Electrical access to antiferromagnetism in metallic and insulating thin films

The discovery of the effect presently known as anomalous Hall dates back to 1881 when Edwin H. Hall observed that a current flowing in a perpendicularly magnetized iron slab produces a transverse voltage one order of magnitude larger than in other non-magnetic metals. A proper understanding of this peculiar property of ferromagnets took more than 70 years, and, today, it is well established that the fundamental origin of the anomalous Hall effect comes from the simultaneous presence of spin-orbit coupling in the solid and broken time inversion symmetry arising from the magnetization. Interestingly, recent observations of large anomalous Hall effect have been reported in antiferromagnetic systems, although the magnetization in antiferromagnets is zero by definition. In these systems, like Mn3Ge or Mn3Sn, the peculiar non-collinear spin structure introduces a topological term in the transverse conductivity, originating the anomalous Hall effect. These findings were not just important as a fundamental achievement in solid state physics but also opened some possible applications in the emerging field of antiferromagnetic spin-electronics (or spintronics), which hopes to realize memory devices faster, more scalable and more robust than their ferromagnetic counterpart. In fact, anomalous Hall effect is one of the possible routes that can be used to read the state of antiferromagnetic memory cells. Metallic non-collinear antiferromagnets, however, represent a rather specific case, where this anomalous component depends on the precise crystal structure and composition. The work presented here proposes instead a more general case, consisting of interfaces between antiferromagnetic and non-magnetic thin films which display a sizable anomalous Hall effect. We demonstrate in fact the possibility to detect electrically the proximity induced magnetization in the non-magnetic layer in contact with the antiferromagnet. The advantage of this method is that it can be exploited independently on the type of ordering (i.e. it works also on collinear antiferromagnets) and is even applicable to insulating antiferromagnets. Several examples are provided demonstrating the generality of this approach, identifying anomalous Hall measurements as a quick and cost-effective way of characterization both in fundamental studies as well as in actual devices. A first instructive example is provided by the platinum/chromium system. The use of simple elemental compounds as in this case allows for precise ab initio calculations of the electronic and magnetic properties of the materials. By doing so, we found indications that antiferromagnetic chromium induces a net magnetization in neighboring platinum layer. With the idea of electrically detecting this induced magnetization, we grew those compounds as thin films by molecular beam epitaxy (MBE). Simple cross structures were patterned by conventional lithographic techniques and ion milling, allowing for a comprehensive electrical characterization of the bilayer as a function of temperature and magnetic field. After having assessed a solid antiferromagnetic behavior up to 290 K, we demonstrated that an anomalous Hall effect can be observed in absence of any external magnetic field. To this scope, we cooled down through the magnetic phase transition in presence of an applied magnetic field to set a preferred magnetic orientation of the Pt/Cr interface. Depending on the writing field direction, different states of transverse resistance can be reproducibly set and read-out at remanence up to the transition temperature of 290 K. We demonstrate that the origin of this anomalous transport comes only from the interface, as shown by repeated experiments with various Cr and Pt thickness, and it is suppressed when a diamagnetic gold interlayer is placed between Cr and Pt. All these results allow to exclude possible competing mechanisms for explaining the observed transport behavior, and definitely point out a specific role of the proximity induced magnetization in Pt. Even though Cr is an excellent demonstrator, since the relationship between electrical transport effect and antiferromagnetism is backed by ab initio calculations, the Hall signal is very small in this case and requires an high-accuracy measurement setup. Nevertheless, the same principle of operation can be generally extended to other antiferromagnetic systems which provide a larger signal and are thus more suitable for applications. For instance, one interesting case covered is the one of Ta/IrMn which presents an Hall signal 4 orders of magnitude larger than Pt/Cr. Multiple states of resistance can be set in Ta/IrMn using different values of the magnetic field applied out of the field plane during the cooling process. Moreover, being the state stored in the antiferromagnet, the read-out can be performed at remanence and it cannot be erased below the transition temperature by external fields (at least up to 9 T). Besides for potential applications, the electrical detection of antiferromagnetic state can be used also for fundamental studies in material science. An example that will be discussed in this context is the one of tetragonal Cr2O3, an uncommon phase of Cr (III) oxide for which no previous reports of antiferromagnetism were given. We demonstrate that thin film of t-Cr2o3 can be stabilized by epitaxy on perovskite BaTiO3. We then show how interface effects in Pt/t-Cr2O3 can be used to verify a magnetic phase transition at about 40 K which can be identified both by measuring the magnetoresistance and the anomalous Hall effect. Finally, we will discuss the case of Pt/TmFeO3. Bulk TmFeO3 belongs to the class of rare-earth orthoferrites which, although they present interesting magnetic properties, are seldom studied in thin films. We cover the optimization of the growth by pulsed laser deposition of TmFeO3 on SrTiO3, and demonstrate through anomalous Hall effect in Pt its antiferromagnetic nature up to room temperature. Supported by all these experimental evidences, we can conclude that proximity effects are a widespread phenomenon at interfaces between certain non-magnetic metals and antiferromagnets, providing at the same time a tool for the electrical detection of the magnetic state in antiferromagnetic systems. We envisage electrical measurements, and anomalous Hall in particular, as powerful methods for the characterization of magnetic properties in unknown or new materials as well as a potential way to read-out information in antiferromagnetic spintronic devices which, free of any ferromagnetic layer, are fully robust against external fields.