Helimagnetic State Research Articles

Anisotropic Applied Field Dependency of Two Successive Magnetic Transitions in LiCu2O2

The anisotropy of two successive transitions of the spin-ladder compound LiCu2O2 is studied by the specific heat (C) under magnetic fields with H// c and H// ab (written as Hc and Hab in the following) up to 14 T. The peak of specific heat at 24.5 K in zero field shifts to lower temperature when the field is increased and the magnitude of the peak is suppressed by the field. On the contrary, the peak of 22.5 K shifts to higher temperature, especially at 14 T. Its magnitude increases in the field of Hc, whereas it decreases in the field of Hab. We calculate the entropy change between 21 K and 63 K. The different influence of the spin ordering by fields of different direction is obtained. Our experimental results suggest a mixed state between the long range incommensurate helimagnetic ground state and the higher-T short-range dimer liquid state. The temperature range of mixed state is shrunk with the increasing field. Possible mechanism is discussed.

Commensurate to Incommensurate Transition in the Chiral Helimagnet CuB2O4

We have studied a commensurate to incommensurate transition in the novel helimagnet CuB 2 O 4 by means of high-frequency ESR. In the commensurate weak ferromagnetic (CWF) state, two ESR modes have been found at low temperatures, which are explained in terms of an electron paramagnetic resonance of the Cu 2+ spins at 8 d -sites and an antiferromagnetic resonance of the magnetically ordered Cu 2+ spins at 4 b -sites with the Dzyaloshinskii–Moriya (DM) interaction. The DM interaction among the Cu 2+ spins at 4 b -sites that carries the CWF state is evaluated quantitatively. The incommensurate helimagnetic state is discussed in terms of the magnetic frustration.

Pressure induced helimagnetism in Fe-based (Y 2Fe 17, Lu 2Fe 17) intermetallic compounds

The helimagnetic ground state was induced in the hexagonal Y 2Fe 17 and Lu 2Fe 17 intermetallics by hydrostatic pressure above 1.05 and 0.3 GPa, respectively. The observed incommensurate helical structure was characterized by a propagation vector (0 0 τ z ) and by a metamagnetic transition at the critical field H c. The new incommensurate magnetic structure has been revealed now in Lu 2Fe 17 at magnetic fields above H c at low temperatures. A detailed pressure-temperature-field evolution of the magnetic structures in Y 2Fe 17 and Lu 2Fe 17 is presented.

ESR measurements of the chiral spin system CuB 2O 4

We report ESR measurements of the chiral spin system CuB 2O 4 at 35 GHz in H ∥ c . The g-value is determined as 2.16 for the c-axis. Anomalies in the resonance field and the line width have been observed around 21 and 9 K, which correspond to the successive phase transitions. In the helimagnetic state, a small hysteretic dip appears in ESR spectrum in addition to a helimagnetic resonance absorption. The dip shifts rapidly to the higher field with decreasing temperature below the transition temperature at 9 K.

High field ESR measurements on the chiral spin system CuB2O4

Magnetically ordered states in the chiral spin system CuB2O4have been studied by ESR measurements. Two resonance absorptions are observed in the helimagnetic state, which imply that the two inequivalent copper sites are in the helimagnetically ordered state and the paramagnetic state with a development of short range order. The frequency-independent dip with hysteresis on the spectrum is also observed in the helimagnetic state. The behavior of the dip can be interpreted by the change of the dielectric properties rather than the magnetic resonance.

Investigation by spherical neutron polarimetry of magnetic properties in BaCo 2(AsO 4) 2

The cobaltite BaCo 2(AsO 4) 2 (BCAO) is a good realization of the quasi-2D system in which the Co 2 + ions form a magnetically frustrated honeycomb lattice with magnetic moments mainly confined in the basal planes. Below T N = 5.35 ± 0.5 K , the ground state structure is described by an incommensurate propagation vector k ≈ ( 0.265 , 0 ,- 1.33 ) and the magnetic excitation spectrum displays a resolution-limited energy gap Δ 0 ≈ 1.45 meV at q = 0 . No magnetic excitations emerging from q = k could be detected, contrary to what would be expected for a simple helimagnetic ground state. In order to understand these puzzling results, we have recently re-investigated the magnetic structure and excitations in BCAO using spherical neutron-polarization analysis.

High-field-magnetization study of an ErMn 6Sn 6 single crystal

In previous investigations, it was shown that the compound ErMn 6Sn 6, that crystallizes in the HfFe 6Ge 6-type of structure, orders in a helimagnetic state at T N = 352 K, with the moments rotating within the [0 0 1] plane. In the helimagnetic state, metamagnetic behavior is observed, the critical field increasing when the temperature decreases from T N to 220 K and then decreasing with further decrease of the temperature from 200 to 75 K. At this latter temperature, the compound orders ferrimagnetically with the moments in the [0 0 1] plane. Very peculiarly, the in-plane magnetocrystalline anisotropy was found to decrease with temperature decreasing from 125 to 4.2 K. The magnetization of ErMn 6Sn 6 single crystals has been measured in magnetic fields up to 51 T. The measurements were performed at 4.2 K in the ferrimagnetic state and at 125 K in the helimagnetic state with the magnetic field applied along the three principal crystallographic directions. The measured magnetic isotherms are discussed in terms of the competition between the Er-Mn exchange interaction and the magnetocrystalline anisotropies of the Er and the Mn magnetic sublattice.

A neutron diffraction study of HfFe 6Ge 6-type YMn 6Sn 6− xIn x compounds (0.03≤ x≤0.72)

The HfFe 6Ge 6-type YMn 6Sn 6− x In x compounds ( x=0.03, 0.10, 0.23, 0.37, 0.72) have been studied by means of magnetization measurements and powder neutron diffraction studies. YMn 6Sn 5.97In 0.03 and YMn 6Sn 5.90In 0.10 display helimagnetic order over their entire ordered temperature range ( T N=330 and 334 K, respectively). YMn 6Sn 5.77In 0.23 and YMn 6Sn 5.63In 0.37 order ferromagnetically ( T c=324 and 323 K, respectively) and undergo a ferro- to helimagnetic transition below T t=308 and 258 K. YMn 6Sn 5.28In 0.72 displays ferromagnetic order over its entire ordered temperature range ( T c=312 K). In the helimagnetic state, unusual thermal variations of the q z component of the propagating vector were observed with Δ q z/Δ T>0 for x=0.03 and 0.10 and Δ q z/Δ T<0 for x=0.23 and 0.37.

Magnetic structure and the anomalous Hall effect of Cu1$minus;xZnxCr2Se4

Abstract To study what roles spin structures of ferromagnets play in determining their Hall resistivity ρH or to see if the spin chiral order really contributes to the unusual behavior of anomalous Hall resistivity, we have carried out transport and neutron diffraction studies on the spinel-type system Cu1−xZnxCr2Se4, which is a collinear ferromagnet at x∼ 0 and in the helimagnetic state at x=1.0. It is established that the sign change of ρH takes place, with decreasing temperature, along with the appearance of the conical spin structure in the region 0

119Sn Mössbauer and magnetization studies of Co2ScSn

Magnetization measurements on the Heusler alloy Co2ScSn show that the system undergoes a transition from a paramagnetic state to a possible helimagnetic state at Tc=238 K, and exhibits spin-glass behavior below 100 K. 119Sn Mössbauer spectra of Co2ScSn are found to be influenced significantly by heat treatment. Single-dip spectra are observed above Tc as expected from the cubic symmetry at the Sn sites. The transferred hyperfine field at the Sn nuclei appears below Tc, increases with decreasing temperature, and has a value of 38 kOe at 4.2 K. The temperature dependence of the hyperfine field at the Sn nuclei shows a small break at 100 K which correlates with the onset of the spin-glass transition.

Transition between helimagnetic states in Mn 1− tCr tAs: phase diagram and effect of strong magnetic field

Mn 1− t Cr t As has been investigated for 0.20 ⩽ t ⩽ 0.45 by X-ray and neutron diffraction, differential scanning calorimetry and magnetometric measurements for external magnetic fields up to 250 kOe. The para- (P) to helimagnetic (H a ) phase transition is classified as weakly first order despite the lack of hysteresis, the P to H c transition as second order and the H a to H c transition as first order (subscripts a and c denoting propagation direction of the spirals). The t, T phase diagram in zero external field is essentially consistent with earlier findings. The H a mode requires only a moderate external field to become spin aligned. The boundary surrounding the H c phase (with rather high critical fields) in the H, T phase diagrams comprises first and second order sections which join at a tricritical point. The main features of the t, T phase diagrams are reproduced by a simple mean field model.

Magnetic structure of MnP as probed by 57Fe.

This study shows that great care has to be taken when alloying small amounts of $^{57}\mathrm{Fe}$ into MnP in order to avoid magnetic inhomogeneities. Indications for these could not be found by x-ray techniques, but they showed up very clearly in M\ossbauer spectra. A well-prepared sample of ${\mathrm{Mn}}_{0.99}^{57}$${\mathrm{Fe}}_{0.01}$P is paramagnetic above 285 K and ferromagnetic between 285 and \ensuremath{\sim}80 K. The transition to the helimagnetic state at \ensuremath{\simeq}80 K is rather indistinct. The magnetic structure at 11 K is best described as a distorted helix. Although neutron diffraction studies also are in favor of a distorted spiral, these descriptions do not comply with the presently found distortions.

Effect of a magnetic field on MnAs 0.88P 0.12 below 80 K

MnAs 0.88P 0.12 has been studied by powder neutron diffraction in external magnetic fields up to 15.2 kOe and temperatures down to 4.2 K. MnAs 0.88P 0.12 takes the MnP type atomic arrangement and exists in para-, ferro- and different (essentially) helimagnetic states. The observation of a double 000 ± satellite at 4.2 K < T ⪷ 70 K adds further evidence to the chain of arguments for distinction between the helimagnetic states H' a (4.2 K < T < T S,1 ≈ 70 K) and H a ( T S,2 ≈ 180 K < T < T N = (243 ± 5) K). External magnetic fields at 4.2 K < T < 70 K evoke a new magnetic state, which is also characterized by a satellite doublet, and is tentatively designated H' a,fan.

Degenerate helimagnetic states, lines of soft modes and absence of long-range order in three dimensions

The triangular and hexagonal classical Heisenberg model with suitable competing intraplane interactions have infinitely many ground states having inequivalent helical order of different pitch Q. The spin wave energy vanishes along a line in k space, the locus of the Q vectors. Helimagnetic long-range order is destroyed at finite temperature also in three dimensions. The existence of a new phase is suggested: an isotropic helimagnet that leads to a characteristic ring scattering. The effect of quantum corrections and of easy plane anisotropy is also considered.

Inelastic neutron scattering investigation of the magnetic excitations in the helimagnetic state of NiBr2

Inelastic neutron scattering investigation of the magnetic excitations in the helimagnetic state of NiBr2