Magnetoresistive detection of strongly pinned uncompensated magnetization in antiferromagnetic FeMn

We observed and studied pinned uncompensated magnetization in an antiferromagnet using magnetoresistance measurements. For this, we developed antiferromagnet-ferromagnet spin valves (AFSVs) that consist of an antiferromagnetic layer and a ferromagnetic one, separated by a nonmagnetic conducting spacer. In an AFSV, the uncompensated magnetization in the antiferromagnet affects scattering of spin-polarized electrons giving rise to giant magnetoresitance (GMR). By measuring angular dependence of AFSVs' resistance, we detected pinned uncompensated magnetization responsible for the exchange bias effect in an antiferromagnet-only exchange bias system Cu/FeMn/Cu. The fact that GMR measured in this system persists up to 110 kOe indicates that the scattering occurs on strongly pinned uncompensated magnetic moments in FeMn. This strong pinning can be explained if this pinned uncompensated magnetization is a thermodynamically stable state and coupled to the antiferromagnetic order parameter. Using the AFSV technique, we confirmed that the two interfaces between FeMn and Cu are magnetically different: The uncompensated magnetization is pinned only at the interface with the bottom Cu layer.

Figure 1

Magnetic hysteresis loop of the $\mathrm{Ta}(5\text{-nm)}/[\mathrm{Cu}{\left(5\text{-nm)/FeMn}\left(5\text{-nm}\right)\right]}_{10}/\mathrm{Ta}\left(5\text{-nm}\right)$ stack measured at 10 K after a 70-kOe-magnetic-field cooling. The bottom-right inset shows the central part of the loop with a clear indication of exchange bias shift. The exchange bias field is $H_E=-985\mathrm{Oe}$. The top-left inset depicts the pinned and unpinned uncompensated magnetic moments inside the FeMn layer; the compensated antiferromagnetic spin structure is not shown.

Figure 2

Angular dependences of resistance of the Ta(5-nm)/Cu(5-nm)/Py(2-nm)/Cu(5-nm)/FeMn(5-nm)/Cu(5-nm)/Ta(5-nm) AFSV measured in 200 Oe (black line/open dots) and 70 kOe (black line/solid dots) after the AFSV was cooled down from 300 K in (a) $H_{\mathrm{COOL}}=70\mathrm{kOe},{\mathrm{\Theta }}_{\mathrm{COOL}}=0^{\circ }$; (b) $H_{\mathrm{COOL}}=70\mathrm{kOe},{\mathrm{\Theta }}_{\mathrm{COOL}}={90}^{\circ }$; (c) $H_{\mathrm{COOL}}=0\mathrm{kOe}$. The zero angle corresponds to the orientation of the AFSV where the current is parallel to the external magnetic field. (d) and (e) are schematics illustrating the structure of the AFSV and scattering of polarized electrons on pinned uncompensated magnetic moments, respectively.

Figure 3

The difference in resistances measured at 180° and 0°, $\mathrm{\Delta }{R}_{\uparrow \downarrow }$ at $T=10\mathrm{K}$ after field cooling at ${\mathrm{\Theta }}_{\mathrm{COOL}}=0^{\circ }$ for the Ta(5-nm)/Cu(5-nm)/Py(2-nm)/Cu(5-nm)/FeMn(5-nm)/Cu(5-nm)/Ta(5-nm) AFSV as a function of magnetic-field $H_{\mathrm{MEAS}}$ applied during the measurement.

Figure 4

(a) Angular dependence of the resistance of the Ta(5-nm)/Cu(5-nm)/FeMn(5-nm)/Cu(5-nm)/Py(2-nm)/Cu(5-nm)/Ta(5-nm) AFSV measured in 70 kOe after the AFSV was cooled down in $H_{\mathrm{COOL}}=70\mathrm{kOe}$ applied at ${\mathrm{\Theta }}_{\mathrm{COOL}}=0^{\circ }$. The inset in the top-right corner shows the magnified peaks. (b) and (c) are schematics illustrating the scattering of polarized electrons at the Cu/FeMn interface and the structure of the AFSV, respectively.