Canted Antiferromagnetic Phase Research Articles

Pressure induced magneto-structural phase transitions in layered RMn2X2 compounds (invited)

We have studied a range of pseudo-ternaries derived from the parent compound PrMn2Ge2, substituting for each constituent element with a smaller one to contract the lattice. This enables us to observe the magneto-elastic transitions that occur as the Mn-Mn nearest neighbour distance is reduced and to assess the role of Pr on the magnetism. Here, we report on the PrMn2Ge2−xSix, Pr1−xYxMn2Ge2, and PrMn2−xFexGe2 systems. The pressure produced by chemical substitution in these pseudo-ternaries is inherently non-uniform, with local pressure variations dependent on the local atomic distribution. We find that concentrated chemical substitution on the R or X site (e.g., in Pr0.5Y0.5Mn2Ge2 and PrMn2Ge0.8Si1.2) can produce a separation into two distinct magnetic phases, canted ferromagnetic and canted antiferromagnetic, with a commensurate phase gap in the crystalline lattice. This phase gap is a consequence of the combination of phase separation and spontaneous magnetostriction, which is positive on transition to the canted ferromagnetic phase and negative on transition to the canted antiferromagnetic phase. Our results show that co-existence of canted ferromagnetic and antiferromagnetic phases depends on chemical pressure from the rare earth and metalloid sites, on local lattice strain distributions and on applied magnetic field. We demonstrate that the effects of chemical pressure bear close resemblance to those of mechanical pressure on the parent compound.

Origin of spin-glass and exchange bias in La1∕3Sr2∕3FeO3−γ nanoparticles

The structure and the magnetic properties of nanopowdered samples of La1∕3Sr2∕3FeO3−γ with average particles size d in the range of 67-367 nm prepared by a sol-gel method were investigated in detail. The samples were characterized by X-ray diffraction, scanning electron microscopy, specific heat, Mössbauer spectroscopy, ac susceptibility, and magnetization measurements. Exchange bias with vertical magnetization shift was found in all samples. Charge ordering and antiferromagnetism were observed close to 200 K for large particles (d ≥ 304 nm) samples, while for particles with intermediated and smaller values (d ≤ 156 nm) a cluster-glass like behaviour and a short range charge ordering were seen near 115 K and 200 K, respectively. The spin-glass like and exchange bias behaviour in nanopowdered samples of La1∕3Sr2∕3FeO3−γ are associated to compact Fe3+ antiferromagnetic (AF) clusters caused by an oxygen deficiency, which was found to be higher in the samples with the smallest average particles sizes. The effect of exchange bias and vertical magnetization shifts are explained by a simple model involving the interaction of one AF phase with a canted AF phase.

Phases and collective modes of Rydberg atoms in an optical lattice

We chart out the possible phases of laser-driven Rydberg atoms in the presence of a hypercubic optical lattice. We define a pseudospin degree of freedom whose up (down) components correspond to the excited (ground) states of the Rydberg atoms and use them to demonstrate the realization of a canted Ising antiferromagnetic (CIAF) Mott phase of the atoms in these systems. We also show that, on lowering the lattice depth, the quantum melting of the CIAF and density-wave Mott states (which are also realized in these systems) leads to supersolid phases of the atoms. We provide analytical expressions for the phase boundaries and collective excitations of these phases in the hard-core limit within mean-field theory and discuss possible experiments to test our theory.

Pressure Induced Crossover between a Ferromagnetic and a Canted Antiferromagnetic State for [Bis(pentamethylcyclopentadienyl)-iron(III)][Tetracyanoethenide], [FeCp2*][TCNE

The reversible hydrostatic pressure dependent DC magnetic behavior of the ferromagnetically ordered electron transfer salt [Fe(III)Cp2*](•+)[TCNE](•-) (Cp* = pentamethylcyclopentadienide; TCNE = tetracyanoethylene) was studied up to 12.2 kbar. A significant departure from the ambient pressure ferromagnetic behavior was observed under pressure. The temperature dependent magnetization data were typical of a ferromagnet at ambient pressure but exhibited an extreme reduction with increasing applied pressure, while metamagnetic-like behavior was evident in the field dependent magnetization data at 4.2 kbar and above. Hence, the decrease of the intermolecular separations due to increasing pressure enhances the nearest neighbor couplings, leading to an increase in magnetic ordering temperature, T(c). Furthermore, the presence of a metamagnetic-like behavior suggests an increase of the antiferromagnetic contribution to the interchain interactions. The low field magnetization data indicate that spin canting is induced by pressure, leading to a canted antiferromagnetic phase with a much lower magnetization than the low-pressure ferromagnetic state. This unprecedented magnetic behavior is consistent with the field, temperature, and pressure dependences of the magnetization below 20 K.

Specific heat and magnetocaloric effect of Pr1−x Ag x MnO3 manganites

This paper presents experimental results of the specific heat and magnetocaloric effect in Pr1−x Ag x MnO3 (x = 0.05–0.25) manganites. A maximum value of magnetic entropy change for the sample with x = 0.15 was observed in magnetic field of 18 kOe at T C = 125 K and was ΔS max ≈ 2.96 J/kg K. In Pr0.95Ag0.05MnO3 sample, a sign inversion of the magnetocaloric effect was observed at low temperatures (~80 K) in low magnetic fields of 750 Oe. The coexistence of ferromagnetic and canted antiferromagnetic phases with closely spaced critical temperatures was found to force the magnetocaloric effect into abrupt sign inversion.

Magnetic entropy change plateau in a geometrically frustrated layered system: FeCrAs-like iron-pnictide structure as a magnetocaloric prototype

Monte Carlo modeling suggests that the magnetothermal features of the Fe2P-structured FeCrAs-like compound offer a promising route for the design of magnetocaloric materials. The prototype structure is modeled as antiferromagnetically coupled layered Heisenberg systems mimicking the distorted Kagome/triangular stacked architecture of FeCrAs iron-pnictide. The magnetic entropy change ΔSm(T) presents a plateau-like behavior which can be tailored by tuning either the JCr−Fe/JCr−Cr exchange energy ratio or the magnetic field. The plateau is defined by cooperative spin ordering within a ferrimagnetic region which exists between two critical temperatures separating at the lower bound () a canted antiferromagnetic phase and at the upper bound () the thermally disordered phase. The refrigerant capacity and adiabatic change of temperature are and A(H)Tp/Cm respectively, with , A(H) an increasing positive function of the field defining the height of the plateau and Cm the magnetic specific heat, whose critical behavior is related to the values.

Nambu–Goldstone modes and the Josephson supercurrent in the bilayer quantum Hall system

An interlayer phase coherence develops spontaneously in the bilayer quantum Hall system at the filling factor $\nu =1$. On the other hand, the spin and pseudospin degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor $\nu =2$. There emerges a complex Nambu-Goldstone mode with a linear dispersion in the zero tunneling-interaction limit for both cases. Then its phase field provokes a Josephson supercurrent in each layer, which is dissipationless as in a superconductor. We study what kind of phase coherence the Nambu-Goldstone mode develops in association with the Josephson supercurrent and its effect on the Hall resistance in the bilayer quantum Hall system at $\nu=1,2$, by employing the Grassmannian formalism.

Nematic phase and phase separation near saturation field in frustrated ferromagnets

We study the effects of quantum fluctuations in magnetic properties of quantum frustrated ferromagnets in a magnetic field. It is shown that a non-classical phase or a phase separation appears due to quantum fluctuations below the saturation field in a parameter range close to the classical zero-field phase boundary between ferromagnetic and antiferromagnetic phases, for the case that the classical antiferromagnetic state is not an eigenstate of the quantum model. As an example to which this argument is applicable, we study the S=1/2 J1-J2 Heisenberg model with ferromagnetic J1 (J1<0) on the bcc lattice using a dilute Bose gas approach. For -1.50097 < J1/J2 < -1.389, magnons form f-wave two-magnon bound states, leading to a spin nematic phase, and for -1.389 < J1/J2 < -0.48 a canted coplanar antiferromagnetic phase appears accompanied with a phase separation below the saturation field.

Spin supercurrent in the canted antiferromagnetic phase

The spin and layer (pseudospin) degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor $\nu =2$. There emerges a complex Goldstone mode describing such a combined degree of freedom. In the zero tunneling-interaction limit ($\Delta_{\text{SAS}}\rightarrow 0$), its phase field provokes a supercurrent carrying both spin and charge within each layer. The Hall resistance is predicted to become anomalous precisely as in the $\nu =1$ bilayer system in the counterflow and drag experiments. Furthermore, it is shown that the total current flowing in the bilayer system is a supercurrent carrying solely spins in the counterflow geometry. It is intriguing that all these phenomena occur only in imbalanced bilayer systems.

Spontaneous reorientation of the electric polarization in Eu1 − x Ho x MnO3 multiferroics

The magnetic, dielectric, and ferroelectric properties of Eu1 − x Ho x MnO3 single crystals (0 < x ≤ 0.5), where magnetic ordering can be varied from the canted antiferromagnetic phase to modulated spin structures, have been studied. It has been found that a ferroelectric state appears at x ≥ 0.2 and low temperatures. As the temperature decreases and the holmium content increases, the electric polarization in this state is reoriented from the a axis to the c axis. It has been shown that the polarization is reoriented owing to a change in the spin rotation plane in the cycloidal phase from the ab to cb plane because of the stabilization of the latter upon an increase in the rare-earth contribution to the anisotropy energy. The T-x phase diagram of magnetic and ferroelectric states has been constructed.

Phase diagrams of strained Ca1–xCexMnO3 films

The transport and magnetic properties were examined of thin films of the electron-doped manganite Ca1−xCexMnO3 (CCMO; 0 ≤ x ≤ 0.1) deposited on three different single-crystalline oxide substrates: YAlO3, NdAlO3, and LaSrAlO4, which subject the films to a compressive strain, a practical lattice matching, and a tensile strain, respectively. The temperature dependence of the activation energy extracted from the resistivity–temperature curves clearly indicated the phase transitions in the CCMO films. A comparison with magnetization measurements revealed that regardless of the substrates, the temperature-driven phase transition of the CCMO films changed from the G-type antiferromagnetic transition at x = 0 to the canted antiferromagnetic transition and to the charge- and/or orbital-ordered transition, as x was increased. In the canted antiferromagnetic phase, the epitaxial strains have significant impact not only on the transport properties but also on the magnetic properties such as the spontaneous magnetization and the magnetic anisotropy, confirming that the competition between antiferromagnetic super-exchange and ferromagnetic double-exchange interactions results in the canted antiferromagnetic state in the electron-doped CCMO films. Based on the results of the transport and magnetic measurements, the temperature versus band-filling phase diagrams of the strained CCMO films on three different substrates were established.

XBaTiO3–(1−x)BiFeO3 strained epitaxial thin film with enhanced magnetization

Strained epitaxial xBaTiO3–(1−x)BiFeO3 (x=0–0.2) thin films were grown on STO (001) substrate by radio frequency magnetron sputtering. The crystal structure of the as-deposited thin films were investigated using the combination of powder X-ray diffraction (XRD) and high resolution X-ray diffraction reciprocal space mapping (HRXRD–RSM). The relationship of the crystal structure and magnetic properties of the as-deposited thin films was also investigated. The larger c/a values provide the iron ion with a non-vanished orbital quenching effect, which produces the strong spin-orbital coupling that suppresses the spiral spin structure and initiates a canted antiferromagnetic phase with larger magnetizations at room temperature.

Edge excitations of the canted antiferromagnetic phase of theν=0quantum Hall state in graphene: A simplified analysis

We perform a simplified analysis of the edge excitations of the canted antiferromagnetic (CAF) phase of the $\nu=0$ quantum Hall state in both monolayer and bilayer graphene. Namely, we calculate, within the framework of quantum Hall ferromagnetism, the mean-field quasiparticle spectrum of the CAF phase neglecting the modification of the order parameter at the edge. We demonstrate that, at a fixed perpendicular component $B_\perp$ of the magnetic field, the gap $\Delta_\text{edge}$ in the edge excitation spectrum gradually decreases upon increasing the parallel component $B_\parallel$, as the CAF phase continuously transforms to the fully spin-polarized ferromagnetic (F) phase. The edge gap closes completely ($\Delta_\text{edge}=0$) once the F phase, characterized by gapless counter-propagating edge excitations, is reached at some finite $B_\perp$-dependent value $B_\parallel^*$ and remains closed upon further increase of $B_\parallel$. This results in an gradual insulator-metal transition, in which the conductance $G \sim (e^2/h) \exp(-\Delta_\text{edge}/T)$ grows exponentially with $B_\parallel$ in the range $0<B_\parallel<B_\parallel^*$, while in the gapped CAF phase, and saturates to a metallic value $G \sim e^2/h$ in the F phase at $B_\parallel>B_\parallel^*$. This unique transport feature of the CAF phase provides a way to identify and distinguish it from other competing phases of the $\nu=0$ quantum Hall state in a tilted-field experiment.

Canted Antiferromagnetic Phase of theν=0Quantum Hall State in Bilayer Graphene

Motivated to understand the nature of the strongly insulating ν=0 quantum Hall state in bilayer graphene, we develop the theory of the state in the framework of quantum Hall ferromagnetism. The generic phase diagram, obtained in the presence of the isospin anisotropy, perpendicular electric field, and Zeeman effect, consists of the spin-polarized ferromagnetic (F), canted antiferromagnetic (CAF), and partially (PLP) and fully (FLP) layer-polarized phases. We address the edge transport properties of the phases. Comparing our findings with the recent data on suspended dual-gated devices, we conclude that the insulating ν=0 state realized in bilayer graphene at lower electric field is the CAF phase. We also predict a continuous and a sharp insulator-metal phase transition upon tilting the magnetic field from the insulating CAF and FLP phases, respectively, to the F phase with metallic edge conductance 2e(2)/h, which could be within the reach of available fields and could allow one to identify and distinguish the phases experimentally.

Experimental charge density of hematite in its magnetic low temperature and high temperature phases

Structural parameters of hematite (α-Fe(2)O(3)), including the valence electron distribution, were investigated using convergent beam electron diffraction (CBED) in the canted antiferromagnetic phase at room temperature and in the collinear antiferromagnetic phase at 90K. The refined charge density maps are interpreted as a direct result of electron-electron interaction in a correlated system. A negative deformation density was observed as a consequence of closed shell interaction. Positive deformation densities are interpreted as a shift of electron density to antibinding molecular orbitals. Following this interpretation, the collinear antiferromagnetic phase shows the characteristic of a Mott-Hubbard type insulator whereas the high temperature canted antiferromagnetic phase shows the characteristic of a charge transfer insulator. The break of the threefold symmetry in the canted antiferromagnetic phase was correlated to the presence of oxygen-oxygen bonding, which is caused by a shift of spin polarized charge density from iron 3d-orbitals to the oxygen ions. We propose a triangular magnetic coupling in the oxygen planes causing a frustrated triangular spin arrangement with all spins lying in the oxygen planes. This frustrated arrangement polarizes the super-exchange between iron ions and causes the spins located at the iron ions to orient in the same plane, perpendicular to the threefold axis.

Structural expansion and suppression of spiral spin state in Pb-doped BiFeO3 (00l) epitaxial thin films

We demonstrate that Pb-doped BiFeO3 thin films grown on a strain relaxed SrRuO3 bottom layer on SrTiO3 (STO) substrate undergo a structural expansion effect that is highly dependent on growth temperature, TS. Highly symmetric cubic-like or low symmetric tetragonal structures can be grown at lower TS (= 650 °C) and higher TS (= 700 °C), respectively. The structural expansion and changes in symmetry provide the iron ion with a non-vanished orbital quenching effect, which produces the strong spin-orbital coupling that suppresses the spiral spin structure and initiates a canted antiferromagnetic phase with a larger net saturated magnetic moment at room temperature.

Magnetic properties of rare earth HoCrO 3 chromites

The temperature dependence of the magnetic properties was systemically studied by dc/ac magnetization and specific heat measurement for heavy rare earth HoCrO 3 chromites. The results revealed the existence of complex phase coexistence and competitive magnetic behavior in HoCrO 3 chromites. It was found that, in the region of higher temperature above 141.0 K, HoCrO 3 behaved as a typical Curie-Weiss paramagnetic (PM). And in the region of low temperature, a novel magnetization behavior was observed with negative magnetization (diamagnetism-like) characteristics under an external field of 100 Oe and M-T curves exhibited two symmetrical branches for field cooling (FC) and zero field cooling (ZFC) modes. This behavior indicated the coexistence of canted antiferromagnetic (CAFM) and weak ferromagnetic (FM) phase. These also exhibited the existence of competition mechanism below characteristic temperature T N1=141.0 K and the magnetic order of Ho ion below 7.5 K. The current complex magnetization might be attributed to the interaction between paramagnetic Ho 3+ moments and canted Cr 3+ moments.

Anisotropic magnetic order of the Eu sublattice in single crystals of EuFe2−xCoxAs2(x=0,0.2) studied by means of magnetization and magnetic torque

Here, we present a combination of magnetization and magnetic torque experiments to investigate the magnetic orders in undoped EuFe2As2 and Co doped EuFe1.8Co0.2As2 single crystals. Although at low temperatures typical results for an antiferromagnetic (AFM) state in EuFe2As2 were found, our data strongly indicate the occurrence of a canted antiferromagnetic (C-AFM) order of the Eu2+ moments between 17 K and 19 K, observed even in the lowest studied magnetic fields. However, unlike in the parent compound, no low-field and low-temperature AFM state of the Eu^2+ moments was observed in the doped EuFe1.8Co0.2As2. Only a C-AFM phase is present at low fields and low temperatures, with a reduced magnetic anisotropy as compared to the undoped system. We present and discuss for both, EuFe2As2 and EuFe1.8Co0.2As2, the experimentally deduced magnetic phase diagrams of the magnetic ordering of the Eu^2+ sublattice with respect to the temperature, the applied magnetic field, and its orientation to the crystallographic axes. It is likely that the magnetic coupling of the Eu and the Fe sublattice is strongly depending on Co doping, having detrimental influence on the magnetic phase diagrams as determined in this work. Their impact on the occurrence of superconductivity with higher Co doping is discussed.

Novel Magnetization Induced by Phase Coexistence in Multiferroic HoCrO3 Chromites

Single-phase HoCrO3 samples were prepared using the conventional solid-state reaction. The temperature dependence of dc magnetic properties show that the ZFC and FC magnetization curves are irreversible strongly and nearly symmetrical below 141.0 K under 100 Oe. Moreover, there appears a novel magnetization behavior with negative magnetization (diamagnetism-like) characteristics. This behavior exhibits the coexistence of canted antiferromagnetic and weak ferromagnetic phase and the existence of competition mechanism below Tc1 = 141.0 K. In addition, it is also proved that Ho ion ordered weak CAFM below 7.5 K. The current novel magnetization can be interpreted from the interaction between paramagnetic Ho3+ moments and canted Cr3+ moments.

Magnetocaloric effect in Pr1 − x Ag x MnO3 manganites

The magnetocaloric effect in alternating magnetic fields has been investigated in Pr1-xAgxMnO3 manganites with x=0.05-0.25. The stepwise reversal of the sign of the magnetocaloric effect has been revealed in a weakly doped sample (x=0.05) at low temperatures (~80 K). This reversal is attributed to the coexistence of the ferromagnetic and canted antiferromagnetic phases with different critical temperatures.