Memory Effect in Magnetic Nanowire Arrays

www.advmat. de www.MaterialsViews. com Xiaoming Kou , Xin Fan , Randy K. Dumas , Qi Lu , Yaping Zhang , Hao Zhu , Xiaokai Zhang , Kai Liu , and John Q. Xiao* Magnetic materials are widely used for information storage because of their large capacity and low cost. [ 1 ] Storage medium technologies have evolved from analog recording with mag- netic tapes to high ﬁdelity digital recording with magnetic hard disks. Nevertheless, both techniques use a magnetic medium consisting of magnetic particles, whose sizes have also evolved from micrometers in magnetic tapes to nanometers in modern hard disks. In analog recording, signals are converted into mag- netic ﬁelds which change the magnetization of a group of mag- netic particles (bit). The magnetization variations represent the stored information which can subsequently be read out. The magnetization, and therefore the stored information, could be changed by an external magnetic ﬁeld and/or thermal effects. In digital recording, the bit magnetization can be aligned either left or right in parallel recording or up and down in perpen- dicular recording. [ 2 ] The information is stable as long as the medium is not subjected to a magnetic ﬁeld higher than the coercivity, or a temperature higher than the superparamagnetic limit, of the constituent magnetic particles. In order to clearly distinguish one bit from another it is advantageous to minimize the dipolar interaction among magnetic particles, which is typi- cally achieved by creating boundaries between particles. Since the magnetic dipolar interaction is particularly pronounced in a collection of magnetic entities, such as magnetic particles and nanowires, it is scientiﬁcally interesting to question whether such a degree of freedom can be exploited in order to create additional memory functions. To answer this question, one needs a magnetic system with a sizable and preferably control- lable dipolar interaction. The magnetic nanowire array is an ideal system for this purpose. Magnetic nanowire arrays embedded in an insulating Al 2 O 3 matrix have been intensively studied. [ 3–12 ] When the magnetoc- rystalline anisotropy is negligible, the magnetization direction of the nanowires is preferably aligned along the length of the nanowire because of the shape anisotropy. When nanowires are very close to each other, dipolar interactions play a signiﬁcant X. Kou, Dr. X. Fan, Q. Lu, Y. Zhang Prof. J. Q. Xiao Department of Physics and Astronomy University of Delaware Newark, DE, 19716, USA E-mail: jqx@udel.edu Dr. R. K. Dumas, Prof. K. Liu Department of Physics University of California Davis, CA, 95616, USA Dr. H. Zhu, Dr. X. Zhang Spectrum Magnetics LLC, 1210 First State Blvd, Wilmington, DE, 19804, USA DOI: 10.1002/adma.201003749 Adv. Mater. 2011, 23, 1393–1397 role in the magnetic behavior of the nanowire array, leading to rich physical phenomena and great application potentials. [ 7–12 ] Recently, it was demonstrated that the dipolar interaction among magnetic nanowires could provide zero ﬁeld ferromagnetic res- onance (FMR) tunability, which has potential applications in a variety of microwave devices. A double FMR feature caused by the dipolar interaction in a magnetic nanowire array was also predicted [ 13 ] and veriﬁed. [ 14–17 ] In this manuscript, we demon- strate how dipolar interactions can induce an analog memory effect in magnetic nanowire arrays. Through this effect, the magnetic nanowire array has the ability to ‘memorize’ the maximum magnetic ﬁeld that the array has been exposed to. A novel, low cost, and robust electromagnetic pulse detecting method is proposed based on this memory effect. Nanowire arrays of Ni 90 Fe 10 and Ni were synthesized by elec- trodeposition into anodized alumina templates. The diameter, center-to-center interpore distance, and length of the nanowires are 35 nm, 60 nm, and 30 μ m, respectively. Figure 1 a shows the hysteresis loop, with a coercivity of 1080 Oe, of a Ni 90 Fe 10 nanowire array with a magnetic ﬁeld parallel to the wire (open squares). The loop with the ﬁeld perpendicular to the wire is shown in the inset. Clearly, a well deﬁned easy axis exists along the wire axis because of the dominant shape anisotropy. The memory effect was demonstrated using a vibrating sample magnetometer. The Ni 90 Fe 10 nanowire array was satu- rated along the wire prior to the measurement. The magnetic moment of the array was monitored as a series of magnetic ﬁeld pulses were applied parallel to the nanowires. Figure 1b displays the series of magnetic pulses with different magni- tudes and directions. The corresponding change of the mag- netic moment is illustrated in Figure 1c. We ﬁnd that the magnetic moment decreases monotonically as the magnitude of the negative pulses increases, while the moment remains the same after the positive pulses. This demonstrates that the maximum negative magnetic ﬁeld can be recorded into the nanowire array. However, this is violated for the 800 and 900 Oe ﬁeld pulses, and this discrepancy will be explained later. The result is also plotted in the magnetic moment verses applied ﬁeld ( M – H ) graph, displayed in Figure 1d. Similar prop- erties are also observed in Ni nanowire arrays. This phenomenon is attributed to the dipolar interactions among the nanowires. Previously, using a theoretical model, two assumptions were proposed. [ 13 ] First, each nanowire is a single domain cylinder with a uniform magnetization pointing up or down parallel to the wire. The second assumption is that the number of nanowires with up magnetizations ( N ↑ ) and down magnetizations ( N ↓ ) is determined by the total magnetization M(H) , i.e. (N ↑ – N ↓ )/(N ↑ + N ↓ ) = M(H)/M s , where M s is the saturation magnetization. According to these assumptions, the dipolar ﬁeld among the nanowires can be written as [ 13 ] © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com COMMUNICATION Memory Effect in Magnetic Nanowire Arrays