The Dynamics of Artificial Spin Ice in Real and Reciprocal Space

Artificial Spin Ice (ASI) have been fabricated from NiFe in the square geometry over areas 1-2mm^2. They have small enough volumes to be thermally active within an experimental temperature range. The dynamic hysteresis has been measured using a SQUID-VSM magnetometer. We found that larger dipolar interaction can stabilise the superparamagnetic behaviour and lead to increased order at low temperature. Variation in the observed average blocking temperatures, recorded via hysteresis and zero- field-cooled (ZFC) experiments, is attributed to the distribution of island sizes and interaction effects.

A new synchrotron capability at the I10 beamline in Diamond Light Source (UK) has been implemented and used to measure islands as small as 30 × 70 × 8 nm^3. At this size, measuring the exact microstates of the individual nanomagnets is challenging. Although this method is a scattering process, it maintains sensitivity to the microstates of the system by using coherent X-rays to produce a speckle pattern. Dynamics were observed in the speckle pattern in the temperature range 180 - 250 K. The
characteristic relaxation time was fitted to a Vogel- Fulcher law, with an activation temperature of 40±10 K and freezing temperature of 178±5 K. From magnetometry measurements and simulations we attribute the activation temperature as originating from the non-uniform magnetic structure at the ends of the islands. The freezing temperature relates well to the energy scale of the interaction.

Finally, a full-field magnetic microscopy method to probe dynamic ASI has been demonstrated; transmission X-ray microscopy (TXM). The method uses circularly-polarised soft X-rays to probe the magnetic orientation of the individual nanoislands. We have developed an on-membrane heater and thermometer, which is capable of temperatures in excess of ≈ 700 K. We have used it to heat the ASI whilst tracking the individual vertex states. We have been able to measure the creation and propagation of emergent monopole excitations and observed increased avalanche velocities and magnetic mobilities at higher temperatures. The largest change in the magnetic
mobility was found for the most strongly interacting array, increasing by 1.7±0.7mm2A−1s−1 for ∆T ≈ 30 K.