Influence of structural disorder on the magnetic properties and electronic structure of ${\mathrm{YCo}}_2$

In this paper the changes of magnetic properties with increasing disorder in the exchange-enhanced Pauli paramagnet ${\mathrm{YCo}}_2$ are discussed. The structural disorder is initially introduced by rapid quenching, while further changes on a micro- or nanoscale are caused by a high-pressure torsion (HPT). Values of the magnetic moment determined for the plastically deformed ribbons reach $0.10{\mu }_{\mathrm{B}}/\mathrm{Co}$ atom (for a sample subjected to the deformation at a pressure of 4 GPa) and $0.25{\mu }_{\mathrm{B}}/\mathrm{Co}$ (6 GPa) at 2 K. The magnetic moment arises not only from the surface of nanocrystals but also from volume. Ab initio calculations explained the influence of chemical disorder and different types of structural defects on the electronic structure and magnetic properties of ${\mathrm{YCo}}_2$-based Laves phases. The calculated magnetic ground states are in qualitative agreement with experimental results for all considered structures with point defects.

Figure 1

Crystal structure of the cubic ${\mathrm{MgCu}}_2$-type Laves phase with the large green balls representing Mg atoms and the small red balls indicating Cu atoms.

Figure 2

X-ray diffraction patterns of ${\mathrm{YCo}}_2$ ribbons in as-quenched state and after a plastic deformation (HPT—high-pressure torsion) at 4 and 6 GPa.

Figure 3

The continuous heating differential scanning calorimetry curves of ${\mathrm{YCo}}_2$ ribbons in as-quenched form and after plastic deformation (HPT—high-pressure torsion) at 4 and 6 GPa measured at the heating rate of 20 K/min.

Figure 4

Temperature dependence of the magnetization of ${\mathrm{YCo}}_2$ ribbons in the as-quenched form and after plastic deformation (HPT—high-pressure torsion) at 4 and 6 GPa.

Figure 5

Magnetic field dependences of magnetization of ${\mathrm{YCo}}_2$ ribbons in as-quenched form and after plastic deformation (HPT—high-pressure torsion) at 4 and 6 GPa measured at (a) 2 K and (b) 300 K.

Figure 6

The total energies calculated with FP-LAPW-PBE method versus lattice parameter. Results for FiM and NM solutions of the bulk ${\mathrm{YCo}}_2$ are presented.

Figure 7

Total and spin projected densities of states (DOS) for bulk ${\mathrm{YCo}}_2$ phase calculated within the PBE approximation. Rows: (1) NM state; (2) FiM state—majority spin; (3) FiM state—minority spin. Columns: (a) total DOS; (b) $\mathrm{Y}4d$ orbitals; (c) $\mathrm{Co}3d$ orbitals.

Figure 8

Total and spin-projected DOS for ${\mathrm{YCo}}_2$. (a) NM state, (b) FiM state, (c) FiM state with $a=8.0\AA$, (d) v@Y—FiM state, (e) Ti@Y—FiM state. Systems designations are explained in the text.

Figure 9

Spin projected total DOS for ${\mathrm{YCo}}_2$ systems with point defects. (a) v@Y, (b) v@Co, (c) Ti@Y, (d) Ti@Co, (e) Nb@Y, (f) Nb@Co. Systems designations are explained in the text.

Figure 10

The supercell of ${\mathrm{YCo}}_2$ with a vacancy in the place of Y atom v@Y (${\mathrm{Y}}_7{\mathrm{Co}}_{16}$). The Co atoms are separated into two types with the spin magnetic moments $m_{\mathrm{Co}}=1.13{\mu }_{\mathrm{B}}$ (blue) and $1.44{\mu }_{\mathrm{B}}$ (red).

Figure 11

Face views of ${\mathrm{YCo}}_2$ structures with point defects: (a) v@Y; (b) v@Co; (c) Ti@Y; (d) Ti@Co. The colors of Co atoms correspond to different values of magnetic moments; see Table 3. Each representation is centered on a specific defect. The considered structures with Nb substitutions do not differ noticeably from the Ti substituted ones; thus they are omitted in the graph.