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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.

MMM 2020

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

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 temperature dependence of magnetic properties in thin films are important for understanding the underlying physics and enabling the creation of novel devices. Whilst the temperature dependence of saturation magnetization Ms, exchange stiffness A and anisotropy K have been widely investigated [1, 2, 3], research on the interfacial Dzyaloshinskii–Moriya interaction (DMI) remains limited [4]. The DMI exists in perpendicularly magnetized ultrathin films with interfaces to heavy metals and is critical to the formation of magnetic skyrmions. We present a study of the temperature dependence of DMI in a Ta/Pt/CoFeB/Ru/Pt/CoFeb/Ru/Pt heterostructure hosting skyrmions, based on extensive characterization by bulk magnetometry and x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM). Ms, anisotropy field Hk, zero-field domain width W and skyrmion radius R are extracted directly from the measurements. Through Ms and Hk, the strength of uniaxial anisotropy Ku is determined. Then, we adapt the bulk Bloch law to the thin film geometry to extract A from the temperature dependence of Ms (Fig. 1). Furthermore, we combine Ms, W, R, Ku and A in the static multilayer domain energy model [5] and the static skyrmion size model [6], where the DMI values are calculated at different temperatures (Fig. 2). We find a large spatial variation of the DMI strength between the one derived by the spatial domains and that at skyrmion sites. Besides, the DMI value shows a linear correlation with A and Ku. The possible physical origins are discussed. Finally, according to our results, we argue that the skyrmion size should remain constant over an extensive temperature range [7].References: [1] Atxitia, U., et al., Phys. Rev. B 82, 134440 (2010) [2] Moreno, R., et al., Phys. Rev. B 94, 104433 (2016) [3] Sato, H., et al., Phys. Rev. B 98, 214428 (2018) [4] Schlotter, Sarah, Parnika Agrawal, and Geoffrey SD Beach, Appl. Phys. Lett. 113, 092402 (2018) [5] Lemesh, Ivan, Felix Büttner, and Geoffrey SD Beach, Phys. Rev. B 95, 174423 (2017) [6] Wang, X. S., H. Y. Yuan, and X. R. Wang, Commun Phys 1, 31 (2018) [7] Zhou, Yifan, et al., Phys. Rev. B 101, 054433 (2020)

Temperature dependence of the Dzyaloshinskii-Moriya interaction in ultrathin films

The magnetic proximity effect (MPE) in nonmagnetic heavy metal/magnetic heterostructures has received much recent attention. The MPE is believed to play a role in the spin Hall magnetoresistance (SMR) and may also be leveraged to control the emerging phenomena associated with topological insulators. Pt is most commonly used due to its vicinity to the Stoner criterion, and its use as a source of spin-orbit torques in spintronic measurements. However, much is still unknown about the properties of the MPE, such as the temperature dependence and the coupling to magnetic layers when multiple magnetic sublattices are present. To explore this, we examined pulsed laser deposited 25 nm thick dysprosium iron garnet (DyIG)/sputtered 10 nm Pt heterostructures using polarized neutron reflectometry (PNR), above (200K) and below (15 K) the magnetic compensation temperature (Tcomp = 190 K)1. Below Tcomp, the Dy3+ sublattice dominates the net moment, and above Tcomp, the net Fe3+ dominates. The MPE magnetization has only small effects on the fit to the data, but the results are consistent with a small Pt moment antiparallel to the net DyIG moment at 15 K, i.e. the Pt moment is parallel to the net Fe3+ moment, whereas at 200 K the Pt magnetization is negligible. The MS obtained from the PNR fits is 605 kA/m at 15 K (literature value = 622 kA/m) and 10 kA/m at 200 K (VSM value = 10 kA/m) 1. The scattering length density profile suggests interdiffusion at the GGG substrate interface over a 2-3 nm region, consistent with previous studies. This is accounted for with a layer of composition intermediate between DyIG and GGG. A dip in density at the DyIG/Pt interface is also present, indicating the presence of surface contamination due to the ex situ Pt deposition, or a translational growth region.References: J.J. Bauer, E.R. Rosenberg, S. Kundu, K.A. Mkhoyan, P. Quarterman, A.J. Grutter, B.J. Kirby, J.A. Borchers, and C.A. Ross, Adv. Electron. Mater., Vol. 6, 1900820 (2020).

Temperature Dependence of the Magnetic Proximity Effect in Magnetic Insulator/Platinum Heterostructures by Polarized Neutron Reflectometry

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

Magnetic skyrmions are compact spin textures that show remarkably unique, particle-like properties. They are widely regarded as promising candidates for emergent magnetic devices as they are small in size and easily maneuverable [1]. The Dzyaloshinskii-Moriya interaction (DMI) is often critical to the generation and manipulation of skyrmions. However, there has been a fundamental lack of understanding of the microscopic origin of DMI or the mechanisms by which DMI stabilizes skyrmions in magnetic bilayers. Little has been known of the material parameters that determine the value of DMI. This knowledge is critical for rational design of skyrmion-hosting materials and further development of skyrmion technology [2]. Here, we address this knowledge gap and present a model to explain the microscopic origin of DMI in heavy metal/ferromagnet (HM/FM) bilayers. We calculate the DMI values in a series of HM/FM bilayers and explain the trend of DMI values observed [3]. We present a new theoretical model that explains the microscopic origin of DMI in these bilayers. We show that the value of DMI depends on two parameters, namely, interfacial hybridization and orbital contributions of the heavy metal. Using these parameters, we explain the experimentally observed DMI trends. Additionally, we report four new materials systems predicted to demonstrate giant DMI. In HM/FM bilayers, the size and stability of DMI-stabilized-skyrmions is decided by the interplay between the DMI, the exchange constant, the out-of-plane anisotropy and dipolar fields. Due to their giant DMI, our newly discovered materials are promising for hosting small sized skyrmions at room-temperature, enabling ultra-dense, ultra-low power skyrmion devices. Together, our results put forth a new understanding of DMI, discover promising materials and suggest pathways for the controlled generation of skyrmions.References: 1. A. Fert, V. Cros, and J. Sampaio, Nature nanotechnology, Vol. 8, p. 152 (2013) 2. W. Jiang et al., Physics Reports, Vol. 704, p. 1-49 (2017). 3. P. Jadaun, L. F. Register and S. K. Banerjee, npj Computational Materials, Vol. 6, p.88 (2020)

Design of giant DMI and its microscopic origin in magnetic bilayers

Tuning the perpendicular magnetic anisotropy (PMA) in magnetic thin films has been an interest for several decades owing to its potential in magnetic recording applications. For device-level applications, it is preferable to employ a single film layer with PMA instead of a multilayer to evade the complicated fabrication process. Therefore, there is a search for materials exhibiting strong PMA in a single layer. Of the several materials exhibiting PMA, rare-earth transition metal-based Gd-Fe thin films have attracted special attention from both the fundamental and technological viewpoints. Gd-Fe films exhibit tunable magnetic anisotropy that can be tailored by modifying the composition, thickness, ion-irradiation and annealing conditions, etc.[1]. Although substantial work has been carried out in understanding the magnetic anisotropy of these films, studies on magnetic transport are still elusive. In the present study, we have grown 150 nm thick Gd-Fe thin film capped with 5nm Cr on Si and Si/SiO2 substrates by electron-beam evaporation under vacuum. Films were found to be amorphous in nature due to the large difference between the atomic radius of Gd and Fe atoms and high cooling rate attained during the deposition process. Magnetization measurements and domain imaging studies indicated the anisotropy axis is tilted in between in-plane and out-of-plane direction. This inherent tilt anisotropy makes it a unique material for sensing vector fields. Magneto-transport measurement on this film showed positive magneto-resistance along both in-plane (IP) and out-of-plane (OOP) directions. This resistance change was mainly contributed by the scattering at domain walls [2].References: [1] A. Talaptra, J. Arout Chelvane and J. Mohanty, AIP Advances., Vol. 8, p. 056327 (2018) [2] M. Tofizur Rahman, et al, Journal of applied physics., Vol. 97, p. 10C515 (2005)

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

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Temperature dependence of the Dzyaloshinskii-Moriya interaction in ultrathin films

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