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Low-dimensional quantum magnets for thermal management

Final Report Summary - LOTHERM (Low-dimensional quantum magnets for thermal management)

Thermal management in innovative technologies is a major problem to be solved, especially in novel electronic devices [1], such as further miniaturized microchips, hard disks and interfaces between biological structures (e.g. nerve cells) and semiconductor microstructures [2]. This is reflected in current technological strategy documents by the European Commission [3], where “exploiting new materials and design approaches to manage energy and heat dissipation at the nano level“ has been identified as an important long term research target in order to sustain the growth of technology in Europe. This is exactly the scientific goal of our project and the basis for the research training in this ITN.

About 10 years ago and in agreement with theoretical predictions [4], a new highly efficient mode of thermal conduction was discovered, namely heat transport by magnetic excitations in quasi one-dimensional (1D) insulating quantum-magnetic materials [5,6]. The magnetic conduction is highly anisotropic, dwarfing the usual lattice contribution (by up to a factor of 50) and has mostly been studied in novel copper based oxides with one-dimensional magnetic structures (so-called “spin” structures) such as the spin chain materials Sr2CuO3 and SrCuO2 and the “ladder” compound (La,Sr,Ca)14Cu24O41. The crucial point of this project is that around room temperature the magnetic heat conductivity kappa-mag, i.e. the heat conductivity that arises from magnetic excitations is of the order of 100 1/Wm*1/K. This is as efficient as metallic heat conduction, e.g. that of pure iron. However, compared to conventional materials with high thermal conductivity these novel compounds offer the following advantages:

• They are electrically insulating and can therefore be used to simultaneously electrically insulate and carry away heat.
• Heat is conducted primarily along one crystal axis, hence the material can thermally insulate in two directions and carry away heat along the other.
• Heat is carried by localised magnetic moments which can be manipulated with magnetic fields or light. Thus, an electrical insulator with tunable heat conductivity at room temperature could emerge.

The aim of LOTHERM was thus to explore the exploitation of these low dimensional magnetic compounds for technological applications, in particular thermal management. In order to tackle this scientific challenge in a multidisciplinary approach (physics, chemistry, materials science, computational science, electronic media), the fellows in our ITN have been trained in cutting edge experimental and theoretical techniques. We furthermore applied a state-of-the-art training through electronic media for reinforcing the effectiveness and availability of training within this ITN, using a newly developed e-learning platform. Members of the project have been trained on using the platform; they used the platform for their daily work frequently.

The main scientific-technological objective of this project was to provide the knowledge and excellence which is necessary to exploit low-dimensional quantum magnets for advanced and innovative thermal management. During the course of the project we have worked on the fundamental steps towards achieving our primary objectives. On the experimental side we have grown high purity single crystals of prototype materials with high kappa-mag (such as SrCuO2, Sr2CuO3, La5Ca9Cu24O41, and La2CuO4). Furthermore, we have successfully grown single crystals of new materials, either impurity doped variants of the above listed compounds (e.g. Ca-substituted Sr2CuO3), or new materials featuring at present unexplored low-dimensional quantum magnets such as the five-leg ladder compound La8Cu7O19.

Basic characterization of the materials shows their in-principle long-term applicability at high temperatures. Thermal conductivity measurements on undoped Sr2CuO3 single crystals of SrCuO2 and yield unprecedentedly high kappa-mag values (up to ~1000 1/Wm*1/K) and provide evidence for ballistic heat transport in Heisenberg spin chains. On the other hand, artificial doping-induced disorder leads to a drastic suppression of kappa-mag and induces a peculiar spin gap in the formerly gapless ground state of S=1/2 Heisenberg spin chains. In addition, we have started successfully to grow thin films of these materials – a crucial step towards technical exploitation. The films were characterized using advanced thermal analysis and imaging techniques.

Considerable progress has been made in the development and refinement of novel experimental techniques for studying the magnetic transport and dynamics: We obtained for the very first time high-quality high-temperature heat conductivity data up to about 800K. Two novel experimental approaches to study dynamic heat transport have been introduced: time resolved microthermal imaging and the fluorescent flash method. Furthermore, an ultrafast spatially resolved microscopy method to study heat and particle diffusion on micron length scales and picosecond time scales has been developed. These techniques allowed us for the first time to determine the phonon-spin equilibration time, i.e. the time that a magnetic excitation needs to decay into a lattice vibration, in the spin-ladder compound.

We have also done cutting-edge theoretical and numerical work for understanding the magnetic transport and dynamics at a microscopic level, and performed simulations for elucidating the thermal management capabilities of high kappa-mag materials. This includes design simulations of heat flow in order to consider realistic electronic device configurations as well as new concepts of so-called thermal rectification. Finally, based on crucial requirements for heat channelling we designed, fabricated and characterized a device, which is based on a high kappa-mag material and which demonstrates anisotropic heat distribution on the surface and thus the extraordinary heat conduction properties of these novel compounds.

Coordinator contact details:

Dr. Christian Hess
Leibniz Institute for Solid State and Materials
Research Dresden (IFW Dresden)
Helmholtzstrasse 20
01069 Dresden
Tel.: +49-(0)351-4659533
Fax: +49-(0)351-4659313

[1] K. Watari, S. L. Shinde, MRS Bulletin 26, 440 (2001).
[2] P. Fromherz, Physica E 16, 24 (2003).
[3] Vision 2020: Nanoelectron. at the centre of change, Europ. Comm. ISBN 92-894-7804-7 (2004).
[4] X. Zotos, F. Naef, P. Prelovšek, Phys. Rev. B 55, 11029 (1997).
[5] C. Hess, et al. Phys. Rev. B 64, 184305 (2001).
[6] N. Hlubek, et al., Phys. Rev. B 81, 020301(R) (2010)