Spin-flop Field Research Articles

Synthesis, nuclear, and magnetic structures and magnetic properties of [Mn3(OH)2(SO4)2(H2O)2

[Mn(3)(OH)(2)(SO(4))(2)(H(2)O)(2)] and its deuterated analogue were synthesized by a hydrothermal technique and characterized by differential thermal analysis, thermogravimetric analysis, and IR spectroscopy. Its nuclear structure, determined by single-crystal X-ray analysis and Rietveld analysis of neutron powder-diffraction data, consists of a 3D network of chains of edge-sharing Mn(1)O(6), running along the c axis, connected by the apices of Mn(2)O(6) and SO(4) units. It is isostructural to the nickel analogue. Determination of the magnetic structure and measurements of magnetization and heat capacity indicate the coexistence of both magnetic long-range ordering (LRO) and short-range ordering (SRO) below a Néel temperature of 26 K, while the SRO is retained at higher temperatures. The moments of the two independent Mn atoms lie in the bc plane, and that of Mn(1) rotates continuously by 54 degrees towards the c axis on decreasing the temperature from 25 to 1.4 K. While the SRO may be associated with frustration of the moments within a Mn(3) trimer, the LRO is achieved by antiparallel alignment of the four symmetry-related trimers within the magnetic unit cell. A spin-flop field, measured by dc and ac magnetization on a SQUID, is observed at 15 kOe.

Size effects in the spin–flop transition of hematite nanoparticles

The spin–flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Both the Morin transition temperature (TM) and the spin-flop transition field (Hsf) have been found to increase for increasing particle size. However, the size dependence of Hsf(T=0) is faster than that observed for TM, indicating the occurrence of a modification of both the anisotropy and exchange fields induced by thermal treatments. By contrast, the increase of TM with increasing size should be mainly ascribed to the effect of microstructure on the sole anisotropy field.

Antiferromagnetic resonance studies in molecular magnetic materials

EPR spectroscopy has become an increasingly powerful tool to examine the spin states and dynamics of single-molecule magnets, but has not been exploited to probe bulk magnetically ordered phases of molecular magnets. In this article, we review the EPR spectra of antiferromagnets and canted antiferromagnets below T N with particular reference to our own studies on the canted antiferromagnet, p-NCC 6F 4CNSSN ( T N=36 K). The antiferromagnetic resonance experiment allows the saturation of the sublattice magnetisation to be probed. In addition, the exchange and anisotropy fields ( H e and H a), the spin-flop field ( H sf) and for canted antiferromagnets, the Dzyaloshinskii–Moriya field ( H DM), which gives rise to the spontaneous moment, can be determined.

Dzialoshinskii-Moriya interaction in the organic superconductorκ−(BEDT−TTF)2Cu[N(CN)2]Cl

The authors report ${}^{13}\mathrm{C}$ NMR and magnetization measurements on the magnetic state of oriented single crystals of the organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}.$ To understand these data a spin Hamiltonian based on the Pnma symmetry of the crystal is developed. When interpreted in the context of this Hamiltonian, the measurements provide a detailed picture of the spin ordering. It is found that the Dzialoshinskii-Moriya (DM) interaction is largely responsible for the details of the ordering above the spin-flop field. Of particular note, the interplane correlations are determined by the intraplane DM interactions and the direction of the applied field.

Magnetic study of CMR perovskite manganite (Pr,Ca)MnO 3− δ: spin-flop in antiferromagnetic state and anomalous microwave response with the charge ordering

The magnetization and the microwave response of (Pr,Ca)MnO 3− δ were measured in the temperature range from 4.2 to 270 K under the magnetic field up to 14 T. A small magnetization jump due to the spin-flop was newly observed below T N. Temperature dependence of the spin-flop field was analyzed with the two-sublattice model. The antiferromagnetic exchange field, H E=18.5 T was obtained. The microwave response at 34 GHz showed large anomalies with charge ordering, which may be related to not only resistive change but also dielectric change due to charge redistribution.

Antiferromagnetic resonance inλ−(BEDT−TSF)2FeCl4

From microwave cavity transmission measurements at 23-110 GHz on λ-(BEDT-TSF) 2 FeCl 4 , we have observed an antiferromagnetic resonance (AFMR) in the antiferromagnetic insulating state below the metal-to-insulator transition temperature of T M I =8.3 K. The frequency-field dependence is fundamentally reproduced with a two-sublattice model for a biaxial anisotropy. The easy axis is confirmed to be the direction tilted by 30° from c to b*, and the hard axis is found to be close to a*. The spin-flop field is identified to be 1.3 T through the frequency-field dependence for the easy axis. The AFMR is interpreted to be caused by spin waves excited on the π spins, exclusively due to the large π-π exchange interaction estimated by theoretical calculations. The exchange field and anisotropy fields for π spin systems at 1.2 K are evaluated to be H E ∼330 T, H 1 A∼ 1.5×10 - 3 T for the intermediate axis and H 2 A∼5.0×10 - 3 T for the hard axis, respectively.

Elastically induced mechanism of magnetization reversal in orthoferrites

In weak ferromagnets-orthoferrites in magnetic fields, which are considerably less than the spin-flop field, a new mechanism of magnetization reversal has been experimentally revealed. This is shown to be accompanied by the nonsteady motion of the domain wall whose velocities exceed the limiting velocity.

Canted Antiferromagnetism in an Organo-modified Layered Nickel Phyllosilicate

The hydrothermal reaction of 3-aminopropyltriethoxysilane (APTES) and nickel acetate tetrahydrate at 170 °C results in a layered nickel silicate, Ni3Si(C3H6NH3)F0.65O1.9(OH)4.45(CH3COO)1.1·xH2O, x = 0.38, with an interlayer spacing of 21 Å. The molecular structure is deduced from powder X-ray diffraction, transmission electron microscopy, X-ray absorption and IR spectroscopies, and a structure is proposed. The structure consists of edge-sharing Ni(O,OH)6 layers separated by aminopropyl silicate and acetate. The magnetic behavior is described by layers of ferromagnetically coupled nickel ions (C = 1.28 emu·K·mol-1 and θ = +35 K) coupled antiferromagnetically at a Néel temperature of 21 K. A small spontaneous magnetization is observed due to canting of the sublattice magnetizations amounting to 0.7°. Isothermal magnetization measurements at 2 K on oriented samples reveal that the easy axis is perpendicular to the layers and a spin-flop field is observed at 20 kOe.

Impurity-induced antiferromagnetic ordering in the spin gap systemTlCuCl3

The magnetization measurements have been performed on the doped spin gap system TlCu_{1-x}Mg_xCl_3 with x <= 0.025. The parent compound TlCuCl_3 is a three-dimensional coupled spin dimer system with the excitation gap Delta/k_B = 7.7 K. The impurity-induced antiferromagnetic ordering was clearly observed. The easy axis lies in the (0,1,0) plane. It was found that the transition temperature increases with increasing Mg^{2+} concentration x, while the spin-flop transition field is almost independent of x. The magnetization curve suggests that the impurity-induced antiferromagnetic ordering coexists with the spin gap for x <= 0.017.

Cp*W(dmit)2]•: An Organometallic 15-Electron Neutral Radical (d1) with Optical Limiting Properties in Solution and an Antiferromagnetic Ground State in the Solid State

The synthesis of the organometallic d2 [Cp*W(dmit)2]1– complex (where Cp* is pentamethylcyclopentadienyl and dmit is 1,3-dithiole-2-thione-4,5-dithiolate), and its oxidation to the paramagnetic d1 [Cp*W(dmit)2]• species, is described and their X-ray crystal structures given. Geometrical evolutions upon oxidation, characterized by a variable folding of the WS2C2 metallacycles along the S–S hinge, are rationalized by density functional theory (DFT) calculations and by comparison with the molybdenum analogs; as is also the evolution in the UV-vis-NIR absorption spectra. In solution, only the d1 complexes exhibit positive optical density variations in transitory nanosecond spectroscopy after 10 ns laser pulses. A weak optical limiting effect was observed on these d1 species, stronger in the W than in the Mo complex. In the solid state, the interacting, paramagnetic [Cp*W(dmit)2]• species (θCurie–Weiss = –20 K) orders antiferromagnetically below TNéel = 4.5 K with a spin-flop field, BSF(W) of 8000 G. Compared with the molybdenum analog, the weaker θCurie–Weiss(W) and TNéel(W) values, and larger BSF(W) values reflect weaker intermolecular interactions due to a decreased spin density on the dithiolene ligands and stronger spin–orbit coupling with the W atom, as confirmed by DFT calculations on the d2 and d1 Mo and W complexes.

Low-temperature domain-wall dynamics in weak ferromagnets

The nonlinear and multidimensional supersonic dynamics of domain walls in weak ferromagnets—orthoferrites and iron borate—are investigated experimentally at low temperatures in magnetic fields much less than the spin-flop field. A qualitative physical model of the observed effects is presented, and an elastically induced spin-reorientation mechanism of magnetization reversal is observed and investigated. This mechanism is accompanied by the unsteady motion of the domain walls, the velocity of which is considerably higher than the limiting velocity for their steady-state motion.

Ｃｏ／Ｃｄ多層膜におけるスピンフロップ現象と膜面垂直磁気抵抗効果

Current-perpendicular-to-plane magnetoresistance (CPP-MR) was measured for ferrimagnetic Co/Gd multilayers, which have twisted spin structure accompanied by spin-flop transition. The influence of the twisted spin structure on the electronic transport was exclusively obtained, eliminating the effect of anisotropic magnetoresistance. MR curves showed a downturn around the spin-flop field, suggesting that the formation of the twisted spin structure leads to a negative resistance change. This is possibly caused by the GMR-like mechanism whereby the resistance decreases in the process of the spin structure change from antiparallel to parallel alignment between Co and Gd moments.

Magnetic resonance in dilute quasi-one-dimensional antiferromagnet CsNi1−x MgxBr3

The effect of alloying with nonmagnetic Mg2+ ions on the low-frequency branch of resonance of a noncollinear quasi-one-dimensional CsNiBr3 antiferromagnet is investigated experimentally. It is found that a weak dilution (x=2 to 4%) leads to a considerable (up to 15%) reduction of the resonant gap and of the spin-flop field. The results agree with the theory of Korenblit and Schender, according to which the small parameter of perturbation of the initial system is \(x\sqrt {J/J'}\) rather than the impurity concentration x; i.e., a quasi-one-dimensional amplification coefficient exists, which is equal in this case to approximately six.

Current-perpendicular-to-plane magnetoresistance in Co/Gd multilayers with twisted spin structure

Current-perpendicular-to-plane magnetoresistance was measured for ferrimagnetic Co/Gd multilayers which have twisted spin structure accompanied with spin-flop transition. The influence of twisted spin structure on the electronic transport was exclusively obtained, eliminating the effect of anisotropic magnetoresistance. MR curves showed a downturn around the spin-flop field, suggesting that the formation of twisted spin structure leads to a negative resistance change.

Effects of Dy on spin-flop and magnetostriction in exchange-coupled Tb-Dy-Fe/Fe-Cu-Nb-Si-B multilayers

Magnetostrictive changes arising from magnetic transitions are investigated in Tb-Dy-Fe/Fe-Cu-Nb-Si-B multilayer systems. The spin-flop behavior of the magnetization is clearly observed in the samples with in-plane anisotropy. The magnetostriction measured in the longitudinal (transverse) direction shows an up and down (down and up) change associated with the spin-flop. It is found that the spin-flop field decreases and susceptibility increases with increasing the Dy content.

ESR Measurements of (DI-DCNQI)2Ag Single Crystals

X-band and millimeter wave ESR measurements have been performed on (DI-DCNQI) 2 Ag single crystals. X-band ESR measurements were carried out with the static magnetic field up to 1 T. Electron paramagnetic resonance was observed above 5 K. The one-dimensionality of this compound is discussed from the behavior of electron paramagnetic resonance. The millimeter wave ESR measurements in the frequency region from 40 to 315 GHz were carried out with the pulsed magnetic field up to 16 T in the temperature region from 1.8 to 70 K. Antiferromagnetic resonance (AFMR) modes with an easy axis type anisotropy were observed at 1.8 K. The analyses of AFMR modes showed that the easy axis is the c axis and the spin-flop field is 1.3 T.

Localized spin flop transition in a ladder structure with nonmagnetic impurities

A classical two-leg spin ladder with an antiferromagnetic exchange interaction and easy axis anisotropy in an external magnetic field exhibits a spin flop transition. A single nonmagnetic impurity introduced into this system allows for the formation of a metastable localized spin flop state at a field above the bulk spin flop field. This state will be symmetrically pinned to the impurity. Two neighboring impurities on the same leg will also result in the stabilization of a localized state pinned to the impurities, but in this case the state is not centered on the impurities.

Exchange magnetostriction in TbFe/FeCuNbSiB multilayers

The magnetostrictive changes arising from the magnetic transition have been investigated in TbFe/FeCuNbSiB multilayer systems. The spin-flop behavior of the magnetization has clearly been observed in the samples which possess in-plane anisotropy. The magnetostriction measured in the longitudinal (transverse) direction shows an up and down (down and up) change associated with the spin-flop. The spin-flop field has been found to decrease with increasing the Tb–Fe layer thickness. Furthermore, the spin-flop field decreases and the magnetic softness represented by susceptibility and coercivity are improved on annealing.

Magnetic spin-flop and magnetic saturation in Ag 2FeGeSe 4, Ag 2FeSiSe 4 and Cu 2MnGeSe 4 semiconductor compounds

Measurements of magnetization ( M) at helium temperatures and with pulsed magnetic fields up to 32 T, have been made on polycrystalline samples of the three compounds Ag 2FeGeSe 4, Ag 2FeSiSe 4 and Cu 2MnGeSe 4. All three compounds have orthorhombic symmetry, with the wurtz–stannite structure, and have semiconductor properties. The resulting curves of M versus B have been used to give information on the magnetic spin–flop and magnetic saturation transitions. The observed values of the spin-flop field B f differ appreciably for the three cases, with mean values of 0.35 T for Ag 2FeSiSe 4, 4.0 T for Cu 2MnGeSe 4 and 13.5 T for Ag 2FeGeSe 4. The saturation behavior is also very different from one compound to another. In the case of Ag 2FeSiSe 4, the magnetization curve at low fields (<3 T) is quite different for the cases of pulsed field and steady field measurements. This is attributed to domain effects, with Ag 2FeSiSe 4 showing weak ferromagnetic behavior because of crystallographic spin-canting. This effect can also be the cause for the very low value of B f in this compound.

Antiferromagnetic resonance spectrum in LaMnO3: Interrelation of the orbital structure and the magnetic properties

The relation between the orbital ordering and magnetic structure of the crystal LaMnO3 is investigated. The dependence of the exchange parameters on the angle Φ of the orbital structure is determined. When the isotropic exchange interaction and the single-ion anisotropy, which depends on the angle Φ and the rotational distortions, are introduced into the spin Hamiltonian, a four-sublattice structure (AX, FY, GZ) is obtained with orientation of the magnetic moments of the sublattices near the long axis of the orthorhombic cell of the crystal in the basal plane of the crystal (AX ≫ GZ, FY). The effect of a magnetic field on the magnetic structure and the antiferromagnetic resonance spectrum are investigated taking account of the nonequivalent, anisotropic, orbitally-dependent g tensors. The spin-flop and spin-flip transition fields are calculated.