Antiferromagnetic Layer Research Articles

Effect of Laser-Induced Heating of the Antiferromagnetic IrMn Layer on Generation of Terahertz Pulses in Co/WSe2-Based Spintronic Emitters

Effect of Laser-Induced Heating of the Antiferromagnetic IrMn Layer on Generation of Terahertz Pulses in Co/WSe2-Based Spintronic Emitters

Spin-anomalous-Hall unidirectional magnetoresistance in light-metal/ferromagnetic-metal bilayers

Nonreciprocal magnetotransport is one of the central topics in spintronics because of its importance for electrically probing magnetic information. Among numerous electrical probes used to read magnetic orders, unidirectional magnetoresistance (UMR), characterized by sign changes upon reversal of either current or magnetization, is currently a matter of great interest and has been identified in various spin–orbit-coupled bilayer systems composed of an (anti)ferromagnetic layer and a nonmagnetic layer with strong spin Hall effect. A recent theoretical work predicts that a spin-anomalous-Hall (SAH) UMR in those metallic conducting bilayers can originate from the spin-anomalous-Hall effect of the ferromagnetic layer and the structural inversion asymmetry. However, this type of UMR has not been reported experimentally. Here, we give the experimental evidence of spin-anomalous-Hall UMR in the light-metal/ferromagnetic-metal Cu/Co bilayers, where the emergence of net nonequilibrium spin density is attributed to the interfacial spin leakage asymmetry due to the spin memory loss effect at the Cu/Co interface and multiple spin reflections. We also show a highly tunable UMR in the Cu/Co/CuOx trilayer by varying the Cu thickness, which is due to the competition between the orbital Rashba effect in Co/CuOx and the spin-anomalous-Hall effect in Cu/Co. Our work widens the material choice for UMR device applications and provides an alternative approach to detect in-plane magnetization without an external spin polarizer.

Medium dynamic field range linear bipolar spin valve sensor through soft pinning the sensing layer

Magnetic sensor with spin valve-GMR technology with medium dynamic range is designed for a diversity of applications, including linear and rotary position measurements, proximity switches, and current sensors. For this, the sensing layer (SL) of the spin valve stack was modified by a soft pinning layer (SPL) through an exchange bias field created by an antiferromagnetic layer which has a lower blocking temperature than the one that is kept adjacent to the pinned layer. Numerical simulation was carried out to control the bias field by keeping a non-magnetic Ru spacer layer between the SPL and SL layers and the results were experimentally verified. The magnetic sensor was fabricated with linear operating field range of the order ±100 Oe having a sensitivity of the order of 0.1 m V V−1 Oe−1 near zero field. The thermal performance confirms that the device can be operated in the temperature range of −40 ∘C to 125 ∘C and it has a thermal coefficient of voltage around 15 µV V−1∘C−1.

Emergence of exchange bias in van der Waals magnetic alloy CrxPt1−xTe2

CrxPt1−xTe2 is a recently developed van der Waals magnetic alloy noted for its stability under ambient conditions. Here, we report the emergence of an exchange bias effect in CrxPt1−xTe2, without typical exchange bias sources such as an adjacent antiferromagnetic layer. We find that the exchange bias is present for x = 0.45 and absent for x = 0.35, which is correlated to the presence of a Cr modulation where the Cr concentration alternates each vdW layer (modulation period of 2 layers) for x ≥ 0.4. We perform Monte Carlo simulations utilizing exchange parameters from first-principles calculations, which recreate the exchange bias in hysteresis loops of Cr0.45Pt0.55Te2. From our simulations, we infer the source of exchange bias to be magnetic moments locked into free energy minima that resist magnetization reversal. This work presents a way to introduce desirable magnetic properties to van der Waals magnets.

Even-odd-layer-dependent symmetry breaking in synthetic antiferromagnets

In this work we examine synthetic antiferromagnetic structures consisting of two, three, and four antiferromagnetic coupled layers, i.e. bilayers, trilayers, and tetralayers. We vary the thickness of the ferromagnetic layers across all structures and, using a macrospin formalism, find that the nearest neighbor exchange interaction between layers is consistent across all structures for a given thickness of the ferromagnetic layer. Our model and experimental results demonstrate significant differences in how the static equilibrium states of even and odd-layered structures evolve as a function of the external field. Even layered structures continuously evolve from a collinear antiferromagnetic state to a spin canted non-collinear magnetic configuration that is mirror-symmetric about the external field. In contrast, odd-layered structures begin with a ferrimagnetic ground state; at a critical field, the ferrimagnetic ground state evolves into a non-collinear state with broken symmetry. Specifically, the magnetic moments found in the odd-layered samples possess stable static equilibrium states that are no longer mirror-symmetric about the external field after a critical field is reached.

Macroscopic tunneling probe of Moiré spin textures in twisted CrI3

Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI3 due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking—with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii–Moriya or dipole–dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI3 bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI3 under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at ≈1° twist angle, with a different global tunneling resistance that can be switched by magnetic field.

Full electrical manipulation of perpendicular exchange bias in ultrathin antiferromagnetic film with epitaxial strain

Achieving effective manipulation of perpendicular exchange bias effect remains an intricate endeavor, yet it stands a significance for the evolution of ultra-high capacity and energy-efficient magnetic memory and logic devices. A persistent impediment to its practical applications is the reliance on external magnetic fields during the current-induced switching of exchange bias in perpendicularly magnetized structures. This study elucidates the achievement of a full electrical manipulation of the perpendicular exchange bias in the multilayers with an ultrathin antiferromagnetic layer. Owing to the anisotropic epitaxial strain in the 2-nm-thick IrMn3 layer, the considerable exchange bias effect is clearly achieved at room temperature. Concomitantly, a specific global uncompensated magnetization manifests in the IrMn3 layer, facilitating the switching of the irreversible portion of the uncompensated magnetization. Consequently, the perpendicular exchange bias can be manipulated by only applying pulsed current, notably independent of the presence of any external magnetic fields.

Atomic-scale visualization of a cascade of magnetic orders in the layered antiferromagnet GdTe3

GdTe3 is a layered antiferromagnet which has attracted attention due to its exceptionally high mobility, distinctive unidirectional incommensurate charge density wave (CDW), superconductivity under pressure, and a cascade of magnetic transitions between 7 and 12 K, with as yet unknown order parameters. Here, we use spin-polarized scanning tunneling microscopy to directly image the charge and magnetic orders in GdTe3. Below 7 K, we find a striped antiferromagnetic phase with twice the periodicity of the Gd lattice and perpendicular to the CDW. As we heat the sample, we discover a spin density wave with the same periodicity as the CDW between 7 and 12 K; the viability of this phase is supported by our Landau free energy model. Our work reveals the order parameters of the magnetic phases in GdTe3 and shows how the interplay between charge and spin can generate a cascade of magnetic orders.

Modeling of the effect of the thickness of the antiferromagnetic layer on the energy of interfacial exchange interaction in Ferromagnetic/Antiferromagnet films

By means of thermodynamic and configuration averaging of the energy of the exchange interaction between the magnetic spin moments adjacent through the ferromagnetic/antiferromagnetic interphase boundary, the expression for the energy of the interphase exchange interaction Eex is obtained. The dependence of the energy Eex on the temperature and the characteristic dimensions of the ferromagnetic/antiferromagnetic two-phase film is modeled. The experimentally observed regularity of the growth of the energy of the interfacial exchange interaction due to the increase of the thickness of the antiferromagnetic layer is confirmed. A method for the estimation of the constants of the interfacial exchange interaction Jint and the exchange interaction in antiferromagnetics JAFM is proposed based on the agreement of the experimental and theoretical curves of the temperature dependence of the exchange bias field HebT.

Dual mode spin to charge conversion using inverse spin Hall effect in NiFe/FeMn/NiFe multilayer thin films

Microwave devices with more than one operating frequency and their ease of tunability are of great importance for high-frequency information processing. Magnetic thin films offer an unparalleled advantage of engineering different microwave bands that can be precisely tailored and reconfigured externally. Here, novel trilayer structures consisting of NiFe/FeMn/NiFe with varying antiferromagnetic FeMn layer thickness have been investigated by exploring their ferromagnetic resonance (FMR) properties and inverse spin Hall effect (ISHE) responses. Two-step magnetic hysteresis loops are observed for higher FeMn thickness (t ⩾ 12 nm), where the bottom NiFe layer shows a comparatively more significant shift due to the presence of strong interfacial exchange coupling. FMR study reveals two resonant modes associated with the two ferromagnetic layers, which are distinguishable for higher thicknesses of FeMn or at high excitation frequencies. The choice of FeMn thickness determines the operating frequencies, which can be finely tailored by optimizing the FeMn thickness. Gilbert damping parameter is found to be in the range of 0.009–0.012 where the presence of exchange bias adds to the the scattering mechanisms. Prominent ISHE responses are obtained from the bottom NiFe layer as compared to the top NiFe layer. Variation of FeMn thickness also shows a strong influence on the spin pumping ( Vsp ), and perpendicular anisotropic magnetoresistance ( VAMR⊥ ) components. The results are correlated with the efficiency of spin flow and spin-to-charge conversion of the FeMn layer. Our systems can be used as an emerging alternative for microwave detectors and microwave energy harvesters.

Antiferromagnetic droplet soliton driven by spin current

We demonstrate that a spin current flowing through a nanocontact into a uniaxial antiferromagnet with first- and second-order anisotropy can excite a self-localized dynamic magnetic soliton, known as a spin-wave droplet in ferromagnets. The droplet nucleates at a certain threshold current with the frequency of the Néel vector precession laying below the antiferromagnetic resonance. The frequency exhibits nonlinear behavior with the increase in the applied current. At the high value of the applied torque, the soliton mode transforms, and the oscillator emits spin waves propagating in the antiferromagnetic layer.

Anisotropic magnetoresistance probed by non-resonant microwave absorption in exchange-biased NiFe/IrMn structures

Sharp change of non-resonant microwave absorption was found in exchange-biased NiFe/IrMn and NiFe/Cu/IrMn structures possessing anisotropic magnetoresistance (AMR) and planar Hall effect (PHE). Angular dependence of the microwave detected AMR and contribution of the domain expansion to AMR microwave signal were analyzed. Decrease in the microwave absorption signal caused by Cu spacer between ferromagnetic NiFe and antiferromagnetic IrMn layers was observed, when NiFe and IrMn layers become exchange decoupled. Additional characterization of the electric transport was proved by asymmetry of resonant line. Sensitivity of the microwave AMR method is enough high to detect magnetic fields of microTesla level, that provides remote detection with a sensitivity corresponding to standard galvanic sensors. Developed technique is promising for wireless AMR sensor technology providing coordinate and angular field measurements through remote microwave absorption.

Antiferromagnetic Cr-Mn alloys: Crystal structure, magnetic anisotropy, and exchange bias in magnetron-sputtered polycrystalline thin films

In this paper we study the formation of the antiferromagnetic Cr-Mn alloys with body-centered cubic crystal structure, and their capability to act as a pinning layer for the exchange bias effect in magnetron-sputtered polycrystalline films. By means of X-ray diffractometry, executed in different geometries, we perform a thorough analysis of Ta/Cr-Mn/Ta films, which results in the construction of a room-temperature phase diagram of Cr-Mn. These results combined with magnetic measurements of films with adjacent antiferromagnetic Cr-Mn and ferromagnetic Fe20Ni80 layers allow us to establish the conditions for the observation of the exchange bias effect, the maximum blocking temperature Tb of which can be as high as 540 K. Employing a specialized measurement protocol, we estimate the effective anisotropy constant Keff, whose values help to explain thickness dependence of the exchange bias field in structures with a Cr-Mn pinning layer.

Antiferromagnetic magnetostriction of IrMn detected by angular dependent exchange bias

The manipulation of Néel vector of antiferromagnetic (AFM) layer by an applied stress has attracted considerable attention due to the technical importance for AFM-based spintronic devices. Here, we fabricated CoFeB/IrMn and Ni/IrMn exchange bias (EB) bilayers on PMN-PT(011) to quantitatively study the AFM magnetostrictive behaviors of the IrMn layer. Numerical calculations based on the Stoner–Wohlfarth model show that the response of the Néel vector to compressive stress can be detected by measuring the angular dependent EB. The hysteresis loops experimentally measured with applying an electric field on PMN-PT show significantly different shape changes due to the opposite magnetostriction between CoFeB and Ni. The non-vanished EB field obtained at 90∘ indicates the rotation of Néel vector under compressive stress, suggesting the positive magnetostriction of IrMn. The numerical fitting of the angular dependent EB indicates that the Néel vectors in both samples deviate from the initial direction by the same −5∘. The AFM magnetostriction coefficient of IrMn is estimated to be 238 ppm, which is in good agreement with the theoretically predicted value.

Harnessing Van der Waals CrPS4 and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in Fe3GeTe2.

Two-dimensional van der Waals (vdW) heterostructures are an attractive platform for studying exchange bias due to their defect-free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnetic material, possesses uncompensated magnetic spins in a single layer, rendering it a promising candidate for exploring exchange bias phenomena. Recent findings have highlighted that naturally oxidized vdW ferromagnetic Fe3GeTe2 exhibits exchange bias, attributed to the antiferromagnetic coupling of its ultrathin surface oxide layer (O-FGT) with the underlying unoxidized Fe3GeTe2. Anomalous Hall measurements are employed to scrutinize the exchange bias within the CrPS4/(O-FGT)/Fe3GeTe2 heterostructure. This analysis takes into account the contributions from both the perfectly uncompensated interfacial CrPS4 layer and the interfacial oxide layer. Intriguingly, a distinct and nonmonotonic exchange bias trend is observed as a function of temperature below 140 K. The occurrence of exchange bias induced by a "preset field" implies that the prevailing phase in the polycrystalline surface oxide is ferrimagnetic Fe3O4. Moreover, the exchange bias induced by the ferrimagnetic Fe3O4 is significantly modulated by the presence of the van der Waals antiferromagnetic CrPS4 layer, forming a heterostructure, along with additional iron oxide phases within the oxide layer. These findings underscore the intricate and complex nature of exchange bias in van der Waals heterostructures, highlighting their potential for tailored manipulation and control.

Dyakonov surface polaritons in bi-hyperbolic metamaterial

A periodically layered metamaterial (MM) structured with antiferromagnetic (AF) and ionic-crystal (IC) layers was proposed, which exhibits bi-hyperbolicity with effective hyperbolic permeability and permittivity. We investigated Dyakonov surface polaritons (DSPs) localized at this bi-hyperbolic metamaterial surface. For such a complicated material system, we analytically acquired one concise dispersion equation and corresponding wave solutions. From them, we found four DSPs, or DSP-I, DSP-II, DSP-III, and DSP-IV, and meanwhile one Dyakonov surface magnon and one Dyakonov surface phonon were found in this bi-hyperbolic metamaterial. The ATR spectra numerically calculated clearly proved the actual existence of the DSPs acquired from the dispersion equation and offered the path to excite and observe the DSPs. The DSP’s energy propagating direction is parallel to the air/MM interface, but not along the wave vector direction, and further it can approximatively be vertical to the wave-vector.

Spatially reconfigurable antiferromagnetic states in topologically rich free-standing nanomembranes

Antiferromagnets hosting real-space topological textures are promising platforms to model fundamental ultrafast phenomena and explore spintronics. However, they have only been epitaxially fabricated on specific symmetry-matched substrates, thereby preserving their intrinsic magneto-crystalline order. This curtails their integration with dissimilar supports, restricting the scope of fundamental and applied investigations. Here we circumvent this limitation by designing detachable crystalline antiferromagnetic nanomembranes of α-Fe2O3. First, we show—via transmission-based antiferromagnetic vector mapping—that flat nanomembranes host a spin-reorientation transition and rich topological phenomenology. Second, we exploit their extreme flexibility to demonstrate the reconfiguration of antiferromagnetic states across three-dimensional membrane folds resulting from flexure-induced strains. Finally, we combine these developments using a controlled manipulator to realize the strain-driven non-thermal generation of topological textures at room temperature. The integration of such free-standing antiferromagnetic layers with flat/curved nanostructures could enable spin texture designs via magnetoelastic/geometric effects in the quasi-static and dynamical regimes, opening new explorations into curvilinear antiferromagnetism and unconventional computing.

Exchange Bias Modulated by Antiferromagnetic Spin-Flop Transition in 2D Van der Waals Heterostructures.

Exchange bias is extensively studied and widely utilized in spintronic devices, such as spin valves and magnetic tunnel junctions. 2D van der Waals (vdW) magnets, with high-quality interfaces in heterostructures, provide an excellent platform for investigating the exchange bias effect. To date, intrinsic modulation of exchange bias, for instance, via precise manipulation of the magnetic phases of the antiferromagnetic layer, is yet to be fully reached, owing partly to the large exchange fields of traditional bulk antiferromagnets. Herein, motivated by the low-field spin-flop transition of a 2D antiferromagnet, CrPS4, exchange bias is explored by modulatingthe antiferromagnetic spin-flop phase transition in all-vdW magnetic heterostructures. The results demonstrate that undergoing the spin-flop transition during the field cooling process, the A-type antiferromagnetic ground state of CrPS4 turns into a canted antiferromagnetic one, therefore, it reduces the interfacial magnetic coupling and suppresses the exchange bias. Via conducting different cooling fields, one can select the exchange bias effect switching among the "ON", "depressed", and "OFF" states determined by the spin flop of CrPS4. This work provides an approach to intrinsically modulate the exchange bias in all-vdW heterostructures and paves new avenues to design and manipulate 2D spintronic devices.

Anisotropy constant of antiferromagnetic Pt50Mn50

We have measured the anisotropy constant of polycrystalline PtMn thin films deposited on different seed layer materials: Pt, Ru and Nb. Values as high as (2.5±0.5)⋅107ergcm−3 were achieved for samples deposited on Pt. The films can be crystallised into the antiferromagnetic, face-centred-tetragonal phase on Ru and Pt seed layers at annealing temperatures compatible with back-end-of-line conditions of up to 400 ∘C for one to three hours. Additionally these antiferromagnetic layers, 8 nm thick, are highly thermally stable with median blocking temperatures above 200 ∘C. The effect of diffusion on the stoichiometry of the PtMn layers is discussed with regards to the different seed layer materials.

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.