Ultrafast giant magnetic cooling effect in ferromagnetic Co/Pt multilayers

Abstract

The magnetic cooling effect originates from a large change in entropy by the forced magnetization alignment, which has long been considered to be utilized as an alternative environment-friendly cooling technology compared to conventional refrigeration. However, an ultimate timescale of the magnetic cooling effect has never been studied yet. Here, we report that a giant magnetic cooling (up to 200 K) phenomenon exists in the Co/Pt nano-multilayers on a femtosecond timescale during the photoinduced demagnetization and remagnetization, where the disordered spins are more rapidly aligned, and thus magnetically cooled, by the external magnetic field via the lattice-spin interaction in the multilayer system. These findings were obtained by the extensive analysis of time-resolved magneto-optical responses with systematic variation of laser fluence as well as external field strength and direction. Ultrafast giant magnetic cooling observed in the present study can enable a new avenue to the realization of ultrafast magnetic devices.

Role of non-thermal electrons in ultrafast spin dynamics of ferromagnetic multilayer

“…G ee is an energy exchange coefficient between non-thermal and thermal electrons. The K l term describes the thermal diffusion of energy via the lattice, which is modeled to be proportional to the third power of the temperature increase of the lattice system 12,33 .…”

“…Photoinduced spin dynamics allows us to control magnetic moments on a femtosecond time scale simply by optical pulses 4,5 , as well as by external field or spin current 6,7 . Since the seminal work on a Ni single layer by Beaurepaire et al 8 , numerous studies have investigated the underlying mechanism of ultrafast photoinduced spin dynamics [9][10][11][12][13] . It has been generally accepted that energy from photons is transferred first into the electron sub-system within a few tens of femtoseconds and hot electrons then transfer their energies into the spin and the lattice sub-systems, leading to a final equilibrium state among electron, spin, and lattice sub-systems [8][9][10] .…”

Accelerating ultrafast magnetization reversal by non-local spin transfer

“…Such spin cooling and spin heating of magnetization can be observed when the external source of angular momentum is a magnetic field 23 , spin-polarized hot electrons 24 – 26 , the spin angular momentum coming from one sublattice in ferrimagnets 27 – 29 or a Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange coupling 30 , 31 . In these cases, however, magnetization was either far from quenched or CSD was still present (see the supporting information in ref.…”

Ultrafast Orbital‐Oriented Control of Magnetization in Half‐Metallic La0.7Sr0.3MnO3 Films

“…[2][3][4] One of the key issues related to the ultrafast spin dynamics of magnetic materials is the demagnetization process. In the transition 3d ferromagnetic metals, [5][6][7][8] the magnetic order was found to be quenched in a sub-picosecond timescale and then re-magnetized in a longer timescale of several picoseconds (ps). In half metals, [9][10][11] such as Sr2FeMoO6, CrO2 and La0.6Sr0.4MnO3, experimental observations have shown that the ultrafast demagnetization involves two steps: an initial instantaneous decrease within a ps and a subsequent slow decreasing response lasting for several hundreds of ps.…”

“…However, the transient enhancement of the magnetization observed with s-polarized probe light cannot be expected from the thermal process, which should lead to an instantaneous drop rather than increase in magnetization as previously reported. [8] Considering the strongly coupled degrees of freedom between the spin and orbital in the LSMO film, we expect that the orbital orientation plays a role in the magnetization enhancement. To verify this assumption, we have investigated the effect of the probe light polarization orientation on the pump-induced spin dynamics.…”