A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d10–d0 cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr2Cu(Te0.5W0.5)O6, where the isostructural end phases Sr2CuTeO6 and Sr2CuWO6 are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr2Cu(Te0.5W0.5)O6 is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr2Cu(Te0.5W0.5)O6—similar to several spin liquid candidates—a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr2Cu(Te0.5W0.5)O6 highlight the role of disorder in addition to magnetic frustration in spin liquid physics.

Herein, by studying a stepwise phase transformation of 23 nm FeO-Fe3O4 core-shell nanocubes into Fe3O4, we identify a composition at which the magnetic heating performance of the nanocubes is not affected by the medium viscosity and aggregation. Structural and magnetic characterizations reveal the transformation of the FeO-Fe3O4 nanocubes from having stoichiometric phase compositions into Fe 2+ deficient Fe3O4 phases. The resultant nanocubes contain tiny compressed and randomly distributed FeO sub-domains as well as structural defects. This phase transformation causes a tenfold increase in the magnetic losses of the nanocubes, which remains exceptionally insensitive to the medium viscosity as well as aggregation unlike similarly sized single-phase magnetite nanocubes. We observe that the dominant relaxation mechanism switches from Néel in fresh core-shell nanocubes to Brownian in partially oxidized nanocubes and once again to Néel in completely treated nanocubes. The Fe 2+ deficiencies and structural defects appear to reduce the magnetic energy barrier and anisotropy field, thereby driving the overall relaxation into Néel process. The magnetic losses of the particles remain unchanged through a progressive internalization/association to ovarian cancer cells. Moreover, the particles induce a significant cell death after being exposed to hyperthermia treatment. Here, we present the largest heating performance that has been reported to date for 23 nm iron oxide nanoparticles under cellular and intracellular conditions. Our findings clearly demonstrate the positive impacts of the Fe 2+ deficiencies and structural defects in the Fe3O4 structure on the heating performance under cellular and intracellular conditions.

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La1.5Sr0.5CoMnO6 stands out as presenting the largest ZEB reported so far, while for La1.5Ca0.5CoMnO6 the exchange bias field (HEB) is one order of magnitude smaller. Here we show that La1.5Ba0.5CoMnO6 also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate HEB value in respect to Ca-and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La2CoMnO6 was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca-and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co 2+ /Co 3+ and Mn 4+ /Mn 3+ are already present at the La2CoMnO6 parent compound, and that Ba 2+ /Ca 2+ /Sr 2+ partial substitution at La 3+ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the L-edge XAS indicate the presence of oxygen vacancies in all samples (0.05≤ δ ≤0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn. arXiv:1909.05287v1 [cond-mat.mtrl-sci]

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor J1 and next-nearest-neighbor J2 interactions (J2/J1 ≈ 0.5). The double perovskites Sr2CuTeO6 and Sr2CuWO6 are isostructural spin-1/2 square-lattice antiferromagnets with Néel (J1 dominates) and columnar (J2 dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr2Cu(Te1-xWx)O6 (0 ≤ x ≤ 1) tunable from Néel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the x = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of x ≈ 0.1-0.6. This coincides with significant T-linear terms in the lowtemperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated J2/J1 ≈ 0.5 region square-lattice Heisenberg model.Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in x = 0.5.

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