Bottom Magnetic Layer Research Articles

Stabilization of CsPbBr3 Nanowires Through SU-8 Encapsulation for the Fabrication of Bilayer Microswimmers with Magnetic and Fluorescence Properties.

All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals have drawn great interest because of their excellent photophysical properties and potential applications. However, their poor stability in water greatly limited their use in applications that require stable structures. In this work, a facile approach to stabilize CsPbBr3 nanowires is developed by using SU-8 as a protection medium; thereby creating stable CsPbBr3/SU-8 microstructures. Through photolithography and layer-by-layer deposition, CsPbBr3/SU-8 is used to fabricate bilayer achiral microswimmers (BAMs), which consist of a top CsPbBr3/SU-8 layer and a bottom Fe3O4 magnetic layer. Compared to pure CsPbBr3 nanowires, the CsPbBr3/SU-8 shows long-term structural and fluorescence stability in water against ultrasonication treatment. Due to the magnetic layer, the motion of the microswimmers can be controlled precisely under a rotating magnetic field, allowing them to swim at low Reynolds number and tumble or roll on surfaces. Furthermore, CsPbBr3/SU-8 can be used to fabricate various types of planar microstructures with high throughput, high consistency, and fluorescence properties. This work provides a method for the stabilization of CsPbBr3 and demonstrates the potential to mass fabricate planar microstructures with various shapes, which can be used in different applications such as microrobotics.

Investigation of magnetization dynamics in trilayer width-modulated nanowires

We have investigated the magnetization reversal processes and dynamic behavior of trilayered Py(50 nm)/Pd(tPd)/Py(20 nm) nanowires with periodic width modulation as a function of spacer layer thickness tPd in the range from 0 to 10 nm and compared them with single-layer nanowires. The ferromagnetic resonance spectra show more than three modes that result from a non-uniform demagnetizing field in width-modulated nanowires. We observe that the spacer layer thickness influenced the ferromagnetic resonance spectra, which showed different numbers and values of modes and frequencies due to the different magnetization configurations for different spacer layer thicknesses. We also found that the two ferromagnetic layers are exchange-coupled for tPd = 2 nm nanowire arrays, showing the sharp switching of magnetization from the static measurements and sharp frequency jump from 13.6 to 14.7 GHz around −18 mT from the dynamic measurements. However, for tPd = 10 nm, the two layers switch at different fields, indicating a gradual decrease in magnetization as the reversal is mediated through dipolar coupling. The origin of modes is well explained from the spatial mode profiles of top and bottom magnetic layers. The dynamic responses in this spin-valve-type structure are useful for designing microwave-based spintronic devices.

Field-free spin–orbit torque switching in ferromagnetic trilayers at sub-ns timescales

Current-induced spin torques enable the electrical control of the magnetization with low energy consumption. Conventional magnetic random access memory (MRAM) devices rely on spin-transfer torque (STT), this however limits MRAM applications because of the nanoseconds incubation delay and associated endurance issues. A potential alternative to STT is spin-orbit torque (SOT). However, for practical, high-speed SOT devices, it must satisfy three conditions simultaneously, i.e., field-free switching at short current pulses, short incubation delay, and low switching current. Here, we demonstrate field-free SOT switching at sub-ns timescales in a CoFeB/Ti/CoFeB ferromagnetic trilayer, which satisfies all three conditions. In this trilayer, the bottom magnetic layer or its interface generates spin currents with polarizations in both in-plane and out-of-plane components. The in-plane component reduces the incubation time, while the out-of-plane component realizes field-free switching at a low current. Our results offer a field-free SOT solution for energy-efficient scalable MRAM applications.

Theoretical and experimental study of anomalous Nernst effect in biphasic magnetic system

We perform a theoretical and experimental investigation of magnetic properties and anomalous Nernst effect in a NiFe/Cr/NiFe trilayer. Our system presents a biphasic magnetic behavior, a response owed to distinct structural properties of the bottom and top magnetic layers. We verify the thermomagnetic results strongly depend on the anisotropies and magnetic response of the biphasic system. Moreover, the thermomagnetic results seem to be much more sensitive to the anisotropies than the magnetometry ones. We show the anomalous Nernst coefficient found in our trilayer is quite comparable with those verified for high-efficient thermomagnetic systems. We also disclose an enhancement if compared with the ones observed for single layers. Beyond, our results highlight the role of each ferromagnetic layer on the thermomagnetic response. Our experimental and theoretical findings show that the top and bottom layers contribute differently to the thermomagnetic signal due to the Cr spacer in the trilayer system.

Bipolar spin Hall nano-oscillators

We demonstrate a spin Hall nano-oscillator (SHNO) that allows for efficient tuning of magnetic auto-oscillations over an extended range of gigahertz frequencies, using bipolar direct currents at constant magnetic fields. This is achieved by stacking two distinct ferromagnetic layers with a platinum interlayer. In this device, the orientation of the spin polarized electrons accumulated at the top and bottom interfaces of the platinum layer is switched upon changing the polarity of the direct current. As a result, the effective anti-damping required to drive large amplitude auto-oscillations can appear either at the top or bottom magnetic layer. Tuning of the auto-oscillation frequencies by several gigahertz can be achieved by combining two materials with sufficiently different saturation magnetizations. Here, we show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced synchronization capabilities to neuromorphic computing applications.

Anisotropic giant magnetoresistive effect in the sandwich based FexNi1−x (x ≈ 0.5) and Cu

The anisotropic effect of a giant magnetoresistance (GMR) in the three-layer FexNi1−x/Cu/FexNi1−x/Sub (x ≈ 0.5, Sub is the substrate) magnetically ordered film is analyzed experimentally and theoretically using the phenomenological approach. It is shown that in the case when the direction of the current density vector j coincides with the direction of the local magnetization vector M (j∥M) in magnetic layers, the consideration of the resistance anisotropy results in a decrease in the magnitude of the GMR effect; and while the vectors are mutually perpendicular in the film plane (j⊥M), the GMR value increases. The analysis of the size dependence (dependence of the top magnetic layer on the thickness dm2) of the magnetoresistance ratio (MR ratio) shows that, in the case of the isotropic GMR effect, when inequalities dm2 > dm1) (dm1 is the thickness of the bottom magnetic layer) hold, the indicated effect is negligible due to shunting of the top layer resistance by the base layer resistance (shunting of the base layer resistance by the top-layer resistance). In the case when the magnetic layer thicknesses are comparable in size (dm1 ~ dm2), the magnitude of the anisotropic giant magnetoresistance (AGMR) acquires its maximum (amplitude) value because of the absence of the shunting effect. The method for calculating the asymmetry parameter αlj=l0j+∕l0j– (l0js is the spin-dependent free path of charge carriers in the s = ±th spin channel of the jth magnetic layer), which characterizes the difference in the free paths of electrons in the spin conduction channels, is proposed for the first time.

Spin currents and spin-orbit torques in ferromagnetic trilayers.

Magnetic torques generated through spin-orbit coupling1-8 promise energy-efficient spintronic devices. For applications, it is important that these torques switch films with perpendicular magnetizations without an external magnetic field9-14. One suggested approach 15 to enable such switching uses magnetic trilayers in which the torque on the top magnetic layer can be manipulated by changing the magnetization of the bottom layer. Spin currents generated in the bottom magnetic layer or its interfaces transit the spacer layer and exert a torque on the top magnetization. Here we demonstrate field-free switching in such structures and show that its dependence on the bottom-layer magnetization is not consistent with the anticipated bulk effects 15 . We describe a mechanism for spin-current generation16,17 at the interface between the bottom layer and the spacer layer, which gives torques that are consistent with the measured magnetization dependence. This other-layer-generated spin-orbit torque is relevant to energy-efficient control of spintronic devices.

Micromagnetic Calculation for Magnetic Tunnel Junctions With Equal Parallel and Antiparallel Thermal Stability

In magnetic tunnel junctions (MTJs) with perpendicular magnetic easy axes, the retention times for parallel and antiparallel states are different because of dipolar coupling with the reference layer (RL). To make retention times symmetrical, we model three types of MTJs that have single or synthetic ferrimagnetic (SyF) RLs with stepped structures. Micromagnetic calculations are performed on the basis of the Landau–Lifshitz–Gilbert equation of motion to evaluate the stray fields from the RLs of each design. A SyF structure with a step on the bottom magnetic layer has the smallest stray field of 2 mT at lateral dimensions of 30 nm × 30 nm in the layer. Symmetric thermal stability for parallel and antiparallel states should help to realize high-density nonvolatile magnetic random-access memory and logic chips.

Fabrication of package embedded spiral inductors with two magnetic layers for flexible SIP point of load converters in Internet of Everything devices

Inductive DC-DC converters are key elements in power delivery units for the development of Internet of Everything (IoE) architectures made of interconnected networks of smart devices, self-powered from different energy sources. The ultra-low power levels involved in IoE devices in addition to cost reduction of final products requires the development of new technologies for the miniaturization of all the active and passive components. In this paper, a low temperature, cheap and simple two-steps fabrication process of embedded planar inductors with two magnetic layers on a very thin, flexible and lightweight FR4 organic substrate is demonstrated for flexible System in package (SIP) based point of load DC-DC converters in IoE devices. The process uses standard printed wiring board (PWB) copper etching to define the square geometry of the planar inductors and trenches underneath the inductor copper traces. Very thick top and bottom magnetic layers are deposited using stencil printing of an FR4 compatible epoxy-NiZn ferrite composite magnetic material on top of the copper traces and into the trenches. At 30MHz, the permittivity is 3.4 and 6.23 while the dielectric loss tangent is 0.014 and 0.009 for the epoxy and composite magnetic material respectively. The composite material has a permeability of 8.46, a loss tangent value of 0.1 at a frequency of 30MHz and saturates around 0.13 Tesla. The DC resistance varies from 136mΩ to 386mΩ while at 30MHz the AC resistance varies from 1.9Ω to 10.1Ω for an inductance value from 180nH to 715nH, corresponding to an inductance density between 5.69nH/mm2 and 12.47nH/mm2 and a quality factor between 13 and 17. Flexibility and its effect on the electrical parameters of the fabricated inductors is demonstrated through bending tests. A variation of <3% for the DC resistance is obtained for 5mm bend radius. At 30MHz and for a bend radius (both upward and downward) from 3.7mm to 5mm, the flat state (unbent) AC resistance and inductance values (1.9Ω to 10.1Ω for 180nH to 715nH) decrease by 6.5% to 18% and by 2% to 4.4% respectively which increase the self-resonant frequency by 3% to 3.4% and the quality factor by 2.2% to 20%. The characterized inductors are modeled in ANSYS HFSS electromagnetic simulator and simulation results show a good correspondence with the measured inductances and quality factors, which allows a reliable prediction of the inductors behavior for DC-DC converter design and implementation for IoE devices.

Observation of superparamagnetism in coexistence with quantum anomalous Hall C\u2009=\u2009\xb11 and C\u2009=\u20090 Chern states

Simultaneous transport and scanning nanoSQUID-on-tip magnetic imaging studies in Cr-(Bi,Sb)2Te3 modulation-doped films reveal the presence of superparamagnetic order within the quantum anomalous Hall regime. In contrast to the expectation that a long-range ferromagnetic order is required for establishing the quantum anomalous Hall state, superparamagnetic dynamics of weakly interacting nanoscale magnetic islands is observed both in the plateau transition regions, as well as within the fully quantized C = ±1 Chern plateaus. Modulation doping of the topological insulator films is found to give rise to significantly larger superparamagnetic islands as compared to uniform magnetic doping, evidently leading to enhanced robustness of the quantum anomalous Hall effect. Nonetheless, even in this more robust quantum state, attaining full quantization of transport coefficients requires magnetic alignment of at least 95% of the superparamagnetic islands. The superparamagnetic order is also found within the incipient C = 0 zero Hall plateau, which may host an axion state if the top and bottom magnetic layers are magnetized in opposite directions. In this regime, however, a significantly lower level of island alignment is found in our samples, hindering the formation of the axion state. Comprehension and control of superparamagnetic dynamics is thus a key factor in apprehending the fragility of the quantum anomalous Hall state and in enhancing the endurance of the different quantized states to higher temperatures for utilization of robust topological protection in novel devices.

Manipulation of magnetization switching and tunnel magnetoresistance via temperature and voltage control.

Magnetization switching between parallel and antiparallel alignments of two magnetic layers in magnetic tunnel junctions (MTJs) is conventionally controlled either by an external magnetic field or by an electric current. Here, we report that the manipulation of magnetization switching and tunnel magnetoresistance (TMR) in perpendicularly magnetized CoFeB/MgO/CoFeB MTJs can be achieved by both temperature and voltage. At a certain range of temperature, coercivity crossover between top and bottom magnetic layers is observed in which the TMR ratio of the MTJs is almost unmeasurable. Furthermore, the temperature range can be tuned reversibly by an electric voltage. Magnetization switching driven by the voltage reveals an unconventional phenomenon such that the voltage driven coercivity changes with temperature are quite different for top and bottom CoFeB layers. A model based on thermally-assisted domain nucleation and propagation is developed to explain the frequency and temperature dependence of coercivity. The present results of controlling the magnetization switching by temperature and voltage may provide an alternative route for novel applications of MTJs based spintronic devices.

Magnetization reversal in asymmetric trilayer dots: effect of the interlayer magnetostatic coupling

The spin structure and magnetization reversal in Co/insulator/Fe trilayer nanodots are investigated by micromagnetic simulations. The vortex and C-state are found and the magnetization reversal is dominated by the shape asymmetry of the dots, which is produced by cutting off a fraction of the circular dot. The vortex chirality is thus controlled by the magnetic field direction. On the other hand, the magnetostatic interaction between the top and bottom magnetic layers has interesting influence on the dot reversal process, where the magnetocrystalline anisotropy direction of the Co layer is allowed to vary within the layer plane. The combined effect of these two aspects is discussed on the base of dot coercivity, remanent magnetization, vortex nucleation and annihilation, and the bias of the Fe layer hysteresis loop. While leading to a new S-state in circle dots, the interlayer interaction facilitates the formation of C-state in asymmetric dots, which reduces the vortex nucleation field. The bias effect of all dots is decreased with the deviation of the Co layer easy axis from the field direction. Unlike the circle and semicircle dots, the field range of the vortex state in other asymmetric dots increases with the angle between the cutting direction and the Co layer anisotropy. Additionally, vortex ranges in less asymmetric dots even larger than that in the circle dots are evidenced unexpectedly. Therefore, the control of the vortex chirality and enhancement of the vortex range are found simultaneously.

Electric-field control of CoFeB/IrMn exchange bias system

The electrically controlled spintronic devices based on magnetism at the interface is a key challenge today. We have studied the top sub./CoFeB/IrMn and bottom sub./IrMn/CoFeB pinned exchange bias systems as a function of electric field at room temperature deposited on the (011)-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) piezoelectric substrate. It was found that the electric-field tuning of exchange bias was very small although both the structures show good regular magnetoelectric coupling. We propose an alternative way to control the exchange bias via electric field in the multilayered structure by inserting a metallic spacer layer between exchange bias bilayer and bottom free magnetic layer, i.e., PMN-PT/CoFeB/Cu/CoFeB/IrMn. We successfully tuned the exchange bias of such multilayer structure as function of electric field at room temperature. Our results show a step forward in utilizing electrically controlled multiferroic systems for practical applications.

FePt-Based Exchange-Coupled Composite Perpendicular Recording Media

In order to demonstrate the strong and proper exchange coupling between bottom magnetic layer and capping magnetic layer in FePt-based exchange-coupled composite (ECC) perpendicular recording media, we have investigated the thermal stability, writability, degree of exchange coupling, and ECC gain factor of FePt-based ECC media as a function of the thickness of the capping FePt layer. With increasing the thickness of capping FePt layer, both thermal stability and media writability increase simultaneously as a result of the change from strong exchange coupling to proper exchange coupling. The proper exchange-coupled FePt ECC is a promising candidate for future high-density perpendicular recording.

Tuning exchange coupling by replacing CoFe with amorphous CoFeB in the CoFe/Ru/CoFe synthetic antiferromagnetic structure

We studied exchange coupling in the CoFe/Ru/CoFe synthetic antiferromagnetic structure with systematical replacement of the crystalline CoFe with amorphous CoFeB. Antiferromagnetic exchange coupling intensity decreases with an increase in the replacement in the bottom magnetic layer, which indicates that exchange coupling intensity could be tuned by the replacement. The origin of weakening antiferromagnetic exchange coupling is attributed to the amorphous CoFeB replacement inducing incomplete crystallization and disordered orientation in the Ru layer.

Bistable control of ferromagnetic resonance frequencies in ferromagnetic trilayered dots

The ferromagnetic resonance (FMR) properties of submicron-scale Ni81Fe19/Cu/Ni81Fe19 trilayered dots were investigated for application in programable microwave absorbers for broadband filter devices. The trilayered dot shows binary FMR frequencies, depending on the relative orientation of magnetization in the top and the bottom magnetic layers. A parallel configuration of magnetization, which exhibits a lower FMR frequency than that for an antiparallel configuration, can be metastable as the lateral aspect ratio of the dot is larger than the threshold value governed by the Cu layer thickness. The difference in the FMR frequencies between the parallel and antiparallel configurations is gradually decreased with elongation of the dot. This is associated with the suppression of the magnetostatic coupling energy that stabilizes the antiparallel configuration. It was also found that the magnetic configurations can be programed by the selective application of easy- and hard-axis magnetic fields used to saturate the magnetization. This is an appropriate manner to realize the antiparallel configuration of magnetization in the case of a dot with a plural stack of magnetic layers with the same coercive field.

Improvement of recording performances through exchange coupling in triple-layered PMR media

Improved recording performances with exchange coupling between bottom magnetic layer and medium magnetic layer in triple-type perpendicular magnetic recording (PMR) media are presented. Fundamentally, triple-type PMR media with exchange coupling between magnetic layers reduces DC erase noise and improves writability. Furthermore, triple-type PMR media could be expected clear magnetic cluster as well as relatively sharp transition pattern. Consequently, better spectral integral signal-to-noise ratio (SpiSNR), improved OW and about 1.5–2.0 order better bit error rate (BER) performances in triple-type PMR media with exchange coupling between magnetic layers were attributed to the reduction of DC erase noise and sharp transition as well as improved writability. In addition, work of micro-magnetic modeling on SNR profiles could help to verify the understanding on the role of exchange coupling between magnetic layers in triple-type PMR media.

Oscillatory tunnel magnetoresistance in double barrier magnetic tunnel junctions

We report an unconventional oscillatory tunnel magnetoresistance as a function of the applied bias in double barrier magnetic tunnel junctions that were made of two Al2O3 barriers sandwiched by three ferromagnetic layers. When the center ferromagnetic layer is aligned antiparallel to the top and bottom magnetic layers, a distinct magnetoresistance oscillation appears with respect to the increase of the bias voltage at 4.2 K and at room temperature. The period of the oscillation is about 1.6 mV.

Composite-anisotropy amorphous magnetic materials for high-frequency devices

In-plane biaxial anisotropy in permeability, desired for integrated inductor cores, was achieved using a composite-anisotropy sandwiched structure. With Tb-doped CoTaZr alloy as the material for magnetic layers, the permeability of the composite film was close to 50% of that of blanket film along the easy axes of both top and bottom magnetic layers, and the ferromagnetic-resonance frequencies were about 2GHz. Simulations show that the integrated inductors with the composite film as their magnetic cores have inductances near a half of that for the ideal isotropic permeability case.

Magnetoresistance effects on La 0.67Ca 0.33MnO 3/YBa 2Cu 3O y/Ni trilayers

La 0.67Ca 0.33MnO 3 (LCMO)/YBa 2Cu 3O y (YBCO)/Ni trilayers were fabricated with varying thickness of the mid-superconducting YBCO layer. Two different deposition systems were used to avoid oxidation of the magnetic metal Ni and contamination of the deposition systems. Transport properties show that the temperature dependence of resistance goes from a metallic-to-metal–insulator transition (manganite-like behavior) to a superconducting behavior (M–M–S) with increase of the thickness of the YBCO layer. The magnetoresistance (MR) ratio is very small in comparison with LCMO/YBCO bilayers at low temperature. Such phenomenon may depend on the charge transfer and the magnetic spin interaction between the bottom and cap magnetic layers.