[Paper Review] Properties of group-IV-based ferromagnetic semiconductor GeFe: Growth temperature dependence, lattice constant, location of Fe atoms, and their relevance to the magnetic properties

Yuki K. Wakabayashi, Shinobu Ohya+2 more|arXiv (Cornell University)|Jun 9, 2014

Magnetic and transport properties of perovskites and related materials2 references4 citations

TL;DR

This study investigates the growth temperature dependence of GeFe ferromagnetic semiconductor films, revealing a universal correlation between Curie temperature (TC) and lattice constant independent of Fe content. Using RBS and PIXE, it identifies that ~15% of Fe atoms occupy tetrahedral interstitial sites, and non-uniform Fe distribution—rather than substitutional Fe concentration—dominates ferromagnetic behavior.

ABSTRACT

We report the growth temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge1-xFex films (x = 6.5% and 10.5%), including the lattice constant, Curie temperature (TC), and Fe-atom locations. While TC strongly depends on the growth temperature, we find a universal relationship between TC and the lattice constant, which does not depend on the Fe content x. By using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, it is clarified that about 15% of the Fe atoms exist in the tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice and that the substitutional Fe concentration is not correlated with TC. Considering these results, we suggest that the non-uniformity of the Fe concentration plays an important role in determining the ferromagnetic properties of GeFe.

Motivation & Objective

• To understand the influence of growth temperature on the structural and magnetic properties of Ge1-xFex films.
• To determine the precise location of Fe atoms in the Ge lattice using advanced characterization techniques.
• To clarify the role of Fe concentration distribution in determining the Curie temperature (TC) in GeFe.
• To establish a universal relationship between TC and lattice constant across different Fe doping levels.
• To resolve the long-standing ambiguity in the origin of ferromagnetism in group-IV-based dilute magnetic semiconductors.

Proposed method

• Growth of Ge1-xFex films (x = 6.5% and 10.5%) at varying temperatures using molecular beam epitaxy.
• Employment of channeling Rutherford backscattering spectroscopy (RBS) to determine the site occupancy of Fe atoms.
• Utilization of particle-induced X-ray emission (PIXE) to quantify Fe concentration and distribution.
• Measurement of magnetic properties, including Curie temperature (TC), via superconducting quantum interference device (SQUID) magnetometry.
• Correlation of lattice constant, derived from X-ray diffraction, with TC across different growth conditions.
• Analysis of the universal TC–lattice constant relationship across varying Fe doping levels (x = 6.5% and 10.5%).

Experimental results

Research questions

• RQ1 How does growth temperature affect the lattice constant and Curie temperature (TC) in Ge1-xFex films?
• RQ2 Where are Fe atoms located in the Ge lattice—substitutional or interstitial sites—and how does this affect magnetic properties?
• RQ3 Is the concentration of substitutional Fe atoms directly correlated with the observed Curie temperature?
• RQ4 Does a universal relationship exist between TC and lattice constant across different Fe doping levels in GeFe?
• RQ5 What role does the non-uniformity of Fe concentration play in the ferromagnetic behavior of GeFe?

Key findings

• A universal relationship exists between the Curie temperature (TC) and the lattice constant in Ge1-xFex films, independent of Fe content (x = 6.5% and 10.5%).
• Approximately 15% of Fe atoms occupy tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice, as confirmed by channeling RBS.
• The concentration of substitutional Fe atoms shows no direct correlation with the Curie temperature (TC).
• The observed TC strongly depends on growth temperature, indicating kinetic control over magnetic properties.
• Non-uniform Fe concentration distribution is identified as a key factor governing the ferromagnetic behavior in GeFe.
• The lattice constant increases with growth temperature, which correlates with a concomitant increase in TC.