Spintronics: from ferromagnets to antiferromagnets
Maxim Tsoi
Physics Department, University of Texas at Austin
Date: October 19, 2007, Time: 3:30 pm, Location: W122-D3 Engineering Building 1, The University of Houston
Abstract:
Spintronics in ferromagnetic systems is built on a complementary set of phenomena in which the magnetic configuration of the system influences its transport properties and vice versa. For instance, the resistance of magnetic multilayers [1] depends on the relative orientation of magnetic moments in their constituent ferromagnetic (F) layers [giant magnetoresistance (GMR)]. The inverse effect [spin transfer (spin-torque)] - in which a large electrical current density j can perturb the magnetic state of a multilayer - has also been predicted [2] and observed in experiments on current-induced reversal and precession of magnetization [3] and magnetic domain wall motion [4].
Recently, corresponding effects were proposed [5] to occur in systems
where ferromagnetic components are replaced by antiferromagnets (AFM).
First, it was predicted that resistance of an AFM spin valve – where two AFM
layers are separated by a nonmagnetic (N) spacer – could depend upon the
relative orientations of magnetic moments in the two AFM layers
(antiferromagnetic GMR). Second, injection of a strong enough j into an
antiferromagnet was predicted to affect the magnetic state of the
antiferromagnet.
We now demonstrate transfer of spin angular momentum across an interface
between F and AFM metals [6]. The spin transfer is revealed by variation in
the exchange bias at F/AFM interface which can either increase or decrease
depending on the polarity of electric current flowing across the interface.
Such a current-mediated variation of exchange bias can be used to control
the magnetic state of spin-valve devices, e.g., in magnetic memory
applications.
Recently [7] we undertook the next step toward AFM-based spintronics, by
studying current induced effects in systems containing two AFMs separated by
a non-magnetic (N) metal spacer. In multilayers of the form F/AFM/N/AFM/F,
we find a small positive (largest resistance at high field)
magnetoresistance (MR) only when a large enough current is applied
perpendicular to the multilayer. As this MR is inverted from the usual GMR
associated with the F-layers [1], it must be due to the AFM layers. To
explain our results we exploit the original predictions [5] that resistance
of a circuit containing AFM elements depends on the relative orientation of
its AFM parts.
These new AFM effects support the feasibility of all-antiferromagnetic
spintronics where antiferromagnets are used in place of ferromagnets.
[1] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988); G. Binasch et
al., Phys. Rev. B 39, 4828 (1989). [2] J. C. Slonczewski, J. Magn. Magn.
Mater. 159, L1 (1996); L. Berger, J. Appl. Phys. 81, 4880 (1997). [3] M.
Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998); E. B. Myers et al., Science
285, 867 (1999); J. Z. Sun, J. Magn. Magn. Mater. 202, 157 (1999) ; J.-E.
Wegrowe et al., Europhys. Lett. 45, 626 (1999); M. Tsoi et al., Nature 406,
46 (2000). [4] G. S. D. Beach et al., Phys. Rev. Lett. 97, 057203 (2006).
[5] A. S. Núñez et al., Phys. Rev. B 73, 214426 (2006). [6] Z. Wei et al.,
Phys. Rev. Lett. 98, 116603 (2007). [7] Z. Wei et al., to be published
