This is the accepted version of the article: Robbennolt, Shauna; Yu, Pengmei; Nicolenco, Aliona; [et al.]. «Magneto- ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally nanocoated with HfO2». Nanoscale, Vol. 12, Issue 10 (March 2020), p. 5987-5994. DOI 10.1039/C9NR10868H This version is available at https://ddd.uab.cat/record/233367 under the terms of the license ARTICLE Magneto-ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally Received 00th January 20xx, nanocoated with HfO2 Accepted 00th January 20xx Shauna Robbennolt,*a Pengmei Yu,b Aliona Nicolenco,a,c Pau Mercier Fernandez,a Mariona Coll,b DOI: 10.1039/x0xx00000x and Jordi Sort*a,d Advances in nanotechnology require of robust methods to fabricate new types of nanostructured materials whose properties can be controlled at will using simple procedures. Nanoscale composites can benefit from actuation protocols that involve mutual interfacial interactions on the nanoscale. Herein, a method to create nanoscale composite thin films consisting of mesoporous cobalt ferrite (CFO) whose pore walls are nanocoated with HfO2 is presented. Porous CFO films are first prepared by sol-gel. Atomic layer deposition is subsequently used to conformally grow a HfO2 layer at the surface of the pore walls, throughout the thickness of the films. The magnetic properties of uncoated and HfO2-coated CFO mesoporous films are then modulated by applying external voltage, via magneto-ionic effects. The CFO-HfO2 composite films exhibit an enhanced magnetoelectric response. The magnetic moment at saturation of the composite increases 56% upon the application of –50 V (compared to 24% for CFO alone). Furthermore, dissimilar trends in coercivity are observed: after applying –50 V, the coercivity of the composite film increases by 69% while the coercivity of the CFO alone decreases by 25%. The effects can be reversed applying suitable positive voltages. This two-oxide nanocomposite material differs from archetypical magneto-ionic architectures, in which voltage-driven ion migration is induced between fully-metallic and oxide counterparts. The synthesized material is particularly appealing to develop new types of magnetoelectric devices with a highly tunable magnetic response. Introduction oxygen-containing solvent (sol) and allowing the metal ions to As devices continue to miniaturize, there is an increasing bind to the available oxygen creating a network of metal-oxygen demand for precise methods to fabricate and characterize bonds (gel). This solution can be deposited onto a substrate and nanoscale materials, as well as to control their properties at will. then annealed to form a crystallized metal-oxide film. These While robust methods exist to prepare nanoscale materials, films can be made nanoporous by introducing a sacrificial creating solid-state composite materials that are properly templating agent (e.g., surfactants or micelle-forming mixed at the nanoscale remains a challenge. Nanoporous thin polymers) to the solution which is then burned out during films supported on substrates are ideal for integration into annealing leaving an open-cell porous framework. There are devices because they benefit from the unique nanoscale many methods that can be used to deposit another material properties of the material while the substrates allow handling inside the nanoporous framework. Among them, atomic layer and manipulating them easily on the macroscale. Furthermore, deposition (ALD) is one of the most appealing because it uses a nanoporous thin films with an open pore structure provide the gas phase, hence allowing the material to be easily infiltrated optimal framework to create nanoscale composites if the pores and deposited throughout the porous material, not just onto its volume is filled with another material.1-6 outer surface. 3, 6, 17-19 There are several methods that can be used to fabricate There is a significant research effort aimed at controlling nanoporous thin films. These include metal dealloying, sintering magnetism on the nanoscale with an applied electric field micro- or nanoparticles and wet-chemistry techniques.7-16 Sol- (converse magnetoelectric effect) with the purpose of gel is a method that involves dissolving metal salts into an enhancing energy efficiency. Traditionally, devices use external magnetic fields to control the magnetic behaviour of materials. a. Departament de Física, Universitat Autònoma de Barcelona, E-08193 Cerdanyola del Vallès, Spain. E-mail: S.R.: shauna.r@outlook.com, J.S.: jordi.sort@uab.cat b. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, E- 08193 Cerdanyola del Vallès, Barcelona, Spain c. Institute of Applied Physics, MD-2028 Chisinau, Moldova d. Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, E-08010 Barcelona, Spain †Electronic Supplementary Information (ESI) is available. ARTICLE Nanoscale These magnetic fields are generated by passing electric current through a wire (e.g., electromagnets). This procedure is unsuitable at the nanoscale due to resistive (Joule) heating energy losses.6, 20-22 An alternative to this is to use an applied electric field, not current, to control magnetism. In this case, electrons remain static and no Joule heating effect (due to electrons in movement, i.e., when electric currents are present) takes place. Among the various mechanisms that can result in voltage-driven magnetic actuation, the so-called magneto-ionic effect is one of the most convenient since it typically renders drastic changes in the magnetic properties in films whose thickness is not necessarily limited to few-nm range (as opposed to other mechanisms such as electric surface charge accumulation).21,23 Here, ions (typically O2- anions) migrate through the material in response to the applied electric field, Figure 1. Schematic representation of the in-situ application of hence resulting in variations in the magnetic behavior.22-25 voltage during magnetic measurements. The Pt layer under the Previously, we demonstrated modulation of the magnetic sample is connected as the working electrode and the counter- properties in nanoporous cobalt ferrite (CFO) thin films electrode is a Pt wire. The sample and counter-electrode are in a (without filling the pores with a second phase) via a magneto- small Eppendorf tube filled with propylene carbonate treated with ionic mechanism, i.e., due to partial reduction/oxidation Na to remove water (leaving ~ 5 ppm Na+). The Eppendorf is reactions.26 Voltage has been also shown to be a very effective attached to the sample holder in the VSM which vibrates vertically. means to control the properties of nanoporous Fe2O3, particularly the saturation magnetization.27 In these systems, and also in nanoporous metallic alloys,28 the occurrence of room temperature overnight and was never found to be cloudy porosity increases the total surface area which is contact with at this point. Simultaneously, 10 mg of PEO-PB was dissolved in the electrolyte, thus allowing the electric field to be more 1 mL of ethanol and allowed to stir overnight. The next day, the effectively applied as compared to fully dense films of the same two solutions were combined and allowed to stir for an thickness. additional hour. The substrates were prepared by sputtering 70 nm of Pt onto wafers of Si (100). The solution was deposited In this work, we fabricate a two-phase oxide composite material onto the chips by dip-coating with a withdrawal rate of 300 by means of ALD. This technique is used to deposit a layer of mm/min after which the samples were transferred to a hot- HfO2 inside the pores of mesoporous CFO thin films previously plate at 80 °C for 10 min. The films were then heated in air to grown by a sol-gel method. It is observed that, compared to the 600 °C for 3 h before cooling to room temperature. nanoporous CFO-alone, the nanocomposite films show an enhanced magnetoelectric response. This study differs from Deposition of HfO2 most previous works in the literature on magneto-ionics in The HfO2 was deposited by ALD using a Cambridge NanoTech which the effects are induced in metal/metal oxide bilayer films Savannah 100 reactor. HfO2 deposits were prepared by (not in two-phase oxide thin films).22,28,29 In addition, migration alternate pulsing of tetrakis(dimethylamido)hafnium(IV) of O2- anions is accomplished without need of external heat (TDMAH) purchased from Sigma-Aldrich and heated at 75 ºC treatment (in contrast to previous works30,31), hence resulting in and miliQ water as co-reactant. High purity nitrogen gas was a highly energy-efficient procedure. used as carrier and purging gas (20 sccm). The chamber temperature was set at 200 ºC. Film thickness was set to 7 nm by modifying the number of the ALD cycles and was validated Experimental section by X-ray reflectivity measurements on silicon (100) reference Preparation of porous CFO samples. X-ray diffraction (XRD) was used to check the Cobalt(II) nitrate hexahydrate (99.99%), iron(III) nitrate amorphicity of HfO2. nonahydrate (99.999%) and 2-methoxyethanol (99.8%) were purchased from Sigma-Aldrich. Poly(ethylene oxide)-b- Characterization Poly(butylene) with Mn: PEO(4800)-PB(5800), was obtained High-resolution transmission electron microscopy (HRTEM) from Polymer Source. All chemicals were used without further images and selected area electron diffraction patterns (SAED) purification. were acquired using a JEM-2011 microscope operating at 200 The initial sol-gel solution was prepared following a previously kV using a CCD Gatan camera. Cross sections were prepared published recipe.32 Co(NO3)2·6H2O (220 g) and Fe(NO3)3·9H2O mechanically. The magnetic properties were measured using a (620 mg) were dissolved in 1 mL of 2-methoxyethanol and 1 mL MicroSense (LOT-QuantumDesign) vibrating sample of ethanol. This solution was mixed using magnetic stirring at magnetometer (VSM). The voltage was applied in-situ using an 2 | Nanoscale, 2020, 00, XXXX This journal is © The Royal Society of Chemistry 2020 Nanoscale ARTICLE Agilent B2902A power supply. Figure 1 shows a schematic of the electrochemical cell that was built to be used during VSM measurements. Briefly, the sample and a Pt counter-electrode were placed in an Eppendorf tube and then filled with an anhydrous liquid electrolyte (i.e., propylene carbonate treated with metallic sodium to remove any traces of water). This left a small amount of Na+ and OH– ions (≈ 5 ppm Na+ as determined by inductively coupled plasma (ICP) spectroscopy). The use of a liquid electrolyte is advantageous because it allows the generation of ultra-high electric fields (of the order of hundreds of MV/cm), without electric pinholes, by applying moderate (a few V) voltages.28 Such large electric field is created due to the ultra-small thickness of the electric double layer (typically below 2 nm) that forms around the pore walls when voltage is applied.33 All magnetic measurements were acquired in an in- plane configuration at room temperature. Results and discussion Nanoporous thin films of cobalt ferrite (CFO) with a thickness of 130 nm were deposited on Pt-coated Si substrates (1 × 3 cm2) by dip coating and then a thin layer of HfO 2 was deposited at Figure 2. High-resolution transmission electron microscopy the surface of the CFO pore walls via ALD. Figure 2 shows a images of a cross-section of the composite thin film. Panels a–c cross-sectional HRTEM image of the CFO-HfO2 composite. The show the same film at different magnifications. Panel a is the porous structure of the films is well evidenced in Figure 2a and most zoomed-out and shows the thin film on the Pt layer. Panel 2b, and one can see that a 7 nm layer of HfO 2 was deposited b shows the CFO and HfO2 coating as well as the pore structure. fairly uniformly onto CFO throughout the thickness of the film. Panel c shows a single ligament where the crystallographic Figure 2c shows an HRTEM image in which the crystallographic planes of the CFO can be seen surrounded by a thin amorphous planes of the CFO are clearly visible. It is worth noting that there HfO2 nanocoating. Paned d shows the selected area electron are no crystallographic planes seen in the HfO 2 which is likely to diffraction pattern of CFO-HfO2 after being treated at –50 V. The be amorphous since the samples were not post-annealed after red rings unambiguously correspond to CFO crystallographic the HfO2 deposition. The lack of crystallographic peaks for a 40 nm thick HfO2, grown by ALD under the same conditions, was planes, the blue ones to hexagonal Co. The non-indexed spots also evidenced by grazing incidence XRD (Supplementary Figure match reflections of both CFO and Co planes which are virtually S1). superimposed. Indexation of the diffraction peaks was done according to the Crystallography Open Database. Figure 3. Room-temperature magnetic hysteresis loops of the CFO-alone sample (left) and the CFO-HfO2 composite (right) as a function of voltage. Each voltage value was applied for 2h. The upper insets are schematic representations of the films. The y-axis is normalized to facilitate comparison between the films and each axis is set such that the magnetic moment at saturation for the initial sample (0 V) is 100%. This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 00, XXXX | 3 ARTICLE Nanoscale Figure 4. Dependence of the coercivity (left) and relative saturation magnetization (right) on the applied negative voltage extracted from the magnetic hysteresis loops shown in Figure 3. The filled blue symbols represent the trends for the uncoated CFO and the empty black dots correspond to CFO-HfO2 composite film. Therefore, eventual ferroelectric properties of HfO 2 (which exist 1.06 Å most likely correspond to the (100), (102) and (112) only in the orthorhombic Pca21 crystallographic phase34) can be planes of hexagonal-closed packed Co, which are not ruled out.That is, any eventual effect due to voltage application superimposed to any reflection from CFO. This indicates the cannot be attributed to the coupling between ferroelectric and formation of metallic Co phase upon the negative voltage magnetostrictive (i.e., CFO) phases. application. Additional spots, not indexed in the SAED pattern The magnetoelectric response of the as-prepared material was of Figure 2d, match positions of CFO (sometimes superimposed investigated using the setup described above (Figure 1). Figure to Co) reflections. It should be noted that the outer red ring with 3 shows the magnetic hysteresis loops both before (panel a) and interatomic distance around 0.92 Å could also be attributed to after (panel b) the HfO2 deposition. Figure 4 shows the the (310) crystallographic plane of metallic Fe. However, one dependences of coercivity and saturation magnetization as a should take into account that the energy needed to reduce Fe function of applied voltage. It should be noted that the in the spinel ferrites is larger than that of Co (or Ni). 35 In fact, composite sample was smaller in lateral size than CFO alone our previous study on magneto-ionic effects of pure (uncoated) and, therefore, it gives a lower magnetic signal during the CFO films reveal that the oxidation state of Co tends to be more measurements. This is why the hysteresis loops of the CFO-HfO2 sensitive to that of Fe to the applied electric field.26 samples are noisier. To account for this, the y-axes in (a) and (b) Prior to voltage application, the coercivity (H C) of the composite were normalized, so that comparison between the effects of films is lower than for the CFO-alone sample (see Figure 3), voltage in the two samples is easier. Remarkably, the magnetic which means that the ALD deposition causes some changes on moment at saturation increases for both films as more negative the magnetic properties of the samples. Grain growth in CFO voltages are applied. From 0 V to –50 V, the magnetic moment associated with the ALD heating could lead to a reduction of HC. at saturation for the CFO-alone sample increases 24% while that However, the single-domain size for CFO was determined to be of the composite sample increases by 56%. Hence, the ALD around 40 nm36 and the average size of the CFO crystallites in process exacerbates the magnetic effects caused by voltage. our sample is clearly smaller (see Figure 2c). Hence, the changes This is consistent with a magneto-ionic mechanism.26 The in HC after ALD, rather than being due to changes in crystallite formation of reduced metal atoms or metal clusters indeed size, are probably related to variations in the amount of oxygen explains the increase in magnetic moment at saturation seen in vacancies. both samples when a negative voltage is applied because Co, Fe In CFO, the presence of oxygen vacancies results in broken and CoFe all have higher MS values than cobalt ferrite. Indeed, oxygen bonds which are linked superexchange interactions, the SAED pattern obtained on the CFO-HfO2 nanocomposite leading to local spin canting and a larger energy associated to treated at –50 V (Figure 2d) shows the presence of metallic Co magnetization reversal (i.e., higher H C32,37). During ALD the (and eventually Fe) phases. Since the magnetoelectric effects (in amount of surface oxygen vacancies is likely to be decreased particular, magneto-ionic) occur mainly at the interface, the both due to the high-temperature treatment in an oxygen-rich treated sample still retains a large fraction of the CFO. Thus, the atmosphere and the progressive formation of the HfO 2 defects four labelled red rings in the Figure 2d arise from planes with interatomic distances 2.92 Å, 2.49 Å, 1.46 Å and 0.92 Å, which correspond to the (220), (311), (440) and (840) crystallographic planes of CFO, respectively. Nevertheless, the three blue rings corresponding to the interatomic distances of 2.17 Å, 1.48 Å and 4 | Nanoscale, 2020, 00, XXXX This journal is © The Royal Society of Chemistry 2020 Nanoscale ARTICLE Figure 5. Room-temperature magnetic hysteresis loops of the CFO-alone sample (left) and the CFO-HfO2 composite (right) showing the reversibility with negative and positive voltages. The measurements were taken in the following order: initial sample (0 V, solid black), after waiting for 90 min at –50 V (blue), 0 V after 14 hours (dotted black), after waiting for 90 min at +50 V. The initial 0 V and the +50 V overlap in the CFO-alone sample. The y-axis is normalized to facilitate comparison between the films and each axis is set such that the magnetic moment at saturation for the initial sample (0 V) is 100%. in nanoporous CFO would also decrease when CFO becomes dependence of HC on voltage not straightforward. First, the chemically bonded to HfO2.38 number of oxygen vacancies in CFO will tend to increase in both nanocoating. This probably leads to the observed reduction of systems (uncoated and ALD-coated CFO). This should cause an HC. The surface anisotropy and the amount of surface structural increase of coercivity.32,37 Secondly, magnetic metallic clusters By applying –50 V, the coercivity of the CFO-alone sample (e.g., Co) form (Figure 2d). Depending on their size, such clusters decreases from 994 Oe to 741 Oe (25% decrease) while the can be superparagnetic (with zero coercivity), single-domain coercivity of the composite film increases from 295 Oe to 481 (with large coercivity) or multi-domain (with lower coercivity). Oe (69% increase). This result is to some extent counterintuitive Thirdly, if partial chemical reduction occurs at the CFO grain as one might expect the coercivity in both systems to either boundaries, the effective crystallite size of CFO may decrease increase or decrease when the overall oxygen content in the (as it was observed in other oxide materials subject to negative samples is reduced. Indeed, in magneto-ionically manipulated voltages23). Since the size of the CFO crystallites in our oxide materials that are immersed in electrolytes, the applied mesoporous films is already below the single-domain critical negative voltage typically causes O2- ion migration from the size (40 nm), a further reduction of the crystallite size will tend sample to the electrolyte and vice versa for positive to reduce HC. voltages.26,27 The solubility of oxygen in propylene carbonate is Depending on which of the effects described above rather limited39. Hence, formation of oxygen bubbles could be predominates, either an increase or decrease of H C can occur expected. However, in the voltage window utilized in our when negative voltages are applied. Since the amount of oxygen experiments, bubbling was not observed. We observed some vacancies in the pristine CFO-HfO2 composite sample is bubbling in the past during magnetoelectric measurements in presumably lower than for uncoated CFO (due to the other oxide materials (e.g., Co3O4) using the same electrolyte passivation effect induced by HfO2)40, application of voltage is but applying much larger voltages, exceeding –200 V. Thus, in expected to render larger magneto-ionic effects in this case, as the uncoated CFO sample, when negative voltage is applied, O 2- observed experimentally. Diffusion of O2- ions from CFO to HfO2 ions are likely to be partially released to the propylene will create large amounts of oxygen vacancies at the surface of carbonate. In turn, upon negative voltages applied in the ALD- coated sample, the O2- ions might migrate from CFO into the structure of HfO2, which typically contains large amounts of oxygen vacancies in the as-grown state.22,34,36-38 Partial migration of O2- ions from HfO2 to the electrolyte could also occur for strong enough negative voltages. As the amount of oxygen in CFO decreases, several competing effects will simultaneously occur which might lead to dissimilar trends in coercivity. This makes the interpretation of the overall This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 00, XXXX | 5 ARTICLE Nanoscale system may be less able to contract/expand due to electrostriction effects. Voltage-induced strain in the oxygen- deficient CFO could have some influence on the magnetic properties since CFO is magnetostrictive.44 However, effects from electrostriction are believed to be a second-order effect and cannot fully explain our results, in particular the drastic changes in magnetization and the dissimilar trends in coercivity. The changes in the magnetic properties induced by applying a negative voltage can be fully reversed with the application of a positive voltage, as shown in Figure 5. The initial hysteresis loop for each sample is shown in solid black lines and the loops after –50 V are shown in blue. For both samples, the voltage was then removed overnight (14 hours) and then a new “0V” loop was measured, shown in dotted black lines. The CFO-alone sample began to recover when the voltage was removed, likely because oxygen stored at the surface or in the solvent had time to begin diffusing back into the sample. However, the changes in the Figure 6. Change in CFO-HfO2 composite film magnetization (vs. composite sample were permanent and even after 14 hours at the initial state) during the first 5 cycles measured at 15 kOe. 0V, the hysteresis loop matches the loop acquired at –50 V. Each cycle took a total of 120 min: 60 min at −10 V followed by This suggests that the presence of the HfO 2 layer impedes the 60 min at +10 V. The red line is the trendline and the red dots are migration of oxygen back into the sample either because the the maxima and minima in magnetization. oxygen is stable and stored in the HfO 2 or because the oxygen is stored in the electrolyte and the HfO 2 layer physically blocks it from accessing the CFO. Interestingly, after applying +50 V the CFO pore walls, thereby increasing H C. However, such O2- (red lines), both samples recover, and the +50 V loops are vacancies would tend to slightly decrease the saturation almost identical to the initial hysteresis loops. Finally, cyclability magnetization,35 which is not observed experimentally. experiments (applying –10 V and +10 V several times, Therefore, the formation of metallic magnetic clusters (Co and consecutively, for 60 min each) were performed (Figure 6) to eventually Fe), as confirmed by electron diffraction and the study the endurability of the investigated effects. Remarkably, increase of magnetic moment at saturation, is an additional within this voltage window, the effects are fully reversible at mechanism that plays a role in the coercivity. The larger least during 5 cycles. increase of saturation magnetization in the CFO-HfO2 compared to uncoated CFO, together with the increase of coercivity for negative voltages, indicate that the fraction of Conclusions superparamagnetic metallic clusters in the composite film is Here we have shown that nanoscale cobalt ferrite CFO-HfO2 smaller than for uncoated CFO. composites can be fabricated by using atomic layer deposition This is in agreement with a more pronounced magneto-ionic to coat nanoporous CFO previously grown using sol-gel effect, which would result in larger metallic clusters in the CFO- methods. Transmission electron microscopy cross-section HfO2 composite system. In addition, application of negative images reveal that the HfO2 is deposited uniformly at the voltage to the CFO-HfO2 composite sample could also cause surface of the CFO pore walls throughout the film thickness. We oxygen to be partially released from HfO2 to the electrolyte but then investigated the magnetoelectric properties of both the this should be a second-order effect, which should not nanoporous CFO-alone and the CFO-HfO2 composite. The drastically influence the resulting magnetic properties of the application of negative voltages induces an increase in the composite sample since both Hf and HfO 2 are essentially non- magnetic moment at saturation for both samples while the magnetic. Namely, while the saturation magnetization of CFO coercivity is found to decrease in the CFO-alone sample and has been reported to be around 455 emu/cm3,41 some authors increase in the nanocomposite. The different magneto-ionic have claimed that HfO2 with large amounts of oxygen vacancies mechanisms responsible for the observed effects are discussed can become slightly ferromagnetic,42 with a saturation in detail. Furthermore, the changes induced in the composite magnetization of at most 30 emu/cm3 (i.e., at most, 6.5% the sample are found to be more permanent than those induced in value of CFO). Some authors have even ascribed this magnetic CFO-alone because the voltage-driven magnetic effects at –50 moment in HfO2 to impurities or contamination.43 In any case, V remain unaltered after several hours at 0 V. In both types of the eventual ferromagnetic properties of HfO 2 cannot explain samples, the initial magnetic properties can be essentially the large variations observed in the magnetic moment at recovered by the application of a positive voltage and a good saturation, which exceed 55% for the CFO-HfO2 sample. Finally, endurability is demonstrated during voltage cycling it is possible that the HfO2 nanocoating could also play a role in experiments. Interestingly, contrary to some other magneto- induced changes in the mechanical properties of mesoporous ionic studies (which require of high temperatures) here all CFO when voltage is applied. Namely, the CFO-HfO2 composite changes in coercivity and saturation magnetization are induced 6 | Nanoscale, 2020, 00, XXXX This journal is © The Royal Society of Chemistry 2020 Nanoscale ARTICLE at room temperature. The large surface area of the investigated 12 P. Innocenzi and L. Malfatti, Chem. Soc. Rev., 2013, 42, 4198– material together with the ultra-high electric fields generated 4216. 13 E. Ortel, T. Reier, P. Strasser and R. Kraehnert, Chem. Mater., due the formation of the very narrow electric double layer, 28 are 2011, 23, 3201–3209. probably the main reasons that allow the magnetoelectric 14 T. Brezesinski, J. Wang, J. Polleux, B. Dunn and S. H. Tolbert, J. effects to be achieved without need of thermal annealing Am. Chem. Soc., 2009, 131, 1802–1809. treatments. 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Rev., 2011, 40, 5242– achieve an optimum performance (maximized energy 5253 products). 20 C. Song, B. Cui, F. Li, X. Zhou and F. Pan, Prog. Mater. Sci., 2017, 87, 33–82. 21 A. Quintana, J. Zhang, E. Isarain‐Chávez, E. Menéndez, R. Conflicts of interest Cuadrado, R. Robles, M. D. Baró, M. Guerrero, S. Pané, B. J. Nelson, C. M. Müller, P. Ordejón, J. Nogués, E. Pellicer and J. There are no conflicts to declare. Sort, Adv. Func. Mater., 2017, 27, 1701904. 22 K. Duschek, A. Petr, J. Zehner, K. Nielsch and K. Leistner, J. Mater. Chem. C, 2018, 6, 8411–8417. 23 A. Quintana, E. Menéndez, M. O. Liedke, M. Butterling, A. Acknowledgements Wagner, V. Sireus, P. Torruella, S. Estradé, F. Peiró, J. This work was supported by the European Research Council Dendooven, C. Detavernier, P. D. Murray, D. A. Gilbert, K. Liu, under the SPIN-PORICS 2014-Consolidator Grant, Agreement № E. Pellicer, J. Nogues and J. 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