Electronic structure of niobium doped CaMn1-xNbxO3 (x = 0.02, 0.04, 0.06 and 0.08) powder and fresh chip manganites was studied by x-ray absorption near edge spectroscopy ()CANES) at Mn K-edge, and by x-ray photoelectron spectroscopy (XPS) of Mn 3s -, Mn 2p -, O 1s -, Ca 2p- and Nb 3d-core levels. These XPS spectra for all the studied perovskites are very similar. The O 1s and Ca 2p spectra for powders differ from those for fresh chips because of the quick oxidation and carbonization of powder surface. The Mn 3s, Mn 2p and Nb 3d spectra for the powders and fresh chips are almost the same. According to Mn 3s spectra the formal valence of surface manganese ions is 3.82 for all studied oxides indicating the presence of oxygen vacancies in the samples. Niobium is in pentavalent state as evidenced by XPS measurements, but the negative chemical shift of the Nb 3d(5/2) core level with respect to Nb2O5 oxide is observed. High similarity of the XPS spectra of the samples with different Nb doping is probably a result of the self-organization of terminal layers. (C) 2019 Elsevier B.V. All rights reserved.

Abstract Meyer-Neldel behaviour of the conductivity of phase separated La1−xCaxMnO3 manganite system in the low Ca-doping range has been investigated. Evolution of the isokinetic temperature of the conductivity, modified by Ca-doping, hydrostatic pressure and current bias has been determined. In addition, the evolution of the isokinetic temperature with ageing has also been studied. It is found that the Meyer-Neldel behaviour of the manganite system stems from multi-excitation entropy mechanism. The isokinetic temperatures estimated from pressure and doping effects coincide but differ from those determined using current and ageing controlled conductivity changes. It is concluded that in the presence of a detailed theoretical model of the excitations coupling in manganites, the investigations of the Meyer-Neldel effect may became a powerful tool for characterization and investigation of transport mechanisms in phase separated manganites.

An unusual exchange bias (EB) effect in single-crystalline GdCrO3 compensated ferrimagnet (fM), composed of two antiferromagnetically coupled Gd and Cr sublattices, presenting two opposite ferromagnetic (FM) moments, is reported. It is shown that the temperature- or field-induced fast reversal of the net FM moment in GdCrO3 may be identified as a switching between two opposite spin configurations, attendant with abrupt drop in Zeeman energy approximate to k(B)(7 x 10(-4) K) per formula unit. It was found that the FM moment reversal is exchange biased in the narrow temperature interval above the compensation temperature T-comp = 144 K. Namely, the EB field emerges and diverges upon approaching T-comp at temperatures T > T-comp while unexpectedly it collapses to zero below T-comp. This is in contrast to experimental results obtained for compensated fMs RFeO3 (R = Nd, Sm, Er) orthoferrites showing EB in a narrow range, both above and below T-comp. We suppose that breakdown of EB in GdCrO3 may be linked to the lack of anisotropy of the spin-only Gd S-ion magnetic moment dominating below T-comp.

Recent experiments showed magnetic moment reversal and pronounced exchange-bias (EB) effect in the vicinity of the compensation temperature T-comp of orthoferrites RFeO3 (R = Nd, Sm, Er). Although different orthoferrites exhibit diverse R-Fe interactions, T-comp values, and spin-reorientation temperatures, the EB field in the like manner emerges and diverges upon approaching T-comp and changes its sign with crossing T-comp. In order to explain these observations, a mean-field theory approach for the representative ErFeO3 orthoferrite is proposed. The general case of two sublattices, antiferromagnet with exchange anisotropy and rare-earth-iron interactions, is considered. A small applied magnetic field appears to be a source of additional anisotropy, resulting from canting of sublattice moments. This anisotropy leads to an imbalance of free energy for two different types of magnetic domains, causing a spin jump near the T-comp. The suggested approach allows reproduction of magnetization reversal and main features of the coercive and exchange-bias fields observed in ErFeO3.

The unconventional exchange-bias (EB) effect in single crystals of $R\mathrm{Fe}{\mathrm{O}}_{3}(R=\mathrm{Nd},\phantom{\rule{0.28em}{0ex}}\mathrm{Sm})$ compensated ferrimagnets (fMs), composed of two antiferromagnetically (AFM) coupled $R$ and Fe sublattices with opposite ferromagnetic (FM) moments, exhibiting the FM moment reversal via a fast spin switching is reported. In ${\mathrm{NdFeO}}_{3}$, the EB anisotropy field emerges and diverges upon approaching the compensation temperature ${T}_{\mathrm{comp}}^{\mathrm{Nd}}=9.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and changes sign with crossing ${T}_{\mathrm{comp}}^{\mathrm{Nd}}$, in similarity to the behavior observed recently in ${\mathrm{ErFeO}}_{3}$. In contrast, ${\mathrm{SmFeO}}_{3}$ exhibits a substantial EB, not only at its ${T}_{\mathrm{comp}}^{\mathrm{Sm}}=4.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, but also at higher temperatures up to 100 K, and the EB changes its sign with increasing cooling field. It is shown that in all known $R{\mathrm{FeO}}_{3}$ ($R=\mathrm{Nd},\phantom{\rule{0.28em}{0ex}}\mathrm{Sm},\phantom{\rule{0.28em}{0ex}}\mathrm{Er}$) compensated fMs the field-induced FM moment reversal is similarly exchange biased near ${T}_{\mathrm{comp}}$.

The Meyer-Neldel behavior of conductivity of low-doped manganite La1-xCaxMnO3 single crystals has been investigated. The evolution of the isokinetic temperature of conductivity, modified by Ca-doping, hydrostatic pressure, and current bias has been determined. In addition, the evolution of isokinetic temperature with ageing has also been studied. The Meyer-Neldel behavior of the manganite system stems from the multiexcitation entropy mechanism. The isokinetic temperature turned out to be a sensitive parameter characterizing changes in the transport properties of mixed valence manganites, which in the presence of a detailed theoretical model of the excitations coupling in manganites could become a powerful tool for the characterization and investigation of transport properties of manganites.

The exchange-bias (EB) effect with sign reversalwas found in LuFe0.5Cr0.5O3 ferrite-chromite, which is a weak ferrimagnet below T-N = 265K, exhibiting antiparallel orientation of the ferromagnetic (FM) moments of the Fe and Cr sublattices due to opposite sign of the Fe-Cr Dzyaloshinskii vector, as compared to that of the Fe-Fe and Cr-Cr. The weak FM moments of the studied compound compensate each other at temperature T-comp = 230 K, leading to the net magnetic moment reversal and to observed negative magnetization, at moderate applied fields, below T-comp. Both vertical and horizontal shifts from the origin were gotten in the field-cooled magnetization hysteresis loops. The EB sign was found to be positive below T-comp and negative above T-comp, with nonmonotonic dependence on cooling field H-cool. It sharply increases at small values of magnetic fields up to H-cool similar to 1 kOe, then remains almost unchanged in the range 1-30 kOe and strongly decreases with further increase of H-cool. This unusual behavior results from the competition of various Dzyaloshinskii-Moriya interactions between Fe3+ and Cr3+ ions.

Exchange bias effect in CaMn1-xRexO3 (x ≤ 0.1) has been investigated. The effect is very small in the samples doped at x = 0.02 and 0.04, but increases monotonously with further increase in Re doping. For x = 0.1, both vertical and horizontal shifts in hysteresis loop of field cooled sample decrease monotonously with increasing temperature and vanish above 70 K, while coercivity disappears only above 90 K upon approaching the Néel temperature. Exchange bias field, coercivity, and remanence asymmetry depend sensitively on temperature and maximal measuring field. Magnetic training effect has been studied for x = 0.06, 0.08, 0.1 samples and analyzed using a spin relaxation model. The observed exchange bias is attributed to the low-temperature phase separation into ferromagnetic clusters and the G-type and/or C-type antiferromagnetic matrix.

Magnetic and structural properties of CaMn1-xRexO3 (0.02 <= x <= 0.1) have been investigated. Substitution of Re5+ ion for the Mn4+ site of CaMnO3 generates Mn3+ ions according to the chemical formula CaMn1-2x4+ Mnx3+Rex5+O3, accompanied by an increase of lattice parameters and unit-cell volume with increasing x. With increasing doping level x, the magnetic ground state evolves from an antiferromagnetic (AFM) with a weak ferromagnetic (FM) component, for x = 0.02 - 0.06, to the charge ordered C-type AFM state at x = 0.1. Spontaneous magnetization at T = 10 K increases quickly with increasing x, approaches the maximum value of 3.5 emu/g for x = 0.04, and then decreases rapidly to 0.2 emu/g for x = 0.1. Anomalous negative magnetization (NM) for x = 0.02 has been observed in the zero-field-cooled and field-cooled (FC) magnetization below the magnetic transition temperature. Exchange bias (EB) effect, manifested by horizontal shift in the hysteresis loops of FC samples, has also been observed. This effect is very small for x = 0.02, almost zeroes for 0.04, and monotonously increases with increasing x. The EB appears due to low-temperature phase separation into FM clusters and charge-ordered AFM phases. The effect of hydrostatic pressure for all samples revealed a significant increase of the FM phase volume under pressure, linked to both suppression of NM in x = 0.02 sample and reduction of the EB effect in all samples.

Exchange bias effect in CaMn1-xRexO3 (x <= 0.1) has been investigated. The effect is very small in the samples doped at x = 0.02 and 0.04, but increases monotonously with further increase in Re doping. For x = 0.1, both vertical and horizontal shifts in hysteresis loop of field cooled sample decrease monotonously with increasing temperature and vanish above 70 K, while coercivity disappears only above 90 K upon approaching the N ' eel temperature. Exchange bias field, coercivity, and remanence asymmetry depend sensitively on temperature and maximal measuring field. Magnetic training effect has been studied for x = 0.06, 0.08, 0.1 samples and analyzed using a spin relaxation model. The observed exchange bias is attributed to the low-temperature phase separation into ferromagnetic clusters and the G-type and/or C-type antiferromagnetic matrix. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

The effect of hydrostatic pressure on complex magnetic phase diagram of LuFe2O4 has been investigated by various magnetic measurements in the range of 0-12 kbar. The temperature of the ferrimagnetic (fM) transition (T-N approximate to 245 K) practically does not change with increasing pressure, while the temperature of the appearance of mixed fM and antiferromagnetic (AFM) state at similar to 200 K increases with the rate dT(C)/dP approximate to 0.8 K/kbar. Moreover, an applied pressure suppresses the magnetization, enhances the field of metamagnetic transition, and extends the range of existence of mixed fM and AFM state. Suppression of the ordered magnetic moment under pressure induces a transition from a counterclockwise (CCW) thermal hysteresis below T approximate to 220 K to a clockwise (CW) thermal hysteresis in a magnetic field H = 1 kOe. Nevertheless, field induced metamagnetic transition from mixed fM and AFM state to fM state results in a narrowing of the temperature range of thermal hysteresis and in restoring of CCW type of thermal hysteresis. As a consequence of metamagnetic transition, the thermal hysteresis completed at H = 10 kOe remains one of the CCW type under applied pressure.

In this chapter, mainly the papers devoted to the exchange-bias (EB) effect in bulk perovskite manganites that were published during last 5–6 years are reviewed. The basic magnetic properties of mixed-valence perovskite manganites are discussed. These compounds exhibit very rich phase diagrams, since several phases, differing in magnetic, structural, and electronic properties, may coexist in the same sample. It is stressed that the most relevant feature leading to an appearance of EB is an intrinsic phase separation that occurs in manganites. Most relevant scenario for the EB effect in bulk manganites is the coexistence of ferromagnetic clusters distributed in an antiferromagnetic matrix, as strength of the EB effect strongly depends on the size of ferromagnetic clusters. It is shown that EB may be effectively tuned by chemical substitutions and/or by hydrostatic pressure. The relation between magnetization-reversal phenomenon (consisting in an alignment of spins in the direction opposite to an applied magnetic field) and tunable EB is discussed. The EB effect in double perovskites is discussed, with particular attention to a spontaneous hysteresis loop shift after zero-field cooling (so-called spontaneous EB). Comparison of the EB effect in manganites and in other perovskites (cobaltites, ruthenites, and orthoferrites) is made.

Papers devoted to exchange-bias (EB) effect in manganite nanostructures published recently, that is, within last 5–6 years, are reviewed. It is shown that when the size of magnetic structures is reduced to the nanometer scale, many of their basic magnetic properties, for example, spontaneous magnetization, the magnetic transition temperature, and coercivity, differ significantly from the bulk values and become strongly dependent on the particle size. With decreasing particle size, ferromagnetic clusters appear on the surface of basically antiferromagnetic particles that result in natural interface and in the appearance of the EB effect. It is shown that the EB effect significantly varies with particle downsizing—basic parameters, for example, exchange-bias field, H EB, and coercive field, H C, are strongly dependent on the particle size. However, variation of these parameters with reduction of particle size may be non-monotonic. One of the characteristic features of the EB systems, the so-called "training effect" (reduction in the strength of the EB effect with increasing number of subsequent hysteresis loops recorded at the same temperature), is discussed. In addition, examples of the EB effect in manganite thin films, heterostructures, nanosheets, and nanowires are reviewed.

An exchange bias (EB) effect was observed inmixed valent La-x Ba1-x FeO3 (x = 0.125, 0.25, 0.33) perovskites exhibiting the antiferromagnetic (AFM) helical order among Fe4+ ions coexisting with the ferromagnetic (FM) cluster phase in the ground state. The La3+ ions for Ba2+ site substitution, associated with increase in number of the AFM coupled Fe3+ -Fe4+ pairs as well as some Fe3+ -Fe3+ pairs, leads to strengthening of the AFM phase and consequently to the alteration of the EB characteristics, which depend on level of the La doping x. At low doping x <= 0.25, an abnormal dependence of the EB field, H-EB, on the cooling field, H-cool, was found. The HEB increases rapidly with increasing cooling field at low Hcool, but it falls suddenly at cooling fields higher than 20 kOe, reducing by an order of magnitude at 90 kOe. The suppression of EB is caused by the field-induced increased volume of the FM phase, due to the transformation of the AFM helical spin structure into the FM one. Thus, low-doped La x Ba1-x FeO3 demonstrates a competition of two alternate cooling-field-induced effects, one of which leads to the EB anisotropy and another one to the enhanced ferromagnetism. In contrast, the x = 0.33 sample, having a strong AFM constituent, shows no field-induced FM and no drop in the EB field. Accordingly, the HEB vsHcool dependence was found to be well explained in the framework of a model describing phase-separated AFM-FM systems, namely, the model assuming isolated FM clusters of size similar to 4 nm embedded in the AFM matrix.

Robust random telegraph conductivity fluctuations have been observed in La0.86Ca0.14MnO3 manganite single crystals. At room temperatures, the spectra of conductivity fluctuations are featureless and follow a 1/f shape in the entire experimental frequency and bias range. Upon lowering the temperature, clear Lorentzian bias-dependent excess noise appears on the 1/f background and eventually dominates the spectral behavior. In the time domain, fully developed Lorentzian noise appears as pronounced two-level random telegraph noise with a thermally activated switching rate, which does not depend on bias current and applied magnetic field. The telegraph noise is very robust and persists in the exceptionally wide temperature range of more than 50 K. The amplitude of the telegraph noise decreases exponentially with increasing bias current in exactly the same manner as the sample resistance increases with the current, pointing out the dynamic current redistribution between percolation paths dominated by phase-separated clusters with different conductivity as a possible origin of two-level conductivity fluctuations.

Magnetic and structural properties of CaMn1-xRexO3 (0.02 ≤ x ≤ 0.1) have been investigated. Substitution of Re5+ ion for the Mn4+ site of CaMnO3 generates Mn3+ ions according to the chemical formula CaMn1-2x4+Mnx3+ Rex5+O3, accompanied by an increase of lattice parameters and unit-cell volume with increasing x. With increasing doping level x, the magnetic ground state evolves from an antiferromagn...

Magnetic properties of antiferromagnetic (AFM) electron doped manganite Sm0.1Ca0.7Sr0.2MnO3 have been investigated, focusing mainly on the exchange bias (EB) effect and associated training effect. The studied compound exhibits the ground state with heterogeneous spin configuration, consisting of the C-type antiferromagnetic phase with the Neel temperature TN-C approximate to 120 K, the G-AFM phase with the Neel temperature TN-G approximate to 60 K, and ferromagnetic-like phase with a very weak spontaneous magnetic moment. Measurements of hysteresis loops have shown that the exchange bias field monotonously decreases with increasing temperature and vanishes above 40 K, while the coercivity disappears only above 70 K. The temperature variation of the exchange bias field has been successfully described by an exponential decay form. The stability of EB has been evaluated in the studies of the training effect, which has been discussed in the frame of the spin relaxation model, elucidating the important role of the AFM domain rearrangement at the interface. The complex phase separation and possible contributions from different interfaces between coexisting magnetic phases to the EB effect have also been discussed. (C) 2016 Elsevier B.V. All rights reserved.

An exchange-bias (EB) effect observed in single crystal ErFeO3 compensated ferrimagnet, exhibiting the EB field H-EB increasing and diverging upon approaching compensation temperature T-comp = 45 K, and changing sign with crossing Tcomp, is reported. The EB sign may be changed to the opposite one by varying the field-cooling protocol, depending on whether T-comp is crossed with decreasing or increasing temperature. Namely, a different EB sign with the same vertical bar HEB vertical bar and coercive field H-C values is obtained approaching a given T with increasing and decreasing temperature and the H-EB(T) dependence completed in one way is a mirror image of that completed in another way.