Ferromagnetic Interlayer Coupling in CrSBr Crystals Irradiated by Ions.

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly, the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggests that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

Growth and martensitic transformation of ferromagnetic Co-Cr-Ga-Si epitaxial films.

During cooling, conventional martensitic transformation can only be realized from austenite to martensite. Recently, a so-called reentrant martensitic transformation attracted much interest due to an additional transformation from martensite to austenite during further cooling. Obviously, materials with this reentrant transformation will increase the number of physical effects and possible applications. However, until now, only bulk samples have been available, which are not suitable for applications in micro-devices. In this work, we focus on the Co-Cr-Ga-Si system and examine the suitability of this system for the growth of thin films. We observed that the films grow epitaxially on MgO (100) substrates and exhibit a martensitic transformation if deposited at a sufficiently high temperature or with an additional heat treatment. Films within the austenite state are ferromagnetic while films within the martensitic state just exhibit a very low ferromagnetic order.

Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films.

Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of a 50-nm-thick film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the inhomogeneously strained films can persist up to 100 °C, rendering Cr2O3 relevant for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Defect Nanostructure and its Impact on Magnetism of α-Cr2 O3 Thin Films.

Thin films of the magnetoelectric insulator α-Cr2 O3 are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr2 O3 films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, the impact of the defect nanostructure, including sparse small-volume defects and their complexes is studied on the magnetic properties of Cr2 O3 thin films. By tuning the deposition temperature, the type, size, and relative concentration of defects is tailored, which is analyzed using the positron annihilation spectroscopy complemented with electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen-vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a domain size in the sub-micrometer range. Despite their influence on the domain configuration, neither small open-volume defects nor grain boundaries in Cr2 O3 thin films affect the Néel temperature in a broad range of deposition parameters. The results pave the way toward the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

Highly Tunable Magnetic and Magnetotransport Properties of Exchange Coupled Ferromagnet/Antiferromagnet-Based Heterostructures.

Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical film thickness and measurement temperature, bimetallic Mn-based alloys and transition-metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM γ-IrMn3 thin films with coexisting AFM phases into complex exchange coupled MgO(001)/γ-Ni3Fe/γ-IrMn3/γ-Ni3Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the thermally disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetotransport properties of the exchange coupled heterostructures. We further reveal that the persistence of thermally disordered and reversible AFM moments is crucial for achieving highly tunable magnetic properties and multilevel magnetoresistance states. We anticipate that the presented approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM moments at the nanoscale.

Synthesis of Mg and Zn diolates and their use in metal oxide deposition.

The synthesis of complexes [M(OCHMeCH2NMeCH2)2] (5, M = Mg; 7, M = Zn) is described. Treatment of MeHNCH2CH2NMeH (1) with 2-methyloxirane (2) gave diol (HOCHMeCH2NMeCH2)2 (3), which upon reaction with equimolar amounts of MR2 (4, M = Mg, R = Bu; 6, M = Zn, R = Et) gave 5 and 7. The thermal behavior and vapor pressure of 5 and 7 were investigated to show whether they are suited as CVD (= chemical vapor deposition) and/or spin-coating precursors for MgO or ZnO layer formation. Thermogravimetric (TG) studies revealed that 5 and 7 decompose between 80-530 °C forming MgO and ZnO as evidenced by PXRD studies. In addition, TG-MS-coupled experiments were carried out with 7 proving that decomposition occurs by M-O, C-O, C-N and C-C bond cleavages, as evidenced from the detection of fragments such as CH4N+, C2H4N+, C2H5N+, CH2O+, C2H2O+ and C2H3O+. The vapor pressure of 7 was measured at 10.4 mbar at 160 °C, while 5 is non-volatile. The layers obtained by CVD are dense and conformal with a somewhat granulated surface morphology as evidenced by SEM studies. In addition, spin-coating experiments using 5 and 7 as precursors were applied. The corresponding MO layer thicknesses are between 7-140 nm (CVD) or 80 nm and 65 nm (5, 7; spin-coating). EDX and XPS measurements confirm the formation of MgO and ZnO films, however, containing 12-24 mol% (CVD) or 5-9 mol% (spin-coating) carbon. GIXRD studies verify the crystalline character of the deposited layers obtained by CVD and the spin-coating processes.