Antiferromagnetic Metal Research Articles

Na-vacancies-induced magnetism in NaxMnO2

The magnetism of layered NaxMnO2 with space group P63/mmc is investigated by first-principles calculation and magnetic measurement. Results indicate that NaMnO2 without any Na-vacancies posses the antiferromagnetic metal state. And along with Na-vacancies formed, the magnetism and electronic properties of NaxMnO2 show rich change, which undergo the change of AFM → FM → AFM and metal → semiconductor, respectively. Magnetic measurement of Na0.806MnO2−δ with space group P63/mmc also shows that have thin hysteresis and remanent under 5 K. Hence, Na0.806MnO2−δ presents the ferromagnetic metal state under low temperature rooting in the Mn3+-O2−-Mn4+ double exchange, which is agreement with theoretical results.

Antiferromagnetic Resonance in GdB6

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB6 antiferromagnet (TN = 15.5K) and in the antiferromagnetic state (T < TN). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd3+ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ0H0 ≈ 1.9 T to μ0H0 ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω0(H0) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H 0 2 +2HAHE)1/2, where HE is the exchange field and HA is the anisotropy field. This provides an estimate for the anisotropy field, HA ≈ 800 Oe. This value can result from the dipole−dipole interaction related to the mutual displacement of Gd3+ ions, which occurs at the antiferromagnetic transition.

Persistent Large Anisotropic Magnetoresistance and Insulator-to-Metal Transition in Spin-Orbit-Coupled Sr2(Ir1−xGax)O4 for Antiferromagnetic Spintronics

Antiferromagnetic (AFM) spintronics, where magneto-transport is governed by an antiferromagnet instead of a ferromagnet, opens fascinating new perspectives for both fundamental research and device technology, owing to their intrinsic appealing properties like rigidness to magnetic field, absence of stray field, and ultrafast spin dynamics. One of the urgent challenges, hindering the realization of the full potential of AFM spintronics, has been the performance gap between AFM metals and insulators. Here, we demonstrate the insulator-metal transition and persistently large anisotropic magnetoresistance (AMR) in single crystals Sr2(Ir1-xGax)O4 (0<x<0.09) which host the same basal-plane AFM lattice with strong spin-orbit coupling. The non-doped Sr2IrO4 shows the insulating transport with the AMR as big as ~16.8% at 50 K. The Ga substitution of Ir allows a gradual reduction of electrical resistivity, and a clear insulator-to-metal transition is identified in doped samples with x above 0.05, while the AMR can still have ~1%, sizable in comparison with those in AFM metals reported so far. Our experiments reveal that all the samples have the similar fourfold AMR symmetry, which can be well understood in the scenario of magnetocrystalline anisotropy. It is suggested that the spin-orbit coupled antiferromagnets Sr2(Ir1-xGax)O4 are promising candidate materials for AFM spintronics, providing a rare opportunity to integrate the superior spintronic functionalities of AFM metals and insulators.

Impact of thermal fluctuations on transport in antiferromagnetic semimetals

Recent demonstrations on manipulating antiferromagnetic (AF) order have triggered a growing interest in antiferromagnetic metal (AFM), and potential high-density spintronic applications demand further improvements in the anisotropic magnetoresistance (AMR). The antiferromagnetic semimetals (AFS) are newly discovered materials that possess massless Dirac fermions that are protected by the crystalline symmetries. In this material, a reorientation of the AF order may break the underlying symmetries and induce a finite energy gap. As such, the possible phase transition from the semimetallic to insulating phase gives us a choice for a wide range of resistance ensuring a large AMR. To further understand the robustness of the phase transition, we study thermal fluctuations of the AF order in AFS at a finite temperature. For macroscopic samples, we find that the thermal fluctuations effectively decrease the magnitude of the AF order by renormalizing the effective Hamiltonian. Our finding suggests that the insulating phase exhibits a gap narrowing at elevated temperatures, which leads to a substantial decrease in AMR. We also examine spatially correlated thermal fluctuations for microscopic samples by solving the microscopic Landau-Lifshitz-Gilbert equation finding a qualitative difference of the gap narrowing in the insulating phase. For both cases, the semimetallic phase shows a minimal change in its transmission spectrum illustrating the robustness of the symmetry protected states in AFS. Our finding may serve as a guideline for estimating and maximizing AMR of the AFS samples at elevated temperatures.

Spin transfer torques and spin-dependent transport in a metallic F/AF/N tunneling junction

We study spin-dependent electron transport through a ferromagnetic-antiferromagnetic-normal metal tunneling junction subject to a voltage or temperature bias, in the absence of spin-orbit coupling. We derive microscopic formulas for various types of spin torque acting on the antiferromagnet as well as for charge and spin currents flowing through the junction. The obtained results are applicable in the limit of slow magnetization dynamics. We identify a parameter regime in which an unconventional damping-like torque can become comparable in magnitude to the equivalent of the conventional Slonczewski's torque generalized to antiferromagnets. Moreover, we show that the antiferromagnetic sublattice structure opens up a channel of electron transport which does not have a ferromagnetic analogue and that this mechanism leads to a pronounced field-like torque. Both charge conductance and spin current transmission through the junction depend on the relative orientation of the ferromagnetic and the antiferromagnetic vectors (order parameters). The obtained formulas for charge and spin currents allow us to identify the microscopic mechanisms responsible for this angular dependence and to assess the efficiency of an antiferromagnetic metal acting as a spin current polarizer.

Tailoring colossal magnetoresistance and magnetoresistive memory effect by two-dimension-like phase competition in electron-doped manganite superlattices

Colossal magnetoresistance and giant magnetoresistive memory effect have been demonstrated in the electron-doped manganite superlattices composed of alternatively of G-type antiferromagnetic insulator (G-AFI) layers of CaMnO3 and C-type antiferromagnetic insulator (C-AFI) layers of Ca0.92Ce0.08MnO3. The transport properties of the superlattices strongly depend on the periodicity and the unit cells of each layer. The charge transfer through the interface of the constituent insulators induces appearance of the canted G-type antiferromagnetic metal (cG-AFM) phase in the superlattices. Enhanced colossal magnetoresistance and memory effect emerge in the superlattices composed of the thinner G-AFI layers, which can be attributed to the artificially generated two-dimension-like phase competition between cG-AFM and C-AFI in the charge transfer region. The effective charge transfer through interface provides us an opportunity to control phase separation and thus tailoring electronic properties of electron-doped manganites.

Fragility of Charge Order Near an Antiferromagnetic Quantum Critical Point.

We investigate the interplay between charge order and superconductivity near an antiferromagnetic quantum critical point using sign-problem-free Quantum MonteCarlo simulations. We establish that, when the electronic dispersion is particle-hole symmetric, the system has an emergent SU(2) symmetry that implies a degeneracy between d-wave superconductivity and charge order with d-wave form factor. Deviations from particle-hole symmetry, however, rapidly lift this degeneracy, despite the fact that the SU(2) symmetry is preserved at low energies. As a result, we find a strong suppression of charge order caused by the competing, leading superconducting instability. Across the antiferromagnetic phase transition, we also observe a shift in the charge order wave vector from diagonal to axial. We discuss the implications of our results to the universal phase diagram of antiferromagnetic quantum-critical metals and to the elucidation of the charge order experimentally observed in the cuprates.

Tuning the Pairing Interaction in a d-Wave Superconductor by Paramagnons Injected through Interfaces.

Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. Here we fabricate hybrid superlattices consisting of alternating atomic layers of the heavy-fermion superconductor CeCoIn_{5} and antiferromagnetic (AFM) metal CeRhIn_{5}, in which the AFM order can be suppressed by applying pressure. We find that the superconducting and AFM states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting properties. An analysis of upper critical fields reveals that, upon suppressing the AFM order by applied pressure, the force binding superconducting electron pairs acquires an extreme strong-coupling nature. This demonstrates that superconducting pairing can be tuned nontrivially by magnetic fluctuations (paramagnons) injected through the interface.

Exploring the energy landscape of resistive switching in antiferromagnetic Sr3Ir2O7

We study the resistive switching triggered by an applied electrical bias in antiferromagnetic Mott insulator Sr(3)Ir(2)O(7). The switching was previously associated with an electric-field driven structural transition. Here we use time-resolved measurements of the switching to probe the energy barrier associated with the transition. We quantify the changes in the energy barrier height with respect to the applied bias and find a linear decrease of the barrier with increasing bias. Our observations support the potential of antiferromagnetic transition metal oxides for spintronic applications.

Development of half metallicity within mixed magnetic phase of Cu1−xCoxMnSb alloy

Cubic half-Heusler Cu1−xCoxMnSb () compounds have been investigated both experimentally and theoretically for their magnetic, transport and electronic properties in search of possible half metallic antiferromagnetism. The systems (Cu,Co)MnSb are of particular interest as the end member alloys CuMnSb and CoMnSb are semi metallic (SM) antiferromagnetic (AFM) and half metallic (HM) ferromagnetic (FM), respectively. Clearly, Co-doping at the Cu-site of CuMnSb introduces changes in the carrier concentration at the Fermi level that may lead to half metallic ground state but there remains a persistent controversy whether the AFM to FM transition occurs simultaneously. Our experimental results reveal that the AFM to FM magnetic transition occurs through a percolation mechanism where Co-substitution gradually suppresses the AFM phase and forces FM polarization around every dopant cobalt. As a result a mixed magnetic phase is realized within this composition range while a nearly HM band structure is developed already at the 10% Co-doping. Absence of T2 dependence in the resistivity variation at low T-region serves as an indirect proof of opening up an energy gap at the Fermi surface in one of the spin channels. This is further corroborated by the ab initio electronic structure calculations that suggests that a nearly ferromagnetic half-metallic ground state is stabilized by Sb-p holes produced upon Co doping.

Room-Temperature Skyrmions in an Antiferromagnet-Based Heterostructure.

Magnetic skyrmions as swirling spin textures with a nontrivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in non-centrosymmetric B20 materials, the recent effort has focused on exploring room-temperature skyrmions in heavy metal and ferromagnetic heterostructures, a material platform compatible with existing spintronic manufacturing technology. Here, we report the surprising observation that a room-temperature skyrmion phase can be stabilized in an entirely different class of systems based on antiferromagnetic (AFM) metal and ferromagnetic (FM) metal IrMn/CoFeB heterostructures. There are a number of distinct advantages of exploring skyrmions in such heterostructures including zero-field stabilization, tunable antiferromagnetic order, and sizable spin-orbit torque (SOT) for energy-efficient current manipulation. Through direct spatial imaging of individual skyrmions, quantitative evaluation of the interfacial Dzyaloshinskii-Moriya interaction, and demonstration of current-driven skyrmion motion, our findings firmly establish the AFM/FM heterostructures as a promising material platform for exploring skyrmion physics and device applications.

Bound magnetic polaron in Zn-rich cobalt-doped ZnSe nanowires

The micro-luminescence spectra of the diluted magnetic semiconductor (DMS) can reflect the spin–exciton interaction and related relaxation process. Here the micro-photoluminescence (micro-PL) spectra and PL lifetime measurements have been done on an individual ferromagnetic (FM)-coupled cobalt (Co) doped zinc selenide (ZnSe) nanowire. There occurs a double-peak profile in its near bandedge emission spectrum: the first peak is from free exciton (FX) and the second comes from magnetic polaron (MP). In their temperature dependent PL spectra, the MP emission peak demonstrates obviously temperature-independent behavior, in contrast to the behaviors of FX and reported exciton MP in nanobelt. It is found that in this Co(II) doped ZnSe nanowires, this MP’s temperature-independent emission is related to the coupling between exciton and a FM nanocluster (↑↑↓). The nanocluster is likely due to the interaction of Se vacancies of the wide bandgap semiconductors with the antiferromagnetic (AFM) arrangement transition metal (TM) ions in these Se-deficient Co doped ZnSe nanowires. These results reflect that the AFM coupling TM ions pair can give rise to FM behavior with the involvement of positive charge defect, also indicating that the micro-luminescence detection can be used to study the magnetic coupling in DMS.

A Computational Model of the Longitudinal Magneto-Optic Kerr Effect of Co/Cu/Co Synthetic Antiferromagnetic Thin Films

Event Abstract Back to Event A Computational Model of the Longitudinal Magneto-Optic Kerr Effect of Co/Cu/Co Synthetic Antiferromagnetic Thin Films Cyerra L. Prevo1*, Kathleen B. Oolman2* and Virginia O. Lorenz2* 1 Grambling State University, United States 2 University of Illinois at Urbana-Champaign, United States We develop a computational model of the longitudinal magneto-optic Kerr Effect (L-MOKE) of a Co/Cu/Co synthetic antiferromagnet. The model is based on analytic formulae including the magneto-optic Fresnel coefficients and multiple reflections, written in MATLAB. This project continues work done to obtain a picture of spin-orbit interaction induced magnetization switching in thin multi-layered systems using normal and synthetic antiferromagnetic metals. We first replicate the L-MOKE response of a 10-layer 5.58Co/5Cu/5.58Co ferromagnetic system from the literature to verify accuracy. The model is then adjusted to calculate the L-MOKE from a tri-layer synthetic antiferromagnetic system composed of Co/Cu/Co. This model gives an approximation of the L-MOKE response of synthetic antiferromagnetic thin films. The results of this project will be useful in the determination of the origins of spin-orbit--interaction-based phenomena by comparing with experimental results, towards the development of next-generation spintronic devices. Acknowledgements NSF Illinois Materials Research Science and Engineering Center (I-MRSEC)

Tuning the metamagnetism in a metallic helical antiferromagnet

The antiferromagnetic (AFM)-ferromagnetic (FM) conversion in martensite was observed in Mn/Ni-substitution upon FM elements, such as Fe or Co, in MnNiGe helical antiferromagnets. Here, we report an AFM-FM conversion and consequently a sharp magnetic-field-driven metamagnetic martensitic transformation from paramagnetic (PM) austenite to FM martensite in the Ni- and Mn-substituted MnNiGe alloys with indium, a non-magnetic and large-sized main group element. Accordingly, a giant magnetocaloric effect such that a twofold increase of the magnetic entropy change in MnNi0.92GeIn0.08 and even a nearly threefold increase in the Mn0.92NiGeIn0.08 alloy is obtained with respect to the MnNiGe0.95In0.05 alloy. The origin of AFM-FM conversion and resultantly sharp magnetic-field-induced PM-FM metamagnetic transformation is discussed based on the first-principles calculations and X-ray absorption spectroscopic results.

Magnonic triply-degenerate nodal points

We generalize the concept of triply-degenerate nodal points to non-collinear antiferromagnets. Here, we introduce this concept to insulating quantum antiferromagnets on the decorated honeycomb lattice, with spin-1 bosonic quasiparticle excitations known as magnons. We demonstrate the existence of magnonic surface states with constant energy contours that form pairs of magnonic arcs connecting the surface projection of the magnonic triple nodal points. The quasiparticle excitations near the triple nodal points represent three-component bosons beyond that of magnonic Dirac, Weyl, and nodal-line cases. They can be regarded as a direct reflection of the intrinsic spin carried by magnons. Furthermore, we show that the magnonic triple nodal points can split into magnonic Weyl points, as the system transits from a non-collinear spin structure to a non-coplanar one with a non-zero scalar spin chirality. Our results not only apply to insulating antiferromagnets, but also provide a platform to seek for triple nodal points in metallic antiferromagnets.

Ferromagnetism and antiferromagnetism coexistence in Sr1-xLaxRuO3 induced by La-doping

Recently, a spin-glass state appeared experimentally along with suppression of ferromagnetism in polycrystalline La-doped Sr1-xLaxRuO3. Here, we investigated the structural, magnetic and electronic properties of Sr1-xLaxRuO3 (x = 0, 0.125, 0.25, 0.5, 1) based on generalized gradient approximation plus U (GGA + U) by first-principles calculations. The entire series of Sr1-xLaxRuO3 (x = 0, 0.125, 0.25, 0.5, 1) stabilize in the orthorhombic perovskite structure. The spin-polarized calculations give a ferromagnetic half-metal ground state at 0 ≤ x ≤ 0.25, coexistence of a ferromagnetic half-metal state and antiferromagnetic insulating state at x = 0.5 and an antiferromagnetic metal state at x = 1, in good agreement with experimental results. La substitution for Sr decreases RuORu bond angles, resulting in stronger GdFeO3 distortion. The stronger GdFeO3 distortion reduces density of states at the Fermi level, with broadening of Van-hove singularity near the Fermi level. Therefore, suppression of magnetism in Sr1-xLaxRuO3 becomes stronger with increasing x. La-doping weakens a Stoner mechanism for ferromagnetism in SrRuO3. These theoretical results interpret well experiments.

Magnetic excitations in metallic antiferromagnets Fe0.5Mn0.5 and Fe0.7Mn0.3

Magnetic excitations in the metallic antiferromagnet Fe0.5Mn0.5 were investigated using the inelastic neutron scattering technique at low temperatures well below the antiferromagnetic transition temperatures (TN). The observed peak positions in low energies well sit on the antiferromagnetic dispersion relation reported in the early study. However, the peak positions in higher energies shift to higher energies than that of the antiferromagnetic spin wave dispersion relation. This behavior is the same as one of the behaviors suggesting that the spin waves merge with the continuum of the individual particle-hole excitations at higher energies in Fe0.7Mn0.3.

Mode-dependent damping in metallic antiferromagnets due to intersublattice spin pumping

Damping in magnetization dynamics characterizes the dissipation of magnetic energy and is essential for improving the performance of spintronics-based devices. While the damping of ferromagnets has been well studied and can be artificially controlled in practice, the damping parameters of antiferromagnetic materials are nevertheless little known for their physical mechanisms or numerical values. Here we calculate the damping parameters in antiferromagnetic dynamics using the generalized scattering theory of magnetization dissipation combined with the first-principles transport computation. For the PtMn, IrMn, PdMn and FeMn metallic antiferromagnets, the damping coefficient associated with the motion of magnetization ($\alpha_m$) is one to three orders of magnitude larger than the other damping coefficient associated with the variation of the N\'eel order ($\alpha_n$), in sharp contrast to the assumptions made in the literature.

Weyl fermions in antiferromagnetic Mn3Sn and Mn3Ge

The anomalous Hall effect in the noncollinear antiferromagnetic metals Mn3Ge and Mn3Sn has been observed after a theoretical prediction made by us (Kübler J. and Felser C., EPL, 108 (2014) 67001). The experimental values of the anomalous Hall conductivities (AHC) are large as are the theoretical values. Recently measured thermoelectric properties mirror the large AHC and clearly show that the transport is by quasiparticles at the Fermi energy. We here make an attempt to unravel the origin of the large AHC and show that both Mn3Sn and Mn3Ge host Weyl points, which were recently discovered in semimetals. For this purpose we determine the electronic structure ab initio in the local spin-density functional approximation. The Weyl points are found to occur below the Fermi energy and we argue that spots of large Berry flux (“hot spots”) that are seen at the Fermi surface are produced by the Weyl nodes.

Structural stability, magneto-electronics and spin transport properties of triangular graphene nanoflake chains with edge oxidation

Tuning the magneto-electronic properties of graphene-based structures into distinguished performance is an interesting but challenging work. To address this issue, we here construct several chain-like 1D nanostructures by stitching zigzag-edged triangular graphene nanoflakes with different manners and subsequently oxidized at edges. The high stability of these structures is identified by the calculated edge adsorption energy, phonon spectrum, and molecular dynamics simulations. Unlike edge-hydrogenation case, termination oxygen atoms here are highly magnetized and can control such 1D chain magneto-electronic features substantially. The calculations predict that such 1D chains are very versatile, and behaviors are sensitive to geometry, likely acting as a ferromagnetic metal and half-metal and bipolar magnetic semiconductor, or an antiferromagnetic metal and semiconductor. In particular, such a ferromagnetic half-metallic feature can occur in the ground state and possesses a wide bandgap, suggesting that they are excellent magnetic materials. The calculated spin transport characteristics reveal that such a chain-based device promises not only a perfect double spin-filtering effect, but also an excellent dual spin diode feature and a giant magnetoresistance (GMR) effect. The advantages over graphene nanoribbons are thus expected.