Structural, electronic, and magnetic investigation of magnetic ordering in MBE-grown Cr_xSb_2-xTe₃ thin films

We report the structural, electronic, and magnetic study of Cr-doped Sb₂Te₃ thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of x = 0.42 (in Cr_xSb_2-xTe₃), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from ~2 μ_Bto almost 3 μ_B per Cr ion. The transition temperature (T_c) depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phase-pure films of x = 0.42 ,a T_c of 125 K was determined. Electric transport measurements find surface-dominated transport below ∼10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.