Nanomagnetism and spintronics of Cr2O3 magnetoelectric antiferromagnets

Thin film antiferromagnets (AF) have potential to revolutionize spintronics due to their inherently magnetic-field stable magnetic order and high-frequency operation. To explore their application potential, it is necessary to understand modifications of the magnetic properties and magnetoelectric responses of AF thin films with respect to their bulk counterparts. Considering grainy morphology of thin films, questions regarding the change of the intergranular exchange, criticality behavior and switching of the order parameter need to be addressed.
Our approach is based on the electron transport characterization of magnetic responses of thin film antiferromagnets [1-4]. This task is difficult as minute uncompensated surface magnetization of antiferromagnets needs to be detected, which imposes strict requirements to the sensitivity of the method. We will outline our developments of zero-offset anomalous Hall magnetometry [1] applied to study the physics of conventional metallic IrMn and insulating magnetoelectric Cr2O3 antiferromagnets. To build a reliable description of the material properties, the analysis of the transport data is backed up by structural characterization and real space imaging of AF domain patterns using NV microscopy [2,5]. Based on this unique and novel combination, we for the first time observe the formation of nanoscale antiferromagnetic domains in thin films of Chromia (Cr2O3) across its ordering temperature at ~300 K. Our quantitative results yield a detailed understanding of the domain formation process in Cr2O3 and allow us to determine the efficiency of inter-granular magnetic exchange coupling [5]. This coupling strength has proven decisive in the decades long development of ferromagnetic memory media and will be of equal importance for future antiferromagnetic spintronics technologies, for which we here present a powerful new development tool.
The fundamental understanding of the magnetic microstructure of magnetoelectric α-Cr2O3 thin films and the possibility to read-out its antiferromagnetic order parameter all-electrically enabled the entirely new recording concept where a magnetoelectric memory cell can be addressed without using a ferromagnet. With this approach, we opened an appealing topic of purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) [2].
By exploring the interaction of antiferrmagnetic domain walls with morphological structures prepared on the surface of Cr2O3 single crystals, we access the nanoscale mechanics of AF domain walls. We propose to employ nanoscale patterns as engineered pinning centers for AF domain walls,
where binary information is encoded by the direction of the Neel vector. Our results bear significant potential for technological exploitation be it in the form of the proposed antiferromagnetic memory devices, or ultimately for the realisation of DW logic using antiferromagnets.
These recent developments on the fabrication and characterization of Cr2O3-based functional elements will be discussed in this presentation.
REFERENCES
[1] T. Kosub, M. Kopte, F. Radu, O. G. Schmidt, D. Makarov, “All-Electric access to the Magnetic-Field-Invariant Magnetization of Antiferromagnets”, Phys. Rev. Lett. 115, 097201 (2015).
[2] T. Kosub, M. Kopte, R. Hühne, P. Appel, B. Shields, P. Maletinsky, R. Hübner, M. O. Liedke, J. Fassbender, O. G. Schmidt, and D. Makarov, “Purely antiferromagnetic magnetoelectric random access memory”, Nature Communications 8, 13985 (2017).
[3] R. Schlitz, T. Kosub, A. Thomas, S. Fabretti, K. Nielsch, D. Makarov, and S. T. B. Goennenwein, “Evolution of the spin hall magnetoresistance in Cr2O3/Pt bilayers close to the Neel temperature”, Appl. Phys. Lett. 112, 132401 (2018).
[4] P. Muduli, R. Schlitz, T. Kosub, R. Hübner, A. Erbe, D. Makarov, and S. T. B. Goennenwein, “Local and nonlocal spin Seebeck effect in lateral Pt-Cr2O3-Pt devices at low temperatures”, Appl. Phys. Lett. Materials 9, 021122 (2021).
[5] P. Appel, B. J. Shields, T. Kosub, R. Hübner, J. Fassbender, D. Makarov, and P. Maletinsky, “Nanomagnetism of magnetoelectric granular thin film antiferromagnets”, Nano Letters 19, 1682 (2019).
[6] O. V. Pylypovskyi, A. V. Tomilo, D. D. Sheka, J. Fassbender, and D. Makarov, “Boundary conditions for the Neel order parameter in a chiral antiferromagnetic slab”, Phys. Rev. B 103, 134413 (2021).
[7] N. Hedrich, K. Wagner, O. V. Pylypovskyi, B. J. Shields, T. Kosub, D. D. Sheka, D. Makarov, and P. Maletinsky, “Nanoscale mechanics of antiferromagnetic domain walls”, Nature Physics (2021). https://doi.org/10.1038/s41567-020-01157-0.