Noncollinear Antiferromagnetic Structure Research Articles

Microscopic Insights on the Multiferroic Perovskite-Like [CH3 NH3 ][Co(COOH)3 ] Compound.

The characterization of the crystal structure, phase transitions, magnetic structure and dielectric properties has been carried out on [CH3 NH3 ][Co(COOH)3 ] (1) perovskite-like metal-organic compound through variable-temperature single-crystal and powder neutron and X-ray diffraction and relative permittivity measurements. The paraelectric to antiferroelectric-like phase transition observed at around 90 K is triggered by a structural phase transition; the structural studies show a change from Pnma space group at RT (1A) to P21 /n space group at low temperature (1B). This phase transition involves the occurrence of small distortions in the framework and counterions. Neutron diffraction studies have shown a magnetic order showing spontaneous magnetization below 15 K, due to the occurrence of a non-collinear antiferromagnetic structure with a weak ferromagnetic component, mainly due to the single-ion anisotropy of the CoII ions.

Piezomagnetism and magnetoelastic memory in uranium dioxide

The thermal and magnetic properties of uranium dioxide, a prime nuclear fuel and thoroughly studied actinide material, remain a long standing puzzle, a result of strong coupling between magnetism and lattice vibrations. The magnetic state of this cubic material is characterized by a 3-k non-collinear antiferromagnetic structure and multidomain Jahn-Teller distortions, likely related to its anisotropic thermal properties. Here we show that single crystals of uranium dioxide subjected to strong magnetic fields along threefold axes in the magnetic state exhibit the abrupt appearance of positive linear magnetostriction, leading to a trigonal distortion. Upon reversal of the field the linear term also reverses sign, a hallmark of piezomagnetism. A switching phenomenon occurs at ±18 T, which persists during subsequent field reversals, demonstrating a robust magneto-elastic memory that makes uranium dioxide the hardest piezomagnet known. A model including a strong magnetic anisotropy, elastic, Zeeman, Heisenberg exchange, and magnetoelastic contributions to the total energy is proposed.

Field-induced magnetic states in holmium tetraboride

A study of the zero field and field induced magnetic states of the frustrated rare earth tetraboride, HoB$_4$, has been carried out using single crystal neutron diffraction complemented by magnetisation measurements. In zero field, HoB$_4$ shows magnetic phase transitions at $T_{\mathrm{N1}}$ = 7.1 K to an incommensurate state with a propagation vector ($\delta$, $\delta$, $\delta'$), where $\delta$ = 0.02 and $\delta'$ = 0.43 and at $T_{\mathrm{N2}}$ = 5.7 K to a non-collinear commensurate antiferromagnetic structure. Polarised neutron diffraction measurements in zero field have revealed that the incommensurate reflections, albeit much reduced in intensity, persist down to 1.5 K despite antiferromagnetic ordering at 5.7 K. At lower temperatures, application of a magnetic field along the $c$-axis initially re-establishes the incommensurate phase as the dominant magnetic state in a narrow field range, just prior to HoB$_4$ ordering with an up-up-down ferrimagnetic structure characterised by the $(h,k,\frac{1}{3})$-type reflections between 18 and 24 kOe. This field range is marked by the previously reported $M/M_{\mathrm{sat}}$= $\frac{1}{3}$ magnetisation plateau, which we also see in our magnetisation measurements. The region between 21 and 33 kOe is characterised by the increase in the intensity of the antiferromagnetic reflections, such as (100), the maximum of which coincides with the appearance of the narrow magnetisation plateau with $M/M_{\mathrm{sat}}\approx$ $\frac{3}{5}$. Further increase of the magnetic field results in the stabilisation of a polarised state above 33 kOe, while the incommensurate reflections are clearly present in all fields up to 59 kOe. We propose the $H-T$ phase diagram of HoB$_4$ for the $H \parallel c$ containing both stationary and transitionary magnetic phases which overlap and show significant history dependence.

Static and dynamic magnetic properties of two synthetic francisites Cu3La(SeO3)2O2X (X = Br and Cl)

The formation of long-range magnetic order at low temperatures was established in francisite—type compounds Cu3La(SeO3)2O2X (X = Br and Cl) through measurements of magnetic susceptibility, magnetization, specific heat and X-band electron spin resonance. The significantly enhanced critical index p = 1.0 ± 0.1 in Cu3La(SeO3)2Br and p = 0.8 ± 0.1 in Cu3La(SeO3)2Cl in the temperature dependence of the width of ESR signal evidence the reduced dimensionality of the kagome-type francisite’s magnetic subsystem. Under action of external magnetic field, the presumably non-collinear six-sublattices antiferromagnetic structure of these compounds experiences the first-order metamagnetic transformation. The B–T magnetic phase diagrams were established from the positions of singularities in temperature and field dependences of thermodynamic properties. Contrary to pristine mineral Cu3Bi(SeO3)2Cl, no signature of structural phase transition was detected.

Ab initio investigation of competing antiferromagnetic structures in low Si-content FeMn(PSi) alloy

The antiferromagnetic structures of a low Si-content FeMn(PSi) alloy were investigated by first principles calculations. One possible antiferromagnetic structure in supercell along the c-axis was revealed in FeMnP0.75Si0.25 alloy. It was found that atomic disorder occupation between Fe atom on 3f and Mn atoms on 3g sites is responsible for the formation of antiferromagnetic structures. Furthermore the magnetic competition and the coupling between possible AFM supercells along the c and a-axis can promote a non-collinear antiferromagnetic structure. These theoretical investigations help to deeply understand the magnetic order in FeMn(PSi) alloys and benefit to explore the potential magnetocaloric materials in Fe2P-type alloys.

Vectorial mapping of noncollinear antiferromagnetic structure of semiconducting FeSe surface with spin-polarized scanning tunneling microscopy

Antiferromagnetic semiconductors gain increasing interest due to their possible application in spintronics. Using spin polarized scanning tunneling microscopy operating in a vector field, we mapped the noncollinear antiferromagnetic spin structure of a semiconducting hexagonal FeSe surface on the atomic scale. The surface possesses an in-plane compensated Néel structure which is further confirmed by first-principles calculations.

Magnetic dipole interactions in crystals

The influence of magnetic dipole interactions (MDIs) on the magnetic properties of local-moment spin systems is investigated. The eigenvalues and eigenvectors of the MDI tensor are determined for a wide variety of spin lattices and magnetic wavevectors in the magnetically ordered state with collinear moment alignments. The predicted easy axes in the ordered state are compared with experimental results for antiferromagnetic (AFM) compounds. The theory is generalized to include noncollinear AFM structures and compared with experiments. Many properties caused or influenced by MDIs in the ordered and paramagnetic states are calculated within Weiss molecular field theory.

Frustrated Triangular Magnetic Structures of Mn3ZnN: Applications in Thermal Expansion

One of the specific subjects in frustrated magnetic systems is the phenomenon coupled with noncollinear magnetism, such as zero or negative thermal expansion (ZTE or NTE) in antiperovskite compounds. The first-principles calculations and neutron powder diffraction (NPD) are used to reveal the control of the noncollinear Γ5g antiferromagnetic (AFM) structure and corresponding thermal expansion properties in Mn3Zn0.875X0.125N (X = Mn, Ge, and Sn). Based on the optimal exchange-correlation functional, our results demonstrate that X (X = Mn, Ge, and Sn) doping at Zn site could stabilize the noncollinear Γ5g AFM structure and produce magnetovolume effect (MVE). The predictions of Γ5g AFM ground state and MVE is further verified by the NPD results of Mn3Zn0.83Mn0.15N0.99. Intriguingly, this special magnetic structure with strong spin–lattice coupling can be tunable to achieve ZTE behavior. On the basis of these results we suggest that frustrated magnetic systems with noncollinear Γ5g AFM structure of Mn atoms i...

Effect of complex magnetic structure on the magnetocaloric and magneto-transport properties in GdCuSi

GdCuSi has been studied by structural, magnetic, magneto-thermal, and magneto-transport measurements. The compound crystallizes in the Ni2In-type hexagonal structure. Magnetic measurements show antiferromagnetic ordering at T N = 14.2 K and a magnetic anomaly near 5 K, which are confirmed by different measurements. Magnetocaloric effect (MCE) has been estimated from both magnetization and heat capacity data and it shows a change in sign below T N. Similar to MCE, magnetoresistance (MR) data also show sign change below T N. The compound shows large MCE and MR near its ordering temperature. The sign change in MCE and MR is attributed to the non-collinear antiferromagnetic structure of the compound. The theoretical calculations suggest competition of ferromagnetic and antiferromagnetic interactions, which causes non-collinear magnetic structure in this compound.

Antiferromagnetism inEuCu2As2andEuCu1.82Sb2single crystals

Single crystals of EuCu2As2 and EuCu2Sb2 were grown from CuAs and CuSb self-flux, respectively. The crystallographic, magnetic, thermal and electronic transport properties of the single crystals were investigated by room-temperature x-ray diffraction (XRD), magnetic susceptibility \chi versus temperature T, isothermal magnetization M versus magnetic field H, specific heat Cp(T) and electrical resistivity \rho(T) measurements. EuCu2As2 crystallizes in the body-centered tetragonal ThCr2Si2-type structure (space group I4/mmm), whereas EuCu2Sb2 crystallizes in the related primitive tetragonal CaBe2Ge2-type structure (space group P4/nmm). The energy-dispersive x-ray spectroscopy and XRD data for the EuCu2Sb2 crystals showed the presence of vacancies on the Cu sites, yielding the actual composition EuCu1.82Sb2. The \rho(T) and Cp(T) data reveal metallic character for both EuCu2As2 and EuCu1.82Sb2. Antiferromagnetic (AFM) ordering is indicated from the \chi(T), Cp(T), and \rho(T) data for both EuCu2As2 (T_N = 17.5 K) and EuCu1.82Sb2 (T_N = 5.1 K). In EuCu1.82Sb2, the ordered-state \chi(T) and M(H) data suggest either a collinear A-type AFM ordering of Eu+2 spins S=7/2 or a planar noncollinear AFM structure, with the ordered moments oriented in the tetragonal ab plane in either case. This ordered-moment orientation for the A-type AFM is consistent with calculations with magnetic dipole interactions. The anisotropic \chi(T) and isothermal M(H) data for EuCu2As2, also containing Eu+2 spins S=7/2, strongly deviate from the predictions of molecular field theory for collinear AFM ordering and the AFM structure appears to be both noncollinear and noncoplanar.

Electronic properties of a distorted kagome lattice antiferromagnetDy3Ru4Al12

Electronic properties of ${\mathrm{Dy}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ (hexagonal crystal structure, Dy atoms form distorted kagome nets) are studied on a single crystal by means of magnetization, neutron diffraction, specific heat, and resistivity measurements. The onset of a long-range magnetic order of Dy moments occurs at 7 K through a first-order phase transition. The compound has a noncollinear antiferromagnetic structure with a propagation vector (1/2 0 1/2). The configuration of the Dy moments is consistent with the monoclinic Shubnikov group ${C}_{c}2/c$. The \ensuremath{\gamma} coefficient in the temperature linear term of the specific heat is strongly enhanced to 500 mJ ${\mathrm{mol}}^{\ensuremath{-}1}$ ${\mathrm{K}}^{\ensuremath{-}2}$ taking into account the localized nature of Dy magnetism. An additional contribution originates from spin fluctuations induced in the $4d$ subsystem of Ru by the exchange field acting from the Dy $4f$ moments. In an applied magnetic field ${\mathrm{Dy}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ displays magnetization jumps along all crystallographic directions. All the metamagnetic transitions are accompanied by large positive magnetoresistance. The maximum effect (125%--140%) is attained for current along the [100] axis and field along the [120] or [001] axes. The large positive effect is explained by changes in the conduction electron spectra through the jumps as the conduction electrons interact with localized magnetic moments.

Physical properties of EuPd2As2 single crystals

The physical properties of self-flux grown EuPd2As2 single crystals have been investigated by means of magnetization M, magnetic susceptibility χ, specific heat Cp, and electrical resistivity ρ measurements versus temperature T and magnetic field H. The crystal structure was determined using powder x-ray diffraction measurements, which confirmed the ThCr2Si2-type body-centered tetragonal structure (space group I4/mmm) reported previously. The ρ(T ) data indicate that EuPd2As2 is metallic. The χ(T ) data indicate that the Eu+2 moments have spin S = 7/2 with g = 2. Long-range antiferromagnetic (AFM) ordering is apparent from the χ(T ), Cp(T ), and ρ(T ) measurements. For H∥c the χ(T ) indicates two transitions at TN1 = 11.0 K and TN2 = 5.5 K, whereas for H⊥c only one transition is observed at TN1 = 11.0 K. Between TN1 and TN2 the anisotropic χ(T ) data suggest a planar noncollinear AFM structure, whereas at T < TN2 the χ(T ) and M(H, T ) data suggest a spin reorientation transition in which equal numbers of spins cant in opposite directions out of the ab plane. We estimate the critical field at 2 K at which all Eu moments become aligned with the field to be about 22 T. An upturn in ρ at T < TN1 suggests superzone energy gap formation below TN1. This behavior of ρ(T < TN1) is not sensitive to applied magnetic fields up to H = 12 T.

Anomalous giant positive magnetoresistance and heavy fermion like behaviour in Mn11Ge8

Mn11Ge8 undergoes long range ferromagnetic ordering at 274 K followed by non-collinear antiferromagnetic structure below 150 K. Our investigations indicate the existence of large positive magnetoresistance which changes sign depending upon the applied field and temperature. In the temperature variation of resistivity and heat capacity, the sample shows very large coefficients of quadratic and linear terms, respectively. This indicates the existence of giant spin fluctuation despite the fact that the sample is in a magnetically ordered state. The Mn-moments possibly have both localized and itinerant character and the anomaly appears to be associated with the local moment spin fluctuations mediated via itinerant electrons. The large positive magnetoresistance is believed to be originated from the possible development of short range antiferromagnetic clusters on application of magnetic field.

Frustrated pentagonal Cairo lattice in the non-collinear antiferromagnet Bi4Fe5O13F

We report on the crystal structure and magnetism of the iron-based oxyfluoride Bi${}_{4}$Fe${}_{5}$O${}_{13}$F, a material prototype of the Cairo pentagonal spin lattice. The crystal structure of Bi${}_{4}$Fe${}_{5}$O${}_{13}$F is determined by a combination of neutron diffraction, synchrotron x-ray diffraction, and transmission electron microscopy. It comprises layers of FeO${}_{6}$ octahedra and FeO${}_{4}$ tetrahedra forming deformed pentagonal units. The topology of these layers resembles a pentagonal least-perimeter tiling, which is known as the Cairo lattice. This topology gives rise to frustrated exchange couplings and underlies a sequence of magnetic transitions at ${T}_{1}=62$ K, ${T}_{2}=71$ K, and ${T}_{N}=178$ K, as determined by thermodynamic measurements and neutron diffraction. Below ${T}_{1}$, Bi${}_{4}$Fe${}_{5}$O${}_{13}$F forms a fully ordered non-collinear antiferromagnetic structure, whereas the magnetic state between ${T}_{1}$ and ${T}_{N}$ may be partially disordered according to the sizable increase in the magnetic entropy at ${T}_{1}$ and ${T}_{2}$. Bi${}_{4}$Fe${}_{5}$O${}_{13}$F reveals unanticipated magnetic transitions on the pentagonal Cairo spin lattice and calls for a further work on finite-temperature properties of this strongly frustrated spin model.

Magnetic Structure and Properties of the Na2CoP2O7 Pyrophosphate Cathode for Sodium-Ion Batteries: A Supersuperexchange-Driven Non-Collinear Antiferromagnet

The crystal and magnetic structure and properties of the Na(2)CoP(2)O(7) Na(+)-ion battery cathode material have been characterized by magnetic susceptibility, specific heat, and variable-temperature neutron powder diffraction measurements. Na(2)CoP(2)O(7) crystallizes in the orthorhombic space group Pna2(1) with a = 15.4061(3) Å, b = 10.28854(9) Å, and c = 7.70316(15) Å, having a layered structure with slabs of [CoP(2)O(7)](∞) separated by Na cations. The magnetic property measurements and neutron diffraction data analysis reveal that the material undergoes long-range ordering to a noncollinear antiferromagnetic G-type structure below T(N) ≈ 6.5 K. The magnetic structure is rationalized as a result of supersuperexchange between Co(2+) atoms linked by phosphate groups.

Interplay of localized and itinerant character of Ru ions: Tl2Ru2O7versus Hg2Ru2O7

We carry out a comparative study of the electronic structure of two pyrochlore ruthenate compounds, Tl2Ru2O7 and Hg2Ru2O7, in terms of first principles calculations. Our study reveals the Ru d electrons in Hg2Ru2O7 to be much more delocalized compared to that in Tl2Ru2O7. The subtle change in the Ru-d bandwidths in the two compounds, triggered by the differences in Hg 5d-Ru 4d hybridization compared to that of Tl 5d-Ru 4d, bring in the observed differences in behavior. Our study further shows that the development of long range noncollinear antiferromagnetic structure at low temperature is sufficient to produce the insulating solution in Hg2Ru2O7, in line with the prediction from recent nuclear magnetic resonance study.

Magnetization and magnetization reversal in epitaxial Fe/Cr/Co asymmetric spin-valve systems

We have investigated asymmetric Fe/Cr/Co/Cr superlattices with two magnetic layers of Fe and Co, which are different with respect to their magnetic properties: magnetization, coercivity, and magnetic anisotropy. The magnetic layers are weakly coupled via a mediating Cr spacer layer providing an antiferromagnetic alignment of adjacent layers. The magnetic structure of these spin-valve-like Fe/Cr/Co/Cr superlattices was analyzed from the remanent state up to saturation via polarized neutron scattering and polarized neutron reflectivity (PNR). Furthermore, the domain structure in remanence was imaged via polarized x-ray photoemission electron microscopy (XPEEM). This analysis reveals that the Co magnetization strongly affects the Fe domain structure, while the layer magnetization is collinear from the remanent antiparallel state up to the ferromagnetic saturation state. However, for certain Co layer thicknesses, the as-grown remanent state exhibits a noncollinear antiferromagnetic spin structure, which cannot be recovered after applying a magnetic field. However, the noncollinear structure is reproducible with freshly grown superlattices.

Single-crystal study of the kagome antiferromagnet U3Ru4Al12

A ternary intermetallic compound, U${}_{3}$Ru${}_{4}$Al${}_{12}$ in a single-crystal form, was studied by measurement of susceptibility, magnetization, electrical resistivity, magnetoresistivity, thermopower, and heat capacity, as well as neutron diffraction, revealing that this aluminide is an antiferromagnetically ordered (${T}_{\mathrm{N}}$ $=$ 9.5 K) dense Kondo system (${T}_{\mathrm{K}}$ \ensuremath{\approx} 30 K) with a considerable enhancement of the Sommerfeld coefficient $\ensuremath{\gamma}$(0) of about $200\phantom{\rule{0.28em}{0ex}}{\mathrm{mJ}\phantom{\rule{0.28em}{0ex}}\mathrm{mol}}_{\mathrm{U}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$. The electrical resistivity and thermopower of U${}_{3}$Ru${}_{4}$Al${}_{12}$ show characteristic features of the interplay between anisotropic magnetic exchange, Kondo, and crystal field effects. A pronounced in-plane anisotropy and a unique noncollinear antiferromagnetic structure in the (a,b) hexagonal plane, found by neutron diffraction, are discussed in view of the effect of geometrical frustration caused by the distorted kagome lattice of a Gd${}_{3}$Ru${}_{4}$Al${}_{12}$-type crystal structure. Thus, the overall behavior of U${}_{3}$Ru${}_{4}$Al${}_{12}$ resembles that observed for the orthorhombic compound UCu${}_{5}$In, reported earlier as a spin density wave material.

NMR determination of noncollinear antiferromagnetic structure in TbCoGa5

We report NMR studies of TbCoGa${}_{5}$, which has the tetragonal HoCoGa${}_{5}$ structure and exhibits two antiferromagnetic (AF) transitions at ${T}_{\mathrm{N}1}=36.2$ K and ${T}_{\mathrm{N}2}=5.4$ K. From a symmetry analysis of internal magnetic fields at orthorhombic Ga sites, we have successfully determined the magnetic structures in the AF-I (${T}_{\mathrm{N}2}lTl{T}_{\mathrm{N}1}$) and AF-II ($Tl{T}_{\mathrm{N}2}$) phases. The AF-I phase is a collinear AF order with a propagation vector $\mathbit{q}=[1/2,0,1/2]$ and ordered moments parallel to the [001] direction. In the AF-II phase, on the other hand, we found a noncollinear AF structure described by double propagation vectors ${\mathbit{q}}_{\mathbf{1}}=[1/2,0,1/2]$ and ${\mathbit{q}}_{\mathbf{2}}=[0,1/2,1/2]$, where the moments tilt away from the [001] direction toward [100], keeping a constant value along the [001] direction. In the context of these results, we discuss the possible presence of magnetic frustration in this system.

Giant magnetoresistance in IrMn based antiferromagnetic spin valve

According to the parameter-free first principles calculations, we investigate the spin polarized transport in antiferromagnetic spin valve (AFSV) based on noncollinear IrMn. The giant magnetoresistance (GMR) in Co/IrMn/Cu(111) with L12-type IrMn shows three-fold rotational symmetry, which is easy to be distinguished from the GMR of normal ferromagnetic spin valves. Moreover, GMR based on -phase IrMn with current-in-plane (CIP) structure shows that GMR is 7.7%, around two time larger than that in current-pendicular-to-plane (CPP) structure (3.4%). Our study demonstrates that the AFSV CIP structure possesses a larger GMR effect than the CPP structure, and the GMR effect in AFSV based on noncollinear antiferromagnetic structure is larger than that based on collinear antiferromagnetic structure such as FeMn.