Magnetization-Direction-Dependent Local Electronic Structure Probed by Scanning Tunneling Spectroscopy

Understanding the role of spin-orbit coupling (SOC) has been crucial to controlling magnetic anisotropy in magnetic multilayer films [1][2][3][4]. It has been shown that electronic structure can be altered via interface SOC by varying the superlattice structure, resulting in spontaneous magnetization perpendicular or parallel to the plane [5,6]. In lieu of magnetic thin films, we study the similarly anisotropic helimagnet Cr 1/3 NbS2, where the spin polarization direction, controlled by the applied magnetic field, can modify the electronic structure. As a result, the direction of spin polarization can modulate the density of states, and in turn affect the in-plane electrical conductivity. In Cr 1/3 NbS2, we found an enhancement of in-plane conductivity when the spin polarization is out-of-plane, as compared to in-plane spin polarization. This is consistent with the increase of density of states near the Fermi energy at the same spin configuration, found from first principles calculations. We also observe unusual field dependence of the Hall signal in the same temperature range. This is unlikely to originate from the non-collinear spin texture, but rather further indicates strong dependence of electronic structure on spin orientation relative to the plane. PACS numbers:Despite the fact that its typical energy scale in 3d ferromagnetic metals is small compared to other relevant scales such as band widths, SOC mixes the nature of the spin and orbital components of the Bloch state in a nontrivial way and leads to a variety of electrical transport phenomena e.g. the anomalous Hall effect (AHE), anisotropic magnetoresistance (AMR), and the planar Hall effect. In addition, the recent work in non-collinear magnetically ordered states and the related topological Hall effect [7][8][9] not only has renewed the pivotal role of SOC through the Dzyaloshinskii-Moriya (DM) interaction [10][11][12], but also has presented a possibility to employ these findings for functional components in magnetic devices [13,14]. Non-collinear magnetic ordering is also suggested to possibly manifest spin-orbit coupling in a complex manner, through the DM interaction [12,15,16]. Consequently, the modification of electronic structure by spin-orbit coupling is expected to make in-plane electrical transport sensitive to the magnetization orientation relative to the plane.Cr 1/3 NbS 2 has a layered crystalline structure, in which 3d transition metal Cr atoms are intercalated in the hexagonal 2H-type NbS 2 matrix as trivalent ions and magnetically order at T C = 133 K. The ferromagnetic layers of Cr 3+ lie coplanar with the crystallographic abplanes and the magnetic helix propagates along the caxis with a long pitch of 48 nm, corresponding to 40 unit cells [17]. Its helimagnetic ordering is attributed to the DM interaction, which originates from a broken inversion symmetry shared by all members of space group P 6 3 22 [17][18][19].

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Out-of-plane spin-orientation dependent magnetotransport properties in the anisotropic helimagnetCr1/3NbS2

et al. 2015

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“…Therefore, they do not affect the position or the measured width of magnetic domain walls or magnetic vortices [21]. Moreover, it has been demonstrated that even a non-magnetic probe tip may be able to image domain walls since the local electronic structure is modified within the domain wall due to the local change of the magnetization direction and the accompanied change of the spin-orbit coupling between electronic states [22,23].To summarize this part, SP-STS allows us to enter a new regime of magnetic domain and domain wall observation at sub-nanometer scale spatial resolution which is not accessible by any other magnetic imaging technique up to now. …”

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Mapping spin structures on the atomic scale

Wiesendanger¹

2007

Europhysics News

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A fundamental understanding of magnetic and spin-dependent phenomena requires the determination of spin structures and spin excitations down to the atomic scale. The direct visualization of atomic-scale spin structures [1][2][3][4] has first been accomplished for magnetic metals by combining the atomic resolution capability of Scanning Tunnelling Microscopy (STM) [5,6] with spin sensitivity, based on vacuum tunnelling of spin-polarized electrons [7]. The resulting technique, SpinPolarized Scanning Tunnelling Microscopy (SP-STM), nowadays provides unprecedented insight into collinear and non-collinear spin structures at surfaces of magnetic nanostructures and has already led to the discovery of new types of magnetic order at the nanoscale [8]. More recently, the detection of spin-dependent exchange and correlation forces has allowed a first direct real-space observation of spin structures at surfaces of antiferromagnetic insulators [9]. This new type of scanning probe microscopy, called Magnetic Exchange Force Microscopy (MExFM), provides a powerful new tool to investigate different types of spin-spin interactions based on direct-, super-, or RKKY-type exchange down to the atomic level. By combining SP-STM with inelastic electron tunnelling spectroscopy [10] or by performing MExFM together with high-precision measurements of damping forces [9] localized or confined spin excitations in magnetic systems of reduced dimensions now become experimentally accessible. Principles of Spin-Polarized Scanning Tunnelling Microscopy (SP-STM) and Spectroscopy (SP-STS)The technique of SP-STM is based on vacuum tunnelling of spin-polarized electrons as can be observed in STM-type tunnel junctions involving a magnetic tip and a magnetic sample (Fig. 1). In this case, the tunnelling current does not only depend on the tip-surface separation and the applied bias voltage between tip and sample, but also on the spin polarization of the electronic states near the Fermi level of both electrodes as well as on the relative orientation of the local magnetization of the sample and the magnetic moment at the tip apex. Early experiments based on planar tunnel junctions involving magnetic electrodes have been performed by Jullière in the midseventies [11]. The invention of the STM by G. Binnig and H. Rohrer [5] allowed the replacement of the ill-defined tunnel barrier of planar tunnel junctions by a well defined vacuum barrier and the replacement of one of the planar electrodes by a freely movable and positionable probe tip. Magnetic tips have first been employed in scanning probe microscopy to perform spatially resolved measurements of magnetic dipole forces [12]. The corresponding technique, Magnetic Force Microscopy (MFM), has become a routine method for magnetic domain imaging [13]. However, the spatial resolution of MFM is limited to 10 -20 nm due to the long-range nature of the magnetic dipole forces [6]. In order to improve the spatial resolution of magnetic-sensitive scanning probe microscopy down to the atomic scale, two different a...

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“…The most likely mechanism for the TAMR is that the s-o interaction causes a mixing of 3d minority and majority spin states, as well as 4s and 3d orbitals of the same spin channel, resulting in an anisotropic tunneling DOS at the Fermi level. 13,14 A detailed theoretical analysis would require spin density functional calculations, which is clearly beyond the scope of the present work. Nevertheless, a qualitative understanding of the large difference in temperature dependence between TMR and TAMR can be gained by considering the magnetocrystalline anisotropy.…”

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Magnetoresistance studies on Co∕AlOX∕Au and Co∕AlOX∕Ni∕Au tunnel structures

Liu

Canali²,

Samuelson

et al. 2008

Applied Physics Letters

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We report on magnetoresistance (MR) studies on Co∕AlOX∕Au and Co∕AlOX∕Ni∕Au magnetic tunnel junctions. In spite of the fact that the difference between the two samples is merely a 3nm thick Ni layer, there is a sharp contrast in MR behavior indicating that the electronic structure at the interface between the ferromagnetic electrodes and the insulating barrier dominates the MR signal. The former sample exhibits a clear tunneling anisotropic MR (TAMR), with the characteristic correlation between resistance and current direction, in contrast to the latter sample which displays a conventional tunneling MR (TMR) dominated by the relative orientation between the magnetization directions of the two electrodes. In addition, the TAMR has a much stronger temperature dependence than the TMR, indicating a much faster drop-off of the tunneling density of states anisotropy than the tunneling electron spin polarization with increasing temperature. Finally, we propose a possible simple way to distinguish TAMR from normal TMR by measuring the resistance of the device at different angles of the external magnetic field.

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Magnetization-Direction-Dependent Local Electronic Structure Probed by Scanning Tunneling Spectroscopy

Cited by 126 publications

References 18 publications

Out-of-plane spin-orientation dependent magnetotransport properties in the anisotropic helimagnetCr1/3NbS2

Out-of-plane spin-orientation dependent magnetotransport properties in the anisotropic helimagnetCr1/3NbS2

Mapping spin structures on the atomic scale

Magnetoresistance studies on Co∕AlOX∕Au and Co∕AlOX∕Ni∕Au tunnel structures

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