Temperature Dependence of the Spin Seebeck Effect in a Mixed Valent Manganite

Abstract

We report on temperature dependent measurements of the Longitudinal Spin Seebeck Effect (LSSE) in the mixed valent manganite La0.7Ca0.3MnO3. By disentangling the contribution arising due to the Anisotropic Nernst effect, we observe that these two thermally driven phenomena vary disparately with temperature. In a narrow low temperature regime, the LSSE exhibits a T 0.55 dependence, which matches well with that predicted by the magnon-driven spin current model. Across the double exchange driven paramagnetic-ferromagnetic transition, the LSSE exponent is significantly higher than the magnetization one. These observations highlights the importance of individually ascertaining the temperature evolution of different mechanisms which contribute to the measured spin Seebeck signal.

Large Thermo-Spin Effects in Heusler Alloy-Based Spin Gapless Semiconductor Thin Films

“…According to the theory of thermally generated magnon-driven interfacial spin pumping mechanism, simultaneous application of a vertical ( z -axis) temperature gradient and an external transverse dc magnetic field ( x -axis) across the MgO/CoFeCrGa (95 nm)/Pt film gives rise to transverse spin current pumping from the CoFeCrGa layer into the Pt layer with the interfacial spin current density: at the CoFeCrGa/Pt interface, where G ↑↓ , ℏ, γ, M S , and V a are the interfacial spin-mixing conductance, the reduced Planck’s constant , the gyromagnetic ratio, the saturation magnetization of CoFeCrGa, and the magnon coherence volume, respectively. ,, The magnetic coherence volume is expressed as , where ζ is the Riemann Zeta function and D is the spin-wave stiffness constant. , This transverse spin current, , is then converted into charge current along the y -axis via the inverse spin Hall effect (ISHE), where e , θ SH Pt , and are the electron charge, the spin Hall angle of Pt, and the spin polarization vector, respectively. The corresponding voltage along the y -axis can be expressed as , where R y , L y , λ Pt , and t Pt are the electrical resistance between the voltage leads, the distance between the voltage leads, the spin diffusion length of Pt, and the thickness of the Pt layer (=5 nm), respectively. Since CoFeCrGa is a spin gapless semiconductor with soft ferromagnetic behavior, , concomitant application of the temperature gradient ( z -axis) and dc magnetic field ( x -axis) also generates a spin-polarized current in the CoFeCrGa layer along the y -axis due to ANE, which gives rise to an additional contribution ( V CoFeCRGa ANE ) to the total voltage signal measured across the Pt layer in the MgO/CoFeCrGa (95 nm)/Pt heterostructure.…”

“…On the other hand, the longitudinal spin Seebeck effect (LSSE) refers to the thermal generation of magnonic spin current in a ferromagnetic (FM) material by the concurrent applications of a temperature gradient and an external magnetic field across a FM/heavy metal (HM) bilayer structure and injection of that spin current to the adjacent HM layer with strong SOC, which is then converted into electrically detectable charge current in the HM layer via the inverse spin Hall effect (ISHE). ,, The bilayer structure consists of the ferrimagnetic insulator Y 3 Fe 5 O 12 (YIG), and Pt is known as the benchmark system for generating pure spin current and hence LSSE. ,− Apart from YIG, other magnetic insulators, for example, the compensated ferrimagnetic insulator Gd 3 Fe 5 O 12 , the noncollinear antiferromagnetic insulator LuFeO 3 , etc., have also emerged as promising spin caloritronic materials. Nevertheless, observation of LSSE is not only restricted to magnetic insulators, but it has also been observed in metallic, half-metallic, − and semiconducting ferromagnets …”

Spin Seebeck Effect: Sensitive Probe for Elementary Excitation, Spin Correlation, Transport, Magnetic Order, and Domains in Solids

“…The longitudinal configuration (Figure 1b) has been mainly employed in recent studies owing to its simple and straightforward nature (1,2), enabling systematic and quantitative investigations of SSEs in various magnetic insulators. Note that when a magnetic conductor is used in this configuration, anomalous Nernst effects may overlap with the longitudinal SSE (LSSE) signal (1,2,12,13,14). Materials used for LSSE measurements are listed and (e) inverse spin Hall effect (ISHE).…”

Cryogenic spin Seebeck effect

“…The spin current is injected into the metal contact by spin pumping [4], but the signal is usually dominated by the currents that are generated by temperature gradients in the bulk of the magnet [12]. The reported signals are, in general, proportional to the applied temperature differences T. However, several recent studies of the SSE at low temperatures [13][14][15][16][17][18][19][20] did not address a fundamental issue of thermal transport at ultralow temperatures. Linear response is valid when the perturbation is sufficiently small, but the properly normalized driving force is not T but T /T (or ∂T /T ), i.e., the temperature difference divided by the average one [21].…”