Magnetic sensor based on efficient spin injection into semiconductors

A magnetic sensor based on efficient spin injection of spin-polarized electrons from ferromagnets into semiconductors and rotation of electron spin under a magnetic field. Previous spin injection structures suffered from very low efficiency (less than 5). A spin injection device with a semiconductor layer sandwiched between δ-doped layers and ferromagnets allows for very high efficient (close to 100%) spin polarization to be achieved at room temperature, and allows for high magneto-sensitivity and very high operating speed, which in turn allows devising ultra fast and sensitive magnetic sensors.

FIELD OF THE INVENTION

This invention relates generally to spintronics. In particular, the invention relates generally to a magnetic sensor, a magnetic read nanohead, based on efficient room temperature injection of spin polarized electrons into semiconductors and rotation of their spin under action of a magnetic field.

BACKGROUND OF THE INVENTION

Over the past decade a pursuit of solid state ultra fast scaleable devices, such as magnetic sensors of nanosize proportions, based on both the charge and spin of an electron has led to a development of new fields of magnetoelectronics and spintronics. The discovery of giant magnetoresistance (GMR) in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phenomenon where a relatively small change in magnetism results in a large change in the resistance of the devices.

The phenomenon of a large tunnel magnetoresistance (TMR) of ferromagnet-insulator-ferromagnet structures is a focus of product development teams in many leading companies. TMR is typically observed in F1-I-F2 structures made of two ferromagnetic layers, F1 and F2, of similar or different materials separated by the insulating thin tunnel barrier I with thickness typically ranging between 1.4–2 nm. The tunnel current through the structure may differ significantly depending on whether the magnetic moments are parallel (low resistance) or anti parallel (high resistance). For example, in ferromagnets such as Ni80Fe20, Co—Fe, and the like, resistance may differ by up to 50% at room temperature for parallel (low resistance) versus antiparallel (high resistance) moments on ferromagnetic electrodes.

It is worth mentioning recent studies of the giant ballistic magnetoresistance of Ni nanocontacts. Ballistic magnetoresistance is observed in Ni and some other nanowires where the typical cross-section the nanocontacts of the nanowire is a few square nanometers. The transport in this case is through very short constriction and it is believed to be with conservation of electron momentum (ballistic transport). The change in the contact resistance can be close to 10 fold (or about 1000%).

All magnetic sensors proposed and developed to present day, including the read heads, are based on variations of magnetic configurations, domain structures, in ferromagnets under externally applied magnetic fields. This mechanism does not ensure operating speed and sensitivity required of ultra fast sensors.

Interest has been particularly been keen on the injection of spin-polarized carriers, mainly in the form of spin-polarized electrons into semiconductors. This is large due to relatively large spin-coherence lifetime of electrons in semiconductors, possibilities for use in ultra fast devices. One such device in the works is an ultra fast and sensitive magnetic sensor such as a magnetic read head.

The possibility of spin injection from ferromagnetic semiconductors (FMS) into nonmagnetic semiconductors has been demonstrated in a number of recent publications. However, the Curie temperature (the temperature above which a material becomes non-magnetic) of magnetic semiconductors is substantially below room temperature. The low Curie temperature limits possible applications. Room-temperature spin injection from ferromagnets (FM) into semiconductors also has been demonstrated, but it remains a difficult task and the efficiency is very low. The low spin injection efficiency makes it very difficult, if not impossible, to develop an ultra fast magnetic sensor operable at room temperature.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a magnetic sensor includes first and second ferromagnetic layers, a semiconductor layer formed between the first and second ferromagnetic layers, a first δ-doped layer formed between the first ferromagnetic layer and the semiconductor layer, and a second δ-doped layer formed between the second ferromagnetic layer and the semiconductor layer.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structure have not been described in detail so as not to unnecessarily obscure the present invention.

The density of states (“DOS”) is one of main characteristics of electrons in solid states, in particular, in magnetic materials, such as ferromagnetic Ni, Co, and Fe. DOS is referred to as g(E)dE), which is the number of electrons per unit volume in an energy interval (E,E+dE).FIG. 1Aillustrates the DOS in ferromagnetic Ni, where arrows indicate the DOS for majority (spin up ↑) and minority (spin down ↓) electrons. Note that the DOS have high peaks for both spin-up and spin-down electrons at certain energy intervals.

FIG. 1Billustrates the density of electronic states (DOS) of ferromagnetic Ni, but at a higher resolution than inFIG. 1A. The energy origin is chosen at the Fermi level EF, i.e., E=EF=0. As shown, there is a very large difference in the density of states of minority and majority d-electrons at E>0 (states above the Fermi level). The peak in the DOS of minority d-electron states is positioned at E=Δ0, which for Ni, Δ0≈0.1 eV. Similar region at E>0 exists in Co and Fe. Note that near E≈Δ0, the DOS of the majority d-electrons and DOS of s and p electrons are all negligible when compared the DOS of minority d-electrons. Thus, if electrons are injected from the ferromagnetic material with energies E≈Δ0, the electrons would be almost 100% polarized.

FIG. 2Aillustrates an exemplary magnetic sensor200according to an embodiment of the present invention. As shown, the sensor200may include first and second ferromagnetic layers210and230, respectively, and a semiconductor layer220formed therebetween. The sensor200may also include first and second δ-doped layers215and225, which are located between the first ferromagnetic layer210and the semiconductor layer220and between the semiconductor layer220and the second ferromagnetic layer230, respectively. The sensor200may further include a substrate240, preferably a metal, formed below the first ferromagnetic layer110.

In addition, the sensor200may include electrodes260and250electrically connecting, respectively, to the ferromagnetic layer230and the ferromagnetic layer210or the metal substrate240. The electrodes260and250are not strictly necessary because the ferromagnetic layers210and230may directly play the role of electrodes.

The first and second ferromagnetic layers210and230may each be formed from various magnetic materials, preferably Ni, Fe and Co, as well as various magnetic alloys, which may include one or a combination of Fe, Co, Ni. The semiconductor layer220may be formed from various semiconductor materials including Si, Ge, GaAs, ZnTe, GaSb, GaP, InAs, CdSe, InP, InSb, CdTe, CdS, ZnS, ZnSe, AlP, AlAs, AlSb and also alloys and combinations of these materials. In general, it is preferred that the semiconductor layer220be formed from materials with relatively large time of electron spin relaxation τS(as will be shown below) such as GaAs, Ge, ZnSe and ZnCdSe. The semiconductor layer220is also preferred to be negatively doped.

Donor concentrations Nd1and Nd2of the first and second δ-doped layer215and225are preferred to be greater than the donor concentration Nsof the semiconductor layer220, i.e., Nd1, Nd2>>Nsshould hold. In one embodiment, one or both of the first and second δ-doped layers215and225may be formed by heavily doping of portions of the semiconductor layer220with electron rich materials. The magnetic sensor200thus formed may be described as having a FM−n+−n−n+−FM structure. Examples of electron rich materials include P, As, and Sb, which are typically used to dope semiconductors Ge and Si, and Ge, Se, Te, Si, Pb and Sn, which are typically used to dope semiconductor GaAs.

The inventors have discovered that if the following conditions are satisfied, efficient spin-injection of current from the ferromagnetic layers210and230into the semiconductor layer220may take place at room temperature:

Nd1⁢l+12≈2⁢ɛ0⁢ɛ⁡(Δ-Δ0)q2,Nd2⁢l+12≈2⁢ɛ0⁢ɛ⁡(Δ-Δ0)q2⁢(1)l+1≤t0=ℏ22⁢m*⁡(Δ-Δ0),l+2≤t0=ℏ22⁢m*⁡(Δ-Δ0)(2)
where Nd1and Nd2represent donor concentrations of the first and second δ-doped layers215and225, respectively; l+1and l+2represent the thicknesses of the first and second δ-doped layers215and225, respectively; so represents the permittivity of free space; ε represents a relative permittivity of the semiconductor layer220; Δ represents a height of the Schottky barrier (as measured from the Fermi level of the ferromagnetic layers210and230) on the boundaries between the first ferromagnetic layer210and the first δ-doped layer215and between the second ferromagnetic layer230and the second δ-doped layer225; Δ0represents the height of the lower and wider potential barrier in the semiconductor layer220(also as measured from Fermi level of the ferromagnetic layers210and230); q represents elementary charge; and h is the Planck's constant. For GaAs semiconductor layer, m*≈0.07 m0where m0is the mass of free electron and t0≈1 nm. Similarly for Si, the corresponding values are ≈0.2 m0and ≈0.5 nm, respectively.

Under the conditions of Equations (1) and (2), the δ-doped layers215and225become “transparent” for tunneling electrons. In other words, electrons with energy E≧Δ0may easily traverse the δ-doped layers215and225.

As noted above, when the barrier height Δ0corresponds to the peak in the DOS for minority d ↓ electrons in the ferromagnetic layers210and230(seeFIGS. 1A and 1B), the electrons are almost 100% polarized. In other words, P1and P2are both almost unity, where P1and P2represent degrees of polarization of injected electrons from the first ferromagnetic layer210to the semiconductor layer220, the first FM-S interface, and from the second ferromagnetic layer230to the semiconductor layer220, the second FM-S interface, respectively. It is preferred that the condition of Equation (1) is satisfied to the extent that a dispersion of Δ0is equal to the width of the peak in DOS shown inFIGS. 1A and 1B. Typically, this occurs if Equation (1) is accurate within 20 percent.

In another embodiment, one or both of the first and second δ-doped layers215and225may be formed by growing a n+-doped epitaxial layer on one or both sides of the n-doped semiconductor layer220(this structure may also be referred as a FM−n+−n−n+−FM heterostructure). It is preferred that the epitaxially grown δ-doped layers215and225be doped heavily as practicable and be as thin as practicable. Preferably, one or both of the first and second δ-doped layers215and225have a narrower energy band gap than the energy band gap of the semiconductor layer220, i.e., Egδ1<Egand Egδ2<Eg, and at that electron affinities of the δ-doped layers215and225be greater than an electron affinity of the semiconductor layer220by a value close to Δ0.

If one or both of the first and second δ-doped layers215and225are formed by epitaxial growth of a very thin heavily doped (i.e., n+doped) and narrower energy band gap semiconductor layer, the parameters of the layers215and225, i.e., their respective donor concentrations Nd1and Nd2and their thicknesses l+1and l+2should satisfy the following conditions for efficient spin injection:

Examples of such heterostructures include FM1-GaAs—Ga1-xAlxAs—GaAs-FM2(i.e., n+-δ-doped layers are formed from GaAs and n-doped semiconductor layer is formed from Ga1-xAlxAs), FM1-GexSi1-x—Si—GexSi1-x-FM2, and FM1-Zn1-xCdxSe—ZnSe—Zn1-xCdxSe—FM2, where x and 1−x quantities refer to the composition of the respective materials. Regarding the n+-δ-doped layers (e.g., GaAs, GexSi1-x, and Zn1-xCdxSe), their thickness l+1and l+2should be sufficiently thin such that corresponding FM-S interfaces become transparent for electron tunneling. The conditions of Equations (3) and (4) may be satisfied, for example, if the δ-doped layers215and225are such that the thickness l+1,2≦1 nm and the donor concentration N+d1,2≧1020cm−3.

It is possible that one of the first and second δ-doped layers215and225are formed from heavily doping a portion of the semiconductor layer220and the other of the first and second δ-doped layers215and225be formed by epitaxial growth.

The substrate240may be formed preferably from metals such as Ta, Cu, Ag, Au, and Pl. In addition, the electrodes250and260may be formed from highly conductive materials such as metals, doped silicon, and doped polysilicon.

FIG. 2Billustrates an exemplary magnetic sensor200-2according to another embodiment of the present invention. The sensor200-2is similar to the sensor200shown inFIG. 2A. The sensor200-2differs from the sensor200in that first and second antiferromagnetic layers270and280may be present. As shown inFIG. 2B, the first antiferromagnetic layers270may be placed between the first ferromagnetic layer210and electrode250and may play the role of the substrate. Similarly, the second antiferromagnetic layer280may be placed between the second ferromagnetic layer230and electrode260. The antiferromagnetic layers270and280fix more rigidly magnetizations M1and M2within ferromagnetic layers210and230, respectively. The first and second antiferromagnetic layers270and280may be formed from various materials including FeMn, IrMn, NiO, MnPt (L10), α-Fe2O3, and combinations therefrom.

FIGS.3A1and3A2illustrate an exemplary diagram of the magnetic sensor200,200-2along the line II—II shown inFIGS. 2A and 2Bat equilibrium (FIG.3A1) and under bias (FIG.3A2), wherein the first and second δ-doped layers215and225are both formed by heavily doping portions of the semiconductor layer220.

FIGS.3B1and3B2illustrate an exemplary diagram of the magnetic sensor200,200-2along the line IV—IV shown inFIGS. 2A and 2Bat equilibrium (FIG.3B1) and under bias (FIG.3B2), wherein the first and second δ-doped layers215and225both have energy band gaps that are less than the energy band gap of the semiconductor layer220.

With references to FIGS.3A1–3B2, the operation of the magnetic sensors200,200-2will be explained. First, under bias at room temperature, spin-polarized electrons from the first ferromagnetic layer210are injected into the semiconductor layer220through the first δ-doped layer215using the concept of efficient room temperature injection described above (see also below).

A potential Schottky barrier for electrons always forms at the boundary of any metal-semiconductor interface, such as at the boundary between the first ferromagnetic layer210and the first δ-doped layer215. If the donor concentration Nd1is sufficiently high and the thickness l+1is sufficiently small, the first δ-doped layer215is “transparent” for tunneling electrons, and the electrons from the first ferromagnetic layer210may easily traverse the first δ-doped layer215.

The electrons that tunnel through the first δ-doped layer215meet another lower and wider potential barrier formed in the semiconductor layer220. It is preferred that the width d of the semiconductor layer220be wide enough. When this occurs, electrons with energies below the barrier height are effectively filtered, and essentially, only the electrons with energies above the barrier height Δ0will be able to traverse the semiconductor layer220.

It is preferred that the height of the barrier in the semiconductor layer220is approximately equal to EF+Δ0where EFis the Fermi level at equilibrium. Note that the potential barrier in the semiconductor layer220may be manipulated to a desired value by controlling the characteristics of the magnetic sensor200,200-2, for example by controlling the donor concentration N, of the semiconductor layer220. As previously noted, the DOS of minority d↓ electrons of a ferromagnet reaches maximum at energy level E≈EF+Δ0(seeFIGS. 1A and 1B). For simplicity, origin is chosen such that EF=0. Then the maximum DOS of minority d↓ electrons exceeds, by more than an order of magnitude, the DOS of other type electrons (s↑, s↓, p↑, p↓ and d↑) in the first ferromagnetic layer210at E≈Δ0.

Thus, if the potential barrier height of the semiconductor layer220is such that it coincides with Δ0, then the electrons from the first ferromagnetic layer210tunneling through the first δ-doped layer215and traverse the length d of the semiconductor layer220will be composed of almost all minority d↓ electrons. In other words, the injected current will be almost completely spin-polarized.

Further, the Δ0height of the barrier is preferred to be substantially maintained throughout the width d of the semiconductor layer220at equilibrium (see FIGS.3A1and3B1). The height of the barrier may be substantially maintained if the donor concentration Nsand the width d of the semiconductor layer220substantially satisfy the following conditions:

Ns≤2⁢ɛ0⁢ɛΔ0q2⁢d2(5)d>dmin=aB⁡(1kB⁢T)⁢m0⁢EB⁢Δ0me*(6)
where aB=0.05 nm and EB=13.6 eV are the Bohr parameters; m0is mass of free electron; T is the sensor temperature and kBis the Boltzmann constant. For example, dmin≈6 nm for GaAs and dmin≈3 nm for Ge when Δ0=0.1 eV and T=300K. Under these circumstances, if d>10 nm and Ns≦1017cm−3the conditions specified by Equations (5) and (6) would be satisfied.

Due to the peculiarity of the DOS in the ferromagnet, the end result is that practically only the minority d↓ electrons are injected from the first ferromagnetic layer210into the semiconductor layer220. In other words, the degree of spin polarization P1of the injected electrons from the first ferromagnetic layer210into the semiconductor layer220is close to 1 (or close to unity). The same is true of the degree of spin polarization P2of electrons injected from the second ferromagnetic layer230into the semiconductor layer220.

It is preferred that the electrons conserve their spin orientation during transit through the semiconductor layer220, i.e., spin ballistic transport is desired. For spin ballistic transport to occur, a transit time τdof the electrons through of the semiconductor layer220of the width d should be substantially equal to or less than the spin-coherence time τSof the electrons in the semiconductor layer220.

The transit time τdis determined by the diffusion and the drift of the electrons under the electrical field Ē inside the semiconductor layer220and is equal to τd=d/vdwhere vd=μnĒ+Dn/d, μnand Dnare the mobility and the diffusion constant of the electrons. Thus, spin ballistic transport occurs when:
d<dmax=√{square root over (DnτS)}(7)

According to Equations (6) and (7), the width d of the semiconductor layer220should satisfy the following condition:

In comparison with earlier proposed magnetic sensors, the sensors200,200-2shown inFIGS. 2A and 2Bpossess an additional degree of freedom resulting from spin rotation of the injected spin-polarized electrons under the action of an external magnetic field during transit through the semiconductor layer220in the spin ballistic transport, τd≦τS.

The electron spin rotation occurs with frequency ω=γH where γ=1.76×107Oe−1s−1is the gyromagnetic ratio and H is the magnetic field component normal to the spin (see FIG.3A2). Thus, the angle of the spin rotation is θH=γHτdand its maximum is θHmax=γHτS. Therefore, the total angle θ between the electron spin σ and the magnetization M2of the second ferromagnetic layer230is θ=θ0+θHwhere θ0is the angle between the magnetizations M1and M2of the first and second ferromagnetic layers210and230. The theoretical calculations and experimental studies show that the longest values for spin-coherence time τScan be realized in negative doped semiconductors (n-semiconductors) and can reach up to 1 ns in ZnSe and GaAs at room temperature. Hence it follows that θ=π can be obtained at H in several hundred Oersteds.

A conductivity G of the second FM-S interface, i.e., between the second ferromagnetic layer230and the semiconductor layer220, change with the angle θ between the electron spin σ and the magnetization M2of the second ferromagnetic layer230. Note that the magnetic sensor200-2as shown inFIG. 2Bhas an advantage in that antiferromagnetic layers270and280may be used to fix magnetizations M1and M2of the first and second ferromagnetic layers210and230, respectively. The conductivity G of the structures shown inFIGS. 2A and 2Bmay be written as:
G=G0(1+P1P2cos θ)  (9)
where P1and P2are degrees of polarizations first and second FM-S interfaces as described previously. It was also described above that the variation of the angle θ between the electron spin σ and the magnetization M2inside the second ferromagnetic layer230can exceed π when the magnetic field is several hundred Oersteds. Thus, the maximum magnetoresistance variation can reach:

As mentioned above, P1and P2for the sensors200and200-2shown inFIGS. 2A and 2Bare nearly unity. Therefore, the variation of resistance of the magnetic sensor200,200-2can reach several orders of magnitude. In other words, the magnetic sensors200,200-2may be extremely sensitive.

FIGS. 4A and 4Billustrate the embodiments of magnetic sensors200and200-2in operation. As shown, injection of spin-polarized electrons occurs from the first ferromagnetic layer210into the semiconductor layer220. The electron spin is rotated under action of a magnetic field and it causes change in resistance of the second ferromagnetic layer230, which is measured.

The magnetic sensors200and200-2may be ultra fast. It was noted above that the transit time τdis essentially less than or equal to the spin-coherence time τSof the electrons. It was also noted above that for n-doped semiconductor220the spin-coherence time τScan be less than or equal to 1 ns. This means that the transit time τd<τS<1 ns. In other words, the effect of spin injection from the first ferromagnetic layer210and the spin rotation may manifest as a change in the resistance in the second ferromagnetic layer in less than 1 ns (or less than a billionth of a second). It should be noted that 1 ns is the maximum time at room temperature. Typically, τS=0.1–0.01 ns.

FIGS. 5A–5Dillustrate an exemplary method of manufacturing the sensor200shown inFIG. 2A. As shown inFIG. 5A, the substrate240may be formed. The substrate240may be planarized. Then the first magnetic layer210may be formed on the substrate240. Material to form the first magnetic layer210may be deposited, sputtered, fired on the substrate240. The first magnetic layer210may also be planarized.

Then as shown inFIG. 5B, the first and second δ-doped layers215and225and the semiconductor layer220may be formed. In one embodiment, the first δ-doped layer215may be formed by epitaxial or molecular growth. The first δ-doped layer215may also be deposited, sputtered, or fired onto the first magnetic layer215. Then the semiconductor layer220may be deposited, fired, or sputtered onto the first δ-doped layer215. Then the second δ-doped layer225may be formed by epitaxial or molecular growth, or may be deposited, sputtered, or fired onto the semiconductor layer. Note that each of the first and second δ-doped layers215and225and the semiconductor layer220may be planarized. Also, the first and second δ-doped layers215and225may be doped more heavily as compared to the semiconductor layer220.

In another embodiment, the semiconductor layer220may be formed on the first ferromagnetic layer210and the first and second δ-doped layers215and225may be formed by heavily doping appropriate portions of the semiconductor220.

Then as shown inFIG. 5C, the second ferromagnetic layer230may be formed, again by epitaxial growth, or may be deposited, sputtered, or fired onto on the second δ-doped layer225. The second magnetic layer230may be planarized.

Then as shown inFIG. 5D, the first and second electrodes250and260may be formed by sputtering, firing, or depositing materials on the first and second ferromagnetic layers210and230, respectively.

FIGS. 6A–6Dillustrate an exemplary method of manufacturing the sensor200-2shown inFIG. 2B. The method to manufacture sensor200-2is similar to the method illustrated inFIGS. 5A–5D. Thus the details need not be repeated. However, the methods do differ in first and second antiferromagnetic layers270and270are formed and the substrate240is not.

What has been described and illustrated herein are preferred embodiments of the invention along with some of its variations. The terms, descriptions, and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.