Heusler alloys: A hot topic

Heusler alloys: A hot topic

A-Heusler there matey

Heusler materials are becoming increasingly interesting for use in spintronics.  These materials have X2YZ stoichiometry (where Y is a magnetic ion) and a Heusler phase structure.  The wide choice of Heusler materials offers alloys with a range of properties relevant for different types of spintronics experiments and applications, and it is important to characterise many of these alloys to allow researchers to choose the material that best suits their needs.

 Counting the caloritronics

Spin caloritronics is a developing area of research that studies the link between spin and heat, in the same way that spintronics studies the link between spin and charge.  Spin caloritronics is especially exciting as a possible means to practically harvest what would otherwise be waste heat that is generated in devices that utilise charge currents in their operation.  The paper that is the focus of this week’s Pick of the Week, “Spin-related thermoelectric conversion in lateral spin-valve devices with single-crystalline Co2FeSi electrodes” in Applied Physics Express by Yamasaki et al., investigates spin caloritronics phenomena in the crystalline Heusler alloy Co2FeSi.


Perhaps the most well studied phenomena in spin caloritronics is the spin Seebeck effect (SSE) which has been observed in magnetic metals, semiconductors, and insulators.  The SSE is (simplistically) the generation of a spin current that results from a temperature gradient within a material.  More recently its Onsager reciprocal effect was discovered.  Dubbed the spin Peltier effect (SPE), this phenomenon is the generation of a temperature gradient in a material due to the injection of a spin current.  A study last year suggested that the polycrystalline ferromagnetic alloy CoFeAl has a thermal spin injection efficiency that is 20 times greater than Py, which obviously makes it appealing from a spin caloritronics applications perspective.  This high efficiency was attributed to its unique band structure giving the spin-dependent Seebeck coefficient to be opposite in sign for up and down electronics, which is not the case for Py.  Crystalline materials offer a more controllable route to controlling the band structure, especially in the Heusler class where there is much choice for stoichiometric composition.  There has, however, yet to be any reports of SSE or SPE in crystalline Heusler allows, and this gap in literature is the motivation for the work of Yamasaki and colleagues.


Heat seeking

Yamasaki et al. chose Co2FeSi as their Heusler alloy as it has previously been shown to have a high spin-injection efficiency for pure spin currents generated electrically.  A 25nm thick sample was grown by molecular beam epitaxy, with the RHEED signal indication high quality crystalline growth.  The sample was process by e-beam lithography into a lateral spin valve (LSV) configuration where a non-magnetic Cu connects two Co2FeSi electrodes.  This LSV arrangement allows for non-local voltage measurements to be made on one electrode for a spin/heat current generated at the interface of Cu with the other Co2FeSi electrode.


For the SSE measurements, a temperature gradient between one of the Co2FeSi electrodes and Cu was established via Joule heating from an AC current.  An external magnetic field was applied to the device and swept which reversed the magnetization orientation of Co2FeSi.  The resulting switch in the non-locally detected voltage (along with several other experimental observations) confirmed the presence of the SSE.  By applying a 1D spin model to the experimental data, the spin-dependent Seebeck coefficient was found to be opposite in sign for the spin up and spin down electrons, as is the case for CoFeAl.  While the overall spin-dependent Seebeck coefficient of Co2FeSi is large compared to Py, it is still smaller than CoFeAl, most likely due to its smaller density of states gradient at the Fermi level.


For the SPE measurements, a pure spin current was generated electrically at one of the Co2FeSi electrodes and thus injected into the Cu.  A thermocouple placed at the other Co2FeSi electrode detected the resulting temperature gradient at the interface between the Cu and the other Co2FeSi electrode which was found to be on the order of tens of mK.  Again various checks confirmed the SPE origin of the temperature gradient, while also supporting the scenario of different spin-dependent Seebeck coefficient signs for up and down spins.


SSE: A renewable energy provider

The observation of sizeable SSE and SPE signals in a crystalline Heusler alloy are a promising start when considering these materials for potential application.  The focus should now be on further characterisation of the spintronic and spin caloritronic properties of other crystalline Heusler alloys, each with unique spin resolved band structure.  Such studies will not only reveal further information on how band structure effects spin caloritronic phenomena to allow for greater material optimisation, but it will also establish a database of Heusler materials for researchers to pick from.  An ideal scenario would be a combined spintronic/spin caloritronic device that offers duel functionality.  For example, electrical currents could generate spin currents that torque the moments of the Heusler material (which could have memory applications), while the waste heat from this process could be recycled via the SSE to yield another spin current (which could be used to torque a separate magnetic region of the device).  The compatibility of Hesuler alloys for both spintronic and spin caloritronic experiments would help to keep the sample layer structure and processing steps fairly simple.


Image: Rob Stuck, some rights reserved



Kento Yamasaki1, Soichiro Oki1, Shinya Yamada1, Takeshi Kanashima1, and Kohei Hamaya1 (2015). Spin-related thermoelectric conversion in lateral spin-valve devices with single-crystalline Co2FeSi electrodes
Applied Physics Express, 8