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1. Quantum theory of spin alignment in a circular magnetic nanotube

   Bergmann, G (January 2015, European physical journal. B, Condensed matter and complex systems)

   When electron spin and momentum couple in a solid, one generally obtains intriguing and unexpected phenomena. Metallic ferromagnetic nanotubes of cobalt with circular magnetization, which have been prepared by us and others, are a particularly interesting system. Here the spins of the conduction electrons are frustrated. They would like to align parallel to the magnetic field of the magnetization, but as the electrons move quickly around the tube the spins cannot follow the magnetization direction. In a previous short theoretical paper we solved the spin dynamics using a classical model. Here we generalize our work to a quantum mechanical model. The surprising result is that the spin of most conduction electrons is not parallel or anti-parallel to the circumferential magnetization but mostly parallel or anti-parallel to the axis of the nanotube. This result means that such a cobalt nanotube is a different ferromagnet from a cobalt film or bulk cobalt. 

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2. Magnetic properties and spin reorientation of hexagonal HoFeO3 thin films

   https://doi.org/https://doi.org/10.1016/j.tsf.2021.138596  

   R.C. Rai, C. Horvatits (April 2021, Thin solid films)

   null (Ed.)

   We present structural, magnetic, and optical properties of hexagonal HoFeO3/Al2O3 thin films deposited by Magnetron Sputtering. The x-ray diffraction patterns of HoFeO3 thin films show the c-planes of a hexagonal structure. The magnetization data display an antiferromagnetic transition temperature, TN∼120 ± 5 K and the magnetization-field hysteresis loops were measured below 100 K, confirming a weak ferromagnetism arising from a spin canting of the Fe3+ moments. The magnetization data also show an anomaly around ∼40 K due to a spin-reorientation transition caused by the Ho3+- Fe3+ interactions. We observed comparable magnetization along the ab plane and c axis although the spin canting of Fe3+ sites has a preferential component along the c axis, suggesting that the Ho3+- Fe3+ interactions dominate in the low temperature magnetic structures of hexagonal-HoFeO3. The observed electronic excitations at ∼2.29, 2.87, 3.82, 4.79, and 6.53 eV have been assigned to the Fe3+ d to d on-site as well as O 2p to Fe 3d, Ho 6s, and 5d charge-transfer excitations, respectively. The room temperature energy band gap of the hexagonal-HoFeO3 thin film was measured to be ∼1.99 ± 0.04 eV. 

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3. Temperature‐Dependent Surface Anisotropy in (110) Epitaxial Rare Earth Iron Garnet Films

   https://doi.org/10.1002/smll.202407381  

   Song, Yixuan; Lasinger, Katharina; Tang, Hao; Li, Ju; Beach, Geoffrey_S_D; Ross, Caroline_A (October 2024, Small)

   Abstract Ferrimagnetic oxide thin films are important material platforms for spintronic devices. Films grown on low symmetry orientations such as (110) exhibit complex anisotropy landscapes that can provide insight into novel phenomena such as spin‐torque auto‐oscillation and spin superfluidity. Using spin‐Hall magnetoresistance measurements, the in‐plane (IP) and out‐of‐plane (OOP) uniaxial anisotropy energies are determined for a thickness series (5–50 nm) of europium iron garnet (EuIG) and thulium iron garnet (TmIG) films epitaxially grown on a gadolinium gallium substrate with (110) orientation and capped with Pt. Pt/EuIG/GGG exhibits an (001) easy plane of magnetization perpendicular to the substrate, whereas Pt/TmIG/GGG exhibits an (001) hard plane of magnetization perpendicular to the substrate with an IP easy axis. Both IP and OOP surface anisotropy energies comparable in magnitude to the bulk anisotropy are observed. The temperature dependence of the surface anisotropies is consistent with first‐order predictions of a simplified Néel surface anisotropy model. By taking advantage of the thickness and temperature dependence demonstrated in these ferrimagnetic oxides grown on the low symmetry (110) orientations, the complex anisotropy landscapes can be tuned to act as a platform to explore rich spin textures and dynamics. 

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4. Spherically symmetric Earth models yield no net electron spin

   https://doi.org/10.1103/PhysRevD.111.015015  

   Clayburn, N B; Glassford, A; Leiker, A; Uelmen, T; Lin, J F; Hunter, L R (January 2025, Physical Review D)

   Terrestrial experiments that use electrons in Earth as a spin-polarized source have been demonstrated to provide strong bounds on exotic long-range spin-spin and spin-velocity interactions. These bounds constrain the coupling strength of many proposed ultralight bosonic dark-matter candidates. Recently, it was pointed out that a monopole-dipole coupling between the Sun and the spin-polarized electrons of Earth would result in a modification of the precession of the perihelion of Earth. Using an estimate for the net spin polarization of Earth and experimental bounds on Earth’s perihelion precession, interesting constraints were placed on the magnitude of this monopole-dipole coupling. Here we investigate the spin associated with Earth’s electrons. We find that there are about $6\times {10}^{41}$spin-polarized electrons in the mantle and crust of Earth oriented antiparallel to their local magnetic field. However, when integrated over any spherically symmetric Earth model, we find that the vector sum of these spins is zero. In order to establish a lower bound on the magnitude of the net spin along Earth’s rotation axis we have investigated three of the largest breakdowns of Earth’s spherical symmetry: the large low shear-velocity provinces of the mantle, the crustal composition, and the oblate spheroid of Earth. From these investigations we conclude that there are at least $5\times {10}^{38}$spin-polarized electrons aligned antiparallel to Earth’s rotation axis. This analysis suggests that the bounds on the monopole-dipole coupling that were extracted from Earth’s perihelion precession need to be relaxed by a factor of about 2000. Published by the American Physical Society2025 

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5. Spin-torque devices with hard axis initialization as Stochastic Binary Neurons

   https://doi.org/10.1038/s41598-018-34996-2  

   Ostwal, Vaibhav; Debashis, Punyashloka; Faria, Rafatul; Chen, Zhihong; Appenzeller, Joerg (November 2018, Scientific Reports)

   Abstract Employing the probabilistic nature of unstable nano-magnet switching has recently emerged as a path towards unconventional computational systems such as neuromorphic or Bayesian networks. In this letter, we demonstrate proof-of-concept stochastic binary operation using hard axis initialization of nano-magnets and control of their output state probability (activation function) by means of input currents. Our method provides a natural path towards addition of weighted inputs from various sources, mimicking the integration function of neurons. In our experiment, spin orbit torque (SOT) is employed to “drive” nano-magnets with perpendicular magnetic anisotropy (PMA) -to their metastable state, i.e. in-plane hard axis. Next, the probability of relaxing into one magnetization state (+m_i) or the other (−m_i) is controlled using an Oersted field generated by an electrically isolated current loop, which acts as a “charge” input to the device. The final state of the magnet is read out by the anomalous Hall effect (AHE), demonstrating that the magnetization can be probabilistically manipulated and output through charge currents, closing the loop from charge-to-spin and spin-to-charge conversion. Based on these building blocks, a two-node directed network is successfully demonstrated where the status of the second node is determined by the probabilistic output of the previous node and a weighted connection between them. We have also studied the effects of various magnetic properties, such as magnet size and anisotropic field on the stochastic operation of individual devices through Monte Carlo simulations of Landau Lifshitz Gilbert (LLG) equation. The three-terminal stochastic devices demonstrated here are a critical step towards building energy efficient spin based neural networks and show the potential for a new application space. 

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