Dr Feodor Ogrin
Senior Lecturer
Extension: 4116
Telephone: 01392 724116
Electromagnetic Materials Group
Our research focus is on the interaction between light and matter, where we have a particularly strong interest in the fundamental study of electromagnetic materials that incorporate structure from the nanometre to centimetre scale.
Magnetic nanobots: construction and study of microscopic swimming machines.
The demand for miniaturisation in many areas of technology and medicine faces a number of challenges. How to deliver a precise amount of active agents to a localised target in the human body? How to design and implement complex spatio-temporal cascades of processes in e.g. microfluidic devices, such as timely delivery and homogenisation of reagents and transporting out of undesirable products in liquid volumes of the order of several nanoliteres? These are just a few questions which so far have proved difficult to solve. It is not surprising, therefore, that considerable efforts have and are being made to understand the physics behind the swimming of autonomous microscopic devices able to self-propel in viscous environments.
In this study we aim to address the question of dynamics of swimming at low Re (Reynolds number), by suggesting a novel actuation mechanism generating the necessary shape sequence and providing the requisite energy for the motion. The main objective is to construct a microscopic magnetically driven device, which will generate linear motion by using non-reciprocal displacement of its components. The proposed system employs pairs of particles, one with “hard” and the other with “soft” magnetic properties. The alternating uniform external field can “switch” the magnetisation of the soft particle while leaving the state of the hard particle the same. The interaction of the particles with two different moments (parallel and anti-parallel) provides the required driving mechanism to activate linear motion of the system.
Recently we have developed a numerical model demonstrating swimming of such a device in glycerine. The simulations show that a device based on two ~1 micron diameter bids can generate a directional displacement with velocities exceeding 100 microns per second. This performance is well exceeding the natural microscopic swimmers such E-coli bacterium or sperm cells. The model also predicts that the motion of the magnetic swimmers can be controlled. In particular, the orientation of swimming depends on a number of parameters, including the frequency and the amplitude of the magnetic actuation. By changing either of these the swimmer can change direction within a broad range of angles, thus providing a simple mechanism to manipulate the motion.
Following the theoretical work we have constructed the first experimental prototype (see the attached film) of a magnetic swimmer based on the principles proposed. The device consists of two particles, a ‘hard’ neodymium boron iron particle, typically 0.03 – 0.2 mm3 in volume and a soft ‘iron’ particle 0.01- 0.1 mm3 in volume connected by a latex ring 1 to 3 mm in diameter. To observe the motion of the device it was floated on a glycerol/air interface and its trajectories determined from analysis of video images acquired through a low power microscope. The device was activated by an alternating uniform magnetic field produced by two parallel Helmholtz coils. The field amplitude and the frequency were varied between 0-100 G and 20 to 2000 Hz respectively. The motion consisted of two components, rapid oscillation of each particle with an associated deformation of the elastic ring, and translation of the centre of mass (‘swimming’). The details of the translational motion depended on the dimensions of the device and the applied field. In particular, as predicted by the model, it was possible to control its direction and speed by varying the amplitude and frequency of the applied field. Translational velocities of up to 500 microns/second were produced in a field of 100 G, thus demonstrating swimming in the low-Re domain (Re ~ 0.0005).
The demonstrated mechanism of actuation can be utilised in a number of applications. This includes a range of microfluidic devices: such as microscopic pumps, stirrers and mixers, and other applications where locally induced forces are required. Our group is currently looking at the possibility to utilise these principles in tactile devices, where the dipole-pairs would be playing a role of touch sensors/stimulators.
Theoretical aspects of this work were published in Physical Review Letters, vol 100 Article number 218102 (May 2008), which can be downloaded here.
This download is made available for personal use only. Any other use requires prior permission of the author and the copyright holder. Copyright (2008) American Physical Society. The article above may also be found on the journal's website.
Magnetic Materials and Dynamics
Key personnel: Prof Rob Hicken, Dr Feodor Ogrin, Dr Volodymyr Kruglyak
Today most research into magnetic materials is driven by the needs of the data storage industry where there is an insatiable demand for devices such as disk drives that are able to record and retrieve larger quantities of data on ever shorter timescales. Advances in nano-fabrication have led to the development of new materials with tunable magnetic properties and to the miniaturization of the components of the disk drive system. In Exeter we investigate the basic physical properties of nanoscale magnetic materials and are particularly interested in dynamic magnetic phenomena that may occur on femtosecond through to geological timescales. These include precessional switching of the macroscopic magnetisation, spin wave excitations, and thermally activated reversal of the magnetization.Picosecond precessional magnetization dynamics are fundamental to the operation of microwave frequency devices and high bit rate data storage systems. We use time resolved optical techniques to explore these dynamics. The sample magnetization is 'pumped' with an optically triggered magnetic field pulse that has arise time in the range 10 to 100 ps. The response is 'probed' with a Magneto Optical Kerr effect (MOKE) measurement made with a 100 fs laser pulse. The technique is non-resonant and well suited to the study of non-linear effects and relaxation processes. We study Ferromagnetic Resonance and large angle magnetization switching in thin film elements and the measurements may be used to characterize the magnetic parameters of the sample including the Gilbert damping constant.
The principle of operation of the time resolved MOKE measurements is shown in the schematic diagram above. An ultrafast Titanium-sapphire laser supplies pulses of light, less than 100 fs in duration at a repetition rate of 80 MHz. Each pulse is divided into two parts and the first part is used to "pump" the sample. The second is time delayed before being used to "probe" the sample at a later time. Every pair of pulses pump and probe the same process. By averaging for just 1 second, 80 million events are collected, making for an excellent signal to noise ratio. The time delay can be varied by reflecting the probe beam off a mirror mounted on a translation stage, so that the sample response can be mapped out as a function of delay time. The MOKE measurement senses the instantaneous magnetic state of the sample. The sample magnetization causes the plane of polarization of the probe beam to be only slightly rotated, but an optical "bridge" detector consisting of a polarizer and two photodiodes allows the rotations of down to 1 microdegree to be resolved. The schematic shows how a pulsed magnetic field may be generated by an optically triggered Au/GaAs photoconductive switch and delivered to the sample by a coplanar strip transmission line.
Time resolved MOKE measurements may also be performed using commercial pulse generators or microwave synthesizers to excite the sample. In this case measurements are performed on a microwave probe station on which high frequency probes deliver the exciting waveform to a coplanar waveguide structure into which the sample structures are integrated. A more advanced version of the optical bridge detector employs 2 quadrant photodiodes (see photo) and allows all 3 components of the magnetization vector to be detected simultaneously.
Synchroton radiation sources also have a pulsed time structure that is ideally suited to time resolved measurements. Specifically we are using soft X-rays to measure X-ray magnetic circular dichroism (XMCD) in transition metal ferromagnets. XMCD has the advantage that is it element specific and can resolve both the orbital and spin contributions to the magnetic moment. Secondary electrons generated by X-ray absorption may be collected with an electron microscope column and used to generate magnetic images of the sample. Dynamic measurements are performed by synchronizing either an exciting electrical waveform or an ultrafast laser with the X-ray pulses.
