United States

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, YFeO&lt;sub&gt;3&lt;/sub&gt;, 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 Y&lt;sub&gt;Fe&lt;/sub&gt; 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 CoFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; embedded epitaxially in the YFeO&lt;sub&gt;3&lt;/sub&gt;matrix 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.

2022 Joint MMM-INTERMAG

Defect mechanism for ferroelectricity in orthoferrites

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.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

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This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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.

technical paper

Exceptional tunability in the electronic and magnetic properties makes Heusler compounds very promising from both the fundamental and applied perspectives. Half-metallic ferromagnetism, half-metallic fully-compensated ferrimagnetism, spin-gapless semiconducting behavior, and fully-compensated ferrimagnetic spin-gapless semiconducting behavior have been predicted or demonstrated experimentally in Heusler compounds which makes them suitable for applications in a number of novel spintronic devices1,2,3,4,5. Half-metallic Co-based Heusler alloys are among the most attractive systems due to their high Curie temperatures, high spin polarization and the structural similarity to binary semiconductors6,7,8. Spin-gapless semiconductors can be regarded as a combination of gapless semiconductors and half-metallic ferromagnets where the conducting electrons or holes are not only 100% spin polarized but also easily excited. The realization of spin-gapless semiconducting behavior in Heusler alloys is expected to fulfill the needs for semiconductor spintronics. In this work, we have introduced the substitution of low valence transition metal atoms Y = Ti, V, Cr, Mn, and Fe to Co atoms in the parent Co2FeGa system, and examined the properties of quaternary Heusler compounds Co2-xYxFeGa (x = 0.50), both experimentally and theoretically to get a global overview of the electronic, magnetic, electron transport and mechanical properties. All single-phase alloys exhibit Heusler like L21 ordering, as corroborated by X-ray diffraction. The low-temperature saturation magnetic moments agree fairly well with the values expected from a Slater-Pauling rule for half metals (Fig.1). Electrical transport measurements are performed to explain the electronic structure of the alloys. The temperature dependence of electrical resistivity for Mn substituted alloy shows semiconducting nature in the complete temperature regime (5-400)K (Fig.2). Ab initio calculations are also performed to understand the experimental findings.

![](https://assets.underline.io/uploads/markdown_image/1/image/14f7b59db99b038cf74a4718f8f85273.jpg)

The variation of saturation magnetic moment with valence electron counts per formula units with element Y.

![](https://assets.underline.io/uploads/markdown_image/1/image/4d76d8c1a433798c5183033756315811.jpg)

Temperature dependence of electrical resistivity in Co1.50Mn0.50FeGa at zero magnetic field showing semiconducting behavior.

Possible half-metallic and spin-gapless semiconducting behavior in quaternary Heusler compounds Co2-xYxFeGa (Y = Ti, V, Cr, Mn, Fe, and Co, x = 0.50)

Quantized electronic transport in magnetic topological insulators is intertwined with spontaneous magnetic ordering, as magnetization controls band topology through the exchange interaction. We show that considering the exchange gaps at the mean-field level is inadequate to predict phase transitions between electronic states of distinct topology. Thermal spin fluctuations that disturb the magnetization can act as frozen disorders, which strongly scatter electrons and thus reduce the onset temperature of quantized transport appreciably even in the absence of structural impurities. This effect, which has hitherto been overlooked, provides an alternative explanation of recent experiments on magnetic topological insulators.

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Spin Fluctuations in Quantized Transport of Magnetic Topological Insulators

Inorganic chiral crystals have attracted much recent interest for their potential topological quantum properties1. In particular, β-Ag2Te was shown to be a bulk insulator hosting a topological surface state (TSS) of highly anisotropic Dirac dispersion2. On the other hand, in chiral materials, an electric current along the helical axis produces a collinear spin polarization; this is known as chirality-induced spin selectivity (CISS)3. This raises the intriguing possibility that Ag2Te may be a material that simultaneously exhibits TSS and CISS. Here we report on electrical transport measurements on oriented films of nonmagnetic Ag2+δTe. The films were grown by electron-beam evaporation on quartz, which were shown to be highly textured by XRD. With decreasing temperature, the film resistivity shows semiconducting behavior at high temperatures, and plateaus below ~50 K, consistent with the predominance of a TSS (Fig. 1). The in-plane field magnetoresistance (MR) of the samples exhibits qualitatively different behavior for the longitudinal and transverse configurations. The transverse MR is positive and of a similar magnitude to polycrystalline Ag2Te samples4. In contrast, the longitudinal MR is negative in the semiconducting temperature range, and remains substantial at 300 K. At lower temperatures, the longitudinal MR shows a non-monotonic behavior, with sign reversals for the slope and even magnitude with increasing field (Fig. 2). The unusual MR in the oriented films of Ag2+δTe implies two competing mechanisms behind the magnetotransport properties of Ag2+δTe, possibly reflecting the coexistence of a TSS on the surface and CISS in the bulk. *Work supported by NSF grant DMR-1905843

![](https://assets.underline.io/uploads/markdown_image/1/image/3dbdf24c4831393f5bc0415e2f16b80a.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/b148479fb729dd5560b9b113714769b9.jpg)

Sign Reversal and Large Anisotropy in the Magnetoresistance of Oriented Thin Films of Ag2+δTe

Sign Reversal and Large Anisotropy in the Magnetoresistance of Oriented Thin Films of Ag2+δTe

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

Defect mechanism for ferroelectricity in orthoferrites

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