Search

Skyrmions are particle-like vortices of magnetization with non-trivial topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions (DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd- and Eu-based skyrmion-lattice (SkL) hosts with net-zero DMI, where both the SkL stabilization mechanisms and magnetic ground states remain controversial. We address these by investigating both static and dynamic spin properties of the centrosymmetric SkL host Gd$_2$PdSi$_3$ using muon spectroscopy ($\mu$SR). We find that spin fluctuations in its non-coplanar SkL phase are highly anisotropic, implying that spin anisotropy plays a prominent role in stabilizing this phase. We also observe strongly-anisotropic spin dynamics in the ground-state (IC-1) incommensurate magnetic phase of the material, indicating that it is a meron-like multi-$q$ structure. In contrast, the higher-field, coplanar IC-2 phase is found to be single-$q$ with nearly-isotropic spin dynamics., Comment: Main text: 7 pages, 3 figures. Supplemental Material: 6 pages, 5 figures

Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar noncollinear magnetic structure with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-$\mu^+$SR) measurements on a stack comprising [Ta/CoFeB/MgO]$_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from N\'eel-like near the surface to Bloch-like deeper within the sample. LE-$\mu^+$SR reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the $\mu^+$SR results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT., Comment: 9 pages, 4 figures

Mechanical straining of skyrmion hosting materials has previously demonstrated increased phase stability through the expansion of the skyrmion equilibrium pocket. Additionally, metastable skyrmions can be generated via rapid field-cooling to form significant skyrmion populations at low temperatures. Using small-angle x-ray scattering and x-ray holographic imaging on a thermally strained 200 nm thick FeGe lamella, we observe temperature-induced strain effects on the structure and metastability of the skyrmion lattice. We find that in this sample orientation (H || [1 1 0]) with no strain, metastable skyrmions produced by field cooling through the equilibrium skyrmion pocket vanish from the sample upon dropping below the well known helical reorientation temperature. However, when strain is applied along [110] axis, and this procedure is repeated, a substantial volume fraction of metastable skyrmions persist upon cooling below this temperature down to 100 K. Additionally, we observe a large number of skyrmions retained after a complete magnetic field polarity reversal, implying that the metastable energy barrier protecting skyrmions from decay is enhanced.

We present magnetization and muon-spin spectroscopy measurements of N\'{e}el skyrmion-host GaV$_4$S$_8$ under the application of hydrostatic pressures up to $P=2.29$ GPa. Our results suggest that the magnetic phase diagram is altered with pressure via a reduction in the crossover temperature from the cycloidal (C) to ferromagnetic-like state with increasing $P$, such that, by 2.29 GPa, the C state appears to persist down to the lowest measured temperatures. With the aid of micromagnetic simulations, we propose that the driving mechanism behind this change is a reduction in the magnetic anisotropy of the system, and suggest that this could lead to an increase in stability of the skyrmion lattice., Comment: 9 pages, 6 figures

Isotropic helimagnets are known to host a diverse range of chiral magnetic states. In 2016, F.N. Rybakov et al. theorized the presence of a surface-pinned stacked spin spiral phase [F.N. Rybakov et al., 2016 New J. Phys. 18 045002], which has yet to be observed experimentally. Here we present experimental evidence for the observation of this state in lamellae of FeGe using resonant x-ray holographic imaging data and micromagnetic simulations. The identification of this state has significant implications for the stability of other coexisting spin textures, and will help complete our understanding of helimagnetic systems.

We report small-angle neutron scattering from the conical state in a single crystal of Zn-substituted Cu2OSeO3. Using a 3D vector-field magnet to reorient the conical wavevector, our measurements show that the magnitude of the conical wavevector changes as a function of crystallographic direction. These changes are caused by the anisotropic exchange interaction (AEI), whose magnitude transitions from a maxima to a minima along the <111> and <100> crystallographic directions respectively. We further find that the AEI constant undergoes a change of sign from positive to negative with decreasing temperature. Unlike in the related compound FeGe, where similar behaviour of the AEI induces a reorientation of the helical wavevector, we show that the zero field helical wavevector in (Cu0.98Zn0.02)2OSeO3 remains along the <100> directions at all temperatures due to the competing fourth-order magnetocrystalline anisotropy becoming dominant at lower temperatures.

The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations that demonstrate the materials are half-metals. There is also a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta), with the gap-like feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($\mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $\mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase., Comment: 6 pages, 4 figures

We report muon spin relaxation and magnetometry studies of bulk Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and MnNiGa, two materials which have recently been proposed to host topological magnetic states in thin lamella (antiskyrmions for Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and biskyrmions for MnNiGa), and show spin reorientation transitions in bulk. These measurements shed light on the magnetic dynamics surounding the two magnetic phase transitions in each material. In particular, we demonstrate that the behaviour approaching the higher temperature transition in both samples is best understood by considering a slow decrease in the frequency of dynamics with temperature, rather than the sharp critical slowing down typical of second order transitions. Furthermore, at low temperatures the two samples both show spin dynamics over a broad range of frequencies that persist below the spin reorienation transition. The dynamic behavior we identify gives new insight into the bulk magnetism of these materials that may help underpin the stabilization of the topologically non-trivial phases that are seen in thin lamellae., Comment: 9 pages, 6 figures

We present longitudinal-field muon-spin relaxation (LF $\mu$SR) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu$_2$OSeO$_3$, and Co$_x$Zn$_y$Mn$_{20-x-y}$ for $(x,y)~=~(10,10)$, $(8,9)$ and $(8,8)$. We find that the SkL phase of Cu$_2$OSeO$_3$ exhibits emergent dynamic behavior at megahertz frequencies, likely due to collective excitations, allowing the SkL to be identified from the $\mu$SR response. From measurements following different cooling protocols and calculations of the muon stopping site, we suggest that the metastable SkL is not the majority phase throughout the bulk of this material at the fields and temperatures where it is often observed. The dynamics of bulk Co$_8$Zn$_9$Mn$_3$ are well described by $\simeq~2$ GHz excitations that reduce in frequency near the critical temperature, while in Co$_8$Zn$_8$Mn$_4$ we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase., Comment: 9 pages, 5 figures

The intermetallic Gd2PdSi3 has recently generated a lot of excitement after reports that it hosts chiral magnetic nano-skyrmions despite its centrosymmetric crystal structure. Using magnetic-field-dependent polarized resonant elastic x-ray scattering (REXS), we find than an unexpected incommensurate charge density wave (CDW) appears below the ordering transition with a wavevector equal to that of the magnetic textures. Furthermore, we show these incommensurate magnetic textures in Gd2PdSi3 are highly anisotropic, with the vast majority of the spin modulation lying within the hexagonal ab-plane. This observation is not compatible with the previously suggested non-coplanar magnetic textures and coplanar alternatives are discussed. Our results thus refute the interpretation of the observed large anomalous Hall and Nernst effects in Gd2PdSi3 as arising from topologically-nontrivial magnetic skyrmions., Comment: We have recently collected new measurements which conflict the main claims of this paper. We will reinterpret our old results together with the new data in due course and will report our findings once the analysis is complete and a better understanding of the complete picture has been achieved

Magnetic skyrmions are nano-sized topological spin textures stabilized by a delicate balance of magnetic energy terms. The chemical substitution of the underlying crystal structure of skyrmion-hosting materials offers a route to manipulate these energy contributions, but also introduces additional effects such as disorder and pinning. While the effects of doping and disorder have been well studied in B20 metallic materials such as Fe$_{1-x}$Co$_x$Si and Mn$_{1-x}$Fe$_x$Si, the consequences of chemical substitution in the magnetoelectric insulator Cu$_2$OSeO$_3$ have not been fully explored. In this work, we utilize a combination of AC magnetometry and small angle neutron scattering to investigate the magnetic phase transition dynamics in pristine and Zn-substituted Cu$_2$OSeO$_3$. The results demonstrate that the first order helical-conical phase transition exhibits two thermally separated behavioural regimes: at high temperatures, the helimagnetic domains transform by large-scale, continuous rotations, while at low temperatures, the two phases coexist. Remarkably, the effects of pinning in the substituted sample are less prevalent at low temperatures, compared to high temperatures, despite the reduction of available thermal activation energy. We attribute this behaviour to the large, temperature-dependent, cubic anisotropy unique to Cu$_2$OSeO$_3$, which becomes strong enough to overcome the pinning energy at low temperatures. Consideration and further exploration of these effects will be crucial when engineering skyrmion materials towards future applications.

We present an investigation of the influence of low-levels of chemical substitution on the magnetic ground state and N{\' e}el skyrmion lattice (SkL) state in GaV$_4$S$_{8-y}$Se$_y$, where $y =0, 0.1, 7.9$, and $8$. Muon-spin spectroscopy ($\mu$SR) measurements on $y=0$ and 0.1 materials reveal the magnetic ground state consists of microscopically coexisting incommensurate cycloidal and ferromagnetic environments, while chemical substitution leads to the growth of localized regions of increased spin density. $\mu$SR measurements of emergent low-frequency skyrmion dynamics show that the SkL exists under low-levels of substitution at both ends of the series. Skyrmionic excitations persist to temperatures below the equilibrium SkL in substituted samples, suggesting the presence of skyrmion precursors over a wide range of temperatures., Comment: 6 pages, 4 figures

We report scanning transmission X-ray microscopy of mixed helical and skyrmion magnetic states in thin FeGe lamellae. This imaging of the out-of-plane magnetism allows clear identification of the different magnetic states, and reveals details about the coexistence of helical and skyrmion states. In particular, our data show that finite length helices are continuously deformable down to the size of individual skyrmions and are hence topologically equivalent to skyrmions. Furthermore, we observe transition states between helical and skyrmion states across the thickness of the lamella that are evidence for frozen Bloch points in the sample after field cooling., Comment: Further analysis by the authors suggests that the evidence we presented for the zipping transition between skyrmions and helices was flawed. This mistaken interpretation resulted from slight misalignment between positive and negative circularly polarized images used to produce the XMCD STXM images

We report small angle X-ray scattering (SAXS) measurements of the skyrmion lattice in two 200~nm thick Cu$_2$OSeO$_3$ lamellae aligned with the applied magnetic field parallel to the out of plane [110] or [100] crystallographic directions. Our measurements show that the equilibrium skyrmion phase in both samples is expanded significantly compared to bulk crystals, existing between approximately 30 and 50~K over a wide region of magnetic field. This skyrmion state is elliptically distorted at low fields for the [110] sample, and symmetric for the [100] sample, possibly due to crystalline anisotropy becoming more important at this sample thickness than it is in bulk samples. Furthermore, we find that a metastable skyrmion state can be observed at low temperature by field cooling through the equilibrium skyrmion pocket in both samples. In contrast to the behavior in bulk samples, the volume fraction of metastable skyrmions does not significantly depend on cooling rate. We show that a possible explanation for this is the change in the lowest temperature of the skyrmion state in this lamellae compared to bulk, without requiring different energetics of the skyrmion state., Comment: 10 pages, 8 figures

Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures. While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the sample. Study of this skyrmion tube (SkT) state is highly relevant for investigating skyrmion metastability and for implementation in recently proposed magnonic computing. However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the first real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, confirming their extended structure. The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigations into this unexplored dimension of the skyrmion spin texture.

We present a detailed magnetic study of the triangular antiferromagnet ErMgGaO$_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the Er$^{3+}$ moment along the c axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.5~K and no evidence of residual entropy as temperature approaching zero. Zero field (ZF) $\mu$SR measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu$SR measurements exhibit persistent spin fluctuations down to our lowest temperature of 25~mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ErMgGaO$_4$.

Rare earth garnets are an exciting playground for studying the exotic magnetic properties of the frustrated hyperkagome lattice. Here we present a comprehensive study of the single ion and collective magnetic properties of the garnet Er$_3$Ga$_5$O$_{12}$. Using inelastic neutron scattering, we find a crystal field ground state doublet for Er$^{3+}$ with strong Ising anisotropy along local [100] axes. Magnetic susceptibility and heat capacity measurements provide evidence for long-range magnetic ordering with $T_N$~$=$~0.8~K, and no evidence for residual entropy is found when cooling through the ordering transition. Neutron powder diffraction reveals that the ground state spin configuration corresponds to the six-sublattice, Ising antiferromagnetic state ($\Gamma_3$) common to many of the rare earth garnets. However, we also found that $\mu$SR appears to be insensitive to the ordering transition in this material, in which a low-temperature relaxation plateau was observed with no evidence of spontaneous muon precession. The combined muon and neutron results may be indicative of a dynamical ground state with a relatively long correlation time. Despite this potential complication, our work indicates that Er$_3$Ga$_5$O$_{12}$ is an excellent model system for studying the complex metamagnetism expected for a multi-axis antiferromagnet.

We report small angle neutron scattering (SANS) measurements of the skyrmion lattice in (Cu$_{0.976}$Zn$_{0.024}$)$_2$OSeO$_3$ under the application of an electric field. These measurements show an expansion of the skyrmion lattice stability region with electric field similar to that seen in pristine Cu$_2$OSeO$_3$. Furthermore, using time-resolved SANS, we observe the slow formation of skyrmions after an electric or magnetic field is applied, which has not been observed in pristine Cu$_2$OSeO$_3$ crystals. The measured formation times are dramatically longer than the corresponding skyrmion destruction times after the external field is removed, and increase exponentially from 100~s at 52.5~K to 10,000~s at 51.5~K. This thermally activated behaviour indicates an energy barrier for skyrmion formation of 1.57(2)~eV, the size of which demonstrates the huge cost for creating these complex chiral objects., Comment: 8 pages, 8 figures

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase., Comment: 8 pages, 6 Figures

Previous observations of metastable magnetic skyrmions have shown that close to the equilibrium pocket the metastable state has a short lifetime, and therefore rapid cooling is required to generate a significant skyrmion population at low temperatures. Here, we report that the lifetime of metastable skyrmions in Cu$_2$OSeO$_3$ is extended by a factor of 50 with the introduction of only 2.5% zinc doping, allowing over 50% of the population to survive when field-cooling at a rate of just 1 K/min. Our systematic study suggests that the lifetime enhancement is due to the removal of spins by the non-magnetic dopant, which entropically limits the number of skyrmion decay pathways. We expect that doping can be exploited to control the lifetime of the metastable SkL state in other chiral magnets, offering a method of engineering skyrmion materials towards application in future devices., Comment: 9 Pages, 7 Figures, submission in progress

We present an investigation into the structural and magnetic properties of Zn-substituted Cu$_{2}$OSeO$_{3}$, a system in which the skyrmion lattice (SkL) phase in the magnetic field-temperature phase diagram was previously seen to split as a function of increasing Zn concentration. We find that splitting of the SkL is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behaviour. No such multiphase behaviour is observed in single crystal samples., Comment: 5 pages, 6 figures, includes supplemental material

A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($\mu$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$\mu$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ spins undergo spin freezing into a disordered ground state below 4~K, with a characteristic internal field strength of 140~G. Persistent Ni spin dynamics are present to our lowest temperatures (75~mK), a feature characteristic of many geometrically frustrated magnetic systems.

Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states. It is thus important to understand the routes towards such dilute-carrier correlated states. One established pathway is through Kondo effect in metallic non-magnetic analogues. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low carrier-density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-f-shell counterpart, Lu3Ir4Ge13. Through magnetotransport, optical conductivity and thermodynamic measurements, we establish that the correlated semimetallic state of Yb3Ir4Ge13 below its Kondo temperature originates from the Kondo effect of a low carrier conduction-electron background. In addition, it displays fragile magnetism at very low temperatures, which, in turn, can be tuned to a non Fermi liquid regime through Lu-for-Yb substitution. These findings are connected with recent theoretical studies in simplified models. Our results open an entirely new venue to explore the strong correlation physics in a semimetallic environment.

The magnetic structure of the in-plane skyrmions in epitaxial MnSi/Si(111) thin films is probed in three dimensions by the combination of polarized neutron reflectometry (PNR) and small angle neutron scattering (SANS). We demonstrate that skyrmions exist in a region of the phase diagram above at temperature of 10 K. PNR shows the skyrmions are confined to the middle of the film due to the potential well formed by the surface twists. However, SANS shows that there is considerable disorder within the plane indicating that the magnetic structure is a 2D skyrmion glass.

Neutron diffraction and muon spin relaxation ($\mu$SR) studies are presented for the newly characterized polymorph of NiNb$_2$O$_6$ ($\beta$-NiNb$_2$O$_6$) with space group P4$_2$/n and $\mu$SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4$_2$/n form was determined from neutron diffraction using both powder and single crystal data. Powder neutron diffraction determined an ordering wave vector $\vec{k}$ = ($\frac{1}{2},\frac{1}{2},\frac{1}{2}$). Single crystal data confirmed the same $\vec{k}$-vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni$^{2+}$ layers perpendicular to the c-axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related tri-rutile phases, NiSb$_2$O$_6$ and NiTa$_2$O$_6$. $\mu$SR data finds a transition temperature of $T_N \sim$ 15 K for this system, while the columbite polymorph exhibits a lower $T_N =$ 5.7(3) K. Our $\mu$SR measurements also allowed us to estimate the critical exponent of the order parameter $\beta$ for each polymorph. We found $\beta =$ 0.25(3) and 0.16(2) for the $\beta$ and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent $\beta =$~0.28(3) for $\beta$-NiNb$_2$O$_6$, in agreement with the $\mu$SR value. While both systems have $\beta$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to well-described by $S = 1$ spin chains, the interchain interactions in the $\beta$-polymorph are likely much larger., Comment: 10 pages, 12 figures

We present muon spin rotation ($\mu$SR) measurements on the noncentrosymmetric superconductor PbTaSe$_2$. From measurements in an applied transverse field between $H_{c1}$ and $H_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. This data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero field measurements show no evidence for a time reversal symmetry breaking field greater than 0.05~G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that PbTaSe$_2$ is characterized by conventional BCS s-wave superconductivity in multiple bands., Comment: 6 pages, 7 figures

The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic ordering transition at $T_N = 1.6$ K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a non-divergent relaxation rate in $\mu$SR. The form of the heat capacity below $T_N$ suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd$_2$Pt$_2$O$_7$, $T_N = 1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which have been found to order at 1 K or lower. We attribute this enhancement in $T_N$ to the $B$-site cation, platinum, which, despite being non-magnetic, has a filled $5d$ $t_{2g}$ orbital and an empty $5d$ $e_g$ orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd$_2$Pt$_2$O$_7$ is partially "relieved", thereby promoting magnetic order., Comment: 8 pages, 6 figures

We present a comprehensive set of elastic and inelastic neutron scattering measurements on a range of Fe-doped samples of U(Ru$_{1-x}$Fe$_x$)$_2$Si$_2$ with 0.01~$\le~x~\le$~0.15. All of the samples measured exhibit long-range antiferromagnetic order, with the size of the magnetic moment quickly increasing to 0.51~$\mu_B$ at 2.5\% doping and continuing to increase monotonically with doping, reaching 0.69~$\mu_B$ at 15\% doping. Time-of-flight and inelastic triple-axis measurements show the existence of excitations at (1~0~0) and (1.4~0~0) in all samples, which are also observed in the parent compound. While the excitations in the 1\% doping are quantitatively identical to the parent material, the gap and width of the excitations change rapidly at 2.5\% Fe doping and above. The 1\% doped sample shows evidence for a separation in temperature between the hidden order and antiferromagnetic transitions, suggesting that the antiferromagnetic state emerges at very low Fe dopings. The combined neutron scattering data suggests not only discontinuous changes in the magnetic moment and excitations between the hidden order and antiferromagnetic phases, but that these changes continue to evolve up to at least $x$~=~0.15., Comment: 9 pages, 6 figures, 2 tables

We present magnetometry and muon spin rotation ($\mu$SR) measurements of the superconducting dichalcogenide Ir$_{0.95}$Pt$_{0.05}$Te$_2$. From both sets of measurements we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional s-wave model and yiel fitting parameters consistent with a BCS weak coupling superconductor. We therefore see no evidence for exotic superconductivity in Ir$_{0.95}$Pt$_{0.05}$Te$_2$., Comment: 7 pages, 7 figures

We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. muSR measurements reveal the presence of an ordering transition at T_N = 0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k = (000) Gamma_5 ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We compare and contrast this phenomenology with the low temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets., Comment: 10 pages, 6 figures, accepted in Phys. Rev. B

We present muon spin rotation ($\mu$SR) and susceptibility measurements on single crystals of isoelectronically doped URu$_{2-x}$T$_x$Si$_2$ (T = Fe, Os) for doping levels up to 50\%. Zero Field (ZF) $\mu$SR measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment - hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping., Comment: 10 pages, 8 figures

We present data on the optical conductivity of URu$_{2-x}$(Fe,Os)$_{x}$Si$_{2}$. While the parent material URu$_2$Si$_2$ enters the enigmatic hidden order phase below 17.5 K, an antiferromagnetic phase is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as $\omega^2$. We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density-wave-gap.

In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is never reached due to the formation of specific surface chiral modulations with the propagation direction perpendicular to the film surfaces [Wilson et al. Phys. Rev. B 88, 214420 (2013)]. In this paper we show that the occurrence of such chiral surface twists is a general effect attributed to all bulk and con- fined magnetic crystals lacking inversion symmetry. We present experimental investigations of this phenomenon in nanolayers of MnSi/Si(111) supported by detailed theoretical analysis within the standard phenomenological model. In magnetic nanolayers with intrinsic or induced chirality, such surface induced instabilities become sizeable effects and play a crucial role in the formation of skyrmion lattices and other nontrivial chiral modulations.

This paper reports on magnetometry and magnetoresistance measurements of MnSi epilayers performed in out-of-plane magnetic fields. We present a theoretical analysis of the chiral modulations that arise in confined cubic helimagnets where the uniaxial anisotropy axis and magnetic field are both out-of-plane. In contrast to in-plane field measurements (Wilson et al., Phys. Rev. B 86, 144420 (2012)), the hard-axis uniaxial anisotropy in MnSi/Si(111) increases the energy of (111)-oriented skyrmions and in-plane helicoids relative to the cone phase, and makes the cone phase the only stable magnetic texture below the saturation field. While induced uniaxial anisotropy is important in stabilizing skyrmion lattices and helicoids in other confined cubic helimagnets, the particular anisotropy in MnSi/Si(111) entirely suppresses these states in an out-of-plane magnetic field. However, it is predicted that isolated skyrmions with enlarged sizes exist in MnSi/Si(111) epilayers in a broad range of out-of-plane magnetic fields.

Magnetometry and magnetoresistance measurements in MnSi thin films and rigorous analytical solutions of the micromagnetic equations show that the field-induced unwinding of confined helicoids occurs via discrete steps. A comparison between the magnetometry data and theoretical results shows that finite size effects confine the wavelength and lead to a quantization of the number of turns in the helicoid. We demonstrate a prototypical spintronic device where the magnetic field can push or pull individual turns into a magnetic spring that can be read by electrical means.

A detailed investigation of the magnetization processes in epitaxial MnSi thin film reveals the existence of elliptically distorted skyrmion strings that lie in the plane of the film. We provide proof that the uniaxial anisotropy stabilizes this state over extended regions of the magnetic phase diagram. Theoretical analysis of an observed cascade of first-order phase transitions is based on rig- orous numerical calculations of competing chiral modulations, which shows the existence of helicoids, elliptic skyrmions, and cone phases.