Templating Layers For Perpendicularly Magnetized Heusler Films

Abstract

Devices are described that include a multi-layered structure that is non-magnetic at room temperature, and which comprises alternating layers of Co and at least one other element E (that is preferably Al; or Al alloyed with Ga, Ge, Sn or combinations thereof). The composition of this structure is represented by Co.sub.1-xE.sub.x, with x being in the range from 0.45 to 0.55. The structure is in contact with a first magnetic layer that includes a Heusler compound. An MRAM element may be formed by overlying, in turn, the first magnetic layer with a tunnel barrier, and the tunnel barrier with a second magnetic layer (whose magnetic moment is switchable). Improved performance of the MRAM element may be obtained by placing an optional pinning layer between the first magnetic layer and the tunnel barrier.

Claims

1. A device, comprising: a multi-layered structure that is non-magnetic at room temperature, the structure comprising alternating layers of Co and E, wherein E comprises at least one other element that includes Al, wherein the composition of the structure is represented by Co.sub.1-xE.sub.x, with x being in the range from 0.45 to 0.55; and a first magnetic layer that includes a Heusler compound, the magnetic layer being in contact with the structure.

2. The device of claim 1, wherein the magnetic moment of the magnetic layer is substantially perpendicular to the interface between the structure and the magnetic layer.

3. The device of claim 2, wherein the magnetic layer has a thickness of less than 5 nm.

4. The device of claim 2, wherein the magnetic layer has a thickness of less than 3 nm.

5. The device of claim 2, wherein the magnetic layer has a thickness of one unit cell.

6. The device of claim 1, wherein the Heusler compound is selected from the group consisting of Mn.sub.3.1-xGe, Mn.sub.3.1-xSn, and Mn.sub.3.1-xSb, with x being in the range from 0 to 1.1.

7. The device of claim 1, wherein the Heusler compound is a ternary Heusler.

8. The device of claim 7, wherein the ternary Heusler is Mn.sub.3.1-xCo.sub.1.1-ySn, wherein x1.2 and y1.0.

9. The device of claim 1, wherein E is an AlGe alloy.

10. The device of claim 1, wherein E is an AlGa alloy.

11. The device of claim 1, wherein E includes an alloy selected from the group consisting of AlSn, AlGe, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn.

12. The device of claim 1, comprising a substrate underlying the multi-layered structure.

13. The device of claim 12, comprising a tunnel barrier overlying the first magnetic layer, thereby permitting current to pass through both the tunnel barrier and the first magnetic layer.

14. The device of claim 13, comprising a second magnetic layer in contact with the tunnel barrier.

15. The device of claim 14, wherein the tunnel barrier includes Mg and O.

16. A method, comprising: using the device of claim 14 as a memory element.

17. The method of claim 16, wherein the memory element is a racetrack memory device.

18. A method of forming the device of claim 1, comprising: depositing Co and Al, thereby forming a composite layer on a substrate; annealing the composite layer, so that at least one layer of Co and at least one layer of Al are formed from the composite layer, thereby forming the multi-layered structure; and depositing the magnetic layer over the multi-layered structure.

19. The method of claim 18, wherein the annealing takes place at a temperature of at least 400 C.

20. The method of claim 18, wherein the substrate is MgO.

21. A device, comprising: a substrate; a multi-layered structure that is non-magnetic at room temperature, the structure comprising alternating layers of Co and E, wherein E comprises at least one other element that includes Al, wherein the composition of the structure is represented by Co.sub.1-xE.sub.x, with x being in the range from 0.45 to 0.55, wherein the structure overlies the substrate; a first magnetic layer that includes a Heusler compound, the magnetic layer being in contact with the structure; a tunnel barrier overlying the first magnetic layer; and a second magnetic layer in contact with the tunnel barrier, wherein the second magnetic layer has a magnetic moment that is switchable.

22. The device of claim 21, comprising a capping layer in contact with the second magnetic layer.

23. The device of claim 21, wherein the first magnetic layer includes Mn and an element selected from the group consisting of Sn, Sb, and Ge.

24. The device of claim 23, wherein the first magnetic layer further includes Co.

25. A device, comprising: a multi-layered structure that is non-magnetic at room temperature, the structure comprising alternating layers of Co and E, wherein E comprises at least one other element that includes Al, wherein the composition of the structure is represented by Co.sub.1-xE.sub.x, with x being in the range from 0.45 to 0.55; and a first magnetic layer that includes an L1.sub.0 compound, the magnetic layer being in contact with the structure.

26. The device of claim 25, wherein the magnetic moment of the magnetic layer is substantially perpendicular to the interface between the structure and the magnetic layer.

27. The device of claim 25, wherein the L1.sub.0 compound is selected from the group consisting of MnGa, MnAl, FeAl, MnGe, MnSb, and MnSn alloys.

28. A method, comprising using the device of claim 25 as a memory element.

29. The method of claim 28, wherein the memory element is a racetrack memory device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1. Illustration of the atomic templating concept. A and X represent transition metal elements, and E and Z represent main group elements.

[0012] FIG. 2. XRD scans of 300 thick Co.sub.100-xAl.sub.x films on a MgO/MgO(001) substrate. CoAl films were deposited at room temperature and annealed in situ at various temperatures in vacuum.

[0013] FIG. 3. High resolution TEM image of 20 thick Mn.sub.3Sb films on a CoAl templating layer grown on a MgO(001) single crystal substrate.

[0014] FIG. 4. Root mean square (RMS) roughness (r.sub.rms) versus annealing temperature of a CoAl templating layer.

[0015] FIG. 5. Perpendicular magneto-optical Kerr effect (P-MOKE) hysteresis loops of 20 thick Mn.sub.3Ge films on a 300 CoAl templating layer. The CoAl layer was annealed at various temperatures prior to the deposition of Mn.sub.3Ge, which was carried out at 100 C.

[0016] FIG. 6. Perpendicular MOKE signal from a CoAl templating layer grown on a MgO(001) single crystalline substrate.

[0017] FIG. 7. P-MOKE hysteresis loops of 20 thick Mn.sub.2.8Sb and Mn.sub.3Ge films on a CoAl templating layer grown on a MgO(001) single crystalline substrate.

[0018] FIG. 8. Material stack for 20 thick Mn.sub.3Z Heusler films (Z=Ge, Sn, and Sb) grown on an amorphous substrate (Si(001)/250 SiO.sub.2). All thicknesses are in .

[0019] FIG. 9. P-MOKE hysteresis loops for Mn.sub.3Z Heusler films deposited on a CoAl templating layer grown on an amorphous substrate (Si(001)/250 SiO.sub.2).

[0020] FIG. 10. P-MOKE loop for 20 thick L1.sub.0 Mn.sub.2.5Sb films for a CoAl templating layer grown on an amorphous substrate (Si(001)/250 SiO.sub.2).

[0021] FIG. 11 illustrates a magnetic tunnel junction device that incorporates the templating and Heusler layers described herein.

DETAILED DESCRIPTION

[0022] New magnetic materials are needed to allow for scaling of STT-MRAM (spin transfer torque-magnetic random access memories) beyond the 20 nm node. These materials must have very large perpendicular magnetic anisotropy (PMA) and, for integration purposes, be compatible with conventional CMOS technologies. Such magnetic materials form electrodes of magnetic tunnel junction (MTJ) based memory elements. An important mechanism for switching the state of the MTJ element is using spin polarized tunneling currents that are passed through the MTJ. The magnitude of this current is limited by the size of the transistors used to provide the write current. This means that the thickness of the electrode must be sufficiently small that it can be switched by the available current. For magnetization values of 1000 emu/cm.sup.3, the electrode must have a thickness that does not exceed approximately 1 nm. The thickness of electrodes formed from Heusler alloys, to date, far exceeds this value in all cases. The thinnest layers to date are for the Heusler compound Mn.sub.3Ge, for which layers as thin as 5 nm showed perpendicular magnetic anisotropy and reasonably square magnetic hysteresis loops. The magnetic hysteresis loops for thinner layers showed poor squareness and are much less suitable for STT-MRAM.

CoAl Chemical Templating Layer

[0023] Single crystal epitaxial films of Co.sub.1-xAl.sub.x alloy were grown by dc-magnetron sputtering onto MgO buffer layers overlying MgO(001) single crystal substrates, in an ultra-high vacuum (UHV) chamber with a base pressure of 210.sup.9 Torr. The MgO buffer layer was prepared by depositing 20 thick MgO at room temperature using ion-beam deposition (IBD) from a MgO target. Films of 300 thick CoAl were deposited at room temperature and annealed at various temperatures T.sub.AN=200, 300, 400, and 500 C. for 30 minutes. The composition of the CoAl layers was determined to be Co.sub.51Al.sub.49 by Rutherford backscattering measurement.

[0024] X-ray diffraction (XRD) -2 scans were performed on these films. FIG. 2 shows sets of XRD scans of CoAl films annealed at various T.sub.AN for 30 minutes (200 C., 300 C., 400 C., 500 C.). These CoAl films were deposited from a single CoAl alloy target. The data were compared with those taken from a CoAl film which was not annealed. The data show the main CoAl (002) peak at 2=65.5 as well as the CoAl (001) peak at 2=31.4. The existence of the CoAl (001) superlattice peak clearly proves that there is an alternate layering of Co and Al for all annealing temperatures, even without any annealing. However, we find that as-deposited CoAl films grown by co-sputtering from individual targets of Co and Al do not show the CoAl(001) superlattice peak. These films, upon annealing at 400 C. for 30 minutes, form the layered structure. The x-ray diffraction associated with the substrate was observed for all samples and is labeled as the MgO(002) peak. The lattice parameter of the CoAl film is 2.86 , which is close to that of bulk CoAl in B2 structure.

[0025] FIG. 3 is a high resolution transmission electron microscopy (HR-TEM) of a typical 20 Mn.sub.3Sb layer grown on a CoAl templating layer. The stack of this sample was MgO(001)/20 MgO/300 CoAl/20 Mn.sub.3Sb/20 MgO/30 Ta. The CoAl templating layer consists of alternating layers of Co and Al in agreement with the XRD measurements detailed in FIG. 2. Excellent epitaxial matching of 20 thick Mn.sub.3Sb layers with the CoAl templating layer is clearly seen. Furthermore, the chemical ordering within the Heusler layer is evident: The MnSb layers are easily distinguishable from the MnMn layers, showing the alternating atomic layers and even the chemical ordering within each MnSb layer.

[0026] Atomic force microscopy was performed to probe the surface morphology of 300 thick CoAl templating layers. FIG. 4 shows root-mean-squared surface roughness (r.sub.rms) for various annealing temperatures. The films show a very smooth surface independent of annealing temperature, with r.sub.rms<2 , which is smoother than the other seed layers attempted (CoGa, CoSn, and CoGe).

Mn.SUB.3.Z Tetragonal Heusler

[0027] 20 -thick Mn.sub.3Ge films were deposited at 100 C. by ion beam deposition on a CoAl templating layer. The stacks were capped by 20 thick MgO and 30 thick Ta to prevent ambient oxidation of the Heusler layer. The resulting structures are of the form MgO(001)/20 MgO/300 CoAl/20 Mn.sub.3Ge/20 MgO/30 Ta (see FIG. 5; excellent performance was obtained even with CoAl layers having thicknesses of 20 , 50 , 100 , and 200 .). The CoAl layers were deposited at room temperature and then annealed at various temperatures (200 C., 300 C., 400 C., 500 C.) for 30 minutes prior to Mn.sub.3Ge deposition. FIG. 5 shows the perpendicular magneto-optical Kerr effect (P-MOKE) hysteresis loops of Mn.sub.3Ge for different annealing temperatures of the CoAl templating layer. Interestingly, the magnetic properties of Mn.sub.3Ge are independent of the annealing temperature. Excellent PMA is found for the films using a CoAl templating layer with a squareness of 1 even for the film that was not annealed. Abrupt switching of the magnetic moment implies that there is no second phase. FIG. 6 shows the P-MOKE signal from a MgO(001)/20 MgO/300 CoAl/20 MgO/30 Ta sample that was annealed at 400 C. for 30 minutes. The lack of any magnetic field-dependent MOKE signal clearly indicates that the CoAl templating layer is non-magnetic.

[0028] The structural ordering of ultrathin layers is likely due to the distinct chemical properties of the elements Co and Al in the templating layer. As an alternative to Al, Al alloys such as AlSn, AlGe, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn may be employed. Binary (X=Y) and ternary Heusler alloys consist of two or three different types of atoms, respectively. In X.sub.2YZ Heuslers, the Z main group element typically has high chemical affinity for X and Y. In this context, the formation of a chemically ordered structure should take place, irrespective of the choice of Z. FIG. 7 shows the case of Mn.sub.3Z Heusler compounds for Z=Sb and Ge. 20 thick Mn.sub.3Z films were deposited at room temperature on CoAl templating layers (which had been annealed at 400 C. for 30 minutes). The sample structures are of the form MgO(001)/20 MgO/300 CoAl/20 Mn.sub.3Z (Z=Ge, Sb)/20 MgO/30 Ta. Excellent PMA was observed in all cases with squareness 1, but the highest coercivity 2 kOe is found for the Mn.sub.2.8Sb film. The polarity of the Kerr signal for Mn.sub.3Sb is opposite that of the Mn.sub.3Ge Heusler alloys.

Mn.SUB.3.Z Heusler Alloy on Amorphous Substrate

[0029] A CoAl templating layer is capable of inducing chemical ordering in ultra-thin Heusler films even when they are deposited on amorphous Si/SiO.sub.2 substrates at room temperature. FIG. 8 shows the details of a stack grown on a Si(001) substrate having a 250 amorphous layer of SiO.sub.2. The stack consists of Si(001)/250 SiO.sub.2/50 Ta/3 CoFeB/30 MgO/50 CoAl (annealed at 400 C.)/20 Mn.sub.3Z/20 MgO/20 Ta. (All the layers up to and including CoAl were deposited at room temperature and then annealed at 400 C. for 30 minutes.) FIG. 9 shows the P-MOKE hysteresis loops obtained for the structure shown in FIG. 8, for Heusler alloys with Z=Sn, Ge, and Sb. These hysteresis loops, though not sharp, indicate the presence of remanent magnetization in the three cases studied here. Moreover, consistent with our previous observations, the polarity of the Kerr signal for Mn.sub.2.8Sb is opposite that seen for the Mn.sub.3Ge and Mn.sub.3Sn Heusler alloys.

Mn.sub.2.5Sb L1.sub.0 Tetragonal Compound on Amorphous Substrates

[0030] 20 thick Mn.sub.2.5Sb films were also deposited on CoAl layer at room temperature using dc-magnetron sputtering, and the films were capped by 20 thick MgO and 20 thick Ta for structural and magnetic property measurements. The final stack is Si(001)/250 SiO.sub.2/50 Ta/3 CoFeB/30 MgO/50 CoAl (annealed at 400 C.)/20 Mn.sub.2.5Sb/20 MgO/20 Ta. (That is, all the layers up to and including CoAl were deposited at room temperature and then annealed at 400 C. for 30 minutes. See FIG. 8 for an analogous structure that employs Mn.sub.3Sb.) The Mn.sub.2.5Sb layer and the capping layers were subsequently deposited after cooling the samples to 100 C. FIG. 10 includes the P-MOKE hysteresis loops from this sample which shows excellent PMA with a squareness of 1. The AFM measurements indicate a very smooth sample with r.sub.rms of 2 . These results demonstrate that the CoAl templating layer is capable of inducing excellent PMA in Heusler and L1.sub.0 compounds on amorphous substrates without any need for additional annealing. The CoAl templating layer is effective in inducing ordering within the L1.sub.0 compounds, whose constituent elements include one transition metal element and a main group element. Other candidate L1.sub.0 compounds include MnAl alloys, MnGa alloys, MnSn alloys, MnGe alloys, and FeAl alloys.

Applications

[0031] The structures described herein lend themselves to a variety of applications, including MRAM elements and a racetrack memory device, such as that described in U.S. Pat. No. 6,834,005, issued Dec. 21, 2004 and titled Shiftable magnetic shift register and method of using the same, which is hereby incorporated herein. One such MRAM element is shown in FIG. 11. As with MRAM elements generally, a tunnel barrier is situated between two magnetic electrodes, one of which has a fixed magnetic moment and the other of which has a magnetic moment that is switchable, thereby permitting the recording and erasing of data. Unlike MRAM elements of the prior art, however, the magnetic layer of FIG. 11 having the fixed magnetic moment (pinning layer) comprises Heusler films such as those described herein. An optional (second) pinning layer may be advantageously employed for even better performance, as described below.

[0032] Note that in FIG. 11, the pinning layer (Heusler layer, which may be either ferro- or ferri-magnetic) overlies a seed layer which in turn overlies a substrate. As described herein, the respective compositions of the seed layer and the substrate may be advantageously chosen to promote growth of the Heusler layer, whose magnetic moment is aligned perpendicular to the layer plane.

[0033] The templating layer of FIG. 11 is, as described previously herein, a multi-layered structure that is non-magnetic at room temperature, and which comprises alternating layers of Co and at least one other element E (preferably Al; or Al alloyed with Ga, Ge, Sn or any combination thereof, such as AlSn, AlGe, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn). The composition of this structure is represented by Co.sub.1-xE.sub.x with x being in the range from 0.45 to 0.55. Overlying the templating layer is a magnetic layer that includes a Heusler compound, e.g., Mn.sub.3Ge, Mn.sub.3Sn, or Mn.sub.3Sb. (Alternatively, overlying the templating layer is a magnetic layer that includes a L1.sub.0 compound, such as one or more alloys selected from the group consisting of MnAl, MnGa, MnSn, MnGe, and FeAl.) An optional, second pinning layer may be used to increase performance, and may include Fe, a CoFe alloy, or Co.sub.2MnSi.

[0034] The tunnel barrier is preferably MgO (001), although other (001)-oriented tunnel barriers may be used, such as CaO and LiF. Alternatively, MgAl.sub.2O.sub.4 can be used as the tunnel barrier; its lattice spacing can be selected by choosing a MgAl composition that leads to good lattice matching with the underlying Heusler or L1.sub.0 compound. The magnetic electrode overlying the tunnel barrier may comprise Fe, a CoFe alloy, or a CoFeB alloy, for example. The capping layer may comprise Mo, W, Ta, Ru, or a combination thereof. Current may be induced by applying a voltage between the two magnetic electrodes, which are separated by the tunnel barrier.

[0035] Certain structures described herein may also be used in racetrack memory devices. In this case, the racetrack is a nanowire that may include a substrate, an optional seed layer, a templating layer, and a first magnetic layer of a Heusler compound. (See the discussion above with respect to FIG. 11 for possible compositions of these layers. Note that in a racetrack memory device the tunnel barrier and the switchable magnetic layer shown in FIG. 11 would not normally be present; however, in this case the first magnetic layer shown in FIG. 11 would have a magnetic moment that is switchable rather than fixed.) Magnetic domain walls may be moved along this racetrack, as described in U.S. Pat. No. 6,834,005. Data may be read out of (and stored in) the racetrack by interrogating (or changing) the orientation of the magnetic moment of the magnetic material between adjacent domain walls within the racetrack.

[0036] The various layers described herein may be deposited through any one or more of a number of methods, including magnetron sputtering, electrodeposition, ion beam sputtering, atomic layer deposition, chemical vapor deposition, and thermal evaporation.

[0037] The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within that scope.