Magnetocrystalline Anisotropy Constant K1 Research Articles

Multi-cycle low-temperature demagnetization (LTD) of multidomain Fe3O4 (magnetite)

An improved multi-cycle low-temperature demagnetization (MC-LTD) method for multidomain (MD) Fe3O4 (magnetite) by repeating LTD cycles between any two temperatures below 300K is proposed to provide more rigorous constraints on the utilization of the LTD mechanism. MC-LTD was performed for four temperature intervals, 300–150K, 150–125K, (cycling through the isotropic point Tk∼130K, where the first magnetocrystalline anisotropy constant K1 changes sign), 125–95K (cycling through the Verwey transition Tv∼120K, where crystal symmetry changes from cubic to monoclinic) and 95–60K. Results show that the remanence loss during MC-LTD depends on the number of LTD cycles. Above Tv, demagnetization of remanence after MC-LTD is caused by the gradual reorganization of magnetic domains. For the 95–125K interval, the remanence loss results from the changes of easy axes between [001] of the cubic phase above Tv and the c-axis of the monoclinic phase below Tv. These two processes approximately follow the Boltzmann-analog relation, but the latter involves more degree of freedom. Below Tv, thermal equilibrium processes control the minor decay (less than 0.5% of the initial remanence intensity) of remanences.

Temperature dependence of core loss in Co-substituted MnZn ferrites

Substitution of Co2+ for a small part of Fe2+ in (MnZn)Fe2O3 results in much smaller temperature variation in core loss, compared with that of conventional MnZn ferrite used for transformers. This effect appears in the temperature dependence of hysteresis loss rather than two other contributions of loss, eddy current loss, and residual loss. The total loss not exceeding 370 kW/m3 at 100 kHz/200 mT is obtained in the wide temperature range from 0 to 100 °C. However, excess substitution gives rise to a steep increase of loss below room temperature. Both the improvement of temperature dependence and the abrupt changes in loss in a lower-temperature region accompanying Co2+ substitution could result from a change in the magnetocrystalline anisotropy constant K1.

ＭｎＺｎフェライトの鉄損温度依存性に及ぼすＣｏＯ置換効果

The core loss of conventional MnZn ferrite varies with temperature. Therefore, for use in a trans-former, the main composition must be chosen so that the loss is lowest between 80°C and 100°C. Substitution of Co for Fe in an MnO-ZnO-Fe2O3 ternary system gives a smaller variation in loss with temperature. The gradient of the loss-temperature curve in the lower temperature region becomes steadily gentler upon substitution of CoO up to 0.5 mol%, irrespective of the ZnO concentration. However, excess substitution gives rise to a steep increase in the loss as the temperature decreases. According to analysis using the Globus’ model, both the improvement in the temperature dependence and the abrupt change in core loss in the lower temperature region accompanying Co replacement could result from a change in the magnetocrystalline anisotropy constant K1.

Crystal structure and magnetic properties of Gd3(Fe1-x-yCoyTix)29 compounds

A series of Gd3(Fe1-x-yCoyTix)29 (x = 0.011-0.034, y = 0-0.393) samples have been prepared. Single-phase samples assigned as 3:29 compounds with the monoclinic lattice are obtained in the following composition regions: x = 0.011, 0 y0.307; x = 0.022, 0 y0.313; and x = 0.034, 0 y0.393. The solid-solution limit of cobalt in Gd3(Fe1-x-yCoyTix)29 increases with the Ti content. The cell parameters a, b and c show anisotropic decreases with the Co content, i.e. a and b decrease significantly and c decreases slightly in the composition range investigated. The Curie temperature is increased by the substitution of Co for Fe in Gd3(Fe1-x-yCoyTix)29. The saturation magnetization first increases and then decreases with the Co content. The dependence of the intrinsic magnetic properties of Gd3(Fe1-x-yCoyTix)29 on the Co and Ti contents is presented. The compounds exhibit an easy-magnetization direction on the a-b plane corresponding to the CaCu5-type structure at room temperature. The anisotropy field Ha of Gd3(Fe1-x-yCoyTix)29 is decreased and the magnetocrystalline anisotropy constant K1 is increased by the substitution of Co for Fe. No spin reorientation is observed in the alternating-current susceptibility measurement between 77 K and the Curie temperature.

Magnetic anisotropy engineering in in-plane magnetized ultrathin ferromagnetic films (invited)

We have studied the effect of depositing submonolayer quantities of Cu onto the CO exposed Co/Cu(110) system at room temperature using the magneto-optic Kerr effect. Cu overlayers are found to completely reverse the in-plane 90° easy axis switch caused by the CO adsorption, for all Co thicknesses studied up to 40 ML. The Cu reverses the sign of the effective in-plane uniaxial anisotropy KUeff thereby switching the easy axis from the [1-10] to the [001] direction. Two modes of switching are observed depending on the magnitude of the cubic magnetocrystalline anisotropy constant K1 which is in turn dependent on the thickness of the Co films. For sufficiently thick Co films (dCo>15 ML), the easy axis is found to shift gradually from the [1-10] to the [001] direction due to the competition between the cubic and effective uniaxial anisotropy contributions. Therefore, we are able to controllably engineer the direction of the easy axis in this system as a function of Cu overlayer thickness. For thin Co films (dCu<15 ML) K1 tends to zero as revealed by BLS measurements of Hillebrands et al. and the easy axis switch is abrupt. We have engineered an experimental realization of an isotropic two-dimensional XY magnet by depositing submonolayer coverages of Cu onto a CO exposed 5 ML Co/Cu(110) film with a zero cubic anisotropy component K1 at room temperature. For a Cu coverage of 1.02 ML, the uniaxial anisotropy component vanishes also, and we observe a corresponding loss of ferromagnetic order at remanence. Further Cu deposition restores the uniaxial anisotropy and the magnetic order. Therefore we have directly observed the stabilization of ferromagnetic order by magnetic anisotropy in an ultrathin magnetic film, as theoretically predicted.

Hard magnetic properties and microstructure of melt-spun Sm2Fe15−xCuxGa2C (x=0 and 0.5) ribbons

The structure, magnetic properties and microstructure of Sm2Fe15−xCuxGa2C (x=0 and 0.5) ribbons prepared by melt spinning and subsequent heat treatment were studied. A significant increase in coercivity has been found for ribbons prepared with the addition of a small amount of Cu. The maximum coercivity μ0Hc is 1.4 and 2.0 T at room temperature for ribbons with x=0 and 0.5, respectively, after annealing in vacuum at 1043 K for 15 min. A small amount of Cu addition has little effect on the lattice parameters, Curie temperature TC, and magnetocrystalline anisotropy field HA at room temperature, while it decreases the saturation magnetization Ms. The magnetocrystalline anisotropy constants K1 and K2 for the x=0 and 0.5 samples were determined in the temperature range between 50 and 293 K, using magnetically aligned samples. The variations of coercivity, magnetization, and remanence as functions of applied field and temperature are consistent with the view that the magnetization of Sm2Fe15−xCuxGa2C ribbons with x=0 as well as 0.5 are mainly controlled by pinning of domain walls. High-resolution transmission electron microscopy and energy-dispersive x-ray analysis demonstrate that Cu atoms dissolve partially into the 2:17-type matrix phase in Sm2Fe14.5Cu0.5Ga2C ribbons, which is effective in pinning the domain walls and enhancing the coercivity.

HARD MAGNETIC PROPERTIES OF Sm-Fe-Cu-Si-C WITH THE 2∶17-TYPE STRUCTURE

The structural and magnetic properties of Sm2Fe15Si2C and Sm2Fe14CuSi2C were analyzed.The Curie temperature TC and saturation magnetization Ms were measured.The magnetocrystalline anisotropy field HA and anisotropy constant K1 and K2 at temperatures ranging from 1.5K to room temperature were obtained by analyzing the magnetic curve.We can conclude that Cu does not come into the Th2Zn17-type structure,while Cu doping modifies the micro-structure and enhances the coercivity of Sm2Fe14CuSi2C ribbons.The maximum intrinsic coercivity(μ0iHc)around 1.0 and 1.2 T at room temperature for Sm2Fe15Si2C and Sm2Fe14CuSi2C ribbons were achieved, respectively. The microstructural parameter αk and the averaged local effective demagnetization factor Neff were derived from the temperature dependence of coercivity.

Temperature dependence of the magneto-crystalline anisotropy in R2Fe17 (R=Y, Gd, Tb, Dy, Er)

We measured the temperature dependence of the magneto-crystalline anisotropy constants K1 and K2 in R2Fe17, where R=Gd, Tb, Dy, Er and Y. Relatively large single crystals were grown by the Bridgman method under 1.2 atm argon atmosphere. The starting materials were polycrystalline alloys prepared in an arc-melting furnace. The anisotropy constants K1 and K2 were derived by a Sucksmith and Thompson analysis from the magnetization curves of each single crystal which were measured from room temperature to 4.2 K.

The Temperature Dependence of Magnetic Properties of Zn-Ti Substituted Ba-ferrite Particles for Magnetic Recording

It was shown that the morphological and magnetic properties of Zn-Ti doped Ba-ferrite particles are suitable for magnetic recording. In order to clarify the temperature dependence of magnetic properties of these particles, BaFe10.8−2XZnX TiXO19 particles with the nominal composition X= 0.0-0.50 were prepared by synthesis from salt melts. The saturation magnetization Ms, magnetic anisotropy field HA and magnetocrystalline anisotropy constant K1 of above-mentioned particle assembly were estimated by using the law of approach to saturation of polycrystalline ferrite and the Curie temperature Tc was determined by the thermodynamic relation. With an increasing X, the Tc decreased, while the lattice parameter a, c and Ms have an unnoticeable maximum at X=0.20. The Hc(T) curves of samples with different X show that all particles have positive temperature coefficient in the vicinity of room temperature, which decreases with increasing X. The Ms(T), HA(T) and K1(T) were determined and discussed.

Magnetic properties of Sm2Fe17Nx, x = 3.9

Hydrogen decrepitation of Sm2Fe17 followed by treatment in ammonia produces interstitial nitrides with nitrogen concentrations greater than three per formula unit. Measurements of the lattice parameters and magnetic properties are consistent with interstitial nitrogen occupation of the 9e and either the 3b or 18g sites. The effect of the extra nitrogen on the iron sublattice is to reduce the overall moment, particularly of the 18h sites. The magnetocrystalline anisotropy constants K1 and K2 indicate a significant decrease in the easy-axis character compared to Sm2Fe17N3 material.

Magnetic properties and rotational hysteresis of Fe3O4 and γ-Fe2O3 particles ∼ 250 nm in diameter

We report the magnetic properties for dispersed, spherical Fe3O4 and γ-Fe2O3 particles ∼ 250 nm in diameter in the temperature range T = 4.2–294 K. For Fe3O4, hysteresis properties show anomalies at 130 K, where the magnetocrystalline anisotropy constant K1 vanishes; the ratio Hc/Is (Hc = coercive force, Is = saturation magnetization) follows a linear relationship with K1/Is2 between 130 and 294 K. Below the Verwey transition temperature TV (∼ 120 K, the hysteresis properties depend on whether the sample is cooled in a strong magnetic field or in zero field; no magnetic anomalies were observed for γ-Fe2O3 particles, generated by oxidizing the Fe3O4 particles under study. The rotational hysteresis integral at 294 K is ∼ 0.7 for both compounds, pointing to a coherent-type magnetization reversal for assumed uniaxial single-domain particles (SDPs); however, the reduced saturation remanence is only ∼ 0.2, far below that for SDPs. Possible models for the particle magnetization modes are discussed.

Effects of phosphorous doping on the stability of coercivity of cobalt-substituted acicular oxides

The effects of P doping on the coercivity and magnetic-thermal stability of Co-substituted iron oxides have been investigated by means of magnetic measurements, torque curves, and Mössbauer spectra. It is found that the P doping of cobalt-substituted acicular oxides promotes the coercivity and the stability with temperature of the coercivity. Then effects may be attributed to the increase of the cubic magnetocrystalline anisotropy constant K1 and the activation energy E, respectively. The Mössbauer results suggest that P ions are located in the neighborhood of the B-site Fe ions.

Magnetic anisotropy of Rn+1Co3n+5B2n (R  Y, La; n = 1, 2, 3)

Magnetization curves have been measured at T = 4.2 K for field-aligned samples of compounds LaCo4B and Yn + 1Co3n + 5B2n with n = 1, 2 and 3, in a pulsed high magnetic field up to 35 T. Magnetocrystalline anisotropy constants K1 have been estimated and discussed for the compounds mentioned above.

Magnetic alignment in powder magnet processing

A strong magnetic field is used to align single-crystal powder particles in the process of producing sintered powder permanent magnets, including hard ferrites and rare-earth permanent magnets. The applied magnetic field aligns the easy direction of magnetization of each particle, owing to strong crystalline anisotropy. Shape anisotropy, existence of particles containing multigrains, and physical interlock between particles reduce the degree of alignment. This study provides a quantitative analysis of magnetic alignment in powder magnet processing. We assume (1) the powder particle is a single crystal; (2) it has the shape of an oblate spheroid and its short axis is the easy direction of magnetization; and (3) the applied magnetic field is strong enough to overcome the resistance of alignment. By applying the minimum-energy principle, it was concluded that the necessary and sufficient condition for a complete magnetic alignment is that the magnetocrystalline anisotropy constant K1 of the particles is greater than its shape anisotropy constant Ks, provided the applied magnetic field is strong enough. When Ks≳K1+2K2, the angle between the short axis of the oblate particle and the direction of applied magnetic field is 90°, and when K1≤Ks≤K1+2K2, the angle is arcsin√(Ks−K1)/2K2.

Characteristics of the Mn and Co volume-doped acicular γ-Fe2O3 particles

Mn- and Co-doped acicular γ-Fe2O3 particles with a coercivity of 923 Oe and Co content of 3.5 wt % have been prepared by a special process. This involves high-temperature ion diffusion, then step annealing. These particles exhibit a high magnetic thermal stability and low-temperature coefficient of coercivity. The latter demonstrates a striking contrast to the ordinary cobalt-body-doped γ-Fe2O3 particles. The induced uniaxial anisotropy constant Ku and the magnetocrystalline anisotropy constant K1 of the particles were determined by torque measurements. The novel properties of these particles are mainly due to the high uniaxial anisotropy, resulting from the anisotropic arrangements of cobalt ions. With the above characteristics and low Co content, these acicular particles can be one of the promising candidates for particulate high-density magnetic recording materials.

Superposition model analysis of the magnetocrystalline anisotropy of Ba-ferrite

Theoretical analysis of the first magnetocrystalline anisotropy constantK1 of BaFe12O19 is performed. Two contributions toK1 are considered — single ion anisotropy and dipolar anisotropy. ParameterD which determines the magnitude of the single ion contribution is calculated on the basis of the superposition model. It is argued that the disagreement between calculated and observed values ofK1 is most likely connected with the contribution of Fe3+ ions on bipyramidal sites, for which the value ofD is uncertain.

Magnetic and magneto-optic properties of thick face-centered-cubic Co single-crystal films

The present letter describes for the first time the magnetic and magneto-optical properties of thick fcc cobalt single-crystal films (≂1000-Å thickness). The magneto-crystalline anisotropy constants K1 and K2 are about −7.2×10 erg/cc and 2×10 erg/cc at 77 K, respectively. Both decrease in magnitude with increasing temperature in a range from 77 K to about 450 K. The easy axis is found to be 〈111〉. The polar Kerr angle for the fcc phase is much larger than that of the hcp phase by 0.1°–0.2° in a photon energy range from about 1 to 5 eV.

Magnetic properties of praseodymium-substituted iron garnet films

We have investigated the growth conditions, the magnetic properties and low-field ferromagnetic resonance (FMR) properties of praseodymium-substituted garnet films. Films of general composition Y3−x−yPrxLuyFe5O12, were grown by liquid phase epitaxy onto [111]-oriented Gd3Ga5O12 substrates. Vibrating sample magnetometry, torque magnetometry and FMR were used to measure saturation magnetization and the cubic and induced anisotropy constants. It is shown that the uniaxial anisotropy constant Ku and the first order magnetocrystalline anisotropy constant K1, have a strong dependence upon the praseodymium content. The addition of praseodymium in the garnet films improves the soft magnetic properties. The effect of the doping by praseodymium on the saturation magnetization is insignificant at room temperature. In-plane coercivity is about 0.1 Oe.

Measurement of effective magnetic anisotropy of nanocrystalline Fe-Cu-Nb-Si-B soft magnetic alloys

The magnetic anisotropy of a nanocrystalline soft magnetic Fe73.5Cu1Nb3Si13.5B9 alloy has been determined by studying the magnetization process. The experimental results on the approach to magnetic saturation have been used to determine its effective magnetic anisotropy constant 〈K〉. This is about one tenth of the magnetocrystalline anisotropy constant K1 of individual α-Fe (Si) grains.

Magnetocrystalline anisotropy in Y(Co0.85Al0.15)2 with the C15 cubic Laves phase structure

The temperature dependence of the first magnetocrystalline anisotropy constant K1 has been deduced from the analysis of isothermal magnetization curves I(H) for polycrystalline Y(Co0.85Al0.15)2 samples with the C15 cubic Laves phase structure. The magnitude of K1 is equal to 7.6*104 erg cm-3 at T=7 K and falls rapidly with increasing temperature. K1 has a negative sign over the temperature range investigated from 7 to 20 K, i.e. below TC=25 K. The results of the anisotropy study are used to explain the low-field magnetic behaviour of the Y(Co0.85Al0.15)2 alloy.