AVS 67 Session MI-Invited On Demand: Magnetic Interfaces Invited On Demand Session

I will present recent developments and discoveries in atomically resolved magnetic microscopy based on scanning probes (SPM). (a) A novel method we call SPEX microscopy combines spin-polarized scanning tunneling microscopy (SP-STM) with magnetic exchange force microscopy (MExFM), and enables the disentanglement of structural, electronic and magnetic contributions to the SPM signal [1]. This way, we were able to resolve the antiferromagnetic chiral spin spiral structure of a Mn monolayer on W(110) with unprecedented resolution [2]. (b) Our recent development of a dilution-fridge UHV SP-STM allows for the interrogation of in situ MBE-grown magnetic samples down to a temperature of 30 mK and in fields up to 9 T [3].

As a showcase, I will present the first atomic-scale magnetic microscopy results on the surface of the rare-earth element neodymium (Nd), whose complex magnetic ground state is still under debate after more than half a century of experiments. We surprisingly found evidence that Nd manifests the first example of a recently predicted new state of matter referred to as self-induced spin glass [4]. Contrary to the accepted paradigm that a spin glass requires structural disorder (as found in dilute magnetic alloys), self-induced glassiness can also occur in defect-free single crystalline materials. SP-STM experiments on extremely clean MBE-grown Nd(0001) films on W(110), combined with abinitio calculations and atomistic spin-dynamics simulations, reveal very complex non-collinear spin structures with local but no long-range order. By performing measurements at various temperatures (30 mK - 7 K) and as a function of magnetic field, we observed and quantified the aging phenomenon, which distinguishes spin glasses from quantum spin liquids or spin ice. We relate the glassy behavior to the crystalline symmetry, leading to competing magnetic interactions. This not only resolves the long-standing debate on the magnetic ground state of neodymium, but suggests that glassiness may arise in elemental solids without disorder if certain symmetry conditions are met.

1. N. Hauptmann, J. W. Gerritsen, D. Wegner, and A. A. Khajetoorians, Nano Letters 17, 5660-5665 (2017).
2. N. Hauptmann, S. Haldar, T.-C. Hung, W. Jolie, M. Gutzeit, D. Wegner, S. Heinze & A. A. Khajetoorians, Nature Communications 11, 1197 (2020).
3. H. von Allwörden, A. Eich, E. J. Knol, J. Hermenau, A. Sonntag, J. W. Gerritsen, D. Wegner, and A. A. Khajetoorians, Review of Scientific Instruments 89, 033902 (2018).
4. U. Kamber, A. Bergman, A. Eich, D. Iuşan, M. Steinbrecher, N. Hauptmann, L. Nordström, M. I. Katsnelson, D. Wegner, O. Eriksson, and A. A. Khajetoorians, Science 368, 966 (2020).

Perpendicular magnetic tunnel junctions, pMTJs, are extensively used in industry for sensing and data storage applications. pMTJs are multilayered structures composed of several ferromagnetic and non-magnetic thin films, including a free layer with switchable magnetization. Reducing the dimensions of pMTJs, including the thickness of layers and the lateral size, is desirable to enhance the energy efficiency, speed, and data storage density. As the dimensions, mainly the thickness, of the magnetic film decrease, interfacial effects dominate, and the magnetic properties change. Of particular interest are the interfacial perpendicular magnetic anisotropy, PMA, and the exchange interaction of the pMTJ free layer that serves as write media. The reason is: 1) strong PMA is needed to obtain higher thermal stability of the device, and 2) exchange interactions set the length scale for micromagnetic inhomogeneities. Therefore, they both have a significant effect on the spin torque switching dynamics and the switching time^{1, 2}.

Interfacial anisotropy of the free layer drops inversely with its thickness. This implies that any spatial inhomogeneities of the film thickness alter the magnetic energy landscape, such as in the form of a higher-order perpendicular anisotropy. Moreover, the magnetic exchange interaction (Heisenberg model) of the free layer thin film is reduced significantly compared to bulk values. We have observed this effect when we used VSM analysis used to determine the exchange constant of CoFeB thin film. Exchange interaction is reduced further for a composite CoFeB/W/CoFeB free layer in pMTJ stack. This reduction of exchange energy is also observed when dynamic methods such as spin-torque ferromagnetic resonance, ST-FMR, are used.

The higher-order anisotropy and the reduced exchange interaction in the free layer thin film also have significant effects on the spin-torque switching dynamics of the free layer. On the one hand, the presence of higher-order anisotropy reduces the switching time². On the other hand, the reduced exchange interaction results in delayed switching³. I will present some of the implications of the above-mentioned effects on the switching dynamics and switching speed of pMTJ free layers.

Acknowledgments

This research supported by Spin Memory Inc.

References:

1. J. B. Mohammadi, B. Kardasz, G. Wolf, Y. Chen, M. Pinarbasi, and A. D. Kent, ACS Applied Electronic Materials 1 (10), 2025-2029 (2019).

1. M. Lavanant, S. Petit-Watelot, A. D. Kent and S. Mangin, Applied Physics Letters 114 (1), 012404 (2019).

1. J. Beik Mohammadi and A. D. Kent, arXiv:2003.13875, 2020 (2020).

Non-collinear spin textures in ultra-thin film systems are in the focus of ongoing research. Specifically, atomic-scale magnetic skyrmions raise
expectations for their application in information technology as logic spin-electronic devices or in recording media. For their characterization
and manipulation spin-sensitive techniques with ultimate spatial resolution are required. Spin-polarized scanning tunneling microscopy (SPSTM)
can resolve magnetic surface structures down to the atomic-scale. However, based on spin-polarized tunneling of electrons, this
imaging technique is restricted to tip-sample separations of only a few Ångstroms, making the technique very fragile in terms of sensitivity to
vibrations or for accidental, destructive tip-sample collisions. Ångstrom distances are also technically challenging for practical applications in
future spin-electronic devices.

Spin-polarized vacuum resonance states (sp-RS) are unoccupied electronic states in the vacuum gap between a probe tip and a magnetic
sample. As I will show in this talk, these states exhibit the same local spin quantization axis as the spin texture of the underlying sample
surface, even when the spins are rotating on the atomic-scale [1]. In an SP-STM setup, the sp-RS can be addressed by spin-polarized electrons
that tunnel resonantly from the magnetic tip into the surface, resulting in a magnetic image contrast governed by the spin-polarized electron
tunneling into the sp-RS [1]. Our SP-STM experiments on ultrathin films with non-collinear spin textures demonstrate that this resonant
tunneling allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm. This technique provides a
loophole from the hitherto existing dilemma of losing spatial resolution when increasing the tip-sample distance in a scanning probe setup
[2]. Experimental results will be discussed in terms of the sp-RS’ spin-splitting and the magnetic contrast as a function of bias and tip-sample
distance, as well as in terms of the atomic-scale nature of the resonant tunneling condition between the probe tip and the sample.

The tip-sample distances demonstrated in this talk are in the range of present flying heights of read-write heads in data storage devices. In
combination with thermally-assisted spin-transfer torque switching via sp-RS [3], our approach qualifies for a spin-sensitive read-write
technique with ultimate lateral resolution in future spin-electronic applications.

[1] A. Schlenhoff et al., Phys. Rev. Lett. 123, 087202 (2019).
[2] A. Schlenhoff et al., Appl. Phys. Lett. 116, 122406 (2020).
[3] A. Schlenhoff et al., Phys. Rev. Lett. 109, 097602 (2012).

Two-dimensional (2D) magnetic materials are of great interest for fundamental science as well as advanced applications. Besides 2D materials, the dilute magnetic semiconductors (DMS) have also captured worldwide interest by combining magnetism with electronic properties. MnGaN-2D is the ultimately-thin limit of a novel DMS material, but having a densely concentrated and well-ordered structural design. With 1/3 of the monolayer being magnetic manganese atoms, MnGaN-2D shows room-temperature ferromagnetism as demonstrated with spin-polarized scanning tunneling microscopy as a function of applied magnetic field, and as predicted by first-principles theoretical calculations which reveal the origins of the ferromagnetism through its highly spin-split and spin-polarized electronic structure.[1]

These results are backed up by SQUID magnetometry measurements which reveal a high spin-polarization of ∼79% at room temperature as well as perpendicular magnetic anisotropy. But this intriguing system shows even more interesting behavior as it turns out that its electronic structure is strain-dependent leading to the possibility of magneto-elasticity. Theoretical calculations reveal a high sensitivity of the spin-polarized electronic structure to lattice strain, offering one explanation for results from tunneling spectroscopy (dI/dV) measurements, which reveal unexpected variations in the electronic properties.

Simulations, including both isotropic and anisotropic cases, confirm a highly strain-dependent manganese partial density of states. Spin-orbit coupling is included which indicates either out-of-plane perpendicular magnetic anisotropy (PMA) or in-plane magetic anisotropy, dependent on the type of strain whether compressive or tensile. Clear evidence for both compressive and tensile local lattice strains is found by detailed analysis of atomic resolution STM images which reveal a highly non-Gaussian lattice spacing distribution.

[1] A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature, Yingqiao Ma, Abhijit V. Chinchore, Arthur R. Smith, María Andrea Barral, and Valeria Ferrari, Nano Letters 18, 158 (2018).

Multiferroic materials that combine magnetism and ferroelectricity are described by order parameters in real space under the Landau's phenomenological theory. The recent strides in topological order -- described by reciprocal space invariants -- have identified several new classes of materials including topological insulators and Weyl/Dirac semimetals. In this talk I will address the question: how can topological order in reciprocal space be combined with, and possibly controlled by, real space order parameters such as magnetism and ferroelectricty? I will identify the fundamental design rules for their coexistence in several classes of materials and give examples how topological order can be controlled via multiferroic order parameters.