Voltage-Controlled Magnetization in Chromia-Based Magnetic Heterostructures

Spin-electronics or “spintronics” promises a new generation of low-power, high-speed, non-volatile memory and logic devices. Almost all existing and planned spintronic devices operate on controlling magnetization, preferably by electrical means alone. In this context, Cr2O3 (chromia) based heterostructures have emerged as a promising candidate. Chromia is magnetoelectric, which provides a direct coupling between an applied electric field and an induced magnetic moment. Moreover, chromia has an uncompensated moment at its interface, known as boundary magnetization. This magnetization is closely associated with the antiferromagnetic domain state of the chromia which can be electrically switched. The boundary magnetization can therefore be used as a logical bit in spintronic devices. While the equilibrium boundary magnetization of chromia is strictly tied to its domain state, non-equilibrium states may be created. For example, in the presence of an adjacent ferromagnet, exchange coupling at the interface may stabilize the interfacial domain state of the chromia against the rotation of the bulk of the crystal when it is subjected to critical magnetoelectrical switching fields. This results in an antiferromagnetic interface in a nonequilibrium state which is incommensurate with the underlying bulk. Upon magnetically cycling the ferromagnet, the antiferromagnet then relaxes, which in turn is reflected in the exchange bias of the ferromagnetic hysteresis loop. In other cases, the interfacial exchange locks the antiferromagnetic interface to rotate along with the ferromagnet. This causes a competition between the exchange energy at the interface and the anisotropy energy of the crystal. At the crossover point between these two mechanisms there occurs a simultaneous disappearance of exchange bias with a more than twofold increase in coercivity of the ferromagnetic hysteresis loop. Due to its relatively high Néel temperature, chromia has the advantage of potential operation at room temperature, nevertheless, further increasing the Néel temperature would provide more flexibility in applications. It is shown that by boron doping, the Néel temperature of chromia can be controllably tuned from the undoped temperature of T_N=307 K, up to at least 400 K. In addition, boron doped chromia exhibits voltage-controlled and non-volatile Néel vector reorientation in the absence of an applied magnetic field, further expanding its functionality