Jiangwei Cao

Frequency multiplying behavior in a magnetoelectric unimorph

We observe frequency multiplying behavior in a magnetoelectric (ME) unimorph: such a frequency-multiplied signal is generated when the input frequency (fin) of an alternating current magnetic field (Hac) is around 1/n (n denotes integer) of the mechanical resonance frequency (fr) of the device. We observe both odd and even harmonic signals when a direct current magnetic field (Hdc) is applied, whereas only even harmonic signals arise for Hdc = 0. This behavior results from the combined effect of the nonlinear character of the Metglas magnetostriction and a mechanical resonance phenomenon in the magnetoelectric unimorph.

Electric field control of anisotropy and magnetization switching in CoFe and CoNi thin films for magnetoelectric memory devices

We report on the marked change in magnetic anisotropy and magnetization reversal in Co50Fe50/[Pb(Mg1/3Nb2/3O3)]1?x?[PbTiO3]x (PMN?PT) and Co43Ni57/PMN?PT heterostructures under an electric field. For the Co50Fe50/PMN?PT structure, the electric-field-induced magnetic anisotropy field can be as large as 1.2 kOe at 12 kV/cm, corresponding to a magnetoelectric coefficient of 100 Oe cm/kV. In the Co43Ni57/PMN?PT heterostructure, the electric-field-induced anisotropy has a sign opposite to that in Co50Fe50/PMN?PT. As a result, in the [CoNi/Cu/CoFe/Cu]n/PMN?PT heterostructure, the parallel magnetic moment between two magnetic layers in the initial state may become perpendicular under an electric field. On the basis of these discussions, a voltage-write magnetoelectric memory device model is proposed.

On-chip measurement of the Brownian relaxation frequency of magnetic beads using magnetic tunneling junctions

We demonstrate the detection of the Brownian relaxation frequency of 250 nm diameter magnetic beads using a lab-on-chip platform based on current lines for exciting the beads with alternating magnetic fields and highly sensitive magnetic tunnel junction (MTJ) sensors with a superparamagnetic free layer. The first harmonic out-of-phase component of the MTJ response gives the imaginary part of the magnetic bead susceptibility, which peaks at the Brownian relaxation frequency. This work paves the way to on-chip implementation of Brownian magnetorelaxometry in innovative “lab-on-a-bead” assays for biomolecular recognition.

Effects of Ru and Ag cap layers on microstructure and magnetic properties of FePt ultrathin films

The effects of Ru and Ag cap layers on the microstructure and magnetic properties of the FePt ultrathin films have been investigated. The results indicate that i) The Ag cap layer segregates from the FePt/Ag bilayer, lowers the FePt ordering temperature, promotes the FePt thin films to form island structure, and enhances the coercivity; ii) The Ru cap layer increases the FePt ordering temperature, helps to maintain smooth continuous structure film, and restrains the FePt (001) orientation and perpendicular magnetic anisotropy (PMA). The effects become more pronounced for the 3-nm-thick FePt thin films. The effects can be mainly attributed to the different melting point and thermal expansion stress between the cap layer and FePt thin films.

Microwave Permeability of FeCoAlON Ferromagnetic Thin Films with Stripe Domain Structure

We report the observation of multiple permeability peaks for FeCoAlON thin ferromagnetic films with stripe domain structure, in the 100 MHz to 5 GHz range and the space between any two adjacent peaks is equal approximately. This behavior should be correlated with the sinusoidal distribution of the magnetic moments and their magnetic collective excitation in the stripe domain. The multiple peak resonance can be described in terms of the phonon theory

Spin orbit torques induced magnetization reversal through asymmetric domain wall propagation in Ta/CoFeB/MgO structures

The magnetization reversal induced by spin orbit torques in the presence of Dzyaloshinskii-Moriya interaction (DMI) in perpendicularly magnetized Ta/CoFeB/MgO structures were investigated by using a combination of Anomalous Hall effect measurement and Kerr effect microscopy techniques. By analyzing the in-plane field dependent spin torque efficiency measurements, an effective field value for the DMI of ~300 Oe was obtained, which plays a key role to stabilize Néel walls in the film stack. Kerr imaging reveals that the current-induced reversal under small and medium in-plane field was mediated by domain nucleation at the edge of the Hall bar, followed by asymmetric domain wall (DW) propagation. However, as the in-plane field strength increases, an isotropic DW expansion was observed before reaching complete reversal. Micromagnetic simulations of the DW structure in the CoFeB layer suggest that the DW configuration under the combined effect of the DMI and the external field is responsible for the various DW propagation behaviors.

Electric-field manipulation of coercivity in FePt/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructures investigated by anomalous Hall effect measurement

The effect of electric field (E-field) on the magnetism of FePt thin films in FePt/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) heterostructures was investigated by anomalous Hall effect measurement. For FePt films of different thicknesses, the coercivity vs E-field curves show a typical butterfly-like loop behavior. Further results indicate that the coercivity variation is composed of the volatile symmetrical butterfly-like loop and nonvolatile hysteresis loop-like parts, which originate from the volatile and nonvolatile strains induced by the E-field in the PMN–PT(001) substrate, respectively. No significant difference has been observed after inserting a 2 nm W interlayer, suggesting that the charge-mediated coercivity variation is negligible in FePt/PMN–PT heterostructures.

Spin-orbit torque induced magnetization switching in Ta/Co20Fe60B20/MgO structures under small in-plane magnetic fields

Spin-orbit torque (SOT)-induced magnetization switching under small in-plane magnetic fields in as-deposited and annealed Ta/CoFeB/MgO structures is studied. For the as-deposited samples, partial SOT-induced switching behavior is observed under an in-plane field of less than 100 Oe. Conversely, for the annealed samples, an in-plane field of 10 Oe is large enough to achieve full deterministic magnetization switching. The Dzyaloshinskii-Moriya interaction at the Ta/CoFeB interface is believed to be the main reason for the discrepancy of the requisite in-plane magnetic fields for switching in the as-deposited and annealed samples. In addition, asymmetric field dependence behavior of SOT-induced magnetization switching is observed in the annealed samples. Deterministic magnetization switching in the absence of an external magnetic field is obtained in the annealed samples, which is extremely important to develop SOT-based magnetoresistive random access memory.

Enhancement of spin-orbit torques in Ta/Co20Fe60B20/MgO structures induced by annealing

Spin-orbit torques (SOTs) in Ta/CoFeB/MgO structures are studied by harmonic voltage method. The results indicate that both Slonczewski-like (HSL) and field-like (HFL) effective field are enhanced by annealing in the film stacks with various Ta thicknesses. Investigation of the crystallographic phase of the Ta layers and resistance of Hall bar devices suggest that annealing may induce a phase transformation in the Ta layers from the α to the β phase, which results in the enhanced HSL of the annealed samples. Current-induced magnetization switching experiments revealed a corresponding decrease of the switching current in the annealed samples because of their enhanced SOTs.

The E-Field-Induced Volatile and Nonvolatile Magnetization Switching of CoNi Thin Films in CoNi/PMN-PT Heterostructures

In this paper, we report the giant electric-field (E-field)-induced magnetic anisotropy and magnetization switching in CoNi/Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-PT) magnetoelectric heterostructures. For CoNi/PMN-30%PT, the E-field-induced anisotropy shows a volatile behavior; whereas for CoNi/PMN-32%PT, a large and nonvolatile E-field-induced anisotropy field up to 54 kA/m is observed. These behaviors can be understood by measuring the strain versus the E-field curves of two kinds of substrates. On the basis of the E-field-induced nonvolatile magnetic switching, two stable magnetization states defined by applying E-field pulses were demonstrated in CoNi/PMN-32%PT heterostructure, which paves a new way for voltage-write magnetic random memory devices.