Open Access

Show Abstract ▼

Abstract: The ability to represent information using an antiferromagnetic material is attractive for future antiferromagnetic spintronic devices. Previous studies have focussed on the utilization of antiferromagnetic materials with biaxial magnetic anisotropy for electrical manipulation. A practical realization of these antiferromagnetic devices is limited by the requirement of material-specific constraints. Here, we demonstrate current-induced switching in a polycrystalline PtMn/Pt metallic heterostructure. A comparison of electrical transport measurements in PtMn with and without the Pt layer, corroborated by x-ray imaging, reveals reversible switching of the thermally-stable antiferromagnetic Néel vector by spin-orbit torques. The presented results demonstrate the potential of polycrystalline metals for antiferromagnetic spintronics.

Show Abstract ▼

Abstract: The effect of Ce substitution on the structural and magnetic properties of (Nd1-δCeδ)2Fe14B (0 < δ < 1) series was systematically studied using neutron diffraction, 57Fe Mössbauer spectroscopy and X-ray magnetic circular dichroism (XMCD). An anomaly in the lattice parameters was observed in a sample with composition in the range of 0.2 < δ < 0.4, where a phase separation happens and the 2:14:1 isostructural dual-main-phases (IDMPs) appear. Outside this composition range, Ce shows a preferential occupation at the 4g site than the 4f site and a preferable Ce3+ valence state. The average magnetic moments of Nd/Ce and Fe atoms decrease with the increasing of δ. The magnitude of the Fe moments at non-equivalent sites is related to several factors, such as the Fe coordination number, the Wigner-Seitz cell volume, and the Boron atom effect. These results reveal the microscopic mechanisms for the structural and magnetic properties of Ce-substitution Nd2Fe14B, providing perspective on developing the next-generation low-cost and high-performance permanent magnetic materials.

Open Access

Show Abstract ▼

Abstract: Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties. Spatially resolved soft X-ray absorption spectroscopy (XAS) in combination with Atomic Force Microscopy (AFM) and Scanning Helium Ion-Milling Microscopy (SHIM) has been applied to visualise and characterise the behaviour of individual cobalt nanoparticles (CoNPs) supported on two-dimensional substrates (SiOxSi(100) (x < 2) and rutile TiO2(110)) after undergoing reduction–oxidation–reduction (ROR). The behaviour of the Co species is observed to be strongly dependent on the type of support. For SiOxSi a weaker MSI between Co and the support allows a complete reduction of CoNPs although they migrate and agglomerate. In contrast, a stronger MSI of CoNPs on TiO2 leads to only a partial reduction under H2 at 773 K (as observed from Co L3-edge XAS data) due to enhanced TiO2 binding of surface-exposed cobalt. SHIM data revealed that the interaction of the CoNPs is so strong on TiO2, that they are seen to spread at and below the surface and even to migrate up to ∼40 nm away. These results allow us to better understand deactivation phenomena and additionally demonstrate a new understanding concerning the nature of the MSI for Co/TiO2 and suggest that there is scope for careful control of the post-synthetic thermal treatment for the tuning of this interaction and ultimately the catalytic performance.

Show Abstract ▼

Abstract: Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible 180 ∘ switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent 90 ∘ switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.

Show Abstract ▼

Abstract: In antiferromagnetic spintronics the Néel vector corresponding to the staggered magnetization is used to encode information. In the framework of spin-based electronics, antiferromagnets as active elements have a number of advantages compared to ferromagnetic materials: The natural spin dynamics of antiferromagnets in the THz range is of major significance as it makes this class of magnetic materials promising for ultrafast switching as well as for the generation and detection of THz radiation. Furthermore, the lack of net magnetization of antiferromagnets makes their Néel vector oriented state robust with respect to external magnetic fields. This property results in a high stability of the information encoded in the Néel vector orientation, which cannot be destroyed by external magnetic fields. Last but not least, antiferromagnets do not produce stray fields, which are limiting the minimum feasible size of data storage units based on ferromagnets. However, the lack of net magnetization makes the manipulation of the Néel vector orientation much more challenging than the corresponding manipulation of the magnetization of ferromagnets. One of the possible approaches is based on a current induced bulk spin-orbit torque, which was predicted to exist for very specific crystal structures of collinear antiferromagnets, the so-called Neel spin-orbit torque. Only two antiferromagnetic compounds with the required crystal structure are known up to now: Mn2Au, on which this work is focused, and CuMnAs. In my thesis, I investigate the effects of current pulses on the antiferromagnetic domain structure which is modified by a Néel vector reorientation originating from Néel spin-orbit torques and alternative effects. As an alternative Néel vector manipulation method, I also investigate the switching of the Néel vector orientation in Mn2Au by the application of very large high magnetic field pulses causing a spin-flop transition. To investigate current induced Néel vector switching in Mn2Au, I performed electric transport measurements where I measured current-induced changes of the resistance of up to 6% and compared the results with calculations of our collaboration partners. In order to directly visualize the Néel vector reorientation, we performed photoelectron emission microscopy experiments with magnetic contrast combined with transport measurements. During this experiment, massive changes of the AFM domain pattern induced by current pulses were observed. Also the reorientation of the Néel vector induced by magnetic field pulse driven spin-flop transitions was correlated with magnetotransport measurements. Here, persistent changes of the resistance associated with anisotropic magnetoresistance and a transient drop of the resistance in the range of 2% were obtained. These experiments demonstrated Néel vector switching in Mn2Au both by a current induced Néel spin-orbit torque mechanism and by a magnetic field induced spin-flop transition. It was shown that the magnetoresistance of the antiferromagnetic domain walls in Mn2Au plays a crucial role in the understanding of current-induced resistance modifications associated with Néel vector switching processes. Additional to transient effects, there are as well persistent resistance modifications induced by the Neel vector reorientation , which is an essential property for data storage applications.