Spin-flop Phase Research Articles

Discommensurate and inhomogeneous states induced by a strong magnetic field in low-dimensional antiferromagnets

Anisotropic antiferromagnetic systems of dimensionality greater than one in an external field are shown to exhibit a complicated array of ground states depending on the spin structure of the surface. The simplest structure that exhibits these effects is the spin ladder with the surface being the ladder end, which can be either compensated or non-compensated spins. The structure with the compensated end has a surface spin flop phase, the non-compensated end has a discommensurational phase, and the transition to these phases can be either first or second order with a tricritical point.

Magnetic phase diagram ofEu4Ga8Ge16by magnetic susceptibility, heat capacity, and Mössbauer measurements

The magnetic ordering in the ${\mathrm{Eu}}_{4}{\mathrm{Ga}}_{8}{\mathrm{Ge}}_{16}$ clathrate compound was studied using magnetic susceptibility, M\"ossbauer spectroscopy, and heat-capacity measurements. Antiferromagnetic ordering is observed at 8 K. M\"ossbauer spectroscopy shows the Eu atoms to exist as ${\mathrm{Eu}}^{2+}$ with strong room-temperature absorption at -11.8 mm/s vs ${\mathrm{EuF}}_{3}.$ The temperature dependence of the Lamb-M\"ossbauer factor can be fitted to an Einstein model with an effective Einstein temperature of 80 K. In combination with heat-capacity measurements, this result suggests that the Eu atoms behave like isolated oscillators that are effectively decoupled from the lattice. The magnetic susceptibility data are interpreted using the Curie-Weiss law for ordering with ${\ensuremath{\mu}}_{\ensuremath{\parallel},\mathrm{eff}}=8.00\ifmmode\pm\else\textpm\fi{}0.03{\ensuremath{\mu}}_{B}/{\mathrm{Eu}}^{2+}$ ion and $\ensuremath{\theta}=4.43\ifmmode\pm\else\textpm\fi{}0.08\mathrm{K}.$ The positive Weiss constant is rationalized by a nearest-neighbor ferromagnetic exchange interaction within an overall antiferromagnetically ordered system. The spin-flop phase diagram is established and used to extract the next-nearest-neighbor interaction energy. The location of the tri-critical point in H,T space is determined. The magnetic heat capacity ${C}_{m}$ is extracted by modeling the lattice contribution using a weighted Debye-Einstein model with fitted Debye temperature ${\ensuremath{\Theta}}_{D}=314\ifmmode\pm\else\textpm\fi{}4\mathrm{K}$ and using a fixed Einstein temperature ${\ensuremath{\Theta}}_{E}=80\mathrm{K}.$ Integration of ${C}_{m}/T$ gave 15.1 J/K mol ${\mathrm{Eu}}^{2+}$ in fair accordance with expectations based on configurational considerations.

Spin flop transition in doped antiferromagnets

In this paper we compute the mean field phase diagram of a doped antiferromagnet, in amagnetic field and with anisotropic exchange. We show that at zero temperature there isa metamagnetic transition from the antiferromagnetic configuration along thez direction to a spin flop (SF) state. In the spin flop phase the system prefers a commensuratemagnetic order, at low doping, whereas at larger doping the incommensurate phase is favourable.Contrary to the pure Heisenberg case, the spin flop region does not span an infinite area in the(Δ,h) plane, whereΔ is the exchangeanisotropy and h is the external magnetic field. We characterize the magnetic and charge-transportproperties of the SF phase, computing the magnetic susceptibility and the Drudeweight. This latter quantity presents a sudden variation as the SF to paramagnetphase transition line is crossed. This effect could be used as a possible sourceof large magneto-resistance. Our findings may have some relevance for dopedLa2−δSrδCuO4 in a magnetic field.

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb2MnxCd1−xCl4 crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.

Magnetic structures of Laves phase superlattices

We present a determination of the magnetic structuresof three Laves phase superlattice samples of structure[70/30]60,[150/100]50,and [50/70]60,where the structure is given as [t1 Å DyFe2/t2 Å YFe2]N,grown by molecular beam epitaxy. The experiments were performed usingmagnetization measurements and neutron scattering measurements at the NRUreactor at Chalk River in Canada.For the [70/30]60sample, a magnetic field parallel to the scattering vector, Q,was applied to determine the nuclear component of the scattered intensity. Themagnetic structure with the field aligned along the [001] had all the momentsaligned along the field direction and the magnitude of the moment on the iron sitewas temperature independent with a value of 2.3(0.3) μB.The moment on the dysprosium site was found to decreasewith temperature from about 10 μBto a value of 6(0.5) μBat 300 K. When the field was applied in the [11̄0]direction the magnetic moments were found to rotate out of the(110)epitaxial plane towards the [110] direction by an angle ψ = 10°at 300 K, which increased at 4 K to an angle of ψ = 40° close tothe [100]out-of-plane direction.For the [150/100]50sample, when a 6 T magnetic field was applied along the [11̄0]direction, the main peak of the magnetically sensitive (111̄)reflection was found to decrease in intensity while the scatteringat the satellite peaks increased. This change in intensity is due tothe formation of magnetic exchange springs in the ‘soft’ YFe2layers of the superlattice. Detailed measurements around the (111̄)reflection and calculations for an exchange spring model give excellent agreementbetween the model and the experiment.Finally, the [50/70]60sample showed unexpected behaviour because the moments aligned largelyperpendicular to an applied field. This is similar to a spin-flop phase of anantiferromagnet and it is argued that this occurs because the net moments on the DyFe2 andYFe2layers are nearly equal.

Transition from parallel to antiparallel magnetic state at the interface of Fe3O4/Mn3O4 superlattices (invited)

A series of ferrimagnetic/ferrimagnetic Fe3O4/Mn3O4 superlattices are grown on MgO(001) and (011) substrates by plasma-oxygen-assisted molecular beam epitaxy. The thickness of the Fe3O4 layer is 4 nm and the thickness of Mn3O4 is varied from 1 to 16 nm and the total thickness of these films is between 100 and 200 nm. The magnetization as a function of temperature and external field (parallel to the film surface) is measured in the range of 4–300 K and −50–50 kOe. The main magnetic response of the superlattices with thicker Mn3O4 layers show compensation points (∼39 K) and spin-flop phases at a high external field (>15 kOe) indicating the net moments in Fe3O4 and Mn3O4 are antiparallel at the interface. The magnetic response of the superlattices with thinner Mn3O4 layers show regular ferromagnetic hysteresis and temperature curves indicating the net moments in Fe3O4 and Mn3O4 are parallel at the interface. A transition of the magnetic coupling between Fe3O4 and Mn3O4 from antiparallel to parallel is clearly observed when the Mn3O4 layer thickness is less than 4 nm. A domain-wall-like configuration is believed to form at the interface to maintain the antiparallel state of Fe3O4/Mn3O4. When the layer thickness reduces below certain thickness the domain-wall-like configuration no longer exists and a parallel state is preferred. A model of the domain-wall-like configuration between Fe3O4 and Mn3O4 is discussed.

Effect of anisotropy on compensation point in Fe3O4/Mn3O4 grown on MgO(011)

[ Fe 3 O 4 (40 Å)/ Mn 3 O 4 (160 Å)] ×10 superlattice is grown on MgO(110), and its in-plane angular-dependent magnetic response is studied. Magnetic compensation and spin–flop state are observed at ∼42 K and external field >15 kOe when the magnetic measurement is along the [1-10] direction. These features disappear when the measurement is along the [001] direction. This strong anisotropic effect may originate from the tetragonal structure of Mn3O4 whose magnetic easy axis is in the a-b plane and hard axis is perpendicular to the a-b plane. This study demonstrates that the compensation point and spin–flop phase transition both are orientationally dependent in an antiparallel-coupled magnetic layer system if there is at least one constituent component with strong magnetic anisotropy.

Magnetic polaritons in metamagnet thin films

We investigate the magnetic polariton modes which propagate in metamagnet thin films, consisting of ferromagnetically ordered layers, with the intralayer ferromagnetic exchange interactions being much stronger than the weak antiferromagnetic interaction between adjacent layers. We consider also the presence of an external magnetic field H 0 applied perpendicular to the layers. We confine our discussion to the metamagnet materials FeCl 2 and FeBr 2 in which the magnetic anisotropy is sufficiently large compared with the weak antiferromagnetic coupling, so that there is no spin-flop phase intermediate between the AFM and FM phases. The dynamic response of a metamagnetic material is derived within the classical torque equation of motion model. Our theoretical results indicate that there is a larger frequency separation in FeBr 2, compared with FeCl 2 in the surface magnetic polariton spectra, which favor the former material for experimental studies.

Transverse and lateral structure of the spin-flop phase in fe/cr antiferromagnetic superlattices.

Direct evidence of the nonuniformly canted state of the spin-flop phase induced by a magnetic field applied to Fe/Cr(100) superlattices is obtained by polarized neutron reflectometry. It is unambiguously demonstrated that the magnetization of the alternating Fe layers is twisted through the multilayer stack proving a stable noncollinear configuration. The maximal tilt at the end layers progressively reduces towards the center of the multilayer. The set of tilt angles is deduced from a model-free data evaluation employing the supermatrix routine. Spin-flip off-specular scattering is determined by the in-plane magnetization fluctuations and is fitted by a theoretical model of domains.

Structure of multiple spin-flop states inBaCu2Si2O7

The unusual 2-stage spin flop transition in BaCu2Si2O7 is studied by single-crystal neutron diffraction. The magnetic structures of the various spin-flop phases are determined. The results appear to be inconsistent with the previously proposed theoretical explanation of the 2-stage transition.

Order-to-disorder transition in theXY-like quantum magnetCs2CoCl4induced by noncommuting applied fields

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-$\frac{1}{2}$ $\mathrm{XY}$-like antiferromagnet ${\mathrm{Cs}}_{2}{\mathrm{CoCl}}_{4}$ using single-crystal neutron diffraction. In zero-field, interchain couplings cause long-range order below ${T}_{N}=217(5) \mathrm{mK}$ with chains ordered antiferromagnetically along their length and moments confined to the $(b,c)$ plane. Magnetic fields applied at an angle to the $\mathrm{XY}$ planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the $(b,c)$ plane, proposed to be a spin-liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region $2.10<{H}_{\mathrm{SL}}<2.52 \mathrm{T}\mathrm{}\ensuremath{\Vert}a.$ The observed phase diagram is discussed in terms of known results on $\mathrm{XY}$-like chains in coexisting longitudinal and transverse fields.

On the Peculiarities of the Temperature Dependence of the Magnetoelectric Susceptibility of Chromium Oxide

We have shown that in the chromium oxide the components of the tensor of magnetoelectric susceptibility f 33 ( T ) in the original phase and f 31 ( T ) in the spin-flop phase exhibit similar temperature dependencies. It's proved in the present paper that noted above behavior of these components can be explained in the framework of the phenomenological theory if one takes into account invariants of the third and fifth order of magnitude in the components of the antiferromagnetic vector in the phenomenological thermodynamic potential with explicitly separated exchange and relativistic terms. Also, the form of thermodynamic potentials elaborated in this paper suggests that the transition to the spin-flop phase in the chromium oxide is incomplete and f 33 cannot be equal to zero in the spin-flop phase.

Interaction of the spin-flop phase and superconductivity inDyMo6S8single crystals

Interaction of the spin-flop phase and superconductivity inDyMo6S8single crystals

Magnetic behavior of the diluted antiferromagnet Mn 0.39Zn 0.61F 2 at strong fields

The magnetic phase boundaries of the random-field Ising model (RFIM) system Mn 0.39Zn 0.61F 2 are determined using magnetization measurements, under finite DC applied fields ( H). At low fields ( H<12 kOe), our results support a critical phase boundary, separating an ordered antiferromagnetic (AF) phase from the paramagnetic (P) one. For intermediate fields (12< H<18 kOe), the AF ordering coexists with a spin-flop (SF) clustering. For higher H, a spin-flop phase dominates the upper part of the ( H, T) phase diagram.

Magnetoelectric interactions and induced toroidal ordering in Cr 2O 3

It is shown that magnetoelectric interactions play a significant role in the onset of antiferromagnetic ordering in crystals, the form of the linear magnetoelectric tensor fully representing the magnetic point group of the crystal under consideration. When dealing with the off-diagonal components of the magnetoelectric tensor even in the case of uniform antiferromagnetic ordering we propose to treat the situation in the framework of the appearance of toroidal moment — polar vector, changing sign under space inversion and time reversal. By group theoretical analysis it is shown that in uniaxial easy-plane antiferromagnets (due to the irreducible corepresentation of the order parameter) there may occur either toroidal or weak ferromagnetic type of ordering. Following theoretical prediction, the linear transverse magnetoelectric effect, induced by the external magnetic field exceeding the spin-flop value, was investigated in Cr 2O 3 single crystals. The electric polarization components P a and P c versus pulsed magnetic field H, oriented at an angle θ=5 o with respect to c-axis in the ac plane of the crystal in the temperature range 5–300 K were measured. The study of the linear magnetoelectric effect in Cr 2O 3 crystals in the spin-flop phase made it possible to reveal the asymmetry of the magnetoelectric tensor components α 13 and α 31 and thereby to prove the existence of a toroidal moment induced by an external magnetic field.

Anisotropic antiferromagnetism in Ca 9La 5Cu 24O 41

We present specific heat measurements of a Ca 9La 5Cu 24O 41 single crystal in magnetic fields up to 14 T along the three principal crystallographic axes. Anomalies of the specific heat indicating long-range antiferromagnetic order exhibit a strong anisotropy with respect to the field orientation. In addition, no evidence for a spin-flop phase could be found down to 4 K representing an unusual feature for S = 1 2 copper magnetism.

Effects of Random Fields on the Phase Diagram in the Field-Temperature Plane of the Diluted Antiferromagnet MnxMg 1-xTiO 3

The diluted antiferromagnet Mn x Mg 1- x TiO 3 with x ≥0.64 has been studied by magnetization measurements, and the magnetic phase diagrams in the field-temperature ( H - T ) plane have been determined for the samples with different x including pure MnTiO 3 . It is demonstrated that the existence of random fields gives large effects on the phase transitions. The most noticeable result is that the H - T phase diagram of the samples except MnTiO 3 consists of four phases: well known three phases of a paramagnetic, an antiferromagnetic (AF) and a spin-flop (SF) phase, and a new type of phase. The shape of the phase diagram changes systematically as the Mn concentration, x , decreases. It is considered that in this “new phase” an AF like short range order and a SF like short range order coexist and that appearance of the “new phase” is caused by random fields. In this paper, we discuss the effects of random fields on the phase diagram of Mn x Mg 1- x TiO 3 and the x dependence of phase diagram.

High-field magnetic structure and critical phenomena in MnWO 4

The magnetic structure of the high-field phase HF in MnWO 4 was resolved by single-crystal neutron diffraction at 14.5 T and 2.5 K. It is a spin-flop phase with the same magnetic unit cell as the magnetic ground state, but the magnetic moments switched perpendicular to the applied field. The temperature dependence of the propagation vector for the intermediate, incommensurate phase indicates that the coexistence point of HF, the intermediate phase, and the paramagnetic region might be a Lifshitz-point.

Ordering and the nature of the spin flop phase transition in MnPS3

Powder diffraction patterns were measured on a high quality sample of manganese thiophosphate, MnPS3, at temperatures between 2 and 120 K and magnetic fields between 0 and 60 kOe. In zero field, the magnetic moments in MnPS3 are collinear and normal to the ab planes. On applying a sufficiently large magnetic field along the normal to the ab plane, it was found that the moments flop towards the b axis, in agreement with a dipole-dipole interaction model for the anisotropy. The width of this transition was found to be greater than 20 kOe. No evidence for critical fluctuations was seen during the spin flop phase transition.

Phase transitions in antiferromagnetic cobalt fluoride

The phase transitions in cobalt fluoride (at T≪TN=38 K) in a longitudinal magnetic field are investigated theoretically, and it is shown that the field-induced transition of the magnetic subsystem from an antiferromagnetic phase can occur only to a canted phase and that it is a first-order transition. The character of this transition is due to the “sign” of the Dzyaloshinskiı̆ interaction, and this affords an opportunity to determine experimentally the “sign” of the Dzyaloshinskiı̆ interaction. The field of the transition to the spin-flop phase (when the total magnetic moment becomes parallel to the easy axis) coincides with the field of the transition to the spin-flip phase (when the antiferromagnetism vector becomes equal to zero).