M. Kajňaková

Exchange bias in phase-segregated Nd2/3Ca1/3MnO3 as a function of temperature and cooling magnetic fields

Exchange bias (EB) phenomena have been observed in Nd2/3Ca1/3MnO3 colossal magnetoresistance perovskite below the Curie temperature TC ∼ 70 K and attributed to an antiferromagnetic–ferromagnetic (FM) spontaneous phase segregated state of this compound. Field cooled magnetic hysteresis loops exhibit shifts toward negative direction of the magnetic field axis. The values of exchange field HEB and coercivity HC are found to be strongly dependent of temperature and strength of the cooling magnetic field Hcool. These effects are attributed to evolution of the FM phase content and a size of FM clusters. A contribution to the total magnetization of the system due to the FM phase has been evaluated. The exchange bias effect decreases with increasing temperature up to TC and vanishes above this temperature with disappearance of FM phase. Relaxation of a non-equilibrium magnetic state of the compound manifests itself through a training effect also observed while studying EB in Nd2/3Ca1/3MnO3.

Two coordination polymers based on semicarbazone Schiff base and azide: synthesis, crystal structure, electrochemistry, magnetic properties and biological activity

[Mn(L)(μ1,1–N3)2]2 n [Mn(H2O)2(μ1,1–N3)2] n (1) and [Cd(HL)(μ1,1–N3)2] n (2) have been synthesized from HL (HL: pyridine-2-carbaldehyde semicarbazone) and azide ligands, characterized by FT-IR, UV–vis spectroscopy and X-ray crystallography. Single crystal X-ray diffraction revealed that 1 is a coordination polymer consisting of two infinite 1-D chains: chain A with [Mn(L)(μ1,1–N3)2]2 n and chain B with [Mn(H2O)2(μ1,1–N3)2] n . In both chains, Mn centers are connected via two double end-on (EO) azide bridges. 2 is a coordination polymer consisting of a 1-D infinite chain, where Cd centers are connected via two double EO azide bridges. The electrochemistry of HL, 1 and 2 were studied by cyclic voltammetry. Magnetic susceptibility measurements indicate bulk ferromagnetic coupling for 1 below 5 K. Antimicrobial activities of both compounds 1 and 2 were greater than HL, with the strongest effect for 2 consistent with its larger radius and electronegativity of Cd(II) ions.

Low-temperature phase segregation in La2∕3Ba1∕3MnO3: Manifestation of nonequilibrium thermodynamics

Thermodynamic characteristics of the perovskite-like compound La2∕3Ba1∕3MnO3, exhibiting a structural phase transformation of the martensitic type with a characteristic temperature Ts≈200K, are studied in the temperature range 2–340K. Step-like hysteretic temperature behavior of the effective heat capacity is revealed at 150–250K and attributed to the discrete kinetics and a latent heat of the martensitic transformation. The magnetic subsystem is found to exhibit a magnetic glass state below 220K and temperature hysteresis of the magnetic susceptibility clearly pronounced in the 40–100K and 180–230K regions. The Debye and Einstein temperatures, θD=230K and θE=500K, respectively, derived from the experimental Debye–Waller factors for the La∕Ba, Mn, and O sublattices, are used to refine contributions from the structural and magnetic transformations to the heat capacity and to reveal thermodynamically nonequilibrium states.

Exchange bias associated with phase separation in the Nd2/3Ca1/3MnO3 manganite

The exchange bias (EB) phenomenon has been found in Nd2/3Ca1/3MnO3 perovskite. The phenomenon manifests itself as a negative horizontal shift of magnetization hysteresis loops. The EB phenomenon is evident of an interface exchange coupling between coexisting antiferromagnetic (AFM) and ferromagnetic (FM) phases and confirms the phase separated state of the compound at low temperatures. The EB effect is found to be strongly dependent on the cooling magnetic field and the temperature, which is associated with the evolution of spontaneous AFM–FM phase separated state of the compound. Analysis of magnetic hysteresis loops has shown that ferromagnetic moment MFM originating from the FM clusters saturates in a relatively low magnetic field about H ∼ 0.4 T. The obtained saturation value MFM (1 T) ∼ 0.45 μB is in a good agreement with our previous neutron diffraction data.

Direct evidence of the low-temperature cluster-glass magnetic state of Nd2/3Ca1/3MnO3 perovskite

A giant exchange bias is detected in the colossal magnetoresistance of Nd2/3Ca1/3MnO3 perovskite at low temperatures and is evidence of intrinsic exchange coupling in this compound. These phenomena confirm our previous assumption that the low-temperature magnetic structure of this compound consists of small (nanosized) ferromagnetic clusters embedded in a charge-ordered antiferromagnetic matrix. The magnetic behavior of the perovskite Nd2/3Ca1/3MnO3 is consistent with a cluster-glass magnetic state and inconsistent with the classical spin-glass state observed in a variety of disordered magnetic systems. We think that the cluster-glass magnetic behavior of Nd2/3Ca1/3MnO3 originates in a self-organized phase-separated state of the compound. A Cole-Cole analysis of the dynamic susceptibility at low temperatures reveals an extremely broad distribution of relaxation times, indicating that spins are frozen on a “macroscopic” time scale. Slow relaxation of the zero-field-cooled magnetization is also observed exp...

Manifestation of the Jahn–Teller effect in the EPR spectrum of the metalorganic complex [Cu(en)2H2O]SO4

The temperature dependence of the resonance spectrum of [Cu(en)2H2O]SO4 single crystals is investigated in the temperature range 2.3–60 K at a frequency of ∼73 GHz. At a temperature T>15 K the angular dependence of the spectrum is described by a spin Hamiltonian of axial symmetry. At T<15 K a lowering of the symmetry to rhombic and a splitting the resonance line for a field orientation H∥b are observed. These effects are discussed in a model of two nonequivalent Jahn–Teller centers with intercoupled dynamic distortions in the cell. The low-temperature behavior of the resonance line is due to critical broadening in the region of short-range magnetic order.

Two complexes crystallizing from the aqueous system Ni2+ - tacn -[Ni(CN)4]2− ( tacn = 1,4,7-triazacyclononane)

Two new complexes [Ni(tacn)2][Ni(CN)4]·2H2O (1) and Ni(tacn)Ni(CN)4·H2O (2) (tacn = 1,4,7-triazacyclononane) have been synthesized from water and characterized by chemical analysis and infrared spectroscopy. Single-crystal X-ray structure analysis of 1 revealed an ionic structure built up of [Ni(tacn)2]2+ and [Ni(CN)4]2− complex ions without coordinated water. While cationic Ni(II) atom is octahedrally coordinated by two tridentate tacn ligands with Ni–N bonds from 2.101(3)–2.118(3) Å, the anionic Ni(II) atom is square planar with Ni–C bonds from 1.866(4) to 1.880(3) Å. An extended hydrogen bonding system connects complex cations, complex anions and water yielding hydrogen-bonded layers. Magnetic study of 1 revealed a decrease of the effective magnetic moment from 2.90 (300 K) to 2.43 μB at 1.8 K due to zero-field splitting. Fitting of the susceptibility data yielded g = 2.05, D/hc = 3.82 cm–1 and E/hc = 0.23 cm–1. IR spectral data indicate the presence of bridging cyano ligands in the structure of 2.

Electron paramagnetic resonance in powder samples of metalorganic copper compounds

The EPR spectra of powder samples of a number of metalorganic compounds containing Cu2+ ions are investigated. These compounds are characterized by the presence of identical chains of octahedra forming the local environment of Cu2+, but they differ strongly in their interchain ligand structure. The resonance absorption bands permit determination of the components of the effective g factor and linewidth of individual powder particles and also their temperature dependence. It is found that the orbital singlet |x2−y2〉 is the ground state of the copper ion in all the compounds, and the exchange interaction parameters in them are estimated. The maximum exchange is observed in the system with the simplest geometry of the interchain structure in the series of compounds studied.

Nanophase Separation and Magnetic Spin Glass in Nd2/3Ca1/3MnO3

A study of the low temperature magnetic state of polycrystalline colossal magnetoresistance perovskite Nd2/3Ca1/3MnO3 has been carried out. The data obtained, such as strongly divergent ZFC and FC static magnetizations and frequency dependent ac susceptibility, are evident of the glassy magnetic state of the system. Well defined maxima Tmax in the in-phase linear ac susceptibility χ curves were observed, indicating a spin-glass transition. Clear frequency dependence of the cusp temperature Tmax was found. The frequency dependence of Tmax was successfully analyzed by the dynamical scaling theory of a three-dimensional spin glass. Slow relaxation process and variety of relaxation times found imply a cluster glass magnetic state of the compound at low temperatures rather than a canonical spin glass state. The cluster glass state, accompanied by the multiple magnetic transitions of Nd2/3Ca1/3MnO3, might exist due to the competing interaction between the FM clusters and the AFM matrix induced by the complex nanophase segregated state of the compound.

Phase Transformations in Nd2/3Ca1/3MnO3: Effect of Y Substitution

It was found that below the room temperature both parent Nd2/3Ca1/3MnO3 and doped (Nd0.9Y0.1)2/3Ca1/3MnO3 compounds exhibit a sequence of phase transformations: charge ordering, structural transformation of O-O type within the orthorhombic structure, and three magnetic transitions. Three different types of long-range magnetic order co-exist in (Nd0.9Y0.1)2/3Ca1/3MnO3 at low temperatures (as it was earlier found in Nd2/3Ca1/3MnO3): the antiferromagnetic orderings of PCE and DE types existing below ~110 K and ~60 K, respectively, and the ferromagnetic one of B type existing below ~42 K. Charge ordering occurs at 290 K in the doped compound. Diluting of Nd subsystem by Y in the parent perovskite has opposite effects on the temperatures of magnetic and charge orderings: the temperatures of all magnetic transformations are reduced in the doped compound by 20-30 K, while the charge ordering one increases by 80 K. A relationship between evolution of the phase transformation temperatures and crystal and electronic structures of the compound are analyzed.