Engineering of half metallic band structure optimizes thermoelectricity

In recent years, interest in thermoelectric materials has grown due to their potential to convert waste heat into energy. However, traditional materials suffer from low efficiency and high cost, prompting exploration of alternatives like half-metal ferromagnets (HMFs), which exhibit both metallic and insulating properties based on electron spin orientation. In 2015, it was proposed that HMFs could show significant thermoelectric performance in anti-parallel spin valve configurations. Building on this, we used density functional theory (DFT) to simulate HMF systems like Co2MnSi and Fe3O4 and analyze parameters such as the Seebeck coefficient, thermal conductivity, and SVEF. Our findings reveal that opening the band gap in HMFs enhances thermoelectric performance by increasing the Seebeck coefficient and reducing thermal conductivity. This research highlights how manipulating band structures can improve thermoelectric efficiency in HMFs for practical applications.

F.E.M. Rahangiar, A.B. Cahaya, M.S. Muntini, I. Anshori, and E.H. Hasdeo, “Optimal half-metal band structure for large thermoelectric performance”, Phys. Rev. B 110, 035150 (2024)

Dipolar and antisymmetric exchange interactions stabilizes  hybrid magnetic skyrmion

Recently, a type of hybrid skyrmion with intermediate helicity between Bloch and Néel skyrmion, has gained more attraction. It has improved mobility and reduced the skyrmion-Hall effect, which prevents the skyrmions from moving parallel to the current flow. We show that the hybrid skyrmion can stabilize through the interplay of dipolar interaction and two types of antisymmetric Dzyaloshinskii–Moriya interactions (DMI): interfacial DMI and bulk DMI. Since the hybrid skyrmion can be considered as a superposition of the Néel and Bloch skyrmion, it is also a potential candidate as building blocks for quantum bits (qubits) in quantum computers, where information can be stored by utilizing the helicity degree of freedom.

M.P.M. Akhir, E. Suprayoga, A.B.Cahaya, “Stabilization and Helicity Control of Hybrid Magnetic Skyrmion” J. Phys. D Appl. Phys. 57, 165303 (2024)

Hyperfine interactions enable spin-orbit torque on nuclear spin

Hyperfine interactions describe the magnetic interaction between spins of atomic nucleus and electron. The interaction, also referred as hyperfine coupling, consists of Fermi contact and dipolar interactions between dipole magnetic moments of atomic nucleus and electron. We demonstrate that the hyperfine coupling can mediate the application of spin-orbit torques acting on nuclear spins. In quantum computing, nuclear spin is a candidate for qubit. Since qubit has the potential to the miniaturization of memory, the application of nuclear spin by means of nuclear spin orbit torque could lead to further miniaturization of spintronics devices.

A.B. Cahaya, A.O. Leon, and M.H. Fauzi, “Spin-orbit torque on nuclear spins exerted by a spin accumulation via hyperfine interactions”, Nanotechnology 34, 505001 (2023)

Spin-orbit couplings at magnetic interface generate magnetoelectric effect and enable exchange bias

Exchange bias is a unidirectional magnetic anisotropy that often arise from interfacial interaction of a ferromagnetic and antiferromagnetic layers. we show that spin-orbit couplings in magnetic heterostructures, such as La2/3Sr1/3MnO3|LaAlO3|SrTiO3 can induces an exchange bias via an interface magnetoelectric effect. The interface magnetoelectric effect is induced by spin-orbit couplings that arises from the broken symmetry of the system. We demonstrate that the exchange bias can be controlled by electric field.

A.B. Cahaya, A.A. Anderson, A. Azhar, and M.A. Majidi,”Electrically controllable exchange bias via interface magnetoelectric effect”, IEEE Trans. Magn. 59 (11) 1300304 (2023)

Twisted spin density generates anisotropic magnetic interactions.

The interactions of spins -most miniature magnets- can be isotropic or anisotropic, the latter resulting in twisted spin orientations. This article finds a simple explanation of the anisotropic interaction named Dzyaloshiskii-Moriya in metals. The model is based on two atomic spins; one of them is heavy, such as a lanthanide. As a result, a spin orientation emerges in the electrons of the metal and is perpendicular to the atomic spins. We name this as Dzyaloshisnkii-Moriya Spin-Density (DM-SD). When additional magnetic atoms are considered, the DM-SD mediates their interaction, similar to the magnetic field of large magnets’ poles, or the isotropic polarization in metals connecting far spins. The figure shows three spins (one of them, Jf, a lanthanide) and the DM-SD (colormap and small arrows) mediating their exchange.

A.B. Cahaya and A.O.Leon, “Dzyaloshinskii-Moriya spin density by skew scattering”, Phys. Rev. B 106, L100408 (2022)

In chemistry, the mixing of atomic orbitals form orbital hybridization. The orbital hybridization helps to explain molecule shape. In a carbon atom, for example, s and p orbitals form s-p orbital hybridization. In the band theory of solid state, orbital hybridization can also occur. For example, in the Anderson impurity model, an impurity in a sea of conduction electrons is modeled as a localized orbital and an itinerant orbital. The orbital hybridization enables the interaction between the bands. In this article, we study a spin current generation phenomenon at the interface of a magnetic material and a heavy metal. The heavy metal can be described using a generalized Anderson model with s-d orbital hybridization. By comparing the data of various heavy metals, we show that the s-d orbital hybridization enhances the spin current generation.

A.B. Cahaya, R.M. Sitorus, A. Azhar, A.R.T. Nugraha, and M.A. Majidi, “Enhancement of spin-mixing conductance by s-d orbital hybridization in heavy metals”, Phys. Rev. B 105 211438 (2022)

Spin current generation by orbital moment

Due to its low intrinsic damping, rare earth iron garnet is often as spin current generator. It is actually a ferrimagnetic with antiferromagnetically coupled magnetic lattices. Here, rare earth iron garnet is described with two exchange-coupled magnetic sub-lattices. The angular momentum of rare earth reduces spin mixing conductance and magnetization. The orbital angular momentum of rare earth increases gyromagnetic ratio. Spin pumping is proportional to the difference of orbital and spin angular momentum.

A.B. Cahaya, “Enhancement of thermal spin pumping by orbital angular momentum of rare earth iron garnet”, J. Magn. Magn. Mater. 553, 169248 (2022)