Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films

Abstract

Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of a 50-nm-thick film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 μB nm−2. The antiferromagnetic ordering in the inhomogeneously strained films can persist up to 100 °C, rendering Cr2O3 relevant for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Flexomagnetic Effect Enhanced Ferromagnetism and Magnetoelectrochemistry in Freestanding High-Entropy Alloy Films

“…It is noteworthy that both strain and strain gradients are present in our wrinkled film, but the nonuniform strain gradients dominate (Figure S8, Supporting Information) based on the above discussions, and a maximum strain gradient of 25% per micron can be achieved (∇ε = dε/d t , ε and t are the magnitude of strain and film thickness, respectively) . This has also been reported in other inhomogeneously strained films, , which can induce the spin-reorientation following the atomic displacements because of the spin-exchange interaction in the antiferromagnet. − Thus, in our work, these large differences in magnetization of the BCC and FCC phases suggest that the partial structural phase transition caused by the inhomogeneous gradient term (∇ε) is responsible for the observed flexomagnetic coupling behavior.…”

“…Magnetism is also particularly attractive for tuning spin-related electron transfer in electrochemistry, especially in magneto­electrochemistry that couples the external magnetic field with electrocatalysts to control and understand the electrochemical reactions. − Unfortunately, numerous 3 d -transition-metal materials are diamagnetic or paramagnetic at room temperature, which significantly impedes the application of the magnetic boosting strategy . Noteworthily, introducing external inhomogeneous strain gradients (flexomagnetic effect) in electrocatalytic film can lead to phase transition and spin-reorientation following the atomic displacements, − thus modifying the magnetic ground state, and a flexomagnetism-driven magnetization enhancement is expected to be observed. ,, Furthermore, when an external magnetic field is applied to this engineered magnetic film, the enhanced magnetization can contribute to a more substantial electron transfer by weakening the scattering of electrons, − resulting in magnetic responses and a better activity. However, an experimental design for combining flexomagnetism with electrocatalysts is still lacking.…”

“…7 Unlike piezomagnetism (M ∝ ε) and magnetostriction (M 2 ∝ ε) in thin films, which have been used to design sensors and actuators, 12−14 flexomagnetism (M ∝ ∇ε) has attracted few attempts in practical applications, 7−9 in part due to the difficulties in fabricating and controlling the inhomogeneous strain gradients. Moreover, the current research landscape of flexomagnetism is limited to very few experimental results of the emergent ferromagnetism in inhomogeneously strained antiferromagnetic materials, 7,8 so the impact of flexomagnetic coupling on phase transition and spin-reorientation of magnetic materials remains a vastly unexplored area to be investigated. 9 Magnetism is also particularly attractive for tuning spinrelated electron transfer in electrochemistry, especially in magnetoelectrochemistry that couples the external magnetic field with electrocatalysts to control and understand the electrochemical reactions.…”

“…Freestanding thin films are powerful platforms for multifunctional devices and manipulating properties via extreme strain (ε) and strain gradients (∇ε). − For example, in freestanding oxide thin films, extreme uniaxial strain exceeding 8% can be withstood before breaking, , and the strain gradients-induced tunable flexoelectricity has been observed, allowing the study of strain responses not achievable in substrate-supported thin films. Very recently, the coupling between magnetism and strain gradients, i.e., flexomagnetic effect, − has been demonstrated in experiments to induce a spontaneous magnetic moment of rippled GdPtSb alloy films at room temperature . Unlike piezomagnetism ( M ∝ ε) and magnetostriction ( M 2 ∝ ε) in thin films, which have been used to design sensors and actuators, − flexomagnetism ( M ∝ ∇ε) has attracted few attempts in practical applications, − in part due to the difficulties in fabricating and controlling the inhomogeneous strain gradients.…”

“…Very recently, the coupling between magnetism and strain gradients, i.e., flexomagnetic effect, − has been demonstrated in experiments to induce a spontaneous magnetic moment of rippled GdPtSb alloy films at room temperature . Unlike piezomagnetism ( M ∝ ε) and magnetostriction ( M 2 ∝ ε) in thin films, which have been used to design sensors and actuators, − flexomagnetism ( M ∝ ∇ε) has attracted few attempts in practical applications, − in part due to the difficulties in fabricating and controlling the inhomogeneous strain gradients. Moreover, the current research landscape of flexomagnetism is limited to very few experimental results of the emergent ferromagnetism in inhomogeneously strained antiferromagnetic materials, , so the impact of flexomagnetic coupling on phase transition and spin-reorientation of magnetic materials remains a vastly unexplored area to be investigated …”