Epitaxy, exfoliation, and strain-induced magnetism in rippled Heusler membranes

Abstract

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism).

Quantifying Mn Diffusion through Transferred versus Directly Grown Graphene Barriers

“…For CVD graphene growth on Ge (001), we followed growth conditions described in refs and . CVD graphene growth on Cu foils was performed at 1050 °C using ultrahigh-purity CH 4 , as described in ref .…”

“…Our graphene-transfer process is similar to other polymer-assisted wet transfers reported in previous works. , The graphene/Cu foils were cut into 5 by 5 mm pieces and flattened using cleaned glass slides. Approximately, 300 nm of 495 K C2 PMMA (chlorobenzene base solvent, 2% by wt., MicroChem) was spin-coated on the graphene/Cu foil substrate at 2000 rpm for 2 min and left to cure at room temperature for 24 h. Graphene on the backside of the Cu foil was removed via reactive ion etching using 90 WO 2 plasma at a pressure of 100 mTorr for 30 s. The Cu foil was then etched by placing the poly­(methyl methacrylate) (PMMA)/graphene/Cu foil stack on the surface of an etch solution containing 1-part ammonium persulfate (APS-100, Transene) and 3-parts H 2 O.…”

“…This extreme thinness is highly attractive for applications where a nearly electronically transparent interface is desired, for example, spin injection via tunneling . Moreover, recent reports of “remote epitaxy” suggest that it is possible to synthesize epitaxial films through a monolayer graphene barrier, − opening the possibility of epitaxial metal/graphene/semiconductor heterostructures that would otherwise not be stable due to significant metal/semiconductor interdiffusion …”

Pinhole-seeded lateral epitaxy and exfoliation of GaSb films on graphene-terminated surfaces

“…Remote epitaxy of three-dimensional materials on graphene-terminated surfaces 1 promises to circumvent many of the limitations of conventional epitaxy. For example, the weak van der Waals interaction between the film and graphene creates new strain relaxation pathways for highly mismatched growth 2 , 3 and enables exfoliation of free-standing crystalline membranes for flexible devices 1 , 4 and tuning properties via strain in novel geometries 5 . Since the first demonstrations for compound semiconductors 1 , 6 , epitaxy and exfoliation from graphene have been demonstrated for a variety of other materials including transition metal oxides 4 , 7 , simple metals 8 , halide perovskites 3 , and intermetallic compounds 5 .…”

“…For example, the weak van der Waals interaction between the film and graphene creates new strain relaxation pathways for highly mismatched growth 2 , 3 and enables exfoliation of free-standing crystalline membranes for flexible devices 1 , 4 and tuning properties via strain in novel geometries 5 . Since the first demonstrations for compound semiconductors 1 , 6 , epitaxy and exfoliation from graphene have been demonstrated for a variety of other materials including transition metal oxides 4 , 7 , simple metals 8 , halide perovskites 3 , and intermetallic compounds 5 . These films are typically thought to grow via a remote epitaxy mechanism 1 , 6 , in which epitaxial registry between film and substrate is achieved via remote interactions that permeate through graphene.…”