Ferrimagnetic Phase Transition Research Articles

Structural and magnetoelectric properties ofGa2−xFexO3single crystals grown by a floating-zone method

Lattice-structural, magnetic, and magnetoelectric (ME) properties have been investigated for single crystals of prototypical polar ferrimagnet ${\mathrm{Ga}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ $(0.8\ensuremath{\leqslant}x\ensuremath{\leqslant}1.4)$ as melt-grown by a floating-zone (FZ) method. Magnetization measurements show that the saturated magnetization as well as the ferrimagnetic phase transition temperature $({T}_{C})$ increases with as increase of $\mathrm{Fe}$ content $x$, while the coercive force decreases. A neutron powder diffraction study indicates fairly low ordering of $\mathrm{Ga}$ and $\mathrm{Fe}$ arrangement at cation sites, which is likely related to the lower ${T}_{C}$ in the FZ crystals than in the corresponding flux-grown crystals. Coefficients of linear and quadratic ME effects have been obtained with measurements of change in electric polarization induced by sweeping a magnetic field. Electric polarization was largely modulated in a magnetic field applied parallel to the direction of spontaneous magnetization, but not in a field parallel to that of the spontaneous polarization. A simple model to explain the sharp contrast is presented.

Magnetism in Mn-doped ZnO bulk samples prepared by solid state reaction

Contrasting magnetic properties were obtained from bulk Mn-doped ZnO synthesized under different processing conditions. While a ferrimagnetic phase transition was observable in a Zn0.95Mn0.05O sample processed at 1170 K, no such transition was found for a sample with the same composition processed at 1370 K. The detailed magnetic, structural, and spectroscopic studies of these two samples have revealed that the ferrimagnetic transition in the former sample is attributable to the secondary phase, (Mn,Zn)Mn2O4, in the system. For the latter sample processed at higher temperature, no secondary phase was detected and the major feature of the system remained paramagnetic down to 4 K. The implication of the present results for Mn-doped ZnO thin films is discussed.

Radical-Copper Macrocycles and Related Compounds

4-Pyrimidinyl nitronyl nitroxide (4PMNN) was synthesized and characterized, and four 4PMNN-metal(II) chloride complexes were investigated. Hexanuclear macrocyclic [CuCl 2 ·(4PMNN)] 6 shows intermolecular ferromagnetic interaction. MnCl 2 ·(4PMNN) exhibits antiferromagnetic interaction. Mononuclear CoCl 2 ·(4PMNN) 2 shows ferrimagnetic phase transition at ca. 3 K. NiCl 2 ·(4PMNN) 2 are practically diamagnetic below 100 K

Radical-Copper Macrocycles and Related Compounds

4-Pyrimidinyl nitronyl nitroxide (4PMNN) was synthesized and characterized, and four 4PMNN-metal(II) chloride complexes were investigated. Hexanuclear macrocyclic [CuCl 2 ·(4PMNN)] 6 shows intermolecular ferromagnetic interaction. MnCl 2 ·(4PMNN) exhibits antiferromagnetic interaction. Mononuclear CoCl 2 ·(4PMNN) 2 shows ferrimagnetic phase transition at ca. 3 K. NiCl 2 ·(4PMNN) 2 are practically diamagnetic below 100 K

Study on the Molecular Magnetism in Mixed Valence Iron Complexes, M[Fe II Fe III (mto) 3 ](M=( n -C n H 2n+1 ) 4 N, mto=monothiooxalato (C 2 O 3 S))

We have synthesized and investigated the physical properties of a new mixed valence iron complex, ( n -C 3 H 7 ) 4 N[Fe II Fe III (mto) 3 ] (mto=C 2 O 3 S). This complex behaves as a ferrimagnet with T N =38 K and θ= m 117 K. From the analysis of the Mössbauer spectrum of ( n -C 3 H 7 ) 4 N[Fe II Fe III (mto) 3 ] at 298 K, the spin state of Fe II was determined to be the high spin state (S=2), but it was difficult to determine the spin state of Fe III . So that, in order to elucidate the spin state of Fe III , we have investigated ESR spectra, and confirmed the coexistence of high spin state (S=5/2) and low spin state (S=1/2) for the Fe III sites. Moreover, we have synthesized ( n -C 4 H 9 ) 4 N[Fe II Fe III (mto) 3 ]. This complex shows the ferrimagnetic phase transition at 41 K, and Weiss constant is m 76 K. In our previous paper, we have reported that mixed-valence complex ( n -C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ] (dto=C 2 O 2 S 2 ) shows a new type of phase transition connected with charge transfer phase transition and spin-crossover transition around 120 K. In the title complexes, the charge transfer phase transition was not observed. mixed valence complex charge transfer phase transition monothiooxalato

Anomalies of internal friction of ferrimagnetic spinel Li0.5Fe2.5O4 in various structural states

The results of high-temperature (20≤T≤800°C) relaxation and magnetic investigations of ferrite Li0.5Fe2.5O4 in various structural states are given. Anomalies of internal friction caused by the occurrence of ferrimagnetic and structural (ordering type) second-order phase transitions and by vibrations of domain walls of ferrimagnetic and antiphase domains were revealed. It is shown that the 1: 3 ordering of Fe3+ and Li+ ions in the octahedral sublattice of the spinel leads to a decrease in the Curie temperature, a change in the character of the temperature dependence of the low-field magnetization, and a narrowing of the temperature range of the structural phase transition, as well to a substantial weakening of dissipation processes connected with vibrations of ferrimagnetic domain walls in the field of elastic stresses.

Magnetic phase transition in MnCr2-2xIn2xS4 crystals

The influence of In for Cr substitution on the para- to ferrimagnetic phase transition has been investigated for MnCr2-2xIn2xS4 spinel crystals by means of DC magnetization measurements. The critical exponents and calculated from the high field extrapolation of the modified Arrott plots were found to be unaltered with substitution and close to 3D Heisenberg ones. A considerable expansion of the temperature range of the upward curvature of the inverse initial susceptibility of the samples with disorder increase has been found. Non-monotonic temperature dependences of Kouvel-Fisher effective susceptibility exponent * for disordered samples and the increase of the * value with the In concentration growth were also observed.

Ferrimagnetic phase transition in the system Li0.5Fe2.52−x GaxO4 in the vicinity of the multicritical point of the x-T phase diagram

In connection with the problem of identifying magnetic states in the vicinity of x0 (the multicritical point of the x-T diagrams of spin-glass systems) a study has been made of properties that can be exploited to determine the presence of a thermodynamic phase transition at the Curie point TC and the distinctive features of the transition, specifically, the temperature dependence of the magnetic part of the specific heat Cm(T), the temperature dependence of the low-field magnetization σH(T), and (with a view toward examining critical behavior in a magnetic field) the magnetization isotherms σH(T). The investigated object is the system of dilute ferrimagnetic spinels Li0.5Fe2.5−xGaxO4, in which every type of magnetic state has spatially inhomogeneous cluster structures. The results obtained for a sample with x=1.45 indicate that the classical criteria of a ferrimagnetic second-order phase transition at TC=(97±2) K occur for x∼x0. The results of similar investigations for a sample with x=1.6, which exists in the cluster spin-glass state for T Tf, are also given for comparison with the preceding case.

Influence of temperature and magnetic field on ferrimagnetic CePtGe2.

An external magnetic field causes a significant (more than twofold) increase of [gamma] deduced from high-temperature-range data (i.e., the paramagnetic state) showing no saturation even at 7.5 T, and a slight rise of the electronic specific-heat coefficient for the magnetically ordered state up to [similar to]125 mJ/mol K[sup 2] with saturation at this value in a much lower field (2.5 T). The entropy ([Delta][ital S]) associated with the ferrimagnetic phase transition is estimated to be 2.9 J/mol K, which is only 0.35[ital R] or 50% of the theoretically expected [ital R] ln2 value. The heat-capacity and resistivity measurements suggest that CePtGe[sub 2] is a moderately heavy-fermion system.

Novel magnetic phase transition behavior in short period EuTe/PbTe superlattice

[EuTe( ξ)/PbTe( η)] n short period superlattices were grown by molecular beam epitaxy along the [111] growth direction, with ξ denoting the number of monolayers of EuTe separated by η monolayers of diamagnetic PbTe, and 2 ⩽ ξ ⩽ 5 alternated with 6 ⩽ η ⩽ 15. We find that superlattices with even ξ numbers show antiferromagnetic phase transitions; those with odd ξ show ferrimagnetic phase transitions, while bulk EuTe is a simple two-sublattice antiferromagnet. The transition temperatures T c observed for these short period superlattices are all below or close to the bulk phase Neél temperature and T c increases as ξ increases. An antiferromagnetic interlayer correlation along the direction ⊥ to the superlattice planes is found for superlattices with η ⩽ 12. In the paramagnetic regime, the Curie-Weiss law fits well for all superlattices. Positive paramagnetic Curie-Weiss temperatures θ are obtained for superlattices with odd ξ numbers while negative θ are obtained for even ξ.

Specific Heat and Magnetic Susceptibility Studies on Bromobis (1,4‐Diazacycloheptane) Copper(II) Perchlorate, [Cu(dach)2Br](C104), at Low Temperatures

AbstractSpecific heat and magnetization investigations are made on polycrystalline [Cu(dach)2Br](Cl04) at low temperatures. For temperatures above 25 K, the dc magnetization shows a Curie‐Weiss behaviour with a Curie constant of 7.92 × 103 e.m.u. K/g and a paramagnetic Curie temperature of −60.8 K. The downturn of 1/X vs. T below 25 K is indicative of ferrimagnetic system. The specific heat between 4 and 8 K can be described by the relation: C = 11.9T3 mJ/mol K with the cubic term leading to a Debye temperature of θD = 194 K. Below 4 K the specific heat rises very rapidly suggesting magnetic ordering. This is substantiated by the presence of a sharp peak at 0.95 K in the ac susceptibility results which can be interpreted as being due to a ferrimagnetic phase transition. Both the magnetization and specific heat investigations indicate the behavior of the system to be ferromagnetic at low temperatures which is interpreted as the result of the canted antiferromagnetism of sublattice moments.

Polytypism in the cation-deficient iron sulfide,Fe9S10, and the magnetokinetics of the diffusion process at temperatures about the antiferro- to ferrimagnetic (λ) phase transition

The variation of the periodicity along the $C$ axis (${N}^{\ensuremath{'}}C$) as a function of temperature in ${\mathrm{Fe}}_{9}$${\mathrm{S}}_{10}$, where ${N}^{\ensuremath{'}}$ varies between 5 and 1 over the range 300-600 K, has been investigated. The antiferromagnetic to ferrimagnetic ($\ensuremath{\lambda}$) phase transition is explained in terms of a vacancy rearrangement. Using absolute-reaction-rate theory and measurements of magnetization as a function of time at constant temperature around the $\ensuremath{\lambda}$ transition, coefficients of diffusion for iron atoms along the $C$ axis have been determined over the range 440 to 484 K and found to be \ensuremath{\sim}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$/sec. Appropriate mechanisms for the diffusion process are also considered.

Ferrimagnetism in FeCoF5⋅2H2O

The magnetic susceptibility of FeCoF5⋅2H2O was measured over the temperature range from 3 to 296 °K. A ferrimagnetic phase transition was observed at 27.0±0.25 °K. At temperatures between 80 and 296 °K, FeCoF5⋅2H2O was paramagnetic and could be described by the Curie–Weiss law χ=C/(T+ϑ) with ϑ=56 °K and an effective magnetic moment per formula unit of 6.8±0.1 μB. In the ordered region the magnetization did not saturate at fields up to 11 kOe. At applied fields above 500 Oe the magnetization departed from the usual Brillouin behavior suggesting the occurrence of a second ordered phase at temperatures below 10°K.

Mössbauer and magnetic study of the antiferro to ferrimagnetic phase transition in Fe9S10 and the magnetokinetics of the diffusion of iron atoms during the transition

The antiferromagnetic to ferrimagnetic phase transition referred to as the λ transition in Fe9S10 has been explained by many workers on the basis of a structural change without any experimental evidence. Our Mössbauer and X-ray diffraction results support Lotgering’s suggestion that the ferrimagnetism at about 493 K arises from the 4C superstructure. The antiferromagnetic to ferrimagnetic (λ) phase transition is explained in terms of a vancancy rearrangement. Using absolute reaction rate theory and measurements of magnetization as a function of time at constant temperature around the transition, coefficients of diffusion for iron atoms along the C axis have been determined over the range 440 to 484 K and found to be 10−18 cm2/sec.

Ferrimagnetism in Fe2F5⋅2H2O

The magnetic susceptibility of Fe2F5⋅2H2O was measured over the temperature range from 5 to 300°K. A ferrimagnetic phase transition was observed at 48.5±0.25°K. At temperatures between 94 and 300°K, Fe2F5⋅2H2O followed a Curie–Weiss law χ =C/(T+ϑ), with ϑ=−138°K and an effective moment of 7.5±0.1 μB. The magnetization was well behaved below the transition and displayed no hysteresis. No evidence of a second low-temperature phase was observed.