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Controlling changes in magnetic anisotropy across epitaxial film interfaces is an important prerequisite for many spintronic devices. For the canonical dilute magnetic semiconductor GaMnAs, magnetic anisotropy is highly tunable through strain and doping, making it a fascinating model system for exploration of anisotropy control in a carrier-mediated ferromagnet. Here, we have used transmission electron microscopy and polarized neutron reflectometry to characterize the interface between GaMnAs-based layers designed to have anisotropy vectors oriented at right angles from one another. For a bilayer of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ and ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$, we find that the entirety of the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layer exhibits in-plane magnetic anisotropy and that the $majority$ of the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ exhibits perpendicular anisotropy. However, near the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ interface, we observe a thin Mn-rich region of the nominally perpendicular ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ that instead exhibits $in\ensuremath{-}plane$ anisotropy. Using first-principles energy considerations, we explain this sublayer as a natural consequence of interfacial carrier migration.

We report on the evolution of the average and depth-dependent magnetic order in thin-film samples of biaxially stressed and electron-doped EuTiO3 for samples across a doping range < 0.1 to 7.8 × 1020 cm-3. Under an applied in-plane magnetic field, the G-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated Eu2+ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.

Tensile-strained $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films are ferromagnetic, in sharp contrast to the zero-spin bulk, although no clear consensus has emerged as to the origin of this phenomenon. While magnetism has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, structure, magnetism, and transport are studied in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films (10--22-nm thick) on various substrates (from 1.4% compressive to 2.5% tensile strain), using synchrotron x-ray diffraction, scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect. High quality, smooth films are obtained, exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains. Consistent with prior work, ferromagnetism with an approximately 80--85 K Curie temperature is observed under tension; polarized neutron reflectometry confirms a relatively uniform magnetization depth profile, albeit with interfacial dead layer formation. Electrical transport is found to have similar semiconducting nature to bulk, but with reduced resistivity and activation energy. Hall effect measurements, however, reveal a striking inversion of the majority carrier type, from $p$-type in the bulk and under compression to n-type under tension. While thus far overlooked, ferromagnetism in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films is thus directly correlated with $n$-type behavior, providing important insight into the ferromagnetic state in this system. Aided by density functional theory calculations, these results are interpreted in terms of tensile-strain-induced orbital occupation and band structure changes, including a rapid decrease in effective mass at the ${e}_{g}$-derived conduction band minimum, and corresponding increase at the valence band maximum.

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Strain-coupled composite multiferroics, comprised of magnetostrictive and piezoelectric materials, promise tunable control of magnetism in applications such as rf resonators, sensors, and energy harvesters. Using polarized neutron reflectometry, the authors demonstrate that magnetostrictive and weakly magnetostrictive materials in a multilayer can be strain-coupled to an electric field over large length scales that enable designer functionality across a range of device applications.

We demonstrate emergent antiferromagnetic interactions in strained thin films of the mixed valence manganite $(\mathrm{Eu},\mathrm{Sr}){\mathrm{MnO}}_{3}$. Although the composition studied, ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$, will nominally yield a ferromagnetic phase in the bulk, we observe significant suppression of the saturation magnetization in films under both tensile and compressive strain. Despite the magnetization suppression, muon spin rotation spectrometry and polarized neutron reflectometry reveal uniform magnetic ordering in these films. Neutron diffraction and x-ray absorption spectroscopy demonstrate high-temperature G-type antiferromagnetic order in films under tensile strain, in contrast to the A- and C-type orders that are found in the phase diagrams of bulk $(\mathrm{Eu},\mathrm{Sr}){\mathrm{MnO}}_{3}$ and other mixed-valence manganites. These probes also show that compressive strain results in a uniform but suppressed magnetization suggestive of moment canting resulting from competition between ferromagnetism and antiferromagnetism.

Although ferromagnetism is in general a long-range collective phenomenon, it is possible to induce local spatial variations of magnetic properties in ferromagnetic materials. For example, systematic variation of the exchange coupling strength can be used to create systems that behave as if they are composed of virtually independent segments that exhibit ``local'' Curie temperatures. Such localization of thermodynamic behavior leads to boundaries between strongly and weakly magnetized regions that can be controllably moved within the material with temperature. The utility of this interesting functionality is largely dependent on the inherent spatial resolution of magnetic properties, specifically the distance over which the exchange strength and corresponding properties behave locally. To test the degree to which this type of localization can be realized in materials, we have fabricated epitaxial films of ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}$ alloy featuring a nanometer-scale triangular wavelike concentration depth profile. Continuous nanoscale modulation of the local Curie temperature was observed using polarized neutron reflectometry. These results are consistent with mean-field simulations of spin systems that encompass the possibility of delocalized exchange coupling and show that composition grading can be used to localize magnetic properties in films down to the nanometer level. Since this is demonstrated here for an itinerant metal, we assert that for virtually any modulated magnetic material system, collective effects can be suppressed to length scales smaller than about 3 nm, so that magnetic behavior overall can be well described in terms of local material properties.

Electrostatic control using electrolytes such as ionic liquids has emerged as an exciting means to induce ultrahigh carrier densities on material surfaces, to the point where electronic phases can be tuned. In materials such as oxides, the gating mechanisms remain controversial, however. Here, $i\phantom{\rule{0}{0ex}}n$ $o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}o$ x-ray diffraction and neutron reflectometry are used in combination to probe gating mechanisms in ferromagnetic cobaltite films. The results confirm a striking polarity asymmetry, negative gate voltage enabling primarily electrostatic control of magnetism, while positive bias provides electrochemical control via oxygen vacancy formation. Generally, these results highlight the importance of a small number of key factors in determining electrostatic vs. redox response in a given material.

The magnetic structure of the in-plane skyrmions in epitaxial MnSi/Si(111) thin films is probed in three dimensions by the combination of polarized neutron reflectometry (PNR) and small angle neutron scattering (SANS). We demonstrate that skyrmions exist in a region of the phase diagram above at temperature of 10 K. PNR shows the skyrmions are confined to the middle of the film due to the potential well formed by the surface twists. However, SANS shows that there is considerable disorder within the plane indicating that the magnetic structure is a 2D skyrmion glass.

Preventing stray magnetic interactions is important to improving technology that relies upon manipulating an electron's charge and spin. To this end, the authors investigate a binary intermetallic that has an internal magnetic field, yet does not affect the spin of surrounding components. As-grown epitaxial thin films of Mn${}_{3}$Al on GaAs(001) present the the ideal D0${}_{3}$ Heusler phase. This compound should be quite interesting for optimizing tomorrow's spintronic devices.

We observe an induced switchable magnetic moment of $1.6\ifmmode\pm\else\textpm\fi{}0.40\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$ for the nominally antiferromagnetic $\mathrm{LaFe}{\mathrm{O}}_{3}$ extending two to four interface layers into the non--charge transfer system $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}\text{/}\mathrm{LaFe}{\mathrm{O}}_{3}\text{/}\mathrm{SrTi}{\mathrm{O}}_{3}(111)$. Simultaneously a mismatch of oxygen octahedra rotations at the interface implies an atomic reconstruction of reduced symmetry at the interface, reaching two to five layers into $\mathrm{LaFe}{\mathrm{O}}_{3}$. Density functional theory of a structure with atomic reconstruction and different correlation strength shows a ferrimagnetic state with a net Fe moment at the interface. Together these results suggest that engineered oxygen octahedra rotations, affecting the local symmetry, affect electron correlations and can be used to promote magnetic properties.

We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of $172\phantom{\rule{0.16em}{0ex}}\mathrm{kJ}/{\mathrm{m}}^{3}$ at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.

New mechanisms for achieving direct electric field control of ferromagnetism are highly desirable in the development of functional magnetic interfaces. To that end, we have probed the electric field dependence of the emergent ferromagnetic layer at CaRuO_{3}/CaMnO_{3} interfaces in bilayers fabricated on SrTiO_{3}. Using polarized neutron reflectometry, we are able to detect the ferromagnetic signal arising from a single atomic monolayer of CaMnO_{3}, manifested as a spin asymmetry in the reflectivity. We find that the application of an electric field of 600 kV/m across the bilayer induces a significant increase in this spin asymmetry. Modeling of the reflectivity suggests that this increase corresponds to a transition from canted antiferromagnetism to full ferromagnetic alignment of the Mn^{4+} ions at the interface. This increase from 1 μ_{B} to 2.5-3.0 μ_{B} per Mn is indicative of a strong magnetoelectric coupling effect, and such direct electric field control of the magnetization at an interface has significant potential for spintronic applications.

The magnetism of La0.7Ca0.3MnO3 (LCMO) epitaxial thin films grown on SrTiO3 (STO) and BaTiO3 (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.

We have found ferromagnetism in epitaxially grown superlattices of ${\mathrm{CaRuO}}_{3}/{\mathrm{CaMnO}}_{3}$ that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the ${\mathrm{CaMnO}}_{3}$ layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the ${\mathrm{CaMnO}}_{3}$ thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of ${\mathrm{CaMnO}}_{3}$ at each interface. The interfacial moment alternates between the $1{\ensuremath{\mu}}_{B}/\mathrm{\text{interface}}$ Mn ion for even ${\mathrm{CaMnO}}_{3}$ layers and the $0.5{\ensuremath{\mu}}_{B}/\mathrm{\text{interface}}$ Mn ion for odd ${\mathrm{CaMnO}}_{3}$ layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic ${\mathrm{CaMnO}}_{3}$ layers.

Competing exchange interactions can give rise to varying degrees of frustration that manifest itself in noncollinear magnetic moment ordering or canonical geometric frustration in magnets with large ground-state degeneracies. Relieving this frustration has the potential to stabilize ground states inaccessible in the bulk. We demonstrate that heteroepitaxial lattice distortions can modify the strength of exchange interactions in thin films of the frustrated ferrimagnet, CuCr${}_{2}$O${}_{4}$. The reduction of magnetic frustration in CuCr${}_{2}$O${}_{4}$ through lattice distortions results in greater collinear spin ordering in CuCr${}_{2}$O${}_{4}$ thin films and an enhanced magnetization. We identify heteroepitaxial lattice distortions as a method to tune spin functionality and potentially lift ground-state degeneracies more broadly in frustrated magnets.

In the past year, several groups have observed evidence for long-range spin-triplet supercurrent in Josephson junctions containing ferromagnetic (F) materials. In our work, the spin-triplet pair correlations are created by non-collinear magnetizations between a central Co/Ru/Co "synthetic antiferromagnet" (SAF) and two outer thin F layers. Here we present data showing that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane magnetizing field. This surprising result can be explained if the Co/Ru/Co SAF undergoes a "spin-flop" transition, whereby the two Co layer magnetizations end up perpendicular to the magnetizations of the two thin F layers. Direct experimental evidence for the spin-flop transition comes from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry., 4 pages, 4 figures

Interlayer exchange coupling (IEC) between two ${\text{Ga}}_{0.95}{\text{Mn}}_{0.05}\text{As}$ layers separated by Be-doped GaAs spacers was investigated experimentally using magnetization, magnetotransport and neutron-scattering measurements, which all indicated the presence of robust antiferromagnetic IEC when the GaAs spacer is sufficiently thin. We argue that the observed behavior arises from a competition between the interlayer exchange field and magnetocrystalline anisotropy fields intrinsic to GaMnAs layers.

We report the observation of the giant magnetoresistance effect in semiconductor-based GaMnAs/GaAs:Be multilayers. Clear transitions between low-field-high-resistance and high-field-low-resistance states are observed in selected samples with Be-doped nonmagnetic spacers. These samples also show negative coercive fields in their magnetic hysteresis and antiferromagnetic (AFM) splittings in polarized neutron reflectivity. Our data indicate that the AFM interlayer exchange couplings in this system occur over much longer periods than predicted by current theories, strongly suggesting that the coupling in III-V semiconductor-based magnetic multilayers is significantly longer ranged than in metallic systems. © 2010 The American Physical Society.

A unified approach combining polarized neutron and resonant x-ray magnetic reflectometry has been applied to determine the magnetic structure in an ${[\text{Fe}(35\text{ }\text{\AA{}})/\text{Gd}(50\text{ }\text{\AA{}})]}_{5}$ multilayer as a function of temperature and magnetic field. Simultaneous self-consistent refinement of neutron and x-ray data made it possible to resolve the element-specific magnetization profile in the multilayer with unprecedented accuracy. It is shown that the small number of bilayer periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique low-temperature magnetic phases characterized by significant twisting of Fe and Gd magnetic moments and nonuniform distribution of vectorial magnetization within Gd layers. A twisted magnetic state was found to be stable at small magnetic fields and at a low temperature of 20 K, which is well below the compensation temperature of this artificial ferrimagnetic system.

We measured the spin dependence of polarized neutrons reflected by a GaAs sample as it was optically pumped. This dependence was correlated with the helicity of the circularly polarized light and found to oscillate with neutron wave vector transfer. The data provide definitive evidence that optically induced nuclear polarization in GaAs is not uniform with depth. Quantitative analysis of the data shows that nuclear polarization is suppressed for tens of nanometers near the surface of GaAs.

We have used polarized neutron reflectometry to study the structural and magnetic properties of the individual layers in a series of $\mathrm{Al}\mathrm{Ga}\mathrm{As}:\mathrm{Be}∕{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}∕\mathrm{Ga}\mathrm{As}∕{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ multilayer samples. Structurally, we observe that the samples are virtually identical except for the GaAs spacer thickness (which varies from $3\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}12\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$), and confirm that the spacers contain little or no Mn. Magnetically, we observe that for the sample with the thickest spacer layer, the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layer adjacent to the Be-doped AlGaAs cap has a temperature dependent magnetization very different from that of the other ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layer. However, as the spacer layer thickness is reduced, the temperature dependent magnetizations of the two ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layers become progressively more similar---a trend we find to be independent of the crystallographic direction along which spins are magnetized. These results definitively show that ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layers can couple across a nonmagnetic spacer and that such coupling depends on spacer thickness.

Using polarized neutron reflectometry, we obtained separate depth profiles for pinned and unpinned magnetization across the interface of a ferromagnet/antiferromagnet bilayer as a function of the sign of exchange bias. The pinned and unpinned magnetization depth profiles are nonuniform and extend well beyond the chemical interface, suggesting an interfacial region magnetically distinct from its surroundings. A model that includes pinned and unpinned moments in the ferromagnet and antiferromagnet is developed for a complete description of the data.

We have used complementary neutron and x-ray reflectivity techniques to examine the depth profiles of a series of as-grown and annealed Ga{sub 1-x}Mn{sub x}As thin films. A magnetization gradient is observed for two as-grown films and originates from a nonuniformity of Mn at interstitial sites, and not from local variations in Mn at Ga sites. Furthermore, we see that the depth-dependent magnetization can vary drastically among as-grown Ga{sub 1-x}Mn{sub x}As films despite being deposited under seemingly similar conditions. These results imply that the depth profile of interstitial Mn is dependent not only on annealing, but is also extremely sensitive to initial growth conditions. We observe that annealing improves the magnetization by producing a surface layer that is rich in Mn and O, indicating that the interstitial Mn migrates to the surface. Finally, we expand upon our previous neutron reflectivity study of Ga{sub 1-x}Mn{sub x}As, by showing how the depth profile of the chemical composition at the surface and through the film thickness is directly responsible for the complex magnetization profiles observed in both as-grown and annealed films.

We have studied the depth-dependent magnetic and structural properties of as-grown and optimally annealed ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films using polarized neutron reflectometry. In addition to increasing the total magnetization, the annealing process was observed to produce a significantly more homogeneous distribution of the magnetization. This difference in the films is attributed to the redistribution of Mn at interstitial sites during the annealing process. Also, we have seen evidence of significant magnetization depletion at the surface of both as-grown and annealed films.