Xin Fan

Observation of the nonlocal spin-orbital effective field

The spin-orbital interaction in heavy nonmagnetic metal/ferromagnetic metal bilayer systems has attracted great attention and exhibited promising potentials in magnetic logic devices, where the magnetization direction is controlled by passing an electric current. It is found that the spin-orbital interaction induces both an effective field and torque on the magnetization, which have been attributed to two different origins: the Rashba effect and the spin Hall effect. It requires quantitative analysis to distinguish the two mechanisms. Here we show sensitive spin-orbital effective field measurements up to 10 nm thick ferromagnetic layer and find the effective field rapidly diminishes with the increase of the ferromagnetic layer thickness. We further show that this effective field persists even with the insertion of a copper spacer. The nonlocal measurement suggests that the spin-orbital effective field does not rely on the heavy normal metal/ferromagnetic metal interface.

Quantifying interface and bulk contributions to spin–orbit torque in magnetic bilayers

Spin-orbit interaction-driven phenomena such as the spin Hall and Rashba effect in ferromagnetic/heavy metal bilayers enables efficient manipulation of the magnetization via electric current. However, the underlying mechanism for the spin-orbit interaction-driven phenomena remains unsettled. Here we develop a sensitive spin-orbit torque magnetometer based on the magneto-optic Kerr effect that measures the spin-orbit torque vectors for cobalt iron boron/platinum bilayers over a wide thickness range. We observe that the Slonczewski-like torque inversely scales with the ferromagnet thickness, and the field-like torque has a threshold effect that appears only when the ferromagnetic layer is thinner than 1 nm. Through a thickness-dependence study with an additional copper insertion layer at the interface, we conclude that the dominant mechanism for the spin-orbit interaction-driven phenomena in this system is the spin Hall effect. However, there is also a distinct interface contribution, which may be because of the Rashba effect.

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 ]

Spin Pumping in Electrodynamically Coupled Magnon-Photon Systems

We use electrical detection, in combination with microwave transmission, to investigate both resonant and nonresonant magnon-photon coupling at room temperature. Spin pumping in a dynamically coupled magnon-photon system is found to be distinctly different from previous experiments. Characteristic coupling features such as modes anticrossing, linewidth evolution, peculiar line shape, and resonance broadening are systematically measured and consistently analyzed by a theoretical model set on the foundation of classical electrodynamic coupling. Our experimental and theoretical approach paves the way for pursuing microwave coherent manipulation of pure spin current via the combination of spin pumping and magnon-photon coupling.

Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB

The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed.

Tunable ferromagnetic resonance in NiFe nanowires with strong magnetostatic interaction

Magnetic materials with tunable ferromagnetic resonant (FMR) frequencies are highly desirable in microwave devices. In this manuscript, we demonstrate that the natural FMR of Ni90Fe10 nanowire array can be tuned continuously from 8.2 to 11.7 GHz by choosing different remanent states. Theoretical model based on magnetostatic interaction among nanowires has been developed to explain the observed phenomena.

Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co?ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100?K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.

In-situ characterization of rapid crystallization of amorphous CoFeB electrodes in CoFeB/MgO/CoFeB junctions during thermal annealing

We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson–Mehl–Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

Real-time evolution of tunneling magnetoresistance during annealing in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeB∕MgO∕CoFeB junctions during annealing at 380°C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing.

Tunnel Barrier Enhanced Voltage Signal Generated by Magnetization Precession of a Single Ferromagnetic Layer

We report the electrical detection of magnetization dynamics in an Al/AlOx/Ni80Fe20/Cu tunnel junction, where a Ni80Fe20 ferromagnetic layer is brought into precession under ferromagnetic resonance conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed when the contacts in this type of multilayered structure are Ohmic. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal.