Low-field Phase Research Articles

Room-temperature spin reorientation transition and low-temperature significant magnetothermal effect in Mn-doped DyFeO3 single crystals

Rare-earth orthoferrites (RFeO3) promise in spin storage and spin sensing, but the majority of the magnetic phase transition and intriguing properties occur at low temperatures. Searching for higher-temperature or even room-temperature spintronics devices has been a critical challenge in the development of spintronics. We doped Mn ions into Fe sites of DyFeO3 in 10%–50 % ratios to coexist antiferromagnetic and ferromagnetic coupling in the ab plane and elevate their spin reorientation transition (SRT) temperatures. The structural, magnetic, and magnetothermal properties of the Mn-doped DyFeO3 single-crystal system are investigated. With increasing doping ratio, the Néel temperature progressively falls, while the SRT temperature (TSR) continuously rises to the room temperature of 307 K with a 40 % doping ratio, making it promising for room-temperature spin storage devices. When the ratio reaches 50 % (half doping), the magnetic configuration of Dy sublattices changes and a significant magnetothermal effect is observed. Because of low-field metamagnetic phase transition, the magnetic entropy change along the c-axis approaches 10.41 J/kg·K for the half doping crystal at 5 K (0 J/kg·K for DyFeO3, 0.93 J/kg·K for orthorhombic DyMnO3). The large adiabatic temperature change contributed by magnon indicates the exceptional refrigeration efficiency in a direct way. Therefore, it is possible to convert DyFeO3 into a room-temperature spin storage device material or a magnetic refrigeration contender at low temperatures.

Critical behavior and phase diagram of skyrmion-hosting material Co3.6Fe4.4Zn8Mn4 probed by anomalous Hall effect

Critical behaviour in magnetism and condensed matter physics is important owing to it’s possibility for understanding the nature of underlying spin-spin interactions, through determination of the critical parameters in the vicinity of a magnetic phase transition. Conventionally, most of the studies on critical behavior were carried out using a magnetic isotherm-based scaling analysis in the vicinity of the transition temperature. However, this conventional magnetometry technique is insufficient for investigation of critical behavior and/or understanding the nature of spin-spin interactions associated with lowering of dimensions (from bulk to thin films) in mesoscopic magnetic devices or in the presence of unconventional magnetic phases. Here, we introduce a magnetotransport approach using anomalous Hall effect (AHE) and evaluate the critical parameters of a structurally disordered skyrmion hosting itinerant ferromagnet Co3.6Fe4.4Zn8Mn4. Comparison of the evaluated critical parameters (β = 0.377, γ = 1.285, δ = 4.408) using AHE shows excellent agreement to the ones obtained from the conventional magnetic isotherm-based scaling analysis. Our results indicate the prevalence of a short-range 3D Heisenberg model in Co3.6Fe4.4Zn8Mn4 with isotropic interaction strength decreasing as r−4.84. The low-field magnetic phase constructed using the AHE-based and magnetic isotherm-based scaling analysis clearly show remarkable agreement, indicating the feasibility of the proposed magnetotransport based technique. The present magnetotransport approach is universal and might be used to understand the modifications of spin-spin interactions or detect the presence of unconventional magnetic phases all the way from “bulk” to mesoscopic magnetic devices.

Magnetic field induced deformation of the spin density wave microphases in Ca3Co2O6

The frustrated triangular Ising magnet Ca$_3$Co$_2$O$_6$ has long been known for an intriguing combination of extremely slow spin dynamics and peculiar magnetic orders, such as the evenly-spaced non-equilibrium metamagnetic magnetization steps and the long-wavelength spin density wave (SDW) order, the latter of which is essentially an emergent crystal of solitons. Recently, an elaborate field-cooling protocol to bypass the low-field SDW phase was proposed to overcome the extraordinarily long timescale of spin relaxation that impeded previous experimental studies in equilibrium, which may point to a deep connection between the low-temperature slow relaxation and the cooling process passing through the low-field SDW phase. As the first step to elucidate the conjectured connection, we investigate the magnetic field-induced deformation of the SDW state and incommensurate-commensurate transitions, thereby mapping out the equilibrium in-field phase diagram for a realistic three-dimensional lattice spin model by using Monte Carlo simulations. We also discuss Ginzburg-Landau theory that includes several Umklapp terms as well as an effective sine-Gordon model, which can qualitatively explain the observed magnetic field-induced deformation of the SDW microphases.

Ferrielectricity in the Archetypal Antiferroelectric, PbZrO3.

Antiferroelectric materials, where the transition between antipolar and polar phase is controlled by external electric fields, offer exceptional energy storage capacity with high efficiencies, giant electrocaloric effect, and superb electromechanical response. PbZrO3 is the first discovered and the archetypal antiferroelectric material. Nonetheless, substantial challenges in processing phase pure PbZrO3 have limited studies of the undoped composition, hindering understanding of the phase transitions in this material or unraveling the controversial origins of a low-field ferroelectric phase observed in lead zirconate thin films. Leveraging highly oriented PbZrO3 thin films, a room-temperature ferrielectric phase is observed in the absence of external electric fields, with modulations of amplitude and direction of the spontaneous polarization and large anisotropy for critical electric fields required for phase transition. The ferrielectric state observations are qualitatively consistent with theoretical predictions, and correlate with very high dielectric tunability, and ultrahigh strains (up to 1.1%). This work suggests a need for re-evaluation of the fundamental science of antiferroelectricity in this archetypalmaterial.

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr3Si7

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

Ground state and stability of the fractional plateau phase in metallic Shastry\u2013Sutherland system TmB4

We present a study of the ground state and stability of the fractional plateau phase (FPP) with M/Msat = 1/8 in the metallic Shastry–Sutherland system TmB4. Magnetization (M) measurements show that the FPP states are thermodynamically stable when the sample is cooled in constant magnetic field from the paramagnetic phase to the ordered one at 2 K. On the other hand, after zero-field cooling and subsequent magnetization these states appear to be of dynamic origin. In this case the FPP states are closely associated with the half plateau phase (HPP, M/Msat = ½), mediate the HPP to the low-field antiferromagnetic (AF) phase and depend on the thermodynamic history. Thus, in the same place of the phase diagram both, the stable and the metastable (dynamic) fractional plateau (FP) states, can be observed, depending on the way they are reached. In case of metastable FP states thermodynamic paths are identified that lead to very flat fractional plateaus in the FPP. Moreover, with a further decrease of magnetic field also the low-field AF phase becomes influenced and exhibits a plateau of the order of 1/1000 Msat.

Chiral properties of the zero-field spiral state and field-induced magnetic phases of the itinerant kagome metal YMn6Sn6

Applying a magnetic field in the hexagonal plane of YMn$_6$Sn$_6$ leads to a complex magnetic phase diagram of commensurate and incommensurate phases, one of which coexists with the topological Hall effect (THE) generated by a unique fluctuation-driven mechanism. Using unpolarized neutron diffraction, we report on the solved magnetic structure for two previously identified, but unknown, commensurate phases. These include a low-temperature, high-field fan-like phase and a room-temperature, low-field canted antiferromagnetic phase. An intermediate incommensurate phase between the fan-like and forced ferromagnetic phases is also identified as the last known phase of the in-plane field-temperature diagram. Additional characterization using synchrotron powder diffraction reveals extremely high-quality, single-phase crystals, which suggests that the presence of two incommensurate magnetic structures throughout much of the phase diagram is an intrinsic property of the system. Interestingly, polarized neutron diffraction shows that the centrosymmetric system hosts preferential chirality in the zero-field double-flat-spiral phase, which, along with the THE, is a topologically non-trivial characteristic.

Unconventional critical behaviors at the magnetic phase transition of Co3Sn2S2 kagomé ferromagnet

Co3Sn2S2 has generated a growing interest as a rare example of the highly uniaxial anisotropic kagomé ferromagnet showing a combination of frustrated-lattice magnetism and topology. Recently, via precise measurements of the magnetization and AC susceptibility we have found a low-field anomalous magnetic phase (A-phase) with very slow spin dynamics that appears just below the Curie temperature (T C). The A-phase hosts high-density domain bubbles after cooling through T C as revealed in a previous in-situ Lorentz-TEM study. Here, we present further signatures of the anomalous magnetic transition (MT) at T C revealed by a study of the critical behaviors of the magnetization and magnetocaloric effect using a high-quality single crystal. Analyses of numerous magnetization isotherms around T C (≃177 K) using different approaches (the modified Arrot plot, Kouvel–Fisher method and magnetocaloric effect) result in consistent critical exponents that do not satisfy the theoretical predictions of standard second-order-MT models. Scaling analyses for the magnetization, magnetic entropy change and field-exponent of the magnetic entropy change, all consistently show low-field deviations below T C from the universal curves. Our results reveal that the MT of Co3Sn2S2 can not be explained as a conventional second-order type and suggest an anomalous magnetic state below T C.

High-field depinned phase and planar Hall effect in the skyrmion host Gd2PdSi3

For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a new magnetic phase with isotropic value of the critical field for all orientations, where the magnetic ordering vector $\mathbf{q}$ is depinned from its preferred directions in the basal plane. This is in contrast to the highly anisotropic behavior of the low field phases, such as the skyrmion lattice (SkL), which are easily destroyed by in-plane magnetic field. The bulk nature of the SkL and of other magnetic phases was evidenced by specific-heat measurements. Resistivity anisotropy, likely originating from partial gapping of the density of states along $\mathbf{q}$ in this RKKY magnet, is picked up via the planar Hall effect (PHE). The PHE confirms the single-$\mathbf{q}$ nature of the magnetic order when the field is in the hexagonal plane, and allows to detect the preferred directions of $\mathbf{q}$. For field aligned perpendicular to the basal plane, several scenarios for the depinned phase (DP), such as tilted conical order, are discussed on the basis of the data.

Imaging the effect of drive on the low-field vortex melting phenomenon in a Ba0.6K0.4Fe2As2 single crystal

Self-field imaging of current distribution in the $\mathrm{B}{\mathrm{a}}_{0.6}{\mathrm{K}}_{0.4}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ superconductor is used to study the effect of drive on the low-field vortex solid to liquid melting phase transformation. At low fields, the current-induced drive on the vortices aids in thermally destabilizing the low field glassy vortex (solid) phase, thereby shifting the low-field melting phase boundary. We show that the current-induced drive shifts solid-liquid boundaries and prepones the vortex melting phenomenon compared to the equilibrium situation. The analysis shows that, for currents above 50 mA, Joule heating effects shift the melting line whereas, below 50 mA, an effective temperature concept for the driven system viz., a drive-dependent shaking temperature, explains the shift. The observation of a transformation from inhomogeneous to homogeneous current flow in the sample at low fields as a function of the driving force is reconciled via an inverse dependence of the shaking temperature on vortex velocity.

Features of magnetic and magnetoelectric properties, H-T phase diagram of GdCr3(BO3)4

Comprehensive studies of magnetic properties of GdCr3(BO3)4 single crystal have been carried out. The integrals of intrachain (J/k = 4 ± 1 K) and interchain (J΄/k ≈ −0.7 K) exchange interactions in the Cr subsystem have been determined and the strength of Cr-Gd exchange interaction (JCr-Gd/k ~ −0.2 K) has been estimated. The values of the exchange field 2HE ≈ 17 T and the effective magnetic anisotropy field HA ≈ 0.05 T of GdCr3(BO3)4 have been estimated. The electric polarization ΔPa along the a axis in the longitudinal geometry of the experiment has been detected. Correlations between the electric polarization and the magnetization of the studied compound have been found. The spin-reorientation phase transition in the magnetically ordered state has been found. This transition exists for the external magnetic field applied along any crystallographic direction and the transition field depends weakly on the direction of the field. Nature of the spin-reorientation phase transition has been discussed. Magnetic phase diagram has been constructed and spin configurations for the low-field and high-field phases have been proposed.

Two-magnon bound states in the Kitaev model in a [111] field

It is now well established that the Kitaev honeycomb model in a magnetic field along the $[111]$-direction harbors an intermediate gapless quantum spin liquid (QSL) phase sandwiched between a gapped non-abelian QSL at low fields $H< H_{c1}$ and a partially polarized phase at high fields $H> H_{c2}$. Here, we analyze the low field and high field phases and phase transitions in terms of single- and two-magnon excitations using exact diagonalization (ED) and density matrix renormalization group (DMRG) methods. We find that the energy to create a bound state of two-magnons $\Delta_p$ becomes lower than the energy to create a single spin flip $\Delta_s$ near $H_{c2}$. In the entire Kitaev spin liquid $\Delta_p<\Delta_s$ and both gaps vanish at $H_{c2}$. We make testable predictions for magnon pairing that could be observable in Raman scattering measurements on Kitaev QSL candidate materials.

Field-induced magnetic incommensurability in multiferroicNi3TeO6

Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pm\delta$),$\delta\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{IC}$ is found to drift both as function of magnetic field and temperature. Preliminary inelastic neutron scattering reveals that the spin wave gap in zero field has minima exactly at $Q_{IC}$ and a gap of about 1.1 meV consisting with a cross-over around 8.6 T. Our findings excludes the possibility of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magneto-electric coupling earlier observed in this material and advocates for the symmetric exchangestriction as the origin of this effect.

Magnetic domain structure within half-metallic ferromagnetic kagome compound Co3Sn2S2

$\mathrm{C}{\mathrm{o}}_{3}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{S}}_{2}$ has been attracting attention as a model quasi-2D kagome ferromagnet as well as a ferromagnetic Weyl semimetal. Here, we report the evolution of the magnetic domain within $\mathrm{C}{\mathrm{o}}_{3}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{S}}_{2}$ as a function of temperature and external field as studied by in situ Lorentz microscopy. The domain structure typical for anisotropy-controlled material was observed, and domain wall (DW) motion was found to depend strongly on temperature. It was inactive due to strong pinning in a low-temperature range yet activated as a result of thermal depinning with increasing temperature. The activation temperature decreased as the external field increased. In the temperature range just below the Curie temperature, spontaneous domain reorganization associated with DW creep occurred even in the absence of an external field. The low-field anomalous magnetic phase just below the Curie temperature and split between zero-field-cooled and field-cooled magnetization-temperature curves found in our previous study [M. A. Kassem et al., Phys. Rev. B 96, 014429 (2017)] are explained in terms of temperature-dependent DW motion including the DW creep.

Topological Hall effect and emergent skyrmion crystal at manganite-iridate oxide interfaces

Scalar spin chirality is expected to induce a finite contribution to the Hall response at low temperatures. We study this spin-chirality-driven Hall effect, known as the topological Hall effect, at the manganite side of the interface between La$_{1-x}$Sr$_{x}$MnO$_{3}$ and SrIrO$_3$. The ferromagnetic double-exchange hopping at the manganite layer, in conjunction with the Dzyaloshinskii-Moriya (DM) interaction which arises at the interface due to broken inversion symmetry and strong spin-orbit coupling from the iridate layer, could produce a skyrmion-crystal (SkX) phase in the presence of an external magnetic field. Using the Monte Carlo technique and a two-orbital spin-fermion model for manganites, supplemented by an in-plane DM interaction, we obtain phase diagrams which reveal at low temperatures a clear SkX phase and also a low-field spin-spiral phase. Increasing temperature, a skyrmion-gas phase, precursor of the SkX phase upon cooling, was identified. The topological Hall effect primarily appears in the SkX phase, as observed before in oxide heterostructures. We conclude that the manganite-iridate superlattices provide another useful platform to explore a plethora of unconventional magnetic and transport properties.

Even- and Odd-Frequency Superconductivity in Q1D Organic Superconductors Under Magnetic Field

One of the quasi-one-dimensional organic superconductors $$(\hbox {TMTSF})_2X$$ , called Bechgaard salts, has attracted a lot of interest since its discovery in 1980. These compounds have high critical magnetic field $$H_{\mathrm {c}2}$$ in the direction parallel to the layers, and their pairing symmetries seem to be different between low-field phase and high-field phase. In addition, recently, the possibility of the realization of odd-frequency superconductivity was pointed out. Therefore, we study pairing symmetries of Bechgaard salts under magnetic field with frequency-dependent pairing interaction due to spin and charge fluctuation. The model is given by the extended Hubbard model on a quasi-one-dimensional square lattice, and the spin susceptibility and charge susceptibility for the effective pairing interactions are derived by random-phase approximation under magnetic field. The odd-frequency s-wave and even-frequency f-wave regions expand as the magnetic field is increased.

Reversal and non-reversal ferroelectric polarizations in a Y-type hexaferrite

The Cr ion doped Y-type hexaferrite Ba0.5Sr1.5Zn2Fe11.4Cr0.6O22 has been found to be a promising magnetoelectric material for the non-zero spontaneous electric polarization. A newly emerging transition at 183 K separates the low-field ferroelectric phase into two regions with reversal and non-reversal polarization respectively.

Monte Carlo study of the discontinuous quantum phase transition in the transverse-field Ising model on the pyrochlore lattice

The antiferromagnetic Ising model on the pyrochlore lattice exhibits a quantum phase transition in an applied transverse field from the low-field quantum spin-ice phase to the high-field polarized regime. Recent field-theoretical analysis and series expansion results indicate this to be a discontinuous, first-order transition. Here, we explore this transition using quantum Monte Carlo simulations in order to assess this scenario and study the thermodynamic properties in the vicinity of the quantum phase transition. For this purpose, we also consider several variants of extended cluster-update schemes for the transverse field Ising antiferromagnet on frustrated lattices and compare their performance to the conventional bond-based algorithm for the transverse field Ising model on the pyrochlore lattice.

Orbitally limited pair-density-wave phase of multilayer superconductors

We investigate the magnetic field dependence of an ideal superconducting vortex lattice in the parity-mixed pair-density wave phase of multilayer superconductors within a circular cell Ginzburg-Landau approach. In multilayer systems, due to local inversion symmetry breaking, a Rashba spin-orbit coupling is induced at the outer layers. This combined with a perpendicular paramagnetic (Pauli) limiting magnetic field stabilizes a staggered layer dependent pair-density wave phase in the superconducting singlet channel. The high-field pair-density wave phase is separated from the low-field BCS phase by a first-order phase transition. The motivating guiding question in this paper is: what is the minimal necessary Maki parameter $\alpha_M$ for the appearance of the pair-density wave phase of a superconducting trilayer system? To address this problem we generalize the circular cell method for the regular flux-line lattice of a type-II superconductor to include paramagnetic depairing effects. Then, we apply the model to the trilayer system, where each of the layers are characterized by Ginzburg-Landau parameter $\kappa_0$, and a Maki parameter $\alpha_M$. We find that when the spin-orbit Rashba interaction compares to the superconducting condensation energy, the orbitally limited pair-density wave phase stabilizes for Maki parameters $\alpha_M> 10$.

Role of band filling in tuning the high-field phases of URu2Si2

We present a detailed study of the low temperature and high magnetic field phases in the chemical substitution series URu$_2$Si$_{2-x}$P$_x$ using electrical transport and magnetization in pulsed magnetic fields up to 65T. Within the hidden order region (0 $\ x$$\ $ 0.035) the high field ordering is robust even as the hidden order temperature is suppressed. Earlier work shows that for 0.035 $\ x$ $\ $ 0.26 there is a Kondo lattice with a no-ordered state that is replaced by antiferromagnetism for 0.26 $\ x$ 0.5. We observe a simplified continuation of the high field ordering in the no-order $x$-region and an enhancement of the high field state upon the destruction of the antiferromagnetism with magnetic field. These results closely resemble what is seen for URu$_{2-x}$Rh$_x$Si$_2$\footnote{The concentration in this paper is defined as URu$_{2-x}$Rh$_x$Si$_2$ while the chemical formula in the literature is given as U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ [24-26]}, from which we infer that charge tuning uniformly controls the ground state of URu$_2$Si$_2$, regardless of whether s/p or d-electrons are replaced. This provides guidance for determining the specific factors that lead to hidden order versus magnetism in this family of materials.