1. Epitaxial films of tetragonal Mn 3 Ga: magnetism and microstructure F. Casoli 1,*, J. Karel 2, P. Lupo 3, L. Nasi 1, S. Fabbrici 1,4, L. Righi 1,5, F. Albertini 1, C. Felser 2 1 IMEM – CNR, Parma, Italy 2 Max Planck Institute for Chemical Physics of Solids, Dresden, Germany 3 Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 4 MIST E-R Laboratory, Bologna, Italy 5 Dipartimento di Chimica, Università di Parma, Parma, Italy * casoli@imem.cnr.it

2. Tetragonal Mn 3-x Ga is a unique system with an uncommon combination of magnetic and electronic properties:  High uniaxial anisotropy - K 1 ≈ 1 - 3 × 10 6 J/m 3 [Wu F. et al, APL 94, 2009; Mizukami S. et al., PRB 85, 2012; Rode K. et al., PRB 87, 2013]  Low saturation magnetisation - M S = 110 - 440 kA/m depending on composition and growth conditions [Rode K. et al., PRB 87, 2013; Li M. et al., APL 103, 2013]  Curie temperature above 770 K [Krén E., Kádár G., Solid state Communications 8, 1970]  High spin-polarization - predicted value for Mn 3 Ga 88% [Winterlik J. et al., PRB 77, 2008]  Low Gilbert damping - α < 0.015 [Mizukami S. et al, PRL 106, 2011] STT-MRAMs and PERMANENT MAGNETS Motivation & background

5. A few recent works have demonstrated epitaxial growth of this phase with PMA on MgO, SrTiO 3 or Cr, Pt underlayers. Typical thickness was 40 - 100 nm. [Wu F. et al, APL 94, 2009; Mizukami S. et al., PRB 85, 2012; Rode K. et al., PRB 87, 2013; Li M. et al., APL 103, 2013] D0 22 tetragonal phase shows ferrimagnetic order red sites = 2b positions Mn I with μ I = 2.8 μ B green sites = 4d positions Mn II with μ II = 1.6 μ B grey sites = 2a positions Ga [Krén E., Kádár G., Solid state Communications 8, 1970] We focused on films with thickness 10 - 40 nm grown on STO with compositions Mn 3 Ga and Mn 2.33 Ga a (Å)c (Å) Mn 3 Ga - D0 22 3.917.10 MgO4.21= a SrTiO 3 3.905= a Cr 4.07/√ 2 = a Pt3.92= a

6.  RF sputtering on:SrTiO 3 (100) - STO  Growth temperature: 250 °C - 350 °C  Thickness:10, 20, 30, 40 nm  Composition:Mn 0.75 Ga 0.25 Mn 3 Gaand Mn 0.70 Ga 0.30 Mn 2.33 Ga We exploited the alternate layer deposition from 2 targets - Mn and MnGa - to obtain the desired compositions. The result was verified by EDS on a series of Mn x Ga 1-x samples grown on SiO x /Si. Growth

7. Growth T = 300 °Cthickness = 40 nm Optimisation of the growth conditions Mn 3 Ga Mn 2.33 Ga At growth T = 300 °C:  D0 22 phase with c-axis predominately oriented perpendicular to the substrate surface; both for Mn 3 Ga and Mn 2.33 Ga  No mosaicity  Maze-like morphology

8. Growth T = 325 °Cthickness = 40 nmGrowth T = 350 °Cthickness = 40 nm With increasing growth T:  Both D0 22 and D0 19 phases – the second visible only in TEM images  D0 22 phase shows also in- plane c-axis  Disordered morphology

9. Growth T = 350 °C, thickness = 40 nm  PMA and coercivity values between 1 and 2 T  With lower Mn content: lower H C and higher M S  Secondary magnetic component also evident in T growth = 300 °C samples, which show only epitaxial perpendicular D0 22 phase in XRD Growth T = 300 °C, thickness = 40 nm Mn 3 GaMn 2.33 Ga

10. Growth T = 300 °Cthickness = 10, 20, 30 nm 20 nm 30 nm 20 nm 10 nm  The perpendicular D0 22 phase is epitaxial also at reduced thickness  AFM shows maze-like morphology, very similar for Mn 3 Ga and Mn 2.33 Ga; grain size decreases with decreasing thickness  Islands exhibit a quite flat surface - roughness below 1.5 nm; variation in island height is of several nanometers and increases with film thickness Structure and magnetism in thin layers

11.  Very large coercivity values > 2 T  Small amount of secondary magnetic component: in-plane c-axis? Secondary phase? Mn 2b sublattice [K. Rode et al. PRB 87 (2013)] ? Role of faceting on magnetic anisotropy?

12. (001) (110) (020) Growth T = 300 °CMn 3 Gathickness = 20 nm  HAADF images: sharp faceting  EDS map: Pt capping homogeneously covers the Mn 3 Ga grains; good control of composition; no segregation of secondary phases Pt M Mn K Ga K

13. 200 020 110 004 020 Mn 3 Ga (100) STO (100) STO Mn 3 Ga STO Mn 3 Ga Pt  HRTEM: no other phases than D0 22 ; D0 22 is predominately with perpendicular c-axis and epitaxial, but…

14. 220 200 020 2-20 FFT A small fraction of in-plane c-axes regions was found Mn 3 Ga Pt

15.  Growth at 350 o C leads to a polycrystalline film: most of the film shows the D0 22 phase with perpendicular c-axis, but part of the film exhibits the tetragonal c-axis in the film plane or the D0 19 (hexagonal) phase.  After reducing growth temperature to 300 °C at fixed thickness (40 nm) a fraction of in-plane c-axis is still present.  Reducing the film thickness or Mn concentration improves the film quality; the 300 °C Mn 3 Ga 20nm sample shows only a small amount of the c-axis in plane, whereas the Mn 2.33 Ga 40 nm sample exhibits primarily a single crystallographic orientation.  It can prove challenging to identify the presence of additional crystallographic orientations using conventional XRD. TEM is of critical importance in characterizing these materials. Conclusions

17. backup backup K u ≈ 0.9 × 10 6 J/m 3 (BH) max ≅ 8.7 kJ/m 3 K u ≈ 0.9 × 10 6 J/m 3 (BH) max ≅ 8.7 kJ/m 3 T growth =350 °C 40 nm, Mn 3 Ga T growth =300 °C 40 nm, Mn 3 Ga T growth =300 °C 20 nm, Mn 3 Ga T growth =300 °C 40 nm, Mn 2.33 Ga