Low temperature ferromagnetism in CaCu_3Ti_4O_(12)

The low-temperature magnetic order behaviors of perovskite oxide CaCu_3 Ti_4 O_(12)(CCTO) ceramics prepared by different methods are discussed.X-ray diffraction,scanning electron microscope,x-ray photoelectron spectroscopy,and direct current(DC) magnetization are used to characterize the structures,microscopic morphologies,valence states,and magnetic properties of the samples.The results show that the magnetic behaviors of CCTO ceramics are very sensitive to the preparation process.The quenched CCTO ceramic and CCTO powders grown in a molten salt crystal,which contain much more oxygen vacancies and Ti~(3+),show the coexistence of weak ferromagnetic order and antiferromagnetic order below the Neel temperature.It suggests that the bound magnetopolaron formed by oxygen vacancies and Ti~(3+) ion composite defects are responsible for the weak ferromagnetic order at low temperature.     

Magnetic correlations in a layered iridate, Na(2)IrO(3)

Analysis of published data gathered on a sample of Na(2)IrO(3), held deep inside the antiferromagnetic phase at 1.58?K, shows that iridium magnetic dipole moments, measured in resonant x-ray Bragg diffraction, lie in the a-c plane of the monoclinic crystal and enclose an angle ≈118°?with the c-axis. These findings, together with bulk measurements, are united in a plausible magnetic ground state for an iridium ion constructed from a Kramers doublet. A magnetic space group, derived from the chemical space group C2/m (unique axis b), possesses an anti-translation, to accommodate antiferromagnetic order, and an odd, two-fold axis of rotation symmetry on the b-axis, [Formula: see text], placing Ir magnetic dipoles perpendicular to the b-axis. Anapoles (toroidal dipoles) are predicted to be likewise confined to the a-c plane, and magnetic charges forbidden.     

Coexistence of antiferromagnetic and ferromagnetic orders at remanent state in epitaxial multiferroic Bi2FeCrO6 nanostructures

We report the local electronic and magnetic properties of Bi(2)FeCrO(6) nanostructures by element-specific polarized x-ray techniques. Sizable magnetic ordering in the remanent state is observed at room temperature for both Fe and Cr ions. The Bi(2)FeCrO(6) system offers an example of d(5)-d(3) magnetic superexchange interaction with a magnetic order for both Fe and Cr, which are both formally in the +?3 valence state. The results suggest a coexistence of antiferromagnetic and ferromagnetic superexchange interaction between Fe and Cr spins in the nanostructures at the remanent state and at room temperature.     

Structural and magnetic characterization of the complete delafossite solid solution (CuAlO?)?-x(CuCrO?)x

We have prepared the complete delafossite solid solution series between diamagnetic CuAlO(2) and the t(2g)(3)frustrated antiferromagnet CuCrO(2). The evolution with composition x in CuAl(1-x)Cr(x)O(2) of the crystal structure and magnetic properties has been studied and is reported here. The room-temperature unit cell parameters follow the Végard law and increase with x as expected. The μ(eff) is equal to the Cr(3+) spin-only S = 3/2 value throughout the entire solid solution. Θ(CW) is negative, indicating that the dominant interactions are antiferromagnetic, and its magnitude increases with Cr substitution. For dilute Cr compositions, the nearest-neighbor exchange coupling constant J(BB) was estimated by mean-field theory to be 3.0 meV. Despite the sizable Θ(CW), long-range antiferromagnetic order does not develop until x is almost 1, and is preceded by glassy behavior. The data presented here, and those on dilute Al substitution from Okuda et al, suggest that the reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as chemical disorder and dilution of the magnetic exchange. For all samples, the 5 K isothermal magnetization does not saturate in fields up to 5 T and minimal hysteresis is observed. The presence of antiferromagnetic interactions is clearly evident in the sub-Brillouin behavior with a reduced magnetization per Cr atom. An inspection of the scaled Curie plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO(2) above its Néel temperature, consistent with its magnetic frustration. Uncompensated short-range behavior is present in the Al-substituted samples and is likely a result of chemical disorder.     

Mesoscopic spin clusters, phase separation, and induced order in spin-gap magnets: A review

In many low-dimensional systems with antiferromagnetic interactions, a magnetic order of the Néel type is absent. These systems remain in a quantum-disordered (spin-liquid) state down to zero temperature. The disordered state appears to be stable to weak perturbations when magnetic excitations are separated from the ground state by an energy gap. The stability of the spin-liquid ground state is destroyed upon introduction of impurities or in a sufficiently strong magnetic field. This paper presents a review of the main results of the experimental investigations performed in order to reveal and identify mesoscopic spin clusters formed in the vicinity of impurity ions, to determine the spatial structure of an impurity-induced magnetic order in spin-gap systems, and to examine the low-frequency excitation spectra of field-induced antiferromagnetic phases.     

Elastic-stress relaxation in compacted nanocrystalline CuO

The magnetic properties of two types of nanocrystalline antiferromagnetic CuO samples, namely, dense nanoceramics and loose powders, were studied. For nanomaterials with smaller particles, the magnetic susceptibility χ was shown to increase with a decrease in temperature T < T_N. The increase in χ in both series of samples is related to the disordering of Cu²⁺ spins at the surfaces of nanoparticles. The magnetic properties of nanopowders characterize the properties of isolated nanoparticles. In a dense nanoceramic, the size effect is compensated for by the interaction between nanoparticles. The magnetic properties of nanoceramics are determined by elastic stresses induced by an external action. Elastic-stress relaxation results in the recovery of magnetic order and decreases the magnetic susceptibility.     

La$lt;sub$gt;0.4$lt;/sub$gt;Ca$lt;sub$gt;0.6$lt;/sub$gt;MnO$lt;sub$gt;3$lt;/sub$gt;中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La_0.4Ca_0.6MnO₃ 中电荷有序反铁磁基态的调控作用. 磁性质的测量结果表明, 两种离子掺杂均能有效抑制原型样品中的长程电荷有序相, 但是Fe离子掺杂样品均具有反铁磁的基态, 而Cr掺杂样品中则出现了显著的铁磁性. 结合电输运测量结果显示, Cr掺杂引起的铁磁态同时具有金属性, 表明其中是电子双交换作用占主导. 对比两种掺杂离子的电子结构发现, Cr离子空的e_g电子轨道促进了电子双交换作用, 而Fe掺杂则只是引入了不同的自旋交换作用, 导致自旋无序. 关键词： 磁性氧化物 反铁磁     

Spin polarons in nickel sulphide

Using a random formulation of the Hubbard model developed by the authors, thermodynamic and electronic properties of antiferromagnetic nickel sulphide are considered. The transition mechanism is shown to be magnetic in origin and not due to phonons in the high temperature phase as has been accepted. The metallic-like properties of the magnetic phase originate from hole-like spin polarons for non-stoichiometric samples. We obtain good quantitative agreement assuming a leV Ni(3d) bandwidth and predict stoichiometric NiS to behave as an intrinsic semiconductor at low temperatures.     

Spiral terraces and spin frustration in layered antiferromagnetic Cr(001) films

We have successfully fabricated a novel type of high-density spiral terraces on Cr(001) films. The influence of nanoscale spiral terraces on layered antiferromagnetic ordering of Cr(001) films has been studied at room temperature by direct imaging of both topographic and magnetic structures using spin-polarized scanning tunneling spectroscopy. Spin frustration and asymmetric magnetic ordering due to dense spiral terraces are observed. Sizable modification of the layered antiferromagnetic order is found to be originating from the topological asymmetry as confirmed by the continuum micromagnetic simulation.     

Spin–phonon coupling in van der Waals antiferromagnet VOCl

We report magnetization and Raman spectroscopy study on single crystals of VOCl, a van der Waals antiferromagnetic material. Magnetization measurement confirms an antiferromagnetic transition at 79 K and a magnetic easy axis along crystallographic a direction. The temperature-dependent Raman spectrum reveals five peaks at 30 K. Below the Neel temperature TN, the Raman-active modes 247 cm~(-1) and 404 cm~(-1) remarkably deviate from the standard Boltzmann function,which is ascribed to the strong magnetoelastic coupling between spins and phonons. We further observe an anomaly in 383 cm~(-1) mode at around 150 K. This coincides with the broad maximum in VOCl's magnetic susceptibility, suggesting a development of short-ranged magnetic order at this temperature.     

Ground state of finite arrays of magnetic dots in the presence of an external magnetic field

The ground state of an array of magnetic particles (magnetic dots), which are ordered in a square 2D lattice and whose magnetic moment is perpendicular to the lattice plane, in the presence of an external magnetic field has been analyzed. Such a model is applicable for sufficiently small dots with perpendicular anisotropy that are in a single-domain state and for dots in a strongly inhomogeneous vortex state whose magnetic moment is determined by the vortex core. For the magnetic field perpendicular to the system plane, the entire set of the states has been analyzed from the chessboard antiferromagnetic order of magnetic moments in low fields to the saturated state of the system with the parallel orientations of the magnetic moments of all dots in strong fields. In the presence of the border, the destruction of the chessboard order first occurs at the edges of the system, then near the extended sections of the surface, and finally expands over the entire interior of the array. The critical field at which this simplest state is destroyed is much more weakly than the value characteristic of the ideal infinite system. In contrast to this scenario, the destruction of the saturated state with decreasing field always begins far from the borders. Despite such different behaviors, the magnetic structure in the intermediate range of fields that is obtained with both increasing and decreasing field for finite arrays strongly differs from that characteristic of the ideal infinite system. The role of simple stacking faults of the magnetic dot lattice (such as single vacancies or their clusters) in the remagnetization of the system has been analyzed. The presence of such faults is shown to give rise to the appearance of local destructions of the chessboard antiferromagnetic order at fields that are much weaker than those for an ideal lattice.     

High-frequency properties of KNiPO<Subscript>4</Subscript> in alternating magnetic and electric fields

The dependences of the antiferromagnetic resonance frequencies on the constant magnetic field H and constant electric field E are calculated for a KNiPO₄ crystal with spontaneous electric polarization and antiferromagnetic order. It is demonstrated that the KNiPO₄ crystal is characterized by an exchange-enhanced effect of the electric field E on the antiferromagnetic resonance frequencies. This effect is not revealed in the magnetoelectric materials studied earlier. It is established that oscillations of both magnetization and electric polarization exhibit resonance response at antiferromagnetic resonance frequencies. The expressions for these responses in alternating magnetic and electric fields are presented.     

Effect of Cu doping on the magnetic and electrical properties of n=2 Ruddlesden-Popper manganates

A series of polycrystalline Cu-doped n=2 Ruddlesden-Popper manganates La1.2Sr1.8CuzMn（2-x）O7 （x=0, 0.04, 0.13） were synthesized by the solid state reaction method. The effect of Cu doping on the magnetic and transport properties has been studied. It is found that Cu substitution for Mn greatly affects the magnetic and electrical properties of the parent phase La1.2Sr1.8Mn2O7. With the increase of Cu content, the system undergoes a transition from longrange ferromagnetic order to the spin glass state and further to an antiferromagnetic order. A little of Cu dopant can lead to the samples showing semiconductor or insulator behaviour in the whole observed temperature range while the parent phase has a metal-insulator transition. These samples show colossal magnetoresistance at low temperatures and the value of it decreases with increasing Cu content.     

Fragmented and single condensate ground states of spin-1 bose Gas

We show that the ground state of a spin-1 Bose gas with an antiferromagnetic interaction is a fragmented condensate in uniform magnetic fields. The number fluctuations in each spin component change rapidly from being enormous (order N) to exceedingly small (order 1) as the magnetization of the system increases. A fragmented condensate can be turned into a single condensate state by magnetic field gradients. The conditions for existence and method of detecting fragmented states are presented.     

Magnetic moment of iron atoms in Fe-Al body-centered cubic alloys as a function of the nearest environment

The Fe-Al systems in the concentration range from 29 to 44 at. % Al are investigated in terms of the density functional theory. It is shown that, in the system under consideration, there can exist three magnetic states with close energies. Two of these three magnetic states have collinear magnetic moments (the ferromagnetic and antiferromagnetic states), and the third is a spin-spiral state. In collinear magnetic structures, the local magnetic moments are determined by the nearest chemical environment and, in the antiferromagnetic state, the iron atoms surrounded by a large number of aluminum atoms in their environment have a negative magnetic moment. The results obtained substantiate the applicability of modified models of the Jaccarino-Walker type for the interpretation of the experimental data obtained for Fe-Al alloys. The results of the calculations also indicate a significant role of Stoner excitations in the formation of magnetic order in these alloys.     

Is the ground state of AC60 (A = Rb, Cs) antiferromagnetic?

Electron paramagnetic resonance measurements in the metallic and the low-temperature orthorhombic phases of RbC₆₀ and CsC₆₀ powder samples and a RbC₆₀ powder sample aligned by uniaxial pressure have been performed at 34 and 94 GHz. A detailed analysis of the low-temperature signal of all samples allows the assignment of several line components to paramagnetic defects, leaving one broad line component unaccounted for. Although the behavior of this component gives strong indications of magnetic correlations, we do not find typical characteristics of antiferromagnetic resonance that have been reported by other groups at higher fields. We ascribe this to a high degree of structural disorder in the samples which inhibits the development of a long-range antiferromagnetic order. Therefore, we interpret the signal behavior as being due to magnetically coupled spin clusters.     

Crystallographic and magnetic properties of van der Waals layered FePS_3 crystal

The crystallographic and magnetic properties are presented for van der Waals antiferromagnetic FePS_3. High-quality single crystals of millimeter size have been successfully synthesized through the chemical vapor transport method. The layered structure and cleavability of the compound are apparent, which are beneficial for a potential exploration of the interesting low dimensional magnetism, as well as for incorporation of FePS_3 into van der Waals heterostructures. For the sake of completeness, we have measured both direct current(dc) and alternating current(ac) magnetic susceptibility.The paramagnetic to antiferromagnetic transition occurs at approximately T_N 115 K. The effective moment is larger than the spin-only effective moment, suggesting that an orbital contribution to the total angular momentum of the Fe~(2+) could be present. The ac susceptibility is independent of frequency, which means that the spin freezing effect is excluded.Strong anisotropy of out-of-plane and in-plane susceptibility has been shown, demonstrating the Ising-type magnetic order in FePS_3 system.     

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ～ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ～3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.     

Creation of an antiferromagnetic exchange spring

We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models.