Insights into Ultrafast Demagnetization in Pseudogap Half-Metals

Interest in femtosecond demagnetization dynamics was sparked by Bigot’s experiment in 1996, which unveiled the elementary mechanisms that relate the electrons’ temperature to their spin order. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high-spin-polarization metals, and consequently there is broad interest in further understanding the physics of these phenomena. To gain new and relevant insights, we performed ultrafast optical pump-probe experiments to characterize the demagnetization processes of highly spin-polarized magnetic thin films on a femtosecond time scale. Full spin polarization is obtained in half-metallic ferro- or ferrimagnets, where only one spin channel is populated at the Fermi level, whereas the other one exhibits a gap. In these materials, the spin-scattering processes is controlled via the electronic structure, and thus their ultrafast demagnetization is solely related to the spin polarization via a Fermi golden-rule model. Accordingly, a long demagnetization time correlates with a high spin polarization due to the suppression of the spin-flip scattering at around the Fermi level. Here we show that isoelectronic Heusler compounds (${\mathrm{Co}}_2\mathrm{MnSi}$, ${\mathrm{Co}}_2\mathrm{MnGe}$, and ${\mathrm{Co}}_2\mathrm{FeAl}$) exhibit a degree of spin polarization between 59% and 86%. We explain this behavior by considering the robustness of the gap against structural disorder. Moreover, we observe that CoFe-based pseudogap materials, such as partially ordered Co-Fe-Ge and Co-Fe-B alloys, can reach similar values of the spin polarization. By using the unique features of these metals we vary the number of possible spin-flip channels, which allows us to pinpoint and control the half-metals’ electronic structure and its influence on the elementary mechanisms of ultrafast demagnetization.

Popular Summary

Achieving high spin polarizations in magnetic materials and switching their orientations with a high degree of control and high speed are at the heart of the engineering of spin-based electronic devices, such as magnetic hard disks or memory devices. Microscopic processes in a magnetic material involve electrons, their spins, and their interactions with phonons in the material. It has been demonstrated that the interaction of an ultrafast laser pulse with a magnetic, metallic material can not only modify the magnetic state of the material, but also provides an exciting approach into those microscopic processes. The experiments we report in this paper exploit such interactions and demonstrate successful control of spin polarization and spin dynamics on ultrafast time scales in magnetic materials through their electronic structures.

For achieving a systematic variation in electronic structure as a control, we have made a judicious selection of a number of magnetic materials. Some of the materials are so-called half-metals, in the family of the Heusler compounds, and their electronic structures are such that conducting electrons have their spins oriented in only one direction. The materials act as a metal for one and as an insulator for the other spin direction. Others, called pseudogap materials, are close relatives to half-metals, but their electronic structure is robust against structural disorder.

We have observed in the series of Heusler compounds a large range of variation in spin polarization, from 59% to 86%. Interestingly, similar values of spin polarization are seen in the pseudogap materials as well, making them another promising class of high-spin-polarization materials. We have also revealed the effectiveness of this electronic-structure-based spin-flip suppression with the observed long demagnetization times of several hundreds of femtoseconds.

Figure 1

Film optimization. Annealing temperature dependence of ${\mathrm{Co}}_2\mathrm{FeAl}$. Panel (a) depicts the x-ray diffraction 004-peak intensity, (b) shows the demagnetization time (vertical bar marks the value of ${\tau }_m$), the lines are fits using an analytical function derived from a three-temperature model, and (c) exhibits the TMR of the ${\mathrm{Co}}_2\mathrm{FeAl}/\mathrm{MgO}/\mathrm{CoFe}$ junctions.

Figure 2

Ultrafast demagnetization spectra. Femtosecond pump-probe experiments for different materials are sorted by their spin polarization $P$. Heusler compounds are indicated by red data points. The lines are fits using an analytical function derived from a three-temperature model, and the vertical bar marks the value of ${\tau }_m$.

Figure 3

Demagnetization time ${\tau }_m$ versus spin polarization $P$. The half-metallic properties can be classified in a $P$ versus ${\tau }_m$ plot. If the points lie on top of the lines given, spin-flip blocking is the dominant mechanism describing the simultaneous increase of $P$ and ${\tau }_m$. The lines are model calculations using Fermi’s golden-rule approach showing the ${\tau }_{\mathrm{el}\mathrm{\text{−}}\mathrm{sp}}\sim (1-P{)}^{-1}$ behavior. For each material the demagnetization time ${\tau }_m$ taken from Fig. 2 is plotted as a function of the spin polarization $P$. Note the logarithmic scale for ${\tau }_m$. Intersections are $P=0$, ${\tau }_m=\frac{{\tau }_{\mathrm{el},0}}{c^2}=100\mathrm{fs}$ and $P=1$, ${\tau }_m=3\mathrm{ps}$ or 1 ns, respectively. Heusler compounds are given in red. In brackets the value of $P$ and ${\tau }_m$ (fs) is given.

Figure 4

Probing the electronic structure. (a) Tunneling spectroscopy of Heusler-based magnetic tunnel junctions. A gaplike feature of 300 mV is visible in the ${\mathrm{Co}}_2\mathrm{FeAl}$ spectrum. (b) Inelastic electron-tunneling spectra of the Heusler-based and the reference Co-Fe-B junctions. In ${\mathrm{Co}}_2\mathrm{FeAl}$, the magnon peak at negative bias is suppressed (compare Ref. 42).

Figure 5

Energy dependent golden-rule model. (a) Model density of states for a one-spin channel convoluted with temperature-dependent kernel with a width corresponding to $T=600\mathrm{K}$ (approximately 60 meV) given by the derivative of the Fermi distribution $-{\partial }_{E}f(E)$, followed by the according spin polarization $P$. In panels (b)–(d) the influence of the thermal excitations on the spin-flip suppression factor ${\tau }_{\mathrm{el}\mathrm{\text{−}}\mathrm{sp}}\sim (1-P{)}^{-1}$ is calculated in analogy to (a) for three cases: a large gap of 1 eV (b), a small gap of 300 meV (experimental value for ${\mathrm{Co}}_2\mathrm{FeAl}$) in the spin down density of states (c), as well as for a pseudogap, representing the case of Co-Fe-Ge (d). The suppression factor is given on a logarithmic scale.