United States

It is over twenty years since the discovery of a giant magnetocaloric effect (GMCE) in Gd-Si-Ge [1], and in that time two low-cost GMCE materials systems have been pursued towards commercialization: Mn-Fe-P-Si [2] and La-Fe-Si [3]. This relatively small number of target room temperature magnetic refrigerants underscores the need for time- and cost-effective magnetocaloric materials discovery projects. The ideal material candidates for magnetocaloric applications must be non-toxic, inexpensive, and readily manufacturable [4]. High- throughput workflows, guided by machine learning and other statistical methods in material informatics, are essential to streamlining the process of screening new possible materials, making the experimental process as targeted and efficient as possible. This presentation describes two approaches to materials screening based on simulated data. The first approach is a modification of the screening method of Bocarsly et al. [5], wherein lattice deformation across the magnetic phase transition (magnetic deformation) was computed based on the difference between unit cell dimension lengths with and without magnetic interactions in models based on density function theory (DFT). We add a phenomenological parameter calculated from the electronic density of states available from the Materials Project [6]. The addition of this new parameter can enhance the prediction of magnetic field-induced entropy change, ΔSM, with nearly 60% better correlation than using magnetic deformation alone. The second approach is motivated by the relative scarcity of consistent magnetic entropy data for stoichiometric compounds, which limits the ability of researchers to cross-check predicted ΔSM with experiment. We describe a method of scraping the Materials Project database for DFT results on magnetically ordered compounds [7] using the Pymatgen open-source Python library [8]. By analyzing the electronic density of states, computed unit cell volume and other parameters obtained through the Materials Project database for over 20,000 materials, we compile a data subset which we use to propose directions for magnetocaloric material research.References: [1] V. Pecharsky et al. 1997 Phys. Rev. Let. 78 4494 [2] N. H. Dung et al. 2011 Adv. Energy Mater 1 1215-1219. [3] M. Katter et al. US Patent Application No. 2012/0043497 A1 (23 Feb. 2012) [4] N. A. Zarkevich et al. 2018 J. Phys. D: Appl. Phys. 51 024002 [5] J. Bocarsly, et al. 2017 Chem. Mat. 29 (4), 1613-1622 [6] A. Jain, S.P. Ong et al. 2013 APL Materials, 2013, 1(1), 011002 [7] S. P. Ong et al. 2015 Comp. Mat. Sci. 97 209–215 [8] S. P. Ong et al. 2013 Comp. Mat. Sci. 68 314–319

MMM 2020

Applying Materials Informatics Insights to Magnetocaloric Materials Screening and Discovery

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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technical paper

The magnetic proximity effect (MPE) enhances the exchange interaction of nominally non-ferromagnetic materials overcoming the Stoner limit in close contact to ferromagnetic (FM) materials. The paramagnet Pt is a famous example and gets spin polarized by the MPE in Pt/FM bilayers which is of huge importance when studying spin transport phenomena at interfaces such as for the spin Seebeck effect [1], the spin Hall magnetoresistance [2], or spin-orbit torque effects [3]. However, the exact quantitative values for the induced magnetic moment per Pt atom cannot be determined by standard x-ray magnetic circular dichroism (XMCD) experiments on a single thin-film system due to the depth-insensitivity of the XMCD technique at buried interfaces of the sample. Within this contribution, we present the quantitative determination of the Pt magnetic moment induced by the MPE utilizing x-ray resonant magnetic reflectivity (XRMR) [4-6], a favored synchrotron-based technique for spin-depth profiling. Thus, element selectivity and depth resolution can be combined. A detailed data analysis based on asymmetry ratio fits and mapping the goodness of fit parameter Χ2 (see Fig. 1) [3,7] together with an ab-initio factor (conversion from magnetooptic constant Δβ to Pt magnetic moment) [4] provides quantitative reliable results. The obtained quantitative XRMR values are in very good agreement to vibrating sample magnetometry results [3] and XMCD analyses [8] which take the effective magnetic Pt thickness determined by XRMR into account. Therefore, XRMR is a powerful technique to study spin distributions with depth resolution, element selectively and quantitative reliability.References: [1] P. Bougiatioti et al., Phys. Rev. Lett. 119, 227205 (2017) [2] M. Althammer et al., Phys. Rev. B 87, 224401 (2013) [3] A. Moskaltsova et al., AIP Advances 10, 015154 (2020) [4] T. Kuschel et al., Phys. Rev. Lett. 115, 097401 (2015) [5] C. Klewe et al., Phys. Rev. B 93, 214440 (2016) [6] T. Kuschel et al., IEEE Trans. Magn. 52, 4500104 (2016) [7] J. Krieft et al., J. Phys. D: Appl. Phys. 53, 375004 (2020) [8] D. Graulich et al., in preparation

Quantitative determination of the Pt magnetic moment in proximity to ferromagnetic material

Enhancement of ferromagnetic resonance damping due to spin pumping from a ferromagnetic insulator (FI) into adjacent heavy metal (HM) is well-known. The less known phenomenon of a substantial magnetic anisotropy induced in FI at the interface with an HM overlay has not been fully studied. Here, we report a microscopic study of the HF/FI interfacial spin interaction in thin films using scanned ferromagnetic resonance force microscopy (sFMRFM). HM nanostructures, such as 5nm-thick Au pads, are patterned onto 20nm-thick Y3Fe5O12 (YIG) grown on Gd3Ga5O12 substrate using off-axis sputtering technique. Scanning the sFMRFM probe across the boundary separating the bare YIG and YIG/Au bilayer reveals a substantial, 32 Gauss, change of resonance field in the magnetic field out-of-plane geometry. Detailed micromagnetic simulation accurately reproduces the experimental result, demonstrating that a sharp change of uniaxial anisotropy across the boundary induces the observed resonance field shift in YIG/Au. This sharp variation of magnetic anisotropy creates a drastic change of internal field over a short spatial distance, can be highly useful in designing nanoscale spintronic devices. The modification of magnetic properties at the nanoscale is usually achieved by modifying the shape or crystal structure of the FI thin film, such as etching or ion irradiation. Notably, we achieve this modification of magnetic anisotropy by a heavy metal pattern overlayer, thus opening a novel path to fabricate nanoscale magnetic structure with minimal disturbance to the material properties of the FI thin film. Utilizing the ability to vary the magnetic anisotropy in nanometer scale, we propose spintronic devices based on the integration of heavy metal nanostructures on YIG thin films.References: G. Wu, S. P. White, W. T. Ruane Physical Review B, 101(18), 184409. (2020)

Microscopic Study of Magnetic Anisotropy Modification in Nanoscale Heavy Metal/Ferromagnetic Insulator Heterostructures

Dzyaloshinskii-Moriya Interaction (DMI) generated at the interface between the magnetic layer and heavy metal layer plays an important role in studies on current-induced magnetic domain wall motion[1-2]. However, since it is difficult to estimate DMI with existing methods, a simple estimation method is required. Although the magneto-optical Kerr effect (MOKE) measurement is one of the simplified measurement methods, the signal of the MOKE is not high. Recently, Sumi et al. have reported that the MOKE can be enhanced by using multiple interference[3] in thin films. In this study, we have applied the multiple interference effect to MOKE measurement in the magnetic layer / heavy metal layer hetero-structure to enhance the signal from the interface and discuss the contribution of the DMI. Thin films with structures of Pt(5 nm)/GdFeCo(6 nm)/SiNx(10 nm), was deposited on silicon substrates with thermally oxidized SiO2 layer (100 nm). Kerr rotation angle and ellipticity spectra of the samples were measured in an energy range of 1.77–4.13 eV (wavelength range of 300-700 nm) by home-made MOKE spectroscopy[3]. Figure 1 shows MOKE spectra of (a)Pt/GdFeCo. Measured spectra are different from calculated ones in the low photon energy region. This result is quite similar to our previous result of (b)Pt(3 nm)/TbCo(6 nm)/SiNx(10 nm)[4]. Such differences are attributable to the interfacial effect of the DMI at Pt/GdFeCo and Pt/TbCo. We can confirm the differences remain when the ferrimagnet layer was changed. Focuses on ellipticity spectra, the peak of the measured spectrum is in a similar position in the photon energy region as calculated one for (b)Pt/TbCo. On the other hand, the peak of the measured spectrum is located in a larger photon energy region than the calculated one for (a)Pt/GdFeCo. MOKE spectra may have the information of not only the interface effect originated from large DMI of heavy metal, but also the interfacial magnetic structure of ferrimagnet material. This research was supported by JSPS Grant-in-Aids for Scientific Research (20H02185).References: [1] K. Ryu et al., Nature Nanotech. 8, 527 (2013). [2] Yang, S.-H. et al., Nat. Nanotech. 10, 221 (2015). [3] Sumi, S. et al., Sci. Rep. 8, 776 (2018). [4] Iemoto, S. et al., crystals 8, 377 (2018).

Detection of interfacial effects in rare earth metal doped ferrimagnet layer / heavy metal layer using magneto-optical Kerr effect

This research reports a fundamental study of transport and magnetic properties of pulsed laser deposited metallic (film thickness ≈ 25-100 nm) and semiconducting (film thickness ≈ 25 – 10 nm) titanium nitride (TiN) thin films. The study is motivated to discern the critical thickness at which the properties of TiN films digress from bulk-like (3D) to 2D thin films. The study showed that while the crystal structure remains rock salt in the entire thickness range, the films undergo a metal-to-insulator (MI) transition around ~15 nm. The thickness (dimensionality) dependent MI transition, which is free from a structural phase transition, is attributed to a the localization of 3d1 electrons in the valence band, which takes place due to the separation in the energy states larger than the gaps between the valence and conduction bands. Analysis of transport resistivity data in different temperature regions indicated that while the Arrhenius law governs the transport properties for all the TiN films in 300 – 350 K range, the transport properties are governed by a thickness-dependent variable range hopping mechanism below 300 K. The magnetic properties of metallic and semiconducting TiN films were also found to be interesting. A room temperature ferromagnetic behavior was observed for both metallic and semiconducting TiN films, there was a marked correlation between coercivity (Hc) and saturation magnetization (Ms) with TiN film thickness. At room temperature, Hc increased from 66 to134 Oe as the thickness decreased from 100 to 9 nm. Similarly, the Ms (2.8 emu/g) at room temperature for the 9 nm sample was the highest. The maximum values of Hc and Ms are among the best values reported for TiN thin films in the literature. Contrary to previous reported causes of ferromagnetism in TiN due to nitrogen vacancies or oxygen doping, we have attributed the observed magnetic behavior of TiN films to the localized 3d1 electrons in TiN which become more localized with the decrease in film thickness due to the size-dependent enhanced separation of energy states. Thickness dependent ferromagnetism study of TiN thin films can add a new dimension to their current applications and tune the magnetic nature of low dimensional TiN films.

Transport and Magnetic Properties of TiN Thin Films

The deeply buried, yet most important part of any spintronic device is the interface. This is even more interesting and much more complex when soft, light materials like organic molecules are in contact with an inorganic metallic electrode. Hence, exceptional care is required to better understand the phenomena driven by this type of hybrid organic-inorganic interfaces. To this end, ferromagnetic nuclear resonance (FNR) spectroscopy [1] studies were performed to investigate the morphology and magnetic properties of the hybrid organic-inorganic interfaces when zinc tetra-phenyl porphyrin (ZnTPP) molecules are at the vicinity of ferromagnetic metallic cobalt (Co) electrodes. With unique sample architectures, the experimental results of FNR suggest that Co/ZnTPP and ZnTPP/Co interfaces are governed by weak van der Waals interactions in the absence of strong chemical bonds between interfacial ZnTPP molecules and Co atoms. Nevertheless, when ZnTPP is deposited on top of Co the resulting interface (Co/ZnTPP) consists of around 0.3 nm of Co leading to smooth and quite sharp interface morphology. On the contrary, the interface obtained for deposition of Co on top of ZnTPP (ZnTPP/Co) involves much larger amount of Co (0.8nm) resulting in wide and diffusive type of interface. These observations suggest that the two type of interfaces are highly asymmetrical, and that a minimum of 5 nm of Co is required to complete the interface formation. Finally, and more importantly, through the so-called site dependent restoring fields [1] of FNR, the experimental evidence showed that Co/ZnTPP and ZnTPP/Co interfaces are magnetically softer than the bulk of Co of the sample, demonstrating the absence any magnetic hardening effect that is generally present in other organic/inorganic hybrid systems [2,3].References: [1] P. Panissod and C. Meny, Applied Magnetic Resonance, Vol 19, p. 447 (2000) [2] T. Moorsom, M. Wheeler, T. Mohd Khan et al, Physical Review B, Vol 90, p. 125311 (2014) [3] M. Callsen, V. Caciuc, N. Kiselev et al. Physical Review Letters, Vol 111, p. 106805 (2013)

Revealing the Morphology and Magnetic Properties of Single Hybrid Interfaces Embedded in Co/ZnTPP and ZnTPP/Co by FNR

Understanding the formation of magnetic domain patterns in ferromagnetic thin films and their evolution under a magnetic field is useful for application in nanotechnologies such as magnetic storage. We are specifically studying domains patterns in multilayered [Co/Pt]_N thin films with perpendicular magnetic anisotropy (PMA). We are imaging the domain patterns via Atomic and Magnetic Force Microscopy (AFM/MFM) at remanence as well as in the presence of a magnetic field applied in-situ. The films consist of 3 nm thick Co layers and 0.7 nm thick Pt layers, with a number of repeats N, varying from 6 to 20. We are investigating how the number of layers N affects the domain size and therefore the domain density but also the degree of PMA exhibited by the films. We will show images of magnetic domain patterns collected at remanence as well as over a range of field values from about 0.10 T to 0.60 T. The data shows the evolution of the magnetic domain morphology from a maze-like state at remanence and low field values, toward a bubble state at higher field values. The bubble-like domains then disappear as the sample reaches saturation, occurring at about 1 T. When the field is released, the magnetic pattern may remain in a bubble state. This study provides with a unique view of how the thin films react to external magnetic fields and how to optimize domain density.

Magnetic Domain Patterns of Multilayered [Co/Pt] Thin Films

In this work, we demonstrate that the TbVO4 compound can be used as magnetic refrigerant in efficient and ecofriendly magnetic cryocoolers due to its giant magnetocaloric effect (MCE) at low temperature regime. The application of a relatively low magnetic field of 2 T along the easy magnetization plane (ab) gives rise to a maximum entropy change of about 20 J/kg K at 4 K. More interestingly, under sufficiently high magnetic fields, the entropy change (-ΔS) remains approximately constant over a wide temperature range which is highly appreciated from a practical point of view. For example, in the magnetic field change of 7 T, -ΔS that reaches roughly 22 J/kg K remains practically unchanged over about 34 K, leading to an outstanding refrigerant capacity of 823 J/kg. On the other hand, the change in the crystallographic symmetry from the tetragonal to the orthorhombic structure occurring close to 33 K as confirmed by Raman scattering data (Figure 1), results in a strong magnetocrystalline anisotropy. Accordingly, giant thermal effects are obtained simply by spinning the TbVO4 single crystals between the hard (c axis) and easy (ab plane) orientations (Figure 1) in constant magnetic fields. The rotation of TbVO4 crystals between their easy and hard magnetization directions in a constant magnetic field of 2 T, enables to generate entropy changes of about 18 J/kg K around the liquid helium temperature. Such rotating MCEs would open the way for the implementation of TbVO4 in more efficient and compact magnetocaloric coolig systems with a simplified design that could be dedicated to the hydrogen and helium liquefaction.

Giant anisotropy of the magnetocaloric effect in the orthovanadate TbVO4 single crystals

Fe nanoparticles are grown in a TiN/Fe/TiN heterostructure using a pulsed laser deposition (PLD) system. TiN/Fe/TiN multilayers are made by altering the average interlayer distance between two Fe nanoparticles and varying Fe nanoparticle sizes by changing the number of laser pulses impinging on a commercial Fe target. Crystallographic studies carried out using x-ray diffraction (XRD) have indicated crystalline nature of the film with an FWHM of 0.13o. By controlling the nanoparticle size in the confined layers, a superparamagnetic to ferromagnetic phase transition is observed during zero field cooled (ZFC) temperature (T) dependent magnetization (M) measurements and thereby tuned magnetocaloric properties in a controlled manner. A maximum magnetization of ~3.6 × 105 A/m which is 2 order of magnitude higher than our previously reported Fe2Ta thin films is obtained at 0.075 T. Applying the Maxwell relation to the MT curves at various fields, we have calculated vs H the integration of which provides quantitative information about the isothermal entropy change. We have observed a positive MCE with maximum entropy change of ~ 600 J/K-m3 for the magnetic field changing from 0.002 - 0.075 T in TiN/Fe/TiN system. A coexistence of positive and negative ΔS is observed below and above blocking temperature respectively.References: 1. V. Franco, J. Blázquez, B. Ingale and A. Conde, Annual Review of Materials Research., 42, pp. 305-342 (2012). 2. C. W. Miller, D. D. Belyea and B. J. Kirby, Journal of Vacuum Science & Technology A., 32(4), p. 040802 (2014). 3. O. Tegus, E. Brück and K. H. J. Buschow, Nature, 415(6868), pp. 150-152 (2002).

Enhanced Magnetic and Magnetocaloric Properties of Fe Nanoparticles in TiN Thin Film Matrix

The effect of hydrostatic pressure on magnetic entropy change in Fe7Se8 single crystals is determined. The combined magnetic entropy change, due to both magnetic field and pressure application is analyzed experimentally and theoretically. The studied material is a ferrimagnetic metal with Néel temperature (TN) equal 450 K. The first-order spin reorientation transition (SRT) from easy c-axis to easy c-plane is observed in this material at the temperature Tr ≈ 125 K [1]. This effect was shown to originate from the specific dependence of the magnetocrystalline energy on temperature and can be induced by applied hydrostatic pressure and/or by a magnetic field. Under applied hydrostatic pressure, Tr is shifting to the lower temperatures, while the effect of the magnetic field depends on the direction to the c-axis. Magnetic measurements show the decrease of Fe7Se8 magnetization under applied hydrostatic pressure. It is a consequence of iron vacancies redistribution induced by pressure. It has been observed that the entropy change ΔSmag and refrigeration capacity RC, are in strong dependence on Tr and magnetization. Indirect methods, based on magnetization measurements, were used to calculate ΔSmag (Fig.1). It has been shown that the combined application of the magnetic field and hydrostatic pressure can significantly affect entropy change. The sign of the ΔSmag is determined on the direction of the magnetic field H. In conclusion, we have shown, that entropy change in magnetic materials such as Fe7Se8, subjected to simultaneous variation of magnetic field and pressure, reach large values in a wide range of temperatures in the vicinity of Tr. This study was partly financed by the National Centre for Research and Development, Research Project no. PBS2/A5/36/2013References: [1] I. Radelytskyi et al., J. Appl. Phys. 124, 143902 (2018).

Combined Pressure and Magnetic Field Induced Caloric Effects in Fe7Se8 Single Crystals

Scaling laws have been widely applied for the study of second-order phase transition (SOPT) magnetocaloric materials as their magnetic entropy change, ΔSM, collapses onto a single universal curve [1]. However, for non-single phase SOPT cases, distortions in the scaled curves prevent good collapse. These distortions get more evident when the ratio of the different phases and the proximity of their transitions increase [2]. Based on scaling laws, a procedure is proposed and successfully applied to deconvolute the magnetocaloric responses of biphasic Gd80Pd20 composite (formed by Gd and Gd7Pd3 with Curie temperatures separated by ~45 K) [3, 4]. The global ΔSM curves of the composite were firstly rescaled by the typical universal scaling procedure considering separately each phase (i=1, 2), i.e. temperature rescaling using one reference temperature, Tr,i, as θi = (T-Tpk,i)/(Tr,i -Tpk,i) and normalizing the ΔSM with respect to the corresponding peak values, ΔSMpk,i. In this case of highly overlapped phase transitions, the typical power law behavior for SOPT (ΔSMpk ∝ Hn and Tr ∝ Hζ) is not fulfilled. An iterative procedure is then followed to account for the phase overlap, leading to a good agreement between experimental and calculated phase fractions and reconstruction of the experimental ΔSM curves (Fig. 2). This procedure can be applied to extract the response of a pure phase even in cases where the synthesis procedure gives rise to impurity phases, providing a tool to gauge the potential applicability of a material without dedicating large efforts for optimizing its synthesis. Work supported by AEI/FEDER-UE (grants MAT-2016-77265-R and PID2019-105720RB-I00), US/JUNTA/FEDER-UE (grant US-1260179), Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (grant P18-RT-746), Army Research Laboratory under Cooperative Agreement Number W911NF-19-2-0212 and Polish National Science Centre (Project 2019/03/X/ST5/00066).References: [1] V. Franco et al., Int. J. Refrig., 33 465–473 (2010). [2] V. Franco et al., J. Magn. Magn. Mater., 321 1115–1120 (2009). [3] Á. Díaz-García et al., JOM, (2020). https://doi.org/10.1007/s11837-020-04251-z. [4] P. Gebara et al., J. Magn. Magn. Mater., 500 166175 (2020).

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