3814 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012
The Dependence of Vortex Oscillation Frequency on SmallIn-Plane Magnetic Fields in Spin-Valve NanocontactsMoritz Eggeling, Theodoros Dimopoulos, Rudolf Heer, and Hubert Brückl
Austrian Institute of Technology—AIT, Health & Environment, 1220, Vienna, Austria
In this work we investigate the magnetic field dependence of the precession frequency of vortex states in spin-valve nanocontacts withan amorphous CoFeB free layer and an artificial antiferromagnet as polarizer. The nanocontacts have radii between 70 and 90 nm. Weshow that the excitation frequency in these devices responds to small, in-plane magnetic fields along the easy and hard axis directions.The characteristics of the frequency response depend on the generated magnetic configuration under the nanocontact. This, in turn,results from the combined effect of the applied magnetic field and the current-generated Oersted field. Taking also into account therelative large nanocontact radii, a variety of vortex excitation modes can arise with distinctive frequency versus field responses, some ofwhich could be considered for magnetic field sensing applications.
Index Terms—Artificial antiferromagnet, nanocontact, vortex precession.
I. INTRODUCTION
I N a magnetic multilayer, a spin-polarized current cantransfer spin angular momentum, driving steady state
magnetic excitations [1]–[4]. Apart from the broadly inves-tigated coherent dynamic precession of the magnetization[5], [6], another type of excitation, seen both in nanopillar[7] and nanocontact (NC) [8], [9] device architectures, is thegyroscopic precession of a magnetic vortex. It takes placein the subgigahertz to GHz frequency regime, periodicallymodulating the voltage drop of either a giant magnetoresistance(GMR) or tunneling magnetoresistance (TMR) multilayer.Vortex excitation is observed at low current densities and also
without the application of an external magnetic field [10]–[12].The excitation is associated with high emitted power and narrowlinewidth [13]. Interest has grown further with the demonstra-tion of phase-locking in double-point contact [14], [15] andmore recently with the phase synchronization of vortices in anarray of four nanocontacts [16], leading to significant poweramplification compared to a single contact. The precession fre-quency, , of spin-torque excitations is shown to be influencedby the direct current density, , the nanocontact radius, , [17],[18] and the applied magnetic field, . These devices have ahigh technological potential for the realization of nano-sized os-cillation sources [19], for fast and none-volatile memory-cells[20], as well as for magnetic field sensing in the nano-scale [21].In this work we extend our previous investigations [12], [18]
on vortex excitations in spin-valve nanocontacts to include theinfluence of small easy- and hard-axis magnetic fields on theprecession frequency. The magnetic field dependence of the fre-quency is particularly important for magnetic field sensing ap-plications, as it has been proposed in [21].
II. EXPERIMENTAL DETAILS
The NCs were fabricated on top of spin valve mesas,100 100 m in size, of the multilayer sequence: MgO(5)/Ru(30) / Co Fe (3) / Ru(0.8) / Co Fe (2.2) / Cu(5) /
Manuscript received March 02, 2012; revised April 24, 2012; accepted May18, 2012. Date of current version October 19, 2012. Corresponding author:M. Eggeling (e-mail: [email protected]).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.2012.2201456
Co Fe B (2.5)/Ta(2)/Ru(2.5). The subscripts stand forcompositions in atomic percent and the layer thickness denotedin parentheses, in nm. The amorphous CoFeB layer is the freelayer (FL) and the CoFe/Ru/CoFe artificial antiferromagnet(AAF) the magnetically rigid polarizer. The fabrication andmeasurement setup details are presented elsewhere [12]. Theinvestigated devices have NC radii between 70 and 90 nm (nm). These values have been extracted from scanning electronmicroscopy images.
III. RESULTS AND DISCUSSION
In the investigated NCs magnetization excitation takes placeonly for one applied current polarity, the one in which electronsflow from the FL to the AAF [12]. Low critical current den-sity for the excitation onset and reversibility of the dynamicspectra with respect to the current were obtained. The spec-tral power and linewidth have been shown to depend on thein-plane magnetic field, assuming maximum, respectively min-imum, values within the FL’s magnetization switching. For cer-tain field and current windows, metastable dynamic states wereclearly demonstrated as a result of the highly inhomogeneousmagnetization profile below the NC [12]. Such dynamic states,in the form of vortex-antivortex pairs, have been considered forinstance in the work of Berkov and Gorn [24]. The influence ofthe FL’s magnetic moment and of the NC size on the excitationdynamics has been also reported.To investigate the frequency response to the magnetic field
we firstly concentrate on a NC with a radius of nm. Theminor magnetoresistance (MR) loops (FL switching) for fieldapplication along the hard (HA) and easy axis (EA) of the FL’smagnetization are shown as insets in Fig. 2(b) and (c), respec-tively, measured for small applied currents ( 1 mA).In both measurements the AAF net moment is fixed along
the FL’s easy axis. This means that when the FL is switchedalong its HA, the measured R is only half of the one obtainedfor EA switching. The application of larger currents through theNC results to the decrease of the MR effect and a deformationof the MR loop, as the current-generated Oersted field graduallydevelops a vortex-like state beneath the NC.According to the rigid vortex model based on the Thiele equa-
tion, the spin-transfer-torque force counteracts the damping and
EGGELING et al.: DEPENDENCE OF VORTEX OSCILLATION FREQUENCY ON SMALL IN-PLANE MAGNETIC FIELDS 3815
Fig. 1. Sketch of the multilayer stack with an artificial antiferromagnet as po-larizer and a CoFeB free layer.
Fig. 2. (a) Frequency versus power spectral density (PSD) measurements fordifferent magnetic field values in hard-axis directions for an elements with ananocontact radius of 70 nm. Frequency dependence on magnetic fields in hard-(a) and easy- (b) axis directions. The insets show the corresponding hard andeasy-axis minor loops, measured for small currents (I 1 mA).
sets the amplitude of the orbit. On the other hand, the gyrotropicforce counteracts the restoring force caused by the Oersted field.The balance of these two forces determines the frequency ofmotion [13], [22], [23]. In the relatively large NCs studied here,various magnetization states with different degree of nonuni-formity can arise, which may differ considerably from the idealvortex state assumed in the model of Thiele. The applied mag-netic field is adding to the complexity of the system, competing
Fig. 3. Frequency dependence on magnetic fields applied along the easy-axisdirection for an element with a radius of 90 nm (a) and 75 nm (b). The in-sets show the corresponding easy-axis minor loops, measured for small currents(I 1 mA).
with the Oersted field influence which tends to stabilize thevortex state.Fig. 2(a) depicts excitation spectra measured for specific
magnetic field values along the FL’s HA. We observe thatlower excitation power density leads to larger linewidth. Thisis related to the fact that for each applied field, the maximumexcitation power density is obtained for a different currentvalue, whereas the measurements in Fig. 2(a) were all obtainedat mA. The versus dependence is shown inFig. 2(b). It presents a maximum for Oe and de-creases almost linearly away from this field, with slopes of
MHz/Oe. For EA fields f decreases with H in the windowfrom to Oe. The slope is nevertheless smaller than forHA fields. Similar trends, relevant to magnetic field sensing,were also observed in [10] and are about 5 times smaller thanthe ones extracted for nanopillar geometries [20].Although the majority of NCs present the aforementioned
f(H) dependence, the slope of the frequency decrease versusfield can vary considerably from one NC to the other, as canbe seen in Fig. 3(a) for a NC with a radius of 90 nm. Interest-ingly, there are even cases, like the one in Fig. 3(b), where thef(H) dependence is reversed, i.e., the frequency as a functionof the field presents a pronounced minimum at a field value ofapproximately 20 Oe. Moreover, the field regime for which ex-citations are observed can vary considerably, even for NCs ofsimilar size [compare Fig. 2(c) with 3(b)].
3816 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012
Fig. 4. Frequency dependence on magnetic fields applied along the easy-axisdirection for different currents and (a) nm and (b) nm.
If we assume that the different observed trends stem from thecompeting action of the external magnetic field and the Oer-sted field, then we expect that larger currents would decreasethe influence of the external magnetic field on the precessionfrequency.Fig. 4 demonstrates the effect of increasing current on the
dependence for the 70 and 75 nm NC and EA magneticfields. The trend is particularly obvious for the 70 nm NC. The
slope gradually decreases with increasing until finallythe frequency is almost constant over the field. The increase offrequencywith the current is predicted fromThiele’s model. Ad-ditionally, the magnetic field regime for which excitation can beobserved is significantly enhanced with increasing current. Thisis anticipated since the stabilization of the vortex state beneaththe nanocontact with increasing Oersted field will lead to a de-creased interaction with the external field. For the 75 nm NCthe previous observations are applied, although the trend of de-creasing f(H) slope with increasing I can only be safely deducedfor decreasing fields (left arm relative to the minimum).Finally, a comment about the possibility to use these devices
as magnetic field sensors. Today, a signal-to-noise ratio (SNR)in the range of 27–33 dB can be achieved with conventionalMR sensors. Following Braganca’s approach [21] and assumingfor the present devices a frequency shift in the order of 100MHz as response to small, in-plane magnetic fields, a maximumlinewidth in the order of 5 MHz would have to be achieved in
order to reach SNR of 27 dB. A significant improvement of thedevice oscillation quality is therefore necessary for magneticfield sensing.
IV. CONCLUSION
Frequency spectra measurements have been obtained as afunction of the applied current and magnetic field for varioussizes of spin-valve nanocontacts, employing a CoFeB free layerand an artificial antiferromagnet as polarizer. The majority ofnanocontacts show a decreasing tendency of the frequencywith the applied field. However, the slope of the linear decreasevaries considerably from one NC to the other and in somecases the frequency dependence on the field is reversed. Thesedeviations stem from the combined effect of the applied fieldand the Oersted field, which give rise to different vortex-likemagnetization states with different degree of nonuniformitybeneath the NC. We have shown that small variations in thedevice size can lead to drastically different frequency versusfield responses in relatively large NCs. By increasing thecurrent, a the vortex state is stabilized and the field dependenceof the frequency is reduced, while the precession persists forlarger field windows.
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