News and posts

Jakub Zelezny

Spin currents are one of the key concepts of spintronics. In the past, two types of spin currents have been predominantly discussed and utilized: the spin-polarized current in ferromagnetic materials and the spin Hall effect. The spin-polarized current only exist in magnetic materials, has a non-relativistic origin and flows in the same direction as the charge current. In contrast, the spin Hall effect exists also in non-magnetic materials, has typically a relativistic origin and is transverse to the charge current. In recent years it has been discovered, however, that the phenomenology of spin currents is much richer. We have shown that the spin-polarized current can also exist in some antiferromagnetic materials and that a new type of spin Hall effect exists, which has origin in the magnetic order, and occurs in ferromagnetic and some antiferromagnetic materials [1]. This effect is now referred to as the magnetic spin Hall effect and has been recently experimentally demonstrated in non-collinear antiferromagnet Mn3Sn [2]. We have also shown that the conventional spin Hall effect can exist in some non-collinear magnetic systems even in absence of the relativistic spin-orbit interaction [3].
Furthermore, we have found that a non-relativistic magnetic spin Hall effect can exist in a collinear antiferromagnet [4]. Such system is distinct from systems where the transverse spin currents have been previously discussed because in the non-relativistic limit it conserves spin and thus it allows for spin-charge conversion in a spin-conserving system. Here we review the various types of spin currents that can occur in magnetic systems and give general conditions for their existence as well as a symmetry classification. In addition, we present calculations of these novel spin currents in various collinear and non-collinear antiferromagnets.

[1] J. Železný et al., Phys. Rev. Lett. 119, 187204 (2017)
[2] M. Kimata et al., Nature 565, 627–630 (2019)
[3] Y. Zhang et al: New J. Phys. 20 ( 2018 )
[4] R. G. Hernández el., arXiv:2002.07073 (2020)

Yuki Shiomi

Magnetopiezoelectric effect [1,2], which refers to a linear strain response to electric currents and its inverse response in low-symmetric magnetic metals, is a generalization of magnetoelectric effects in insulators to metals. In metallic materials with high conduction-electron densities, static (dc) piezoelectric responses are not allowed, even if the metals have a symmetry group low enough to support a static polarization. This is because the static surface charge density is screened out by bulk conduction electrons. However, it was recently proposed [1] that dynamic distortion can arise in response to electric currents without screening effects in antiferromagnetic metals that simultaneously break time-reversal and spatial-inversion symmetries. Note that another magnetopiezoelectric effect of a topological origin has also been proposed recently [2].
Here, we have experimentally studied the magnetopiezoelectric effect [3-5] in antiferromagnetic conductors with low crystal symmetries: EuMnBi2 [3,5] (TN = 315 K) and CaMn2Bi2 [4] (TN = 150 K). Using laser Doppler vibrometry at low temperatures, we found that dynamic displacements emerge along the [110] direction upon application of ac electric currents in the c direction in EuMnBi2 below TN [3,5]. The displacement signals showing up in response to the electric current increase in proportion to the applied electric currents. We confirmed that such displacements are not observed along the c direction of EuMnBi2 or EuZnBi2 with nonmagnetic Zn ions, consistent with the symmetry requirement of the magnetopiezoelectric effect [1]. As temperature increases from the lowest temperature, the displacement signals decrease monotonically, showing that magnetopiezoelectric signals are larger for higher conductivity states as opposed to the conventional piezoelectric effect.

[1] H. Watanabe and Y. Yanase, Phys. Rev. B 96, 064432 (2017).
[2] D. Varjas, A. G. Grushin, R. Ilan, and J. E. Moore, Phys. Rev. Lett. 117, 257601 (2016).
[3] Y. Shiomi, H. Watanabe, H. Masuda, H. Takahashi, Y. Yanase, and S. Ishiwata, Phys. Rev. Lett. 122, 127207 (2019).
[4] Y. Shiomi, Y. Koike, N. Abe, H. Watanabe, and T. Arima, Phys. Rev. B 100, 054424 (2019).
[5] Y. Shiomi, H. Masuda, H. Takahashi, and S. Ishiwata, Sci. Rep. 10, 7574 (2020).

James M. Taylor

The field of antiferromagnetic spintronics is based on recent developments in the manipulation and detection of antiferromagnetic properties using electrical methods, opening up the possibility of these materials evolving from passive to active components of spintronic devices. Doing so offers a number of advantages, such as improved stability, reduction in stray fields and increased speed of dynamics. However, changes in the orientation of typical antiferromagnets’ Néel vectors do not produce read-out signals of the size required for applications. Topological antiferromagnets may offer the solution.

In this talk, we focus on the particular example of noncollinear antiferromagnets of the type Mn3X, whose chiral spin textures break time- and inversion-symmetries, leading to novel magnetotransport properties driven by momentum-space Berry curvature. These include a large intrinsic anomalous Hall effect [1], anomalous Nernst effect [2], and both intrinsic- [3] and magnetic-spin Hall effects [4].

However, for the utilization of these Berry curvature generated phenomena in topological antiferromagnetic spintronic devices, further exploration of the behavior of Mn3X materials in thin film form is required. We therefore present results from our recent work, where we grow high-quality thin films of such noncollinear antiferromagnets with different crystal structures by exploiting epitaxial engineering. Specifically, we demonstrate the thin film deposition of two distinct varieties of noncollinear antiferromagnet: Mn3Ir, with a cubic structure [5], and Mn3Sn, with a hexagonal structure [6].

The crystal structure of the films is characterized using a combination of x-ray diffraction and transmission electron microscopy, whilst their magnetic properties are studied using vibrating sample magnetometry and x-ray magnetic circular dichroism experiments. In doing so, we illuminate the important role played by uncompensated moments in both materials, exploring how these are affected by sample microstructure and how, in turn, they affect antiferromagnetic domain distribution.
Such chiral domains play a key role in governing topological magnetotransport in these compounds. We elucidate this by measuring the Hall effect in lithographically patterned samples of both Mn3Ir and Mn3Sn, and find very different behavior in both cases. Whilst Mn3Ir shows a small conventional anomalous Hall effect [7], we observe a large anomalous Hall effect in Mn3Sn. Films down to 30 nm in thickness demonstrate an anomalous Hall conductivity of σxy (µ0H = 0 T) = 21 Ω-1cm-1, which we attribute to a Berry curvature mechanism [8]. Following cooling of Mn3Sn below its transition temperature into a glassy ferromagnetic state, we identify a change in transport behavior from anomalous to topological Hall effects.
Finally, we bring noncollinear antiferromagnets closer to functionality by moving to investigate the generation and interaction of spin currents in our thin films. Specifically, we use spin-torque ferromagnetic resonance measurements in Mn3X / NiFe bilayers to quantify their spin Hall angle. Significant charge-to-spin current conversion is identified, which depends intimately on epitaxial growth conditions, thin film magnetic state, and chiral domain structure. We conclude by discussing the origin of these different phenomena, and the potential for Mn3X materials to be used in chiralitronic devices.

[1] S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015)
[2] H. Reichlová et al., Nature Communications 10, 5459 (2019)
[3] W. Zhang, W. Han, S. H. Yang, Y. Sun, Y. Zhang, B. Yan, and S. S. P. Parkin, Science Advances 2, e1600759 (2016)
[4] M. Kimata et al., Nature 565, 627 (2019)
[5] J. M. Taylor et al., Physical Review Materials 3, 074409 (2019)
[6] A. Markou, J. M. Taylor, A. Kalache, P. Werner, S. S. P. Parkin, and C. Felser, Physical Review Materials 2, 051001(R) (2018)
[7] J. M. Taylor et al., Applied Physics Letters 115, 062403 (2019)
[8] J. M. Taylor, A. Markou, E. Lesne, P. K. Sivakumar, C. Luo, F. Radu, P. Werner, C. Felser, and S. S. P. Parkin, Physical Review B 101, 094404 (2020)

Andrew Ross

With spin dynamics in the THz regime, stability in the presence of external magnetic fields, and a lack of stray fields, antiferromagnetic materials are positioned to become key in future low power spintronic devices [1]. Here, we grow and investigate high quality thin films of hematite (α-Fe2O3) (< 500 nm) of different orientations. Through measurements of the spin Hall magnetoresistance in hematite/Pt bilayers, the magnetic anisotropies of the thin films can be extracted, and the critical temperature of the Morin transition from the easy plane to the easy axis antiferromagnetic phase is electrically observed [2, 3]. Whilst a key part of antiferromagnetic spintronics is to encode and read information in the Néel vector, the efficient transfer of information is crucial for integration of antiferromagnets into devices. Recently, we demonstrated that a diffusive magnon current can be carried over micrometres in antiferromagnetic single crystals, but such crystals are not suitable for spintronic devices [4,5]. Despite theoretical works investigating the mechanisms for the long-distance propagation of pure spin currents carried by the antiferromagnetic order [6, 7], studies on thin film antiferromagnets making use of single-frequency or broadband excitations have failed to achieve efficient transport of angular momentum by magnons [8]. Making use of hematite thin films, a robust magnon current can propagate with intrinsic diffusion lengths of hundreds of nanometres. The efficiency of the transport mechanisms can be tuned by field cycling of the domain structure, the growth orientation, and the relative orientations of the magnetic field and magnetic anisotropies. The manner by which the stabilisation of the antiferromagnetic domain structure (see Fig. 1) results in frequency dependent length scales and proves to be critical in the magnon transport will be discussed [9].

[1] T. Jungwirth et al., Nat. Phys. 14, 200-203 (2018)
[2] R. Lebrun et al., Comm. Phys. 2 50 (2019)
[3] A. Ross et al., arXiv:2001.03117 (2020)
[4] R. Lebrun, A. Ross et al., Nature 561, 222-225 (2018)
[5] R. Lebrun et al., arxiv: 2005.14414 (2020)
[6] S. Takei et al., Phys. Rev. B, 90, 94408 (2014)
[7] S. Bender et al., Phys. Rev. Lett. 19, 056804 (2017)
[8] H. Wang et al., Phys. Rev. B 91, 220410 (2015)
[9] A. Ross, R. Lebrun et al., Nano Lett. 20 1, 306-313 (2020)

Kai Litzius

Magnetic skyrmions are topologically stabilized spin configurations that, like domain walls (DWs), can react to external stimuli by collective displacement, which is both physically intriguing and bears promises to realize next generation non-volatile data storage technologies. [1] However, skyrmions in ferromagnets move at an angle with respect to the current direction, which complicates the use of skyrmions in wire devices because the motion component perpendicular to the current can move the skyrmion to a wire edge and thereby annihilate it. [2] Antiferromagnetically coupled systems with compensated angular momentum (such as compensated ferrimagnets and natural antiferromagnets) can reduce this skyrmion Hall effect to zero and could additionally provide high speed dynamics to move spin structures at unprecedented speeds. [3,4] Skyrmions are predicted to move at even higher speeds in these materials, thus making these materials challenging but promising candidates for future spintronic devices.
Besides the compensation of perpendicular motion of skyrmions with respect to the drive, the predictability of their trajectories is also of major importance. Analytical equations of motion describe straight 180° DWs in the one-dimensional (1D) model while rigid, circular bubble domains and skyrmions are predicted to move according to the Thiele equation. [5] However, DWs and skyrmions are often not perfectly straight or circular. Here, we study how strongly deformed DWs and bubble skyrmions move in uncompensated ferrimagnetic Pt/CoGd/W in response to current pulses. We find that all 1D spin textures as well as all fully enclosed spin textures, reach speeds >500 m/s and display identical dynamics. While high speeds are indeed reached, the predicted differences between skyrmion and DW dynamics could not be observed. We attribute this deviation from the commonly used model to significant deformations of the skyrmions during their motion.

[1] K. Everschor-Sitte et al., Journal of Applied Physics 124, 240901 (2018)
[2] W. Jiang et al., Physics Reports 704, 1-49 (2017)
[3] S. Woo et al., Nature Communications 9, 959 (2018)
[4] L. Caretta et al., Nature Nanotechnology 13, 1154 (2018)
[5] F. Büttner, I. Lemesh & G. S. D. Beach, Scientific Reports 8, 4464 (2018)