Research ArticlePHYSICS

Direct light–induced spin transfer between different elements in a spintronic Heusler material via femtosecond laser excitation

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Science Advances  17 Jan 2020:
Vol. 6, no. 3, eaaz1100
DOI: 10.1126/sciadv.aaz1100
  • Fig. 1 Direct light–induced spin transfer in Co2MnGe.

    (A) Representation of spin dynamics in Co2MnGe. Before excitation, Mn atoms (orange arrows) have a 3× larger magnetic moment than Co atoms (blue arrows), which are 2× more abundant in the bcc lattice. The purple arrow represents the net magnetic moment of the compound. Immediately upon excitation by light (within a few femtoseconds), the Mn moment starts to decrease and the Co magnetic moment grows by 10%. Hundreds of femtoseconds later, the Mn and Co atomic spins become disordered, and the angular momentum begins to transfer to the lattice. After 1 to 2 ps, the spins have reached their maximum quenching. (B) Schematic of the experimental setup. Ultrafast femtosecond laser pulses excite the sample, while the element-specific magnetization dynamics are tracked using femtosecond EUV pulses. IR, infrared. (C) Density of states for each element in the half-metal. Note that the minority spin channel is gapped, with no available states at the Fermi level for the minority channel. Critically, this gap is larger for Mn than for Co. After excitation (dominated by transitions from the Mn majority states, marked by a red arrow), the conduction band states are hybridized, as illustrated by the shared red wave function.

  • Fig. 2 Element-resolved ultrafast magnetization dynamics following excitation by a femtosecond laser.

    (A) In the half-metallic B2 phase, the Co magnetization increases as Mn decreases, with the changes happening as soon as light is incident on the material. Here, the open circles are the data points, and the solid lines represent a best fit to the data points. Inset: Dynamics of ultrafast spin transfer. Here, the lines link the actual data points. The blue and orange solid lines plot the magnetization of Mn and Co for a 55-fs (FWHM) pump pulse. The gray dashed lines show the broader response time for a longer 90-fs pump pulse. Note that the location of the peak of the enhancement is shifted in time by ~20 fs (approximately half the difference between the duration of the two pulses), underlining that this process is a direct optical manipulation. (B) Element-resolved ultrafast magnetization dynamics in the non–half-metallic A2 phase. There is no enhancement of the Co magnetization. (C) Atomic structure of the compounds studied. In the B2 phase, the Co atoms are ordered and occupy sites at the edges of the bcc structure, while the centers are randomly interspersed between Mn and Ge. In the A2 phase, the material has formed the ordered bcc structure, but the locations of atoms within the structure are random.

  • Fig. 3 Density of states for Co2MnGe.

    Density of states (DOS) in the (A) B2 phase and (B) A2 phase. Note that the half-metallic character is only present in the B2 phase.

  • Fig. 4 Mechanism for direct light–induced spin transfer in Co2MnGe.

    The probability for exciting a spin-up (majority) versus spin-down (minority) electron from the valence band in the B2 phase for different pump energies in (A) Mn sites. Note that for a 1.55-eV pump, the probability is higher for spin-up electrons to be excited from Mn. (B) Probability for excitations in Co sites. In contrast to the Mn result, the probability is higher for minority electrons to be excited in Co. (C) Illustration of a process that leads to direct optical transfer of spin polarization from Mn to Co. The initial-state wave function is hybridized and composed of both Mn and Co d-states, with a larger contribution from the Mn atom. In the final state, the situation is reversed, and the Co d-states dominate. Hence, when an electron is optically excited from the initial- to the final-state wave function, this is associated with a transfer of spin polarization from Mn to Co.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaaz1100/DC1

    Supplementary Text

    Section S1. Further details of the experimental setup

    Section S2. Sample preparation

    Section S3. Change in reflectivity due to optical excitation in both phases

    Section S4. Element-averaged response of the A2 and B2 phases

    Section S5. Dynamics of Co2MnGe on α-Al2O3

    Section S6. Description of method for calculating the density of states and magnetic moment in Co2MnGe

    Section S7. Method for calculating the transition probabilities in Fig. 4

    Section S8. Results of the calculations of k-conserving transition probabilities for the L21 phase

    Section S9. Results of aLLG simulations

    Fig. S1. Detailed experimental layout used to capture the element specific magnetic response of Co2MnGe using EUV light.

    Fig. S2. Static asymmetry measurements for the two phases of the material.

    Fig. S3. Change in reflectivity measured for both the A2 and B2 phases.

    Fig. S4. Total magnetization of sample plotted with element resolved signal.

    Fig. S5. Element-resolved dynamics of Co2MnGe (B2 phase) on sapphire.

    Fig. S6. Element- and orbital-resolved density of states for the A2 and B2 phases.

    Fig. S7. Transition probabilities for the dipole allowed transitions as a function of photon energy in the B2 phase of Co2MnGe.

    Fig. S8. Transition probabilities for the dipole-allowed transitions in the A2 phase.

    Fig. S9. Transition probabilities for Mn and Co in the A2 phase.

    Fig. S10. K-conserving transition probabilities for the L21 phase compared to non k-conserving transitions.

    Fig. S11. Element-resolved demagnetization in Co2MnGe for the L21, A2, and B2 phases.

    Table S1. Fitting parameters of the double exponential function to fit M(t)/M(0).

    Table S2. Site-resolved Gilbert damping parameter in the L21 phase.

    Table S3. Site-resolved Gilbert damping parameter in the A2 phase.

    Table S4. Site-resolved Gilbert damping parameter in the B2 phase.

    Table S5. Fitting parameters for fits performed to the functions used in the main text.

    References (3542)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Section S1. Further details of the experimental setup
    • Section S2. Sample preparation
    • Section S3. Change in reflectivity due to optical excitation in both phases
    • Section S4. Element-averaged response of the A2 and B2 phases
    • Section S5. Dynamics of Co2MnGe on α-Al2O3
    • Section S6. Description of method for calculating the density of states and magnetic moment in Co2MnGe
    • Section S7. Method for calculating the transition probabilities in Fig. 4
    • Section S8. Results of the calculations of k-conserving transition probabilities for the L21 phase
    • Section S9. Results of aLLG simulations
    • Fig. S1. Detailed experimental layout used to capture the element specific magnetic response of Co2MnGe using EUV light.
    • Fig. S2. Static asymmetry measurements for the two phases of the material.
    • Fig. S3. Change in reflectivity measured for both the A2 and B2 phases.
    • Fig. S4. Total magnetization of sample plotted with element resolved signal.
    • Fig. S5. Element-resolved dynamics of Co2MnGe (B2 phase) on sapphire.
    • Fig. S6. Element- and orbital-resolved density of states for the A2 and B2 phases.
    • Fig. S7. Transition probabilities for the dipole allowed transitions as a function of photon energy in the B2 phase of Co2MnGe.
    • Fig. S8. Transition probabilities for the dipole-allowed transitions in the A2 phase.
    • Fig. S9. Transition probabilities for Mn and Co in the A2 phase.
    • Fig. S10. K-conserving transition probabilities for the L21 phase compared to non k-conserving transitions.
    • Fig. S11. Element-resolved demagnetization in Co2MnGe for the L21, A2, and B2 phases.
    • Table S1. Fitting parameters of the double exponential function to fit M(t)/M(0).
    • Table S2. Site-resolved Gilbert damping parameter in the L21 phase.
    • Table S3. Site-resolved Gilbert damping parameter in the A2 phase.
    • Table S4. Site-resolved Gilbert damping parameter in the B2 phase.
    • Table S5. Fitting parameters for fits performed to the functions used in the main text.
    • References (3542)

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