IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011 1575
Modulation of Individual and Mutually Synchronized
Nanocontact-Based Spin Torque Oscillators
P. K. Muduli, Ye. Pogoryelov, F. Mancoff, and J. Åkerman
Physics Department, University of Gothenburg, 412 96 Gothenburg, Sweden
Materials Physics, Royal Institute of Technology, Electrum 229, 164 40 Kista, Sweden
Everspin Technologies, Inc., Chandler, AZ 85224 USA
We study current modulation of spin torque oscillators (STOs) based on single and double nano-contacts. For single nano-contact
devices, we show modulation for a carrier frequency of 19–40 GHz and modulation frequencies from 50 MHz to 1 GHz. We show that
the behavior of modulation and the asymmetry in the power of the modulation sidebands can be very well modeled, and predicted,
using nonlinear frequency and amplitude modulation (NFAM) calculations based on the free-running STO parameters. Modulation of
synchronized double nano-contact STOs is equally well modeled by NFAM theory and demonstrates the feasibility of communication
applications based on arrays of synchronized STOs.
Index Terms—Modulation, nonlinear frequency and amplitude modulation, spin torque oscillators, synchronization of spin torque
oscillators.
I. INTRODUCTION
S PIN transfer torque (STT) allows for manipulation ofmagnetic moments in nano-sized magneto-resistive de-
vices using direct current instead of magnetic field. In certain
sample geometries and/or applied fields, STT can also maintain
a steady precession of one or more magnetic layers [1], [2].
As a consequence, the device resistance undergoes oscillation,
either through giant magnetoresistance (GMR) or tunneling
magnetoresistance (TMR), producing an output voltage signal
in the microwave frequency range. These so-called spin torque
oscillators (STOs) are typically based either on nano-contacts
on top of GMR stacks [3]–[5], or nano pillars of GMR or
TMR stacks [6]. While the first STOs had both magnetic
layers magnetized in the film plane, a wide range of additional
architectures have recently been proposed [7]–[9] and exper-
imentally realized [10]–[12]. In this study we focus on the
rather traditional nano-contact type GMR-STO devices based
on in-plane magnetized CoFe/Cu/NiFe trilayers.
Interest in STOs is rapidly increasing [13] due to their poten-
tial use for microwave generation over large frequency ranges
[14]–[16], their extremely small footprint, and the straightfor-
ward integration with CMOS technology using well-established
MRAM fabrication and integration processes. STOs can also be
used as analog modulators [17]–[19], and detectors [20], [21],
which are important demonstrations for future communication
applications. In 2005, Pufall et al. [17] first reported frequency
modulation in a nano-contact based STOs. They showed a non-
linear frequency modulation which results in asymmetry of the
power of sidebands. In a recent study [18] we showed that the
quantitative values of asymmetry of the power of sidebands
can be explained by using a combined nonlinear frequency and
Manuscript received September 30, 2010; accepted November 23, 2010. Date
of current version May 25, 2011. Corresponding author: J. Åkerman (e-mail:
johan.akerman@physics.gu.se).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2010.2096463
Fig. 1. Schematic diagram of (a) single nano-contact and (b) two nano-contact
devices. A device is composed of a fixed layer, Co Fe that serves as a polar-
izer (spin polarizing the conduction electrons) and a free layer, Ni Fe whose
magnetization is excited into steady state oscillations. The two-nano contact de-
vices can function as two independent oscillators or a single oscillator depending
on the operating conditions and separation between the nano-contacts (see text).
amplitude modulation (NFAM) theory [22]. The complex dy-
namics of the modulation of STOs can be modeled, and pre-
dicted, using the unmodulated properties of the STOs. In this
work, we extend our previous study [18] to higher modulation
frequencies, higher operating frequency of the STO, and to a
case of mutually synchronized STO pairs. We show modula-
tion of STOs and quantitative agreement with NFAM theory for
modulation frequencies up to 1 GHz and modulation of a STO
operating at 40 GHz. We also show modulation of STO pairs
and demonstrate that mutually synchronized state of the STO
pairs can be treated as a single oscillator.
II. EXPERIMENT
The schematics of the devices studied in this work are
shown in Fig. 1. We will discuss modulation of three dif-
ferent devices. The first two devices are single nano-contact
devices with diameter and 100 nm. The third de-
vice is composed of two circular nano-contacts of 80 nm
diameter each, separated by a distance of 400 nm. The
nano-contacts were fabricated through a SiO insulating
layer, onto a m pseudo-spin-valve mesa using
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