by Alessandro Sola (INRiM)


Spin is the intrinsic angular momentum that elementary particles might exhibit. Particles having spin have magnetic moment too; this can be observed when particles are under the effect of an external magnetic field. Since solids contain particles that may have spins (magnetic moment), some effects may rise in the mesoscopic world, like spin-polarized current (of electrons), pure spin currents or spin waves (magnons), magnetic wall motions or in the macroscopic world in terms of magnetization.



Spin transfer torque

Spin transfer torque is one of the most interesting effects regarding a spin current in a magnetic material. Thanks to this effect, it is possible to modify the magnetization of a thin layer of a magnetic material by a spin current. This effect is electrically detectable by the giant magnetoresistance effect (GMR), a phenomenon that was observed for the first time by Fert and Grunberg (Nobel prize for Physics in 2007). GMR effect consists in the resistance variation of a layered structure formed by the film whose magnetization is changed by the spin current and another layer whose magnetization remains unperturbed.



Spin pumping

Spin pumping is one of the methods of generating a spin current from a magnetic material. The mechanism exploits the precessing magnetization of a ferromagnetic material in condition of resonant absorption of microwaves. A spin current rises in a paramagnetic layer connected to a ferromagnetic layer in these conditions. From an ideal point of view, spin pumping can be considered as the inverse effect of spin transfer torque. In fact, in spin transfer torque a spin current acts on the magnetization of a given material, while in spin pumping a given material properly magnetized is put under conditions to generate a spin current.



Magnetic domain wall

In the framework of the microscopic description of magnetism, domain theory asserts the existence of regions of uniform magnetization that are separated by surfaces in which the magnetization changes direction. If these regions are confined in a low dimension object like a wire, the motion of the domain wall inside it can be used as a logic switch. Such a device is able to do the same work of a field effect transistor but with more efficient speed and power consumption.




The word “spintronics” derives from spin-electronics and it refers to the possibility of using both the charge and the spin in a device that processes data as electronic devices are doing. For this purpose it is necessary to apply some new concepts of solid state physics and magnetism, including the aforementioned spin transfer torque, spin pumping and domain wall motion. Spintronics is promising to lead some advantages, compared to standard electronics, which relies on the presence of an electric current density.In particular a larger efficiency due to the lower energy range required for spin manipulation allows a better scalability . A device that is able to perform some logic operation or store data without the support of electric charge can be smaller and faster as is not affected by the issue of ohmic energy dissipation associated with the motion of charge. Some commercially available devices as Hard disk read heads, and MRAM (magnetoresistive random access memory) are already exploiting these new concepts.




This word refers to the phenomena in which an interaction between heat currents and spins occurs in a given material. A class of phenomena relating the interaction between heat currents and electric currents was already observed since the nineteenth century and is called thermoelectricity. Spin-caloritronics can be considered the “spin counterpart” of thermoelectricity and it refers to a class of phenomena that can be observed in ferrimagnetic insulators like the spin-Seebeck effect. This last was first observed in 2008 and represented a breakthrough for spin-caloritronics with the discovery of many other related effects.



Spin-Seebeck effect

The spin Seebeck effect refers to the rising of a spin accumulation in a magnetic material under a thermal gradient. Spin Seebeck effect has strong importance because it opened the route to the family of spin caloritronics effects and it allows the conversion of a heat current into a pure spin current. This means that spin current is not mediated by other carriers like electrons and is easily observed in insulators but can occur also in metals and semiconductors. The application of this effect in the framework of Spintronics could be a spin current generator that works as a thermopile.



Spin-Hall effect

Among the family of spin effects in solids, a key role is played by the relationship between pure spin current and charge current. As the interplay between heat current and spin current without the mediation of charge dominates the spin Seebeck effect, in materials that exhibit strong spin-orbit coupling it is possible to observe a pure spin current generation as consequence of electric current. The phenomenon is called spin Hall effect and his inverse process (inverse spin Hall effect) is especially useful to make a pure spin current electrically detectable.



Spin-Peltier effect

As happens in thermoelectricity, the inverse of Seebeck effect is the Peltier effect and his spin counterpart is responsible for the presence of a thermal gradient as consequence of a spin current injection in a magnetic material by spin Hall effect. The observation of this effect corresponds to the possibility to build the reciprocity matrix for charge, heat and spins. This model allows the estimation of the currents from the driving forces in a reversible way for a system that contains these three elements.



Thermoelectric generators (based on spin effects)

Thermoelectric generators are devices that can convert thermal gradients into electric energy. The efficiency of such a generator depends on the electrical conductivity and on the thermal resistivity of the material used to fabricate the device. Unfortunately, materials with high electrical conductivity have low thermal resistivity and vice versa, and the compromise is very narrow and limited to a few semiconducting materials; the conversion efficiency of thermoelectric generators is limited by this characteristic. A possible solution to this issue is to have a device in which electrical and thermal transport occur in two separated channels. From this perspective, spin-caloritronics can play a key role since the thermal effects (spin-Seebeck) involve the magnetic insulator with high thermal resistivity and the electrical effect (inverse spin-Hall effect) regard the metal with high spin orbit coupling and high electrical conductivity. These two channels are related by the pure spin current injection from the magnetic insulator to the metal.