layer.

An atomic model calculation of exchange anisotropy and uncompensated spin element in antiferromagnetic layer: An effect of exchange coupling with various ferromagnetic materials

An atomic model of the direct exchange coupling in an antiferromagnetic (AF) and ferromagnetic (FM) bilayer has been investigated. Exchange anisotropy is achieved by different mechanisms in the body-centred-cubic and face-centred-cubic FM layers. For the former, the formation of an AF domain wall directly causes the exchange anisotropy. For the latter, a cooperative effect in the AF domain wall and the asymmetric form of the uncompensated AF spins produces the exchange anisotropy.

Multiorbital physics in Fermi liquids prone to magnetic order

The interplay of spin-orbit-coupling and strong electronic correlations is studied for the single-layer and the bilayer compound of the strontium ruthenate Ruddlesden-Popper series by a combination of first-principles band-structure theory with mean-field rotationally invariant slave bosons. At equilibrium strongly renormalized (spin-orbit-split) quasiparticle bands are traced and a thorough description of the low-energy regime for the nearly ferromagnetic bilayer system in accordance with experimental data is presented. The metamagnetic response of Sr$_3$Ru$_2$O$_7$ in finite magnetic field $H$ is verified and a detailed analysis of the underlying correlated electronic structure provided. Intriguing multi-orbital physics on both local and itinerant level, such as e.g. competing paramagnetic and diamagnetic contributions, is observed with important differences depending on the magnetic-field angle $\theta$ with the crystallographic c axis.

Instability of exchange bias induced by an overlaid superconductor tab in antiferromagnet\ferromagnet bilayers

The effect of a superconducting overlaid layer on exchange bias in antiferromagnet/ferromagnet bilayers is investigated through temperature-dependent electrical transport measurements. It is found that below the transition temperature of the superconductor, the exchange bias field (He) and coercivity (Hc) of the bilayer vary randomly in repeated magnetoresistance measurements, while they remain as constant above the superconductor transition temperature. We attribute this to the instability of spin structure of the antiferromagnet induced by the superconductor, which in turn affects the exchange bias at the antiferromagnet/ferromagnet interface. Micromagnetic modeling is performed to assist the interpretation of the experimental results.

Signature of the long range triplet proximity effect in the density of states

We study the impact of the long range spin-triplet proximity effect on the density of states (DOS) in planar SF${}_{1}$F${}_{2}$S Josephson junctions that consist of conventional superconductors (S) connected by two metallic monodomain ferromagnets (F${}_{1}$ and F${}_{2}$) with transparent interfaces. We determine the electronic DOS in F layers for arbitrary orientation of the magnetizations using the solutions of Eilenberger equations in the clean limit and for a moderate concentration of impurities in ferromagnets. We find that a fully developed long range proximity effect occurs in Josephson junctions with a highly asymmetric ferromagnetic bilayer. For orthogonal magnetizations, the effect manifests itself as an enhancement in DOS and as a dominant second harmonic in the Josephson current-phase relation. Distinctive variation of DOS in ferromagnets with the angle between magnetizations is experimentally observable by tunneling spectroscopy. This can provide an unambiguous signature of the long range spin-triplet proximity effect.

Anisotropic bimodal distribution of blocking temperature with multiferroic BiFeO3 epitaxial thin films

Controlling BiFeO3 (BFO)/ferromagnet (FM) interfacial coupling appears crucial for electrical control of spintronic devices using this multiferroic. Here, we analyse the magnetic behaviour of exchange-biased epitaxial-BiFeO3/FM bilayers with in-plane or out-of-plane magnetic anisotropies. We report bimodal distributions of blocking temperatures similar to those of polycrystalline-antiferromagnet (AF)/FM bilayers. The high-temperature contribution depends on the FM anisotropy direction and is likely related to thermally activated depinning of domain walls in the BiFeO3 single crystal film as opposed to thermally activated reversal of spins in AF grains for polycrystalline AF. In contrast, the low-temperature contribution weakly depends on the anisotropy direction, consistent with a spin-glass origin.

Quasiparticles relaxation processes in Nb/CuNi bilayers

The dynamic instability of the moving vortex lattice at high driving currents has been studied in superconductor (S)/weak ferromagnet (F) bilayer, Nb/Cu0.38Ni0.62. Voltage-current, V(I), characteristics have been acquired as a function of both the temperature, T, and the magnetic field, H, and interpreted in the framework of the model proposed by Larkin and Ovchinnikov. From these analysis the values of the quasiparticle relaxation time, τ, have been estimated. The results confirm the high performance of S/F hybrids in terms of velocity in the energy relaxation process, compared to corresponding single superconducting thin films. Moreover the temperature dependence of τ E is extremely smooth, also if compared with the data reported in literature for other weak ferromagnet S/F based systems. This last result has been tentatively ascribed to the disorder present in the CuNi alloy.

Spin-wave excitation in the antiferromagnetic bilayer ruthenateCa3Ru2O7

We study the spin-wave excitation spectrum of the antiferromagnetic bilayer ruthenate Ca${}_{3}$Ru${}_{2}$O${}_{7}$ through inelastic neutron scattering measurements. The material behaves as a quasi-two-dimensional ferromagnetic bilayer system with very weak interbilayer antiferromagnetic coupling, and its magnetic dispersion can be well described with a nearest-neighbor Heisenberg model. We find that the intrabilayer exchange interaction along the out-of-plane $c$-axis is much stronger than the in-plane intralayer interaction, in sharp contrast to the previously reported bilayer manganite system. And our study reveals a finite lifetime effect of the magnetic excitation, presumably attributable to the existence of a small number of itinerant charge carriers within the planes.

Controllable spin filter composed of ferromagnetic AB-stacking bilayer graphenes

The electronʼs tunneling and spin transport in the normal/ferromagnetic/normal (N/FM/N) AB-stacking bilayer graphene (BLG) junction have been studied using Landauer–Büttiker formula. It is found that the resonant conductance peaks could be split well into spin-up and down ones by the exchange field in its FM barrier, leading to a very large spin polarization. More importantly, if a perpendicular electric field is also applied on the FM barrier, a completely spin-polarized flow can be realized by changing its barrier height, making the N/FM/N AB-stacking BLG junction act as a controllable spin filter.

Exchange-bias in amorphous ferromagnetic and polycrystalline antiferromagnetic bilayers: Structural study and micromagnetic modeling

We study the role of the structure of antiferromagnetic polycrystalline metallic films in determining the magnetic properties of an exchange-coupled amorphous ferromagnetic layer. The bilayers are sputter-deposited, highly textured {111} Ir22Mn78 and Co65.5Fe14.5B20 thin films. We focus on structural characterization of Ir22Mn78 as a function of layer thickness in the range having the strongest influence over the exchange-bias field and training effect. We have used transmission electron microscopy to characterize defects in the form of interface steps and roughness, interdiffusion, twin- and grain-boundaries. Such defects can result in uncompensated magnetic spins in the antiferromagnet, which then contribute to exchange-bias. These experimental results form the basis of a general model, which uses finite element micromagnetic simulations. The model incorporates the experimental structural parameters of the bilayer by implementing a surface integral technique that allows numerical calculations to solve the transition from an amorphous to a granular structure. As a result, a detailed calculation of the underlying magnetic structure within the antiferromagnetic material is achieved. These calculations are in good agreement with micromagnetic imaging using Lorentz transmission electron microscopy and the macro-magnetic properties of these bilayers.

Spin pumping and anisotropic magnetoresistance voltages in magnetic bilayers: Theory and experiment

We investigate experimentally and theoretically the dc voltage generated in ferromagnetic and nonmagnetic metal bilayers under ferromagnetic resonance. The voltage is given by a superposition of the contributions from spin pumping (${V}_{\mathrm{SP}}$) and anisotropic magnetoresistance (${V}_{\mathrm{AMR}}$). A theoretical model is presented that separately determines ${V}_{\mathrm{SP}}$ and ${V}_{\mathrm{AMR}}$ as a function of the applied static field intensity as well the in-plane angle. The model is used to interpret a detailed set of data obtained in a series of ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Pt samples excited by in-plane ferromagnetic resonance. The results show excellent agreement between theory and the measured voltages as a function of the Permalloy and Pt layer thicknesses. Our findings show that the quantitative separation of both effects is crucial to the interpretation of experiments and the determination of the spin Hall angle and spin-diffusion length.

Spin-polarized current and tunneling magnetoresistance in ferromagnetic gate bilayer graphene structures

We study spin transport in bilayer graphene structures where gate electrodes are attached to ferromagnetic graphene. Due to the exchange field in the gated regions, the current becomes spin dependent and can be controlled by tuning the gate voltages. It is shown that thanks to strong resonant chiral tunneling inherent in bilayer graphene, very high spin polarization and tunneling magnetoresistance can be achieved in the considered structures. Different possibilities for controlling the spin current are discussed. The study demonstrates the potential of bilayer graphene structures for spintronic applications with significant improvement over previously predicted results in monolayer graphene structures.