Heusler compounds for future magnetic memory and logic

• Applicants: Professorin Dr. Claudia Felser; Professor Dr. Burkard Hillebrands; Professor Dr. Günter Reiss
• Subject Area: Synthesis and Properties of Functional Materials ; Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263669276

• Final Report Year: 2019

Final Report Abstract

The project’s main objective was to investigate new magnetically perpendicular materials from the Heusler compounds and similar material families with respect to their magnetic properties and their suitability to serve as electrodes in magnetic tunnel junctions. The part of the German partners was the theory and experiment on bulk materials and thin film systems and the simulation of the properties with respect to the integration of the materials in small tunnel junctions. The French partners were involved in the theoretical description of the transport in such tunnel junctions and in the realization of ultrasmall tunnel junctions. The investigations have shown, that the Heusler compounds based on Mn 3X (with X=Ga, Ge) in their tetragonally distorted lattice structure show very promising magnetic properties such as a large magnetocrystalline anisotropy. In addition, it was possible to screen a large amount of ternary compounds (Mn-Co-Pt, Mn-Ga-Fe, Mn-Fe-Ge, Mn-Rh-Sn, …) in a wide range of compositions. All these materials have the potential to form electrodes in magnetic tunnel junctions if the two following issues can be resolved: Roughness: It turned out, that some of the compounds (such as Mn 3Ga) show an extremely large tendency to grow in island form. This results in a large roughness that hinders the formation of a smooth tunneling barrier. - Mn-diffusion and Mn-O formation: The Mn in these compounds has a considerable tendency to diffuse already at relatively low temperature (≈300°C). This is problematic for the formation of a crystalline film structure and for the initiation of the exchange bias. These issues have been simultaneously identified worldwide. In particular the discussions with Japanese researchers showed, that similar results have been also found in the laboratories in Sendai and Tsukuba. Within the project, a two-step procedure for the growth turned out to be a possible mitigation strategy for the roughness issue. In addition, Heusler compounds that have an interface induced anisotropy have been investigated that do not contain Mn (Co2FeAl and Co2FeGe). In this case, films up to a thickness of 1nm did show perpendicular magnetic anisotropy with promising properties concerning magnetic damping and roughness. With respect to applications, these results form a solid base for further work that is necessary to create small magnetic tunnel junctions with high retention time large TMR. The collaboration of the partners is ongoing including additional contacts with companies to realize this goal. The large roughness found for many of the Mn-containing compounds was not expected at the start of the project.

Publications

• Chemical disorder as engineering tool for spin-polarization in MmGa-based Heusler systems Physical Review B 91 (2015) 094203
  S. Chadov, S. W. D'Souza, L. Wollmann, J. Kiss, G. H. Fecher, C. Felser
  
• Titanium nitride as a seed layer for Heusler compounds. Journal of Applied Physics 118, 243904 (2015)
  Alessia Niesen, Manuel Glas, Jana Ludwig, Jan-Michael Schmalhorst, Roshnee Sahoo, Daniel Ebke, Elke Arenholz, Günter Reiss
  
• Termination layer compensated tunneling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy. Nature Communication 7 (2016) 10276
  Jaewoo Jeong, Yari Ferrante, Sergey V. Faleev, Mahesh G. Samant, Claudia Felser, and Stuart S.P.Parkin
  
• Lack of correlation between the spin mixing conductance and the ISHE-generated voltages in CoFeB/Pt,Ta bilayers. Physical Review B 95, 174426 (2017)
  A. Conca, B. Heinz, M. R. Schweizer, S. Keller, E. Th. Papaioannou, B. Hillebrands
  
• Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study. J. Phys. D: Appl. Phys. 50 (2017) 095002
  Yuichiro Matsushita, Galia Madjarova, J. K. Dewhurst, S Shallcross, C. Felser, S. Sharma, and E. K. U. Gross
  
• Perpendicular magnetic anisotropy of TiN buffered Co2FeAl/MgO bilayers. Journal of Applied Physics 121,223902 (2017)
  Alessia Niesen, Jana Ludwig, Manuel Glas, Robin Silber, Jan-Michael Schmalhorst, Elke Arenholz, Günter Reiss
  
• Direct Observation of Spin Diffusion-Enhanced Non-Adiabatic Spin Torque Effects in Rare-Earth-Doped Permalloy. Phys. Rev. B, 98, 214406 (2018)
  P. Krautscheid, R. Reeve, D. Schönke, I. Boventer, A. Conca, A. Chumak, B. Hillebrands, J. Ehrler, J. Osten, J. Fassbender, M. Kläui
  
• Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co2FeAl/MgO interface upon annealing. Journal of Physics D-Appl. Phys 51, 165303 (2018)
  Andres Conca Parra, Alessia Niesen, Günter Reiss and Burkard Hillebrands
  
• Separation of the two-magnon scattering contribution to damping for the determination of the spin mixing conductance. Phys. Rev. B, 98, 214439 (2018)
  A. Conca, S. Keller, M. R. Schweizer, E. Th. Papaioannou, B. Hillebrands
  
• Structural, magnetic, and electrical properties of perpendicularly magnetized Mn4-xFexGe thin films Journal of Applied Physics 123, 113901 (2018)
  A. Niesen, N. Teichert, T. Matalla-Wagner, J. Balluf, N. Dohmeier, M. Glas, C. Klewe, E. Arenholz, J.-M. Schmalhorst, G. Reiss