AU-17

GMR and TMR effects in FeCoGd-based spin valves and

magnetic tunnel junctions

X. J. Bai1, J. Du1,*, J. Zhang1, B. You1, L. Sun1, W. Zhang1, X. S. Wu1, S. L. Tang1, A. Hu1, H.

N. Hu2, S. M. Zhou2

1

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

2

Department of Physics, Fudan University, Shanghai 200433, P. R. China

Abstract

Giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) were studied in spin

valves

of

FeCo/Cu/(FeCo)1-xGdx

and

magnetic

tunnel

junctions

of

FeCo/AlO/(FeCo)1-xGdx, respectively. When the FeCoGd layer is thick enough, both GMR and TMR ratios change their signs from positive to negative at the compensation composition with 0.293 ? x0 ? 0.337, as the Gd content is increased. This scenario is originated from a competition of rare-earth and transition-metal spins in FeCoGd layers with antiferromagnetic coupling. Accordingly, it is deduced that in FeCoGd layer the spin polarizations PN of electrons at Fermi level and tunneling spin polarization PTSP are negative and positive for x < x0, respectively, and vice verse for x > x0.

PACS numbers: 75.47.Np; 75.50.Gg; 75.70.Cn; 85.75.-d

_________________________________________________________________ * Electronic address: jdu@nju.edu.cn

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Spin polarization of ferromagnetic (FM) alloys near the Fermi surface has attracted much attention because of their importance in both magneto-electronic devices [1] and fundamental research [2-4]. First, the magnitude and sign of giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) are strongly related to the spin polarization of the FM alloys. Secondly, the spin polarization of rare-earth transition-metal (RE-TM) alloys is less well understood, compared with transition-metal (TM) and their magnetic alloys. Therefore, exploration of the spin polarization in RE-TM alloys is encouraged.

Measurement of TMR in magnetic tunnelling junctions (MTJs) is a convenient and powerful technique to probe the spin polarization. According to Jullier’s model [5], TMR = 2PTSP1PTSP2/(1-PTSP1PTSP2), where PTSP1 and PTSP2 are the values of the tunnelling spin polarization of electrodes. If TMR ratio is positive (negative), the spin polarizations of both ferromagnetic (FM) electrodes have identical (opposite) signs. In contrast to TMR effect, GMR effect is caused by the spin-dependent scattering, which includes both the extrinsic and intrinsic contributions [2]. Only the intrinsic one is determined by the spin polarization PN of the FM layers [2], where PN = [N(?)-N(?)]/[N(?)+N(?)], and N(?) and N(?) are the density of states (DOS) at the Fermi level for majority spins and minority ones, respectively. Therefore, in combinations with the GMR and TMR data in SVs and MTJs consisting of the same FM layers, the magnitude and especially the sign of the spin polarization at Fermi level in FM layers can be known. In this work, GMR effects in SVs of Fe50Co50 (=FeCo)/Cu/(Fe50Co50)xGd1-x(=FeCoGd) and TMR effects in MTJs of FeCo/AlO/FeCoGd were studied simultaneously. It is found that GMR and TMR ratios change their signs at almost the same x and PTSP and PN always have opposite signs at all x.

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SVs of Si/NiO(20 nm)/FeCo(2 nm)/Cu(4 nm)/FeCoGd (t nm)/NiO(2 nm) and MTJs of glass/Ta(5 nm)/FeCoGd (30 nm)/AlO/FeCo(28 nm)/Ta(5 nm) were fabricated by magnetron sputtering. The FeCoGd layer was deposited from a composite target with a couple of small Gd chips on FeCo target. The composition of the FeCoGd layer was measured by X-ray fluorescence (XRF). A magnetic field of about 250 Oe was applied parallel to the film plane to induce a uniaxial anisotropy in the FM layers during deposition. In SVs, the top thin NiO layer is used as a specular reflection layer to enhance the GMR effect [6] and to prevent the sample from oxidation. Fabrication of AlO barrier has been described elsewhere [7]. The MR ratio, measured using standard dc four-point method, is defined as (RAP-RP)/RP, where RP (RAP) is the resistance when the magnetic moments of the two neighboring FM layers in SVs or MTJs are parallel (antiparallel) to each other. When RAP is larger (smaller) than RP, normal (inverse) GMR or TMR effect occurs. All the measurements have been performed at room temperature.

Figures 1 (a) - 1 (d) show typical GMR curves of SVs with various FeCoGd layer thickness t at x = 0.238 and 0.451. Apparently, an antiparallel alignment of magnetizations in two FM layers and a plateau in each R-H curve are formed in Fig.1, because the coercivity of the bottom FeCo layer is enhanced by the NiO pinning layer [8]. Therefore, the resistance change as a function of the external magnetic field is induced by spin-dependent scattering. For x = 0.238, the GMR ratio is positive for all t. For x = 0.451, however, it is positive for t = 8.1 nm and negative for t = 18 nm. Therefore, the dependence of the GMR ratio on t is strongly related to the value of x.

Figure 2 shows the GMR ratio in SVs as a function of t with various x. For all samples,

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as t is increased, the GMR ratio initially increases sharply to reach a maximum value. For samples with x ? 0.293, the GMR ratio keeps positive and decreases slowly due to the current shunting effect. For samples with x ? 0.337, the GMR ratio decreases abruptly and then turns to negative. The crossover value of t where the GMR effect changes from normal to inverse is within the range from 12 nm to 15 nm. The effect of t on the GMR ratio depends on the value of x. Similar results have also been observed in Fe/Cu/CoGd SVs [9].

The GMR effect in SVs is generally assumed to come from both interfacial and bulk spin-dependent scattering. It can also be seen from the fact that for all x the GMR ratio changes with t in two distinct stages. At the initial stage, the GMR ratio increases sharply with t. It is indicated that the interfacial spin-dependent scattering is dominant and its contribution to the GMR ratio is always positive for all x. The corresponding value of asymmetrical scattering factor ? is always larger than 1.0 for either high or low x. It has been found that ? is always larger than 1.0 both within FeCo layer and at FeCo/Cu interface [10]. At the second stage, the bulk spin-dependent scattering effect begins to participate and may even be dominant if it is large enough as t is further increased. Apparently, the bulk contribution is positive for low x and negative for high x. This means that ? is smaller than 1.0 for high x and larger than 1.0 for low x. Since ? is proportional to N(?)/N(?) [2], N(?) is larger than N(?) for high x and vice versa for low x. In a word, with thick enough FeCoGd layers the negative (positive) GMR ratio for high (low) x means the positive (negative) sign of PN in the FeCoGd layer. Finally, it is deduced that PN is negative for x ? 0.293 and positive for x ? 0.337.

Since TMR effect is essentially an interfacial tunneling behavior and mainly determined by the FM-insulator bonding, it will not be more informative to study the effect of t on the

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TMR ratio. Therefore, we have studied the TMR ratio in FeCoGd (30 nm)/AlO/FeCo (28 nm) MTJs as a function of x. Figure 3 shows that the TMR ratio is positive for x ? 0.293 and negative for x ? 0.337. Since it is well known that the PTSP is positive at FeCo/AlO interface, the FeCoGd layer should offer a negative (or positive) PTSP if x ? 0.337 (or x ? 0.293) in terms of Julliere’s model [5, 11].

It is instructive to compare the GMR and TMR ratios as a function of x. As shown in Fig. 4, the GMR and TMR ratios are positive for low x and negative for high x. Here, t in SVs is chosen to be 22 nm, thick enough for the bulk spin-dependent scattering to prevail over the interfacial one. In this way, the GMR ratio contributed from the bulk spin-dependent scattering has the same sign as the TMR ratio for all x. As analyzed above, for low x PN is negative and PTSP is positive, and vice verse for high x. Apparently, PN and PTSP always have opposite signs at all x. Secondly, PN and PTSP both change their signs concurrently with x. As discussed below, this salient future is caused by the magnetization compensation in the FeCoGd layer at room temperature.

It is well known that the atomic magnetic moment of Gd is antiferromagnetically coupled to those of Fe and Co atoms. At a specific temperature, for x higher or lower than that of compensation composition, the net magnetization of RE-TM alloys is parallel and antiparallel to those of RE (TM) atoms and TM (RE) atoms, respectively. In our experiments, we found that as x is increased from 0.293 to 0.337, the sign of Kerr rotation is reversed. Apparently, one has 0.293 ? x0 ? 0.337 at the compensation composition at room temperature. Fortunately, sign changes of PN and PTSP, and the compensation composition are almost located at the same Gd content of x0.

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In MTJs employing RE-TM as electrode, the tunneling current is considered to consist of spin-polarized currents from FeCo and Gd sub-networks [12]. The entire tunneling spin polarization

in

the

FeCoGd

electrode

consists

of

two

components,

i.e.,

PTSP?sgn(x0?x)[PFeCo1?xx?PGd], where PFeCo and PGd are the tunneling spin

1?x?x/rx?(1?x)rpolarization of pure FeCo and Gd, respectively, x0 is the Gd content at compensation composition of FeCoGd layer at room temperature, and r is the ratio of the tunneling probability from the FeCo as compared to the Gd sites. Since the second term in PTSP can only change its sign when x > 0.80, the sign of PTSP in our experiments is only controlled by the first term, i.e. sgn(x0-x). Therefore, it is easily understood that PTSP changes its sign at x0.

As for PN, it affects the spin-dependent scattering probability in terms of the electronic band structure. Because the effect of spin-dependent scattering from Gd atoms is very weak due to strong shielding effect of 4f electrons [9], the spin-dependent scattering of the FeCoGd alloy is originated mainly from Fe and Co atoms. Moreover, the magnetization of FeCo sub-network parallel or anti-parallel to the applied magnetic field depends on x, which will alter the category of majority or minority spins. Therefore, PN also changes its sign at x0.

It is generally accepted that the s-character electrons are dominant in the tunnelling process by bonding effect at the FM-AlO interface for Fe, Co, Ni, and their alloys [4]. Accordingly, PTSP mainly reflects the spin polarization of s electrons. Moreover, in general, the DOS of s electrons at Fermi level is much smaller than that of d electrons and thus PN approximately represents the spin polarization of d electrons. Therefore, according to the above results, it is suggested that the spin polarization of s electrons has opposite sign to that of the spin polarization of d electrons in FeCoGd layer at all x.

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In summary, GMR and TMR effects have been investigated in FeCo/Cu/FeCoGd SVs and FeCo/AlO/FeCoGd MTJs. For SVs with all x, the contribution of interfacial spin-dependent scattering to the GMR ratio is positive. The contribution of bulk spin-dependent scattering is positive (negative) for x smaller (larger) than x0 of the compensation composition, respectively. Very interestingly, TMR ratio also changes its sign from positive to negative at x0. It is deduced that in FeCoGd layer PN and PTSP are positive and negative for low x and vice verse for high x, respectively.

Acknowledgement This work was supported by the National Science Foundation of China (Grant Nos. 10174014, 60271013, 10321003, and 60490290), and National Basic Research Program of China (No. 2007CB925104).

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Figure captions

Figure 1. GMR curves at room temperature for representative spin valves NiO(20 nm)/FeCo (2 nm)/Cu(4 nm)/FeCoGd(t nm)/NiO(2nm) with x = 0.238 (left column) and t = 6.3 nm (a), 17.5 nm (b), and with x = 0.451 (right column) and t = 8.1 nm (c), 18 nm (d).

Figure 2. GMR ratio of SVs versus t with various x at room temperature. The lines serve as a guide to the eye.

Figure 3. TMR curves in typical FeCoGd /AlO/FeCo MTJs at room temperature with x = 0.238 (a), 0.293 (b), 0.337 (c), and 0.451 (d).

Figure 4. Dependence of GMR and TMR ratios on x in NiO(20nm)/FeCo(2 nm)/Cu(4 nm)/FeCoGd(22 nm)/NiO(2 nm) SVs and FeCoGd(30 nm)/AlO/FeCo(28 nm) MTJs, respectively. The lines serve as a guide to the eye.

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