Magnetism of atomically laminated MAX phase Mn2GaC and (Cr0.5Mn0.5)2GaC films

Abstract

MAX phases have attracted interest due to their anisotropic atomically layered structure resulting in a combination of ceramic and metallic properties. The correlation between the complex crystal structure with the here reported magnetic properties in these compounds can become an important element in the development of microelectronic devices of the future. This thesis presents an experimental study of the magnetic properties of (Cr0.5Mn0.5)2GaC and Mn2GaC MAX phase films measured by ferromagnetic resonance and magnetometry. Both compounds exhibit a complex magnetism due to competing magnetic interactions and its laminar structure. The (Cr0.5Mn0.5)2GaC films with a thickness of 20.8 to 156 nm show soft magnetic properties at temperature T = 100 K with a narrow hysteresis of less than 4 mT and a magnetization M of about 200 kA/m. No dependence of M on the film thickness was observed. The films display a broad magnetic phase transition to a paramagnetic (PM) state above the transition temperature Tt of about 230 K. The 12.5 nm film’s surface exhibits a discontinuity in the surface morphology, which results in a magnetic anisotropy increase comparable to the thicker films. The ferromagnetic resonance field μ0Hr shows a nearly thickness-independent behavior for the 20.8 – 156 nm films. The 20.8 nm film demonstrated magnetic stability at ambient conditions without any capping layer within the time span of 1 year. This stability makes the material a candidate for low-temperature electronic devices used in oxidation-harsh conditions. The Mn2GaC film exhibits two magnetic phase transitions within the range of 50 K ≤ T ≤ 850 K. A magneto-structural transition from noncollinear ferromagnetic (FM) to antiferromagnetic (AFM) configuration occurs at Tt = 214 K. A second magnetic transition from an AFM configuration to a presumably PM state appears at 507 K. Theoretically predicted competing FM and AFM interactions were observed in the field-dependent magnetization as metamagnetic transitions and as an opening of the hysteresis loop. The magneto-structural transition is accompanied by a large uniaxial magnetostriction effect of 450 ppm along the c-axis. The magnetostriction changes sign above and below the transition, which leads to a compression or an expansion of the lattice spacing along the c-axis. The magnetoresistance showed a similar behavior also with a change of sign. It reaches 3 % at T = 300 K. By cooling the Mn2GaC with an applied magnetic field across the Tt, μ0Hr decreases to about 30 % and the linewidth decreases to about 40 %. Such response can be explained by a smaller canting angle between the magnetic moments as they align along the applied magnetic field while experiencing magneto-structural transformations. A negative perpendicular magnetic anisotropy constant K2⊥ indicates an “easy-plane” magnetic anisotropy, with no preferred orientation within the basal plane of the film. The results were published in 2 peer-reviewed journals and were presented at 10 international conferences as oral and poster contributions.