Temperature Dependence Of Hc Research Articles

Structural and magnetic properties of Sb substituted LTP Mn-Bi annealed ribbons

We optimized the composition of Mn-Bi alloy, examined the effects of third element substitution of Sn, Cu, Al, and Sb on the magnetic properties, and investigated the microstructures and magnetic properties of Sb substituted Mn-Bi annealed ribbons. The optimized composition was Mn54Bi46, and it was found that the Sb substitution had the most positive effects on the magnetic properties among the substitution elements. Thus, we accomplished a detailed investigation on Mn54Bi46-xSbx (0.0 ≤ x ≤ 5.0) annealed ribbons. The fracture planes of the annealed ribbons revealed that the grain size drastically decreased from a few tens of microns to submicrons close to the single domain size, which substantially enhanced the coercivity (Hc), remanent magnetization (Mr), and maximum energy product ((BH)max). As a result, as the Sb content changed from 0 to 1.5, the Mr, Hc, and (BH)max of randomly oriented samples increased significantly from 14.7 emu/g, 0.50 kOe, and 0.15 MGOe to 37.2 emu/g, 8.15 kOe, and 3.27 MGOe, respectively. Furthermore, the temperature dependence of Hc on the Sb substitution showed significant enhancement over a wide temperature range from 223 to 378 K, suggesting that Mn-Bi alloy can be used in electrical vehicles.

Diameter dependent structural and magnetic properties of CoNi alloy nanotubes

Cobalt-Nickel (CoNi) alloy nanotubes (NTs) with diameters 50 nm (D1), 100 nm (D2), 150 nm (D3), and 200 nm (D4) have been fabricated using Anodic Aluminum Oxide (AAO) membranes. X-ray diffraction spectroscopy (XRD), field-emission scanning electron microscopy (FE-SEM), vibrating-sample magnetometer (VSM), and physical property measurement system (PPMS) analysis methods were employed to systematically investigate the structural, morphological and magnetic properties of CoNi NTs samples. Magnetization reversal mechanism has been explained by angular dependence of coercivity. Low temperature studies have shown that coercivity Hc of nanotubes follows the thermal activation model. Temperature dependence of Hc follows the 3/2 power law for the field dependence of energy barriers.

Magnetic Properties and Mössbauer Investigations of La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) Perovskites

We report magnetic phase separation in pure La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) ceramic samples. Rhombohedral crystal structures obtained with R3C space group show no significant changes in the lattice structures due to the close values of ionic radii of Fe3+ and Mn3+ ions. However, significant changes occur in the magnetic properties due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions because of the presence of Fe3+ substitution. The FM double exchange interactions (DE) (Mn3+–O–Mn4+) appears suppressed by AFM superexchange interactions (Fe3+–O–Mn4+, Fe3+–O–Fe3+, Mn4+–O–Mn4+). The saturation magnetization for x = 0 has the highest value of approximately 80 emu/g at 2 K, which dropped significantly to approximately 5 emu/g at 2 K for the x = 0.5 sample. The coercivity (Hc) for x = 0.5 increased from 161 Oe at 300 K to 786 Oe at 2 K compared with that of x = 0 which varied from 110 to 150 Oe. A value of Hc(0) = 1.5 ± 0.04 kOe was extrapolated from the model fit to the temperature dependence of Hc. The temperature-dependent 57Fe Mossbauer spectroscopy measurements show the presence of Fe in two different environments that have an AFM Fe3+–O–Mn4+ and Fe3+/2+–O–Fe3+/2+ coordination. The unequal fraction population of Fe ions at both AFM environments seems to increase the exchange bias effect.

High pressure effects on isotropic Nd2Fe14B magnet accompanying change in coercive field

We investigated the effects of hydrostatic pressure on an isotropic Nd2Fe14B magnet (the exact chemical formula is Nd2.0Fe14.1B) consisting of nanocrystals, with the size of approximately 30 nm, by magnetization measurements at pressures (P's) up to 9.3 GPa and structural analyses up to 4.3 GPa. Magnetization curves were measured by using a miniature diamond anvil cell made of Ti alloy with spatially uniform magnetization. The initial value of coercive field Hc at 300 K is 840 kA/m (=10.6 kOe), and Hc initially increases to approximately 1180 kA/m (=15.0 kOe) almost linearly against the pressure. The increase in Hc, however, saturates at around P = 3 GPa. The change in Hc is understood by the decrease in the saturation magnetization Ms within the framework of the constant anisotropy of the single domain phase. The crystalline strain increases for P < 1 GPa. Afterward, the crystalline size (D) starts to decrease with increasing pressure, and the reduction tends to saturate at above approximately 3 GPa. Furthermore, the change in Ms is actually related with both the change in strain and that in D. The data on the temperature dependence of Hc at P = 0, 6.6, and 9.3 GPa exhibit pressure-induced suppression of the Curie temperature. The maximum energy product decreases with increasing pressure over the whole temperature range.

Analysis of the Critical Magnetic Field and the Dielectric Susceptibility for the Smectic A- Smectic C* Transition in DOBAMBC

The temperature dependences of the critical magnetic field Hc and the dielectric susceptibility χ(at constant magnetic fields) are analyzed close to the smectic A- smectic C* transition in DOBAMBC using the experimental data according to a power-law formula. These analyses are performed below Tc in the smectic C* phase. Values of the critical exponent δ for the critical magnetic field are determined in a given temperature interval (Tc- T) with increasing and decreasing Hc. Also, values of the critical exponent γ for the dielectric susceptibility χ (dielectric constant) are determined in the two different temperature intervals (Tc- T) for constant magnetic fields ranging from 0 to 6 T. From our analysis of the temperature dependences of Hc and χ, we extract the values of the critical exponent Φ to describe the magnetic field dependence of the dielectric susceptibility close to the AC* transition in DOBAMBC.

On the nature of magnetic state in the spinel Co2SnO4

In the spinel Co2SnO4, coexistence of ferrimagnetic ordering below TN ≃ 41 K followed by a spin glass state below TSG ≃ 39 K was proposed recently based on the temperature dependence of magnetization M(T) data. Here new measurements of the temperature dependence of the specific heat CP(T), ac-susceptibilities χ′(T) and χ″(T) measured at frequencies between 0.51 and 1.2 kHz, and the hysteresis loop parameters (coercivity HC(T) and remanence MR(T)) in two differently prepared samples of Co2SnO4 are reported. The presence of the Co2+ and Sn4+ states is confirmed by x-ray photoelectron spectroscopy (XPS) yielding the structure: Co2SnO4 = [Co2+][Co2+Sn4+]O4. The data of CP versus T shows only an inflection near 39 K characteristic of spin–glass ordering. The analysis of the frequency dependence of ac-magnetic susceptibility data near 39 K using the Vogel–Fulcher law and the power-law of the critical slowing-down suggests the presence of spin clusters in the system which is close to a spin–glass state. With a decrease in temperature below 39 K, the temperature dependence of the coercivity HC and remanence MR for the zero-field cooled samples show both HC and MR reaching their peak magnitudes near 25 K, then decreasing with decreasing T and becoming negligible below 15 K. The plot of CP/T versus T also yields a weak inflection near 15 K. This temperature dependence of HC and remanence MR is likely associated with the different magnitudes of the magnetic moments of Co2+ ions on the ‘A’ and ‘B’ sites and their different temperature dependence.

Particle-diameter dependence of the coercive field in FePt nanoparticles with a face-centered tetragonal structure

It has recently been reported that, in FePt nanoparticles (NPs) protected by poly(N-vinyl-2-pyrrolidone) (PVP), which possess a particle diameter (D) near the critical size (2–3 nm) for the emergence of the face-centered tetragonal (fct) structure, the coercive field (Hc) at room temperature increases with increasing D, whereas the Hc value below 10 K increases with decreasing D. We investigated the fct-ordering parameter (S) for PVP-coated FePt NPs with three mean D values (=2.0, 2.6, and 3.4 nm) in order to gain a better insight into the unique behavior of Hc. Furthermore, the temperature dependencies of Hc, saturation magnetization (Ms), and their product (HcMs) were also investigated. The behavior of Hc at high temperature is attributed to the decrease in the magnetic switching volume (V), while the enhancement of Hc at low temperatures with decreasing D is thought to originate from the decrease in the saturation magnetization, Ms, with decreasing D. The decrease in Ms with decreasing D is related to the decrease in the fct-ordering.

Enhancement of coercivity in Al and Cu added Nd-Fe-B sintered magnets by high feld annealing

For the purpose of enhancement of coercivity (Hc) in sintered Nd-Fe-B magnets, we study the effects of high magnetic felds of up to 140 kOe applied during the annealing. We investigated the annealing temperature (Ta) dependence of Hc systematically. Coercivity enhancement phenomena induced by the magnetic annealing were observed only when the sample contains a small amount of Al and Cu additives, and the annealing temperature is in the vicinity of Ta = 500°C or 550°C. DSC measurements suggest that these temperatures can be interpreted as the eutectic points of Nd70Cu30 and Al17Cu83 phase, respectively. Temperature dependence of Hc and maximum energy product (BH)max in the coercivity-enhanced samples were measured up to 250°C. We have found that a sample with Hc = 14.6 kOe at 20°C gives a twice larger (BH)max value at 125°C as compared with those in the sample with Hc = 9.8 kOe at 20°C.

Annealing effect on magnetostatic properties of nanocomposite Fe‐Zr‐N films

AbstractResults on the investigation of the magnetic properties and microstructure of the as‐deposited and annealed at T = 200‐700 °C Fe‐Zr‐N films are presented. The Fe‐Zr‐N films with the 0.7‐µm thickness were deposited by RF sputtering onto glass square substrates. Structural properties of the films were studied by means of the X‐ray diffraction technique. The bulk magnetic characteristics of the films have been measured by a vibrating sample magnetometer (VSM). The study of the near‐surface magnetic properties of the Fe‐Zr‐N samples was carried out employing magneto‐optical micromagnetometer with a surface sensitivity of about 20 nm of the thickness depth. The strong annealing effect on the values of the coercivity HC and the saturation field HS of the films was revealed. The values of HC and HS of the films, annealed at T = 450 and 500 °C, were found to be minimal. The temperature dependences of HC and HS were explained by microstructure changes of the studied films after their post‐deposition heat treatment. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exchange‐spring behavior of nanocrystalline (NdPr)4Fe71Co5Cu0.5Nb1B18.5

AbstractAn amorphous alloy of composition (NdPr)4Fe71Co5Cu0.5Nb1B18.5 prepared by a melt spinning technique in an argon atmosphere has been studied to observe exchange‐spring behavior in the nanocrystalline state. Hysteresis loop measurements at room temperature revealed that the highest value of the coercivity, Hc, of 4.08 kOe has been obtained for the sample annealed at 913 K for 10 min. At this annealing temperature (BH)max and Mr/Ms have been found to be 11.06 MGOe and 0.716, respectively. Steep recoil curves of the samples annealed at 873 and 973 K show typical characteristics of the exchange‐spring mechanism having smaller loop area. The temperature dependences of Hc, Mr/Ms and (BH)max in the range of 5 to 380 K generally decrease with the increase of temperature. From the temperature dependence of the hysteresis loop parameters, it has been found that at 5 K the values of Hc and (BH)max are 6.89 kOe and 13.72 MGOe respectively for the sample annealed at 873 K for 10 min and 6.69 kOe and 13.74 MGOe respectively for the sample annealed at 923 K for 10 min. A change in the shape of the hysteresis loops was observed at temperatures below 100 K, related to the spin reorientation in the hard magnetic phase. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin reversal in small magnetic elements as a function of orientation in external magnetic field

Monte Carlo simulations are used to investigate magnetization reversal mechanisms and temperature dependence of Hc,ψ. Hc,ψ is the switching field when the external field is directed at an angle ψ to the easy axis. ψ is defined as the acute angle the external field makes with the easy axis, positive ψ corresponding to a counter-clockwise turn from the easy axis. We find different magnetization reversal mechanisms for different ψ and show the role of edge domains. For ψ=−π∕4 and ψ=0 switching occurs by the nucleation of vortices while for ψ=+π∕4 switching occurs by a skewed stretching of the edge domains. Differences in the location and growth of the vortices are reported. Vortices can facilitate switching as evidenced by the low values of Hc,−π∕4. Comparison with known temperature dependencies of Hc,+π∕4 and Hc,0 obtained via theory, simulation and experiment shows that (i) values of Hc,−π∕4 are consistently lower than Hc,+π∕4 and Hc,0 at all temperatures considered and (ii) Hc,−π∕4 has the same linear temperature dependence as Hc,0 but different from the T2 dependence Hc,+π∕4.

化学合成したＦｅＰｔ微粒子媒体の静磁気特性と結晶構造

We have investigated magnetic properties and crystal structures of chemically synthesized FePt nanoparticle media. The average grain diameter estimated as 4.4 nm and σ/D 13 % from TEM bservation. We found that each anoparticle is not coalesced after annealing for 750°C-30 min. We were confirmed L10 structure of FePt nanoparticles form the electron diffraction patterns. Diffraction rings for (001) and (110) are confirmed clearly. Hk and Ku V values were estimated from the temperature dependence of Hc. Ku V/kT value was 41 at room temperature.

The Influence of Temperature on the Coercivity and Kerr Rotation for MnBiRE(RE=Ce, Pr, Nd and Sm)Thin Films

MnBiRE thin films posess large magneto-optical(MO) Kerr rotation angle θk1). We investigated the influence of temperature on the coercivity Hc and MO Kerr rotation angle θk for MnBiRE (RE=Ce, Pr, Nd and Sm) thin films and their writing characteristics. The temperature dependence of Hc and θk of MnBiRE thin films as the functions of the temperature are studied. The thermo-magnetic writing was carryied out. The readout and writing chariceristics of the samples are discussed.

Critical behaviour in the temperature dependence of the coercivity for nanocrystalline soft-magnetic materials

The Curie temperature T am C of the residual amorphous phase and the temperature dependence of the coercivity Hc have been studied for nanocrystalline Fe91Zr7B2 prepared by primary crystallization of a melt-spun amorphous precursor. The temperature dependence of Hc for the nanocrystalline Fe91Zr7B2 near T am C is well fitted by the following relation: Hc(T)alpha(T am C-T)-6beta, where beta = 0.36. This result indicates that the effective exchange stiffness in the nanocrystalline sample is governed by the square of the spontaneous magnetization in the intergranular amorphous region.

Shift in the magnetic percolation threshold of phase separated Co-rich CoAg very thin films due to reduced dimensionality

An investigation of the structural and magnetic properties of phase separated Co-rich CoAg very thin films (5 nm–50 nm) as a function of film thickness and Co concentration is presented. In the as-deposited state the films are very fine grained and the coercive field is relatively low (Hc<20 Oe). However, after annealing at 420 °C for half an hour in high vacuum, grain growth is promoted and a drastic change in the magnetic properties is observed. The coercivity now has a strong thickness dependence with a maximum at approximately 15 nm. The largest room temperature value of coercivity, Hc=850 Oe, was found for a CoAg 70:30 volume percent alloy. The concentration of maximum coercivity is usually associated with the percolation threshold, xp, of the ferromagnetic element and is much higher than the ∼50 vol % value normally observed in thick film and bulk granular ferromagnets. Values of xp≲0.65 were found for 30 nm thick films increasing up to xp∼0.75 for 10 nm films. This behavior is explained as a shift in the percolation limit to higher values associated with the reduced dimensionality of very thin films. This shift implies that the volume fraction of the ferromagnetic component should be larger than its bulk counterpart if granular ferromagnets are to be used as thin film media for magnetic recording. The temperature dependence of Hc was also studied and coercivities exceeding 1800 Oe have been found at 50 K. The presence of a thin antiferromagnetic oxide layer is manifested as a shift in the low temperature hysteresis loops due to the anisotropic exchange coupling interaction with ferromagnetic Co.

Magnetic Hysteresis Properties of Fine Particles of Monoclinic Pyrrhotite Fe7S8.

The magnetic hysteresis properties have been studied of synthetic Fe7S8 separated into a suite of 8 particle size fractions in the range 1 to 30μm. Between-196°C to about 260°C, both saturation isothermal remanence (Mrs) and coercive force (Hc) can be represented as analytical functions of particle size (L) of the form (Mrs, Hc) ∝ L-n or (Mrs, Hc) ∝ exp(-kL1+2) although the particle size dependence of Mrs is much weaker than that of Hc. The parameters n and k show a small but systematic fall in values as temperature rises, but may be acceptably constant to furnish analytic functions for physical models of more complex magnetic properties such as thermoremanent magnetization or domain dynamics. The temperature dependence of Hc below room temperature is weaker than that above, possibly due to an increasing single-ion contribution to magnetocrystalline anisotropy. The inferred temperature dependence of magnetocrystalline anisotropy above room temperature up to about 250°C suggests that temperature-induced expulsion or nucleation of domain walls is not expected in this temperature range. The temperature dependence of Mrs and Hc can be expressed as (1 - T/Tc)q where q shows a small but systematic fall as particle size increases in the range 1-30μm. The several properties of the finest, monodomain, fraction (

Comment on ‘‘The pinning force between a Bloch wall and a planar pinning site in MnAlC’’

Pinning of a domain wall in uniaxial material by a thin layer defect is discussed. A coercivity Hc≂(2/3(3)1/2) (‖ακ−1‖/α) WfGM(K/M)((K/A)1/2) is found, where A, K, and M represent the exchange, anisotropy, and saturation magnetization of the bulk, and αA, and κK are those for the layer of thickness W≪((A/K))1/2. The Gaunt-Mylvaganam thermal excitation factor fGM≂1–0.40 K1/2A−3/2kT. This model is shown to be capable of explaining both the magnitude and temperature dependence of Hc in MnAlC for plausible values of α and κ for the layer, identified as grain boundary. Objections to the more complex treatment by Chen and Gaunt are discussed, in particular violation of exchange torque continuity.

Magnetic Properties of Plastically Deformed Iron Single Crystals II

Magnetization curves of single crystals (picture-frame shapes) elongated along the [001] and [011] axes under various conditions were measured at low temperatures down to -196°C. It was found that their coercive forces Hc increase as the temperature is decreased. If the origin of Hc is some type of induced anisotropy caused by a magnetoelastic interaction, the temperature dependence of Hc would mainly be due to the magnetostriction.

The effect of annealing on the structure of cobalt modified iron oxide particles. Part II

A new high coercivity cobalt modified iron oxide particle series has been prepared by annealing in a nitrogen atmosphere. The dependence of coercivity (Hc), Hc-aging, temperature dependence of Hc, and magnetic tape properties have been studied as a function of annealing temperature. X-ray photoelectron spectroscopy (XPS) has been used to correlate the changes in magnetic properties to the cobalt concentration gradient near the particle surface. As in Part I of the study, most of the changes observed are consistent with diffusion of cobalt from the surface into the particle core.

Temperature dependence of coercivity of epitaxial magnetic garnet films

The temperature dependence of the coercivity Hc, magnetization, anisotropy, and domain wall energy was measured for SmGaYIG, SmCaGeYIG, and SmLuCaGeYIG epitaxial bubble garnet films from 100 K up to the Néel temperature TN≳400 K. Data were analyzed together with results published on similar samples on the basis of the theory of domain wall pinning by a random array of uniform defects and a good fit was observed in the form of H1/2c=H1/20 −CT2/3, where H0 and C are constants. The maximum average interaction force between defects and walls is 2.9×10−7 dyne, corresponding to the minimum interaction range equal to the wall width. The density of defects is in the range between 1014 and 1016 cm−3. The size and number of defects derived from the temperature dependence of Hc is in agreement with the microstructure of epitaxial garnets revealed by transmission electron microscopy.