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The LaFe&lt;sub&gt;13-x&lt;/sub&gt;Si&lt;sub&gt;x&lt;/sub&gt; compounds have been studied for nearly two decades for their potential to serve as materials for magnetocaloric cooling. They exhibit either first- or second-order phase transitions depending on x(Si). Here we present the discovery of a naturally formed two-phase composite at the LaFe&lt;sub&gt;2&lt;/sub&gt;Si stoichiometry, which exhibits strongly responsive behaviors without mechanical degradation that can be exploited in weak magnetic fields. Some of the unusual properties include a two-step, nearly anhysteretic first-order phase transformation between ferromagnetic (FM) and paramagnetic (PM) states, instead of a singular phase change present in conventional LaFe&lt;sub&gt;13-x&lt;/sub&gt;Si&lt;sub&gt;x&lt;/sub&gt;, that is preserved in magnetic fields as high as 100 kOe. In addition to the giant magnetocaloric effect, the title material exhibits large, plateau-like positive magnetoresistance, both extending over a much wider temperature range when compared to phase-pure LaFe&lt;sub&gt;13-x&lt;/sub&gt;Si&lt;sub&gt;x&lt;/sub&gt;. Finally, the composite also shows a Griffiths-like phase associated with short-range FM clustering in the PM state (Pathak et. al., Acta Mat. 215 (2021) 117083). We will also discuss the effect of impurities on magnetic, magnetocaloric, and transport behaviors of LaFe&lt;sub&gt;2&lt;/sub&gt;Si.&lt;br/&gt;The work at Ames Laboratory was supported by the Division of Materials Science and Engineering, the Office of Basic Energy Sciences of the DOE. Ames Laboratory is operated for the U.S DOE by Iowa State University under Contract No. DE-AC02-07CH11358. Work at SUNY, Buffalo State was supported by the startup fund from SUNY Buffalo State College and the Undergraduate Research Office, Buffalo State.

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Unusual magnetic, magnetocaloric, and transport properties in phase-separated LaFe2Si

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The LaFe13-xSix compounds have been studied for nearly two decades for their potential to serve as materials for magnetocaloric cooling. They exhibit either first- or second-order phase transitions depending on x(Si). Here we present the discovery of a naturally formed two-phase composite at the LaFe2Si stoichiometry, which exhibits strongly responsive behaviors without mechanical degradation that can be exploited in weak magnetic fields. Some of the unusual properties include a two-step, nearly anhysteretic first-order phase transformation between ferromagnetic (FM) and paramagnetic (PM) states, instead of a singular phase change present in conventional LaFe13-xSix, that is preserved in magnetic fields as high as 100 kOe. In addition to the giant magnetocaloric effect, the title material exhibits large, plateau-like positive magnetoresistance, both extending over a much wider temperature range when compared to phase-pure LaFe13-xSix. Finally, the composite also shows a Griffiths-like phase associated with short-range FM clustering in the PM state (Pathak et. al., Acta Mat. 215 (2021) 117083). We will also discuss the effect of impurities on magnetic, magnetocaloric, and transport behaviors of LaFe2Si. The work at Ames Laboratory was supported by the Division of Materials Science and Engineering, the Office of Basic Energy Sciences of the DOE. Ames Laboratory is operated for the U.S DOE by Iowa State University under Contract No. DE-AC02-07CH11358. Work at SUNY, Buffalo State was supported by the startup fund from SUNY Buffalo State College and the Undergraduate Research Office, Buffalo State.

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Artificial spin ice (ASI) [1] systems are two-dimensional arrays of single-domain nanomagnetic islands that are designed to study collective behavior of interacting magnetic systems. These nanoislands usually have a stadium shape so that the magnetization orients along the longer axis, i.e., the moments are Ising-like. Most research on ASI systems focuses on different arrangements of nanoisland arrays [2], but, recently, some studies have focused on modifying the shapes of nanoislands [3,4]. Here, we introduce a new design of nanoislands, the Y-shape island, which has three equal-length arms joining at the center, with 120° separation. This island has six possible moment configurations in which the magnetization on each arm points either toward the center or away from it. We fabricated such nanoislands from permalloy, arranged in a hexagonal lattice geometry (Fig 1). Since permalloy has little magnetocrystalline anisotropy, the magnetization of the nanoislands can be well-controlled by utilizing shape anisotropy. We annealed such samples above the Curie temperature and examined the moment configurations using magnetic force microscopy after the samples cooled back to room temperature. We find strong correlations among the complex moment arrangements of these islands, suggesting the possibility of new types of moment ordering and new phenomenological ways of describing these novel magnetic objects associated with their unusual symmetry. Work at Yale University was funded by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Grant No. DE-SC0020162. Work at the University of Minnesota was supported by NSF through Grants No. DMR-1807124 and DMR-2103711.

![](https://assets.underline.io/uploads/markdown_image/1/image/2edd3e78dd111803b2e2fb7c7778f707.jpg)

Figure 1. (a) A scanning electron microscopy image of Y-shape nanoislands in hexagonal geometry. (b) A magnetic force microscopy image taken after annealing at 550 degree C. The white and black dots correspond to the poles of the island moments. (c) The corresponding moment schematic.

Artificial Spin Ice with Y-shaped Nanomagnets

The fundamental building blocks of artificial spin ice (ASI) systems are magnetic nanoislands designed for Ising-like single domain configurations magnetized along the nanoisland length due to shape anisotropy. The dipolar interaction between these macrospins serve as the basis for investigating geometric frustration in ASIs with a variety of array geometries ranging from square-like arrangements to quasicrystals. Using X-ray photoemission electron microscopy (X-PEEM) to perform magnetic domain imaging, we studied thermally demagnetized La0.7Sr0.3MnO3-based brickwork ASIs and showed that non-Ising domains, which we call complex spin textures (CSTs), can be stabilized through appropriate selection of nanoisland width, W, and ASI lattice parameter, a. While W-dependent formation of CSTs can be explained due to the dominance of shape anisotropy, the a-dependence was unexpected for nanoislands possessing identical W, suggesting that inter-island effects play an important role. MuMax3 micromagnetic simulations indicated that an intermediate range of 100 nm ≤ W &lt; 150 nm and 600 nm ≤ a ≤ 700 nm exists where the formation of Ising states are preferred over CSTs as shown in Fig. 1. For larger values of W and a that reside above the black border in the energy diagram of Fig. 1, CST formation becomes energetically favorable over an arrangement of Ising states in higher energy vertex configurations. Energy analysis suggests that while magnetostatic energy dominates the energetics, the small contribution from exchange energy determines whether nanoislands can exist in Ising or CST states. The nanoisland magnetic state changes the magnetostatics arising from both intra-island shape anisotropy and inter-island dipolar interaction. These results suggest the potential to create tunable nanoisland domain states, providing a new direction in the development of ASI systems with exotic phase transitions as well as novel nanomagnetic logic architectures.

![](https://assets.underline.io/uploads/markdown_image/1/image/12c68623b9c39a06d7d5874092f9e981.jpg)

Energetics of complex spin textures in LSMO artificial spin ice structures

Energetics of complex spin textures in LSMO artificial spin ice structures

Collective dynamics in lithographically-defined artificial spin ices offer insights into emergent correlations and phase transitions of geometrically-frustrated Ising spin systems. Their temporal and spatial evolution observed in experiments are also often simulated using kinetic Monte Carlo (kMC) simulations. As we recently showed, simplified models that describe the switching barriers, which are necessary to obtain predictive simulations results, overlooked a chiral contribution that leads to a splitting of barrier energies and results in faster relaxation [1,2]. As consequence of the chiral barrier splitting, the relaxation pathways are modified. In this work, we study the evolution from the field-saturated state to the ground-state order of artificial square ice with kMC simulations; and compare the effect of commonly-used mean-field barriers to the chiral split-barrier description at different interaction strengths and temperatures. We assess general measures that describe the dynamic evolution, such as vertex populations and net magnetisation as well as spatial correlations. By taking into account the chiral barrier splitting we observe faster and more efficient evolution to the ground state. Compared to the mean-field barrier model, for which the demagnetisation is mainly dominated by the nucleation and propagation of ground-state strings (for strong interactions) or random events (for weak interactions), the chiral barrier splitting promotes a richer diversity of switching events. This leads to a variety of behaviours, such as breaking up strings without the generation of charged defect vertices, the formation of coral-like domain walls, and efficient annihilation of domain walls in the later stages of the evolution to the ground state. Our results demonstrate that the rich demagnetisation dynamics of artificial square ice, as is also observed experimentally, can be partially explained even in the absence of additional disorder.

Role of Chiral Switching Barriers on Relaxation Kinetics in Artificial Square Ice

Bi-component magnonic crystals have shown promising applications in magnonics and spintronics [1,2]. Arrays of two-dimensional exchange and magnetostatically coupled bi-component nanotriangles (BNTs) comprising of alternating permalloy (Py) and cobalt (Co) in hexagonal arrangement were fabricated by self-aligned stepwise nanosphere lithography [3]. The separation (S) between constituent NTs was varied to investigate the effects of exchange and magnetostatic interaction. Static magnetic properties of BNTs were investigated by OOMMF simulations and compared to homogeneous Py NTs [4]. Fig 1(a) shows simulated M-H loops for BNTs during hard axis magnetization with different S values. The loops exhibit distinct two-step switching corresponding to magnetization reversal of Py and Co NTs at different fields. Py NTs are observed to switch at a lower field shown in Fig 1(d) for S=0 nm. For homogeneous Py NTs in Fig 1(b), multiple kinks were also observed for S=0 & -60 nm (overlapping NTs) but these represent the domain changes in individual NTs. For S= -60nm, antiparallel alignment was displayed in each NT pair during reversal. For S=60nm, the NTs react independently due to the lack of strong exchange coupling. Hence switching is random and independent of location. For S=0, the NTs within the inner ring switched at a lower field, as shown in Fig 1(e). Fig 2 shows MFM images of BNTs with S=0 during hard axis field reversal. The magnetic domains of BNTs at remanence after saturation is in good agreement with simulation in Fig 1(c). In contrast to Fig 1(d), Co NTs were observed to reverse at a lower field. This is attributed to the smaller fabricated Co size compared to Py NTs. We will discuss the effect of material difference and separation between constituent NTs in detail via OOMMF simulation and field reversal study.

![](https://assets.underline.io/uploads/markdown_image/1/image/dae6f6f4d1da11df0bb2e846d9d8ecf2.jpg)

Fig. 1 Simulated M-H loops for S= -60, 0, +60 nm for (a) Co/Py NTs; (b) Py/Py NTs. Simulated spin states for (c-d) BNTs; (e) Py/Py NTs at different fields. Colored arrows indicate spin orientations.


![](https://assets.underline.io/uploads/markdown_image/1/image/8285bc51588ea41eb64cd46369774d8f.jpg)

Fig. 2 (a) AFM; (b-d) MFM of BNTs with S=0 nm at remanence after applying field of 3500, -400 and -600 Oe. White arrow indicates the applied field direction. Colored circles indicate a change in magnetic domain.

Static Magnetic Properties of Bi-component Nanotriangles in Hexagonal Arrangement Fabricated by Stepwise Nanosphere Lithography

We have studied the use of self-assembled block copolymers to pattern multilayers of Co and Pd on silicon wafers. Stacks ranging from 4 to 12 bilayers of Co(0.3 nm)/Pd(0.8 nm) were sputtered onto Ta/Pd seed layers and capped with 3 nm of Ta, and were found to have perpendicular magnetic anisotropy as-deposited. The block copolymer, PS-PFS, was dissolved in toluene and spun onto the wafers. After heat treatment to phase-separate the polymers, the PS was removed by oxygen plasma ashing. The remaining polymer, PFS, formed self-assembled spheres that were used as masks to ion-mill the Co/Pd multilayers into nanopillars [1]. To study the effect of etch time and etch angle on the coercivity distribution, we synthesized samples in a “design of experiments” grid in these two variables. Scanning electron micrographs showed nanopillars ranging from 10 to 30 nm in diameter, depending primarily on etch time (Fig. 1). M-H loops measured on both patterned and unpatterned wafers showed an increase of up to 113% in coercivity upon patterning. First Order Reversal Curves (FORC) were measured, and the resulting FORC distributions showed different features for the nanopatterned vs. unpatterned films. Fig. 2 shows the FORC curves and the resulting FORC distribution for an unpatterned multilayer [Co/Pd] plotted by (1) FORC+ (MagVis.org), which shows the exact measured FORC density without averaging and (2) the more often used averaged color map, in which information is lost. Both displays show a large positive peak with coercivity (Hc) about 500 Oe, with a “dipole tail” characteristic of switching by domain nucleation and wall motion [2]. In the FORC+ plot, the tail consists of a string of negative (blue, on the left) and positive (orange, on the right) pixels – the variation of density along the tail is smaller in the averaged curve, but this comes at the expense of greatly overestimating the separation between the positive and negative tails.

![](https://assets.underline.io/uploads/markdown_image/1/image/fd0faa405db6b295f8638cd9fef73451.jpg)

Fig. 1. SEM image of a patterned [Co(0.3 nm)/Pd(0.8 nm)]12 film: 20 nm diameter pillars.

![](https://assets.underline.io/uploads/markdown_image/1/image/5959a0e19128eea71ab4aa949fd706fa.jpg).

Fig. 2. Top: FORC curves M(H). Bottom right: FORC+ distribution. Orange is positive, blue negative Left: Smoothed FORC distribution, on the same scale but displaced to left, with color legend.

FORC Analysis of Nanopatterned vs. Unpatterned Films: Coercivity and Switching Mechanisms

Non-collinear antiferromagnetic orders are of strong interest in the fields of spintronics, topological magnetism and multiferroicity [1]. However, the magnetic imaging of these materials is particularly challenging due to their zero net magnetization. Dy epitaxial films exhibit an antiferromagnetic helical phase and strong X-ray magnetic scattering at the Dy L3 edge [2] These characteristics make it a perfect candidate to fully vectorial reconstruction of a non-collinear magnetic order, enabled by the new combination of Resonant X-ray Magnetic Scattering (RXMS) [3] and Coherent Diffraction Imaging (CDI) [4]. In order to implement this technique, micro-objects are required as model samples. The micropatterning of Dy nanofilms (top-down approach) is however particularly challenging due to the high reactiveness of Dy. In this work, we present the results of this microfabrication process, from the growth of Dy epitaxial nanofilms to the patterning and full characterization of the µ-disks. The high crystalline quality of the Dy(0001) films, grown on A-plane sapphire/(110) Nb templates, is kept after the optical lithography followed by Ion Beam Etching. X-ray diffraction (Fig.1) indicates a moderate decrease in coherence length (10%) and an increase in mosaicity (17%) in 2 mm disks compared with continuous films. However, an FCC phase is formed, likely due to hydrogenation during the patterning process [5] SQUID measurements and RXMS (BM28-ESRF) confirm the occurrence of the helical phase in the Dy µ-disks, with similar Néel and Curie temperatures as in continuous films, but with a slight increase in critical fields. The temperature dependence of (002-t) magnetic satellites reveals that helical turn angles in the patterned samples do not differ significantly from those observed in epitaxial films [6](Fig. 2). The effect of the buffer layer (Nb or Nb/Y) is also studied and correlated to the role of epitaxial strains.

![](https://assets.underline.io/uploads/markdown_image/1/image/8129cbfe6c8f62f2fcbd865b2993bdcd.jpg)

(0 0 L) scans at the Dy L3 edge at 130 K measured for Nb/Dy (green), Nb/Y/Dy (orange) and their respective µ-disks. Grey region indicates the magnetic satellites. Insert: SEM tilted view of Dy µ-disks.

![](https://assets.underline.io/uploads/markdown_image/1/image/45d553476adcd2f5d6198955784f0389.jpg)

Turn angle of the Dy helical phase as a function of temperature (same samples as in Fig.1).

Development of Dy µ-disks for imaging of helimagnetic domains

Spin glasses, generally defined as disordered systems with randomized competing interactions that result in an extensively degenerate ground state1, 2, are a widely investigated complex system. Theoretical models describing spin glasses are broadly used in other complex systems, such as those describing brain function3, error-correcting codes4, or stock-market dynamics5. This wide interest in spin glasses provides strong motivation to generate an artificial spin glass within the framework of artificial spin ice systems6. Here, we present the first experimental realization of an artificial spin glass, consisting of dipolar coupled single-domain Ising-type nanomagnets arranged onto an interaction network that replicates the aspects of a Hopfield neural network7. Using cryogenic x-ray photoemission electron microscopy (XPEEM), we performed temperature dependent imaging of thermally driven moment fluctuations within these networks and observed characteristic features of a two-dimensional Ising spin glass. Specifically, the temperature dependence of the spin glass correlation function follows a power law trend predicted from theoretical models on two-dimensional spin glasses. Furthermore, we observe clear signatures of the hard to observe rugged spin glass free energy1 in the form of sub-aging, out of equilibrium autocorrelations1 and a transition from convergent to divergent dynamics1, 8.

![](https://assets.underline.io/uploads/markdown_image/1/image/b3ae2199ab934834ddc1bb2571df82f4.jpg)

Direct observation of a Dynamical Glass Transition in a Nanomagnetic Artificial Hopfield Network

The MnTX (T = Ni, Co; X = p-block elements) compounds have received considerable attention due to the coupled first-order magnetic and structural phase transitions (MST) they exhibit. Large magnetocaloric effects are often associated with these phase transitions. The Mn1-xFexNiSi1-yAly system is a member of the MnTX family for which a large magnetic entropy change of 23 J kg-1 K-1 has been reported at T= 210K, ΔH = 2T. This value was reported for x = 0.5, y = 0.07 and for x = 0.5, y = 0.05 the peak value decreased to 17 J kg-1 K-1 (T = 315K, ΔH = 2T) (Biswas et. al. Acta Mat. 180 (2019) 341). For x = 0.4, y = 0.7, a peak magnetic entropy change of 9.3 J kg-1 K-1 was recently observed near 247 K (Ghosh et. al. JMMM 500 (2020) 166345). These studies suggested that the Mn1-xFexNiSi1-yAly system exhibits the maximum entropy change for x = 0.5. It is interesting to study the effect of Fe and Ni concentration variation on the magnetocaloric properties of the Mn0.5Fe0.5-xNi1-xSi1-yAly, which has not been reported yet. Therefore, in this work, we present an experimental study on the effects of partially replacing Fe with Ni on the structural, magnetic, and magnetocaloric properties of Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 (0.025 ≤ x ≤ 0.10). The results of x-ray diffraction and scanning electron microscopy (SEM) showed that the samples exhibited a single-phase and energy dispersive spectroscopy analysis showed that the resulting compositions were close to the nominal compositions. Maximum magnetic entropy changes of – ΔSM = 22 and 57 J kg-1K-1 for ΔH = 2 and 5 T, respectively have been realized for x = 0.1 around 322 K. The observed large MCEs are due to the MST exhibited by the materials from low-temperature FM orthorhombic phase to high-temperature PM hexagonal phase. Work at SUNY, Buffalo State was supported by the startup fund from SUNY Buffalo State College and Undergraduate Summer Research Fellowship award, Undergraduate Research Office, Buffalo State.

The variation of Fe/Ni concentration and its effect on the magnetic and magnetocaloric properties of Mn0.5Fe0.5-xNi1+xSi0.94Al0.06

The M5XB2 materials system has been widely studied as permanent magnetic materials[1] [2]. As permanent magnetic materials, their coercivity and magnetic energy product are lower than those of the mainstream permanent magnetic materials. Xie et al.[3] and Cedervall et al.[4] proposed that the Mn5PB2 and (Fe1-xCox)5PB2 compounds have the prospect of being applied as magnetocaloric materials due to their near room temperature Curie temperature. In this work, the structure and magnetocaloric properties of the Mn5(SiP)B2 compounds of the M5XB2 material system were studied. According to a refinement of the XRD data the Mn5(Si1-xPx)B2 compounds all crystallize in the Cr5B3–type body-centered tetragonal structure, with a small amount of Mn2P as a secondary phase (less than 7%). The lattice parameters and the unit-cell volume of the compounds change linearly with the increase in P content. These experimental results are consistent with calculated DFT results. The Curie temperature of the compounds can continuously be adjusted between 305 and 411 K by changing the Si/P ratio. The introduction of P also caused a decrease in saturation magnetization. The magnetic phase transition of these compounds was determined using Arrot plots and the field exponent n for the magnetic entropy change based on Landau's theory. The studied compounds all show a second-order magnetic phase transition. Since the compounds show a second-order phase transition, the magnetic entropy change caused by the phase transition was not large: 1.9 and 1.4 J/kgK for Mn5SiB2 and Mn5PB2, respectively. However, the advantage of this series of compounds is that the Curie temperature can be adjusted continuously around room temperature.

![](https://assets.underline.io/uploads/markdown_image/1/image/1d9940cac68d7c5951d382a4b1969178.jpg)

Fig. 1. Temperature dependence of magnetization of Mn5(Si1-xPx)B2(0 ≤ x ≤ 1) in a field of 0.01T.


![](https://assets.underline.io/uploads/markdown_image/1/image/a3a6abe7d32a72ba069affbbfed1f01d.jpg)

Fig. 2. TC as a function of P content for Mn5(Si1-xPx)B2 (0 ≤ x ≤ 1).

Magnetocaloric Properties of a New Type of Magnetocaloric Materials - M5XB2 System

La(Fe, Si)13 is a promising candidate for magnetocaloric based technology such as magnetic refrigeration or energy harvesting [1-3]. This alloy family consists of a low content of rare earth (7.1 at. %) and high content of iron, resulting in a cheap and non-toxic compound. La(Fe, Si)13 based alloys exhibit a large magnetocaloric effect (MCE) at the transition between ferromagnetic and paramagnetic states, which occurs at Curie temperatures (Tc) ranging between 200-259K [4]. The magnetic properties (Tc, magnetization, MCE), the crystal structure (cubic or tetragonal) and the type of transition can be tuned by adjusting the Si content. Furthermore, the Curie temperature can be increased to above room temperature by doping and hydrogenation [5]. Recently our group reported for the first time the possibility to fabricate thick films of La(Fe, Si)13 [6]. Here we present a study on compositionally graded films of La-Fe-Si. The films were fabricated by triode co-sputtering of three targets (La-Fe, Fe, Si-Fe) onto thermally oxidised stationary Si substrates (100 mm in diameter). The composition of the as-deposited films was mapped by EDX, as shown in Fig. 1. The films were annealed ex-situ in a rapid thermal annealing furnace at 900°C. For each film, we selected samples from specific regions for characterization using optical and scanning electron microscopy, VSM-SQUID magnetometry and X-ray spectroscopy. Fig. 2 displays M(T) measurements on four La-Fe-Si samples of 5 μm thickness, taken from the regions specified in Fig. 1. The presentation will report results from films of nominal thickness ranging between 150nm – 5μm. The presentation will give details about how the structural and magnetic properties vary as a function of film composition, thickness and annealing conditions. Prospects for further developments and possible applications will also be discussed. Acknowledgement: This research has received funding from the French National Research Agency under the HiPerTherMag project (ANR-18-CE05-0019).

![](https://assets.underline.io/uploads/markdown_image/1/image/e47b4016b72ccd38f00fea3375cd00d9.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/a06f84bea07fbcfb4eb5845d9b87a6ea.jpg)

Compositionally graded La-Fe-Si films

Unusual magnetic, magnetocaloric, and transport properties in phase-separated LaFe2Si

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Unusual magnetic, magnetocaloric, and transport properties in phase-separated LaFe2Si

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