Chirality in Magnetic Multilayers Probed by the Symmetry and the Amplitude of Dichroism in X-Ray Resonant Magnetic Scattering

Abstract

Chirality is central to understand many fundamental mechanisms in various domains of physics and chemistry. In condensed matter, a large variety of physical phenomena hinge on the emergence of these complex chiral windings of order parameters, their observation and subsequently their control especially in magnetism and spintransport at the nanoscale. The ability to probe the nature of these chiral magnetic textures has now become a crucial element of modern magnetism and is therefore essential to gain a deeper understanding of these mechanisms. Spin-polarized scanning tunneling microscopy (SP-STM) revealed that magnetic textures with a cycloidal configuration of the magnetization and Néel domain walls are stabilized in ultra-thin magnetic films (one or a few atomic layers) on heavy metal substrates [1] . It was realized that these magnetic textures are stabilized by Dzyaloshinskii-Moriya (DM) interaction [2,3], (∑ 〈 〉• × , with Si and Sj two neighboring spins) which is the anti-symmetric analog of the Heisenberg interaction favoring a curling magnetization textures around the DM vector . The DM interaction requires spin-orbit coupling (SOC) and broken inversion symmetry, found either in specific crystalline structures, such as B20 materials [4], or at film interfaces [5,6].In this letter, we demonstrate that XRMS experiments (Figure 1a) allow us to reveal the actual magnetic textures existing in ultrathin magnetic multilayers with perpendicular anisotropy and large interfacial chiral interaction. Importantly, this determination is straightforward and does not require any assumptions or careful comparison to parameter-dependent micromagnetic simulations. Moreover, the circular dichroism in XMRS enables to identify directly not only the direction but also the sense of the magnetic winding and therefore of the actual sign of DM interaction as we proved by a thorough analysis. This information has a far-reaching impact in particular for spin-orbitronics and spin-orbit torque (SOT) studies. Indeed, SOT is a recent very promising approach to move efficiently domain walls [7] and magnetic skyrmions along magnetic race-tracks and shows therefore a great potential for future spintronic devices. Yet, the detailed texture of the domain walls plays a major role in their motion [8]: the DW high speed is related to its nature (Néel or Bloch) while its direction of motion depends on its chirality [7]. Besides, DM interaction can play an important role for existing technological applications in spintronics already using ultrathin magnetic multilayers with perpendicular magnetic anisotropy such as MRAM or detectors, in which the presence and the impact of non-negligible DM interaction has been largely overlooked (see e.g. [9,10]), emphasizing thus the need to probe its consequences on magnetic textures. XRMS presents several advantages that might soon turn it to a standard way to characterize and quantify the DM interaction. First, XMRS is a non-perturbative technique contrary to the approach based on magnetic domain analysis th...

Chiral asymmetry detected in a 2D array of permalloy square nanomagnets using circularly polarized x-ray resonant magnetic scattering

“…The effect is entirely due to the term of intensity related to the interference between the charge and the magnetic scattering. This contrasts with the observations made in other magnetic systems in which their out-of-plane magnetization component has a significant magnitude, whose asymmetries in their magnetization distribution were detected solely by the pure magnetic scattering term [17,22]. The demonstrated sensitivity of CXRMS to the chirality of 2D magnetic vortex opens the door to the study of more complicated systems where the potential capabilities of the technique, like non-invasive detection, element sensitivity, time resolved dynamics and/or magnetic superlattice order sensitivity can be exploited to the study of 2D magnet arrays even at smaller scales than the presented in this experiment.…”

Reciprocal space tomography of 3D skyrmion lattice order in a chiral magnet

“…By changing the energy of the incident X-rays, we varied systematically the attenuation depth to perform a quasi-tomographic mapping of the magnetic state. This goes well beyond previous depth-dependent REXS studies, which pursued chemical modifications and the identification of the presence of magnetic phases only (12,14,45).…”

“…X-rays are perfectly matched to the length scales of the magnetic periodicities, suggesting that studies of the spin structure with increasing depth as controlled by the scattering angle or the photon energy should be possible (12). In addition, representing chiral spin textures, the dichroism of soft X-ray scattering permits reconstruction of the full 3D spin order, providing information on gradual variations (13,14). Last but not least, high-quality single crystals are available as a key precondition for meaningful experimental results.…”

Observation of magnetic skyrmions in unpatterned symmetric multilayers at room temperature and zero magnetic field

“…The iDMI emerges in HM/FM interfaces owing broken spatial inversion symmetry which determines its sign and direction, whilst the iDMI magnitude depends on the SOC [14][15][16]. In this sense, different combinations of HM/FM interfaces have been investigated to obtain strong iDMI amplitudes and distinct signs to stabilize skyrmions and define their chirality [17][18][19]. Although isolated skyrmions have been observed recently at room temperature in this kind of systems, their nucleation and stabilization in most cases require external magnetic field and/or electrical current [20][21][22].…”