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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.
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)

   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.
3. Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

   https://doi.org/10.1038/s41598-019-44602-8  

   Shivaram, B. S. ; Holleis, Ludwig ; Ulrich, V. W. ; Singleton, John ; Jaime, Marcelo ( June 2019 , Scientific Reports)

   Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt₃with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which forθ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8–30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T forθ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs atθ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt₃formore »all magnetic field values.

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4. 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)

   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 nodemore »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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5. Wind-fed GRMHD simulations of Sagittarius A*: tilt and alignment of jets and accretion discs, electron thermodynamics, and multiscale modelling of the rotation measure

   https://doi.org/10.1093/mnras/stad837  

   Ressler, S. M. ; White, C. J. ; Quataert, E. ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   Wind-fed models offer a unique way to form predictive models of the accretion flow surrounding Sagittarius A*. We present 3D wind-fed magnetohydrodynamic (MHD) and general relativistic magnetohydrodynamic (GRMHD) simulations spanning the entire dynamic range of accretion from parsec scales to the event horizon. We expand on previous work by including non-zero black hole spin and dynamically evolved electron thermodynamics. Initial conditions for these simulations are generated from simulations of the observed Wolf–Rayet stellar winds in the Galactic Centre. The resulting flow tends to be highly magnetized (β ≈ 2) with an ∼r−1 density profile independent of the strength of magnetic fields in the winds. Our simulations reach the magnetically arrested disc (MAD) state for some, but not all cases. In tilted flows, standard and normal evolution (SANE) jets tend to align with the angular momentum of the gas at large scales, even if that direction is perpendicular to the black hole spin axis. Conversely, MAD jets tend to align with the black hole spin axis. The gas angular momentum shows similar behaviour: SANE flows tend to only partially align while MAD flows tend to fully align. With a limited number of dynamical free parameters, our models can produce accretion rates,more »230 GHz flux, and unresolved linear polarization fractions roughly consistent with observations for several choices of electron heating fraction. Absent another source of large-scale magnetic field, winds with a higher degree of magnetization (e.g. where the magnetic pressure is 1/100 of the ram pressure in the winds) may be required to get a sufficiently large rotation measure with consistent sign.

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