The influence of the mechanical strain and chemical substitution on the magnetic anisotropy energy (MAE) of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ is studied using first-principles calculations. Dzyaloshinskii-Moriya interaction contributes substantially to MAE by inducing spin canting when the antiferromagnetic order parameter is not aligned with the hexagonal axis. Nearly cubic crystal field results in a very small MAE in pure ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ at zero strain, which is incorrectly predicted to be negative (in-plane) on account of spin canting. The MAE is strongly modified by epitaxial strain, which tunes the crystal-field splitting of the ${t}_{2g}$ triplet. The contribution from magnetic dipolar interaction is very small at any strain. The effects of cation (Al, Ti, V, Co, Fe, Nb, Zr, Mo) and anion (B) substitutions on MAE are examined. Al increases MAE thanks to the local lattice deformation. In contrast, the electronic configuration of V and Nb strongly promotes easy-plane anisotropy, while other transition-metal dopants have only a moderate effect on MAE. Substitution of oxygen by boron, which has been reported to increase the N\'eel temperature, has a weak effect on MAE, whose sign depends on the charge state of B. The electric field applied along the (0001) axis has a weak second-order effect on the MAE.

https://link.aps.org/accepted/10.1103/PhysRevMaterials.3.034405

Influence of strain and chemical substitution on the magnetic anisotropy of antiferromagnetic Cr 2 O 3 : An ab-initio study

The use of molecular modification on magnetism has gained considerable interest in the development of multifunctional molecular spintronics. Such hybrid structures of nonmagnetic molecules and ferromagnetic metals manifest great promises of producing novel electric and magnetic features. The external electric field effect on the magnetism of C6H6-adsorbed Fe3O4(001) surface is elucidates by density functional theory calculations. The reduced magnetic moments of partial octahedral Fe atoms in the first layer break the spherical spatial spin density distribution. Such modification that is independent of the direction of electric field can be attributed to the charge redistribution as a result of screening effect, which changes orbital occupancy in unpaired octahedral Fe-d electrons near EF accompanied by a spin flip. Furthermore, octahedral Fe atom underneath C atom changes only as the applied field is large enough. Additionally, it is shown that the study of modulation on surface magnetism through external...

Electric Field Tunable Magnetism at C6H6-Adsorbed Fe3O4(001) Surface

The influence of the mechanical strain and chemical substitution on the magnetic anisotropy energy (MAE) of Cr$_2$O$_3$ is studied using first-principles calculations. Dzyaloshinskii-Moriya interaction contributes substantially to MAE by inducing spin canting when the antiferromagnetic order parameter is not aligned with the hexagonal axis. Nearly cubic crystal field results in a very small MAE in pure Cr$_2$O$_3$ at zero strain, which is incorrectly predicted to be negative (in-plane) on account of spin canting. The MAE is strongly modified by epitaxial strain, which tunes the crystal-field splitting of the $t_{2g}$ triplet. The contribution from magnetic dipolar interaction is very small at any strain. The effects of cation (Al, Ti, V, Co, Fe, Nb, Zr, Mo) and anion (B) substitutions on MAE are examined. Al increases MAE thanks to the local lattice deformation. In contrast, the electronic configuration of V and Nb strongly promotes easy-plane anisotropy, while other transition-metal dopants have only a moderate effect on MAE. Substitution of oxygen by boron, which has been reported to increase the Neel temperature, has a weak effect on MAE, whose sign depends on the charge state of B. The electric field applied along the (0001) axis has a weak second-order effect on the MAE.

Influence of strain and chemical substitution on the magnetic anisotropy of antiferromagnetic Cr$_2$O$_3$: an ab-initio study

We present the design and micromagnetic simulations of lenses for spin waves with anisotropic dispersion. Spin waves are attractive candidates for on-chip wave-computing devices. These, in general, require means of generation, manipulation, and detection of spin waves; here, we address manipulation of spin waves. Most wave-computing algorithms proposed so far require isotropic wave propagation, since it is the easiest to handle, and well-established concepts from optics can be used. Spin waves in in-plane magnetized magnetic films are, in general, anisotropic, and this magnetization direction is often favorable from practical perspectives. We investigated the possibility of using anisotropic spin waves in designs based on the Fourier transforming property of a lens.

Lens Design for Computing With Anisotropic Spin Waves

We investigate the second harmonic generation (SHG) signal in strained Cr2O3 clusters. We show that the SHG signal generated by nanometric Cr2O3 clusters embedded in MgO varies under an applied electric field, at room temperature. The variation of the intensity follows a Langevin law as a function of the electric field, which is consistent with a super-paraelectric clusters assembly. This reveals the presence of a weak spontaneous electric dipole in Cr2O3 when in the shape of highly strained epitaxial clusters, whereas this material does not posses any permanent electric dipole in the bulk phase. These results indicate that the multiferroic state recently observed at low temperature in those clusters, which was associated to a giant magneto-electric effect, might still exist at room temperature: this opens the way to new applications based on chromium oxide strained nanoparticles.

https://iopscience.iop.org/article/10.1088/1361-648X/aa68f8/pdf

Evidence of a permanent electric polarisation in highly strained Cr 2 O 3 clusters measured by a second harmonic generation technique

Magnetic properties of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ epitaxial clusters inserted in an Fe/MgO/Fe tunnel barrier are revealed by their tunnel magnetoresistance signature. The cluster assembly has been shown in a previous work to behave as a superparamagnet when a magnetic field was applied in the plane of the tunnel junction. We here demonstrate that an external large out-of plane electric field (in the order of 0.5 GV/m) favors in-plane magnetization orientation. This is due to an electric-field-induced magnetic anisotropy along the normal to the plane, corresponding to large anisotropy fields reaching up to 2 T. The assembly of clusters is thus strictly speaking not superparamagnetic and its magnetization cannot be exactly described by a Langevin law. This is attributed either to a strain-induced enhanced magnetoelectric effect or to a voltage-induced change of the magnetic anisotropy at interfaces with MgO.

Mohamad Hamieh

Papers

Controlling the magnetic anisotropy in epitaxial Cr 2 O 3 clusters by an electric field