Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets

Abstract

Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared to their ferromagnetic counterparts. While nanoscale thin film materials are key to the development of future antiferromagnetic spintronics technologies, experimental tools to explore such films on the nanoscale are still sparse. Here, we offer a solution to this technological bottleneck, by addressing the ubiquitous surface magnetisation of magnetoelectic antiferromagnets in a granular thin film sample on the nanoscale using single-spin magnetometry in combination with spin-sensitive transport experiments. Specifically, we quantitatively image the evolution of individual nanoscale antiferromagnetic domains in 200-nm thin-films of Cr 2 O 3 in real space and across the paramagnet-to-antiferromagnet phase transition. These experiments allow us to discern key properties of the Cr 2 O 3 thin film, including the mechanism of domain formation and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into Cr 2 O 3 's magnetic ordering mechanism and establishes single spin magnetometry as a novel, widely applicable tool for nanoscale addressing of antiferromagnetic thin films. arXiv:1806.02572v1 [cond-mat.mes-hall]

Search for the Magnetic Monopole at a Magnetoelectric Surface

“…If the only contribution to the internal field at the muon site were the monopolar field from the magnetoelectric response, we would expect the shifts in both cases to be in the same direction, since both sets of measurements are performed on the same magnetoelectric domain. It is known, however, that Cr 2 O 3 thin films can have stray spins caused by defects at the interface with the Al 2 O 3 substrate, as well as impurity spins at the interface, which have been shown to be susceptible to small magnetic fields in thin film samples 28,30 . (Note that the intrinsic surface spin density resulting from the termination of the antiferromagnetic magnetoelectric [31][32][33] is not reversed under the conditions of our experiment, since this would require the reversal of the full antiferromagnetic domain 33 .…”

Seeing is believing: visualization of antiferromagnetic domains

“…For example, the stray fields of the single layer uncompensated spins on the surface of Cr 2 O 3 were imaged by scanning nitrogen-vacancy (NV) center microscopy, which allowed to measure the reorientation of the AFM order parameter across the domain wall with sub-100 nm spatial resolution. 94 This impressive sensitivity to the local magnetization is achieved by attaching a diamond nanocrystal to the atomic force microscopy tip. The NV centers exhibit a Zeeman splitting according to the local magnetic field, which can thus be measured with high magnetic and spatial resolution.…”

(111)-oriented, single crystal diamond tips for nanoscale scanning probe imaging of out-of-plane magnetic fields

“…4(c)&(d)). This behavior is in strong contrast to magnetometry with non-OOP oriented scanning NV centers [5,6,8,9], where the qualitative behavior of B NV near sample edges depends strongly on the orientation of the edge (Fig. 4(b)).…”

“…Our results will be impactful for scanning NV magnetometry in general and for applications in spintronics and the investigation of thin film magnets in particular.Scanning probe magnetometry using nitrogen-vacancy (NV) center electronic spins in diamond offers a unique combination of spatial resolution, magnetic field sensitivity and quantitative magnetic imaging [1,2]. These combined performance characteristics have led to room-temperature imaging of single electron spins [3], nanoscale domains in multiferroics [4] and antiferromagnets [4,5], and to cryogenic experiments addressing superconductors [6-8] and magnetism in atomically thin crystals [9]. These and other studies have demonstrated how scanning NV magnetometers can yield valuable insights into materials of high scientific and technological interest, beyond the capacity of existing nanoscale imaging methods.Magnetometry based on NV center spins [10] builds on the extraordinary properties of this point defect in diamond that consists of a nitrogen atom adjacent to a lattice vacancy ( Fig.…”

“…All-diamond scanning probes hosting individual NV spins in nanopillars ( Fig. 1(b)) offer a particularly powerful approach to perform nanoscale NV magnetometry [1] and have been the basis of key achievements in the field [3][4][5][6][7][8][9]12]. All reported implementations of such scanning probes thus far consisted of diamond tips fabricated from (100)-oriented starting material -the most commonly available crystal orientation for high purity diamonds.…”