Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns

TL;DR

This study investigates how growth temperature and Mn content influence the structure and magnetism of Ge(1-x)Mn(x) nano-columns, revealing multiple magnetic phases and nanostructure formations in the films.

Contribution

It provides detailed insights into the relationship between growth parameters, nanostructure morphology, and magnetic properties in Ge-Mn thin films, highlighting the coexistence of various magnetic phases.

Findings

Formation of Mn-rich nanostructures across all growth conditions.

Growth temperature determines nanostructure type and size.

Multiple magnetic phases including paramagnetic, superparamagnetic, and ferromagnetic states.

Abstract

We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures (Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and 11 %. Their crystalline structure, morphology and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese rich nanostructures embedded in a nearly pure germanium matrix. Growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover we show in this paper the influence of growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns size and density. For temperatures higher than 180deg C, we observed the formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns and nanoclusters coexist. Combining high resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc nanocolumns (120 K < Tc < 170 K), (iii) high-Tc nanocolumns (Tc> 400 K) and (iv) Ge3Mn5 clusters.