Spectroscopic studies on the electronic and magnetic properties of Cr-based ferromagnetic thin films

概要

The two-dimensional (2D) magnetic materials attract lots of attention in the research
field because intrinsic long-range magnetic order and large magnetic anisotropy can be
manipulated by controlling thickness and carrier doping. Therefore, 2D magnets have
a high potential for developing next-generation spintronic devices. We focus on both
van der Waals (vdW) and non-vdW 2D magnets and in particular focus on the Cr-based
materials which exhibit high Curie temperature (TC). They are an excellent platform to
realize the magnetism in both bulk and thin-film forms.
2. XAS and XMCD results of (CrxSb1-x)2Te3
Magnetic doping in a topological insulator (TI) leads to long-range ferromagnetic
(FM) order with a gradual temperature (T)-dependent energy gap at the Dirac-point in
topological surface states (TSS). For ferromagnetic TI with highest Curie-temperature
(TC) (CrxSb1-x)2Te3, the magnetic gap remains elusive. We carried out the T-dependent
X-ray absorption spectroscopy and magnetic circular dichroism (XAS-XMCD) on
(CrxSb1- x)2Te3 to probe the magnetic gaps in the unoccupied dopant states and TSS. The
magnetic gaps track the T-dependent XMCD intensity and bulk magnetization. Angleresolved photoemission spectroscopy (ARPES) reveals that linear band dispersion of
TSS is retained for high doping in the occupied states.
3. Cr2Se3 thin film
We have successfully fabricated an ultrathin film of Cr2Se3 by molecular beam
epitaxy method (MBE), and characterized its crystal structure by reflection high energy
electron diffraction (RHEED) and low-energy electron diffraction (LEED). In order to
clarify the electronic structure related to the ferromagnetism of 2D Cr2Se3, we have
carried out Cr L-edge x-ray absorption spectroscopy and confirmed a Cr3+ configuration,
which is consistent with the cluster model calculation. The temperature dependent
ARPES measurements have shown a characteristic change in the valence-band
structure associated with the FM transition.
4. Conclusion
Overall, the spectroscopic studies of Cr-doped Sb2Te3 showed the direct link
between unoccupied Cr 3d dopant states and Te 5p topological surface state leading to
the magnetic gap formation in the FM state. Ultrathin film of Cr2Se3 showed
ferromagnetism unlike the case of bulk crystal showing antiferromagnetism. We have
concluded that both ferromagnetic Cr-doped Sb2Te3 and ML Cr2Se3 are useful for
application in spintronic devices owing to their high TC nature. ...