Micromagnetometry of two-dimensional ferromagnets

Abstract

The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism 1-9 that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials 10-17 . Here we employ ballistic Hall micromagnetometry 18,19 to study magnetization of individual 2D ferromagnets. Our devices are multilayer van der Waals (vdW) heterostructures 20 comprising of an atomically-thin ferromagnetic crystal placed on top of a Hall bar made from encapsulated 21 graphene. 2D ferromagnets can be replaced repeatedly, making the graphene-based Hall magnetometers reusable and expanding a range of their possible applications. The technique is applied for the quantitative analysis of magnetization and its behavior in atomically thin CrBr 3 . The compound is found to remain ferromagnetic down to a monolayer thickness and exhibit high out-of-plane anisotropy. We report how the critical temperature changes with the number of layers and how domain walls propagate through the ultimately thin ferromagnets. The temperature dependence of magnetization varies little with thickness, in agreement with the strongly layered nature of CrBr 3 . The observed behavior is markedly different from that given by the simple 2D Ising model normally expected to describe 2D easy-axis ferromagnetism. Due to the increasingly common usage of vdW assembly, the reported approach offers vast possibilities for investigation of 2D magnetism and related phenomena.Research on magnetism in strongly layered (vdW) materials is only a couple of years old but has already revealed a number of interesting phenomena including, for example, unexpected changes in magnetic properties as a function of the number of layers 2,17 and the possibility to control magnetism by electric and chemical doping [12][13][14][15][16]22 . Of particular interest are ferromagnetic semiconductors such as Cr 2 Ge 2 Te 6 and CrI 3 , in which a magnetization-dependent optical response and switching of a magnetization direction by applied electric field have been reported [12][13][14][15][16] . A number of different techniques have been employed to study magnetic properties of the above compounds at a few-layer thickness, including magneto-optical Kerr effect 1,2,15 , circular dichroism

Topological spin texture in Janus monolayers of the chromium trihalides Cr(I, X)3

“…, both J 1 and J 2 yield negative values, which imply ferromagnetism (FM) for all investigated systems, which is consistent with the measured ferromagnetism of CrX 3 , with X = Cl, Br and I11,24,26 . The J 1 , K 1 and A zz parameters all decrease in magnitude when the X ion of Cr(I,X) 3 varies from I to Cl, via Br.…”

“…A non-negligible Kitaev coefficient has also been previously identified in CrI 3 and CrGeTe 3 and was found to be crucial for determining and understanding the different magnetic anisotropies of these two latter compounds 13 . A Kitaev term was also needed in order to reproduce the temperature evolution of magnetization in CrBr 3 few layers 24 and is the key ingredient to create quantum spin liquid states (when the J parameter is vanishing 25 ).…”

Anomalous Hall effect mechanisms in the quasi-two-dimensional van der Waals ferromagnet Fe0.29TaS2

“…It is noted that similar TC of ≈ 80 K are obtained for 14-nm, 24-nm and bulk single-crystal Fe0.29TaS2 devices ( Fig. S6 in Supplemental Material [25]), consistent with other 2D ferromagnets with similar TC when the thickness is larger than approximately ten layers, such as 2D Fe3GeTe2 and Cr2Ge2Te6 [14,48]. The results of * vs measured on various thickness Fe0.29TaS2 devices are summarized in Fig.…”

Magnetic Proximity Effect in Graphene/CrBr3 van der Waals Heterostructures

“…The Cr 3+ ions are configured in a honeycomb network, and the green arrows represent the spin direction of Cr atoms, which are found to exhibit a strong ferromagnetic coupling. [ 39–44 ] The Raman spectra of Figure 1c indicates that the monolayer graphene still preserves a high crystal quality when heterostructured with CrBr 3 layer. The optical image in Figure 1d displays the as‐fabricated graphene/CrBr 3 heterostructures, along with the H‐shape Hall bar structure for conducting non‐local measurements.…”

“…Moreover, other possible source of producing non‐local signal like ohmic contribution were analyzed and excluded (Figure S5, Supporting Information). As to the finite V nl at 0 T, it may be due to the spin Hall effect caused by spin polarization which is induced by the exchange field under 0 T, [ 13,37 ] since the magnetic hysteresis of 2D CrBr 3 [ 44 ] and EuS [ 52 ] is different, CrBr 3 has a non‐zero remanence but EuS is zero at zero applied field (Text S6 and Table S1, Supporting Information). Therefore, this feature enables the prospect of utilizing novel devices without external magnetic field which cannot be generated on nano spatial scales.…”