Antiferromagnetic resonance in ferroborate NdFe3(BO3)4

The AFMR spectra of the NdFe₃(BO₃)₄ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields H_a1=1.14 kOe and H_a2=60 kOe and magnetic excitation gaps Δν₁=101.9 GHz and Δν₂=23.8 GHz. It is shown that commensurate-incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width.At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.     

Numerical simulation of magnetic interactions in polycrystalline YFeO3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO₃ sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (H_E=5590 kOe), anisotropy field (H_A=0.5 kOe) and Dzyaloshinsky–Moriya antisymmetric field (H_D=149 kOe) are in good agreement with previous reports on this system.     

Magneto-optical measurement on the layer type magnet (CH2)2(ND3)2MnCl4

The linear birefringence (LB) of the antiferromagnet (CH₂)₂(ND₃)₂MnCl₄ has been measured as a function of temperature and in magnetic fields up to 100 kOe. The temperature dependence of the LB points to a pronounced two dimensional magnetic behaviour. No anomaly corresponding to the effect of three dimensional ordering could be detected at T_N. In theffield dependent measurements the spin flop at H_SF = 33.6 ± 1 kOe (T = 4K) could clearly be detected.     

Magnetocaloric effect at room temperature in manganese perovskite La0.65Nd0.05Pb0.3MnO3 with double resistivity peaks

Series of polycrystalline manganese perovskite oxides La_0.7−xNd_xPb_0.3MnO₃ (x=0, 0.05, and 0.1) are prepared by the sol-gel technique, La_0.65Nd_0.05Pb_0.3MnO₃ were representatively investigated because the peculiar double resistivity peaks were found; the maximum magnetic entropy change ΔS_H=−2.03 J/kg K and its good refrigerant capacity 71.05 J/kg around room temperature were obtained under 9 kOe magnetic field variation. The expected double peaks of magnetocaloric effect had not occurred since magnetic entropy change originated from the differential coefficient of magnetic moment to temperature; the relatively well refrigerant capacity possibly results from the faint magnetic inhomogeneity mixed in the double exchange strong magnetic signal.     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.     

Visual investigation of the intermediate state in the region of metamagnetic phase transition in ErFeO3

We carried out the first observation of the intermediate state (IS) in the region of metamagnetic phase transition (MPT) in ErFeO₃: an external magnetic field H∥c (H=0–4 kOe), the temperature range 2.15–1.6 K. The ranges of the IS existence and the domain structure (DS) features were determined. Likely mechanisms of the IS thermodynamic stabilization and conditions for its visualization in ErFeO₃-plates with various crystallographic orientations were analyzed. The thermodynamic model of IS was offered: the intermediate state was stabilized by the mechanical stress that accompanies MPT. Excellent agreement with experiment was achieved.     

Mössbauer characterization of tin dopant ions in the antiferromagnetic ilmenite MnTiO3

Mössbauer parameters of ¹¹⁹Sn diamagnetic dopant cations in an antiferromagnetic compound having the ilmenite structure are for the first time reported. The spectra reveal a well resolved hyperfine splitting pattern of combined magnetic and quadrupole interactions (at 5 K, δ=0.19 mm/s, H₁=52.5 kOe, eV_ZZQ_3/2=−0.80 mm/s, θ≈0°). This spectral component whose contribution (A₁=82%) represents more than four fifths of the total amount of the dopant (Sn/(Mn+Ti)=1/200) is assigned to Sn(IV) ions located in the bulk of MnTiO₃, on the Mn(II) site, and with a Mn(II) vacancy in their nearest surrounding. Two spectral components with minor contributions are also observed: one of them (H₂≈25 kOe, A₂=8%) can be assigned to Sn(IV) ions, in the MnTiO₃ lattice as well, on a site where they exhibit a weaker spin polarization (this site could be the Ti(IV) one) and the other (H₃=0 kOe, A₃=10%) to SnO₂ or/and Ti_1−xSn_xO₂ clusters. The Néel temperature of MnTiO₃ probed by the ¹¹⁹Sn dopant (T_N=69±2 K) agrees well with the values previously provided by ESR and antiferromagnetic resonance measurements. Variation of H₁ with temperature follows close the Brillouin function for S=5/2. No perturbation appears in the Mössbauer spectra around T=90 K where a broad peak, characteristic of 2D magnetic interactions, is observed on the static magnetic susceptibility curve.     

Sign of the electric field gradient at 57Fe nuclei in Eu3Sc2Fe3O12 garnet

The ⁵⁷Fe Mössbauer effect in conjunction with an external magnetic field H_ext has been used to measure the signs of quadrupole coupling constants Δ at the octahedral (a) and tetrahedral (d) iron sites in the Eu₃Sc₂Fe₃O₁₂ garnet. It is shown that application of H_ext to the sample at temperatures T above the Néel temperature T_N slows down the relaxation effects, whose influence on the spectra depends upon the temperature difference T-T_N. By comparing a spectrum measured at 203.0 K and in H_ext=62.4 kOe with theoretical spectra computed for different combinations of Δ(a), Δ(d) signs, it is concluded that Δ(d) is negative. The sign of Δ(a) could not be determined.     

High field magnetization of Y2Co7 and YCo3 hydrides

The magnetization curves for the Y₂Co₇H_x and the YCo₃H_x systems have been measured at 4.2 K in the pulsed high magnetic fields up to 280 kOe. The metamagnetic transition is observed in the β and the γ hydrides except for Y₂Co₇H₃ and YCo₃H₁, which is interpreted in terms of the itinerant electron metamagnetism.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

Anisotropy of Hc2 in single crystal Nb3Sn and V3Si at high magnetic fields: Limitations of linear chain model

The anisotropy of H_c2 for single crystal Nb₃Sn and V₃Si in the low temperature tetragonal phase was measured in high magnetic fields. At ～ 8K, δ = [H_c2[100]-H_c2[110])/H_c2] ≈0.05 for Nb₃Sn and δ ≈ 0.02 for V₃Si. The data do not show evidence of a highly anisotropic Fermi surface for Nb₃Sn or V₃Si.     

Structural first-order transformation in La2/3Ba1/3MnO3: ESR study

We have studied by the electron-spin resonance (ESR) and static magnetic field techniques, the La_2/3Ba_1/3MnO₃ perovskite, which was previously shown to exhibit a martensitic phase transformation in the vicinity of T_s∼200 K [Physical Review B 68, 054109 (2003)], leading to its structural phase-segregated state. Resonant absorptions reveal that in the temperature interval from 100 K to 340 K the compound represents a mixture of two ferromagnetic phases possessing different magnetizations, in varying proportions depending on the temperature, and a small amount of a paramagnetic phase. The results agree well with the previous neutron diffraction study. Applied in the ESR experiments, magnetic fields (2–6 kOe) strongly affect the magnetization curves: even magnetic field as high as 700 Oe modifies the anomaly in the phase transformation region and removes the difference between the zero-field cooled and field-cooled magnetization curves, which implies that the difference in the magnetic susceptibility of the coexisting phases is small and the magnetic domain configuration can be easily changed.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

Magnetic entropy change in perovskite manganites La0.6Pr0.1Pb0.3MnO3 with double metal-insulator peaks

Following the double metal-insulator peaks found in series of perovskite manganites La_0.7−xPr_xPb_0.3MnO₃ (x=0, 0.05, 0.1), the magnetic entropy change of La_0.6Pr_0.1Pb_0.3MnO₃ was carefully investigated as a representative. The maximum magnetic entropy change (ΔS_H=−1.7 J/kg K at 300 K) and the expanded refrigerant capacity (about 123.8 J/kg) had been obtained under 10 kOe magnetic field variation, though the double peak of maximum magnetic entropy change had not occurred since the comparative faint magnetic signal from the Pr ions inhomogeneity existed in the octahedral frame submerged in the strong magnetic signal originated from the dominating octahedral frame both in the double exchange mechanism, but the width at half maximum in the magnetic entropy change comparatively broadened.     

Effect of refractory metals addition on the structural and magnetic properties of Pr3(Fe, Co, Ti)29 system

Polycrystalline refractory metal-substituted Pr₃(Fe_0.6M_0.1Co_0.3)_27.5Ti_1.5 (M=V, Ti, Zr, Mo, Nb,Cr) and Pr₃(Fe_0.5Co_0.5)_27.5Ti_1.5 have been studied for high-temperature permanent magnetic materials. X-ray diffraction showed the main phase to be the 3:29 phase. We observed the highest reported T_C (Curie temperature) of 640 °C for the 3:29 system in the Pr₃(Fe_0.5Co_0.5)_27.5Ti_1.5. In the refractory metal-substituted systems, the highest T_C of 480 °C was observed for the Nb-substituted alloy. SEM measurements showed that Ti in Pr₃(Fe_0.6Ti_0.1Co_0.3)_27.5Ti_1.5 is deposited near the grain boundary. H_A (anisotropy energy) of V-substituted alloy is as high as 72 kOe, the highest reported in the 3:29 system and is ∼200% higher than 24 kOe observed in Pr₃(Fe_0.7Co_0.3)_27.5Ti_1.5. Cr and Ti substitutions show an increase of 65% (40 kOe) and 45% (35 kOe) in H_A respectively. M_S (saturation magnetization) values were ∼100 emu/g and are lower than that observed in Pr₃(Fe_0.7Co_0.3)_27.5Ti_1.5.     

Phase diagrams of CuCl2·2NC5H5 and CsCuCl3

The phase diagrams of two quasi one-dimensional,$\text{S =}\text{1}\text{2}$ quantum systems CuCl₂·2NC₅H₅ and CsCuCl₃, have been determined experimentally for fields between 0 and 90 kOe. The results show an increase of the Néel temperature as a function of the field, in accordance with earlier observations on manganese systems.     

The relation between magnetoresistance and magnetocaloric effect in La0.85Ag0.15MnO3 manganite

We present the temperature dependence of La_0.85Ag_0.15MnO₃ resistivity in the temperature interval between 77 and 340 K and magnetic fields up to 26 kOe. We offer a method of separating tunnel magnetoresistance from total magnetoresistance. A change in both the magnetic entropy, which is caused by the magnetocaloric effect (MCE), and the magnetoresistance are shown to be connected through a simple relationship to La_0.85Ag_0.15MnO₃.     

Mössbauer investigation of hexagonal tungsten bronze type FeIII fluorides: (H2O)0.33 FeF3 and anhydrous FeF3

The new HTB (Hexagonal Tungsten Bronze) phases of FeF₃ and (H₂O)_0.33 FeF₃ have been characterized by ⁵⁷Fe Mössbauer spectroscopy; they have saturation hyperfine fields of (577 ±3) and (560±3) kOe and magnetic ordering temperatures (97±2) and (128.7 ± 0.5) K, respectively. The magnetic ordering temperature and the electric hyperfine interactions on iron are sensitive to the presence of zeolitic water in the system. Hydrolysed samples have also been investigated.     

The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.