Paramagnetic Order Research Articles

Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle

Mn doped TiO2 nanoparticles are synthesized by sol–gel method. Incorporation of Mn shifts the diffraction peak of TiO2 to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO2 show peaks at g = 1.99 and 4.39, characteristic of Mn2+ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO2, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn3O4. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn2+ ions via oxygen vacancy (F+ center). Increase in Mn concentration increases the fraction of Mn3O4 phase and thereby increases the paramagnetic ordering.

Magnetic structure and glassiness in [formula omitted

This work explores the magnetic properties of Fe0.5Ni0.5PS3. The system shows pronounced hysteresis in the magnetic phase transition temperature as a function of the direction of the change in temperature. Field cooled/zero field cooled hysteresis is not pronounced. However, the transition temperature between antiferromagnetic and paramagnetic order occurs at approximately 97K on cooling, but at 138K on warming, whether the warming is after zero field or field cooling.This is indicative of magnetic glassiness, and made all the more unusual because all measurements exhibit a transition to a third magnetic phase existing at temperatures below ∼14K. The intermediate phase relaxes on a laboratory time scale of the order of 48min, into an antiferromagnetic state whose magnetic structure is, from neutron diffraction, indistinguishable from the low temperature state. This low temperature state shows magnetic ordering consistent with that observed in CoPS3 and NiPS3. Analysis of the neutron measurements shows that the direction of moments cannot be along the b-axis. It is also shown that the moments are unlikely to lie in the c⁎ direction. Therefore, we suggest that the moments lie along the a-axis.

Structural, magnetic and electric behavior of the new Ba2TiMoO6 material

We report synthesis and characterization of the structural, morphologic and ferroelectric behavior of the complex perovskite Ba2TiMoO6. Samples of Ba2TiMoO6 were synthesized through standard solid state reaction methods. Crystalline structure was studied by means of X-ray diffraction experiments and Rietveld-like analysis. Results reveal that material crystallizes in a tetragonal structure, space group P4/mnm (#123), with cell parameters a=3.8557Å and c=11.8678Å. The tolerance factor of perovskite was determined to be 1.04. Surface morphology was examined using Scanning Electron Microscopy, which shows the micrometric granular character of samples with 1.0–5.0μm mean grain size. Ferroelectric response of material was established from curves of polarization as a function of applied electric field. Our results reveal that Ba2TiMoO6 double perovskite evidences a ferroelectric hysteretic behavior at ambient temperature and paramagnetic ordering. © 2011 Elsevier Science. All rights reserved.

Spin density waves in a semiconductor superlattice in a tilted magnetic field

The ground state of a semiconductor superlattice (SL) placed in a tilted magnetic field is shown to exhibit a spin-density wave structure when the energy spectrum favors crossings between opposite-spin Landau minibands. The SL is modeled as an array of infinitely attractive quantum wells, whose single energy level is broadened into a miniband of width $\ensuremath{\Delta}$ when weak interwell tunneling is considered. In the presence of the Coulomb interaction, by tailoring the relationship between $\ensuremath{\Delta}$ and the cyclotron and Zeeman energies, the system transitions between paramagnetic, ferromagnetic, spin-density wave (SDW), and ferromagnetic-paramagnetic stripe ordering. These results are obtained by solving numerically a spin-density-wave gap equation derived at $T=0$ K in a self-consistent formalism. We find that for a given value of the difference between the Landau energy and the Zeeman splitting, the initial paramagnetic or ferromagnetic order becomes unstable with respect to the formation of a SDW for $\ensuremath{\Delta}$ within a certain range. At larger $\ensuremath{\Delta}$, the system exhibits alternate ferromagnetic-paramagnetic stripes. In the SDW regime, the fractional polarization is up to the order of several tens of percent.

Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe2O4

Neutron diffraction measurements on multiferroic LuFe${}_{2}$O${}_{4}$ show changes in the antiferromagnetic (AFM) structure characterized by wave vector $q=(\frac{1}{3}\frac{1}{3}\frac{1}{2})$ as a function of electric field cooling procedures. The increase of intensity from all magnetic domains and the decrease in the two-dimensional (2D) magnetic order observed below the N\'eel temperature are indicative of increased ferroelectric charge order (CO). The AFM order changes the dynamics of the CO state, and stabilizes it. It is determined that the increase in electric polarization observed at the magnetic ordering temperature is due to a transition from paramagnetic 2D charge order to AFM three-dimensional charge order.

Phase diagram of the one-dimensional Hubbard-Holstein model at quarter filling

We derive an effective Hamiltonian for the one-dimensional Hubbard-Holstein model, valid in a regime of both strong electron-electron (e-e) and electron-phonon (e-ph) interactions and in the nonadiabatic limit ($t/{\ensuremath{\omega}}_{0}\ensuremath{\le}1$), by using a nonperturbative approach. We obtain the phase diagram at quarter filling by employing a modified Lanczos method and studying various density-density correlations. The spin-spin AF (antiferromagnetic) interactions and nearest-neighbor repulsion, resulting from the e-e and the e-ph interactions, respectively, are the dominant terms (compared to hopping) and compete to determine the various correlated phases. As e-e interaction $(U/t)$ is increased, the system transits from an AF cluster to a correlated singlet phase through a discontinuous transition at all strong e-ph couplings $2\ensuremath{\le}g\ensuremath{\le}3$ considered. At higher values of $U/t$ and moderately strong e-ph interactions ($2\ensuremath{\le}g\ensuremath{\le}2.6$), the singlets break up to form an AF order and then to a paramagnetic order all in a single sublattice; whereas at larger values of $g$ ($>$$2.6$), the system jumps directly to the spin disordered charge-density-wave (CDW) phase.

Electron spin resonance (ESR) of Eu2+ in type-I clathrate Eu8Ga16Ge30

Abstract We report temperature dependent X-band ( ν ~ 9.5 GHz ) electron spin resonance (ESR) experiments in type-I clathrate Eu 8 Ga 16 Ge 30 . This system presents interesting thermoelectric properties in its paramagnetic state and orders ferromagnetically below T C = 35 K . Although the Eu 2 + ions sits on two inequivalent sites in the type-I clathrate structure, we found in the paramagnetic state a single Dysonian Eu 2 + ESR line with a nearly temperature independent g ~ 2 is observed, and its linewidth follows a Korringa-like behavior as a function of temperature. The microscopic details of the exchange coupling between the Eu 2 + local moments and the conduction-electrons (c-e) in this compound are investigated through the obtained Korringa rate ( Δ H / Δ T ~ 1 ( 1 ) Oe / K ) and g-shift ( Δ g ~ 0.01 ( 1 ) ) from ESR experiments combined with the magnetic susceptibility and specific heat data.

Exchange-Driven Collective Behavior in a 3D Array of Nanoparticles

A Monte Carlo simulation is performed in a cubic lattice of interacting identical Stoner-Woldfarth nanoparticles. The model system is a randomly-anisotropic Heisenberg spin system with a small anisotropy-to-exchange ratio D/J = 3.5. The dc susceptibility, chi(dc)(T), shows a Curie-Weiss-like transition at a temperature T-C/J approximate to 1.5, followed by a low-temperature glassy behavior manifested by cusps in both the zero-field-cooled and the field-cooled curves. The ac susceptibility, chi(ac) (T, omega), at various frequencies, w, shows that with decreasing temperature, a non-Arrhenius dispersive peak occurs at T-b(omega), succeeded by another dispersionless peak at T-g/J approximate to 1.20 in the in-phase part, chi'(T, omega), of chi (T, omega) while the out-of-phase part, chi '' (T, omega), shows only one peak. A dynamic scaling analysis shows that the system exhibits a critical slowing-down at T-g with a quite small exponent zv approximate to 1.65. However, no universal collapse is seen for the fully-scaled data of chi '' (T, omega). These observed behaviors are interpreted under the droplet-like hypothesis that the formation and development of exchange-induced correlated clusters drive ensembles of nanoparticles undergoing a transition from a paramagnetic order to a short-range order (SRO) at T-C, followed by a transition at T-g to the magnetic state in which a magnetic glassy order and a magnetic quasi-long-range order (QLRO) coexist. In addition, our simulation shows that the onset of the latter transition, which is peculiarly manifested by the dispersionless peak, occurs only for those ensembles possessing the anisotropy strength in the region 1.0 <= D/J <= 5.0.....

Interesting magnetic behavior from reduced titanium dioxide nanobelts

The magnetic behavior of titanium dioxide nanobelts has been investigated with and without Co doping. Room temperature ferromagnetism was observed when the Co-doped anatase TiO2 nanobelts were prepared via vacuum annealing of 2.5at.% Co-doped titanate nanobelts, while annealing them in air resulted in paramagnetic ordering. Interestingly, by vacuum annealing the undoped titanate nanobelts under the same conditions, superparamagnetic ordering was observed in the resulting anatase TiO2 nanobelts. The electron paramagnetic resonance of this latter sample shows a strong symmetrical signal at g=2.003 suggesting some sort of exchange interactions among the localized electrons’ spin moments from single electron trapped in oxygen vacancies.

Ordering mechanism and spin fluctuations in a geometrically frustrated heavy-fermion antiferromagnet on the Kagome-like lattice CePdAl: AAl27NMR study

$^{27}\mathrm{Al}$ NMR measurements have been performed to investigate the ordering mechanism and spin fluctuations in a geometrically frustrated heavy-fermion antiferromagnet on the Kagome-like lattice CePdAl. This compound shows a partially ordered state below ${T}_{\mathrm{N}}$ $(2.7\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, in which $1∕3$ of the Ce moments remain paramagnetic. Whether the remaining paramagnetic moments order at lower temperature or not has been controversial. Our NMR measurements show that the nuclear magnetic relaxation rate ${T\phantom{\rule{0.2em}{0ex}}}_{1}^{\ensuremath{-}1}$ is proportional to the temperature below $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and there is no other phase transition down to $30\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The simulation of the spin echo spectra shows that the observed spectra are compatible with the partially ordered state and an incommensurate $k$ vector along the $c$ axis down to the lowest temperature. These results demonstrate that the paramagnetic Ce moments in the partially ordered state are in a heavy-fermion state and consequently the partially ordered state is considered to be stable down to $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of ${T\phantom{\rule{0.2em}{0ex}}}_{1}^{\ensuremath{-}1}$ also shows that antiferromagnetic correlations develop from much higher temperatures above ${T}_{\mathrm{N}}$.

Magnetic ordering in relaxor ferroelectric (1 − x)Pb(Fe2/3W1/3)O3–xPbTiO3 single crystals

Single crystals of the perovskite solid solution (1 − x)Pb(Fe2/3W1/3)O3–xPbTiO3, with x = 0, 0.07, 0.27, and 0.75, have been synthesized by the high-temperature solution growth using PbO as flux and characterized by x-ray diffraction and dielectric and magnetic measurements. The crystal structure at room temperature changes from a pseudocubic to a tetragonal phase with the PbTiO3 (PT) content increasing to x ⩾ 0.27. As the amount of PT increases, the relaxor ferroelectric behavior of Pb(Fe2/3W1/3)O3 (PFW) is transformed toward a normal ferroelectric state with sharp and nondispersive peaks of dielectric permittivity at TC. Two types of magnetic orderings are observed on the temperature dependence of the magnetization in the crystals with x ⩽ 0.27. This behavior is explained based on the relationships among the magnetic ordering, perovskite structure, composition, and relaxor ferroelectric properties. Furthermore, the macroscopic magnetization of the system was measured under the application of a magnetic field, which demonstrates different magnetic behavior associated with the weakly ferromagnetic, antiferromagnetic, and paramagnetic ordering in the temperature range of 2 to 390 K. Interestingly, the low-temperature ferromagnetism is enhanced by the addition of ferroelectric PT up to x = 0.27.

Order–disorder transitions of t2g‐orbitals of V3+ ions and incommensurate structural deformations in the metallic spinel CuV2S4

AbstractStructural transitions of the spinel CuV2S4 have been studied by means of X‐ray diffraction using synchrotron radiation. The temperature dependence of reflections forbidden in a cubic spinel structure of Fd3m suggests that there are order–disorder transitions of 3d2 (t2g) orbitals of V3+ ions at about 75 and 210 K. An incommensurate superlattice reflection of (2+3/4, 2, 2+3/4) shows the structural phase transition occurring at about 90 K, in addition to the superlattice structural modulation at about 30–35 and 55 K. A reduced wavevector q = (3/4–δ, 0, 3/4–δ) shows the temperature dependence of the δ value from 0.015 to –0.025. A lock‐in of δ = 0.0 and a crossover of the δ value occur in the region of 40 to 50 K. The incommensurate structural modulation along [110], [011], and [101] directions appearing below about 90 K affects a paramagnetic spin order of V3+ ions at about 30–35 and 55 K, in addition to the 90 K structural transition. The paramagnetic behavior is interpreted mainly by the structural deformation and the (dxy , dyz , dzx ) orbitals in the VS6 octahedral chains sited along [110], [011], and [101] directions. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Light-induced ESR in polycrystalline and thin films of [formula omitted

We studied on a chromium oxide Cr 2 O 3 to investigate near-infrared (NIR) light-induced magnetism function at room temperature. A X-band electron-spin resonance (ESR) spectrometer and a SQUID magnetometer were employed to clarify magnetic behavior in the Cr 2 O 3 under the illumination of NIR light ( λ = 1064 nm ). The light-induced ESR signal appears above 250 K and is remarkably enhanced around room temperature. Contrary, with visible green light ( λ = 532 nm ), the ESR profile does not show any change. The light-induced ESR is strongly dependent on the irradiation light wavelength. We conclude the light-induced ESR below T N may come from light-excited carriers associating with a reentrance from antiferromagnetic to paramagnetic spin order. Thin films were successfully prepared by a RF magnetron sputtering and a sol–gel method. The T N of the thin film is increased from 308 to about 400 K. We discuss intriguing properties on light-induced effect for both polycrystalline and thin films and state the chromate is a candidature to be applied to NIR detection industrial devices.

The Overlap of First- and Second-Order Phase Transitions and Related Magnetic Entropy Changes in Ni$_2 + x$Mn$_1 - x$Ga Heusler Alloys

From application point of view, offstoichiometric Ni2MnGa is of interest and therefore many studies on offstoichiometric Ni2MnGa have been reported. Special focus has been paid on Ni rich Ni2MnGa, where substitution of Ni for Mn results in an increase in TM and decrease of TC, resulting in an overlap of the two transitions for certain Mn concentration [1]. As mentioned earlier, the martensitic transition is a first order transition where as the transition at TC from ferromagnetic order to a paramagnetic order is a second order transition. It is found that the overlap of TM and TC results in a first order magnetostructural transition from the paramagnetic cubic phase to ferromagnetic martensitic phase. Recent investigations show that this first order magnetostructural transition, due to the overlap of TM and TC, results in a fairly large magnetic entropy change, SM, in offstoichiometric Ni2MnGa. In this paper we present a study on the gradual overlap of the first order and second order phase transitions and the related magnetic entropy changes of polycrystalline Ni2+xMn1-xGa (x = 0.16, 0.17, 0.18, 0.19, 0.20). Ingots of approximately 5gm of polycrystalline Ni2+xMn1-xGa (x = 0.17, 0.18, 0.19, 0.20) were fabricated by conventional arc melting in an argon atmosphere using Ni, Mn, and Ga or 4N purity. The samples were annealed in vacuum for 144 hours at 800°C, and slowly cooled down to room temperature.

Competing magnetic phases in La0.7Sr0.3MnO3 as deduced from Mn site hyperfine parameters

Time differential perturbed angular correlation measurements using the181Ta probein La0.7Sr0.3Mn0.995Hf0.005O3 reveal the presence of two distinct hyperfine components, identified with probeatoms occupying Mn sites which are rich and deficient in hole concentration. TheMn4+ richzones exhibit ferromagnetic ordering at all temperatures below 360 K, the bulk Curie temperature. In thecase of Mn4+ deficient zones, the paramagnetic order is seen to evolve into a canted antiferromagneticordering below 360 K, that becomes ferromagnetic below 250 K. Concomitantly, there is achange in the fractions below 250 K. The implications of these results are discussed in termsof electronic phase separation.

ELECTRONIC STRUCTURE AND MAGNETISM OF Mn1-xPdx ALLOYS

An ab initio linearized augmented plane-wave method has been employed to calculate the electronic and magnetic structures of both face-centered-cubic and body-centered-cubic Mn 1-x Pd x alloys with x=0, 0.25, 0.5, 0.75 and 1, respectively. The calculated result for fcc Mn 1-x Pd x alloys at different concentrations shows that fcc Mn 1-x Pd x alloys with x=0.5, 0.75 is ferromagnetic ordering. With low Pd concentration and pure Mn , it presents ferrimagnetic and antiferromagnetic ordering, respectively. As expected, pure Pd is paramagnetic ordering. For bcc Mn 1-x Pd x alloys with x=0, 0.5 and 0.75, the calculated result indicates that these alloys are ferromagnetic. For the Mn -rich alloy and pure Pd , however, it presents ferrimagnetic and paramagnetic ordering, respectively.

Antiferromagnetism and Atomic Disorder in Fe2VSi

To expand our knowledge of Fe 2 VSi, band calculations (LMTO-ASA) have been performed for paramagnetic (P), ferromagnetic (F) and antiferromagnetic (AF) orderings. The total-energy results show the...

Magnetisation studies on TmMn 6Sn 5.85Ga 0.15 single crystal

Single crystals of TmMn 6Sn 5.85Ga 0.15 were obtained using a flux method and studied by means of X-ray diffraction and magnetisation measurements as a function of temperature (10–500 K) and magnetic fields (0–9 Tesla) applied both parallel and perpendicular to the main crystallographic directions. The compound orders ferrimagnetically below 44 K then undergoes a ferri to antiferromagnetic transition on heating followed by a new transition to paramagnetic order at 350 K. In the 50–350 K temperature range, metamagnetic behaviour may be observed.

Spin fluctuation in single-crystalline terbium probed by temperature-dependent magnetic EXAFS

Magnetic extended x-ray absorption fine structure (MEXAFS) is a helicity-dependent counterpart of the well-established EXAFS technique. By means of MEXAFS it is possible not only to analyze the local magnetic structure but also to learn about magnetic fluctuations. Here temperature-dependent MEXAFS was employed to study the spin fluctuation of a terbium single crystal in a wide temperature range from 10 to 250 K covering two magnetic transitions. As compared with EXAFS, the MEXAFS signal intensity decreases more dramatically with increasing temperature, especially when spin orientation changes from ferromagnetic order to helical structure and paramagnetic order. The strong temperature dependence of MEXAFS is related to the atomic thermal vibrations (probed by EXAFS) and to the reduction of the magnetization at increasing temperatures. To analyze the individual scattering contributions to the MEXAFS and the EXAFS, ab initio calculations have been performed. The strongly enhanced magnetic multiple scattering was observed at low temperature and interpreted on the basis of the exchange interaction model.

The case for phase separation in URu2Si2

Motivated by experiment, we review the case for phase inhomogeneity in URu2Si2.In this scenario, the paramagnetic hidden order phase coexists with smalldistinct domains of antiferromagnetism whose volume fraction increases withpressure. The implications for the nature of the hidden order are discussed.