Magnetic properties of LaNi <Emphasis Type="Bold">1-</Emphasis><Emphasis Type="Bold">x</Emphasis>Mn <Emphasis Type="Bold">x</Emphasis>O <Emphasis Type="Bold">3+δ</Emphasis> perovskites

Magnetic measurements have been carried out in different LaNi_1-xMn_xO _{3 + δ} samples with 0.1 ⩽ x ⩽ 0.9. All these samples show two magnetic anomalies, one at relatively high temperature characteristic of a ferromagnetic ordering and the other at low temperature, typical of magnetic relaxation phenomena. Neutron diffraction patterns indicate that long-range ferromagnetic ordering is only achieved for x ≥ 0.5. Neutron patterns of LaNi_0.5Mn_0.5O _{3 + δ} samples show an ordered arrangement of Ni and Mn atoms in the perovskite lattice. LaNi_0.5Mn_0.5O _{3 + δ} is then, a double perovskite A₂BB'O₆ whereas Ni and Mn atoms are randomly distributed for the rest of the samples. X-ray magnetic circular dichroism experiments confirm the presence of collinear ferromagnetism in LaNi_0.5Mn_0.5O _{3 + δ} . The role of competitive magnetic interactions, structural disorder, magnetic anisotropy and magnetic disaccommodation is also discussed Received 19 July 2002 / Received in final form 23 October 2002 Published online 31 December 2002     

Preparation, magnetoresistance and magnetocaloric effect of two-layered perovskite La 2.5- x K 0.5+ x Mn 2 O 7+δ

Polycrystalline two-layered perovskite La_2.5-xK_0.5+xMn₂O _{7 + δ} (0 < x < 0.5) samples have been prepared by a modified sol-gel method and their magnetoresistance and magnetocaloric effects have been studied. A large deviation between the metal-insulator (MI) transition temperature (T _ρ ) and the magnetic transition temperature (T_C) is observed. Large magnetoresistance (MR) effects with Δρ/ρ of ∼40% at 12 kOe are obtained in wide temperature ranges. The maximum of the magnetic entropy change peaks at its Curie temperature (T_C), far above its MI transition temperature (T _ρ ). The large magnetic entropy change (∼1.4 J/kg.K) is obtained in the sample La_2.5-xK_0.5+xMn₂O _{7 + δ} (x = 0.35) upon 10 kOe applied magnetic field. Received 2 May 2002 / Received in final form 1st October 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: wzhong@ufp.nju.edu.cn     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001     

Induced ferromagnetism and colossal magnetoresistance by Ir-doping in Pr 1 - x Ca x MnO 3

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_1-xCa_xMnO₃ ( 0.4 ? x ? 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180 K and CMR ratio in 7 T as large as 10⁴. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state. Received 14 May 2001 and Received in final form 2 July 2001     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

Transport properties of La 1 - x Ca x MnO 3 ( 0.5 ? x < 1)

The transport properties of the La_1-xCa_xMnO₃ ( 0.5 ? x < 1) system in magnetic fields up to 14 T were studied. We found that the relationship between the charge ordering temperature T _CO and Mn⁴⁺ content n _{Mn4 +} obeys the formula T _CO / T _max = 1 - a ( n _{Mn4 +} - n ₀ ) ² , here n₀ and a are constants and T _max is the maximum of T _CO . For x = 0.65, T _CO arrives at the maximum value of 249.5 K in zero magnetic field, while the charge ordered (CO) state is most stable around x = 0.75. For x = 0.5 when H < 6 T the resistivity displays Mott's variable-range hopping (VRH) behavior, when 6 < H < 12 T it is suggested that two kinds of conduction mechanism, i.e., VRH and magnetic polarons, coexist in the material, and when H > 12 T the resistivity shows metallic-like behavior and the transport mechanism is attributed to coexistence of magnetic polarons and free carriers. For x = 0.95, the conduction mechanism accords with the coexistence of VRH and magnetic polarons. Received 4 January 2002 and Received in final form 28 January 2002     

Charge states of transition metal in “Cr, Co and Ni” doped Ln0.5Ca0.5MnO3 CMR manganites

The XAS study at Cr, Co, Ni and Mn K-edges was performed for the doped CMR manganites Ln_0.5Ca_0.5Mn_1-xB_xO₃ with Ln=La, Nd, Sm and B= Cr, Co, Ni (), on the samples that were studied previously for their ferromagnetic-metallic to antiferromagnetic-insulator transition. We observed that the formal charges of the doping elements are Ni²⁺, Co²⁺ and Cr³⁺. It is also evidenced that the average formal charge of the manganese is increased after doping, in agreement with the charge compensation keeping “O₃” stoichiometry. These results suggest that the doping elements participate directly to the band structure. Received: 9 January 1998 / Received in final form: 6 April 1998 / Accepted: 7 April 1998     

The electronic structure of the doped one-dimensional transition metal oxide Y 2 - x Ca x BaNiO 5 studied using X-ray absorption

A strong anisotropic distribution of the holes in Ni 3 d and O 2 p orbitals is observed in the polarization dependent O 1 s and Ni 2 p _3/2 X-ray absorption spectroscopy of the linear-chain nickelate Y_2-xCa_xBaNiO₅ (x = 0, 0.05, 0.1, 0.2), which demonstrates the one-dimensional nature of the electronic state in these compounds. The holes introduced by Ca-doping occupy both O 2 p and Ni 3 d orbitals along the NiO₅ chains. By comparing the experimental Ni 2 p _3/2 absorption spectra of Y_2-xCa_xBaNiO₅ to those from charge transfer multiplet calculations we can derive the orbital character of the additional holes to be of ∼60% O2 p and ∼40% Ni 3 d. Received 7 January 2002     

Ferromagnetism in lacunar perovskite manganites Pr 0.7 Sr 0.3 - x x MnO 3 and Pr 0.7 - x x Sr 0.3 MnO 3

Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite oxides Pr_0.7Sr_{0.3-x x}MnO₃ ( 0 ? x ? 0.3) and Pr_{0.7-x x}Sr_0.3MnO₃ ( 0 ? x ? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite structure and R c space group for x ? 0.2 in strontium deficient samples and for x ? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for x = 0 to 90 K for x = 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x = 0 to 315 for x = 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ? x ? 0.1) in the strontium lacunar ones when the temperature decreases. Received 12 April 2000 and Received in final form 8 January 2001     

Structural stability and structural imitation of (M = Mn,Mo, Nb,Ti, V,Al, Cr,Ni and Si)

The structural stability of La₂Co_17-xM_x (M = Mn, Mo, Nb, Ti, V, Al, Cr, Ni and Si) based on the interatomic potential has been studied. The calculated site preference of the third element M is found to be the 6c site, which is in agreement with the experiments. In the calculations, if the crystal cohesive energy of La₂Co₁₆Mn is taken as the highest one in the crystallization of La₂Co_17-xM_x, the lowest content x of the third element M (M = Mn, Mo, Nb, Ti and V) required to stabilize La₂Co_17-xM_x, is near that found in the experiments. The differences of the cell parameters between the calculated and the experimental values are less than 0.4%. The differences of the atomic parameters for Co (or M) between the calculated and the experimental values are about or even smaller than 1%, and that of La is about 3%. Because the energies of La(Co_1-xAl_x)₁₃ are lower than those of La₂(Co_1-xAl_x)₁₇, La₂(Co_1-xAl_x)₁₇ could not be formed in the experiments. In the calculations, with either a range of deformation of the structure or the reconstruction of the initial structure La₂Co₁₇ from LaCo₅, the same results including the cohesive energy curves and the crystallographic parameters can be retrieved after the action of the interatomic potentials. Received 1st November 2002 / Received in final form 17 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: hchang@aphy.iphy.ac.cn     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001     

Magnetic phases in Mn<Subscript>1</Subscript>_<Subscript>x</Subscript>Fe<Subscript>x</Subscript>WO<Subscript>4</Subscript> studied by neutron powder diffraction

The magnetic structures of Mn_1-xFe_xWO₄ with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn²⁺- and Fe²⁺-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO₄ and FeWO₄. Received 9 October 2002 Published online 14 March 2003     

Orbital correlations in doped manganites

We review our recent X-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO₃. For x=0.25, we find an orbital structure indistinguishable from the undoped structure and long-range orbital order at low temperatures. For dopings 0.3≤x≤0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short-range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn³⁺ sublattice with neutron-scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transition, with a correlation length of 1–2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO₃, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron–phonon coupling. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Evolution of charge ordering in manganites

Charge ordering phenomena in the manganites Ca_1-xSm_xMnO₃ have been studied for , using electron diffraction and lattice imaging, completed by magnetic and transport measurements. Three domains can be distinguished, depending on the nature of the structural transitions with temperature. For , the structural transition from a pseudo-tetragonal to a monoclinic form, with decreasing temperature, coincides with the competition between ferromagnetism and antiferromagnetism that is characterized by the temperature T_peak on the M ( T ) curves; short-range charge ordering is observed for manganites. For the second domain, , a structural transition from an orthorhombic to a long-range charge ordered state is clearly observed with decreasing temperature. The corresponding temperature T_CO coincides with the temperature T_peak deduced from magnetic measurements. This long range charge ordering, which appears along a, is either commensurate or incommensurate depending on the x value, with a modulation vector, q being close to x. These modulated superstructures correspond to a stacking of single Mn³⁺ stripes with multiple Mn⁴⁺ stripes along a, either in a commensurate or in an incommensurate manner. The third domain , is characterized by a transition to a charge ordered state with commensurate superstructure at low temperature. The latter can be described as a “partially” charge ordered state in which single “Mn³⁺” stripes alternate with mixed “Mn³⁺/Mn⁴⁺” stripes. Received 17 June 1998     

Charge ordering-disordering in the Th-doped CaMnO3

The structural transitions that appear in the manganites Ca_1-xTh_xMnO₃ versus temperature are studied in connection with their magnetic and transport properties, and compared to those of the Ca_1-xLn_xMnO₃ manganites. An orthorhombic to monoclinic transition is observed for low x values (;this structural distortion, also observed for Ln-doped oxides, is related to the magnetoresistance properties. For higher x values (), modulated commensurate and incommensurate phases are obtained at low temperature, with , b =2 a p and , which are related to Mn³⁺/Mn⁴⁺ charge ordering (CO) phenomena. T values, determined from electron diffraction, are in agreement with those determined from the M ( T ) curves. The low temperature electron microscopy shows that the CO in those oxides is more complex than in Ln-doped manganites. In particular, the destabilisation of CO and consequently of the antiferromagnetic interactions is evidenced as the thorium content increases which may explain the appearance of a spin-glass like behavior for higher x values not seen for Ca_1-xSm_xMnO₃ phases . Received 2 November 1998     

A lower bound for the volume-averaged mean-square magnetostatic stray field

Based on a micromagnetics model, we develop a method through which quantitative information on the volume-averaged mean-square magnetostatic stray field 〈|H ^b _d|²〉 _v due to non-zero divergences of the magnetization M within the bulk of a ferromagnetic body can be obtained by analysis of magnetic-field-dependent small-angle neutron scattering data. In the limit of high applied magnetic field H _a, when the direction of M deviates only sligthly from H _a, we have estimated a lower bound for 〈|H ^b _d|²〉 _v as a function of the external field, and we have applied the method to bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The root-mean-square magnetostatic stray field, which is inherent to a particular magnetic microstructure, shows a pronounced field dependence, with values ranging from about 5 to 50mT. Even at applied fields as large as 1.7T, the quantity μ〈|H ^b _d|²〉^1/2 _v of nanocrystalline Co is still 24mT, which suggests that contributions to the total magnetostatic field originating from the bulk are significant in nanocrystalline ferromagnets; therefore, 〈|H ^b _d|²〉 _v cannot be ignored in the interpretation of e.g. measurements of magnetization or spin-wave resonance. A comparison of 〈|H ^b _d|²〉 _v with the volume-averaged mean-square anisotropy field reveals that both quantities are of comparable magnitude. Received 25 April 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: anmi@nano.uni.saarland.de     

Phase transitions of MnO3 compounds revealed by nonlinear magnetooptics

Magnetic second-harmonic generation (SHG) and three-photon difference-frequency generation (DFG) are used to investigate the magnetic phase diagrams of MnO₃ ^3- compounds with five- and sixfold coordination of the Mn³⁺ (3d⁴) ion. In the hexagonal manganites, the six basic antiferromagnetic structures following from a one-dimensional representational analysis of the hexagonal unit cell are clearly distinguished on the basis of their different selection rules for SHG. All structures break the geometric frustration of the unit cell by triangular antiferromagnetic ordering of the Mn spins. The stability and the mutual interactions of in-plane and inter-plane exchange and the in-plane anisotropy are investigated. The three quantities act as almost decoupled degrees of freedom, which leads to independent in-plane and inter-plane reorientations of the Mn spin lattice. DFG was observed in the orthorhombic manganites Pr_1-xCa_xMnO₃ and Nd_1-xSr_xMnO₃. The DFG reflects the fourfold anisotropy of the quasicubic crystal, and the independent tensor elements for DFG are determined quantitatively. A metal–insulator transition with a two-order-parameter coupling to antiferromagnetism and charge ordering leads to additional difference-frequency contributions, which allow us to observe the formation of ∼100 μm magnetic domains. The variety of results gained from the two vastly different groups of manganite compounds, with spectral and spatial resolution used as additional degrees of freedom, demonstrates the versatility of nonlinear magnetooptical experiments for the investigation of magnetic structures and symmetries. Received: 16 October 2001 / Published online: 2 May 2002     

Hole doping by Li substitution and antiferromagnetism in YBa Cu O studied by neutron powder diffraction measurements

The magnetic structure of tetragonal insulating YBa₂Cu_3-xLi_xO_y has been studied as a function of x and y. The Néel temperature and the mean ordered magnetic moment on the Cu2 sites were determined by neutron powder diffraction measurements. The decrease of these two parameters as compared to YBa₂Cu₃O₆ is much stronger for lithium than for zinc substitution. The difference is quantitatively explained by the presence of holes created in the CuO₂ planes. These holes arise from the substitution of plane Cu²⁺ by Li⁺. We suggest an explanation why such holes are not seen for the same substitution of plane Cu²⁺ by Li⁺ in orthorhombic superconducting YBa₂Cu_3-xLi_xO _{7 - δ} . Received 31 October 2001 and Received in final form 6 March 2002 Published online 25 June 2002     

Magnetism of alloyed models of Cr(Mn) V and Mn Cr overlayers on V (001) substrates

The magnetism is calculated for substitutional alloyed of Cr_x(Mn_x)V_1-x monolayers (MLs) and Cr_xMn_1-x on V (001) surface with a variety of concentrations (x = 0.25, 0.50 and 0.75). The substitutional surface alloys were treated by an artificial super cell construction. Parallel magnetic ordering is obtained for all the considered structures. The surface net magnetization increases in terms of Cr (Mn) concentration in Cr_x(Mn_x)V_1-x/V (001) system, while no serious variations occur in Cr_xMn_1-x/V (001). Vanadium atoms at the interfacial layers acquire appreciable magnetic moments antiferromagneticlly (AF) coupled with the surface moments. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: jkalifa@sci.ju.edu.jo