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1. In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2

   https://doi.org/10.1038/s41467-020-19421-5  

   Liu, Panpan; Klemm, Mason L.; Tian, Long; Lu, Xingye; Song, Yu; Tam, David W.; Schmalzl, Karin; Park, J. T.; Li, Yu; Tan, Guotai; et al (December 2020, Nature Communications)

   null (Ed.)

   Abstract A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe 2 As 2 , which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature T s . Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature T N ( <  T s ) and create in-plane resistivity anisotropy above T s . Here we use neutron polarization analysis to show that such a strain on BaFe 2 As 2 also induces a static or quasi-static out-of-plane ( c -axis) AF order and its associated critical spin fluctuations near T N / T s . Therefore, uniaxial pressure necessary to detwin single crystals of BaFe 2 As 2 actually rotates the easy axis of the collinear AF order near T N / T s , and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system. 

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2. Tetrairon( ii ) extended metal atom chains as single-molecule magnets

   https://doi.org/10.1039/d1dt01007g  

   Nicolini, Alessio; Affronte, Marco; SantaLucia, Daniel J.; Borsari, Marco; Cahier, Benjamin; Caleffi, Matteo; Ranieri, Antonio; Berry, John F.; Cornia, Andrea (June 2021, Dalton Transactions)

   null (Ed.)

   Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon( ii ), halide-capped complexes [Fe 4 (tpda) 3 Cl 2 ] ( 1Cl ) and [Fe 4 (tpda) 3 Br 2 ] ( 1Br ), obtained by reacting iron( ii ) dihalides with [Fe 2 (Mes) 4 ] and N 2 , N 6 -di(pyridin-2-yl)pyridine-2,6-diamine (H 2 tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χ M T at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron( ii ) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy ( D = −11 to −19 cm −1 , | E / D | = 0.05–0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy ( D = 8–10 cm −1 , | E / D | = 0.06–0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1–Fe2 and Fe3–Fe4 interactions and antiferromagnetic Fe2–Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field. 

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3. Field-free deterministic switching of all–van der Waals spin-orbit torque system above room temperature

   https://doi.org/10.1126/sciadv.adk8669  

   Kajale, Shivam N; Nguyen, Thanh; Hung, Nguyen Tuan; Li, Mingda; Sarkar, Deblina (March 2024, Science Advances)

   Two-dimensional van der Waals (vdW) magnetic materials hold promise for the development of high-density, energy-efficient spintronic devices for memory and computation. Recent breakthroughs in material discoveries and spin-orbit torque control of vdW ferromagnets have opened a path for integration of vdW magnets in commercial spintronic devices. However, a solution for field-free electric control of perpendicular magnetic anisotropy (PMA) vdW magnets at room temperatures, essential for building compact and thermally stable spintronic devices, is still missing. Here, we report a solution for the field-free, deterministic, and nonvolatile switching of a PMA vdW ferromagnet, Fe₃GaTe₂, above room temperature (up to 320 K). We use the unconventional out-of-plane anti-damping torque from an adjacent WTe₂layer to enable such switching with a low current density of 2.23 × 10⁶A cm⁻². This study exemplifies the efficacy of low-symmetry vdW materials for spin-orbit torque control of vdW ferromagnets and provides an all-vdW solution for the next generation of scalable and energy-efficient spintronic devices. 

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4. Symmetry-controlled SrRuO ₃ /SrTiO ₃ /SrRuO ₃ magnetic tunnel junctions: spin polarization and its relevance to tunneling magnetoresistance

   https://doi.org/10.1088/1361-648X/ad765f  

   Samanta, Kartik; Tsymbal, Evgeny Y (September 2024, Journal of Physics: Condensed Matter)

   Abstract Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO₃/SrTiO₃/SrRuO₃(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO₃(001) electrodes and evanescent states in the SrTiO₃(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO₃/SrTiO₃/SrRuO₃(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs. 

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5. Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

   https://doi.org/10.1038/s41535-024-00651-5  

   Chen, Lebing; Teng, Xiaokun; Hu, Ding; Ye, Feng; Granroth, Garrett E; Yi, Ming; Chung, Jae-Ho; Birgeneau, Robert J; Dai, Pengcheng (May 2024, npj Quantum Materials)

   Abstract We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe₃. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe₃is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming acrossT_N, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations aboveT_N. These results suggest that the first-order AF phase transition in FePSe₃may arise from the competition betweenC₃symmetric AF andC₁symmetric FM spin fluctuations aroundT_N, in place of a conventional second-order AF phase transition. 

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