Strong Domain Wall Pinning Research Articles

An investigation of time-dependent domain wall pinning effects in Tb/Fe multilayer thin films

Reverse domain nucleation time measurements have been performed on two Tb/Fe multilayer magneto-optic films exhibiting different degrees of domain wall pinning. A linear relationship between ln (reverse domain nucleation time) and the applied field has been predicted and observed for a sample exhibiting weak domain wall pinning. This is in agreement with theoretical work presented which addresses time dependence in systems possessing weak domain wall pinning. A non-linear relationship applicable over a restricted field range has been derived for a sample exhibiting strong domain wall pinning. Experimental results have indicated that this relationship is also valid.

Nanoscale magnetic-domain structure in colossal magnetoresistance islands

Magnetic force microscopy reveals the nature of local magnetic structure in submicron islands of colossal magnetoresistance (CMR) thin films. The evolution of domains in a magnetic field reveals the importance of shape anisotropy as well as magnetostriction in determining the micromagnetics in such small CMR structures. At room temperature, a characteristic multidomain structure with perpendicular orientation, predicted by theory, is observed. The magnetization reversal of the islands in magnetic fields perpendicular and parallel to the substrate is dominated by strong domain wall pinning. Strong domain wall pinning in conjunction with geometrical confinement in these submicron CMR structures gives rise to reproducible multidomain structure.

Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography

Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd 2Fe 14B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning.

Single‐domain‐like behavior in a 3‐mm natural single crystal of magnetite

We have observed single‐domain (SD) like behavior in a 3‐mm natural single crystal of magnetite following low‐temperature demagnetization (LTD), which consists of zero‐field cycling through the Verwey transition to erase remanence carried by pinned domain walls. We compared stepwise alternating field (AF) and thermal demagnetization curves of 1‐mT total thermoremanent magnetization (TRM), 1‐mT partial TRM (pTRM) acquired between the Curie point (TC = 575°C) and 565°C, and saturation isothermal remanent magnetization (SIRM) measured with and without prior LTD. AF demagnetization curves of untreated TRM and SIRM decreased exponentially with increasing AF. SIRM was more resistant to demagnetization than was TRM, a multidomain (MD) result of the Lowrie‐Fuller test. After LTD the TRM and SIRM memories had AF demagnetization curves with sigmoid shapes and initial plateaus below 8 mT in which little or no demagnetization occurred. Both features are reminiscent of SD behavior. During thermal demagnetization, untreated TRM and SIRM decreased almost linearly with increasing temperature up to ≈500°C. Such distributed unblocking temperatures TUB are expected for pinned walls in MD grains. The remaining 60% of TRM and 40% of SIRM were lost over a narrow temperature interval concentrated between 560°C and TC. The TRM memory after LTD was very stable against thermal demagnetization. There was no decrease in remanence below 550°C and very little change until 565°C, only 10°C below TC. This very high TUB fraction of TRM seems to have SD‐like character. SIRM has similar behavior. In the case of pTRM, both the untreated remanence and the memory after LTD have almost entirely high TUBs, identical to the range of pTRM blocking temperatures TB, 565°C–TC. TUB = TB is a basic property of SD partial TRM. The pTRM memory fraction is also larger than that of total TRM or SIRM. These observations suggest that pTRM(TC, 565°C) isolates an SD‐like fraction of remanence similar to that constituting TRM and SIRM memory. The high AF coercivities of the TRM and SIRM memories indicate that the source of SD‐like behavior may be unusually strong domain wall pinning by crystal defects formed at monoclinic twin boundaries as a result of crystal distortion from cubic to monoclinic structure in passing through the Verwey transition.

Magnetisation reversal in thin Cu/Ni/Cu/Si(0 0 1) films

A step wedged structure of the form 30 Å Cu/50 Å Ni/ hÅ Cu/Si(0 0 1) ( h = 600, 1000, 1500, 2000 A ̊ ) grown by MBE has been characterised by in situ RHEED and ex situ polar MOKE magnetometry and MOKE microscopy. The magnetisation reversal process was investigated through measurements of hysteresis loops, magnetisation relaxation and domain images. The sample exhibits a strong magnetic aftereffect and the macroscopic relaxation measurements were fitted by a model which assumes thermally activated magnetisation reversal through domain nucleation and domain-wall propagation. Magnetic imaging confirms that domain-wall propagation dominates over domain nucleation at all Cu buffer layer thicknesses. For the 600 Å Cu buffer layer, strong domain-wall pinning occurs on a micron scale, whereas for the 2000 Å Cu a domain-wall can propagate freely over hundreds of microns. The reduced pinning causes the coercive field to decrease as the Cu buffer thickness increases. The Barkhausen length deduced from fits to the relaxation data increases with Cu buffer thickness. The correlation length at the Cu buffer and Ni overlayer surface as obtained from the width of the RHEED streaks also increases, suggesting an interdependence between the crystallographic properties of the buffer and overlayer and the magnetic properties.

Interface anisotropy and magnetization reversal in epitaxial Fe(001)/Pd double layers

Static and dynamic magnetic properties of epitaxial Fe(001) films with Pd overlayers have been measured by means of Brillouin light scattering and magneto-optic Kerr effect magnetometry. From low energy electron diffraction studies, the initial growth of Pd [dPd<2 monolayer (ML)] is identified as a self-formation process of quasione-dimensional Pd structures. Auger electron spectroscopy examinations proved that for dPd⩾2 ML the Pd overlayers cover the Fe films almost completely. The measured frequencies of the dipolar spin wave mode and the derived anisotropy constants show a linear change with increasing dPd up to 5 ML with following saturation. The M(H) curves show pronounced anomalies in the coercive field for dPd<2 ML accompanied by large tails in the hysteresis loops. The results are interpreted as a strong pinning of domain walls caused by large local changes in the surface anisotropy due to the inhomogeneous Pd cover layer in this regime.

High coercive field and film stress for epitaxial monolayers of Fe on W(110)

Elastic and magnetic properties of ultrathin iron films on tungsten (110) are investigated. In situ film stress measurements during growth show stress values of 25 GPa per deposited monolayer. Our experiments indicate that the tremendous film stress triggers the formation of a misfit dislocation network at a coverage of 1.5 monolayers. The effect of the film stress and its spatial variation on the magnetic behavior are discussed. We find a high coercivity of order 0.3 T for 1.5 monolayer thick films. A model is presented that explains the high coercivity in terms of strong domain wall pinning.

Magnetic viscosity and activation volume in domain wall pinning

When domain wall pinning is due to random arrays of inhomogeneities, the dynamical process of unpinning could be described by the weak or the strong domain wall pinning models. Based on these two models, the values of activation volume V/sub w/ (for weak pinning) and V/sub s/ (for strong pinning) of bowed domain wall of thermally annealed iron-rich FeAu specimens were estimated. The results were then used to calculate their corresponding fluctuation held H/sub fw/ (using V/sub w/) and H/sub fs/ (using V/sub s/), for each of the specimens. Comparing the values of H/sub fw/ and H/sub fs/ with the Barbier's plot which relates magnetic viscosity to the coercive field H/sub 0/ of a wide variety of magnetic materials, it was found that H/sub fw/ of the low age specimens plotted against their H/sub 0/ exhibited similar behaviour as in the Barbier's plot. However, when values of H/sub fs/ were used in the plotting, similarity with the Barbier's result was only found in specimens after prolonged aging. It is also found that when the localized demagnetizing effect on the bowed portion of the domain wall due to the presence of the surface free-poles is to be considered in the derivation, the fluctuation field is smaller than that without such consideration by a factor of F/2F/sub 0/ in the case when the material has a weak pinning regime, where F and F/sub 0/ are the pinning force per unit area with and without the demagnetizing consideration respectively, and by a factor of 1/4 in the case of strong pinning. >

Magnetic hysteresis in Fe-rich rare-earth amorphous alloys

We investigate the hysteresis behavior of R 15Fe 77B 8 amorphous alloys with R = Sm, Tb, Dy and Er in the temperature range 4.2–250 K and their — extrapolated to T = 0 K — coercive fields in the light of some existing models. A small-fluctuations model does not explain the H c values at T = 0 K and a large-fluctuations model seems to be more appropriate. A strong domain-wall pinning exists in the Tb alloy in the temperature range studied, yet a domain-wall diffusion model fairly works up to ≈ 100 K in this alloy. This same model works better for the Sm alloy up to 250 K with a domain-wall thickness of about one lattice spacing.

A strong pinning model for the coercivity of die-upset Pr-Fe-B magnets

We have measured the temperature dependence of the intrinsic coercivity Hci(T) between 5 and 565 K in a die-upset Pr-Fe-B magnet. Over a very wide temperature range up to 477 K, Hci(T) is in excellent agreement with a model for strong domain-wall pinning by a random array of pinning sites proposed by Gaunt [P. Gaunt, Philos. Mag. B 48, 261 (1983)]. The model includes both the temperature dependence of the intrinsic magnetic properties of the Pr2Fe14B phase and the effects of thermal activation of domain walls over the pinning barrier. The pinning sites are modeled as nonmagnetic planar inhomogeneities at the boundaries between platelet-shaped Pr2Fe14B grains. We develop an expression for the maximum pinning force per site, f, and derive the model prediction that (Hci/γHA)1/2 varies linearly with (T/γ)2/3, where HA and γ are the magnetocrystalline anisotropy field and the domain-wall energy per unit area of the Pr2Fe14B phase, respectively. Significant deviations from the model are observed only at high temperature, suggesting that the strong pinning model is no longer valid very close to the Curie temperature (565 K). The present result agrees with the model fit obtained for a die-upset Nd-Fe-B magnet.

Magnetic viscosity in Ba-ferrite

Results are presented on magnetic viscosity measurements performed at room temperature on Ba-ferrite pressed powder samples of different particle sizes. Maximum values of viscosity coefficients ( S) occur around coercive fields, except for a sample with an intrinsic coercive force of 4639 Oe, in which the maximum is at a much higher field (5706 Oe). Activation volumes are of the order of 10 −17 cm 3 and decrease along with particle size (except, again, for the high coercivity sample). Experimental values of coercive fields and viscosity parameters ( kT/ q), used in connection with Gaunt's theory, suggest that the viscosity mechanism in Ba-ferrite, for particle sizes from 7 to 0.7 > m is the strong pinning of domain walls.

Coercivity of die upset NdFeB magnets: A strong pinning model

We show that the temperature dependence of the intrinsic coercivity Hci(T) between 5 and 600 K in a die-upset NdFeB magnet is in good agreement with a model for strong domain wall pinning by a random array of pinning sites proposed by Gaunt [P. Gaunt, Phil. Mag. B48 (1983) 261]. The model includes both the temperature dependence of the intrinsic magnetic properties of the Nd2Fe14B phase and the effects of thermal activation of domain walls over the pinning barrier. The pinning sites are modeled as nonmagnetic planar inhomogeneities at the boundaries aetween platelet-shaped Nd2Fe14B grains. We develop an expression for the maximum pinning force per site, f, and derive the model prediction that (Hci/γHA) 12 varies linearly with (T/γ)23, where HA and γ are the magnetocrystalline anisotropy and the domain wall energy per unit area of the Nd2Fe14B phase, respectively. The model is in good agreement with the observed Hci values over a broad temperature range from 200 to 477 K. Deviations from the model below 200 K are an artifact of the axial-to-conical spin reorientation in Nd2Fe14B at low temperature. Deviations at high temperature most likely occur because the strong pinning model is no longer valid close to the Curie temperature (585 K).

Magnetisation mechanisms and magnetic viscosity in NdFeB alloys

Initial magnetization, demagnetization, and magnetic viscosity data are presented for sintered and melt-spun NdFeB over a range of temperatures. A model based on strong domain wall pinning is used to investigate the energy barriers in operation in each case. Temperature is observed to have little effect on magnetization of the material, whereas the demagnetization process is found to be greatly affected. This seems to imply that very different mechanisms are at work in the magnetization and demagnetization processes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mean-field simulation of field-induced domains and hysteretic behavior in dilute Ising antiferromagnets

A mean-field free-energy surface of a dilute Ising antiferromagnet on two-, three-, and four-dimensional lattices is studied numerically as a function of temperature and an applied magnetic field. In the presence of the field parallel to spins, the dilute Ising antiferromagnet is believed to map onto the random-field Ising model providing a convenient experimental realization. From the hysteretic behavior we identify three temperature regions: (1) a high-temperature equilibrium region above the hysteresis boundary ${T}_{H}$ where an accessed state is independent of the sample history; (2) a low-temperature region below ${T}_{c}$ where the antiferromagnetic state has the lowest free energy; (3) an intermediate-temperature region ${T}_{c}&lt;T&lt;{T}_{H}$ where the accessed state depends on the sample history, but the domain state has a lower free energy than the antiferromagnetic state. When the sample is cooled in a finite field, the domains are formed and stabilized below ${T}_{H}$. Thus the antiferromagnetic state is inaccessible in our simulation. In contrast, once the antiferromagnetic order is prepared at low temperatures and the temperature is raised in the finite field, the sample remains in the antiferromagnetic state until ${T}_{c}$ and then the domains nucleate above ${T}_{c}$, and finally the sample recovers the high-temperature equilibrium state at ${T}_{H}$. In recent neutron scattering experiments on three-dimensional (3D) Ising antiferromagnets, the analogous behavior is observed. Using the simulation results, we argue that strong domain-wall pinning prevents long-range order from occurring in 3D samples when cooled with $H\ensuremath{\ne}0$, despite the theoretical consensus that the lower critical dimension in equilibrium is 2 for the random-field Ising model.

Magnetic viscosity in a C uMn spin glass as evidence of strong domain wall pinning

The magnetic viscosity parameter of a 0.243% CuMn spin glass rises to a maximum value in an applied field Hm below the glass temperature. Hm increases as the temperature is lowered. We find that H1/2m is a linear function of T2/3. Hm is interpreted as the coercive force of the most prevalent fraction of a magnetically ordered portion containing domain walls and showing strong pinning effects. The magnitude of this most probable fraction of the distribution increases by a factor of 208 on cooling from 3.26 to 1.36 K.

Relation between coercive force and microstructure of sintered SmCo5 permanent magnets

The coercive force of a sintered SmCo5 permanent magnet is found to depend reproducibly on the temperature of heat treatment. In order to find out the origin of this, the mechanism of the demagnetization was studied by magnetic-domain observation, using the polar Kerr effect. It appears that the highest coercive forces which are obtained by heat treating at 850–950 °C and quenching are due to difficult nucleation of reverse domains and strong pinning of domain walls by the grain boundaries. After quenching from higher temperature (∼1100 °C) the coercive force decreases owing to weaker pinning of the domain walls. The drop of the coercive force which is found to occur upon heating at lower temperature (∼700 °C) comes from an easy nucleation of reverse domains. Indirect evidence is presented which supports the assumption that this easy reverse-domain nucleation occurs on Sm2Co17 precipitates. The Sm2Co17 precipitation is governed by the Co-rich SmCo5 phase boundary and its metastable extension below the eutectoid temperature of SmCo5. The state of the grain boundary which may be responsible for the strong pinning is discussed.