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1. Experimental realization of linearly polarized x-ray detected ferromagnetic resonance

   https://doi.org/10.1088/1367-2630/ac465f  

   Klewe, C. ; Emori, S. ; Li, Q. ; Yang, M. ; Gray, B. A. ; Jeon, H-M ; Howe, B. M. ; Suzuki, Y. ; Qiu, Z. Q. ; Shafer, P. ; et al ( January 2022 , New Journal of Physics)

   We present the first theoretical and experimental evidence of time-resolved dynamic x-ray magnetic linear dichroism (XMLD) measurements of GHz magnetic precessions driven by ferromagnetic resonance in both metallic and insulating thin films. Our findings show a dynamic XMLD in both ferromagnetic Ni₈₀Fe₂₀and ferrimagnetic Ni_0.65Zn_0.35Al_0.8Fe_1.2O₄for different measurement geometries and linear polarizations. A detailed analysis of the observed signals reveals the importance of separating different harmonic components in the dynamic signal in order to identify the XMLD response without the influence of competing contributions. In particular, RF magnetic resonance elicits a large dynamic XMLD response at the fundamental frequency under experimental geometries with oblique x-ray polarization. The geometric range and experimental sensitivity can be improved by isolating the 2ωFourier component of the dynamic response. These results illustrate the potential of dynamic XMLD and represent a milestone accomplishment toward the study of GHz spin dynamics in systems beyond ferromagnetic order.
2. Quaternary cerium( iv ) containing fluorides exhibiting Ce ₃ F ₁₆ sheets and Ce ₆ F ₃₀ frameworks

   https://doi.org/10.1039/d0dt00616e  

   Ayer, Gyanendra B. ; Klepov, Vladislav V. ; Pace, Kristen A. ; zur Loye, Hans-Conrad ( May 2020 , Dalton Transactions)

   A series of new Ce( iv ) based fluorides with two different compositions, Cs 2 MCe 3 F 16 (M = Ni 2+ , Co 2+ , Mn 2+ , and Zn 2+ ) and Na 3 MCe 6 F 30 (M = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) were synthesized as high quality single crystals via a mild hydrothermal route. The compounds with the composition Cs 2 MCe 3 F 16 (M = Ni 2+ , Co 2+ , Mn 2+ , and Zn 2+ ) crystallize in the hexagonal crystal system with space group P 6 3 / mmc and are isotypic with the uranium analogs, whereas the Na 3 MCe 6 F 30 (M = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) compounds crystallize in the trigonal space group P 3̄ c 1 and are isotypic with the uranium and thorium analogs Na x MM′ 6 F 30 (M′ = Th, U). The Cs 2 MCe 3 F 16 compounds exhibit a complex 3D crystal structure constructed of edge-sharing cerium trimers, in which all three Ce atoms share a common μ 3 -F unit.more »The Na 3 MCe 6 F 30 compounds are constructed of edge- and vertex-sharing cerium polyhedra connected to each other to form Ce 6 F 30 6− framework, which can accommodate only relatively smaller trivalent cations (M 3+ = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) as compared to uranium and thorium analogs. Magnetic susceptibility measurements were carried out on the samples of Cs 2 MCe 3 F 16 (M = Ni 2+ and Co 2+ ), which exhibit paramagnetic behavior.« less
3. Local cation order and ferrimagnetism in compositionally complex spinel ferrites

   https://doi.org/10.1063/5.0123728  

   Wang, Xin ; Musicó, Brianna L. ; Kons, Corisa ; Metz, Peter C. ; Keppens, Veerle ; Gilbert, Dustin A. ; Zhang, Yuanpeng ; Page, Katharine ( December 2022 , APL Materials)

   We present an exploration of a family of compositionally complex cubic spinel ferrites featuring combinations of Mg, Fe, Co, Ni, Cu, Mn, and Zn cations, systematically investigating the average and local atomic structures, chemical short-range order, magnetic spin configurations, and magnetic properties. All compositions result in ferrimagnetic average structures with extremely similar local bonding environments; however, the samples display varying degrees of cation inversion and, therefore, differing apparent bulk magnetization. Additionally, first-order reversal curve analysis of the magnetic reversal behavior indicates varying degrees of magnetic ordering and interactions, including potentially local frustration. Finally, reverse Monte Carlo modeling of the spin orientation demonstrates a relationship between the degree of cation inversion and the spin collinearity. Collectively, these observations correlate with differences in synthesis procedures. This work provides a framework for understanding magnetic behavior reported for “high-entropy spinels,” revealing many are likely compositionally complex oxides with differing degrees of chemical short-range order—not meeting the community established criteria for high or medium entropy compounds. Moreover, this work highlights the importance of reporting complete sample processing histories and investigating local to long-range atomic arrangements when evaluating potential entropic mixing effects and assumed property correlations in high entropy materials.
4. Reversal of the magnetoelectric effect at a ferromagnetic metal/ferroelectric interface induced by metal oxidation

   https://doi.org/10.1038/s41524-021-00679-2  

   Chen, Zhaojin ; Yang, Qiong ; Tao, Lingling ; Tsymbal, Evgeny Y. ( December 2021 , npj Computational Materials)

   Multiferroic materials composed of ferromagnetic and ferroelectric components are interesting for technological applications due to sizable magnetoelectric coupling allowing the control of magnetic properties by electric fields. Due to being compatible with the silicon-based technology, HfO₂-based ferroelectrics could serve as a promising component in the composite multiferroics. Recently, a strong charge-mediated magnetoelectric coupling has been predicted for a Ni/HfO₂multiferroic heterostructure. Here, using density functional theory calculations, we systematically study the effects of the interfacial oxygen stoichiometry relevant to experiments on the magnetoelectric effect at the Ni/HfO₂interface. We demonstrate that the magnetoelectric effect is very sensitive to the interface stoichiometry and is reversed if an oxidized Ni monolayer is formed at the interface. The reversal of the magnetoelectric effect is driven by a strong Ni−O bonding producing exchange-split polarization-sensitive antibonding states at the Fermi energy. We argue that the predicted reversal of the magnetoelectric effect is typical for other 3dferromagnetic metals, such as Co and Fe, where the metal-oxide antibonding states have an opposite spin polarization compared to that in the pristine ferromagnetic metals. Our results provide an important insight into the mechanism of the interfacial magnetoelectric coupling, which is essential for the physics and application of multiferroic heterostructures.
5. Emergent long-range magnetic order in ultrathin (111)-oriented LaNiO3 films

   https://doi.org/10.1038/s41535-021-00345-2  

   Kane, Margaret M. ; Vailionis, Arturas ; Riddiford, Lauren J. ; Mehta, Apurva ; N’Diaye, Alpha T. ; Klewe, Christoph ; Shafer, Padraic ; Arenholz, Elke ; Suzuki, Yuri ( May 2021 , npj Quantum Materials)

   The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO₃(LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO₆octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition att = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni²⁺ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni²⁺–Ni³⁺ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.