Antiferromagnetic Materials Research Articles

Modulation of magnetic damping in antiferromagnet/CoFeB heterostructures

The modulation of antiferromagnetic (AFM) material and thermal annealing treatment on the magnetic damping of various AFM/CoFeB (CFB) samples have been systematically studied with the time-resolved magneto-optical Kerr effect. It is found the saturated magnetic damping factor αs increases considerably after introducing a thin MnIr or MnPt AFM layer. As a thin Al spacer of tAl = 0–2 nm is inserted, αs is found to decrease rapidly, reaching nearly the same value as that of the single CFB film at tAl = 2 nm. The result suggests that the AFM layer is not a good spin sink material and the surprisingly strong decrease in the damping factor is mainly attributed to the reduced direct exchange coupling between CoFe and Mn spins at the AFM/FM interface. Moreover, in spite of the exchange bias effect occurring or not, a similar monotonic increasing trend of αs with the increasing AFM layer thickness is observed for the as-deposited and annealed AFM/CFB samples, indicating that the enhanced magnetic damping at an elevated annealing temperature is mainly related to the increased interface roughness and atomic diffusion. These findings provide deeper insights into the role of the AFM/FM interface in magnetization dynamics, which will be helpful for future spintronic applications.

A new Bi0.7Fe1.3O1.5F1.7 phase: Crystal structure, magnetic properties, and cathode performance in fluoride-ion batteries

We synthesize a Bi0.7Fe1.3O1.5F1.7 (BFOF) phase via a non-topochemical reaction with a fluorination agent. The crystal structure is refined by Rietveld refinement on the neutron diffraction patterns as a hexagonal lattice in the R3¯ space group, along with the defect structure. The sudden decrease in magnetic susceptibility below 250 K and the linear relationship between the magnetization and the magnetic field indicate that BFOF is an antiferromagnetic material. When BFOF is used as a cathode in fluoride-ion batteries (FIBs), a discharge (charge) capacity of 360 (225) mAh/g is achieved at 140 °C. Magnetization and x-ray diffraction measurements confirm that the F ions are transferred from the cathode to the Pb counter electrode during discharge and in the opposite direction during charge, in a manner analogous to the transfer of lithium (Li) ions in Li-ion batteries. These findings contribute to the development of quaternary oxyfluorides serving as FIB cathodes.

A comparative study of spin Hall magnetoresistance in Fe2O3-based systems

Spin Hall magnetoresistance (SMR) provides a unique opportunity for the signal readout of magnetic ordering, especially for magnetic insulators. However, the magnitude of SMR signals is quite small, generally lower than one in a thousand, which is a persistent challenge in the field of spintronics. Here, we investigate SMR at room temperature in α-Fe2O3/Pt and γ-Fe2O3/Pt bilayers as well as the α-Fe2O3/Pt/γ-Fe2O3 trilayer, where α-Fe2O3 and γ-Fe2O3 are antiferromagnetic and ferrimagnetic materials, respectively. In contrast to the positive SMR in the γ-Fe2O3/Pt bilayer, an unprecedentedly large negative SMR with the magnitude of ∼0.35% is observed in the α-Fe2O3/Pt bilayer due to the spin-flop. The α-Fe2O3/Pt/γ-Fe2O3 trilayer shows features of a combination of positive and negative SMR. A comparative study of SMR in the Fe2O3 family not only enriches spin physics, but also makes Fe2O3 a versatile candidate in spintronic devices.

Physical properties and magnetic structure of a layered antiferromagnet PrPd0.82Bi2**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0302901 and 2016YFA0300604), the National Natural Science Foundation of China (Grant No. 11774399), Beijing Natural Science Foundation, China (Grant No. Z180008), the K. C. Wong Education Foundation (Grant No. GJTD-2018-01), the DAAD-PPP programme, and the joint German-Sino

We report the physical properties, crystalline and magnetic structures of singe crystals of a new layered antiferromagnetic (AFM) material PrPd0.82Bi2. The measurements of magnetic properties and heat capacity indicate an AFM phase transition at TN ∼ 7 K. A large Sommerfeld coefficient of 329.23 mJ⋅mol−1⋅K−2 is estimated based on the heat capacity data, implying a possible heavy-fermion behavior. The magnetic structure of this compound is investigated by a combined study of neutron powder and single-crystal diffraction. It is found that an A-type AFM structure with magnetic propagation wavevector k = (0 0 0) is formed below TN. The Pr3+ magnetic moment is aligned along the crystallographic c-axis with an ordered moment of 1.694(3) μB at 4 K, which is smaller than the effective moment of the free Pr3+ ion of 3.58 μB. PrPd0.82Bi2 can be grown as large as 1 mm×1 cm in area with a layered shape, and is very easy to be cleaved, providing a unique opportunity to study the interplay between magnetism, possible heavy fermions, and superconductivity.

Spin transmission in IrMn through measurements of spin Hall magnetoresistance and spin-orbit torque

Understanding the transport of spin current in antiferromagnetic materials is indispensable to develop antiferromagnetic spintronic devices. In this work, we study the spin current transmission through an antiferromagnetic IrMn insertion layer in a W/IrMn($t$)/CoFeB structure by measuring the spin Hall magnetoresistance (SMR) and spin-orbit torque (SOT). The temperature dependences of SMR and SOT effective fields indicate that the spin current transmission reaches its maximum at the N\'eel temperature of IrMn. The enhancement is ascribed to the increase in the interfacial spin mixing conductance, which is related to the maximum magnetic susceptibility of the IrMn layer at the N\'eel temperature. The spin transmission decreases monotonically as a function of the IrMn thickness, which is different from the case with an insulating antiferromagnetic NiO insertion layer in previous works. In addition, we found that the spin transmission through an antiferromagnetic IrMn layer is independent of the exchange bias orientation. Our results suggest that the spin current transmission through the IrMn layer (from W layer to CoFeB layer) is mainly mediated by spin-polarized electrons rather than magnons, which is likely due to the absence of the effective excitation of magnons in the IrMn layer by the spin-polarized current.

Large Spin Hall Angle and Spin-Mixing Conductance in the Highly Resistive Antiferromagnet Mn2Au

Antiferromagnetic (AFM) materials recently have shown interest in the research in spintronics due to its zero stray magnetic field, high anisotropy, and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn attention because it exhibits unique properties and its Neel temperature is very high. Here, we report spin pumping and inverse spin Hall effect investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance. We found large spin Hall angle {\theta}_SH = 0.22

Numerical Methods for Antiferromagnets

Compared with ferromagnetic counterparts, antiferromagnetic materials are considered as the future of spintronic applications since these materials are robust against the magnetic perturbation, produce no stray field, and display ultrafast dynamics. There are (at least) two sets of magnetic moments in antiferromagnets (with magnetization of the same magnitude but antiparallel directions) and ferrimagnets (with magnetization of the different magnitude). The coupled dynamics for the bipartite collinear antiferromagnets is modeled by a coupled system of Landau-Lifshitz-Gilbert equations with an additional term originated from the antiferromagnetic exchange, which leads to femtosecond magnetization dynamics. In this paper, we develop three Gauss-Seidel projection methods for micromagnetics simulation in antiferromagnets and ferrimagnets. They are first-order accurate in time and second-order in space, and only solve linear systems of equations with constant coefficients at each step. Femtosecond dynamics, N\'{e}el wall structure, and phase transition in presence of an external magnetic field for antiferromagnets are provided with the femtosecond stepsize.

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Recent research works have shown that the magnetic order in some antiferromagnetic materials can be manipulated and detected electrically, due to two physical mechanisms: Neel-order spin-orbit torques and anisotropic magnetoresistance. While these observations open up opportunities to use antiferromagnets for magnetic memory devices, different physical characterization methods are required for a better understanding of those mechanisms. Here we report a magnetic field induced rotation of the antiferromagnetic Neel vector in epitaxial tetragonal CuMnAs thin films. Using soft x-ray magnetic linear dichroism spectroscopy, x-ray photoemission electron microscopy, integral magnetometry and magneto-transport methods, we demonstrate spin-flop switching and continuous spin reorientation in antiferromagnetic films with uniaxial and biaxial magnetic anisotropies, respectively. From field-dependent measurements of the magnetization and magnetoresistance, we obtain key material parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, spin-flop and exchange fields.

Spin-orbit torque induced electrical switching of antiferromagnetic MnN

Electrical switching and readout of antiferromagnets allows to exploit the unique properties of antiferromagnetic materials in nanoscopic electronic devices. Here we report experiments on the spin-orbit torque induced electrical switching of a polycrystalline, metallic antiferromagnet with low anisotropy and high N\'eel temperature. We demonstrate the switching in a Ta / MnN / Pt trilayer system, deposited by (reactive) magnetron sputtering. The dependence of switching amplitude, efficiency, and relaxation are studied with respect to the MnN film thickness, sample temperature, and current density. Our findings are consistent with a thermal activation model and resemble to a large extent previous measurements on CuMnAs and Mn$_2$Au, which exhibit similar switching characteristics due to an intrinsic spin-orbit torque.

Realizing spin Hamiltonians in nanoscale active photonic lattices.

Spin models arise in the microscopic description of magnetic materials and have been recently used to map certain classes of optimization problems involving large degrees of freedom. In this regard, various optical implementations of such Hamiltonians have been demonstrated to quickly converge to the global minimum in the energy landscape. Yet, so far, an integrated nanophotonic platform capable of emulating complex magnetic materials is still missing. Here, we show that the cooperative interplay among vectorial electromagnetic modes in coupled metallic nanolasers can be utilized to implement certain types of spin Hamiltonians. Depending on the topology/geometry of the arrays, these structures can be governed by a classical XY Hamiltonian that exhibits ferromagnetic and antiferromagnetic couplings, as well as geometrical frustration. Our results pave the way towards a scalable nanophotonic platform to study spin exchange interactions and could address a variety of optimization problems.

Magnetisation Processes in Geometrically Frustrated Spin Networks with Self-Assembled Cliques

Functional designs of nanostructured materials seek to exploit the potential of complex morphologies and disorder. In this context, the spin dynamics in disordered antiferromagnetic materials present a significant challenge due to induced geometric frustration. Here we analyse the processes of magnetisation reversal driven by an external field in generalised spin networks with higher-order connectivity and antiferromagnetic defects. Using the model in (Tadić et al. Arxiv:1912.02433), we grow nanonetworks with geometrically constrained self-assemblies of simplexes (cliques) of a given size n, and with probability p each simplex possesses a defect edge affecting its binding, leading to a tree-like pattern of defects. The Ising spins are attached to vertices and have ferromagnetic interactions, while antiferromagnetic couplings apply between pairs of spins along each defect edge. Thus, a defect edge induces frustrated triangles per n-clique participating in a larger-scale complex. We determine several topological, entropic, and graph-theoretic measures to characterise the structures of these assemblies. Further, we show how the sizes of simplexes building the aggregates with a given pattern of defects affects the magnetisation curves, the length of the domain walls and the shape of the hysteresis loop. The hysteresis shows a sequence of plateaus of fractional magnetisation and multiscale fluctuations in the passage between them. For fully antiferromagnetic interactions, the loop splits into two parts only in mono-disperse assemblies of cliques consisting of an odd number of vertices n. At the same time, remnant magnetisation occurs when n is even, and in poly-disperse assemblies of cliques in the range . These results shed light on spin dynamics in complex nanomagnetic assemblies in which geometric frustration arises in the interplay of higher-order connectivity and antiferromagnetic interactions.

Impact of Samarium on the Growth of Epitaxial Bismuth Ferrite Thin Films

Doping of rhombohedral bismuth ferrite (BFO) with rare earth elements has been widely investigated as a pathway to extract ferromagnetic response from an otherwise antiferromagnetic material. However, increased level of such doping, in conjunction with the ability of BFO to accommodate large strain, has also resulted in nontrivial changes in the structure, i.e., transition to orthorhombic structure and phase separation to form vertically aligned columns. Herein, epitaxially grown and single crystalline samarium oxide (Sm2O3) and doped BFO films are used to investigate the structural evolution. Thin films are grown from undoped (BFO and Sm2O3) and doped targets, (0.2,0.5)Sm2O3–(0.8,0.5)BFO. In addition, the in‐plane strain, imposed by the lattice mismatch between film and substrates, is used to demonstrate the stability of the structures formed in the doped films. Interestingly, the resultant orthorhombic structures are found to be largely independent of the underlying substrates. In‐depth structural and nanoscopic measurements are conducted to investigate the structures. Ordered columnar structures, reminiscent of phase separation, are successfully obtained albeit driven by spontaneous ordering of differently oriented crystals.

Weak ferromagnetic component in goethite (α-FeOOH) and its relation with microstructural characteristics

Goethite is an antiferromagnetic material that frequently presents a superimposed weak ferromagnetic component. In this research, two goethite samples were prepared using distinct synthesis routes, being later subjected to heat treatment. Magnetization dynamics were analyzed as a function of applied magnetic field and temperature. Moreover, the magnetic measurements were correlated with the resulting microstructures, obtained through scanning and transmission electron microscopy. The results indicate an enhanced ferromagnetic behavior in those goethite samples, with greater presence of defects and imperfections. Besides that, it was observed a reduction of magnetic fluctuations after the healing of the samples via heat treatment. Despite what is vastly proposed in the literature to explain the origin of the magnetic component in goethite, our analysis showed that, for the studied samples, this effect is caused by a magnetic mismatch in the antiferromagnetic lattice. This mismatch is possibly caused by defects as grain boundaries, voids and grain rotations, which may lead to dislocations and consequently to magnetic mismatches in the lattice.

Fabrication of spin torque devices by using quasi-antiferromagnetic materials

The devices we use on a day-to-day basis require a substantial amount of energy to power, but even more energy is left unused and goes to waste. What if our devices were (at least nearly) completely energy efficient? This would help pave the way towards a greener and more energy smart future. This concept is something one team of Japanese researchers is working on, by using quasi antiferromagnetic (AFM) materials which work successfully at the nanoscale. Professor Hiroma Yuasa is based at Kyushu University, Japan, where her laboratory is working on spintronics research. Currently, her focus includes spin current physics and spin torque in artificial magnetic structures, including the applications of these, such as in energy harvesting and ultra-low power consumption devices, which could help in achieving a greener future.

Magneto-electric antiferromagnetic spin–orbit logic devices

As electronic integrated circuits are scaled to ever smaller sizes, they run into the obstacle of excessive power dissipation. Spintronic devices hold the promise of alleviating this problem via improved energy efficiency. Research effort around a promising class of such devices based on antiferromagnetic materials and magnetoelectric switching is reviewed.

Magnetic dynamics of quasi-antiferromagnetic layer fabricated by 90° magnetic coupling

In this study, we fabricated quasi-antiferromagnetic (AFM) layers and investigated the magnetic dynamics of quasi-AFM in both an experiment and simulation. Quasi-AFM has multi domains with alternating antiparallel magnetization, which can be realized by using 90° magnetic coupling between two ferromagnetic layers. In magnetic resonance measurement, a frequency and damping constant of CoFe-based quasi-AFM are higher than those of conventional CoFe. Likewise, in micromagnetic simulation, we obtained the spin torque oscillation in Quasi-AFM with high frequency. This suggests the possibility of a novel material with the high frequency wave generator without a stray field, like antiferromagnetic materials.

Ligand-Stabilized CoO and NiO Nanoparticles for Spintronic Devices with Antiferromagnetic Insulators

Cobalt oxide (CoO) and nickel oxide (NiO) nanoparticles, coordinated by oleylamine, oleic acid, and trioctylphosphine oxide, were prepared as antiferromagnetic materials in 1-octanol or 1-dodecanol...

Controlling vertical magnetization shift by spin–orbit torque in ferromagnetic/antiferromagnetic/ferromagnetic heterostructure

We report the control of the vertical magnetization shift (VMS) and exchange bias through spin–orbit torque (SOT) in Pt/Co/Ir25Mn75/Co heterostructure devices. The exchange bias accompanying with a large relative VMS of about 30% is observed after applying a single pulse of 40 mA in a perpendicular field of 2 kOe. Furthermore, the field-free SOT-induced variations of VMS and exchange bias are also observed, which would be related to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. The SOT-induced switched fraction of out-of-plane interfacial spins shows a linear dependence on relative VMS, indicating that the number of uncompensated pinned spins is proportional to the switched interfacial spins. Our finding offers a comprehensive understanding for electrically manipulating interfacial spins of antiferromagnetic materials.

Electrical manipulation of the magnetic order in antiferromagnetic PtMn pillars

Antiferromagnets are magnetically ordered materials without a macroscopic magnetization. As a result, they could be of use in the development of memory devices because data cannot be erased by external magnetic fields. However, this also makes it difficult to electrically control their magnetic order (Neel vector). Here, we show that pillars of antiferromagnetic PtMn, which are grown on a heavy-metal layer and have diameters down to 800 nm, can be reversibly switched between different magnetic states by electric currents. The devices are based on materials that are typically used in the magnetic memory industry, and we observe switching down to a current density of ~2 MA cm−2. Furthermore, by varying the amplitude of the writing current, multilevel memory characteristics can be achieved. Micromagnetic simulations suggest that the different magnetic states may consist of domains separated by domain walls with vortex and anti-vortex textures that move in response to current, modifying the average Neel vector. Pillars of antiferromagnetic PtMn, grown on a heavy-metal layer, can be reversibly switched between different magnetic states by electric currents, illustrating the potential of silicon-compatible antiferromagnetic materials in the development of memory and computing devices.

Silicon-compatible antiferromagnets switch on

An electric current can be used to switch the magnetization of an antiferromagnetic material that could be integrated with silicon technology.