Short-range Magnetic Order Research Articles

Short-Range and Long-Range Order in AFM–FM Exchange Coupled Compound LiCu2(VO4)(OH)2

The presence of both strong interchain and intrachain interactions of an antiferromagnetic as well as an ferromagnetic nature may lead to the appearance of a gap in the magnetic excitation spectrum of LiCu2(VO4)(OH)2 as evidenced by the hump in magnetic susceptibility χ at T* ≈ 30 K. The temperature range of short-range magnetic order is terminated by the onset of long-range magnetic order at TN = 10 K, which is triggered by substantial interchain exchange interactions. This observation is corroborated by the specific heat, Cp, singularity, electron spin resonance, and nuclear magnetic resonance measurements. The latter reveals a broad distribution of the resonance fields ascribed to the formation of a helix magnetic structure. First-principles calculations allow estimations of both intrachain and interchain exchange interaction parameters, suggesting the implementation of a strongly coupled scenario with competing inter- and intrachain interactions.

µSR Study of the Dynamics of Internal Magnetic Correlations in Tb(Bi)MnO3 Multiferroic in Magnetically Ordered and Paramagnetic States

The dynamics of internal magnetic correlations in Tb0.95Bi0.05MnO3 multiferroic in the temperature range of 10–290 K has been studied. Separation into two phases with different relaxations of the polarization of muons in the basic matrix of the crystal and the phase separation regions has been detected for the first time both in a magnetically disordered state at T < TN = 40 K and in a paramagnetic state in a transverse magnetic field of 290 G in a temperature range of 80–150 K. A muon ferromagnetic complex (Mn3+-Mu-Mn4+), where the hyperfine interaction in a muonium depolarizes a muon in a time less than 10−8 s, is formed at T < 40 K in phase separation regions containing pairs of Mn3+ and Mn4+ ions, as well as electrons that recharge them. In the matrix of the original crystal containing only Mn3+ ions, a muonium is formed with a broken hyperfine bond. In this case, muons are depolarized at a high rate because of their interactions with the local magnetic fields of a cycloid. At temperatures of 80–150 K, one phase in the phase separation regions constitutes approximately 50% and is characterized by long relaxation times about 10 µs (described by the Gaussian relaxation function). The other phase is formed by Mn3+-Mn3+ correlations in the short-range magnetic order regions in the matrix of the original crystal, which are weakly sensitive to a magnetic field of 290 G.

Physics of integer-spin antiferromagnetic chains: Haldane gaps and edge states

The antiferromagnetic Heisenberg spin chain with integer spin has short-range magnetic order and an excitation energy gap above the ground state. This so-called Haldane gap is proportional to the exchange coupling $J$ of the Heisenberg chain. We discuss recent results about the spin dependence of the Haldane gap and conjecture an analytical formula valid asymptotically for large spin values. We next study the robustness of the edge states of the spin one chain by studying by the DMRG algorithm a spin one ladder. We show that the peculiar hidden topological order of the spin one chain disappears smoothly by increasing the ladder rung coupling without any intervening phase transition. This is evidence for the fragile character of the topological order of the spin one chain.

The Model of Decomposition of a Fe–Cu Alloy with Concentration-Depending Interatomic Interactions

The thermodynamics and kinetics of decomposition of a Fe–Cu alloy are investigated in the context of a simple ab initio parameterization model taking into account the concentration dependence of the magnetic contribution to the free energy. It is shown that taking into account a difference between the short-range and long-range magnetic orders near the Curie temperature is very important for calculating the solubility of copper in iron. The solubility of copper in bcc iron and the stability limit of a homogeneous state (physical spinodal) are evaluated via the kinetic Monte Carlo method. The impact of structural defects (dislocations and grain boundaries) on these curves, as well as on the kinetic time–temperature—transformation (TTT) diagram, is also discussed.

Slow Relaxation of Magnetic Susceptibility in Polycrystalline GdBaCo2O5.53 above the Curie Point and the Effect of High Hydrostatic Pressures

A slowly decaying anomaly of magnetic susceptibility (characteristic time τ ~ 103 s) is revealed in studying the ferromagnetic–paramagnetic phase transition in polycrystalline cobaltite GdBaCo2O5.53 that occurs due to the remaining short-range magnetic order at temperatures above the Curie point (TC = 275 K). High hydrostatic pressures (up to 1.48 GPa) are found to have a weak effect on TC with baric coefficient dTC/dP = 2.1 K/GPa.

Magnetic field driven critical behavior in bulk Gd

Gadolinium (Gd) has been proven to be such a complex magnetic system that its static universality class varies as a function of the reduced temperature (|ɛ| = |T−TC|/TC). Though many works have been done, physical mechanisms of this phenomenon have not been fully elucidated. In this work, we analyzed the static magnetization data, M(H), around the ferromagnetic-paramagnetic phase transition temperature (TC) based on Banerjee's criteria, the modified Arrott plot and Kouvel–Fisher techniques, the critical isotherm analysis, and the scaling hypothesis to reconsider the phase transition, magnetic order, and isotropic and uniaxial behaviors of a polycrystalline Gd bulk. The M(H) data analyses with H variations up to 140 kOe indicated that Gd still keeps the nature of a second-order phase transition and short-range magnetic order. In the critical region of |ɛ| = 3.4 × 10−3–6.9 × 10−2, we observed the critical-behavior transition from a mixed isotropic and uniaxial ferromagnet for H = 3–10 kOe through an isotropic 3D Heisenberg ferromagnet for H = 10–100 kOe to a uniaxial 3D Ising ferromagnet for H = 100–140 kOe. The results demonstrate the possibility of using high fields to identify the universality class of Gd. The fluctuations and preferred reorientation of spins in the hexagonal close-packed lattice at temperatures T &amp;gt; 230 K are thought to cause these interesting phenomena.

Dimensional crossover in a layered ferromagnet detected by spin correlation driven distortions

Magneto-elastic distortions are commonly detected across magnetic long-range ordering (LRO) transitions. In principle, they are also induced by the magnetic short-range ordering (SRO) that precedes a LRO transition, which contains information about short-range correlations and energetics that are essential for understanding how LRO is established. However these distortions are difficult to resolve because the associated atomic displacements are exceedingly small and do not break symmetry. Here we demonstrate high-multipole nonlinear optical polarimetry as a sensitive and mode selective probe of SRO induced distortions using CrSiTe3 as a testbed. This compound is composed of weakly bonded sheets of nearly isotropic ferromagnetically interacting spins that, in the Heisenberg limit, would individually be impeded from LRO by the Mermin-Wagner theorem. Our results show that CrSiTe3 evades this law via a two-step crossover from two- to three-dimensional magnetic SRO, manifested through two successive and previously undetected totally symmetric distortions above its Curie temperature.

Phase diagram of the Kondo model on the zigzag ladder

The effect of next-nearest-neighbor hopping $t_{2}$ on the ground-state phase diagram of the one-dimensional Kondo lattice is studied with density-matrix renormalization-group techniques and by comparing with the phase diagram of the classical-spin variant of the same model. For a finite $t_{2}$, i.e., for a zigzag-ladder geometry, indirect antiferromagnetic interactions between the localized spins are geometrically frustrated. We demonstrate that $t_{2}$ at the same time triggers several magnetic phases which are absent in the model with nearest-neighbor hopping only. For strong $J$, we find a transition from antiferromagnetic to incommensurate magnetic short-range order, which can be understood entirely in the classical-spin picture. For weaker $J$, a spin-dimerized phase emerges, which spontaneously breaks the discrete translation symmetry. The phase is not accessible to perturbative means but is explained, on a qualitative level, by the classical-spin model as well. Spin dimerization alleviates magnetic frustration and is interpreted as a key to understand the emergence of quasi-long-range spiral magnetic order which is found at weaker couplings. The phase diagram at weak $J$, with gapless quasi-long-range order on top of the two-fold degenerate spin-dimerized ground state, competing with a nondegenerate phase with gapped spin (and charge) excitations, is unconventional and eludes an effective low-energy spin-only theory.

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A\u2009=\u2009In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn2Mo3O8 have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li2AMo3O8 (A = In or Sc) is such a compound where spin-1/2 Mo3O13 clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the A site. Li2InMo3O8 undergoes conventional 120° long-range magnetic order below TN = 12 K whereas isomorphic Li2ScMo3O8 exhibits no long-range magnetic order down to 0.5 K. Here, we report exotic magnetisms in Li2InMo3O8 and Li2ScMo3O8 investigated by muon spin rotation (μSR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li2InMo3O8 and Li2ScMo3O8 show completely different behaviors observed in both μSR and INS measurements, representing their different ground states. Li2InMo3O8 exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the 120° spin structure. In contrast, Li2ScMo3O8 undergoes short-range magnetic order below 4 K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

Magnetic Ground State Crossover in a Series of Glaserite Systems with Triangular Magnetic Lattices.

The magnetic properties are reported for three members of the glaserite series of compounds, Na2BaM(VO4)2, M = Mn, Mn0.6Co0.4, and Co. Large single crystals are grown using a high-temperature hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M2+O6 octahedra are interconnected with nonmagnetic (VO4)3- groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group P3̅ m1); however, at lower temperatures both Na2BaMn(VO4)2 and Na2BaMn0.6Co0.4(VO4)2 undergo a structural transition to lower symmetry (monoclinic, C2/ c). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na2BaMn0.6Co0.4(VO4)2 does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na2BaMn(VO4)2 and Na2BaCo(VO4)2 were determined using neutron powder diffraction. At zero magnetic field, Na2BaMn(VO4)2 possesses an antiferromagnetic structure with the moments ordered in a Néel-type arrangement and aligned along the C4 axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na2BaCo(VO4)2 represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the c-axis direction. The relationships between these structures and magnetic properties are discussed.

Role of external magnetic fields in determining the thermodynamic properties of iron carbides in steel

The experimental results indicated that a 12 T magnetic field effectively promoted the precipitation of metastable χ-Fe5C2 iron carbides in low temperature tempered steel. A straightforward approach was proposed to analyze the thermodynamic mechanisms of iron carbides considering the combined effects of self-magnetism and external-field-induced magnetism. Our calculations revealed that the self-magnetic structure characteristics originated from the trigonal prism, which was composed of a lattice of six iron atoms surrounding a carbon atom positioned in the center of the framework. The external-field-induced magnetic exchange coupling increased the long and short-range magnetic order, which was inherently connected with changes in the Curie temperature, thereby altering the peak of magnetic entropy and enthalpy. The external-field-induced magnetism determined magnetic Gibbs free energy change, and further influenced on the precipitation stability of the iron carbides, which agreed well with the experimental results. These findings effectively illuminate the key magnetism problem for the Tokamak of the well-known ITER project for fusion energy and other Fe-based alloys and compounds in extreme conditions.

Модель распада сплава Fe-Cu с концентрационно-зависящими межатомными взаимодействиями

AbstractThe thermodynamics and kinetics of decomposition of a Fe–Cu alloy are investigated in the context of a simple ab initio parameterization model taking into account the concentration dependence of the magnetic contribution to the free energy. It is shown that taking into account a difference between the short-range and long-range magnetic orders near the Curie temperature is very important for calculating the solubility of copper in iron. The solubility of copper in bcc iron and the stability limit of a homogeneous state (physical spinodal) are evaluated via the kinetic Monte Carlo method. The impact of structural defects (dislocations and grain boundaries) on these curves, as well as on the kinetic time–temperature—transformation (TTT) diagram, is also discussed.

Quantum critical nature of the short-range magnetic order in Sr2−xLaxIrO4

We have investigated the nature of the short-range magnetic phase region of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{IrO}}_{4}$ ($0\ensuremath{\le}x\ensuremath{\le}0.14$). This so-called spin-orbit Mott insulator's predicted transition to superconductivity with electron doping has remained elusive. Instead, short-range magnetism prevails over an extensive doping range up to the La solubility limit. We find evidence for quantum critical-like fluctuations of the short-range magnetic phase, in both a scaling relation of the ac susceptibility $\ensuremath{\chi}{T}^{\ensuremath{\alpha}}$ and the magnetization $M{T}^{\ensuremath{\alpha}\ensuremath{-}1}$ with $\ensuremath{\alpha}=0.4--0.5$, as well as in the time-field scaling ($t/{H}^{\ensuremath{\gamma}}$) of the $\ensuremath{\mu}\mathrm{SR}$ asymmetry function. Our combined study of ac susceptibility and $\ensuremath{\mu}\mathrm{SR}$, on the one hand, uncovers a two-stage weak symmetry breaking to spin freezing or proximate spin glass and to a subsequent spin glass within the short-range magnetic phase. On the other hand, we demonstrate that magnetic correlations become more static above $x=0.06$ in spite of an apparent dilution of magnetic moments, indicative of enhanced inhomogeneities in a higher La-doping regime.

Transverse and longitudinal spin-fluctuations in INVAR Fe0.65Ni0.35

The presence of spin-fluctuations deep within the ordered state of ferromagnetic alloy has long been suspected but seldom directly observed. Inhomogeneities of one type or another have been cited as important in stabilizing behaviour—either longitudinal spin-fluctuations associated with the -state (local environment) model or transverse magnetisation arising from non-collinear spin structures. In this study we employ small-angle neutron scattering with neutron polarization analysis to distinguish between the two possibilities. Surprisingly we in fact find evidence of dominant but uncorrelated longitudinal spin-fluctuations coexisting with transverse magnetisation which exists in short-range clusters of size ~. This finding supports recent first principles calculations of in which both longitudinal spin-fluctuations and magnetic short-range order are identified as important ingredients in reproducing the equilibrium lattice.

Substantial magnetoelectric coupling in nanocrystalline-Fe2TeO6 at room temperature

Nanocrystalline Fe2TeO6 (FTO) is synthesized following the sol-gel process. The nanoaspect of FTO is confirmed from transmission electron microscopy, where the particles of sizes 10–40 nm are seen. Magnetic measurement on nano-FTO confirms the antiferromagnetic nature of particles having Nèel temperature TN = 167 K, which is very much lower than that of bulk (210 K). Polarization verses electric field loop measurement at room temperature (RT) gives the remanent polarization (Pr) value of 0.098 μC/cm2, confirming ferroelectricity in FTO. A nonmonotonous increase in the remanent polarization is noticed when an external magnetic field is applied on the sample. This is a clear indication of prevailing substantial magnetoelectric (ME) coupling in the sample at room temperature. The quantification of magnetoelectricity is done by directly measuring the ME voltage (V) in the presence of varying dc magnetic field (H), and the ME coefficients are obtained using a quadratic relation in H. The values so obtained for the first order (α/d), second order (β/d), and third order (γ/d) ME coefficients are ∼0.22 mV cm−1Oe−1, ∼−1.70 × 10−2 mV cm−1 Oe−2, and ∼0.72 × 10−6 mV cm−1 Oe−3, respectively. The presence of ferroelectricity and magnetoelectricity above the Nèel temperature is a very unusual phenomenon in this material. The presence of short range magnetic ordering which prevails even up to RT, much higher than that of long range magnetic ordering temperature, is suspected to be responsible for this.

Magnetic short-range order in Fe and Ni above the Curie temperature

Abstract We study magnetic short-range order in Fe and Ni above the Curie temperature using the nonlocal dynamic spin-fluctuation theory with self-consistent calculation of the self-energy. We calculate the spatial correlation function of Fe and Ni at nearest neighbours, next-nearest neighbours, etc., in a wide range of temperatures. Our results are in agreement with neutron scattering experiments but give a larger short-range order compared to classical spin models. We show that the spatial spin correlator decreases ten times at distances of about 6 A in Fe and 8 A in Ni at T / T C = 1.1 and short-range order remains well above the Curie point.

Evidence of spin-phonon coupling in CrSiTe3

We present the Raman scattering results on layered 2D semiconducting ferromagnetic compound CrSiTe$_3$. Four Raman active modes, predicted by symmetry, have been observed and assigned. The experimental results are supported by DFT calculations. The self-energies of the $A_g^3$ and the $E_g^3$ symmetry modes exhibit unconventional temperature evolution around 180 K. In addition, the doubly degenerate $E_g^3$ mode shows clear change of asymmetry in the same temperature region. The observed behavior is consistent with the presence of the previously reported short-range magnetic order and the strong spin-phonon coupling.

Probing short-range magnetic order in a geometrically frustrated magnet by means of the spin Seebeck effect

Competing magnetic interactions in geometrically frustrated magnets give rise to new forms of correlated matter, such as spin liquids and spin ices. Characterizing the magnetic structure of these states has been difficult due to the absence of long-range order. Here, we demonstrate that the spin Seebeck effect (SSE) is a sensitive probe of magnetic short-range order (SRO) in geometrically frustrated magnets. In low temperature (2 - 5 K) SSE measurements on a model frustrated magnet \mathrm{Gd_{3}Ga_{5}O_{12}}, we observe modulations in the spin current on top of a smooth background. By comparing to existing neutron diffraction data, we find that these modulations arise from field-induced magnetic ordering that is short-range in nature. The observed SRO is anisotropic with the direction of applied field, which is verified by theoretical calculation.

Magnetoelectric memory effect of paramagnetic nonpolar phase in Co4Nb2O9

A polarization memory effect of the paramagnetic nonpolar phase is observed in magnetoelectric antiferromagnet Co4Nb2O9, which has a magnetic field induced polarization in the antiferromagnetic phase. Following magnetoelectric poling in the polar phase, the nonpolar paramagnetic state retains a strong memory of polarization. When reentering the polar phase without applying the electric field, a polarization along the initial poling direction is recovered. If the applied magnetic field while staying in the paramagnetic phase is weakened, the memory effect is enhanced. With reversing this magnetic field, the polarization is also reversed when reentering the polar phase. The memory effect can be attributed to the ferroelectric seeds forming in the nonpolar phase due to short-range magnetic ordering as evidenced by magnetic entropy data.

Ising-like behaviour of mesoscopic magnetic chains

We demonstrate an experimental realization of the short range magnetic order in a one-dimensional Ising chain using fabricated mesospins. We confirm an excellent agreement between the experimental findings and simulations obtained using the original Ising model. In particular, we are able to show that the thermal behaviour of the mesoscopic Ising chain dominates over the thermal behaviour of the individual mesospins themselves, confirming that fabricated mesospins can be viewed as artificial magnetic atoms.