Anisotropy Constant Decreases Research Articles

High-permeability and high-Curie temperature NiCuZn ferrite

High-permeability and high-Curie temperature NiCuZn ferrite was developed for use in telecommunication. In the development process, attention was focused on the amount of CuO contained in NiZn ferrite and a small additive of MoO3 added in the NiCuZn ferrite. It was confirmed that the initial permeability of the core increased and the Curie temperature only decreased a little with 4mol% CuO content. This was mainly attributed to the presence of Cu ions activating the sintering processes in ferrites and leading to increase in density; the decrease of magnetocrystalline anisotropy constant also does some contribution. By optimizing the MoO3 additive, the initial permeability could get a sharp increase because of enhancement of grain size. As a result, we were able to develop the NiCuZn ferrite with a permeability of 2480 and Curie temperature of around 118°C.

Magnetic and Structural Properties of Fe3Pt Thin Films

The hcp ordered phase of Fe3Pt alloy thin films is for the first time successfully synthesized onto Al2O3 (00.1) substrates by electron beam evaporation technique. It exhibits the basal-plane 6-fold magnetic anisotropy of the order of 105 erg/cc. The magnetic anisotropy varies with deposition temperature in the similar way as the intensity of hcp (00.1) superlattice peak changes. This result implies the close correlation between the basal-plane magnetic anisotropy and the occurrence of the hcp structure. The inplane magnetic anisotropy constant decreases with temperature from 2.0×105 erg/cc at 80K to 1.4×105 erg/cc at 300K.

FMR studies of Y 3Fe 5O 12/Gd 3Ga 5O 12 (YIG/GGG) superlattices and YIG thin films

We have employed ferromagnetic resonance to obtain magnetic anisotropy information in a series of YIG/GGG superlattices and a single YIG thin film with temperature variations. The Curie temperature of these samples increases with the YIG layer thickness, reaching saturation beyond a specific YIG thickness. As temperature rises, the magnitude of the calculated magnetocrystalline anisotropy constant decreases while the uniaxial anisotropy constant exhibits growing behavior.

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.

Short-range order effects in amorphous FexSi1-x/Si multilayers induced by preparation conditions

Magnetic properties of compositionally modulated Fe x Si 1-x /Si amorphous multilayers are reported. The concentration x was 0.68≤x≤0.82 for two nominal modulation lengths λ of 4 and 8 A. The samples were prepared by pulsating two-cathode sputtering. From SQUID magnetometry and transverse magneto-optic Kerr effect, we detected a noticeable decrease in both magnetization and relative reflectivity (δ=ΔR/R) as λ increases from 4 to 8 A. Also, for a fixed value of x, it was found that both coercive field and in-plane uniaxial anisotropy constant decrease systematically for increasing λ. The two pulses used in the preparation method causes slight chemical short range effects,'altering also the magnetic properties by changing the number and nature of nearest neighbors of a given Fe atom. The above short-range effects are present even when no multilayer structure is detected by x-ray diffraction.

Micromagnetic modelling of multilayer media

A micromagnetic model for the calculation of magnetization processes in magnetic multilayer systems has been developed using a finite element method. Shell elements which account for the multilayer structure and a magnetic vector potential to treat long-range dipolar interactions are the essential features of the algorithm. Micromagnetic calculations of hysteresis properties and domain structures in Co/Pt multilayer systems show a qualitative agreement with magnetic images obtained from field emission Lorentz microscopy. The nucleation field of Co/Pt multilayers increases with improving texture. Spatial fluctuations of the magneto-crystalline anisotropy energy create barriers for domain wall motion. The pinning field of domains increases with increasing grain size. The domain wall structure depends on he local magnetocrystalline anisotropy. The wall structure changes from a Bloch-type to a Néel-type wall as the local anisotropy constant decreases. The quality of texture and the grain size significantly influence the jaggedness of domains in Co/Pt multilayers. Large grains and strong deviations of the easy directions from the film normal deteriorate the smoothness of domains and increase the transition width.

Fe-C films grown by ion beam sputtering and their magnetocrystalline anisotropy constants

Fe-C films are grown epitaxially on a GaAs (100) single crystal substrate by ion beam sputtering and the magnetocrystalline anisotropy constants of the Fe-C films are evaluated. The magnetocrystalline anisotropy constant of the obtained Fe film is 40 kJ/m 3, which is 15% smaller than that reported for bulk Fe. The magnetocrystalline anisotropy constant decreases with the addition of C to Fe, and is 32 kJ/m 3 at a C concentration of 3.9 at%. However, this decrease is not large enough to explain the improvement in soft magnetic properties of Fe film obtained by adding C. This supports the idea that the improvement comes from fine crystallites of Fe-C films.

Magnetic properties of BaFe12−2xCoxTixO19 fine particles prepared by coprecipitation tempering

The magnetic properties of BaFe12−2xCoxTixO19 fine particles prepared by the coprecipitation tempering method were studied. The experimental data shows that the specific saturation magnetization σs decrease with an increase in the substitution x is not significant and the anisotropy constant K1 decrease is obvious. We believe that the Co2+ ions preferentially occupy 12k sites in the structure of magnetoplumbite, and the tendency of Co2+ ions to enter lattice sites has some temperature dependence.

Influence of nitrogen, oxygen, and water on magnetic properties of dc magnetron sputtered GdTbFe films

Amorphous GdTbFe thin films were produced by dc magnetron sputtering in argon atmosphere at varying partial pressures of nitrogen, oxygen, and water. The compensation temperature Tcomp, the Curie temperature TC, and the uniaxial magnetic anisotropy constant Ku decrease strongly with increasing impurity concentration in the films. The influence of nitrogen on Tcomp and Ku is about a factor of 3 less than that of oxygen and water. The strong decrease of Tcomp and Ku is due to the preferred reaction of the impurities with the rare-earth (RE) atoms. This leads to a preferred deactivation of the latter resulting in a decrease of the rare-earth (RE) sublattice magnetization. Moreover, the uniaxial magnetic anisotropy is primarily based on the single ion anisotropy of the Tb atoms. If these are deactivated Ku decreases. At increasing impurity concentrations the selectivity of deactivation decreases and increasing amounts of Fe are affected. Thus, the steepest decrease of Tcomp and Ku is observed at low concentrations. For Tcomp the mean values of the slopes at concentrations below 1 at. % are 35 K per at. % for nitrogen, 85 K per at. % oxygen (Ar/oxygen atmosphere), or 125 K per at. % oxygen (Ar/water atmosphere). Ku is reduced to half its original value at concentrations of 4 at. % nitrogen, 1.2 at. % oxygen (Ar/O2), and 0.9 at. % oxygen (Ar/H2O) in the films. The Curie temperature TC decreases at a rate of about 10 K per at. % impurity. This is due to the diminishing magnetic interactions at increasing impurity incorporation.

Magnetic Properties of Alnico Alloy Phases and Temperature Instability of Permanent Magnets

Analysis of the structural constituents of Alnico alloys with high coercive force showed that the saturation magnetization of the α′ phase is quite high, and for Alnico VIII and Alnico VS55 is close to the saturation of binary Fe–Co (Is∼1800 emu) At the same time, the saturation magnetization of the α phase is only several scores of emu. The relative volume of α′-phase decreases from Alnico V to Alnico VIII and VS55. This result is in qualitative agreement with the corresponding decrease of Is and increase in Hc for these alloys. Comparison of the Ku(T) and Hc(T) experimental curves for these alloys reveals their correspondence: a maximum in Ku and Hc in the temperature interval of +20°-–+100°C and a relatively small decrease of Ku and Hc over a wide temperature range. Calculation of the Ku(T) dependence from the equation Ku = ½(Is1 − Is2)2N·ν1·ν2, taking into account the temperature dependence of Is1 and Is2, shows good agreement between the experimental and calculated curves. These results prove that it is the shape anisotropy of the strongly magnetic phase particles that explains the high coercive force of the Alnico alloys. Anisotropy fields HA=(2Ku/Is) were calculated using the Ku and Is values of the alloys measured at room temperature. Comparison of the HA values with the coercive force shows that the anisotropy field is three-times higher than the coercive force values. Causes of the variation in anisotropy field in various alloys may be explained by analyzing the data. The increase in HA in Alnico VIII in comparison with Alnico V is explained by changing alloy-phase characteristics: an increase in ΔIs causes a 35% increase in HA, and an increase in ΔN causes a 17% increase; an increase in ν1+ν2 results in a 9% increase of HA, and decreased saturation Is causes a 9% increase in HA. All this results in a value of HA for Alinco VIII 70% higher than for Alnico V. When comparing corresponding characteristics of Alnico VIII and VS55, one can see that the ΔIs value is very small, and the increase in ΔN is negligible. The increase in HA is mainly due to a decrease in Is in Alnico VS55 in comparison with Alinco VIII, which increases HA by 30%. The decrease in Is is mainly due to a smaller ν1 (relative volume of the α′-phase). Thus, the Ku and HA values achieved in single-crystal alloys Alnico VIII and VS55 are nearly as large as possible because of the following: (a) further increase in Is (at a given composition) and increase in ΔN are practically impossible; (b) Is2 values are low enough so that further decrease has only a small effect. Part II. Special features of temperature changes in the induction of permanent magnets are qualitatively explained on the basis of analysis of results of investigation of the temperature dependence of the main alloy magnetic properties: saturation magnetization, coercive force and uniaxial anisotropy constant. Irreversible induction losses during the first temperature cycles may be related to the process of irreversible rotation in some precipitates of strongly magnetic phase under the influence of the internal field because of a decrease of anisotropy constant. Reversible magnet induction changes may be accounted for by the following: (a) Reversible temperature dependence of alloy saturation; (b) Temperature dependence of reversible rotation of magnetization in single-domain particles in the internal demagnetizing field. Induction changes when cooling magnets in the interval of +100° to −180°C are probably caused by two opposite effects, namely: (a) an increase in the saturation of the alloy results in an increase in induction, and (b) increasing reversible rotation of magnetization of strongly magnetic phase particles decreases the induction.1 Thus, a maximum is observed on the curve of reversible induction vs temperature. Reversible induction changes with temperatures above +100° may be accounted for by the influence of the following factors: (a) the decrease in alloy saturation results in a decrease of induction; (b) diminishing reversible rotation of particle magnetization causes some increase in induction. Because of the second mechanism, reversible temperature changes in magnet induction are always less than relative temperature changes in alloy saturation. It can be shown that the higher the internal demagnetizing field, the greater is this effect. This explains, first, the decrease in ΔB/B rev and temperature coefficient of induction with lowering the magnet working point, and second, the relatively high stability (i.e., small ΔB/B rev and temperature coefflcient values) of Alnico VIII and VS55 magnets. The (BH)max point for these high coercive-force alloys corresponds to high values of the internal demagnetizing field. This causes a significant reduction in the decrease of induction with temperature. Both parts of this abstract will be published in the I.E.E.E. Transactions on Magnetics as individual papers.