Characterization of a Disordered above Room Temperature Skyrmion Material Co8Zn8Mn4.

Topologically nontrivial spin textures host great promise for future spintronic applications. Skyrmions in particular are of burgeoning interest owing to their nanometric size, topological protection, and high mobility via ultra-low current densities. It has been previously reported through magnetic susceptibility, microscopy, and scattering techniques that Co8Zn8Mn4 forms an above room temperature triangular skyrmion lattice. Here, we report the synthesis procedure and characterization of a polycrystalline Co8Zn8Mn4 disordered bulk sample. We employ powder X-ray diffraction and backscatter Laue diffraction as characterization tools of the crystallinity of the samples, while magnetic susceptibility and Small Angle Neutron Scattering (SANS) measurements are performed to study the skyrmion phase. Magnetic susceptibility measurements show a dip anomaly in the magnetization curves, which persists over a range of approximately 305 K-315 K. SANS measurements reveal a rotationally disordered polydomain skyrmion lattice. Applying a symmetry-breaking magnetic field sequence, we were able to orient and order the previously jammed state to yield the prototypical hexagonal diffraction patterns with secondary diffraction rings. This emergence of the skyrmion order serves as a unique demonstration of the fundamental interplay of structural disorder and anisotropy in stabilizing the thermal equilibrium phase.

Magnetism in Mixed Valence, Defect, Cubic Perovskites: BaIn1-x Fe x O2.5+δ, x = 0.25, 0.50, and 0.75. Local and Average Structures.

The series BaIn1-x Fe x O2.5+δ, x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba2In2O5 (BaInO2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba2Fe2O5 (BaFeO2.5) crystallizes in a complex monoclinic structure, P21/c, showing seven Fe3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm3̅m, with long-range disorder on the small cation and oxygen sites. 57Fe Mössbauer studies indicate a mixed valency, Fe4+/Fe3+, for both the air-fired and argon-fired samples. The increased Fe3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm3̅m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ'(max) and spin glass behavior, while for x = 0.75, χ'(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe3+-Fe3+ antiferromagnetic exchange and ferromagnetic Fe3+-Fe4+ exchange. The PDF and 57Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm3̅m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe3+ sites are largely fourfold-coordinated and Fe4+ sites are fivefold- or sixfold-coordinated.

Emergent bound states and impurity pairs in chemically doped Shastry-Sutherland system.

Impurities often play a defining role in the ground states of frustrated quantum magnets. Studies of their effects are crucial in understanding of the phase diagram in these materials. SrCu2(BO3)2, an experimental realization of the Shastry-Sutherland (SS) lattice, provides a unique model system for such studies using both experimental and numerical approaches. Here we report effects of impurities on the crystals of bound states, and doping-induced emergent ground states in Mg-doped SrCu2(BO3)2, which remain stable in high magnetic fields. Using four complementary magnetometry techniques and theoretical simulations, a rich impurity-induced phenomenology at high fields is discovered. The results demonstrate a rare example in which even a small doping concentration interacts strongly with both triplets and bound states of triplets, and thus plays a significant role in the magnetization process even at high magnetic fields. Our findings provide insights into the study of impurity effects in geometrically frustrated quantum magnets.

Structure and Magnetic Properties of KRuO4.

The crystal structure of KRuO4 is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, I41/a, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru7+ ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d1(e1). Curie-Weiss parameters are near free ion values for the effective moment and θ = -77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected R ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, J1, and the next-nearest-neighbor, J2, exchange constants are AF and of similar magnitude. A spin dimer analysis finds J1/J2 ≈ 25, indicating weak frustration, and a (dz2)1 ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the c axis. The Ru7+ moment is estimated to be 0.57(7) µB, reduced from an expected value near 1 µB. A recent computational study of isostructural, isoelectronic KOsO4 predicts a surprisingly large orbital moment due to spin-orbit coupling (SOC). However, the free ion SOC constant for Ru7+ is only ∼30% that of Os7+, so it is unclear that this effect can be implicated in the low ordered moment for KRuO4. The origin of the short-range spin correlations is also not understood.

Cubic Re6+ (5d1) Double Perovskites, Ba2MgReO6, Ba2ZnReO6, and Ba2Y2/3ReO6: Magnetism, Heat Capacity, µSR, and Neutron Scattering Studies and Comparison with Theory.

Double perovskites (DP) of the general formula Ba2MReO6, where M = Mg, Zn, and Y2/3, all based on Re6+ (5d1, t2g1), were synthesized and studied using magnetization, heat capacity, muon spin relaxation, and neutron-scattering techniques. All are cubic, Fm3̅m, at ambient temperature to within the resolution of the X-ray and neutron diffraction data, although the muon data suggest the possibility of a local distortion for M = Mg. The M = Mg DP is a ferromagnet, Tc = 18 K, with a saturation moment ∼0.3 bohr magnetons at 3 K. There are two anomalies in the heat capacity: a sharp feature at 18 K and a broad maximum centered near 33 K. The total entropy loss below 45 K is 9.68 e.u., which approaches R ln 4 (11.52 e.u.) supporting a j = 3/2 ground state. The unit cell constants of Ba2MgReO6 and the isostructural, isoelectronic analogue, Ba2LiOsO6, differ by only 0.1%, yet the latter is an anti-ferromagnet. The M = Zn DP also appears to be a ferromagnet, Tc = 11 K, µsat(Re) = 0.1 µB. In this case the heat capacity shows a somewhat broad peak near 10 K and a broader maximum at ∼33 K, behavior that can be traced to a smaller particle size, ∼30 nm, for this sample. For both M = Mg and Zn, the low-temperature magnetic heat capacity follows a T3/2 behavior, consistent with a ferromagnetic spin wave. An attempt to attribute the broad 33 K heat capacity anomalies to a splitting of the j = 3/2 state by a crystal distortion is not supported by inelastic neutron scattering, which shows no transition at the expected energy of ∼7 meV nor any transition up to 100 meV. However, the results for the two ferromagnets are compared to the theory of Chen, Pereira, and Balents, and the computed heat capacity predicts the two maxima observed experimentally. The M = Y2/3 DP, with a significantly larger cell constant (3%) than the ferromagnets, shows predominantly anti-ferromagnetic correlations, and the ground state is complex with a spin frozen component Tg = 16 K from both direct current and alternating current susceptibility and µSR data but with a persistent dynamic component. The low-temperature heat capacity shows a T1 power law. The unit cell constant of B = Y2/3 is less than 1% larger than that of the ferromagnetic Os7+ (5d1) DP, Ba2NaOsO6.