Commensurate Antiferromagnetic Order Research Articles

Unusually large magnetic moment and tricritical behavior of the CMR compound NaCr2O4 revealed with high resolution neutron diffraction and SR

The mixed valence Cr3+/Cr4+ compound NaCr2O4, hosts a plethora of unconventional electronic properties. In the present study, muon spin rotation/relaxation (SR) and high-resolution time-of-flight neutron powder diffraction measurements were carried out on high-quality samples to clarify the complex magnetic ground state of this unique material. We identified a commensurate canted antiferromagnetic order (C-AFM) with a canting angle of the Cr spin axial vector equal to , and an estimated Cr moment . Such an unusually large value of is compatible with the existence of high-spin Cr sites created by the presence of an unconventional negative charge transfer state in NaCr2O4. In addition to the C-AFM structure, a novel magnetic supercell was also revealed. Such supercell display an incommensurate (IC)-AFM propagation vector (0 0 ), having a Cr moment . It is suggested that the C-AFM and IC-AFM modulations have two different electronic origins, being due to itinerant and localized contributions to the magnetic moment respectively. Finally, the direct measurement of the magnetic order parameter for the C-AFM structure provided a value of the critical exponent , suggesting a non conventional critical behavior for the magnetic phase transition in NaCr2O4.

Spin gap in the weakly interacting quantum spin chain antiferromagnet KCuPO4·H2O

The $S=1/2$ Heisenberg linear chain antiferromagnet is the simplest spin model; nevertheless it serves as a platform for various quantum many-body phenomena. Here, we report the magnetic behavior of a quasi-one-dimensional antiferromagnet ${\mathrm{KCuPO}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$. A long-range commensurate antiferromagnetic order with ordered moment 0.31(1) ${\ensuremath{\mu}}_{\mathrm{B}}$ per spin occurs at ${T}_{\mathrm{N}}=11.7(1)$ K. Above ${T}_{\mathrm{N}}$, the inelastic neutron excitation is characterized by a two spinon continuum. The intrachain interaction $J$ and interchain interaction $|{J}^{\ensuremath{'}}|$ are estimated to be 172 and 4.25(4) K, respectively; thus the ratio of $|{J}^{\ensuremath{'}}|/J=0.0247(3)$. At lower energies, below ${T}_{\mathrm{N}}$, a spin gap is observed in the dispersive excitations. These results are consistent with characteristics observed in a weakly interacting $S=1/2$ Heisenberg chain system.

Incommensurate magnetic order in rare earth and transition metal compounds with local moments

Within the framework of the s–d(f) exchange model in the mean-field approximation for square, simple cubic, body-centered and face-centered cubic lattices, the formation of a ferromagnetic, spiral, and commensurate antiferromagnetic (AFM) order is investigated. The possibility of the formation of inhomogeneous states (magnetic phase separation), which necessarily arises during first-order phase transitions in the electron filling parameter, is taken into account. The saturation of the AFM and spiral states is studied depending on the parameters of the model. The results obtained include a rich variety of magnetic structures and phase transitions, allowing the interpretation of magnetic properties of semiconducting and metallic systems containing magnetic atoms.

Martensitic transition assisted modification of the antiferromagnetic ordering in Ge-site doped MnNiGe alloys

The magnetic equiatomic alloys of the nominal compositions ${\mathrm{MnNiGe}}_{0.9}{\mathrm{Al}}_{0.1}$ and ${\mathrm{MnNiGe}}_{0.928}{\mathrm{Ga}}_{0.072}$ have been investigated through the neutron powder diffraction (NPD) technique. Both these alloys undergo a martensitic phase transition from the high-temperature hexagonal (${\mathrm{Ni}}_{2}\mathrm{In}$ type, with space group $P{6}_{3}/mmc$) austenite phase to the low-temperature orthorhombic (TiNiSi type, with space group $Pnma$) martensite phase. Our NPD analysis confirmed an incommensurate antiferromagnetic ordering of the low-temperature martensitic phase with helical modulation. The incommensurate propagation vector $\mathbf{k}$ is found to be (0.2065(1),0,0) and (0.2088(4),0,0) for Al and Ga doped alloys, respectively, at 1.5 K and shows a monotonic increase with increasing sample temperature. On the other hand, a commensurate antiferromagnetic ordering was observed for the high-temperature austenite phase. The incomplete martensitic transition allowed both these incommensurate and commensurate antiferromagnetic structures to coexist down to the lowest temperature of measurement (1.5 K).

Neutron Diffraction Magnetic and Mossbauer Spectroscopic Studies of Pb0.8Bi0.2Fe0.728W0.264O3 and Pb0.7Bi0.3Fe0.762W0.231O3 Ceramics

We report on high-temperature crystallographic structure, magnetic and physical properties of chemically B-site disordered lead-bismuth iron tungstate, (PFW)1−x(BFO)x (which can be written as Pb0.8Bi0.2Fe0.728W0.264O3 (0.8PFW–0.2BFO) for x = 0.2 and Pb0.7Bi0.3Fe0.762W0.231O3 (0.7PFW–0.3BFO) for x = 0.3) or PBFW), solid solutions through neutron diffraction (ND), magnetization, electron paramagnetic resonance, and Mössbauer spectroscopic studies. From the high temperature magnetic susceptibility measurement, it is observed that increase antiferromagnetic to paramagnetic phase transition around TN = 435 K (Pb0.8Bi0.2Fe0.728W0.264O3) and 504 K (Pb0.7Bi0.3Fe0.762W0.231O3), compared to pure PFW. Room-temperature crystallographic study confirms the formation of pseudo cubic structure with Pm-3m space group, whereas the magnetic structure is commensurate G-type antiferromagnetic ordering. The obtained temperature dependent structural parameters from the ND, evidenced to existence of strong spin-lattice coupling around TN for both the compounds. The discontinuity in the Pb/Bi–O bond length around ferroelectric transition (TC) indicates the presence of magnetoelectric coupling. Interestingly, microscopic 1:1 B-site ordered nanoclusters of PBFW exhibits the ferrimagnetic clusters along with antiferromagnetic order and it observed through the opening of M vs H hysteresis curves in the lower field regime. The EPR and Mössbauer spectroscopic studies well support the magnetic property and also reveal the Fe+3 state, and the weak signal in EPR and broader linewidth in the Mössbauer spectra exhibit the B-site disorderliness.

Magnetic phase diagram of the quantum spin chain compound SrCo2V2O8: a single-crystal neutron diffraction study

We explore magnetic order in the quantum spin chain compound SrCo2V2O8 up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector = (0, 0, 1) develops below TN ≃ 5.0 K. Applying an external magnetic field (H∥c axis) destabilizes this C-AFM order, leading to an order-disorder transition between TN and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by = (0, 0, 1 ± δl). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo2V2O8 appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo2V2O8, our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.

Frustration of square cupola in Sr(TiO) Cu4(PO4)4

The structural and magnetic properties of the square-cupola antiferromagnet Sr(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$ are investigated via x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance experiments on polycrystalline samples, as well as density-functional band-structure calculations. The temperature-dependent unit cell volume could be described well using the Debye approximation with the Debye temperature of $\theta_{\rm D} \simeq $ 550~K. Magnetic response reveals a pronounced two-dimensionality with a magnetic long-range-order below $T_{\rm N} \simeq 6.2$~K. High-field magnetization exhibits a kink at $1/3$ of the saturation magnetization. Asymmetric $^{31}$P NMR spectra clearly suggest strong in-plane anisotropy in the magnetic susceptibility, as anticipated from the crystal structure. From the $^{31}$P NMR shift vs bulk susceptibility plot, the isotropic and axial parts of the hyperfine coupling between $^{31}$P nuclei and the Cu$^{2+}$ spins are calculated to be $A_{\rm hf}^{\rm iso} \simeq 6539$ and $A_{\rm hf}^{\rm ax} \simeq 952$~Oe/$\mu_{\rm B}$, respectively. The low-temperature and low-field $^{31}$P NMR spectra indicate a commensurate antiferromagnetic ordering. Frustrated nature of the compound is inferred from the temperature-dependent $^{31}$P NMR spin-lattice relaxation rate and confirmed by our microscopic analysis that reveals strong frustration of the square cupola by next-nearest-neighbor exchange couplings.

Domain imaging across the magneto-structural phase transitions in Fe1+yTe

The investigation of the magnetic phase transitions in the parent compounds of Fe-based superconductors is regarded essential for an understanding of the pairing mechanism in the related superconducting compounds. Even though the chemical and electronic properties of these materials are often strongly inhomogeneous on a nanometer length scale, studies of the magnetic phase transitions using spatially resolved experimental techniques are still scarce. Here, we present a real space spin-resolved scanning tunneling microscopy investigation of the surface of Fe1+yTe single crystals with different excess Fe content, y, which are continuously driven through the magnetic phase transition. For Fe1.08Te, the transition into the low-temperature monoclinic phase is accompanied by the appearance of a chevron-patterned structural ordering due to the four 90° rotational domains of the monoclinic lattice. Each of the structural domains contains locally commensurate nanoscale diagonal double stripe antiferromagnetic spin order domains with π-phase slips accross domain boundaries. In the low-temperature phase of Fe1.12Te, on the other hand, the chevron pattern gets rather narrow and less well-defined, and an additional 90° rotated component of the spin-order with local plaquette order emerges. The simultaneous imaging of spin and structural order we show here gives valuable insights into the nature of the magneto-structural domains of Fe1+yTe near the tricritical point, which presumably add to the understanding of the mechanism of superconductivity in the related Fe1+yTexSe1−x material.

Ordered aeschynite-type polar magnets RFeWO6 ( R=Dy , Eu, Tb, and Y): A new family of type-II multiferroics

We report the discovery of magnetoelectric multiferroicity in a family of oxides, $R\mathrm{FeW}{\mathrm{O}}_{6}$ ($R=\mathrm{Dy}$, Eu, Tb, and Y) that crystallize in a polar aeschynite-type structure ($Pna{2}_{1}$) with an ordered arrangement of $\mathrm{F}{\mathrm{e}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions. Magnetization and analysis of neutron-diffraction data of $\mathrm{DyFeW}{\mathrm{O}}_{6}$ reveal a commensurate noncollinear antiferromagnetic ordering of $\mathrm{F}{\mathrm{e}}^{3+}$ spins (${T}_{\mathrm{N}}^{\mathrm{Fe}}\ensuremath{\sim}18\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), which induce Dy spins to order at the same temperature. A sudden change in electric polarization $(\mathrm{\ensuremath{\Delta}}P)$ appears in all the compounds at ${T}_{\mathrm{N}}^{\mathrm{Fe}}=15--18\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The electric polarization is sensitive to an applied magnetic field, and the coupling between different magnetic $R$-ion and Fe-ion moments suppresses the polarization to a different extent. Although the measured polarization in polycrystalline $\mathrm{DyFeW}{\mathrm{O}}_{6}$ at 3.5 K is about $3\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$, a simple calculation of the ionic contribution to polarization with formal charges is $75\phantom{\rule{0.16em}{0ex}}560\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ and it is of the form $\mathbf{P}=({p}_{x},0,{p}_{z})$ where, ${p}_{x}$ comes from magnetic ordering and ${p}_{z}$ is associated with the polar structure in the paramagnetic state. These findings open up an avenue to explore further new polar magnets with rare-earth/transition-metal ions in the ordered aeschynite-type structure.

Fermi Surface Reconstruction and Drop in the Hall Number due to Spiral Antiferromagnetism in High-T_{c} Cuprates.

We show that a Fermi surface reconstruction due to spiral antiferromagnetic order may explain the rapid change in the Hall number as recently observed near optimal doping in cuprate superconductors [Badoux etal., Nature (London) 531, 210 (2016)]. The single-particle spectral function in the spiral state exhibits hole pockets which look like Fermi arcs due to a strong momentum dependence of the spectral weight. Adding charge-density wave order further reduces the Fermi surface to a single electron pocket. We propose quantum oscillation measurements to distinguish between commensurate and spiral antiferromagnetic order. Similar results apply to certain metals in which topological order replaces antiferromagnetic order.

Zeeman spectrum, magnetic neutron diffraction pattern, and Dirac multipoles for a multiferroic materialCuB2O4

Zeeman spectra, dichroic signals, and neutron Bragg diffraction patterns generated by copper ions in magnetically ordered copper metaborate $(\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4})$ are investigated within a minimal model of Cu atomic states. A theory platform, common to understanding optical spectra and neutron diffraction patterns, affords the immediate benefit of a unified description of the experimental probes in terms of electronic multipoles. Results for dichroic signals illustrate a nontrivial use of a general, quantum mechanical theory of photon absorption couched in terms of Dirac multipoles that are magnetic and polar. Anapoles (Dirac dipoles) are predicted to generate Bragg spots in magnetic neutron diffraction that are not indexed by the motif of conventional (axial) magnetic-dipole moments. The minimal model of Cu states is informed by magnetic symmetry, derived from an established commensurate antiferromagnetic order, with a sparse number of parameters that comply with available empirical evidence.

Magnetic–crystallographic p,T‐phase diagram of Fe1.141Te: A high‐pressure neutron diffraction study

The crystal and magnetic structures of Fe1.141Te have been studied by neutron powder diffraction in the temperature range from 5 to 106 K and pressures in the range from ambient to ≈2.7 GPa. The p,T‐phase diagram contains three phases with monoclinic, orthorhombic, and tetragonal symmetry. The monoclinic phase was found to be stable for T ≲ 57 K and p < 0.4 GPa while the orthorhombic phase is stable for T ≲ 63 K and 0.4 ≲ p ≲ 2.16 GPa. The tetragonal phase is stable at high temperatures and becomes stable down to the lowest measured temperatures for p > 2.16 GPa. The monoclinic phase shows commensurate bicollinear antiferromagnetic order with propagation vector k = (½ 0 ½), while the orthorhombic phase is incommensurately antiferromagnetically ordered with propagation vector k = (½‐δ 0 ½). The δ−parameter increases linearly with the pressure for 0.4 ≲ p ≲ 2.16 GPa. Pressure‐induced ferromagnetic order was observed in the tetragonal phase for p ≳ 2.1 GPa and temperatures less than ∼68 K, depending on the pressure.

Multiferroicity Broken by Commensurate Magnetic Ordering in Terbium Orthomanganite.

TbMnO3 is an important multiferroic material with strong coupling between magnetic and ferroelectric orderings. Incommensurate magnetic ordering is suggested to be vital for this coupling in TbMnO3 , which can be modified by doping at the site of Tb and/or Mn. Our study shows that a self-doped solid solution Tb1-x Mny MnO3 (y≤x) can be formed with Mn doped into the site of Tb of TbMnO3 . When y is small Tb1-x Mny MnO3 shows both ferroelectric and incommensurate magnetic orders at low temperature, which is similar to TbMnO3 . However, if y is large enough, a commensurate antiferromagnetic ordering appears along with the incommensurate magnetic ordering to prevent the appearance of multiferroicity in Tb1-x Mny MnO3 . That is to say, the magnetoeletric coupling can be broken by the co-existence of a commensurate antiferromagnetic ordering. This finding may be useful to the study of TbMnO3 .

Magnetic order in the 2D Heavy-Fermion system CePt2In7 studied by μ+SR

The low-temperature microscopic magnetic properties of the quasi-2D heavy- fermion compound, CePt2In7 are investigated by using a positive muon-spin rotation and relaxation (μ+SR) technique. Clear evidence for the formation of a commensurate antiferromagnetic order below TN ≈ 5.40 K is presented. The magnetic order parameter is shown to fit well to a modified BCS gap-energy function in a strong-coupling scenario.

Synthesis, structure, and magnetic properties of (Tb(1-x)Mn(y))MnO(3-δ).

Two compounds (Tb(1-x)Mn(y))MnO(3-δ) (W1, x = 0.089, y = 0.063; W2, x = 0.122, y = 0.102) have been synthesized by solid-state method and characterized using neutron diffraction and magnetic measurements. They crystallize in space group Pnma at room temperature and Pna21 at low temperature. W1 shows a sinusoidal antiferromagnetic order, and W2 shows both sinusoidal and canted commensurate antiferromagnetic orders. The magnetic moments of the commensurate antiferromagnetic order for W2 are antiferromagnetically coupled along the a- and c-axes, and ferromagnetically coupled along the b-axis in the Pna21 setting. Strong ferromagnetic response is induced by doping more Mn into the Tb site of (Tb(1-x)Mn(y))MnO(3-δ).

Theory of Antiferromagnetic Order in High-Tc Oxides: An Approach Based on Ginzburg–Landau Expansion

The mean-field phase diagram of antiferromagnetic order in t-J model has been examined, using the free energy obtained by Ginzburg-Landau (GL) expansion. We extended the usual GL theory in two ways: First, we have included higher order terms with respect to the spatial derivative (or wave number) to incorporate the incommensurate antiferromagnetic order. Second, we have also included higher order terms with respect to the order parameter amplitude, in order to treat the first order phase transition between paramagnetic and antiferromagnetic phase, which appears at some doping rates. We found the possibility of tricritical point and critical endpoint in the magnetic phase diagram of the high-Tc oxides associated with the commensurate and incommensurate antiferromagnetic order. The possible effects of thermal fluctuations and randomness (spin glass) are also discussed qualitatively based on the GL free energy.

Spin gap and the nature of the 4d3magnetic ground state in the frustrated fcc antiferromagnet Ba2YRuO6

The geometrically frustrated double perovskite Ba2YRuO6 has magnetic 4d3 Ru5+ ions decorating an undistorted face-centered cubic (FCC) lattice. This material has been previously reported to exhibit commensurate long-range antiferromagnetic order below T_N = 36K, a factor f = 15 times lower than its Curie-Weiss temperature Theta_CW = -522 K, and purported short-range order to T* = 47K. We report new time-of-flight neutron spectroscopy of Ba2YRuO6 which shows the development of a 5 meV spin gap in the vicinity of the [100] magnetic ordering wavevector below T_N = 36K, with the transition to long-range order occurring at T* = 47K. We also report spin waves extending to 14 meV, a surprisingly small bandwidth in light of the large Theta_CW. We compare the spin gap and bandwidth to relevant neutron studies of the isostructural 4d1 material Ba2YMoO6,and discuss the results in the framework of relatively strong spin-orbit coupling expected in 4d magnetic systems.

Magnetic phase diagram of Sr3Fe2O7−δ

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe^{4+}-containing perovskite-related phase Sr_3Fe_2O_7-x as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully-oxygenated compound (x=0) exhibits a charge-disproportionation transition at T_D = 340 K, and an antiferromagnetic transition at T_N = 115 K. For temperatures T \leq T_D, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe^{4+} moments. The fully-deoxygenated compound (x=1) is highly insulating, and its Fe^{3+} moments exhibit commensurate antiferromagnetic order below T_N ~ 600 K. Compounds with intermediate x exhibit different order with lower T_N, likely as a consequence of frustrated exchange interactions between Fe^{3+} and Fe^{4+} sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.

Emergent phases of nodeless and nodal superconductivity separated by antiferromagnetic order in iron-based superconductor (Ca4Al2O6)Fe2(As1−xPx)2:75As- and31P-NMR studies

We report ${}^{31}$P- and ${}^{75}$As-NMR studies on (Ca${}_{4}$Al${}_{2}$O${}_{6}$)Fe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ with an isovalent substitution of P for As. We present the novel evolution of emergent phases that the nodeless superconductivity (SC) in $0\ensuremath{\le}x\ensuremath{\le}0.4$ and the nodal one around $x=1$ are intimately separated by the onset of a commensurate stripe-type antiferromagnetic (AFM) order in $0.5\ensuremath{\le}x\ensuremath{\le}0.95$, as an isovalent substitution of P for As decreases a pnictogen height ${h}_{Pn}$ measured from the Fe plane. It is demonstrated that the AFM order takes place under a condition of $1.32\phantom{\rule{4pt}{0ex}}\AA{}\ensuremath{\le}{h}_{Pn}\ensuremath{\le}1.42\phantom{\rule{4pt}{0ex}}\AA{}$, which is also the case for other Fe pnictides with the Fe${}^{2+}$ state in (Fe$Pn$)${}^{\ensuremath{-}}$ layers. This novel phase evolution with the variation in ${h}_{Pn}$ points to the importance of electron correlation for the emergence of SC as well as AFM order.

Pressure-induced ferromagnetism in antiferromagnetic Fe1.03Te

The magnetic properties of Fe1.03Te under hydrostatic pressure up to p ~ 5.7 GPa were investigated by means of muon spin rotation, dc magnetization, and neutron depolarization measurements. With increasing pressure the antiferromagnetic ordering temperature TN decreases continuously from 70 K at ambient pressure towards higher pressures. Surprisingly, the commensurate antiferromagnetic order of FeTe enters a region of incommensurate and dynamical magnetic order before at p ~ 1.7 GPa the system turns ferromagnetic. The ferromagnetic ordering temperature TC increases with increasing pressure.