Strongly anisotropic spin dynamics in magnetic topological insulators

Alfonsov, A.; Facio, Jorge I.; Mehlawat, Kavita; Moghaddam, Ali G.; Ray, Rajyavardhan; Zeugner, Alexander; Richter, Manuel; Brink, Jeroen van den; Isaeva, Anna; Büchner, B.; Kataev, V.

doi:10.1103/physrevb.103.l180403

Cited by 21 publications

(18 citation statements)

References 50 publications

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“…While the early synchrotron ARPES studies reported 7,12,14 that the DP gap in MnBi 2 Te 4 persists well above the Néel temperature, recent laser-ARPES data show about 40% reduction of the DP gap (from 65 to 40 meV) upon heating from below T N up to 35 K 71 . An incomplete closing of the gap seems to be consistent with strong short range order effects that persist in MnBi 2 Te 4 up to about 50-60 K, as observed by electron spin resonance, ferromagnetic resonance, and antiferromagnetic resonance experiments 7,72,73 . The measured magnetization data 52 , revealing that MnBi 2 Te 4 is not in the paramagnetic limit even at T ≈ 50 K, confirm this observation.…”

Section: Discussionsupporting

confidence: 71%

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Garnica¹,

Otrokov²,

Aguilar³

et al. 2021

Preprint

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The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi2Te4 is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi2Te4 Dirac cone. Here, we study the crystalline and electronic structure of MnBi2Te4(0001) using scanning tunneling microscopy/spectroscopy (STM/S), micro(µ)-laser angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. Our topographic STM images clearly reveal features corresponding to point defects in the surface Te and subsurface Bi layers that we identify with the aid of STM simulations as BiTe antisites (Bi atoms at the Te sites) and MnBi substitutions (Mn atoms at the Bi sites), respectively. X-ray diffraction (XRD) experiments further evidence the presence of cation (Mn-Bi) intermixing. Altogether, this affects the distribution of the Mn atoms, which, inevitably, leads to a deviation of the MnBi2Te4 magnetic structure from that predicted for the ideal crystal structure. Our transport measurements suggest that the degree of this deviation varies from sample to sample. Consistently, the ARPES/STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples/sample cleavages. Our DFT surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, MnBi defects can cause a strong reduction of the MnBi2Te4 Dirac point gap, given the recently proved antiparallel alignment of the MnBi moments with respect to those of the Mn layer. Our results provide a key to puzzle out the MnBi2Te4 Dirac point gap mystery.

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Section: Discussionsupporting

confidence: 71%

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Garnica¹,

Otrokov²,

Aguilar³

et al. 2021

Preprint

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“…Spin fluctuations can form in magnetic TIs and preserve the out-of-plane spin anisotropy even above Curie/Neel temperature. As was shown for such fluctuations exist up to 50–60 K which is 2–3 times higher than the magnetic transition temperature in this material 35 , 36 . Concerning (ii), it means that the bulk magnetic measurements cannot be directly used for the analysis of the surface magnetic properties modification.…”

Section: Resultssupporting

confidence: 53%

“…There are several attempts to solve the problem of unclosed KP/DP gap. Namely, (i) spin fluctuations and (ii) increased surface transition temperature 35 , 36 . Spin fluctuations can form in magnetic TIs and preserve the out-of-plane spin anisotropy even above Curie/Neel temperature.…”

Section: Resultsmentioning

confidence: 99%

Non-monotonic variation of the Kramers point band gap with increasing magnetic doping in BiTeI

Shikin

Rybkina

Estyunin

et al. 2021

Sci Rep

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Polar Rashba-type semiconductor BiTeI doped with magnetic elements constitutes one of the most promising platforms for the future development of spintronics and quantum computing thanks to the combination of strong spin-orbit coupling and internal ferromagnetic ordering. The latter originates from magnetic impurities and is able to open an energy gap at the Kramers point (KP gap) of the Rashba bands. In the current work using angle-resolved photoemission spectroscopy (ARPES) we show that the KP gap depends non-monotonically on the doping level in case of V-doped BiTeI. We observe that the gap increases with V concentration until it reaches 3% and then starts to mitigate. Moreover, we find that the saturation magnetisation of samples under applied magnetic field studied by superconducting quantum interference device (SQUID) magnetometer has a similar behaviour with the doping level. Theoretical analysis shows that the non-monotonic behavior can be explained by the increase of antiferromagnetic coupled atoms of magnetic impurity above a certain doping level. This leads to the reduction of the total magnetic moment in the domains and thus to the mitigation of the KP gap as observed in the experiment. These findings provide further insight in the creation of internal magnetic ordering and consequent KP gap opening in magnetically-doped Rashba-type semiconductors.

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“…In this field geometry both in-plane and out-of-plane fluctuations should contribute to the line broadening while for H c * the in-plane fluctuations make a dominant contribution (see, e.g., Supplemental Material in Ref. [17] for details). One can conjecture that a wipeout of the signal for the out-of-plane orientation of H at T < T N might be a consequence of the sharp boosting of the in-plane spin fluctuations upon establishment of the long-range magnetic order.…”

Section: B Transition To the Afm Ordered Statementioning

confidence: 99%

“…Recently a detailed nuclear magnetic resonance (NMR) study of single crystals of Ni 2 P 2 S 6 demonstrated the sensitivity of the 31 P nuclear spin probe to quasi-2D correlations [15]. However, a direct addressing the electronic spin system of Ni 2 P 2 S 6 with electron spin resonance (ESR) spectroscopy has not been attempted so far whereas this method was recently successfully applied for studies of magnetic van der Waals topological insulators providing interesting insights onto the relationship between the spin dynamics and the electronic properties [3,[16][17][18].…”

Section: Introductionmentioning

confidence: 99%

Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$

Mehlawat,

Alfonsov,

Selter

et al. 2021

Preprint

Self Cite

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The quasi-two-dimensional antiferromagnet Ni2P2S6 belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni2P2S6. The results enable to reliably determine the positive, 'easy'-plane type of the single ion anisotropy of the Ni 2+ ions with the upper limit of its magnitude 0.7 meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the Néel temperature TN = 158 K and extending in the antiferromagnetically ordered state down to low temperatures. There, a new low-energy magnon excitation gapped from the ground state by only ∼ 1.5 meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in a latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of Ni2P2S6.

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Strongly anisotropic spin dynamics in magnetic topological insulators

Cited by 21 publications

References 50 publications

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

Non-monotonic variation of the Kramers point band gap with increasing magnetic doping in BiTeI

Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$

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