United States

Magnetic materials in which it is possible to control the topology of their magnetic order in real space or the topology of their magnetic excitations in reciprocal space are highly sought-after as platforms for alternative data storage and computing architectures. Here we show that multiferroic insulators, owing to their magneto-electric coupling, offer a natural and advantageous way to address these two different topologies using laser fields. We demonstrate that via a delicate balance between the energy injection from a high-frequency laser and dissipation, single skyrmions—archetypical topological magnetic textures—can be set into motion with a velocity and propagation direction that can be tuned by the laser field amplitude and polarization, respectively. Moreover, we uncover an ultrafast Floquet magnonic topological phase transition in a laser-driven skyrmion crystal and we propose a new diagnostic tool to reveal it using the magnonic thermal Hall conductivity.

![](https://assets.underline.io/uploads/markdown_image/1/image/7db0278173bbc5cba45a88101bb6611a.jpg)

Real- and reciprocal-space topology can be controlled by lasers. (a) The topological spin structure of a skyrmion carrying in-plane electric polarization Ρ undergoes translational motion under a circularly polarized laser. (b) An ultrafast topological phase transition occurs in the Floquet magnon band structure of skyrmion crystals due to the effective magnetic field induced by circularly polarized laser irradiation.



2022 Joint MMM-INTERMAG

Laser-Controlled Real and Reciprocal Space Topology in Multiferroic Insulators

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INTERMAG 2021

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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The search for magnetic topological semimetals has been one of the most intriguing issues in the field of spintronics because they exhibit anomalous transport properties that can be utilized for novel spintronic devices. Quite recently, it was theoretically predicted that an inverse spinel compound VMg2O4 is a good magnetic Weyl semimetal [1,2]. The Weyl points are found just above the Fermi energy in the eg-band formed by the 3d-orbitals of the tetravalent V on the diamond lattice. Stimulated by these previous works, here we study the electronic states and transport properties of another spinel compound MnMg2O4, in which V is totally substituted for Mn with a smaller ionic radius. The significant difference between V and Mn is that, if they maintain the nominal tetravalent configuration, V has one electron in the eg-orbital while Mn also has one hole in the eg-orbital in the low-spin state or one electron in the t2g orbital in the high-spin state. Here we focus on how the topological properties found in VMg2O4 can be affected by the orbital symmetry by first-principles calculations using WIEN2k code. The band structure and the density of states of MnMg2O4 are shown in Figure 1 (a) and (b), respectively. We find that the Fermi energy lies at the t2g band and high-spin state is realized in the 3d electrons of Mn. The resulting band structure is half-metallic with the minority band gap is found to be 3eV. We note that, unlike the eg-band lies around 1eV below the Fermi level, there is no obvious linear crossing in the t2g band. We also compute the intrinsic anomalous Hall conductivity (AHC) within the linear-response theory in the clean limit, and the energy dependence of the AHC is shown in Fig. 2. As expected from the band structure in Fig. 1(a), the AHC is very small at the Fermi energy, shown with black line. We also examine the effects of the compressive strain on the electronic structures and AHC of the systems. Lifting the degeneracy of the t2g band, the Weyl points appears slightly above the Fermi energy, resulting in a large AHC ( -144 S/cm ).

![](https://assets.underline.io/uploads/markdown_image/1/image/b9452ae2fe6ff8055dc1e70c5a891907.jpg)

Fig.1
(a) The band structure and (b) the density of states of MnMg2O4.

![](https://assets.underline.io/uploads/markdown_image/1/image/d084731d45c35b3bbf4ae0675c47bd19.jpg)

Fig.2
The energy dependences of the anomalous Hall conductivity with several compressive strains.

Ab initio study on the possible magnetic topological semimetallic state in MnMg2O4.

The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. While much of the attention in this context has been centered on the heterogeneous stacking of exfoliated flakes, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is developing as an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy (MBE) to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe2) and a topological semimetal (ZrTe2).1 We characterize the crystal structure and stoichiometry of our MBE-grown thin films via scanning transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. We also carry out in-vacuo angle-resolved photoemission spectroscopy to determine the band structure of the 1T-CrTe2 films and find good agreement with first-principles band structure calculations. Temperature-dependent anomalous Hall measurements reveal that robust ferromagnetic order persists in the true 2D limit of one unit-cell (u.c.) thick 1T-CrTe2 grown epitaxially on ZrTe2. In thicker samples (12 u.c. thick CrTe2), the anomalous Hall effect exhibits a magnetic field dependence that may arise from real-space Berry curvature. Finally, in ultrathin CrTe2 (3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device. This work was supported by NSF Grant Nos. DMR-1539916, DMR-2011401, ECCS-2025124, DOE EFRC grant DE-SC0019331, and SMART, one of seven centers of nCORE, a Semiconductor Research Corporation program, sponsored by NIST.

ZrTe2/CrTe2: an epitaxial van der Waals platform for spintronics

The transition metal dichalcogenides are a class of layered materials with a van der Waals terminated surface that provides novel bonding characteristics for the preparation of nanometer scale metallic films. Furthermore, in single layer form, systems such as MoS2 act as a semiconducting analog to graphene, making it of interest to understand the formation of metal contacts on MoS2 for developing optoelectronic and spintronic device structures. We have investigated the electronic, magnetic, and structural properties of the Ni/MoS2 and Ni/WSe2 interfaces for nanometer scale films using a combination of scanning probe microscopy and density functional calculations. Our experiments show that Ni grows in a very different manner than other FCC metals like Au, Ag, and Pd. Despite a very large lattice mismatch, these three metals form quantized nanostructures with atomically flat surfaces. Nanometer scale Ni films show no signs of such quantized features, instead forming grains as might be expected for metallic growth on an inert system with poor lattice matching. The difference in growth patterns arises from a significantly larger bonding energy at the Ni/dichalcogenide interface. While the enhanced bonding also increases the impact of strain on film growth, it also leads to stronger hybridization between the Ni and dichalcogenide molecules. According to our calculations, this induces spin-polarized metallic behavior in the uppermost molecular layer of the otherwise semiconducting dichalcognide substrate. This confinement to the uppermost molecular layer creates an atomically abrupt magnetic interface within the dichalcogenide substrate. At the same time, the electronic and magnetic properties of Ni are nearly unaffected by the presence of WSe2 and MoS2, except a small reduction of magnetic moment at the interfacial Ni atoms. This research is supported by Grant No. DE-SC0020334 funded by the U.S. Department of Energy, Office of Science.

Electronic, magnetic, and structural properties of Ni/MoS2 and Ni/WSe2 interfaces

Magnetic two-dimensional (2D) van der Waals (vdW) materials show promising magnetic properties, which can be utilized to build spintronic devices with improved performance and new functionalities. However, most vdW magnets have Curie temperatures below 300 K. With molecular beam epitaxy (MBE) [1], we recently synthesized 1T-phase single-crystal CrTe2 thin films [2], which show ferromagnetism and strong perpendicular magnetic anisotropy at room temperature. We have also studied the magnetic domain evolutions and the anomalous Hall effect (AHE) in these samples at different temperatures and magnetic fields. As shown in Fig. 1, MFM has revealed clear magnetic stripe domains with out-of-plane polarization in the 16 ML CrTe2 film at room temperature. The magnetic stripe domains still exist upon applying an out-of-plane magnetic field, but the width becomes narrower at +1.2 kOe and wider at -1.2 kOe, respectively. We have patterned the samples into Hall bars and measured electron transport properties at different temperatures. Both the 8 ML and 16 ML CrTe2 films show AHE up to room temperature. However, the AHE changes sign between 100 K and 150 K. This indicates intriguing magnetic domain evolution at lower temperatures. Further work has been carried out toward a deeper understanding of the magnetization in CrTe2. Our study has highlighted CrTe2 as a unique platform for studying vdW magnetism and opened a new avenue for developing large-scale 2D magnet-based spintronics devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/56f0f120d0aa30f2da3b95b65bbc1a07.jpg)

Fig.1: MFM images showing stripe domains for the 16 ML CrTe2 at H = 0 (a), +1.2 kOe (b) and -1.2 kOe (c), respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/8ef809c914f5e6a06754ccaa6457d940.jpg)

Fig.2: a. Anomalous Hall resistance vs. H at different temperatures for 8 ML CrTe2. b, c. Dependence of anomalous hall resistance (RAHE) (b) and coercivity (c) vs. temperature for 8 ML and 16 ML CrTe2.

Magnetization evolution in room-temperature van der Waals magnet CrTe2

Anomalous Nernst effect (ANE) has drawn special attention during the last few years because of its possibility in thermal energy harvesting and fundamental physics involved [1]. Reports on ANE in perovskite oxides are relatively few compared to ferromagnetic metallic alloys. The ANE was previously reported in Sr substituted LaCoO3 [2]. Because of the smaller size of Ca2+ cation compared to Sr2+, magnetic interactions are expected to be modified. Hence, we studied the ANE in La0.5Ca0.5CoO3 for the first time, which shows ferromagnetic ordering at 147 K (Fig. 1(a)). The non-saturating nature of the field dependence of magnetization and its smaller value shown in the inset of Fig. 1(a) indicates the existence of competitive FM-AFM interactions that largely depends on the spin states of the Co3+ and Co4+ ions[2-4]. Such a system can be very interesting to study ANE as the aforementioned competitive effect may dramatically affect the spin-dependent deflection of thermally driven charge carriers. The presence of the ANE in La0.5Ca0.5CoO3 is confirmed from the field dependences of the Nernst thermopower (Sxy) as shown in Fig. 1(b), which shows hysteresis like the magnetization. In this preliminary work, we have carried out ANE measurements only at four selected temperatures. Sxy nearly saturates above 20 kOe at 40 K but not at 100 K, where it has a maximum value at the highest field. The Sxy shows behavior similar to superparamagnetic at 160 K and linear behavior at 200 K with much-reduced values. Magnetic, electrical transport, thermoelectric measurements, and ANE at multiple temperatures will be carried out to realize the mutual effects of magnetism, charge, and heat currents to establish a better relationship between them. This work was supported by the Ministry of Education, Singapore (Grant no. R144-000-422-114 and R144-000-442-114).

![](https://assets.underline.io/uploads/markdown_image/1/image/70f4701c2741c601a6cc175160e1bb04.jpg)

Fig. 1: (a) Temperature dependence of magnetization (M) in La0.5Ca0.5CoO3 . Inset: Field dependence of (b) M at 5 K. and (b) Nernst thermopower (Sxy) measured at T = 40, 100, 160 and 200 K.

Anomalous Nernst effect in perovskite La0.5Ca0.5CoO3

Controlling ion migration of transition metal oxide(TMO) to recast crystal structure and macroscopic properties is current research emphasis [1, 2], which has potential applications in solid fuel cells, resistance switching etc. Among cobalt-based TMO, B-site atom determines functionality, but oxygen atom is hard to ignored. Electronic structure of B-site atom is profoundly affected by oxygen content due to coupling of lattice, charge, orbit, and spin. Vacuum annealing is a stable method to extract O2--ions [3, 4]. 40nm La0.5Sr0.5CoO3(P-LSCO) film are grown on LSAT(100) substrate. Then films are respectively annealed at 400 centigrade for 2 hours(B-LSCO) and 500 centigrade for 35 hours(S-LSCO) to extract O2--ions. Fig. 1a shows soft X-ray absorption spectrum(XAS) for cobalt L-edge, and an obvious peak shift to low energy (L3-peak of S, B and P-LSCO appear at 782.2eV, 781.5eV and 780.4eV respectively) indicates continuous decrease in average valence states of Co and oxygen content during O2--ions extracting process. For XAS of oxygen K-edge (Fig. 1b), the A and C peaks, representing hybridization of Co-2p/3d and O-2p orbital, gradually decrease in three samples. While B-peak, which represents content of oxygen vacancy, exhibits an increasing trend, consistent with above results. As a result, dramatic magnetism transformation was observed in film upon annealing. Fig. 1c and 1d show magnetic moment-temperature(M-T) curves and magnetic moment- magnetic field (M-H) curves for three samples, demonstrating ferromagnetism(FM) with Tc~220K, antiferromagnetism(AFM) and ferromagnetism with Tc&gt;300K in P-LSCO, B-LSCO and S-LSCO, respectively. In sum, we achieved giant magnetic transformation from FM to AFM to FM accompanying with O2--ion migration in LSCO film by vacuum annealing, which provides idea for regulating physics properties by ion migration engineering.

![](https://assets.underline.io/uploads/markdown_image/1/image/2f9d13eeec602e127c33a77c98f889f5.jpg)

Fig. 1 XAS for a) cobalt L-edge, b) oxygen K-edge, c) M-T curve, d) M-H curve

Oxygen ion migration regulates magnetism in La0.5Sr0.5CoO3 film

Nowadays, chromium-based normal spinel oxides ACr2O4 are one of the most studied materials in the condensed matter community due to the interplay between its magnetic, electric and structural properties as well as to its potential application to different key industry sectors. In these compounds, several physical effects have been observed, which include magnetostriction, colossal magnetoresistance, multiferroic, spin frustration and more [1-3]. In particular, for MnCr2O4, the ground state magnetic structure is still controversial because the magnetic structures reported by different groups and investigated by independent techniques are inconsistent [1-6]. The magnetic structure of this compound was reinvestigated by magnetization, specific heat and neutron diffraction experiments at different temperatures. The results suggested that a new magnetic phase, not previously reported, is developed under 18 K when the sample is synthesized under a reductive atmosphere. The magnetic phases present in this sample were: long-range ferrimagnetic order below TC = 45 K; incommensurate conical spin order with propagation vector kS1 = (0.62(1), 0.62(1), 0) below TS1 = 20 K; and incommensurate conical spin order with propagation vector kS2 = (0.660(3), 0.600(1), 0.200(1)) below TS2 = 18 K. Using the superspace group formalism [7, 8], the symmetry of the nuclear and magnetic structures is described. The presence of transverse conical magnetic structures in the lower-temperature phases implies the existence of multiferroicity. Using simple theoretical calculations, we derive the directions along which the electric polarization lies for each magnetic phase.

![](https://assets.underline.io/uploads/markdown_image/1/image/ab2df24cc036f478f8e3383173532d06.jpg)

Incommensurate Magnetic Phases of the Multiferroic Compound MnCr2O4 Described with the Superspace Formalism

Incommensurate Magnetic Phases of the Multiferroic Compound MnCr2O4 Described with the Superspace Formalism

Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO3, a perovskite-structured canted antiferromagnet [1]. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density functional theory calculations reveals that the presence of YFe antisite defects facilitates a non-centrosymmetric distortion promoting ferroelectricity. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. Furthermore, a vertically aligned nanocomposite consisting of pillars of a magnetoelastic oxide CoFe2O4 embedded epitaxially in the YFeO3matrix exhibits both robust ferroelectricity and ferrimagnetism at room temperature, as well as a noticeable strain-mediated magnetoelectric coupling effect [2]. Our work uncovers the distinctive role of antisite defects in providing a novel mechanism for ferroelectricity in a range of magnetic orthoferrites and further augments the functionality of this family of complex oxides for multiferroic applications.

Defect mechanism for ferroelectricity in orthoferrites

One of the most promising ways of improvement of thermoelectric energy conversion leads via band gap engineering towards the enlargement of the density of states at the Fermi level, e.g. via a co-doping to induce a resonant level in the electric transport relevant band [1]. Cr in PbTe is a particularly interesting example since Cr atoms form mixed valence Cr2+/3+ donor centers resonant with the PbTe conduction band [2]. We report on detailed temperature and magnetic field dependence of magnetization of PbTe doped with transition metal Cr2+/3+. The material is studied by a SQUID magnetometer in order to quantitatively determine the contribution of single substitutional Cr2+/3+ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about 1019 cm-3 paramagnetic Cr ions, of which 2/3 are the Cr3+ ions formed via self-ionization of Cr2+ resonant donors. The magnetic finding agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr2+/3+ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr2Te3 and Cr5Te8 are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibit (blocked) superparamagnetic properties. Also HR-TEM confirmed the presence of numerous single nanometer size precipitates of Cr-Te compounds. The magnetic susceptibility of both n- and p-type PbTe in the temperature range 100 &lt; T &lt; 400 K has been established. These magnitudes are essential to properly account for the high temperature magnetic susceptibility of PbTe:Cr [3]. This study has been supported by the National Science Centre (Poland) through project OPUS (UMO - 2017/27/B/ST3/02470) and by the National Science Centre for Development (Poland) through grant TERMOD No TECHMATSTRATEG2/408569/5/NCBR/2019 and by the Foundation of Polish Science through the IRA Programme co-financed by EU within SG OP.

Magnetic Constitution of PbTe:Cr - Experimental Aspects

We find the magnetic susceptibility of NH4Al(SO4)2-12H2O below its ferroelectric Curie temperature of TC = 58 K can be tuned to become positive or negative simply by controlling the cooling speed. We explain this by taking into consideration that reorienting NH4+ ions possess weak magnetic moments that are linearly coupled to dipole moments which in turn are aligned by the crystal field with cooling but at the same time experiencing competition from geometric frustration. With fast cooling such as a rate of 1.0 K/min, the magnetic moments are not able to become ordered in time so the system remains mostly diamagnetic. In contrast, with a slow cooling rate of 0.1 K/min, the magnetic moments are allowed to access a more ordered state so the magnetic susceptibility becomes positive. From such, four -ferroic states can be accessed below TC by applying either a positive or negative bias field and cooling the sample slow or fast to obtain polarization and magnetization of different polarities. Specific heat and dielectric constant measurements near TC indicate the origin of multiferroic behavior as originating from proton orbital degeneracies. Our findings identify an entire new class of multiferroic materials in the ammonium alums and possibly other hydrogen-bonding compounds.

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