Influencia de la porosidad en las propiedades magnéticas de arreglos de nanohilos de Ni

Abstract

RESUMENSe estudiaron las propiedades de histéresis magnética de arreglos ordenados de nanohilos de Ni, electrodepositados éstos en una membrana de alúmina con un arreglo hexagonal de poros cilíndricos. Se analizó el efecto de la geometría de los nanohilos individuales (diámetro y longitud) y del ordenamiento y porosidad del arreglo sobre la coercitividad y la remanencia del conjunto. Las membranas de alúmina auto-ensambladas se sintetizaron por anodizado en dos pasos, usando una celda electroquímica y potencial eléctrico constante. Se variaron los diámetros de poro entre 20 nm y 40 nm (no así la distancia media entre ellos) sumergiendo las membranas en ácido fosfórico al 5% durante distintos tiempos. Estas membranas se caracterizaron por microscopía de barrido (SEM). Los nanohilos de Ni se obtuvieron por electrodeposición AC, a 20 V, 200 Hz y temperatura ambiente. Una vez sintetizados, los arreglos de nanohilos se caracterizaron estructuralmente por SEM y magnéticamente usando un SQUID. Se observa que las propiedades magnéticas de los arreglos son fuertemente dependientes de la relación largo/diámetro de los nanohilos, la que regula el valor de su anisotropía de forma, y también de la porosidad, que determina la magnitud del campo de interacción dipolar que percibe cada nanohilo por la presencia de sus vecinos en el arreglo. En particular, se encontró que la coercitividad decrece con la porosidad y que el parámetro ε, que regula la intensidad de las barreras de energía del sistema, debe ser variable y depender de la porosidad del arreglo para ajustar bien los datos experimentales. Palabras clave: Nanohilos, Propiedades magnéticas, Níquel ABSTRACTThe hysteresis properties of ordered arrays of Ni nanowires, electrodeposited into cylindrical nanoporous alumina membranes were investigated. The effect of the nanowire geometry (diameter and length), the array order and the nanowires' volume fraction on the array coercivity and remanence were analyzed. Porous alumina membranes were produced by a two-step anodization process in a potentiostatic electrochemical cell. The pores' diameters were varied from 20 nm to 40 nm by immersing the membranes in a 5% phosphoric acid solution for different times. The membranes were characterized using a scanning electron microscope (SEM) and both the pore size distribution as well as their separation were determined. These membranes were used as templates for the AC electrodeposition of Ni nanowires, using a voltage of 20 V and a frequency of 200 Hz, at room temperature. The Ni nanowire arrays were structurally and magnetically characterized by SEM and with a SQUID magnetometer, respectively, at room temperature. It was found that the magnetic properties of the array strongly depend on the nanowires aspect ratio -length/diameter ratio-which regulates the wire shape anisotropy and also on the porosity, which determines the magnitude of the dipolar interaction field acting on the array. Particulary, it was found that coercitivity decreases with porosity and that the parameter ε, which is...

Temperature dependence of the effective anisotropy in Ni nanowire arrays

“…In sample DC200, coercivity decreases as temperature increases. On the contrary, an unexpected behavior of μ 0 H C is observed in arrays with small diameters, AC33 and DC65, in which coercivity increases with T. Besides, at low temperatures, μ 0 H C goes through an inflexion point at about 50-100 K, while sample DC200 slightly decreases for increasing T. As indicated before, similar results in Ni NW systems with granular structure and irregular surfaces were observed by other authors [5,[20][21][22].…”

“…1a and referred to as "home-made" hereafter) were prepared in our lab, following a well-known two-step procedure [25], as described in Refs. [5,26].…”

“…The hysteresis behavior of ferromagnetic Ni nanowire (NW) arrays, grown by electrodeposition inside the pores of anodic aluminum oxide (AAO) templates, has been extensively studied [1][2][3][4][5][6][7][8][9][10][11]. The array properties depend on the structure of the individual wires, the dipolar interactions between these wires and also on the extent of the mechanical interaction between the Al substrate, the alumina template and the metallic nanowires, arising from internal thermal stresses and the large magnetostriction of Ni.…”