The chalcogenides GeTe [1] and SnTe [2] are the prototypes of ferroelectric Rashba semiconductors (FERSC), a class of materials in which ferroelectricity is coupled to the Rashba spin texture, enabling the non-volatile ferroelectric control of spin-to-charge conversion [3]. While the properties of GeTe as FERSC have been widely investigated [4, 5], SnTe is mostly known as topological material, probably due to the low Curie temperature (100 K) as bulk ferroelectric. However, attempts to bring the critical temperature of ferroelectric SnTe to room temperature [6] are motivated by the prediction of giant intrinsic spin Hall conductivity in its Rashba phase [7], with the possibility of controlling the spin-to-charge conversion ferroelectrically. Here we propose to identify some conditions leading to stable ferroelectric SnTe at room temperature by exploiting the dipolar interaction with ferroelectric GeTe. First, we study the growth of epitaxial thin SnTe(111) films on GeTe(111)/Si(111) by molecular beam epitaxy. Then, by spin and angular photoemission spectroscopy experiments, we access the band dispersion of the SnTe in such heterostructure as a function of temperature. The work shows the coexistence of topological and Rashba states in SnTe(111), suggesting that the ferroelectric distortion of SnTe on GeTe survives up at room temperature, possibly thanks to the dipolar interaction with the ferroelectric GeTe underneath. We find a nice agreement with density functional theory calculations [2] which also predict the tunability of topological and Rashba bands with the orientation of the ferroelectric polarization. These preliminary results open the way to the investigation of SnTe/GeTe for reconfigurable spin-based transistors based on the ferroelectric control of the spin transport. Acknowledgements. C.R. and S.P. acknowledges the project TWEET, grant no. 2017YCTB59 by MIUR.

**References:**

[1] D. Di Sante et al., Adv. Mater., Vol. 25, p. 509 (2013). [2] E. Plekhanov, S. Picozzi et al., Phys. Rev. B, Vol. 90, p. 161108(R) (2014) [3] Paul Noël et al., Nature, Vol. 580, p. 483-486 (2020) [4] C. Rinaldi et al., Nano Letters, Vol. 18, p. 2751 (2018). [5] C. Rinaldi et al., APL Mater., Vol. 4, p. 032501 (2016). [6] K. Chang et al., Science, Vol. 353, p. 274 (2016) [7] H. Wang et al., npj Comput. Mater., Vol. 6, p. 7 (2020)

**Images:**

 Coexistence of topological and Rashba states in ferroelectric SnTe

Antiferromagnetic spintronics is on the rise as it could provide advantages over ferromagnetic-based conventional spintronics in terms of scalability, reliability and speed, opening the route to extend the working frequency of memory and logic devices from the current GHz range up to THz [1]. The electrical read-out of the magnetic state in antiferromagnetic materials, however, presents additional challenges due to the lack of a net magnetization which prevents the use of the typical detection schemes employed for ferromagnets. The possibility to access the magnetic configuration state in antiferromagnets by electrical methods would then provide a versatile characterization tool and is fundamental for the development of spintronic devices. At variance with ferromagnets, only few antiferromagnetic conductors have been demonstrated to show anisotropic magnetoresistance [2,3] allowing for a direct electric detection of the magnetic configuration by resistive measurements. In IrMn, the most widely used metallic antiferromagnet in applications, a small anisotropic magnetoresistance [4] has been demonstrated, whereas tunneling anisotropic magnetoresistance has been used to infer its magnetic state [5]. Here we follow a different approach exploiting interfacial effects between the antiferromagnet and a suitable neighboring metal layer. This approach, already employed for the insulating antiferromagnet Cr2O3 [6,7], underlines the critical role of interfaces in the determination of the transport properties related to the antiferromagnetic state. In this contribution we report the electrical detection of the antiferromagnetic state of IrMn through anomalous Hall measurements in Ta/IrMn heterostructures, grown by DC magnetron sputtering on a SiO2/Si substrate. The magnetic state is set in the antiferromagnet through out-of-plane field cooling and detected electrically by transverse resistance measurements in Hall bar structures, without the need of any ferromagnetic layer. An offset compensation system has been used to isolate the transverse resistance from spurious contributions (longitudinal resistance and magnetoresistance) [8]. The signal goes to zero at the Néel temperature of the antiferromagnet (TN), above which the system becomes paramagnetic. TN is found out to be 110 K for our 4 nm thick IrMn film, smaller than in the bulk (650 K) because of the finite thickness. The amplitude of the signal increases with the magnetic field applied during the cooling (whereas the value of TN is not affected) and is enhanced by the proximal interface with the Ta layer. We ascribe this enhancement to spin Hall magnetoresistance at room temperature and above, and to magnetic proximity which is the leading term at lower temperatures. From the temperature dependence of the effect and the comparison between Ta/IrMn and Ru/IrMn interfaces, we propose an explanation of such readout based on the simultaneous occurrence of spin-Hall magnetoresistance and magnetic proximity effect in Ta. These findings highlight how interface effects could be generally employed for the investigation of antiferromagnetic materials as well as for the electrical readout of the antiferromagnetic state.

**References:**

[1] T. Jungwirth, J. Sinova, A. Manchon et al, Nat. Phys, Vol. 14, p. 200 (2018) [2] S.Y. Bodnar, L. Smejkal, I. Turek et al, Nat. Commun., Vol. 9, p. 348 (2018) [3] P. Wadley, B. Howells, J. Zelezný et al, Science, Vol. 351, p. 587 (2016) [4] T. Kosub, M. Kopte, F. Radu et al, Phys. Rev. Lett., Vol. 115, p. 097201 (2015) [5] D. Petti, E. Albisetti, H. Reichlová et al, Appl. Phys. Lett., Vol. 102, p. 192404 (2013) [6] J.M. Taylor, E. Lesne, A. Markou et al, Appl. Phys. Lett., Vol. 115, p. 062403 (2019) [7] M. Asa, C. Autieri, R. Pazzocco, et al., Phys. Rev. Research, vol. 2, p. 043394 (2020) [8] M. Asa, C. Rinaldi, R. Pazzocco, et al., J. Appl. Phys., Vol. 128, p. 053904 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/32c3006792ed14c06dba33f4b5099a58.jpg)
(a) Transverse resistance (RXY) in Ta (2 nm)/IrMn (4 nm) /Ta (2 nm) Hall cross at remanence resulting from different field cooling processes. In the inset a sketch of the sample is reported. (b) Anomalous Hall resistance difference (ΔRXY) at remanence between states set with upward (+Bset) and downward (-Bset) field cooling. The value of Bset is reported beside each curve. The inset reports the value of ΔRXY at 10 K as function of the absolute value of the writing field Bset. The red line is a guide for the eye.

 Electrical Readout of the Antiferromagnetic State of IrMn through Anomalous Hall Effect

Ferroelectric switching of spin-to-charge conversion in GeTe

The spin to charge interconversion has been intensively studied in 5d materials such as platinum. In these non-magnetic systems, the relation between the charge current, the spin current and its polarization is strongly constrained by symmetries. Recently, a strong spin to charge interconversion efficiency has been obtained in 3d ferromagnets with strong spin orbit interactions such as NiCu. In ferromagnetic materials, the symmetry breaking induced by the presence of the magnetization lifts the geometrical constraints on the relation between charge and spin currents. However, the link between the ferromagnetic order and spin to charge interconversion remains unclear. By using spin pumping measurements, we injected a spin current from a CoFeB layer into the weak ferromagnets Ni60Cu40 and Ni50Cu50. The charge current generated by the spin to charge interconversion was subsequently measured across the Curie temperatures (Tc) of these alloys. A change of sign of the generated spin current was observed across the Tc when the CoFeB and NiCu layers are in direct contact. However, this sign change disappears when inserting a copper layer between the two. This indicates that two ferromagnetic materials can be coupled and influence each other at the Ferromagnetic Resonance (FMR), thus questioning the results of studies claiming the observation of spin fluctuation and spin-charge interconversion using FMR measurements. Most importantly, the fact that the generated charge current does not change across the Curie temperature of NiCu alloys indicates that the spin-charge interconversion does not depend on the magnetic phase [1]. The independence of the interconversion on the magnetic phase shows either that the band structure of NiCu is not affected, or that its change across the Tc does not influence the spin-charge interconversion. These results consequently provide an important contribution to both the experimental and theoretical understanding of the spin to charge interconversion in ferromagnetic materials.

Christian Rinaldi

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