Additive manufacturing is a novel manufacturing paradigm which has numerous potential applications in industry and research. PolyJet technology allows printing of extremely complex geometrical structures with high precision and smooth surface. New engineering polymers with diverse characteristics should be developed to expand PolyJet applications. Bismaleimides (BMIs) are very attractive polymers due to their excellent thermal, mechanical, and chemical stability, and their good dielectric properties. However, BMIs are currently not available as inks for PolyJet technology. Low‐viscosity aliphatic BMIs are used to develop a novel UV‐curable ink. The UV reactivity and ink viscosity are optimized by addition of an environmentally friendly diluent and a mixture of photoinitiators. Optimization of the jetting and printing conditions allows for the first ever production of 3D thermosetting BMI objects by PolyJet technology. Thermal post curing is used to enhance mechanical properties and thermal stability of the printed material. It is found to have a thermal decomposition temperature, T5%, of about 400 °C, low dielectric constant, high dielectric strength, and low moisture absorption. The resulting properties of the printed BMI material open a wide range of potential applications in fields such as robotics, electronics, automotive, aerospace, and space technology.
We show that shape anisotropy induces effective single domain behavior in elliptical structures of thin permalloy films with long axis ranging between several microns to several millimeters, provided that the ratio of the film long and short axes is large enough. We also show that the thin film elliptical structures exhibit a wide range of effective anisotropy fields, from less than 10 Oe up to more than 100 Oe. We discuss the advantage of shape anisotropy in the fabrication of planar Hall effect sensors with high field resolution. V
We study the thermal stability of the magnetization states in permalloy microstructures in the form of two crossing elongated ellipses, a shape which yields effective bi-axial magnetic anisotropy in the overlap area. We prepare the structure with the magnetization along one of the easy axes of magnetization and measure the waiting time for switching when a magnetic field favoring the other easy axis is applied. The waiting time for switching is measured as a function of the applied magnetic field and temperature. We determine the energy barrier for switching and estimate the thermal stability of the structures. The experimental results are compared with numerical simulations. The results indicate exceptional stability which makes such structures appealing for a variety of applications including magnetic random access memory based on the planar Hall effect.
We study magnetization reversal of nanostructures of the itinerant ferromagnet SrRuO3 (Tc∼ 150 K). We find that down to 10 K the magnetization reversal is dominated by thermal activation. From 2 − 10 K, the magnetization reversal becomes independent of temperature, raising the possibility for reversal dominated by macroscopic quantum tunneling (MQT). A 10 K crossover to MQT is consistent with the extremely large anisotropy field (∼ 7 T) of SrRuO3. PACS numbers:Quantum tunneling through a potential barrier is one of the most remarkable manifestations of quantum behavior. While there is good understanding and many experimental realizations of this phenomenon when the tunneling object is microscopic, the extension of this behavior to macroscopic objects poses one of the most intriguing theoretical and experimental challenges. A promising route, which we adopt here, is to look for signatures of MQT in magnetization reversal of ferromagnetic nanoparticles.At elevated temperatures, the magnetization reversal of ferromagnetic nanoparticles is commonly described in the framework of the Néel-Brown model [1][2][3]. In its simplest form, the model describes a thermally activated process of coherent rotation at a temperature T over an energy barrier E b , and it predicts an average waiting time τ given by τ = τ 0 e E b /kB T , where τ 0 is a sample specific constant linked to Larmor frequency with a typical value around 10 −9 s [4]. However, in the low temperature limit, a crossover is theoretically expected from a thermally activated reversal to a temperature-independent magnetization reversal dominated by MQT [5,6].A low-temperature crossover to MQT-dominated reversal has been demonstrated using the magnetic molecules Mn 12 [7]and [Fe 8 O 2 (OH) 12 (tacn) 6 ] 8+ [8], both with a spin ground state of S = 10 with crossover temperatures of 0.35 and 0.6 K, respectively. The crossover was manifested in temperature independent hysteresis loops with a series of steps separated by plateaus indicating resonant tunneling.Consisting of at least hundreds of spins, nanoparticles have an energy level spacing which is too small to be identified by resonant tunneling in hysteresis loops. Thus reports on MQT of nanoparticles are mainly based on the identification of a crossover from a thermally activated reversal to a temperature-independent reversal. These reports include temperature independence below 5 K of switching field distribution of nickel nanowires [9] and temperature independence below 0.35 K of two level fluctuations of self-assembled ErAs quantum wires and dots in semi-insulating GaAs matrix [10]. In addi-tion, it has been shown that the magnetization reversal of BaFe 12−2x Co x Ti x O 19 deviates from Néel-Brown model below 0.4 K [11] and the crossover temperature depends on the direction of the reversing field, in agreement with theoretical predictions for MQT.Here we study patterned nanostructures of thin films of an extremely hard ferromagnet, SrRuO 3 , and show that when a reversing field that does not yield immediate rev...
Patterned nanostructures on the order of 200 nm × 200 nm of the itinerant ferromagnet SrRuO3 give rise to superparamagnetic behavior below the Curie temperature (∼ 150 K) down to a sampledependent blocking temperature. We monitor the superparamagnetic fluctuations of an individual volume and demonstrate that the field dependence of the time-averaged magnetization is well described by the Langevin equation. On the other hand, the rate of the fluctuations suggests that the volume in which the magnetization fluctuates is smaller by more than an order of magnitude. We suggest that switching via nucleation followed by propagation gives rise to Langevin behavior of the total volume, whereas the switching rate is determined by a much smaller nucleation volume.PACS numbers: 75.60.Jk, The magnetization of ferromagnetic nanoparticles commonly exhibit thermally induced fluctuations known as a superparamagnetic behavior at a temperature interval below the Curie temperature. Superparamagnetism has been known for decades [1]; however, the interest in this fundamental phenomenon has increased in recent years in connection with a wider use of spintronic devices consisting of nanoscale magnetic components [2]. Although the best way to study superparamagnetism is by exploring the superparamagnetic behavior of an individual nanoparticle, so far due to technical challenges the study of superparamagnetism has been mainly performed with ensembles of magnetic nanoparticles where the fluctuations are not observed directly but inferred from the field and temperature dependence of the average magnetization of the ensemble [3][4][5][6][7][8].The magnetic fluctuations of an individual superparamagnetic nanoparticle are described in the framework of the Néel-Brown model [9][10][11]. In its simplest form, the model describes a thermally activated process of coherent rotation of a single magnetic domain particle with uniaxial magnetic anisotropy at a temperature T over an energy barrier E b , and it predicts an average waiting time for reversal τ given by τ = τ 0 e E b /kB T , where τ 0 is a sample specific constant linked to Larmor frequency with a typical value on the order of 10 −9 s [12]. The temperature below which the waiting time exceeds the relevant measuring time (commonly on the order of 100 s) is defined as the blocking temperature T b given by T b = 25K u V /k B , where K u , V , and k B are the anisotropy constant, the volume of the sample, and Boltzmann constant, respectively.The field dependence of the average magnetization − → M is described by the Langevin equation, where µ 0 − → H is the magnetic field. The application of the Langevin equation to describe the magnetization curves of ensembles of nanoparticles is not straightforward due to variations in the volume and shape of the nanoparticles. Therefore, any fit requires making assumptions regarding the volume distribution [13]. On the other hand, in the few reports where superparamagnetic fluctuations of individual superparamagnetic nanoparticles were monitored [14][15][1...
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