United States

We combined XMLD, XMCD and MOKE measurements in order to follow the magnetic properties of epitaxial CoO(111)/Fe(110) [1] and NiO(111)/Fe(110) [2] bilayers. We find that in both studied cases FM sublayer plays a dominant role and determines the magnetic state of the neighboring AFM, however completely different interaction mechanisms are involved.&lt;br/&gt;In CoO/Fe bilayers the AFM spins are frozen and their orientation is imprinted by magnetization of Fe layer when the system passes the Neel temperature of CoO. Once the Fe layer grafts the particular magnetic anisotropy (MA) into the CoO overlayer, it later remains frozen and insensitive to external factors like external magnetic field or Fe magnetization direction [3]. Specifically, choice of particular magnetic state of Fe sublayer, when passing Néel temperature of CoO, determines both the axis and direction of interfacial antiferromagnetic spins after the sample is cooled and allows for imprinting their +/- 90° and 0/180° alignment within the sample plane. For example, particular direction of frozen AFM spins determines the corresponding sign of the shift field of exchange biased magnetic hysteresis loop (Fig.1).&lt;br/&gt;For NiO/Fe bilayers, due to the weak intrinsic MA of NiO, the AFM spins are rotatable and always follow the reorientation of Fe magnetization that can be controlled by external magnetic field or via the temperature and thickness driven SRT of Fe(110). Recently we showed that in a uniform thickness NiO(111)/Fe(110) system, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO [2]. Field-free, reversible switching between these AFM states was demonstrated. Here we present results of systematic investigations of such field-free, temperature induced switching of AFM NiO spins. Fig. 2 shows temperature dependence of R&lt;sub&gt;L2&lt;/sub&gt; ratio for various thicknesses of Fe. Critical temperature of SRT in AFM NiO can be tuned in wide temperature window ~ (250 – 380 K) for field-free, reversible switching of AFM spins. XML(C)D-PEEM results obtained on individual self-organized NiO/Fe nanostripes will be also presented.

![](https://assets.underline.io/uploads/markdown_image/1/image/ea6cc84785bb0485bc069d8c09366d06.jpg)

Fig.1. Memory of frozen AFM spins in CoO(111)/Fe bilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/46723238dbaaa7708138289f718efd3d.jpg)

Fig.2. Temperature dependence of XMLD (RL2 ratio) for NiO/Fe [4].

2022 Joint MMM-INTERMAG

Memory of frozen and rotatable antiferromagnetic spins in epitaxial CoO(111)/Fe and NiO(111)/Fe bilayers.

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INTERMAG 2021

We combined XMLD, XMCD and MOKE measurements in order to follow the magnetic properties of epitaxial CoO(111)/Fe(110) [1] and NiO(111)/Fe(110) [2] bilayers. We find that in both studied cases FM sublayer plays a dominant role and determines the magnetic state of the neighboring AFM, however completely different interaction mechanisms are involved. In CoO/Fe bilayers the AFM spins are frozen and their orientation is imprinted by magnetization of Fe layer when the system passes the Neel temperature of CoO. Once the Fe layer grafts the particular magnetic anisotropy (MA) into the CoO overlayer, it later remains frozen and insensitive to external factors like external magnetic field or Fe magnetization direction [3]. Specifically, choice of particular magnetic state of Fe sublayer, when passing Néel temperature of CoO, determines both the axis and direction of interfacial antiferromagnetic spins after the sample is cooled and allows for imprinting their +/- 90° and 0/180° alignment within the sample plane. For example, particular direction of frozen AFM spins determines the corresponding sign of the shift field of exchange biased magnetic hysteresis loop (Fig.1). For NiO/Fe bilayers, due to the weak intrinsic MA of NiO, the AFM spins are rotatable and always follow the reorientation of Fe magnetization that can be controlled by external magnetic field or via the temperature and thickness driven SRT of Fe(110). Recently we showed that in a uniform thickness NiO(111)/Fe(110) system, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO [2]. Field-free, reversible switching between these AFM states was demonstrated. Here we present results of systematic investigations of such field-free, temperature induced switching of AFM NiO spins. Fig. 2 shows temperature dependence of RL2 ratio for various thicknesses of Fe. Critical temperature of SRT in AFM NiO can be tuned in wide temperature window ~ (250 – 380 K) for field-free, reversible switching of AFM spins. XML(C)D-PEEM results obtained on individual self-organized NiO/Fe nanostripes will be also presented.

![](https://assets.underline.io/uploads/markdown_image/1/image/ea6cc84785bb0485bc069d8c09366d06.jpg)

Fig.1. Memory of frozen AFM spins in CoO(111)/Fe bilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/46723238dbaaa7708138289f718efd3d.jpg)

Fig.2. Temperature dependence of XMLD (RL2 ratio) for NiO/Fe [4].

technical paper

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The thermal Hall effect, sometimes also called Righi-Leduc effect, appears as transversal heat currents induced by thermal gradients. Magnons, the magnetic excitations in long-ranged ordered systems, transport spin and heat with no charge currents. In this talk I demonstrate that the transversal transport coefficient in a two-dimensional honeycomb antiferromagnet features clear signs of magnetic and topological phase transitions. An external magnetic field drives the system from the antiferromagnetic phase via the noncollinear spin-flop phase into the field-polarized phase. Additionally, topological phase transitions are found in the spin flop phase. Both types of phase transitions manifest themselves in prominent and distinguishing features in the thermal conductivities; depending on the temperature, the conductivity changes by several orders of magnitude, providing a means to identify experimentally these phase transitions.

![](https://assets.underline.io/uploads/markdown_image/1/image/13ae6acc84046eacddf6d5ad89e32051.jpg)

Thermal Hall Effect of Magnons in Collinear Antiferromagnetic Insulators: Signatures of Magnetic and Topological Phase Transitions

The intrinsic anomalous Hall effect (AHE) present in noncollinear Mn3NiN shows boosted values in spite of the vanishing magnetisation and modest spin-orbit coupling present in the material, while the related anomalous Nernst effect (ANE) has been predicted to be an order of magnitude greater than Mn3Sn [1,2]. In addition, recent reports suggest an unconventional component in the spin-to-charge conversion process in the related material Mn3GaN due to the magnetic ordering symmetry [3]. The shared origin of these effects lies in the Berry curvature of the band structure. In this talk I will review our current progress with this system, growth of epitaxial films by pulsed laser deposition on various perovskite substrates and comparison of properties to literature reports. Our recent measurements are taken on Mn3NiN films grown on STO and BTO substrates, and I will discuss their properties in light of the magnetic-order biaxial in-plane strain phase diagram that we have established previously [4]. I will discuss our recent experimental measurement of spectroscopic MOKE, AHE and ANE compared to the theoretical prediction calculated using spin density functional theory (SDFT) and linear response approximation, highlighting where we find strikingly good agreement. These materials are also predicted to host giant piezomagnetism and strong sensitivity of the Berry phase curvature to strain. These different properties do not necessarily add constructively and I will present our current views on this based on the variation of the AHE in films grown on piezoelectrically controlled BTO.

![](https://assets.underline.io/uploads/markdown_image/1/image/c92f36e3b266ca0502d83be0d150abc5.jpg)

Spectroscopic MOKE and other Berry phased boosted properties of ferrimagnetic and antiferromagnetic Mn3NiN

Spectroscopic MOKE and other Berry phased boosted properties of ferrimagnetic and antiferromagnetic Mn3NiN

Antiferromagnets (AFMs) are magnetically ordered materials without a macroscopic magnetization. There is accelerating interest in using them for memory applications, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects [1-2]. Here we show that micrometer-scale bits of the antiferromagnet PtMn, grown on a Ta or Pt heavy metal (HM) layer, can be switched reversibly between different magnetic states by electric currents, with readout through a second harmonic voltage technique [3]. The device exhibits multiple states, which can be set using different current amplitudes and may find application in neuromorphic computing. Micromagnetic simulations suggest that these metastable states consist of vortex and anti-vortex textures that move in response to current. We also demonstrate, using a modified device layout, direct voltage measurements to read out the electrically-induced switching of noncollinear PtMn3. The modified layout and measurement methodology, which was recently demonstrated using the IrMn3/Pt material system [4], is utilized here to distinguish current-induced magnetic and nonmagnetic switching signals in PtMn3/Pt devices. We also realize switching down to 10 µs pulse width, which shows the potential for fast device applications. The effect of temperature and external magnetic field (up to 15 T) are also studied. Our results pave the way towards practical PtMn and PtMn3 antiferromagnetic memories integrated on silicon, and indicate the importance of micromagnetic textures in the static and dynamic behavior of antiferromagnetic devices.

Electrical manipulation of antiferromagnetic order in PtMn and PtMn3 pillars

In antiferromagnetic topological insulator MnBi2Te4, the interplay between topological electrons and magnetization dynamics in the form of spin torques and charge pumping remains an open question. Using the Berry phase theory, we develop a universal formalism to quantify the spin-orbit torques (SOTs) and their reciprocal effects in layered MnBi2Te4, where the symmetry of SOTs exhibits an even-odd layer-number pattern. The dynamical consequences of our finding are demonstrated by the SOT-driven magnetic resonances. In particular, we identify an anomalous exchange (AE) mode in the tri-septuple layer (SL) case, where the magnetic moments from the top and bottom SL rotate with a pi phase difference while those in the middle SL stay static. Unlike the ordinary exchange mode and the acoustic mode, the AE mode does not react to a microwave electromagnetic field. Therefore, the AE resonance can only be excited by the peculiar SOTs we find, providing a unique way to verify the SOTs in MnBi2Te4. Our study lays the theoretical foundation of current-induced spin dynamics in intrinsic antiferromagnetic topological insulators.

![](https://assets.underline.io/uploads/markdown_image/1/image/4045560a7cd6d072b78a10061469d722.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/d50337c92d0bcff8e7d7da23d0825a66.jpg)

Anomalous Exchange Resonance Driven by Spin-Orbit Torques in Intrinsic Antiferromagnetic Topological Insulator

Spin pumping has been an efficient way of spin generation, which converts coherent spin precessions in a magnet into pure spin currents in an adjacent metal. Recently, spin pumping has been demonstrated in easy-axis antiferromagnets (AFs) that operate at the terahertz frequency range, opening the exciting possibility of signal generation and processing at ultrafast speed. However, easy-plane AFs which are experimentally easier to prepare than their easy-axis counterparts, were considered to be bad candidates for spin pumping because the N\'{e}el vector is linearly polarized due to the easy-plane anisotropy, placing a major restriction on the choice of materials for practical applications. Here, using NiO and α-Fe2O3 (hematite) as two archetypal easy-plane AFs, we theoretically challenge this seemingly established conclusion by showing that easy-plane AFs can pump DC spin currents with the help of either the Dzyaloshinskii-Moriya interaction or an applied magnetic field without inducing the spin-flop transition. The magnitude of spin pumping turns out to be as strong as that in easy-axis AFs, which significantly broadens the materials pool for antiferromagnetic spintronics. Furthermore, we find that in the presence of a magnetic field, NiO can function as a sub-terahertz spin-torque oscillator at a highly reduced threshold current density.

![](https://assets.underline.io/uploads/markdown_image/1/image/0487d1f128d8b2c5f538c85b9f9d5307.jpg)

DC output as a function of the applied magnetic field for different values of DM interaction in α-Fe2O3

![](https://assets.underline.io/uploads/markdown_image/1/image/c322020ac034c68f560001032918d42e.jpg)

DC output as a function of the applied magnetic field along the in-plane easy axis direction below the spin-flop transition for different driving frequencies in NiO

Coherent Spin Pumping in Easy-Plane Antiferromagnets

The spin-1/2 of an electron makes it an archetypal two-level system and inspires the description of other two-level systems using an analogous pseudospin. The quantized spin excitations of an ordered antiferromagnet can be viewed as pairs of spin-up and -down magnons and can be characterized by a magnonic pseudospin. The similarity between electronic spin and magnonic pseudospin has triggered the prediction of exciting phenomena like emergent spin-orbit coupling and topological states in antiferromagnetic magnonics. In the last year, first experimental observations of the associated dynamics of antiferromagnetic pseudospin have been reported [1,2]. Based on these findings, we will expand the concept of magnon pseudospin and the description of magnon pseudospin dynamics [3,4] and discuss the influence of dimensionality on the magnon Hanle effect. Additionally, we show our recent experiments demonstrating control of magnon spin transport and pseudospin dynamics in thin films with varying thickness of the antiferromagnetic insulator hematite (α-Fe2O3) utilizing two Pt strips for all-electrical magnon injection and detection [2] . We observe an oscillation in polarity of the magnon spin signal at the detector as a function of the applied magnetic field, which we quantitatively explain in terms of diffusive magnon transport. In particular, we observe a coherent precession of the magnon pseudospin caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. Moreover, we find peculiar changes in the magnon spin signal for thicker hematite layers. Our results are paramount in unnlocking the high potential of antiferromagnetic magnonics towards the realization of electronics-inspired phenomena. Financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy - EXC-2111 - 390814868 and project AL2110/2-1, and the Research Council of Norway through its Centers of Excellence funding scheme, project 262633, "QuSpin" is gratefully acknowledged.

Antiferromagnetic Magnon Pseudospin Dynamics and the Magnon Hanle Effect in Hematite Thin Films

Antiferromagnetic materials (AFMs) provide a pathway to overcome the limitations of ferromagnet-based spintronics, enabling fast and high density memory devices, as well as terahertz detectors and sources. This has fueled intensive research on the electrical manipulation of metallic AFMs using interfacial spin-orbit torque (SOT) from a heavy metal (HM) layer [1-4]. However, the threshold current density in HM/AFM bilayers is typically large, thus creating non-magnetic contributions to resistive switching signals, complicating their interpretation. Here, we present a new device structure and measurement protocol based on differential voltage measurements, allowing in situ reference measurements and to systematically separate the switching signals originating in the AFM and HM layers of a HM/AFM heterostructure [5]. The proposed device consists of a double-cross HM structure with six terminals, where the AFM, patterned in a micropillar shape, is placed on one of the two crosses. We successfully use this method in devices formed by Pt and the non-collinear AFM IrMn3. The results provide clear evidence of electrical switching of the AFM order parameter in the IrMn3 pillar by SOT in the range of writing current densities from 36 to 50 MA/cm2 with no appreciable switching signal from the Pt layer. We discuss the role of different switching configurations (180° and 90° writing currents), as well as the role of thermal activation on the switching characteristics. Control experiments in Pt-only double-cross samples without an AFM pillar reveal HM-dominated switching at higher current densities ~60 MA/cm2. We discuss active readout of the switching signals using a differential amplifier, with large peak-to-peak output voltage ~100 mV. The structure allows for reliable electrical measurement of resistive signatures of AFM switching induced by SOT, which will be important not only for the fundamental understanding of current-induced AFM domain dynamics, but also as an electrical readout method for practical applications in memory and computing devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/9f7b7e5713bbb79e6a1b7f3436e814c5.jpg)

Fig. 1. Design of double-cross device for differential voltage measurements and the measured differential voltage for the 180° switching configuration with 20 mA write pulses.

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

Antiferromagnetic materials have recently attracted much interest due to a number of advantages including high resonance frequencies up to the terahertz. Further, the possibility to electrically modulate the magnetic properties such as the anisotropy makes them an excellent candidate for energy-efficient devices at ultra-high frequencies. In this study, we theoretically explored one such application in the dynamic manipulation of electromagnetic responses with a broad tunable spectral range. The materials considered were dielectric antiferromagnets with strong magneto-electric or –elastic coupling which can achieve robust control in the perpendicular magnetic anisotropy. The metallic counterparts were not suitable due to the anticipated large loss. A theoretical analysis based on the Landau-Lifshitz equation was used to derive the frequency-dependent permeability and the resonant frequency of the antiferromagnet for different biaxial anisotropy profiles and incident electromagnetic wave polarizations. Numerical calculations based on typical parameters available in the literature (such as NiO) strongly indicated that the resonance frequency modulation as much as 10’s of percent may be possible in the sub-mm wavelength range, along with the corresponding changes in the permeability. Application to specific devices such as a bandpass filter further illustrated the validity of the concept. Funding Acknowledgement This work was supported, in part, by the U.S. Army Research Office (No. W911NF-16-1-0472).

![](https://assets.underline.io/uploads/markdown_image/1/image/6ffd93f402fd8b4c21123e86d0bbb698.jpg)

Fig.1 Schematic of the magnetic-piezoelectric composite. Electric field along out-of-plane direction causes magneto-elastic effects and influences eigen-frequency by modifying in-plane magnetic anisotropy. EM wave is incident along x-axis.


![](https://assets.underline.io/uploads/markdown_image/1/image/e677976b7cbcf549791f319205ea030e.jpg)

Fig.2 Data showing a high electric field tunable AFMR frequency shift with external voltage at 0, -V, and +V=8kV/cm (Not the final results)

Voltage-Controlled Dynamic Modulation of Electromagnetic Response in Antiferromagnetic Materials at Terahertz Frequencies

Recently antiferromagnetic materials have been gathering increasing attention from the spintronics community, as detecting and manipulating antiferromagnetic (AFM) order electrically is an important milestone for realizing antiferromagnetic devices. Characterizing the type and amplitude of the spin-orbit torques (SOT) is crucial for understanding and predicting AFM dynamics. One important technique to identify and measure SOTs is to perform harmonic Hall measurements in an AFM/heavy metal (HM) layer. In this study, we report the full angular dependence of 1st and 2nd harmonic voltage measurements in epitaxial c-axis oriented α-Fe2O3/Pt bilayer heterostructures (Fig. 1a). By modeling the harmonic resistance signals together with known material parameters such as the exchange field, anisotropy and DMI constant (Fig. 1b), we extract the amplitudes of field-like and anti-damping-like spin-orbit torques from field-dependent measurements in XY, XZ and YZ plane scans (Fig. 1a). Our model suggests a small angle tilt of the easy-plane and indicates that the amplitude of the field-like torques are significantly larger than anti-damping-like torques. Additionally, we show that there is an anomalous Hall-effect contribution in out-of-plane scans and that the Oersted fields are negligible compared to field-like torques. Our work provides a straight-forward way of characterization of spin-orbit torques in AFM/HM heterostructures.

![](https://assets.underline.io/uploads/markdown_image/1/image/87a94b08c94b21f9137b16231524bc27.jpg)

a) Transverse measurement configuration showing the Pt Hall cross with 5 µm channel width on AFM α-Fe2O3 layer. An AC current and the transverse voltage is measured with a lock-in amplifier. The thicknesses of α-Fe2O layer and Pt layers are 30 nm and 5 nm respectively. Next to it, geometry of magnetic field scans in the 3 principal planes (XY, XZ and YZ) is shown. b) Model geometry: D represents both the DMI vector and the hard axis direction. n is the Néel vector which lies in the easy plane. B is the external magnetic field. Alternating current is in x-direction and the spin accumulation in the y-direction. The Néel vector is perpendicular to both D and B.

Quantifying Spin-Orbit Torques in AFM/HM Heterostructures

Antiferromagnetic materials with non-collinear spin structures (NC-AFMs) offer significant potential for spintronic applications[1]. They have vanishingly small magnetization and ultrafast spin dynamics. In addition, they allow for the presence of important counterparts to the effects considered to be exclusive to materials with net magnetization, such as the anomalous Hall effect (AHE)[2], magneto-optical Kerr effect[3] or anomalous Nernst effect (ANE)[4]. One of the promising candidates of the NC-AFM class are Mn antiperovskites, with a typical representative being Mn3NiN. In this contribution we present ANE measurements on epitaxial thin films of Mn3NiN prepared on SrTiO3 substrate with spin structure lying in the (111) plane. Previous theoretical predictions suggest that despite the modest spin-orbit coupling, the magnitude of the ANE in Mn3NiN ought to exceed that of Mn3Sn NC-AFM due to the Berry phase boost[5]. We implement two complementary methods to detect the ANE response of the Mn3NiN films. First, the global Nernst response is measured using a micro-patterned Hall bar device with platinum resistive heaters generating an in-plane thermal gradient. The measured anomalous Nernst data (Fig.1(a)) are in good agreement with the AHE measurements[5], indicating a similar physical origin of the two effects. Second, to add a spatial resolution, scanning thermal gradient microscopy[4] is used. A laser beam incident on a micro-patterned device induces thermal gradients. The method reveals the sample’s domain structure. We find a signature of an ANE component that we can control with the external field due to the unsaturated moment present lying in the (111) plane, different from the previously measured Mn3Sn. This small moment provides a means for manipulating magnetic order, and even writing magnetic domain-like structures (Fig.1(b)). These two methods provide fruitful insight into the magnetic behaviour of NC-AFMs.

![](https://assets.underline.io/uploads/markdown_image/1/image/e3391f086f9d4953ae3f5db220e7247b.jpg)

Fig.1. Anomalous Nernst effect in Mn3NiN. (a) Global ANE as a function of the out-of-plane magnetic field at different base temperatures. (b) Scanning thermal gradient microscopy at T = 100 K. The domain-like structure was written by high-intensity laser beam and 500 mT in-plane magnetic field.

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Thermal Hall Effect of Magnons in Collinear Antiferromagnetic Insulators: Signatures of Magnetic and Topological Phase Transitions

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Tailoring the magnetic anisotropy and controlling the spin orientation in antiferromagnetic NiO(111) films on Fe(110).

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Thermal Hall Effect of Magnons in Collinear Antiferromagnetic Insulators: Signatures of Magnetic and Topological Phase Transitions

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Tailoring the magnetic anisotropy and controlling the spin orientation in antiferromagnetic NiO(111) films on Fe(110).

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