Anisotropic Magnetoresistance and Anisotropic Tunneling Magnetoresistance due to Quantum Interference in Ferromagnetic Metal Break Junctions

Abstract

We measure the low-temperature resistance of permalloy break junctions as a function of contact size and the magnetic field angle, in applied fields large enough to saturate the magnetization. For both nanometer-scale metallic contacts and tunneling devices we observe large changes in resistance with angle, as large as 25% in the tunneling regime. The pattern of magnetoresistance is sensitive to changes in bias on a scale of a few mV. We interpret the effect as a consequence of conductance fluctuations due to quantum interference. PACS numbers: 72.25.Ba; 73.63.Rt; 75.75.+a The magnetoresistance properties of nanometer-scale magnetic devices can be quite different from those of larger samples. One aspect of this difference has been explored extensively in previous experiments -the resistance of magnetic domain walls created when the magnetic moment direction in one magnetic electrode is rotated relative to the moment in a second electrode [1,2,3,4,5,6,7,8,9]. Here we focus on a different aspect of the physics of magnetoresistance in nanoscale magnetic contacts -the anisotropic magnetoresistance (AMR) that arises when the magnetization throughout a device is rotated uniformly so as to change the angle between the direction of current flow and the magnetic moment. Our measurements are motivated by predictions of increased AMR for atomic-sized ballistic conductors [10] and indications of enhanced AMR in Ni contacts [8]. By making detailed studies of resistance as a function of field angle using mechanically-stable permalloy contacts, we show that the size of the AMR signal at low temperature can increase dramatically as the contact cross section is narrowed to the nanometer-scale regime. Even more strikingly, we find that point contacts which are completely broken, so as to enter the tunneling regime, also exhibit a tunneling anisotropic magnetoresistance effect (TAMR) as large as 25% when the magnetic-moment directions in the two contacts are rotated together while remaining parallel.Magnetostriction and magnetostatic forces can alter the geometry of nanoscale junctions as the magnetic field is varied, and produce artifacts in the resistance, so experiments must be designed to minimize these effects [5,6,7]. For this reason, our contacts are firmly attached to a non-magnetic silicon substrate and are measured entirely at low temperature to suppress thermally-driven surface diffusion of metal atoms. Similar structures have proven [8,9] to be much more mechanically-stable than previous samples which were measured at room temperature. We fabricate our devices using aligned steps of electron beam lithography to first pattern 20-nm-thick gold contact pads and then 30-nm-thick magnetic permalloy (Py = Ni 80 Fe 20 ) point contacts [9]. Each contact con- sists of two elongated electrodes which are connected by a 100-nm-wide bridge ( Fig. 1(b)). The magnetic field B is applied using a 3-coil vector magnet capable of 0.9 T in any direction and up to 7 T along one axis (the x axis, defined below) with the other two coils t...

Anisotropic magnetoresistance in nanocontacts

“…1,2 The transport counterpart of MAE is anisotropic magnetoresistance ͑AMR͒, i.e., the dependence of the resistance on the angle between the magnetization and the current flow. Whereas AMR in bulk was known back in the 19th century and is a rather small effect, the recent observation of AMR in a variety of low dimensional systems, [3][4][5][6][7][8][9][10][11][12] largely exceeding bulk values, has opened a new research venue in the field of spin-polarized quantum transport. Very large AMR has been reported in planar tunnel junctions ͓tunneling anisotropic magnetoresistance ͑TAMR͔͒ with a variety of electrode and barrier materials.…”

“…Very large AMR has been reported in planar tunnel junctions ͓tunneling anisotropic magnetoresistance ͑TAMR͔͒ with a variety of electrode and barrier materials. [3][4][5][6][7][8] Enhanced AMR has also been observed in atomic sized contacts, both in the tunnel regime ͑TAMR͒ and in the contact ͑or ballistic 13 ͒ regime ͓ballistic anisotropic magnetoresistance ͑BAMR͔͒, 14 for Py, 9 Fe, 10 Ni, 11 and Co. 12 Additionally, GaMnAs islands in the Coulomb blockade regime show electrically tunable AMR. 15 Thus, a wide range of nanostructures made from different materials display enhanced AMR.…”

“…On the other hand, nanocontacts allow us to study AMR going from the contact ͑BAMR͒ to the tunnel ͑TAMR͒ regime in the same system, as shown in the case of both Ni and Py. 9,18 Ni nanocontacts have also been used as electrodes to explore the Coulomb blockade and the Kondo regimes. 19 The crux of the matter is to identify the necessary and sufficient conditions to expect large values of AMR in quantum transport.…”

Conductance fingerprints of noncollinear magnetic states in single-atom contacts: A first-principles Wannier-functions study

“…[22][23][24] Finally, the effect of spin-orbit coupling on the conductance needs to be considered, 16,25 which leads to novel transport phenomena such as the ballistic anisotropic magnetoresistance 16,26 or the tunneling anisotropic magnetoresistance. 27,28 Recently, the transition regime from tunneling to contact in a spin-polarized STM geometry has been studied based on density functional theory in order to explain, e.g., the conductance of a single magnetic atom, 5 and to analyze the contribution from different conduction channels. 21 As a magnetic STM tip approaches a single magnetic atom on a surface to measure the distance-dependent conductance (as in Ref.…”

Anisotropic magnetoresistance in single electron transport

“…Recently, the concept of Tunneling Anisotropic Magnetoresistance (TAMR) has been proposed theoretically [5] and independently verified in experiments [6,7,8] in tunnel junctions with GaAsMn electrodes. The related concept of Ballistic Anisotropic Magneto Resistance (BAMR) has been proposed theoretically [9] and observed in atomic sized Nickel [10] and Iron nanocontacts [11]. As opposed to TMR and BMR, where the high and low resistance states are related to variations in Ω 1 · Ω 2 , TAMR and BAMR effects occur for Ω 1 = Ω 2 ≡ Ω and depend on the angle between the transport direction and Ω, which is controlled by an external field.…”