arXiv subjects

Hossein Sepehri-Amin

Publications and source records attributed to Hossein Sepehri-Amin.

25 records · Page 2Linked to original sources

arXiv2023

Observation of nonvolatile magneto-thermal switching in superconductors

Applying a magnetic field to a solid changes its thermal-transport properties. Although such magneto-thermal-transport phenomena are usually small effects, giant magneto-thermal resistance has recently been observed in spintronic materials1,2 and superconductors3,4, opening up new possibilities in thermal management technologies. However, the thermal conductivity conventionally changes only when a magnetic field is applied due to the absence of nonvolatility, which limits potential applications of thermal switching devices5,6. Here, we report the observation of nonvolatile thermal switching that changes the thermal conductivity when a magnetic field is applied and retains the value even when the field is turned off. This unconventional magneto-thermal switching, surprisingly, arises in commercial Sn-Pb solders and is realized by phase-separated superconducting states and resultant nonuniform magnetic flux distributions. This result confirms the versatility of the observed phenomenon and aids the development of active solid-state thermal management devices.

cond-mat.mtrl-sci↗

arXiv2022

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation

Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect (AEE). In addition to ANE/AEE, we discovered unique and superior material properties in these alloys: the amorphous structure, low thermal conductivity, and large in-plane coercivity and remanent magnetization. These properties make it advantageous over conventional materials to realize heat flux sensing applications based on ANE, as our Sm-Co-based films can generate thermoelectric output without an external magnetic field. Importantly, the amorphous nature enables the fabrication of these films on various substrates including flexible sheets, making the large-scale and low-cost manufacturing easier. Our demonstration will provide a pathway to develop flexible transverse thermoelectric devices for smart thermal management.

cond-mat.mtrl-sci↗

arXiv2019

Observation of anomalous Ettingshausen effect and large transverse thermoelectric conductivity in permanent magnets

This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo$_5$-type magnet exhibits the large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo$_5$-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that the large AEE originates from the intrinsic transverse thermoelectric conductivity of SmCo$_5$. This finding makes a connection between permanent magnets and thermal energy engineering, providing the basis for creating "thermoelectric permanent magnets."

cond-mat.mtrl-sci↗

arXiv2019

Coercivity enhancement of selective laser sintered NdFeB magnets by grain boundary infiltration

Laser powder bed fusion is a well-established additive manufacturing method that can be used for the production of net-shaped Nd-Fe-B sintered magnets. However, low coercivity has been one of the drawbacks in the laser powder bed fusion processed Nd-Fe-B magnets. In this work, we have demonstrated that the grain boundary diffusion process using low-melting Nd-Cu, Nd-Al-Ni-Cu, and Nd-Tb-Cu alloys to the selective laser sintered NdFeB magnets can results in a substantial enhancement of coercivity from 0.65 T to 1.5 T. Detailed microstructure investigations clarified formation of Nd-rich grain boundary phase, introducing Tb-rich shell at the surface of Nd2Fe14B grains, and maintaining the grain size in nano-scale are responsible for the large coercivity enhancement.

physics.app-ph↗

arXiv2017

Back hopping in spin-transfer-torque devices, possible origin and counter measures

The effect of undesirable high-frequency free-layer switching in magnetic multilayer systems, referred to as back hopping, is investigated by means of the spin-diffusion model. A possible origin of the back-hopping effect is found to be the destabilization of the pinned layer which leads to perpetual switching of both layers. The influence of different material parameters on the critical switching currents for the free and pinned layer is obtained by micromagnetic simulations. It is found that the choice of a free-layer material with low polarization $β$ and saturation magnetization $M_s$, and a pinned-layer material with high $β$ and $M_s$ leads to a low free-layer critical current and a high pinned-layer critical current and hence reduces the likelihood of back hopping. While back hopping was observed in various types of devices, there are only few experiments that exhibit this effect in perpendicularly magnetized systems. However, our simulations suggest, that this is likely to change due to loss of pinned-layer anisotropy when decreasing device sizes.

physics.comp-ph↗

arXiv2017

Significant reduction of critical currents in MRAM designs using dual free layer with perpendicular and in-plane anisotropy

One essential feature in MRAM cells is the spin torque efficiency, which describes the ratio of the critical switching current to the energy barrier. Within this paper it is reported that the spin torque efficiency can be improved by a factor of 3.2 by the use a of dual free layer device, which consists of one layer with perpendicular crystalline anisotropy and a second layer with in-plane crystalline anisotropy. Detailed simulations solving the spin transport equations simultaneously with the micromagnetics equation were performed in order to understand the origin of the switching current reduction by a factor of 4 for the dual layer structure compared to a single layer structure. The main reason could be attributed to an increased spin accumulation within the free layer due to the dynamical tilting of the magnetization within the in-plane region of the dual free layer.

physics.comp-ph↗

arXiv2016

Field- and damping-like spin-transfer torque in magnetic multilayers

We investigate the spin-transfer torque in a magnetic multilayer structure by means of a spin-diffusion model. The torque in the considered system, consisting of two magnetic layers separated by a conducting layer, is caused by a perpendicular-to-plane current. We compute the strength of the field-like and the damping-like torque for different material parameters and geometries. Our studies suggest that the field-like torque highly depends on the exchange coupling strength of the itinerant electrons with the magnetization both in the pinned and the free layer. While a low coupling leads to very high field-like torques, a high coupling leads to low or even negative field-like torques. The dependence of the different torque terms on system parameters is considered very important for the development of applications such as STT MRAM and spin-torque oscillators.

physics.comp-ph↗