Tunable ferromagnetic resonance in coupled trilayers with crossed in-plane and perpendicular magnetic anisotropies

Abstract

A novel approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni 80 Fe 20 (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo x film with perpendicular magnetic anisotropy (PMA) through a non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect (TMOKE), alternating gradient magnetometry (AGM) as well as vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both Co concentration and Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic field is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than ±70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.The magnetic properties of thin films and multilayers exhibiting stripe domains have been investigated extensively in both experiment and theory since their discovery more than half a century ago 1 . In recent years, research results on stripe domains have triggered the prospect of employing their unique properties in future microwave, magnonic, and spintronic devices with novel functionalities. The formation of stripe domains is the result of energy minimization as well as the competition between PMA (K ⊥ ) and shape anisotropy ( 1 2 µ 0 M 2 S ), which favor out-of-plane and in-plane magnetization, respectively. The ratio Q = 2K ⊥ /µ 0 M 2 S , known as reduced anisotropy or quality factor 2 , is commonly used to describe the extent of stripe domains. For moderate (Q < 1) to weak (Q 1) PMA, the magnetization tends to lie in the plane, but above a critical film thickness d cr , a ground state with stripe domains emerges. The latter is characterized by a perpendicular magnetization component alternating between up and down within a period λ . The critical thickness d cr is typically in the range of 20 -40 nm for moderate Q value materials such as amorphous NdCo alloys 3,4 , whereas for materials like Py with small values of Q, generally larger values of d cr = 170 -300 nm are found 5-8 . Intimately linked to the presence of stripe domains is the occurrence of a pseudo-uniaxial or rotatable anisotropy 9,10 , which is the result of the in-plane magnetization being aligned along the stripe direction. The latter, however, is not fixed as it can be reoriented by applya) Electronic

Control of Dynamics in Weak PMA Magnets

“…However, NdCo x alloys (due to its rare-earth content) have a very broad resonance which prevents its use as a good material for dynamics. We have recently shown how the growing of hybrid systems can help in surpassing this difficulty [36]. If another material with the appropriate dynamic properties (Permalloy in our case) is grown on top of NdCo x , the stripe domain structure of the latter is, at least partially, imprinted on Permalloy.…”

“…This distribution is visualized in Figure 2 where the three magnetization components of a 65 nm thick NdCo 5 layer in the (0YZ) plane transverse to the stripe direction (0X) are shown. Only one full wavelength of the stripe domains, λ*, is considered at the remanent (MFM measurements reveal a stripes wavelength~140 nm [36]). Figure 2a shows the cartesian axes used through this work and the direction used for the saturating external field.…”

“…Figure 3 summarizes the magnetization state of the hybrid when a 10 nm thick Permalloy is coupled to a 65 nm thick NdCo 5 film through a 2.5 nm thick Al separator. We have selected this Al thickness among the experimental ones used [36] because here, the effects of the NdCo 5 stray field on Permalloy are the biggest ones. Before being plugged to NdCo x, Permalloy is magnetized completely in-plane (due to its IMA).…”

“…We will only focus on the case where H DC is applied along the NdCo x in-plane hard axis: experimentally, the biggest effects have been found for this particular case [36]. Following the procedure reported in the methods section, we keep the magnetic system under linear response by applying a low amplitude h pulse : 1mT.…”

Stripe domains reorientation in ferromagnetic films with perpendicular magnetic anisotropy

“…In the following years, the phenomenon has been found and deepened in different kinds of thin films with moderate PMA, such as FeGa [35][36][37][38], FeN [31,39,40], FeSiB [41,42], CoFeB [43], FeTaN [44], GdFe [45], NdCo [46], FeCoZr [23], La x Sr 1−x MnO 3 [47] and multilayers with moderate perpendicular anisotropy [48][49][50]. Very recently, it has been shown [51] that the formation of stripe domains can be induced in a Py film, even far below the critical thickness, by coupling the Py film to a NdCo one, characterized by a moderate PMA.…”