Ferromagnetic Insulator Research Articles

Phase-controllable nonlocal spin polarization in proximitized nanowires

We study the magnetic and superconducting proximity effects in a semiconducting nanowire (NW) attached to superconducting leads and a ferromagnetic insulator (FI). We show that a sizable equilibrium spin polarization arises in the NW due to the interplay between the superconducting correlations and the exchange field in the FI. The resulting magnetization has a nonlocal contribution that spreads in the NW over the superconducting coherence length and is opposite in sign to the local spin polarization induced by the magnetic proximity effect in the normal state. For a Josephson-junction setup, we show that the nonlocal magnetization can be controlled by the superconducting phase bias across the junction. Our findings are relevant for the implementation of Majorana bound states in state-of-the-art hybrid structures.

Abundant valley-polarized states in two-dimensional ferromagnetic van der Waals heterostructures

Manipulating the valley degree of freedom provides a novel paradigm for future valleytronics. Here we use first-principles approaches and the $kp$ model analyses to carry out a comparative study of ten van der Waals (vdW) heterostructures constructed by transition-metal dichalcogenides and recently discovered two-dimensional (2D) ferromagnetic (FM) insulators. Our calculations show that the $\mathrm{MoT}{\mathrm{e}}_{2}/\mathrm{CrB}{\mathrm{r}}_{3}$ heterostructure hosts a large valley splitting of $\ensuremath{\sim}28.7\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which is much larger than that observed in the $\mathrm{WS}{\mathrm{e}}_{2}/\mathrm{Cr}{\mathrm{I}}_{3}$ heterostructure $(\ensuremath{\sim}3.5\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$. Specifically, the spin-orbital coupling can induce band inversions in $\mathrm{WS}{\mathrm{e}}_{2}/\mathrm{CrB}{\mathrm{r}}_{3}$ and $\mathrm{MoT}{\mathrm{e}}_{2}/\mathrm{CrB}{\mathrm{r}}_{3}$ heterostructures due to strong charge transfer across the interfaces of these two systems. Most strikingly, $\mathrm{WS}{\mathrm{e}}_{2}/\mathrm{CrB}{\mathrm{r}}_{3}$ is characterized as a valley-polarized quantum anomalous Hall effect system with Chern number $C=\ensuremath{-}1$ at the $K$ point while $C=0$ at the ${K}^{\ensuremath{'}}$ point. All those findings together with recent progress in 2D FM insulators point out an experimentally achievable scheme for exploration of abundant valley-polarized states in 2D FM vdW heterostructures.

Ferromagnetism and superconductivity in twisted double bilayer graphene

Twisted double bilayer graphene is a highly tunable moir\'e system to study strongly interacting physics. Here, the authors present a theory of competing ferromagnetic and superconducting orders in this system. They show that the single-particle moir\'e bands can be drastically modified by a perpendicular electric displacement field. As a result, the ferromagnetic insulating gap driven by Coulomb interaction has a dome shape dependence on the layer potential difference, as observed in several experiments. The stability of the ferromagnetic insulator against collective excitations, including spin and valley magnons, is theoretically verified. Furthermore, the authors investigate the possibility of phonon-mediated intervalley equal-spin pairing, using the property that the acoustic phonon mediated attraction respects an enlarged SU(2)\ifmmode\times\else\texttimes\fi{}SU(2) symmetry.

Supercurrent vortices and Majorana zero modes induced by an in-plane Zeeman field on the surface of a three-dimensional topological insulator

A nonuniform in-plane Zeeman field can induce spontaneous supercurrents of spin-orbit-coupled electrons in superconducting two-dimensional systems and thin films. In this work it is shown that current vortices can be created at the ends of a long homogeneously magnetized strip of a ferromagnetic insulator, which is deposited on the surface of a three-dimensional topological insulator. The $s$-wave superconductivity on its surface is assumed to have an intrinsic origin, or to be induced by the proximity effect. It is shown that vortices with the odd vorticity can localize Majorana zero modes. The latter may also be induced by sufficiently narrow domain walls inside the strip, which opens a way for manipulating these modes by moving the walls. It is shown that the vorticity can be tuned by varying the magnetization and width of the strip. A stability of the strip magnetization with respect to the Berezinsky-Kosterlitz-Thouless transition has been analyzed.

Quantum Oscillations of Gilbert Damping in Ferromagnetic/Graphene Bilayer Systems

We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange coupling constant to be determined. The results demonstrate in theory that the ferromagnetic resonance measurements may be used to detect the spin resolved electronic structure of the adjacent materials, which is critically important for future spin device evaluations.

Anomalous Josephson Hall effect charge and transverse spin currents in superconductor/ferromagnetic-insulator/superconductor junctions

Interfacial spin-orbit coupling in Josephson junctions offers an intriguing way to combine anomalous Hall and Josephson physics in a single device. We study theoretically how the superposition of both effects impacts superconductor/ferromagnetic insulator/superconductor junctions' transport properties. Transverse momentum-dependent skew tunneling of Cooper pairs through the spin-active ferromagnetic insulator interface creates sizable transverse Hall supercurrents, to which we refer as anomalous Josephson Hall effect currents. We generalize the Furusaki-Tsukada formula, which got initially established to quantify usual (tunneling) Josephson current flows, to evaluate the transverse current components and demonstrate that their amplitudes are widely adjustable by means of the spin-orbit coupling strengths or the superconducting phase difference across the junction. As a clear spectroscopic fingerprint of Josephson junctions, well-localized subgap bound states form around the interface. By analyzing the spectral properties of these states, we unravel an unambiguous correlation between spin-orbit coupling-induced asymmetries in their energies and the transverse current response, founding the currents' microscopic origin. Moreover, skew tunneling simultaneously acts like a transverse spin filter for spin-triplet Cooper pairs and complements the discussed charge current phenomena by their spin current counterparts. The junctions' universal spin-charge current cross ratios provide valuable possibilities to experimentally detect and characterize interfacial spin-orbit coupling.

Magnon-mediated superconductivity on the surface of a topological insulator

We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next nearest neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case, as compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.

Scattering of Dirac electrons from a skyrmion: Emergence of robust skew scattering

We study electron scattering from a closed magnetic structure embedded in the top surface of a topological insulator (TI). Outside the structure there is a uniform layer of ferromagnetic insulator (FMI), leading to a positive effective mass for the Dirac electrons. The mass inside can be engineered to be negative, leading to a skyrmion structure. The geometric shape of the structure can be circular or deformed, leading to integrable or chaotic dynamics, respectively, in the classical limit. For a circular structure, the relativistic quantum scattering characteristics can be calculated analytically. For a deformed structure, we develop an efficient numerical method, the multiple multipole method, to solve the scattering wavefunctions. We find that anomalous Hall effect as characterized by strong skew scattering can arise, which is robust against structural deformation due to the resonant modes. In the short (long) wavelength regime, the resonant modes manifest themselves as confined vortices (excited edge states). The origin of the resonant states is the spin phase factor of massive Dirac electrons at the skyrmion boundary. Further, in the short wavelength regime, for a circular skyrmion, a large number of angular momentum channels contribute to the resonant modes. In this regime, in principle, classical dynamics are relevant, but we find that geometric deformations, even those as severe as leading to fully developed chaos, have little effect on the resonant modes. The vortex structure of the resonant states makes it possible to electrically ``charge'' the skyrmion, rendering feasible to manipulate its motion electrically. In the long wavelength regime, only the lowest angular momentum channels contribute to the resonant modes, making the skew scattering sharply directional. These phenomena may find applications for information storage and in Hall devices based on dynamic skyrmion.

Ellipticity and dissipation effects in magnon spin valves

We consider alignment-dependent spin and heat transport across a magnon spin valve in the tunneling regime, i.e., a junction consisting of two weakly coupled ferromagnetic insulators. We determine the difference in spin and heat conductance between the parallel and antiparallel configuration of the magnetization direction. The dependence of these conductances on both the Gilbert damping and ellipticity is studied. We find that both magnon ellipticity and dissipation open channels for magnons to tunnel through in the antiparallel configuration. Our results highlight an important difference between electronic and magnon spin transport in spin-valve structures and may be important for the development of devices based on magnetic insulators.

Axion insulator state in ferromagnetically ordered CrI3/Bi2Se3/MnBi2Se4 heterostructures

Realizing axion insulator state with a uniform magnetization considerably facilitates experimental explorations of the intriguing topological magnetoelectric effect, a hallmark of three-dimensional (3D) topological insulators (TIs). Through density functional theory calculations and four-band model studies, we find that magnetic ions Cr3+ in monolayer CrI3 and Mn2+ in septuple-layer MnBi2Se4 have opposite exchange couplings to the topological surface states of 3D TI Bi2Se3. As an exciting result of such opposite exchange couplings, axion insulator state is realized by a uniform magnetization in CrI3/Bi2Se3/MnBi2Se4 heterostructure. Our work opens up opportunities for exploring topological magnetoelectric effect realized by the uniform magnetization induced axion insulator state in heterostructures of 3D TIs and two-dimensional van der Waals ferromagnetic insulators.

Interfacial Dzyaloshinskii-Moriya interaction between ferromagnetic insulator and heavy metal

Recent demonstration of the interfacial Dzyaloshinskii-Moriya interaction (DMI) between a heavy metal and a magnetic insulator provides the possibility to manipulate chiral spin textures in the magnetic insulator for the extremely low power consumption devices. However, the origin and strength of the interfacial DMI remain in dispute in this system. We used the electrical transport measurements to determine the DMI strength to be ∼0.040 pJ/m at room temperature in Pt/Tm3Fe5O12 (TmIG) bilayers. The TmIG saturation magnetization and DMI strength exhibit different temperature dependences, which is attributed to the DMI being mainly contributed by Fe ions instead of Tm ions. With a Cu layer inserted between Pt and TmIG, the DMI strength is reduced to ∼0.012 pJ/m and the topological Hall effect vanishes, strongly suggesting that the Pt/TmIG interface has important contribution to the DMI.

Magneto-optical Kerr effect in spin split two-dimensional massive Dirac materials

Two-dimensional (2D) massive Dirac electrons possess a finite Berry curvature, with Chern number 1/2, that entails both a quantized dc Hall response and a subgap full-quarter Kerr rotation. The observation of these effects in 2D massive Dirac materials such as gapped graphene, hexagonal boron nitride or transition metal dichalcogenides (TMDs) is obscured by the fact that Dirac cones come in pairs with opposite sign Berry curvatures, leading to a vanishing Chern number. Here, we show that the presence of spin–orbit interactions, combined with an exchange spin splitting induced either by diluted magnetic impurities or by proximity to a ferromagnetic insulator, gives origin to a net magneto-optical Kerr effect in such systems. We focus on the case of TMD monolayers and study the dependence of Kerr rotation on frequency and exchange spin splitting. The role of the substrate is included in the theory and found to critically affect the results. Our calculations indicate that state-of-the-art magneto-optical Kerr spectroscopy can detect a single magnetic impurity in diluted magnetic TMDs.

Recent progress in the research of quantum anomalous <?A3B2 ACK?>Hall effect

<p indent="0mm">The quantum anomalous Hall (QAH) effect is a quantum Hall effect that can occur without external magnetic field. The experimental realization of this effect in magnetically doped topological insulator thin films not only provides an ideal platform for research and application on chiral quantum Hall state, but also paves the way for constructing many other exotic topological quantum phases. But the realization temperature of QAH is quite low. The non-uniform magnetic disorder could be one of main reasons. To increase the uniformity of magnetic order, different element was doped, like V instead of Cr, or Cr-V co-doping, which enhance the magnetic anisotropy in the system. In Cr-V co-doping system, the realization temperature of the QAH was ten times enhanced. Also, a penta-layer structure with heavily doped sub-surface layer was formed to effectively increase the QAH temperature. In this structure, the modulation-doping originates from the suppression of the disorder in the surface state interacting with the rich magnetic moments. In the future, with combined element doping and modified structure, we may get much improved QAH temperature. To elevate the working temperature of the QAH effect, a TI sandwiched by two ferromagnetic insulator layers is also applied because of the more ordered structure and potentially high Curie temperature. Although several works on FMI/TI heterostructure reports the proximity ferromagnetic in TI, the QAH was observed at quite a low temperature in an all-telluride base heterostructure. A strong coupling between the surface state of TI and ferromagnetic layer could be required. In a magnetically doped TI system, it is natural to ask how a QAH system evolves in a magnetization reversal process. Two groups observed the zero plateau of Hall conductance around the coercive field independently, indicating transition between two QAH phases occurs via an ordinary insulator phase, which could be understood in a multi-magnetic domain structure. A similar zero plateau of Hall conductance have been observed in magnetic-doped TI (MTI) sandwich structure MTI/TI/MTI but with opposite out-of-plane magnetization direction on top and bottom surface. The system behaves like a normal insulator in a <italic>DC</italic> transport measurement but will have topological magnetoelectric effect in <italic>AC</italic> measurement, which is still a challenge. The QAH insulator is important not only as a rare topological quantum phase that has been unambiguously experimentally realized, but also as a solid and versatile building block to construct many other topological quantum states. The latter role requires a QAH system to be engineered or incorporated into various kinds of heterostructures, on which several exciting experimental progresses have been made in the past years: The QAH insulator-normal insulator heterostructures simulated high Chern number QAH insulator has been experimental reported. But it is still quite challenging because the multilayer structure acquires accurate growth parameter control; a QAH insulator acquiring proximity superconductivity from an adjacent <italic>s</italic> wave superconductor (SC) layer can show the properties of a chiral topological SC. Majorana bound states (MBSs) are expected to appear in the magnetic vortices of the topological SC and could be used to compose topological qubits which are presumably robust against decoherence with topological protection. A better-defined QAH-SC interface is crucial for further explorations in this direction to avoid disturbance of interface disorders; the first intrinsic anti-ferromagnetic TI material MnBi₂Te₄ with intercalation ferromagnetic layer MnTe was found. By alternating MBE growth of Bi₂Te₃ and MnTe, MnBi₂Te₄ thin film was formed and gapless Dirac surface state from ARPES in over 2 SL films indicate the “topology” of this material. Recently the quantized Hall resistance was reported in exfoliated thin flake of MnBi₂Te₄ with high magnetic field. The results establish that MnBi₂Te₄ could be an idea platform for further exploring topological phenomena. The recent progresses on QAH show us big opportunities to reach various novel topological states of matter experimentally. The explorations call for more exquisitely designed heterostructures and higher sample quality. With these efforts, the fantastic world of topological quantum matter is getting more and more realistic.

Exchange-biased nanocomposite ferromagnetic insulator

Magnetic insulating and exchange bias (EB) are two basic magnetic states with actual and potential applications in spintronic and magnonic devices. However, ferromagnetic insulators (FMIs) are rare, and it is even more challenging to obtain a FMI with large EB. In this paper, high performance of FMIs and EB in $\mathrm{LaMn}{\mathrm{O}}_{3}$:NiO nanocomposite epitaxial thin films is reported. The antiferromagnetic NiO nanoparticles, randomly embedded in a ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_{3}$ matrix, enhance the robustness of the insulating state and result in an EB field as large as 1100 Oe. The simultaneously enhanced resistivity and EB are attributed to localized oxygen vacancy, preserved strain state, and increased ferromagnetic-antiferromagnetic interface ratio. This paper provides not only a material candidate for spintronics but also a referential strategy to design artificial materials with multifunctionality.

Spin-split valence bands of the ferromagnetic insulator Cr2Ge2Te6 studied by angle-resolved photoemission spectroscopy

The low-energy electronic structure of a ferromagnetic insulator Cr2Ge2Te6 was studied by angle-resolved photoemission spectroscopy (ARPES). A broadening of the ARPES spectra, which we ascribe to a splitting of the valence band, has been observed at low temperature. Through temperature dependent measurements, we found that the band splitting presented a transition at ∼60–70 K, which is consistent with the ferromagnetic transition temperature of Cr2Ge2Te6. Therefore, we suggested that the splitting of the valence band is due to the spin polarization effect of Te 5p states by Cr 3d states, which is compatible with the superexchange coupling through Cr-Te-Cr to form long-range ferromagnetic order in Cr2Ge2Te6.

Ferromagnetic dual topological insulator in a two-dimensional honeycomb lattice

Magnetic topological insulators (TIs), including the quantum anomalous Hall effect and antiferromagnetic TIs, have attracted significant attention owing to the exotic properties they give rise to, however, ferromagnetic TIs with gapless surface/edge states and a nonzero topological invariant have not been reported so far.

Exploring the structural, electronic, and magnetic properties of cation-ordered 3d−5d double-perovskite Bi2FeReO6 and Bi2FeIrO6 thin films from first principles

We report a first-principles study of Bi-based 3$d$-5$d$ ordered double perovskite oxides (A$_2$BB$^\prime$O$_6$) with a 3$d$ atom (Fe) at the B-site and 5$d$ atoms (Re,Ir) at the B$^\prime$-site while keeping highly polarizable ions (Bi$^{3+}$) at the A-site. We find that, under coherent heteroepitaxy, Bi$_2$FeReO$_6$} exhibits a strain-driven anti-ferromagnetic insulator to ferrimagnetic semi-metal transition, while Bi$_2$FeIrO$_6$ shows correlation driven ferromagnetic insulator to ferrimagnetic half-metal transition with calculated magnetic moments of 5 $\mu_B$/f.u. and 3 $\mu_B$/f.u., respectively. These properties along with the low band gaps in the insulating phases make the compounds appealing for spintronics applications. Furthermore, in Bi$_2$FeIrO$_6$, the conduction and valence states are localized on different transition metal sublattices implying more efficient electron-hole separation upon photoexcitation, a desirable feature for photovoltaic applications.

Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene

Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices.

Temperature dependence of the magnon-phonon energy relaxation time in a ferromagnetic insulator

We have used the Boltzmann kinetic equation for the phonon distribution function to analyze the relaxation kinetics of the spin system of a ferromagnetic insulator (F) lying on a massive dielectric substrate with high thermal conductivity. Under periodic heating of the spin system, the relaxation depends on the thickness of the F layer and on the frequency of the thermal source $\ensuremath{\omega}$. When the thickness of the F layer is much greater than the phonon-magnon scattering length, the magnon temperature dependence on the frequency has two features related to specific characteristic times of the system. One of them determines the dependence in the low-frequency regime and is related to the average phonon escape time from the F layer to the substrate ${\ensuremath{\tau}}_{\mathrm{es}}$. In turn, the high-frequency behavior is determined by the magnon-phonon collisions time ${\ensuremath{\tau}}_{mp}$. From the latter, the time of phonon-magnon collisions ${\ensuremath{\tau}}_{pm}$ can be found. In contrast, the response of effectively thin F layers is characterized by just one feature, which is determined by the time ${\ensuremath{\tau}}_{mp}$. Thus, based on the obtained theoretical results, the times ${\ensuremath{\tau}}_{\mathrm{es}},{\ensuremath{\tau}}_{mp}$, and ${\ensuremath{\tau}}_{pm}$ can be deduced from experiments on the parametric excitation of spin waves by electromagnetic radiation modulated at frequency $\ensuremath{\omega}$.

Semi-quantized Spin Pumping and Spin-Orbit Torques in Topological Dirac Semimetals

We study the time-development processes of spin and charge transport phenomena in a topological Dirac semimetal attached to a ferromagnetic insulator with a precessing magnetization. Compared to conventional normal metals, topological Dirac semimetals manifest a large inverse spin Hall effect when a spin current is pumped from the attached ferromagnetic insulator. It is shown that the induced charge current is semi-quantized, i.e., it depends only on the distance between the two Dirac points in momentum space and hardly depends on the disorder strength when the system remains in the topological Dirac semimetal phase. As an inverse effect, we show that the electric field applied to the topological Dirac semimetal exerts a spin torque on the local magnetization in the ferromagnetic insulator via the exchange interaction and the semi-quantized spin Hall effect. Our study demonstrates that the topological Dirac semimetal offers a less-dissipative platform for spin-charge conversion and spin switching.