High pressure neutron scattering of the magnetoelastic Ni-Cr Prussian blue analogue

This paper summarizes 0 GPa to 0.6 GPa neutron diffraction measurements of a nickel hexacyanochromate coordination polymer (NiCrPB) that has the face-centered cubic, Prussian blue structure. Deuterated powders of NiCrPB contain ~100 nm sided cubic particles. The application of a large magnetic field shows the ambient pressure, saturated magnetic structure. Pressures of less than 1 GPa have previously been shown to decrease the magnetic susceptibility by as much as half, and we find modifications to the nuclear crystal structure at these pressures that we quantify. Bridging cyanide molecules isomerize their coordination direction under pressure to change the local ligand field and introduce inhomogeneities in the local (magnetic) anisotropy that act as pinning sites for magnetic domains, thereby reducing the low field magnetic susceptibility

Local-Ising type magnetic order and metamagnetism in the rare-earth pyrogermanate Er2_2Ge2_2O7_7

The recent discoveries of proximate quantum spin-liquid compounds and their potential application in quantum computing informs the search for new candidate materials for quantum spin-ice and spin-liquid physics. While the majority of such work has centered on members of the pyrochlore family due to their inherently frustrated linked tetrahedral structure, the rare-earth pyrogermanates also show promise for possible frustrated magnetic behavior. With the familiar stoichiometry $RE_2$Ge$_2$O$_7$, these compounds generally have tetragonal symmetry with a rare-earth sublattice built of a spiral of alternating edge and corner sharing rare-earth site triangles. Studies on Dy$_2$Ge$_2$O$_7$ and Ho$_2$Ge$_2$O$_7$ have shown tunable low temperature antiferromagnetic order, a high frustration index and spin-ice like dynamics. Here we use neutron diffraction to study magnetic order in Er$_2$Ge$_2$O$_7$ (space group $P4_{1}2_{1}2$ ) and find the lowest yet Ne\'el temperature in the pyrogermanates of 1.15 K. Using neutron powder diffraction we find the magnetic structure to order with $k = (0,0,0)$ ordering vector, magnetic space group symmetry $P4_{1}^{'}2_{1}2^{'}$ and a refined Er moment of $m = 8.1 \mu_B$ - near the expected value for the Er$^{3+}$ free ion. Provocatively, the magnetic structure exhibits similar 'local-Ising' behavior to that seen in the pyrocholres where the Er moment points up or down along the short Er-Er bond. Upon applying a magnetic field we find a first order metamagnetic transition at $\sim$ 0.35 T to a lower symmetry $P2_{1}^{'}2_{1}^{'}2$ structure. This magnetic transition involves an inversion of Er moments aligned antiparallel to the applied field describing a class I spin-flip type transition, indicating a strong local anisotropy at the Er site - reminiscent of that seen in the spin-ice pyrochlores.Comment: 11 pages, 8 fig

Magnetic neutron diffraction study of Ba(Fe1-xCox)2As2 critical exponents through the tricritical doping

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for $x$ = 0.039, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the ($x$,$T$) plane, $x_{tr} \approx$0.022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long range antiferromagnetic order that may be described near the transition by simple power laws, with $\beta$ =~0.306$\pm$0.060 for $x$ =~0.039, $\beta$ =~0.208$\pm$0.005 for $x$ =~0.022, and $\beta$ =~0.198$\pm$0.009 for $x$ =~0.021. For the $x$ =~0.039 sample, the data are reasonably well described by the order parameter exponent $\beta$ =~0.326 expected for a 3D Ising model while the $x$ =~0.022 and $x$ =~0.021 samples are near the $\beta$ =~0.25 value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions

Magnetic Neutron Scattering of Thermally Quenched K-Co-Fe Prussian Blue Analogue Photomagnet

Magnetic order in the thermally quenched photomagnetic Prussian blue analogue coordination polymer K0.27Co[Fe(CN)6]0.73[D2O6]0.27 1.42D2O has been studied down to 4 K with unpolarized and polarized neutron powder diffraction as a function of applied magnetic field. Analysis of the data allows the onsite coherent magnetization of the Co and Fe spins to be established. Specifically, magnetic fields of 1 T and 4 T induce moments parallel to the applied field, and the sample behaves as a ferromagnet with a wandering axis

Interplay of frustration and magnetic field in the two-dimensional quantum antiferromagnet Cu(tn)Cl2_2

Specific heat and ac magnetic susceptibility measurements, spanning low temperatures ($T \geq 40$ mK) and high magnetic fields ($B \leq 14$ T), have been performed on a two-dimensional (2D) antiferromagnet Cu(tn)Cl$_{2}$ (tn = C$_{3}$H$_{10}$N$_{2}$). The compound represents an $S = 1/2$ spatially anisotropic triangular magnet realized by a square lattice with nearest-neighbor ($J/k_{B} = 3$ K), frustrating next-nearest-neighbor ($0 < J^{\prime}/J < 0.6$), and interlayer ($|J^{\prime \prime}/J| \approx 10^{-3}$) interactions. The absence of long-range magnetic order down to $T =$ 60 mK in $B = 0$ and the $T^{2}$ behavior of the specific heat for $T \leq 0.4$ K and $B \geq 0$ are considered evidence of high degree of 2D magnetic order. In fields lower than the saturation field, $B_{\text{sat}} = 6.6$ T, a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for the ideal square lattice, except that $B_{\text{sat}}$ is shifted to values lower than expected. Potential explanations for this observation, as well as the possibility of a Berezinski-Kosterlitz-Thouless (BKT) phase transition in a spatially anisotropic triangular magnet with the N\'{e}el ground state, are discussed.Comment: 8 pages, 9 figure

Spin stripe order in a square planar trilayer nickelate

Trilayer nickelates, which exhibit a high degree of orbital polarization combined with an electron count (d8.67) corresponding to overdoped cuprates, have been identified as a promising candidate platform for achieving high-Tc superconductivity. One such material, La4Ni3O8, undergoes a semiconductor-insulator transition at ~105 K, which was recently shown to arise from the formation of charge stripes. However, an outstanding issue has been the origin of an anomaly in the magnetic susceptibility at the transition and whether it signifies formation of spin stripes akin to single layer nickelates. Here we report single crystal neutron diffraction measurements (both polarized and unpolarized) that establish that the ground state is indeed magnetic. The ordering is modeled as antiferromagnetic spin stripes that are commensurate with the charge stripes, the magnetic ordering occurring in individual trilayers that are essentially uncorrelated along the crystallographic c-axis. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related quasi-2D oxides including manganites, cobaltates, and cuprates, these orders uniquely appear simultaneously, thus demonstrating a stronger coupling between spin and charge than in these related low-dimensional correlated oxides.Comment: 34 pages, 10 figures (including SI). Accepted by Physical Review Letter

Magnetic field induced quantum phase transition of the S=1/2S = 1/2 antiferromagnet K2NaCrO8K_2NaCrO_8

The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and inelastic neutron scattering data have been acquired over a wide range of temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic interactions are quasi-two-dimensional prior to long-range ordering, where $T_N = 1.66$ K in $H = 0$. In the $T \to 0$ limit, the magnetic field tuned antiferromagnetic-ferromagnetic phase transition suggests a critical field $H_c = 7.270$ T and a critical exponent $\alpha = 0.481 \pm 0.004$. The neutron data indicate the magnetic interactions may extend over intra-planar nearest-neighbors and inter-planar next-nearest-neighbor spins.Comment: 6 pages, with 7 figures and 1 table include

Localized Singlets and Ferromagnetic Fluctuations in the Dilute Magnetic Topological Insulator Sn0.95_{0.95}Mn0.05_{0.05}Te

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, Sn$_{0.95}$Mn$_{0.05}$Te, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our Sn$_{0.95}$Mn$_{0.05}$Te sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations ($<$~70 $\mu$eV), indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with $J_{\rm AF}=460$~$\mu$eV\@. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers

Excitations in the field-induced quantum spin liquid state of alpha-RuCl3

The Kitaev model on a honeycomb lattice predicts a paradigmatic quantum spin liquid (QSL) exhibiting Majorana Fermion excitations. The insight that Kitaev physics might be realized in practice has stimulated investigations of candidate materials, recently including alpha-RuCl3. In all the systems studied to date, non-Kitaev interactions induce magnetic order at low temperature. However, in-plane magnetic fields of roughly 8 Tesla suppress the long-range magnetic order in alpha-RuCl3 raising the intriguing possibility of a field-induced QSL exhibiting non-Abelian quasiparticle excitations. Here we present inelastic neutron scattering in alpha-RuCl3 in an applied magnetic field. At a field of 8 Tesla, the spin waves characteristic of the ordered state vanish throughout the Brillouin zone. The remaining single dominant feature of the response is a broad continuum centered at the Gamma point, previously identified as a signature of fractionalized excitations. This provides compelling evidence that a field-induced QSL state has been achieved.Comment: 32 pages, 7 figure

Competing magnetic interactions in the antiferromagnetic topological insulator MnBi2_{2}Te4_{4}

The antiferromagnetic (AF) compound MnBi$_{2}$Te$_{4}$ is suggested to be the first realization of an antiferromagnetic (AF) topological insulator. Here we report on inelastic neutron scattering studies of the magnetic interactions in MnBi$_{2}$Te$_{4}$ that possess ferromagnetic (FM) triangular layers with AF interlayer coupling. The spin waves display a large spin gap and pairwise exchange interactions within the triangular layer are frustrated due to large next-nearest neighbor AF exchange. The degree of frustration suggests proximity to a variety of magnetic phases, potentially including skyrmion phases, that could be accessed in chemically tuned compounds or upon the application of symmetry-breaking fields.Comment: 5 pages, 4 figure