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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. 

Abstract GeI 2 is an interesting two-dimensional wide-band gap semiconductor because of diminished edge scattering due to an absence of dangling bonds. Angle-resolved x-ray photoemission spectroscopy indicates a germanium rich surface, and a surface to bulk core-level shift of 1.8 eV in binding energy, between the surface and bulk components of the Ge 2p 3/2 core-level, making clear that the surface is different from the bulk. Temperature dependent studies indicate an effective Debye temperature ( θ D ) of 186 ± 18 K for the germanium x-ray photoemission spectroscopy feature associated with the surface. These measurements also suggest an unusually high effective Debye temperature for iodine (587 ± 31 K), implying that iodine is present in the bulk of the material, and not the surface. From optical absorbance, GeI 2 is seen to have an indirect (direct) optical band gap of 2.60 (2.8) ± 0.02 (0.1) eV, consistent with the expectations. Temperature dependent magnetometry indicates that GeI 2 is moment paramagnetic at low temperatures (close to 4 K) and shows a diminishing saturation moment at high temperatures (close to 300 K and above). 

Abstract Because of its compatibility with semiconductor-based technologies, hafnia (HfO 2 ) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO 2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO 3 ) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO 2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material. 

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.