Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

Abstract

We have used inelastic neutron scattering measurements to study the magnetic excitations of Co core/CoO shell nanoparticles for energies from 0-50 meV. Above the blocking temperature TB, broad quasielastic scattering is observed, corresponding to the reorientation of the Co core moments and to paramagnetic CoO scattering. Below TB, two nearly dispersionless inelastic peaks are found, whose energies increase with decreasing temperature as order parameters, controlled by the nanoparticle Néel temperature TN =235 K, and saturating as T→0 at 2.7 meV and 6.7 meV, respectively. Similar excitations were observed in a powdered single crystal of CoO, indicating that both are intrinsic excitations of CoO, resulting from the exchange splitting of single ion states for T≤TN . Pronounced finite size effects are observed for the scattering from the CoO nanoparticle shells, whose thicknesses range from 1.7-4.5 nm. These include an enhanced excitation linewidth, as well as a response that is not only spread over a much wider range of wave vectors, but is also significantly more intense in the nanoparticles than in bulk CoO.

Neutron scattering investigation of thed−dexcitations below the Mott gap of CoO

“…We therefore have no evidence for the low-energy spin excitations claimed to be seen in Ref. 40 below the gapped (∼25 meV) onemagnon excitations described previously.…”

Ordered magnetism in the intrinsically decorated jeff=12α−CoV3O8

“…A key difference between MnF 2 and both α-CoV 3 O 8 and FeF 2 is the presence of strong crystal field effects and spin-orbit coupling in the latter two compounds 133,175 . It is also worth noting that unlike the case of pure CoO 81,91,94,[181][182][183][184] where the large and far reaching exchange constants result in a significant and ultimately problematic entanglement of spin-orbit levels 94 , in the case of α-CoV 3 O 8 , the exchange constants are weak and the Weiss temperature is near 0 K. Both observations suggest that the presence of both strong crystal field effects and spin-orbit coupling with well-separated j eff manifolds, as is the case for α-CoV 3 O 8 , may be central to making the dynamics robust against strong disorder.…”

Strong magnon softening in tetragonal FeCo compounds

“…It is found that above some threshold magnetic field the uniform precessional mode, i.e., the k = 0 magnons decay into nonuniform magnons (k = 0), i.e., the Zeeman energy stays in the magnetic subsystem and scatters between magnon modes. [17][18][19][20] However, in ultrafast magnetization reversal the high-energy electrons generated by the laser field decay into the lower-energy magnon excitations. [22][23][24] In both cases spin-orbit coupling (SOC) is responsible for the transfer of the angular momentum to the lattice through different scattering mechanisms such as magnon-magnon, magnon-phonon, magnon-impurity scattering, and so on, where each process has a different relaxation time.…”

“…12 In current devices the switching speeds have reached a point where dynamical effects are becoming very important [12][13][14][15][16] Magnons are created in fast (field driven) as well as ultrafast (laser induced) magnetization reversal processes. [17][18][19][20][21][22][23][24][25][26] The former case is of particular interest for current device applications. It is found that above some threshold magnetic field the uniform precessional mode, i.e., the k = 0 magnons decay into nonuniform magnons (k = 0), i.e., the Zeeman energy stays in the magnetic subsystem and scatters between magnon modes.…”