Antiferromagnetic Layer Research Articles

Canted antiferromagnetic phases in the candidate layered Weyl material EuMnSb2

${\mathrm{EuMnSb}}_{2}$ is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline $^{151}\mathrm{Eu}$ M\"ossbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below ${T}_{{\text{N}}_{\text{Mn}}}=323(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Mn magnetic moment ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below ${T}_{{\text{N}}_{\text{Eu1}}}=23(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Eu magnetic moment ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ canted away from the layer normal and ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ retaining its higher temperature order. ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$ with the chemical and magnetic unit cells having the same dimensions. Cooling below ${T}_{{\text{N}}_{\text{Eu2}}}=9(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ reveals a third AFM phase where ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ remains unchanged but ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ develops an additional substantial in-plane canting. This phase has MSG $P11\frac{{2}_{1}}{{a}^{\ensuremath{'}}}$. We also find some evidence of short-range magnetic correlations associated with the Eu between $12\phantom{\rule{4pt}{0ex}}\text{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}30\phantom{\rule{4pt}{0ex}}\text{K}$. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in ${\mathrm{EuMnSb}}_{2}$ and their potential for tuning topological properties.

Ultrafast optical control of surface and bulk magnetism in magnetic topological insulator/antiferromagnet heterostructure

Optical control of the magnetic properties in topological insulator systems is an important step in applying these materials in ultrafast optoelectronic and spintronic schemes. In this work, we report the experimental observation of photo-induced magnetization dynamics in the magnetically doped topological insulator (MTI)/antiferromagnet (AFM) heterostructure composed of Cr-(Bi,Sb)2Te3/CrSb. Through proximity coupling to the AFM layer, the MTI displays a dramatically enhanced magnetism, with robust perpendicular magnetic anisotropy. When subjected to intense laser irradiation, both surface and bulk magnetism of the MTI are weakened by laser-induced heating of the lattice, however, at the surface, the deleterious heat effect is compensated by the strengthening of Dirac-hole-mediated exchange coupling as demonstrated by an unconventional pump-fluence-dependent exchange-bias effect. Through theoretical analyses, the sizes of exchange coupling energies are estimated in the MTI/AFM bilayer structure. The fundamentally different mechanisms supporting the surface and bulk magnetic order in MTIs allow a novel and distinctive photo-induced transient magnetic state with antiparallel spin configuration, which broadens the understanding of the magnetization dynamics of MTIs under ultrashort and intense optical excitation.

Fe3O4 thin films epitaxially growth model on TiO2-terminated SrTiO3(100)

The advanced interface of Fe 3 O 4 /SrTiO 3 , popular in spintronic, has attracted considerable attention. Some experiments have given controversial results on the structure at the Fe 3 O 4 /SrTiO 3 (100) interface. One opinion suggests the formation of interfacial antiferromagnetic FeO layers, while another opinion suggests that are γ-Fe 2 O 3 layers. Here, we propose a theoretical model that what kind of iron oxide stacking sequence forming on the substrate depends on the substrate termination charge. We give a tentative and easy-to-understand growth model for epitaxy, which agrees with the opinion of γ-Fe 2 O 3 layers formation at the Fe 3 O 4 /SrTiO 3 (100) interface. This theoretical model agrees well with the experimental curve and gives a prediction for the different Fe ions concentration depending on thickness. The calculation results of the Fe ions concentration adjacent to the substrate surface show that there are almost no B-site Fe ions at the interface. It indicates that there are unusual electric and magnetic properties at interface. Our growth model illustrates that the possible magnetically dead layer at the interface is responsible for the decrease of the magnetization in the thin films. • This growth mode give credit to the results of interfacial ferrimagnetic γ-Fe 2 O 3 layer in Fe 3 O 4 /SrTiO 3 (100). • The theoretical model curve agrees with the experimental curve, gives prediction for the Fe ions concentration depending on thickness. • The calculation result shows there are almost no B-site Fe at interface. This indicates that there are unusual electric and magnetic properties at interface. The present growth illustrates there exists possible magnetically dead layer at the interface. • It is proposed that the growth mode and the oxidation level are determined by the TiO 2 -termination charge. • The phenomenon that magnetite film grown on SrTiO 3 (001) exhibited vertical compressive and lateral tensile strain, which contradicted the expected behavior, has been explained.

Inertial domain wall characterization in layered multisublattice antiferromagnets

The motion of a Néel-like 180∘ domain wall induced by a time-dependent staggered spin–orbit field in the layered collinear antiferromagnet Mn2Au is explored. Through an effective version of the two sublattice nonlinear σ-model which does not take into account the antiferromagnetic exchange interaction directed along the tetragonal c-axis, it is possible to replicate accurately the relativistic and inertial traces intrinsic to the magnetic texture dynamics obtained through atomistic spin dynamics simulations for quasistatic processes. In the case in which the steady-state magnetic soliton motion is extinguished due to the abrupt shutdown of the external stimulus, its stored relativistic exchange energy is transformed into a complex translational mobility, being the rigid domain wall profile approximation no longer suitable. Although it is not feasible to carry out a detailed follow-up of its temporal evolution in this case, it is possible to predict the inertial-based distance travelled by the domain wall in relation to its steady-state relativistic mass. This exhaustive dynamical characterization for different time-dependent regimes of the driving force is of potential interest in antiferromagnetic domain wall-based device applications.

Spin waves in layered antiferromagnets with honeycomb structure

We develop a description of spin waves in a $3D$ quantum $XY$ antiferromagnet (AFM) in terms of macroscopic variables, magnetization and N\'eel vector densities. We consider a layered AFM with spins located on the honeycomb lattice. In the discussed system, the spectrum of spin waves consists of four modes, all well captured by our macroscopic description. The gapless mode of the spin waves, i.e., magnons, is described by a system of equations, which has a structure general for the Goldstone mode in AFMs. We demonstrate that the parameters in the spin Hamiltonian can be evaluated by fitting the experimental data with the results obtained for the four modes using the macroscopic variable approach. The description of AFM in terms of macroscopic variables can be easily extended to the case when the lattice of the magnetic substance is deformed by an external strain or acoustic wave.

Magnetic phase diagram of the solid solution LaMn2(Ge1−xSix)2 (0 ≤ x ≤ 1) unraveled by powder neutron diffraction

The structural and magnetic properties of the ThCr2Si2-type solid solution LaMn2(Ge1−xSix)2 (x = 0.0 to 1.0) have been investigated employing a combination of X-ray diffraction, magnetization and neutron diffraction measurements, which allowed establishing a magnetic composition-temperature phase diagram. Substitution of Ge by Si leads to a compression of the unit cell, which affects the magnetic exchange interactions. In particular, the magnetic structure of LaMn2(Ge1−xSix)2 is strongly affected by the unit cell parameter c, which is related to the distance between adjacent Mn layers. Commensurate antiferromagnetic layers and a canted ferromagnetic structure dominate the Si-rich part of the solid solution, whilst an incommensurate antiferromagnetic flat spiral and a conical magnetic structure are observed in the Si-poor part.

Controlling domain wall and field-free spin–orbit torque switching in synthetic antiferromagnets

Perpendicular magnetization switching driven by spin–orbit torques plays an increasingly important role for spintronic devices toward practical applications but is also hindered by the well-known technical challenge that an external in-plane magnetic field is required for deterministic switching. Here, we show that the deterministic switching can be achieved in synthetic antiferromagnets through the flexible domain control in the absence of external magnetic fields. Specifically, we have observed that the domain wall (DW) distorts under an applied electric current in contrast to the conventional rigid DW motion in a single ferromagnet. More importantly, the distorted DWs can be precisely controlled under zero magnetic field, leading to the deterministic switching. Our results indicate that the critical technical challenge may be addressed by employing a synthetic antiferromagnetic layer through the DW motion dominated field-free switching.

Unique surface polaritons and their transitions in metamaterials.

We investigated surface polaritons in a metamaterial composed of polar-crystal layers and antiferromagnetic layers. In a specific geometry, two surface polaritons were predicted, which are a unique ghost surface polariton (GSP) and surface hybrid-polarization polariton (SHP). The two surface polaritons occupy different segments of one smooth dispersion curve and are magnetically tunable. An external magnetic field along the antiferromagnetic easy axis can bring about the switch or transition between the two surface polaritons and meanwhile performs the necessary condition for the existence of two surface polaritons. In the metamaterial, either surface polariton consists of two branch waves. The branch waves of the GSP are coherent and have the same amplitude and different phases, but those of the SHP are not coherent and have different amplitudes and phases. The main characteristic of the GSP is that its fields oscillate and attenuate with the distance away from the metamaterial surface and exhibit interferent fringes on the plane normal to the surface.

Angle-dependent switching in a magnetic tunnel junction containing a synthetic antiferromagnet

The angle dependence of field-induced switching was investigated in magnetic tunnel junctions with in-plane magnetization and a pinned synthetic antiferromagnet reference layer. The 60 × 90 nm2 elliptical nanopillars had sharp single switches when the field was applied along the major axis of the ellipse, but even with small (20°) deviations, reversal occurred through an intermediate state. The results are interpreted with a model that includes the external applied field and the effective fields due to shape anisotropy and the fringe field of the synthetic antiferromagnet and used to extract the magnetization direction at various points in the magnetoresistance loop. The implications for faster spintronic probabilistic computing devices are discussed.

Critical analysis of proximity-induced magnetism in MnTe/Bi2Te3 heterostructures

An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in $\mathrm{MnTe}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ layer. Furthermore, the interface between MnTe and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is not abrupt, but extending over $\ensuremath{\sim}2.2$ nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.

Competing orders and cascade of degeneracy lifting in doped Bernal bilayer graphene

Motivated by recent experiments [H. Zhou et al., Science 375, 774 (2022) and S. C. de la Barrera et al., arXiv:2110.13907], here we propose a general mechanism for valley and/or spin degeneracy lifting of the electronic bands in doped Bernal bilayer graphene, subject to electric displacement ($D$) fields. A $D$-field induced layer polarization (LP), when accompanied by a Hubbard repulsion-driven layer antiferromagnet (LAF) and next-nearest-neighbor repulsion-driven quantum anomalous Hall (QAH) orders, lifts the fourfold degeneracy of electronic bands, yielding a quarter metal for small doping, as also observed in ABC trilayer graphene. With the disappearance of the QAH order, electronic bands recover twofold valley degeneracy, thereby forming a conventional or compensated (with majority and minority carriers) half metal at moderate doping, depending on the relative strength of LP and LAF. At even higher doping and for a weak $D$ field only LAF survives and the Fermi surface recovers fourfold degeneracy. We also show that a pure repulsive electronic interaction mediated triplet $f$-wave pairing emerges from a parent correlated nematic liquid or compensated half metal when an in-plane magnetic field is applied to the system.

Magnetoelastic resonance as a probe for exchange springs at antiferromagnet-ferromagnet interfaces

In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}\ensuremath{-}{\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{20}$, we are able to associate the magnetoelastic resonance with the presence of the exchange spin-spirals in both the ferromagnetic and antiferromagnetic layer. Our findings offer a complementary, integrative insight into emergent magnetic materials for applications of noncollinear spin textures in view of low-energy-consumption spintronic devices.

On the elastic tensors of ultra-thin films: A study of ruthenium

Critical Dimensions (CD) of electronic device patterns have been reduced to the order of a few nanometers due to the extreme miniaturization of devices, setting the thickness of some of its constituting materials at the (sub-)nanometer level. At these length scales, the mechanical integrity of the constituting films begin to deviate strongly from their bulk material values, introducing a set of undesired reliability side-effects such as mechanical failure, delamination, and strain enhanced diffusion. Understanding the fundamentals behind these aspects is critical to further sustain technology developments. We use Density Functional Theory (DFT) to quantify how the mechanical properties of sub-10 nm thick films depend on their thickness and surface roughness, specifically for Ru [001], which is currently being investigated for CMOS interconnects, an antiferromagnetic layer, and an etch stop layer in Magnetic Random Access Memories (MRAM). Our findings underline that overlooking the impact of film dimensions and surface roughness on the mechanical properties at these CDs, hampers the optimization of device performances and challenges the validity of many experimental and modeling techniques relying on the assumption that bulk mechanical properties are preserved.

Complex Study of Magnetization Reversal Mechanisms of FeNi/FeMn Bilayers Depending on Growth Conditions.

Magnetization reversal processes in the NiFe/FeMn exchange biased structures with various antiferromagnetic layer thicknesses (0–50 nm) and glass substrate temperatures (17–600 °C) during deposition were investigated in detail. Magnetic measurements were performed in the temperature range from 80 K up to 300 K. Hysteresis loop asymmetry was found at temperatures lower than 150 K for the samples with an antiferromagnetic layer thickness of more than 10 nm. The average grain size of FeMn was found to increase with the AFM layer increase, and to decrease with the substrate temperature increase. Hysteresis loop asymmetry was explained in terms of the exchange spring model in the antiferromagnetic layer.

Anomalous Critical Field Dependence of Cr-Nb-Cr Trilayers on Niobium Thickness

Cr-Nb-Cr trilayer thin films were deposited using DC/RF sputtering with Cr layer thickness maintained at 7.5 nm and Nb thickness varying from 30 nm to 229 nm, in order to study the interplay of antiferromagnetism and superconductivity. Various viewpoints have been expressed in the literature with respect to analyzing the anomalous properties that have been identified in superconductor/magnetic thin films. The study of Cr-Nb-Cr thin films were taken up to identify the existence of similar anomalous superconducting properties on account of antiferromagnetic-superconductor interfaces. X-ray reflectivity measurements showed high quality of thin film structure with minimum surface and interface roughness. Transport measurements down to 2K were carried out in the presence of magnetic fields up to 2 T applied perpendicular to the trilayers to deduce Tc. The suppression of Tc due to Cr layer is larger than proximity effect of similar metal films. It could be explained by proximity effect using antiferromagnetic Cr layer. Upper critical field measurements show a distinct nonmonotonic dependence of upper critical magnetic field and the slope on Nb layer thickness. The analysis due to WHH theory to deduce upper critical fields for the trilayer thin films studied did not match with experimental values. Although studies performed on Cr-Nb-Cr trilayers did not show any anomaly in Tc, it clearly showed a depression of Tc much largerthan proximity effect, non monotonic behavior in Bc2(0) and dBc2/dTc behavior with Nb layer thickness.

Enhancement of current to spin-current conversion and spin torque efficiencies in a synthetic antiferromagnetic layer based on a Pt/Ir/Pt spacer layer

We investigated the current-induced spin-orbit torque (SOT) originating from the spin Hall effect in stack systems with perpendicularly magnetized Co/Pt/Ir/Pt/Co synthetic antiferromagnetic (AFM) structures which exhibit nearly compensated magnetization and interlayer exchange coupling of ${J}_{\mathrm{ex}}=0.11\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$. The results were compared with ferromagnetic stack systems with perpendicularly magnetized Pt/Co and (Ir/Pt)-multilayer/Co structures. The magnetizations of the two Co layers in the Co/Pt/Ir/Pt/Co synthetic AFMs can be switched between two antiparallel states simultaneously by SOT. By this switching mechanism, the current to spin-current conversion and spin-torque efficiencies in a synthetic AFM layer stack based on a Pt/Ir/Pt spacer layer are about twice as high as those of a ferromagnetic stack using a Pt heavy metal electrode. The efficient switching of compensated synthetic AFMs would advance magnetic memory devices with high density, high speed, and low power consumption. We expect the Pt/Ir/Pt spacer layer paves the way to AFM spintronics based on multilayer systems.

Beating the limitation of the Néel temperature of FeO with antiferromagnetic proximity in FeO/CoO

In our study, we investigate the influence of the proximity of an antiferromagnetic CoO layer on magnetic properties of ultrathin wüstite (FeO) films. Comparative Mössbauer spectroscopy measurements for MgO/FeO/MgO(001) and MgO/FeO/CoO/MgO(001) show that the neighboring CoO layer can significantly enhance the ordering temperature (TN) of wüstite. Importantly, we find that the proximity of antiferromagnetic CoO strongly influences the exchange interaction at the Fe/FeO interface in the Fe/FeO/CoO heterostructure. We observe a 500% enhancement in the exchange bias field and a double increase in the blocking temperature compared to the Fe/FeO bilayer. Our results show that the limitation of the low ordering temperature of a seemingly application-useless antiferromagnet can be overcome by antiferromagnetic proximity.

Theory of Antiferromagnet-Based Detector of Terahertz Frequency Signals

We present a theory of a detector of terahertz-frequency signals based on an antiferromagnetic (AFM) crystal. The conversion of a THz-frequency electromagnetic signal into the DC voltage is realized using the inverse spin Hall effect in an antiferromagnet/heavy metal bilayer. An additional bias DC magnetic field can be used to tune the antiferromagnetic resonance frequency. We show that if a uniaxial AFM is used, the detection of linearly polarized signals is possible only for a non-zero DC magnetic field, while circularly polarized signals can be detected in a zero DC magnetic field. In contrast, a detector based on a biaxial AFM can be used without a bias DC magnetic field for the rectification of both linearly and circularly polarized signals. The sensitivity of a proposed AFM detector can be increased by increasing the magnitude of the bias magnetic field, or by by decreasing the thickness of the AFM layer. We believe that the presented results will be useful for the practical development of tunable, sensitive and portable spintronic detectors of THz-frequency signals based of the antiferromagnetic resonance (AFMR).

Metals, fractional metals, and superconductivity in rhombohedral trilayer graphene

Combining mean-field and renormalization group analyses, here we unveil the nature of recently observed superconductivity and parent metallic states in chemically doped rhombohedral trilayer graphene, subject to external electric displacement fields ($D$) [H. Zhou, \emph{et al.}, Nature (London) {\bf 598}, 434 (2021)]. We argue that close to the charge neutrality, on site Hubbard repulsion favors layer antiferromagnet, which when combined with the $D$-field induced layer polarization, produces a spin-polarized, but valley-unpolarized half-metal, conducive to the nucleation of spin-triplet $f$-wave pairing (SC2). At larger doping valence bond order emerges as a prominent candidate for isospin coherent paramagent, boosting condensation of spin-singlet Cooper pairs in the $s$-wave channel (SC1), manifesting a "selection rule" among competing orders. Responses of these paired states to displacement and in-plane magnetic fields show qualitative similarities with experimental observation. With the onset of the quantum anomalous Hall order, the valley degeneracy of half-metal gets lifted, forming a quarter-metal at lower doping [H. Zhou, \emph{et al.}, Nature (London) {\bf 598}, 429 (2021)].

Study of training effect by Kerr microscopy measurement in Fe thin film implanted with F-ions

The training effect and the evolution of magnetic domain during the magnetization irreversible process of Fe thin film which exhibits exchange bias after F-ion implantation have been studied. Kerr microscopy measurements were carried out at different temperatures of the exchange bias F-implanted Fe thin film after the field-cooling from room temperature. With increasing anti-ferromagnetic (AFM) FeF2 layer thickness, the value of exchange bias increases and the training effect becomes weaker. Temperature-dependent study of exchange bias field indicates that with increasing F-ion fluences the disorder decreases at the Fe/FeF2 interface. Specimen of higher ion-fluence indicates a behavior that is changing from non-equilibrium state to equilibrium state and behavior of training effect is suggesting step-like.