Iron germanide (FeGe) is an

intermetallic compound An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic bonding, metallic alloy that forms an ordered solid-state Chemical compound, compoun ...

, a

germanide A germanide is any binary compound of germanium and a more electropositive element. The composition of most germanides is analogous to that of the corresponding silicides and does not follow formal valence rules. The germanides of alkali and alkali ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

. At ambient conditions it crystallizes in three polymorphs with

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

and cubic structures. The cubic polymorph has no

inversion center In geometry, a point reflection (point inversion, central inversion, or inversion through a point) is a type of isometry of Euclidean space. An object that is invariant under a point reflection is said to possess point symmetry; if it is invari ...

, it is therefore helical, with right-hand and left-handed chiralities.

Magnetism

FeGe is extensively studied for its unusual magnetic properties. Electron spins in this material show dissimilar, yet regular spatial arrangements at different values of applied magnetic field. Those arrangements are named

helical Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...

, skyrmion lattice, and conical. They can be controlled not only by temperature and magnetic field, but also by

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

, and the current density required for manipulating skyrmions (~10⁶ A/m²) is approximately one million times smaller than that needed for moving

magnetic domain A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction. When c ...

s in traditional ferromagnets. As a result, skyrmions have potential application in ultrahigh-density

magnetic storage Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is acc ...

devices. The helical, conical and skyrmion structures are not unique to FeGe; they are also found in

MnSi Brownleeite is a silicide mineral with chemical formula Manganese monosilicide, MnSi. It was discovered by researchers of the Lyndon B. Johnson Space Center, Johnson Space Center in Houston while analyzing the Pi Puppids, Pi Puppid particle shower ...

, MnGe and similar compounds, but contrary to those materials, the observation of magnetic ordering patterns in FeGe does not require cryogenic cooling. The disadvantage of FeGe over MnSi is its polymorphism, which hinders the growth of large homogeneous crystals.

Synthesis

Polycrystalline FeGe

Polycrystalline FeGe is produced by

vacuum arc remelting Vacuum arc remelting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. The VAR process has revolutionized the specialty traditional metallurgica ...

spark plasma sintering Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), or plasma pressure compaction (P2C) is a sintering technique. The main characteristic of SPS is that the pulsed or u ...

, or high-pressure high-temperature treatment of a mixture of elemental iron and germanium. Single crystals of FeGe ca. 1 mm in size can be grown from the powder using a

chemical transport reaction In chemistry, a chemical transport reaction describes a process for purification and crystallization of non- volatile solids. The process is also responsible for certain aspects of mineral growth from the effluent of volcanoes. The technique is ...

and

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

as transporting agent. The source temperature is maintained at 450 °C and the temperature gradient at ca. 50 °C across the reaction tube, over 1–2 weeks.

FeGe thin film

FeGe films can be epitaxially grown on Si (111) using MBE. The thin film FeGe is polycrystalline with ± 30° in-plane rotations around 11out-of-plane axis. Theoretical simulations indicate that FeGe thin film can hold skyrmion cylinder or chiral bobber phases, which were recently imaged in a 35 nm plan-view FeGe thin film using Lorentz STEM/TEM.

Structure

Iron germanide is a

non-stoichiometric compound In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); m ...

where the Ge:Fe ratio often deviates from 1. The Fe₂Ge₃ compound is a

Nowotny phase In inorganic chemistry, a Nowotny chimney ladder phase (NCL phase) is a particular intermetallic crystal structure found with certain binary compounds. NLC phases are generally tetragonal and are composed of two separate sublattices. The first is ...

exhibiting a chimney ladder structure. It is a

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

with a

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...

or 0.03 eV.

References

{{Iron compounds Iron compounds Germanides Iron monosilicide structure type