Heusler alloy
Encyclopedia
A Heusler alloy is a ferromagnetic metal
alloy
based on a Heusler phase. Heusler phases are intermetallics
with particular composition and face-centered cubic crystal structure. They are ferromagnetic—even though the constituting elements are not—as a result of the double-exchange mechanism
between neighboring magnetic ions. The latter are usually manganese
ions, which sit at the body centers of the cubic structure and carry most of the magnetic moment of the alloy. (See the Bethe-Slater curve
for more info on why this happens.)
mining engineer and chemist
Friedrich Heusler
, who studied such an alloy in 1903. It contained two parts copper
, one part manganese, and one part tin
, that is Cu2MnSn, and has the following properties. Its magnetism varies considerably with heat treatment and composition. It has a room-temperature saturation induction of around 8,000 gauss, which exceeds that of the element nickel
(around 6100 gauss) but is smaller than that of iron
(around 21500 gauss). For early studies see. In 1934, Bradley and Rogers showed that the room-temperature ferromagnetic phase was a fully ordered structure of the L21 type. This has a primitive cubic lattice of copper atoms with alternate cells body-centered by manganese
and aluminium
. The lattice parameter is 5.95 Ångström
s. The molten alloy has a solidus
temperature of about 910 °C. As it is cooled below this temperature, it transforms into disordered, solid, body-centered cubic beta-phase. Below 750 °C, a B2 ordered lattice forms with a primitive cubic copper
lattice, which is body-centered by a disordered manganese-aluminium sublattice. Cooling below 610 °C causes further ordering of the manganese and aluminium sub-lattice to the L21 form. In non-stoichiometric alloys, the temperatures of ordering decrease, and the range of anealing temperatures, where the alloy does not form microprecipitates, becomes smaller than for the stoichiometric material.
Oxley found a value of 357 °C for the Curie temperature, below which the alloy becomes ferromagnetic. Neutron diffraction and other techniques have shown that a magnetic moment of around 3.7 Bohr magnetons resides almost solely on the manganese atoms. As these atoms are 4.2 Angstroms apart, the exchange interaction, which aligns the spins, is likely indirect and is mediated through conduction electrons or the aluminium
and copper atoms.
Electron microscopy
studies demonstrated that thermal antiphase boundaries (APBs) form during cooling through the ordering temperatures, as ordered domains nucleate at different centers within the crystal lattice and are often out of step with each other where they meet. The anti-phase domains grow as the alloy is annealed. There are two types of APBs corresponding to the B2 and L21 types of ordering. APBs also form between dislocations if the alloy is deformed. At the APB the manganese atoms will be closer than in the bulk of the alloy and, for non-stoichiometric alloys with an excess of copper
(e.g. Cu2.2MnAl0.8), an antiferromagnetic layer forms on every thermal APB. These antiferromagnetic layers completely supersede the normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy. This significantly modifies the magnetic properties of the non-stoichiometric alloy relative to the stoichiometric alloy which has a normal domain structure. Presumably this phenomenon is related to the fact that pure manganese is an antiferromagnet although it is not clear why the effect is not observed in the stoichiometric alloy. Similar effects occur at APBs in the ferromagnetic alloy MnAl at its stoichiometric composition.
Another useful Heusler alloy is the class of materials known as ferromagnetic shape memory alloys. These are generally composed of nickel, manganese and gallium and can change their length by up to 10% in a magnetic field.
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
based on a Heusler phase. Heusler phases are intermetallics
Intermetallics
Intermetallics or intermetallic compounds is a term that is used in a number of different ways. Most commonly it refers to solid-state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term...
with particular composition and face-centered cubic crystal structure. They are ferromagnetic—even though the constituting elements are not—as a result of the double-exchange mechanism
Double-exchange mechanism
The double-exchange mechanism is a type of a magnetic exchange that may arise between ions in different oxidation state. First proposed by Clarence Zener, this theory that predicts the relative ease with which an electron may be exchanged between two species, and has important implications for...
between neighboring magnetic ions. The latter are usually manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
ions, which sit at the body centers of the cubic structure and carry most of the magnetic moment of the alloy. (See the Bethe-Slater curve
Bethe-Slater curve
Bethe-Slater curve is a graphical representation of exchange energy for transition metals as a function of the ratio of the interatomic distance a to the radius r of the 3d electron shell....
for more info on why this happens.)
Discovery and properties
The term is named after a GermanGermany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
mining engineer and chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...
Friedrich Heusler
Friedrich Heusler
Friedrich Heusler was a German mining engineer and chemist. He discovered a special group of intermetallics now known as Heusler phases, which are ferromagnetic though the constituting elements are not ferromagnetic.-Biography:...
, who studied such an alloy in 1903. It contained two parts copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, one part manganese, and one part tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, that is Cu2MnSn, and has the following properties. Its magnetism varies considerably with heat treatment and composition. It has a room-temperature saturation induction of around 8,000 gauss, which exceeds that of the element nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
(around 6100 gauss) but is smaller than that of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(around 21500 gauss). For early studies see. In 1934, Bradley and Rogers showed that the room-temperature ferromagnetic phase was a fully ordered structure of the L21 type. This has a primitive cubic lattice of copper atoms with alternate cells body-centered by manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
. The lattice parameter is 5.95 Ångström
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
s. The molten alloy has a solidus
Solidus (chemistry)
In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
temperature of about 910 °C. As it is cooled below this temperature, it transforms into disordered, solid, body-centered cubic beta-phase. Below 750 °C, a B2 ordered lattice forms with a primitive cubic copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
lattice, which is body-centered by a disordered manganese-aluminium sublattice. Cooling below 610 °C causes further ordering of the manganese and aluminium sub-lattice to the L21 form. In non-stoichiometric alloys, the temperatures of ordering decrease, and the range of anealing temperatures, where the alloy does not form microprecipitates, becomes smaller than for the stoichiometric material.
Oxley found a value of 357 °C for the Curie temperature, below which the alloy becomes ferromagnetic. Neutron diffraction and other techniques have shown that a magnetic moment of around 3.7 Bohr magnetons resides almost solely on the manganese atoms. As these atoms are 4.2 Angstroms apart, the exchange interaction, which aligns the spins, is likely indirect and is mediated through conduction electrons or the aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
and copper atoms.
Electron microscopy
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
studies demonstrated that thermal antiphase boundaries (APBs) form during cooling through the ordering temperatures, as ordered domains nucleate at different centers within the crystal lattice and are often out of step with each other where they meet. The anti-phase domains grow as the alloy is annealed. There are two types of APBs corresponding to the B2 and L21 types of ordering. APBs also form between dislocations if the alloy is deformed. At the APB the manganese atoms will be closer than in the bulk of the alloy and, for non-stoichiometric alloys with an excess of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
(e.g. Cu2.2MnAl0.8), an antiferromagnetic layer forms on every thermal APB. These antiferromagnetic layers completely supersede the normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy. This significantly modifies the magnetic properties of the non-stoichiometric alloy relative to the stoichiometric alloy which has a normal domain structure. Presumably this phenomenon is related to the fact that pure manganese is an antiferromagnet although it is not clear why the effect is not observed in the stoichiometric alloy. Similar effects occur at APBs in the ferromagnetic alloy MnAl at its stoichiometric composition.
Another useful Heusler alloy is the class of materials known as ferromagnetic shape memory alloys. These are generally composed of nickel, manganese and gallium and can change their length by up to 10% in a magnetic field.
List of Heusler alloys
- Cu2MnAl, Cu2MnIn, Cu2MnSn,
- Ni2MnAl, Ni2MnIn, Ni2MnSn, Ni2MnSb, Ni2MnGa
- Co2MnAl, Co2MnSi, Co2MnGa, Co2MnGe
- Pd2MnAl, Pd2MnIn, Pd2MnSn, Pd2MnSb
- Co2FeSi, Fe3Si
- Fe2VAl
- Mn2VGa, Co2FeGe