Collinear Antiferromagnetic Phase Research Articles

Magnetic phase diagram ofα-NdIr2Si2

Magnetism in $\ensuremath{\alpha}$-NdIr${}_{2}$Si${}_{2}$ single crystals has been probed through specific-heat, magnetization, magnetic susceptibility, and electrical resistivity measurements as a function of temperature, under an applied magnetic field, and for the magnetization curves under high hydrostatic pressures. Neutron diffraction experiments performed with powder and single-crystal samples were focused on microscopic aspects. Two collinear (Nd moments along the $c$ axis) antiferromagnetic phases are found at low temperatures: a longitudinal sine-modulated structure with the magnetic propagation vector k $=$ (0 0 5/6) at temperatures 18.2(2) K $lTl$ 32.3(3) K ($+{T}_{N}$) and a simple AF1-type structure; k $=$ (0 0 1/2) for $Tl$ 18.2(2) K ($+{T}_{t}$). Both of the magnetically ordered phases can be destabilized by applying a strong magnetic field along the $c$ axis. The character of the unusual magnetic-field-induced phases is discussed, and the complex magnetic phase diagram is constructed. Applied hydrostatic pressure yields an increase of both ${T}_{N}$ and ${T}_{t}$. The determined physical properties of $\ensuremath{\alpha}$-NdIr${}_{2}$Si${}_{2}$ are discussed in the context of behavior of $\ensuremath{\alpha}$-PrIr${}_{2}$Si${}_{2}$.

Second-order quantum corrections for the frustrated spatially anisotropic spin-12Heisenberg antiferromagnet on a square lattice

The effects of quantum fluctuations due to directional anisotropy and frustration between nearest neighbors and next-nearest neighbors of the quantum spin-1/2 Heisenberg antiferromagnet on a square lattice are investigated using spin-wave expansion. We have calculated the spin-wave energy dispersion in the entire Brillouin zone, renormalized spin-wave velocities, and the magnetization up to second order in 1/S expansion for the antiferromagnetic Neel and collinear antiferromagnetic stripe phases. It is shown that the second-order corrections become significant with increase in frustration. With these corrections magnetizations and spin-wave velocities for both the phases become zero at the quantum critical points as expected from other numerical and analytical methods. We have shown that the transition between the two ordered phases are always separated by the disordered paramagnetic phase.

The ground state phase diagram of the quantumJ1–J2 spin-1/2 Heisenberg antiferromagnet on an anisotropic square lattice

We have studied the ground state phase diagram of the quantum spin-1/2 frustrated Heisenberg antiferromagnet on a square lattice by using theframework of the differential operator technique. The Hamiltonian is solvedby using an effective-field theory for a cluster with two spins (EFT-2). Themodel is described using the Heisenberg Hamiltonian with two competingantiferromagnetic interactions: nearest neighbor (NN) with different coupling strengthsJ1 andJ1′ alongthe x and y directions and next nearest neighbor (NNN) with couplingJ2. We propose a functional for the free energy (similar to the Landau expansion)and using Maxwell construction we obtain the phase diagram in the (λ, α) space,where λ = J1′/J1 and α = J2/J1. We obtain three different states depending on the values ofλ andα: antiferromagnetic (AF), collinear antiferromagnetic (CAF)and quantum paramagnetic (QP). For an intermediate regionλ1 < λ < 1 we observe a QP state between the ordered AF and CAF phases, which disappears forλ above some critical value . We find a second-order phase transition between the AF and QP phases and a first-ordertransition between the CAF and QP phases. The boundaries between these orderedphases merge at the quantum triple point (QTP). Below this QTP there is again adirect first-order transition between the AF and CAF phases, with a behaviorapproximately described by the classical line .

Collinear antiferromagnetic state in a two-dimensional Hubbard model at half filling

In a half-filled Hubbard model on a square lattice, the next-nearest-neighbor hopping causes spin frustration, and the collinear antiferromagnetic (CAF) state appears as the ground state with suitable parameters. We find that there is a metal-insulator transition in the CAF state at a critical on-site repulsion. When the repulsion is small, the CAF state is metallic, and a van Hove singularity can be close to the Fermi surface, resulting in either a kink or a discontinuity in the magnetic moment. When the on-site repulsion is large, the CAF state is a Mott insulator. A first-order transition from the CAF phase to the antiferromagnetic phase and a second-order phase transition from the CAF phase to the paramagnetic phase are obtained in the phase diagram at zero temperature.

The magnetic order of GdMn2Ge2 studied by neutron diffraction and x-ray resonant magnetic scattering

The magnetic structure of GdMn2Ge2 (tetragonal I4/mmm) has been studied by hot neutron powder diffraction and x-ray resonant magneticscattering techniques. These measurements, along with the results of bulk experiments,confirm the collinear ferrimagnetic structure with moment direction parallel to thec-axisbelow TC = 96 K and the collinear antiferromagnetic phase in the temperature regionTC < T < TN = 365 K. Inthe antiferromagnetic phase, x-ray resonant magnetic scattering has been detected at Mn K and GdL2 absorption edges. The Gd contribution is a result of an induced Gd 5d electron polarizationcaused by the antiferromagnetic order of Mn-moments.

Zero Temperature Phases of the Frustrated J 1–J 2 Antiferromagnetic Spin-1/2 Heisenberg Model on a Simple Cubic Lattice

At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg antiferromagnet on a simple cubic lattice with competing first and second neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave theory. We find existence of two phases: a two sublattice Neel phase for small J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain the sublattice magnetizations and ground state energies for the two phases and find that there exists a first order phase transition from the AF-phase to the CAF-phase at the critical transition point, pc = 0.28. Our results for the value of pc are in excellent agreement with results from Monte-Carlo simulations and variational spin wave theory. We also show that the quartic 1/S corrections due spin-wave interactions enhance the sublattice magnetization in both the phases which causes the intermediate paramagnetic phase predicted from linear spin wave theory to disappear.

High field properties of S = 1/2 square lattice J1-J2 magnetic compounds

We discuss the two-dimensional S = 1/2 J1-J2 frustrated Heisenberg model on a square lattice. This model provides a good description of two classes of quasi-two-dimensional vanadates, namely the Li2VOXO4 (X = Si,Ge) and AA′VO(PO4)2 (A,A′ = Pb, Zn, Sr,Ba,Cd) compounds. We use the finite-temperature Lanczos method and spin wave analysis to study the high-field properties of this model in the whole J1-J2 plane.Of particular interest is the saturation field of the low-temperature magnetization. Together with the estimates for the average effective exchange constant from the zero-field heat capacity and ΘCW = (J1 + J2)/kB from the uniform magnetic susceptibility, the saturation field determines the position of a particular compound in the phase diagram uniquely.We give a summary of the results for the experimentally known compounds and show that our numerical findings agree very well with the experimental data for the magnetic susceptibility and the field dependence of the magnetization of the new compound BaCdVO(PO4)2. This compound is close to the quantum-critical point of the spin-nematic and collinear antiferromagnetic phase of the J1-J2 model as witnessed by a pronounced nonlinear magnetization curve.

Two magnetic orderings and a spin-flop transition in spin-1 systemSrNi2(PO4)2

Magnetic properties of ${\text{SrNi}}_{2}{({\text{PO}}_{4})}_{2}$ are investigated by means of susceptibility, magnetization, heat-capacity, and $^{31}\text{P}$ NMR measurements. Our experimental results show that ${\text{SrNi}}_{2}{({\text{PO}}_{4})}_{2}$ is a three-dimensional antiferromagnet with two magnetic orderings at $\ensuremath{\sim}23$ and $\ensuremath{\sim}10\text{ }\text{K}$. It is suggested that canted antiferromagnetic ordering at $\ensuremath{\sim}23\text{ }\text{K}$ may correspond to a N\'eel noncollinear spin arrangements of ${\text{Ni}}^{2+}$ ions, and such noncollinear spin arrangements further induce spin rotations into a steady collinear antiferromagnetic phase below $\ensuremath{\sim}10\text{ }\text{K}$ with decreasing temperature. Also, spin-flop transition is observed at a critical field below $\ensuremath{\sim}10\text{ }\text{K}$, showing magnetic anisotropy in the system.

Two-dimensional quantum antiferromagnet with a fourfold degenerate dimer ground state

We study the competition between antiferromagnetic order and valence bond solid formation in a two-dimensional frustrated spin-1/2 model. The J1-J2 model on the square lattice is further frustrated by introducing products of three-spin projectors which stabilize four dimer-product states as degenerate ground states. These four states are reminiscent of the dimerized singlet ground state of the Shastry-Sutherland model. Using exact diagonalizations, we study the evolution of the ground state by varying theratio of interactions. For a large range of parameters (J2 > 0.25J1), our model shows a direct transition between the valence-bond-solid phase and the collinear antiferromagnetic phase. For small values of J2, several intermediate phases appear which are also analyzed.

Collinear Order in Frustrated Quantum Antiferromagnet on Square Lattice (CuBr)LaNb2O7

Magnetic susceptibility, heat capacity, high-field magnetization and neutron diffraction measurements have been performed on a two-dimensional S ¼ 1=2 square-lattice system (CuBr)LaNb2O7, prepared by a topotactic ion-exchange reaction of a nonmagnetic double-layered perovskite RbLaNb2O7. (CuBr)LaNb2O7 exhibits a second-order magnetic transition at 32 K, in marked contrast to a spin-singlet nature for its Cl-based counterpart (CuCl)LaNb2O7, despite nearly identical structural parameters. The magnetic structure is a novel collinear antiferromagnetic (CAF) ordering characterized by a modulation vector q ¼ð �; 0 ;� Þ with a reduced moment of 0.6� B. Mixed ferromagnetic nearest-neighbor (J1) and antiferromagnetic second-nearest-neighbor (J2) interactions are of comparable strength (J1=kB ¼ � 35:6 K and J2=kB ¼ 41:3 K), placing the system in a more frustrated region of the CAF phase than ever reported.

Nematic order in square lattice frustrated ferromagnets

We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose–Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic J 1 – J 2 model. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.

Canted phase of an antiferromagnetic Anderson lattice

On the basis of extended periodic Anderson model with antiferromagnetic (AFM) exchange and s–f exchange interactions the energy spectrum of heavy-fermion (HF) AFM metals is calculated. The quasiparticle band structure in collinear AFM phase is described by four splitted bands one of which is very narrow (order of exchange interaction constant) and lies in the vicinity of a localized level. Application of an external magnetic field causes a canting of the sublattice magnetization and due to s–f exchange interaction strongly modifies the HF band picture. Thus eight different quasiparticle bands appear. Their evolution when temperature and magnetic field change is studied.

Nematic Order in Square Lattice Frustrated Ferromagnets

We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose-Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored in some detail for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long-range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic model J1-J2. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.

The magnetization process in the 2-dimensional J 1 -J 2 model

We study the alpha = J_2/J_1-dependence of the magnetization process in the J_1-J_2 model on a square lattice with frustrating couplings J_2 along the diagonals. Perturbation expansions around alpha=J_2/J_1=0 and 1/alpha=0$ yield an adequate description of the magnetization curve in the antiferromagnetic and collinear antiferromagnetic phase, respectively. The transition from one phase to the other (0.5 < alpha < 0.7) leaves pronounced structures in the longitudinal and transverse structure factors at p=(pi,pi) and p=(0,pi).

Roughness of layer interfaces and phase diagram of magnetic multilayer structures

The dependence of the type of magnetic ordering in multilayer structures on the magnitude of the exchange interaction between the magnetic layers and on the degree of roughness of the interfaces between the layers is investigated. In the case of a three-layer system the regions of existence of the domain structure, of noncollinear ordering of the homogeneous magnetic layers, and of the collinear ferromagnetic phase and collinear antiferromagnetic phase are found.

Noncollinear Phase of Magnetic Multilayers

ABSTRACTThere is considered the dependence of magnetic ordering type in multilayer structures on the magnitude of exchange interaction between magnetic layers and the interlayer surfaces roughness. With a simple example of the three-layered structure we found the areas of the existence of such states as: (i) domain structure, (ii) noncollinear ordering of homogeneous magnetic layers, (iii) collinear ferromagnetic phase, and (iiii) collinear antiferromagnetic phase.

Magnetic incommensurability in hexagonal HoGa2

Specific heat, powder neutron diffraction experiments and magnetization measurements on a single crystal of the hexagonal HoGa2 compound are presented. Below TN = 7.6 K two collinear antiferromagnetic phases (M∥a of the orthohexagonal cell) are observed: (i) a simple collinear antiferromagnetic structure with Q1 =(0, 1, 0) (in the orthohexagonal cell) below Tt = 6.5 K; (ii) a mixing of this phase (about 36%) and of an incommensurate amplitude modulated structure (about 64%) with Q2 =(0.123, 1, 0) for Tt <T <TN. Below TN smooth metamagnetic processes are observed along both directions of the easy basal plane. The second order crystal field parameter A02, deduced from the shift of the reciprocal susceptibilities along and perpendicular to c, is significantly smaller than in the other RGa2 compounds.

Magnetic phase transition in ϵ-(Mn 1− xFe x) 3+δGe

A pseudo-binary system of ϵ-(Mn 1− x Fe x ) 3+δGe (hexagonal DO 19 type structure) has a collinear antiferromagnetic phase in a composition range near x = 0.2 at low temperatures. A field-induced transition from the collinear antiferromagnetic phase to the triangular antiferromagnetic phase (higher temperature phase) was observed in high fields of about 100 kOe.

Magnetic field induced transitions in Mn5Si3

The magnetic properties of the intermetallic compound Mn5Si3 with D88-type crystallographic ordering have been investigated at high fields and low temperatures. Magnetisation measurements in fields up to 12 T reveal that it is possible to induce transitions from the low temperature non-collinear antiferromagnetic phase by the application of a critical field which reduces in an approximately quadratic fashion with increasing temperature. The transition takes place over a field range of about 4 T wide and is accompanied by a large increase in susceptibility and hysteresis. At higher fields the behaviour seems to be unsaturated and similar to that observed for the collinear antiferromagnetic phase which is stable in the range 70–95 K. In addition, for both phases another transition which appears to be of second order occurs at a much smaller field involving a decrease in susceptibility for the non-collinear phase and an increase in susceptibility for the collinear phase.

Magnetic transitions in Pr2NiO4 single crystal

The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first-order character and involves both the out-of-plane and the in-plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first-order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out-of-phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field-induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high-field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4.