Drop Of Coercivity Research Articles

Structure and magnetic properties of N-doped Fe–C granular films

As-deposited Fe–C granular films doped with N fabricated using facing-target sputtering at room temperature are amorphous. Annealing at temperatures ranging from 200 °C to 600 °C causes the crystallization of metal granules, the enlargement of the particle size from ∼2 nm to ∼50 nm and the decrease of the N atomic fraction. With the increasing nitrogen partial pressure (PN), the dominant phases of the particles in 600 °C annealed films develop from N-poor phases to N-rich phases, and the CN matrix still retains the amorphous state after the annealing. Meanwhile, high-temperature annealing turns superparamagnetism of the as-deposited films synthesized at PN < 0.1 Pa and paramagnetism at PN ≥ 0.1 Pa into ferromagnetism. The saturation magnetization of 600 °C annealed films decreases with the increasing PN, and both their saturation magnetization and coercivity drop with the rise of measuring temperature. In addition, the coercivity of the films increases with increasing annealing temperature initially and then declines due to the contribution of phase segregation and interparticle interactions.

Magnetoelastic and magnetostatic interactions in exchange-spring multilayers

The macroscopic or average aggregate behavior of Kneller's exchange-spring mechanism is well established and widely examined; less is known about its microscopic details. In the present study, the microscopic nature of exchange spring was investigated using $\mathrm{TbFe}∕\mathrm{FeCo}$ multilayers and correlated with their aggregate behavior (magnetostriction, torque, and magnetization curves). Results show that, as predicted, the exchange-coupled geometry reduces the switching field by increasing the average magnetization and decreasing the average anisotropy of the multilayers. However, it is the magnetostatic interactions between domain walls in adjacent layers that lead to a reduction in coercivity. Stray fields emanating from domain walls in a given layer magnetostatically lock in with the stray fields from walls in adjacent layers, giving rise to low energy, low coercivity ``twin'' walls. Since in a multilayer a wall can magnetostatically lock in with walls in layers both above and below it for optimum flux closure, a sharp drop in coercivity is observed when the number of bilayers exceeds two. Preliminary results on Fe-Pd based shape memory alloy (SMA) thin films and multilayers are also given to further emphasize the efficacy of the exchange-spring mechanism as well as to highlight a key micromagnetic difference between magnetostrictive and magnetic SMA films. In particular, the number of martensite variants is greatly reduced because variants with easy axis normal to the plane of the film are prohibited due to magnetostatic considerations.

Microstructure characterization of ball milled Sm(CoCuFeZr) 2:17 powders

The microstructure evolution and coercivity development in ball milled Sm(Co,Cu,Fe,Zr)z powders have been investigated by TEM, SEM, HRTEM, and VSM. The fully heat-treated Sm(Cobal,Fe0.1,Cu0.08,Zr0.03)7.5 and Sm(Cobal,Fe0.1,Cu0.08,Zr0.03)8.3 alloys were ball milled to powders with different ball milling rates and times. At a low ball milling rate, both the particle size and coercivity decrease slowly and monotonically with increasing milling time. Even after 48h milling, both powders with an average particle size of about 2μm still retain about 40%–50% of their initial coercivity. Both the cellular and lamellar structures are still observed in the two powders even after 48h milling, although crystal lattice distortion and defects are introduced into the cellular and lamellar structures in both powders. At a high milling rate, both the particle size and the coercivity drop sharply with milling time for both powders, and even 2min milling leads to the loss of 50% of the initial coercivity; the mean particle size was measured to be 5μm, whereas the initial cellular structure was significantly destroyed and highly strained. Further ball milling leads to sample oxidation and the formation of amorphous phase.

Simulation of magnetization behaviour in nanocrystalline Pr2Fe14B by micromagnetic finite element method

Magnetization behaviour in the nanocrystalline Pr2Fe14B has been simulated by micromagnetic finite element method. The decrease of anisotropy in an interlayer leads to the drop of coercivity and enhancement of remanence, while the decline of intergrain exchange coupling increases the coercivity and decreases the remanence. The experimental coercivity and remanence may be simultaneously obtained by selecting proper parameters for the interlayer. Unfortunately, it is difficult to simulate the demagnetization curve of the ribbons before many efforts mustbe paid in future.

Coercivity mechanism in mold-cast Nd 60Fe xCo 30− xAl 10 bulk amorphous alloys

The structural and magnetic properties of as-cast Nd 60Fe x Co 30− x Al 10 (5≤ x≤30) bulk amorphous rods were investigated. The samples exhibit inhomogeneous microstructure consisting of Fe-rich spherical regions separated by Nd-rich amorphous phase. The as-cast samples show large room temperature coercivities of ∼0.4 T. Thermomagnetic curves reveal two Curie temperatures T C1=32–52 K and T C2=470–500 K suggesting the presence of two magnetic phases. The coercivity increases monotonically with decreasing temperature up to T C1, and falls sharply with further decreasing temperature. Domain wall pinning mechanism for the temperature dependence of coercivity is proposed, which states that the phase with Curie temperature T C1 pins the domain walls at temperatures T> T C1. When the temperature decreases below T C1, the pinning centers order ferromagnetically resulting in a strong drop in coercivity. The presence of blocking temperatures T b in the zero-field-cooled (ZFC) samples suggests the presence of thermally activated magnetization processes connected with the domain wall pinning coercivity mechanism.

Difference in coercivity between Co/Fe and Fe/Co bilayers

A study of the deposition order and film thickness dependence on the coercivity of ferromagnetic bilayers, ∥Co/Fe and ∥Fe/Co, is presented (the sign, “∥,” is for indicating glass or Si substrate position). The magnetization of the Co layer is aligned with the in-plane direction during rf sputter deposition. The thickness is controlled in the range of 3–22 nm. Since there exists a strong exchange interaction between the two ferromagnetic layers, the magnetization reversal process occurs cooperatively. ∥Fe/Co shows an isotropic and hard-magnetic behavior, whereas ∥Co/Fe shows an anisotropic and soft-magnetic behavior. A sudden drop of coercivity in ∥Fe/Co observed at the Fe layer thickness below 5 nm is caused by a decrease in the saturation magnetization of the Fe layer. Due to the surface roughness, the bilayer on the glass substrate possesses a higher coercivity than that of the bilayer deposited on the silicon substrate. The magnetization reversal process of the ferromagnetic bilayers is discussed.

Magnetic and structural properties of FeCoB thin films

The magnetic and structural properties of FeCoB thin films as a function of boron content have been investigated for applications requiring soft, high moment materials. The films were either co-sputtered from separate FeCo-35 and B targets or from a (Fe/sub 65/Co/sub 35/)/sub 90/B/sub 10/ alloy target. A peak in the coercivity was observed for small amounts of boron. Measurements of coercivity with fields applied along the radial and circumferential directions of the Si wafer showed isotropic magnetic behavior. The coercivity and saturation induction decreased with increasing amounts of boron above 2 at.%. High angle X-ray diffraction showed a broadening of the [110] peak with increasing boron, indicative of either increased microstrains or smaller average grain size perpendicular to the film. The [110] peak, however, did not shift with added boron, suggesting that on average the boron does not expand the FeCo lattice. The drop in coercivity around 10 at.% B was identified with a transition to a primarily amorphous state. Since these alloys have a high positive magnetostriction, their magnetic properties are particularly sensitive to film stress/strain. Using sputtering gas pressure as a variable, differences in coercivity were also correlated with changes in film strain.

Shock compaction of SmCo5 particles

Fine ferromagnetic particles (1–10 μm) of SmCo5 were compacted using shock pressures of 1.7–27 GPa (17–270 kbar). One-piece disks were produced with pressures in the range 2–9 GPa. X-ray-diffraction measurements revealed no changes in the composition of the compacted SmCo5 for shock pressures below 15 GPa. Magnetic properties of the shocked samples were measured with a superconducting quantum interference magnetometer. In the pressure range 2–9 GPa, the shocked specimens exhibit high intrinsic coercivity Hci. Below 2 GPa, lower values of coercivity were observed, presumably caused by incomplete bonding of particles. Above 10 GPa the intrinsic coercivity drops suddenly and levels off at higher pressures. The remnant magnetization Br, exhibits a broad peak at 6 GPa and then falls off rapidly at higher pressures. The saturation magnetization Ms, has a peak near 10 GPa that correlates with the drop in coercivity. The hardness coefficient a, derived from a fit of the phenomenological ‘‘law of approach’’ formula to the hysteresis curve, shows a linear increase with pressure near 8 GPa. This suggests an increase of inclusions in the samples and correlates with the drop in Hci and the peak in Ms. There is a non-negligible linear contribution to the magnetization given by κH. The nonzero value of κ suggests the possibility of incomplete saturation or paramagnetic impurities. Maximum values in coercivity Hci, remnant magnetization Br, and saturation magnetization Ms are found at about 9 GPa.

Superparamagnetic behavior and giant magnetoresistance in oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) epitaxial films (abstract)

We have studied the magnetic and magnetotransport properties of oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) films and have observed superparamagnetic behavior in these films. The superparamagnetism is indicated by (i) the increasing spread between zero field cooled (ZFC) and field cooled (FC) magnetization curves as temperature decreases, (ii) a sharp drop of the ZFC magnetization at low temperature, (iii) enhanced Curie–Weiss constant above the ferromagnetic temperature, and (iv) at low temperature a sharp drop of coercivity (Hc) with increasing temperature. These findings strongly suggest that all oxygen deficient manganites are magnetically inhomogeneous; i.e., ferromagnetic clusters exist. We estimate the spin cluster diameter to be 7–10 nm at low temperature (T&amp;lt;30 K). We discuss a possible origin of the giant magnetoresistance effect based on the existence of spin clusters.

Effect of composition and microstructure on reversible losses of magnetic properties in FeNdB magnets

Temperature characteristics of coercivity and remanence for FeNdB magnets having different compositions and microstructures have been evaluated. Improved performance of a magnet at elevated temperatures can be achieved by increasing its coercivity. Higher Nd content and a proper concentration of boron, with combination of additional elements such as Dy, Co or Al increase room and elevated temperature coercivities. High coercivity is also decisive for good temperature characteristics of the remanence. Due to the intrinsic nature of the reversible losses, the relative drop of coercivity is greater for magnets having higher initial coercivity which is mainly related to the phenomena in the area of grain boundaries.

Low-temperature behavior of single-domain through multidomain magnetite

We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.

Effects of cobalt on sintering characteristics of iron-neodymium-boron alloys: K. Majima et al (Osaka University, Japan), J Japan Soc Powder and Powder Metallurgy, Vol 39, No 10, 1992, 855–859. (In Japanese)

The structural and magnetic properties of as-cast Nd60FexCo30−xAl10 (5≤x≤30) bulk amorphous rods were investigated. The samples exhibit inhomogeneous microstructure consisting of Fe-rich spherical regions separated by Nd-rich amorphous phase. The as-cast samples show large room temperature coercivities of ∼0.4 T. Thermomagnetic curves reveal two Curie temperatures TC1=32–52 K and TC2=470–500 K suggesting the presence of two magnetic phases. The coercivity increases monotonically with decreasing temperature up to TC1, and falls sharply with further decreasing temperature. Domain wall pinning mechanism for the temperature dependence of coercivity is proposed, which states that the phase with Curie temperature TC1 pins the domain walls at temperatures T>TC1. When the temperature decreases below TC1, the pinning centers order ferromagnetically resulting in a strong drop in coercivity. The presence of blocking temperatures Tb in the zero-field-cooled (ZFC) samples suggests the presence of thermally activated magnetization processes connected with the domain wall pinning coercivity mechanism.

Magnetic properties of oblique-evaporated Co-Ni thin films

Co 1 - x Ni x ( x = 0, 0.2 and 0.3) thin films of thickness about 1500 Å were electron-beam evaporated onto silicon and polymide substrates at various oblique angles α. In-plane coercivities and squareness ratios both along and transverse to the incidence plane were examined. Also, the angular variations of coercivity of films prepared at α = 0 ° to 85 ° were investigated. The magnetic anisotropy changes from an in-plane anisotropy with the easy axis perpendicular to the incidence plane to an out-of-plane anisotropy parallel to the incidence plane, the transition occuring at about 60 °. Also discussed is the effect of the substrate temperature on the magnetic properties and columnar microstructure of the oblique-evaporated films. At room temperature, there is a small drop in coercivity at α = 60 ° before a sharp rise in coercivities to 1400 Oe as the oblique angle increases.