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1. The High-Temperature Polymorph of LiBF ₄

   https://doi.org/10.1021/acs.jpclett.3c02961  

   Sonnenberg, Laura A.; Chandra Paul, Shujit; Wunder, Stephanie L.; Zdilla, Michael J. (January 2024, The Journal of Physical Chemistry Letters)

   The single-crystal-to-single-crystal phase transition is determined using X-ray crystallography on LiBF4, resolving a longstanding ambiguity in the existence of a high-temperature polymorph of LiBF4. LiBF4 possesses an endothermic phase change at 28.2 °C with ΔH = 1180 J mol-1 and ΔS = 3.92 J mol-1K-1 based on DSC. Single-crystal X-ray diffraction shows that the low temperature phase collected at 200K is a twinned trigonal P system with a twin law indicating reflection through the 110 plane. The same crystal collected above the phase transition temperature at 313 K is a C-centered orthorhombic system describable as the superposition of the two low-temperature twin geometries undergoing interconversion. The geome-tries of the high- and low-temperature phases are consistent with the calorimetry experiments, and with previous NMR find-ings indicating BF4 geometric reorientations above 300 K. 

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2. Perpendicular and in-plane hole asymmetry in a strained NiFe ₂ O ₄ film

   https://doi.org/10.1088/1361-648X/abf0c5  

   Knut, R; Malik, R S; Kons, C; Shoup, J E; Radu, F; Luo, C; Kvashnin, Y O; Gupta, A; Karis, O; Arena, D A (May 2021, Journal of Physics: Condensed Matter)

   Abstract Strained materials can exhibit drastically modified physical properties in comparison to their fully relaxed analogues. We report on the x-ray absorption spectra (XAS) and magnetic circular dichroism (XMCD) of a strained NiFe 2 O 4 inverse spinel film grown on a symmetry matched single crystal MgGa 2 O 4 substrate. The Ni XAS spectra exhibit a sizable difference in the white line intensity for measurements with the x-ray electric field parallel to the film plane (normal incidence) vs when the electric field is at an angle (off-normal). A considerable difference is also observed in the Fe L 2,3 XMCD spectrum. Modeling of the XAS and XMCD spectra indicate that the modified energy ordering of the cation 3 d states in the strained film leads to a preferential filling of 3 d states with out-of-plane character. In addition, the results point to the utility of x-ray spectroscopy in identifying orbital populations even with elliptically polarized x-rays. 

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3. Magnetotransport Anomaly in Room‐Temperature Ferrimagnetic NiCo ₂ O ₄ Thin Films

   https://doi.org/10.1002/adma.201805260  

   Chen, Xuegang; Zhang, Xiaozhe; Han, Myung‐Geun; Zhang, Le; Zhu, Yimei; Xu, Xiaoshan; Hong, Xia (November 2018, Advanced Materials)

   Abstract The inverse spinel ferrimagnetic NiCo₂O₄presents a unique model system for studying the competing effects of crystalline fields, magnetic exchange, and various types of chemical and lattice disorder on the electronic and magnetic states. Here, magnetotransport anomalies in high‐quality epitaxial NiCo₂O₄thin films resulting from the complex energy landscape are reported. A strong out‐of‐plane magnetic anisotropy, linear magnetoresistance, and robust anomalous Hall effect above 300 K are observed in 5–30 unit cell NiCo₂O₄films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence that peaks around room temperature, and reverses its sign at low temperature in films thinner than 20 unit cells. The scaling relation between the anomalous Hall conductivity and longitudinal conductivity reveals the intricate interplay between the spin‐dependent impurity scattering, band intrinsic Berry phase effect, and electron correlation. This study provides important insights into the functional design of NiCo₂O₄for developing spinel‐based spintronic applications. 

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4. Anomalous Tunneling Magnetoresistance Oscillation and Electrically Tunable Tunneling Anisotropic Magnetoresistance in Few‐Layer CrPS ₄

   https://doi.org/10.1002/apxr.202400052  

   Fu, ZhuangEn; Huang, Hong‐Fei; Samarawickrama, Piumi; Watanabe, Kenji; Taniguchi, Takashi; Wang, Wenyong; Ackerman, John; Zang, Jiadong; Yu, Jie‐Xiang; Tian, Jifa (July 2024, Advanced Physics Research)

   Abstract 2D van der Waals (vdW) magnets with layer‐dependent magnetic states and/or diverse magnetic interactions and anisotropies have attracted extensive research interest. Despite the advances, a notable challenge persists in effectively manipulating the tunneling anisotropic magnetoresistance (TAMR) of 2D vdW magnet‐based magnetic tunnel junctions (MTJs). Here, this study reports the novel and anomalous tunneling magnetoresistance (TMR) oscillations and pioneering demonstration of bias and gate voltage controllable TAMR in 2D vdW MTJs, utilizing few‐layer CrPS₄. This material, inherently an antiferromagnet, transitions to a canted magnetic order upon application of external magnetic fields. Through TMR measurements, this work unveils the novel layer‐dependent oscillations in the tunneling resistance for few‐layer CrPS₄devices under both out‐of‐plane and in‐plane magnetic fields, with a pronounced controllability via gate voltage. Intriguingly, this study demonstrates that both the polarity and magnitude of TAMR in CrPS₄can be effectively tuned through either a bias or gate voltage. The mechanism behind this electrically tunable TAMR is further elucidated through first‐principles calculations. The implications of the findings are far‐reaching, providing new insights into 2D magnetism and opening avenues for the development of innovative spintronic devices based on 2D vdW magnets. 

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5. Single crystal neutron and magnetic measurements of Rb ₂ Mn ₃ (VO ₄ ) ₂ CO ₃ and K ₂ Co ₃ (VO ₄ ) ₂ CO ₃ with mixed honeycomb and triangular magnetic lattices

   https://doi.org/10.1039/C9DT03389K  

   Smith Pellizzeri, Tiffany M.; Sanjeewa, Liurukara D.; Pellizzeri, Steven; McMillen, Colin D.; Garlea, V. Ovidiu; Ye, Feng; Sefat, Athena S.; Kolis, Joseph W. (April 2020, Dalton Transactions)

   null (Ed.)

   Two new alkali vanadate carbonates with divalent transition metals have been synthesized as large single crystals via a high-temperature (600 °C) hydrothermal technique. Compound I , Rb 2 Mn 3 (VO 4 ) 2 CO 3 , crystallizes in the trigonal crystal system in the space group P 3̄1 c , and compound II , K 2 Co 3 (VO 4 ) 2 CO 3 , crystallizes in the hexagonal space group P 6 3 / m . Both structures contain honeycomb layers and triangular lattices made from edge-sharing MO 6 octahedra and MO 5 trigonal bipyramids, respectively. The honeycomb and triangular layers are connected along the c -axis through tetrahedral [VO 4 ] groups. The MO 5 units are connected with each other by carbonate groups in the ab -plane by forming a triangular magnetic lattice. The difference in space groups between I and II was also investigated with Density Functional Theory (DFT) calculations. Single crystal magnetic characterization of I indicates three magnetic transitions at 77 K, 2.3 K, and 1.5 K. The corresponding magnetic structures for each magnetic transition of I were determined using single crystal neutron diffraction. At 77 K the compound orders in the MnO 6 -honeycomb layer in a Néel-type antiferromagnetic orientation while the MnO 5 triangular lattice ordered below 2.3 K in a colinear ‘up–up–down’ fashion, followed by a planar ‘Y’ type magnetic structure. K 2 Co 3 (VO 4 ) 2 CO 3 ( II ) exhibits a canted antiferromagnetic ordering below T N = 8 K. The Curie–Weiss fit (200–350 K) gives a Curie–Weiss temperature of −42 K suggesting a dominant antiferromagnetic coupling in the Co 2+ magnetic sublattices. 

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