United States

Magnetocaloric effect is a new nonconventional method that can be used to provide cooling power with several benefits compared to conventional gas-compression based cooling. Heusler alloys are promising candidates for micro-magnetic cooling applications thank their desirable behaviour. Preparation of Heulser alloys in a form of glass-coated microwires by the Taylor-Ulitovsky technique allows easy and fast production of microwires with protective glass coating and many additional advantages such as scalability of the size of cooling material.&lt;br/&gt;Previous works show the possibility of tuning the structural and magnetic transformation temperature of glass coated Heusler microwires by changing the chemical composition and valence electron concentration ratio &lt;i&gt;e/a&lt;/i&gt;. This is one of the most favourable properties of glass coated Heusler microwires. Combining structural and magnetic transformation may enhance the amplitude of the magnetocaloric effect when both transformations overlap, the evaluation of their contribution may become complicated.&lt;br/&gt;Therefore, we would like to present the possible analyses which can provide information about the magnetocaloric effect or the transformation temperatures including both structural and magnetic transitions. Direct measurements of magnetocaloric effect, indirect measurement of magnetocaloric effect, resistivity measurements, Arrott plot method, universal curves, method of critical exponent &lt;i&gt;n&lt;/i&gt;, Kouvel-Fisher method with Arrott-Noakes plots and many others can be used to study the magnetocaloric effect and transformations on a presented group of alloys.&lt;br/&gt;This research was supported by Slovak Grant Agency VEGA 1/0053/19, Slovak Grant Agency grant number APVV-16-0079.

2022 Joint MMM-INTERMAG

Magnetocaloric Effect Study on Heusler Based Glass-Coated Microwires

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INTERMAG 2021

Magnetocaloric effect is a new nonconventional method that can be used to provide cooling power with several benefits compared to conventional gas-compression based cooling. Heusler alloys are promising candidates for micro-magnetic cooling applications thank their desirable behaviour. Preparation of Heulser alloys in a form of glass-coated microwires by the Taylor-Ulitovsky technique allows easy and fast production of microwires with protective glass coating and many additional advantages such as scalability of the size of cooling material. Previous works show the possibility of tuning the structural and magnetic transformation temperature of glass coated Heusler microwires by changing the chemical composition and valence electron concentration ratio e/a. This is one of the most favourable properties of glass coated Heusler microwires. Combining structural and magnetic transformation may enhance the amplitude of the magnetocaloric effect when both transformations overlap, the evaluation of their contribution may become complicated. Therefore, we would like to present the possible analyses which can provide information about the magnetocaloric effect or the transformation temperatures including both structural and magnetic transitions. Direct measurements of magnetocaloric effect, indirect measurement of magnetocaloric effect, resistivity measurements, Arrott plot method, universal curves, method of critical exponent n, Kouvel-Fisher method with Arrott-Noakes plots and many others can be used to study the magnetocaloric effect and transformations on a presented group of alloys. This research was supported by Slovak Grant Agency VEGA 1/0053/19, Slovak Grant Agency grant number APVV-16-0079.

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Self-Assembled Magnetic Nanoparticle (MNP) structures have a wide range of applications in industry, chemistry, and medicine[1]. It has been shown through experiments and computational simulation that various patterns of MNPs can be deposited on a substrate [2,3]. One mechanism for self-assembly is the gradient of an applied field exerting a force on the MNPs. For example, a single perpendicular transition of magnetization (from up alignment to down) in a computer hard drive can produce a gradient sufficient to promote a single line with a width of multiple MNPs assembling on the substrate. Therefore, the chosen magnetic structure of the substrate is a determining factor in creating the desired end structure. While simulations have been performed to understand this assembly, they lack key features found in experimental systems. This work builds upon existing computational models [3] by including small reversed regions in the magnetization, multiple transition regions in the simulated area, and changes to the total magnetic field created by the addition of a soft underlayer found in hard disk drives, as seen in Fig. 1. Along with new geometry, we also investigate the effect of varying the transition separation (distance d in Fig. 1) on the final structure. Our simulations investigate the trajectories of 27 nm diameter magnetite nanoparticles using a model based on Langevin dynamics including translation and rotation. The simulation results match those found experimentally both qualitatively and also in quantitative measures of the hexagonal ordering of assembled MNPs.

![](https://assets.underline.io/uploads/markdown_image/1/image/c5671c00cf66a50726252709d063f817.jpg)

Fig. 1. The magnetic profile of the hard and soft layers of the hard drive. With constants t1, t2, a, and d being the hard layer thickness, soft layer thickness, width of reversed magnetization, and separation of regions respectively. Above the substrate is the deposited nanoparticle structure found through simulation results using this geometry.

![](https://assets.underline.io/uploads/markdown_image/1/image/1d865be38544da32adc5e3193282044a.jpg)

Fig. 2. Scanning Electron Microscope image of magnetic nanoparticles assembled on substrate with spacing of 500 nm between the center of the reversed magnetized regions.

Self-Assembled Magnetite Nanoparticles above a hard drive: the effect of the Soft Underlayer and Changing the Separation of Reversed Magnetic Regions

Pt/Co/Ir-based synthetic antiferromagnets (SAFs) are key building blocks in high performance magnetic memory devices due to their large perpendicular magnetic anisotropy (PMA) and interlayer exchange coupling (IEC) energies, respectively, and tolerance to thermal processing [1-2]. Fortuitously, the IEC field and PMA field both can be enhanced significantly via judicious tuning of the Ir thickness [3]. Further gains in IEC and PMA may be required in emerging memory devices. And while the materials system may appear to be well optimized, additional refinement could be realized through the exploration of alternative processing approaches. In particular, the replacement of Argon with a heavier gas during sputtering (e.g. Krypton) is known to enhance the PMA of Co/Pt multilayers due to reduced bombardment of the growing film from gas species during deposition, reduced intermixing of layers and improved columnar film growth, overall enhancing the PMA [4]. To the best of our knowledge, this remains an unexplored approach for SAFs in general, and particularly not for the Pt/Co/Ir system. We investigated the role of sputter gas in the properties of a SAF structure grown by DC magnetron sputtering in an ultrahigh vacuum chamber (base pressure &lt; 10-7 Pa) on a thermally oxidized Si wafer with the following film stacking structure (thicknesses in parentheses in nm): Ta(5)/Pt(5)/[Co(0.4)/Pt(0.6)]x5 /Co(0.4)/Ir(t Ir )/[Co(0.4)/Pt(0.6)]x3 /Ru(2)/Ta(3). We varied the Ir spacer thickness between 0.3 nm and 0.7 nm to cover the first IEC peak around 0.5 nm. The sample sputtered in an 6 mTorr Kr environment exhibited a peak IEC field of 1.7 T, exceeding its Ar counterpart by over 35% (see Figure 1). This same thickness is also associated with the largest PMA (µ0HK = 5.4 T), which exceeded the Ar-sputtered counterpart by over 30% (see Figure 2). A 400°C post-annealing treatment in vacuum (&lt; 4 x 10-6 Pa) only causes a moderate reduction in the IEC field to 1.5 T. These results suggest that using Krypton as the sputtering gas could lead to superior performing SAFs for emerging memory devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/3890c9e476423eca20805db63c667207.jpg)

Out-of-plane magnetic hysteresis curves for Pt/Co/Ir SAF sputtered in 6 mTorr (a) Ar or (b) Kr

![](https://assets.underline.io/uploads/markdown_image/1/image/6b14d60039618d0b815c660023e772ea.jpg)

In-plane magnetic hysteresis loops for Pt/Co/Ir SAF

Perpendicular Magnetic Anisotropy, Interlayer Exchange Coupling and Annealing Stability in Krypton-sputtered Pt/Co/Ir Synthetic Antiferromagnets

Transition-metal oxide (TMO) based heterostructures are the fertile playground to explore or functionalize novel quantum states. In this regard, the combination of 3d and 5d TMOs have gained special interest because of the strong competition between strong spin-orbit coupling and electron correlation at the interface of those heterostructures. Additionally, the inversion symmetry breaking may result in a distinct interfacial Dzyaloshinskii-Moriya interaction (DMI) and the formation of non-collinear magnetic texture in case of magnetic TMOs. Among the 5d TMOs, SrIrO3 (SIO) has gained significant attention in recent years. It displays large spin-orbit coupling and the semi-metallic ground state is highly susceptible to structural distortions. In this study, we have grown crystalline trilayer structures consisting of SIO and a 3d ferromagnetic insulator (FMI), i. e., (SIO/FMI/SIO or FMI/SIO/FMI). In-situ observation of RHEED oscillations and x-ray diffraction indicate layer-by-layer epitaxial growth. We will report in detail on the electronic transport and magnetic properties of the these heterostructures.

Magnetotransport of SrIrO3 based symmetric heterostructure

In this work we study the influence of interface parameters on the perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (DMI) of Pd(111)/Co/CoO/Pd thin epitaxial films with different oxidation depths. Epitaxial Si(111)/Cu(2 nm)/Pd(2 nm)/Co(1 nm) films were grown by molecular beam epitaxy in ultrahigh vacuum chamber. After deposition of ferromagnetic layer, films were exposed the procedure of artificial oxidation with dry oxygen. We varied the oxide thickness and this way we changed the remanent ferromagnetic thickness. Determination of the thickness of the magnetic layers carried out by measuring the saturation magnetic moment with vibrating samples magnetometry. We also used SQUID-magnetometer for low-temperature measurements. Magnetic structure was investigated by a magneto-optical Kerr effect (MOKE) microscope which was equipped with a hand-made coil applying out-of-plane magnetic fields and an in-plane electromagnet. Significant elastic strains appear on Pd/Co interface lead to induce strong perpendicular magnetic anisotropy [1]. Artificial oxidation of magnetic layer leads to symmetry breaking on the bottom and top magnetic interfaces in epitaxial films, which reduces compensation of interfacial DMI. It should be noted that including of oxide layer is eliminate the asymmetrical contribution of chiral dumping in comparing with symmetrical Pd/Co/Pd [2] structures, it following from symmetrical parabolic shape of domain walls velocity curves as shown on figure 1(a) and allow determine the DMI field by minimal velocity field Hmin[3]. Moreover, Pd/Co/CoO thin films demonstrate strong exchange bias in out-of-plane magnetic field at a low temperature, as shown on figure 1(b). The reported study was partially funded by Council for Grants of the President of the Russian Federation (project - MK-1384.2021.1.2), the Russian Ministry of Science and Higher Education (the state task 0657-2020-0013) and grant of the Government of the Russian Federation (Project Proposal No.2020-220-08-4899) .

![](https://assets.underline.io/uploads/markdown_image/1/image/065806aae110d0b3e47020289f3a6c9f.jpg)

Dzyaloshinskii-Moriya Interaction and exchange bias in epitaxial Pd/Co/CoO films

Dzyaloshinskii-Moriya Interaction and exchange bias in epitaxial Pd/Co/CoO films

The formation of hybrid ferromagnetic/molecule heterostructures can lead to strong modifications of the magnetic properties of the ferromagnetic layer[1]. Such modifications, like magnetic anisotropy easy axis reorientation[2, 3] and magnetic hardening[4] are linked to the hybridization at the interface between the molecules p-orbitals and the ferromagnetic surface d-orbitals[1][2]. Here we report a novel finding: an interface dependent of NMR spectra of 7 nm polycrystalline Co thin films capped with two different organic molecules, Fullerene C60 and Gallium-quinoline Gaq3. Cobalt layers are grown simultaneously on two Al2O3(0001) substrates at RT and UHV by e-beam evaporation. The molecular layers are then grown without breaking the vacuum, by thermal evaporation at similar conditions. The molecular layer thickness is 25 nm, in order to guarantee a uniform coverage of the metallic surface. Cobalt surface characterization is performed by ex-situ AFM on a bare 7nm Co ultra-thin film (Fig. 1), showing a uniform, atomically flat and granular surface (RMS of (0.3 ± 0.1) nm). Ex-situ AFM characterization of the molecular layers shows a complete coverage, and RMS less than 1nm. NMR characterization is performed at T=77°K in zero external applied field. 59Co spectra where taken in the frequency range 190-235 MHz. For each sample, different spectra were acquired with different transmitted RF power, in order to obtain the enhancement factor [5]. We obtain an enhancement factor ratio of 2 between the Co/Gaq3 and the Co/C60 systems. This is in agreement with L-MOKE characterization at T= 77°K, showing that the Co/Gaq3 system is magnetically harder than the Co/C60. The corrected NMR spectra, (Fig. 2), show the presence of both FCC and HCP phases. Remarkably, their spectral weights is different for the two samples, depending on the molecular species deposited over the Cobalt surface. These results can be explained as due to an interface-induced modification of the hyperfine field felt by the 59Co nuclei, implying a profound impact of the interface on the magnetic properties of a significant portion of the ferromagnetic structure.

![](https://assets.underline.io/uploads/markdown_image/1/image/c869a8e80820732a32b196f9bff82f4d.jpg)

Fig.1 AFM image of 7 nm Co on Al2O3(0001).

![](https://assets.underline.io/uploads/markdown_image/1/image/3d5728d48b592cd7c66676f9a176ec72.jpg)

Fig.2 NMR spectra of Co(7nm)/C60 and Co(7nm)/Gaq3.

Interface dependence of NMR spectra of 59Co with different molecular overlayers

Modification and control of magnetic anisotropy is crucial for the development of new permanent magnet materials, high density information storage, and many advanced spintronic applications. Enhanced anisotropy is often achieved via shape modification, stabilization of non-cubic crystal structures, or proximity effects such as exchange bias. An unusual proximity effect occurs at the interfaces of thin Co films adjacent to π-conjugated C60 fullerene films where initial field hysteresis loops exhibit a substantial asymmetry and considerable broadening at reduced temperatures [1]. We investigate these effects with temperature-dependent broadband ferromagnetic resonance (FMR). A key question in these studies is whether the C60-derived anisotropy affects the dynamical properties of the Co / C60 system including resonant fields, damping, and spectroscopic g-factors. We examine the magnetometry and FMR response of a Co [12 nm] / C60 [35 nm] / Nb [16 nm] sample as a function of temperature and field history. Conventional field hysteresis loops identify two main effects: the asymmetry is present only for the initial loop and the enhanced coercive field increases monotonically for temperatures below 200 K. Temperature-dependent FMR from 4 – 35 GHz confirm these observations. The effective magnetization (μ0Meff) tracks the increased anisotropy of the Co / C60 upon cooling, but following field cycling μ0Meff is reduced. The difference (Δμ0Meff) is as large as 26 mT at 10 K and Δμ0Meff drops to zero above 200 K. These effects are not observed in control samples with a Cu spacer layer inserted between the Co and C60 layers. Gilbert-type damping is slightly increased in the Co / C60 system at all temperatures, however, the enhanced anisotropy prior to field cycling has little effect on damping.

Anisotropy Enhancement in Co / C60 Fullerene Bi-Layers Determined by Ferromagnetic Resonance

The interest in the magnetic cooling devices has led to an intensive search for suitable well-performing magnetocaloric materials over the last 25 years. However, to this date only a few material systems have been found to be suitable for commercial applications. Computational screening makes possible sieving through thousands of known compounds without the need to perform time-consuming measurements and therefore can play an important role in detecting novel magnetocaloric materials. To identify such systems of interest we selected several screening parameters using properties of various well-known MCE materials as a reference. Along with magnetic properties, other factors important for practical applications were taken into consideration including price, availability, and toxicity of candidate materials. Combining these criteria, we implemented an algorithm for the screening process as a set of automated computational tools. It relies on information readily available in the structural databases to perform detailed modelling via ab-initio calculations. Two computational databases (Aflowlib and Materials Project) and two experimental databases (COD and ICSD) are used as initial source of information totaling to 800k candidate compositions. As a main metric to predict the magnetocaloric performance we use the magneto-elastic response of the candidate. This is done by computing magnetic moment of the material across a range of deformations, both volumetric and uniaxial. This parameter shows good correlation with ΔSM measured in known magnetocaloric materials. A step-by-step application of initial screening parameters to sort out unsuitable materials before performing more computationally heavy assessments allows us to quickly process large number of candidates. This resulted in a shortlist of promising compounds to serve as a guide for experimental research. The flexibility of the computational toolset makes it possible to easily build in additional screening parameters to further adjust or refine the search process.

An Efficient Strategy for the Discovery of Novel Magnetocaloric Materials

In recent years, magnetic refrigeration (MR) has gained much attention, rather than conventional gas refrigeration, being an energy efficient and eco-friendly technology. For that purpose, the current research for exploring potential magnetocaloric material is focused on the material exhibiting first-order magnetostructural transition (MST) along with large change in magnetization and hence large magnetocaloric effect (MCE) [1]–[3]. Recently, Ni-Co-Mn-Ti-based all d-metal Heusler alloy is found to be an excellent candidate for magnetic refrigeration which exhibits multicaloric properties such as MCE [4], elastocaloric effect [5], and giant barocaloric effect [6]. In the present work, we explore the effect of Sn doping at Ti-site and also heat treatment condition on MCE properties of Ni35Co15Mn37Ti13-xSnx (x = 0,1 and 2) alloys through thermal and magnetic measurements. The observed MST temperature (TM) shifts toward lower temperature with Sn doping whereas, the sharpness of the transition enhances with annealing time which effectively increases the MCE response as shown in fig.1 (a)). The sample with x = 1 exhibits a plateau-like magnetic entropy change (ΔSM) with a maximum peak value of ΔSM ~ 5.7 Jkg-1K-1 due to a magnetic field change of 50 kOe. It is interesting to note that ΔSM value remains almost the same with value ~ 4 Jkg-1K-1 for a temperature region of 300 K-350 K shown in fig.1 (b). Large ΔSM in a wide temperature interval is attributed to the existence of inter-martensite transition along with MST. The large ΔSM value with wide tunable operating temperature region in the vicinity of room temperature and the estimated large relative cooling power (RCP) of ~ 384 J/kg enable the material a suitable candidate for solid-state MR technology.

![](https://assets.underline.io/uploads/markdown_image/1/image/0742db6c7fcbb9ed5a3e2fcb08c45c88.jpg)

Figure 1: (a) Magnetization vs. T for x = 1 and 2alloys under different annealing temperatures. Inset shows DSC heat-flow vs. T with a ramp rate of 10 K/m of x = 1 sample.

![](https://assets.underline.io/uploads/markdown_image/1/image/2d9c91f1787e1efc24138fdba6d61ecd.jpg)

Figure 1: (b) ΔSM vs. T at different magnetic field for x = 1 alloy.

Large plateau-like Magnetic entropy change in Sn doped Ni-Co-Mn-Ti all d metal Heusler alloy

Gadolinium (Gd) films have been considered for micro and nano-scale application, because of their high magnetic moment and a Curie point close to room temperature (RT) [1]. Recently, free-standing thick films have been pointed out as promising active substance in energy conversion micro-devices [2] due to their magnetocaloric properties (MC). Indeed, as put in [2], good MC properties, surface quality, absence of substrate, and flexibility are the main features to get a caloric membrane to be deployed in a micro-electromechanical system (MEMS). Unfortunately, a systematic study of the optimal preparation parameters to obtain the aforementioned properties, is still lacking. Actually, film thickness, growth temperature, and the choice of the buffer layer, are all key features affecting the final result after substrate removal. Here, polycrystalline Gd films are prepared by sputtering on a silicon substrate with a thermally oxidized layer. Thickness of the film is varied from 100 nm to 17 μm, exploring growth temperatures in an interval between RT up to 666 K. Buffer layers of tungsten (W) and tantalum (Ta) are used. More than 45 Gd thick films with various growth temperatures, thicknesses and buffer layers have been prepared. The first result is that notwithstanding the growth temperature and thickness, tungsten buffer makes the substrate removal more difficult, particularly over large film surfaces (e.g. tens millimetres size). Increasing the growth temperature improves the magnetic properties [3] and changes the surface morphology. For instance, surface roughness is 10 nm and 121 nm for films prepared at 543 K and 666 K, respectively. Using tantalum, wide area free-standing films of various thicknesses from 5 to 17 μm have been obtained, with a flexibility depending on the growth temperature. Thinner free-standing films, namely 3 μm Ta/Gd/Ta have also been obtained showing smaller sizes (i.e. few millimeters size). In summary, the main result here is the identification of optimal preparation conditions (buffer layer, growth temperature and thickness) to obtain large, smooth and flexible Gd films to be used in MEMS device as a magnetocaloric membrane.

Producing gadolinium free-standing and flexible films, with high quality surface for energy applications: the role of thickness and growth temperature

ABO3 type compounds with perovskite structure are known to exhibit a very rich variety of phase transitions ranging from ferroelectric, antiferroelectric, ferrielectric, antiferrodistortive octahedral tilt, ferromagnetic, antiferromagnetic, ferrimagnetic, John-Teller, metal-insulator and superconducting transitions [1]. The richness of these phenomena is further enhanced by introducing substitutional disorder at the A or B sites leading to relaxor ferroelectricity, dipole-glass, spin-glass and quantum phase transitions [2]. The technological applications of these perovskites is due to the functional properties imported by such phase transitions. In this context, the B-site disordered iron-niobate family has evinced considerable interest due to the temperature stable giant dielectric constant and multiferroicity. The low temperature magnetic transitions and the ground state of these complex perovskites is now fully understood [3,4]. However, understanding of the high temperature behaviour is still lacking. In the present investigation, we report high temperature phase transitions in Sr(Fe1/2Nb1/2)O3 (SFN) using dc magnetization and x-ray diffraction measurements. Our dc magnetization M (T) study reveals two magnetic transitions around 620 K and 700 K for the first time. The magnetic transition occurring around 620 K is linked with the structural phase transition as confirmed by temperature dependent x-ray diffraction studies. The integrated intensity of the strongest superlattice reflection (ISL) and the octahedral tilt angle (φ) decrease continuously with increasing temperature and vanishes across the structural phase transition. We have fitted temperature variation of both ISL and φ using the expressions B(TC-T)β and B(TC-T)2β, respectively, where B is constant, TC is the structural phase transition temperature and β is critical exponent. The value of β comes out to be 0.24 which suggests the tricritical nature of the phase transition. Another transition around 700 K in the M (T) plot occurs close to the peak temperature in the loss tangent (tanδ). We believe that this transition is most likely driven by oxygen vacancies.

![](https://assets.underline.io/uploads/markdown_image/1/image/6e631abc07abd6eec3192e6391959901.jpg)

DC magnetization versus temperature plot of Sr(Fe1/2Nb1/2)O3 at 100 Oe field

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Self-Assembled Magnetite Nanoparticles above a hard drive: the effect of the Soft Underlayer and Changing the Separation of Reversed Magnetic Regions

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