Temperature dependence of anisotropic magnetoresistance and atomic motion in ferromagnetic break junctions
We fabricate ferromagnetic nanocontacts with cross-sections a few atomic-diameters wide using electromigration. These devices exhibit a large anisotropic magnetoresistance (AMR) signal as a function of the angle of an applied magnetic field. We measure a strong temperature dependence of this effect at cryogenic temperatures, in quantitative agreement with the expectation for a quantum-interference mechanism. In the course of making these measurements, we also observed two-level resistance fluctuations as a function of time, associated with reconfigurations of the atomic structure, which depend sensitively on the magnetic-field angle.
Sufei Shi, Kirill I. Bolotin, Ferdinand Kuemmeth
Recent experiments  have found that the anisotropic magnetoresistance (AMR) of nanometer-scale ferromagnetic contacts at low temperature can be much larger than that of bulk samples, and can exhibit more complicated variations as a function of sample bias and the angle of an applied magnetic field than in the bulk case. Here we test a proposal that quantum interference of electrons may explain these results, by measuring the temperature dependence of the AMR signals in nanometer-scale contacts made from permalloy, nickel, and cobalt.
We fabricate our devices by electron beam lithography to define a 100-nm-wide, 30-nm-thick ferromagnetic wire connected to gold contact pads. By using electromigration  with active feedback, we form a constriction in the ferromagnetic wire and reduce the contact cross section at 4.2 K while monitoring the resistance. We can reliably achieve nanometer-scale contacts within approximately 10% of a desired resistance value. We then rotate the angle of a large magnetic field in the plane of the sample, and use a lock-in amplifier to measure the differential resistance. We could control the measurement temperature by using a resistive heater inside our cryostat.
When any of our devices is narrowed to the point that the resistance is greater than about 1 kOhm, we observe enhanced AMR variations at T = 4.2 K as a function of field angle and bias, in agreement with our previous study. When we increase the device temperature from 4.2 K, the AMR variations decrease significantly in amplitude and variations as a function of bias voltage smoothen out (Fig. 1). The temperature dependence fits well to the convolution of the low-temperature conductance with the derivative of the Fermi distribution at finite temperature (Fig. 2). This is the type of strong temperature dependence that is expected for a quantum-interference effect.
In the course of exploring the temperature-dependent transport properties of the ferromagnetic contacts, we also observed abrupt changes in the conductance at particular angles of magnetic field in about 10% of samples (Fig. 3). When we set the field angle to values close to the abrupt steps, we observed two-level conductance fluctuations as a function of time due to atomic motion. The duty cycle of the fluctuations changed from 0 to 100% within a small interval of angle (Fig. 4). The observation of these time-dependent fluctuations at fixed magnetic-field angle demonstrates that the abrupt jumps in Fig. 3 are due to mechanical instabilities, rather than being an intrinsic electronic effect as has been claimed by other groups. We find that mechanical instabilities and two-level conductance fluctuations become increasingly common in all of our contacts above a few 10s of degrees Kelvin, and in a few samples are present even at 4.2 K. This suggests that magnetic nanocontacts measured at even higher temperatures are likely to be highly dynamic, unstable structures.
Figure 1: AMR of a Ni nanocontact as a function of magnetic field angle. Inset: SEM image of the Ni wire before electromigration.
Figure 2: Temperature dependence of the resistance of a Ni nanocontact as a function of bias.
Figure 3: Abrupt conductance changes as a function of magnetic-field angle for a Ni electrode at 4.2 K.
Figure 4: Time-resolved conductance fluctuations for magnetic field angles in the vicinity of the abrupt step in conductance.
- K. I. Bolotin et al., Anisotropic Magnetoresistance and Anisotropic Tunneling Magnetoresistance due to Quantum Interference in Ferromagnetic Metal Break Junctions, Phys. Rev. Lett. 97, 127202 (2006).
- H. Park et al., Fabrication of metallic electrodes with nanometer separation by electromigration, Appl. Phys. Lett. 75, 301 (1999).
Last updated: 11-July-2007