Canted Phase Research Articles

Element-selective magnetometry in ferrimagnetic erbium iron garnet

The emergence of a field induced canted phase below a critical temperature is one of the characteristic properties of ferrimagnets with two inequivalent antiferromagnetically coupled sublattices. Using x-ray magnetic circular dichroism at the Fe $K$ edge, we have performed element selective magnetometry in ferrimagnetic erbium iron garnet in fields up to 30 T. The signal from the tetrahedral Fe sites at 70 K allows the detection of the two transitions at 10 and 23 T bounding the canted phase and the direct observation of the reversal of the Fe-sublattice magnetization within this phase.

Spin-12Heisenberg antiferromagnet on an anisotropic kagome lattice

We use the coupled cluster method to study the zero-temperature properties of an extended two-dimensional Heisenberg antiferromagnet formed from spin-1/2 moments on an infinite spatially anisotropic kagome lattice of corner-sharing isosceles triangles, with nearest-neighbor bonds only. The bonds have exchange constants $J_{1}>0$ along two of the three lattice directions and $J_{2} \equiv \kappa J_{1} > 0$ along the third. In the classical limit the ground-state (GS) phase for $\kappa < 1/2$ has collinear ferrimagnetic (N\'{e}el$'$) order where the $J_2$-coupled chain spins are ferromagnetically ordered in one direction with the remaining spins aligned in the opposite direction, while for $\kappa > 1/2$ there exists an infinite GS family of canted ferrimagnetic spin states, which are energetically degenerate. For the spin-1/2 case we find that quantum analogs of both these classical states continue to exist as stable GS phases in some regions of the anisotropy parameter $\kappa$, namely for $0<\kappa<\kappa_{c_1}$ for the N\'{e}el$'$ state and for (at least part of) the region $\kappa>\kappa_{c_2}$ for the canted phase. However, they are now separated by a paramagnetic phase without either sort of magnetic order in the region $\kappa_{c_1} < \kappa < \kappa_{c_2}$, which includes the isotropic kagome point $\kappa = 1$ where the stable GS phase is now believed to be a topological ($\mathbb{Z}_2$) spin liquid. Our best numerical estimates are $\kappa_{c_1} = 0.515 \pm 0.015$ and $\kappa_{c_2} = 1.82 \pm 0.03$.

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.

From the Cooper Problem to Canted Supersolids in Bose-Fermi Mixtures

We calculate the phase diagram of the Bose-Fermi Hubbard model on the 3d cubic lattice at fermionic half filling and bosonic unit filling by means of single-site dynamical mean-field theory. For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce s-wave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the interspecies coupling strength. In contrast, slow bosons favor fermionic charge density wave structures for attractive fermionic interactions. These competing instabilities lead to a rich phase diagram within reach of cold gas experiments.

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.

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.

Monte Carlo and variational calculations of the magnetic phase diagram of CuFeO2

Monte-Carlo and variational calculations are used to revise the phase diagram of the magnetically- frustrated material CuFeO2. For fields 50 T < H < 65 T, a new spin flop phase is predicted between a canted three-sublattice phase and the conventional conical spin-flop phase. This phase has wavevector Q (0.8 , 0.43 ) and is commensurate in the x direction but incommensurate in the y direction. A canted five-sublattice phase is predicted between the multiferroic phase and either a collinear five-sublattice phase for pure CuFeO2 or a canted three-sublattice phase for Al- or Ga-doped CuFeO2.

Zero- and finite-temperature mean field study of magnetic field induced electric polarization in Ba2CoGe2O7: Effect of the antiferroelectric coupling

We investigate the spin induced polarization in the multiferroic compound Ba${}_{2}$CoGe${}_{2}$O${}_{7}$ using variational and finite-temperature mean field approaches, with the aim to reproduce the peculiar behavior of the induced polarization in a magnetic field observed experimentally in Murakawa et al. [Phys. Rev. Lett. 105, 137202 (2010)]. The compound is usually described by a spin-3/2 Heisenberg model extended with easy-plane anisotropy and Dzyaloshinskii-Moriya (DM) interaction. By applying a magnetic field parallel to the $[110]$ axis, three phases can be distinguished in this model: (i) At high magnetic field, we find a partially magnetized phase with spins parallel to the fields and uniform polarization. (ii) Below a critical field, the ground state is a twofold-degenerate canted antiferromagnet, where the degeneracy can be lifted by a finite DM interaction. (iii) At zero field, a U$(1)$ symmetry-breaking phase takes place, exhibiting a Goldstone mode. We find that extending the Hamiltonian with an antiferroelectric term results in the appearance of a canted ferrimagnetic phase for $h\ensuremath{\lesssim}1$ T. This phase is characterized by a finite staggered polarization, as well as by a magnetization closing a finite angle with the applied field leading to torque anomalies.

Phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice in an applied magnetic field

The Heisenberg antiferromagnet on a two-dimensional triangular lattice is a paradigmatic problem in frustrated magnetism. Even in the classical limit $S\ensuremath{\rightarrow}\ensuremath{\infty}$, its properties are far from simple. The ``120-degree'' ground state favored by the frustrated antiferromagnetic interactions contains a hidden chiral symmetry and supports two distinct types of excitation. And, famously, in an applied magnetic field, three distinct phases, including a collinear one-third magnetisation plateau, are stabilized by thermal fluctuations. The questions of symmetry breaking raised by this model are deep and subtle, and after more than thirty years of study many of the details of its phase diagram remain surprisingly obscure. In this paper we use modern Monte Carlo simulation techniques to determine the finite-temperature phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice in an applied magnetic field. At low to intermediate values of the magnetic field, we find evidence for a continuous phase transition from the paramagnet into the collinear one-third magnetization plateau, belonging to the three-state Potts universality class. We also find evidence for conventional Berezinskii-Kosterlitz-Thouless transitions from the one-third magnetization plateau into the canted ``$Y$ state'' and into the 2:1 canted phase found at high fields. However, the phase transition from the paramagnet into the 2:1 canted phase, while continuous, does not appear to fall into any conventional universality class. We argue that this, like the chiral phase transition discussed in the zero field case, deserves further study as an interesting example of a finite-temperature phase transition with compound order-parameter symmetry. We comment on the relevance of these results for experiments on magnetic materials with a triangular lattice.

Novel Magnetic Phases Revealed by Ultra-High Magnetic Field in the Frustrated Magnet ZnCr2O4

The Faraday rotation technique is used to map out the finite-temperature phase diagram of the prototypical frustrated magnet ZnCr 2 O 4 , in magnetic fields of up to 190 T generated by the single-turn coil method. We find evidence for a number of magnetic phase transitions, which are well-described by the theory based on spin–lattice coupling. In addition to the 1/2 plateau and a 3:1 canted phase, a 2:1:1 canted phase is found for the first time in chromium spinel oxides, which has been predicted by a theory of Penc et al. to realize in a small spin–lattice coupling limit. Both the new 2:1:1 and the 3:1 phases are regarded as the supersolid phases according to a magnetic analogy of Matsuda and Tsuneto, and Liu and Fisher.

A frustrated quantum spin-s model on the Union Jack lattice with spins s &gt; 1/2

The zero-temperature phase diagrams of a two-dimensional frustrated quantum antiferromagnetic system, namely the Union Jack model, are studied using the coupled cluster method (CCM) for the two cases when the lattice spins have spin quantum number $s=1$ and $s=3/2$. The system is defined on a square lattice and the spins interact via isotropic Heisenberg interactions such that all nearest-neighbour (NN) exchange bonds are present with identical strength $J_{1}>0$, and only half of the next-nearest-neighbour (NNN) exchange bonds are present with identical strength $J_{2} \equiv \kappa J_{1} > 0$. The bonds are arranged such that on the $2 \times 2$ unit cell they form the pattern of the Union Jack flag. Clearly, the NN bonds by themselves (viz., with $J_{2}=0$) produce an antiferromagnetic N\'{e}el-ordered phase, but as the relative strength $\kappa$ of the frustrating NNN bonds is increased a phase transition occurs in the classical case ($s \rightarrow \infty$) at $\kappa^{\rm cl}_{c}=0.5$ to a canted ferrimagnetic phase. In the quantum cases considered here we also find strong evidence for a corresponding phase transition between a N\'{e}el-ordered phase and a quantum canted ferrimagnetic phase at a critical coupling $\kappa_{c_{1}}=0.580 \pm 0.015$ for $s=1$ and $\kappa_{c_{1}}=0.545 \pm 0.015$ for $s=3/2$. In both cases the ground-state energy $E$ and its first derivative $dE/d\kappa$ seem continuous, thus providing a typical scenario of a second-order phase transition at $\kappa=\kappa_{c_{1}}$, although the order parameter for the transition (viz., the average ground-state on-site magnetization) does not go to zero there on either side of the transition.

Exchange Bias Effect Induced by the Intrinsic Inhomogeneity in La1−xCaxMnO3 (0.55≤x≤0.95) Compounds

The exchange bias (EB) effect of La1−xCaxMnO3 (0.55≤x≤0.95) compounds is closely related to ferromagnetic (FM) phases induced by the electronic phase separation in antiferromagnetic (AFM) background, and shows a maximum about 2200 Oe around x=0.60, which is mainly affected by the AFM anisotropy and exchange energy at the FM/AFM interface. For x≥0.85, where the spin canted G-type AFM phase emerges, the competition between the reduced C-type and increased G-type AFM fractions results in another EB-effect peak near x=0.90. This intrinsic inhomogeneity-induced EB effect is important for designing related devices.

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.

Magnetic order on a frustrated spin-12Heisenberg antiferromagnet on the Union Jack lattice

We use the coupled cluster method (CCM) to study the zero-temperature phase diagram of a two-dimensional frustrated spin-half antiferromagnet, the so-called Union Jack model. It is defined on a square lattice such that all nearest-neighbor pairs are connected by bonds with a strength ${J}_{1}&gt;0$, but only half the next-nearest-neighbor pairs are connected by bonds with a strength ${J}_{2}\ensuremath{\equiv}\ensuremath{\kappa}{J}_{1}&gt;0$. The bonds are arranged such that on the $2\ifmmode\times\else\texttimes\fi{}2$ unit cell they form the pattern of the Union Jack flag. Alternating sites on the square lattice are thus four-connected and eight-connected. We find strong evidence for a first phase transition between a N\'eel antiferromagnetic phase and a canted ferrimagnetic phase at a critical coupling ${\ensuremath{\kappa}}_{{c}_{1}}=0.66\ifmmode\pm\else\textpm\fi{}0.02$. The transition is an interesting one, at which the energy and its first derivative seem continuous, thus providing a typical scenario of a second-order transition (just as in the classical case for the model), although a weakly first-order transition cannot be excluded. By contrast, the average on-site magnetization approaches a nonzero value ${M}_{{c}_{1}}=0.195\ifmmode\pm\else\textpm\fi{}0.005$ on both sides of the transition, which is more typical of a first-order transition. The slope, $dM/d\ensuremath{\kappa}$, of the order parameter curve as a function of the coupling strength $\ensuremath{\kappa}$, also appears to be continuous, or very nearly so, at the critical point ${\ensuremath{\kappa}}_{{c}_{1}}$, thereby providing further evidence of the subtle nature of the transition between the N\'eel and canted phases. Our CCM calculations provide strong evidence that the canted ferrimagnetic phase becomes unstable at large values of $\ensuremath{\kappa}$, and hence we have also used the CCM with a model collinear semistripe-ordered ferrimagnetic state in which alternating rows (and columns) are ferromagnetically and antiferromagnetically ordered, and in which the spins connected by ${J}_{2}$ bonds are antiparallel to one another. We find tentative evidence, based on the relative energies of the two states, for a second zero-temperature phase transition between the canted and semistripe-ordered ferrimagnetic states at a large value of the coupling parameter around ${\ensuremath{\kappa}}_{{c}_{2}}\ensuremath{\approx}125\ifmmode\pm\else\textpm\fi{}5$. This prediction, however, is based on an extrapolation of the CCM results for the canted state into regimes where the solutions have already become unstable and the CCM equations based on the canted state at any level of approximation beyond the lowest have no solutions. Our prediction for ${\ensuremath{\kappa}}_{{c}_{2}}$ is hence less reliable than that for ${\ensuremath{\kappa}}_{{c}_{1}}$. Nevertheless, if this second transition at ${\ensuremath{\kappa}}_{{c}_{2}}$ does exist, our results clearly indicate it to be of first-order type.

Colossal magnetoresistance mechanism in doped manganites based on the canted ferromagnetic phase

It is demonstrated that in the doped manganites the antiferromagnetic phase transforms into the ferromagnetic canted phase under the action of the mobile electron hopping. It is shown that the colossal magnetoresistance effect can be conditioned by the change in the magnetization canting angle at the application of a constant magnetic field. The evaluation of the magnetoresistance value in the ferromagnetic canted phase model is carried out and it is shown that it qualitatively and quantitatively agrees with the experiment.

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.

Polar properties and phase sequence inEu0.8Y0.2MnO3

In this work, we have studied in detail the temperature dependence of the electric polarization ofEu0.8Y0.2MnO3 aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperaturemagnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinearspiral-antiferromagnetic phase, stable in that temperature range. Contrary toresults published in previous works, our experimental data clearly show that theweak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsicallyferroelectric. The misinterpretation, regarding the polar character of the lowertemperature magnetic phases, stems from the existence of an induced polarizationoccurring below 30 K. The mechanisms associated with polar and magnetic properties,and their correlation with both spin and lattice structures are also discussed.

H-Bonding Supramolecular Assemblies of PTCDI Molecules on the Au(111) Surface

Using a combination of scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) and a systematic theoretical method based on considering all possible hydrogen bond connections between molecules with subsequent density functional theory (DFT) calculations, we studied supramolecular assemblies of highly symmetrical rectangular PTCDI molecules on the Au(111) surface. We show, using a systematic prediction procedure followed by ab initio density functional calculations, that just over 10 monolayer structures are possible assuming two molecules in the primitive cell, some of which would appear indistinguishable in the STM images. By breaking down these structures into distinct assemblies, we predict six possible phases. Two of these had been observed previously: a canted phase seen on a number of surfaces including Au(111) and a brick wall phase seen so far only on the NaCl(001) surface. Using STM imaging of PTCDI molecules on the Au(111) surface in ultrahigh vacuum, we discovered a completely new “domino...

Static and dynamic double-exchange effects in doped manganites

It is shown that antiferromagnetic ordering in doped manganites with strong double-exchange interaction is transformed into ferromagnetic canted ordering with residual antiferromagnetic behavior in the basal plane as a result of hopping of mobile electron. The canting angle between the core magnetiztions is controlled by the competition of the Heisenberg antiferromagnetic exchange and double exchange. The temperatures of the paramagnet-antiferromagnet and paramagnet-canted ferromagnetic phase transitions are calculated. The results on the dependence of the magnetization in the canted phase and critical temperatures on the doping degree are in qualitative agreement with experiment. The form of uniform oscillations of core magnetiztions in the canted ferromagnetic phase of a doped manganite sample with hopping conduction is analyzed with and without allowance for relaxation of mobile electrons to the lattice. We propose a mechanism for the ferromagnetic resonance broadening and its resonance frequency shift in a ferromagnetic conducting sample (hopping conduction) of doped manganite due to double exchange. The resonance frequency shift and the ferromagnetic resonance damping constant (linewidth) are calculated in this model. In contrast to other relaxation mechanisms, the model is based on the fact that mobile electrons rapidly relax to the lattice (over a time on the order of the precession period).