A First Principle Study on Magneto-Optical Effects and Magnetism in Ferromagnetic Semiconductors Y$_3$Fe$5$O${12}$ and Bi$_3$Fe$5$O${12}$ (original) (raw)
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Physical Review B, 2019
We investigate and compare the electronic structure of a bulk single crystal of Y 3 Fe 5 O 12 garnet [YIG, a high-T C (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature T C = 380 K, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe 2p HAXPES spectrum of the bulk single crystal exhibits a purely trivalent Fe 3+ state for octahedral and tetrahedral sites. The Fe 3s spectrum shows a clear splitting which allows us to estimate the on-site Fe 3s-3d exchange interaction energy. The valence band HAXPES spectrum shows Fe 3d, O 2p, and Fe 4s derived features and a band gap of ∼2.3 eV in the occupied density of states, consistent with the known optical band gap of ∼2.7 eV. Fe Ledge XAS identifies the octahedral Fe 3+ and tetrahedral Fe 3+ site features. XMCD spectra at the Fe L 2,3 edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral-and tetrahedral-site spins. The calculated Fe 2p HAXPES, Fe Ledge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite Fe 2+ contribution and a reduced Fe 3+ local spin moment. The Fe 2+ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of ∼6 nm. The presence of Fe 2+ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.
arXiv (Cornell University), 2021
In the present work Fe 3+ sublattice spin reversal and Fe 3+ spin-canting across the magnetic compensation temperature (TComp) are demonstrated in polycrystalline Y1.5Gd1.5Fe5O12 (YGdIG) by means of in-field 57 F e Mössbauer spectroscopy measurements. Corroborating in-field 57 F e Mössbauer measurements, both Fe 3+ & Gd 3+ sublattice spin reversal has also been manifested with x-ray magnetic circular dichroism (XMCD) measurement in hard x-ray region. Moreover from in-field 57 F e Mössbauer measurements, estimation and analysis of effective internal hyperfine field (H eff), relative intensity of absorption lines in a sextet elucidated unambiguously the signatures of Fe 3+ spin reversal, their continuous transition and field induced spin-canting of Fe 3+ sublattices across TComp. Further, Fe K-(Gd L3-) edge XMCD signal is observed to consist of additional spectral features, those are identified from Gd 3+ (Fe 3+) magnetic ordering, enabling us the extraction of both the sublattices (Fe 3+ & Gd 3+) information from a single edge analysis. The evolution of the magnetic moments as a function of temperature for both magnetic sublattices extracted either at the Fe K-or Gd L3-edge agree quite well with values that are extracted from bulk magnetization data of YGdIG and YIG (Y3Fe5O12). These measurements pave new avenues to investigate how the magnetic behavior of such complex system acts across the compensation point.
Absence of spin-mixed states in ferrimagnet Yttrium iron garnet
Journal of Applied Physics, 2022
The spectroscopic g-factor of epitaxial thin film Yttrium Iron Garnet (YIG) has been studied using a combination of ferromagnetic resonance spectroscopy and x-ray magnetic circular dichroism. The values obtained by the two techniques are found, within experimental error, to be in agreement using Kittel's original derivation for the g-factor. For an insulating material with an entirely Fe 3+ configuration, a spin mixing correction to Kittel's derivation of the spectroscopic g-factor, as recently shown by Shaw et al. [Phys. Rev. Lett. 127, 207201 (2021)] for metallic systems, is not required and demonstrates that the spin mixing parameter is small in YIG due to negligible spin-orbit coupling.
Advanced electronic materials, 2016
include recently discovered phenomena such as the quantum anomalous Hall effect [2] in magnetic topological insulators (TIs), [3] spin transfer torque [4,5] effects in nonmagnetic metal/ferromagnetic metal/ oxide heterostructures, and spin orbit torque (SOT) effects in heterostructures that include ferromagnetic metals/heavy metals, [6,7] magnetically-doped TIs, [8] and FMI/heavy metals where the FMI is a garnet [9] or Ba hexaferrite. [10] To realize novel circuit devices based on these effects a variety of magnetic materials and heterostructures has still to be developed in which the magnetic properties and interfacial spin transport can be controlled. FMIs with PMA are of particular interest in spintronics. In FMI/heavy metal or FMI/TI heterostructures, current is limited to the metal or to the surface layer of the TI which reduces the conductivity (and potentially the power consumption) compared to all-metallic structures, and avoids the possibility of direct spin transfer torque from current flow in the FMI layer. This facilitates the study of proximity effects, SOT and other exotic phenomena occurring at the interfaces, enabling for example an identification of the various contributions to spin orbit torques. Moreover, the presence of PMA in the FMI leads to stray-field-induced interface effects even at remanence. Domain walls in PMA films also have qualitatively different structures and dynamics compared to those of FMIs with in-plane magnetic anisotropy, which is relevant to racetrack memory or logic devices. Thus there is considerable interest in developing PMA FMI materials and heterostructures. One of the most prominent classes of FMI is that of ferrimagnetic iron garnets, of which the best studied is Y 3 Fe 5 O 12 (YIG). The ultralow damping [11] and magneto-optical properties [12,13] of YIG are well known. The former makes YIG a suitable candidate for spin wave logic [14] and signal transmitters [15] due to the extremely large magnon propagation length of several tens of millimeters. YIG/heavy metal (e.g., Pt, W, Ta) and YIG/topological insulator heterostructures have demonstrated proximity effects, spin pumping, spin Seebeck, and other spintronic phenomena. [15-19] However, YIG films generally have an in-plane easy axis dominated by shape anisotropy due to their weak With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film.
Tm3 Fe5 O12 /Pt Heterostructures with Perpendicular Magnetic Anisotropy for Spintronic Applications
Advanced Electronic Materials, 2016
include recently discovered phenomena such as the quantum anomalous Hall effect [2] in magnetic topological insulators (TIs), [3] spin transfer torque [4,5] effects in nonmagnetic metal/ferromagnetic metal/ oxide heterostructures, and spin orbit torque (SOT) effects in heterostructures that include ferromagnetic metals/heavy metals, [6,7] magnetically-doped TIs, [8] and FMI/heavy metals where the FMI is a garnet [9] or Ba hexaferrite. [10] To realize novel circuit devices based on these effects a variety of magnetic materials and heterostructures has still to be developed in which the magnetic properties and interfacial spin transport can be controlled. FMIs with PMA are of particular interest in spintronics. In FMI/heavy metal or FMI/TI heterostructures, current is limited to the metal or to the surface layer of the TI which reduces the conductivity (and potentially the power consumption) compared to all-metallic structures, and avoids the possibility of direct spin transfer torque from current flow in the FMI layer. This facilitates the study of proximity effects, SOT and other exotic phenomena occurring at the interfaces, enabling for example an identification of the various contributions to spin orbit torques. Moreover, the presence of PMA in the FMI leads to stray-field-induced interface effects even at remanence. Domain walls in PMA films also have qualitatively different structures and dynamics compared to those of FMIs with in-plane magnetic anisotropy, which is relevant to racetrack memory or logic devices. Thus there is considerable interest in developing PMA FMI materials and heterostructures. One of the most prominent classes of FMI is that of ferrimagnetic iron garnets, of which the best studied is Y 3 Fe 5 O 12 (YIG). The ultralow damping [11] and magneto-optical properties [12,13] of YIG are well known. The former makes YIG a suitable candidate for spin wave logic [14] and signal transmitters [15] due to the extremely large magnon propagation length of several tens of millimeters. YIG/heavy metal (e.g., Pt, W, Ta) and YIG/topological insulator heterostructures have demonstrated proximity effects, spin pumping, spin Seebeck, and other spintronic phenomena. [15-19] However, YIG films generally have an in-plane easy axis dominated by shape anisotropy due to their weak With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film.
Magnetism and spin transport in rare-earth-rich epitaxial terbium and europium iron garnet films
Rare-earth iron garnet thin films with perpendicular magnetic anisotropy (PMA) have recently attracted a great deal of attention for spintronic applications. Thulium iron garnet (TmIG) has been successfully grown and TmIG/Pt heterostructures have been characterized. However, TmIG is not the only rare-earth iron garnet that can be grown with PMA. We report the growth, magnetic, and spintronic properties of epitaxial terbium iron garnet (TbIG) and europium iron garnet (EuIG) thin films with PMA. Reciprocal space mapping shows the films are lattice matched to the substrate without strain relaxation, even for films up to 56 nm thick. The lattice strain and magnetostriction coefficient produce PMA in certain cases. TbIG grows on (111) gadolinium gallium garnet (GGG) with PMA due to the in-plane compressive strain, whereas TbIG on (111) substituted GGG (SGGG) is in tension and has an in-plane easy axis. EuIG grows with PMA on and GGG substrates, which facilitates the investigation of spintronic properties as a function of orientation. Both garnets have excess rare earth, which is believed to occupy Fe octahedral sites and in the case of TbIG is associated with an increase in the compensation temperature to 330 K, higher than the bulk value. Anomalous Hall effect (AHE) measurements of Pt/EuIG Hall crosses show that the spin mixing conductance of Pt/ (111) and (100) EuIG is similar. AHE measurements of Pt/TbIG Hall crosses reveal a sign change in the AHE amplitude at the compensation point analogous to all-metallic systems.
Large magneto-optical effects and magnetic anisotropy energy in two-dimensional Cr2Ge2Te6
Physical Review B
Atomically thin ferromagnetic (FM) films were recently prepared by mechanical exfoliation of bulk FM semiconductor Cr2Ge2Te6. They provide a platform to explore novel two-dimensional (2D) magnetic phenomena, and offer exciting prospects for new technologies. By performing systematic ab initio density functional calculations, here we study two relativity-induced properties of these 2D materials [monolayer (ML), bilayer (BL) and trilayer (TL) as well as bulk], namely, magnetic anisotropy energy (MAE) and magneto-optical (MO) effects. Competing contributions of both magneto-crystalline anisotropy energy (C-MAE) and magnetic dipolar anisotropy energy (D-MAE) to the MAE, are computed. Calculated MAEs of these materials are large, being in the order of ∼0.1 meV/Cr. Interestingly, we find that the out-of-plane magnetic anisotropy is preferred in all the systems except the ML where an in-plane magnetization is favored because here the D-MAE is larger than the C-MAE. Crucially, this explains why long-range FM order was observed in all the few-layer Cr2Ge2Te6 except the ML because the out-of-plane magnetic anisotropy would open a spin-wave gap and thus suppress magnetic fluctuations so that long-range FM order could be stabilized at finite temperature. In the visible frequency range, large Kerr rotations up to ∼1.0 • in these materials are predicted and they are comparable to that observed in famous MO materials such as PtMnSb and Y3Fe5O12. Moreover, they are ∼100 times larger than that of 3d transition metal MLs deposited on Au surfaces. Faraday rotation angles in these 2D materials are also large, being up to ∼120 deg/µm, and are thus comparable to the best-known MO semiconductor Bi3Fe5O12. These findings thus suggest that with large MAE and MO effects, atomically thin Cr2Ge2Te6 films would have potential applications in novel magnetic, MO and spintronic nanodevices.
Physical Review B, 2008
We have studied the influence of the stoichiometry on the structural, magnetic, and magneto-optical properties of bismuth iron garnet ͑Bi 3 Fe 5 O 12 ͒ thin films grown by pulsed laser deposition. Films with different stoichiometries have been obtained by varying the Bi/Fe ratio of the target and the oxygen pressure during deposition. Stoichiometry variations influence the Curie temperature T C by tuning the ͑Fe͒-O-͓Fe͔ geometry: T C increases when the lattice parameter decreases, contrary to what happens in the case of stoichiometric rare-earth iron garnets. The thermal variation of the magnetization, the Faraday rotation, and the Faraday ellipticity have been analyzed in the frame of the Néel two-sublattice magnetization model giving energies of −48 K ͑4.1 meV͒, −29 K ͑2.5 meV͒, and 84 K ͑7.3 meV͒ for the three magnetic exchange integrals j aa , j dd , and j ad , respectively. Magneto-optical spectroscopy linked to compositional analysis by Rutherford backscattering spectroscopy shows that Bi and/or Fe deficiencies also affect the spectral variation ͑between 1.77 and 3.1 eV͒. Our results suggest that bismuth deficiency has an effect on the magneto-optical response of the tetrahedral Fe sublattice, whereas small iron deficiencies affect predominantly the magneto-optical response of the octahedral sublattice.
Thin Solid Films, 2011
Ferrimagnetic bismuth substituted yttrium iron garnet Bi x Y 3 − x Fe 5 O 12 (BiYIG) films with x = 1 and 2 pulsed laser deposited onto (111) Gd 3 Ga 5 O 12 (GGG) substrates were studied using magneto-optical (MO) Kerr spectroscopy in the photon energy range of 1.8-5 eV at both polar and longitudinal magnetizations. The interference at lower photon energies provided the refined film thicknesses ranging between 70 and 200 nm. The films were grown under compressive strain and displayed saturation magnetizations (μ 0 M s ) lower than that of their bulk counterparts due to the presence of nanograins forming BiYIG layers and/or magnetically dead interface layers. The trends in the MO spectra agree with those deduced from the published permittivity tensor data for BiYIG using a transfer matrix model applied to a film (BiYIG)-substrate (GGG) system. Due to the reduced μ 0 M s the predicted amplitudes are typically higher. The agreement was improved using effective medium approach or by incorporating into the model MO passive interface layers. The information on MO activity at longitudinal magnetization in the garnet layers below 100 nm presents interest for MO imaging and magnetophotonic devices. The results suggest that the MO Kerr spectroscopy combined with MO Kerr magnetometry may represent a valuable, cheap and nondestructive tool for the characterization of magnetic garnet films less than 200 nm thick.