Magnetic moment impact on spin-dependent Seebeck coefficient of ferromagnetic thin films

Abstract

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µI), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µI influence on the spin-dependent S of model ferromagnetic thin films, allowing µI thermal fluctuations, ordering, and density variation influence to be independently investigated. µI influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

Influence of the Mn5Ge3/Ge ohmic-contact interface on the Seebeck coefficient of the Mn5Ge3/Ge bilayer

“…99.99% pure Ge and 99.9% pure Mn targets were sputtered on l = 1.5 × L = 2.5 cm 2 (Fig. 4 a) glass substrates in a commercial magnetron sputtering system with a base vacuum of 10 −8 Torr 55 , 58 , 59 . Ge and Mn deposition rates were calibrated by measuring the thickness of different films deposited in different conditions on oxidized Si substrate using X-ray reflectivity.…”

“…The chosen interface is the contact between a polycrystalline thin film of the metallic compound Mn 5 Ge 3 and a polycrystalline thin film of p -type Ge. Mn 5 Ge 3 is known to be n -type 55 and to form an ohmic contact with Ge if Ge is p -type or a Schottky contact if Ge is n -type 56 , 57 . An n -type metal/ p -type semiconductor interface allows possible carrier injection through the interface to be more easily evidenced.…”

“…An n -type metal/ p -type semiconductor interface allows possible carrier injection through the interface to be more easily evidenced. Mn and Ge were deposited at room temperature by magnetron sputtering on glass substrate 55 , 58 , and the Mn 5 Ge 3 /Ge bilayer was grown by reactive diffusion thanks to ex situ annealing under vacuum ( P ~ 10 −7 mbar) at 400 °C for 10 min 55 , 59 , 60 . The use of thin films allows the study of a single interface, as well as the volume of the phases to be precisely controlled.…”

“…( 1 ), leading to complex variations of S eff versus temperature, since temperature plays differently on carrier trapping/releasing, carrier recombination, and carrier injection. Neglecting the effect of the interface on the concentration of electrons in metallic Mn 5 Ge 3 , and assuming that the main effect of the interface is the modification of the hole concentration ( p ) and the electron concentration ( n ) in the semiconductor, one can roughly assume S h ∝ p −2/3 , S e ∝ n −2/3 5 – 8 , 30 , 32 , 44 , σ h ∝ p , and σ e ∝ n 6 , 30 , 55 , leading to the approximation …”

Impact of Spin-Entropy on the Thermoelectric Properties of a 2D Magnet

“…On the basis of thermodynamic considerations for an electronic system, this term is directly related to the average entropy transported per charge carrier , using the Maxwell equation true( ∂ μ ∂ T true) N = prefix− true( ∂ S ∂ N true) T , where N is the mean time-averaged population of the system and S is the electronic entropy. − This implies that all mechanisms that increase the entropy per carrier can enhance the Seebeck coefficient. In particular, the spin degrees of freedom of carriers in magnetic materials can lead to such increased entropy, ,− as these correspond to more phase space for the same number of electrons (especially the magnetically active d electrons). The different types of entropy will affect the electronic states (band energies and electron density) and the chemical potential, which then modifies the Seebeck coefficient, as shown in eq .…”