Patent Publication Number: US-8525602-B2

Title: Magnetic device with weakly exchange coupled antiferromagnetic layer

Description:
TECHNICAL FIELD 
     The disclosure relates to magnetic devices, such as magnetic devices that may be used in spin torque oscillators. 
     BACKGROUND 
     Some spin torque oscillators (STOs) may detect the presence and strength of a magnetic field due to changes in a magnetic oscillation of the STO that is induced in a free layer by a spin torque of the magnetic field. 
     SUMMARY 
     In one example, a magnetic device is provided that comprises a free layer having a magnetic anisotropy, wherein the magnetic anisotropy is at least partially non-uniform. The magnetic device further comprises an antiferromagnetic layer adjacent to and weakly exchange coupled with the free layer, wherein the weak exchange coupling reduces the non-uniformity of the magnetic anisotropy of the free layer. 
     In another example, a method for manufacturing a magnetic device is provided. The method may comprise forming a free layer having at least a partially non-uniform magnetic anisotropy and forming an antiferromagnetic layer adjacent to the free layer, wherein the antiferromagnetic layer reduces the non-uniformity of the magnetic anisotropy. 
     According to another example, a system is provided that comprises a circuit and a spin torque oscillator coupled to the circuit. The spin torque oscillator may comprise a free layer having at least a partially non-uniform magnetic anisotropy. The spin torque oscillator may further comprise at least one antiferromagnetic layer weakly exchange coupled to the free layer, wherein the weak exchange coupling causes the antiferromagnetic layer to reduce the non-uniformity of the magnetic anisotropy. A current may induce a magnetic field in the free layer, and the spin torque oscillator provides a signal related to the magnetic field to the circuit. 
     The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating one example of a device comprising a spin torque oscillator including a magnetic device comprising at least one antiferromagnetic layer weakly exchange coupled to a free layer. 
         FIGS. 2A-2D  are block diagrams illustrating example magnetic devices including at least one antiferromagnetic layer weakly exchange coupled to a free layer. 
         FIG. 3  is a flowchart illustrating an example method for manufacturing a magnetic device including at least one antiferromagnetic layer weakly exchange coupled to a free layer. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale and are drawn to emphasize features relevant to the present disclosure. Like reference characters denote like elements throughout the figures and text. 
     DETAILED DESCRIPTION 
     In some magnetic devices, the introduction of current through the device causes magnetic moments in a free layer to reverse, such that the magnetization of the free layer oscillates. This reversal of magnetic moments may be referred to as spin precession. Spin precession of the free layer can generate power, which can be used for various purposes, such as to support relatively high bandwidth wireless communication. Uneven precession of the free layer can reduce the power output of the magnetic device. Example magnetic devices described herein include at least one antiferromagnetic layer that may help improve the uniformity of the precession of the free layer, which may help increase power output of the magnetic devices. The antiferromagnetic layer can be positioned adjacent to the free layer, such as above, below, or along an outer perimeter of the free layer, and, as described below, is weakly exchange coupled to the free layer. The presence of the antiferromagnetic layer can improve the uniformity of the magnetization of the free layer, which supports a more uniform precession. In this way, the antiferromagnetic layer may cause the spin precession of the free layer to be more even and steady compared to examples in which the magnetic device does not include the weakly exchange coupled antiferromagnetic layer. 
     Spin torque oscillators can be subject to errors, including timing jitter and phase noise with respect to induced and desired spin-precession response. These errors may result from edge pinning effects in the free layer of the STO. The timing jitter indicates cycle-to-cycle time lags and variability in the oscillation of the magnetization. This variability can degrade the desired oscillation performance and power transfer characteristics. These errors and non-uniformities may decrease the reliability and accuracy of the STO. 
     In addition to increasing the power output of the magnetic device, an improvement in the uniformity in the spin precession of the free layer can reduce phase noise. The phase noise may result from different atoms of the free layer rotating magnetic moments out of synch. For example, some atoms of the free layer may lead the rotation of magnetic moment, while other atoms may lag. The introduction of the at least one antiferromagnetic layer that is weakly exchange coupled to the free layer may help the atoms of the free layer rotate in synch, such that the rate of precession of the free layer is more constant. In addition, increasing the uniformity of spin precession of the free layer may also reduce timing jitter. In some examples, a reduction in timing jitter and phase noise may be achieved by defining the material structure and geometry of the magnetic device to support a single, dominant magnetization-reversal and precessional mode. 
     In some examples, the uniformity of precession of the free layer is improved through the definition of an easy-axis canting in the free layer. For example, a bias (easy-axis canting) is defined, for example, for an in-plane structure or for a perpendicular structure (e.g., a free layer) of the magnetic device. This easy-axis canting can help improve coherent precession and magnetization swing of the structure with more uniform anisotropy and weakly exchange coupling the free layer to an antiferromagnetic layer than in magnetic devices without an antiferromagnetic layer weakly exchange coupled to a free layer. In some examples, the magnetic device includes a circular, elliptical, or cylindrical geometry, which may reduce demagnetization. In addition, in some examples, the magnetic device includes a perimeter layer positioned around an outer perimeter of the device, which may help reduce pinning and coupling at the periphery of the device to reduce the likelihood or strength of secondary precession modes. 
       FIG. 1  is a block diagram illustrating one example of a system  2  comprising a spin torque oscillator (STO)  4  including a magnetic device  6  comprising at least one antiferromagnetic layer  8  weakly exchange coupled to a free layer  10 . System  2  may further include a first circuit  12 , a second circuit  14 , and one or more processors  16 , and a current source  18 . In some examples, system  2  may be one or more devices. Examples of system  2  may include any computing device (e.g., a desktop personal computer (PC), a mobile device, a tablet PC, and the like), a wireless device (e.g., a cell phone, radio, and the like), a satellite communications device, a radar device, as well as any other device or system where STO  4  may be utilized. 
     Spin torque oscillator  4  may detect the presence of a magnetic field via magnetic device  6 . Magnetic device  6  may detect a magnetic field through a change in precessional oscillation frequency of a magnetization of free layer  10 . In some examples, magnetic device  6  may be a magnetoresisitve sensor, including at least one of a giant magnetoresistive (GMR) device or a tunneling magnetoresistive (TMR) device. In other examples, magnetic device  6  may be any other type of magnetic device or sensor. 
     In one example, antiferromagnetic (AFM) layer  8  of magnetic device  6  is adjacent to free layer  10 . As used herein, adjacent layers may refer to two layers at least partially, approximately physically contacting each other, such as directly adjacent to each other. AFM layer  8  may be composed of an antiferromagnetic material. Antiferromagnetic materials are materials in which the magnetic moments of atoms, ions, or molecules in the material tend to assume an ordered arrangement in the absence of an applied magnetic field. In some examples, antiferromagnetic layer  8  may be a weak antiferromagnetic layer. 
     In some examples, free layer  10  is composed of a material having low coercivity, such that free layer  10  may have an easily rotatable or moveable magnetic moment which responds to an external magnetic field (e.g., changes direction in the presence of). In some examples, free layer  10  may be composed of a ferromagnetic material having soft magnetization. 
     Free layer  10  may be a magnetic free layer having a magnetic anisotropy. A material with magnetic anisotropy may have magnetic properties that are directionally dependent. In one example, the magnetic anisotropy is approximately perpendicular to a biasing direction of free layer  10 . A biasing direction may align a magnetic moment along a certain orientation. In other examples, the magnetic anisotropy of free layer  10  may be at another angle with respect to the biasing direction. In some examples, the magnetic anisotropy of free layer  10  is at least partially non-uniform. For example, there may be local variations in the magnetic anisotropy of free layer  10  due to the partially non-uniform magnetic anisotropy. In such an example, non-uniformity of the magnetic anisotropy may arise due to the proximity of other materials to free layer  10 . For example, a pinned layer adjacent to free layer  10  may cause edge pinning effects in free layer  10 , which can distort the magnetic anisotropy of free layer  10  along the surface proximate to the pinned layer. 
     In one example, antiferromagnetic layer  8  is weakly exchange coupled to free layer  10 . Exchange coupling may result when a hard magnetization of an antiferromagnetic material causes a shift in a soft magnetization of a ferromagnetic material. For example, exchange coupling antiferromagnetic layer  8  to free layer  10  results in shifts in the magnetization of free layer  10  along an interface between antiferromagnetic layer  8  and free layer  10 . In one example, weakly exchange coupling antiferromagnetic layer  8  to free layer  10  reduces non-uniformity of the anisotropy in free layer  10 , such as by reducing the dispersion effects of the magnetization of free layer  10 . The exchange coupling between antiferromagnetic layer  8  and free layer  10  is selected to be relatively weak, such that the anisotropy of free layer  10  is maintained. In another example, weakly exchange coupling antiferromagnetic layer  8  to free layer  10  improves the non-uniformity of the anisotropy in free layer  10 . Any two layers described herein as being weakly exchange coupled are weakly exchange coupled similar to antiferromagnetic layer  8  and free layer  10 . 
     A weak exchange bias due to the weak exchange coupling may be present at an interface between antiferromagnetic layer  8  and free layer  10 . For example, non-uniformity of the magnetic anisotropy of free layer  10  may result in a hysteresis loop of free layer  10  having some local variations, non-linearity, or asymmetries. In such an example, the weak exchange coupling may reduce the non-uniformity of the magnetic anisotropy, resulting in a smoother hysteresis loop, without significantly shifting the hysteresis loop. In another example, the weak exchange bias may affect the magnetic anisotropy of free layer  10 . In one example, the weak exchange coupling does not significantly reduce the effects of thermal energy on the anisotropy/or hysteresis switching of free layer  10 . In another example, the weak exchange coupling does not induce magnetic anisotropy in free layer  10 . In a further example, the weak exchange coupling between AFM layer  8  and free layer  10  may not substantially change the anisotropy of free layer  10 . The antiferromagnetic properties of AFM layer  8  can be selected to achieve any of these results of the weak exchange coupling. 
     In one example, a hysteresis loop in a switching field of free layer  10  may not be significantly altered by the weak exchange coupling. In one example, the weak exchange coupling may shift an antiferromagnetic exchange field (H ex ) by anywhere between approximately −1 to 1 Oersted (Oe). In another example, the weak exchange coupling may shift H ex  anywhere between approximately −5 to 5 Oe. In another example, the hysteresis loop is centered around approximately 0 Oe. In other examples, the hysteresis loop is centered around other magnetic field strength values. In one example, the weak exchange coupling between antiferromagnetic layer  8  and free layer  10  may not result in pinning any interfacial spins of free layer  10 . In some examples, the canting angle may be as large as possible without switching the magnetization. 
     Current source  18  may provide a current to STO  4 . Current source  18  may be a battery or any other current source. In one example, the current that is applied to magnetic device  6  is a spin polarized current  19 . STO  4  detects magnetization induced by spin polarized current  19  by using magneto-resistance and magneto-resistive current effects. In one example, spin polarized current  19  is not spin polarized until the current from current source  18  is applied to a layer in magnetic device  6 . In one example, spin polarized current  19  may drive a precession of a magnetization in free layer  10 . The amount of spin polarized current  19  may be selected to compensate for any energy lost in the precession. In another example, the direction of spin polarized current  19  may be selected to influence the direction of the precession of the magnetization in the free layer  10 . 
     The effects of weakly exchange coupling AFM layer  8  with free layer  10  results in improved precession of the magnetization of free layer  10  in the presence of spin polarized current  19 , which can subsequently improve communications in such implementations. In one example, STO  4  may assist in chip-to-chip communications in system  2 , for example, between first circuit  12  and second circuit  14 . As used herein, “chip-to-chip” may refer to any implementation where a number of devices or components within a device are electrically coupled together. Examples of a device may include any circuit, such as first circuit  12  and second circuit  14 , chip, memory, processor, micro-controller, video card, or the like. In some examples, system  2  employs at least one high rate device, for example, with an operating frequency measured in giga-Hertz (GHz) or higher. 
     STO  4  may be a local oscillator for first circuit  12 . In one example, STO  4  may output a signal related to the detected magnetic field to first circuit  12 . In one example, the signal related to the detected magnetic field is a clock signal  11 . First circuit  12  may transfer a signal  13  to second circuit  14 , where clock signal  11  is used as a carrier frequency for signal  13 . In one example, the weak exchange coupling between antiferromagnetic layer  8  and free layer  10  improves the consistency and reliability of clock signal  11 . In one example, clock signal  11  may have a narrow bandwidth, low jitter, and good phase noise properties. In some examples, clock signal  11  may have a bandwidth up to approximately 100 Megahertz (MHz). In one example, clock signal  11  has a bandwidth between approximately 100 kilohertz (kHz) to approximately 100 MHz. Using STO  4  for clocking may eliminate the need to have a local oscillator (LO) circuit coupled to first circuit  12 . 
     In other examples, system  2  may include additional circuitry. For example, system  2  may include more than one spin torque oscillator  4 . For example, system  2  can include a second STO that is coupled to second circuit  14  and used to decode the carrier frequency. In some examples, STO  4  is a spin torque nano-osciallator (STnO). The example of system  2  shown in  FIG. 1  is merely one example of a system that can include magnetic device  6  that includes AFM layer  8  weakly exchange coupled to free layer  10 . For example, in another example of a system that includes magnetic device  6 , current source  18  is located external to system  2 . 
       FIGS. 2A-2D  are block diagrams illustrating example magnetic devices including at least one antiferromagnetic layer weakly exchange coupled to a free layer. Schematic cross-sectional views of the magnetic devices are depicted in  FIGS. 2A-2D . The magnetic devices may have any suitable shape.  FIG. 2A  depicts one example of a magnetic device  20  which may comprise an antiferromagnetic layer  8  weakly exchange coupled to a free layer  10  along an interface  23  between layers  8 ,  10 . Because antiferromagnetic layer  8  may be weakly exchange coupled to free layer  10 , the magnetic anisotropy in free layer  10  is more uniform compared to magnetic devices that do not include an AFM layer  8  or an AFM layer  8  weakly exchange coupled to free layer  10 . In one example, edge pinning effects on free layer  10  are reduced along interface  23  due to the weak exchange coupling with AFM layer  8 . The reduction of the edge pinning effects on free layer  10  may enable free layer  10  to precess more freely, which may help increase the uniformity of the precession of free layer  10 . 
     In one example, an easy-axis is defined which may be canted to support uniform magnetization and coherent reversal and precession for in-plane or perpendicular magnetization. In one example, a magnetization in free layer  10  is canted, either out of the plane or rotated in the plane of the free layer  10 , or any angle between. In another example, a canted easy-axis may be defined in a pinned layer. In one example, the weak AFM coupling may be applied to any orientation of the magnetization. 
     In one example, free layer  10  may be a permalloy, that is, composed at least partially of nickel-iron (NiFe). For example, free layer  10  may be approximately 80% Ni and 20% Fe. Additional examples of free layer  10  may be composed of other NiFe compositions or of other suitable materials. In one example, free layer  10  may be approximately 1 to 5 nanometers (nm) thick. In other examples, free layer  10  may be other thicknesses. 
     Examples of antiferromagnetic layer  8  may be composed of manganese (Mn), platinum manganese (PtMn), iridium manganese (IrMn), nickel manganese (NiMn), iron manganese (FeMn), palladium platinum manganese (PdPtMn), or combinations thereof. In other examples, AFM layer  8  may be composed of other materials. In some examples, AFM layer  8  may be approximately 1 to 5 nm thick. In other examples, AFM layer  8  may be other thicknesses. 
     Magnetic device  20  may further comprise a pinned layer  22 . Pinned layer  22  may be a reference layer having an approximately fixed magnetic moment. Examples of pinned layer  22  may be composed of at least one of nickel-iron (NiFe), nickel-iron alloys, iron cobalt (FeCo), nickel iron cobalt (NiFeCo), or combinations thereof. Other suitable materials can also be used for pinned layer  22 . In some examples, the magnetization of free layer  10  is biased with respect to the fixed magnetic field of pinned layer  22 . Magnetically biasing free layer  10  enables the magnetization of free layer  10  to precess due to spin torque oscillation. Pinned layer  22  can have a greater thickness than free layer  10  in some examples. In one example, pinned layer  22  may be approximately 1 to 20 nm thick, such as approximately 3 to 20 nm thick. In other examples, pinned layer  22  may be other thicknesses. In some examples, the thickness of pinned layer  22  may be selected for the strength of the switching field; a thicker pinned layer  22  leads to a greater switching field. 
     In the example of  FIG. 2A , free layer  10  is located between antiferromagnetic layer  8  and pinned layer  22 . That is, antiferromagnetic layer  8  may be located on a first side of free layer  10  and pinned layer  22  may be located on a second side of free layer  10  opposite the first side. In one example, antiferromagnetic layer  8  may be kept relatively far from pinned layer  22  within the magnetic device  20 , e.g., relative to free layer  10 , in order to help reduce the possibility that antiferromagnetic layer  8  will be coupled to pinned layer  22 . In one example, the fixed magnetic moment of pinned layer  22  is further stabilized by another antiferromagnetic layer. 
     In some examples, magnetic device  20  may also comprise a spacer layer  24  formed over the pinned layer  22 . One example of spacer layer  24  may be a non-magnetic spacer layer. One example of a non-magnetic spacer layer  24  may be an electrically conductive spacer layer when magnetic system  20  is a giant magnetoresistive (GMR) sensor. As another example, spacer layer  24  may be a non-magnetic, electrically insulating barrier layer when the source of magnetoresistance of magnetic system  20  is based on a tunneling magnetoresistive (TMR) effect. In one example, spacer layer  24  may be approximately 1 nm thick. In other examples, spacer layer  24  may be other thicknesses. 
       FIG. 2B  depicts one example of a tunneling magnetoresistive (TMR) device  30 . TMR device  30  may comprise a tunnel barrier layer  36  and an antiferromagnetic layer  8  weakly exchange coupled to a free layer  10 . TMR device  30  may further include a second pinned layer  32  in addition to pinned layer  22 . Tunnel barrier layer  36  may provide TMR functionality to TMR device  30 . In one example, tunnel barrier layer  36  may be one of aluminum oxide (AlOx) or magnesium oxide (MgOx), or any other suitable material. 
     In one example, pinned layer  22  and second pinned layer  32  may have opposite magnetization and are placed on opposite sides of free layer  10  to supply spin-polarized currents of the desired polarity. The inclusion of two pinned layers having substantially opposite magnetization can help control the precession direction of free layer  10 . In some examples, pinned layer  22  and second pinned layer  32  may be located either adjacent to or some distance from the TMR device  30 . 
       FIG. 2C  is a block diagram of one example of a giant magnetoresistive (GMR) device  40 . In this example, GMR device  40  may have a first antiferromagnetic layer  26  and a second antiferromagnetic layer  44 . First antiferromagnetic layer  26  may be weakly exchange coupled to a first side  25  of free layer  10 . Second antiferromagnetic layer  44  may be weakly exchange coupled to a second side  27  of free layer  10 . In this example, first side  25  is opposite second side  27 . Both antiferromagnetic layers  26  and  44  improve the uniformity of the magnetic anisotropy of free layer  10 . Positioning an AFM layer  26  or  44  on the top or bottom of free layer  10  results in exchange coupling through horizontal surfaces  25  or  27 , respectively. The addition of weak AFM exchange coupled layers  26  and  44  to free layer  10  may reduce dispersion effects to the magnetization of free layer  10 . Because spins are throughout free layer  10 , including along sides  25  and  27 , any precession may be influenced by defects in the spins along the sides of free layer  10 . Having an antiferromagnetic layer  26  or  44  on both sides  25  and  27  of free layer  10  further regulates the precession to improve its uniformity. 
     GMR device  40  may include a non-magnetic layer  46  as a spacer layer. In some examples, non-magnetic layer  46  may be made of copper (Cu), or the like. Another example of non-magnetic layer  46  may be a non-magnetic, electrically insulating barrier layer. Non-magnetic layer  46  may provide giant magnetoresistive functionality to GMR device  40 . 
     GMR device  40  may also include a synthetic antiferromagnetic (SyAF) bilayer  43  as a pinned layer. In one example, a current is applied to SyAF bilayer  43 . In one example, the current in SyAF bilayer  43  is spin polarized. GMR device  40  may further include a metalization layer  48  that may be a capping layer formed over the sensor stack of GMR device  40 . One example of metallization layer  48  may be tantalum (Ta), although any material suitable for capping may be used. 
       FIG. 2D  is a block diagram of an example of a magnetic sensor  50 . As described herein, magnetic sensor  50  has the same structure as magnetic device  20  ( FIG. 2A ), with the addition of a perimeter layer  52 . Perimeter layer  52  is positioned adjacent to at least one of free layer  10  or pinned layer  22  to suppress edge pinning effects. In the example shown in  FIG. 2D , perimeter layer  52  is formed at least partially along an outer perimeter  54  of free layer  10 . In examples in which magnetic sensor  50  is shaped as an elliptical or circular device, layers  8 ,  10 ,  22 , and  24  may be layers having an approximately elliptical or circular cross-sectional shape in a plane that is substantially perpendicular to the plane of the image shown in  FIG. 2D . Perimeter layer  52  may be formed around some or all of layers  8 ,  10 ,  22 , and  24 . In one example, perimeter layer  52  is formed only around the outer perimeter  54  of free layer  10 . Perimeter layer  52  further improves the uniformity of precession of a magnetization of free layer  10 . 
     In one example, perimeter layer  52  may be an antiferromagnetic material. AFM perimeter layer  52  may be weakly exchanged coupled to at least free layer  10 . In such an example, AFM perimeter layer  52  reduces edge pinning effects along perimeter  54  of free layer  10 . 
     In another example, perimeter layer  52  may be a soft layer. A soft perimeter layer  52  may be a material that is highly permeable. When the magnetization of free layer  10  is canted, the magnetization may be influenced by magnetic poles forming on the edge(s) of free layer  10  as the magnetization precesses. The magnetic poles along perimeter  54  of free layer  10  can cause a non-uniform demagnetization field. Soft perimeter layer  52  may draw in any magnetization from these surface poles because of the permeability of the soft layer. Likewise, soft perimeter layer  52  may not emit a stray magnetic field. This may allow material properties to dominate the precession of the magnetization so it is more uniform. 
     In another example, perimeter layer  52  may be a decoupling or non-pinning layer. The decoupling or non-pinning layer may make the stoichiometry (chemical composition) of materials in contact with it more uniform. For example, during deposition, the composition of the deposited materials may differ between the edges and the bulk of the material. The decoupling or non-pinning layer can improve the uniformity of these layers by homogenizing the chemical composition of the material. For example, decoupling or non-pinning layer perimeter layer  52  may homogenize the stoichiometry of free layer  10  along perimeter  54 . 
     Any of the layers as described herein with respect to  FIGS. 2A-2D  (such as pinned layer  22 , for example) may be a single layer or a structure of more than one layer or partial layers. Furthermore, any of the layers or features described in any of the examples of  FIGS. 2A-2D  may be combined with each other for additional examples. Magnetic devices  20 ,  30 ,  40 , or  50  may also have additional layers or structures. For example, any of magnetic devices  20 ,  30 ,  40 , or  50  may include a magnetic shield layer. In such an example, a magnetic shield layer may be an electrically conductive, magnetic material, such as nickel-iron (NiFe). In further examples, the layers of the devices  20 ,  30 ,  40 , and  50  may be any type of structure, for example, polycrystalline, monocrystalline, amorphous, or the like. 
       FIG. 3  is a flowchart illustrating an example method  60  for manufacturing a magnetic device comprising at least one antiferromagnetic layer weakly exchange coupled to a free layer. Method  60  may include forming a free layer (e.g., free layer  10 ) having at least partially non-uniform magnetic anisotropy ( 62 ). Method  60  may further include forming a first antiferromagnetic layer (e.g., antiferromagnetic layer  8 , antiferromagnetic layer  26 , antiferromagnetic layer  34 , or antiferromagnetic layer  44 ) adjacent to the free layer ( 64 ). In such an example, the first AFM layer may reduce the at least partially non-uniformity of the magnetic anisotropy of the free layer. 
     Method  60  may further include weakly exchange coupling the first AFM layer to the free layer ( 66 ). In some examples, the first AFM layer may be weakly exchange coupled to the free layer based on the material properties of at least the first AFM layer, such that upon forming the first AFM layer adjacent to the free layer, the first AFM layer is weakly exchange coupled to the free layer. In other examples, the first AFM layer may be weakly exchange coupled to the free layer through performing a thermal anneal after deposition of the AFM layer. The temperature or duration of the thermal anneal may be selected to achieve a desired strength of the exchange coupling. Some examples of the temperature and duration of the thermal anneal may be between from approximately 100 to approximately 500 degrees Celsius (° C.) and the duration may be approximately 0.1 hours to 5 hours. In one example, the thermal anneal is performed for approximately 1 hour at approximately 250° C. In other examples, alterations to the deposition process or configuration of the magnetic device may be made in order to achieve a weak exchange coupling of a desired strength. 
     In one example, method  60  may include forming a second antiferromagnetic layer adjacent to the free layer, wherein the second antiferromagnetic layer is located on a side of the free layer opposite the first antiferromagnetic layer. In other examples, method  60  may further include forming a perimeter layer at least partially around the free layer, wherein the perimeter layer further reduces the non-uniformity of the magnetic anisotropy of the free layer. In yet another example, method  60  may include forming a pinned layer, wherein the free layer is located between first antiferromagnetic layer and the pinned layer. 
     Any method of deposition or fabrication now known or later developed may be used to implement method  60 . For example, magnetic sputtering or molecular beam epitaxy (MBE) may be used to deposit the layers. Chemical or physical deposition methods may be used. In some examples, thin film deposition techniques are used to perform method  60 . 
     Some existing spin torque oscillators are subject to timing jitter, phase noise, and sub-optimal or sub-maximum precession. Weakly exchange coupling an antiferromagnetic layer to a free layer reduces these effects and allows precession to be more uniform. In one example, the precession of magnetization in the free layer is increased. In another example, edge pinning effects are reduced by weakly exchange coupling one or more antiferromagnetic layers to the free layer. In one example, the weak AFM exchange coupling reduces noise effects due to temperature variation of magnetic device  6 . 
     Examples of the present disclosure may reduce the power needed to run system  2 . Further examples of the present disclosure may reduce the overall size of system  2  by increasing the power that is output by magnetic device  6 . A magnetic device, according to examples of the present disclosure, may provide a higher frequency broad-frequency range tunable oscillator with low jitter, low phase noise, and high output power. 
     In the discussion and claims herein, the term “on” used with respect to two materials, one “on” the other, means at least some contact between the materials, while “over” means the materials are in proximity, but possibly with one or more additional intervening materials such that contact is possible but not required. Neither “on” nor “over” implies any directionality as used herein. The term “about,” “approximate,” or the like indicates that the value listed may be somewhat altered, as long as the alteration does not result in nonconformance of the process or structure to the illustrated example. 
     Terms of relative position as used in this disclosure are defined based on a plane parallel to the conventional plane or working surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “horizontal” or “lateral” as used in this disclosure is defined as a plane parallel to the conventional plane or working surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal. Terms such as “on,” “side” (as in “sidewall”), “higher,” “lower,” “over,” “top,” and “under” are defined with respect to the conventional plane or working surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate. 
     Various aspects of the disclosure have been described. Aspects or features of examples described herein may be combined with any other aspect or feature described in another example. These and other examples are within the scope of the following claims.