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1. Spin fluctuation driven magnetoresistance, domain Re-distribution and anomalous Hall effect in helical antiferromagnetic Eu metal thin films

   https://doi.org/10.1063/9.0000861  

   Shrestha, Narendra; Tang, Jinke (March 2025, AIP Advances)

   Europium (Eu) metal has a body centered cubic crystal structure which, upon a paramagnetic-to-helical magnetic phase transition, undergoes a body centered tetragonal distortion. The magnetic helix appears below a Néel temperature (TN) of ∼90 K, and an applied magnetic field gives rise to conical magnet structure. We have prepared Eu metal thin films on Si (001) substrates using Eu metal as a target by pulsed laser deposition and studied the transport properties by a four-probe method. The resistance shows a sudden slope change at TN of 88 K. The magnetoresistance (MR) is positive at temperatures below 30 K and exhibits negative values above that. Our analyses show that the positive MR at low temperatures originates from magnetic field induced spin fluctuation, and the negative MR at higher temperature is a result of suppression of critical spin fluctuation of the Eu spins by the magnetic field. The Eu film also shows hysteretic MR behaviors in mid field range, which is a result of re-distribution of the helical antiferromagnetic domains by the magnetic fields. We have also studied the transverse magnetotransport in the Eu thin films. The observed anomalous Hall effect is believed to be associated with the magnetic moment induced by the field or due to the helical spin structure of Eu itself. 

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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. Stoichiometry‐Induced Ferromagnetism in Altermagnetic Candidate MnTe

   https://doi.org/10.1002/adfm.202405829  

   Chilcote, Michael; Mazza, Alessandro R; Lu, Qiangsheng; Gray, Isaiah; Tian, Qi; Deng, Qinwen; Moseley, Duncan; Chen, An‐Hsi; Lapano, Jason; Gardner, Jason S; et al (June 2024, Advanced Functional Materials)

   Abstract The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (T_N ≈ 310 K) and semiconducting properties. The results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films are presented. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle‐resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first‐principles calculations for altermagnetic spin‐splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn‐richness that is intrinsic to the MBE‐grown samples. When combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications. 

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4. The Phase Diagram and Exotic Magnetostrictive Behaviors in Spinel Oxide Co(Fe1−xAlx)2O4 System

   https://doi.org/10.3390/ma12101685  

   Zhou, Chao; Zhang, Azhen; Chang, Tieyan; Chen, Yusheng; Zhang, Yin; Tian, Fanghua; Zuo, Wenliang; Ren, Yang; Song, Xiaoping; Yang, Sen (May 2019, Materials)

   We report the magnetic and magnetostrictive behaviors of the pseudobinary ferrimagnetic spinel oxide system (1−x)CoFe2O4–xCoAl2O4 [Co(Fe1−xAlx)2O4], with one end-member being the ferrimagnetic CoFe2O4 and the other end-member being CoAl2O4 that is paramagnetic above 9.8 K. The temperature spectra of magnetization and magnetic susceptibility were employed to detect the magnetic transition temperatures and to determine the phase diagram of this system. Composition dependent and temperature dependent magnetostrictive behaviors reveal an exotic phase boundary that separates two ferrimagnetic states: At room temperature and under small magnetic fields (∼500 Oe), Fe-rich compositions exhibit negative magnetostriction while the Al-rich compositions exhibit positive magnetostriction though the values are small (<10 ppm). Moreover, the compositions around this phase boundary at room temperature (x = 0.35, 0.4, 0.45, 0.5) exhibit near-zero magnetostriction and enhanced magnetic susceptibility, which may be promising in the applications for magnetic cores, current sensors, or magnetic shielding materials. 

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5. Excited-state spin-resonance spectroscopy of V$${}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$ defect centers in hexagonal boron nitride

   https://doi.org/10.1038/s41467-022-30772-z  

   Mathur, Nikhil; Mukherjee, Arunabh; Gao, Xingyu; Luo, Jialun; McCullian, Brendan A.; Li, Tongcang; Vamivakas, A. Nick; Fuchs, Gregory D. (June 2022, Nature Communications)

   Abstract The recently discovered spin-active boron vacancy (V$${}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$ ${}_B^{-}$) defect center in hexagonal boron nitride (hBN) has high contrast optically-detected magnetic resonance (ODMR) at room-temperature, with a spin-triplet ground-state that shows promise as a quantum sensor. Here we report temperature-dependent ODMR spectroscopy to probe spin within the orbital excited-state. Our experiments determine the excited-state spin Hamiltonian, including a room-temperature zero-field splitting of 2.1 GHz and a g-factor similar to that of the ground-state. We confirm that the resonance is associated with spin rotation in the excited-state using pulsed ODMR measurements, and we observe Zeeman-mediated level anti-crossings in both the orbital ground- and excited-state. Our observation of a single set of excited-state spin-triplet resonance from 10 to 300 K is suggestive of symmetry-lowering of the defect system fromD_3htoC_2v. Additionally, the excited-state ODMR has strong temperature dependence of both contrast and transverse anisotropy splitting, enabling promising avenues for quantum sensing. 

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