Helimagnetism is a form of magnetic ordering where spins of neighbouring magnetic moments arrange themselves in a spiral or helical pattern, with a characteristic turn angle of somewhere between 0 and 180 degrees. It results from the competition between ferromagnetic and antiferromagnetic exchange interactions.[ citation needed ] It is possible to view ferromagnetism and antiferromagnetism as helimagnetic structures with characteristic turn angles of 0 and 180 degrees respectively. Helimagnetic order breaks spatial inversion symmetry, as it can be either left-handed or right-handed in nature.
Strictly speaking, helimagnets have no permanent magnetic moment, and as such are sometimes considered a complicated type of antiferromagnet. This distinguishes helimagnets from conical magnets, (e.g. Holmium below 20 K [1] ) which have spiral modulation in addition to a permanent magnetic moment.
Helimagnetism was first proposed in 1959, as an explanation of the magnetic structure of manganese dioxide. [2] Initially applied to neutron diffraction, it has since been observed more directly by Lorentz electron microscopy. [3] Some helimagnetic structures are reported to be stable up to room temperature. [4] Like how ordinary ferromagnets have domain walls that separate individual magnetic domains, helimagnets have their own classes of domain walls which are characterized by topological charge. [5]
Many helimagnets have a chiral cubic structure, such as the FeSi (B20) crystal structure type. In these materials, the combination of ferromagnetic exchange and the Dzyaloshinskii–Moriya interaction leads to helixes with relatively long periods. Since the crystal structure is noncentrosymetric even in the paramagnetic state, the magnetic transition to a helimagnetic state does not break inversion symmetry, and the direction of the spiral is locked to the crystal structure.
On the other hand, helimagnetism in other materials can also be based on frustrated magnetism or the RKKY interaction. The result is that centrosymmetric structures like the MnP-type (B31) compounds can also exhibit double-helix type helimagnetism where both left and right handed spirals coexist. [6] For these itinerant helimagnets, the direction of the helicity can be controlled by applied electric currents and magnetic fields. [7]
Material | Temperature range | Space group |
---|---|---|
β-MnO2 [2] [8] | < 93 K | P42/mnm |
FeGe, [4] | < 278 K | P213 |
MnGe [9] | < 170 K | P213 |
MnSi, [10] | < 29 K | P213 |
FexCo1−xSi (0.3 ≤ x ≤ 0.85) [11] [12] | P213 | |
Cu2OSeO3 [13] | < 58 K | P213 |
FeP [6] | < 120 K | Pnma |
FeAs [14] | < 77 K | Pnma |
MnP [15] | < 50 K | Pnma |
CrAs [16] | < 261 K | Pnma |
FeCl3 [17] | < 9 K | R3 |
NiBr2 [18] | < 22 K | R3m |
NiI2 [19] | < 75 K | R3m |
Cr1/3NbS2 [20] [21] | < 127 K | P6322 |
Tb [22] | 219–231 K | P63/mmc |
Dy [23] | 85–179 K | P63/mmc |
Ho [24] | 20–132 K | P63/mmc |
Spintronics, also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. The field of spintronics concerns spin-charge coupling in metallic systems; the analogous effects in insulators fall into the field of multiferroics.
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism. The phenomenon of antiferromagnetism was first introduced by Lev Landau in 1933.
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers, practical magnetic semiconductors would also allow control of quantum spin state. This would theoretically provide near-total spin polarization, which is an important property for spintronics applications, e.g. spin transistors.
Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties in the same phase:
Exchange bias or exchange anisotropy occurs in bilayers of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exchange bias phenomenon is of tremendous utility in magnetic recording, where it is used to pin the state of the readback heads of hard disk drives at exactly their point of maximum sensitivity; hence the term "bias."
A spin ice is a magnetic substance that does not have a single minimal-energy state. It has magnetic moments (i.e. "spin") as elementary degrees of freedom which are subject to frustrated interactions. By their nature, these interactions prevent the moments from exhibiting a periodic pattern in their orientation down to a temperature much below the energy scale set by the said interactions. Spin ices show low-temperature properties, residual entropy in particular, closely related to those of common crystalline water ice. The most prominent compounds with such properties are dysprosium titanate (Dy2Ti2O7) and holmium titanate (Ho2Ti2O7). The orientation of the magnetic moments in spin ice resembles the positional organization of hydrogen atoms (more accurately, ionized hydrogen, or protons) in conventional water ice (see figure 1).
Spin pumping is the dynamical generation of pure spin current by the coherent precession of magnetic moments, which can efficiently inject spin from a magnetic material into an adjacent non-magnetic material. The non-magnetic material usually hosts the spin Hall effect that can convert the injected spin current into a charge voltage easy to detect. A spin pumping experiment typically requires electromagnetic irradiation to induce magnetic resonance, which converts energy and angular momenta from electromagnetic waves to magnetic dynamics and then to electrons, enabling the electronic detection of electromagnetic waves. The device operation of spin pumping can be regarded as the spintronic analog of a battery.
Gallium manganese arsenide, chemical formula (Ga,Mn)As is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide,, and readily compatible with existing semiconductor technologies. Differently from other dilute magnetic semiconductors, such as the majority of those based on II-VI semiconductors, it is not paramagnetic but ferromagnetic, and hence exhibits hysteretic magnetization behavior. This memory effect is of importance for the creation of persistent devices. In (Ga,Mn)As, the manganese atoms provide a magnetic moment, and each also acts as an acceptor, making it a p-type material. The presence of carriers allows the material to be used for spin-polarized currents. In contrast, many other ferromagnetic magnetic semiconductors are strongly insulating and so do not possess free carriers. (Ga,Mn)As is therefore a candidate as a spintronic material.
Ferromagnetic superconductors are materials that display intrinsic coexistence of ferromagnetism and superconductivity. They include UGe2, URhGe, and UCoGe. Evidence of ferromagnetic superconductivity was also reported for ZrZn2 in 2001, but later reports question these findings. These materials exhibit superconductivity in proximity to a magnetic quantum critical point.
The term magnetic structure of a material pertains to the ordered arrangement of magnetic spins, typically within an ordered crystallographic lattice. Its study is a branch of solid-state physics.
Superexchange or Kramers–Anderson superexchange interaction, is a prototypical indirect exchange coupling between neighboring magnetic moments by virtue of exchanging electrons through a non-magnetic anion known as the superexchange center. In this way, it differs from direct exchange, in which there is direct overlap of electron wave function from nearest neighboring cations not involving an intermediary anion or exchange center. While direct exchange can be either ferromagnetic or antiferromagnetic, the superexchange interaction is usually antiferromagnetic, preferring opposite alignment of the connected magnetic moments. Similar to the direct exchange, superexchange calls for the combined effect of Pauli exclusion principle and Coulomb's repulsion of the electrons. If the superexchange center and the magnetic moments it connects to are non-collinear, namely the atomic bonds are canted, the superexchange will be accompanied by the antisymmetric exchange known as the Dzyaloshinskii–Moriya interaction, which prefers orthogonal alignment of neighboring magnetic moments. In this situation, the symmetric and antisymmetric contributions compete with each other and can result in versatile magnetic spin textures such as magnetic skyrmions.
Mitsutaka Fujita was a Japanese physicist. He proposed the edge state that is unique to graphene zigzag edges. Also, he theoretically pointed out the importance and peculiarity of nanoscale and edge shape effects in nanographene. The theoretical concept of graphene nanoribbons was introduced by him and his research group to study the nanoscale effect of graphene. He was an associate professor at Tsukuba University, and died of a subarachnoid hemorrhage on March 18, 1998. His posthumous name is Rikakuin-Shinju-Houkou-Koji (理覚院深珠放光居士) in Japanese.
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Mohindar Singh Seehra is an Indian-American Physicist, academic and researcher. He is Eberly Distinguished Professor Emeritus at West Virginia University (WVU).