High-temperature ferromagnetism in alloyed MnxSi1–x (0.5 < x < 0.55) films D. Averyanov*, Yu. Matveyev†, V. Ryl’kov*, A. Semisalova††, A. Zenkevich† †Moscow Institute of Physics and Technology *National Research Center “Kurchatov institute” ††Lomonosov Moscow State University

Motivation Semiconductor spintronics is in need of: homogeneous material combining semiconducting and ferromagnetic properties at room temperature which can possibly be integrated into silicon technology …There are several way for achievement this aim – to develop….But there are principal problems on these way… These problems are discussed in detail by Dr. Zhou in this paper.

History (1) 2005: “Dilute” Si:Mn (similar to GaAS:Mn): MnxSi1-x (x ≈ 0.001-0.01) “Isolated MnSi1.7 clusters” in Si matrix 2011: “concentrated” MnxSi1-x (x ≈ 0.35) too many (> 5) Mn silicide phases – non-repeatable

c-MnSi (B2) phase is ferromagnetic, Tc ≈ 250 K History (2) 2012: Epitaxial stabilization of c-MnSi (B2) on Si c-MnSi (B2) phase is ferromagnetic, Tc ≈ 250 K 2006: Epitaxial stabilization of c-FeSi (x ≈ 0.54) Non-stoichiometric FexSi1-x (x ≈ 0.52-0.55)  c-FeSi (B2)  FM @ RT!

History (3) 1996-2007: non-stoichiometric c-FeSi (x ≈ 0.52 - 0.54) R. Mantovan and A. Zenkevich, unpublished (2007) M. Walerfang et al. PRB 73 214423 (2006) Non-stoichiometric FexSi1-x (x ≈ 0.52-0.55)  c-FeSi (B2)  FM @ RT!

“Recent history” 2012: FM above RT in non-stoichiometric MnxSi1–x (x ≈ 0.52-0.55) polycrystalline MnxSi1–x (x ≈ 0.52-0.55): FM above RT Ms≈1.1-1.4 µB per Mn atom “macroscopically”- B20 structure (XRD)

“Recent history” (2012): anomalous Hall effect Anomalous HE Normal HE For sample with x  0.52 the value of the AHE remains nearly the same in the temperature range 6-200K. At T = 300K the maximum AHE is observed for sample with x=0.52! This slide shows the behavior of Anomalous Hall effect verses magnetic field at different temperatures. I will remind that the Hall resistance in magnetic materials consists usually from two components, the first of which is related to the Lorentz force and is proportional to the magnetic induction B. Whereas the second term is due to the anomalous Hall effect, which is proportional to the magnetization and carriers polarization. You can see that in all studied samples we observed a clearly pronounced AHE dominating over the normal Hall effect. Magnetic field dependence of the Hall effect resistivity for samples with x ≈ 0.52, 0.53 and 0.55 at T = 6 and 197 K. The inset shows ρH(B) curves at room temperature for the samples with x ≈ 0.52, 0.53 and 0.55.

Motivation for the eMS: Summary: unlike stoichiometric ε-MnSi (B20) phase, the non-stoichiometric polycrystalline MnхSi1-х (x  0.52) films exhibit RT ferromagnetic properties retain B20 macroscopic crystalline structure origin of FM: exchange interaction of (Si) vacancy defects with Mn in ε-MnSi? or the nuclei of another (B2) phase? Motivation for the eMS: to correlate composition vs. micro-structure vs. local magnetic properties of polycrystalline non-stoichiometric MnхSi1-х (x > 0.5)

Growth of alloyed MnxSi1–x (x ≈ 0.52) thin films Sapphire substrate Small changes of Mn/Si ratio across the sample! → all measurements are on stripes ~(2-5)x10 mm Buffer gas (Kr) P ~ 10-2 mbar Focused laser beam s.c.-MnSi target Heater, T~ 350 C Growth by Pulsed Laser Deposition technique Kr buffer gas (P ≈ 10-2 mbar) and shadow geometry are employed to exclude droplets graded composition across the sample is built-in the growth process Substrate: sapphire Growth T=340 C Mn1,025Si (x=0.505) Mn1,07Si (x=0.517) Composition of Al2O3/MnxSi1–x samples as derived from RBS spectra

Macroscopic structure of as grown MnxSi1–x (x ≈ 0.52) sample Random (black) vs. channeling (red) RBS spectra: the difference indicates partial ordering of the crystalline grains normal to the surface. θ-2θ XRD scan of MnxSi1–x film grown on sapphire XRD analysis reveals polycrystalline ε-MnSi phase (B20 structure) RBS in the channeling mode indicates partial orientation of grains

Magnetic properties of as grown MnxSi1–x samples (SQUID) MnxSi1–x , off-stoichiometry x SQUID measurements of M(T) for several of MnxSi1–x samples with slightly different composition ferromagnetic properties persist above RT (expected Tc ~ 400⁰C) apparent contributions from 2 phases: ε-MnSi (Tc=30 K) + “high-Tc”

MnxSi1–x samples at hand d = 70 nm x = 0,52 d = 270 nm x = 0.505 12 mm 2 mm 8 4 Sample #1 1 2 3 5 Sample #3 (MnSi single crystal) 8 mm d = 25 nm x = 0,525 d = 100 nm x = 0.505 12 mm 15 mm Sample #2 Sample #4 (MnSi single crystal) 5 mm

Planned experiments RT(?) eMS measurements on ferromagnetic MnхSi1-х (stripe) vs. reference paramagnetic stripe or single crystal ε-MnSi In situ annealing  measure across ordering transition in non- stoichiometric alloys information on the role of defects and the annealing kinetics “measure in applied magnetic field, quenching and time-delayed experiments to address defects kinetics” (Roberto) eMS measurements on the reference c-FeSi sample? Also to check correlation between CEMS and eMS data …..