Antiparallel Magnetization States Research Articles

Tuning of the ferroelectric and optical properties of ZnSnO3 with transition metals: Mn, Cr and Co

Using an approach based on the density functional theory, the electronic, polarization, structural, and optical properties of Co, Cr and Mn doped ZnSnO3 were studied. The analysis of the structural properties shows that the introduction of magnetic ions produces antiparallel magnetic states and reduces the band gap as compared with the gap of pristine ZnSnO3. Additionally, the results reveal the persistence of polarization after doping, reaching a maximum polarization value of 44.94 μC/cm2 for 12.5 % Co-doped ZnSnO3. These results highlight the importance of ZnSnO3 as a candidate material for solar energy harvesting, by combining a reduced gap and significant spontaneous polarization.

Competition between the superconducting spin-valve effect and quasiparticle spin-decay in superconducting spin-valves

In a ferromagnet/normal metal/ferromagnet spin-valve, spin dependent scattering causes a difference in resistance between antiparallel (AP) and parallel (P) magnetization states. The resistance difference, ΔR = R(AP) − R(P) is positive due to increased scattering of majority and minority spin-electrons in the AP-state. If the normal metal is substituted for a superconductor, the superconducting spin-valve effect occurs: in the AP-state the net magnetic exchange field acting on the superconductor is lowered and the superconductivity is reinforced meaning R(AP) decreases. For current-perpendicular-to-plane spin-valves, existing experimental studies show that the normal state effect dominates (ΔR > 0) over the superconducting spin valve effect (ΔR < 0). Here however, we report a crossover from giant magnetoresistance (ΔR > 0) to the superconducting spin-valve effect (ΔR < 0) in current-perpendicular-to-plane ferromagnet/superconductor/ferromagnet spin-valves as the superconductor thickness decreases below a critical value.

Spin rectification by planar Hall effect in synthetic antiferromagnets

We study spin rectification effects in NiFe/Ru/NiFe synthetic antiferromagnets at the saturated, non-collinear and antiparallel magnetic states. The change in magnetization orientation at these states allows us to study the angle dependence of spin rectification phenomena by only changing the applied field. This approach avoids the need of changing the sample orientation within the experiment for this kind of studies. Our results show large planar Hall effect contributions to the measured spin rectified voltage with interesting features in the non-collinear and antiparallel states, which bring new possibilities for device design and applications. The results improve the understanding of electrically detected ferromagnetic resonance signals, not only for synthetic antiferromagnets, but also for other samples where the signals coming from the anisotropic magneto resistance effect and the inverse spin hall effect are superimposed.

Van der Waals Magnetic Heterojunctions with Giant Zero‐Bias Tunneling Magnetoresistance and Photo‐Assisted Magnetic Memory

AbstractVan der Waals (vdW) magnetic heterostructures can offer much improved performance in logical operation and information storage technology compared with conventional ones. However, it is still challenging to achieve the perfect spin‐filtering capability in the vdW magnetic devices, a major obstacle to the advancement of low‐dimensional magnetic information storage. Herein, this study reports two newly designed vdW magnetic multilayers, ZrTe2/CrOCl/CrOX/ZrTe2 (X = Cl, Br), where the CrOCl/CrOX bilayer acts as the spin‐filter tunnel barriers. With the vdW interfacial engineering and the doubly spin‐filtering effect of the CrOCl/CrOX bilayer, the vdW four‐layer heterostructures can potentially function as a perfect zero‐bias spin filter with giant spin‐filter energy‐dependent tunnel magnetoresistance up to 48 000%. Importantly, these calculations also show that the CrOCl/CrOBr bilayer without the 1T‐ZrTe2 stacking can produce intensive and weak photo‐carrier transmission at parallel and antiparallel magnetization states, respectively. As such, the two contrasting photoelectric responses can be implemented for encoding the digital information as “‘1”’ and “‘0”’ via flipping the interlayer magnetic states. These novel functionalities not only endow the CrOCl/CrOX bilayer as a promising candidate for spin‐based vdW devices but also facilitate the future development of atomically thin magnetic information storage.

Magnetization dynamics in synthetic antiferromagnets: Role of dynamical energy and mutual spin pumping

We investigate magnetization dynamics in asymmetric interlayer exchange coupled Py/Ru/Py trilayers using both vector network analyzer-based and electrically detected ferromagnetic resonance techniques. Two different ferromagnetic resonance modes, in-phase and out-of-phase, are observed across all three regimes of the static magnetization configurations, through antiparallel alignment at low fields, the spin-flop transition at intermediate fields, and parallel alignment at high fields. The nonmonotonic behavior of the modes as a function of the external field is explained in detail by analyzing the interlayer exchange and Zeeman energies and is found to be solely governed by the interplay of their dynamical components. In addition, the linewidths of both modes were determined across the three regimes and the different behaviors of the linewidths versus external magnetic field are attributed to mutual spin pumping induced in the samples. Interestingly, the difference between the linewidths of the out-of-phase and in-phase modes decreases at the spin-flop transition and is reversed between the antiparallel and parallel aligned magnetization states.

Manipulation of free-layer bias field in giant-magnetoresistance spin valve by controlling pinned-layer thickness

The manipulation of the bias field of the free-layer in giant magnetoresistance spin-valves is of great importance in sensor applications because this feature dominantly affects the low-field sensitivity of magnetoresistance. In this study, it is demonstrated that the bias field of the free-layer can be manipulated by controlling the thickness of the pinned-layer deposited afterward. The key to success is the utilization of the magnetostatic interactions between the free-poles formed on the Néel walls in both free- and pinned-layers. Magnetostatic interactions play a role in stabilizing the antiparallel magnetization state and hence in suppressing the magnetization switching of the free-layer from an antiparallel to a parallel state. A nearly zero bias field is achieved for a Ta-buffered sample with a pinned-layer thickness of 1.75 nm, where a very high low-field sensitivity of 7.7 mV/mA·Oe is obtained.

Numerical analysis of the spin dynamics in bilayer nanodots with interlayer antiferromagnetic coupling

Synthetic antiferromagnetic nanodots with perpendicular magnetic anisotropy are promising candidates for improving the performance of magnetic random-access memory or spin torque nano-oscillators; however, the mechanism for the interlayer antiferromagnetic coupling is still not completely understood. Therefore, we numerically investigated the ferromagnetic resonance characteristics of perpendicularly magnetized bilayer nanodots with interlayer antiferromagnetic coupling. The results show that the resonance frequency strongly depends on the interlayer antiferromagnetic coupling intensity and the individual layer thickness. It was found that external fields induce opposite resonance peak shifts, reflecting the contradicting Zeeman energy effect on individual layers with opposite magnetization directions. The resonance properties were successfully reconfigured by adjusting the uniaxial anisotropy and coupling intensity. Moreover, bistable (parallel and antiparallel) magnetization states were controlled by applying an external field sweep. The difference between the resonance frequencies of two bistable states was enhanced by decreasing the layer thickness and increasing the antiferromagnetic coupling intensity. Our numerical results demonstrate the potential ability of currently available strong interlayer exchange coupling for further increasing of high resonance frequencies in the synthetic antiferromagnet system with perpendicular anisotropy.

Tailoring of magnetic properties of giant magnetoresistance spin valves via insertion of ultrathin non-magnetic spacers between pinned and pinning layers

The low-field sensitivity of a giant magnetoresistance (GMR) spin valve can be enhanced by tailoring the bias field of the free layer because this sensitivity and bias field are known to show a strong correlation. In this study, the free-layer bias field is reduced considerably to almost zero via the insertion of an ultrathin nonmagnetic spacer between the pinned layer and the pinning layer. The spacer promotes an increase in the density of Néel walls in the pinned layer. This increase, in turn, induces domain-wall-induced magnetostatic interactions of the free poles formed on the Néel walls inside the free and pinned layers. The magnetostatic interactions result in the formation of flux closures that act as pinning sites during the magnetization reversal process and stabilize the antiparallel magnetization state between the free layer and the pinned layer by suppressing the switching of the free layer from the antiparallel state to the parallel state. Furthermore, the spacer offers an additional advantage of increasing the GMR ratio by inducing a specular scattering effect at its top and bottom interfaces. A highly improved low-field sensitivity of 12.01 mV/mA·Oe is achieved in the sample with a Cu/Pt dual spacer.

Electrical detection of multiple ferromagnetic resonance modes in interlayer exchange coupled Fe20Ni80/Ru/Fe20Ni80 multilayers

We report the electrical detection of multiple ferromagnetic (FM) resonance modes in multilayers of Pt/Fe20Ni80/Ru/Fe20Ni80 using microwave spin pumping and the inverse spin Hall effect (ISHE) as a function of Ru layer thickness tRu. The electrically detected voltage signals were analyzed for the anti-parallel magnetization state as well as the “spin-flop” magnetization state of the two Fe20Ni80 layers. When the two FM layers are antiferromagnetically (AFM) coupled (tRu = 1 nm), both the optic and acoustic modes were observed. The amplitude of the acoustic mode voltage signals obtained increases with frequency. However, when the two magnetic layers are ferromagnetically coupled (tRu = 1.4 nm), only acoustic mode was observed whose voltage signals decreases as frequency increases. We demonstrate the tunability of the sign of the ISHE voltage in the AFM coupled system by controlling the polarization of the spin currents.

Telegraphic switching signals by magnet tunnel junctions for neural spiking signals with high information capacity

Magnetic tunnel junctions (MTJs) operating in the superparamagnetic regime produce telegraphic signals that emulate neural spiking signals. Previous studies have characterized the random spiking signals produced by MTJs in terms of the percentage of time spent in the anti-parallel (AP) magnetization state (referred to as the “AP rate”) but ignore the switching rate of the MTJ. In this work, we demonstrate that with proper tuning of both an external bias field and a bias voltage, we can control the average dwell time in the AP-state and P-state pulses separately. Our data show that the AP rate can be tuned with bias voltages ranging from 310 mV to 460 mV and bias fields from −200 Oe to −230 Oe. The average dwell times in each state ranged from 225 ns to 285 μs and could be controlled separately. This suggests that neural spiking signals produced by MTJs can be decoded by both the spike rate and the spike count, which creates the possibility for increasing the information capacity in the rate coding scheme.

Tailoring the magnetic and magneto-transport properties of Pd/Co multilayers and pseudo-spin valve antidots

Magnetic multilayers and pseudo-spin valves (PSVs) based on Pd/Co were tailored to present well-defined antiparallel magnetization states while maintaining a high perpendicular magnetic anisotropy (PMA). The magneto-electrical properties were measured as a function of intermediate Cu spacers to determine the values that optimize magnetoresistance and the Hall effect. Magnetic multilayers were also deposited on flat porous alumina membranes, forming antidots. The flat antidots shows good PMA and PSVs were studied in these systems for the first time, showing improved magnetoresistance and the extraordinary Hall effect (EHE). Anomalous peaks were obtained in the EHE curves of the PSVs on both continuous films and 40 nm antidots, with four well-defined remnant Hall resistance states.

A multi-state synthetic ferrimagnet with controllable switching near room temperature

Ferrite composites with temperature-induced magnetization reversal, and synthetic ferrimagnets and antiferromagnets have been of great interest to the scientific community due to their uncommon thermal properties and potential applications in magnetic storage, spintronic devices, and several other fields. One of the advantages of these structures is the strong antiferromagnetic coupling, which stabilizes the magnetization state and gives access to interesting static and dynamical magnetic behaviors. Some of their drawbacks lie in that it is difficult to induce temperature-induced magnetization reversal at room temperature in composites, and that the strong interaction makes it difficult to induce a parallel magnetization state (and thus a high magnetic moment). In this work, we study numerically the magnetization behaviour of a Cu(1 0 0)/Ni/Pt/[Co/Pt]4 synthetic ferrimagnet and show that is possible to revert the sign of its magnetization by varying the temperature in ranges around room temperature. We also show that the four parallel and antiparallel magnetization states are stable at temperatures up to 360 K, and demonstrate that it is possible to change deterministically between these states by increasing the temperature of the device and/or applying a magnetic field, showcasing simultaneous non-hysteretic and hysteretic switching processes induced by temperature. Thus, this structure opens the possibility to have reconfigurable magnetic devices with multiple purposes based on the nature of the different switching events and the interplay between them.

Spin-valve Josephson junctions for cryogenic memory

Josephson junctions containing two ferromagnetic layers are being considered for use in cryogenic memory. Our group recently demonstrated that the ground-state phase difference across such a junction with carefully chosen layer thicknesses could be controllably toggled between zero and $\ensuremath{\pi}$ by switching the relative magnetization directions of the two layers between the antiparallel and parallel configurations. However, several technological issues must be addressed before those junctions can be used in a large-scale memory. Many of these issues can be more easily studied in single junctions, rather than in the superconducting quantum interference device (SQUID) used for phase-sensitive measurements. In this work, we report a comprehensive study of spin-valve junctions containing a Ni layer with a fixed thickness of 2.0 nm and a NiFe layer of thickness varying between 1.1 and 1.8 nm in steps of 0.1 nm. We extract the field shift of the Fraunhofer patterns and the critical currents of the junctions in the parallel and antiparallel magnetic states, as well as the switching fields of both magnetic layers. We also report a partial study of similar junctions containing a slightly thinner Ni layer of 1.6 nm and the same range of NiFe thicknesses. These results represent the first step toward mapping out a ``phase diagram'' for phase-controllable spin-valve Josephson junctions as a function of the two magnetic layer thicknesses.

Current control of time-averaged magnetization in superparamagnetic tunnel junctions

This work investigates spin transfer torque control of time-averaged magnetization in a small 20 nm × 60 nm nanomagnet with a low thermal stability factor, Δ ∼ 11. Here, the nanomagnet is a part of a magnetic tunnel junction and fluctuates between parallel and anti-parallel magnetization states with respect to the magnetization of the reference layer generating a telegraph signal in the current versus time measurements. The response of the nanomagnet to an external field is first analyzed to characterize the magnetic properties. We then show that the time-averaged magnetization in the telegraph signal can be fully controlled between +1 and −1 by voltage over a small range of 0.25 V. NIST Statistical Test Suite analysis is performed for testing true randomness of the telegraph signal that the device generates when operated at near critical current values for spin transfer torque. Utilizing the probabilistic nature of the telegraph signal generated at two different voltages, a prototype demonstration is shown for multiplication of two numbers using an artificial AND logic gate.

Magnetic aftereffects in CoFeB/Ta/CoFeB spin valves of large area

Magnetic aftereffect measurements on a synthetic antiferromagnet with strong perpendicular anisotropy are presented. Two types of magnetic transitions are observed in the CoFeB/Ta/CoFeB bilayer where the two ferromagnetic CoFeB layers are antiferromagnetically coupled: the transition from a parallel to an antiparallel CoFeB layers magnetization alignment and the transition between two possible antiparallel magnetization states. Magnetic relaxation measurements show a complete reversal in the case of the transition between the two antiferromagnetic configurations. The experimental data can be well fitted in the frame of extended exponential relaxation using the Fatuzzo-Labrune model. Consequently, the domain nucleation and domain-wall propagation can be studied as a function of temperature and applied field.

Electrical detection of spin accumulation and relaxation in p -type germanium

We report on electrical measurements of spin-dependent transport of holes in all-epitaxial CoFe/$p$-type germanium $(p\text{\ensuremath{-}}{\mathrm{Ge})/\mathrm{Fe}}_{3}\mathrm{Si}$ spin valves, where the hole concentration $({p}_{\mathrm{h}})$ of the $p$-Ge layer is estimated to be $\ensuremath{\sim}{10}^{18}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Spin-accumulation output voltages can electrically be detected in the antiparallel magnetization state between CoFe and ${\mathrm{Fe}}_{3}\mathrm{Si}$ ferromagnetic electrodes. The room-temperature spin lifetime of holes in the $p$-Ge layers can tentatively be discussed in terms of the theory by Fert and Jaffr\`es. We propose that the use of (111)-oriented $p$-Ge with a hole concentration of $\ensuremath{\sim}{10}^{18}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ enables the transport of spin-polarized holes in bulk Ge even at room temperature.

First-Principle Study of Effects of Magnesium Oxide Adsorption in SiCNT-Based Magnetic Tunnel Junction

We attempt to study the spin-dependent transport properties of a magnetic tunnel junction (MTJ) which consists of a magnesium oxide (MgO)-adsorbed SiCNT tunnel barrier in between two half-metallic ferromagnetic (HMF) CrO2 electrodes. The I–V characteristic curves for both parallel and anti-parallel magnetization states are obtained. The comparison of the results with other reported studies suggests that a very large value of tunnel magneto-resistance (TMR) and a perfect spin filtration is obtained majorly due to the adsorption of MgO molecules and also due to the use of HMF electrodes. For bias voltages above 0.4 V, a high TMR at ∼100% is obtained. The TMR is also high at ∼97.89% for lower bias voltages. Also, a perfect spin filtration of almost 100% is obtained at all bias voltages. These results suggest that this device can prove to be of significant use in spin valves, MRAMs, and other spintronics applications. The spin-dependent non-equilibrium transport properties are also investigated by analyzing the bias-dependent transmission coefficients.

Simulation Study on Understanding the Spin Transport in MgO Adsorbed Graphene Based Magnetic Tunnel Junction

First principles calculations of spin-dependent electronic transport properties of magnetic tunnel junction (MTJ) consisting of MgO adsorbed graphene nanosheet sandwiched between two CrO2 half-metallic ferromagnetic (HMF) electrodes is reported. MgO adsorption on graphene opens bandgap in graphene nanosheet which makes it more suitable for use as a tunnel barrier in MTJs. It was found that MgO adsorption suppresses transmission probabilities for spin-down channel in case of parallel configuration (PC) and also suppresses transmission in antiparallel configuration (APC) for both spin-up and spin-down channel. Tunnel magneto-resistance (TMR) of 100% is obtained at all bias voltages in MgO adsorbed graphene-based MTJ which is higher than that reported in pristine graphene-based MTJ. HMF electrodes were found suitable to achieve perfect spin filtration effect and high TMR. I–V characteristics for both parallel and antiparallel magnetization states of junction are calculated. High TMR suggests its usefulness in spin valves and other spintronics-based applications.

Understanding the Spin Transport in H2O-Adsorbed Graphene-Based Magnetic Tunnel Junction

First-principles calculations of spin-dependent electronic transport properties of a magnetic tunnel junction consisting of a H2O (water)-adsorbed graphene nanosheet sandwiched between two CrO2 half-metallic ferromagnetic (HMF) electrodes is reported. H2O adsorption on graphene opens a bandgap in the graphene nanosheet which makes it more suitable for use as a tunnel barrier in magnetic tunnel junctions. It was found that H2O adsorption suppresses transmission probabilities for a spin-down channel in the case of parallel configuration (PC) and also suppresses transmission in antiparallel configuration (APC) for both spin-up and spin-down channels which result in higher tunnel magnetoresistance (TMR) at higher bias voltages in these structures. HMF electrodes were found suitable to achieve the perfect spin filtration effect and high TMR. I-V characteristics for both parallel and antiparallel magnetization states of junction are calculated. A high value of TMR ∼100 % is obtained at all bias voltages in the range of 0 to 1.2 V. High TMR suggests its usefulness in spin valves and other spintronics-based applications.

Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature

We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800Oe at room temperature with a coercive field around 600Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_\mathrm{eff} = 0.41\,\mathrm{mJ}/\mathrm{m}^2$ and $K_\mathrm{eff} = 37\,\mathrm{kJ}\,/\,\mathrm{m}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds $325^\circ$C, however the median blocking temperature in the limit of thick MnN is $160^\circ$C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.