Magnetic exchange interactions in monolayer CrI3 from many-body wavefunction calculations

The marked interplay between the crystalline, electronic, and magnetic structure of atomically thin magnets has been regarded as the key feature for designing next-generation magneto-optoelectronic devices. In this respect, a detailed understanding of the microscopic interactions underlying the magnetic response of these crystals is of primary importance. Here, we combine model Hamiltonians with multireference configuration interaction wavefunctions to accurately determine the strength of the spin couplings in the prototypical single-layer magnet CrI3. Our calculations identify the (ferromagnetic) Heisenberg exchange interaction J = -1.44 meV as the dominant term, being the inter-site magnetic anisotropies substantially weaker. We also find that single-layer CrI3 features an out-of-plane easy axis ensuing from a single-ion anisotropy A = -0.10 meV, and predict g-tensor in-plane components gxx=gyy=1.90g(zz) = 1.92. In addition, we assess the performance of a dozen widely used density functionals against our accurate correlated wavefunctions calculations and available experimental data, thereby establishing reference results for future first-principles investigations. Overall, our findings offer a firm theoretical ground to recent experimental observations.

Microfluidic magnetic particles manipulation

Amar Rida

In this thesis work we propose new concepts of transporting and manipulating magnetic particles on microfluidic scale. The objective is to propose solutions to the problems related to the integration of the magnetic inductive components within a microfluidic network for magnetic particle manipulation. A second part of this work is the study of the physics of magnetic particle pattern formation when a suspension of the particles is subjected to an external magnetic field. The first concept that we propose is based on the dynamic motion of a selfassembled structure of ferromagnetic beads that are retained within a microfluidic flow using a local alternating magnetic field. We show that an alternating magnetic field induces a rotational motion of the magnetic particles, thereby strongly enhancing the fluid perfusion through the magnetic structure that behaves as a dynamic random porous medium. The result is a very strong particle-liquid interaction that can be controlled by adjusting the magnetic field frequency and amplitude, as well as the liquid flow rate. This fact is at the basis of very efficient liquid mixing. We anticipate that the intense interaction between the fluid and magnetic particles with functionalized surfaces holds large potential for the development of future bead-based assays. The second concept is the transport of magnetic particles on microfluidic scales over long-range distances using an array of simple planar coils placed in large static magnetic field. This magnetic field imposes a permanent magnetic moment to the microparticles, so that a very small magnetic field gradient of a simple planar coil is sufficient to displace the microparticles. We demonstrate that the repulsive and attractive magnetic forces generated by adjacent strongly overlapped coils, allow an efficient and well controlled magnetic particle displacement along one-dimensional or two dimensional paths. Finally, we study the dynamics of supraparticles structure (SPS) patterns of a ferromagnetic particle suspension in an alternating magnetic field. We show that the application of an alternating magnetic field to a ferromagnetic particle suspension results in a phase separation of the supraparticles structures (SPS) patterns into periodic structures of columns. Moreover, we investigate the dynamic motion, relative to a microfluidic flow, of such magnetic SPS. The latter are locally manipulated with an alternating magnetic field that is transverse to a microfluidic channel. We show that the SPS in the microchannel are composed of weakly aggregated and open columnar-like structures, allowing large particles surfaces to be in contact with the fluid flow.

EPFL2004

Magnetic anisotropy of Fe and Co ultrathin films deposited on Rh(111) and Pt(111) substrates: An experimental and first-principles investigation

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We report on a combined experimental and theoretical investigation of the magnetic anisotropy of Fe and Co ultrathin layers on strongly polarizable metal substrates. Monolayer (ML) films of Co and Fe on Rh(111) have been investigated in situ by x-ray magnetic circular dichroism (XMCD), magneto-optic Kerr effect, and scanning tunneling microscopy. The experiments show that both magnetic adlayers exhibit ferromagnetic order and enhanced spin and orbital moments compared to the bulk metals. The easy magnetization axis of 1 ML Co was found to be in plane, in contrast to Co/Pt(111), and that of 1 ML Fe out of plane. The magnetic anisotropy energy (MAE) derived from the magnetization curves of the Fe and Co films is one order of magnitude larger than the respective bulk values. XMCD spectra measured at the Rh M2,3 edges evidence significant magnetic polarization of the Rh(111) surface with the induced magnetization closely following that of the overlayer during the reversal process. The easy axis of 1–3 ML Co/Rh(111) shows an oscillatory in-plane/out-of-plane behavior due to the competition between dipolar and crystalline MAE. We present a comprehensive theoretical treatment of the magnetic anisotropy of Fe and Co layers on Rh(111) and Pt(111) substrates. For free-standing hexagonally close-packed monolayers the MAE is in plane for Co and out of plane for Fe. The interaction with the substrate inverts the sign of the electronic contribution to the MAE, except for Fe/Rh(111), where the MAE is only strongly reduced. For Co/Rh(111), the dipolar contribution outweighs the band contribution, resulting in an in-plane MAE in agreement with experiment while for Co/Pt(111) the larger band contribution dominates, resulting in an out-of-plane MAE. For Fe films however, the calculations predict for both substrates an in-plane anisotropy in contradiction to the experiment. At least for Fe/Pt(111) comparison of theory and experiment suggests that the magnetic structure of the adlayer is more complex than the homogenous ferromagnetic order assumed in the calculations. The angular momentum and layer-resolved contributions of the overlayer and substrate to the MAE and orbital moment anisotropy are discussed with respect to the anisotropic hybridization of the 3d, 4d, and 5d electron states and vertical relaxation. The role of technically relevant parameters such as the thickness of the surface slab, density of k points in the Brillouin zone, and electron-density functionals is carefully analyzed.

2010

Crystal Structure and Magnetic Properties of Two New Antiferromagnetic Spin Dimer Compounds; FeTe3O7X (X = Cl, Br)

Helmuth Berger, Jia Liu, Dong Zhang

Two new isostructural layered oxohalides FeTe3O7X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic susceptibility and magnetization measurements, and also by density functional theory (DFT) calculations of the electronic structure and the spin exchange parameters. FeTe3O7X crystallizes in the monoclinic space group P2(1)/c with the unit cell parameters a = 10.7938(5), b = 7.3586(4), c = 10.8714(6) angstrom, beta = 111.041(5)degrees, Z = 4 for FeTe3O7CI, and a = 11.0339(10), b = 7.3643(10), c = 10.8892(10) angstrom, beta = 109.598(10)degrees, Z = 4 for FeTe3O7Br. Each compound has one unique Fe3+ ion coordinating a distorted [FeO5] trigonal bipyramid. Two such groups share edges to form [Fe2O8] dimers that are isolated from each other by Te4+ ions. The high-temperature magnetic properties of the compounds as well as spectroscopic investigations are consistent with an isolated antiferromagnetic spin dimer model with almost similar spin gaps of similar to 35 K for X = Cl and Br, respectively. However, deviations at low temperatures in the magnetic susceptibility and the magnetization data indicate that the dimers couple via an interdimer coupling. This interpretation is also supported by DFT calculations which indicate an interdimer exchange which amounts to 25% and 10% of the intradimer exchange for X = Cl and Br, respectively. The magnetic properties support the counterion character and a weak integration of halide ions into the covalent network similar to that in many other oxohalides.