Local moment formation and magnetic coupling of Mn dopants in Bi2Se3: A low-temperature ferromagnetic resonance study
Abstract
We compare the magnetic and electronic configuration of single Mn atoms in molecular beam epitaxy (MBE) grown Bi2Se3 thin films, focusing on electron paramagnetic (ferromagnetic) resonance (EPR and FMR, respectively) and superconducting quantum interference device (SQUID) techniques. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) reveal the expected increase of disorder with increasing concentration of magnetic guest atoms, however, Kikuchi patterns show that disorder consists majorly of mu m-scale 60 degrees twin domains in the hexagonal Bi2Se3 structure, which are promoted by the presence of single unclustered Mn impurities.
Ferromagnetism below T-C similar to (5.4 +/- 0.3) K can be well described by critical scaling laws M(T)similar to(1-T/T-C)(beta) with a critical exponent beta = (0.34 +/- 0.2), suggesting 3D Heisenberg class magnetism instead of e.g. 2D-type coupling between Mn-spins in van der Waals gap sites. From EPR hyperfine structure data we determine a Mn2+ (d(5), S = 5/2) electronic configuration with a g-factor of 2.002 for -1/2 -> +1/2 transitions.
In addition, from the strong dependence of the low temperature FMR fields and linewidth on the field strength and orientation with respect to the Bi2Se3 (0001) plane, we derive magnetic anisotropy energies of up to K-1 = -3720 erg/cm(3) in MBE-grown Mn-doped Bi2Se3, reflecting the first order magneto-crystalline anisotropy of an in-plane magnetic easy plane in a hexagonal (0001) crystal symmetry. We observe an increase of K-1 with increasing Mn concentration, which we interpret to be correlated to a Mn-induced in-plane lattice contraction.
Across the ferromagnetic-paramagnetic transition the FMR intensity is suppressed and resonance fields converge the paramagnetic limit of Mn2+ (d(5), S = 5/2).