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Thermoelectric power (TEP) along with temperature dependent resistivity of Ti doped Gd-Sr manganite (Gd0.7Sr0.3Mn1-xTixO3 (x = 0, 0.1 and 0.15) denoted by G, G0.1 and G0.15 respectively) are studied systematically. Significant Seebeck coefficients (S) are exhibited by all the three samples which is a most desirable quality of a good thermoelectric material. In the case of sample G, highest attained value of S is −35.57mVK−1 at 91 K. For G0.1 and G0.15 highest attained values are −31.60 mVK−1 at 114 K and –33.76 mVK−1 at 133 K respectively. This reveals that Ti doping shifts the highest value of S to higher temperatures. Diffusion and magnon drag contribute to TEP at lower temperatures. Temperature dependence of thermopower at comparatively higher temperature region gives an insight of combined SPH and VRH models. Temperature dependent electrical resistivity is successfully analyzed within the interesting framework of variable range hopping of small polarons.

Future applications in energy storage are greatly dependent on a material with multifunctional property. In this respect transition metal oxide has shown special attention, especially nickel oxide. A unique synthesis (coprecipitation) and multifaceted properties of NiO has been studied systematically. The result from XRD confirms Bunsenite structure having average size of crystallite ∼ 30 nm, lattice strain from W-H plot is ∼ 0.00255 along with other structural parameters are reported. The electron density distribution profile of Ni-O was computed using Rietveld method. Particle size from SEM was about 52 nm, peak at 419 cm−1 of FTIR confirms Ni-O stretching frequency and band gap of 2.95 eV are all confirmation for fine crystallinity of the prepared product under 600 °C calcination. 2.6102 m emu Saturation magnetization (Ms), Coercivity (Hc) of 198.35 Gauss and Retentivity (Mr) 55.172 μemu affirms paramagnetism from VSM study. The two magnon band intensity in raman spectra decreases rapidly for smaller crystallite size. This effect was clearly observed in the decrease of antiferromagnetic spin correlations and lead to paramagnetic phase. Electrochemical Impedance spectroscopy calculations illustrated aphase angle of 86° which indicates the capacitive behaviour.

Helium diffusion in metals is one of the key atomic processes for bubble nucleation. Ferromagnetic effects play important roles on microstructural evolution in BCC iron. The spin-lattice dynamics simulations are performed to study the ferromagnetic effects on the non-Arrhenius diffusion behavior of single interstitial helium solute in BCC iron. It is found that the ferromagnetic effects significantly reduce the helium diffusivity at low temperatures, but give rise to a statistically negligible influence at high-temperatures. In addition, we propose a coarse-grained formula to describe the non-Arrhenius behavior, and three diffusion modes are well-defined depending on the competition between thermal energy of helium gained from the host atoms’ collisions and its migratory barrier, for instant, helium diffusion reveals Arrhenius- and Einstein-diffusion modes in the low- and high-temperature limits, respectively. Further, the effects of quantum statistics of both phonons and magnons in BCC Fe are also discussed, which is found to give rise to the non-linear temperature dependent thermal energy thus the strong non-Arrhenius behavior in the region of Arrhenius-diffusion mode. The above-mentioned non-Arrhenius diffusion feature of helium in BCC Fe is consistent with the cases of helium diffusion in BCC W [Wen et al., J. Nucl. Mater. 493 (2017) 21; Woo et al., Phys. Rev. E 96 (2017) 032133], and the diffusion parameters are in agreement with the experimental measurement and other calculations results. The current work could help to get a complete understanding of ferromagnetic effects on irradiation damage accumulation in structural materials of fusion reactors.

Critical phenomenon at the phase transition reveals the universal and long-distance properties of the criticality. We study the ferromagnetic criticality of the pyrochlore magnet Lu$_2$V$_2$O$_7$ at the ferromagnetic transition ${T_\text{c}\approx 70\, \text{K}}$ from the isotherms of magnetization $M(H)$ via an iteration process and the Kouvel-Fisher method. The critical exponents associated with the transition are determined as ${\beta = 0.32(1)}$, ${\gamma = 1.41(1)}$, and ${\delta = 5.38}$. The validity of these critical exponents is further verified by scaling all the $M(H)$ data in the vicinity of $T_\text{c}$ onto two universal curves in the plot of $M/|\varepsilon|^\beta$ versus $H/|\varepsilon|^{\beta+\gamma}$, where ${\varepsilon = T/T_\text{c} -1}$. The obtained $\beta$ and $\gamma$ values show asymmetric behaviors on the ${T < T_\text{c}}$ and the ${T > T_\text{c}}$ sides, and are consistent with the predicted values of 3D Ising and cubic universality classes, respectively. This makes Lu$_2$V$_2$O$_7$ a rare example in which the critical behaviors associated with a ferromagnetic transition belong to different universality classes. We describe the observed criticality from the Ginzburg-Landau theory with the quartic cubic anisotropy that microscopically originates from the anti-symmetric Dzyaloshinskii-Moriya interaction as revealed by recent magnon thermal Hall effect and theoretical investigations., Comment: 6 pages, 4 figures, submit on the behalf of Dr. J-G Cheng. First two authors contributed equally

In this work, we propose a concept of a microwave to optical photon converter for applications in Quantum Information (QI) that is based on travelling magnons in a thin magnetic film. The converter employs an epitaxially grown Bi-substituted yttrium iron garnet (Bi-YIG) film as the medium for propagation of travelling magnons (spin waves). The conversion is achieved through coupling of magnons to guided optical modes of the film. We evaluate the conversion efficiency for this device theoretically. Our prediction is that it will be larger by at least four orders of magnitude than experimentally obtained in a similar process exploiting a uniform magnetization precession mode in a YIG sphere. By creating an optical resonator of a large length from the film (such that the traveling magnon decays before forming a standing wave over the resonator length) one will be able to further increase the efficiency by several orders of magnitude, potentially reaching a value similar to achieved with opto-mechanical resonators. An important advantage of the suggested concept of the QI devices based on travelling spin waves is a perfectly planar geometry compatible with the optical lithography process and a possibility of implementing the device as a hybrid opto-microwave chip.

The effect of aluminum on (La0.7Ba0.3MnO3)1-x/(Al2O3)x composites is investigated based on their crystal structure, thermoelectric power and magnetic properties. The X-ray diffraction process reveals that the composite still exhibits a rhombohedral perovskite structure R 3 ¯ C of all composites. The thermoelectric power data satisfies the Magnon and Phonon-drag models in the low-temperature range, and the smaller Polaron conduction mechanism in the high-temperature range. The power factor values decrease with Al content and the x = 0.025 record the highest value. All composites undergo ferromagnetic to paramagnetic transition at a certain temperature TC. The AC susceptibility values decrease with the increasing Al content.

We report doping at B-Site in TmFeO3 with In3+ ion of varying concentration (x = 0.02, 0.05, 0.07, 0.1) to explore the effect of In3+ ion substitution on structural, magnetic and vibrational properties. XRD study revealed that all the synthesized samples exhibit distorted perovskite orthorhombic (Pbnm) crystal symmetry. The immediate observable effect was an increase in lattice parameters with increasing doping up from x = 0.0 to 0.1 as evidenced by diffraction peaks shifted towards lower angle. Spin reorientation transition (Γ4→Γ2) in all prepared samples occurs at temperature (∼85–92 K) to be a process involving the continuous rotation of Fe3+ spins. M-H curves display the coexistence of weak ferromagnetism and antiferromagnetic ordering at all the measured temperatures with reduction in coercive field and remanence values. Varied magnetic behaviors were observed in this series of compounds, which are believed to be associated with the different interactions of Fe and rare earth ions. The occurrence of two-magnon and two-phonon modes in Raman spectra at temperature around 100 K attributed to spin-reorientation transition exhibited by TmFe1-xInxO3. This might be unambiguous evidence for spin-phonon coupling in the prepared compounds. The coupling and possible presence of two phonon and magnon pair scattering and other energy carriers have been used to explain several recently discovered thermally driven spin transport phenomena.

Nonlinear effects of spin pumping have been investigated by numerically calculating the temporal evolution of the magnon number affected by three- and four-magnon interactions. We showed that the three-magnon interaction gives rise to the enhancement of spin-pumping spin current. We also found that the four-magnon interaction in combination with the three-magnon interaction can be responsible for the experimentally observed low-frequency oscillation of spin current.

The spin Hall conductivity, σxy(ω) is studied in the quantum frustrated two-dimensional bilayer anisotropic Heisenberg model in the triangular lattice in an external magnetic field employing linear spin waves. Our results show a spin Hall conductivity nonzero when the external field hz is different from zero. Moreover, we present the study of the thermal transport by spin excitations for the two-dimensional frustrated monolayer Heisenberg model in the triangular lattice. We also employ the linear spin wave theory to investigate the dependence on T of the thermal Drude weight in this case. We present our results for the dependence on T of the thermal Drude weight in the gapless regime. Our results show an ideal thermal conductivity for T > 0 as expected for gapless spin systems, and an insulator thermal transport at T = 0 .

We systematically compared the local and nonlocal spin Seebeck effect (SSE) in YIG/Pt nano-thick films. Their dependences on heating current, magnetic field and temperature were studied. Both the local and nonlocal SSE are quadratically related to heating current while their dependences on the magnetic field are distinguishing. The local SSE is seldomly affected but the nonlocal one is obviously suppressed by high magnetic field. Besides, local SSE gradually enhances to maximum around 150 K and then quickly decreases. However, nonlocal SSE monotonously grows up from 50 to 300 K. The transport properties of the associated spin current in local and nonlocal SSE are found to be different which is possibly due to that the local signal is directly related to the temperature difference at the interface whereas the nonlocal one is mainly driven by the spin chemical potential.

Formalism is proposed for studying of the thermodynamic properties of ferromagnetic nanowires, on the basis of which it is established that at temperatures below the Debye temperature the contribution of a domain wall to the heat capacity of ferrite-garnet nanowires can exceed the phonon and magnon contributions. The conditions for realization of this phenomenon are determined. It is shown that this effect has a nanoscale nature. It is found that by varying the magnitude of the nanowire magnetization it is possible to influence its temperature and dimensional characteristics.

We investigate theoretically the combined effect of phonons and magnons caused by finite temperatures on the electrical resistivity of nonstoichiometric half-Heusler NiMnSb alloy. The coherent potential approximation within the alloy analogy model is employed for an efficient treatment of chemical impurities, atomic displacements, and magnetic disorder. Spin fluctuations of local Mn moments are described by two models: (i) uncompensated disordered local moment approach and (ii) tilting of the moments. The calculated resistivity agrees with experimental data, the agreement is good up to 600 K. We show that a strong magnetic disorder leads to a violation of the Matthiessen’s rule for the resistivity. We also discuss the spin polarization of the electrical current which exceeds 90% at room temperature but it is dramatically reduced by the magnetic disorder for higher temperatures approaching the Curie point ( T C = 730 K).

Neutron resonant spin echo spectroscopy has been applied to measure the splitting of the magnon bands due to the magnetic dipole interaction in the two dimensional antiferromagnet MnPS3. The splittings measured are more than two orders of magnitude smaller than energies of the magnons. Simulation of the magnon energies involving the calculation of the magnetic dipole interaction in MnPS3 predict splittings a little higher than those measured. The comparison confirms that the magnetic dipole interaction provides most of the anisotropy which stabilises the antiferromagnetic order in MnPS3. We present the first evidence for the splitting of magnon bands due to the magnetic dipole interaction in a two dimensional antiferromagnet.

We used the Self-Consistent Harmonic Approximation (SCHA) to study the thermodynamics of the precession magnetization in a two-dimensional isotropic ferromagnet. The SCHA treats the Hamiltonian in terms of the canonically conjugate operators S z and φ (the azimuth angle) including renormalized temperature dependent parameters to take into account higher order interactions. It is well-known that in right conditions, a dynamic magnetic field is able to provide spin pumping and drives the system to a magnon Bose-Einstein condensation. The magnon condensate is a coherent state that presents minimal uncertainty for the S z and φ operators. Consequently, 〈 S z 〉 and 〈 φ 〉 should constitute natural fields to describe the model, which justifies the SCHA formalism. The results obtained are consistent with other theoretical and experimental works.

We have shown that as the combined effect of alloy and temperature increases, the magnons created in the C o x P t 1 − x ∕ P t multilayers undergo a transition in nature. From a purely quantum behavior at low temperature, these magnetic excitations acquire a behavior of the classical limit of localized particles (their associated wavelength is very low) at high temperature. The magnetization per spin calculated M c a l z ( T , x ) using an extended average number of particles forming the mixture of these two types of magnons created shows, indeed, a change of slope of the tangent to its curve at a temperature T = T r ( x ) . This would reflect a spin reorientation transition (SRT) from out-of-plane to in-plane observed experimentally in this type of materials. The comparison of M c a l z ( T , x ) with the experiment is indeed very satisfactory.

Through the simple electrical measurement, the spin injection to magnetic insulators via large DC currents applied in heavy metals is reported. The measured nonlocal voltages are divided into two types: the electrical-generated magnons (Vodd) and the thermal-generated magnons (Veven) according to the different symmetry with respect to the applied current. With the current increasing, a nonlinear behavior of Vodd is observed. The dependences of Vodd on the current and temperature are compared, excluding the effect of current-induced heating on the nonlinear transport. The field dependence of the nonlocal voltages is also systematically studied which indicates the frequency-dependent mechanism needs to be introduced to explain the nonlinearity. It is possible that the frequency change of the dominate magnons when the current is large enough to drive the magnetization out of thermal equilibrium induces the deviation from the linear relationship in the low current region.

In this article we report the effect of charged impurity doping on the thermal spin-wave excitations of 2D Lieb lattice using the longitudinal/transverse and in-plane magnonic heat transport properties in a systematically Green’s function approach and the Born approximation. We present numerical results for XY Heisenberg model. Accordingly, the flow of magnonic current is enhanced depending on the impurity concentration, in which (i) at very low and very high concentrations, in-plane magnon conductivity becomes negative, (ii) the magnon Seebeck coefficient decreases (increases) before (after) a critical impurity concentration for both longitudinal/transverse and in-plane components, (iii) the longitudinal/transverse magnon thermal conductivity increases slightly with impurity concentration, while the in-plane one becomes flat, and (iv) both longitudinal/transverse and in-plane magnonic heat transport efficiencies decrease (increase) at low (high) temperatures with low impurity concentrations, whereas become flat (decrease) at high impurity concentrations. Our results are useful for the implementation of a new spin energy conversion and capable spin-filter devices for the spintronic community.

In the reentrant (RE) state at low temperatures, the functional dependence of magnetization on temperature at fixed magnetic fields, M H ( T ) , in the Cr70Fe30 thin films deviates considerably from the well-known Bloch T 3 / 2 power law, predicted by the spin-wave (SW) theory for a conventional three-dimensional (3D) ferromagnet. Strong departures from the SW predictions are shown to be a manifestation of an anomalous softening of magnon modes in the RE regime. The model that invokes the Bose–Einstein condensation (BEC) of magnons provides a satisfactory explanation for this unusual behavior. The magnon BEC transition temperature, T c ( H ) , the volume, V ( H ) , over which the condensate wavefunction retains its phase coherence, the chemical potential, μ ( T , H ) , and the average number of magnon condensates in the ground state, 〈 n 0 ( T , H ) 〉 , are accurately determined from M H ( T ) using a self-consistent approach. Reduction in the film thickness from 978 nm to 21 nm enhances the BEC transition temperature at zero field, T c (H = 0), from 0.1 K to 0.33 K but drastically reduces the phase coherence length of the BE condensate wavefunction and the spin-wave stiffness at T = 0 and H = 0. The variation of T c with magnetic field has the form that is characteristic of the magnon BEC in a 3D spin system. Regardless of the film thickness, the BE condensate fraction at H = 0, 〈 n 0 ( H = 0 ) 〉 , turns out to be zero at T ≃ 2 K ≫ T c ( H = 0 ) , as expected. The ‘zero-field’ quantities M 0 ( H = 0 ) , D 0 ( H = 0 ) , V ( H = 0 ) and T c ( H = 0 ) are found to vary with the film thickness as M 0 ( H = 0 ) ∼ t 3 / 4 , D 0 ( H = 0 ) ∼ t - 3 / 4 , V ( H = 0 ) ∼ t 2 / 5 and T c ( H = 0 ) ∼ t - 1 / 3 . These power laws assert that the film thickness is the fundamental length scale so far as the magnon BEC phenomenon in the Cr70Fe30 thin films is concerned.

Longitudinal spin Seebeck effect (LSSE) refers to generate spin accumulation in ferromagnetic material by thermal gradient, and the spin accumulation can transfer to the attached heavy metal and convert to charge current through inverse spin Hall effect (ISHE). However, the origin of LSSE has been under debate for a long time. In this paper, we prepared the YIG/Pt heterostructure by pulsed laser deposition and magnetron sputter, and studied the relationship between temperature and the voltage of inverse spin Hall effect ( V ISHE ). It is found that the V ISHE vs T curve measured under zero-field (ZFC) and nonzero-field (FC) condition are not coincident with each other, meaning there might exist some spin glass states in the system which may come from the YIG/Pt interface. Moreover, we also compare the experimental data and magnon spin current theory prediction which considers both the bulk magnon spin current effect and the interfacial spin pumping process in the interface. It is found that there is a big deviation between them, meaning that the interface effect may play a crucial role in the temperature-dependent LSSE, especially in low temperature. This work will provide deep insight into the spin transport behavior of the YIG/Pt heterostructure.

Published magnon dispersion data are discussed, showing that magnon dispersions of bulk magnets are essentially as for linear spin chains, i.e. one-dimensional. This applies to all propagation directions in cubic magnets, in layered two-dimensional magnets and in axial magnets. Propagation of magnons seems to be restricted to the individual magnetic domain. Within each domain the spin structure is collinear (one-dimensional). Perfect collinear spin structures up to ordering temperature, in spite of local exchange anisotropies, cannot be understood on the basis of atomistic models. As we have argued earlier, collinear spin structures are caused by a one-dimensional boson field. The bosons are essentially magnetic dipole radiation emitted by the precessing spins. Since emission is by stimulated emission, perfect one-dimensional boson fields result. The one-dimensional boson fields are considered as responsible for the magnon dispersions of linear chain type. However, magnon dispersions follow sine function of wave vector (for antiferromagnets) and sine function squared (for ferromagnets) not perfectly. At low q-values, magnon dispersions often deflect from sine function (or from sine function squared) towards a magnon gap at q≡0. This part of the magnon dispersion is excellently described by gap energy plus qx power function of wave vector. The two analytically different sections of wave vector have different temperature dependencies. The gap closes for T→Tc, while the sine function persists nearly unperturbed into the paramagnetic phase. In contrast to the sine functions, the qx term is universal that is, independent of spin structure but depends on whether the spin is integer or half-integer. It is concluded, that for all q-values, the functional wave-vector dependence of the magnon dispersions (not their absolute energies) is determined by boson fields with well defined dimensionalities.

The effect of transition to disordered phase on AC conductivity in the two-dimensional frustrated ferrimagnetic model in the square lattice at T = 0 is investigated. The spin conductivity, σ reg ( ω ) is determined for different values of frustration parameters J 2 S and J 2 s that separates the different phases such as the ferrimagnetic phase and the collinear phase. We obtain a small influence of this phase transition on spin conductivity. Furthermore, we present the behavior of the half-width of S ( q → , ω ) that gives the information about the scattering of magnons at long wavelength range.

We report on some thermal phenomena related to the transport of magnons in insulating ferromagnetic systems. The influence of the magnon-phonon dissipation in the magnon thermal conductivity and in the spin Seebeck effect is investigated through the Boltzmann’s theory. The spin Peltier effect is also discussed, where the intensity of the effect as a function of the magnetic layer thickness is highlighted. For both effects, our results are compared with some recent reported experimental data for the magnetic insulator yttrium iron garnet (YIG), and the bilayer YIG/Pt. A good agreement between our calculations and experimental data was observed.

Spectrum of spin waves (magnons) in the two-dimensional bilayer antiferromagnet based on the VSe 2 is studied theoretically. The vanadium atoms within individual layers are coupled ferromagnetically, while the exchange coupling between V atoms located in different planes is antiferromagnetic. The magnon dispersion relation and its dependence on magnetic anisotropy is analyzed in the regime of weak external magnetic field. The spin-wave spectra are derived within the spin-wave theory of antiferromagnets in terms of the Holstein-Primakoff transformation combined with the Bogolubov diagonalization scheme. The magnon dispersion features are discussed in case of the T-type stacking of the VSe2 bilayer.

We propose a dual gravitational theory corresponding to isotropic ferromagnetic systems based on the holographic duality, and establish the holographic dictionary between physical quantities in ferromagnets and the gravitational theory. Utilizing the holographic dictionary, the holographic calculation in the dual gravitational theory is shown to reproduce thermodynamics of ferromagnets including ferromagnetic phase transitions and low-temperature behaviors originating from magnons and conduction electrons. We further derive the equation for magnetization dynamics in ferromagnets from the equations for scalar fields and non-abelian gauge fields on charged black holes in the dual gravitational theory. From these results, the holographic duality is expected to give a new guiding principle to explore new phenomena in spintronics and magnonics based on the gravitational perspective.

For many magnets with a three-dimensional spin, experimental data of the magnetic ordering temperature, Tc, and of the magnon energy at the zone boundary, EZB (the near-neighbor interaction strength) are compared. It is observed that the lower the spin quantum number is, the lower is Tc in comparison with EZB/kB. This is explained by the fact that the magnetic ordering transition is the ordering transition of a boson field and occurs largely decoupled from the exchange interactions between the spins. Due to a strong interaction with the bosons, the spins order at the same temperature as the boson field. The bosons are essentially magnetic dipole radiation, emitted through stimulated emission by the precessing spins. Surprisingly, in the critical range (and below), the bosons dominate the dynamics of the spins totally, and prevent the local exchange interactions from performing a phase transition into a long-range ordered state. Condition for the ordering transition of the boson field is that the density of the bosons, travelling along those crystallographic directions along which domains will be formed below Tc, is sufficiently high, such that the threshold for the spontaneous onset of stimulated emission is reached. In the ordered state of the boson field, the bosons are in a one-dimensional and coherent state. This is realized in each magnetic domain. Since in magnets with a low spin quantum number the bosons get generated with a low rate, the critical density, necessary for the ordering transition of the boson field is reached at a correspondingly low temperature only.

Interactions between quasiparticles of different nature, such as magnons and phonons in a magnetic medium, significantly change the spectrum of both types of quasiparticles. In regions where their dispersion curves intersect, forming an avoided crossing, the properties of the two subsystems mix and hybrid states are born, whose characteristics, such as velocity, lifetime and scattering efficiency, are determined by the magnon and phonon portion of their composition. In particular, the influence of the elastic component of quasiparticles allows for the manipulation of the position and even the number of spectral minima of the magnon gas in the wavevector space. Using Brillouin light scattering spectroscopy, we experimentally investigated such a system populated by the parametric pumping of magnons in a film of yttrium iron garnet. We found that the processes of thermalization of exchange and dipole magnons lead to their accumulation in different areas at the bottom of the spin-wave spectrum separated by the magnon–phonon hybridization region. The localization of quasiparticles in the wavevector space eventually leads to their separation in real space.

In this work, thulium iron garnet (Tm3Fe5O12 – TmIG (20 nm)/Tungsten(W)(t) bilayers, sputtered on top of gadolinium gallium garnet (1 1 1) substrate, were used to investigate spin pumping (SP) line broadening mechanism in Ferromagnetic Resonance (FMR). The TmIG, films prior and after tungsten cap layer deposition, were investigated employing FMR and X-ray diffraction techniques. The TmIG films showed (1 1 1) orientation and perpendicular magnetic anisotropy (PMA). Due to the interface TmIG/W, when the TmIG magnetization is in resonance a spin current is pumped out the TmIG into the W layer, increasing the damping of the magnetization. Measuring the out-of-plane angular dependence of the FMR resonance field and linewidth, we were able to obtain solely the SP contribution to the line broadening, filtering Gilbert, mosaicity, and two magnon scattering mechanisms.

Procedure of determination of magnetic part of the specific heat of ferromagnetic (FM) solid from the results of the low-temperature measurements is proposed and applied to the number of non-stoichiometric Ni50Mn50-xInx alloys. It is shown that the disregard of magnetic contribution to the specific heat of FM solid results in the noticeable underestimation of Debye temperature and overestimation of the specific heat of electron subsystem of FM solid. The magnetic contribution to the low-temperature specific heat of antiferromagnetic (AFM) solid is negligibly small if Neel temperature exceeds the Debye temperature. In the opposite case the magnetic part of the specific heat of AFM solid may be comparable in value with non-magnetic part, but separation of these parts is an open problem.

In order to simplify the conversion between slow light and fast light, we ameliorate the cavity-magnon coupling system and theoretically analyze the characteristics of group delay of the output field. The system under consideration consists of a microwave cavity and N yttrium iron garnets (YIGs), of which the quantity of YIG spheres is designed to be controlled by an external magnetic field. It is found, most importantly, that the conversion between fast and slow light can be realized by adjusting the number of YIG spheres that the external magnetic field acts on. Significantly, through adjusting the quantity N of magnons, the group delay can be stabilized at a required value even though the magnons are slightly detuned. Moreover, adjusting the quantity of YIG spheres can also alter the frequency corresponding to the fast-slow light effect, signifying that the conversion can be expanded to the frequency domain. Meanwhile, the cavity-magnon coupling strength required to generate the fast-slow light can be also effectively reduced by increasing the quantity of YIG spheres. Furthermore, we find that the cavity-magnon coupling strength and the cavity-field loss rate are complementary in terms of the group delay, which provides a feasible alternative for experimental design. The factors determining the slow-light limit have also been summarized based on their influence on the group delay. Our results have theoretical significance for the improvement of the generation and conversion of the fast-slow light, upgrading and innovating the information technology, especially in the field of all-optical networks.

We study spin transport at zero temperature in the two-dimensional spin-one Heisenberg antiferromagnet on a square lattice with easy-plane single-ion anisotropy in the Neel phase. The regular part of the spin conductivity is calculated at zero temperature using the SU(3) Schwinger boson formalism and the Kubo theory. Three magnon processes provide the dominant contribution to the spin conductivity. We find evidence of the Higgs mode contribution to the spin conductivity.

In continuation of our studies on the binary RPd compounds (orthorhombic CrB-type structure, Pearson symbol oS8, space group Cmcm, No. 63), we report here the magnetic properties of the iso-structural PrPd and SmPd. The two compounds order ferromagnetically with the Curie temperature, TC, of nearly 12 K and 22 K, respectively (inferred from the peak temperature of the heat capacity at the paramagnetic to ferromagnetic transition). Anomalies in the magnetization and the electrical resistivity are also observed in correspondence of the bulk magnetic transition. The ferromagnetic nature of the magnetic transition is clearly indicated by the temperature and field dependence of the magnetization. The coercive fields of the two compounds are measured to be 160 Oe and 12 kOe at 2 K, respectively. The temperature dependence of both the heat capacity and electrical resistivity indicates a gapped ferromagnetic magnon spectrum with the gaps of ≈11 K and ≈40 K in PrPd and SmPd, respectively. The data obtained in current work are compared and discussed together with those previously reported for CePd and NdPd; the Curie temperatures of the four RPd iso-structural compounds follow the de Gennes scaling.

An immediate need of information technology is designing fast, small and low-loss devices. One of the ways to design such devices is using the bosonic quasiparticles, such as magnons, for information transfer/processing. This is the main idea behind the field of magnonics. When a magnon propagates through a magnetic medium, no electrical charge transport is involved and therefore no energy losses, creating Joule heating, occur. This is the most important advantage of using magnons for information transfer. Moreover the mutual conversion between magnons and the other carriers e.g. electrons, photons and plasmons shall open new opportunities to realize tunable multifunctional devices. Magnons cover a very wide range of frequency, from sub-gigahertz up to a few hundreds of terahertz. The magnon frequency has an important impact on the performance of magnon-based devices (the larger the excitation frequency, the faster the magnons). This means that the use of high-frequency (terahertz) magnons would provide a great opportunity for the design of ultrafast devices. However, up to now the focus in magnonics has been on the low-frequency gigahertz magnons. Here we discuss the feasibility of using terahertz magnons for application in magnonic devices. We shall bring the concept of terahertz magnonics into discussion. We discuss how the recently discovered phenomena in the field of terahertz magnons may inspire ideas for designing new magnonic devices. We further introduce methods to tune the fundamental properties of terahertz magnons, e.g. their eigenfrequency and lifetime.

We consider electron drag in a system of two ferromagnetic layers separated by an insulating interface. The source of it is expected to be magnon-electron interactions. Namely, we assume that the external voltage is applied to the "active" layer stimulating electric current through this layer. In its turn, the scattering of the current-carrying electrons by magnons leads to a magnon drag current within this layer. The 3-magnons interactions between magnons in the two layers (being of non-local nature) lead to magnon drag within the "passive" layer which, correspondingly, produce electron drag current via processes of magnon-electron scattering. We estimate the drag current and compare it to the phonon-induced one., Comment: 20 pages, 1 figure

We report on X-ray diffraction, electrical transport, heat capacity and magnetocaloric effect measurements of a polycrystalline sample of Pr 2 Pt 2 In . The compound forms in the tetragonal Mo 2 FeB 2 type structure and orders ferromagnetically at T C =9 K. In the ordered state, its thermodynamic and electrical transport properties are dominated by magnon contributions with an energy gap of about 8 K in the spin-wave spectrum. The magnitude of magnetocaloric effect is similar to the values reported for most rare-earth based intermetallics. Characteristic behavior of the isothermal magnetic entropy change maximum points to a second-order character of the ferromagnetic phase transition in the compound studied.

Magnetic excitations and exchange interactions in multiferroic NdFe3(BO3)4 were studied by inelastic neutron scattering in the phase with commensurate antiferromagnetic structure. The observed spectra were analyzed in the frame of the linear spin-wave theory. It was shown that only the model, which includes the exchange interactions within eight coordination spheres, describes satisfactorily all observed dispersion curves. The calculation showed that the spin-wave dynamics is governed by the strongest antiferromagnetic intra-chain interaction and three almost the same inter-chain interactions. Other interactions, including ferromagnetic exchange, appeared to be insignificant. The overall energy balance of the antiferromagnetic inter-chain exchange interactions, which couple the moments from the adjacent ferromagnetic layers as well as within a layer, stabilizes ferromagnetic arrangement in the latter. It demonstrates that the pathway geometry plays a crucial role in forming of the magnetic structure.

We consider the phenomenon of Bose-Einstein condensation of quasi-equilibrium magnons which leads to a spin superfluidity, the coherent quantum transfer of magnetization in magnetic materials. These phenomena are beyond the classical Landau-Lifshitz-Gilbert paradigm. The critical conditions for excited magnon density for ferro- and antiferromagnets, bulk and thin films are estimated and discussed. The BEC should occur in the antiferromagnetic hematite at much lower excited magnon density compared to the ferromagnetic YIG., 0 figures

Conversion of thermal energy into magnonic spin currents and/or effective electric polarization promises new device functionalities. A versatile approach is presented here for generating and controlling open circuit magnonic spin currents and an effective multiferroicity at a uniform temperature with the aid of spatially inhomogeneous, external, static electric fields. This field applied to a ferromagnetic insulator with a Dzyaloshinskii-Moriya type coupling changes locally the magnon dispersion and modifies the density of thermally excited magnons in a region of the scale of the field inhomogeneity. The resulting gradient in the magnon density can be viewed as a gradient in the effective magnon temperature. This effective thermal gradient together with local magnon dispersion result in an open-circuit, electric field controlled magnonic spin current. In fact, for a moderate variation in the external electric field the predicted magnonic spin current is on the scale of the spin (Seebeck) current generated by a comparable external temperature gradient. Analytical methods supported by full-fledge numerics confirm that both, a finite temperature and an inhomogeneous electric field are necessary for this emergent non-equilibrium phenomena. The proposal can be integrated in magnonic and multiferroic circuits, for instance to convert heat into electrically controlled pure spin current using for example nanopatterning, without the need to generate large thermal gradients on the nanoscale., Comment: to appear in Phys. Lett. A

Antiferromagnets are promising for future spintronic applications owing to their advantageous properties: They are magnetically ordered, but neighboring magnetic moments point in opposite directions, which results in zero net magnetization. This means antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, they show intrinsic high frequency dynamics, exhibit considerable spin–orbit and magneto-transport effects. Over the past decade, it has been realized that antiferromagnets have more to offer than just being utilized as passive components in exchange bias applications. This development resulted in a paradigm shift, which opens the pathway to novel concepts using antiferromagnets for spin-based technologies and applications. This article gives a broad perspective on antiferromagnetic spintronics. In particular, the manipulation and detection of antiferromagnetic states by spintronics effects, as well as spin transport and dynamics in antiferromagnetic materials will be discussed. We will also outline current challenges and future research directions in this emerging field.

We have studied theoretically (including computer simulations) dynamical magnetic properties of aggregates of ferromagnetic amorphous nanogranules in the presence of direct exchange between the neighboring granules and random anisotropy fields. It was shown that in such structures there still exist delocalized magnetic excitations similar to magnons in the bulk materials, although at much lower frequencies. The excitations at high frequencies are localized within the corresponding granules. The details of spectrum of these excitations (including structural factors) are discussed.

Current induced spin wave excitations in spin transfer torque nano-contacts are known as a promising way to generate exchange-dominated spin waves at the nano-scale. It has been shown that when these systems are magnetized in the film plane, broken spatial symmetry of the field around the nano-contact induced by the Oersted field opens the possibility for spin wave mode co-existence including a non-linear self-localized spin-wave bullet and a propagating mode. By means of micromagnetic simulations, here we show that in systems with strong perpendicular magnetic anisotropy (PMA) in the free layer, two propagating spin wave modes with different frequency and spatial distribution can be excited simultaneously. Our results indicate that in-plane magnetized spin transfer nano-contacts in PMA materials do not host a solitonic self-localized spin-wave bullet, which is different from previous studies for systems with in plane magnetic anisotropy. This feature renders them interesting for nano-scale magnonic waveguides and crystals since magnon transport can be configured by tuning the applied current., Comment: 6 pages, 3 figures

The field of magnon spintronics offers a charge current free way of information transportation by using spin waves (SWs). Compared to forward volume spin waves for example, Damon-Eshbach (DE) SWs need a relatively weak external magnetic field which is suitable for small spintronic devices. In this work we study DE SWs in Co 2 MnSi, a half-metallic Heusler alloy with significant potential for magnonics. Thin films have been produced by pulsed laser deposition. Integrated coplanar waveguide (CPW) antennas with different distances between emitter and detection antenna have been prepared on a Co 2 MnSi film. We used a vector network analyzer to measure spin wave reflection and transmission. We observe spin wave propagation up to 100 μm, a new record for half-metallic Heusler thin films.

The squeezed states of dressed magnons in ferromagnets have been investigated. No effective Debye cutoff frequency has been assumed unlike what has been done hitherto. Instead, the results have been expressed throughout in terms of the reduced temperature. The effect of dressed magnon-phonon interactions on the formulation of these states has been studied. It has been shown that the magnon-phonon interactions play a significant role in determining the squeeze factor and the variation of the dressed magnon effective mass with temperature.

We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe2As2 and BaFe2As2 in pure form for umklapp scattering with scattering wave vector Q = ( 1 , 1 ) , in the unit of π/a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe2As2 and BaFe2As2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.

We report the magnetoresistance studies on Dy 1−x Y x (x ≤ 0.05) alloys across the first order helimagnetic to ferromagnetic phase transition. These alloys exhibit multiple magnetic phases on varying the temperature and magnetic field. The magnetoresistance studies in the hysteresis region shows irreversibility in forward and reverse field cycles. The resistivity values at zero field for these alloys after zero field cooling to the measurement temperatures, are different in both forward and reverse field cycles. The path dependence of magnetoresistance suggests the presence of helimagnetic phase as the supercooled metastable state which transforms to the stable ferromagnetic state on increasing the field. At high magnetic fields negative magnetoresistance following a linear dependence with field is observed which is attributed to the magnon scattering.

We study the excitations of breathers and rogue wave in a classical Heisenberg spin chain with twist interaction, which is governed by a fourth-order integrable nonlinear Schrodinger equation. The dynamics of these waves have been extracted from an exact solution. In particular, the corresponding existence conditions based on the parameters of perturbation wave number K , magnon number N , background wave vector k s and amplitude c are presented explicitly. Furthermore, the characteristics of magnetic moment distribution corresponding to these nonlinear waves are also investigated in detail. Finally, we discussed the state transition of three types nonlinear localized waves under the different excitation conditions.

The investigation of the fundamental excitations of magnetically ordered systems has been at the core of the scientific career of Sergio Rezende. Here, I briefly summarize some of our own recent work that often took inspiration from his research work. In particular, I discuss how heat transported by magnons can give rise to measurable spin currents and how magnons may hybridize with other dynamic excitations. Both of these topics are currently at the core of larger research efforts within the field of magnetism.

We reveal the multiple-path microwave interference induced microwave transmission in a cavity-magnonic system composed of a yttrium-iron-garnet (YIG) film and a microstrip cavity circuit. By changing the position of the YIG in the circuit, we identify that the magnon in YIG is driven by the standing microwave from the cavity or by the traveling wave through the microstrip. We observe that the microwave transmission spectra show an anti-crossing dispersion named mode repulsion and an anomalous dispersion named mode attraction, respectively. The characteristic of anti-cross dispersion spectra is theoretically predicted using the coupling between the magnon and the cavity. Without coupling, the anomalous dispersion in our experiment is well interpreted by the multiple-path microwave interference between the indirect paths through the cavity and the magnon and the direct path through the traveling wave. And the anomalous dispersion is caused by the redistribution of the microwave transmission intensity. We also find the phases of the magnon in two cases changes π , which can be the key physical factor to understand the change of microwave transmission characteristics. Our work highlights the role played by the multiple-path microwave interference in magnonic microwave devices, which will be an important issue in designing microwave circuits based on the magnon.

We report on the anisotropy of the effective damping (αeff) in the sputtered β-Ta(6 nm)/2D-epitaxial-Py(tPy = 3–10 nm) bilayers. In-plane field orientation dependent FMR measurements revealed a prominent anisotropy in αeff, which is ascribed to the spin wave induced spin pumping from Py to β-Ta in β-Ta/epi-Py system. The results especially suggest that at lower tPy a relatively larger αeff and its anisotropic nature are present in β-Ta/epi-Py bilayers as compared to bare epi-Py, which are unlikely at higher tPy. This is possibly due to both spin pumping which stems from in-plane spin waves associated with two magnon scattering mechanism and interface induced anisotropy from heavy metals like β-Ta at β-Ta/Py interface. The in-plane Hr vs. φ reveals clear anisotropy behavior at tPy ≤ 4 nm.

Summary While progress in thermoelectric materials based on the engineering of electronic and phononic characteristics is reaching a plateau, the addition of the spin degree of freedom has the potential to open a new landscape for alternative thermoelectric materials. Here, we present the concepts, current understanding, and guidelines for designing spin-driven thermoelectrics. We show that the interplay between the spin and heat currents in entropy transport via charge carriers can offer a path to enhance the electronic thermopower. The classical antiferromagnetic semiconductor manganese telluride (MnTe) is chosen as the case study due to its significant spin-mediated thermoelectric properties. We show that, although the spin-disorder scattering reduces the carrier mobility in magnetic materials, spin entropy, magnon, and paramagnon carrier drags can dominate and significantly enhance the thermoelectric power factor, and hence zT. Finally, several guidelines are drawn based on the current understanding for designing high-performance spin-driven thermoelectric materials.

The dispersion of a broadband electromagnetic pulse in a dielectric was theorized by Sommerfeld and Brillouin to separate into parts, with two groups of frequencies traveling faster—or “running ahead”—of the main signal from a Lorentz-like media. The observation of one such group, the Brillouin precursor (BP), arising from magnon and phonon polaritons generated in a random-walk ultra-small particle ferrite media is reported for the first time. Upon input of a broadband low-frequency radio wave in the form of a very low frequency (VLF) square wave to the ferrite particle dielectric core of the Random Walk Antenna (RWA), BP was generated and detected by the VLF antenna. The BP observation propagating in free space and water is supported by two salient percussor features (1) temporal Bessel-like waveform, and (2) algebraic, rather than exponential, attenuation with distance. The Brillouin precursor observed here in the VLF region can lead to precursors in the THz and optical regime with respect to the resonance frequency of the quasi-particles of optical phonons and excitons, respectively