Direct measurement of anisotropic conductivity in a nanolaminated (Mn0.5Cr0.5)2GaC thin film

Appl. Phys. Lett. 115, 094101 (2019)

Tim Flatten, Frank Matthes, Andrejs Petruhins, Ruslan Salikhov, Ulf Wiedwald, Michael Farle, Johanna Rosen, Daniel E. Bürgler, Claus M. Schneider

The individual control of the probe positions of the LT Nanoprobe 4-probe STM allows to perform the direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic M_n+1AX_n (MAX) phase film.

The ternary carbides and nitrides with the general formula M_n+1AX_n (MAX) with n =1, 2, 3 (M is an early transition metal, A is an A-group element mostly of the main groups 13–16, and X is C or N) represent a material class with nanolaminated and anisotropic atomic structures, but with predominantly covalent bonds both within the two-dimensional building blocks formed by M-X-M planes and between them. As a consequence, MAX phases combine metallic and ceramic properties in a unique manner. MAX phases are electrically and thermally conductive, thermally stable, elastically stiff, light-weight, and readily machinable. Great potential of MAX phases in applications ranging from electrical contacts, magnetic sensors, spintronics devices to coating materials in aerospace technology has triggered the development of an environmentally friendly, sustainable, and cheap MAX phase synthesis scheme that can be scaled up to industrial scale.

We employed the LT Nanoprobe multi-probe STM is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities ρ_ab and ρ_c. A 155 nm thick (Cr_0.5Mn_0.5)₂GaC MAX phase film shows a large anisotropy ratio 𝜌_c/𝜌_ab = 525 ±49. This is a consequence of the complex bonding scheme of MAX phases with covalent M–X and metallic M–M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes.

Figure: 4-probe transport measurements of a 155 nm thick (Cr_0.5Mn_0.5)₂GaC thin film.(a) SEM image of an in-line configuration with equidistant probe spacing s. (b) V-Icurve for s=15 µm. Red dots are measured values (including error bars), and theblue line is a linear fit yielding R= (1.7738 ± 0.0005) Ω and V₀=(51.0±0.2) µV.(c) Measured resistances R (red) vs equidistant probe spacing s. The blue curve is a fit to the data yielding 𝜌_c/𝜌_ab = 525 ± 49. Black curves show for comparison the isotropic behavior (solid line) and anisotropic cases for anisotropy ratios 10² (dotted) and 10⁴ (dashed).