Ferromagnetic Junctions Research Articles

Spin- and valley-dependent transport through arrays of ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions in silicene through arrays of barriers, of width $d$, in the presence of an exchange field $M$ and a tunable potential of height $U$ or depth $-U$. The spin- and valley-resolved conductances as functions of $U$ or $M$, exhibit resonances away from the Dirac point (DP) and close to it a pronounced dip that becomes a gap when a critical electric field $E_z$ is applied. This gap widens by increasing the number of barriers and can be used to realize electric field-controlled switching of the current. The spin $p_s$ and valley $p_v$ polarizations of the current near the DP increase with $E_z$ or $M$ and can reach 100\% for certain of their values. These field ranges widen significantly by increasing the number of barriers. Also, $p_s$ and $p_v$ oscillate nearly periodically with the separation between barriers or wells and can be inverted by reversing $M$.

Wiedemann-Franz law for magnon transport

One of the main goals of spintronics is to improve transport of information carriers and to achieve new functionalities with ultra-low dissipation. A most promising strategy for this holy grail is to use pure magnon currents created and transported in insulating magnets, in the complete absence of any conducting metallic elements. Here we propose a realistic solution to this fundamental challenge by analyzing magnon and heat transport in insulating ferromagnetic junctions. We calculate all transport coefficients for magnon transport and establish Onsager relations between them. We theoretically discover that magnon transport in junctions has a universal behavior, i.e. is independent of material parameters, and establish a magnon analog of the celebrated Wiedemann-Franz law which governs charge transport at low temperatures. We calculate the Seebeck and Peltier coefficients which are crucial quantities for spin caloritronics and demonstrate that they assume universal values in the low temperature limit. Finally, we show that our predictions are within experimental reach with current device and measurement technologies.

Spin wave scattering and interference in ferromagnetic cross

Magnetostatic spin wave scattering and interference across a CoTaZr ferromagnetic spin wave waveguide cross junction were investigated experimentally and by micromagnetic simulations. It is observed that the phase of the scattered waves is dependent on the wavelength, geometry of the junction, and scattering direction. It is found that destructive and constructive interference of the spin waves generates switching characteristics modulated by the input phase of the spin waves. Micromagnetic simulations are used to analyze experimental data and simulate the spin wave scattering and interference.

Electromagnetic effect on disordered surface of topological insulators

We theoretically study electromagnetic effects due to magnetization on disordered surface of topological insulators with attaching a ferromagnetic insulator junction by using the result of the magnetization dynamics induced charge flow. We find that the electric polarization is induced by not only the magnetization but also the spatial derivative of the magnetization with the diffusion on the disordered surface.

Magnetically Controlled Electronic Transport Properties of a Ferromagnetic Junction on the Surface of a Topological Insulator* *Supported by National Natural Science Foundation of China under Grant Nos. 11174088, 11175067, 11274124

We have investigated the transport properties of the Dirac fermions through a ferromagnetic barrier junction on the surface of a strong topological insulator. The current-voltage characteristic curve and the tunneling conductance are calculated theoretically. Two interesting transport features are predicted: observable negative differential conductances and linear conductances tunable from unit to nearly zero. These features can be magnetically manipulated simply by changing the spacial orientation of the magnetization. Our results may contribute to the development of high-speed switching and functional applications or electrically controlled magnetization switching.

Spin filter and spin valve in ferromagnetic graphene

We propose and demonstrate that a EuO-induced and top-gated graphene ferromagnetic junction can be simultaneously operated as a spin filter and a spin valve. We attribute such a remarkable result to a coexistence of a half-metal band and a common energy gap for opposite spins in ferromagnetic graphene. We show that both the spin filter and the spin valve can be effectively controlled by a back gate voltage, and they survive for practical metal contacts and finite temperature. Specifically, larger single spin currents and on-state currents can be reached with contacts with work functions similar to graphene, and the spin filter can operate at higher temperature than the spin valve.

Perfect GMR effect in gapped graphene-based ferromagnetic–normal–ferromagnetic junctions

We investigate the quantum transport property in gapped graphene-based ferromagnetic/normal/ferromagnetic (FG/NG/FG) junctions by using the Dirac–Bogoliubov–de Gennes equation. The graphene is fabricated on SiC and BN substrates separately, so carriers in FG/NG/FG structures are considered as massive relativistic particles. Transmission probability, charge, and spin conductances are studied as a function of exchange energy of ferromagnets (h), size of graphene gap, and thickness of normal graphene region (L) respectively. Using the experimental values of Fermi energy in the normal graphene part (EFN ∼ 400 meV) and energy gap in graphene (260 meV for SiC and 50 meV for BN substrate), it is shown that this structure can be used for both spin-up and spin-down polarized current. The latter case has different behavior of gapped FG/NG/FG from that of gapless FG/NG/FG structures. Also perfect charge giant magnetoresistance is observed in a range of .

Polarized spin and valley transport across ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions through silicene barriers, of width d, with an exchange field M and metallic gates above them that provide tunable potentials of height U. Away from the Dirac point (DP), the spin- and valley-resolved conductances, as functions of U, exhibit resonances and close to it a pronounced dip that becomes a transport gap when an appropriate electric field Ez is applied. The charge conductance gc of such a junction changes from oscillatory to a monotonically decreasing function of d beyond a critical Ez. This change of gc can be used to realize electric-field-controlled switching. The field M splits each resonance of gc in two spin-resolved peaks. The spin ps and valley pv polarizations of the current near the DP increase with Ez or M and become nearly perfect above certain of their values. We also show that ps and pv can be inverted either by reversing the polarity of U or the direction of M. For two barriers, there is no splitting in gc when the fields M are in opposite directions. Most of these phenomena have no analogs in graphene.

In-plane anisotropy of a nano-scaled magnetic tunnel junction with perpendicular magnetic easy axis

We investigate magnetic properties of a 100-nm-diameter CoFeB/MgO magnetic tunnel junction (MTJ) with perpendicular magnetic easy axis by homodyne-detected ferromagnetic resonance (FMR) and junction resistance measurements. The resonant frequency depends clearly on the direction of the in-plane magnetic field, which is also the case for the angle dependence of the junction resistance. A good correspondence between the two independent measurements indicates the presence of unintentionally introduced in-plane magnetic anisotropy in the present MTJ.

Observation of 0–π transition in SIsFS Josephson junctions

The 0–π transition in Superconductor-Insulator-superconductor-Ferromagnet-Superconductor (SIsFS) Josephson junctions (JJs) was investigated experimentally. As predicted by theory, an s-layer inserted into a ferromagnetic SIFS junction can enhance the critical current density up to the value of an SIS tunnel junction. We fabricated Nb′ | AlOx | Nb | Ni60Cu40 | Nb JJs with wedge-like s (Nb) and F (Ni60Cu40) layers and studied the Josephson effect as a function of the s- and F-layer thickness, ds and dF, respectively. For ds = 11 nm, π-JJs with SIFS-type jc(dF) and critical current densities up to jcπ=60 A/cm2 were obtained at 4.2 K. Thicker ds led to a drastic increase of the critical current decay length, accompanied by the unexpected disappearance of the 0–π transition dip in the jc(dF) dependence. Our results are relevant for superconducting memories, rapid single flux quantum logic circuits, and solid state qubits.

Topological insulator in junction with ferromagnets: Quantum Hall effects

The ferromagnet–topological insulator–ferromagnet (FM–TI–FM) junction exhibits thermal and electrical quantum Hall effects. The generated Hall voltage and transverse temperature gradient can be controlled by the directions of magnetizations in the FM leads, which inspires the use of FM–TI–FM junctions as electrical and as heat switches in spintronic devices. Thermal and electrical Hall coefficients are calculated as functions of the magnetization directions in ferromagnets and the spin-relaxation time in TI. Both the Hall voltage and the transverse temperature gradient decrease but are not completely suppressed even at very short spin-relaxation times. The Hall coefficients turn out to be independent of the spin-relaxation time for symmetric configuration of FM leads.

Domain wall pinning in ultra-narrow electromigrated break junctions

The study of magnetic domain walls in constrained geometries is an important topic, yet when dealing with extreme nanoscale magnetic systems artefacts can often dominate the measurements and obscure the effects of intrinsic magnetic origin. In this work we study the evolution of domain wall depinning in electromigrated ferromagnetic junctions which are both initially fabricated and subsequently tailored in-situ in clean ultra-high vacuum conditions. Carefully designed Ni80Fe20 (Permalloy) notched half-ring structures are fabricated and investigated as a function of constriction width by tailoring the size of the contact using controlled in-situ electromigration. It is found that the domain wall pinning strength is increased on reducing the contact size in line with a reduction of the wall energy in narrower constrictions. Furthermore, the angular dependency and symmetry of the depinning field is measured to determine the full pinning potential for a domain wall in a system with a narrow constriction.

Heat transport of graphene-based normal metal–ferromagnetic barrier-superconductor junctions

In this paper, the effect of ferromagnetic barrier (FB) on the thermal conductance of graphene-based normal metal/ferromagnetic barrier/s- and d-wave superconductor junction has been studied theoretically. The results show that the exchange energy (h) in the ferromagnetic barrier has a strong effect on the amplitude, phase and period of the thermal conductance oscillations in terms of the FB length. Also, we have discussed novel thermal conductance oscillations in a graphene-based NFBS junction that happened for maximum superconducting orientation α=π/4. This finding is arisen from the quantum nature of interference between spin states, up and down quasiparticles in FB region. Interestingly, at α=π/4 the h has also the highest effect on the thermal conductance and by increasing the exchange energy h, the thermal conductance enhances at this angle. The exchange energy h also affects the thermal conductance as a function of temperature for both s- and d-wave superconductor. At last, we propose an experimental setup to detect our predicted effects.

Josephson effect in normal and ferromagnetic topological-insulator junctions: Planar, step, and edge geometries

We investigate Josephson junctions on the surface of a three-dimensional topological insulator in planar, step, and edge geometries. The elliptical nature of the Dirac cone representing the side surface states of the topological insulator results in a scaling factor in the Josephson current in a step junction as compared to the planar junction. In edge junctions, the contribution of the Andreev bound states to the Josephson current vanishes due to spin-momentum locking of the surface states. Furthermore, we consider a junction with a ferromagnetic insulator between the superconducting regions. In these ferromagnetic junctions, we find an anomalous finite Josephson current at zero phase difference if the magnetization is pointing along the junction (and perpendicular to the Josephson current). An out-of-plane magnetization with respect to the central region of the junction opens up an exchange gap and leads to a non-monotonic behavior of the critical Josephson current for sufficiently large magnetization as the chemical potential increases.

Magnetoresistance in half-metallic/metallic ferromagnetic junction through silicon

We report spin transport through the silicon in novel magnetic junction with half metallic as free layer and metallic as pinned layer. We used La0.7Sr0.3MnO3 as free layer, FeCo as pinned layer and studied the magnetoresistance through silicon as spacer layer. We fabricated this magnetic tunnel junction using RF/DC sputtering technique over SrTiO3 substrate. Tunneling magnetoresistance (TMR) measurement for this junction at room temperature was found to be 1.1 %. At 2 K, we found a large magnetoresistance of 396 %. TMR found to be increased with decreasing temperature. The results are discussed.

Valley and spin thermoelectric transport in ferromagnetic silicene junctions

We have investigated the valley and spin resolved thermoelectric transport in a normal/ferromagnetic/normal silicene junction. Due to the coupling between the valley and spin degrees of freedom, thermally induced pure valley and spin currents can be demonstrated. The magnitude and sign of these currents can be manipulated by adjusting the ferromagnetic exchange field and local external electric field, thus the currents are controllable. We also find fully valley and/or spin polarized currents. Similar to the currents, owing to the band structure symmetry, tunable pure spin and/or valley thermopowers with zero charge counterpart are generated. The results obtained here suggest a feasible way of generating a pure valley (spin) current and thermopower in silicene.

Giant magnetoresistance in the junction of two ferromagnets on the surface of diffusive topological insulators

We reveal the giant magnetoresistance induced by the spin-polarized current in the ferromagnet (F${}_{1}$)/ topological insulator (TI)/ferromagnet (F${}_{2}$) junction, where two ferromagnets are deposited on the diffusive surface of the TI. We can increase and reduce the value of the giant magnetoresistance by tuning the spin-polarized current, which is controlled by the magnetization configurations. The property is intuitively understood by the nonequilibrium spin-polarized current, which plays the role of an effective electrochemical potential on the surface of the TI.

Dual Control Theory of Spintronics

For explaining about the quantized Barkhausen jump that is observed in iron nanowires system, we introduced the two type spin Josephson junction models. One type of these system are composed of the sandwich structure by FM (ferromagnet)/AM (anti ferromagnet)/FM junction, and its dual junction is consists of the sandwich structure by AF/FM/AF junction. In order to apply to spintronics technology these spin junction models, we constructed a dual control theory by pseudo-spin quantum Hall effect. [doi:10.2320/matertrans.MAW201406]

Sd-Exchange emission in ferromagnetic junctions

The spin-injection emission in a ferromagnetic junction at terahertz frequencies is theoretically analyzed. It is demonstrated that the efficiency of the emission caused by the sd-exchange interaction of the injected spin in which the electromagnetic field is involved strongly depends on the orientation angle of the magnetization of the active region relative to the magnetization direction of the injecting region. The fact that the calculated radiation frequency and power are close to the experimental results shows that the sd-exchange emission must be taken into account in the interpretation of the observed terahertz emission in magnetic junctions.

Anisotropic spin-dependent thermopower and current in ferromagnetic graphene junctions

We study the spin-dependent thermoelectric transport through two-dimensional normal/ferromagnetic/normal/ferromagnetic/normal graphene (NG/FG/NG/FG/NG) junctions. It is found that both charge and spin thermopowers depend on the FGʼs magnetization direction and exhibit an anisotropic behavior. Interestingly, the spin thermopower can be as large as the charge thermopower and even can exceed the latter in magnitude. Moreover, the pure spin thermopower and spin current emerge in this device. The results obtained here suggest a feasible way of enhancing thermospin effects and generating the pure spin current in two-dimensional graphene. • Spin-dependent thermopower in ferromagnetic graphene junctions is studied. • Spin and charge thermopowers are anisotropic. • Spin thermopowers can be as large as the charge thermopower. • A pure spin thermopower (spin current) is obtained.