Antiferromagnetic Layer Research Articles

Terahertz-frequency oscillator driven by spin–orbit torque in NiF2/Pt bilayers

Exploration and manipulation of terahertz signal generators are crucial steps in the creation of numerous applications. Antiferromagnets can boost output signal frequency to the terahertz range. We propose a nanometer-scale generating device that produces terahertz signals by DC-exciting in a bilayer structure. The structure comprises a heavy metal layer (Pt) and a non-collinear antiferromagnetic layer (NiF2), where the magnetic moments in NiF2 with single-ion anisotropy are excited by the spin current from the Pt layer through spin–orbit torque. The inhomogeneous dynamic behaviors of the magnetic moments of NiF2 are calculated by the Landau–Lifshitz–Gilbert equation. It is found that terahertz-frequency AC can be reliably output from the bilayer structure, with the frequency that can reach to 1.82 terahertz. The oscillator shows the best performance when the polarized direction of the spin current is along the hard-axis of NiF2. The frequency and the amplitude of the AC can be adjusted by the current density, thickness and damping constant of the NiF2 layer. The threshold currents for exciting and maintaining the stable oscillation increase with the thickness and damping constant of the NiF2.

Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers.

THz radiation emitted by ferromagnetic/non-magnetic bilayers is a new emergent field in ultra-fast spin physics phenomena with a lot of potential for technological applications in the terahertz (THz) region of the electromagnetic spectrum. The role of antiferromagnetic layers in the THz emission process is being heavily investigated at the moment. In this work, we fabricate trilayers in the form of Co/CoO/Pt and Ni/NiO/Pt with the aim of studying the magnetic properties and probing the role of very thin antiferromagnetic interlayers like NiO and CoO in transporting ultrafast spin current. First, we reveal the static magnetic properties of the samples by using temperature-dependent Squid magnetometry and then we quantify the dynamic properties with the help of ferromagnetic resonance spectroscopy. We show magnetization reversal that has large exchange bias values and we extract enhanced damping values for the trilayers. THz time-domain spectroscopy examines the influence of the antiferromagnetic interlayer in the THz emission, showing that the NiO interlayer in particular is able to transport spin current.

Co/NM/Pt]n-based seedless synthetic anti-ferromagnetic layer design for preventing MgO crystallinity degradation from Pt diffusion

Co/NM/Pt]n-based seedless synthetic anti-ferromagnetic layer design for preventing MgO crystallinity degradation from Pt diffusion

Ferrovalleytricity in a two-dimensional antiferromagnetic lattice.

Control over and manipulation of valley physics via ferrovalleytricity is highly desirable for advancing valleytronics. Current research focuses primarily on two-dimensional ferromagnetic systems, while antiferromagnetic counterparts are seldom explored. Here, we present a general mechanism for extending the ferrovalleytricity paradigm to antiferromagnetic lattices to achieve spin control over valley physics. Our symmetry analysis and k·p model reveal that by introducing a Zeeman field aroused by the proximity effect, spin-switchable non-uniform potential is imposed on the two sublattices of an antiferromagnetic lattice. This enables spin control over the anomalous valley Hall effect, thereby realizing ferrovalleytricity. This mechanism is confirmed in a CrBr3-MnPSe3-CrBr3 heterotrilayer from first principles, where the spin-switchable non-uniform Zeeman effect is exerted on two Mn sublattices when the antiferromagnetic MnPSe3 layer is sandwiched between ferromagnetic CrBr3 layers. Such a non-uniform Zeeman effect combined with valley physics guarantees spin control over the anomalous valley Hall effect, i.e., ferrovalleytricity, in the MnPSe3 layer. Our work will shed light on potential applications of valley physics in antiferromagnetic systems.

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Terahertz emission characterization of silicon based ferromagnetic heterostructures

Terahertz spectroscopy and imaging have many applications, so the generation of broadband terahertz radiation is very important, but now it faces some challenges. Opto-spintronic terahertz emitters, composed of nanometer-thin magnetic multilayer, can produce high-quality broad-band terahertz pulses. Integration of opto-spintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices. In this work, Ta/CoFeB/Ir heterostructures are deposited on thermally oxidized silicon wafers by dc magnetron sputtering. Under the illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate, a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir. Consequently, the spin current can be converted into the charge current due to the strong spin-orbit coupling. The sub-picosecond transient charge current gives rise to the terahertz radiation that enters into the free space. The terahertz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the heterostructure. The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1–2.5 THz frequency range with a maximum value of about 0.64 THz. We also investigate the dependence of THz peak-to-peak value on the pump fluence. The THz emission is found to be saturated at a pump fluence of ~0.73 mJ/cm<sup>2</sup>. Our results demonstrate the existence of the strong spin-orbit coupling in the heavy metal Ir. Furthermore, we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by changing the thickness of Ir layer. According to the thickness dependence of THz emission from the heterostructure, the propagation length of the spin current at THz frequencies is extracted to be about 0.59±0.12 nm, which is shorter than the GHz experimental measurement (~1.34 nm). Our experimental observation is consistent with that in the antiferromagnet IrMn layer, which may be attributed to different transport regimes. Theoretically, the optimized thickness values for CoFeB and Ir layers are 2.4 nm and 1.1 nm, respectively.

Single-crystal growth, structure and thermal transport properties of the metallic antiferromagnet Zintl-phase β-EuIn2As2.

Zintl-phase materials have attracted significant research interest owing to the interplay of magnetism and strong spin-orbit coupling, providing a prominent material platform for axion electrodynamics. Here, we report the single-crystal growth, structure, magnetic and electrical/thermal transport properties of the antiferromagnet layer Zintl-phase compound β-EuIn2As2. Importantly, the new layered structure of β-EuIn2As2, in rhombohedral (R3̄m) symmetry, contains triangular layers of Eu2+ ions. The in-plane resistivity ρ(H, T) measurements reveal metal behavior with an antiferromagnetic (AFM) transition (TN ∼ 23.5 K), which is consistent with the heat capacity Cp(H, T) and magnetic susceptibility χ(H, T) measurements. Negative MR was observed in the temperature range from 2 K to 20 K with a maximum MR ratio of 0.06. Unique 4f7J = S = 7/2 Eu2+ spins were supposed magnetically order along the c-axis. The Seebeck coefficient shows a maximum thermopower |Smax| of about 40 μV K-1. The kink around 23 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the electron band structure, while the pronounced thermal conductivity peak at around 10 K is attributed to the phonon-phonon Umklapp scattering. The results suggest that the Eu2+ spin arrangement plays an important role in the magnetic, electrical, and thermal transport properties in β-EuIn2As2, which might be helpful for future potential technical applications.

Heusler-alloy-based magnetoresistive sensor with synthetic antiferromagnet

Heusler alloy has been widely utilized in magnetoresistive sensors to enhance the device performance. In this work, we theoretically investigate the performance of Heusler-alloy-based magnetoresistive sensors with a synthetic antiferromagnet (SAF) layer. The atomistic model combined with the spin accumulation model will be used in this work. The former is used to construct the reader stack and investigate the magnetization dynamics in the system. The latter is employed to describe the spin transport behavior at any position of the structure. We first perform simulations of the exchange bias (EB) phenomenon in the IrMn/ (CFS) system providing a high EB field. Then, a realistic reader stack of IrMn/CFS/Ru/CFS/Ag/CFS is constructed via an atomistic model. Subsequently, the resistance–area product (RA) and magnetoresistance (MR) ratio of the reader can be calculated by using the spin accumulation model. As a result of the spin transport behavior in the Heusler-alloy-based reader stack including SAF structure at 0 K, an enhancement of the MR ratio up to 120 and RA can be observed. This study demonstrates the important role of the Heusler alloy and SAF layer in the development of magnetoresistive sensors for the application of readers in hard disk drives with an areal density beyond 2 Tb in−2.

Growth and characterization of the magnetic topological insulator candidate Mn2Sb2Te5

We report a new member of topological insulator (TI) family i.e. Mn2Sb2Te5, which belongs to MnSb2Te4 family and is a sister compound of Mn2Bi2Te5. An antiferromagnetic layer of (MnTe)2 has been inserted between quintuple layers of Sb2Te3. The crystal structure and chemical composition of as grown Mn2Sb2Te5 crystal is experimentally visualized by single crystal x-ray diffractometer and field emission scanning electron microscopy. The valence states of individual constituents i.e., Mn, Sb and Te are ascertained through x-ray photo electron spectroscopy. Different vibrational modes of Mn2Sb2Te5 are elucidated through Raman spectroscopy. Temperature-dependent resistivity ρ(T) of Mn2Sb2Te5 resulted in metallic behavior of the same with an up-turn at below around 20 K. Further, the magneto-transport ρ(T) vs H of the same exhibited negative magneto-resistance (MR) at low temperatures below 20 K and small positive at higher temperatures. The low Temperature −ve MR starts decreasing at higher fields. The magnetic moment as a function of temperature at 100 Oe and 1 kOe showed anti-ferromagnetism (AFM) like down turn cusps at around 20 K and 10 K. The isothermal magnetization showed AFM like loops with some embedded ferromagnetic/paramagnetic (PM) domains at 5 K and purely PM like at 100 K. The studied Mn2Sb2Te5 clearly exhibited the characteristics of a magnetic TI.

Engineering Magnetic Phases of Layered Antiferromagnets by Interfacial Charge Transfer.

Van der Waals heterostructures composed of distinct layered materials can display behaviors entirely different from those of each individual layer due to interfacial coupling. Here we investigate the manipulation of magnetic phases in two-dimensional magnets through interfacial charge transfer in heterostructures of magnetic and nonmagnetic layers. This is demonstrated by first-principles calculations, which unveil a transition toward the ferromagnetic phase by stacking antiferromagnetic bilayer CrSBr on graphene. Using an effective model consisting of two electronically coupled single layers, we show that the antiferromagnetic to ferromagnetic magnetic phase transition occurs due to interfacial charge transfer, which enhances ferromagnetism. We further reveal that the magnetic phase transition can also be induced by electron and hole carriers and demonstrate that the phase transition is a spin-canting process. This allows for precise gate-control of noncollinear magnetism on demand. Our work predicts interfacial charge transfer as a potent mechanism to tune magnetic phases in van der Waals heterostructures and creates opportunities for spintronic applications.

Electrically Controllable Exchange Bias via Interface Magnetoelectric Effect

Exchange bias is a unidirectional magnetic anisotropy that often arise from interfacial interaction of a ferromagnetic and antiferromagnetic layers. In this article, we show that a metallic layer with spin-orbit coupling can induces an exchange bias via an interface magnetoelectric effect. In linear response regime, the interface magnetoelectric effect is induced by spin-orbit couplings that arises from the broken symmetry of the system. Furthermore, we demonstrate that the exchange bias can be controlled by electric field.

Influence of alloying elements on various properties of face-centered-cubic iron

Austenitic stainless steel 316 L, a typical face-centered-cubic (FCC) iron alloy, demonstrates great potential as structural materials in nuclear plant. However, liquid metal (Pb and Bi) embrittlement (LME) constitutes the major bottlenect in its wide application. In this work, first-principles calculations are conducted to reveal the influences of Cr, Mo, Ni, Pb, and Bi on magnetic property, surface energy, stacking fault energy, ductility, as well as electronic structure of FCC Fe under the magnetic ground state of antiferromagnetic double layer (AFMD). It is discovered that the addition of Ni could considerably increase the ductility of FCC Fe, while the effects of Cr and Mo on the increase of ductility of FCC Fe seem much smaller. On the contrary, both Pb and Bi significantly reduce the ductility of FCC Fe. Such a large decrease is mainly attributed to the more reduction of surface energy than that of unstable stacking fault energy as well as a more directional and weaker chemical bonding. The derived results could provide insight into the influence of alloying elements on the various properties of FCC Fe.

Terahertz magnetic excitations in non-collinear antiferromagnetic Mn3Pt: Atomistic-scale dynamical simulations

Current-induced spin–orbit torque (SOT) enables highly efficient manipulation of magnetic orders. Non-collinear antiferromagnets exhibit ultrafast spin dynamics by SOT with much higher oscillation frequency than ferromagnets, offering potential applications in terahertz spintronic devices. In this study, we used atomistic scale Landau-Lifshitz-Gilbert simulation approach to demonstrate different magnetic excitation modes in W/Mn3Pt heterostructures with varying crystalline direction. When the spin polarization is perpendicular to the Kagome plane of Mn3Pt, all the magnetic Mn atoms oscillate periodically within the Kagome plane around the normal direction. In contrast, when the spin polarization is parallel to the Kagome plane, only two neighboring Mn atoms rotate while another neighboring Mn atom remains immobile. Interestingly, both of these magnetic excitation modes have the same frequencies and occur in the terahertz range. Additionally, the frequency is proportional to current density and inversely proportional to the antiferromagnetic layer thickness. These findings provide a deeper understanding of magnetic excitation properties in non-collinear antiferromagnets, which can inform the design of terahertz spintronic devices.

Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations, we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.

Large exchange bias effect and interfacial magnetic structure in FePt3 (partially ordered)/FePt3 (disordered) multilayer film

Stoichiometric FePt3 thin film with modulated chemical order is a good system for the study of the magnetic exchange coupling due to distinctive structural and electronic compatibility at the interface. In this study, FePt3(partially ordered)/FePt3 (disordered) multilayer films are fabricated using magnetron sputtering. The interface, magnetic depth profile of each layer, and the magnetic properties of the film are investigated via polarized neutron reflectivity and AC susceptibility measurements. At low temperature, an exchange bias field up to 430 Oe is observed in the film. Exchange bias and coercivity of the film are subjected to a fast increase as temperature goes below 100 K suggesting that Q2 antiferromagnetic (AF) order may be induced in the film. First-principles calculations indicate, in partially ordered stoichiometric FePt3, there may be competitions among Q1, Q2 and ferromagnetic (FM) phases at low temperature due to some extent of chemical disorder. The large exchange bias in the sample may be attributed to the appearance of a small amount of FM phase and AF Q2 phase in AF layers and at the interfaces between FM and AF layers.

Towards skyrmion crystal stabilization in the antiferromagnetic triangular lattice at ambient conditions

We explore possibilities of stabilizing a skyrmion lattice (SkX) phase in the frustrated antiferromagnetic (AF) Heisenberg model with the Dzyaloshinskii–Moriya interaction on a triangular lattice at ambient conditions. We show that by stacking multiple AF layers and coupling them with strong ferromagnetic (F) interaction the SkX phase stability temperature window can be extended by several times. The transition temperature increase is supported by the increasing number of layers and interlayer interaction and thus room-temperature SkX phase could be achievable by proper tailoring of the multilayer structure. Further, we demonstrate that in the sandwich heterostructure, obtained by inserting a stiff F layer in between two AF layers, the required external magnetic field can be completely replaced by the effective field imposed by the F layer. Thus, one can design a heterostructure with the SkX phase stabilized at relatively high temperatures and zero field - the environment favorable for technological applications.

Stochastic generation in a Josephson-like antiferromagnetic spin Hall oscillator driven by a pure AC current

We numerically demonstrate that a pure time-harmonic bias AC current of a specific amplitude τf and angular frequency ωf can excite the chaotic magnetization dynamics in a Josephson-like antiferromagnetic (AFM) spin Hall oscillator (SHO) with biaxial magnetic anisotropy of an AFM layer. The nature of such a stochastic generation regime in a Josephson-like AFM SHO could be explained by the random hopping of the working point of the SHO between several quasi-stable states under the action of an applied AC current. We reveal that depending on the ωf/τf ratio several stochastic generation regimes interspersed with regular generation regimes can be achieved in an AFM SHO, which can be used in spintronic random signal sources and various nano-scale random signal devices, including the spintronic p-bit device considered in this work. The obtained results are important for the development and optimization of spintronic devices capable of generating and processing (sub-)THz-frequency random signals, which are promising for ultra-fast probabilistic computing, cryptography, secure communication, etc.

Phase transitions driven by magnetoelectric and interfacial Dzyaloshinskii-Moriya interaction

In this paper, we study phase transitions in a magnetoelectric bilayer with two order parameters using Monte Carlo simulation. We investigate the effects of temperature, magnetoelectric coupling, and anisotropy in the multiferroic bilayer formed by a frustrated antiferromagnetic layer with Heisenberg spins and a triangular lattice ferroelectric layer. We show that magnetic frustration induces creation of skyrmions and a skyrmion lattice at moderate values of Dzyaloshinskii-Moriya interaction and interface magnetoelectric coupling with adjacent ferroelectric layer. The skyrmion crystal is stable up to relatively high temperatures. The effect of anisotropy is to cause only a shift of the critical temperature without changing the type of the phase transition. The phase of the skyrmion lattice becomes more resistant and stable to temperature fluctuations in the case of a highly anisotropic frustrated film.

High efficient field-free magnetization switching via exchange bias effect induced by antiferromagnetic insulator interface

Spin–orbit torque induced deterministic magnetization switching typically requires the assistance of an external magnetic field for symmetry breaking. However, achieving field-free switching in perpendicular magnetized layers is crucial for expanding the market of high-density memory. Previous reports have utilized exchange bias, an antiferromagnetic interfacial effect, to realize field-free magnetization switching. However, metallic antiferromagnetic layers will introduce shunting effects that reduce switching efficiency and the Néel vector becomes unstable when current flows through the antiferromagnetic layer. In this study, we achieved the zero-field magnetization switching in NiO/Pt/Co/Pt multilayers. Simulation results demonstrate higher efficiency compared to metallic antiferromagnetic IrMn-based structures. Our findings highlight that the insulator antiferromagnetic can provide an exchange bias field, eliminating the need for an external magnetic field. By avoiding shunting effects, our designed structure offers a more efficient approach for spintronic devices.

Challenges in electrical detection of spin-orbit torque in Ir20Mn80/Pt hetero-structures

Manipulation of antiferromagnetic sublattice orientations, a key challenge in spintronic device applications, requires unconventional methods such as current induced torques including Spin Transfer Torque (STT) and Spin-Orbit Torque (SOT). In order to observe the deviation of the Néel vector from the anisotropy axis, one of the simplest approaches is the electrical detection, whereby one monitors the change in resistance as a function of applied current. In this work, we have investigated the conditions under which an ultra-thin metallic antiferromagnet, Ir20Mn80 becomes susceptible to SOT effects by studying antiferromagnetic layer structure and thickness dependence in antiferromagnetic metal (Ir20Mn80)/heavy metal (Pt) superlattices. Our electrical measurements reveal that in bilayer structures there exists a shallow range of Ir20Mn80 thicknesses (∼1–2 nm) for which SOT driven control of spins is apparent, whereas for lower thicknesses incomplete sublattice formation and for higher thicknesses improved thermal stability prohibits vulnerability to spin currents. Furthermore, in multilayers, structural changes in Ir20Mn80 layer quenches local torques due to stronger (111) magnetocrystalline anisotropy. These results suggest that an exhaustive optimization of the antiferromagnet parameters is crucial for the successful deployment of spintronic devices.