It had been shown recently that the calculated magnonic spectra of two-dimensional periodic ferromagnetic composites can present frequency ranges forbidden for the propagation of magnon excitations throughout the composite. However, those forbidden energy gaps were found to be highly sensitive to the exchange contrast between the component ferromagnetic materials but were very weakly sensitive to the contrast in spontaneous magnetizations of the two materials. Accordingly, in this paper we introduce a new mathematical definition of the exchange field acting in inhomogeneous medium. With this new definition the present theory gives magnonic spectra reasonably sensitive to magnetization contrast, as they should be from the physical viewpoint; moreover, the magnetization contrast now becomes a gap-creating factor as well.
The single-ion rare earth anisotropy was investigated in ThMn_{12}-type compounds. For this purpose the crystal electric field parameter values were studied. In these compounds, described by formula RFe_{12-x}T_{x}, R = rare earth, T = Ti, V, Cr, Mo, W and Si, the T atoms have strong crystallographic site preference changing the local crystal electric field potential which "sees" the rare earth ion. The crystal electric field potentials A^{0}_{2} were calculated considering this site preference. The summations were performed taking into account the nearest neighborhood of the rare earth ion according to the recent results of band structure calculations. The charges of the surrounding Fe and T atoms were established applying the chemical bond model proposed by Pauling. The absolute value of A^{0}_{2} decreases when the content of vanadium increases in 8(i) position, which is in agreement with experimental data. Localization of Si atoms in 8(j) and 8(f) causes a decrease in A^{0}_{2}. The ^{155}Gd Mössbauer spectroscopy data confirm this fact. Miedema's "macroscopic" atom model of cohesion in alloys was applied for interpretation of the role of T atoms in the isomer shift and volume effects.
The hyperfine field anisotropy tensors, obtained from analysis of ^{57}Fe NMR spectra for iron sites in various magnetic oxides (barium hexaferrite, lithium spinel, magnetite and yttrium-iron garnet) were used to investigate the relation between the hf anisotropy and surrounding ionic arrangement. While a reasonable correlation has been found when the distribution of the nearest cations was considered, no correlation was obtained between the hyperfine field anisotropy and the positions of the nearest oxygen anions.
Ground state properties of Tb^{3+} ion were investigated in the tetragonal ThΜn_{12}-type TbNi_{10}Si_{2} compound on the basis of temperature dependence of the paramagnetic susceptibility and magnetization measurements. The rare earth temperature dependence of the susceptibility was calculated using the Van Vleck equation. The Γ_{t5}^{(1)} doublet is suggested to be a ground state separated by 9 K from the first excited singlet Γ_{t1}^{(2)}. The magnetic moment associated with the ground state doublet agrees with saturation magnetization data at 4.2 K. The overall crystal field splitting is estimated to be 105 K.
^{57}Fe NMR spectra were measured on thin films of yttrium iron garnet with divalent (Ca^{2+}) and tetravalent (Ge^{4+}) cation substitutions using the spin echo technique at 4.2 K. A series of Y_{3-x}Ca_{x}Fe_{5-x}Ge_{x}O_{12} samples with 0 ≤ x ≤ 0.0 5 was prepared by an epitaxial growth from BaO/B_{2}O_{3}/BaF_{2} flux on gadolinium gallium garnet substrate. Satellite lines corresponding to the iron ions having the Ge^{4+} and Ca^{2+} in their neighbourhood were identified. The sample with the germanium content of x(Ge) = 0.004 and (110) substrate orientation was used for a detailed study: NMR spectra of this sample were measured in an external magnetic field of 0.1 T applied along several directions in the plane of the film. The modification of the isotropic part and the anisotropy tensor of the hyperfine interaction caused by the Ge^{4+} substituent were obtained from the experimental data. The results were compared with those previously reported for trivalent substitution (Ga^{3+}).
A study of crystalline structure, magnetic and magnetocaloric properties of Ho(Co_{1-x}Feₓ)₂ (x = 0.09, 0.12) intermetallic compounds has been undertaken. Phase composition was controlled by X-ray diffraction analysis. Magnetic properties were measured within the temperature range 4.2-350 K in magnetic fields up to 7 T. Magnetic ordering temperatures corresponding to paramagnetic-ferrimagnetic phase transitions were found to be 199 K and 258 K respectively. Temperature dependences of heat capacity for these compounds have been inferred for the temperature interval 77-340 K. Comparison of magnetocaloric effect (MCE) values determined by direct measurement and by calculation was carried out as well. It was found that significant MCE peak broadening occurs for higher iron concentration in the compound.
The structural and magnetic properties of Dy_{1_{x}}Er_{x}Fe_{10}Si_{2} are investigated. X-ray analysis reveals that these compounds are of the tetragonal ThMn_{12} structure. In this structure the rare earth atoms occupy one crystallographic position 2(a). The unit cell contains 26 atoms. The spin reorientation temperature, T_{SR}, was measured from the temperature dependence of the initial susceptibility using an ac bridge of mutual inductance of the Harsthorn type. Dy^{+3} and Er^{+3} have opposite contributions to the entire magnetic anisotropy. The spin reorientation temperature is found to be about 271 K in DyFe_{10}Si_{2} and 48 K in ErFe_{10}Si_{2}. The values are discussed applying the crystal field model. The value of the rare earth-transition metal exchange coupling constant J_{RFe}/k_{B} derived from the mean-field model analysis of the Curie temperature is about -13 K. The Fe-Fe exchange integral is much higher and is equal to about 75 K.
In a stoichiometric oxide the energy for the magnetic ordering is due to superexchange. This depends on the virtual transfer of a d electron from the transition ion to the neighbouring oxygen. When the oxide is p-doped there are compensating holes on the oxygen or the transition ion becomes mixed valent. The oxide may then conduct. The same transfer integral enters both the expression for the antiferromagnetic superexchange and the band width of the mobile carriers. Thus materials with a large antiferromagnetic exchange energy will be expected to have a relatively wide conduction band in the doped state and hence to have a high conductivity. In this paper the difference is explored between the materials in which there is true antiferromagnetism and those which are ferrimagnetic. In the antiferromagnets the carriers must destroy the magnetic order as they move. This behaviour is well known from the cuprates. In ferrimagnets the carriers may be able to move entirely on one sublattice. This occurs in Fe_{3}O_{4} and probably in the doped garnets. In the case where motion is on one sublattice then doping does not destroy the magnetism and there is a relatively small magnetoresistance. An interesting feature is that unlike the cuprates the ferrimagnets do not become good metals at any doping but exhibit hopping conductivity. We explain the apparent paradox that the best conductivity is actually observed in materials where the conduction is only allowed when the antiferromagnetism has been quenched and that the conductivity in ferrimagnets is low.
A series of samples of the system Co_{1-x}Zn_{x}Fe_{2}O_{4} (x = 0.0, 0.3, 0.4, 0.5 and 0.8) have been prepared by the usual ceramic technique. X-ray analysis shows that they are cubic spinel (single phase). The lattice parameter a and the bulk density D are measured for the samples. The substitution experiments replacing Co^{2+} by Zn^{2+} furnish new significant fact concerning the magnetic behaviour of ferrites. The magnetic strength of the magnetic ions on the Band A-sites can be varried relative to each other by the substitution of Zn^{2+}. The increase of density of the composition increases the magnetic interaction of the dipole moment at the B-sites.
The magnetic and magneto-optical properties of the typical representatives of three rare-earth iron garnets (RIG) groups: with heavy rare-earth elements Yb, Er, Dy, Tb; with elements from the middle of lanthanide series Gd, Sm, and with light rare-earth element Nd are presented. In contradistinction to other work on the Faraday rotation, which were done only at 1152 nm (8696 cm^{-1}), here we present FR spectra obtained in the energy region 5500-20000 cm^{-1} with high optical resolution. The investigations have been done at temperatures of 5, 82, 130, 295 K using magnetic field up to 25 kOe applied parallel to the [111] crystallographic axis of the crystals. It has been shown that the contribution proportional to the magnetic field and independent of temperature to the mixing of the ground state multiplets exceeds the paramagnetic contribution in YbIG, ErIG, GdIG, SmIG. In Tb and Dy iron garnets contributions from the two mechanisms have opposite signs, and the paramagnetic mechanism gives the greatest contribution to the Faraday rotation. Nevertheless, the contribution of the diamagnetic mechanism, caused by the influence of the exchange field in the iron sublattices on rare-earth ions, is significant, and it is necessary to take it into account. Anomalously large magneto-optical activity is observed in NdYIG. This is the result of contributions of the same sign and approximately equal in magnitude from the paramagnetic and diamagnetic mechanisms.
A new configuration of cobalt magnetic moments with respect to rare-earth ones on local level, named as parimagnetism, surviving above the Curie temperature was determined few years ago for ErCo_{2}. Occurrence of parimagnetism has recently been confirmed also for HoCo_{2}. In this paper we present results manifesting considerable composition and hydrostatic-pressure influence on characteristic temperatures of magnetism in Ho(Co_{1-x}Si_{x})_{2} compounds and draw a preliminary scenario analyzing the exchange interactions involved.
This work is devoted to the study of the impact of the substitution of iron by Eu on the properties of magnetically soft Ni_{0.3}Zn_{0.7}Eu_{x}Fe_{2-x}O_4 ferrites aimed at enhancement of the knowledge related to the behavior of such materials in dependence on the amount of iron substitution. Our latest studies focused on the materials having similar chemical composition gave a hint of some possible approaches to manage the resulting magnetic properties in a precisely controlled way by the combination of several factors; choice of initial chemical composition of non-substituted ferrite, selection of substituted and substituting element, variations of final chemical composition, modifications of fabrication technology (sintering temperature and time), etc.
The specific heat of HoCo_2 and Ho(Co_{0.95}Si_{0.05})_2 was measured as a function of temperature in several constant magnetic fields up to 8 T. A data analysis allowed us to determine the isothermal entropy change and the magnetocaloric effect in a wide temperature range. The considerable values of the magnetocaloric effect in the vicinity of the magnetic ordering transition are qualifying both compounds as suitable for magnetic refrigeration purposes. The magnetic phase transition temperature (T_C) increases from 77 K for HoCo_2 to 103 K for Ho(Co_{0.95}Si_{0.05})_2 while the large magnetocaloric effect in the vicinity of TC is maintained, which demonstrates possible ways of tuning the operating temperatures of the magnetic refrigerant.
This paper presents results of investigation of stress influence on magnetic characteristics of ferrite ring cores for inductive electronics components. Ring-shaped cores were made of Ni-Zn ferrite designed for anti-lung filter cores. The results of the study indicate a significant impact of external forces stresses on the magnetic properties of ferrite cores. Such changes are very important due to their influence on the correct operation of the induction components.
The short-range parimagnetic configurations, observed in the heavy rare-earth RCo_{2} compounds in paramagnetic range far above the Curie temperature T_{C}, consist in formation of ferromagnetic cobalt clusters antiferromagnetically coupled to the neighboring R magnetic moments. The characteristic temperature of the onset of parimagnetism T_{f} is very sensitive to changes of external conditions like pressure or composition. The pressure coefficients ∂T_{f}/∂p and ∂T_{C}/∂p are comparable, indicating a close connection of underlying mechanisms. Our recent measurements indicate systematic evolution of the pressure coefficients with spanning the rare-earth series from Dy through Tm in RCo_{2} with a specific case TmCo_{2}.
At room temperature, the normal oxide spinel NiCr₂O₄ is tetragonally distorted and crystallizes in the I4₁/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni²⁺. The ferrimagnetic Curie temperature (T_{C}) for NiCr₂O₄ is 74 K. The magnetic moments of NiCr₂O₄ are composed of a ferrimagnetic (longitudinal) and an antiferromagnetic (transverse) component. Exchange interaction between the magnetic cations influences the overall magnetic properties of the compound. Present work focuses on the modification of structural and magnetic properties upon substituting Fe at Cr site in NiCr₂O₄ with the motivation of changing the magnetic exchange interaction. In order to do so, single phase Ni(Cr_{0.5}Fe_{0.5})₂O₄ samples were prepared by co-precipitation techniques, while controlling the pH of precipitation. Upon Fe substitution, crystal structure was not affected much contrary to the earlier reports. In order to determine the oxidation state of each elements X-ray photoelectron spectroscopy (XPS) was performed. T_{C} was found to increase dramatically above 300 K, confirmed both from temperature and field dependent dc-magnetization studies.
The paper presents the results of investigation of the temperature influence on the inductance and power losses in ferrite cores. Such effect can significantly influence the utility parameters of electronic devices, particularly in precision equipment. For example slight parameter change in this type of components in measuring devices can cause significant changes in output parameters. It is also equally important for mobile devices where increase of losses can limit the duration of service. Special measurement system composed of hysteresis graph, cryostat and PC was utilized to perform the experiment. The cores used during the investigation had closed magnetic circuit. In order to perform measurements of magnetic properties of the material, two sets of windings (magnetizing and sensing) were made on each core. The cores were placed in a cryostat, which was used to set the temperature value within the range from -20 to +50°C. The magnetic properties were measured by the hysteresis graph, to assess the influence of the temperature on the functional parameters.
We present a study of the magnetic properties of a mixed ferro-ferrimag-netic ternary alloy of the type AB_p C_{1-p} on a cubic lattice consisting of three different Ising spins S_A =3/2, S_B = 2, and S_C = 5/2. We employ the mean-field approximation and Monte Carlo simulation to find the compensation temperatures of the system for selected values of the parameters in the model Hamiltonian. In particular, the relation between considered mixed ferro-ferrimagnetic model and magnetic properties of the ternary metal Prussian blue analog such as (Fe_p^{II}Mn_{1-p}^{II})_1.5 [Cr^{III}(CN)_6] · nH_2O is discussed.
The drop of coercivity, while preserving the saturation magnetic polarisation J_{S} of substituted M-type Ba hexaferrites with composition BaFe_{12-2x}(Me_{1}Me_{2})_{x}O_{19}, was studied. Divalent Me_{1}=Ni, Zn, Sn and tetravalent Me_{2}=Ru, Sn ionic combinations were used in various compounds. Mössbauer spectroscopy was used to reveal sites of replacement of the Fe^{3+} ions by Ru^{4+} and Sn^{4+}. The Ni^{2+}, Zn^{2+} and Sn^{2+} occupation sites were identified as well. Strong drop of coercivity difference Δ H_{c}(x) near x=0.1 and slight decrease of Δ H_{c}(x) close to x=0.3 was obtained for suitable coupling of Me_{1}^{2+ }and Me_{2}^{4+} ions.
We present the study of pressure effect on magnetic properties of TM^{2+}_3[Cr^{III}(CN)_6]_2·nH_2O ferrimagnets and ferromagnets (TM = Cr and Co) under pressures up to 0.9 GPa. Applied pressure strengthens super-exchange interaction in Cr^{2+}-prussian blue analogues with dominant antiferromagnetic interaction J_{AF} leading to increase in the Curie temperature T_C (ΔT_c/Δp = 29.0 K/GPa) and reduces T_C of Co^{2+}-prussian blue analogues with dominant ferromagnetic interaction J_F (ΔT_c/Δp = -1.8 K/GPa). The rise of J_{AF} interaction is attributed to the enhanced value of the single electron overlapping integral S. On the other hand, the applied pressure slightly affects bonding angles between magnetic ions mediated by the cyano-bridge and reduces the strength of magnetic coupling. Changes of the magnetization curve with pressure can be attributed to changes of magnetic anisotropy. The reduction of magnetization with pressure observed on Cr^{2+}-prussian blue analogues can be explained by pressure induced transition from Cr^{2+} high spin state to Cr^{2+} low spin state. All pressure induced changes are reversible.
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