Iron germanide (FeGe) is an

intermetallic compound An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Inte ...

, 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 alkalin ...

iron Iron is a chemical element; it has symbol Fe () 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, forming much of Earth's o ...

. 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 Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

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 is d ...

and cubic structures. The cubic polymorph has no

inversion center In geometry, a point reflection (also called a point inversion or central inversion) is a geometric transformation of affine space in which every point is reflected across a designated inversion center, which remains fixed. In Euclidean or ...

, 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, 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 flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

, 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 Manganese monosilicide (MnSi) is an intermetallic compound, a silicide of manganese. It occurs in cosmic dust as the mineral brownleeite. MnSi has a cubic crystal lattice with no inversion center; therefore its crystal structure is helical, wit ...

, 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 (physics), homogeneity for highly demanding applications. The VAR process has revolutionized the specialty S ...

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; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid 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 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); most often, in s ...

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 with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

with a

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

or 0.03 eV.

References

{{Iron compounds Iron compounds Germanides Iron monosilicide structure type