Strong Exchange Anisotropy Research Articles

Resonant inelastic X-ray scattering as a probe of Jeff\u2009=\u20091/2 state in 3d transition-metal oxide

The state with effective total moment Jeff = 1/2 stabilized by the spin-orbit coupling is known to suppress Jahn-Teller distortions and may induce a strong exchange anisotropy. This in turn may lead to the formation of an elusive spin-liquid state in real materials. While recent studies have demonstrated that such a situation can be realized in 3d transition-metal compounds such as those based on Co2+ and Cu2+, diagnosis of Jeff = 1/2 state remains challenging. We show that resonant inelastic X-ray scattering is an effective tool to probe this state and apply it to CuAl2O4, material where Cu2+ ions were previously proposed to be in the Jeff = 1/2 state. Our results unambiguously demonstrate that, contrary to previous expectations, a competitive (to Jeff = 1/2) Jahn-Teller state realizes in this compound.

Spatial coexistence of multiple modes in a nanogap spin Hall nano-oscillator with extended Pt/Ni/Fe trilayers

We experimentally study the microwave generation characteristics of a spin Hall nano-oscillator driven by a local spin current injected into a nanoscale region of extended Pt/Ni/Fe trilayers due to the spin Hall effect in the Pt layer and the interfacial Rashba effect at the Pt/Ni interface. The dependence of the generated microwave spectra on the exciting current and on the magnitude and angle of the magnetic field indicates that multiple spin-wave modes with either higher frequencies than the ferromagnetic resonance frequency ${f}_{\mathrm{FMR}}$ at low out-of-plane tilting angles and small fields or lower frequencies than ${f}_{\mathrm{FMR}}$ at high tilting angles and large fields are excited by local spin currents. Furthermore, the temperature dependence of the generated spectra in the extended Pt/Ni/Fe trilayers demonstrates that the thermally activated mode transition between these distinct dynamical modes is significantly suppressed. These results suggest that the observed multiple modes are physically separated in different local potential wells created by the spatial inhomogeneity of the internal field in the asymmetric ferromagnetic bilayer with strong interfacial exchange coupling and anisotropy fields. Finally, the very weak dependence of the minimum linewidth on temperature further confirms the lack of thermal-fluctuation-induced mode coupling and mode transition between these individual dynamical modes.

Dirac Magnons in a Honeycomb Lattice Quantum XY Magnet CoTiO3

The discovery of massless Dirac electrons in graphene and topological Dirac-Weyl materials has prompted a broad search for bosonic analogues of such Dirac particles. Recent experiments have found evidence for Dirac magnons above an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome lattice magnet and in the van der Waals layered honeycomb crystal CrI$_3$, and in a 3D Heisenberg magnet Cu$_3$TeO$_6$. Here we report on our inelastic neutron scattering investigation on large single crystals of a stacked honeycomb lattice magnet CoTiO$_3$, which is part of a broad family of ilmenite materials. The magnetically ordered ground state of CoTiO$_3$ features ferromagnetic layers of Co$^{2+}$, stacked antiferromagnetically along the $c$-axis. We discover that the magnon dispersion relation exhibits strong easy-plane exchange anisotropy and hosts a clear gapless Dirac cone along the edge of the 3D Brillouin zone. Our results establish CoTiO$_3$ as a model pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum XY magnet.

Magnetic Relaxation Experiments in CNT-Based Magnetic Nanocomposite

In this work, we discuss the relaxation of the magnetic moments in a novel carbon nanotube (CNT)-based nanocomposite synthesized by using chemical vapor deposition process. The material consists of a matrix of CNT filled by Fe-based nanoparticles. This structure is seen clearly by scanning and transmission. X-ray diffraction and Raman spectroscopy are used to detect the predominant Fe3C phase and the CNT presence in the sample, respectively. The results obtained from both hysteresis cycles, M(H), and zero field cooled-field cooled (ZFC-FC) measurements confirm that the material is characterized by both a strong ferromagnetic exchange and random magnetic anisotropy. For the first time, we have been able to fit the magnetic relaxation data, M(t), by using both the two distributions of nanoparticles data deduced from the ZFC-FC data and the temperature dependence of the magnetic anisotropy obtained from the law of approach to saturation in random magnets.

Magnetocaloric properties of a spin-1 anisotropic ferromagnetic Heisenberg system within the pair approximation

The thermodynamics properties of a ferromagnetic Heisenberg model with the exchange anisotropy of a spin-1 system in presence of single-ion anisotropy (D) and external magnetic field (h) are studied here using the pair approximation approach. We treat the model in three-dimensional lattice with coordination number z = 6. Other coordination numbers have been also examined. Within this approximation based on Gibbs free energy, we investigate the effect of both single-ion and exchange coupling anisotropies on the thermodynamic quantities such as magnetization, entropy, heat capacity, susceptibility and entropy change as in-process measures. The objective is to describe magnetocaloric effect (MCE) and relative cooling power aiming to optimize them. We find some interesting phenomena in these quantities due to external magnetic field, exchange anisotropy and single-ion anisotropy. For conflicting interactions, re-entrance behavior can occur. Interestingly, high spin size as well as strong exchange or single-ion anisotropy can improve the MCE performances.

Generic nearest-neighbor kagome model: XYZ and Dzyaloshinskii-Moriya couplings with comparison to the pyrochlore-lattice case

The kagome lattice is a paragon of geometrical frustration, long-studied for its association with novel ground-states including spin liquids (SLs). Many recently synthesized kagome materials feature rare-earth ions, which may be expected to exhibit highly anisotropic exchange interactions. The consequences of this combination of strong exchange anisotropy and extreme geometrical frustration are yet to be fully understood. Here, we establish a general picture of the interactions and resulting ground-states (GSs) arising from nearest neighbour exchange anisotropy on the kagome lattice. We determine a generic anisotropic exchange Hamiltonian from symmetry arguments. In the high-symmetry case where reflection in the kagome plane is a symmetry of the system, the generic nearest-neighbour Hamiltonian can be locally defined as a XYZ model with out-of-plane Dzyaloshinskii-Moriya interactions. We proceed to study its phase diagram in the classical limit, making use of an exact reformulation of the Hamiltonian in terms of irreducible representations (irreps) of the lattice symmetry group. This reformulation in terms of irreps naturally explains the three-fold mapping between SLs recently studied on kagome by the present authors [Nat Commun 7, 10297 (2016)]. In addition, a number of unusual states are stabilised in the regions where different forms of GS order compete, including a stripy phase with a local Z8 symmetry and a classical analogue of a chiral SL. This generic Hamiltonian also turns out to be a fruitful hunting ground for coexistence of different forms of magnetic order, or of order and disorder, which we find is a particular property of the kagome lattice arising from the odd number of spins per frustrated unit. These results are compared and contrasted with those obtained on the pyrochlore lattice, and connection is made with recent progress in understanding quantum models with S = 1/2

Ising exchange interaction in lanthanides and actinides

The Ising exchange interaction is a limiting case of strong exchange anisotropy and represents a key property of many magnetic materials. Here we find the necessary and sufficient conditions to achieve Ising exchange interaction for metal sites with unquenched orbital moments. Contrary to current views, the rules established here narrow much the range of lanthanide and actinide ions that can exhibit Ising exchange interaction. It is shown that the Ising interaction can be of two types: (i) coaxial, with magnetic moments directed along the anisotropy axes on the metal sites and (ii) non-coaxial, with arbitrary orientation of one of the magnetic moments. These findings will contribute to purposeful design of lanthanide- and actinide-based materials.

Supersolid magnetic phase realization in strongly anisotropic easy-plane antiferromagnet with Ising-like exchange interaction in the transverse magnetic field

Phase states of strongly anisotropic easy-plane spin-1 antiferromagnet with Ising-like exchange interaction have been investigated at transverse orientation of the external magnetic field. It is shown that the supersolid magnetic phase does not realize at this orientation of the magnetic field, while it does at the longitudinal orientation of the external magnetic field. The strong single-axis exchange anisotropy has essential influence on the dynamic and the static properties of the system.

Phase transitions in S=1 antiferromagnet with Ising-like exchange interaction and strong easy-plane single-ion anisotropy

Spin states of strongly anisotropic easy-plane antiferromagnet with Ising-like exchange interaction in the external magnetic field are investigated within the mean-field approximation. It is shown that the influence of strong single-axis exchange anisotropy considerably changes both the dynamic and static properties of the system. The types of the phase transitions and the phase diagram are determined.

Critical properties of generalized four-state clock model on square lattices

We study the finite-temperature transition to the 1/2 magnetization plateau in a model of interacting S = 1/2 spins with longer range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. In previous studies, it was obtained from Monte Carlo calculations that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, while a single phase transition takes place in the purely Ising limit[l]. To understand these behaviors, we introduce the generalized four-state chiral clock model and perform Monte Carlo calculations for this model. By comparing the phase diagrams of the two models, we find that the topology of the thermal phase diagram is the same each other - the criticality of the thermal phase transition in the original model can be well explained by that of the generalized four-state chiral clock model.

Effective quantum pseudospin-1/2 model for Yb pyrochlore oxides

An effective quantum pseudospin-1/2 Hamiltonian for Yb2TM2O7 (TM = Ti and Sn) is obtained in terms of the atomic Kramers ground doublet of the LS coupling and the crystalline electric field, which is almost described with Jz = ±1/2. It is calculated microscopically as the sum of Anderson's superexchange interaction and the magnetic dipole interaction. It is found that it shows a strong exchange anisotropy.

Finite-temperature phase transition to them=12plateau phase in the spin-12XXZmodel on the Shastry-Sutherland lattices

We study the finite-temperature transition to the $m=1/2$ magnetization plateau in a model of interacting $S=1/2$ spins with longer range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. This model was shown to capture the qualitative features of the field-induced magnetization plateaus in the rare-earth tetraboride, ${\rm TmB_4}$. Our results show that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, whereas a single phase transition takes place in the purely Ising limit. To better understand these behaviors, we perform Monte Carlo simulations of the classical generalized four-state chiral clock model and compare the phase diagrams of the two models. Finally, we estimate a parameter set that can explain the magnetization curves observed in ${\rm TmB_4}$. The magnetic properties and critical behavior of the finite-temperature transition to the $m=1/2$ plateau state are also discussed.

Giant Magnetic Hardness in the Synthetic Mineral Ferrimagnet K2CoII3(OH)2(SO4)3(H2O)2

We present the synthesis, single-crystal X-ray (173 K) and powder neutron (2−30 K) structures and its thermal, optical and magnetic properties of K2CoII3(OH)2(SO4)2(H2O)2. It is a ferrimagnet (TC = 29.7 K) constructed of Co3(OH)2 diamond chains connected by sulfate and it displays hysteresis loops ranging from being soft with nearly zero coercivity between 29 and 10 K to very hard reaching coercive field exceeding 70 kOe at 1.8 K. This dramatic change is associated with the changes in domain shape due to the strong exchange anisotropy. Considerable frequency dependence of the ac-susceptibilities is observed in the ordered state. Measurements on a single crystal have established the magnetic axes to be a-axis (easy), b-axis (intermediate), and c-axis (hard).

Low Magnetization and Anisotropy in the Antiferromagnetic State of Undoped Iron Pnictides

We examine the magnetic phase diagram of iron pnictides using a five-band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Γ in photoemission experiments.

Systematic Study for Magnetization Dependence of Exchange Anisotropy Strength in Mn-Ir/FM (FM${=}$Ni-Co, Co-Fe, Fe-Ni) Bilayer System

The unidirectional anisotropy constant, JK, of the Mn76Ir24/FM (FM =Ni-Co, Co-Fe, Fe-Ni) bilayers was investigated, systematically changing the FM layer composition. On the (111)-oriented face centered cubic (fcc) Mn-Ir layer, the FM layers were grown in the respective crystal structure, corresponding to their composition. The phase boundaries between fcc and body centered cubic (bcc) structures existed around Co80Fe20 and Fe70Ni30, roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, Ms, of the FM layer showed a broad peak around Fe60Co40 and a shallow dip at Fe70Ni30 , corresponding well with the Slater-Pauling curve of bulk alloys. On the other hand, the JK reached a maximum around Co70Fe30 and showed different variation from that of the Ms against the FM layer composition. Therefore, the correlation plot between JK and Ms did not provide any clear relationship. From the experimental results, we found a general feature that bcc structured FM layers are more likely to induce strong exchange anisotropy than fcc structured FM layers.

Spin-orbit effects inNa4Ir3O8: A hyper-kagome lattice antiferromagnet

We consider spin-orbit coupling effects in ${\text{Na}}_{4}{\text{Ir}}_{3}{\text{O}}_{8}$, a material in which ${\text{Ir}}^{4+}$ spins form an hyper-kagome lattice, a three-dimensional network of corner-sharing triangles. We argue that both low-temperature thermodynamic measurements and the impurity susceptibility induced by dilute substitution of Ti for Ir are suggestive of significant spin-orbit effects. Because of uncertainties in the crystal-field parameters, we consider two limits in which the spin-orbit coupling is either weak or strong compared to the noncubic atomic splittings. A semi-microscopic calculation of the exchange Hamiltonian confirms that indeed large antisymmetric Dzyaloshinskii-Moriya (DM) and/or symmetric exchange anisotropy may be present. In the strong spin-orbit limit, the Ir-O-Ir superexchange contribution consists of unfrustrated strong symmetric exchange anisotropy, and we suggest that spin-liquid behavior is unlikely. In the weak spin-orbit limit, and for strong spin-orbit and direct Ir-Ir exchange, the Hamiltonian consists of Heisenberg and DM interactions. The DM coupling is parametrized by a three-component DM vector (which must be determined empirically). For a range of orientation of this vector, frustration is relieved and an ordered state occurs. For other orientations, even the classical ground states are very complex. We perform spin-wave and exact diagonalization calculations, which suggest the persistence of a quantum spin liquid in the latter regime. Applications to ${\text{Na}}_{4}{\text{Ir}}_{3}{\text{O}}_{8}$ and broader implications are discussed.

Control of Magnetism in Cobalt Nanoparticles by Oxygen Passivation

We report on the preparation of ferromagnetic cobalt nanospheres with antiferromagnetic oxide capping layer and its implication for the variation in magnetic property. The hcp cobalt nanospheres were prepared by thermal decomposition of cobalt carbonyl in the presence of organic surfactants. The spherical nanoparticles thus prepared were oxidized to grow antiferromagnetic layers of varying composition and thickness on top of cobalt spheres. High-resolution transmission electron microscopy confirmed growth of Co3O4 in one case and CoO in another case. Strong exchange anisotropy and enhanced coercive field was observed due to the core−shell structure in the Co−CoO system. On the other hand only a marginal improvement was seen in the Co−Co3O4 system. A low-temperature paramagnetic behavior was also observed that is interpreted in the framework of crystal defects in the oxide shell.

Interface effect on the magnetic anisotropy of CoPt clusters

We study the magnetic anisotropy of CoPt clusters produced by condensation of a stoichiometric vapor with an inert gas (helium) and co-deposited with various matrices. From electron transmission microscopy we show that the clusters have a mean diameter of about 2 nm with a narrow size distribution and present the FCC A1 disordered phase. At high temperature, the clusters display a superparamagnetic behavior. The transition to the blocked state enables us to determine the clusters magnetic anisotropy energy (MAE). From a careful analysis, we show in the one hand that CoPt clusters present a higher volume and intrinsic surface MAE than pure Co ones. In the other hand, the presence of platinum at the interface in CoPt clusters decreases the strong interfacial exchange anisotropy observed for Co clusters embedded in MgO.

Influence of surface anisotropy on the magnetization precessional switching in nanoparticles

A study of surface anisotropy effects on the precessional switching of nanoparticles is presented. Spherical nanoparticles with uniaxial anisotropy in the bulk and radial anisotropy for spins on the boundary are considered. The multispin dynamics is found by using the Landau–Lifshitz equation with the effective field derived from a Heisenberg-type Hamiltonian. The expressions for critical magnetic fields that guarantee the precessional switching are derived analytically for the case of very strong exchange and weak surface anisotropy. These analytical results are also used to test the numerical approach that is applied to the general case of the problem. The distinct features of the precessional switching in nanoparticles are examined and their dependence on various parameters of the problem is discussed.

Electron spin resonance studies on magnetoresistive (La 0.67Ca 0.33)MnO 3 compound

Electron spin resonance (ESR) measurements have been carried out on colossal magneto-resistive (La 0.67Ca 0.33)MnO 3 thin films deposited on single-crystalline alumina substrate by laser ablation technique. Both, the resonance field value and line shape in ESR spectra were observed to exhibit strong temperature dependence between 4 and 300 K. Significant absorptions appeared at low fields as well. The ESR spectra at low temperature were seen to consist of overlapping low-energy spin wave resonance (SWR) modes excited across opposite surfaces of the film. The SWR data were analyzed by using a magnetic energy density including usual Zeeman and exchange terms contained both in bulk and surface anisotropy energies. The magnetic parameters, such as magnetization, exchange stiffness, relaxation and exchange anisotropy energy, have been deduced as a function of temperature between 4 and 300 K by fitting the simulated spectra to the experimental ones. It was found that quite strong exchange anisotropy is induced consistent with spins canting to give smaller magnetization due to the Dzyaloshinsky–Morya (D–M) interaction in perhaps spin glass-like phase at low temperature.