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1. Imaging Local Effects of Voltage and Boron Doping on Spin Reversal in Antiferromagnetic Magnetoelectric Cr ₂ O ₃ Thin Films and Devices

   https://doi.org/10.1002/adfm.202408542  

   Erickson, Adam; Shah, Syed_Qamar_Abbas; Mahmood, Ather; Buragohain, Pratyush; Fescenko, Ilja; Gruverman, Alexei; Binek, Christian; Laraoui, Abdelghani (August 2024, Advanced Functional Materials)

   Abstract Chromia (Cr₂O₃) is a magnetoelectric oxide that permits voltage‐control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Néel Temperature (T_N≈307 K) and the need of a symmetry‐breaking applied magnetic field to achieve reversal of the Néel vector. Recently, boron (B) doping of Cr₂O₃films led to an increaseT_N>400 K and allowed the realization of voltage magnetic‐field free controlled Néel vector rotation. Here, the impact of B doping is directly imaged on the formation of AFM domains in Cr₂O₃thin films and elucidates the mechanism of voltage‐controlled manipulation of the spin structure using nitrogen‐vacancy (NV) scanning probe magnetometry. A stark reduction and thickness dependence of domain size in B‐doped Cr₂O₃(B:Cr₂O₃) films is found, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray‐field maps, a qualitative distinction between the undoped and B‐doped Cr₂O₃films is found, manifested by the histogram distribution of the AFM ordering, that is, 180°domains for pure films, and 90°domains for B:Cr₂O₃films. Additionally, NV imaging of voltage‐controlled B‐doped Cr₂O₃devices corroborates the 90°rotation of the AFM domains observed in magnetotransport measurement. 

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2. The spin polarization of palladium on magneto-electric Cr ₂ O ₃

   https://doi.org/10.1088/1361-648X/acc710  

   Komesu, Takashi; Echtenkamp, Will; Binek, Christian; Dowben, Peter A. (April 2023, Journal of Physics: Condensed Matter)

   Abstract While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr₂O₃(0001) is possible because of the boundary polarization at the Cr₂O₃(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr₂O₃(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr₂O₃(0001) at 310 K, i.e. above the Néel temperature of bulk Cr₂O₃. While Cr₂O₃(0001) has surface moments that tend to align along the surface normal, for Pd on Cr₂O₃, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr₂O₃(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr₂O₃is provided by measurement of the exchange bias fields in Cr₂O₃/Pd(buffer)/[Co/Pd]_nexchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm. 

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3. Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

   https://doi.org/10.1038/s41598-020-71619-1  

   Vu, N. M.; Luo, X.; Novakov, S.; Jin, W.; Nordlander, J.; Meisenheimer, P. B.; Trassin, M.; Zhao, L.; Heron, J. T. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract The manipulation of antiferromagnetic order in magnetoelectric Cr 2 O 3 using electric field has been of great interest due to its potential in low-power electronics. The substantial leakage and low dielectric breakdown observed in twinned Cr 2 O 3 thin films, however, hinders its development in energy efficient spintronics. To compensate, large film thicknesses (250 nm or greater) have been employed at the expense of device scalability. Recently, epitaxial V 2 O 3 thin film electrodes have been used to eliminate twin boundaries and significantly reduce the leakage of 300 nm thick single crystal films. Here we report the electrical endurance and magnetic properties of thin (less than 100 nm) single crystal Cr 2 O 3 films on epitaxial V 2 O 3 buffered Al 2 O 3 (0001) single crystal substrates. The growth of Cr 2 O 3 on isostructural V 2 O 3 thin film electrodes helps eliminate the existence of twin domains in Cr 2 O 3 films, therefore significantly reducing leakage current and increasing dielectric breakdown. 60 nm thick Cr 2 O 3 films show bulk-like resistivity (~ 10 12 Ω cm) with a breakdown voltage in the range of 150–300 MV/m. Exchange bias measurements of 30 nm thick Cr 2 O 3 display a blocking temperature of ~ 285 K while room temperature optical second harmonic generation measurements possess the symmetry consistent with bulk magnetic order. 

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4. Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure

   https://doi.org/10.1002/adma.202001460  

   Pan, Lei; Grutter, Alexander; Zhang, Peng; Che, Xiaoyu; Nozaki, Tomohiro; Stern, Alex; Street, Mike; Zhang, Bing; Casas, Brian; He, Qing_Lin; et al (July 2020, Advanced Materials)

   Abstract Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall effect in a magnetically‐doped topological insulator grown on the antiferromagnetic insulator Cr₂O₃. The exchange coupling between the two materials is investigated using field‐cooling‐dependent magnetometry and polarized neutron reflectometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr₂O₃and the magnetic topological insulator, manifested as an exchange bias when the sample is field‐cooled under an out‐of‐plane magnetic field, and an exchange spring‐like magnetic depth profile when the system is magnetized within the film plane. These results identify antiferromagnetic insulators as suitable candidates for the manipulation of magnetic and topological order in topological insulator films. 

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5. Voltage controlled Néel vector rotation in zero magnetic field

   https://doi.org/10.1038/s41467-021-21872-3  

   Mahmood, Ather; Echtenkamp, Will; Street, Mike; Wang, Jun-Lei; Cao, Shi; Komesu, Takashi; Dowben, Peter A.; Buragohain, Pratyush; Lu, Haidong; Gruverman, Alexei; et al (December 2021, Nature Communications)

   null (Ed.)

   Abstract Multi-functional thin films of boron (B) doped Cr 2 O 3 exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H . Toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. The boundary magnetization associated with the Néel vector orientation serves as state variable which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B:Cr 2 O 3 film. Switching of the Hall voltage between zero and non-zero values implies Néel vector rotation by 90 degrees. Combined magnetometry, spin resolved inverse photoemission, electric transport and scanning probe microscopy measurements reveal B-dependent T N and resistivity enhancement, spin-canting, anisotropy reduction, dynamic polarization hysteresis and gate voltage dependent orientation of boundary magnetization. The combined effect enables H  = 0, voltage controlled, nonvolatile Néel vector rotation at high-temperature. Theoretical modeling estimates switching speeds of about 100 ps making B:Cr 2 O 3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications. 

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