Magnetization at the interface of Cr2O3 and paramagnets with large stoner susceptibility

The spin polarization of palladium on magneto-electric Cr2O3

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.

Voltage controlled Néel vector rotation in zero magnetic field

Multi-functional thin films of boron (B) doped Cr2O3 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:Cr2O3 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 TN 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:Cr2O3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications.

Parasitic Magnetism in Magnetoelectric Antiferromagnet

Parasitic magnetism plays an important role in magnetoelectric spin switching of antiferromagnetic oxides, but its mechanism has not been clearly investigated. Unlike the widely obtained surface boundary magnetization in magnetoelectric Cr₂O₃ antiferromagnet, we previously reported that Al doping could produce volume-dependent parasitic magnetism (M_para) in Cr₂O₃ with the remaining magnetoelectric effect and antiferromagnetic properties. In this work, we systematically investigated the magnetic properties of M_para in Cr₂O₃ through its different exchange coupling characteristics with the ferromagnet at various conditions. The columnar grain boundaries cause an antiferromagnetic sublattice breaking to produce uncompensated spins and thus are considered to be responsible for M_para in both undoped and Al-doped Cr₂O₃. Finally, a model was proposed for the formation mechanism of the parasitic magnetism in Cr₂O₃, which explains the reported magnetic characteristics of Cr₂O₃, and some current topics such as the domain formation and motion in Cr₂O₃ during magnetoelectric spin switching. This work contributes to a deep understanding of antiferromagnetic spintronics and provides a method to realize the low-energy operation of antiferromagnetic-based magnetic random access memory.

Evidence of the Topological Hall Effect in Pt/Antiferromagnetic Insulator Bilayers

The topological Hall effect has been a primary indicator of nontrivial spin textures in magnetic materials. We observe the evidence of the topological Hall effect in Pt/Cr_{2}O_{3} bilayers grown on Al_{2}O_{3}(0001) and Al_{2}O_{3}(112[over ¯]0), where the Cr_{2}O_{3} epitaxial film is an antiferromagnetic insulator. The Pt/Cr_{2}O_{3} bilayers exhibit topological Hall resistivity for Cr_{2}O_{3} thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of Cr_{2}O_{3}, revealing the key role of thermal fluctuations in the formation of spin textures. The similarity of topological Hall signals in (0001)- and (112[over ¯]0)-oriented Cr_{2}O_{3} films indicates that the emergence of spin textures is insensitive to crystalline orientation.