Intersublattice Exchange Interaction Research Articles

Determination of the intersublattice exchange interactions in GdCo12-xFexB6 (x = 0-3) intermetallic compounds by high field magnetization measurements

High field magnetization measurements up to 60 T on free powder samples from GdCo12-xFexB6 (x = 0-3) compounds are reported. The data were used to evaluate the microscopic exchange interaction integral, JGd-3d, between Gd and 3d (Co,Fe) spins. The systems are ferrimagnets; they order magnetically between TC = 95 K for x = 3 and TC = 165 K for x = 0. The low temperature magnetization curves as well as the temperature dependence of intrinsic magnetic parameters are determined by magnetic measurements in pulsed magnetic field. The average magnetic moment ⟨μCo+Fe⟩ per mean transition metal atom (Co + Fe) is small and increases with increasing Fe concentration from 0.44 μΒ for x = 0 to 0.51 μΒ for x = 3 at T = 4 K. From high field magnetization curves, a value of JGd-3d/kB = −4.65 K is derived for x = 0, whereas mean field approximation yields a much larger 3d-3d exchange integral of JCo-Co/kB = 105 K. The obtained results reveal an increase of −JGd-3d/kB with Fe concentration. For x = 0.5, the intersublattice coefficient nGd-3d is found to keep an almost constant value of 5.87 ± 0.13 T*f.u.*μB−1 whatever the temperature in the 2 to 60 K range.

Dielectric and magnetic properties of Mg–Ti substituted barium hexaferrite

Abstract Barium hexaferrites with general chemical formula BaFe 12−2 x Mg x Ti x O 19 ( x =0, 1, 1.5, 2) were synthesized by the solid state diffusion method. X-ray diffraction (XRD) confirmed the formation of single phase magnetoplumbite with large crystallite size. The lattice parameter a decreased with the increase in x , leading to the increase in X-ray density. Formation of well defined grains was observed through scanning electron microscopy (SEM). The dielectric properties were studied by impedance measurements as a function of frequency (in the range 100 Hz–10 MHz) and temperature (in the range 30 °C–345 °C). The effect of composition, frequency and temperature on the dielectric constant ( e′ ), dielectric loss (tan δ ), and conductivity ( σ ) have been explained on the basis of hopping of electrons between Fe 2+ and Fe 3+ ions. The variation of magnetization ( M ) with applied magnetic field ( H ) and temperature ( T ) were studied with a vibrating sample magnetometer (VSM). The coercivity ( H c ) and saturation magnetization ( M s ) decreased with the substitution level due to the occupancy of Ti 4+ ions at octahedral site and of Mg 2+ ions at tetrahedral site. Superparamagnetic behavior was observed in the compounds with x =1.5 and 2. Curie temperature decreased with the increase in x as some intersublattice exchange interactions strongly decreased with Mg and Ti addition.

The magnetic properties of oxide spinel Li0.5Fe2.5−2xAlxCrxO4 solid solutions

The exchange interactions (JBB and JAB are the intra and the inter-sublattice exchange interactions between neighbouring spins, respectively) are obtained by using the general expressions of canting angle and critical temperature obtained by mean field theory of Li0.5Fe2.5−2xAlxCrxO4. The expression of magnetic energy of Li0.5Fe2.5−2xAlxCrxO4 is obtained for different spin configurations and dilution x. The saturation magnetisation of Li0.5Fe2.5–2xAlxCrxO4 is obtained with different values of dilution x. The magnetic phase diagram of Li0.5Fe2.5–2xAlxCrxO4 materials is obtained by high temperature series expansions (HTSEs). The critical exponent associated with the magnetic susceptibility of Li0.5Fe2.5−2xAlxCrxO4 is deduced.

Magnetic properties of a DyFe 5Al 7 single crystal

A single crystal of DyFe 5Al 7 (tetragonal ThMn 12 structure) has been studied. DyFe 5Al 7 orders ferrimagnetically at T C = 231 K and has a compensation of the Dy and Fe sublattices at T comp = 93 K. Magnetization isotherms measured along the principal axes of the single crystal indicate that the magnetic moments lie in the basal plane of the tetragonal lattice. Strong magnetic anisotropy is observed not only between the basal plane and the [0 0 1] axis but also within the basal plane, with the [1 0 0] axis being the easy magnetization direction with the spontaneous magnetic moment 2.1 μ B. A very high coercivity with H c values up to 3 T was found at 2 K. In the temperature range 75–100 K, the compound exhibits a field-induced magnetic transition along the easy [1 0 0] direction. Temperature dependence of the critical field H cr of the transition is very strong, the value of H cr evidently exceeds 14 T below 75 K. A model for the magnetization process of a two-sublattice ferrimagnet with a weak intersublattice exchange interaction has been developed to explain the magnetic transition. The model reproduces the temperature behavior of H cr near the compensation point.

Magnetism of LiMn2O4 manganite in structurally ordered and disordered states

The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally ordered Li[Mn2]O4 and disordered Li1 − δMnδ[Mn2 − δLiδ]O4 (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally disordered state of the manganite was achieved under irradiation by fast neutrons (E eff ≥ 1 MeV) with a fluence of 2 × 1020 cm−2 at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure, and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes in the structural and magnetic states of the LiMn2O4 manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice due to the appearance of intersublattice exchange interactions.

Effect of neutron irradiation on the long-wavelength magnetic excitations in the Zn0.60Ni0.40Fe2O4 ferrite

An energy analysis of small-angle neutron scattering in the Zn0.60Ni0.40Fe2O4 ferrite nonirradiated and irradiated by fast neutrons has been performed. It has been revealed that the inelastic neutron scattering in the initial ferrite below TC is due to not only the intradomain spin waves but also the significant contribution of the intraboundary long-wavelength magnetic excitations (w) and those localized near the domain boundaries (nw). The number of these magnetic excitations in the irradiated sample is significantly smaller. The elevated density of states of the w and nw magnetic excitations is assumed to be related to the competition between the intersublattice and intrasublattice exchange interactions, which is significantly weakened in the irradiated ferrite, where the corresponding exchange integrals are related by the inequality JAB ≫ JBB.

Towards the theory of ferrimagnetism: II

The present paper is a sequel to the paper by Karchev (2008 J. Phys.:Condens. Matter 20 325219). A two-sublattice ferrimagnet, with spin-s1 operatorsS1i at the sublatticeA site and spin-s2 operators S2i at the sublattice B site, is considered. Renormalized spin-wave theory, which accounts for themagnon–magnon interaction, and its extension are developed to describe the two ferrimagnetic phases(0,T*) and(T*,TN) in the system, and to calculate the magnetization as a function of temperature.The influence of the parameters in the theory on the characteristic temperaturesTN andT* is studied. It is shown that, increasing the inter-sublattice exchange interaction, the ratioTN/T*>1 decreases approaching one, and above some critical value of the exchange constant there is only onephase TN = T*, and the magnetization–temperature curve has the typical Curie–Weissprofile. When the intra-exchange constant of the sublattice with strongerintra-exchange interaction increases the Néel temperature increases whileT* remainsunchanged. Finally, when the magnetic order of the sublattice with smaller magnetic order decreases,T* decreases. The theoretical predictions are utilized to interpret the experimentally measuredmagnetization–temperature curves.

Magnetic properties of the ferrimagnetic spinels systems CoFe2–2xCr2xO4

The magnetic properties of a diluted ferrimagnetic spinels system CoFe2–2xCr2xO4 are studied using the high-temperature series expansions method in the range 0 ≤ x ≤ 1. The inter-sublattice exchange interactions (JCo Cr or JCo–Fe) are calculated using the probability law. The high-temperature series expansions, together with the Padé-approximants method, are applied to the spinels systems CoFe2–2xCr2xO4 to determine the magnetic phase diagram, i.e., TC versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. PACS Nos.: 75.30.Et; 75.40.Cx; 74.25.Ha, 75.30.Cr

A high-energy inelastic neutron scattering investigation of the Gd–Fe exchange coupling in GdFe 11TiD x ( x = 0 and 1)

The Gd–Fe intersublattice exchange interaction is probed by high-energy inelastic neutron scattering in GdFe 11Ti and GdFe 11TiD compounds. Deuterium insertion within the crystal structure considerably increases the Curie temperature and modifies the magnetisation of the GdFe 11Ti phase whereas the Gd–Fe intersublattice exchange interaction is much less sensitive to the presence of D. The J ex exchange coupling is almost insensitive to the D content but the Gd–Fe exchange field is significantly larger for the doped sample. The influence of deuterium insertion on the exchange interaction between the Gd and Fe sublattices is compared to that of nitrogen.

Physical reasons for the emergence of jumps during rotation of the magnetization of a two-sublattice ferrimagnet at low temperatures

It is shown that the contribution of the intersublattice exchange interaction to the magnetic anisotropy energy of a two-sublattice ferrimagnet can come only from higher order constants satisfying the condition 2K 2+3K 3+⋯>0. For this reason, for different signs of the first anisotropy constants of the sublattices, this contribution may cause a spontaneous spin-reorientation second-order transition, but not first-order transitions and jumps during magnetization rotation, which are associated with such a transition. Such jumps can appear only when the opposite inequality is satisfied, and the corresponding contribution to anisotropy can be ensured only by a fairly strong magnetoelastic interaction.

Effect of Si content on magnetic properties and intersublattice exchange interactions in GdFe 11− xSi xTi compounds

Induction melted GdFe 11− x Si x Ti alloys ( x⩽2) with ThMn 12-type structure have been investigated. The decrease of Curie temperature, magnetic moment and anisotropy constant are ruled by Si 3p electron transfer into Fe 3d band. The increase of the exchange interaction parameter between Gd and Fe sublattice shows that indirect 3d–4f interaction is enhanced.

The effect of Al-content and crystallinity on the magnetic properties of goethites

This paper reviews a profound Mossbauer study on the magnetic properties of goethites with varying crystallinity and Al-content. It has been found that both the cluster-ordering and the superferromagnetic models reproduce the temperature dependencies of the average hyperfine fields quite well. However, the magnetic transition temperatures predicted by the cluster-ordering model are in better agreement with those measured from thermoscan experiments. On the other hand, it has been found that the hyperfine field of maximum probability at low temperatures follows the spin–wave law for antiferromagnets. Linear relationships between the saturation hyperfine fields, the inter-sublattice exchange interactions, and the Neel temperatures to the Al-content and crystallinity could be derived. On the other hand, applied-field Mossbauer spectroscopy seems to suggest that the anisotropy field can differentiate between poor crystallinity and Al-content, and that a certain degree of ferrimagnetism occurs at higher Al-contents.

Anomalous singular-point-detection peak of monocrystalline YCo 4Al

High pulsed-field measurements were performed on a monocrystalline YCo 4Al sample using the singular-point-detection (SPD) method, in the temperature range 5–300 K. At 5 K, the anisotropy field value determined from the SPD method shows good agreement with that obtained from quasi-static high-field measurements in the Amsterdam high-field installation. In particular, besides the SPD peak that corresponds to the anisotropy field, an additional peak appears at lower field for all results obtained. We propose an explanation based on the two-sublattice model. One sublattice is composed of the Co atoms at the 2c site and the other of the Co atoms at the 3g site. Because of the competition between the magnetocrystalline anisotropy and the intersublattice exchange interaction, the two sublattices do not rotate coherently in magnetic fields. Calculation shows agreement with the experimental results.

Full magnetization process of 3d−4f hard magnetic materials in ultrahigh magnetic fields (an example: RFe11Ti)

The prospects of using the free-powder high-field magnetization method for a quantitative study of inter-sublattice exchange interaction in 3d−4f hard magnetic materials are analyzed. Such analysis is stimulated by the availability of pulsed magnetic fields ∼103 T generated by implosion. Particular attention is paid to effects due to magnetic anisotropy, essential for these materials. The 3d−4f ferrimagnets where both sublattices contribute positively to the easy-axis anisotropy are shown to be suitable objects of study by the free-powder method, because (i) anomalies in their low-temperature magnetizatization curves are sharp and (ii) anisotropic effects can be allowed for without quantitative knowledge of the anisotropy constants. Moreover, these “good” hard magnetic materials can be brought into metamagnetic regime by diluting the rare earth sublattice with nonmagnetic yttrium; then, regardless of the anisotropy constants, the magnetization curve at low temperatures has just one steplike anomaly, the threshold field being equal exactly to the molecular field acting on the rare earth.

Effects of Al substitution on intersublattice-exchange interaction in GdCo 5- xAl x

The strength of the intersublattice Gd-Co exchange-coupling parameter, A GdCo, and the magnetic moment of the Co sublattice, M Co , are reported for the GdCo 5- x Al x ( x = 0, 0.5, 1.0, 1.5 and 1.8) compounds. As the Al content increases, a tendency to decrease is found for both A GdCo and M Co. In comparison with a previous study of Gd-Co-B series, a systematic influence of 3d-p hybridization on the 4f-3d exchange coupling is evident. These results are discussed in terms of the magnetic valence model.

Studies on the exchange interactions in R 2Fe 14B, R 2Fe 14C and R 2Co 14B by molecular field theory

The intra- and inter-sublattice exchange interactions in three series of permanent magnetic materials (R 2Fe 14B, R 2Fe 14C and R 2Co 14B, R = rare earth elements) have been studied on the basis of molecular field theory. The effective exchange coupling constant ( J CoCo) of Co sublattices in R 2Co 14B is about 16.61 × 10 −22J, which is nearly three times the values ( J FeFe) of Fe sublattices in both R 2Fe 14B and R 2Fe 14C. The effective exchange coupling constants ( J RT) between rare earth (R) and transition metal (T) are not constant across the series but dependent on the nature of R atoms, which is different from the usual assumption that the J RT are constant across the series due to the similarities of band structures for all R atoms. The molecular fields acting on the rare earth ions are also calculated. The maximum molecular fields are found for R = Sm in each series.

The intersublattice exchange interactions in the R 2Co 7B 3 compounds (R = heavy rare earth)

The experimental values of Curie temperatures of hexagonal R 2Co 7B 3 (R = heavy rare earth) compounds are used to calculate the effective intersublattice exchange interactions, J RCo, between R and Co sublattices. A mean field approximation is used. The compounds belong to the R n+1 Co 3 n+5 B 2 n family. The R 2Co 7B 3 is a combination of CaCu 5 and CeCo 3B 2-type structures. The J RCo has a tendency to decrease with increasing atomic number of the rare earth partner. A similar tendency has been found for other Fe and Co-based compounds.

Pressure effect on magnetic phase transitions in RCo 4B

The pressure dependence of magnetic phase transition in YCo 4B and GdCo 4B are studied. It turns out from the results obtained that the 3d magnetocrystalline anisotropy depends strongly on the pressure and the 4f-3d intersublattice exchange interaction increases with decreasing interatomic distances.

Magnetic and Mössbauer study of hydrogen-amorphized GdFe2Hx ferrimagnet

Changes in the magnetic properties of the intermetallic GdFe2 compound during hydrogen-induced amorphization have been studied. A sharp decrease in energy of the GdFe exchange interaction and an increase in the magnetic moment of iron from 1.6 μB in the initial compound to 2.2 μB in an amorphous hydride were established. The results obtained were interpreted from the assumption that a heterophase nanostructure occurs, in which the main amorphous ferrimagnetic phase is enriched in Fe relative to the stoichiometry and is characterized by the non-uniform intersublattice exchange interaction.

Mesoscopic oscillations of the magnetization near Ising impurity centers in strong magnetic fields

The frequency of orientational quantum oscillations of the magnetization near impurity-ion clusters with Ising properties in a saturated magnetic crystal is calculated. It is noted that in compounds of the type HoxY3−x Fe5O12, where magnetic phase transitions are observed, additional magnetization reversal and magnetic resonance features due to mesoscopic oscillations of the magnetization can be observed at low concentrations x<0.001 and cryogenic temperatures in fields comparable to the intersublattice exchange interaction field.