Completely inverted hysteresis loops: Inhomogeneity effects or experimental artifacts C. Song, B. Cui, H. Y. Yu, and F. Pan Citation: J. Appl. Phys. 114, 183906 (2013); doi: 10.1063/1.4830011 View online: http://dx.doi.org/10.1063/1.4830011 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i18 Published by the AIP Publishing LLC.
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JOURNAL OF APPLIED PHYSICS 114, 183906 (2013)
Completely inverted hysteresis loops: Inhomogeneity effects or experimental artifacts C. Song,1,a) B. Cui,1 H. Y. Yu,2 and F. Pan1,b) 1
Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2 Center for Testing and Analyzing of Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
(Received 8 September 2013; accepted 28 October 2013; published online 12 November 2013) Completely inverted hysteresis loops (IHL) are obtained by the superconducting quantum interference device with large cooling fields (>10 kOe) in (La,Sr)MnO3 films with self-assembled LaSrMnO4, an antiferromagnetic interface. Although the behaviours of measured loops show many features characteristic to the IHL, its origin, however, is not due to the exchange coupling between (La,Sr)MnO3/LaSrMnO4, an often accepted view on IHL. Instead, we demonstrate that the negative remanence arises from the hysteresis of superconducting coils, which drops abruptly when lower cooling fields are utilized. Hence the completely inverted hysteresis loops are experimental artifacts rather than previously proposed inhomogeneity effects in complicated C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4830011] materials. V
I. INTRODUCTION
The hysteresis loop is at the heart of ferromagnetism, which is characterized by two important parameters, the remanence Mr, and the coercive field HC with positive values. The remanence reflects that the magnetization remains after applying a large magnetic field to a sample and then removing it. The coercive field represents the field that must be applied to a specimen in order to reverse its magnetization. These well-established features are challenged by the observation of inverted hysteresis loops (IHL) with partially inverted loops with negative remanence at low fields and normal loops at high fields in amorphous Gd-Co films.1 This phenomenon was commonly observed in inhomogeneous systems, such as exchange-coupled multilayers2–8 and soft/ hard magnetized materials9,10 while some reports on a simple homogeneous system, like epitaxial Fe films11 and a single domain particle with two competing anisotropies.12,13 The origin of the IHL has been vigorously pursued, and several mechanisms based on coupling effects are proposed: magnetostatic interaction,3 the antiferromagnetic coupling,4 and the competition of two anisotropies.12,13 Note that the hysteresis loops are partially inverted, exhibiting normal (counter-clockwise) loops at high fields and abnormal (clockwise) loops at low fields. Such partially inverted behavior can be explained by the two-phase model.2,3,5 There has been extensive attention toward completely inverted hysteresis loops with a negative remanence, including a single phase ferrimagnet,14 magnetic granular films,15,16 exchange-biased Cr2O3 coated CrO2 particles,17 CoFeAlO films,18 FeSiB nanowires,19 and CoFe-LaCoFeO two-phase films.20 Simultaneously, theoretical models were developed.20,21 The area of the hysteresis loop reflects the energy dissipated during the field cycle, and the negative area of the a)
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completely inverted loops violates the thermomechanical second law.2 Differently, the area integral of the partially inverted loops is positive as discussed above.1–13 However, there is no attempt to distinguish these two plausibly similar but intrinsic different loops. Inhomogeneity effects were persistently employed to explain the completely inverted loops,14–20 in analogy to the understanding of partially inverted ones. Clearly, there is a pressing need for an in-depth reality check of the previous concepts and a systematic measurement on the IHL. We recently reported that the exchange bias (EB) effect can unexpectedly emerge at low temperatures (
