Yunpeng Chen

University of Delaware, Newark, Delaware

Xin Fan

University of Denver, Denver, Colorado

Yunsong Xie, Yang Zhou, and Tao Wang

University of Delaware, Newark, Delaware

Jeffrey D. Wilson and Rainee N. Simons

Glenn Research Center, Cleveland, Ohio

Sui-Tat Chui and John Q. Xiao

University of Delaware, Newark, Delaware

Tunable Magnetic Resonance in Microwave

Spintronics Devices

NASA/TM—2015-218737

April 2015
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Yunpeng Chen

University of Delaware, Newark, Delaware

Xin Fan

University of Denver, Denver, Colorado

Yunsong Xie, Yang Zhou, and Tao Wang

University of Delaware, Newark, Delaware

Jeffrey D. Wilson and Rainee N. Simons

Glenn Research Center, Cleveland, Ohio

Sui-Tat Chui and John Q. Xiao

University of Delaware, Newark, Delaware

Tunable Magnetic Resonance in Microwave

Spintronics Devices

NASA/TM—2015-218737

April 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

2015 International Microwave Symposium

sponsored by the Institute of Electrical and Electronics Engineers

Phoenix, Arizona, May 17–22, 2015
Acknowledgments

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contract number NNX11AQ29A.

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Supplementary Notes

The fi gures are archieved under E–19066.
NASA/TM—2015-218737 1 

Tunable Magnetic Resonance in Microwave Spintronics Devices 

Yunpeng Chen 

University of Delaware 

Newark, Delaware 19716 

Xin Fan 

University of Denver 

Denver, Colorado 80208 

Yunsong Xie, Yang Zhou, and Tao Wang

University of Delaware 

Newark, Delaware 19716 

Jeffrey D. Wilson and Rainee N. Simons 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Sui-Tat Chui and John Q. Xiao

University of Delaware 

Newark, Delaware 19716

Abstract 

Magnetic resonance is one of the key properties of magnetic 

materials for the application of microwave spintronics devices. 

The conventional method for tuning magnetic resonance is to 

use an electromagnet, which provides very limited tuning 

range. Hence, the quest for enhancing the magnetic resonance 

tuning range without using an electromagnet has attracted 

tremendous attention. In this paper, we exploit the huge 

exchange coupling field between magnetic interlayers, which is 

on the order of 4000 Oe and also the high frequency modes of 

coupled oscillators to enhance the tuning range. Furthermore, 

we demonstrate a new scheme to control the magnetic 

resonance frequency. Moreover, we report a shift in the 

magnetic resonance frequency as high as 20 GHz in CoFebased tunable microwave spintronics devices, which is 10X 

higher than conventional methods. 

1.0 Introduction 

Spintronics utilizes electron spins in addition to, and 

sometimes in place of, electron charges to achieve new electron 

device functions. The discovery of giant magnetoresistance 

(GMR) in 1988 revolutionized the data storage in hard drives 

(Ref. 1). More recently, the discovery of tunnel magnetoresistance (TMR) phenomena in magnetic tunnel junctions (MTJs) 

accelerated the development of hard drive technology. Besides 

reading out the static magnetic moment (stored information) of 

a nano-magnet in, for example, a computer hard disk, the 

development of spintronics also offers a new scheme to 

manipulate the magnetic moment by means of spin transfer 

torque (STT). The STT transfers angular momentum from spinpolarized charge carriers to local magnetic moments without 

using an electromagnet. Owing to its fast speed, high density, 

and low energy consumption, STT has been utilized in 

developing the next generation of magnetic random access 

memories (MRAM). Another emerging subfield of spintronics 

involves coupling spin transport to spin dynamics. Since spin 

dynamics usually occurs in the gigahertz frequency range due 

to the intrinsic resonance of magnetic materials, spintronics 

devices have found applications at microwave frequencies, for 

example in microwave generators, detectors, and phase shifters 

(Refs. 2 to 4). 

The typical spintronic devices include spin valves (SVs) 

and MTJs, which are respectively, made of a nonmagnetic 

metal or insulating barrier sandwiched between two 

ferromagnetic (FM) layers as illustrated in Figure 1(a). One of 

the FM layers usually has its magnetic moment (Mfixed) fixed 

and aligned with the adjacent antiferromagnetic (AFM) layer 

through exchange bias. The other layer’s magnetic moment 

(Mfree) is free to rotate with the magnetic fields. The electric 

resistance of the structure is determined by the relative angle 

between Mfixed and Mfree. When a current is flowing through the 

stack, the electron spins are polarized along the direction of 

Mfixed (Fig. 1(a)). If Mfree is not collinear with Mfixed, the electrons 

with polarized spins passing through the free layer will align 

with Mfree due to exchange interaction. By the conservation of 

angular momentum, the change of the carrier spin momentum 

will transfer to Mfree, which is the spin transfer torque. 

To generate the spin current without passing a charge current 

through a magnetic layer, one can pass a spin current through a 

material with strong spin orbital coupling (SOC) such as heavy 

metals (HM) like Pt or topological insulators (Ref. 5). Due to 

SOC, the electrons with opposite spins will be separated in the 
NASA/TM—2015-218737 2 

direction transverse to the electric current (Fig. 1(b)), which is 

known as the spin Hall effect (SHE). The transverse spin 

current diffuses into the adjacent FM layer and manipulates its 

magnetic configuration as illustrated in Figure 1(b). The STT 

(TST) is aligned with the damping torque (Tdamping). Depending 

on the sign of the injected spin, the STT will either enhance the 

damping or act as an anti-damping torque. STT is able to reduce 

the device noise by compensating the damping, exciting 

magnetic resonance and emitting microwaves by sustaining 

magnetization precession. Figure 1(c) illustrates the principle 

of a nano-STT-microwave oscillator. The spintronics devices 

can also work as nano-microwave detectors based on the spin 

rectification effect. As depicted in Figure 1(d), the 

magnetization precession is excited through the interaction of 

microwave magnetic or electric fields. In a SV or an MTJ, the 

rotation of free layer magnetization will cause a time variant 

resistance. The ac resistance rectifies the microwave field, 

resulting in a dc voltage output. Moreover, the microwave field 

can be used to excite a spin wave (the wave of angular 

momentum oscillation) in a spin-wave logic device with low 

operating energy (Fig. 1(e)), which has the potential to be more 

energy efficient than the conventional complementary metal 

oxide semiconductor (CMOS) technology. 
NASA/TM—2015-218737 3 

In conventional tunable microwave spintronic devices, the 

magnetic resonance is controlled by an external applied 

magnetic field, which is generated by a bulky and energy 

inefficient electromagnet. To integrate these devices and to 

lower the power consumption, it is desirable to eliminate the 

use of an electromagnet. Researchers have demonstrated that 

the magnetic resonance in piezoelectric/ferromagnetic 

multiferroic heterostructures can be controlled by an external 

electric field. However, no material satisfies both tunable range 

and microwave performance simultaneously. This paper reports 

a new technique to control the magnetic resonance, which is 

demonstrated by exploiting the huge interlayer exchange 

coupling field present between the device magnetic layers and 

the high frequency modes of coupled oscillators. 

2.0 Magnetic Resonance Modes in 

Exchange Coupled Magnetic 

Bilayer 

Exchange interaction exists inside every magnetic material 

and also at the interface of two different magnetic materials. In 

a typical ferromagnetic material, the exchange field arising 

from the Pauli Exclusion Principle can be as high as 103

 T, 

equivalent to a 28 THz frequency range. The idea of using two 

coupled layers FM1 and FM2 resembles two coupled 

oscillators, which exhibit low frequency acoustic and high 

frequency optical modes, and the sensitivity of the latter will 

depend on the coupling at the interface. We model the interface 

arising from the dominating exchange coupling between two 

nearest neighboring layers and neglect the much smaller dipolar 

interaction. The interlayer exchange energy per unit area is 

described as 

1 2

1 2

Ms Ms

M M E J   

 

 (1) 

where M1

 and M2

 are the magnetization vectors of FM1 and 

FM2, Ms1 and Ms2 are the saturation magnetizations of FM1 and 

FM2, respectively, and J is the interface exchange coupling 

strength. The coupled magnetization dynamics can then be 

described by the modified Landau-Lifshitz-Gilbert (LLG) 

equation as 

  dt

dM M

M

M H H

dt

dM i

i

s i

i

i eff i e i

i

     

      

_

0 _ _ (2) 

where  is the gyromagnetic ratio of 28 GHz/T, Heff _ i

 is the 

effective field due to other atomic layers on the i-th layer 

including the external field, anisotropy field and demagnetizing 

field, He _ i

 is the effective exchange field on the i-th layer, i

is the damping of the i-th layer, and Ms _ i is the saturation 

magnetization of the i-th layer. 

We are able to simulate the permeability spectrum of the 

exchange coupled layer. Indeed, when the interface exchange 

coupling is finite but much weaker than the intralayer exchange 

coupling inside each ferromagnetic layer, the bilayer exhibits 

two microwave-resonance modes as illustrated in Figure 2. The 

two modes are the acoustic mode with the magnetizations in 

two layers precessing in-phase and the optical mode with the 

two magnetizations precessing out-of-phase. In a simplified 

example, where FM1 and FM2 are identical with negligible 

anisotropy, i.e., Ms1  Ms2  Ms and d  d  d 1 2 , the two 

resonance modes can be calculated as 

  Heff Heff  Ms acoustic  0 and (3) 

















 

 



















   eff s

s

eff

s

H M

M d

J H

M d

J

0 0

optical 0

2 2 (4) 

In the above equations Heff is the effective magnetic field. The 

optical mode has a higher resonance frequency than the acoustic 

mode from the contribution of an additional effective exchange 

field J M d 0 s 2  . The resonance frequency of the optical mode 

can be shifted by the change of the interlayer exchange 

coupling. 
NASA/TM—2015-218737 4 

3.0 Electric Field Tuned Magnetic 

Resonance in Multiferroic 

Heterostructures 

The application of an electric field induces strain in 

piezoelectric materials. The strain-induced magnetoelectric 

(ME) interaction was recently demonstrated to be a practical 

method to tune the magnetic resonance in layered multiferroic 

heterostructures (Refs. 8 to 14). A comparison of electric field 

induced resonant field shift in various multiferroic 

heterostructures is summarized in Table I. A typical 

experimental configuration is illustrated in Figure 3(a). FM 

layers were deposited on a piezoelectric substrate, such as 

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3–

PbTiO3 (PZN-PT). The magnetic resonance was detected by 

microwave transmission. In the presence of an electric field, the 

piezoelectric substrate will strain the FM layers, which 

introduces an effective anisotropy field due to the 

magnetostrictive effect. The largest strain-induced effective 

magnetic field of 3500 Oe was achieved in terfenol-D/PZN-PT 

(Ref. 6) because of the high magnetostriction of terfenol-D. 

However, the ferromagnetic resonance (FMR) linewidth was 

also large indicating enormous loss. Besides the 

magnetrostrictive effect, the change of interlayer exchange 

coupling may also contribute to the shift of resonance frequency, 

because the exchange coupling will be affected by the strain. 

We deposited Co10Fe90 (40 nm)/Ni80Fe20 (40 nm) on PZNPT(011). We observed an effective magnetic field of 820 Oe 

when we applied a bias electric field as shown in Figure 3(b) 

and (c). The effective magnetic field is also known as the 

tunable magnetic field or the electric field induced magnetic 

anisotropy field (ΔH). The control sample with a single CoFe 

layer on PZN-PT shows a smaller effective magnetic field of 

600 Oe. The 220 Oe difference is due to the change of interlayer 

exchange coupling. Therefore, the interlayer exchange coupling 

is not efficiently tuned by strain. A much more effective 

controlling method is demonstrated in the next section. 
NASA/TM—2015-218737 5 

4.0 Giant Resonance Frequency 

Tunability Using Exchange 

Coupling 

To efficiently tune the resonance frequency of the optical 

mode, one can adjust the interlayer exchange coupling J by 

engineering the interface. A tantalum (Ta) spacer layer was 

deposited in between two FM layers. The interlayer exchange 

coupling can be adjusted by the thickness of the Ta layer. 

Although the magnetization precessions of the two magnetic 

layers are out-of-phase at the optical mode, it is still suitable for 

the application of spintronics microwave devices, because the 

ferromagnetic layer near the nonmagnetic/insulator interface 

governs the performance of the devices (Ref. 9). We use a spin 

valve based microwave detector to demonstrate the effect of the 

optical mode on microwave detection. As shown in Figure 4(a), 

the detector is a strip of spin valve (SV), in which the free 

magnetic layer is composed of an exchange coupled 

ferromagnetic bilayer with a discontinuous nonmagnetic layer 

of varying thickness in between to tune the strength of exchange 

coupling, Cu (2 nm)/Co10Fe90 (4 nm)/Ta (0.4-2.0 nm)/Ni80Fe20 

(4 nm)/Cu (3 nm)/Co10Fe90 (4 nm)/IrMn (15 nm)/Ta (5 nm). 

The SV is patterned as a part of a coplanar waveguide (CPW), 

which guides microwave fields into the SV detector. The 

microwave magnetic field excites the acoustic mode as well as 

the optical mode in the bilayer. The precession of the 

magnetization in the bilayer results in an SV with ac resistance 

that rectifies the microwave field and generates a dc voltage 

analogous to the spin-torque diode microwave detectors 

(Ref. 7). In this case, only the NiFe layer that is adjacent to the 

3 nm Cu layer contributes to the resistance change in the SV. 

The dc voltage is proportional to the microwave power and 

attains a maximum value at the acoustic mode and the optical 

mode resonances. 

The measured voltage peaks corresponding to the acoustic 

mode and the optical mode with opposite phase are shown in 

Figure 4(b). In this figure the red arrows indicate the optical 

mode and the blue arrows indicate the acoustic mode. By 

varying the thickness of the discontinuous Ta spacer layer, the 

resonance frequency due to the optical mode can be shifted 
NASA/TM—2015-218737 6 

TABLE I.—COMPARISON OF ELECTRIC FIELD INDUCED CHANGE OF THE 

MAGNETIC ANISOTROPY FIELD (ΔH) AND MAGNETOELECTRIC 

COUPLING COEFFICIENT (ΔH/ΔE) IN VARIOUS 

MULTIFERROIC HETEROSTRUCTURES 

Structure H

(Oe) 

H/E 

(Oe cm kV–1) 

Linewidth Reference 

CoFe/NiFe/PZN-PT 820 126 170 Oe at 14.5 GHz This study 

CoFe/PZN-PT 600 96 90 Oe at 13 GHz This study 

Terfenol-D/PZN-PT 3500 580 1200 Oe at 9.3 GHz (6) 

NiFe/PMN-PT 375 60 50 Oe at 9.5 GHz (9) 

FeCoHf/PZN-PT 500 82 25 Oe at 11.85 GHz (10) 

FeGaB/PMN-PT 330 33 50 Oe at 9.5 GHz (11) 

FeGaB/PZN-PT 750 94 50 Oe at 9.6 GHz (12) 

FeO/PZN-PT 600 100 400 Oe at 9.3 GHz (13) 

YIG/PMN-PT 40 5.4 6 Oe at 9.3 GHz (14) 

from 5 to 25 GHz as shown in Figure 4(c). The corresponding 

effective field change is 4000 Oe, which is much larger than the 

strain-induced effective field listed in Table I. The magnitude 

of the resonance peaks scale inversely with the resonance 

frequency following the nature of magnetic resonance, as 

shown in the inset of Figure 4(c). For in-situ tuning of interlayer 

exchange coupling, one can replace the Ta spacer layer with a 

phase transition material, such as NiCu alloy, which is sensitive 

to temperature or mechanical strain (Ref. 8). 

5.0 Conclusion 

We have demonstrated a technique to significantly enhance the 

ferromagnetic resonance frequency by evoking the exchange 

mode in magnetic bilayers. The ferromagnetic exchange 

coupling in magnetic bilayers enables an extremely wide 

frequency tuning range. We report a technique to shift the 

magnetic resonance frequency as high as 20 GHz in CoFe-based 

microwave spintronic devices, which is 10X higher than 

conventional techniques (Ref. 9). Such a wide tuning range is 

normally not achievable with applying an external magnetic field 

or by other known tuning methods. Although the demonstrations 

are done with thin film structures, which are already applicable 

to a plethora of spintronics devices, the concept can be extended 

to thick materials with multiple layers, creating new avenues for 

magnetic materials based microwave devices. 

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