Magnon Drag Effect Research Articles

Interplay between diffusion and magnon-drag thermopower in pure iron and dilute iron alloy nanowire networks

Results of measurements on the thermoelectric power of 45 nm diameter interconnected nanowire networks consisting of pure Fe, dilute FeCu and FeCr alloys and Fe/Cu multilayers are presented. The thermopower values of Fe nanowires are very close to those found in bulk materials, at all temperatures studied between 70 and 320 K. For pure Fe, the diffusion thermopower at room temperature, estimated to be around − 15 upmuV/K from our data, is largely supplanted by the estimated positive magnon-drag contribution, close to 30 upmuV/K. In dilute FeCu and FeCr alloys, the magnon-drag thermopower is found to decrease with increasing impurity concentration to about 10 upmuV/K at 10% impurity content. While the diffusion thermopower is almost unchanged in FeCu nanowire networks compared to pure Fe, it is strongly reduced in FeCr nanowires due to pronounced changes in the density of states of the majority spin electrons. Measurements performed on Fe(7 nm)/Cu(10 nm) multilayer nanowires indicate a dominant contribution of charge carrier diffusion to the thermopower, as previously found in other magnetic multilayers, and a cancellation of the magnon-drag effect. The magneto-resistance and magneto-Seebeck effects measured on Fe/Cu multilayer nanowires allow the estimation of the spin-dependent Seebeck coefficient in Fe, which is about − 7.6 upmuV/K at ambient temperature.

Calculation of the magnon drag force induced by an electric current in ferromagnetic metals

Magnon drag effect induced by an applied electric field in ferromagnetic metals is theoretically studied by a microscopic calculation of the force on magnons arising from magnon emission/absorption and scattering due to driven electrons. It is shown that magnon scattering contribution dominates over the emission/absorption one in a wide temperature regime in good metals with long elastic lifetime $\taue$, as the latter has a relative suppression factor of $(\Deltasd\taue)^{-2}$ due to the electron spin flip by the magnon, where $\Deltasd$ is the $sd$ exchange interaction energy. Spin-transfer efficiency is discussed including the magnon drag effect.

Emergence of asymmetric skew-scattering dominated anomalous Nernst effect in the spin gapless semiconductors Co1+xFe1−xCrGa

Heusler alloy-based spin gapless semiconductors (SGSs) with very high Curie temperatures $({T}_{C})$ have recently gained enormous attention because of their unconventional electronic structures. They exhibit a nonzero band gap in one of the spin channels and a zero band gap in the other spin channel, making them an important class of materials for tunable spin transport. Here, we report the experimental observation of anomalous Nernst effect (ANE) in ${\mathrm{Co}}_{1+x}{\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{CrGa}$ ($x=0$, 0.2, 0.4, and 0.5), which are the emerging quaternary Heusler alloy-based SGSs. While the electron-electron elastic scattering and the disorder-mediated weak localization effect play the major roles in electrical transport for all the samples at low temperatures, the magnon-drag effect was found to dominate the longitudinal thermoelectric transport. The ANE coefficient at room temperature increases from $\ensuremath{\approx}0.018\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $x=0$ to $\ensuremath{\approx}0.063\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $x=0.5$, which is higher than that for ${\mathrm{Ni}}_{81}\phantom{\rule{0.16em}{0ex}}{\mathrm{Fe}}_{19}$ and compressively strained $\mathrm{SrRu}{\mathrm{O}}_{3}$ films. Our analysis indicates that the observed ANE in these samples originates from asymmetric skew scattering of charge carriers.

Antiferromagnetic Magnon Drag Effect and Giant On–Off Ratio in a Vertical Device

AbstractAntiferromagnets offer unique advantages in the development of spintronics because of the absence of stray field, insensitivity to external disturbances and the ultrafast terahertz eigenfrequency, advancing the development of devices toward small‐size and high‐speed. However, the ultralow read‐out signals of antiferromagnetic spintronics are notoriously smaller than their ferromagnetic counterpart, seriously hindering the development of antiferromagnets. Thus, experimental realization of large read‐out signals in antiferromagnets, especially in vertical devices, is intensely pursued. Here the antiferromagnetic magnon drag effect and a giant on–off ratio of ≈1000 are realized in a vertical sandwich structure of heavy metal/antiferromagnetic insulator/heavy metal (Pt/α‐Fe2O3/Pt). The read‐out signal can exist up to room temperature, and the detected signal owing to the transmission and blocking of magnon strongly relies on the relative orientation between spin polarization and Néel vector. Benefiting from the nonvolatile characteristic of Fe2O3, zero‐field vertical magnon transport is achieved. The giant on–off signals in vertical devices that can be tightly packed, can greatly promote the practical application of antiferromagnetic magnon devices toward smaller, faster, and lower energy consumption.

CrTe as a versatile thermoelectromagnetic multi-functional material

Materials with large magnetocaloric effects and high thermoelectric performance are of fundamental interest to all-solid-state refrigeration technology. Herein, we report CrTe-based ferromagnetic semi-metals as an exceptional example showing both desirable magnetic entropy change (−ΔSM) and promising thermoelectric figure of merit (ZT) near room temperature. A proper amount (9 mol. %) of Cr deficiency is found to stabilize the hexagonal structure of CrTe. Cr0.91Te displays a maximum −ΔSM value of ∼2.4 J/Kg·K under an applied magnetic field of 5 T around its Curie temperature (TC) of 332 K and a strikingly large ZT value of 1.2 × 10−2 at 300 K (two to four orders of magnitude larger than any other ferromagnetic compounds reported so far). The exceptional thermoelectric performance in such a semi-metal is believed to originate from the inherently strong electron–phonon–spin interactions: magnon-drag effect mediated Seebeck coefficient enhancement and phonon–spin coupling induced thermal conductivity reduction. The introduction of antiferromagnetic CrSb not only systematically decreases TC of Cr0.91Te down to or even below room temperature but also boosts its thermoelectromagnetic properties. Specifically, Cr0.91Te0.9Sb0.1 obtains a maximum −ΔSM value of ∼2.6 J/Kg·K @ 5 T around TC = 315 K and a ZT value of 2.0 × 10−2 at 300 K, which represent 8% and 75% improvement over pristine Cr0.91Te, respectively. This research highlights the possibility of exploring ferromagnetic semi-metals as advanced thermoelectromagnetic multi-functional materials.

Theory of huge thermoelectric effect based on a magnon drag mechanism: Application to thin-film Heusler alloy

To understand the unexpectedly high thermoelectric performance observed in the thin-film Heusler alloy Fe$_2$V$_{0.8}$W$_{0.2}$Al, we study the magnon drag effect, generated by the tungsten based impurity band, as a possible source of this enhancement, in analogy to the phonon drag observed in FeSb$_2$. Assuming that the thin-film Heusler alloy has a conduction band integrating with the impurity band, originated by the tungsten substitution, we derive the electrical conductivity $L_{11}$ based on the self-consistent t-matrix approximation and the thermoelectric conductivity $L_{12}$ due to magnon drag, based on the linear response theory, and estimate the temperature dependent electrical resistivity, Seebeck coefficient and power factor. Finally, we compare the theoretical results with the experimental results of the thin-film Heusler alloy to show that the origin of the exceptional thermoelectric properties is likely to be due to the magnon drag related with the tungsten-based impurity band.

Effect of bismuth doping on the structural, magnetic, electrical, and thermopower behavior of lanthanum silver manganites

The structural, magnetic, electrical, and thermoelectric behaviors of lanthanum silver manganite and lanthanum bismuth silver manganite were thoroughly studied in the present work. Two polycrystalline samples, viz., La0.835Ag0.165MnO3 (LAMO) and La0.67Bi0.165Ag0.165MnO3 (LBAMO) possessing almost constant ratio of Mn3+/Mn4+ were synthesized using the wet chemical method. Phase formation was identified from powder X-ray diffraction and it has been confirmed through the structural analysis of which samples crystallize into rhombohedral symmetry with R-3C space group. Apparent changes at phase transition temperature were observed in temperature-dependent resistivity, magnetization and thermoelectric power behavior for bismuth-doped samples. In addition, an enhanced magnetoresistance was observed in LBAMO samples and is attributed to the presence of both ferromagnetic and anti-ferromagnetic states. The temperature-dependent resistivity data in the temperature range (50–300 K) were analyzed using the phase separation model. Spin-dependent scattering and electron–electron interactions were considered to explain the resistivity minimum in the low temperature region (T < 50 K). The Seebeck coefficient is positive and enhances around the transition temperature by substituting bismuth into LAMO. The high-temperature thermoelectric power data were explained in the framework of small polaron hopping mechanism while magnon drag effect dominates in the low temperature region.

Electrical properties of NiAs-type MnTe films with preferred crystallographic plane of (110)

NiAs-type manganese telluride (MnTe) films with preferred crystallographic plane of (110) were prepared on Si/SiO2 substrates by pulsed laser deposition. X-ray diffraction (XRD) of the films was studied at different temperatures. The XRD peak of MnTe (110) films shifts to higher angle with decreasing temperature, showing the decrease of the lattice parameter. Resistivity of the films was studied in the temperature range of 2–350 K. The bump between 150 and 250 K was observed in the films, which may be related to the special s-d and p-d overlaps induced by the compressed lattice. The magnon drag effect near its Néel temperature TN and enlarged magnetic-elastic coupling below 100 K were observed and analyzed in details.

Thermopower studies of rare earth doped lanthanum barium manganites

Influence of rare earth doping on electrical, magnetic and thermopower studies of La0.34Re0.33Ba0.33MnO3 compound was investigated. Ferro to paramagnetic transition and metal to insulator transition temperatures decrease with decreasing ionic radius of the dopant ion. Electrical resistivity in the entire temperature range is explained by phase separation model. The magnitude of Seebeck coefficient increases with increasing dopant ionic radius. A cross over from negative to positive sign has also been observed in thermopower data with decreasing A site ionic radius (〈rA〉). The low temperature thermopower data has been explained using a qualitative model containing diffusion; magnon drag and phonon drag effects while the paramagnetic insulating part has been analyzed using small polaron hopping mechanism.

Thermopower Studies of Polycrystalline Ag Doped LaMnO3 Manganites

The effects of silver doping on the magnetotransport and thermopower of La1−x Ag x MnO3 (0.05≤x≤0.30) have been investigated. For the sample with x=0.05, temperature dependent resistivity exhibits an insulating behavior, while thermopower is found to be large and negative over the measured temperature range. An increase in the Ag doping enhances the conductivity and shifts the metal-insulator transition temperature toward high temperature side. The low temperature thermopower data has been explained in terms of diffusion, magnon drag, and phonon drag effects and found that the magnon drag effect dominates in this region. Finally, the electrical transport in the high temperature region has been analyzed by using adiabatic small polaron hopping mechanism.

Magnon-drag thermopile

Thermoelectric effects in spintronics are gathering increasing attention as a means of managing heat in nanoscale structures and of controlling spin information by using heat flow. Thermal magnons (spin-wave quanta) are expected to play a major role; however, little is known about the underlying physical mechanisms involved. The reason is the lack of information about magnon interactions and of reliable methods to obtain it, in particular for electrical conductors because of the intricate influence of electrons. Here, we demonstrate a conceptually new device that enables us to gather information on magnon-electron scattering and magnon-drag effects. The device resembles a thermopile formed by a large number of pairs of ferromagnetic wires placed between a hot and a cold source and connected thermally in parallel and electrically in series. By controlling the relative orientation of the magnetization in pairs of wires, the magnon drag can be studied independently of the electron and phonon-drag thermoelectric effects. Measurements as a function of temperature reveal the effect on magnon drag following a variation of magnon and phonon populations. This information is crucial to understand the physics of electron-magnon interactions, magnon dynamics and thermal spin transport.

The study of magnon-drag effects in the thermopower of CuCr2X4 (X = S, Se and Te)

The electrical resistivity ρ and thermopower S of metallic ferromagnets CuCr2X4 (X=S,Se or Te) are measured at low temperatures and in an external magnetic field of 1.5 Tesla. These properties are dominated by strong scattering effects in their ferromagnetic phase, resulting in (a) ρ(T) ~A T2dependence with very large value of coefficient A ≈ 1 − 3×10−8 Ω-cm/K2 in resistivity and (b) a peak in the thermopower around TC/3 . Thermopower is further increased by crystalline order and by the alignment of magnetic domains in an external magnetic field as has been found for Cu1+x Cr2Te4. We have interpreted these effects as due to strongly coherent momentum conserving electron-magnon scattering giving a large T2 – contribution in resistivity and a large drag effect in thermopower.

Thermal Transport Properties of Bechgaard Salts (TMTSF)2PF6 and (TMTSF)2ClO4: Implication of Spin–Charge Separation

We report thermal transport measurements performed on the quasi-one-dimensional Bechgaard salts (TMTSF) 2 ClO 4 and (TMTSF) 2 PF 6 along the a -direction. For both salts, magnon-drag effects are found to contribute considerably to thermopower above 80 K. These results imply spin–charge separation in the metallic state for both salts. Moreover, a linear temperature-dependent thermal conductivity is found to be unaffected by anion disorder below an anion ordering transition temperature T AO ∼25 K in (TMTSF) 2 ClO 4 .

Magnetic and electrical transport properties of Dy xGd 5−xSi 2Ge 2 ( x=0.0, 1.5, 2.5, 3.0, 3.5, 4.5 and 5.0) compounds

Effect of Dy substitution at the Gd-site on the magnetic and electrical transport properties of the giant magnetocaloric effect material, Gd 5Si 2Ge 2, has been explored. At room temperature, Dy x Gd 5− x Si 2Ge 2 ( x⩽3.5) compounds have the monoclinic Gd 5Si 2Ge 2-type crystal structure (Space group P2 1/a), while those with x>3.5, have orthorhombic Sm 5Ge 4-type structure (Space group Pnma). The Dy substitution contracts the unit cell and also reduces the monoclinic distortion. The compound Gd 5Si 2Ge 2 orders ferromagnetically at 276 K; with increasing Dy substitution, the magnetic transition temperature decreases ( T N=56 K for Dy 5Si 2Ge 2) and the nature of magnetic coupling becomes antiferromagnetic in Dy-rich end. The electrical resistivity shows a marked drop near T C/ T N. The thermoelectric power undergoes a slope change in the vicinity of magnetic transition and the signature of phonon and magnon drag effects is observed at low temperatures. Thus progressive Dy substitution in Gd 5Si 2Ge 2 leads to a class of materials with interesting magnetic ground states, with magnetic ordering temperatures spanning from 56 to 276 K.

Magnetism of R2Ti3Ge4 (R=Sc, Gd, Tb, Dy, Ho, and Er) compounds

Results of magnetization measurements down to 1.8 K on polycrystalline R2Ti3Ge4 (R=Sc, Gd, Tb, Dy, Ho, and Er) compounds of orthorhombic Sm5Ge4-type (space group Pnma) are presented and discussed. Gd2Ti3Ge4 is found to be a ferromagnet below 32 K while other R2Ti3Ge4 compounds, with magnetic rare earths (R=Tb, Dy, Ho, and Er), undergo a low temperature antiferromagnetic transition. The effective magnetic moment (μeff) values obtained from the Curie–Weiss paramagnetic susceptibility correspond to that of R3+ ions. Saturation of magnetization is not observed upto applied fields of 5.5 T at 1.8 K for all R2Ti3Ge4 compounds except Gd2Ti3Ge4, indicating the possible presence of crystal field effects. Electrical resistivity of these compounds is dominated by electron-phonon scattering above 20 K and signature of phonon and magnon drag effects are seen in the low temperature thermopower data.

Thermoelectric power of Na-doped La0.7Ca0.3−yNayMnO3 both in the presence and the absence of magnetic field

Magnetic-field (B=0–1.5 T) dependent thermoelectric power (TEP) of the Na-doped La0.7Ca0.3−yNayMnO3 (0.0⩽y⩽0.3) system has been studied in the temperature range 80–300 K. X-ray diffraction studies indicate a rhombohedrally distorted perovskite structure of the samples. The observed sharp peak in the resistivity (ρ) versus temperature (T) curve falls and shifts to higher temperature with increasing Na concentration (y). In the low-temperature ferromagnetic (FM) regime, thermopower (Seebeck coefficient, S) obeys the expression S=S0+S1.5T1.5+S4T4 over the entire range of y. Electron-magnon scattering is found to dominate the low-temperature resistivity and TEP data. High-temperature TEP data can be well fitted with Mott’s small polaron hopping model. The activation energy (ES) and polaron hopping energy (WH) decrease with increasing Na content. Both ES and WH decrease while polaron radius (rp) increase with the application of a magnetic field. Field-dependent TEP data also indicate the suppression of spin fluctuations in the presence of a magnetic field. In the low-temperature FM region, both magnon drag and phonon drag effects coexist.

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R2Ti3Ge4 (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm5Ge4-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.

Ｍｎ１−ｘＣｒｘＴｅ薄膜のマグノン・ドラッグ効果

The crystal structure and thermoelectric properties of Mn1-xCrxTe (0≤x≤0.3) films prepared on glass substrates by vacuum evaporation were investigated. X-ray diffraction measurements showed that the solubility limit of Cr in the NiAs-type MnTe phase for Mn1-xCrxTe films is x=0.06. For films in which 0≤x≤0.06, the Neel temperature TNopt, which was determined from the deflection point in the temperature dependence curve of the optical band gap, shifts strongly toward higher temperatures from TNopt,=327 K (x=0) to TNopt,=460 K (x=0.06) with an increase in x. The Seebeck coefficient S of the films reaches a maximum around TNopt, indicating that the enhancement of S can be attributed to the magnon-drag effect. On the other hand, for x >0.06, the temperature at which S reaches a maximum does not change with x, and S decreases markedly owing to the presence of a metallic Cr2Te3 phase.

Magnon Drag Thermoelectric Power in Magnetic Graphite Intercalation Compounds

Thermoelectric power (TEP) of first-stage non-magnetic graphite acceptor intercalation compounds drops rapidly to a negligibly small value below 7 K. In contrast to the non-magnetic GICs, the magnetic compounds such as first-stage FeCl 3 - and CoCl 2 -GICs exhibit much larger TEP, which is expected to be due to the magnon drag effect. This prediction is confirmed theoretically in this article. The present calculation is carried out for CoCl 2 -GIC, since its magnetic properties have been extensively studied. The dispersion relation for the long wavelength magnons is phonon-like. This is due to the 2D-character of the spin system. By solving the coupled Boltzmann equation for carriers and magnons the magnon drag TEP is calculated and the calculated temperature dependence is proportional to T at low temperatures, which is consistent with the measurement. The calculated value has the same order of magnitude as the observed one.

Transport properties of CeB 6: Comparison with specific heat data

Electrical resistivity (ϱ), thermal conductivity ( K), and thermoelectric power ( Q) measurements were performed on a high quality single crystal of CeB 6 ( ϱ residual = 0.33μΩ cm ). Comparison is made with specific heat data obtained on another sample. The low temperature maximum in Q(T) which is associated with a large contribution in K(T), a T 4 dependence of ϱ, and a T 3 dependence of C, may be due to magnon drag effects.