Strong Domain Wall Pinning Research Articles

Magnetization reversal and domain wall pinning in nanowires with nonlinear modulation of diameter

Micromagnetic simulation of magnetization reversal is studied on cylindrical tapered nanowire with radii 50nm and 10nm at wider and narrower end respectively. The radius decreases from wider end to the narrower end non-linearly as R-βzδ where δ is the degree of non-linearity. Value of δ is either an integer (n) or reciprocal of integer (1/n). At remanence, a vortex and flower state at the wider and narrower extremes respectively are exhibited combined with ferromagnetic ordering in the majority of volume. When δ is below 1/2, pinning and depinning of domain wall drives the hysteresis following a Modified Kondorsky model. Otherwise, the magnetization reversal proceeds with nucleation and propagation of vortex domain wall according to Kondorsky model. The propagation of domain wall is dependent upon the value of δ which suggests tunability of magnetic characteristics with modulation of diameter. For instance, large values of coercivity is observed for wires that have strong domain wall pinning which is considerably reduced when depinning and propagation of domain wall takes over. A large range (20kA/m to 130kA/m) of coercivity can be obtained by changing the modulation.

Magnetic and electrical properties of postannealed Co2MnSi Heusler alloy films

The Co2MnSi (CMS) Heusler alloy compound with ordered crystalline structure is attractive for a number of applications due to soft ferromagnetic properties, high Curie temperature (TC ≅ 985 K), high saturation magnetization (Ms ≅ 5.1 µB/f.u), and predicted full spin polarization of carriers. In this work, we report preparation and properties of nanocrystalline CMS films with thickness d = 120 nm magnetron sputtered onto Si(100) substrates followed by annealing in vacuum at Ta = 300 ÷ 500 °C. Nanocrystalline structure of partially ordered cubic B2 phase with grain diameter d = 15 ÷ 30 nm increasing gradually with Ta has been indicated after annealing at Ta ≥ 400 °C. The angle-dependent differential susceptibility measurements revealed an interesting evolution of magnetic properties of the films in a course of postdeposition annealing at Ta=500 °C. Formation of isolated FM particles demonstrating the angle-dependent coercivity Hc(α) following the well-known Stoner–Wohlfarth model has been indicated after 0.5 h annealing. However, the Kondorsky low Hc ~ 1/cosα demonstrating a strong pinning of domain walls has been certified for similar films after 1.5 h annealing. At the same time, long-term (~ 5 h) annealing at 500 °C revealed reduced magnetization of the films due to a decomposition of the material. Nevertheless, electrical transport measurements revealed low field magnetoresistance of these films of about 0.1% and 0.05% at 80 and 295 K, respectively, demonstrating tunneling of spin-polarized carriers between nanocrystalline grains.

Structural and electrical properties of the sol-gel derived multiferroic BiFeO3 monolayer and NiTiO3-BiFeO3 bilayer thin films

In this paper, the multiferroic BeFiO3 monolayer and NiTiO3–BiFeO3 bilayer thin films were fabricated by spin-coating method on the SrRuO3/n+-Si substrate. The structural and ferroelectric properties of multiferroic BeFiO3 monolayer and NiTiO3–BiFeO3 bilayer thin films were investigated. Both multiferroic films showed the typical XRD patterns of the perovskite structure without presence of the second phase. The electrical properties, such as leakage current and remnant polarization, of the NiTiO3–BiFeO3 bilayer film were superior to those of BeFiO3 monolayer film, which those values were 1.94 × 10−4 A/cm2 at electric field of 0.75 MV/cm and 14.05 μC/cm2, respectively. This outcome is due to the NiTiO3–BiFeO3 bilayer film with a high Schottky barrier height as well as a top NiTiO3 layer on the BiFeO3 film inducing the strain-induced polarization rotation and forming the strong domain-wall pinning.

Compositional variation of amorphous phase controlled coercivity of Nd60Fe30Al10 alloy

Abstract The coercivity of Nd-Fe-Al amorphous ferromagnetic materials originates from the combination of magnetic interaction and strong pinning of domain walls. Using Nd60Fe30Al10 alloys made under three cooling rates, we discovered an important microstructure parameter that can be used to strengthen interaction and pinning. By tuning the variations of composition difference between Nd-rich and Fe-rich amorphous regions, we enhanced the magnetic interaction and increased maximum restoring force per pin of Nd nanocrystals (with size

Insight into the magnetic behavior of Sr2IrO4: A spontaneous magnetization study

Sr2IrO4 is a weak ferromagnet where the spin arrangement is canted anti-ferromagnetic (AF). Moreover, the spin-structure coupling plays an important role in magnetic behavior of Sr2IrO4. In this concern the magnetization under zero applied field i.e. spontaneous magnetization would be interesting to study and would give insight into the novel magnetic behavior of Sr2IrO4. Sophisticated techniques like neutron diffraction, μSR etc has been used to understand the magnetic behavior of Sr2IrO4 under zero applied field. To understand the magnetic behavior we have performed field and temperature dependent magnetization study. The measured field and temperature dependent magnetic data is analyzed rigorously. We have attempted to understand the temperature dependance of spontaneous magnetization, remanent magnetization and coercive force. We observe that the spontaneous magnetization extracted from Arrott plot shows that the Sr2IrO4 is not an ideal ferromagnet. The temperature dependent coercive field follows Guant's model of strong domain wall pinning. Our investigation explicit the temperature dependence of various magnetic properties shows the magnetic transitions from paramagnetic to ferromagnetic phase with Tc around 225 K and a low temperature evolution of magnetic moment around TM∼90 K.

Orthorhombic-tetragonal phase transition induced by Ta isovalent doping and its effect on the fatigue characteristics of KNL-NSTx ceramics

The effect of Ta addition on the bipolar fatigue characteristics of lead-free KNL-NSTx ceramics (x = 0, 0.04, 0.07 and 0.11mol%) is studied. Bipolar cycling up to 1 × 106 cycles leads to strong degradation of the polarization in unmodified KNL-NS ceramics. This can be explained by the development of strong domain wall pinning, leading to the build-up of high local stresses and consequently microcracking of the material. The addition of Ta reduces the domain wall pinning effect and improves the bipolar fatigue resistance. In order to understand the fatigue mechanism, a model based on oxygen vacancy accumulation is proposed. This model is expected to guide future fatigue studies that are concerned with novel lead-free KNN-based materials.

Coercivity Mechanism of (Nd0.8Ce0.2)2.4Fe12Co₂B Ribbons with Ferromagnetic Grain Boundary Phase.

Understanding the coercivity mechanism has had a substantial impact on developing economically more attractive RE-based (RE = rare earth) permanent materials because of price volatility of key RE metals (i.e., Nd and Dy) in recent years. In this work, we investigated the microstructure and magnetic properties of melt-spun (Nd0.8Ce0.2)2.4Fe12Co2B ribbons and annealed samples at 773 K for 15 min with 1 Tesla (T) magnetic field to better understand the coercivity mechanism. We found hard magnetic grains were surrounded by thin and continuous layers along the grain boundaries (GBs) with a high concentration of ferromagnetic elements (Fe + Co >74 at%). The obvious positive peak in the δM plot and the interaction domain structure observed by Lorentz magnetic microscopy indicate that there is strong exchange coupling interaction through the ferromagnetic GB phase between hard magnetic grains. The annealing in an applied magnetic field of 1 T increases the remanence by enhancing the exchange coupling interaction, leading to a maximum product energy ((BH)max) which is 16% higher than that of melt-spun ribbons. We also studied the temperature dependence of the coercivity in a temperature range of 300–500 K, and proposed that the coercivity of melt-spun (Nd0.8Ce0.2)2.4Fe12Co2B ribbons with ferromagnetic GB phase at room temperature was from the combination of strong domain-wall pinning and nucleation. The same mechanism works in the annealed ribbons.

Low-field magnetoresistance and switching behavior of polycrystalline La0.66Sr0.34MnO3/YSZ(001) films with columnar grain structure

The La0.66Sr0.34MnO3 (LSMO) films with thickness d=180–330nm were grown by radio frequency magnetron sputtering on cubic yttria-stabilized zirconia, YSZ(001), at 750°C. Coexistence of columnar grains with the averaged diameter of about 50nm and (001) and (011) planes of a pseudocubic lattice oriented parallel to the film surface has been certified by transmission electron microscopy and x-ray diffraction investigations. Pole figure x-ray diffraction measurements revealed cube-on-cube growth of the LSMO(001) grains on YSZ(001) with 45° in-plane rotation meanwhile two in-plane orientations with the epitaxial relationships: [001]LSMO//[010]YSZ and [001]LSMO//[100]YSZ have been certified for the LSMO(011) grains. Presence of high-angle grain boundaries between the columnar grains resulted enhanced electrical resistivity and low-field magnetoresistance associated to spin-polarized tunnelling of carriers. Electrical resistance versus magnetic field plots demonstrated the characteristic hysteresis behavior in the low field region (H<80kA/m) with peak-like maxima at H=±Hp. An unusual sharp resistance drop at H≅Hp indicated for the films at T<210K and the observed variation of Hp with the angle between applied field and the film plane (α) defined by the Kondorsky relationship: Hp=Hp0/cosα demonstrate strong pinning of domain walls at grain boundaries.

Combined effects of magnetic interaction and domain wall pinning on the coercivity in a bulk Nd₆₀Fe₃₀Al₁₀ ferromagnet.

Understanding the coercivity mechanism has a substantial impact on developing novel permanent materials. However, the current coercivity mechanisms used widely in permanent alloys cannot explain well the amorphous phase produced hard magnetic behavior of Nd-based bulk amorphous alloys (BAAs). Here, we propose that the coercivity in as-cast Nd60Fe30Al10 alloy is from the combination of magnetic interaction and strong pinning of domain walls. Moreover, the role of domain wall pinning is less affected after crystallization, while the magnetic interaction is dependent on the annealing temperature. Our findings give further insight into the coercivity mechanism of Nd-based bulk ferromagnets and provide a new idea to design prospective permanent alloys with coercivity from the combination of magnetic interaction and pinning of domain walls.

Change in the Magnetic Domain Alignment Process at the Onset of a Frustrated Magnetic State in Ferrimagnetic ${\hbox{La}}_{2}{\hbox{Ni}}({\hbox{Ni}}_{1/3}{\hbox{Sb}}_{2/3}){\hbox{O}}_{6}$ Double Perovskite

We have performed a combined study of magnetization hysteresis loops and time dependence of the magnetization in a broad temperature range for the ferrimagnetic La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ni(Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sub> )O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> double perovskite. This material has a ferrimagnetic order transition at ~100 K and at lower temperatures (~20 K) shows the signature of a frustrated state due to the presence of two competing magnetic exchange interactions. The temperature dependence of the coercive field shows an important upturn below the point where the frustrated state sets in. The use of hysteresis data, magnetization versus applied magnetic field, together with the magnetization versus time data provides a unique opportunity to distinguish between different scenarios for the low temperature regime. From our analysis, a strong domain wall pinning results in the best scenario for the low temperature regime. For temperatures larger than 20 K, the adequate scenario seems to correspond to a weak domain wall pinning.

Imaging the Magnetization Processes in Epitaxial Exchange Coupled SmCo$_{5}$/Fe/SmCo$_{5}$ Trilayers

A detailed understanding of the magnetization processes in an exchange coupled SmCo5/Fe/SmCo5 trilayer is derived from imaging the domain configuration of the sample in a succession of remanent states by magnetic force microscopy. Domain nucleation and domain wall movement processes are identified and are interpreted in terms of the dominating coercivity mechanism. As in single hard magnetic SmCo5 layers, strong pinning of domain walls governs the magnetization reversal and determines coercivity. The coupling to the Fe layer, however, leads to an increased domain size in the thermally demagnetized state and to a larger nucleation density prior to the irreversible magnetization switching. Due to slightly different switching fields of the two SmCo5 layers, a compensated state can be adjusted, in which the moments of the Fe layer form 180° spin spirals with laterally varying chirality. The magnetic contrast arising from these spin spirals is imaged and the irreversible changes in a small in-plane field perpendicular to the easy axis are studied.

Structural, Dielectric, and Piezoelectric Properties of Mn-Doped BaTiO3–Bi(Mg1/2Ti1/2)O3–BiFeO3 Ceramics

Mn-doped (0.9-x)BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–xBiFeO3 (x = 0.6, 0.7, and 0.8) ceramics were studied as candidate lead-free piezoelectric materials. The crystal structures were a pseudo-cubic perovskite at x = 0.6, a rhombohedral perovskite at x = 0.8, and a mixture of the pseudo-cubic and rhombohedral perovskite at x = 0.7. The Curie temperature was found to be 470 °C at x = 0.6 and more than 600 °C at x = 0.7 and 0.8. A strong pinning of domain walls was observed for high-field responses at x = 0.7 and 0.8, which was relaxed by high-temperature annealing and subsequent quenching. Piezoelectric properties increased with decreasing x, and the small-field piezoelectric constants d33 and d31 were 94 and 31 pC/N at x = 0.6, respectively.

Structural, Dielectric, and Piezoelectric Properties of Mn-Doped BaTiO3–Bi(Mg1/2Ti1/2)O3–BiFeO3Ceramics

Mn-doped (0.9-x)BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–xBiFeO3 (x = 0.6, 0.7, and 0.8) ceramics were studied as candidate lead-free piezoelectric materials. The crystal structures were a pseudo-cubic perovskite at x = 0.6, a rhombohedral perovskite at x = 0.8, and a mixture of the pseudo-cubic and rhombohedral perovskite at x = 0.7. The Curie temperature was found to be 470 °C at x = 0.6 and more than 600 °C at x = 0.7 and 0.8. A strong pinning of domain walls was observed for high-field responses at x = 0.7 and 0.8, which was relaxed by high-temperature annealing and subsequent quenching. Piezoelectric properties increased with decreasing x, and the small-field piezoelectric constants d33 and d31 were 94 and 31 pC/N at x = 0.6, respectively.

Perpendicular anisotropy and out-of-plane exchange bias in nanoscale antidot arrays

We report on tailoring the perpendicular anisotropy, and out-of-plane exchange bias in (Co/Pd) multilayer exchange coupled to CoO, by patterning them into ordered arrays of antidots or ‘holes’. The coercivity in the multilayers is enhanced due to strong domain wall pinning in the vicinity of the antidots. We also observed a smaller degree of asymmetry in the reversal of the exchange-biased antidots as compared to the continuous film. Moreover, a strong interplay between thermal activation and CoO domain size confinement results in the exchange bias for the antidots being either smaller or larger than the continuous film.

Co/Pt perpendicular antidot arrays with engineered feature size and magnetic properties fabricated on anodic aluminum oxide templates

Antidot arrays with perpendicular anisotropy have been fabricated by depositing Co/Pt multilayers on the anodic aluminum oxide (AAO) templates prepared on Si wafers. The antidot arrays cover an area of about $10\text{ }{\text{cm}}^{2}$. The antidot size $(D)$ and edge-to-edge $(w)$ separation between two adjacent antidots are varied over a wide range from 7 to 46 nm and 14 to 53 nm, respectively. The magnetic properties and magnetization process exhibit strong dependence on the antidot size and separation. The perpendicular coercivity $({H}_{c})$ increases from 140 Oe (continuous film) with the increase in $D$ and shows a peak at about 1350 Oe for the $D$ of around 30 nm. The ${H}_{c}$ starts to decrease for the further increase in $D$ due to the deteriorated perpendicular anisotropy. The magnetization reversal of the antidot arrays with $D$ of 30 nm or less is governed by the strong domain-wall pinning and the switching field shows less sensitivity to the angular variation from the easy axis. The magnetic domain size decreases with increase in $D$. The evolutions of magnetic properties are ascribed to the pinning effects imposed by the AAO pores. The influence of antidot size on the pinning of domain wall, ${H}_{c}$ and magnetization switching characteristics are also investigated by the micromagnetic modeling.

Magnetization Reversal in Arrays of Magnetic Nanoperforations

Nanoperforated ZrO2 membranes, produced via organic/inorganic self-assembly using block copolymer micelles, serve as template for a Co/Pt multilayer stack with out-of-plane magnetic anisotropy. The deposition of the magnetic film results in an array of magnetic nanodots surrounded by a continuous magnetic film. These magnetic dots are found to be in a single-domain state and appear as magnetically exchange isolated from the surrounding film. The latter observation can be related to the specific morphology of the nanoperforations. In addition, strong domain-wall pinning of the continuous magnetic film at the locations of the nanoperforations was observed. Therefore, this approach is promising to create a percolated magnetic medium, which is considered as future recording media.

Controlling magnetization reversal in Co/Pt nanostructures with perpendicular anisotropy

We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A=pore depth/diameter. Magnetization reversal in composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning, which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change in the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.

Metal-insulator Transition and Magnetic Domains in (Ga,Mn)As Epilayers

ABSTRACTWe investigate the magnetic domain pattern in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. We show that post-growth annealing, besides improving the magnetic and transport properties, also drastically changes the domain pattern. Strong pinning of domain walls along the &lt;110&gt; directions is suppressed and large-size domains are observed.

Electric, thermal, and magnetic properties of CeSix with 1.57&amp;lt;x⩽2.0

It has been shown that CeSix exhibits a variety of interesting physical properties, however the magnetic ground state was unclear due to the possible coexistence of multiple magnetic phases. In this study, the physical properties of CeSix with 1.57&amp;lt;x⩽2.0 have been investigated to clarify the magnetic ordering ground states throughout the series. For Si concentrations greater than 1.80, samples are paramagnetic. Ferromagnetic ordering first appears when Si concentration is decreased to 1.80. For Si concentrations ranging from 1.73 to 1.80, TC=13.8K. As Si concentration is further decreased below 1.73, TC steps down to 12K, remaining constant down to the lowest concentration studied, x=1.57. Interestingly, the latter change in magnetic ground state is accompanied by a structural distortion from the ThSi2-type tetragonal structure to the GdSi2-type orthorhombic phase. In the magnetic regime, zero-field-cooled and field-cooled magnetization data are irreversible, indicating a strong domain-wall pinning effect. Values of TC are corroborated by specific-heat and resistivity measurements.

Highly coercive melt-spun Sm(Co,Fe,Cu,Zr)/sub z/ magnets prepared by simple processing

Summary form only given. It is known that high coercivity in Sm(Co,Fe,Cu,Zr)/sub z/ magnets is obtained after a lengthy and complex heat treatment, which consists of solid solution and isothermal aging treatments, followed by slow cooling. In this work, a new simple method was developed for magnetic hardening of melt-spun Sm(Co,Fe,Cu,Zr), alloys. A coercivity of 2.8 T and a uniform cellular and lamellar microstructure were obtained only by slow cooling the as-spun Sm(Co/sub 0.74/Fe/sub 0.1/Cu/sub 0.12/Zr/sub 0.04/)/sub 7.5/ ribbons from 850/spl deg/C to 400/spl deg/C without the standard solid solution and isothermal aging, which are required for the bulk precipitation hardened 2:17-type SmCo alloy. A high density of lamellar phase with large width was observed by transmission electron microscopy (TEM) in the ribbons subjected to slow cooling. This leads to the fast development of an uniform and fine cellular microstructure, which gives rise to a strong domain wall pinning and therefore to high coercivity. The effect of processing parameters such as temperature range for the cooling regime on magnetic properties at room and high temperatures, microstructure and microchemistry of the phases is discussed.