SEARCH

SEARCH BY CITATION

Keywords:

  • manganites;
  • magnetoresistance;
  • spin-calorics;
  • superlattices

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References

High quality La0.7Sr0.3MnO3/SrRuO3 superlattices with ultrathin individual layers were grown by pulsed laser deposition and were studied by magnetometry and magnetoresistance measurements. Depending on layer thickness and structural quality, exchange biasing could be observed both in magnetization and magnetotransport. In this regime the superlattice magnetization forms an exchange spring that leads to a reversible field dependence of the magnetoresistance and the magnetic work. It is shown that the magnetic work and the magnetocaloric effect can be tuned by the SrRuO3 layer thickness. This opens up the possibility of fabricating spin-caloric devices from wedge-shaped superlattices with self-sustaining Seebeck effect. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References

Manganites are one of the most studied material systems in magnetocalorics 1. Further, La0.7Sr0.3MnO3/SrRuO3 (LSMO/SRO) superlattices were recently investigated, since these have an anisotropic and enhanced magnetocaloric effect 2, 3. These superlattices are of particular interest since they show a structural transition of the SRO layers as a function of layer thickness 4, 5; because SRO is a magnetically hard material, the magnetization curves are drastically modified by this transition. The purpose of the present paper is to explore the use of LSMO/SRO superlattices as a spin-caloric material 6, i.e., especially the effect of the structural transition as well as exchange biasing on the magnetic work is studied. Based on this the concept for a spin-caloric device is proposed.

Experimental and results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References

Experimental

LSMO/SRO superlattices (SLs) were fabricated by pulsed laser deposition (KrF laser). The substrate temperature was 650 °C and oxygen partial pressure 0.14 mbar. Vicinal SrTiO3 (001) and Nb-doped SrTiO3 (001) substrates with a miscut angle of about 0.1°, uniform TiO2-termination and an atomically flat terrace morphology were used. X-ray diffractometry showed superlattice reflections, atomic force microscopy a smooth surface with monatomic steps and transmission electron microscopy (TEM) showed coherent growth. Details on the structural characterization can be found in 7–9. Here the focus is on three samples with LSMO/SRO layer thickness of 1.6/3, 1.6/8 and 2.4 nm/5 nm, denoted in obvious notation by SL1.6/3, SL1.6/8 and SL2.4/5. The layer thickness was determined from TEM measurements and has an uncertainty of about half a unit cell 10. The magnetic properties of the SLs were measured by SQUID magnetometry. The magnetic field was applied parallel to the superlattice. The magnetic work density Wm done on the sample was calculated from the magnetization curves using Wm = μ0H dM, where M denotes the magnetization

and H the internal magnetic field. Magnetoresistance measurements were made with a conventional four-point technique.

Results

Figure 1 shows the magnetization of superlattices SL1.6/3 and SL1.6/8 at 10 K and 100 K as a function of the magnetic field. The magnetization of sample SL1.6/3 shows three distinct magnetization steps with a hysteresis loop around zero field and shifted hysteresis loops centered about ±4.2 T (10 K) and ±3.1 T (100 K). The latter shifted loops are caused by the reversal of the magnetically hard SRO layers, whereas the hysteresis loop centered at zero field is due to a collective reversal of the ferrimagnetically coupled layers 8. In contrast, sample SL1.6/8 shows only a single hysteresis loop at both temperatures. This change in the magnetic field dependence of the hysteresis loop is due to a structural transition of the SRO layers occurring as a function of SRO layer thickness between 5 nm and 8 nm 5. In SL1.6/3 the SRO layers have tetragonal symmetry with in-plane magnetic easy axes, in SL1.6/8 the SRO layers have orthorhombic symmetry with a nearly out-of-plane magnetic easy axis. Sample SL1.6/8 is in a canted magnetization state and this determines the shape of the hysteresis loop that is controlled by domain rotation.

thumbnail image

Figure 1. Magnetization of superlattices SL1.6/3 and SL1.6/8 at (a) 10 K and (b) 100 K.

Download figure to PowerPoint

Since the resistivity of ultrathin SRO layers is much smaller than that of ultrathin LSMO layers, it is not evident that the magnetization reversal of LSMO layers affects the magnetotransport properties of the SLs. Figure 2 shows the longitudinal (Hj) here j denotes the current density) and the transverse (Hj) magnetoresistance of sample SL2.4/5. At 10 K, maxima and minima appear in the magnetoresistance at high fields when the SRO layers reverse. There are no corresponding features at low fields at this temperature, since in this sample the magnetically soft LSMO layers form an exchange spring magnet coupled to the magnetically hard SRO layers 11, such that the LSMO layers rotate gradually with respect to the SRO layers when the applied field is varied 8. This interpretation is in agreement with the observation that the magnetoresistance is reversible during a minor loop, see symbols in Fig. 2(a), since the LSMO magnetization is reversibly rotated. At 100 K the magnetization process is different 8 and corresponds to the magnetization of sample SL1.6/3 shown in Fig. 1(b). Accordingly, magnetoresistance maxima and minima do not only appear at high fields, when the SRO layers reverse, but also at low fields, when the ferrimagnetically coupled SL reverses its magnetization. Thus in these SLs electronic and magnetic features are intimately correlated.

thumbnail image

Figure 2. Longitudinal and transverse magnetoresistance of superlattice SL2.4/5 at (a) 10 K and (b) 100 K. Solid lines show the full hysteresis curves between –7 T and +7 T, symbols show minor loops between –0.9 T and +7 T. The solid and dotted arrows indicate MR features observed at the switching of the SRO and LSMO layers, respectively.

Download figure to PowerPoint

Figure 3 shows the magnetic work per unit mass ΔWm done on the sample during the hysteresis cycle. This was obtained by integration of the magnetization curves, setting the work in the initial state at +7 T to zero. The work was normalized to the sample mass using an average mass density of 6.5 × 103 kg/m3. At 10 K the magnetic work is hysteretic over the full cycle, since energy proportional to the enclosed area of the magnetization cycle is dissipated in the sample by irreversible domain-wall movement. If the hysteresis loop is restricted to a minor loop in the exchange-spring regime (symbols in Fig. 3(a)), the magnetic work is reversible in field. If the sample would be thermally isolated such that the magnetic work is fully converted into heat, the entropy change along this reversible branch between 7 T and 0 T would beΔSm = ΔWm/T ≃ –9 J/kg K of the same order as the magnetocaloric effect at the Curie temperature reported for similar superlattices in 2, 3. At 100 K the exchange spring state is not realized anymore and the magnetic work is hysteretic even for minor loops of the system due to the irreversible switching of the magnetization.

thumbnail image

Figure 3. Magnetic work done on superlattice SL2.4/5 at (a) 10 K and (b) 100 K. Black squares show the full hysteresis curves between –7 T and +7 T, symbols show minor loops with a maximum field of +7 T and minimum fields of –1.25 (10 K), –0.9 T and +0.9 T (100 K), respectively.

Download figure to PowerPoint

In Fig. 4 the magnetic work per unit mass ΔWm done on samples SL1.6/3 and SL1.6/8 during a hysteresis cycle is compared. Clear differences in the magnetic work cycles are seen between the two samples, since – as discussed above – the SRO layers in the samples have different symmetry, thus leading to different magnetocrystalline anisotropy and magnetization reversal mechanisms. At 10 K, coming from 7 T, the difference between the magnetic work of the two samples in zero field is about 24 J/kg. This shows that it is possible to change the magnetic work strongly by tuning the thickness of the SRO layers.

thumbnail image

Figure 4. Magnetic work done on superlattices SL1.6/3 and SL1.6/8 at (a) 10 K and (b) 100 K.

Download figure to PowerPoint

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References

In this work it was shown that the magnetic work done on a LSMO/SRO superlattice can be engineered either by tuning the crystalline symmetry or by using exchange biasing to modify the shape of the hysteresis loop. Based on these findings the following spin-caloric sensor is proposed. Consider a wedge-shaped SL sample – indicated schematically in Fig. 4 – with LSMO layers of constant thickness and SRO layers with a thickness varying along the sample. Depending on the location on the sample the magnetization hysteresis curves should have different shapes; the ones on the thin end should be similar to those of sample SL1.6/3, the ones on the thick end similar to SL1.6/8. Accordingly, when the sample is thermally isolated and the magnetic field is cycled rapidly, different amounts of work should be done on both ends of the sample such that a temperature gradient develops across the sample. The latter might be detected by measuring the Seebeck effect. This device would show a self-sustained Seebeck effect, since there is no active heating element at one end of the sample, but the temperature difference is generated solely by the differences in the magnetic work. Of course, the performance of such a device depends critically on the presence of heat leaks, especially on the thermal coupling to the substrate. Here one might think of free-standing samples as were already realized for single SrRuO3 films 12. A rough estimate of a temperature change would be given by ΔT = ΔWm/C where C denotes the unknown heat capacity of the SL. Using the heat capacity of LSMO at 10 K, C ≃ 0.2 J/kg K 13 as an estimate, one would obtain a temperature difference of the order of ΔT ≃ 120 K. Although this estimate is too simplistic, it shows that the proposed spin-caloric device would have some potential to be realized.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References

This work was supported by the DFG within SFB 762 “Functionality of Oxide Interfaces”. I thank Dr. Ionela Vrejoiu (Max-Planck-Institute of Microstructure Physics, 06120 Halle, Germany) for fabrication and structural characterization of the superlattices.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental and results
  5. Discussion
  6. Acknowledgements
  7. References