Van der Waals epitaxy growth of 2D ferromagnetic Cr_(1+δ)Te₂ nanolayers with concentration-tunable magnetic anisotropy

Cr(1+δ)Te2 are pseudo-layered compounds consisting of CrTe2 transition metal dichalcogenide (TMD) layers with additional (δ) self-intercalated Cr atoms. The recent search for ferromagnetic 2D materials revived the interest into chromium tellurides. Here, Cr(1+δ)Te2 nanolayers are epitaxially grown on MoS2 (0001), forming prototypical van der Waals heterostructures. Under optimized growth conditions, ultrathin films of only two TMD layers with a single intercalated Cr-layer are achieved, forming a 2D sheet with van der Waals surfaces. Detailed compositional and structural characterization by scanning tunneling microscopy, grazing incidence x-ray diffraction, and high-resolution Rutherford backscattering indicate the layer-by-layer growth and that the δ can be tuned by post-growth annealing in a range between ∼0.5 and 1. X-ray magnetic circular dichroism and magnetometry measurements demonstrate that all self-intercalated Cr(1+δ)Te2 nanolayers exhibit strong ferromagnetism with magnetic moments larger than 3μB per Cr-atom. The magnetic properties are maintained in the ultrathin limit of a material with a single intercalation layer. Interestingly, the magnetic anisotropy can be tuned from close to isotropic (δ = 1) to a desirable perpendicular anisotropy for low δ values. Thus, the bottom-up growth of these 2D Cr(1+δ)Te2 sheets is a promising approach for designing magnetic van der Waals heterostructures

Absence of magnetic-proximity effect at the interface of Bi2_2Se3_3 and (Bi,Sb)2_2Te3_3 with EuS

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)$_2$(Se,Te)$_3$ family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of $\sim\!10^{-3}$ $\mu_\mathrm{B}$/at.Comment: 7 pages, 3 figures, published in Physical Review Letter

Systematics of electronic and magnetic properties in the transition metal doped Sb2_2Te3_3 quantum anomalous Hall platform

The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically-doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here we report on a detailed and systematic investigation of transition-metal (TM)-doped Sb$_2$Te$_3$. By combining density functional theory (DFT) calculations with complementary experimental techniques, i.e., scanning tunneling microscopy (STM), resonant photoemission (resPES), and x-ray magnetic circular dichroism (XMCD), we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM-doped TIs. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V- and Fe-doped Sb$_2$Te$_3$ display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity-dependent and can vary from in-plane to out-of-plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM-doped Sb$_2$Te$_3$ in the ferromagnetic state.Comment: 40 pages, 13 figure

Direct observation of multivalent states and charge transfer in Ce-doped yttrium iron garnet thin films

Due to their large magneto-optic responses, rare-earth-doped yttrium iron garnets, Y3Fe5O12 (YIG), are highly regarded for their potential in photonics and magnonics. Here, we consider the case of Ce-doped YIG (Ce-YIG) thin films, in which substitutional Ce3+ ions are magnetic because of their 4f1 ground state. In order to elucidate the impact of Ce substitution on the magnetization of YIG, we have carried out soft x-ray spectroscopy measurements on Ce-YIG films. In particular, we have used the element specificity of x-ray magnetic circular dichroism to extract the individual magnetization curves linked to Ce and Fe ions. Our results show that Ce doping triggers a selective charge transfer from Ce to the Fe tetrahedral sites in the YIG structure. This, in turn, causes a disruption of the electronic and magnetic properties of the parent compound, reducing the exchange coupling between the Ce and Fe magnetic moments and causing atypical magnetic behavior. Our work is relevant for understanding magnetism in rare-earth-doped YIG and, eventually, may enable a quantitative evaluation of the magneto-optical properties of rare-earth incorporation into YIG

Spin re-orientation induced anisotropic magnetoresistance switching in LaCo0.5_{0.5}Ni0.5_{0.5}O3−δ_{3-\delta} thin films

Realization of novel functionalities by tuning magnetic interactions in rare earth perovskite oxide thin films opens up exciting technological prospects. Strain-induced tuning of magnetic interactions in rare earth cobaltates and nickelates is of central importance due to their versatility in electronic transport properties. Here we reported the spin re-orientation induced switching of anisotropic magnetoresistance (AMR) and its tunability with strain in epitaxial LaCo$_{0.5}$Ni$_{0.5}$O$_{3-\delta}$ thin films across the ferromagnetic transition. Moreover, with strain tuning, we could observe a two-fold to four-fold symmetry crossover in AMR across the magnetic transition temperature. The magnetization measurements revealed an onset of ferromagnetic transition around 50 K, and a further reduction in temperature showed a subtle change in the magnetization dynamics, which reduced the ferromagnetic long-range ordering and introduced glassiness in the system. X-ray absorption and X-ray magnetic circular dichroism spectroscopy measurements over Co and Ni L edges revealed the Co spin state transition below the magnetic transition temperature leading to the AMR switching and also the presence of Ni$^{2+}$ and Co$^{4+}$ ions evidencing the charge transfer from Ni to Co ions. Our work demonstrated the tunability of magnetic interactions mediated electronic transport in cobaltate-nickelate thin films, which is relevant in understanding Ni-Co interactions in oxides for their technological applications such as in AMR sensors

Control of oxygen vacancy ordering in brownmillerite thin films via ionic liquid gating

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film’s surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices

Effect of the valence state on the band magnetocrystalline anisotropy in two-dimensional rare-earth/noble-metal compounds

[EN] In intermetallic compounds with zero orbital momentum (L = 0) the magnetic anisotropy and the electronic band structure are interconnected. Here, we investigate this connection in divalent Eu and trivalent Gd intermetallic compounds. We find by x-ray magnetic circular dichroism an out-of-plane easy magnetization axis in two-dimensional atom-thick EuAu2. Angle-resolved photoemission spectroscopy and density-functional theory prove that this is due to strong f-d band hybridization and Eu2+ valence. In contrast, the easy in-plane magnetization of the structurally equivalent GdAu2 is ruled by spin-orbit-split d bands, notably Weyl nodal lines, occupied in the Gd3+ state. Regardless of the L value, we predict a similar itinerant electron contribution to the anisotropy of analogous compounds.Discussions with the late J. I. Cerda are warmly thanked. Financial support from Spanish Ministerio deCiencia e Innovacion (projects MAT-2017-88374-P, PID2020-116093RB-C44 and PID2019-103910GB-I00 funded by MCIN/AEI/10.13039/501100011033/) , the Basque Govern-ment (Grants No. IT-1255-19 and No. IT1260-19) , and the University of the Basque Country UPV/EHU (Grant No. GIU18/138) is acknowledged. L.F. acknowledges funding from the European Union's Horizon 2020 research and in-novation programme through the Marie Skodowska-Curie Grant Agreement MagicFACE No. 797109. We acknowl-edge SOLEIL for provision of synchrotron radiation facilities at CASSIOPEE beamline under proposal 20181362. The XMCD experiments were performed at BOREAS beamline at ALBA Synchrotron with the collaboration of ALBA staff. Computational resources were provided by DIPC