2D Heisenberg Ferromagnet Research Articles

Interfacial Magnetic Features of La2CoMnO6/Pt Bilayers Studied by Using Spin Hall Magnetoresistance

AbstractSpin Hall magnetoresistance (SMR) is used to study interfacial magnetic features in La2CoMnO6 (LCMO)/Pt bilayers. LCMO is a ferromagnetic insulator with a Curie temperature Tc = 250 K and a strong magnetic anisotropy that can be tuned by microstructural strain. However, SMR measurements do not show any dependence on the strained state of the LCMO films. In addition, contrary to what may be expected, SMR signal persists above TC exhibiting a strong magnetic field dependence well above the saturation of the magnetization while the typical hysteresis on crossing H = 0 is absent. All these results suggest that the uppermost LCMO interfacial layers are magnetically uncoupled from the rest of the LCMO film and they do not show long range magnetic ordering. This fact is confirmed by a complete scanning transmission electron microscopy study of the interface that makes evident that the last atomic layers of the LCMO film do not contain Co and have a LaMnO3+δ simple perovskite‐like structure with the last atomic layer being lanthanum oxide. The analysis of the experimental results according to recent SMR theories indicates, in fact, that the LCMO interfacial layer behaves as a 2D Heisenberg ferromagnet.

Intrinsic 2D Ferromagnetism in V5Se8 Epitaxial Thin Films.

The discoveries of intrinsic ferromagnetism in atomically thin van der Waals crystals have opened a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. Currently, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating "originally ferromagnetic (FM)" van der Waals crystals, while a bottom-up approach by thin-film growth technique has demonstrated emergent 2D ferromagnetism in a variety of "originally non-FM" van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is "originally antiferromagnetic". Moreover, thickness-dependence measurements reveal that this newly developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

Skyrmion glass in a 2D Heisenberg ferromagnet with quenched disorder

A 2D Heisenberg ferromagnet with exchange J and random magnetic anisotropy of strength has been studied. Analytical theory for the dependence of the average size of a pinned skyrmion on the magnetic field H, and for stability of such skyrmions on a lattice, has been developed. It has been complemented by numerical studies of 2D lattices containing up to 40 million spins. At low fields the average size of the skyrmion, λ, is determined by the average size of Imry–Ma domains. On increasing the field the skyrmions first shrink, with , and then collapse at fields distributed around . The concentration of the skyrmions goes down with the field as .

Interacting spin droplets and magnetic properties of a low-density two-dimensional electron gas

We argue that the magnetic susceptibility data for the low-density two-dimensional (2D) silicon-based electron gas indicate that magnetically active electrons are localized in spin droplets. The droplets exist in both the insulating and metallic phases, and interact ferromagnetically, forming an effective 2D Heisenberg ferromagnet. Comparing the data with analytical and numerical results for a 2D Heisenberg ferromagnet, we determine parameters of the model and show that the value of the droplet spin is either $S=1/2$ or $S=1$ with very few electrons in the droplet. We discuss the dependence of the magnetic susceptibility and the specific heat on the external magnetic field, which follows from the model, and hence we suggest further experimental tests of the model.

Field‐induced magnetic reorientation of the 2D Heisenberg ferromagnet with S = 1: the influence of an exchange anisotropy

AbstractThe two‐dimensional (2D) Heisenberg model with spin S = 1, with the exchange anisotropy plus a second‐order uniaxial single‐site anisotropy and an external transverse magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson‐Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the expectation values 〈Sα〉 (α = x, z) of the spin components from which the orientation angle θ is obtained as a function of the temperature and of the transverse magnetic field for various parameters of the exchange anisotropy in the (x,y)‐plane. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic phase diagram of the nearly 2D Heisenberg ferromagnet (CH 3NH 3) 2CuCl 4 in a transverse magnetic field

The magnetic phase diagram of the nearly 2D ferromagnet (CH 3NH 3) 2CuCl 4 ( T C=8.85 K) in magnetic fields applied perpendicular to the easy axis is obtained. In this system, the ferromagnetic moments lie in planes ( ab planes) formed by the Cu atoms and point along the a crystallographic direction. For field applied along b-axis (intermediate anisotropy axis), a second-order transition to the paramagnetic phase occurs at the field of 80 Oe at the lowest measured temperature ( T=1.8 K). When the field is applied in a direction perpendicular to the Cu planes ( c-axis) the paramagnetic phase is reached only above 1500 Oe after a double transition. The first transition (∼500 Oe) is suggested to be an order–order transition in which a spin reorientation occurs. The obtained in-plane and out-of-plane anisotropy are H A in=80 and H A out=1500 Oe, respectively.

A new class of nodal stationary states in a 2D Heisenberg Ferromagnet

A new class of nodal topological excitations in a 2D Heisenberg model is studied. The solutions correspond to a nodal singular point of the gradient field of the azimuthal angle. An analytical solution is found for the isotropic case. The effect of in-plane exchange anisotropy is studied numerically. This results in solutions which are analogues of the conventional out-of-plane solitons in 2D magnets.

Reorientation of magnetization with temperature in 2D ferromagnets

We investigated 2D Heisenberg ferromagnet (monolayer) with the account of dipolar forces and uniaxial anisotropy and found a reorientation phase transition in temperature from out-of-plane to in-plane phase. This phase transition is of the first order with hysteresis. We estimated the temperatures of switching both analytically and numerically.

Frustration by Multiple Spin Exchange in 2D Solid3HeFilms

Measurements of the magnetization and heat capacity of the second layer of ${}^{3}\mathrm{He}$ films adsorbed on graphite indicate that the evolution of the exchange from antiferromagnetic to ferromagnetic arises from a tuning of the competing exchange processes. At certain coverages the coexistence of an antiferromagnetic heat capacity with a ferromagnetic magnetization is a clear manifestation, predicted by theory, of frustration. At the ferromagnetic anomaly the system is well described by series expansions for a 2D Heisenberg ferromagnet on a triangular lattice.

Frustration by multiple spin exchange in 2D solid3He films

The heat capacity and the magnetization of the second solid layer of3He adsorbed on graphite have been measured to temperatures below 1mK. Prior to the formation of a fluid overlayer the temperature dependence of both sets of data is well described by high temperature series expansions for a 2D Heisenberg antiferromagnet on a triangular lattice. Differing exchange constants from the two quantities provide a measure of frustration due to multiple spin exchange. The subsequent evolution to ferromagnetic exchange suggests that this arises from variations in the coverage dependence of the frustration. After promotion it is found that an antiferromagnetic heat capacity coexists with a ferromagnetic susceptibility. At the ferromagnetic anomaly, the heat capacity is well described by series expansions for a 2D Heisenberg ferromagnet with effective exchange constant close to that determined from the magnetisation, indicating weak frustration.

Magnetization of 3He on Grafoil in the low-temperature limit.

We have measured the magnetization of the ferromagnetic second layer of pure $^{3}\mathrm{He}$ films adsorbed on Grafoil down to 0.075 mK. The zero-temperature magnetization is linear in second layer density (${\mathrm{\ensuremath{\rho}}}_{2}$) and rises by a factor of \ensuremath{\sim}7 with a 6% increase in ${\mathrm{\ensuremath{\rho}}}_{2}$ and less than a factor of 2 increase in the apparent exchange energy, indicating that the ferromagnetic phase coexists with a lower density phase. This is the first experiment to measure the low-temperature limiting magnetization of a 2D Heisenberg ferromagnet. Our data may also support Elser's model of a registered antiferromagnetic phase at lower ${\mathrm{\ensuremath{\rho}}}_{2}$.

Two-dimensional magnetism in Pd (1.2 at. % Fe) films.

Trilayer sandwiches of Pd/Pd(1.2 at. % Fe)/Pd have been prepared by sputtering onto Si(111) substrates. The dc magnetization of a 2 nm thick film has been studied as a function of sample temperature and applied magnetic field. It is found that for 6.5≤T≤15 K the 2 nm film has a uniform susceptibility which increases as exp(53/T) in qualitative agreement with predictions for a 2D Heisenberg ferromagnet and that the field dependence of M(H,T) follows predicted behavior. At 6.5 K there appears either a phase transition or a rather sharp crossover from 2D Heisenberg model behavior to that of another model

Vortex signature in spin dynamics of a quasi two-dimensional ferromagnet

Spin dynamics of a 2D Heisenberg ferromagnet of K 2CuF 4 with easy plane anisotropy have been investigated with high resolution inelastic neutron scattering above T c. In addition to spin waves, a central peak has been measured and interpreted in terms of the dynamics of free spin vortices.

Experimental observation of a two-dimensional Heisenberg nuclear ferromagnet.

We have measured the magnetization of 2.5 atomic layers of $^{3}\mathrm{He}$ adsorbed on graphite to 0.7 mK. The magnetization is well described by the exact high-temperature 2D Heisenberg expansion for $T>J=2.1$ mK, the highest direct nuclear exchange constant ever reported. At lower temperatures, an exponential increase in susceptibility followed by a linear approach to saturation is seen, consistent with present theories for a 2D Heisenberg ferromagnet. The absence of a phase transition is confirmed for temperatures as low as $\frac{J}{3}$.

Magnetic energy in a dilute two-dimensional Heisenberg ferromagnet K2CuxZn1-xF4: linear optical birefringence measurements

The authors report on linear birefringence (LB) measurements in the dilute archetypal 2D Heisenberg ferromagnet K2CuxZn1-xF4 in the whole concentration range 0<x<1. From the temperature dependence of the LB in the range 1.3-300 K, which is separated into two parts (the lattice contribution and the magnetic contribution), they discover information on the influence of the Jahn-Teller effect and on the change in the magnetic dimensionality with dilution. Moreover, they confirm previous specific heat results at low temperatures. Since the magnetic LB is proportional to the magnetic energy, they determine and explain its dependence on dilution.